The plasticity response of 6H-SiC and related isostructural materials to nanoindentation: Slip vs densification

As part of a programme investigating whether slip (by dislocation motion at some critical resolved shear stress) or densification (by structural collapse at some critical hydrostatic pressure) dominates the plastic deformation response of open-crystal-structured ceramics having the diamond, zinc blende or simply-related structures, transmission electron microscopy (TEM) and high resolution scanning electron microscopy (HRSEM) are being used to characterise the deformation structures in, and around, nanoindentations made over a range of loads in single crystal samples of Si, Ge, SIC and various LII-V and II-VI compound semiconductors. Since SiC is believed to lie close to the boundary between those materials which slip and those which densify as primary plasticity responses to contact with Vickers and Berkovich indenters, this part of the study has focused on establishing the deformation mechanisms of 6H-SiC during nanoindentation. TEM of nanoindentations in (0001) 6H-SiC samples has established that dislocation slip is indeed the sole mechanism of plastic deformation from the nucleation of a few dislocation loops at or near the theoretical strength - to extensive dislocation plasticity. Also, HRSEM observations have revealed slip steps of limited extent in 6H-SiC samples with more extensive slip steps arrays found in all the other compounds. By contrast, both Si and Ge show evidence of heavily deformed - and sometimes extruded - material believed to be a characteristic of structural collapse accommodating at least some part of the indentation strain before any dislocation slip occurs.