α-Sialon ceramics synthesised from a clay precursor by carbothermal reduction and nitridation

Simultaneous carboreduction and nitridation (CRN) of Y(2)O(3) doped SiO(2)/clay or elemental Si/clay mixes at atmospheric pressure were used to prepare low-cost powders of alpha-sialon composition. The synthesised products contained crystalline phase mixtures of alpha- and beta-sialon and Y(3)Si(6)N(11). Subsequent hot pressing of the CRN powders at 1800 degrees C resulted in poorly densified materials from mixtures with low Y(2)O(3), whereas high Y(2)O(3) gave dense ceramics. The addition of SiO(2) to the starting mix resulted in ceramics containing substantial amounts of very fine SiC particles distributed in a matrix of beta-sialon and glass. Solid state MAS NMR indicates that the crystalline SiC observed by XRD 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 become overall more oxygenated, with the conversion of beta-sialon to alpha-sialon and the formation of small-amounts of polytypoid sialons. The hardness, H(V10), and fracture toughness, K(1C), of the densified materials were ca. 18 GPa and 3 MPa m(1/2), respectively. The use of elemental Si produced less SiC, the major phase in these ceramics being alpha-sialon and a residual glassy phase observable by electronmicroscopy. The hardness and toughness of these densified materials was ca. 20 GPa and 3 MPa m(1/2), respectively.