The feasibility of using electrostatic charge condition monitoring for lubricant additive screening

Abstract

International standards require lubricant formulators to develop additive packages that increase fuel economy, reduce environmental impact and minimise wear over ever increasing service intervals. However, additive behaviour and interactions between additives is not well understood. An absence of real-time technology has hindered understanding of additive behaviour and interaction between additives in tribo-contacts. The work presented in this thesis assessed whether electrostatic charge monitoring, which is sensitive to changes in surface chemistry, can offer insight into additive-surface behaviour and how this affects tribological performance. Electrostatic sensors were deployed on tribological test apparatus used to simulate: engine valve-train, manual transmission synchroniser and automatic transmission clutch tribo-contacts. Additive performance in these simulated contacts was assessed by electrostatic surface charge measurements and cross-correlated with friction, wear and off-line surface chemistry analysis. The first study involved electrostatic monitoring of valve-train contacts, which was a continuation of previous electrostatic monitoring work carried out to relate wear and electrostatic charge. During a simulated TU3 cam-follower wear test, charge signals underwent a sign inversion; this was due to a transition between tribocharging of the lubricant under running-in and mild wear, and contact potential difference generated at the onset and progression of adhesive wear. It was found that charge signals differed between different oils, which could not be explained by the wear performance alone; this indicated that lubricant chemistry significantly affected charge generation. Dynamic charge peaks produced by simulated valve-train contacts lubricated with zinc dialkyldithiophosphate (ZnDTP) additive were related to the stripping of the tribofilm. The source of this charge peak was an increase in negative charge, which correlated with a dominance of phosphate and sulphate (anions) compared to zinc (cations), as the film was worn away. When friction modifier (FM) and dispersant additives were combined to lubricate a simulated wet clutch contact, x-ray photo-electron spectroscopy (XPS) analysis and friction data indicated that the dispersant dominated the tribofilm composition; evidenced by nitrogen levels and friction levels similar to the dispersant alone. Electrostatic charge data showed that competition for surface sites is an extremely dynamic process; as indicated by charge levels which alternated between the levels of the FM and dispersant alone. When a potassium borate additive was added to a polysulphide additive containing oil during testing (seeding), the charge data showed a transition from being predominantly positive to predominantly negative. This correlated with the formation of a borate rich (anions) layer on top of the sulphur film. Further novel tribological discoveries were found through investigation into these additive-surface charge behaviours. An underdeveloped ZnDTP tribofilm, which predominantly contained sulphur, was formed at room temperature; the sulphur promoted tribochemical wear and resulted in a pro-wear affect for primary ZnDTP. The combination of FM and dispersant showed a clear antagonism resulting in increased wear compared to the individual additives. XPS of brass and steel simulated synchroniser contacts lubricated with potassium borate and polysulphide identified the affect of surface chemistry on film formation. For the potassium borate additive: potassium (cation) preferentially adsorbed to brass, and borate (anion) preferentially adsorbed to steel. Seeding the borate additive into the oil, and therefore the contact, produced the same film composition and structure as the combination from the start. This inferred that the polysulphide additive drove initial film formation. Seeding is an extremely powerful technique, but its use is almost absent in the literature. It has been shown that lubricant chemistry dominates charge levels even in a wearing contact. Electrostatic monitoring is sensitive to the type of additive-surface adsorption and interaction between additive and additive or contaminant. Also, cross-correlation of surface charge and tribofilm chemical composition demonstrated that electrostatic sensors have the ability to detect tribofilm kinematics. This is a significant finding; no current real-time technique used to monitor tribofilm kinematics derives its measurements from the tribofilm composition. Although interpretation of electrostatic charge data is currently an intensive process, in the long term, lubricant development could see a move towards charge informed formulation.

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ORCID for Robert Wood: orcid.org/0000-0003-0681-9239

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