Study of sintering mechanisms of Ca-doped yttrium aluminum garnet ceramics: From nanostructure to macroscopic behaviour

This paper aims at elucidate the role of calcium as sintering aid for YAG ceramics elaborated by a reactionsintering process. From structural and microstructural analyses, the solubility limit of calcium was found to be close to 0.065 +/- 0.015 at% at 1700 degrees C. For calcium content higher than this value up to 1 at%, CaAl12O19 crystallized precipitates with a melting temperature close to 1850 degrees C were observed. Whatever its content, calcium segregates at grain boundaries as evidenced by STEM and NanoSIMS chemical analyses at nanometric scale, and strongly decreases densification and grain growth kinetics of YAG ceramics. This result was explained by a solute-drag mechanism due to the segregation of complex Ca2+- V..O clusters with low mobility at grain boundaries. Finally, calcium appears as a very efficient sintering additive to limit grain growth in YAG, thus avoiding the formation of intragranular porosity that is a critical point for the manufacturing of highly transparent ceramics.