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.