Abstract
The stracutral transformation of diamond-like carbon (DLC) coatings under lubrication with poly-alpha-olefin (PAO) and PAO added a friction modifier (FM) has been investigated by using a home-built in-situ Raman tribometer. Friction tests were carried out using a ball-on-disk, where a 19 mm diameter tetrahedral amorphas carbon (ta-C) DLC-coated ball was loaded and rubbed against a steel disk immersed in lubricant solution. In situ optical microscopy and Raman spectroscopy were used to monitor the structual transformation of DLC coatings and the wear tracks on the ta-C DLC coated ball. Raman analysis were conducted on the rubbed surface on the rotating DLC coated balls every three minutes of sliding time during friction tests. In this study, the degree of graphitization on ta-C surfaces was estimated by calculating the ID/IG ratio from the Raman spectra of the ta-C film during friction tests in the different lubricants. From the test results, the lubrication with PAO + FM showed lower friction and wear volumes than lubrication with pure-PAO. On the other hand, the structural transformation rate of the ta-C DLC coating lubricated with pure-PAO was much faster than the case of lubrication with PAO + FM. Moreover, for both cases, there were relationships between the ID/IG ratio and the friction coefficient or the wear volumes of the ta-C film. From these results, the structural transformation behavior of the DLC coatings depends on the types of lubricant and relate to the tribological properties of the DLC coating under boundary lubrication.
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