Influence of a commercial electroless Ni–P deposit on the fatigue properties of a notched and unnotched SAE 4140 steel

The effect of a commercial electroless Ni–P deposit on the fatigue properties of an SAE 4140 steel in the quenched and tempered condition, has been investigated when the substrate is in a notched and an unnotched condition. The application of such a coating to the substrate gives rise to a significant reduction of the fatigue life in comparison with the uncoated samples. The coated specimens in the as-deposited condition showed a reduction of approximately 88%, whereas in the notched uncoated and coated conditions it was of approximately 94–95%. The decrease in fatigue properties in the unnotched coated samples is comparable to that reported for the notched uncoated specimens. It has been observed that the dominant crack responsible for the fracture process is nucleated at the root of the notch, regardless of the presence of the deposit. A rough estimate of the fracture toughness of the material has been determined from the results of the experiments conducted with the notched uncoated specimens, together with the values of the critical crack length at different alternating stresses. The material constants involved in the Paris relationship, for the description of the fatigue crack growth rate as a function of the stress intensity factor at the crack tip, were also determined from the results obtained with the notched uncoated samples.