Fatigue behaviour of 7075-T6 aluminium alloy coated with WC–12Co alloy deposited by plasma spray

Abstract

This investigation has studied the effect of a WC–12Co alloy deposited by plasma spray on the fatigue and corrosion fatigue properties of a 7075-T6 aluminium alloy, as part of its validation as a viable replacement of electrolytic hard chromium plating for aircraft applications. It has been determined that coating this substrate alloy with a deposit of ∼50 μm gives rise to a significant decrease in the fatigue and corrosion fatigue properties of the substrate coating system in comparison with the uncoated substrate. Thus, if the substrate coating system is tested in air at maximum alternating stress in the range 219–377 MPa, the fatigue life debit varies between 86 and 73% in comparison with the uncoated substrate. On the other hand, if the substrate coating system is tested in a 3%NaCl solution in the same range of alternating stress, the fatigue life debit varies between 23 and 73%. However, at alternating stresses <200 MPa the coated system behaves as the uncoated substrate. The results of the corrosion fatigue tests and the fractographic analysis conducted in some representative samples have shown that the corrosion fatigue process of the substrate coating system is controlled by the early nucleation of fatigue cracks on the surface of the WC–Co coating, owing to its highly heterogeneous microstructure, and their subsequent propagation within it and along the substrate/coating interface. In this way, once the NaCl solution is able to reach the interface, the fracture mechanism of the coated substrate occurs in a similar manner to that of the uncoated substrate. The evaluation of the static mechanical properties of both the uncoated and coated alloy has indicated that the yield and tensile stress of the substrate coating system decrease by about 27 and 8% respectively in relation to the uncoated substrate. Such results can be accounted for not only by fracture and delamination of the coating during the elastic deformation of the system, but also by a possible overheating of the substrate during deposition that modifies the T6 condition of the substrate alloy. Thus, the decrease in fatigue properties has been partially explained by these two effects as well as the propagation of fatigue cracks from the coating to the substrate.