Super-low friction of ta-C coating in presence of oleic acid

Makoto Kano,Maria Isabel De Barros Bouchet,Jean Michel Martin,Kentaro Yoshida

doi:10.1007/s40544-014-0047-1

Makoto Kano, Maria Isabel De Barros Bouchet + Show 2 more

Open Access

https://doi.org/10.1007/s40544-014-0047-1

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Abstract

Abstract Friction tests have been carried out on bearing steel friction pairs coated with a-C:H and ta-C under lubricated conditions with 1-hexadecene and pure oleic acid, respectively. Main results show that ta-C leads to the super-low regime with oleic acid (friction coefficient below 0.01) and that a-C:H is quite well lubricated by 1-hexadecen. Surface analyses by coupling X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) have been carried out in the case of oleic acid with ta-C and a-C:H coatings. Results show that tribochemical reactions have occurred. OH groups terminate the ta-C surface whereas a-C:H is covered by a thin tribofilm and is terminated by oleic acid monolayers. The superlow regime in the case of ta-C is mainly attributed to the presence of the OH-termination.

Highlights

Reduction of energy loss by mechanical friction especially for automotive engine has been strongly required in recent years for improving fuel economy
These results have a shape of a Stribeck curve showing the different regimes that are encountered, namely elasto-hydrodynamic lubrication (EHL) above 100 mm/s, boundary lubrication (BL) under 1 mm/s and mixed between these two speeds
The different friction coefficients of diamond-like carbon (DLC) below 5 mm/s seemed to be caused under boundary lubrication condition whereas those above 50 mm/s seemed to be caused under the EHL lubrication regime

Summary

Introduction

Reduction of energy loss by mechanical friction especially for automotive engine has been strongly required in recent years for improving fuel economy. The super low friction property below 0.01 of the friction coefficient is the most outstanding advantage of DLC coatings in vacuum or inert gas [1, 2], there is no successfully actual application using this property except the applied technology for the following engine parts. A DLC-coated valve lifter and piston ring have been applied already to the actual mass-produced gasoline engines to reduce engine friction in Japan [1, 3]. This low friction property was obtained by the hydrogenfree DLC, (ta-C) lubricated by an ester containing gasoline engine oil [2]. The ultra-low friction properties using ta-C were obtained

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