Wetting and interfacial phenomena in Ni-HfB2 systems

Non-oxide ceramics, such as carbides, nitrides and borides represent one of the fastest growing classes of new advanced materials. Among them, transition metals ceramic diborides, in particular Titanium, Zirconium and Hafnium diborides, are members of a family of materials with extremely high melting temperatures, high thermal and electrical conductivity, high hardness and chemical inertness. These materials -Ultra High Temperature Ceramics (UHTCs)- constitute a class of promising materials for use in high performance applications, where high temperatures, high thermal fluxes, severe surface stresses are involved. It should be remarked that the possibility to exploit commercially their peculiar characteristics often depends to a great extent on the ability to join the ceramic parts one to the other or to special metallic alloys. As the behaviour of a metal-ceramic joint is ruled by the chemical and the physical properties of the interface, the knowledge of wettability, interfacial tensions and interfacial reactions is mandatory to understand what happens at the liquid metal-ceramic interface during joining processes. However, very few data are available on these properties to day for UHTCs, and in particular for HfB2. This study presents data on the wettability and the interfacial characteristics of systems based on HfB2 ceramics hot-pressed with different sintering aids (B4C, HfSi2) in contact with liquid Ni and its alloys with active metals to promote wettability. The experimental data, obtained by sessile drop tests at 1500 ?C under carefully controlled conditions, are reported and discussed as a function of time, compositions and structure of the ceramic. The specimens are analysed by optical microscopy, SEM and EDS and by means of specific mechanical adhesion tests. A comparison is also made with previous studies on the same boride in contact with lower melting alloys (Cu, Au, Ag based). Thermodynamic models are used to interpret the wetting behaviour and the adsorption/reaction interfacial phenomena involved