Abstract:The synergetic lubrication effects between diamond-like carbon (DLC) coatings and lubricating oils have draw much attention for the past decades, and promising results have been reported between DLC and biodegradable synthetic ester, which might be a potential solution in energy conservation and environmental protection. However, the challenges lie in that the synthetic ester oils exhibit lower oxidation stability than the hydrocarbon oils and are prone to produce organic acids in the aging process. Thus, investigating the tribological behavior of DLC coating in acidified ester oil is meaningful to the long-term reliability of ester-lubricated DLC contacts. In this article, the friction and wear behavior of DLC-DLC, DLC/steel, and steel-steel contacts in acidic ester oils with different total acid values (TANs) is systematically studied. It was found that acidification of ester oil showed certain beneficial tribological effects for the self-mated steel and DLC contacts but could cause severe wear loss on steel counterparts with a special polished appearance for steel-DLC hybrid contacts. By focusing on the properties of the tribofilm formed on the contact surfaces, a possible tribological mechanism was discussed. Finally, it was proposed that the steel-DLC contacts exhibited low tolerance to organic acids and thus the additives that could dissipate the produced organic acids are a critical component in lubricating oils that were tailored for the steel-DLC contacts. 

Key words:DIAMOND-LIKE-CARBON; AMORPHOUS-CARBON; LOW-FRICTION; COATINGS; ADSORPTION; STEEL; ADDITIVES; MECHANISMS; STABILITY; BIODIESEL 

Published in TRIBOLOGY TRANSACTIONS, 58 (5):849-858; 10.1080/10402004.2015.1023410 2015