New strategy for reducing the EHL friction in steel contacts using additive-formed oleophobic boundary films
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ISSN 2223-7690 CN 10-1237/TH
RESEARCH ARTICLE
New strategy for reducing the EHL friction in steel contacts using additive-formed oleophobic boundary films Mitjan KALIN*, Maja KUS Laboratory for Tribology and Interface Nanotechnology, Faculty of Mechanical Engineering, University of Ljubljana, Bogišićeva 8, 1000 Ljubljana, Slovenia Received: 06 April 2020 / Accepted: 15 May 2020
© The author(s) 2020. Abstract: In this study we present a mechanism for the elastohydrodynamic (EHD) friction reduction in steel/steel contacts, which occurs due to the formation of oleophobic surface boundary layers from common boundary-lubrication additives. Several simple organic additives (amine, alcohol, amide, and fatty acid) with different molecular structures were employed as the model additives. It was found that the stronger chemisorption at 100 ℃, rather than the physisorption at 25 ℃, is more effective in friction reduction, which reaches 22%. What is more, EHD friction reduction was obtained in steel/steel contacts without use of the diamond-like carbon (DLC) coatings with their wetting or thermal effect, which was previously suggested as possible EHD friction reduction mechanism; yet about the same friction reduction of about 20% was obtained here—but with much simpler and less expensive technology, namely with the adsorbed oleophobic surface layers. A small variation in the additive’s molecular structure results in significant changes to the friction, indicating good potential in future EHD lubrication technology, where these additives could be designed and well optimised for notable reduction of the friction losses in the EHD regime. Keywords: elastohydrodynamic (EHD); friction; additives; oleophobic layer; boundary slip
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Introduction
In engineering applications many lubricated contacts correspond to conditions of elasto-hydrodynamic (EHD) or hydrodynamic (HD) lubrication regime [1–3]. These are the most desirable lubrication regimes for both friction and wear. In these full-film conditions, film pressure is able to resist the load and separate the contacts, preventing any severe collisions between the surface asperities that enhance the friction and wear. Accordingly, in the HD and EHD full-film regimes, the viscous friction dominates the contact, rather than the solid–solid asperities. Until now, for the vast majority of engineering applications and engineering HD/EHD lubrication
models, the viscosity was considered to be the main parameter of the oil that governs the lubrication [1]. Viscosity changes with the pressure and temperature are critical influences in that respect, since they define the actual thickness of the lubricating film. However, recently, the contacting materials also became of interest for EHD lubrication, in at least two different ways. One of the two suggestions is that higher thermal insulation surfaces (compared to steel), such as diamond-like carbon (DLC), might generate higher surface temperatures, affecting the oil temperature in the contact and so reducing the oil’s viscosity, which then
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