Nanomechanical and Nanotribological Properties of an Antiwear Tribofilm Produced from Phosphorus-Containing Additives on Boundary-Lubricated Steel Surfaces

Nanoindentation and nanotribological experiments were performed on an antiwear tribofilm produced from a blend consisting of gear base oil and a phosphorus-containing additive. Electrical contact resistance measurements were used to determine in situ the formation of the tribofilm on AISI steel surfaces at $100 °C$ under conditions favoring sliding in the boundary lubrication regime. Nanoindentation experiments were carried out with a surface force microscope on a small sector of a disk specimen that contained part of the wear track. A Berkovich diamond tip with a nominal radius of curvature equal to 100 nm was used to perform indentations on and off the wear track under a normal load between 100 and 600 μN. The phosphate tribofilm exhibited an average hardness of 6.0 GPa and reduced elastic modulus of 122.7 GPa compared to 12.5 GPa and 217.6 GPa of the steel substrate, respectively. In addition to the nanomechanical properties, the nanotribological properties of the tribofilm were evaluated in light of nanofriction tests performed with a 20 μm radius of curvature conical diamond tip under a normal load in the range of 100–200 μN. The original steel surfaces exhibited constant friction, whereas the wear track covered by the phosphate tribofilm revealed a higher and more fluctuating coefficient of friction, which is attributed to the roughness of the wear track and the greater plasticity of the tribofilm than the steel substrate.