Rapid consolidation processing of silicon nitride powders
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Rapid consolidation processing of silicon nitride powders J. A. Schneider, S. H. Risbud, and A. K. Mukherjee Division of Materials Science and Engineering, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616-5294 (Received 7 April 1995; accepted 23 October 1995)
Using a Plasma Assisted Sintering (PAS) process, submicron size, silicon nitride powders were consolidated to .99% of the theoretical density (TD) at 1750 ±C in less than 5 min with retention of the a phase and the submicron grain size. The silicon nitride powders were sintered with 5 wt. % Y2 O3 and 5 wt. % Y2 O3 1 5 wt. % MgAl2 O4 additives. The PAS processing method for the silicon nitride additive mixtures is attractive for retention of fine-grained microstructures favorable for superplastic deformation. Post superplastic forming heat treatments to transform the a –Si3 N4 to lath-like, creep-resistant b –Si3 N4 is another feature of the present processing method.
I. INTRODUCTION
Silicon nitride based materials are of interest in high temperature structural applications where lightweight components with high strength properties that are resistant to shock, corrosion, and wear are desired. Due to the low thermal conductivity and low diffusion rates for silicon nitride, consolidation relies on the use of additives which are introduced to form a liquid at the grain boundaries at the sintering temperature. These oxide additives are selected on the basis of their solubility in the SiO2 layer surrounding the silicon nitride starting powders. The oxide additives that react to form the liquid phase during sintering often solidify as a glassy second phase at the grain boundaries or at triple points. Degradation of mechanical properties at elevated temperatures is related to the viscosity and composition of this glassy grain boundary phase since grains can rotate and slide swiftly in this “sand and molasses” type microstructure. To increase the potential for retained strength at high temperatures, much research has been directed at engineering grain boundary phases with an increased refractory nature. Improvements have been noted in the mechanical properties of the silicon nitride system by using rare earth oxide additives1–3 as well as optimizing mixtures of additive oxides.4 Conventional consolidation of silicon nitride with additives is conducted by heating for 2 to 4 h at 1750± 6 100 ±C in conventional radiation furnaces under a controlled nitrogen gas environment. Silicon nitride with varying amounts of additives has been successfully consolidated under load conditions ranging from pressureless up to 5 GPa under Hot Isostatic Pressure (HIP). To reduce the consolidation time, other methods of heating low conductivity ceramic powders are continuing to be investigated. A microwave process has been demonstrated that utilizes dielectric heating 358
http://journals.cambridge.org
J. Mater. Res., Vol. 11, No. 2, Feb 1996
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