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MATERIALS RESEARCH

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Microstructure of a bearing-grade silicon nitride Mingqi Liu Trex Enterprises, San Diego, California 92121-4339

Sia Nemat-Nasser Center of Excellence for Advanced Materials, Department of Mechanical and Aerospace Engineering, University of California at San Diego, La Jolla, California 92093-0416 (Received 16 April 1999; accepted 23 September 1999)

The microstructure of a bearing-grade silicon nitride, prepared by pressureless sintering with Y2O3, AlN, and TiO2 additives and then hot-isostatically pressed, is examined with high-resolution transmission electron microscopy, scanning electron microscopy, and x-ray diffraction. The material consists of large acicular ␤–Si3N4 grains and small equiaxial ␣–Si3N4 grains. An amorphous phase containing the sintering aids is observed at the two-grain boundaries and at the grain pockets. No crystalline boundary phase is identified. The ␣-to-␤ and ␤-to-␤ grain boundaries appear straight and well defined. The dominant crystalline planes observed at the ␤-grain boundaries are (1010) and (1120). The intergranular spacing of the two-grain boundaries (␣-to-␤ and ␤-to-␤) is 1.0 nm when a high-contrast boundary phase is present, and it is 0.8 nm when a low-contrast boundary phase is present, confirming that the film thickness is strongly dependent on the boundary-phase composition. The ␣-to-␣ boundaries are often curved, and the thickness of the amorphous film at these boundaries varies from 0.7 to 1.1 nm. Evidence of near-intimate contact between ␤-grains is also observed.

I. INTRODUCTION

Silicon nitride ceramics have shown promising properties suitable to wear-resistant materials for engineering applications. Their unique physical properties, such as low density, low coefficient of thermal expansion, high elastic modulus, high hardness, high strength at elevated temperatures, and superior chemical stability in corrosive environments, make them particularly attractive candidate materials for advanced bearing systems, especially for applications where conventional lubrication is difficult, impossible, or periodically interrupted. The use of ceramic materials in rolling bearings represents a substantial improvement in bearing technology over all-steel bearings. Under marginal lubrication conditions, all-steel ball bearings experience accelerated wear. Trapped debris particles may lead to further bearing degradation. A silicon nitride ceramic ball, on the other hand, minimizes this effect, resulting in appreciable life gain. Hybrid ceramic bearings, consisting of ceramic rolling elements and specialty steel rings, can be used at temperatures of up to approximately 500 °C, and all-ceramic bearings are capable of operating at temperatures over 1000 °C at very high rotating speeds.1,2 Since the 1980s, extensive efforts have been made to commercialize silicon nitride ceramic bearings. These include examination of the lubrication conditions,3–5 wear and fretting characteristics,4–9 crack initiation behavJ. Mater. Res., Vol. 14, No. 12, Dec 1999