Where are we with the understanding of metal/ceramic interactions: the case of transition metals diborides

The combination of the unique properties of UHTC materials, such as high melting temperatures, high thermal and electrical conductivities, excellent thermal shock resistance, high hardness, and chemical inertness. makes them suitable for the extreme chemical and thermal environments associated with space applications, including hypersonic flight, atmospheric re-entry, and rocket propulsion. Other applications are for refractory linings, electrodes, microelectronics, and cutting tools. Among these UHTCs, the transition metals diborides TiB2, ZrB2 and HfB2 deserve a particular industrial interest; indeed, increasingly large research efforts in many countries are being been devoted to them in recent years. Ceramic/metal joining is of great technological significance because, through this process, the individual characteristics of the two classes of materials can be used to produce new components with improved performances. To this end, liquid phase bonding processes are the most reliable techniques. Thus, the study of metal-ceramic interactions, fundamental to understand the basic mechanisms of materials stability, reactivity and integrity, is also functional to obtain all the necessary input data for production processes for production processes of metal-ceramic composites, joining via brazing procedures, and so on. The design and optimization of these processes require expertise in various research fields, principally: 1) Thermodynamic, kinetic and atomistic modeling; 2) Interfacial science; 3) Microstructural characterisation; 4) Active brazing technology; 5) Mechanical testing and 6) Manufacturing process simulation. This review analyzes the results of the last two decades of research, and presents also some new findings in relation to points 1), 2), 3) and 4), i.e. the characterisation of the interfacial interactions (wetting, chemical reactions etc.) and joining that occur during high temperature metal-ceramic contact when the ceramic phase is one of the Group IV diborides.