Microstructural investigation and indentation response of pressureless-sintered α- and β-SiC

Abstract

The microstructure and indentation response of pressureless-sintered α- and β-SiC were studied using a high-resolution electron microscope and analytical electron microscopy. The materials were manufactured with boron and carbon as sintering aids. It was found that the overall porosity of the materials was very low but a large number of carbon inclusions were present. X-ray diffraction revealed the fabricated β-SiC material was of the same 3C polytype as the initial starting powder; however, electron microscope observations indicated that the material contained a high density of faulting of the α-forms. High-resolution imaging of grain boundaries in these materials indicated that the boundaries were very clean, and when they contained an amorphous intergranular film it was at most 0.5 to 1 nm thick. The presence of boron was not detected. Deformation due to identation took several forms. Firstly, radial cracks extending from the corners of the indent suffered little hindrance from the matrix microstructure, such that transgranular fracture was the dominant mode. Secondly, the deformation zone beneath the indentations showed copious lattice microcracks with some preferred orientation during crack formation and propagation.