ALPHA-SIALON CERAMICS SYNTHESIZED FROM A CLAY PRECURSOR BY CARBOTHERMAL REDUCTION AND NITRIDATION

Simultaneous carboreduction and nitridation (CRN) of Y2O3 doped SiO2/c lay or elemental Si/clay mixes at atmospheric pressure were used to pr epare low-cost powders of alpha-sialon composition. The synthesised pr oducts contained crystalline phase mixtures of alpha- and beta-sialon and Y3Si6N11. Subsequent hot pressing of the CRN powders at 1800 degre es C resulted in poorly densified materials from mixtures with low Y2O 3, whereas high Y2O3 gave dense ceramics. The addition of SiO2 to the starting mix resulted in ceramics containing substantial amounts of ve ry fine SiC particles distributed in a matrix of beta-sialon and glass . Solid state MAS NMR indicates that the crystalline SiC observed by X RD after hot-pressing is also present in an XRD-amorphous form in the prepressed powders, and that hot-pressing causes the Al-N units to bec ome overall more oxygenated, with the conversion of beta-sialon to alp ha-sialon and the formation of small-amounts of polytypoid sialons. Th e hardness, H-V10, and fracture toughness, K-1C, of the densified mate rials were ca. 18 GPa and 3 MPa m(1/2), respectively. The use of eleme ntal Si produced less SiC, the major phase in these ceramics being alp ha-sialon and a residual glassy phase observable by electronmicroscopy . The hardness and toughness of these densified materials was ca. 20 G Pa and 3 MPa m(1/2), respectively.