Yielding, strain hardening, and creep under nanoindentation of precursor-derived si-C-N ceramics

Abstract

The plastic deformation of the precursor-derived Si-C-N ceramics during indentation at room temperature displays permanent densification and creep, the former effect promoting strain hardening. This paper attempts to quantify the above two phenomena in these materials using, (i) reverse analysis of nanoindentation load-displacement curves in terms of yield stress (σy) and strain hardening exponent (n), and (ii) indentation creep analysis in terms of the strain rate sensitivity (m), respectively. The estimated values of n were commensurate with the capacity for densification, and were high among the amorphous Si-C-N materials. The present results indicate the importance of the optimum network connectivity in promoting densification in lieu of shear deformation in amorphous covalent networks. The densification-induced strain hardening led to a load dependence of m, and controlled the indentation size effect (ISE). The evolution of m showed good agreement with the cluster model, which relates the increase in the number density of isolated regions or discontinuities in the microstructure to a corresponding increase in m. Phase separation in these materials promoted the shear mode of plastic deformation and led to a decrease in n, an increase in m, and increased vulnerability to ISE. © 2009 The American Ceramic Society.