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This study aims to investigate the effects of duplex surface treatment consisting of plasma nitriding and DLC coating on the fatigue properties of Ti6A4V alloy. For this investigation, Ti6Al4V samples were plasma nitrided at 650 °C, 700 °C and 750 °C for 1, 2 and 4 h and then DLC films were produced on the plasma nitrided samples. The structural, mechanical and morphological features of the samples were characterized by XRD, SEM, micro hardness tester and scratch tester. Fatigue tests were performed on the samples by using stress life method. On the surface and sub-surface of the samples, a compound layer and a diffusion zone were formed by plasma nitriding, respectively and XRD results revealed that these layers consisted of Ti 2 N and TiN phases. The raising process time and temperature caused to increase the thickness of these layers and they also increased the surface hardness of the samples. On the other hand, DLC coated samples exhibited more surface hardness than untreated and all the plasma nitrided samples. Fatigue analyses revealed that plasma nitriding reduced the fatigue strength of the material in all process conditions. Although plasma nitriding formed hard surface and sub-surface layers, the brittle structure of the layers and the great difference of elastic modulus between the substrate and nitride layers caused to decrease the fatigue strength of the material. Although DLC coating increased the fatigue strength of untreated and plasma nitrided samples at 650 °C for 1 h, 700 °C and 750 °C for 4 h), the fatigue strength of duplex treated samples (plasma nitrided at 650 °C for 1 h, 700 °C and 750 °C for 4 h and DLC coated) was lower than the fatigue strength of untreated material, similar to only plasma nitrided materials. This showed that the fatigue strength of duplex treated Ti6Al4V was controlled by plasma nitriding. • Ti6Al4V alloy was plasma nitrided at different temperatures and times. • DLC films were produced on untreated and plasma nitrided samples. • A reduction was observed in the fatigue strength of the material after plasma nitriding. • DLC films produced on untreated and plasma nitrided samples improved the fatigue strength of the material. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The compact tension (CT) specimens manufactured from AISI 4140 steel were plasma nitrided at 400 °C, 500 °C and 600 °C for 1 and 4 h in a gas mixture of 50% N 2 and 50% H 2 . CT specimens were subjected to fatigue crack growth under constant amplitude loading in order to analyze the effect of plasma nitriding on fatigue crack growth (FCG). The crack sizes of specimens were correctly detected by a digital image monitoring system. Also, the structural and mechanical properties of specimens were characterized by XRD, SEM, optical microscopy and 3D profilometer and microhardness tester. The structural and mechanical examinations showed that the surface hardness, layer thicknesses and surface residual stresses continuously increased with increasing plasma nitriding time and temperature. It was found that plasma nitriding improved the FCG life of the material due to the formation of compressive residual stresses and nitride layers in surface and near-surface regions. Additionally, the highest FCG increase rate was obtained from 500 °C to 4 h plasma nitrided specimen with respect to untreated specimen. The formation of compound layer and braunite layer constrained the continuous increase of FCG life because of increasing embrittlement and cracking tendency of nitride layers. [ABSTRACT FROM AUTHOR]