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 μm thick cladding layers of copper on a 125 μm thick niobium core layer were used to join aluminum oxides at 1150°C or 1400°C, or both. Three microstructurally distinct aluminum oxides were joined—a 25 μm 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 initially continuous copper film evolved via Rayleigh instabilities into an array of discrete copper-rich particles along the interlayer/alumina interface with concurrent increases in the niobium/alumina contact area. Processing conditions (temperature and applied load) and the alumina microstructure (grain size) impacted the extent of film breakup, the morphologies of the copper-rich and niobium-rich phases, the interlayer/alumina interfacial microstructure, and thereby the strength characteristics. Joints possessing a large copper/alumina interfacial area fraction were comparatively weak. Increases in bonding pressure and especially bonding temperature yielded interfaces with higher fractional niobium/alumina contact area. For joined polycrystals, such microstructures resulted in higher and more consistent room temperature fracture strengths. Joined 99.9% alumina polycrystals retained strengths >200 MPa to 1200°C. Relationships between processing conditions, interlayer and ceramic microstructure, and joint strength are discussed.