XPS is a surface-sensitive technique that can quantitatively identify elements and oxidation states on each wear track. In comparing the carbon, oxygen and metal (copper and nickel for ML (Ni) , copper only for ML (Cu) ) contents of the wear scars at both 5N and 25N, little difference is observed: the surface of the wear scar typically contains ~50 at.% carbon, ~30 at.% oxygen and ~20 at.% metal (Figure ). These values are consistent and are not significantly deviated by sliding time or (in the case of ML (Ni) ) the load applied.