Comprehensive study on the effect of SiC and carbon additives on the pressureless sintering and microstructural and mechanical characteristics of new ultra-high temperature ZrB2 ceramics

The combined effects of SiC and carbon additives on the densification, structure and characteristics of pressureless sintered ZrB2–SiC ceramic composites were studied. The ZrB2–SiC (1–10wt%) composite powders were mixed by 1–2wt% carbon. The prepared powder mixtures were then cold-consolidated and sintered in argon environment in the temperature range of 1800–2100°C for 2h. The constituted phases and microstructural evolutions were studied using x-ray diffraction and scanning electron microscope equipped with an energy dispersive x-ray detector. The obtained results concluded that the densification was increased by extending the sintering time and also by prior holding of specimens at 1650°C. This caused by reduction of such oxide impurities of ZrO2, B2O3 and SiO2 on particle surfaces via the formation of new phases in the system. However, this was totally induced the enhancement of sintering phenomenon. The presence of SiC with average grain size smaller than that for ZrB2 revealed to aid the densification by enhancing green specimen density. Furthermore, increment of the carbon content through erosion of milling media inhibited the matrix grain growth. Both of SiC and carbon additives seemed to enhance the reduction of oxides formation. The addition of SiC and carbon was found to be advantageous for densification up to 99.5% of the theoretical density. According to the examinations, it was demonstrated that the specimen strength was varied with additions of SiC and carbon. Indentation toughness of the samples was examined and results exemplified toughness increment with SiC. However, carbon had a detrimental effect on fracture toughness. The microhardness values were increased with increasing SiC and carbon content due to the reduction of porosity volume percentage in specimens.