Effect of Grain Size on the Sliding Wear and Friction of Alumina at Elevated Temperatures

The sliding friction and wear of three different grain‐size aluminas were studied from room temperature through 1000°C. The coefficient of friction revealed two distinct regions of decrease with increased temperature, with a transition at ∼700°C. Below 700°C, the coefficient of friction decreased rapidly with increased temperature (∼10‐3/°C). However, above 700°C, the decrease was more gradual (∼10‐5/°C). This was believed to be related to a brittle‐to‐ductile transition at the wear surface. The coefficient of friction was only weakly dependent on grain size, because the largest grain sizes exhibited slightly higher friction coefficients. However, the specific wear loss of the aluminas increased with increased grain size at room temperature and at 600°C, both below the 700°C transition. The primary mechanism of wear was ascertained to be brittle microfracture along grain boundaries. At 1000°C, above the 700°C transition, the specific wear loss was significantly decreased and appeared to be independent of the alumina grain size. At 1000°C, the wear surfaces developed a thin layer of fine grains formed by dynamic recrystallization. The grain size within the thin layer was in agreement with the previously reported grain‐size/Zener‐Hollomon parameter relationship.