Fatigue behavior of AA7075-T6 aluminum alloy coated with a WC–10Co–4Cr cermet by HVOF thermal spray

Abstract

Rotating bending fatigue experiments, both in air and in a 3% NaCl solution, have been carried out in order to study the fatigue behavior of a 7075-T6 aluminum alloy coated with aWC–10Co–4Cr cermet by high velocity oxygen fuel (HVOF) thermal spray, without any grit blasting prior to the coating deposition. The results indicate that the presence of the coating gives rise to a significant increase in the fatigue strength of the substrate and therefore, that from the fatigue behavior point of view this coating could be a feasible replacement of electrolytic hard chromium plating in aircraft applications. Such an increase in fatigue and corrosion-fatigue strength is believed to be associated with the intrinsic microstructural characteristics, corrosion resistance and mechanical properties of the coating, its compressive residual stresses and the possible compressive residual stress field induced in the substrate during coating deposition. The fractographic analysis of the specimens indicates that the final fatigue fracture could be due either to the joint action of a large number of cracks, which propagate from the substrate–coating interface, or to a single dominant fatigue crack, depending on the maximum alternating stress applied to the coated system. It is shown that the lack of grit blasting prior to HVOF deposition does not seem to compromise the functionality of the coated system, while it avoids the introduction of further sharp stress concentrators for the nucleation of fatigue cracks.