Tribological behavior of hard carbon coatings on steel substrates

Abstract

In this work the tribological behavior of hard carbon films on steel substrates has been studied under moderately high contact stress conditions. Diamond-like carbon (DLC) coatings of 1 μm thickness were deposited by a plasma-enhanced chemical vapor deposition (PECVD) process on hardened substrates made from 4140 alloy steel. The coated flat specimens were tested in an oscillatory pin-on-disk configuration in contact with either uncoated or carbon-coated hardened steel balls. Peak Hertzian contact pressures ranged from about 300 to 500 MPa and mean sliding velocity was 50 mm/s. In some tests, the specimens were lubricated with 10W-30 motor oil. It was found that wear of the carbon coatings was approximately linearly proportional to contact force. For unlubricated tests, wear rates of the coated specimens were about 200 times less than rates of wear of uncoated steel specimens for the same contact force. The contacting steel ball (uncoated) showed even greater wear reduction (more than 3 orders of magnitude) when tested against coated substrates. Friction coefficients were four to five times lower for coated specimens. Coating the ball as well as the flat resulted in a further reduction (about 50%) in wear of both ball and flat. Friction and wear reduction brought about by carbon coating in lubricated tests was less than for unlubricated specimens.