Journal of the European Ceramic Society 27 (2007) 3277–3285 Liquid metal/ceramic interactions in the (Cu, Ag, Au)/ZrB 2 systems Alberto Passerone a,∗ , Maria Luigia Muolo a , Rada Novakovic a , Daniele Passerone b a IENI-GE, CNR, Via De Marini 6, 16149-Genoa (I), Italy b Empa-Mater. Sci. & Techn., ¨ Uberlandstrasse 129, CH-8600 D¨ ubendorf, Zurich (CH), Switzerland Received 16 September 2006; received in revised form 7 December 2006; accepted 16 December 2006 Available online 8 March 2007 Abstract Wetting and spreading experiments on ZrB 2 in contact with liquid Cu, Ag and Au have been performed by the sessile drop technique under a vacuum. The wetting and spreading characteristics and the interfacial reactions are discussed as a function of time and of the metal involved. The interfacial morphologies, analysed by optical microscopy, SEM and EDS show the presence of regular interfaces without macroscopic reaction layers. Gold, to a very large extent and copper are shown to give rise to extensive penetration along grain-boundaries, whereas silver neither wets nor penetrates. Interfacial diffusion/dissolution is taken into account and the consequent changes in liquid metal surface tension and wetting behaviours have been evaluated by means of thermodynamic calculations. Moreover, interfacial energetics at the atomistic level has been investigated by means of pseudopotential-based Density Functional Theory (DFT) technique. It is shown how the calculation of the ideal work of separation on the specific transition metal borides-molten metal systems can be used to interpret the wetting behaviour. Moreover, the dependence of the adhesion behaviour on the electronic structure at the interface and on the interface epitaxy and composition is also briefly discussed. © 2007 Elsevier Ltd. All rights reserved. Keywords: A. Joining; B. Interfaces; C. Wettability; D. Borides 1. Introduction Zirconium diboride is a member of a family of materials – the transition metal diborides – with extremely high melting temperatures, high thermal and electrical conductivity, excellent thermal shock resistance, high hardness and chemical inertness. These materials, also referred to as Ultra High Temperature Ceramics (UHTC’s), constitute a class of promising materials for use in aggressive environments, where, for example high temperatures and high thermal fluxes are involved. 1 Often, to exploit the peculiar characteristics of these ceramic materials the necessity arises to join the ceramic parts one to the other or to special metallic alloys by means of diffusion bond- ing or brazing techniques. In the last case, as the behaviour of a metal–ceramic joint is ruled by the chemical and the physi- cal properties of the interface, the knowledge of the interfacial energetics, i.e. of interfacial tensions, interfacial reactions and wettability is mandatory. ∗ Corresponding author. E-mail address: a.passerone@ge.ieni.cnr.it (A. Passerone). Many efforts have been made to improve wetting in metal–ceramic systems. 2 Two ways are widely used: one is to modify the ceramic surface by some specific coatings, in order to let the braze metals spread on this new surface, the other one is to add reactive elements to the braze which can form intermediate products at the interface more readily wet- ted by the liquid alloy. For example, the addition of Ti to Ag or Cu alloys involves the formation of Ti compounds (TiO x , TiC or TiN on oxides, carbides and nitrides, respectively 3–5 ) at the metal–ceramic interface which, due to their more “metallic” character, are wetted better than the underlying ceramic. Active metal additions can also act through an adsorption process, where the additional element accumulates at the solid–liquid interface lowering the interfacial energy and, as a consequence, the contact angle affecting also the spreading kinetics. 6–8 In the past years the wetting experiments performed on ZrB 2 are quite scarce and from them it is difficult to derive general rules (Table 1). As in the case of nearly all ceramic materials, the reasons for this can be attributed to different factors. First of all, the chemical composition and the surface structure of the solid specimens are hardly specified. Indeed, these materials are obtained by various sintering techniques. 0955-2219/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2006.12.008