Run-in and glaze formation on gray cast iron surfaces

Abstract The chief processes involved in run-in of cast iron are smoothing of the surface and formation of a surface coating derived largely from the graphite in the metal structure and iron oxide (Fe3O4). Much of the smoothing can be attributed to this coating rather than to a wearing off or “squashing” of asperities. It may be almost 1 mil thick and its exact composition varies from point to point along the surface. In the case of a specimen run-in with a non-additive oil under laboratory conditions, the carbon content ranged from 30 to 100% and the remainder could be accounted for by Fe3O4, although on an engine cylinder bore it contains significant amounts of other components. This “glaze” doubtlessly imparts a resistance to scuffing during momentary contacts between the mating surfaces but, possibly more important, by covering surface irregularities it produces a smooth surface which allows higher loads to be carried by a fully hydrodynamic lubricant film.