Fatigue properties of a 4340 steel coated with a Colmonoy 88 deposit applied by high-velocity oxygen fuel

The fatigue behavior of a quenched and tempered AISI 4340 steel has been evaluated in three different conditions: (a) uncoated; (b) grit-blasted with alumina; (c) grit-blasted and coated with a thermal-sprayed Colmonoy 88 deposit 220 mum thick, employing a high-velocity oxygen fuel (HVOF) gun. The results indicate that grit blasting the base steel can lead to a significant reduction in the fatigue properties of the material. The microscopic observation of the fracture surfaces of the blasted samples indicates that the fatigue processes are initiated at the alumina particles that were retained within the matrix near to the surface of the specimens, giving rise to the presence of stress concentrators that act as nucleation sites of the fatigue cracks. Coating the blasted substrate with this kind of deposit leads to a further reduction of the fatigue properties of the base steel. Such a reduction has been associated with the fracture and partial detachment of the coating from the substrate along the substrate-deposit interface and the reduction in the area of the load-carrying segments of the composite material during fatigue testing. SEM observations of the fracture surfaces of coated samples tested at low alternating stresses, that support this view, have been presented. Under some alternating stresses, the HVOF deposits are believed to contribute to endure the stress applied to the material and therefore the evaluation of the fatigue properties of this kind of coated materials must take into account the thickness of the deposit sprayed unto the substrate. The analysis of the fracture surfaces of the coated specimens revealed that, in this case, the crack nucleation sites are also associated with the presence of Al2O3 particles at the substrate-deposit interface. The fatigue performance of the material under the different conditions analyzed has been quantified by determining the Basquin parameters from the fatigue Life curves obtained. (C) 2000 Elsevier Science S.A. All rights reserved.