0'-Phase sialon ceramic product and a method of forming dense ceramic product

A silicon aluminium oxynitride product having the crystal lattice structure of silicon oxynitride but of expanded cell dimensions is produced by sintering a powder mixture of 50 to 60 wt % of Si3N4, 22 to 35 wt % SiO2, 1 to 20 wt % Al2O3 and 1 to 15 wt % of at least one oxide of Y, Li, Mg, Ca, Sc, Ce or another rare earth element, the molar ratio of SiO2 to Si3N4 being greater than 1:1 and the percentages being based on the total weight of the ingredients. Sintering is effected at 1600 DEG to 1900 DEG C. in a non-oxidizing atmosphere so that the Al2O3, said at least one oxide and part of the SiO2 in the mixture react to form a liquid phase from which silicon aluminium oxynitride having an expanded silicon oxynitride crystal lattice is crystallized. The sintered material is cooled to form a product containing said silicon aluminium oxynitride and at least one grain boundary phase containing silicon and the metal(s) of said at least one oxide.