Micro-scale impact testing - A new approach to studying fatigue resistance in hard carbon coatings

Abstract

Improving the fatigue resistance of DLC coatings under highly loaded repetitive contact is an important step to increasing their performance in demanding applications. The nano-impact test has been shown to be effective at highlighting differences in resistance to contact damage in thin hard carbon coatings deposited on hardened steel. A novel micro-scale rapid impact test capability capable of producing repetitive impacts at significantly higher strain rate and energy than in the nano-impact test has been developed recently enabling the study of coating fatigue with less sharp spherical indenters than in the nano-impact test.Results with the new micro-impact technique on two commercial hard carbon coatings (Graphit-iC and Dymon-iC from Teer Coatings) on tool steel are presented. The role of coating mechanical properties on the fatigue resistance and the load-sensitivity of the impact failure mechanism is discussed. The harder coating with higher sp 3 /sp 2 bonded C (Dymon-iC) was found to be significantly less durable under fatigue loading than the softer Graphit-iC. Reasons for the observed differences are discussed.

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

“…Impact testing (on both micro and nano scales) has been an emerging technique used to characterise the performance of coatings [33]. It can be useful to determine the fracture resistance of hard coatings [34,35] but can also be used to assess fatigue resistance under repetitive loading like the impacts of erosive particles [33,34,[36][37][38]. Research is trending towards the measurement of dynamic hardness of surfaces and the use of different length-scales in testing to drive failure faster [30,37,39].…”

Probe geometry and surface roughness effects in microscale impact testing of WC-Co

“…Previous micro-impact tests with this instrumentation have used spheroconical diamond probes with end radii in the range 17-20 m. [17,18,23] The smallest radius probe produced qualitatively similar impact behaviour to the 20 m probe but more rapidly and at a lower load. Higher plasticity in the tests with this probe result in pile-up under repetitive contact and cracking extending further outside the contact circle at higher load.…”

Effect of chromium doping on high temperature tribological properties of silicon-doped diamond-like carbon films

“…The progressive reduction in hardness with doping could be induced by a decrease in residual stresses as well as graphitisation [23,33]. Chromium-doped films displayed the lower H/E ratio, which has previously been related with the higher plasticity of tougher films [48], while a-C-Si displayed a higher H/E ratio often related with improved wear resistance due to a higher elastic strain-to-break [49].…”