An experimental study on the characteristics and delamination of TiN coatings deposited on Al 7075-T6 under fatigue cycling

In this paper, delamination of a titanium nitride (TiN) thin film from an aluminium alloy 7075-T6 substrate has been studied under fatigue loading conditions. TiN coatings of 3 μm in thickness were deposited onto the aluminium substrate using a physical vapour deposition process. Fatigue fracture surfaces of the coated specimens, failed under a range of low to high cyclic loads, were examined by means of scanning electron microscopy (SEM). SEM analyses showed that the coating layer remained well-adhered to the substrate under fatigue loading with maximum stresses less than 200 MPa. However, local delaminations were observed at maximum cyclic stresses higher than 200 MPa. The coated specimens were found to beneficially resist maximum cyclic stresses up to 350 MPa without showing delaminations when subjected to a modified post heat treatment with a high solutionising temperature. This was associated with an average improvement of 27% in the fatigue life of the coated specimens subjected to the post heat treatment in a controlled atmosphere (argon) when compared to uncoated Al 7075-T6 for tested maximum alternating stress levels. Characterisation of TiN coatings confirmed the presence of single phase TiN film onto the substrate without any oxidisation when heat treated in argon atmosphere. Moreover, compressive residual stresses in TiN coatings increased from - 4.54 to - 7.56 GPa after the post heat treatment as a result of thermal stresses introduced during the quenching stage of the heat treatment. The actual lattice parameters were determined using the Cohen-Wagner method and were found to increase from 4.257 (Å) for the as-deposited TiN coatings to 4.262 (Å) for TiN coatings subjected to the post heat treatment.