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United States Patent |
5,000,986
|
Li
|
March 19, 1991
|
Metallized coatings on ceramics for high-temperature uses
Abstract
A method for forming metallized coatings on ceramics for high-temperature
uses above about 630.degree. C. comprising the steps of: preparing a
metallizing composition of mixed ingredients of differing sizes,
proportioning the differing sizes to have nonsegregating qualities when
applied onto the ceramics, coating the metallizing composition on the
ceramics; and heating to form the desired metallized layer.
Inventors:
|
Li; Chou H. (379 Elm Dr., Roslyn, NY 11576)
|
Appl. No.:
|
277672 |
Filed:
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December 14, 1988 |
Intern'l Class: |
B05D 003/02; B23K 031/00; B23K 035/22 |
Field of Search: |
427/62,63,229,383.5,376.7,226,376.6
428/446,432,704,698
228/263.12,122,120,124
|
References Cited
U.S. Patent Documents
3553820 | Jan., 1971 | Sara | 427/431.
|
3981429 | Sep., 1976 | Parker | 228/194.
|
4270691 | Jun., 1981 | Ishii et al. | 228/194.
|
Foreign Patent Documents |
58-181770 | Oct., 1983 | JP.
| |
60-200869 | Oct., 1985 | JP.
| |
60-231471 | Nov., 1985 | JP.
| |
64-788 | Jan., 1989 | JP.
| |
1-167291 | Jun., 1989 | JP.
| |
Other References
Suga, "Current research and future outlook in Japan" in Designing
interfaces for Technological Applications: Ceramic-Ceramic, Ceramic-metal
Joining, Ed by S. D. Peters, (1989) pp. 247-263.
Hashimoto et al "Thermal Expansion Coefficients of high-Tc Superconductors"
Jpn. J. Appl. Phys. 27(2) Feb. 1988 L 214-216.
|
Primary Examiner: Morgenstern; Norman
Assistant Examiner: King; Roy V.
Claims
I claim:
1. A method for coating a ceramic with a strong, adherent, substantially
defect-free, and thermomechanically shock resistant metallized layer, said
layer in its solid form being practically useful at temperatures over
about 630.degree. C., comprising:
selecting a ceramic metallizing composition having a plurality of mixed
powdered metallizing ingredients of differing sizes, said composition when
molten causing reactions between the ingredients and also with the ceramic
to form the metallized layer thereon;
preparing the composition by proportioning the differing sizes of the mixed
ingredients to have gravitationally substantially nonsegregating qualities
when applied onto said ceramic;
coating onto a selected surface of the ceramic a layer of the metallizing
composition,
heating the coated ceramic surface to a temperature at which the
metallizing composition melts to cause said reactions between the
ingredients and with the ceramic thereby achieving ceramic metallization;
and
keeping the composition molten for a sufficiently long time to thereby form
on the ceramic by liquid diffusion the strong, adherent, substantially
defect-free, and thermomechanically shock resistant metallized layer
including a controlled interfacial region of substantial thickness whose
microstructure is substantially free of voids, inclusions, and
microcracks.
2. A method as in claim 1 wherein the metallizing composition is of the
W/Mo-based type; and including providing a metallic shock-absorbing layer
at least five microns in thickness at the interfacial region.
3. A method as in claim 2 wherein said metallic layer is made of an
annealed metal selected from the group consisting of Cu, including thermal
and electrical conductivities since grain boundaries are well-known to
contribute to resistivity.
4. A method as in claim 1 for application with carbon reinforcing fibers
for the manufacture of carbon composites each fiber having a plurality of
strands, and wherein said coating, heating, and keeping steps coat each
strand of every fiber and form flawless, nodular metallized spots along
the length of each fiber at a prespecified periodic distance apart.
5. A method as in claim 1 for metallizing a ceramic wherein said coating
step coats the selected surface of the ceramic with a uniform layer of the
metallizing composition comprising an element selected from the group
consisting of tungsten and molybdenum, said layer in its entirety having a
substantially constant chemical composition; and
said heating and keeping steps keep the coated ceramic heated for at least
five minutes at a temperature at least 50 degrees Centigrade above a
temperature at which the composition melts to thereby form a
metal-containing shock-absorbing liquid-diffused surface layer of graded
chemical composition.
6. A method as in claim 5 wherein said ceramic is a material selected from
the group consisting of diamond and graphite and wherein said heating and
keeping steps are done an atmosphere selected to minimize the loss of
carbon from the ceramic.
7. A method as in claim 5 for metallizing a ceramic in the form of a
graphite fiber and wherein said heating and keeping steps coat the fiber
with a liquid-solidified, pinhole-free and microcrack-free layer of a
metal selected from the group consisting of Cu, Ag, Au, Sn, Zn, Pb, Sb,
Cd, Al, Mg, Ga, In, Th, Bi, Cr, Co, Fe, Mn, Ni, Nb, Pt, Pd, Rh, Ir, Os,
and Ru; and including providing the thus coated surface with an
ambient-resistant, top surface metal layer at least 100 A thick.
8. A method as in claim 5 for use in forming a powder metallurgy product
with powders selected from the group consisting of ceramics, boron,
graphite, diamond, or glass in the range of 0.5 to 200 microns in diameter
and wherein said heating keeping steps provide metallized films of up to
20 microns thick on each powder, and including the additional step of
compacting and sintering the thus surface-metallized powders to
prespecified densities mechanical, and other physical properties.
9. A method as in claim 5 for use in a ceramic fiber-reinforced composite
subjected to cyclic environmental heat-moisture conditions and wherein
said coating step periodically coats all strands of the ceramic fibers
along their lengths with nodular metallized spots at a specific distance
apart to break up the moisture passageways into small compartments between
the nodular metallized spots.
10. A method as in claim 9 wherein said ceramic fibers are in the form of
multi-dimensional weave and including: providing the metallizing
composition in a liquid or paste form; and dipping the multi-dimensional
ceramic fiber weave into the metallizing liquid or paste to preferentially
coat only the intersections of the fibers with the metallizing composition
to thereby form the nodules for stopping deep water penetration; and
controlling the size of the nodular metallized spots by adjusting at least
one parameter of the metallizing liquid or paste composition selected from
the group consisting of viscosity, solid content, and wettability of the
composition.
11. A method as in claim 5 wherein said ceramic is a porous ceramic of
controlled density or porosity and wherein said coating, heating, and
keeping steps coat and mettalize substantially the entire internal surface
of all the pores in the porous ceramic.
12. A method as in claim 1 wherein said selecting step selects a ceramic
metallizing composition of two metallizing ingredients; and including the
additional step of providing one of the ingredients in a solution form
while supplying the other ingredient in powders of a single substance
suspended in said solution thereby ensuring substantially uniform and
reproducible metallizing results.
13. A method as in claim 1 wherein said selecting step selects a ceramic
metallizing composition of at least two metallizing ingredients; and
including the additional step of integrating the at least two ingredients
into a physically inseparable form so that the ingredients will provide a
substantially constant chemical composition throughout the coated layer
thereby ensuring substantially uniform and reproducible metallizing
results.
14. A method as in claim 13 wherein said integrating step consists of
alloying the at least two ingredients into a single substantially
homogeneous alloy form.
15. A method as in claim 13 wherein one of said at least two ingredients is
a solid powder while the other ingredient or ingredients are solid, and
said integrating step consists of coating the surface of said one
ingredient powder with the other ingredient or ingredients to form
physically integrated coated solid powders.
16. A method as in claim 1 wherein said selecting step comprises selecting
the ceramic metallizing composition having two powdered ingredients of
densities and sizes d.sub.1 and D.sub.1, and d.sub.2 and D.sub.2,
respectively, d.sub.1 and D.sub.2 being respectively greater than d.sub.2
and D.sub.1, both powders being suspended in a common suspension medium of
density d.sub.m, and including the additional step of selecting the ratio
of the powder sizes D.sub.1 /D.sub.2 to be at least equal to square root
of (d.sub.1 -d.sub.m)/(d.sub.2 -d.sub.m) final settling velocities of the
two powders in said medium are nearly the same thereby ensuring
substantially uniform and to improve the uniformity and reproducibility of
the metallizing results.
17. A method as in claim 1 wherein said selecting step comprises selecting
the ceramic metallizing composition having a plurality of powdered
ingredients of densities and sizes d.sub.1 and D.sub.1, d.sub.2 and
D.sub.2, . . . , d.sub.i and D.sub.i, . . . , respectively, all powders
being suspended in a common suspension medium of density d.sub.m, and
including the additional step of selecting the powder sizes D.sub.1,
D.sub.2, . . . , D.sub.i, to make the D.sub.i.sup.2 /(d.sub.i -d.sub.m)
substantially constant so that the final settling velocities of all the
powders in said medium are nearly the same thereby ensuring substantially
uniform and reproducible metallizing results.
18. A method as in claim 1 wherein said coating step coats only a single
point on the ceramic while the heating and keeping steps provides a
metallized coating on the ceramic at the single point thereby leaving
substantially the entire back surface of the ceramic uncoated and fully
exposed to the ambient.
19. A method as in claim 18 wherein said ceramic is a gem stone selected
from the group consisting of diamond, sapphire, and quartz.
20. A method of strengthening the surface of a ceramic with a metallized
coated layer, said layer in its solid form being practically useful at a
temperature about 630.degree. C. comprising:
preparing a highly penetrating and wettable ceramic metallizing composition
selected from the group consisting of refractory metals W and Mo which
when molten, is highly penetrating and wettable in the ceramic surface
region particularly relative to the surface defects therein;
coating the composition onto a selected surface of the ceramic;
heating the coated ceramic to a temperature at which the metallizing
composition becomes molten; and
keeping the coated ceramic thus heated for a sufficiently long time so that
the molten metallizing composition becomes highly wettable to thereby
penetrate and seal in the coated ceramic surface region defects at which
fractures normally originate, said defects being selected from the group
consisting of microcracks, pinholes, porosities, and notches.
21. A method as in claim 20 including the additional step of additionally
strengthening the ceramic surface layer at the interfacial region through
formation in the ceramic of microcomposite reinforcement in the form of
precipitated particulates, reinforcing roots, branches, or networks.
22. A method as in claim 20 including the additional step of adding a metal
layer over 100 microns thick to further toughen the ceramic.
23. A method as in claim 20 including the additional step of further
strengthening the ceramic surface layer at the interfacial region through
solution hardening of the ceramic.
24. A method as in claim 20 wherein the penetrating metallizing composition
additionally forms in the ceramic microscopic reinforcement in a matrix of
the ceramic material in the interfacial region.
25. A method in claim 20 wherein said preparing step comprises selecting a
composition which, with the ceramic at the metallizing temperature, forms
a substance which is harder than the ceramic.
26. A method as in claim 25 wherein said selecting step selects the
composition which forms the substance having a Mohr hardness of at least
8.
27. A method as in claim 25 wherein said selecting step selects the
composition which forms the hard substance with the ceramic at the
metallizing temperature.
because of the surface energies of the substance relative to those of the
ceramic grains at the metallizing temperature, said hard substance, being
in liquid form and highly penetrating in the ceramic, forming fibers of
the hard substance located at the intersections of the multiple ceramic
grain boundaries.
28. A method as in claim 25 wherein said selecting step selects the
composition which forms the hard substance with the ceramic at the
metallizing temperature, said substance penetrating substantially into the
ceramic to form a sheet of the hard substance located along the boundary
between two neighboring ceramic grains.
29. A method as in claim 25 wherein said selecting step selects the
composition which forms the hard substance with the ceramic at the
metallizing temperature, said hard substance penetrating substantially
into the ceramic to form branches following the ceramic grain boundaries.
30. A method as in claim 25 wherein said selecting step selects the
composition which forms the hard substance with the ceramic at the
metallizing temperature, said substance alloying substantially into the
ceramic and forming precipitated reinforcing particles inside the ceramic
grains.
31. A method as in claim 25 wherein said selecting step selects the
composition which forms the hard substance with the ceramic at the
metallizing temperature, said hard substance penetrating substantially
into the ceramic to form roots which flow deeply into the grain boundaries
and turn or curve around to form at least a partial network of the hard
substance around a multitude of the ceramic grains.
32. A method as in claim 25 wherein said selecting step comprises selecting
a W/Mo-based metallizing composition which forms the hard substance in the
form of a hard compound of W/Mo, and including:
preparing the metallizing composition at the exact or near stoichiometric
compositions for the W/Mo hard compound;
metallizing at a temperature at least 50.degree. C. above the melting point
of the hard compound; and
varying the metallizing time from five minutes up to allow the
grain-boundary reinforcing W/Mo compound to penetrate to a depth of a
fractional centimeter.
33. A method for coating a high-quality layer of powders of a ceramic onto
a selected substrate, said ceramic being selected from the group
consisting of alumina, zirconia, beryllia, mullite, quartz,
intermetallics, diamond, boron, graphite, carbon, silicon, various other
carbides, nitrides, aluminides, or borides; glasses, machinable glasses,
and oxidized surfaces of reactive metals, comprising:
supplying the substrate onto which the powdered ceramic particles are to be
coated;
providing a layer of a metallizing composition on the substrate, said
composition at a sintering temperature being capable of sintering said
powdered ceramic particles together into a strong ceramic layer and
simultaneously, of coating the sintered ceramic powders onto the
substrate;
providing a layer of the powdered ceramic onto the metallizing-composition
layer;
applying at the substrate surface a first temperature at least about
50.degree. to 150.degree. C. above the melting point of the metallizing
composition; and
applying onto the top surface of the ceramic layer a second temperature
which is above both the first temperature and the sintering temperature of
the ceramic so as to form a temperature gradient across the ceramic layer
and to cause the sintering of the ceramic powders and regrowth of the
ceramic grains;
the first and second temperature being sufficiently high to form a liquid
layer of the metallizing composition, said liquid layer sweeping under the
influence of the temperature gradient across the sintered ceramic
particulate layer from the substrate upward and carrying therewith
undesirable impurities in the ceramic so as to purify the ceramic layer,
to facilitate the sintering, densification, and strengthening of this
ceramic layer, and to orient generally normally of the substrate surface
or along the temperature gradient the sintered and regrown ceramic grains
in columnar forms thereby forming the high-quality coated ceramic layer.
34. A method as in claim 33 wherein said two applying steps sinter, purify,
densify, strengthen, and otherwise improve at least a specified
physicochemical property of the sintered ceramic layer.
35. A method as in claim 34 wherein said improved physicochemical property
is the enhanced electrical conductivity of the resultant ceramic layer
because of the thinned, reduced, purified, and oriented ceramic grain
boundaries.
36. A method as in claim 33 wherein said providing step provides a layer of
a W/Mo-based at selected spots on the substrate.
37. A method as in claim 36 wherein said providing step comprises providing
a layer of a metallizing composition containing MoO.sub.3.
38. A method as in claim 37 wherein said two applying steps comprises
applying at the bottom of the MoO.sub.3 -based layer the first temperature
which is above the melting point of this MoO.sub.3 -based layer while
simultaneously applying on the top of the ceramic powders the second
temperature which is at least 20.degree. C. below the melting point of the
ceramic powders, thereby causing the MoO.sub.3 -based layer to melt and
sweep upward to achieve the desired results.
39. A method as in claim 33 wherein said two applying steps cause most of
the impurities associated with the ceramic powders to be dissolved in the
upward sweeping liquid layer, said liquid layer eventually coming up to
the surface of the ceramic layer to be frozen into a highly impure layer.
40. A method as in claim 39 including the additional step of removing the
highly impure layer swept up on top of the ceramic layer.
41. A method of coating a ceramic with a refractory metal which in solid
form is practically useful over about 630.degree. C. comprising:
selecting a metallizing compound comprising a metallizing substance
selected from the group consisting of refractory metals W and Mo, said
compound at a metallizing or reduction temperature being reducible to the
corresponding metallic substance, said metallic substance having a melting
point above both the reduction or metallizing temperature and the melting
temperature of the metallizing compound;
preparing the metallizing compound composition;
coating the compound of the metallizing substance onto a selected portion
of the ceramic surface;
providing a reducing environment around the coated ceramic surface; and
heating the coated surface to a metallizing temperature above both the
melting and reduction temperatures so that the metallizing compound
becomes molten and is reduced to the metallic substance to thereby form a
refractory metallized layer on the coated surface for practical use at
temperature above the reduction temperature but below the melting point of
the metallizing substance.
42. A method as in claim 41 wherein said selecting step comprises selecting
MoO.sub.3 and said heating step heats to above about 801.degree. C. both
to melt the MoO.sub.3 and to reduce it to Mo so that the metallized
ceramic is thereby useful above 801.degree. C. but below the melting point
of Mo at 2810.degree. C.
43. A method as in claim 41 wherein said selecting step comprises selecting
WO.sub.3 and said heating step heats to above about 1550.degree. C. both
to melt the WO.sub.3 and to reduce it to W so that the metallized ceramic
is thereby useful above the melting point of WO.sub.3 but below the
melting point of W at 3410.degree. C.
44. A method as in claim 41 including laterally cooling the molten
metallizing layer to sweep the impurities therein laterally outward from
the central area of the coated portion.
Description
BACKGROUND
1. Field
This invention relates to ceramic-metal joining, and more particularly
relates to ceramic-metal joining with uniform ceramic metallizing
compositions and specially graded seals to produce reproducibly strong and
thermomechanically shock-resistant and substantially defect-free joints.
By ceramic I mean not only the usual ceramics such as alumina, zirconia,
beryllia, mullite; but also quartz, intermetallics, diamond, boron,
graphite, carbon, silicon, and various other carbides, nitrides,
aluminides, or borides; glasses, machinable glasses, Corning's Vision
glass; and the surface of many metals, particularly reactive metals such
as aluminum, magnesium, chromium, silicon, titanium, or zirconium which
always have oxides or other compounds of reactions of the metal with the
environment.
2. Prior Art
Many problems exist with present ceramic metallizing methods. A serious
problem is the difficulty of achieving uniform metallized layers formed on
the ceramic. Take, for example, the commonly used heavy metal processes,
such as W-yttria (W-Y.sub.2 O.sub.3), W-Fe, or Mo-Mn. In these and many
similar joining methods, segregation of the mixed metal or other powder
takes place due to their differing specific gravities, shapes, sizes,
porosities, and surface smoothness. These segregations occur at all times;
during the mixing of the powders, storing of the power suspensions,
application of the suspensions, settling of the suspended power particles,
and drying of the suspension. Further, these segregations occur so fast as
to be practically uncontrollable, as will be shown shortly.
In general, spherical, heavy, large, smooth, and dense particles settle
first and early in the binder or suspension medium. Upon settling, these
particles tend to roll or move sidewise or downward toward the corners or
boundaries faster and further than odd-shaped, light, small, rough, and
porous particles of otherwise identical characteristics.
Take the W-Y.sub.2 O.sub.3 mixed powders in an organic binder of
nitrocellulose in butyl carbitol acetate with specific gravities of 19.3,
4.5, and 0.98, respectively. Such a suspension, even if perfectly mixed up
by shaking, stirring, roller-milling, or otherwise, will immediately tend
to segregate. More specifically, the initial settling acceleration due to
gravitational minus buoyancy forces on W powders is
980.6.times.(19.3-0.98)/19.3=930.8 cmxcm/sec, while that of Y.sub.2
O.sub.3 powders is only 767.0 cmxcm/sec.
In a mixing, storing, or carrying bottle 10 cm high and containing a
perfectly mixed suspension of these metallizing powders, the time to
completely settle out is only 147 ms (milliseconds) for W powders, if
uniform acceleration is assumed. At the tip of a paint brush having a
suspension drop 0.3 cm in diameter, the complete settling time of these
same W powders is merely 25.4 ms, while on a horizontally painted or
sprayed layer 0.1 cm thick, the same settling time is only 14.7 ms. In all
these cases, the complete settling time for the Y.sub.2 O.sub.3 powders is
always the square root of 930.8/767.0=1.21, or 21% longer.
Note in particular that the powder segregations with uniform accelerations
may be completed within 147 to 14.7 ms. Such short times indicate that the
W-Y.sub.2 O.sub.3 powder segregations are beyond human controls. Painted
or sprayed mixed powder layers are thus always not uniform.
In metallizing onto a horizontal ceramic surface to be metallized, most of
the W powders immediately settles out. The first layers are therefore
always very rich in W, and correspondingly very poor in Y.sub.2 O.sub.3.
These first layers are too refractory for the preset metallizing
temperature (up to about 1550 C) so that the ceramic surfaces are not
sufficiently metallized, or not at all. The last settling layers, on the
other hand, are too rich in the fluxing Y.sub.2 O.sub.3. Again, the
ceramic surfaces are improperly metallized, with only a glassy layer being
formed which is very weak in strength and thermal or thermal shock
resistance.
Thus, common metallizing results on ceramics are often erratic and
uncontrollable. The metallized surface may contain loose and unmetallized
spots with high refractory metal content, as well as non-wettable spots
due to the high flux content. The entire process is critical and involved,
and yet nonuniform. The resultant ceramic-metal joints or ceramic coatings
on metals are weak, costly, nonreproducible, and usually not vacuum-tight,
or temperature-resistant.
Painting or spraying onto vertical or inclined surfaces results in vertical
and additional lateral segregations and gradations, and gives added poor
uniformity, reproducibility, and bonding strength.
While only the effect of gravitational density segregation has been
considered in some detail, the other segregation variables such as powder
shape, size, porosity, and surface roughness are also important.
A second important problem with common joining processes is the lack of
control, or even understanding, or dynamic mismatches of temperatures,
stresses, and strain profiles in the joint region, and their variations
with time. Another aspect of this invention is therefore to describe such
dynamic mismatch phenomena, and to specially tailor-grade the composition
and/or physical property profiles of the joint region so that the maximum
or critical transient mismatch stresses never exceed the local material
strength at any point inside the joint region, at any time during the
heating or cooling of such joints in processing or service.
A third problem results from our poor understanding of the required
microstructural, chemical, and physical properties of the interfacial
regions in the ceramic-metal joints.
Accordingly, an object of this invention is to provide improved
ceramic-metal joints and joining methods;
A further object of this invention is to provide improved ceramic
metallizing methods for these joints;
A broad object of this invention is to minimize gravitational segregations
of the components in the metallizing methods during or prior to the
joining;
Another broad object of the invention is to specially tailor-grade, both in
and normal to the joining plane, the composition and/or property profiles
in the joint regions to ensure that the maximum dynamic or transient
stresses do not exceed the local material strengths at any point and time;
A further object of the invention is to provide a specially microengineered
interfacial region of the optimum characteristics to achieve defect-free,
tough, and very strong joints;
Another object of the invention is to flawlessly coat metals or ceramics
with protective materials;
A yet another object of the invention is to provide substantially
flawlessly coated reinforcements for the manufacture of tough, strong,
thermochemically stable, and thermomechanically shock-resistant
composites;
Further objects and advantages of my invention will appears as the
specification proceeds.
SUMMARY
A method for improving the strength of a ceramic-metal bond comprising:
providing a uniform metallizing composition; and
forming by liquid diffusion between the ceramic and metal a
shock-absorbing, interfacial region whose microstructure is free of voids,
inclusions, microcracks, and excessive dynamic mismatch stresses/strains
and stress gradients.
BRIEF DESCRIPTION
The invention and its further objects and features will be more clearly
understood from the following detailed description taken in conjunction
with the drawings in which:
FIG. 1 shows a system for real-time monitoring of mixed settling powders;
FIG. 2 shows nodular bonding spots on reinforcing carbon fibers in carbon
composites;
FIG. 3 shows a multi-purpose bonding method for high temperature ceramic
superconductors;
FIG. 4 shows newly microengineered microstructures of the bonding
interfacial regions; and
FIG. 5 shows a bonding method for mounting diamond or other gem stones.
DETAILED DESCRIPTION
It will be understood that the specific embodiments described herein are
merely illustrative of the general principles of the invention and that
various modifications are feasible without departing from the spirit and
scope of the invention. That is, the invention is of general applicability
for improving the quality of the ceramic-metal joints or joining methods,
or coatings of ceramics on ceramics, or on metals. It is also evident that
materials, structures, and methods other than those especially described
can be used to practice the invention.
Stokes in 1851 first considered the resistance R which a fluid medium of
density d.sub.m and V viscosity n offers to the movement of a spherical
particle of velocity V diameter D and density d.sub.p suspended in it, and
arrived at the equation R=3.pi.Dvn.
The small sphere settling in the fluid (i.e., gaseous or liquid) suspension
medium is acted on by the force of gravity with gravitational constant g,
.pi.D.sup.3 d.sub.p g/6 acting downward; and by the buoyant force of the
fluid, .pi.D.sup.3 d.sub.m g/6, given by Archimedes' principle and acting
upward. The resultant net gravitational force G is .pi.D.sup.3 (d.sub.p
-d.sub.m)g/6 acting downward, producing a downward acceleration, a.
When the resistance R exactly equals this net gravitational force G, the
acceleration reduces to zero; the final velocity, v.sub.f, becomes
constant. There than results:
3.pi.Dnv.sub.f =.pi.D.sup.3 (d.sub.p -d.sub.m)g/6
Hence, the final velocity is: v.sub.f =(d.sub.p -d.sub.m) g D.sub.2 /18 n,
the equation of Stokes' law which has been shown to be widely valid.
For a given fluid density (d.sub.m) at a specific temperature (viscosity n)
and a given sphere (of density d.sub.p and mass M), the Stokes' equation
gives a velocity constant:
v.sub.c =v.sub.f /D.sup.2 =(d.sub.p -d.sub.m)g/18n
Also, the velocity at any time starting from rest, t, is:
v=(1-exp(-Rr/M)).times.v.sub.f ;
while the settling distance at time t is:
s.sub.t =(t-(1-exp(-Rt/M)).times.M/R).times.G/R
The velocity equation shows that the exact v.sub.f is not reached until
after infinitely long time when the exponential term in the equation turns
to zero and then the velocity reduces to v=v.sub.f, as it should.
With the Stokes' law, one can calculate the velocity constants, v.sub.c in
1/cm-sec, for the settling in water at 20 C (d.sub.m =1.0 and n=0.010) of
various metal or oxide powders, with densities in g/cc in parentheses, as
follows: W (19.35) 100,000, Y.sub.2 O.sub.3 (5.01) 21,900, Fe (7.87)
37,400, Mo (10.2) 50,100, Mn (7.2) 33,800, WO.sub.3 (7.16) 33,600,
Fe.sub.2 O.sub.3 (5.24) 23,100, MoO.sub.3 (4.692) 20,100, and MnO.sub.2
(5.026) 21,900.
Thus, in the W-Y.sub.2 O.sub.3 metallizing process, because the W powders
are 3.9 (19.35/5.01) times heavier than Y.sub.2 O.sub.3, the velocity
constants v.sub.c 's of the two components differ by a factor of
100,000/21,900=4.6 times. That is, for a given powder size D, the final
constant settling velocity v.sub.f of W spheres is 4.6 times greater than
that of Y.sub.2 O.sub.3 spheres. As discussed above, this wide difference
in velocities results in severe gravitational segregation and early
depletion of W particles in the settling mixtures and, therefore, poor
metallizing results.
It can also be seen that the powders in the mixed oxide processes, e.g.,
WO.sub.3 -Fe.sub.2 O.sub.3, are much more uniform, or less varying, in
densities, d.sub.p, than mixed particles of the same metals, e.g., W-Fe.
This, the WO.sub.3 -Fe.sub.2 O.sub.3 process shows density and velocity
constant ratios of 1.366 and 1.455, vs 2.459 and 2.674, respectively, for
the W-Fe process.
Similarly, in the Mo-Mn process, replacing the metal powders by their
respective oxides reduces the differences in the ratios of velocity
constants, v.sub.c, and final velocities, v.sub.f, from 48.2% to only 9.0%
and 19.2% to 4.2%, respectively. In addition, the metal particles, i.e.,
W, Fe, Mo, and Mn when reduced during metallizing from their respective
oxides are smaller than the initial oxide powders. These smaller sizes
further promote homogenizations and metallizing results.
Hence, if we select and mix the WO.sub.3 and Fe.sub.2 O.sub.3 spherical
powders in the size (diameter D) ratio of the square root of
(33,600/23,100=1.455) i.e., 1.206, the final settling velocities of both
these size-ratioed powders will be exactly the same. That is, by simply
making the Fe.sub.2 O.sub.3 powders 20.6% larger than the WO.sub.3
powders, the mixed particles will finally settle in water at 20 C at
exactly the same velocity. This condition leads to metallizing uniformity
due to the uniform composition of the finally deposited layers.
The final settling velocities of the two mixed powders, v.sub.s 's,
however, come only after some settling time, t.sub.s, when a specific
amount, Q, of the mixed powders has already settled out at differing
velocities. From this settling time, t.sub.s, for the specific combination
of component powders, the settled amount Q and material use efficiency can
be computed from the materials remaining after the settling time, t.sub.s.
The materials already settled before t.sub.s is the presettled distances,
s.sub.t, multiplied by the initial material densities. But the already
settled materials are not lost, since they can be recirculated and reused
in subsequent metallizing runs.
In this way, gravitational segregations between, for example, cosettling W
and Fe, Mo and Mn, WO.sub.3 and Fe.sub.2 O.sub.3, or MoO.sub.3 and
MnO.sub.2 powders, are minimized. Naturally, the smaller the percentage of
velocity or useful powder size differences, .DELTA.v and .DELTA.D,
respectively, the lower the material use efficiency on a particular
mixed-powder combination. An engineering compromise must, therefore, be
struck.
It can be seen that the fluid suspension medium may be either a gaseous or
liquid medium. The liquid may be water, alcohol, other inorganic or
organic liquids of fairly constant viscosity at room temperature. A
varying viscosity liquid may also be used, for example, a polymerizing
organic substance containing a polymer and a hardener, a nitrocellulose in
an evaporating solvent such as butyl carbitol acetate, or Duco cement
diluted with rapidly evaporating acetone, to achieve rapidly increasing
viscosity, n. The velocity constant of the settling powders is, as shown
above, inversely proportional to this viscosity. In all cases, the
starting time for achieving nearly equal settling velocities is shortened
by the increasing viscosity due to polymerization or solvent evaporation.
With increasing viscosities, the absolute difference is centimeters per
second between the settling velocities of the two mixed powders of
differing densities then become less, and nearly equal settling conditions
are more easily achieved. The real-time monitoring system to be described
in FIG. 1 is also useful, but the nearly equally settling mixed powders
must be quickly used before much further polymerization or evaporation
takes place.
Apparently, the above technique for minimizing gravitational segregation
through minimized settling differences can be used to handle more than two
types of powders of differing densities.
In practice, we specify that the two settling velocities of the mixed
particles are within a certain prespecified percentage, e.g., 20 or 10%,
of each other. Still, gravitational segregations are minimized.
By repeated iteration or computer simulation, the best mixed-powder
metallizing process for optimal combined metallizing uniformity and
material use efficiency can be systematically determined. Based on these
principles, method and equipment can be developed for controlling the
turn-on time for starting to deposit the mixed powder at nearly equal
final settling velocity, v.sub.f, into metallizing layers with the
size-ratioed powders.
FIG. 1 shows that a system for real-time monitoring of the settling
particles is employed to determine the starting time for collecting the
residual or still unsettled mixed particles to be used for metallization.
This system has a vertical settling cylinder 10. Near the bottom of the
cylinder 10, two pairs of light emitters 11 and detectors 12 are located
at two different heights with emitters on one side and detectors on the
opposite side of a vertical cylinder 10, to sense the settling particles.
The times for the particles to pass the top or bottom emitter/detector
pair determine the particle size or type being monitored, while the times
for the particles to transverse through the vertical distance d between
the heights give their velocities. When the settling velocities of the two
types (and sizes) of the powders are within a specified percentages, a
slide shuttle 14 is moved to catch on the shuttle the residual or
unsettled mixed powder of nearly equal settling velocities. These
equal-settling mixed powders in suspension are separated for immediate
metallizing use while the already settled powders are drained through the
valve 15 for subsequent reuse.
Useful metallizing compositions include the commonly used W:Fe or Mo:Mn
system containing 10 to 30 weight percent of Fe or Mn, or their
derivatives WO.sub.3 :Fe.sub.2 O.sub.3, MoO.sub.3 :MnO.sub.2, or other
non-oxide systems. From the atomic or molecular weights of the elements W,
Mo, Fe, Mn, O, Cl, F, I, Br, . . . or radicals NO.sub.3, SO.sub.4, . . . ,
the weight percentage of the heavy metal W or Mo and the other braze and
melting temperature-lowering metals such as Cu, Zn, Pb, Sn, Bi, Fe, Mn,
Ag, Au, In, . . . used for the past, suspension, or solution metallizing
compositions can be readily determined. Generally, I maintain the same
ratio of 10 to 40 weight percent of braze metal to the 90 to 60 percent of
heavy metal in these compositions.
There are other ways to insure a substantially constant chemical
composition consisting of at least two types of metallizing materials
having different densities and carried in a fluid suspension medium. One
way is to cause the two types of materials to come out of the suspension
medium in a substantially constant chemical composition thereby ensuring
uniformity and reproducibility of the metallizing results. For example,
the two types of materials may be integrated into physically integral and
inseparable forms, such as by alloying the materials into integrated alloy
form, or coating the internal and/or exterior surface of one type of
material particles with the other material to form integrated coated
powders.
Thus, tungsten particles may be alloyed or coated with iron to form
integral or inseparable W-Fe powders. Similarly molybdenum powders may be
alloyed or coated with manganese to form integral Mo-Mn powders that will
not segregate.
Another method to minimize segregation of a single fluxing (e.g., MgO,
Y.sub.2 O.sub.3) or brazing (e.g., Cu, CuO, Zn, ZnO), cometallizing (e.g.,
Mn or MnO2 with Mo or Fe or Fe.sub.2 O.sub.3 with W) material is the use
of an aqueous or other solution of W and/or Mo compounds such as sodium
molybdate or tungstate which is soluble in water, or MoO.sub.3 or WO.sub.3
which is soluble is hot water particularly in the presence of NH.sub.4 OH.
Here, the solution is the settling medium itself and suspended powders
being of a single type cannot segregate. Solutions of compounds of Cu, Zn,
Fe, Mo, . . . used with powders of W, Mo, Wo.sub.3, or MoO.sub.3 achieve
the same results.
to completely eliminate gravitational segregations, solution metallizing is
the ideal process. Many molybdenum and tungsten compounds are soluble in
water, alcohol, acid, or bases. MoO.sub.3, for example, is soluble in hot
or ammoniated water. Oxide, chloride, nitrate, sulfate, halogen, and other
compounds of iron, manganese, nickel, antimony, lead, tin, copper, zinc,
and bismuth are similarly soluble. Mixtures of W/Mo and the other
solutions may be compounded into proper compositions for the metallization
of various ceramics. The use of solutions of compounds, e.g., halides, of
nickel, lead, tin, zinc, and copper allow these metal compounds to be
reduced in a hydrogen or nitrogen/hydrogen atmosphere to supply the braxe
metal. In a single processing step, then, complete metallizing, brazing,
and bonding is possible.
One difficulty of metallizing MACOR, Corning Glass's machinable glass
ceramic, by the solution method is the relatively low, allowable
metallizing temperature of about 950.degree. C. The solubilities of the
metallizing compounds are also restricting factors. Still, many potential
metallizing compounds are soluble or at least partly soluble. Zinc
chloride and sodium molybdate, for example, are soluble up to 432 and 65
grams, respectively, per 100 cc of cold water. Such a composite solution
may be filtered to remove solid particles and used for metallizing various
ceramics.
Useful W/Mo-based metallizing compounds include: X(X=W or Mo), XO.sub.3,
Na.sub.2 XO.sub.4, K.sub.2 XO.sub.4, Li.sub.2XO 4, and XH (H=F.sub.2,
Br.sub.2, Cl.sub.2, and I.sub.2). Useful braze metal compounds include:
many YNO.sub.3, YH (Y=Cu, Ag, Au, Zn, In, Fe, Ni, Mn, Ga, Sn, Pb, Cd, Tl,
. . . , and H=F, Br, Cl, and I). Many of these compounds are soluble in
water, alcohol, or solvents and can, therefore, be used to prepare
metallizing solutions. Knowing the elemental atomic weights, one can
readily compute the weight of metallizing w or Mo or braze metal in each
gram of these chemical compounds.
Another important consideration in making joints between dissimilar
materials relates to thermal mismatch stresses and strains. In any
ceramic-metal joints, or for that matter, any joining of two dissimilar
materials, the matching or mismatch of their thermomechanical
characteristics in general, and thermal expansion coefficients in
particular, is extremely important. From this mismatch of their thermal
expansions, thermal stresses are generated.
Mismatches in other thermomechanical characteristics also result in other
thermomechanical mismatch stresses and strains. The magnitude of these
mismatch stresses and strains determines the failure probability of the
joint.
generally, the thermal expansion mismatch differentials of within 100 ppm
(parts per million) are considered as allowable, according to Hagy and
Ritland's paper on "Viscosity Flow in Glass-to-Metal Seals," J. Amer.
Ceram. Soc., vol. 40, pp. 58-62, 1957. Such thermal expansion coefficients
and differentials relate only to the static or equilibrium case, and may
not truly represent dynamic or transient conditions when the joint is
being heated up or cooled down. Yet such transient conditions often exist
during the services of the joint.
Unlike the commonly used static thermal expansion mismatch, the dynamic
mismatch in thermal expansion coefficients is not constant but varies with
the bonded materials shapes and sizes, physical and surface properties,
and heating or cooling conditions and times.
As can be shown, the dynamic expansion strain mismatch may exceed the yield
point of the ceramic materials, while the dynamic mismatch stress often
exceeds the flexure or even comprehensive strengths of these same
materials. What fails most ceramic-metal joints, or cause most coatings to
crack, peel, flake, or spall, is thus the dynamic, rather than the static,
thermal expansion mismatch.
Using this dynamic mismatch technique, we have been able to determine the
location, magnitude, and occurrence time of the maximum or critical
mismatch stresses, and take measures to reduce the dynamic mismatch
stresses on the relatively weaker ceramic so that the ceramic is no longer
failing from the high stresses.
Dynamic mismatches result partly from the fact that metals and ceramics
have widely different thermal conductivities. The conductivities for
metals range from 0.014 cal/sq. cm/cm/degree C/sec for tellurium, to 1.0
for silver (same unit), while those for ceramics are from 0.0018 for glass
to 1.8 for beryllia.
During heating of a ceramic-metal joint, the ceramic temperature lags
behind that of the metal, often markedly so; while under cooling the
opposite is true. This produces different temperature profiles in the
metal and ceramic at a particular time instant on either heating or
cooling. Dynamic mismatches in temperatures, strains (i.e., expansions on
heating or shrinkages on cooling), and stresses (strains multiplied by
Young's moduli) then result.
Take the special example of the case of a long ceramic rod joined
end-to-end to a similarly sized metal rod. The ceramic may be, for
example, Corning Glass's machinable glass ceramic (MACOR), while the metal
may be SAE 1010 carbon steel. The joint is brazed at 950.degree. C. and
is, for the worst-case condition, suddenly air quenched in a
room-temperature (20.degree. C.) ambient.
The Fourier equation for independent radial heat conduction in long ceramic
and metal cylinders is well known. The solution of the cylindrical heat
conduction problem consists of an infinite series, each term of which is a
product of a Bessel's function and an exponential function, as given in
various textbooks on heat conduction. Data tables and master charts for
cylindrical heat diffusion have been compiled. See, e.g., 1961 Gebhart's
"Heat Transfer," McGraw-Hill, New York). With these equations, data
tables, and master charts, one can determine the temperature profiles at
different locations (e.e., radial positions, r, in a cylindrical
end-to-end joint) at various time instants. From these profiles the
critical temperature profile with the associated, maximum transient
mismatch stresses and strains can be calculated.
The cooling down of a MACOR-metal joint from the brazing to room
temperatures represents one of the most severe thermal changes, because of
the wide temperature range involved. The step-by-step temperature changes,
i.e., u.sub.m and u.sub.s for the temperatures of MACOR and steel,
respectively, at cooling time t in seconds, at the center, (r=0) of the
interfacial regions of a two-inch diameter, rod-type MACOR-steel joint are
given in Table 1. Other tables have also been prepared for rods of
different diameters.
The data used in the computations for Table 1 are: rod diameter D=2 in=5.08
cm, rod radius r=2.54 cm, surface heat transfer coefficient=0.1 per inch
for both steel and MACOR, thermal diffusivities=0.108 cm.sub.2 /sec for
steel and 0.0054 for MACOR, initial temperature of both MACOR and
steel=950.degree. C., and room temperature=20.degree. C.
The computed data in Table 1 show, for the particular case treated, the
maximum temperature differential between MACOR and steel at the axial
center point, (or r=0), i.e., .DELTA.u=u.sub.m -u.sub.s, at different
cooling times t in seconds. Thus immediately upon cooling after brazing
(t=0), this differential is zero because both the MACOR are at the same
brazing temperature of 950.degree. C.
TABLE 1
______________________________________
Nonsteady Heat Transfer Computations
For a 2-inch MACOR-Steel Joint
Cooling from 950.degree. C. to 20.degree. C.
t u.sub.m u.sub.s
u.sub.m - u.sub.s
______________________________________
0.0 950 950 0
6.0 950 947 3
12.0 949 935 14
23.9 949 901 48
35.8 949 867 82
47.8 948 835 113
59.8 948 804 144
89.6 948 731 217
119 947 665 282
239 935 456 478
358 918 316 703
478 901 220 681
598 884 155 729
717 868 112 756
836 851 82 769
956 835 62 773
1200 804 39 765
1792 731 23 708
2390 665 22 643
3580 551 22 528
4780 456 21 436
5980 379 21 358
7170 316 21 296
9560 220 21 199
12000 155 21 134
14300 112 21 91
19100 62 20 42
23900 39 20 19
29900 27 20 7
35800 23 20 3
41800 21 20 1
______________________________________
Subsequently, faster cooling of the steel rod relative to MACOR causes this
differential to increase with time t, until both rods are significantly
cooled when the temperature differential decreases. After 29,900 seconds
(8.3 hours), for example, both rods are within a few degrees of the rooms
temperature at 20.degree. C. The maximum temperature differential reaches
775.degree. C. at about 1,000 seconds after the air cooling, giving rise
to the maximum or critical dynamic mismatch stress and strain. Table 1
also shows that the temperature differential T=u.sub.m -u.sub.s reaches
113.degree., 144.degree., 217.degree., 282.degree., 478.degree., and
703.degree. C. at 47.8, 59.8, 89.6, 119, 239, and 358 seconds,
respectively, after the same cooling from 950.degree. C.
By comparison, the maximum temperature differential of 727.degree. C. at
the axial center point of a one-inch (or r=1.27 cm) diameter MACOR-steel
joint is reached sooner, at about 440 seconds after cooling.
The linear thermal expansion coefficients, f, are defined as the thermal
expansion per unit length per unit degree Centigrade. As given in the
literature, they refer only to the static case. These coefficients are
constants, at least for the respective temperature ranges. Within these
ranges, they are, therefore, independent of the specimen temperature,
cooling or heating rates. In addition, these coefficients do not depend on
the specimen geometries, sizes, diffusivities, surface characteristics,
heating or cooling conditions, and initial and final temperatures. Each
material thus has a singular, unique static expansion coefficient, at
least for a given temperature range.
The static thermal shrinkage (or negative expansion) strain, e, for a given
material is, by definition, the static thermal expansion coefficient, f,
multiplied by the temperature range of cooling, u, i.e.,
e=f.times..DELTA.u. Thus, for the steel rod, this strain is: e.sub.s
=f.sub.s .times..DELTA.u.sub.s, and for the MACOR rod, it is: e.sub.m
.times..DELTA.u.sub.m.
Under equilibrium conditions, the materials of the joint, i.e., MACOR and
steel, are supposed to be in constant thermal equilibrium. That is,
U.sub.m =u.sub.s. Both materials are thus at the same brazing temperature
of u.sub.0 at the beginning of cooling (t=0). Also, at any time t during
the cooling, the cooling temperature range for MACOR and steel are always
the same in the static case. Thus:
.DELTA.u.sub.m =u.sub.0 -u.sub.m =u.sub.0 -u.sub.s =.DELTA.u.sub.s
=.DELTA.u,
and the static expansion mismatch strain between steel and MACOR is:
.DELTA.e=e.sub.s -e.sub.m =(f.sub.s
-f.sub.m).times..DELTA.u=constant.times..DELTA.u.
On the other hand, dynamic thermal expansion coefficients, f*, and the
resultant dynamic mismatch strains, e*, and stresses, s*, strongly depend
on the joint materials, geometries, sizes, physical and surface
properties, and heating or cooling conditions.
Starting with zero strain on cooling from the brazing temperature of
950.degree. C., the dynamic strain in the steel rod is: e*.sub.s =f.sub.s
.times..DELTA.u.sub.s where .DELTA.u.sub.s =950-u.sub.s, while that in the
MACOR rod is:
e*.sub.m =f.sub.m .times..DELTA.u.sub.m where .DELTA.u.sub.m 32 u.sub.m.
The difference in dynamic mismatch strain, i.e.,
.DELTA.e*=f.sub.x s .DELTA.u.sub.s -f.sub.m x.DELTA.u.sub.m
The dynamic mismatch strain reaches a maximum of about 0.0123 at t=1,000
seconds. Such high strains exceed even the yield point of steel joined to
the rigid MACOR ceramic.
The dynamic thermal expansion coefficient mismatch, .DELTA.f*, can be
computed by dividing the dynamic mismatch strain, e*.sub.s -e*.sub.m, by
the average cooling temperature range, i.e., .DELTA.u.sub.v =950-(u.sub.s
+u.sub.m)/2. This dynamic coefficient mismatch, for the 2-inch MACOR-steel
rod joint cooling from 950.degree. C. to 20.degree. C., still depends
greatly on the cooling time t. It reaches a maximum rate of about 29.6
ppm/degree C. at a cooling time of about 90 seconds, but continuously
drops down to less than 5.6 pp/degree C. at t=1,000 seconds, as can be
computed from the data in Table 1. The total dynamic coefficient mismatch
over the temperature range of 930.degree. C. far exceeds the maximum of
100 ppm considered allowable by Hagy and Ritland.
In can also be shown that the dynamic expansion coefficient mismatch,
.DELTA.f*=(f*.sub.s -f*.sub.m)av, for the 2-inch MACOR-steel rod joint
cooling from 950.degree. C. to 20.degree. C., is two to five times greater
than the corresponding mismatches for the static or equilibrium case, for
cooling time t of 10 to 10,000 seconds.
Statically, MACOR only marginally "matches" with a few low-expansion metals
such as Sylvania #4, Dumet, 50% nickel alloys, chrome-iron stainless,
platinum, Sealmet, and titanium, according to Corning Glass. Because of
this two to five times greater dynamic expansion coefficient mismatch
relative to the static coefficient mismatch, we must conclude that,
dynamically, MACOR and steel joints now become totally "mismatched", at
least in so far as the specimen configuration, size, and brazing
conditions given above are concerned.
To approximately compute the dynamic mismatch stresses, one may further
neglect the presence of the braze and the metallized layers, and use a
Timoshenko approach as follows. Consider a portion of the steel specimen
of unit length and unit cross-sectional area, brazed together with a MACOR
specimen of equal length and cross-sectional area. At time t=t after
cooling from the brazing temperature of 950.degree. C., the temperature of
the steel is u.sub.s and .DELTA.u.sub.s =950-u.sub.s, while the
temperature of MACOR is u.sub.m and .DELTA.u.sub.m =950-u.sub.m. The steel
specimen has thus shrunk from unit length to 1-f.sub.s
.times..DELTA.u.sub.s, while the MACOR has shrunk to 1-f.sub.m
.times..DELTA.u.sub.m. The steel has shrunk more than MACOR, since both
f.sub.s and .DELTA.u.sub.s are greater than f.sub.m and u.sub.m. To
maintain joint integrity particularly at the ends the originally
stress-free but overshrunk steel must be stretched with dynamic tensile
stress s.sub.s * by the adjoining MACOR, to length y from length 1-f.sub.s
.times..DELTA.u.sub.s, while the undershrunk MACOR must be compressed with
dynamic compressive stress s.sub.m * by the steel, to the same length y
from length of 1-f.sub.m .times..DELTA.u.sub.m.
Hence, the tensile stress in the steel, s.sub.s *, is
s.sub.s =E.sub.s .times.(y-1+f.sub.s .times..DELTA.u.sub.s)/(1-f.sub.s
.times..DELTA.u.sub.s)
where e.sub.s is the Yound's modulus of steel, i.e., 30,000,000 psi;
while the compressive stress in MACOR, s.sub.m, is
s.sub.m =E.sub.m (1f.sub.m .times..DELTA.u.sub.m -y)/(1-f.sub.m
.times..DELTA.u.sub.m)
where E.sub.m is the Young's modulus of MACOR, i.e., 5,000,000 psi.
Apparently, s.sub.s *=s.sub.m. Hence,
y=((1-f.sub.m .times..DELTA.u.sub.m)E.sub.m +(1-f.sub.s
.times..DELTA.u.sub.s)/(E.sub.s +E.sub.m)
From these equations, we compute the equal dynamic mismatch stresses in
MACOR and Steel, s.sub.m *=s.sub.m, to reach over 52,800 psi, well above
MACOR's flexural strength of 15,000 psi or even its comprehensive strength
of 50,000 psi.
Similarly, dynamic or transient differences in temperatures, thermal
expansion coefficients, thermal expansion strains, and thermal mismatch
stresses have been computed for differently sized cylindrical MACOR-steel
joints, at various radial locations and cooling time instants. The dynamic
mismatch stresses and strains are all unexpectedly high. Measures must
therefore be taken to reduce the dynamic mismatch stresses on the
relatively weak ceramic so that the ceramic is no longer subjected to the
high stresses. This reduction can be achieved by, e.g., absorbing a major
portion of the dynamic mismatch stresses normally present in the ceramic
through the use of a soft, yieldable metallic braze. These measures
prevent the braxed joint failures particularly from these dynamic mismatch
stresses, because residual or actual mismatch stress between the two
joined materials is the theoretical mismatch stress with a portion thereof
absorbed in the metallized or brazed layer.
Specifically, this invention also describes the following methods, for uses
singly or in combination, to minimize or neutralize these high mismatch
stresses and strains:
(1) Using a soft, yieldable metal layer to braze the metallized ceramic to
the metal, and to absorb within the braze layer a large or major portion
of these mismatch stresses so that the relatively weak MACOR or other
ceramic is no longer subjected to high stresses thereby preventing
fractures;
(2) Radially grading, or controllably and gradually changing in (i.e.,
parallel rather than perpendicular to) the joining plane or bonding
interfacial region, the thermal conductivity conductivity (or reciprocal
of thermal resistivity), expansion coefficient, and tensile strength of
the braze metal, to ensure that the maximum residual mismatch stress,
after absorption in the braze or the shock-absorbing interfacial region to
be described below, will not exceed the local material strength in the
ceramic at any point and time;
(3) Axially grading, or controllably changing normally of or perpendicular
to the joining plane or bonding interfacial region, from the ceramic side
toward the metal side, the thermal expansion coefficient of the braxe
layer to minimize direct mechanical interaction between the steel and
ceramic members;
(4) A toughened and strengthened microengineered interfacial region between
the ceramic and metallized layer to absorb thermomechanical shocks; and
(5) A new method to achieve flawless bonding regions.
The first two objectives are achieved by providing a novel composite
metallic braze layer of disc 10. This composite metallic disc joins
together a ceramic body 14 and a metal body 15, as shown in FIGS. 1a and
1b. This disc, lying parallel to and forming part of the bonding
interfacial region, has a central copper core 11 inside an outer copper
alloy ring or washer 12 made of, e.g., 70:30 Catridge brass. The linear
thermal expansion coefficient of pure copper is 16.5 ppm.degree C, while
that of, for example, 70 Cu:30 Zn Catridge brass is 19.9 ppm/degree C.
Also, the tensile strength of the brazing-annealed, soft pure copper is
only 15,000 psi, while that of the 70:30 Catridge brass is over 40,000
psi, or about three times greater.
Hence, the tensile strength and thermal expansion coefficient of the
peripheral region in my composite brake disc is 2.67 times and 1.21 times,
respectively, those of the central pure copper core. The thermal
conductivity of pure copper at 0 degrees C. is 0.920, while those of 11%
and 32% Zn:Cu are 0.275 and 0.260, respectively. Hence, the thermal
conductivity of 30% Zn:Cu Catridge brass is about 0.261. That is, the
thermal conductivity of the peripheral catridge brass in our composite
braze disc is only, 0.261/0.920=28.4%, or much less than 50% or 70% of,
that of the central pure copper core.
These combinations of linear thermal expansion and tensile properties
achieve the required results. In a ceramic-steel joint, the maximum or
critical transient mismatch temperatures, dynamic expansion mismatch, and
thermal strains and stresses occur in the axial centers of the interfacial
regions. I therefore have dead soft, brazing-annealed, pure copper at the
core regions. This copper, with a yield strength less the fracture
strength of the ceramic, is highly and readily yieldable to absorb much or
most of the dynamic mismatch thermal strains and, therefor, stresses. Pure
copper also has relatively low thermal expansion to reduce these mismatch
effects in the first place. In addition, the pure copper is a good thermal
conductor, equalizing the temperature between the centers, as well as
their outer and regions, of the steel-MACOR joint, and thus further
minimizes mismatch strains and stresses.
On the other hand, the outer peripheral regions of the braze disc is made
of relatively highly expansive but the low thermal-conducting brass. At
these peripheral regions, the mismatch temperature differentials are
relatively smaller. The higher tensile strength is even desirable at the
large-area peripheral regions to enhance the joint strength.
This composite braze disc design will thus provide the required radially
tailor-graded profiles of braze composition, thermal expansion
coefficient, braze strength, and thermal conductivity needed to overcome
or minimize the critical dynamic mismatch stresses.
The composite braze discs can be made by, for example, metallurgically
cladding or mechanical press-forming a sphere or disc inside a washer, at
least two concentric tubes, or other combinations of different materials.
Elemental interdiffusion during the braze manufacture, brazing operation,
or special pre- or post-brazing heat-treatment can modify or provide any
reasonable composition profiling in the braze discs for achieving the
desired tailor-grading results.
If all these measures are still insufficient to prevent dynamic thermal
mismatch failures axial elemental grading or sudden composition changes
may be added. One method consists of providing a disc of low-expansive
metals such as Sylvania #4, Dumet, 50% nickel alloy, chrome-iron
stainless, platinum, Sealmet, and titanium placed intermediately between
the steel and the copper braze. In this way, the ceramic MACOR is
mechanically isolated from the highly expansive steel. The desired
elemental profiling can also be achieved through controlled diffusion.
Skilled persons can, of course, select other yieldable metals such as gold,
silver, tin, lead, indium, zinc, or even iron or nickel, or other
materials to replace copper, and select other chemical elements to replace
the copper-strengthening zinc. The resultant new alloys will, of course,
be different in compositions, strengths, diffusivities, thermal
conductivities, melting or softening points, and other properties.
In addition to achieve metallizing uniformity and minimal mismatch
stresses, I have also found it desirable to microengineer the chemical
compositions, microstructures, and mechanical properties of the bonding
interfacial regions between the ceramic and metallized layer. Merely
perfecting the interfaces surfaces alone, as is commonly done, is
inadequate to produce strong and reliable joints for withstanding the
unavoidable, severe mismatches stresses and strains as shown above.
Different physical, chemical, and electrical metallizing or film-forming
methods have been developed. Each has its unique advantages. Some, for
example, are atomically precise. Others thoroughly clean the substate
surfaces for better adhesion. Some other result in crystalline epitaxy,
which is necessary for semiconductor or other devices. Others produce
splat cooling and superfine grains, with resultant enhanced mechanical
properties, for example, increased Young's modulus. Still others are done
at low temperatures to avoid unwanted thermal effects. But non deal
effectively with the critical problem of thermal mismatch stresses and
strains.
For extremely shock-resistant joints or metallized layers, I have found it
absolutely necessary to have a carefully microengineered interfacial layer
between the ceramic and the metallized layer. this layer is designed to
absorb the major portion of the always present mismatch stresses and
strains. Many of my ceramic metallizing processes typically last more than
20 minutes and involve liquid-forming layers containing, directly or
indirectly, MoO.sub.3 which melts at 801.degree. C., and WO.sub.3 which
melts at about 1550.degree. C. but can be further reduced by alloying with
other compounds of metals such as ZnO or PbO. Liquid diffusion is rapid
with diffusion coefficient D.sub.1 =1 E-4 to 1 E-5 cm.sup.2 /sec.
Processing for t=20 minutes gives a diffusion length of up to the square
root of D.sub.1 .times.t=0.35 to 0.11 cm. In addition, a diffused
interfacial layer of graded composition, microstructures, and mechanical
properties is formed which can be highly shock-absorbing.
In contrast, most conventional coating processes involve only solid-state
diffusion. Solid diffusion is slow with diffusion coefficient D.sub.s =1
E-10 to 1 E-20. Even for the same processing or diffusion time t, which
these processes do not have, the diffusion length is only 3.2 microns to
3.2 A, or several orders of magnitude shorter than that in my liquid
diffusion case. The mismatch stress gradient is thus proportionately
steeper.
Plasma spraying does involve liquid droplets in rapid transit. These
extremely high-temperature droplets impact the substrate at very high
velocities resulting in splat cooling with millsecond liquid dwell times.
The resultant diffusion is thus also over three orders of magnitude
shorter than my metallizing or metallizing-brazing case. Splat cooling
gives very fine grains with high Elastic moduli which actually increase
the mismatch stresses. Also, the superheated liquid particles form oxides,
mitrides, or other surface layers during transit preventing perfect
bonding between the particles themselves. Laser, electron, and some other
energetic beam enhanced coating processes also give splat cooling and
solid-duffusion conditions.
Without applying any external pressure to force the joining members
together, I have used metallizing and bonding processes described above to
join various ceramics to metals with pure copper brazes. A typical
metallizing process comprises using a mixture of metallizing composition
such as WO.sub.3 -Fe.sub.2 O.sub.3 or MoO.sub.2 -MnO.sub.2 in suspension
or paste form and applied onto the ceramic, heating for 5 to 25 minutes
the coated ceramic to about 800.degree. C.-1500.degree. C. but under no
applied pressure. The ratio of heavy metal W or Mo to Fe or Mn after
reduction from the compounds is between 9:1 to 6:4. This metallizing may
be followed by or simultaneously done with brazing with, e.g., copper or
its alloys. Hydrogen or forming gases of 10 to 40 volume % of hydrogen is
the desirable metallizing atmospheres.
The metallizing temperatures and times depend on other factors. Diamond,
for example, should not be metallized above about 900.degree. C., to
minimize chemical reactions nor should graphite fibers be treated above
about 750.degree. C. In both cases, a carburizing atmosphere, such as one
containing CH.sub.4 or propane, may be useful to prevent too much loss of
carbon. The ceramic I have already bonded with my W/Mo-based metallizing
methods described here include: alumina, zirconia, silicon carbide,
beryllia, yttria, graphite, quartz, silicon, mullite, cordierite,
Corning's MACOR and Vision glass, diamond, peizoelectric ceramics, and 123
high-temperature superconductors. Useful structural metals for the joints
include copper, nickel, stainless steel, high-nickel or cobalt iron
alloys, or even highly "mismatched" ordinary cold-rolled SAE 1010 carbon
steel. These joining metals can, therefore, be used as the braze metals
themselves for more refractor metals in the joints. Even with the
"mismatch" between ceramic and carbon steel, structural joints brazes with
pure copper can be repeatedly thermal cycled without fractures between
980.degree. C. and ice water followed by mechanical shocks including 8 to
10foot drop tests onto carpeted, wood, or even marble floors.
These results show that with my improved precesses, low-cost "mismatched"
ceramic/metal, carbon-metal, ceramic-ceramic, or ceramic-graphite joints
can be made; that these joints can be mechanically strong and thermally
shock resistent; that the bonding processes, being ceramic
material-limited, need no further improvement for the particular material
combinations and thermal shock requirements; and that these joints are,
after bonding and thermomechanical shocks, free of pores, microcracks,
inclusions, inhomogeneities, and other defects at which fractures
originate. Each of these shocks would multiply the number of defects
exponentially and have failed the joints. These joints, including
particularly the metallized layers, thus compare favorably with, e.g.,
certain ceramic-metal jonts or ceramic materials developed at great cost,
as reported in the literature.
Note that our new joints use only thin lyeres, not bulks, of
tungsten/molybdenum; and generally contain no other strategic and
expensive metals such as nickels, cobalt, or chromium. The metallized
layer adheretly joins to the ceramic. Upon the metallized layer,
tenacious, protective metal or ceramic layers can be brazed or formed
which resist spalling, peeling, and thermomechanical shocks. Improved
corrosion, wear, or frictional properties on these coatings are also
possible by suitable selection of the coating materials. A solid lubricant
system may be made, e.g., comprising graphite, talc, or MoS.sub.2 powders
chemically bonded in copper, bronze, nickel, steel, or cast iron. Also,
carbon-carbon composites with improved strength and oxidation resistance
are possible. Advanced chemically bonded intermetallic compounds and
materials (titanium or hafnium carbide, and titanium or nickel aluminides)
are also made available. The same W/Mo-based metallizing compositions are
even usefusl as almost universal high-temperature adhesives or sealants
for ceramics or metals.
It is even possible to leave only the metallizing molybdenum and/or
tungsten between the materials to be joined without any braze metal layer,
the operating temperature of the joint is then generally limited by the
melting point of the metallized layer.
The flawless and defect-free quality of my ceramic-metal joints or
metallized layers on ceramics or graphite are particularly important for
tough, fatigue-resistant, protective, easily wettable, and
thermochemically stable coatings on, or joints between, ceramic, metals,
or graphite, A metallized or coated graphite fiber, for example, cannot
tolerate a single pinhole or microcrack that allows oxygen to penetrate
and to destroy fiber. Ceramic coatings on metals also cannot have defects
when exposed to chemically reactive, high-intensity ion or plasma, high
temperature, or other extreme environments. High-melting precious metals
such as Pt, Os, and Pd and oxidation resistant metals such as Cr, Al, and
Ni are therefore beneficially applied onto the metallized layer, or be
formed simultaneously with a metallizing-brazing composition in a
single-step metallizing-coating process. Less protective metals such as
gold, copper, magnesium, titaniu, or zirconium may also be applied onto,
formed simultaneously with, the metallizing layer, followed by coating by
electrolytic, electroless, or spraying methods, of the above-mentioned
oxidation resistant metals for oxidation protection.
In addition, the metallized or metallized/brazed layers have good wetting
characteristics. Further, the metallizing or metallized/brazed layer
penetrates and seals all surface pinholes, microcracks, or other defects
to strengthen the ceramic at the bonding region. A thick (over 100 microns
thick) metal layer further toughness the brittle ceramic. Graphite cor
carbon fibers or particles may thus not only be oxidation resistnat but
surface toughened and non-brittle.
Coated with my metallized/brazed films up to 20 microns thick, ceramics,
boron, graphite, diamond, or glass powders 0.5 through 50 to 200 microns
in microns in diameters, are also suited for specific particulate
reinforced composites. Upon compacting and sintering these metal coated
particles to proper densities and mechanical properties, special acoustic
or otherwise damping materials are obtained.
Sapphite, quartz, alumina, or zirconia tubes sealed vacuum tightly to
nibobium tantanum, or other ceramic tubes make useful electronic cavity or
optical windows for services to or over 1300.degree. C. or 1500.degree. C.
My bonding method will also avoid the usual frits seals which are weak,
contaminating, short-lived, deteriorating to electrooptical
characteristics of the component, and otherwise unreliable in operations.
Defect-free or flawless coatings or bonding are also necessary to contain
dangerous materials and should be used to replace weldments which almost
always have bubbles, oxides inclusions, segregation patterns, severe
residual stresses, weak grain boundaries, or other defects.
The strong, defect-free, and thermomechanically shock-resistant quality of
the metallized layers on ceramics, graphite, diamond, and reactive metals
such as titanium zirconium, aluminum, or stainless is especially important
in the manufacture of advanced composites. Here, the reingorcing fibers,
particulates, sheets, or two- or three-dimensional weaves of the ceramics,
graphite, boron, oxides of aluminum or zirconium; and carbides or nitrides
of Ti, Zr, Hf, V, Nb, Ta, Cr, No, or W; borides of carbon or nitrogen;
silicides, aluminides, other intermetallics; diamond; and metals are then
perfectly not only wetted by, but bonded to, the matrix of metals,
ceramics, carbon, borides, carbides, diamond, . . . Good interfacial bond
strengths in, e.g., about 20 volume % graphite, SiC, or Si.sub.3 N.sub.4
fibers or particles in Type 6061 alumina, or zirconia, allow load transfer
to occur between matrix and reinforcing particulates, fibers, or weaves
thereby giving maximum specific moduli and strengths. These defect-free
bondings at the interfaces prevent debondings and allow ideal load
transfer between, within, and along the reinforcing members thereby
achieving maximum strength, prodcution yield, and productivity at minimum
costs.
By replacing the soft, yieldable braze metal pure copper (with melting
point 1083.degree. C.), silver (961.9.degree. C., gold (1064.4.degree.
C.), tin (232.0.degree. C.), zinc (419.6.degree. C.), lead (327.5.degree.
C.), antimony (630.degree. C.), cadmium (320.degree. C.), aluminum
(660.4.degree. C.), magnesium (648.8.degree. C.), gallium (29.8.degree.
C.), indium (156.4.degree. C.), thallium (303.5.degree. C.), bismuth
(271.3.degree. C., . . . , and their alloys with higher-melting metals
such as beryllium, chromium, cobalt, hafnium iridium; iron, manganese,
nickel, niobium, osmium; palladium, platinum, protoactiniu, rhenium,
rhodium; ruthenium, samarium, scandium, silicon, tantalum; thorium,
titanium, uranium, vanadium, yttrium, and zirconium, the allowable
operating temperatures of the joints are raised to near their respective
melting points of 1278, 1857, 1495, 2227, 2410; 1535, 1244, 1455, 2468,
2700; 1554, 1772, 3000, 3180, 1966; 2310, 1300, 1541, 1430, 2996; 1800,
1660, 1130, 1890, 1522, and 1852, Centigrade, respectively.
When molybdenum is used as the metallized layer together with an osmium,
rhenim, platinum, protoactinium, rhenium, and tantalum braze layer, the
melting point of molybdenum, i.e., 2810.degree. c., rather than that of
the braze layer, generally limits the useful temperature of the joint.
Similarly, when tungsten (melting point 3410.degree. C.) and carbon
(melting point 3650.degree. C.) are used as the metallized and brazed
layers for more refractary materials, respectively, the lower tungsten
melting point dominates. A variety of new, metallized fibers or
particulates of, e.g., SiC, Si3N4, Al203, ZrO2, mullite, cordierite,
diamond, glass, quartz, and other ceramics can thus be produced that can
be used as reinforcement in composites for temperatures over 1500, 2000,
2500, 3000.degree. C., or higher.
Matrix-reinforcement chemical reactions are serious problems in composites.
In graphite-aluminum composites, for example, the graphite reinforcement
may react with matrix aluminum to form brittle aluminum carbide. At a
given service, processing, or other operating temperature over about
800.degree. C., the graphite-aluminum interfacial reactions may thus be
intolerable. High-melting metals given above and used as the
metallized/brazed layers on the graphite slow down the elemental diffusion
rates and, therefore, graphite particulate--or fiber-matrix interfacial
reactions. The heavy metals W or Mo and refractory metals slow down even
further. This is because the elemental diffusion rates are functions of
the ratio of the operating temperature to the absolute melting
temperature. At the same operating temperature of, e.g., 550.degree. C.,
this ratio for aluminum directly contacting graphite is
(550+273.1)/(660.4+273.1)=0.882. With nickel braze on the graphite fibers
according to my invention, the interfacial reaction is now between nickel
and graphite, and the same ratio is reduced to 823.1/(1455+273.1)=0.476.
When the graphite fibers are metallized with Mo or W, the same ratios are
further reduced to 0.267 or 0.223, respectively. With a wide available
variety of metallizing alloys (e.g., W-Fe, Mo-Mn, Cr-Ni Cu-Zn, . . . ) and
coated layers on ceramic reinforcing fibers and particulates, these ratios
can be selectively chosen to be less than, e.g., 0.6, 0.5, 0.4, 0.3, 0.22,
or even less. The matrix-reinforcement interfacial chemical reactions are
thereby reduced, weakening of composite strength is minimized and
embrittlement of reinforcement or destruction of composite avoided.
Interfacial chemical reactivity between, e.g., ceramic reinforcement and
the metal matrix, can be further suppressed or totally eliminated by
coating the metallized/brazed layer with chromium or aluminum. Chromium,
aluminum, and their alloys form adherent, dense oxides that resist further
oxygen penetration to, e.g., the underneath graphite fibers. These
specially metallized/coated graphite or carbon fibers are thermochemically
stable in oxygen or other oxidizing atmospheres.
Even mismatch ceramic-metal joints made according to my invention refused
to fail under repeated, rapid and severe thermomechanical shocks. Further,
the final forced fractures occur away from the bonding regions. This shows
that the bonds are free of flaws, microcracks, inclusions, and other
defects. In addition, the bond is actually stronger than the weaker
ceramic member. This is because the liquid layer formed on the ceramic
surface during the metallizing step, generally from 5 to 50 microns thick,
actually seals surface notches and other flaws. The metallizing W/Mo
ingredients, as will be shown, also strengthen the ceramic at the
interfacial region through solution strengthening, or formation of
microcomposite reinforcement in the form of precipitated particulates and
reinforcing roots, branches, or networks. In many composites, weight is a
critical consideration. A very thin W/Mo-based metallized/brazed layer,
down to several atomic layers in thickness, may be used with or without
any copper, nickel, or other braze metal. The formation of a surface
liquid diffusion layer 3 to 30 atomic layers (about 10 to 100 A) takes
only 10E-9 to 10E-7 seconds, if a liquid diffusion coefficient of 10E-5
cm.times.cm/sec is used. The control of such extremely thin layer can
still be achieved by applying a thin layer of the metallizing solution
containing the appropriate amount of molybdate or tungstate compounds.
Another problem with composites is that ceramic, graphite, and carbon
fibers are very difficult to be perfectly wetted by, or bonded to, metals,
other ceramics, or even the epoxy. Because of this difficulty, an airplane
or other vehicle made of these composites often structurally fails under
cyclic environmental heat-moisture conditions. Under capillary attraction
forces, rain or condensed moisture on the composite surface deeply
penetrates, or is sucked in, along the tiny passageways in the unbonded or
poorly bonded interfacial regions between the graphite or other ceramic
fibers and the epoxy, metal, or ceramic matrices. This penetration is
facilitated by air release in, for example, an improperly oriented
one-dimensional reinforcement where water enters from the outside skin and
move freely along the entire length of the fibers, with entrapped air
being forced to leave out of the inner surfaces. This fills the composite
structure with water. When the environment turns cold, the filled water
expands on freezing, disruptively enlarging the passageways and further
debonding the reinforcement from the matrices. Repeated filling-expanding
cycles may destroy the composites. When a high-altitude airplane lands in
a hot humid weather, moisture automatically condenses onto the very cold
composite skin and similarly fill the passageways. The vehicle may take
off again into the same freezing attitude where the filled water also
expands on freezing with disruptive forces. Multiple cycles of landing and
high-altitude flying also also destroy the composite.
By uniformly covering these fibers with flawless metallized W/M0-based
coatings, with or without brazing materials, the bonding between these
coatings and the matrix will also be flawless. Water penetration is then
impossible. Periodic coating of all the strands of these fibers 21 along
their lengths with nodular metallized spots 22 at a specific distance d
apart breaks up the passageways into small compartments of length d (FIG.
2a). Water can now penetrate to no more than the same distance d below the
composite surface. Dipping a two-dimensional or three-dimensional fiber
weave into a W/Mo-based metallizing solution or paste, again with or
without braze, preferentially coats only the intersections of the fibers
with the metallizing compound to thereby form the required nodules for
stopping deep water penetration (FIG. 2b). The size of the nodular
metallized spots can be controlled by adjusting the viscosity and/or solid
content of the solution or paste. Wetting control with the addition of
acetone, alcohol, house detergent (e.g., Wisk) also helps.
The reinforcing graphite or other ceramic fibers selectively but perfectly
bonded at the nodulated or coated spots in the composites achieve
excellent load transfer between fibers, or even along the fibers in
metal-matrix composites, but allow systematically and controlably unbonded
or weakly bonded regions between the nodules, lending to excellent
toughness as well as heat and shock resistances.
The ceramic metallizing processes described in this invention also allow
the brazing or coating of the internal or external surfaces of ceramics of
controlled densities or porosities. More specifically, porous alumina,
zirconia, silicon carbide, mullite, and cordierite have already been
metallized on either the internal pores, external surfaces, or both.
Substantially 100% of the internal surfaces of the porous ceramic can be
metallized by my processes. Ceramic filters for, for example, molten
steel, aluminum, or other metals or materials are already in wide uses.
But the difficulty of perfectly bonding these weak and porous filter
ceramic medium to each other or the metal make their uses difficult,
tricky, unreliable, and often dangerous. By bonding these ceramic filters
to steel wires or plates, as I have don, these handling problems are
minimized.
Multi-stage ceramic filters of alumina, zirconia, silicon carbide, yttria,
mullite, cordierite, glass, or other ceramics strongly bonded to the same
or different ceramic of the same or increasingly finer pore sizes can now
be joined together, one on top of the other. Metal-reinforced multi-stage
filters can also be made for, e.g., added strength through metal
strengthening; multiple-purpose separations of gases, liquids, or solids
from one another through physical means due to size differences;
absorption by carbon; catalytic reaction by platinum; liberation or
desorption of gases such as oxygen, nitrogen, carbon oxides, or hyrogen
from the bonded oxides, nitrides, carbides; hydride for doping or addition
to the molten metals or other materials; separation of substances of the
same gas, liquid, or solid phases; and other special features functions.
Ceramic filters for air, gas, oil, transmission fluids, and cooling water
on automobiles, diesels, power generating equipment, and other machineries
are already available. Similar filters for various other fluids including
molten metals such as steel or aluminum, or catalytic reactors can, with
my bonding method, be strongly attached to internal or external carbon
steel or stainless steel containers, other metallic, carbon, or ceramic
hooks, knobs, holders, fasteners, protrusions, strengtheners, friction
contacts, or springy devices for easy handling or to form fluid-tight
enclosures without fluid by-passings.
Catalytic materials such as platinum alloys may also be coated on the
metallized layer via diffuison coating, brazing, electrolytic or
electroless plating. Reactive materials such as yttria or CaO can also be
made porous by sol gel, or by controlled powder packing and sintering, to
achieve any desired powder sizes and packing or sintered densities. Such
reactive ceramic filters, properly bonded to metal structures, may be
used, for example, to remove weakening sulfur in high-quality tool steel
poured through these filters.
An electric heater may surround, or be embedded in, the porous ceramic
filter for periodical activation with electric ohmic heating to burn to
ashes or gases the materials remaining on the ceramic filtering medium.
This achieves reusable or self-cleaning results.
Many other uses in differing industries of my bonding methods are readily
seen. These include ceramic composites, graphite composites, intermetallic
composites, metal-matrix composites, coatings on ceramics, graphite, or
metals, high-strength chemically bonded ceramics, and self-lubricating
materials containing, e.g., lubricating talc, MoS.sub.2, or graphite
particles in iron, steel, copper, or nickel. The composites may involve
reinforcing fibers or particulates of ceramics, intermetallics, graphite,
or metals in a matrix of ceramic, intermetallic, graphite, or metal.
Using my metallizing methods described above, metallized refractory
metallic compounds can be formed for uses as the matrix or reinforcement
for composites. These compounds include: oxides of Al, Ba, Be, Ca, Cr, Eu,
Gd, La, Mg, Mi, Pu, Ru, Sm, Sc, Si, Th, Ti, U, V, Y, and Zr; carbides of
Al, B, Ba, Ve, Ca, Hf, Mo, Nb, Si, Ta, Th, Ti, U, V, W, and Zr; borides of
Ba, Ca, Ce, Hf, Mo, Ni, Sr, Ta, Th, Ti, U, V, and Zr; Sulfides of Ca, Gd,
Sr, U. and Y; nitrides of Al, Hf, La, Nb, Nd, Sc, Si, Pr, Pu, Ta, Th, Ti,
U, V, Y, and Zr; and aluminides of Fe, Ni, Pt, Be, and Ti. Particularly
attractive among these compounds are: Si.sub.3 N.sub.4, SiAlon, Sic,
Al.sub.2 O.sub.3, mullite, AlN, B.sub.4 C, TiB.sub.2, and BN.
Light, strong, tough, and reliable structural Al, Mg, Be, Ti alloys in
composite forms can thus be made with metallized graphite, SiC, or other
ceramic reinforcement that will operate over 480.degree. C.
Powder of a ceramic, carbon, intermetallics, or reactive metal may be
similarly metallized to achieve flawless and perfectly wetting surface
characteristic so that the sintered powder compacts or liquid metal
infiltrated compostes will form that have unusually high strengths,
densities, and thermal conductivites. Such metallized powders can also be
cast as particulate reinforcements or strengtheners. These same powders
can be cast (by, e.g, hot squeeze method) to achieve near net shape or net
shape into complex structures or components.
A multi-purpose procedure for bonding, sintering, purifying, densifying,
strengthening, and otherwise improving the high temperature 123 ceramic
superconductor is shown in FIG. 3. High temperature superconductors are
superconductors which superconducts at above 90 degrees K (Kelvin). In
this multi-purpose procedure, a layer of a suitable MoO.sub.3 -based
mixture 31 is formed at selected spots on the copper substrate 30, as
shown in FIG. 3a. MoO.sub.3 is the key ingredient in many Mo-based
metallizing operations. It melts at 801.degree. C. but the melting point
can be lowered or raised to selectable temperatures by forming eutectics
or compounds with, e.g., CuO, BaO, and Y.sub.2 O.sub.3, and other oxides
such as AgO, CaO, or TlO (Thallium oxide), or even flourides, chlorides,
or iodides in view of Ovshinsky's promising results on superconducting and
particularly current-carrying capabilities, upon this MoO.sub.3 -based
layer is spread the YBa.sub.2 Cu.sub.3 O.sub.7-x powders 32. A vertical
temperature gradient is applied to the composite so that the top of the
superconductor powders is at least 20.degree. to 50.degree. C. below its
melting point, while the bottom of the MoO.sub.3 -based layer is above the
melting point of this mixture. This mixture layer will melt and sweep
upward (FIG. 3a) to achieve the following highly desirable results:
1. Metallizing and bonding of the bottom layer of 123 superconductor to the
copper substrate;
2. Temperature gradient zone-melting to purify the superconductor
boundaries according to Pfann (See: Zone Melting, Wiley, 1966);
3. Vertically oriented, upward coloumnar grain growth 34;
4. Grain boundary scavenging, oxgenation, or halogen doping;
5. Liquid phase sintering of the superconductor particles for improved
sintering speed, density, mechanical strength, and material stabilities
partly also due to the purified or doped grain boundaries;
6. High critical current density of the purified, thinner, and oriented
grain boundaries;
7. Cushioning or shock-absorbing qualities of the liquid-diffused,
chemically and mechanically graded interfacial layer 33 between the
superconductor film and substrate; and
8. Simple, low-cost, single-stop and mass-producing but potentially
high-yielding flim-making operation.
After this special temperature-gradient multi-purpose operation, most of
the impurities will be dissolved in the sweeping zone. This zone
eventually comes up to the surface to be frozen into a highly impure layer
35. This impure layer can be removed by, e.g., grinding or chemical
etching with mineral acids. See FIG. 3b.
Other high-temperature ceramic superconductors such as Tl.sub.2 Ba.sub.2
Ca.sub.2 Cu.sub.3 O.sub.10 and TlCa.sub.2 Ba.sub.3 Cu.sub.4 O.sub.x can be
similarly bonded or treated for properties improvement with the above
method. The substrate does not have to be pure copper, but can be other
metals such as aluminum, nickel, oriron, glasses, graphite, or diamond. In
addition, other ceramics such as Al203, ZrO2, SiC, carbon glasses,
diamond, or even metals powders or filaments, may be similarly bonded onto
metallic, ceramic, glass, or carbon substrates. The ceramic layer 34 with
thinned, purified, oriented grain boundaries have improved physicochemical
properties including thermal and electrical conductivities since grain
boundaries are well-known to contribute to resistivity.
In ceramic-metal joints other than for superconductor application, however,
the above zone-melting procedure is harmful from the bond strength
viewpoint. This is because the last-solidifying layer, usually of complex
ceramic eutectic compounds, is weak and brittle and reduces the joint
strength. The proper cooling direction after the metallizing here should,
therefore, not be vertical but horizontal. In this way, the last-forming
layer is lateraly swept out of the joint region without harmfully
affecting the joint strength.
According to the above disclosures, I microengineer the ceramic-metal,
ceramic-metallizing layer, and/or metallizing-braze layers by substantial
thickness and, more important, graded composition, thermoconductivity, and
mechanical properties. The W/Mo-based metallized layer may be, for
example, 10 20 or 30 microns containing a graded interfacial layer up to 5
or 10 microns. The effective liuqid diffusion length described above may
range from 5 to the entire 30 microns. These layers are obtained by liquid
diffusion, generally over five minutes but up to one hour. The Cu, Ni, or
alloy braze layers may also be chemically, mechanical, and physically
graded, as described above.
Another important grading of the interfacial layer relates to the
microstructure. Many conventional joints rely on superficial adhesion,
weak and defective chemical bonding, or mechanical anchoring wit roughened
surfaces. Rough surfaces increase surface area by about 41.4% with
45-degree slopes or valleys (FIG. 4a). An important feature of my
invention is the principle of rooting (FIG. 4b), branching (FIG. 4c), and
networking (FIG. 4d). Straight roots of the metallizing materials 41
penetrate, during the metallizing or rapid liquid diffusion period, deep
along the ceramic grain boundaries 40, These roots may be in the form of
fibers located at the intersections of the multiple boundaries, or in the
form of sheets each located between two adjacent ceramic grains. These
fibers and sheets may be straight, extending generally perpendicularly to
the ceramic-metal interface (FIG. 4a). They may form branches following
the grain boundaries (FIG. 4b). These roots may even flow deeply into the
grain boundaries and turn or curve around to form a partial or complete
network (FIG. 4c). The formation of these fibers or sheets depend on the
surface energies of the metallizing compounds relative to those of the
ceramic grains at the metallizing temperature. The depth of penetration
also depends on these energies, but primarily on the metallizing
temperature and time.
Preferably, these penetrating metallizing material form reinforcement in a
matrix of the ceramic material at the interfacial region. This can be
achieved by selecting a W/Mo-based metallizing composition which, with the
ceramic at the metallizing temperature, forms hare (Mohr hardness over 8
or 9 versus less then 7 or 6 for the matrix), tough, and strong compounds.
Useful compounds include PbMoO.sub.4, MgWO.sub.4, CaMoO.sub.4, MnWO.sub.4,
MnMoO.sub.4 and the like. In practice, I simply use pure starting
materials such as MoO3, WO3, PbO, CaO, . . . , prepare the exact or near
stoichiometric compositions for the metallizing compositions, and
metallize at a temperature 50.degree. to 200.degree. C. above the melting
points of these compounds. By varying the metallizing time, the
grain-boundary reinforcing compounds penetrate to different depths,
according to the square root of time diffusion law. For example, for a
liquid diffusion case with a diffusion coefficient of 10E-5
cm.times.cm/sec, metallizing for 5 to 60 minutes gives a diffusion length
or penetration depth of about 0.055 to 0.19 cm. I also achieved
moderately different penetrations of reinforcing particles, fibers, or
sheets of different penetration depths by changing the metallizing
compositions, e.g., from the W-based type to the Mo-based type.
Thus, with my new ceramic-ceramic or ceramic-metal joining methods, new
structural joints, coatings, or surfaces can be produced that have wide
uses due to their hardnesses (diamond, alumina, zirconia), hardness and
resistances to wear (diamond, zirconia) or corrosion (diamond, carbon,
alumina), electrical or thermal conductivity/insulation (zirconia,
beryllia, diamond, silver, stainless steel), catalytic activity
(platinum), and other properties or appearances.
Tool bits of silicon carbon or nitride, alumina, diamond, and other cutting
or abrasive materials can, for example, be metallized with my methods and
joined to steel holders to form cutting, drilling, milling, or other
machining tools. Particles of the same materials, mixed with the W/Mo
metallizing compounds together with copper or nickel brazing alloys, can
be spread onto inexpensive carbon steel sheets 0.010 to 0.250 mils thick.
Upon heating in a reducing atmosphere, a steel sanding sheet or block is
formed. The braze metal may be very thin and merely joins the abrasive
particles to the steel plate. The same braze metal may have a thickness up
to 95% of the size of the particles, to support fully and hold strongly
these particles while still allowing their sharp cutting edges to perform.
Gem stones such as diamond, sapphire, quartz, and the like can be mounted
onto metal holders. Because of the excellent strength of the bond, minimum
contact with holding metals is needed. As shown in FIG. 5, diamond 51 can
now be mounted on the tip of a fine wire 52 so that practically its entire
back surface can be illuminated. Also, different back characteristics
(color, texture, and reflectivity) can now be instantly changed.
The invention, as described above, is not to be construed as limited to the
particular forms disclosed herein, since these are to be regarded as
illustrative rather than restrictive. Various combinations, equivalent
substitutions, or other modifications of the preferred embodiments
described herein are obviously possible in light of the description,
without departing from the spirit of the invention. In particular, other
ceramics such as alumina or zirconia may be used instead of MACOR with the
same or a modified metallizing composition. Accordingly, the invention is
to be limited only as indicated by the scope of the following appended
claims:
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