Creep mechanisms in quasi-plastic silicon nitride by instrumented indentation

Quasi-plastic creep behavior of the commercial, fine-grained silicon nitride grade, ST 1, was investigated using variety of techniques with the focus on the analysis of instrumented indentation. Creep deformation in this material was characterized by high creep rates at temperatures above 1300 °C and failure strains around 20%. It was accompanied by strong oxidation, cracking of the oxide layers, excessive cavitation at multigrain junctions and slight texture formation. Instrumented indentation revealed degradation of indentation moduli in the oxide layers and enhancement of oxidation and elastic moduli degradation during creep. Because of the similarities between the mass transport processes in cavitation, diffusion processes involved in oxidation and similar activation energies, both creep and oxidation occur simultaneously, however, oxidation is enhanced by external stress. Texture formation implied from disappearance of α-silicon nitride and anisotropy of indentation modulus contributes insignificantly (<5%) to total tensile strain. Creep processes in the studied material can be explained within the expanded cavitation creep model of Luecke and Wiederhorn assuming that cavitation is facilitated by low viscosity residual glass and small matrix grain size. Tertiary-like creep is attributed to the gradual increase of the applied stress resulting from the reduction of the effective cross section due to the formation of cracked oxide layers. Size and pre-oxidation effects were predicted and confirmed using creep samples with different gauge size.