Friction reduction in elastohydrodynamic contacts by thin-layer thermal insulation

Reducing friction is of utmost importance to improve efficiency and lifetime of many products used in our daily lives. Thin hard coatings like diamond-like carbon (DLC) have been shown to reduce friction in full-film-lubricated contacts. In this work, it is shown that contrarily to common belief, the friction reduction stems mainly from a thermal phenomenon and not only a chemical/surface interaction one. It is shown that a few micrometer-thin DLC coating can significantly influence the thermal behavior in a lubricated mechanical system. The presented simulations, validated by experiments, show that applying a thin DLC coating to metal surfaces creates an insulating effect that due to the increased liquid lubricant film temperature at the center of the contact, locally reduces lubricant viscosity and thus friction. The results of the investigation show that the addition of thin insulating layers could lead to substantial performance increases in many applications. On a component level, the contact friction coefficient in some common machine components like gears, rolling element bearings, and cam followers can potentially be reduced by more than 40 %. This will most likely open up the way to new families of coatings with a focus on thermal properties that may be both cheaper and more suitable in certain applications than DLC coatingsGodkänd; 2014; Bibliografisk uppgift: Nature | Research Highlights Warm carbon coat reduces friction Nature 505, 264 Published online 15 January 2014 "A coating material made of carbon reduces friction not just by providing a slippery surface, but also by keeping the points of contact warm. Marcus Björling of Luleå University of Technology in Sweden and his team coated steel balls with 'diamond-like carbon' — a material in which the carbon atoms have a bonding pattern similar to that of diamond. They rolled the balls against a metal disk with an oil lubricant in between, and showed that the carbon coating acts as an insulator, lowering the viscosity of the lubricant and thus reducing the friction between the ball and the disk. The findings could encourage the development of lubricant coatings made from insulating materials."; 20140113 (andbra)