New Method for solid‐state Bonding Between Ceramics and Metals

Abstract

A new methorl,for solid-state bonding between ceramics and metals was developed. An intcrluver, which is a composite of the materials to be bonded, is used. The technique can he applied to materials which cannot be bonded by conventional cli~irsion-bonding methods because cf thermal-expansion mismatches. It is exprctrd that the bonded materials will have excellent thermal shock resistance.H E TECHNIQUE of bonding between T c c r a m i c s and metals is an important consideration in the fabrication of hightcmpcraturc structural materials. For high-temperature applications, it is ncccssary for materials to have good thermal shock resistance. Since ceramics and mctals usually have diffcrcnt coefficients of thermal expansion, an internal stress acts across the interface bctwccn the bonded materials. Therefore, failure of bonded matcrials rcsults because of the developnicnt of internal strcss under hightemperature operating conditions. The rcsistance of matcrials to thermal shock is one of the most important factors in dctermining thc rcliability of bonded materials.Two methods can bc used to improve the resistance of bonded ceramic-metal matcrials to thcrmal shock. One method involves choosing a ceramic and mctal with similar thcrmal-cxpansion coefficients, such as niobium with alumina, refractory mctals (Mo, Ta, W, ctc.) with carbides and nitrides (AlN, TiN, TIC, etc.) and transition metals (Fe, Ni, Ti, etc.) with zirconia. In thc sccond method, an appropriate interlayer is formed between the ceramic and mctal. Thc thcrmal-cxpansion coefficient of the interlayer phase is intermcdiatc between those of the constituents of the couple. A well-controlled intcrlayer can be expectcd to relieve stress acting across the bonded interface. However, it is difficult to makc such an intcrlayer with sufficient fracturc strength at elevated temperatures.The purpose of this investigation was to develop a mcthod for producing a strong bond with sufficient thermal shock resistancc. In this mcthod, the interlayer is a composite of the materials to be bondcd themsclvcs or a two-phase mixture of these materials. The authors refer to this as "composite interlaycr bonding." This method does not require the addition of a third elcmcnt. Thrcc types of interlayers are possible, as shown in Fig. I . Type I consists of a homogeneous distribution of both materials; type 11 is a two-phase mixture in which the volume fractions change stepwise; and in type 111, the volume fractions in thc interlayer changc continuously. Excellcnt thcrmomcchanical behavior of the bonded materials is easily achieved by controlling thc volumc fractions of the parent materials and the thickness of the interlayer. It is cxpectcd that the bonded materials formed will have excellent thermal shock resistance. Furthermorc, since this method adds a large interlocking force to the total bonding force, the bonding strength is expected to be greater than that obtained by conventional diffusion bonding. By applying the present method, it is possible to bond ceramic-metal couples...

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