The invention provides a process for preparing a protective layer on a tribological surface of a mechanical component, wherein the layer comprises a metal phosphate and/or metal sulphate having anti-wear and/or anti-fretting properties, and the process comprises subjecting the tribological surface of the component to a mechanical treatment which is carried out with a tool that moves at or along the tribological surface during which treatment the metal phosphate and/or metal sulphate in solid form is provided and the layer is formed on the surface of the mechanical component by means of deposition of the metal phosphate and/or metal sulphate compound.