Role of Sintering Atmosphere

2.4.3. Role of Sintering Atmosphere

The effect of atmosphere on the sintering of nano-crystalline α-silicon carbide, prepared by attri- tion milling, has been studied under vacuum, argon and nitrogen atmospheres between 2000° - 2100°C after doping with boron carbide and carbon. It has been found that the sintering atmosphere has a very critical influence on the sintering of silicon carbide. The vacuum atmosphere helps in the sintering of nano-crystalline silicon carbide doped with boron carbide and carbon to about maximum theoretical density, whereas the nitrogen atmosphere has a retarding effect and does not yield full densification, while the effect of argon atmosphere takes an intermediate role in the sintering process [16].

Silicon carbide, when sintered to about theoretical density, shows a remarkable behaviour in terms of many useful technical applications like ceramic engines, components in aerospace etc. This is due to some interesting and favourable thermo-mechanical properties at high temperatures. Therefore, the sintering behaviour of silicon carbide, particularly of nano particles, assumes great importance [2, 3]. The sintering of such nano-crystalline silicon carbide should not only be studied by using various dopants to enhance the diffusion of both silicon and carbon at different temperatures, but it should be also studied under different atmospheres to see the effect of sintering atmosphere on the overall sintering behaviour [16].

The effect of sintering atmosphere was found to be very important in addition to the sintering additives and temperatures in the microstructural evolution of sintered silicon carbides. The atmos- pheric effects have been studied thoroughly by Prochazka et. al. [46] for sintering of silicon carbides. The work was later supplemented by that of Murata and Smoak [28] who confirmed by the use of the ‘compound’ additives that the diffusion of not only boron, but also of nitrogen and phosphorous may take place simultaneously during the sintering process. One of their significant findings is that the maxi- mum density of sintered silicon carbide was achieved at the concentrations of the maximum ‘solid solubility’ of the additives.

Mirzah et. al. [38] further continued the studies on pressureless sintering in vacuum, Ar, He and N2 atmospheres by using α-silicon carbide. No differences in the densities or microstructures could be detected between vacuum, Ar and He as sintering atmospheres. They found that the samples of sintered silicon carbide in vacuum exhibited a carbon-rich surface layer, which was due to the decomposition of silicon carbide. These workers also observed that the sintering in nitrogen required approximately 150°C higher sintering temperature to achieve a full densification. A thermodynamic analysis was carried out by Venkateswaran and Kim [47] who reported by XPS studies about the formation of a thin layer of boron nitride (BN) when α-silicon carbide was sintered in nitrogen atmosphere. They observed that higher temperature is required for the complete densification of silicon carbide in nitrogen atmosphere.

In the investigation of Datta et al. [16, 18], nano-crystalline SiC particles, meticulously prepared by controlling and eventually optimizing various grinding parameters, have been sintered between 2000° - 2100°C under three different atmospheres, like vacuum, argon and nitrogen, in order to see the effect of atmosphere on the sintering behaviour. The results are explained in terms of a diffusion model (see later in the section - 2.7.3).