RECENTLY there has been much disquieting discussion as to the competitiveness and effectiveness of U.S. technologies. One facet of the discussion concerns how far welding science could contribute to novel technologies. I have been long involved in various aspects of ceramic materials exploitation and am unhappy to observe that metallurgists and ceramists pursue their research too strictly within the confines of their specific disciplines. In the search for new materials for advanced technologies, one focus has been on those fine ceramics having properties which set them apart from alternative, but less opposite materials.
    Ceramics range from diverse silicates to oxides of Al, Ti, Zr, Be, etc., but involve also nonoxides such as carbides, nitrides and borides of the transition elements, and finally, the newest class of all - multiphase composites which maybe totally, or partially, ceramic. Ceramics are attractive for numerous reasons dependent on their properties. They may be electrical insulators, semiconductors or exhibit, conductivity or superconductivity. They may display a large piezoelectric effect, have voltage-sensitive resistance, or their electrical permittivity may change with humidity. They may be good heat conductors, or excellent thermal insulators. In new technologies utilizing high temperatures such as energy conversion, gas turbine engines or grinding and machining ceramics are of potential use because they remain stiff at temperatures intolerable by metals and superalloys. Broadly speaking, the permissible operating temperature of a material is between 0.5 and 0.7 Tm. Only 12 known metals have Tm > 2000º C in contrast to 38 oxides, 34 carbides, 17 nitrides and 54 borides.
    Some authorities predict a considerable growth in the utilization of structural ceramics. An estimate for the world market for the year 2000 reaches between $3 to $12 billion, with 25% of the market captured by ceramic cutting tools, the rest divided between heat engines, industrial processing and energy conversion technologies.
    Although the ability to fabricate ceramic materials that are reliably strong and tough is continuously advancing, the successful utilization of these materials will depend on the ability to assemble simple components into structures that will function effectively, and here the current state of the art stops short of adequate solutions. Ceramists seem to be preoccupied with materials development rather than with the final products, while the welding community is populated largely by metallurgists familiar with ductile metals. Linking of the experience particular to each of these two groups is still minimal.
    As a result, there has been little progress within the last ten years in the relevant ceramic joining technology, and a technical stasis has resulted. Thus, many materials with desirable properties cannot be utilized to build structures and new advanced technologies remain stalled, because the need for novel joining techniques is ignored or unrealized. This situation is paradoxical because, although the opportunities for achieving technical breakthroughs are here, they continue to be neglected.
    Traditional welding methods are not directly applicable to the materials of low strain tolerance encountered among ceramics. In such ceramics, the size of a processing zone surrounding a crack tip is limited to a few micrometers, rather than measurable in centimeters, thus obviating macroscopic deformations. Welding introduces localized stresses proportional to aE (where a is the coefficient of thermal expansion). They are typically 1 MPa/deg so that for a welding temperature of 1500-3000º C these stresses exceed the flexural strength of the ceramic. This accounts for the unsatisfactory results of attempts to weld ceramics by methods already proven in metallurgy. Such results need not be discouraging, some possible solutions are noted here.
    Since any microstructural discontinuity in a ceramic joint will result in destructive residual stresses, welding should be restricted to extremely thin surface regions. High energy beams capable of delivering peak power densities, which could instantaneously melt a narrow layer on the surface are of special interest. While metal welding can be performed by starting at one end of the joint, ceramic joining requires the entire joint region to be brought into a reactive state simultaneously. Preliminary supplemental heating may be needed to prevent thermal shock during welding.
   A transfer of the experience gained in other material fields could prove helpful. Adaptation of physical and chemical deposition methods, surface engineering by laser beams, ion implantation, and chemical doping to improve surface reactivity and solid-state adhesion are just a few of the techniques available for implementation. The development of generic joining technology for ceramic materials is vital and should be pursued in the context of both metallurgical and ceramic sciences. Furthermore, it should be paralleled by an engineering effort to design and construct equipment specific to ceramic joining.
    Extending welding technology to ceramic materials will allow implementation of ad advanced technologies in circumstances where ceramics are the only materials having the requisite properties. Such a development provides an opportunity for the expertise of welding engineers plus venture capital to cash in on solving the underlying basic problems and thus augment the international competitiveness of U.S. products. Surely this opportunity should not be passed by.

Wieslaw A. Zdaniewski PATRIA, Inc.
Published in Welding Journal