This paper describes the film formation properties of polyisobutylene (PIB) lubricant under various surface roughness conditions. PIB lubricant has been proposed as a new type of screw tightening lubricant, however the lubrication mechanism in the screw tightening process is yet to be understood. In this study, the in-situ observation tests were conducted under the starved lubrication condition, in which a thin film lubrication regime was simulated. The results showed that film thickness was greater than the surface roughness. Under the high roughness conditions, the time at which direct contact was prevented was shorter. In addition, the occurrence of wear was related to the lambda ratio Λ and under the high roughness conditions, the friction coefficient increased with time owing to expansion of the wear area. It was also shown that under the fully starved condition, the shear stress did not depend on the shear rate, moreover, its value was constant under all roughness conditions. These results suggest that (i) solid-like PIB film was formed on the contact area under the starved condition and (ii) the formation of a film with a high limiting shear stress potentially inhibits wear. These results were consistent with the results of a tightening test.
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