Wetting at High Temperature

Processes and equipments that must operate at high temperatures require materials able to face high thermal fluxes, severe stresses, high chemical reactivity. Ultra High Temperature Ceramics (UHTC's) represent a class of materials most promising for such applications. In particular it is often necessary, to obtain the best performances, to join these ceramic parts one to the other or to special metallic alloys by means of brazing processes, where liquid alloys realize the bonding between the two phases. Thus, also in relation to metallurgical, crystal growth and composite production processes, the knowledge of wettability, interfacial tensions and interfacial reactions is mandatory to understand what happens when a liquid metal comes into contact with a ceramic surface. In this paper, recent systematic studies, addressing both basic (wettability, interfacial tension, phase equilibria determination) and application (joining) aspects, are critically reviewed with a particular reference to transition metal diborides and silicon carbide. A special attention is paid to studies aimed at elucidating the role that dissolution, chemical reactions, additions of active metal elements to the molten matrix have in the wetting process and on the solid-liquid adhesion, and, eventually, on the mechanical characteristics of the brazed joints, as the main final 'sensitive' parameter for application purposes.