Sintering behavior of anorthite-based composite ceramics produced from natural phosphate and kaolin

In the present work anorthite-TCP composite ceramics was produced for the first time by the solid-state sintering process involving the mixture of local natural materials of phosphate and kaolin. Various samples were prepared by varying the kaolin content from 47 to 57 wt%. The composite ceramics were sintered in air at various temperatures ranging from 1250 degrees C to 1325 degrees C and characterized to determine the phase present, relative density, Vickers microhardness, chemical bonding of molecules and microstructural development. In general, all the samples exhibited a hybrid structure, comprising of anorthite and beta-TCP as the major phases with a concomitant minor phases such as TTCP and/or gehlinite depending on the temperature and kaolin content. In addition, increasing kaolin content and sintering temperature were found to be effective in improving the densification and hardness of the sintered body. In particular, sample containing 57 wt% kaolin exhibited excellent densification at 1300 degrees C and 1325 degrees C, achieving above 97% dense bodies and highest hardness of about 6.5 +/- 0.7 GPa. Microstructural investigation revealed that a dense structure was evident for these samples due mainly to enhanced particle coalescence during the liquid phase sintering, resulting in pore elimination and grain coarsening.