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

Abdel Dorgham,Ardian Morina,Abdullah Azam,Pourya Parsaeian,Chun Wang,Anne Neville

doi:10.1007/s11249-021-01446-6

Abstract

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.

Highlights

Water, which can be present in the lubricating oil of any machinery, can have adverse effect on the overall efficiency of the lubricant and lubricated parts such as bearings in different ways [1]
This study aims at investigating this correlation by examining how humidity can alter the physicochemical characteristics of the formed zinc dialkyldithiophosphate (ZDDP) tribofilm over time using Raman spectroscopy and structure and thickness using atomic force microscopy (AFM)
The tribofilm formed at low relative humidity shows the typical patchy structure, which is significantly different from the dendritic structure formed at high relative humidity

Summary

Introduction

Water, which can be present in the lubricating oil of any machinery, can have adverse effect on the overall efficiency of the lubricant and lubricated parts such as bearings in different ways [1]. Water can increase wear and shorten the life of the lubricated parts [2], due to hydrogen embrittlement and oxidation. The extent of this damage depends on whether water is dissolved [3] or free [4]. Chang et al [5] showed that a 50% increase in relative humidity from ambient at 30% to a mild at 60% had a significant impact on the engine manifested mainly in retarding the combustion phasing along with reducing the NOx concentration. Pekula et al [6] showed that a threefold increase in the relative humidity can cause a reduction of NOx concentration by 15 to 20%

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