An Assessment of the Effect of Relative Humidity on the Decomposition of the ZDDP Antiwear Additive

The effect of relative humidity on the decomposition reaction of zinc dialkyldithiophosphate (ZDDP) additive under boundary lubrication regime has been investigated using Raman spectroscopy and atomic force microscopy (AFM). The ZDDP tribofilms were formed using the pin-on-disc tribometer at 30 and 90% relative humidity, which was controlled using a custom-made humid chamber. The results suggest that relative humidity does not change only the length of the phosphate chains but also their affinity towards the available cations. At low relative humidity, zinc pyrophosphate to polyphosphate chains were formed compared to predominantly shorter iron orthophosphate chains at high relative humidity. In addition, relative humidity altered the structure of the mature tribofilm. At low relative humidity, the tribofilm had a patchy structure with average thickness of 140 nm and fractal dimension of 2.17. However, at high relative humidity, the tribofilm had a thin dendritic structure with average thickness of 85 nm and fractal dimension of 2.34. This means that the structure is more complex with a larger number of nesting self-similar structures at all scales. This is significant in making the transition from descriptive towards quantitative information of the tribofilm structure.