Effect of the amount of additives and post-heat treatment on the microstructure and mechanical properties of Ytrium-alpha-sialon ceramics

The yttrium-sialon ceramics with the composition of Y0.333Si10Al2ON15
and an excess addition of Y2O3 (2 or 5 wt%) were fabricated by hot
isostatic press (HIP) sintering at 1800degreesC for 1 h. The resulting
materials were subsequently heat-treated in the temperature range
1300-1900degreesC to investigate its effect on the alpha-->beta-sialon
phase transformation, the morphology of alpha-sialon grains, and
mechanical properties. The results show that alpha-sialons stabilized
by yttrium have high thermal stability. An adjustment of the
alpha-sialon phase composition is the dominating reaction in the
investigated Y-alpha-sialon ceramics during low-temperature annealing.
Incorporation of excess Y2O3 could effectively promote the formation of
elongated alpha-sialon grains during post-heat-treating at relatively
higher temperature (1700degrees and 1900degreesC) and hence resulted in
a high fracture toughness (K-IC = 6.3 MPa.m(1/2)) via grain debonding
and pullout effects. Although the addition of 5 wt% Y2O3 could promote
the growth of elongated a grains with a higher aspect ratio, the higher
liquid-phase content increased the interfacial bonding strength and
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therefore hindered interface debonding and crack deflection. The heat
treatment at 1500degreesC significantly changed the morphology of
alpha-sialon grains from elongated to equiaxed and hence decreased its
toughness.