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 s tudied from room temperature through 1000 degrees C, The coefficient of fri ction revealed two distinct regions of decrease with increased temperature, with a transition at similar to 700 degrees C, Below 700 degrees C, the co efficient of friction decreased rapidly with increased temperature (similar to 10(-3)/degrees C). However, above 700 degrees C, the decrease was more gradual (similar to 10(-5)/degrees C). This was believed to be related to a brittle-to-ductile transition at the wear surface. The coefficient of fric tion was only weakly dependent on grain size, because the largest grain siz es exhibited slightly higher friction co efficients. However, the specific wear loss of the aluminas increased with increased grain size at room tempe rature and at 600 degrees C, both below the 700 degrees C transition. The p rimary mechanism of wear was ascertained to be brittle microfracture along grain boundaries, At 1000 degrees C, above the 700 degrees C transition, th e specific wear loss was significantly decreased and appeared to be indepen dent of the alumina grain size. At 1000 degrees C, the wear surfaces develo ped a thin layer of fine grains formed by dynamic recrystallization. The gr ain size within the thin layer was in agreement with the previously reporte d grain-size/Zener-Hollomon parameter relationship.