Friction regimes in the lubricants solid-state regime

Friction measurements were performed in the lubricant's solid-state regime to study the transition from full-film lubrication, in which the separation is maintained by a solidified lubricant, to mixed lubrication. Special attention is paid to the influence of temperature (inlet viscosity) and roughness on this transition. The friction measurements showed that in the lubricants solid-state region three lubrication modes can be distinguished: A) full-film lubrication; separation is maintained by a solid film (S-EHL), B) mixed lubrication (ML); the load is carried by the interacting asperities as well as the lubricant (acting like a solid or as a liquid) and C) boundary lubrication (BL). Further, the dependency of the transition S-EHL to ML on temperature (inlet viscosity) depends on the lubricant used. The transition S-EHL to ML depends slightly more on roughness as found for L-EHL to ML transition. Finally, the film thickness formulas developed for EHL contacts in which the lubricant behaves as a liquid become doubtful when operating in the lubricant solid-state regime. This on the basis of the calculated film thickness over roughness ratios for the transition S-EHL to ML.