F. Valenza

A review of transition metals diborides: from wettability studies to joining

The renewed interest in transition metals diborides, classified as ultra high temperature ceramics, for applications at high temperature or in highly aggressive environments has given rise in recent years to an increasing number of research projects particularly devoted to the study of their interfacial characteristics when placed in contact with liquid metals and alloys, which complement and upgrade the limited number of older investigations. This paper critically reviews these systematic studies, addressing both basic (wettability, interfacial tension and phase equilibria determination) and application (joining by brazing) aspects. Particular reference is made 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 in relation to the mechanical characteristics of brazed joints.

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.

Critical Issues for Producing UHTC-Brazed Joints: Wetting and Reactivity

A brief survey is presented of the most important interaction phenomena occurring at the solid-liquid interfaces in metal-ceramic systems at high temperatures, with special attention to the most recent developments concerning wetting and joining transition metals diborides. These phenomena are described and discussed from both the experimental and theoretical points of view in relation to joining ceramic and metal-ceramic systems by means of processes in the presence of a liquid phase (brazing, TLPB etc.). It is shown that wetting and the formation of interfacial dissolution regions are the results of the competition between different phenomena: dissolution of the ceramic in the liquid phase, reaction and formation of new phases at the solid-liquid interface, and drop spreading along the substrate surface. We emphasize the role of phase diagrams to support both the design of the experiments and the choice of active alloying elements, and to interpret the evolution of the system in relation to temperature and composition. In this respect, the sessile-drop technique has been shown to be helpful in assessing critical points of newly calculated phase diagrams. These studies are essential for the design of joining processes, for the creation of composite materials, and are of a particular relevance when applied to UHTC materials.

Wettability of SiC and graphite by Co-Ta alloys: evaluation of the reactivity supported by thermodynamic calculations

Within the context of the design of high-temperature brazing process for C and SiC-based composites, a basic study is presented here about the wetting and interfacial reactivity of Co–Ta alloys in contact with graphite or SiC. Wetting results are presented for the first time showing that Co–Ta alloys wet C or SiC fairly or excellently depending on the relative amount of Ta. The final interfacial microchemistry and microstructures are the result of the interplay between the typical interfacial phenomena of the pure elements. Specifically, depending on the alloy composition, dissolution of the ceramic phase by Co or formation of a continuous interfacial layer of TaC that prevents dissolution prevails. The discussion about the interfacial reactivity between liquid Co–Ta alloys and graphite or SiC, as well as the interpretation of solidification phenomena and the formation of interfacial phases, is supported by making reference to new multi-component phase diagrams calculated by CALPHAD method.

Interactions between Superalloys and Mould Materials for Investment Casting of Turbine Blades

The need of increased efficiency of industrial gas turbines comes also through the improvement of the composition of superalloys (addition of new solutes) and of the manufacturing technologies involved in the investment casting process of the turbine blades. Thus, the knowledge of the interactions between the ceramic materials used for casting and the molten superalloys must be deepened in order to minimize the formation of internal defects, to improve the casting surface and to optimize finishing and casting operations. In this work, a study of the wetting behaviour of some Ni- or Co -based superalloys, used for the fabrication of turbine blades, has been performed with reference to the interactions of these alloys in the molten state with the silica-aluminate based ceramic materials forming the shell or the core in the casting process. Wettability tests have been performed by means of the sessile drop method at 1500°C; the characterization of the interfaces between the molten drop and the substrates has been made on solidified sessile drop samples by SEM/EDS analysis to check the final characteristics of the interfaces. The results are discussed in terms of chemical interactions in relation to the processing parameters and as a function of the surface and interfacial energetic properties of the systems.

High Temperature Solid-Liquid Interactions in Metal-Ceramic Brazing: A Critical Review

Solid-liquid interactions in metal-ceramic systems are extremely important in high temperature brazing processes. These interfacial phenomena are reviewed here, from both the thermodynamic and microstructural viewpoints. At high temperature, the wetting characteristics and the adhesion properties of the joints are strongly related to the high atomic mobility of the different phases, giving rise to different phenomena, ranging from the dissolution of the ceramic in the liquid phase, reactions, formation of new phases and reprecipitation at the solid-liquid interface. The role phase diagrams in guiding the choice of the filler alloys composition and to optimize the brazing procedures is emphasized. In particular, it is shown that the computation of new diagrams and the critical use of the existing ones is essential to understand how to suppress the substrate dissolution and to interpret the evolution of the system. Experimental data are presented and discussed concerning the interactions between liquid metals with early-transition-metal diborides (TiB2, ZrB2, HfB2) as a typical example involving the joining of Ultra-High Temperature Ceramics (UHTCs). Overall, these studies represent the basic step linking the chemical and structural information to the design of industrial processes involving a liquid phase at high temperature, such as the production of metal-ceramic joints or composite materials to be used in highly demanding applications.