Origin and forming mechanism of coking substance in ester aviation lubricating oil

Coking substance in ester aviation lubricating oil has a great effect on the flight safety of aircraft, however its origin and forming mechanism is complicated and controversial. Herein, the coking mechanism of ester lubricating oil is investigated deeply based on the crankcase simulation test and thin film oxidation evaluation. Besides, its structural and compositional change is also detected through in situ thermal infrared spectroscopy and gas chromatography-mass spectrometer analysis. The results reveal that oil film can be detected when the oxidation time extends to 40.31 min during the thin film oxidation test at 613 K. Whereas, the weight of carbonaceous deposits increases obviously until the oxidation time extends to 4 h in thick oil oxidation at 613 K. The thin film oxidation is evidently identified as the origin for the formation of coking substance, and its formation process involves the orderly breakage of C-O and C–C bands, the formation, polymerization and carbonization of RCOO• and peroxides. More importantly, the tribological performance of ester lubricating oil before and after oxidation and the real characteristics of coking substance are also discussed contrastively, which providing a foundation for safely using this type of lubricating oil.