Nanopowder processing of ultrafine Si3N4 with improved wear resistance

The processing, microstructure, and material properties of monolithic Si3N4 were investigated by using two amorphous Si3N4 nanopowders doped with (1) 6 wt% Y2O3 and (2) 6 wt% Y2O3 + 8 wt% Al2O3. The orthorhombic Si2N2O-based phase was found in the two as-sintered bulks. Carbothermal reduction treatment (CRT) was thus applied at 1400 °C for 10 h with 3 wt% and 6 wt% carbon black added to the precursor powders of the Si–Y–O–N and Si–Y–Al–O–N systems, respectively, resulting in an increased nitrogen-to-oxygen (N:O) ratio and elimination of Si2N2O within the sintered bulks. The possible mechanisms of nucleation and grain growth of Si3N4 are discussed during CRT and the sintering process. The best wear resistance was achieved in ultrafine Si3N4 doped with Y2O3, which has good hardness, indentation toughness and a protective tribo-film.