STRUCTURE AND MECHANICAL-PROPERTIES OF KAOLIN BASED CERAMICS

ii series of kaolin based composites, Remblend china clay a pottery mi xture, a brick clay composition, and a clay-fly ash composition have b een sintered in air at various temperatures and the resulting products characterised with respect to chemical and mineralogical composition, thermal analysis, mechanical properties and microstructures. Sinterin g of the clay based ceramics changed the microstructure from a point b onded to a dense glass-ceramic microstructure whose composition depend ed on the original clay composition. Structure flaws existing where gl ass formation had started but was insufficient to produce a strong net work, and sand particles, in well developed glass bonded structures, c ould act as sites of fracture initiation. Generally, kaolin and potter y mixtures had similar microstructures after sintering at temperatures differing by similar to 200 ti. Brick clay mixtures showed an increas e up to 1150 degrees C, but any further temperature increase resulted in melting. Kaolin and pottery specimens sintered at temperatures high er than the optimum showed reduced properties as a result of increased porosity and bloating. Modulus of rupture (MOR), fracture toughness K -1c, and modulus of elasticity E were found to be closely related to t he microstructure of the ceramics. Those ceramics where glass formatio n had started but was insufficient to produce a well developed glass b onded structure, with large flaws present such as sand particles which act as sites of fracture initiation, had medium values of E (similar to 10-20 GN m(-2)), MOR (similar to 15-20 MN m(-2)), and K-1c (0.4-0.7 MN m(-3/2)). Well developed glass bonded microstructures without majo r paws (e.g. kaolin sintered at 1200 and 1300 degrees C. or pottery mi xture sintered at 1100 degrees C) had relatively high E (similar to 30 -40 GN m(-2)), MOR (similar to 35-40 MN m(-2)), and K-1c (0.7-0.9 MN m (-3/2)) values. Kaolin compositions sintered at 1400 degrees C reached their maximum properties while pottery mixture specimens attained opt imum properties after sintering at 1150 degrees C. (C) 1996 The Instit ute of Materials.