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

Simultaneous carboreduction and nitridation (CRN) of Y2O3 doped SiO2 /clay or elemental Si/clay mixes at atmospheric pressure were used to prepare low-cost powders of α-sialon composition. The synthesised products contained crystalline phase mixtures of α- and β-sialon and Y3Si6N11 . Subsequent hot pressing of the CRN powders at 1800 °C resulted in poorly densified materials from mixtures with low Y2O3 , whereas high Y2O3 gave dense ceramics. The addition of SiO2to the starting mix resulted in ceramics containing substantial amounts of very fine SiC particles distributed in a matrix of β-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 pre-pressed powders, and that hot-pressing causes the Al–N units to become overall more oxygenated, with the conversion of β-sialon to α-sialon and the formation of small amounts of polytypoid sialons. The hardness,HV10 , and fracture toughness, K1C , of the densified materials were ca. 18 GPa and 3 MPa m1/2 , respectively. The use of elemental Si produced less SiC, the major phase in these ceramics being α-sialon and a residual glassy phase observable by electron microscopy. The hardness and toughness of these densified materials was ca. 20 GPa and 3 MPa m1/2 , respectively.