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Transmission electron microscopy of worn zirconia surfaces W. M. Rainforth Department of Engineering Materials, Mappin Street, University of Sheffield, Sheffield, S1 3JD, United Kingdom

R. Stevens School of Materials, University of Bath, Bath, United Kingdom (Received 6 March 1995; accepted 19 June 1997)

The dry sliding wear behavior of 3 mol % tetragonal zirconia polycrystals (3Y-TZP) and a composite containing 20 vol. % SiC whiskers have been examined by transmission electron microscopy. High wear rates for the TZP were associated with dramatic microstructural changes. The extreme outer ,400 nm consisted of an amorphous surface layer containing both alumina and zirconia. Below this, the t-ZrO2 grain size was an order of magnitude smaller than in the starting material. At a depth of 1–2 mm the tetragonal grains had become elongated, with a maximum aspect ratio of 30 : 1. The first monoclinic zirconia was found at a depth of 5 mm. In contrast, the composite exhibited a wear rate 5 orders of magnitude lower, associated with minor microstructural changes.

I. INTRODUCTION

The potential of ceramics as wear-resistant materials is well recognized. Zirconia ceramics offer a combination of enhanced toughness, high hardness, and chemical inertness, which should confer excellent wear resistance. However, the published literature provides conflicting information. Some reports indicate that zirconia ceramics have good wear resistance both in the laboratory1–3 and in field trials,4–6 while other workers have found poor wear resistance,7–11 even under mild sliding conditions. The role of transformation toughening in the wear of zirconia ceramics remains controversial. The grinding resistance of zirconia ceramics is greater than would be expected from the hardness, as a result of the surface compressive stresses generated by transformation.13 In contrast, sliding wear experiments which promote strong adhesive forces between the two counterfaces (for example, by unlubricated sliding against a metal counterface) generate transformation of tetragonal to monoclinic zirconia which can lead to increased wear rates,7,14,15 believed to be a result of surface fracture, initiated by transformation-induced microcracking. Transformation toughening is strongly temperature dependent, its effectiveness decreasing as the temperature is raised. A further possible explanation for the wide range in the wear data reported is the difference in sliding speed utilized, which would cause a variation in temperature generated at the surface. Rainforth and Stevens7 demonstrated that temperature rises generated by sliding a Mg-PSZ against a steel counterface at 0.24 mys were sufficient to reduce significant transformation to within a thin (200–400 nm) layer. Similarly, Woydt and Habig16 and Woydt et al.17 demonstrated that the wear rate of zirconia depended sensitively on the sliding speed and 396

http://journals.cambridge.org

J. Mater. Res., Vol. 13, No. 2, Feb 1998

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