Transient-liquid-phase bonding of HfC-based ceramics

Transient-liquid-phase (TLP) bonding enables joining at a lower temperature compared to more traditional bonding techniques and since it preserves the potential for high-temperature applications, it is particularly attractive for ultra-high-temperature ceramics (UHTCs) such as carbides and borides. The feasibility of a TLP joint between pure carbides has been recently demonstrated.[1] Following those results the present work studies the interactions occurring during the TLP bonding of pure HfC or HfC doped with MoSi2 using a Ni/Nb/Ni multilayer interlayer. Bonding is performed at 1400°C for 30 min in a high-vacuum furnace equipped with refractory metal (W, Mo) elements and shields to minimize the potential for contamination. a contaminant-free atmosphere. Crack-free joints have been obtained with pure HfC and HfC doped with 5vol% MoSi2, whereas a higher amount of dopant (15vol %) led to fracture of the joint within the reaction layer. SEM-EDS characterization shows that the reaction layer formed at the interlayer/ceramic interface contains mixed carbides and Ni-Nb-Si alloys. When the reaction layer thickness exceeds a certain value, as in the case of the bond obtained with HfC doped with 15vol% MoSi2, residual stresses arise leading to the formation of cracks and eventually to the fracture of the bond. [1] L. Silvestroni, D. Sciti, L. Esposito and A.M. Glaeser, "Joining of ultra-refractory carbides" J. Eur. Ceram. Soc., 32 (2012) 4469-4479