The effect of refractive index on the friction coefficient of DLC coated polymer substrates

This paper describes the enhanced mechanical performance that can be achiev ed by the application of diamond-like carbon (DLC) coatings to polymer subs trates. The polymers coated are silicone and polyethylene, and the effect o n the friction coefficient is studied. Film adhesion is found to depend on the DLC film refractive index (n), whereas the friction is largely independ ent of kn in the range studied. Films were deposited from a He/C2H2 mixture at 20 Pa (0.15 Torr) on to the polymer substrates placed on a 10-cm-diamet er electrode driven at 13.56 MHz. Film growth was monitored by in-situ elli psometry (at 675 nm), which was performed on a glass slide placed near the polymer substrate. Friction measurements were obtained using a pin-on-disk tribometer, and measurements were carried out using a stainless-steel pin a t a linear speed of 6 cm s(-1) Film adhesion was evaluated using a pull-adh esion tester. It was found that DLC coatings adhere well to the polymer sub strates and can significantly reduce the friction coefficient of polymers s uch as silicone. Higher refractive index films (which are harder and have a higher mechanical strength) were found to have a poorer adhesion and provi de a slightly increased friction on the polymer surface when compared to lo wer-index films. This study indicates that DLC may be used to enhance the t ribological properties of polymers with potential applications in the bioma terials and light-engineering industries. (C) 1999 Elsevier Science S.A. Al l rights reserved.