Tribological response of MoS₂ coated and oxy-nitrided samples with alternative extreme pressure and anti-wear additives

Oxy-nitriding and MoS₂ coatings are widely used surface modification techniques in industry to improve the friction and wear characteristics of components and interacting surfaces. To ensure optimum performance of components within tribological environments, it is crucial to ensure compatibility between surfaces and lubricants, as the break-down of the lubricating film could lead to seizure between interacting components. This study analyses the interaction of extreme pressure and anti-wear additives with two modified surfaces. The friction and tribofilm formation behaviour were investigated using a Mini Traction Machine fitted with a 3D Spacer Layer Imaging Method. The chemical composition of the species formed after testing was analysed using X-ray photoelectron spectroscopy. This study found that the properties of the modified surfaces and their interaction with the various lubricant additives impact their tribological performance. With the oxy-nitrided samples, no real effect on friction was observed when using the sulphurised olefin or tricresyl phosphate additives, mostly due to lack of interaction with the less reactive iron nitride layer and their roles as anti-wear additives. However, when using the sulphurised olefin additive with the MoS₂-coated sample, a significant reduction in friction was observed with time. This was mostly likely due to the combined effect with the formation of FeS and MoS₂ within the tribofilm

Investigation of the Effect of a Diamine-Based Friction Modifier on Micropitting and the Properties of Tribofilms in Rolling-Sliding Contacts

The effect of N-Tallow-1,3-DiaminoPropane (TDP) on friction, rolling wear and micropitting has been investigated with the ultimate objective of developing lubricants with no or minimal environmental impact. A Mini Traction Machine (MTM-SLIM) has been utilised in order to generate tribofilms and observe the effect of TDP on anti-wear tribofilm formation and friction. Micropitting was induced on the surface of specimens using a MicroPitting Rig (MPR). The X-ray Photoelectron Spectroscopy (XPS) surface analytical technique has been employed to investigate the effect of TDP on the chemical composition of the tribofilm while Atomic Force Microscopy (AFM) was used to generate high resolution topographical images of the tribofilms formed on the MTM discs. Experimental and analytical results showed that TDP delays the Zinc DialkylDithioPhosphate (ZDDP) anti-wear tribofilm formation. TDP in combination with ZDDP induces a thinner and smoother anti-wear tribofilm with a modified chemical structure composed of mixed Fe/Zn (poly)phosphates. The sulphide contribution to the tribofilm and oxygen-to-phosphorous atomic concentration ratio are greater in the bulk of the tribofilm derived from a combination of TDP and ZDDP compared to a tribofilm derived from ZDDP alone. Surface analysis showed that utilising TDP effectively mitigates micropitting wear in the test conditions used in this study. Reduction of micropitting, relevant to rolling bearing applications, can be attributed to the improved running-in procedure, reduced friction, formation of a smoother tribofilm and modification of the tribofilm composition induced by TDP

Development of valve train rig for assessment of cam/follower tribochemistry.

Component bench tests are a crucial part of a tribology assessment experimental programme for most engines and subsystems. This is because they test the components under conditions simulating the operating characteristics of the system. These have become very important as they shed more light into the friction, wear, lubrication and importantly for this study, the tribochemistry of valve train systems. This work outlines the procedure for the development of a single cam rig (SCR) from a 1.25L FORD Zetec (SE) engine. Friction plots were used to validate the data obtained from the newly developed single cam rig with Mn-phosphate coated and polished follower against a cast iron camshaft. The tribofilm formed using normal and mid Sulphated Ash, Phosphorus and Sulphur (SAPS) 5W-30 oils were evaluated and correlated to the friction and wear properties of the tribopair.Raman and FIB-SEM/EDX investigations of the tribochemical films showed that the normal SAPS oil produced patchy, thick (80-100. nm) and well dispersed tribofilm with better wear prevention capabilities. It was observed that Mid SAPS oil had lower wear prevention due to loosely dispersed and thin tribofilms. Absence of tribofilms at the centre of the insert with this oil also suggests that formation and removal processes are an integral part of the wear mechanisms in highly loaded cam follower systems

Friction and wear mechanisms in boundary lubricated oxy-nitrided treated samples

Oxy-nitriding has been found to be an effective surface treatment technique, used widely in industrial applications due to its ability to prolong the performance lifetime of components. However there has been little work focussing on the potential synergistic effects on tribological behaviour between the treated layers after oxy-nitriding and anti-wear lubricant additives to optimise performance. The friction and wear behaviour of oxy-nitrided (QPQ), MoS2 coated and plain steel samples were analysed. Scanning Electron Microscope (SEM), X-ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy were employed to identify the morphologies and chemical compositions of the treated surfaces before and after testing. QPQ samples exhibited a lower friction coefficient and volume loss. The main mechanisms of these effects were attributed to the mechanical properties of the compound layer formed after the QPQ process and the possible influence of the greater concentration of FeS2 within the tribofilm

Development of a new mechano-chemical model in boundary lubrication

A newly developed tribochemical model based on thermodynamics of interfaces and kinetics of tribochemical reactions is implemented in a contact mechanics simulation and the results are validated against experimental results. The model considers both mechanical and thermal activation of tribochemical reactions instead of former thermal activation theories. The model considers tribofilm removal and is able to capture the tribofilm behaviour during the experiment. The aim of this work is to implement tribochemistry into deterministic modelling of boundary lubrication and study the effect of tribofilms in reducing friction or wear. A new contact mechanics model considering normal and tangential forces in boundary lubrication is developed for two real rough steel surfaces. The model is developed for real tribological systems and is flexible to different laboratory experiments. Tribochemistry (e.g. tribofilm formation and removal) and also mechanical properties are considered in this model. The amount of wear is calculated using a modified Archard’s wear equation accounting for local tribofilm thickness and its mechanical properties. This model can be used for monitoring the tribofilm growth on rough surfaces and also the real time surface roughness as well as changes in the λ ratio. This model enables the observation of in-situ tribofilm thickness and surface coverage and helps in better understanding the real mechanisms of wear

Corrosive-Abrasive Wear Induced by Soot in Boundary Lubrication Regime

Soot is known to induce high wear in engine components. The mechanism by which soot induces wear is not well understood. Although several mechanisms have been suggested, there is still no consensus. This study aims to investigate the most likely mechanism responsible for soot-induced wear in the boundary lubrication regime. Results from this study have shown that previously suggested mechanisms such as abrasion and additive adsorption do not fully explain the high wear observed when soot is present. Based on the results obtained from tests conducted at varying temperature and soot levels, it has been proven that the corrosive–abrasive mechanism was responsible for high wear that occurred in boundary lubrication conditions

Effects of Using Alternative Extreme Pressure (EP) and Anti-Wear (AW) Additives with Oxy-Nitrided Samples

Oxy-nitriding is a widely used industrial process aiming to improve the tribological properties and performance of components. Previous studies have shown the effectiveness of the treatment with friction and wear performance, but very few have focussed on optimising this behaviour. The lubrication properties of several EP and AW additives were examined to investigate their effectiveness in improving the tribological properties of the layers formed after treatment. Previous studies showed the presence of an oxide layer on the sample could improve the effectiveness of the sulphurised olefin (SO) and tricresyl phosphate (TCP) additives. The friction and wear behaviour of oxy-nitrided samples were analysed using a tribometer and surface profiler. Scanning electron microscope, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy were employed to identify the morphologies and chemical compositions of the treated surface before and after testing. No real effect on friction was observed when using the SO or TCP additives, mostly due to lack of interaction with the less reactive iron nitride layer and their roles as anti-wear additives. However, when the zinc dialkyldithiophosphate-containing lubricant was used, a higher friction coefficient was observed. Greater improvements in anti-wear properties with the presence of additives in comparison with only using base oil were reported, with the TCP additive producing the lowest wear rates. The study effectively demonstrated that the additive package type used could impact the tribological and tribochemical properties of oxy-nitrided surfaces

Tribochemical Study of Micropitting in Tribocorrosive Lubricated Contacts: The Influence of Water and Relative Humidity

Water ingress into the lubricant as a contaminant affects performance leading to an alteration in wear, corrosion and fatigue behaviour of the tribological components especially in the rolling element bearings. The current study addresses the tribochemical phenomena involved in micropitting in tribocorrosion systems where different levels of dissolved-water are present in a model lubricant. In this study the effect of different temperatures, water concentrations and relative humidities have been investigated on micropitting under rolling-sliding contacts. The influence of free and dissolved water on tribocorrosive micropitting is clarified. The tribochemical change of the reaction films is studied using X-ray Photoelectron Spectroscopy (XPS) which confirmed that the (poly)phosphate chain length and tribofilm thickness are reduced with increased dissolved-water level

A simple deterministic plastoelastohydrodynamic lubrication (PEHL) model in mixed lubrication

Most power transmitting components operate under mixed lubrication conditions. Concentrated pressures and smaller lubricant film thickness may cause surface and subsurface stresses to exceed the material yield limit causing permanent geometrical changes. A model was developed to include elastoplastic behaviour within a deterministic unified mixed lubrication framework. Model details are presented and the model is validated against published simulation data. A parametric study to address the effect of material yielding on the contact parameters is performed. It is found that the model successfully produces all the key features of the PEHL contact. The model provides a valuable tool to analyse the PEHL contacts with minimal increase in computational effort and complexity