Cr/CrN coating effect on the fatigue crack nucleation period of Ti-6Al-4V alloy

Abstract

Ti-6Al-4V alloy is applied in the production of jet engines and other aeronautical components. The Cr/CrN multilayer coating improves the wear, corrosion and oxidation properties of titanium alloys. However, the influence of hard coatings on fatigue properties needs to be investigated to ensure structural integrity. The present work aims to estimate the fatigue crack nucleation and propagation periods of a Cr/CrN coated Ti-6Al-4V alloy at high-stress and low-stress levels. The material used in this work was the Ti-6Al-4V alloy with an equiaxed microstructure and a yield strength of 1000 MPa. The coating was deposited through the DC sputtering method with 8 bilayers of chromium (Cr) and chromium nitride (CrN), achieving 16 layers and a total thickness of about 5 micrometers. Axial fatigue tests were performed at room temperature for the uncoated and coated titanium alloy. Scanning electron microscopy observation of the fracture surfaces showed that the cracks nucleated at the coating and propagated without deflections to the substrate. The Paris law was applied to estimate the fatigue crack propagation period of the Ti-6Al-4V alloy substrate, considering an initial crack with the size of the coating thickness. The results showed that the crack propagation period for the coated samples represented more than 90% of the fatigue life at the high-stress level of 1000 MPa, while it accounts only for about 1% of the fatigue life at the low-stress level of 700 MPa. The fatigue life debit of the Cr/CrN coated material with 8 bilayers and 5 micrometers can be attributed to the significant crack nucleation period reduction due to the brittle layers of CrN compared to the uncoated Ti-6Al-4V alloy. The detrimental coating effect was more substantial at the high-cycle fatigue regime when the crack nucleation period accounts for the majority of the fatigue life.