Microstructure and preparation of an ultra-fine-grained W-Al2O3 composite via hydrothermal synthesis and spark plasma sintering

Metal-matrix composite powders of W with Al2O3 reinforcements were synthesized by a novel hydrothermal route, which includes calcination and hydrogen reduction steps. It was found that the pH value and reactant concentration in the hydrothermal reaction had a significant influence on the morphology and particle size of the powders produced. The precursor powder particle size was the smallest for pH values of < 1.0 and a reactant concentration of 0.5 g/ml. The addition of Al2O3 led to significant refinement of W particles produced via this route, and these particles gradually become more spherical with increasing Al2O3 content. Subsequently, bulk Al2O3-reinforced W composites were fabricated by spark plasma sintering. The Al2O3 content was varied from 0 to 15 vol%, and sintering was performed under 30 MPa pressure for 5 min at temperatures of 1800-2100 °C. The density of the sintered samples increased with sintering temperature, reaching a value of about 97% of the theoretical value for a sintering temperature of 1900 °C. An increase in the Al2O3 content from 0 to 6 vol% resulted in an increase in the micro-hardness of the sintered composite samples from 278 to 423 HV and a decrease in the W grain size from 4.25 to 2.78 μm.