The Effect of Surface Oxides During Hot Pressing of TiB₂

Abstract

Hot pressing of TiB2 has been investigated with particular emphasis on the evolution of secondary phases originating from the initial surface oxide layer on the TiB2 powders. Carbothermal reduction of the surface oxides during sintering was also investigated by adding carbon to the TiB2 powder. TiO1−xCx was shown to be the main secondary phase in hot‐pressed TiB2, and carbon was shown to strongly influence on sintering process and the amount, composition and distribution of the secondary phase TiO1−xCx. The formation of TiO1−xCx is discussed in relation to volatile boron oxide, which reacts with the graphite die to produce CO gas, which further may cause transport of carbon into TiB2 during sintering before pore closure. Finally it was demonstrated that the density could be controlled by addition of carbon to the TiB2 powder.

Pulsed electric current, in situ synthesis and sintering of textured TiB2 ceramics

“…2(e)) indicated that the bulk phase is TiB 2 whereas the smaller grains contain Ti, O and C. This secondary phase was assumed to be TiO 1−x C x which was also reported to be present in hot pressed TiB 2 . 21 Volatile boron oxide was claimed to be reduced by graphite surrounding the sample to produce CO gas, which transported carbon into the densifying powder compact to form TiO 1−x C x . 21 The titanium oxide and boron oxide were present on the surface of the ZrH 2 and B starting powders, whereas the carbon interdiffused from the graphite die and punches.…”

“…21 Volatile boron oxide was claimed to be reduced by graphite surrounding the sample to produce CO gas, which transported carbon into the densifying powder compact to form TiO 1−x C x . 21 The titanium oxide and boron oxide were present on the surface of the ZrH 2 and B starting powders, whereas the carbon interdiffused from the graphite die and punches. The traces of bright atomic number contrast WC, clearly visible in Fig.…”

Microstructure and mechanical properties of pulsed electric current sintered B4C–TiB2 composites

“…3 and the thermodynamic analysis of Jensen et al [25], it can be concluded that the oxide impurities in the B 4 C and TiB 2 powders can be reduced and completely volatized at temperatures around 2000 • C in vacuum before full densification of the sintering compact is obtained. When using PECS cycles C1 and C2, as shown in Fig.…”

“…According to the thermodynamic calculations, an initial weight loss of TiB 2 powder is expected at relatively low temperatures due to the volatilization of B 2 O 3 . Above approximately 1200 • C, however, the main volatile oxide species is B 2 O 2 and BO at higher temperature [25]. For the residual TiO 2 impurities, the titanium oxide is gradually reduced to TiO as the temperature increases until gaseous TiO and Ti eventually evaporate at temperatures around 2000 • C [25].…”

“…Recently, hot pressing of pure TiB 2 powder and thermodynamic analysis were performed with particular emphasis on the evolution of secondary phases originating from the initial surface oxide layers on the TiB 2 powder [25]. According to the thermodynamic calculations, an initial weight loss of TiB 2 powder is expected at relatively low temperatures due to the volatilization of B 2 O 3 .…”