Overview on Wetting and Joining in Transition Metals Diborides

The ultra high temperature performance of ceramic-based complex structures may require the development of liquid-assisted joining techniques; this in turn requires the definition of the wettability of these materials by various metals over a wide range of compositions and temperatures. After a short description of the relevant experimental aspects of wettability studies at high temperatures, a discussion is presented on how these results can be used to derive chemical and structural information on the solid-liquid interactions. Reference is made mainly to metal-ceramic systems; a summary of the results of sessile drop tests under carefully controlled conditions is given in relation to the wettability and the interfacial characteristics of systems based on transition metals(Zr, Hf) diboride ceramics in contact with liquid Ag, Cu, Au and Ni and of some of their alloys with Ti, Zr, Hf and B to promote/control wettability. In particular, the utilization of phase diagrams is discussed, as one of the most powerful tool to design the filler alloy compositions for the optimization of joining (brazing) processes.