Effect of Impurities on Microstructure and Mechanical Properties of Si₃N₄-TiC Composites

Abstract

Attainment of desired properties in dispersoid-modified composites strongly depends on the type and degree of chemical interaction between the matrix and the dispersoid. The Si₃N₄-TiC (SNT) composites which have been developed for cutting tool and wear applications combine the high fracture toughness and thermal stability of the Si₃N₄ matrix with the hardness of the TiC dispersoid. The resultant material exhibits outstanding resistance to abrasive wear. Properties relevant to performance in the severe environment of high speed metal removal--strength, hardness, and fracture toughness--may be altered by the chemical reaction between the matrix and the dispersoid. Densification of the composite proceeds through liquid phase sintering. Formation of the liquid, consisting of SiO₂ (which is present in Si₃N₄ powder), added sintering aids (Y₂O₃, Al₂O₃), and TiO₂ (which originates from the TiC surface) marks the beginning of densification and initiates a number of chemical reactions. Impurities, mainly glass modifiers, concentrate in the liquid phase. The type of impurity varies, depending on the powder production process. Common, however, are transition metals, especially iron and Group II (Ca, Mg) elements. In addition, powders such as those prepared by carbothermal reduction of SiO₂ may contain free carbon.