Elastic degradation of an advanced silicon nitride during tensile creep

Tensile creep tests were conducted on an advanced silicon nitride at 1400°C in air. During the creep tests, an intermediate layer with precipitates was formed under the subsurface oxidation layer. Microstructure observation revealed the presence of numerous microcracks, which developed perpendicular to the tensile stress axis. The local elastic properties of the cross-section of crept specimens were examined by the micro-indentation technique and elastic degradation was found in the subsurface layers. While the creep curves exhibit transient and steady-state-like stages, the creep rate gradually increases in the steady-state-like stage. The origin of this slightly accelerated stage is discussed.