Liquid-film-assisted formation of alumina/niobium interfaces

Alumina has been joined at 1400degreesC using niobium-based interlayers. Two different joining approaches were compared: solid-state diffusion bonding using a niobium foil as an interlayer, and liquid-film-assisted bonding using a multilayer copper/niobium/copper interlayer. In both cases, a 127-mum-thick niobium foil was used; approximate to1.4-mum- or approximate to3-mum-thick copper films flanked the niobium. Room-temperature four-point bend tests showed that the introduction of a copper film had a significant beneficial effect on the average strength and the strength distribution. Experiments using sapphire substrates indicated that during bonding the initially continuous copper film evolved into isolated copper-rich droplets/particles at the sapphire/interlayer interface, and extensive regions of direct bonding between sapphire and niobium. Film breakup appeared to initiate either at niobium grain boundary ridges or at asperities or irregularities on the niobium surface that caused localized contact with the sapphire.