Effect of atmospheric gas on ZDDP tribofilm formation

The effect of atmospheric gas on zinc dialkyldithiophosphate (ZDDP) tribofilm formation was investigated. Macroscale friction tests and in-situ atomic force microscopy (AFM) observations were carried out in both normal air and argon. In argon, the tribofilms grew slowly although the initial formation stages were similar to those of ones formed in normal air. Furthermore, after prolonged friction, the tribofilms gradually became smooth and wore away. Oxygen in the lubricant is thus important for ZDDP tribofilm formation, and a sufficient supply of oxygen is necessary to maintain the anti-wear performance of ZDDP. •The effect of atmospheric gas on ZDDP tribofilm formation was investigated.•ZDDP tribofilm formation in argon was visualised using in-situ AFM observation.•ZDDP tribofilms formed in argon were prone to wear.•They had longer polyphosphate chains and a high sulphur rate.•A sufficient supply of oxygen is necessary to maintain the anti-wear property of ZDDP.