Densification, Microstructure, and Tribomechanical Performance of SPS-Processed 27R-SiAlON Polytype-Reinforced AlN: A Comparison Between Continuous and Pulsed Direct Current Mode

The effects of current conduction modes of spark plasma sintering (SPS) (within the temperature range of 1800 °C to 2000 °C) on densification, microstructure, and tribomechanical properties of indigenously processed, additive-free AlN reinforced with 27R-SiAlON polytype (prepared by carbothermal-reduction-nitridation technique) have been reported. Sintering curves and density data of sintered specimens under pulsed direct current mode (PM) show improved densification over continuous direct current mode. The reason for enhanced density under PM may be due to the enhanced electrical discharge within the capacitor banks formed on the insulating surface of SiAlON particles. Low-temperature sintered specimen principally contained equiaxed grains, while higher sintering temperature promoted elongation in grains. The denser composite (maximum being sintered at 2000 °C) offered much improved Vickers hardness and indentation fracture toughness in comparison to those for less dense specimens. Composites sintered under PM show the existence of elongated 27R-SiAlON grains. These elongated grains facilitate enhanced energy dissipation through crack bridging and deflection, which are the key factors behind obtaining the higher toughness of the dense composite. Unlubricated linear scratch experiments also indicated better wear resistance of the higher temperature sintered specimen. The present study establishes the suitability of using PM during SPS processing of AlN composite reinforced with 27R-SiAlON polytype that offers satisfactory tribomechanical performance.