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Diamond-like carbon coatings have previously been studied as a protective coating for fretting wear protection providing low friction and low wear. H/E ratio has been used as a metric to rank coating performance in sliding wear, but this has not been applied to gross-slip fretting. Three DLC coating systems (a-C:H, Si-a-C:H, a-C:H:W top layers) on hardened M2 tool steel were studied using a bespoke electrodynamic shaker with a 10 mm 52100 steel ball as the counterface. This work has shown that H/E ratio can be used to predict wear performance in gross-slip fretting; the highest H/E ratio a-C:H performed best with low friction and wear.

Since different properties of coating systems influence their friction and wear at different length scales contact size can play a critical role in microtribological experiments. In this study the behaviour of 3 different types of coating system which vary in terms of their thickness, substrate and mechanical properties has been investigated. The coatings were chosen for either their industrial relevance in automotive or MEMS applications, or as model coating systems. A wide range of nano/microtribological tests have been performed with different indenter geometries (tip sharpness), including single and repetitive scratch tests with unidirectional contact, and reciprocating wear tests, with depth and friction evolution monitored so that the relationships between failure mechanism and friction in coating systems with differing mechanical properties could be explored. The influence of surface topography on friction has been shown in ramped and constant load scratch tests. When fracture occurred resulting in a sudden increase in probe depth there was an abrupt decrease in friction which is ascribed to a contact area effect. In contrast, where deformation progressed through micro-wear a more gradual increase in depth can be associated with higher contact area and higher friction.

Reciprocating contacts occur in a wide variety of practical wear situations including hip joints and electrical contacts. In developing tribological tests for candidate materials with improved durability in these contacts it is beneficial that the contact conditions (e.g. sliding speed) can be reproduced. Hence, a fully instrumented capability for rapid high-cycle linear reciprocating nano-scale wear tests has been developed. It is multi-sensing with high data acquisition measurements of probe displacement data, friction, cumulative frictional energy dissipation and electrical contact resistance. In comparison with other nanoindenters the design has the high level of lateral rigidity which provides sufficient stability to perform nano- or micro-scale wear tests for extended duration (e.g. several hours, up to 300 m sliding). In this study, reciprocating nano-wear tests with diamond probes have been performed on the biomedical alloys Ti6Al4V and 316L stainless steel, and with electrically conductive metallic probes on gold and silver alloys. The stainless steel exhibited a ductile response with low friction throughout the load range. At higher loads on Ti6Al4V, there was an abrupt transition to higher friction and fracture-dominated wear after ~20 cycles. Improved detection of the onset of wear and the subsequent failure mechanisms sliding against conductive probes was possible by a multi-sensing approach simultaneously monitoring friction and electrical contact resistance (ECR). Changes in ECR exhibited a complex correlation with changes to the measured friction. The reciprocating tests of noble metal-noble metal contacts (Au–Au and Ag–Ag) showed much longer endurance than gold vs. steel contacts although occasional isolated failures were observed. A new approach for the analysis of repetitive nano-scratch test data was also developed enabling improved data mining.

© 2020 Elsevier B.V. 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 performance 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 mechanical 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.