Fatigue properties of an AISI 1045 steel coated with an electroless Ni-P deposit

Abstract

The fatigue behavior of samples of an AISI 1045 steel plated with electroless Ni-P (EN) deposits, both on an industrial and laboratory scale, has been evaluated under completed reverse bending or rotating bending conditions. The fatigue experiments were conducted in air at a frequency of 50 Hz, in the stress range of 292–346 MPa. On the laboratory scale, the deposition was carried out in a alkaline solution and the coatings were 17 μm thick, had a P content in the range of 8.4–9.4 wt.% and were post-heat treated in two different ways: at 473 K for 1 h and at 473 K for 1 h followed by 673 K for 1 h. On an industrial scale, the deposition was conducted in an acid bath, the deposits were 25 μm thick, had a P content in the range of 10.4–11.3 wt.% and were post-heat treated at 673 K for 1 h. Both deposits are observed to reduce the fatigue life of the coated components in relation to the uncoated substrate. The reduction in fatigue life is more severe for the samples coated at the laboratory scale, post-heat treated at 673 K. In general, the reduction in fatigue life is associated with the passage of fatigue cracks nucleated in the deposit to the substrate material rather than the nucleation of cracks at the deposit-substrate interface. The nucleation and propagation of such cracks from the surface of the EN coating is enhanced by the microstructural characteristics at the surface of the deposits, by residual stresses present in the coatings and by degradation of the fatigue properties of the coating from the interface to surface. An increase by approximately 2 wt.% in the P content of the deposit, compensates for the reduction in fatigue life induced by an increase in the coating thickness of 8 μm. The parameters involved in the relationship put forward earlier by Basquin, to describe the change in the number of cycles to failure in terms of the alternating stress, are reported for all the conditions investigated.