Probing fatigue resistance in multi-layer DLC coatings by micro- and nano-impact: Correlation to erosion tests

DLC coatings have seen recent use as protective coatings for flow control devices in the oil and gas industries. Improving fatigue resistance for multi-layered DLC coatings on hardened steel is key for improving their per-formance in this harsh environment of highly loads repetitive contact. This has been studied directly by micro- scale repetitive impact tests at significantly higher strain rate and energy than in the nano-impact test, enabling the study of coating fatigue with spherical indenters and dry erosion testing. Nano-impact has also been used to assess the initial fatigue behaviour of the coatings. Good correlation between micro-impact results and erosion results was found. Hard multi-layered a-C:H and Si-a-C:H coatings were found to be significantly less durable under fatigue loading than a-C:H:W. The influence of the coating mechanical properties and structure on these differences is discussed. The results of this study provide further strong evidence that in highly loaded me-chanical contact applications requiring a combination of load support and resistance to impact fatigue, the optimum lifetime of coated components may be achieved by designing the coating system to combine these properties rather than by solely aiming to maximise coating hardness as this may be accompanied by brittle fracture and higher wear.