The Stribeck curve and lubrication design for non-fully wetted surfaces

Abstract Tailoring the properties of tribological interfaces is difficult, but it can be a very efficient way to reduce wear and friction and so increase service life – providing it can be done successfully. However, when considering the interface properties it is necessary not only to take into account the solid–solid contacts, we also need to consider the solid–liquid interfaces, which can have a very significant impact on the micro- and macro-contacts, and which cannot be neglected. In this investigation we show that it is essential to consider the wetting, or slip, properties of the non-fully wetted surfaces when designing the lubrication, and that this is also to a large extent predictable and relatively simple to implement. We discuss the differences between several types of contacts, using steel and DLC as model materials for fully wetted and non-fully wetted surfaces, respectively. We studied steel/steel, steel/DLC and DLC/DLC pairs, and we present the experimental evidence for how the Stribeck curve changes if one or two surfaces in the contact are non-fully wetted. Firstly, in the EHL regime the friction of non-fully wetted surfaces decreases significantly; in our case by around 15% and by more than 20%, respectively, while at minimum friction (the EHL → ML transition) the reduction in friction is lower, but still about 6%, and as high as 9%. Secondly, the film thickness of the non-fully wetted contacts is reduced, which causes a shift in the minimum friction (the Stribeck curve) to higher vη / F values. This suggests a lower bearing capacity for these contacts and the potential danger of a transition in the lubrication regime and, as a result, increased wear. However, we have derived several “ready-to-use” diagrams that are generally valid for any oil and surface properties and which allow us to define the film thickness, the lambda parameter and the regime transition for different wetting/slip properties. These diagrams can be used directly to improve the prediction and design of the lubrication quality and the regimes of the macro-scale contacts by employing a currently used, conventional methodology and equations. Moreover, based on the experimental results and the “ready-to-use” diagrams derived in this investigation for non-fully wetted contacts, the slip at the solid–liquid interface, i.e., the DLC-PAO oil, was estimated independently from the data of two Stribeck curves to be about 20%.

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