Wetting borides

Figure 107. Optical micrograph of a TiB2-Ni composite, pressurelessly sintered at 1600C. Note the incomplete wetting of TiB2 by liquid Ni and the formation of Ni borides in the matrix phase.

Sch] Schenk, H., Steinmetz, E., GohUce, R., Contribution to the Study of the Chemical Activity of the Elements P, S, Si, Cu and Cr in Molten Iron Solutions Saturated with Carbon (in German), Zrc/ . Eisenhuettenwes.,37(12), 919-924 (1966) (Thermodyn., Calculation, 30) [1966Yas] Yasinskaya, G.A., Wetting of the Refractory Carbides, Borides and Nitrides by Molten Metals (in Russian), Poroshk. Metall. (Kiev), 43(7), 53-55 (1966) (Interface Phenomena, Experimental, 5)... 

Like the carbon black, fine amorphous boron is poorly wetted with salt melts and emerges to the surface of the salt immediately after the salt is molten, while the refractory metal powder is left on the bottom. Thus, the first stage of the S5mthesis is the formation of lithium or calcium borides. 

Due to the fact that TiB2 is an expensive material, which is difficult to sinter, very brittle, and sensitive to mechanical and thermal shock, one has also tested composite materials, such as TiB2 mixed with carbon or borides (ZrB2, BN, etc.) or carbides (TiC, SiC, etc.). Composites can be sintered into various shapes or be applied as coatings on regular carbon blocks. Wettable cathodes (or inert cathodes) are wetted by liquid aluminium, so that a thin film of aluminium can be maintained on the cathode surface [4-7],... 

Generally, wetting between a material and a molten metal depends on various conditions such as the surface conditions of the metal and the material (e.g., the presence of oxidized films). According to a study of the wetting between hBN and molten A1 (211), interfacial reactions such as the formation of nitride or boride films of metal components in contact control the... 

The liquid phase at the sintering temperature was presumably constituted by Mo/Ta-Si-C-0, and non-wetted interfaces were found when either MoSij or TaSi were added to ZrC, HfC or TaC. It can be hypothesized that the liquid formed in the carbides at around 1400°C had high viscosity and tended not to completely flow through the grains leaving clean grain boundaries. On the contrary, borides of Zr, Hf and Ta sintered with addition of MoSij or TaSi displayed wetted interfaces, owing to the presence of boron in the liquid phase, which lowered the viscosity (Karlsdottir, 2007). 

This chapter summarises the recent studies on the wetting behaviors of ceramics which include carbides, oxides, nitrides, and borides at a wide temperature range and under various atmospheres. Also, their joining experiments, mainly by brazing and reaction joining methods, are reviewed. The typical and fundamental physical, mechanical, and microstructural examinations, such as contact angle, joint strength, and interlayer structure, are also presented. 

The research in the wetting of borides by metals and alloys are summarized in Table 3, in which the following regularities are obvious ... 

With the more interesting in the UHTCs, transition metal borides have received much attention and quickly become one of the hottest research topics in recent years. Although the wetting experiments conducted on borides, predominantly on ZrB and HfB, are hmited in the past ten years, they are important to explore and extend the potential applications of the boride-based ceramic by means of joining, such as diffusion bonding and brazing method. 

Presently, there are primarily the wetting tests of pure metals to borides, such as Ag, Au, Cu and Ni. The wetting of binary alloy is just at the beginning step. Much work is required to clarify the intrinsic wettabihty of the diborides. 

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