Effects of processing on microstructure and indentation response of AlN doped SiC

Sintering of silicon carbide (SiC) requires high temperature and pressure due to the covalent nature of its bonding. Therefore, sintering additives are used to lower the sintering temperature and to control the microstructure. In this work, role of aluminium nitride (AlN) and carbon in pressure assisted densification is studied as the literature was not clear on whether AlN always induces liquid phase sintering (LPS). It is shown that mixing suffices to produce green bodies in which AlN is present as individual particles. When heated above 1700⁰C the AlN redistributes to grain boundaries and triple junctions through vapour transport and grain boundary diffusion, which causes the onset of densification. Addition of AlN and carbon together leads to microstructure more consistent with solid state sintering (SSS) than with LPS which was induced with the addition of yttria and AlN. Nano-indentation was used to measure hardness as a function of strain rate and temperature. It was found that hardness increases 0.8 GPa per decade strain rate and decreases with increasing temperature. Since in the absence of cracking hardness is a function of stiffness and yield stress, nano-indentation was used to calculate the Peierls stress (12 ± 1 GPa), activation energy (1.1 ± 0.3 eV) and the activation volume (1.44 x 10⁻²⁹ m³) of dislocation glide in SiC. Grain size was found to have a minimal effect on plasticity of the material when indents are small. Consistent with widely reported trends, the hardness was found to decrease when higher loads are used. It is argued that this decrease in hardness is due to an increase in crack length relative to the indent size. An empirical model, based on dimensional analysis, describes the observed decrease in hardness rather well. Observations of a new damage mechanism after unloading of large load indents are presented and a mechanism is proposed.EThOS - Electronic Theses Online ServiceGBUnited Kingdo