Effect of oxygen content on the sintering behaviour and mechanical properties of SiC ceramics

In the current work, SiC powders with various levels of oxygen content were obtained by two ways: acid leaching and heat treatment; and these SiC powders were densified by a pressureless sintering technique with boron carbide and carbon additives. The techniques of X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, transmission electron microscope, and mechanical testing were combined to examine the effects of oxygen impurities on the densification process of SiC ceramics. The experimental analyses indicated that high oxygen content is detrimental to the final sintering and mechanical properties of SiC ceramics. As the oxygen content increases from 0.55 to 2.10 wt%, the relative density and the hardness both decrease: the relative density drops from 98.53% to 70.55% (a 28% decrease), and the hardness drops from 27.1 +/- 0.7 GPa to 6.7 +/- 1.1 GPa (a 75% decrease). Furthermore, oxygen impurities can accelerate the abnormal growth and coarsening of SiC grains during sintering. In order to impede abnormal grain growth and achieve high densities of SiC ceramics, it is proposed that the oxygen content of the powders should be limited to less than 1.30 wt%.