All the tribometer experiments used through-hardened AISI 52100 steel balls and disks from PCS Instruments (Acton, UK). For the Hertz pressure and shear stress calculations, the elastic modulus was taken as 207 GPa and 0.3 was used for the Poisson's ratio. The ball had a rootmean-square (RMS) roughness, Rq = 8.5 nm, while for the disk, Rq = 6.0 nm. The steel ball and disc were soaked in toluene for at least 24 hours before a test and then ultrasonically cleaned in toluene, followed by Analar acetone, before being mounted in the tribometer.