Ceramic joining IV. effects of processing conditions on the properties of alumina joined via Cu/Nb/Cu interlayers

Multilayer copper/niobium/copper interlayers consisting of 3 mum thick clad ding layers of copper on a 125 mum thick niobium core layer were used to jo in aluminum oxides at 1150 degreesC or 1400 degreesC, or both. Three micros tructurally distinct aluminum oxides were joined-a 25 mum grain size 99.5% pure alumina, a submicron grain size 99.9% pure alumina, and single crystal sapphire. Two-phase interlayer microstructures containing both copper-rich and niobium-rich phases developed during bonding. In some cases, the initi ally continuous copper film evolved via Rayleigh instabilities into an arra y of discrete copper-rich particles along the interlayer/alumina interface with concurrent increases in the niobium/alumina contact area. Processing c onditions (temperature and applied load) and the alumina microstructure (gr ain size) impacted the extent of film breakup, the morphologies of the copp er-rich and niobium-rich phases, the interlayer/alumina interfacial microst ructure, and thereby the strength characteristics. Joints possessing a larg e copper/alumina interfacial area fraction were comparatively weak. Increas es in bonding pressure and especially bonding temperature yielded interface s with higher fractional niobium/alumina contact area. For joined polycryst als, such microstructures resulted in higher and more consistent room tempe rature fracture strengths. Joined 99.9% alumina polycrystals retained stren gths > 200 MPa to 1200 degreesC. Relationships between processing condition s, interlayer and ceramic microstructure, and joint strength are discussed. (C) 2001 Kluwer Academic Publishers.