Fatigue behavior of a 4140 steel coated with a Colmonoy 88 alloy applied by HVOF

The fatigue behavior of a quenched and tempered AISI 4140 steel has been investigated, both uncoated and after coating with a deposit of NiCrBSiW alloy (Colmonoy 88) of approximately 240–250 μm in thickness, applied by HVOF thermal spraying. Previous to the deposition of the Colmonoy 88 alloy, an intermediate layer of a NiMoAl alloy known commercially as Metco 447 of approximately 60–70 μm was also applied by means of HVOF thermal spraying, in order to avoid grid blasting of the steel substrate with alumina particles. It has been determined that coating this steel substrate with such a deposit leads to a significant reduction in fatigue properties in comparison with the uncoated substrate. The reduction in fatigue life, evaluated in the alternating stress range of 544–598 MPa has been found to achieve up to 99.8%, whereas, the decrease in fatigue limit was approximately 43%, that is to say 220 MPa below the fatigue limit of the uncoated substrate. It has been shown that such a decrease in fatigue properties can be attributed almost entirely to the presence of both the intermediate bonding layer and coating, rather than to the inclusion of the coating thickness in the computation of the alternating stress applied to the material. The correction of the specimen diameter by subtracting the coating thickness indicates that the drop in fatigue life is still of approximately 99.2%, whereas, the fatigue limit is decreased by 37%. The interpretation of the stress–life curves, together with fractographic analyses conducted on selected samples tested at both low and elevated alternating stresses indicates that the fatigue cracks are nucleated at the surface of the coating and subsequently transferred to the intermediate bonding layer and the substrate. It is believed that the substrate-bonding layer interface, such cracks bifurcate and propagate both along the interface and towards the substrate. The intersection of the cracks, that run along the interface with other circumferential cracks leads to extensive delamination, fracture and complete separation of the coating from the substrate in some sections of the samples.