Study of the Lubrication Mechanism of Thin Film Lubricated Concentrated Steel Contacts.

The lubrication mechanism of fully submerged sliding point contacts of AISI 52100 steel has been studied as a function of surface roughness, type of additive and type of cover gas. Strong indications are found that, at relatively low values of normal load and sliding speed, a thin lubricant film incompletely separates the surfaces. This thin film survives asperity contacts because newly-formed contacts ozidize rapidly. Collapse of the lubricant film occurs if oxidation cannot longer keep ahead of the formation of new asperity contacts. Increasing surface roughness and decreasing oxygen content of the lubricant both cause a reduction in load carrying capacity. At low speed of sliding collapse of the thin lubricant film leads to boundary lubrication. At high values of load, breakdown of this boundary film occurs which leads to severe wear scuffing. This effect may perhaps be associated with a metallurgical transformation in the steel at a particular value of the conjunction temperature. Surface roughness and oxygen content of the lubricant do not influence the load carrying capacity of the boundary film. At relatively high values of sliding speed collapse of the thin lubricant filn directly leads to severe wear. In an oxygen containing lubricant, zinc dialkyldithiophosphate improves the load carrying capacity of both the lubricant film and of the boundary film. Dobenxyl disulfide only manifests itself in the boundary region. In an oxygen free lubricant the role of the additive is much more limited.