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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: The effects of Mo ion implantation into SiC monocrystalline substrates and succeeding annealing treatments on the interfacial characteristics of 6H–SiC were investigated by means of wettability tests. Ni (reactive) and the Ni–56Si alloy (non-reactive) have been used as liquid phases as a means to evidence the influence of surface and interfacial tensions on solid–liquid interactions. The variations of contact angle as a function of Mo implantation and of temperature are analyzed and discussed. The results indicated that a significant improvement of wettability was mainly related to the increase of surface energy ( ) of SiC substrate and to the presence of Mo, rather than to the surface roughness of the ceramic substrate. With increasing annealing temperature, the contact angle of the Ni–56Si/SiC system increased as a result of the crystallization of the SiC amorphous surface and the loss of Mo at the substrate surface due to oxidation and diffusion/vaporization. The influence of these phenomena on technological processes such as metal–ceramic brazing is also taken into consideration. [Copyright &y& Elsevier]

Pure ZrB and ZrB-SiC composites were joined to Ti6Al4V at 1100 °C using B-Ni50 (at.%) as a filler alloy. The brazing medium and the processing parameters were chosen on the basis of specific wetting tests which showed the good adhesion properties of the B-Ni alloy with both the ceramic and the Ti alloy; interfacial reactions were foreseen and interpreted by phase diagram analysis. A multilayer metal-ceramic interfacial structure was observed in the joints and a key role was played by Ti coming from Ti6Al4V: it worked as the active element enhancing the adhesion of the liquid to the ceramic and segregated at the interface forming TiB. A satisfactory mechanical performance was obtained for ZrB-SiC/Ti6Al4V joints, which exhibited a room temperature shear strength of 74 MPa. [ABSTRACT FROM AUTHOR]

Metal-ceramic joints represent an important choice for biomedical devices, in orthopaedic field or as micro-stimulators. ZrO 2 -Ag-HEA joint is here proposed to replace the currently used Ti6Al4V by new Ti-based refractory HEAs. To evaluate the system reactivity, wettability of ZrO 2 by different Ag-based alloys at 1000 °C and their interfacial layers were studied, for the first time. Interpretation and discussion of systems reactivity were supported by thermodynamic calculations, according to the CALPHAD approach and the ad-hoc Ag-Ti-Zr-O thermodynamic database, implemented in this work. A good wettability was reached at 1000 °C adding Ti to Ag: Ag-4Ti and Ag-8Ti reached contact angles of 81–85°. Ag-4Ti-2Zr showed the best wettability (77°) and the most complex interfacial microstructure. A continuous hcp (Ti,O), ~ 20 µm thick, promoted the wettability of Ag-4Ti and Ag-8Ti. In both cases, AgTi was the only intermetallic compound found in the bulk. Ag-4Ti-2Zr formed a thicker interfacial layer of hcp-(Ti,Zr,O) + bcc-(Ti,Zr) + a less compact Ag(Ti,Zr) 2 layer. Here, a low amount of Ag(Ti,Zr) and Ag(Ti,Zr) 2 was found in the Ag matrix. Since Ag resulted a promising filler to braze Ti-rich substrates to ZrO 2 at 1000 °C, preliminary HEA-Ag-ZrO 2 joints were characterized. A sound and defects-free interface was observed, with a first layer, mainly formed by Ag, Ti and Zr (HV ~ 600), and a Ag-based interface (HV ~ 250), extending until the HEA. Further works envisage mechanical and electrochemical evaluations of the system to pave the way for the production of novel HEA-Ag-ZrO 2 joints to be used in biomedical applications. [Display omitted] • Ti addition to Ag promoted ZrO 2 wettability at 1000 °C forming hcp-(Ti,O) at the interface. • Ag-4Ti-2Zr tested on ZrO 2 at 1000 °C showed the best wettability (ϑ<80°). • An ad-hoc Ag-O-Ti-Zr database was assembled and used to support reactivity discussion. • Ag-4Ti-2Zr showed an hcp-(Ti,O)+bcc-(Ti,Zr)+Ag(Ti,Zr) 2 interfacial layer formed at 1000 °C. • Ti 1.5 ZrTa 0.5 Hf 0.5 Nb 0.5 HEA was successfully brazed to ZrO 2 by using a Ag filler at 1000 °C. [ABSTRACT FROM AUTHOR]

A composite joining technique, using a Ni–56Si filler alloy and Mo as interlayers, was used to join SiC to SiC and to Kovar. The wetting of the Ni–Si alloy on SiC ceramic was studied in a vacuum at 1,350 °C by the sessile drop technique as a function of time; the non-reactive wetting characteristics in the Ni–Si/SiC system were confirmed, with an equilibrium contact angle of about 23°. SiC/SiC joints were fabricated by two processes using a Ni–Si/Mo/Ni–Si structure as the interlayer. SiC/Kovar joints were produced by means of a multilayer structure: molybdenum, which is used as the interlayer, was joined to Kovar on one side by means of transient liquid phase bonding and to SiC on the other side, using a Ni–Si coating as a filler alloy. The resulting joints were analyzed and discussed in terms of joint morphology and microstructure, joint strength, and fracture behavior. Two interfacial layers form at the Kovar/Mo and the Mo/Ni–Si interfaces due to dissolution and interdiffusion phenomena between the metallic elements, without there being any observable reactions with the SiC component. The type of joining process and the experimental conditions used play a key role in determining the joint microstructure and composition, the joint strength and its fracture behavior. [ABSTRACT FROM AUTHOR]

In this work, a systematic study of the wetting behavior of two Ni-based and one Co-based superalloys, used, in particular, for the fabrication of turbine blades, is presented with reference to different ceramic substrates: sapphire, polycrystalline alumina, zirconia, and mullite. 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 performed by SEM/EDS analysis in order to check the final characteristics of the solidified 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, morphological and energetic properties of the systems. [ABSTRACT FROM AUTHOR]

Wetting of molten Ni–56 at.% Si alloy on different substrates (SiC ceramic, Ni- and Co-based superalloys, Kovar, and Mo) are performed under different experimental conditions by the sessile drop technique. Temperature, atmosphere, and substrate composition play the key roles in determining the wettability, the spreading characteristics, and the interfacial morphology of the final interfaces. The non-reactive wetting characteristics in Ni–Si/SiC system are confirmed, with a spreading rate increasing with temperature increasing. In the Ni–Si/metal systems the spreading process is determined by the competition between spreading along the substrate surface and the interfacial interactions. Excellent wettability and fast spreading are found in the Ni–Si/Co-based superalloy, Ni–Si/Kovar, and Ni–Si/Mo systems at both the temperatures (1100 and 1200 °C). These results can be used as a reference guide for joining SiC to these metallic components, or to itself, using the Ni–Si alloy as filler metal. [ABSTRACT FROM AUTHOR]

Abstract: A renewed interest in the effects of oxygen on the wetting of solid oxides has recently risen in connection to the development of the technique of “air brazing” which makes use of the strong effect of oxygen to increase the wettability of oxides by means of processes running in air or in atmospheres with high oxygen content. Adsorption of oxygen not only at the liquid–vapour surface but also at the solid–liquid interface has been postulated by many previous researches, mainly on the basis of thermodynamic considerations. Along the same line, new results of the wetting behaviour in the system Ag/α-Al2O3 as a function of oxygen partial pressure are presented, with the simultaneous measurement of the liquid surface tension. These results are compared with the existing ones, and discussed on the basis of thermodynamic principles. The resulting work of adhesion is compared with the “work of separation” computed by the density functional theory (DFT) approach. DFT calculations are also employed, at variance with previous models, to investigate the structures that are formed at the interface upon addition of oxygen in different sites energetics, atomic and electronic properties of this oxygen-rich interface are discussed together with the connection with experiments. [Copyright &y& Elsevier]

Several industrial processes involving SiC coupling to Al–Ti alloys (e.g. metallization of SiC components, brazing of SiC parts) require an in-depth knowledge of Al–Ti/SiC interactions occurring at high temperatures. To this end, the surface reactivity between SiC and Al–Ti alloys (Al 3 Ti and (Al + Al 3 Ti) systems) was analyzed by specific experiments (wetting, DSC, microstructural examinations) as well as by a thermodynamic approach (CALPHAD method). An Al–C–Si–Ti thermodynamic database was successfully established to calculate several sections and projections in order to compare the computed, expected solid phases formed at the interface with those characterized in wetting experiments. In this way, the change in liquid and solid phases was interpreted and discussed, defining the Ti 3 (Al,Si)C 2 mixed MAX-phase as the main interfacial product created by the chemical reaction, as a function of temperature and alloy composition. This work constitutes a guide for the choice of operating parameters in processes such as brazing or SiC metallization in microelectronic applications, in which the control of interfacial products is one of the most delicate production steps. The approach proposed to monitor the pathway of liquid composition with time and temperature during liquid/solid interaction, successfully applied to interpret the microstructure obtained in wetting experiments, is a promising method for interpreting more complex cases such as homogeneous or even heterogeneous brazing processes. Image 1 • Interpretation of AlTi/SiC reactivity by an experimental/thermodynamic approach. • Sequence of interfacial phases defined by a new Al–C–Si–Ti thermodynamic database. • Al3Ti/SiC reaction forms a liquid at a temperature lower than the Al 3 Ti melting point. • The MAX-phase is demonstrated to form more easily at higher temperatures. • Formation of TiC at the interface explained by means of diffusion-limited phenomena. [ABSTRACT FROM AUTHOR]

The wetting of transparent YAG (Y 3 Al 5 O 12 ) by alloys of the AgCuTi system has been studied for the first time. The role of the activity of Ti in determining the interactions between the brazing alloys and the YAG substrate has been analyzed by CALPHAD method. The low contact angle values ensure very good (at 950 °C) and fair (at 850 °C and 820 °C) wettability conditions. A significant reactivity occurred at the solid–liquid interfaces, with the formation of new phases. TiY 2 O 5 was found at Ag–Ti/YAG interfaces, while a second layer, identified as “M 4 X”, associated to the significant wettability improvement, was specifically found only in the AgCuTi/YAG samples tested at 950 °C. All these phases were identified by the newly calculated Ag–Cu–Ti phase diagram and were confirmed by SEM and EDS analyses. The obtained good wettability and the maintained transparency of the YAG are promising for the production of brazed optical windows to be used under extreme conditions. [ABSTRACT FROM AUTHOR]

Abstract: Good wetting and low reactivity of metal/ceramic couples are key factors in many technological processes, in particular in metal/ceramic joining, to avoid degradation of ceramics and to achieve the desired properties during service. Silicon carbide is a covalent material of great technological interest due to its excellent overall properties. Starting from a survey of the surface energies of SiC and liquid metals, the reactivity and wettability of pure metal/SiC systems, as well as the wetting behavior and mechanisms of (liquid metal+Si)/SiC systems are reviewed for understanding the interfacial bonding and for supporting the development of application-oriented processes like non-reactive brazing. Silicon chemisorption and interactions between the molten alloys and SiC at the metal/substrate interface and the intrinsic properties of the alloys or of the pure metals are considered to play the key roles in interfacial bonding. In particular, additions of Si can limit or even suppress the substrate dissolution leaving the solid–liquid interface nearly undisturbed. At the same time, oxidation–deoxidation processes at the SiC surface are the basic mechanisms to be controlled in order to allow the liquid phases to contact a “pure” SiC surface. The need of further investigations, covering basic interfacial phenomena including both experiments and ab-initio modeling of the solid–liquid interfaces is strongly underlined. [Copyright &y& Elsevier]

Abstract: This study presents data on the wettability and the interfacial characteristics of HfB2 ceramics, in contact with liquid Ni with B additions to promote/control wettability. The experimental data, obtained by sessile drop tests at 1130, 1200 and 1520 ∘C under carefully controlled conditions, are reported and discussed as a function of time, compositions and metal–ceramic micro-structure. In order to provide a basis for the correct interpretation of the experimental results, the ternary sections of the Ni–Hf–B system computed by CALPHAD methods are compared, with a good agreement, with the behaviour of the liquid Ni(B) phase in contact with the solid boride. By this way, the complex phenomena occurring at high temperatures between the molten phase and the diboride ceramic substrate (dissolution, reaction, intergranular penetration) and the formation of new phases during the solidification process have found a sound theoretical explication. The evolution of contact angles with time (spreading kinetics), which reached in all cases a nearly complete wetting, has also been interpreted in terms of reaction–dissolution interfacial processes. [Copyright &y& Elsevier]

Abstract: The wettability and reactivity between polycrystalline hot-pressed HfB2 and liquid Ni, Ni–Ti and Ni–B alloys have been investigated by the sessile drop method up to 1520°C. Independently of the sintering aids (B4C, HfSi2), high-temperature interactions led to the formation of a bimodal interface profile, due to the competition between the strong dissolution of HfB2 in the liquid phase and drop spreading along the substrate surface. Ni demonstrates good wetting with the HfB2 substrates (θ =20°). Compared to Ni, high Ti-containing alloys reveal much faster wettability kinetics with a final contact angle below 10°, accompanied by the formation of an interfacial Ti-rich reaction product. The eutectic NiB alloy shows complete wetting and fast spreading on HfB2 and minor dissolution of the ceramic into the liquid, a clear indication that higher B contents could optimize the interfacial structure. These results are of practical interest for liquid-assisted composite synthesis and joining of HfB2-based ceramics. [Copyright &y& Elsevier]

The wetting and spreading of molten Cu on SiC substrates with or without Co–Si(–Mo) coatings at 1120 °C were investigated by the sessile drop technique. The Co–Si(–Mo) coatings on the SiC substrate were prepared by liquid phase sintering process under a vacuum. The interfacial behaviors of the coating/substrate systems and the Cu/coated SiC wetting couples were analyzed. The experimental results indicated that the final contact angle of the Cu/SiC system at 1120 °C, for a holding time of 10 min, decreased from ~142° without coating to 12°, 15–27° and 7° with the corresponding Co–Si, Co–Si–10Mo and Co–Si–20Mo coatings. This result was closely related with the interactions between the Cu drop and the coatings. No reaction layer was observed at any of the coating/SiC interfaces before the wetting tests. However, a thin Mo–Co–Cu–Si layer and a graphitization layer with different thicknesses formed at the Cu/Co–Si–10Mo coated SiC and Cu/Co–Si–20Mo coated SiC interfaces. Moreover, the graphitization layer disappeared at the Cu/Co–Si–10Mo coated SiC interface when the thickness of the Co–Si–10Mo coating increased to ~60 μm. [ABSTRACT FROM AUTHOR]

Highlights: [•] Melting equilibria occurring in the Ni-rich part of the B Ni Zr system were studied. [•] A combined experimental (DTA, XRD) and computational (CALPHAD) approach was used. [•] A solubility range in the Zr2(Ni21 − x Zr x )B6 phase was found to exist for 0.14⩽ x ⩽0.68. [•] The obtained results can help in interpreting data coming from wetting tests. [ABSTRACT FROM AUTHOR]