Fatigue crack growth behavior of DLC coated AISI 4140 steel under constant and variable amplitude loading conditions

In this study, the effect of DLC (diamond-like carbon) coating on fatigue crack growth (FCG) behavior of AISI 4140 alloy was investigated under constant and variable loading conditions. For this purpose, DLC films were deposited on compact tension (CT) test specimens made of AISI 4140 steel using physical vapor deposition (PVD) method. The influence of DLC coating on structural properties of material was studied by means of XRD, SEM, scratch and microhardness tester. Afterwards, untreated and DLC coated CT specimens were subjected to fatigue loads by a servo-hydraulic fatigue testing system. The FCG behavior was monitored and correctly evaluated via digital monitoring system. Constant amplitude load, overload (OL), underload (UL) and their sequences were applied for tests. The results showed that DLC coating extended the FCG life of the material under constant amplitude loading due to increasing compressive residual stress and microhardness. FCG life of both untreated and DLC coated specimens increased after the application of OL. However, after OL application, a reduction in the increased FCG life of DLC coated material was observed because of decreasing crack tip plasticity. Also, FCG rate of untreated material decreased after the application of UL and DLC coating almost suppressed this behavior.