The mechanical properties and microstructural evolution of metal-ceramic bonds produced using a transient liquid phase (TLP) are described. Alumina (Al2O3) was joined at 500 degrees C, 600 degrees C, and 700 degrees C using a multilayer In/Cusil-ABA (R) (commercial copper-silver eutectic brazing alloy)/In interlayer. The introduction of thin In cladding layers allows the system to bond at much lower temperatures than those typically used for brazing with Cusil-ABA (R), thereby protecting temperature-sensitive components. After chemical homogenization, the interlayers retain an operating temperature range similar to that of the brazed joints. TLP bonds made at 500 degrees C, 600 degrees C, and 700 degrees C with holding times ranging from as low as 1.5 h to 24 h had average fracture strengths above 220 MPa. The effects of bonding temperature and time on fracture strength are described. Preliminary analysis of the interlayers shows that the Ag-In or Cu-In intermetallic phases do not form. Considerations unique to systems with two-phase core layers are discussed. Experiments using single-crystal sapphire indicate rapid formation of a reaction layer at 700 degrees C, suggesting the possibility of making strong bonds using lower temperatures and/or shorter processing times.

Transient-liquid-phase (TLP) bonding was used to join high-strength, high-purity Al2O3 ceramic. This method uses a multilayer interlayer (B/A/B sandwich structure), which forms thin transient-liquid layers between the Al2O3 and the refractory core layer (A), then isothermally solidifies through a diffusive mechanism. The presence of thin liquid layers allow interfacial gaps and voids to be filled, while allowing bonding times comparable to those used for conventional brazing. It was shown that TLP bonding produces high-strength joints with re-melt temperatures that are significantly higher than the bonding temperatures used. This study explores the interrelationships between the processing conditions, fracture strengths of the joints, wetting behavior of the TLP, and the diffusion/isothermal solidification kinetics. In particular, when Ni/Nb/Ni interlayers were utilized, four-point bend tests revealed that the fracture strengths of the joints matched those of the monolithic Al2O3, even after a 5-min holding time at 1400°C, the bonding temperature. The resulting interlayer consists of >99% Nb, which has a melting point >2000°C. Sessile-drop wetting experiments revealed that the Ni-Nb liquid alloy formed during bonding exhibits relatively low contact angles (≈90°) on Al2O3, which enables the formation of nearly flaw-free interfaces. An analytical diffusion calculation model was also used to describe the isothermal-solidification and homogenization kinetics, and will be used in future studies to aid new interlayer designs.