Analysis of the tribochemical alterations in automotive lubricating oils subjected to the four‐ball wear test using Raman spectroscopy

Automotive lubricating oils (ALOs) play a critical role in maintaining the performance and durability of engines operating under severe conditions. This study used Raman spectroscopy to evaluate changes in ALOs submitted to shear forces during the four-ball wear test. Three commercial samples of mineral (MN), semi-synthetic (SS), and synthetic (ST) ALOs were essayed in a Four-Ball Tester at temperatures of 25 °C and 75 °C. Aliquots of oils were taken from the packaging without test (NS) and from the holder (ball cup) after the test and analyzed by Raman spectroscopy. Average wear scar diameter (WSD) were measured on the stationary balls, where ST produced the smallest scars, consistent with anti-wear/boundary-lubrication of the synthetic compounds. The mean Raman spectra of the three ALO samples, before (NS) and after the four-ball test at 25 °C and 75 °C (FB25 and FB75) showed the spectral features related to the oil’s composition (hydrocarbons and additives). The difference spectra showed features that suggest cleavage of long aliphatic segments, shear-induced molecular reorganization, generation of peroxides, carboxylates and paraffin compounds, and additive-derived tribochemical transformations. Principal component analysis (PCA) scores confirmed that the spectral changes were consistent with a shear‑induced chain scission, and the Score 3 of ST oil exhibited temperature-dependent changes, with oxidation (carboxylate) features at 75 °C following the test. PCA revealed differences between NS and post-test samples that were consistent with the observed decrease in the measured WSD in the three ALOs. These results demonstrated molecular changes and tribochemical alterations of ALOs after tribological tests.