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

M. Kalin, I. Velkavrh, J. Vižintin

Wear 267 (2009) 1232–1240.

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, andwhich cannot be neglected. In this investigation we show that it is essential to consider the wetting, or slip, properties of the non-fully wettedsurfaces 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 v/F values. This suggests a lower bearing capacity for these contacts and the potential danger of a transition in the lubricationregime 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 lambdaparameter 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 %.

URL: http://www.sciencedirect.com/science/article/pii/S0043164809001008

Keywords: DLC, Stribeck, tribologija, trenje, DLC prevleke, mazanje, slip, oils, wetting, olja, Lambda

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