Influence of Oxygen Content on the Mechanical Properties of In-Situ Formed Si₃N₄-SiC Nanocomposites Prepared by Carbon Coating Method

Abstract

The purpose of the present work is to propose a toughening method of Si₃N₄ ceramics by forming a nanocomposite grain-boundary phase of SiC nanoparticles in the weak grain-boundary phase. The SiC nanoparticles are formed by in-situ reaction between an oxide phase on the surface of Si₃N₄ powder and carbon, which is coated on Si₃N₄ powder by thermal decomposition of methane (CH₄) gas. The surface oxygen content can increase by an oxidation process or decrease by washing process. The sintered bodies are obtained by hot-pressing at 1800°C for 1h in N₂ atmosphere under 30 MPa of applied pressure. By increasing the carbon content, both of the fracture toughness and the bending strength first increase, then reach the maximum value at the same amount of carbon, and decrease. TEM observation showed that the precipitated SiC particles dispersed in the grain-boundary phase, at the Si₃N₄-Si₃N₄ interface and inside of the Si₃N₄ grain. The precipitated SiC content depends on the coated carbon and surface oxygen content on the Si₃N₄ powder surface. The reacted carbon content shifts to higher carbon content as oxygen content increases. When the reacted carbon content increases, the resultant SiC content also increases. Therefore, the carbon content at which the fracture toughness and bending strength exhibit the maximum values increases with increasing the surface oxygen content. A surface oxidized Si₃N₄ powder is generally regarded as an inferior powder; however, the oxidized powder can be used to produce high quality Si₃N₄ ceramics by applying the present carbon coating method.