Testing the Friction Between a Hydrogenated DLC Coating and Polypropylene Homopolymer

Abstract

This study examines dry sliding friction between a hydrogenated amorphous carbon (a-C:H) diamond-like carbon (DLC) coating and a polypropylene homopolymer (PPH). The coating was deposited on a quenched and tempered 42CrMo4 steel specimen (39 ± 1 HRC). Friction tests were performed at three sliding speeds (v = 5, 50, and 500 mm/min) and three surface pressures (p = 2, 4, and 6 MPa). From the recorded friction-force curves, the static friction coefficient (μ₀) was taken as the peak value at the onset of motion, while the dynamic friction coefficient (μ_dyn) was evaluated in the near steady-state (gross sliding) region. Both μ₀ and μ_dyn increased with sliding speed: μ₀ rose from about 0.24 to 0.33 and μ_dyn from about 0.22 to 0.32 in the investigated range. Surface pressure had a smaller effect: μ₀ showed no clear pressure trend, whereas μ_dyn increased with pressure at a given speed. A two-factor analysis confirms that sliding speed is the dominant factor for both μ₀ and μ_dyn, and it also indicates a weak non-linear speed dependence in this parameter range. Overall, the friction coefficient of the PPH–DLC pair remained approximately within 0.20–0.35 for v = 5–500 mm/min and p = 2–6 MPa.