The influence of contact conditions on the initiation and progression of micropitting damage

Abstract

Micropitting is a form of rolling contact fatigue which acts at the level of the asperities in rolling sliding, lubricated contacts of hardened steels. It is characterised by small pits connected by a crack network, which can cover appreciable areas and lead to significant material loss and eventual component failure. The main aim of the work described in this thesis was to investigate and better understand the effects of different contact parameters including surface roughness, specific film thickness, hardness, traction and contact pressure on the initiation and progression of micropitting damage. In addition, the transition from micropitting to macro-pitting damage was explored and finally, the experimental results were used to validate and improve a numerical model for prediction of micropitting. A rigorous test method was employed in this investigation to better isolate the influence of a single contact parameter at a time. The test method, which was shown to generate repeatable results, was conducted on a triple-disc contact fatigue rig (known as ‘PCS MPR’) consisting of three counterface discs equally spaced around a single test roller specimen. The method applies tight tolerances on surface roughness and hardness (and hardness differential) of test specimen and counterface discs. This is found to be crucial to obtaining valid trends given the sensitivity of micropitting to these parameters. The effect of frictional heating on specimen bulk temperature rise was considered using a thermal network model of the rig; this allowed for improved predictions of oil film thickness at different conditions. A custom-made oil consisting of a PAO-base oil with ZDDP anti-wear additive was used throughout the testing. This formulation has previously been shown to control well the wearing-in of the disc counterfaces which is an important factor in micropitting. Using a less hard specimen against significantly harder counterfaces ensures damage is mainly accumulated on the roller specimen and aids in preserving the roughness of the counterface discs. The inlet temperature and entrainment speed were kept constant when exploring the influence of other parameters and film thickness was varied by using different viscosities of the same PAO base oil. Micropitting was shown to consist of an incubation phase, during which a network of microcracks is formed across the surface, and a micropitting wear phase, which occurs once the network of cracks has formed and during which a significant amount of material is continuously removed from the surface. Evidence from this investigation suggests the mechanisms behind the material loss in the micropitting wear phase is dependent on the formation of the crack network and that material loss is generated through wear, micropit generation and plastic extrusion of material on the trailing edge of the pit. The effect of specific film thickness (Λ ratio), commonly used as the main parameter to assess the risk of micropitting, was isolated from the effect of surface roughness by changing the film thickness 3 through use of different PAO viscosities while keeping the root mean square surface roughness (Rq) constant. Similarly, the effect of changing Rq surface roughness was studied at a constant Λ by changing the film thickness via different viscosities of PAO in line with the changes in roughness. Increasing the surface roughness at a constant Λ ratio was shown to increase the amount of micropitting. Increasing the Λ ratio at constant surface roughness was shown to reduce the amount of micropitting as may be expected, with trends at very low Λ ratios being somewhat complicated by the simultaneous presence of adhesive wear. Micropitting was shown to be more sensitive to surface roughness than Λ ratio. Together these results suggest that using the Λ ratio alone without specifying the surface roughness is inadequate to predict the risk of micropitting damage. A finding of practical importance was that there exists a surface roughness level below which micropitting was prevented over a wide range of Λ ratios within a reasonable design life. The effects of Hertzian pressure (p0), specimen and counterface hardnesses, traction, presence of ZDDP and presence of black oxide coating were also investigated. Micropitting wear rates significantly reduced with decreasing p0, reducing traction and using black oxide coated counterface discs. An explicit relationship between micropitting severity and the hardness of the contacting bodies was unobtainable as these tests were dominated by the effect of wearing-in. The experimental results from these investigations were combined to create micropitting severity maps by plotting the Λ ratio against a parameter combining surface roughness, p0 and specimen hardness. These maps are able to show conditions where a high severity of micropitting may be expected and conditions where micropitting is unlikely or low in severity, and as such can aid in design process of mechanical components at risk of micropitting. Roughness parameters which are based on the characteristics of only the peaks of the roughness distribution, such as Rpk, and ideally also include some measure of the wavelength content, such as β* (correlation length) may correlate better with micropitting than simple and commonly used root mean square roughness (Rq). The thesis also explores the transition between micropitting damage and the occurrence of the more severe surface fatigue damage mode of macro-pitting. The aim of this was to provide some practical guidance on which set of conditions would lead to one or the other failure mode as this is important when designing components such as gears for example. Conditions that were seen to clearly promote macro-pitting over micropitting were higher values of Hertz pressure and high amounts of wear at the start of operation which acts to reduce the counterface roughness, such as in cases where no effective anti-wear additive was present or the counterface discs were coated with black oxide. The occurrence of macro-pitting was correlated with deeper asperity stress fields and increasingly similar asperity and Hertzian stress magnitudes. In contrast, lower Λ ratios and/or higher counterface roughness promoted occurrence of micropitting over macro-pitting. Under such conditions, it appears that the removal of material through micropitting and wear acts to hinder the formation of longer cracks that propagate deeper into the subsurface which are a necessary precursor to macro-pitting. Finally, a numerical model for prediction of the onset and progression of micropitting was developed. The model uses measured roughness and accounts for oil film thickness to obtain stress history for each element of material as two rough surfaces roll-slide over each other. This was then combined with the Dang Van fatigue criterion and the Palmgren-Miner linear damage accumulation rule applied to asperity stress history to predict when a segment of material may fail through surface fatigue and hence form a micropit. A wear model based on Archard’s wear equation is also included to account for the interaction between adhesive wear and micropitting which was earlier shown to be important. The general modelling approach employed here is similar to some existing micropitting models, but the present model could be extensively validated against the large amounts of experimental data obtained in this PhD as described above providing a high level of confidence in its predictions.