Non-uniform microstructure evolution in transparent alumina during dwell stage of high-pressure spark plasma sintering

Abstract

Applying pressure during sintering of ceramics is a common practice, allowing for significant reduction in processing temperature, and fabrication of dense specimens with fine-grained microstructures. However, sintering under relatively high pressure can accentuate stress-related phenomena that occur during the final stage of sintering. The present study describes transparent alumina microstructure evolution during the dwell period of high-pressure spark plasma sintering (HP-SPS) at 1100 °C under 500 MPa. It was observed that the grain size, pore morphology and transparency changed rapidly with dwell time and varied between the center and periphery of the samples. Microstructural evidence suggests that microstructure evolution is related to creep during the dwell period under relatively high pressure and low temperature. Temperature measurements in the HP-SPS setup revealed a small temperature gradient (ΔT=~10 °C) in the opposite direction to the grain size gradient. Based on grain growth kinetics at different regions, we propose that a stress gradient exists and the stress at the sample center is roughly twice as high than at the periphery. Thus, the applied pressure is the dominant factor governing densification and grain growth and is responsible for non-uniform microstructure evolution during HP-SPS. Furthermore, analysis of optical in-line transmittance allowed to decouple the scattering effects of porosity and birefringence. The analysis revealed significant pore growth between dwell time of 5 to 15 min, though, birefringence remained the main scattering source. Understanding these aspects of pressure-assisted sintering will help gain better control of final microstructures, which determine physical, mechanical, and optical properties of ceramics.