Fatigue properties of a 316L stainless steel coated with different ZrN deposits

The fatigue properties of a 316L stainless steel coated with three different under stoichiometric ZrN x films, deposited by PVD magnetron sputtering are investigated and compared with those of the uncoated substrate. Such a steel can be safely coated with these ZrN x films in order to improve some of its surface properties, without compromising its fatigue behavior. On the contrary, the excellent adherence of such films to the substrate, together with the elevated compressive residual stresses and mechanical strength give rise to a significant improvement of the fatigue performance of the base steel. The fractographic evidence indicates that the ZrN x coatings applied remain well adhered to the substrate even after severe plastic deformation of the coating–substrate system. A crude estimate of the strength of the deposits, assuming the validity of a simple law of mixtures for the description of the yield strength of the coated specimens, indicates that the strength of films varies between approximately 22.5 and 34.6 GPa, with the apparent trend to achieve higher values as the nitrogen content of the compound increases. The computation of the constants involved in the simple parametric relationship employed for the description of the stress–life curves of the uncoated and coated specimens allowed an estimation of the percentage of increase in fatigue life due to the presence of the coatings. Such an increase is observed to range between approximately 406 and 1192% when the samples were tested at stresses of the order of 435–480 MPa. Also, the fatigue limit is observed to increase between approximately 6.6 and 9.1% in comparison with the uncoated substrate. The fractographic analysis conducted on the fracture surfaces indicates that the fracture process at low alternating stresses is dominated by the propagation of a single crack, whereas at elevated stresses two cracks can some times be observed. Both at low and elevated alternating stresses, the coatings endure severe damage on the plane of fracture. Also at elevated stresses, longitudinal and circumferential cracks are observed to be present at the surface of the coated specimens. The stress–life results and the fractographic analysis allows the conclusion that the fracture process of the coated samples during cyclic loading is dominated by the nucleation of cracks at the surface of the coating and their propagation throughout its thickness, until the coating–substrate is reached.