Effect of oxygen on sintering plasmachemical powders of nonoxide high-melting compounds

Abstract

Electron microscope studies and X-ray structural microanalysis have shown that sintering in the temperature range 800–2000°C proceeds in the solid phase. The granular structure of specimens is due to secondary recrystallization. Development of TiC and TiN recrystallization shows common features but also differences. A common feature is that the centers of grain growth in sintering are not initial nanodispersed powder particles but monocrystalline formations based on pore-free aggregates. The formations are due to coalescence in aggregates at T sint>1000°C, caused by the activation of diffusional mobility in TiC and TiN. The latter is due to the oxygen dissolution (which is always present in plasmachemically synthesized powders) in TiC and TiN and the development of neutral highly mobile complexes which include ions of Ti and O in combination with Ti vacancies. At the initial stage of TiC and TiN recrystallization pores are always present at the grain boundaries. Differences are found in the initial recrystallization temperature ( T r) and the influence of grain boundary pores on its development. For TiN T r is about 1300°C, while for TiC it is about 1400°C. In growing TiN grains, the separation of pores from boundaries takes place at T=1400°C. This causes the anomalous grain growth. At T⩾1400°C the grain growth takes its normal development. In the case of TiC, in the whole range of sintering temperature, the boundaries migrate with pores.