Fatigue Endurance Assessment of DLC Coatings on High-Speed Steels at Ambient and Elevated Temperatures by Repetitive Impact Tests

Diamond-like carbon (DLC) coatings are used, among other contexts, on machine parts subjected to repetitive impact loads at ambient and elevated temperatures. Hereupon, the film load capacity may not be consistent when the operational temperature increases. The paper introduces experimental and analytical results, at room and elevated temperatures, for assessing the fatigue endurance to repetitive impacts of a DLC coating deposited on high-speed steel (HSS) specimens, corresponding to cases such as of coated valves and similar parts. The mechanical properties of the coating and its substrate were determined up to 400 °C via nanoindentations and a finite element (FE)-supported evaluation method. The coating’s fatigue endurance was assessed via the repetitive impact test which simulates sufficiently the operational conditions of valves and other machine elements subjected to rebound impact loads. According to these results, the mechanical properties of the DLC coating remain almost stable up to a temperature of 350 °C. However, the coating fatigue endurance to repetitive impact loads progressively worsens up to around 150 °C due to its increasing deformation imposed by the substrate strength deterioration. Over this temperature, prevailing failure mechanisms are the developed gradual coating decomposition, which increases the brittleness practically without hardness losses up to 350 °C. The paper elucidates the mechanisms leading to the attained experimental results and it presents temperature-dependent coating fatigue endurance stresses determined via finite element method (FEM)-supported calculations.