Pressureless-sintered Al 2O 3-TiC composites

Abstract

Titanium carbide (TiC) (typically 30 wt.% TiC is used in cutting tools) dispersed in an alumina (Al 2O 3) matrix improves the hardness over that of an Al 2O 3 cutting tool, principally by limiting Al 2O 3 grain growth. Al 2O 3-TiC composites have been traditionally processed by intimately mixing the oxide and carbide powders and subsequently hot pressing at temperatures near 1700 °C and pressures 35 MPa. Pressureless sintering of Al 2O 3-30wt.%TiC was accomplished by adding titanium hydride (TiH 2) and heating at moderately high rates (40–50°C min −1) in a graphite furnace under inert gas. Both free titatnium and a eutectic liquid, formed by the carbothermal reduction of Al 2O 3, aided sintering. Conventional heating rates permit free titanium to combine with TiC at temperatures below the melting point of titanium and therefore do not take advantage of the liquid phase. Short sintering times above 1850°C are necessary owing to the high vapor pressure of the phases produced by the carbothermal reduction of Al 2O 3. A density greater than 99% of the theoretical density, a hardness in excess of 22 GPa and a fourpoint flexural strenght higher than 600 MPa were measured on pressureless-sintered-and-hot-isostatistically-pressed bars. These properties were comparable with those measured on composites made by hot pressing. There were no observable differences (microstructural or mechanical) between the hot-pressed and the pressureless-sintered parts. Machining tests showed that the performance of the pressureless-sintered composites was comparable with that of conventional tools. It was also shown that Al 2O 3-30wt.%TiC powders synthesized by the aluminothermic reduction of titania in the presence of carbon could be pressureless sintered without TiH 2 additions. The mechanical properties of these composites were comparable with those of hot-pressed Al 2O 3-TiC composites.