Observation of Residual Stress and Fatigue Behavior of Structurally Integrated Thermally Sprayed Nickel Coatings

Fatigue behavior is strongly correlated with the residual stress state within thermal spray coatings, with neutral or compressive residual stresses being favorable for fatigue-sensitive applications. However, determination of the coating’s residual stress state is predominately made prior to subjection of the coating to cyclic loading and does not give insight into possible changes to the residual stress state once in service. In this work, high-velocity, oxy-fuel nickel coatings were subjected to a partial fatigue loading regime, via both rotating bend fatigue and cantilever fatigue, targeting 99% of the total system fatigue life. Neutron diffraction was used to measure changes between the initial compressive residual stresses and after partial fatigue loading and was compared with the residual stress measurements made via beam curvature techniques during deposition. Results indicate that the fatigue credit typically associated with compressive residual stress coatings was partially dependent on the mode of loading and that there was a change in the residual stress magnitude due to changes during fatigue cycling. Additionally, metallographic assessment of the fracture surface was used to determine final fatigue failure within the substrate and crack propagation within the coating crossing through the substrate interface into the substrate.