Fatigue Life of Layered Metallic and Ceramic Plasma Sprayed Coatings

The application of thermally sprayed coatings can significantly enhance properties of coated parts such as thermal and wear resistance or biocompatibility. For example coatings prepared by HVOF are used for airplane landing gear parts and plasma sprayed coatings form heat shield on surface of turbine blades and vanes, several types of coatings are used in bone implants. In these application fields the fatigue behavior of coated components is of a paramount importance. The intrinsic properties of the deposited coating (modulus, microstructure, porosity etc.) play an important role. At the same time, the coating process can influence fatigue live through defects and residual stresses introduced to the substrate during spraying and associated preparation steps such as grit-blasting. The influence of substrate surface preconditioning and effect of individual layers of composite coatings on fatigue life were characterized. Plain substrates, grit-blasted substrates, and plasma sprayed specimens with one to three layers of coating were studies. The layered coatings were composed of alternating sequence of ceramic (Cr2O3) and metallic (Ni10wt%Al) layers. Deflection controlled resonance bending (R = -1) fatigue test of flat specimens was performed. The deformation amplitude was 1.3 milistrain in crack initiation site and loading frequency was around 80 Hz. The significance of the effect of surface treatment on fatigue properties was examined statistically using Wilcoxon test. From the obtained data the effect of individual layers was deduced. In order to explain the observed fatigue behavior, the fractographic analysis and other means of coating and substrate characterization were performed. Strain hardening in substrate was characterized by micro/nanohardness measurement and EBSD analysis. Residual stress in substrate was measured using neutron diffraction.