Volume 5, Issue 1 (January 2008)
Tribochemistry of Multiply Alkylated Cyclopentane Oils on Diamond-like Carbon (DLC) Coated Thrust Bearings
In contrast to typical diamond-like carbon (DLC) coatings, hydrogenated DLC (H-DLC) coatings have demonstrated extremely low friction in vacuum and dry atmospheres. This behavior suggests the coatings’ potential importance for spacecraft applications. However, most studies have concentrated on sliding wear testing, with relatively few concerning their use on ball bearings. We have conducted a preliminary study of H-DLC-coated steel thrust bearings, lubricated with a multiply alkylated cyclopentane (MAC) oil, either unformulated (NSO2001A), or formulated with lead naphthenate (Pbnp) or an aryl phosphate ester mixture (NSO2001). To determine if the additives could provide life enhancement when using H-DLC coatings, post-test analyses of the bearing surfaces were conducted using electron microscopy and Auger electron spectroscopy, and analysis of the residual lubricant was conducted using Fourier-transform infrared spectroscopy. Minimal additive-based tribofilm formation was detected on the surfaces of the H-DLC coatings in the wear tracks relative to that with steels. For example, using Pbnp resulted in considerably thicker Pb-containing tribofilms on the uncoated bearing surfaces compared to the H-DLC-coated surfaces. The results indicate that additives suitable for steels may not be appropriate for H-DLC coatings. The high roughness of the bearings used in this study contributed to local spallation of H-DLC at asperities in the wear track. When tested with NSO2001 or NSO2001A oil, several H-DLC-coated bearings endured for significantly longer than uncoated bearings (>10 Mcycles) tested with the same oils. However, the variation and magnitude of the bearing roughness contributed to statistical uncertainty regarding the endurance enhancement of H-DLC coatings. Future studies are planned with higher quality bearings.