Tribological properties of hard carbon films produced by the pulsed vacuum arc discharge method

Abstract

Hard diamond-like carbon (DLC) films were deposited on silicon substrates using a pulsed vacuum arc discharge method. Several methods were used for coating characterization. The film composition was analysed using secondary ion mass spectrometry (SIMS), Rutherford backscattering spectroscopy and forward recoil spectroscopy. The film contained about 0.5 at.% oxygen, 0.3 at.% chlorine, 0.3 at.% copper and 1 at.% hydrogen. The tribological properties of the carbon films were studied using pin-on-disc tests. The counterpart materials employed were alumina and hardened steel (AISI 52100 and M50) pins, which were slid against the coated substrates. The friction coefficient was measured and the wear surfaces were studied. The sliding speeds were varied in the range 0.02–0.6 m s −1 and the load in the range 5–20 N. The tests were generally carried out in air with a relative humidity of 50±3%. The effect of relative humidity on wear and friction behaviour was also studied in the range 10–75% relative humidity. The test results show that the DLC coatings produced for this study generally had a coefficient of friction (μ) of about 0.2. The wear resistance of the coatings was good, provided that the adhesion to the substrate was sufficient. The sliding distance influenced the friction and wear behaviour of the pin-disc system. When the sliding distance was longer, the coefficient of friction increased. When the steel counterpart materials were used, the wear rates showed a decreasing trend. This phenomenon was probably caused by formation of a thick and rough reaction layer on the pin wear surface. With the alumina pins, no visible reaction layer formation was detected and the wear rate of the DLC coating rose when the sliding distance was increased. The wear surfaces were studied in detail using SIMS depth profiling and elemental imaging techniques. The relative humidity had a minor effect on the wear of these coatings in the range studied. The coefficient of friction, however, was higher at lower humidity levels and decreased when the relative humidity was increased. The comparative tests with TiN coatings showed that DLC films have a lower coefficient of friction and are considerably more wear resistant than TiN coatings.