Surface- and Microanalytical Investigations of the Chemical Constitution of the Grain Boundary Phase in Dense Silicon Nitride

Abstract

Ceramic materials based on silicon nitride exhibit a greater high temperature strength for temperatures above 1000 °C than conventional super alloys employed at present for turbine engine design. As suitable structural components for high temperature application two groups of Si₃N₄ materials are of special interest: 1) dense hot-pressed Si₃N₄ (HPSN) and sintered Si₃N₄ (SSN) showing strength data between 550 and 900 MN/m² and 2) reaction-bonded Si₃N₄ (RBSN) characterized by a residual porosity from 15 to 30% and maximum strength of 350 MN/m². Further development of dense Si₃N₄ focuses mainly on the optimization of high-temperature properties. Beside the short-term strength, outstanding properties are the oxidation behavior and the long-term strength under mechanical and cyclic thermal load which can be improved by controlling the microstructure and, in particular, by improving the chemical and structural constitution of the mostly amorphous grain boundary phase. Dense Si₃N₄ can only be achieved via liquid-phase sintering where in the case of HPSN and SSN MgO, Y₂O₃ or Y₂O₃ + A1₂O₃ respectively are usually added as sintering aids. In Fig. 1 the evolution of the liquid-phase sintering process for MgO-fluxed HPSN is demonstrated schematically as proposed by Ref.¹. At temperatures below 1400 °C MgO reacts with the amorphous silica layers on the surfaces of α-Si₃N₄ grains of the starting powder forming magnesium silicate forsterite Mg₂SiO₄.