Fig. 1. Scheme of the testing configuration of the pin-on-disc machine.

Fig. 2. Upper surface of FSP zone of Al/Al₂O₃ surface nano-composite layer produced by four FSP passes.

Fig. 3. (a) Optical and (b) SEM micrographs showing perfect bonding between surface composite and aluminum alloy substrate.

Fig. 4. SEM images of the Al/Al₂O₃ surface composite layers produced by (a) one, (b) three, and (c) four FSP passes.

Fig. 5. Optical micrographs showing grain size of (a) as-received 6082 Al and (b) Al 6082 after four FSP passes. SEM images (c) and (d) showing the microstructure of the Al/Al₂O₃ surface composite layer produced by four FSP passes, (d) is enlargement inside a circle for (c).

Fig. 6. Typical variation of the microhardness Hv distributions of the friction stir processed 6082 Al alloy (no Al₂O₃) and surface composite layers.

Fig. 7. Change in the reduction in pin weight with sliding distance for as-received Al and surface nano-composite layer produced by four FSP passes.

Fig. 8. Change in the rate of wear with sliding distance for as-received Al and surface nano-composite layer produced by four FSP passes.

Fig. 9. Change in the coefficient of friction with sliding distance for as-received Al and surface nano-composite layer produced by four FSP passes.

Fig. 10. Scanning electron micrograph of the worn out track of (a) as-received Al, and (c) surface nano-composite layer produced by four FSP passes against steel disk. (b) and (d) are high magnified view of (a) and (c), respectively.

Chemical composition of 6082 Al alloy (in wt.%).

Summary of the average Al₂O₃ cluster size, the average Al matrix grain size and microhardness for surface composite layers produced by various number of FSP passes.

The individual Al₂O₃ size is 50 nm and the initial 6082 Al extruded bar grain size is 120 μm.