Effect of Metal Composition on Mechanical Properties of Tribofilms

Abstract

Low-speed pre-ignition (LSPI), where hot spots in the combustion chamber can cause fuel ignition to occur, can result in increased pressure in the combustion chamber, causing damage to the system and negatively affecting emissions. These hot spots have been identified as calcium (Ca) deposits, causing a push to reduce Ca in engine oils. For this study, tribofilms were generated with an amount of Ca so that LSPI did not occur. Zinc dialkyldithiophosphate (ZDDP), Ca, and magnesium (Mg) detergents were kept constant, with the metal in the tribofilm adjusted by varying the surfactants (friction modifiers) in the engine oil, ensuring enough ZDDP and detergent to be able to protect the engines. Energy dispersive x-ray spectroscopy (EDS) and atomic force microscopy (AFM) revealed three distinct tribofilm components: smooth, streaky films, deeper worn steel, and a mixed region of rougher tribofilm growth. Tribofilms with higher Ca had lower bulk friction than those with higher Zn, while AFM friction maps of the different sections showed that the friction in the mixed region was similar in magnitude to macro-scale friction, but film and substrate COF were much lower. Similarly, AFM modulus mapping of the mixed region showed higher Ca contributing to increased modulus when compared to Zn. Nanoindentation across the entire tribofilm revealed that higher modulus in the film and mixed regions contributed to more effective tribofilm coverage. Overall, Ca was shown to be critical to the wear and friction performance of tribofilms and their mechanical properties even when reduced to avoid LSPI.