Effect of nano-size oxy-nitride starting precursors on spark plasma sintering of calcium sialons along the alpha/(alpha + beta) phase boundary

Several compositions of calcium stabilized sialon ceramics were synthesized by using nano-size starting powder precursors and spark plasma sintering technique at a relatively low temperature of 1500 °C. The formation of calcium alpha/beta-sialons was investigated for the compositions represented by Ca _m/2 Si _12-(m+n) Al _m+n O _n N _16-n where m and n values were varied from 0.6 to 1.6 and 0.4–1.6, respectively. Phase analysis of the selected compositions helped in developing the phase boundary between alpha and alpha/beta phase regimes. Effect of m and n values on the evolution of the final phase(s), densification and mechanical properties were evaluated. All samples yielded densified ceramics with density values (3.13–3.19 g/cm ³ ) comparable with those reported in the literature for similar compositions and synthesized at temperatures greater than 1700 °C via conventional sintering techniques. Vickers hardness value HV ₁₀ of 20 GPa was measured for the Ca _0.5 Si _10.6 Al _1.4 O _0.4 N _15.6 composition (with m = 1.0 and n = 0.4) synthesized at 1500 °C. An increase in n value (higher oxide content) was observed to facilitate the formation of beta-sialon and AlN polytype phases leading to an increase in fracture toughness but a decrease in Vickers hardness.