Influence of arc evaporated TiAlN based coatings on the fatigue life of Ti-6Al-4V

Abstract

Ti-6Al-4V is widely used in the aerospace industry because of its remarkable combination of high specific strength, exceptional corrosion resistance and validated long-term data. Nevertheless, its lack of wear resistance can be problematic when used in components subject to high-temperature tribological interactions. To address this concern, TiAlN based coatings have proven to be an effective surface strengthening technique. However, previous studies emphasized conflicting conclusions on the influence of physical vapor deposited TiAlN on the fatigue behavior of an underlying Ti-6Al-4V alloy. Therefore, this work investigates the effects of arc evaporated TiAlN based thin films on the fatigue life of fully equiaxed Ti-6Al-4V in order to improve the fatigue strength and reliability of this material-combination.Overall, five different coating systems were deposited on custom made Ti-6Al-4V fatigue specimens in an arc evaporation coating system. Subsequently, single cantilever high-cycle fatigue tests were performed using a dynamic mechanical analyzer. As a result, the residual stress-state of the thin film was found to be the critical property. Premature coating fracture resulted in reduced fatigue performance when the residual compressive stress of the arc evaporated thin film was too low. However, once the residual stress-state was large enough, the high-cycle fatigue strength of the material-pairing increased with higher residual compressive stress. Compared to the fatigue limit of the uncoated Ti-6Al-4V, the depositions of TiyAly-1XN (X=transition metal) resulted in improvements of up to 43 %. Moreover, X-ray nanodiffraction experiments have shown, that the addition of a thin metallic interlayer has significantly altered the residual stress gradient of the substrate near the interface. Consequently, the introduction of a metallic interlayer offers new opportunities to modify the residual stress and thus further improve the fatigue performance of TiyAly-1XN coated Ti-6Al-4V.