Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
1995-06-08
2002-07-02
Barr, Michael (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S193000, C427S201000, C427S203000, C427S376700, C427S419700
Reexamination Certificate
active
06413589
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field
This invention relates to ceramic coating and bonding methods, and more particularly relates to fusion-formed, ceramic coating and bonding methods with uniform ceramic metallizing compositions and specially graded, microscopically substantially perfectly defect-free bonded regions to produce reproducibly strong and thermomechanically shock-resistant ceramic coatings or bonds.
“Ceramic” means not only the usual ceramics such as alumina, zirconia, beryllia, mullite, cordierite, silicon carbide; but also quartz, intermetallics, diamond, boron, graphite, carbon, silicon, and various other carbides, nitrides, aluminides, or borides, glasses, machinable glasses, Corning's Vision glass; but also the surface of many reactive metals such as aluminum, magnesium, chromium, silicon, titanium, or zirconium which always have oxides, nitrides, hydrides, or other compounds of reactions of the metal with the environment.
2. Prior Art
Various methods have been developed to coat ceramic or metal with, or to join metal to, ceramics. But none gives inexpensive, stable, strong, and temperature resistant products. Reliable ceramic coatings or joints are not commercially available worldwide at any cost, even for small joint sizes.
Under a well-coordinated intensive effort on ceramic-metal bonding, Japan has been the most successful country in the development and commercialization of products involving metal-ceramic bonds. They already have successfully: (1) used a ceramic turbocharger (NGK, Nissan), (2) produced an all ceramic swirl chamber for diesel engines (Mazda, NGK), and (3) prototyped a ceramic turbomolecular pump (Mitsubuishi and Japan Atomic Energy Research Institute). But according to Prof. T. Suga of the University of Tokyo in his 1989 review paper on the “Future Outlook in Japan” (Exhibit A), the practical useful temperature of the best Japanese ceramic joints to special “matching” metal alloys is only 600° C. Further, the bond strength decreases rapidly with temperature, because the reaction products in their bonded regions become weak and brittle under thermal stresses. They consider the improvement of the thermomechanical shock resistance of their brazed ceramic joints to be an urgent task.
The European effort, mainly in Germany and France, has been even less successful. Germany failed to reach their goal after the first ten-year (1974-1983) program and its follow-up in 1983-1986. Their present program (1985-1994) merely emphasizes on reproducible mechanical properties and component reliability. The US Department of Energy supports much of US ceramic joining R&D. It also had to renew annually the ceramic automotive program after 10-year, 50-million intensive work, mainly producing a specification for automatic ceramic-metal joints.
Each metal-ceramic joint or bond must be specially designed. The factors in joint design include metal and ceramic composition, joint failure modes, parts shapes and sizes, thermal and other demands. The requirements for the National Aero-Space Plane (NASP) is totally different from those of the diamond heat sinks or fusion reactors. A ceramic-metal bond designed for maximum mechanical strength is usually not the best for thermal conductances, which is critical in heat sinks. What is best for one application (e.g., for preventing rapid heating failures) may even be precisely the worst for another (e.g., for preventing severe quenching failures), as shown by the functional grading technique described in this application. On the NASP, for example, the best titanium-Si
3
N
4
joint for the turbine subjected to rapid heating should not be used for the wings of the same plane subjected to possible ice quenching failures. A joining method for many conditions may not be the best for any application.
Different physical, chemical, and electrical metallizing or film-forming methods have been developed for metal-ceramic bonding. Each method has its unique advantages. Some, for example, are atomically precise. Others thoroughly clean the substrate surfaces for better adhesion. Some others result in crystalline epitaxy, which is necessary for semiconductor devices. Still 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 none deal effectively with the many critical problem to be addressed in this invention.
Most ceramic-metal joints have bonding regions that are not microscopically perfect or 100% dense, severely damaging the joint mechanical strength and thermal or electrical conductivities. Sintered, solid-state formed, hot or cold pressed, diffusion bonded, or even liquid infiltrated bonding layers cannot be fully dense, no matter how high the vacuum, external pressure, or processing temperature. This is because trapped gases cannot be compressed to zero volume, particularly if they are sealed off by initial densifications. Evaporated, sputtered, plasma, and electrolytic or electroless deposits generally are packed plates. Packed particles can never be 100% dense. The maximum density in packed spheres is only about 74% for the idealized close-packed, face-centered or hexagonal packing structure. Ceramic metallizing with mixed W/Mo and Fe/Mn powders have voids and segregations initially already present in the coated layers. These defects, generally remain after high-temperature processing, because of contamination, inadequate melting and fluxing, and diffusion voids, and other chemical reactions. Repeated metallizing, sintering, nickeling, as suggested by, e.g., the U.S. Pat. No. 3,901,772, do not solve the basic problems.
Achieving full density in chemical vapor deposition (CVD) or physical vapor deposition (PVD) requires not only complete absence of dust, contamination, inclusion, and trapped gas, but also special ambient such as excellent vacuum, not gaseous ambient under atmospheric pressure. Deposited films also require perfect cleaning and optimal nucleation and crystal growth. Nucleation and crystal growth is still not a science. The later, in particular, requires unknown but continuously varying growth rates and temperature profiles. After billions of dollars of CVD work (e.g., in electronics), defects in CVD films are still prevalent. Pores, for example, often reach up to 10 or 20% in even the most studied diamond films, according to a 1990 DTIC report referred to elsewhere. This is so regardless of whether high or low-pressure, high or low-temperature, plasma or laser enhanced or not, or the type of equipment, carrier gases or reactants used. Unless ultra-high vacuum is used, active metal bonding methods employing Ti, Zr, Nb, Cr, . . . always contains surface oxides, nitrides, carbides, which lead to pores or cracks (from mismatch between, e.g., oxide or metal) and refractory, non-wetted or non-bonded areas.
The metal powders used in the common ceramic metallizing processes are limited to 325 or 400 mesh sizes, or still tens of microns in diameters. Finer powders are costly, and generally surface contaminated. These fine mixed powders are always segregated, and cannot produce thin metallized layers one micron or 100 Angstroms (A) thick, nor with thickness accuracies of less than 1,000 or 100 A.
Hence, most ceramic-metal joints are not substantially perfectly bonded, not only microscopically but even macroscopically. By “macroscopically or microscopically substantially perfect wetting or bonding”, it is meant that no defects are visible in the form of voids, cracks, excessive fluxes, non-wetted, or non-bonded areas under the microscope or on microphoto at 3-20 or 100-1,000 times magnification, respectively. Microphotos such as those in
FIGS. 2 and 3
(at 1,000X magnification) of the Li's “Diamond Metallization” paper given in Ref. E mentioned elsewhere show microscopically perfect bonding with none of the defects mentioned above. These microphotos are available to the public since 1990 via the SDIO Final Report, Ref. 17, in the “Diamond Metal
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