Wear analysis of hydrogen-free diamond-like carbon coatings under a lubricated condition

Hydrogen-free diamond-like carbon (DLC) coatings were formed using four different deposition methods: trigger-type arc ion plating, laser-discharge-induced arc ion plating, filtered cathodic vacuum arc ion plating and magnetron sputtering. Wear tests were conducted on the coatings under a lubricated condition to investigate the effect of their surface hardness on wear, as well as the phase transformation from sp3 to sp2 and the influence of their surface properties. The results showed that DLC coating wear tended to decrease with higher hardness and that surface droplets (particles) formed during the deposition process tended to have the effect of increasing wear. Further wear testing was done using thicker coating samples to investigate the effect of the droplets in more detail. The results showed that abrasive wear tended to occur accompanying the falling out of droplets from inside the coating. An examination of such droplets by electron energy-loss spectroscopy using a transmission electron microscope (TEM-EELS) revealed that they originated from sputtered graphite debris that was not completely melted during the coating deposition process and served as the nucleus for droplet growth. Additionally, the structure of the droplet-coating interface was found to be a weak sp2-rich layer.

► Wear behavior of H-free DLC coatings was investigated under a lubricated condition.
► Droplets can increase the amount of wear beyond the order of coating hardness.
► TEM-EELS yielded evidence of a droplet interface consisting of an sp2-rich layer.
► Droplets tended to fall out from the weak interface due to sliding contact.