Deformation characteristics in 6H-silicon carbide : effects of loading rate, length scale and irradiation

Silicon carbide (SiC) is a crystalline material and exhibits superior mechanical properties and excellent chemical inertness. Consequently, it has been used in various applications which require high durability and strength. Despite the extensive efforts made in investigating the mechanical behaviour of SiC, there are several research gaps which require continuous attention and further research effort. Particularly, the failure mechanism, impact-induced plasticity and deformation behaviour across length scales are not fully understood for SiC. In addition, SiC was proposed as a candidate structural material for nuclear fusion reactor core, and therefore it is of importance to study its resistance to high-dose irradiation damage. The aim of this research was to provide a systematic understanding of deformation characteristics of SiC using various experimental techniques and microscopy tools, with focus on the effects of loading rate, varying length scales and irradiation. Firstly, quasi-static compression tests were conducted to illustrate the basic failure mechanism and to obtain the mechanical properties such as elastic modulus and compressive strength of the material. To explore the strain rate effect, dynamic compression tests were carried out using split Hopkinson pressure bar, and the rate-dependent compressive strength and fragmentation were studied with the aid of post-test microscopic analyses. In order to characterise the mechanical properties of SiC across length scale, indentation experiments were carried out at different load levels, with a specific interest in grain orientation and indentation size effects. To study the potential application of SiC in nuclear fusion reactor core, focused ion beam (FIB) was used as an efficient and time-saving technique to introduce irradiation damage to the material, followed by nanoindentation and microscopic characterisation. The study revealed that both compressive strength and the fragmentation of SiC were dependent on the applied loading rate. In addition, dislocation-associated plastic deformation was observed under high strain rate loading conditions. On the other hand, the mechanical behaviour of SiC was also found dependent on the grain orientation, including the force-displacement response, fracture toughness as well as the hardness and elastic modulus. In addition, size and length scale effects were observed during the indentation experiments performed at a range of load levels. Furthermore, for SiC subjected to irradiation damage, increased hardness and embrittlement were seen for nanoindentations made at low load levels; whereas an overall decreased hardness and strengthened toughness were observed for nanoindentations made at high load levels. To summarise, evident strain rate sensitivity was observed for SiC, with plastic deformation believed to be inertially induced due to geometric confinement during dynamic loading. Grain orientation played an intrinsic role in defining the mechanical response of SiC at nanoscale. In addition, the size effect was attributed to two deformation mechanisms which are associated with strain gradient and defect populations. The response of SiC subjected to irradiation damage was found to be determined by the microstructural variations caused by irradiation, represented by a shallow Ga ion layer on the surface, an amorphous layer in the middle and the substrate underneath.