The accessible loads correspond to a maximum Hertz pressure range of 2.5-3.5 GPa in the steel/steel ball-on-disk contact. 16 The tribofilm thickness was determined every 5-15 minutes using spacer-layer imaging (SLIM). During the tests, the ball was raised from the disk, motion was halted, and the ball was loaded upward, within the test chamber, against a coated glass window with a load of 100 N. White light was then shone into the resulting contact, where some is reflected by a semi-reflective coating on the underside of the glass. The remaining light passes through a silica spacer layer and the tribofilm formed on the ball, before being reflected back from the steel substrate. The two beams undergo optical interference depending on their optical path difference, which corresponds to the tribofilm thickness. An interference image was captured as a red-green-blue (RGB) pixel map using a high-resolution camera. A calibration chart of RGB colour versus optical path difference obtained using ultra-thin film interferometry was then used to convert this RGB pixel map to a tribofilm thickness map. A refractive index of 1.6 was assumed for the tribofilm. A representative set of SLIM images and the corresponding tribofilm thicknesses are shown in Figure .