The Relationship between the Spark Plasma Sintering Temperature and Mechanical Properties of Combustion-Synthesized α- and β-SiAlON

Combustion-synthesized Y-alpha-SiAlON and Ca-alpha-SiAlON powders were consolidated by spark plasma sintering (SPS) at 1300-1450 degrees C for 10 min, and the mechanical properties of the consolidated bulk samples were investigated. XRD analysis revealed that alpha-SiAlON partially transforms into beta-SiAlON during the SPS and a bulk mixture of alpha/beta-SiAlON was obtained. The fraction of beta-SiAlON increased with the increase in sintering temperature and the a to beta transformation ratio was higher for Y-alpha-SiAlON than for Ca-alpha-SiAlON. The hardness of the consolidated bulk increased with sintering temperature, and after reaching a maximum at 1350 degrees C, the hardness gradually decreased with temperature in both the Y-alpha-SiAlON and Ca-alpha-SiAlON. The increase in hardness with temperature arises from the increased density of the sintered body, while the decrease in hardness results from grain growth due to an increase in temperature. The fracture toughness tended to increase with temperature and did not show a maximum for either alpha- or beta-SiAlON, although the Y-alpha-SiAlON always exhibited a greater toughness than the Ca-alpha-SiAlON. The greater toughness of the Y-alpha-SiAlON is attributable to its higher fraction of transformed beta-SiAlON, because the elongated shape of the beta-SiAlON leads to the prevention of crack propagation.