Tribological tests, using the high-frequency reciprocating rig (HFRR), were carried out on commercial diesel fuels with a range of lubricities as well as on solutions of methyl stearate and stearic acid in ultra-low-sulphur, additive-free diesel. The surfaces of discs used in the HFRR tests were measured with micro-Raman and time-of-flight secondary ion mass spectrometry (ToF-SIMS). These measurements showed that amorphous carbon was formed during reciprocating sliding, the structure of which was related to the lubricity as measured by the average wear scar on the ball. Magnetite (Fe3O4) and hematite (α-Fe2O3) were also detected on the surfaces. In most cases, the detection of hematite in the wear track was also associated with low wear. However, hematite was found in the wear particles of all samples. The build up of a film, measured by electrical resistance, was attributed to the presence of hematite, which has a much lower electrical conductivity than magnetit
