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

Abstract

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 microimpact 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.

Influence of Si- and W- doping on micro-scale reciprocating wear and impact performance of DLC coatings on hardened steel

“…This was confirmed by nanoindentation tests to ~25-30% of the total coating thickness that revealed little change in hardness from tests at much lower depth. Although the hardened substrate improved impact resistance somewhat, there was cracking after a few impacts on a-C:H and Si-a-C:H. The SEM images in figures 7-8 show delamination, consistent with cracking starting at or near the interfaces, as was observed in the studies above, in FIB sections of impact craters on Si-a-C:H [79] and on graded a-C:H under similar test conditions [39].…”

“…show that there was cohesive cracking for a-C:H:W deposited without a CrN sublayer but again interfacial delamination was not observed [79]. Ramirez and co-workers reported enhanced resistance to contact fatigue for W-doped DLC with ~7.5 GPa hardness on hardened tool steel [35].…”

Crack propagation of a thin hard coating under cyclic loading: Irreversible cohesive zone model

“…Although the model has not been validated under cyclic conditions, several experimental studies with sharper and blunter probe geometries [10][11][12][37][38][39][40] have shown that hard PVD coatings can display a level of fatigue behaviour under cyclic loading. In addition to the coating mechanical properties, the type of cracking observed depends on a range of factors including probe geometry, coating thickness, severity of loading and number of cycles.…”

“…The location of the crack at the top surface at the periphery of the contact predicted by the model has also been seen in experimental studies of repetitive micro-scale impact by spherical indenters with smaller end radii (17-20 µm). For example, Beake, Liskiewicz and co-workers used the recently developed micro-impact technique to investigate the micro-scale impact resistance of hard carbon coatings on hardened tool steel [10,12,37] and PVD nitrides on cemented carbide [11,12,38].…”

Dynamic contact behavior of graphite-like carbon films on ductile substrate under nano/micro-scale impact

“…The former can provide much closer simulation of actual service environment of materials under highly loaded intermittent contact by performing repetitive impact at a particular location, and evaluate the behavior of impact wear, adhesive failure and fatigue fracture qualitatively by recoding the evolution of impact depth-time curves in real time (Fig. 1b) [16][17][18]. In general, a single rapid increase in penetration depth may represent the adhesive failure of the coatings, and a series of several smaller jumps in the penetration depths corresponds to a more gradual cohesive failure.…”