Effect of contact load and lubricant volume on the properties of tribofilms formed under boundary lubrication in a fully formulated oil under extreme load conditions

The effect of contact load and volume of lubricant in the time to tribofilm breakdown and extent of wear was examined in fully formulated oil with zinc dialkyl dithiophosphate as the main anti-wear additive. The mechanism of tribofilm formation and breakdown was followed carefully by monitoring the friction coefficient over the duration of the test. Tribological tests conducted with limited lubricant exhibited a direct relationship between the amount of lubricant and time to tribofilm breakdown with tests with smaller amounts of lubricant exhibiting shorter lifetimes. In addition the number of cycles for breakdown of the tribofilm is inversely proportional to the applied load at a fixed amount of lubricant. It is also shown that higher contact loads (405 N with maximum Hertzian contact pressure of 2.77 GPa) resulted in premature breakdown of the tribofilm while lower contact loads (297 N with maximum Hertzian contact pressure of 2.5 GPa) resulted in no failure even after 100,000 cycles and the presence of a stable tribofilm. Focused ion beam cross sections of the tribofilm reveal that at higher contact loads the tribofilm is patchy with local regions as thick as 1 μm while at lower contact loads (297 N) the tribofilms are typically 100–200 nm thick. Transmission electron microscopy analysis of the wear debris indicates a larger fraction of the crystalline particles at higher contact loads of 405 N are Fe2O3. This is true even though the duration of the test is quite short indicating higher friction and higher contact temperature results in rapid oxidation of the wear debris to Fe2O3. Fe2O3 is significantly more abrasive and results in rapid bread down of the tribofilm. Analyses of wear debris of tests conducted at 297 N indicate smaller number of oxide crystallites with a chemistry of Fe3O4 within a non-crystalline matrix. XANES analysis indicates that at loads of 405 N sulfur is present as sulfates both at the surface and interior of tribofilms while at loads of 297 N it is present as sulfates at the surface and a mixture of sulfates and sulfides in the interior. Phosphorous K-edge XANES spectra indicate phosphorous is present as a mixture of short chain Zn and Fe phosphates at both loads. Fe L-edge spectra indicates that at 405 N the Fe is present largely as FeSO4 and FePO4 with some oxides of Fe while at 297 N it is present as a mixture of FeS and iron phosphates together with Fe oxides. The Zn L-edge spectra indicate that it is present primarily as phosphates and not as either oxides or sulfides in the tribofilms.