Densification and microstructural evolution in a reactive silicon nitride/alumina platelet system

摘要

Silicon nitride materials have been used as cutting tool inserts for turning cast iron and Ni-based superalloys. However, steel machining with Si{sub}3N{sub}4 has not been successful due to interdiffusion phenomena between the insert material andthe metal chip, resulting in high wear rates. The composite approach is one of the possibilities for modifying this behaviour by using as reinforcement a compound inert with respect to steel. Oxides, such as alumina, are known to resist chemical wear buttheir fracture toughness is poor. Oxide inserts early break during machining operations. Si{sub}3N{sub}4/Al{sub}2O{sub}3 composites could be promising materials to withstand both mechanical and chemical solicitations.Si{sub}3N{sub}4 materials with 0-30vol.% of Al{sub}2O{sub}3 platelets were investigated. Dense composites were obtained with 30MPa of applied pressure from T=1400°C upwards. The dispersoid phase partially reacted with the matrix.β-sialon was formed, thus modifying the amount and composition of the intergranular glass. The microstructure developed by a selection of additive systems was analysed by stereological methods in order to quantify dispersoid preservation. Computer assisted pressure dilatometryallowed for the characterization of the densification behaviour of the materials.α/β-sialon transformation was compared to the densification kinetics in order to discuss the relationship between both phenomena. Vickers hardness was also used to tracethe mechanical quality in relation to both the final density and phase transformation. The differences between the matrix free of reinforcing phase and the composites were analysed and rationalised with respect to the interaction between matrix anddispersoid, theα/β-sialon conversion and glassy phase content and composition.