Correlation Between Engine Oil Degradation, Tribochemistry, and Tribological Behavior with Focus on ZDDP Deterioration

This work aimed to bridge the gap between engine oil degradation, in particular zinc dialkyl dithiophosphate (ZDDP) deterioration, its effect on tribofilm formation, and its eventual impact on friction and wear. Artificial oil alteration of a commercial engine oil SAE 0W-20 was used to produce a series of altered oils that were subjected to high resolution mass spectrometry in order to identify ZDDP degradation products and their abundances over time. It was found that ZDDP was depleted at an early stage of alteration by the replacement of its sulfur atoms by oxygen atoms and the release of the alkyl side chains. As final products of ZDDP deterioration, sulfuric and phosphoric acid were identified. Selected oils were subjected to tribometrical experiments consisting of an oscillating ball-on-disc contact. ZDDP concentration as well as the type and abundance of ZDDP degradation products in the oils directly influenced tribofilm formation and consequently friction and wear. However, the superior tribological performance of the fresh engine oil could not be regained with any of the artificially altered oils. X-ray photoelectron spectroscopy revealed that sulfur occurred mainly in sulfide state in the tribofilm formed from fresh oil. With increasing degree of oil degradation, the sulfate state became more dominant. By trend, the amounts of sulfur, phosphorus, zinc and calcium declined with increasing degree of oil degradation, indicating that tribofilm formation had become increasingly difficult.