Interactions and Wetting of Refractory Diborides by Liquid Metals

Abstract

Transition elements diboride ceramics are a class of promising materials for high temperature and highly aggressive applications. However, the possibility to exploit their peculiar characteristics often depends, to a great extent, on the ability to join the ceramic parts one to the other or to special 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 the joining process. Data on the wettability and the interfacial characteristics of different metal-ceramic systems, and in particular of (Ti,Zr,Hf)B2 in contact with liquid non-reactive metals Cu, Ag, Au and with "reactive " Ni and Ni alloys, is reported and critically discussed. These data, obtained in our laboratory by an advanced sessile-drop technique, are discussed in terms of chemical and thermodynamic properties of the various phases in contact and of their surface properties. Moreover, we show how an ab initio approach can be performed in order to interpret the wetting behaviour and the adsorption/reaction interfacial phenomena involved. Interfacial energetics at the atomistic level is being increasingly investigated by means of sophisticated modelling techniques like pseudopotential-based Density Functional Theory (DFT). Given the complexity calculations, the study is limited to the ideal work of separation, i.e., with plastic and diffusional degrees of freedom suppressed. Moreover, the dependence of the adhesion behaviour on the electronic structure at the interface and on the interface epitaxy and composition is discussed. In particular, the inspection of the electronic density of states projected on selected interface atoms gives a justification of the particularly strong adhesion between Au and ZrB2. This microscopic analysis will ultimately allow forecasting and designing a novel class of tailored materials