Effects of thin hard film deposition on fatigue strength of AA7075-T6

Abstract

Thanks to their favorable strength-to-weight ratio, Aluminum-Zinc alloys are often used in high-performance industrial sectors. These alloys are characterized by poor erosion resistance, which could be increased by the deposition of thin films. For example, PVD (Physical Vapor Deposition) films can reduce wear and ensure appreciable performance in aggressive environments. This paper describes the effects of thin hard film deposition on the strength of AA7075 under cyclic loading. Uncoated samples and samples with a DLC (Diamond-Like Carbon) surface layer were tested with a rotating bending fatigue machine within the range of ${10}^5$ – ${10}^7$ cycles. Two regression models were tested to study the relationship between fatigue strength and number of cycles within the range considered. For the samples without the film, even a linear regression can be suitable. With regard to the fatigue strength of the samples with surface layer, a piecewise linear regression is suggested, although a high scatter of experimental data can be observed. Finite element (FE) modelling was used to calculate the stresses induced by bending moment. The bending stresses were added to the residual stresses induced by the film deposition. The maximum value was found beneath the sample surface. Therefore, crack nucleation is expected under the surface of samples with film. The fatigue cracks initiate at the surface in untreated samples.