Surface active antiwear (AW) additives, such as zinc dialkyl dithiophosphate (ZDDP), are included in lubricant formulations to protect engine surfaces by building a thick, protective, oxide tribofilm. While many researchers have shown this formation mechanism in laboratory tests, few have shown data on parts taken from actual engine tests that would help prove the same mechanism occurs. Worn engine components are examined using x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Fresh and used engine oil samples have also been tested in laboratory high-frequency reciprocating rig (HFRR) wear tests. Contrary to the popularly proposed mechanism, the engine components had no or little evidence of a classical thick tribofilm, even though they came from engine tests with little overall wear. The HFRR tests show that as the lubricant ages during engine operation, tribofilm forming ability decreases. We conclude that, though tribofilms may form and be protective, they are not always necessary for low friction and low wear. All evidence suggests that oxidation, which occurs naturally in an engine with time, reduces the ability of a formulation to form any tribofilm.

Tribofilms on real engine surfaces

248th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.