Comparing the content of oxidized copper on the surface of ML (Ni) (5N) and ML (Cu) (5N) sliding tracks reveals that the copper in nickel-topped ML (Ni) is less susceptible to oxidation than ML (Cu) (Figure ). On average, the Cu 2+ to Cu 0 ratio for ML (Ni) is approximately 0.25:1, whereas for ML (Cu) this increases to close to 1:1, which constitutes an increase of oxidized copper on the sliding surface by 150% in the absence of nickel. The role of nickel is explained when observing the oxidized-to-metal ratios of ML (Ni) at 5 and 25N sliding loads (Figure ). For ML (Ni) sliding at 5N loads, the Ni 2+ /Ni 0 ratio is approx. 4:1the majority of the nickel is oxidized, but both oxidized and non-oxidised nickel remains present on the sliding surface. At 25N sliding load, the Cu 2+ /Cu 0 ratio is less than 0.2:1 up to 12,800 revs, whereas the nickel oxide content increases significantly over this period to > 90% oxide (combined with the reduction of nickel content in the sliding track identified before). At 20,000 revs sliding, the nickel has become fully oxidized (~100% Ni 2+ vs Ni 0 content), and only after this has happened, does the copper sliding surface begin to oxidise, shown by an increase of Cu 2+ /Cu 0 ratio to 1:1; this level of copper-oxidation is similar to those seen during ML (Cu) 5N sliding.