Green superlubrication by hydrogen-free amorphous carbon with human friendly lubricants

Abstract

Recently, many abnormal weather phenomena have been observed throughout the world. One reason for this is thought to be increasing amount of carbon dioxide gas in the atmosphere. Fuelsaving technologies have been increasingly required, especially for automobiles, to avoid global environmental destruction by stringently reducing the amounts of carbon dioxide gas generated and global resource depletion. Additionally, poisonous components such as phosphorus, sulfur, and chlorine in industrial lubricants are required to be decreased or eliminated so that they do not harm human health. Technologies for reducing friction by using environment-friendly materials are direct ways of minimizing environmental problems and improving human health. For automotive engine lubrication, the viscosity of the engine oil has been lowered to improve fuel economy. As the viscosity of engine oil decreases, the requirement of wear resistance for sliding engine components such as the cam follower and piston is becoming stricter. Therefore, diamond-like carbon (DLC) which has several advantages, such as being harmless to the human body because it consists of mainly elemental carbon, low friction properties, high wear resistance, and high corrosion resistance, is increasingly applied to the actual sliding parts in automotive engines. In particular, hydrogen-free amorphous carbon, tetrahedral amorphous carbon (ta-C), is becoming popular for application to the valve lifters and piston rings of mass-produced gasoline engines, because it exhibits a lower friction coefficient under engine oil lubrication compared with hydrogen-containing amorphous carbon (a-C:H). Additionally, ta-C shows superlubricity with human-friendly lubricants, such as oleic acid and glycerol. In this article, recent topics in the technology for applying DLC coatings onto automotive engine components are reviewed. The properties of reducing friction for each application are discussed briefly. The effect of engine oil additives on the friction properties of DLC coatings is also explained. Then, the superlowfriction properties achieved by combining DLC coatings and environment-friendly lubricants are introduced as a promising technology for future engine components. A superlow friction coefficient below 0.01 was obtained by a self-mated ta-C coating lubricated with oleic acid; a-C:H coating and the noncoated bearing steel (SUJ2) did not function well. These results suggest that automotive engine fuel economy can be improved markedly by using completely environmentfriendly materials. Recent fundamental research on the mechanism of superlubrication is introduced briefly. Judging from these advanced technical results, DLC coating technologies to address global environmental problems are promising for use with various components for many industries in addition to the automotive industry.