Low-temperature spark plasma sintering of calcium stabilized alpha sialon using nano-size aluminum nitride precursor

Calcium stabilized alpha-sialon ceramic was synthesized using spark plasma sintering (SPS). The size effect of aluminum nitride precursor (1 μm and 50 nm) on the phase analysis, microstructural and mechanical characteristics of the processed sialon was studied. Alongside the size of AlN precursor, holding time and sintering temperature were also varied in order to assess the degree of densification, phase transformation and its subsequent effect on the mechanical properties of these materials. The development of alpha phase was found to be strongly dependent on the particle size of the aluminum nitride (AlN) precursor. Interestingly, this novel approach of changing particle size from micro to nano-level combined with the SPS process resulted in the formation of alpha-sialon ceramic at much lower than previously reported sintering temperature i.e. 1500 °C as compared to 1700/1800 °C. Surface oxide layer associated with AlN particles resulted in the formation of additional beta phase along with major alpha phase in the final product. The sialon product synthesized from micron-size AlN particles (1 μm) at a sintering temperature of 1500 °C exhibited Vickers hardness value HV₁₀, of 18 GPa and moderate fracture toughness of 5.6 MPa √ m. However, sialon materials synthesized from nano-size AlN particles (50 nm) showed a hardness value of 14 GPa while displaying an improved fracture toughness of 7 MPa √ m. Increase in holding time resulted in a slight increase in densification of these materials, however its influence on mechanical properties was less significant as compared to the effect of particle size of aluminum nitride staring powder precursor.