Interfacial phenomena in molten metals-refractory borides systems

Non-oxide ceramics, such as carbides, nitrides and borides represent one of the fastest growing classes of new advanced materials. Among them, transition metals ceramic diborides, in particular Titanium, Zirconium and Hafnium diborides, are members of a family of materials with extremely high melting temperatures, high thermal and electrical conductivity, excellent thermal shock resistance, high hardness and chemical inertness. These materials -Ultra High Temperature Ceramics (UHTCs)- constitute a class of promising materials for use in high performance applications, where high temperatures, high thermal fluxes, severe surface stresses are involved. However, the possibility to exploit commercially their peculiar characteristics often depends to a great extent on the ability to join the ceramic parts one to the other or to special metallic 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 joining processes. Provided that a large number of ceramic materials are not wet (or poorly wet) by pure liquid metals, their wettability by liquid-metal systems can be significantly modified by using either non-reactive metallic solutes capable of adsorption at the metal-ceramic interface, or reactive elements, so that the energetic contribution coming from reaction (and dissolution) free energy release could contribute to lower the total interfacial energy of the solid-liquid system, increasing, at the same time, and thermodynamic adhesion. Recent 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 Ag and its alloys (Cu, Ti, Zr, Hf) are reported and discussed as a function of time, compositions and structure of the ceramic and of the alloy involved. In particular new data are shown about the interactions of Ag, Cu and Au in contact with ZrB2. Models are also used to interpret the wetting behaviour and the adsorption/reaction interfacial phenomena involved

Ricerche sulle interazioni di bagnabilit? tra piombo fuso e vetro "E"

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Interazioni tra metalli liquidi e ceramici ad elevate prestazioni per la realizzazione di giunzioni metallo-ceramiche operanti in condizioni estreme

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Studio delle propriet? di interfaccia metallo-ceramico e processi di giunzione

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Diamond-metal interfaces in cutting tools: A review

This paper reviews studies undertaken on diamond cutting tools, with particular regard to the characteristics and performance of diamond/metal interfaces. The affinity of carbon to metals, as well as the wettability of diamond by molten metals and the advantage of using coated diamonds under certain cutting conditions, are described. The choice of the appropriate metallic matrix in the field of both impregnated and brazed diamond tools, is discussed in terms of the diamond/alloy interface, mechanical properties of the segment, diamond wear speed and desired cutting performance. The effect of several principal elements and elements added in minor amounts to the metallic matrix is critically evaluated. Relevant open questions, related to the optimization of cutting tools performance, are outlined, with special attention directed towards the need for advanced fundamental studies on the functional link between Work of Adhesion and Work of Fracture

ZrB 2-SiC/Ti6Al4V joints: Wettability studies using Ag- and Cu-based braze alloys

In this paper, a study is presented on the wettability of ZrB 2 ceramics (pure ZrB 2 and ZrB 2-SiC composite) and of the Ti6Al4V alloy by pure Cu and Ag and their alloys with Ti to be used in subsequent brazing processes. Wettability tests aimed at elucidating the interfacial interactions and the role of active metal additions (i.e. Ti) in the brazing procedure, performed in a temperature range between 950 and 1150 \ub0C, showed that Cu wets pure ZrB 2 quite well (contact angle \u3b8 = 70\ub0), while Ag does not. The composite was wetted in a similar way by Cu, while for Ag a \u3b8 = 97\ub0 was found. The addition of Ti improved the wetting (\u3b8 between \u3030\ub0 and 20\ub0) and segregation at the metal- ceramic interface occurred. As expected, both Cu(Ti) and Ag(Ti) wetted the Ti6Al4V alloy very well. In particular, it was shown that Cu can already produce a liquid phase at 950 \ub0C with partial dissolution of the solid phase. These results are discussed in terms of interfacial reactions and taking into account the sintering aids used for the ceramic body. This study is aimed at defining a brazing process that leads to reliable metal-ceramic joints as a result of chemical interactions of the liquid phase with and diffusion into the adjoining solids. Exploratory joining tests were performed that showed, by means of shear tests coupled with microstructural and microchemical description of the interfaces, the possibility to design sound ZrB 2-SiC/Ti6Al4V joints. \ua9 Springer Science+Business Media, LLC 2012

Effetto ternario del germanio sulla crescita di fasi all\u27interfaccia tra leghe liquide Ag-O e silice

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High temperature interfacial interactions in Ni-X/HfB2 systems

This study presents data on the wettability and the interfacial characteristics of systems based on HfB2 ceramics hot-pressed with different sintering aids (B4C, HfSi2) in contact with liquid Ni-X alloys (X= Ti, B) to promote/control wettability. The experimental data, obtained by sessile drop tests at 1500 ?C under carefully controlled conditions, are reported and discussed as a function of time, compositions and ceramic micro-structure. The specimens are analysed by optical microscopy, SEM, EDS, X-ray diffraction and by means of specific mechanical adhesion tests. The main results, at present, are: 1. Liquid Ni first wets and then penetrates polycrystalline HfB2 substrate via grain boundaries. 2. During high-temperature interaction, two phenomena take place that are responsible for wettability kinetics and mass transfer in the Ni/HfB2 couples, i.e. I substrate dissolution, resulting in the displacement of the liquid/substrate interface, II drop spreading, resulting in fast movement of the triple line along the substrate surface. 3. The "competition" between these two phenomena affects the final shape of the drop/substrate interface, i.e., the drop has a lens-like shape if the substrate dissolution is a dominant factor while it forms a thin layer on the substrate if kinetics of spreading is fast but substrate dissolution is negligible. 4. Metallurgical factors, but not interfacial reactions, are responsible for the final structure of drop/substrate interface that is formed during cooling, but not at the test temperature, mainly because, during cooling, recrystallization of HfB2 through the liquid state takes place. 5. Other precipitates (e.g. HfC, Ni intermetallics) are evidenced which can be formed by two processes: ? in situ directly in the drop by chemical reaction between Hf, dissolved in liquid Ni from HfB2, and Ni itself and/or C, dissolved from B4C or from carbon present as a main impurity ? due to chemical reaction between Hf, dissolved in liquid Ni from HfB2, and residual B4C grains, opened at and released from the substrate during dissolution of surrounding HfB2 grains. The unusual shape of drop/substrate interface with well-distinguished two-regions, the specific spreading kinetics curves and the influence of interfacial structure on wetting and adhesion (also mechanical), as well as the need of further research aiming at optimising the ceramic/ceramic joining procedures, will be discussed

Interazioni metallo ceramiche

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Wetting and interfacial phenomena in Ni-HfB2 systems

Non-oxide ceramics, such as carbides, nitrides and borides represent one of the fastest growing classes of new advanced materials. Among them, transition metals ceramic diborides, in particular Titanium, Zirconium and Hafnium diborides, are members of a family of materials with extremely high melting temperatures, high thermal and electrical conductivity, high hardness and chemical inertness. These materials -Ultra High Temperature Ceramics (UHTCs)- constitute a class of promising materials for use in high performance applications, where high temperatures, high thermal fluxes, severe surface stresses are involved. It should be remarked that the possibility to exploit commercially their peculiar characteristics often depends to a great extent on the ability to join the ceramic parts one to the other or to special metallic 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 joining processes. However, very few data are available on these properties to day for UHTCs, and in particular for HfB2. This study presents data on the wettability and the interfacial characteristics of systems based on HfB2 ceramics hot-pressed with different sintering aids (B4C, HfSi2) in contact with liquid Ni and its alloys with active metals to promote wettability. The experimental data, obtained by sessile drop tests at 1500 ?C under carefully controlled conditions, are reported and discussed as a function of time, compositions and structure of the ceramic. The specimens are analysed by optical microscopy, SEM and EDS and by means of specific mechanical adhesion tests. A comparison is also made with previous studies on the same boride in contact with lower melting alloys (Cu, Au, Ag based). Thermodynamic models are used to interpret the wetting behaviour and the adsorption/reaction interfacial phenomena involved