Fatigue properties of a SAE 4340 steel coated with TiCN by PAPVD

Abstract

An investigation has been carried out in order to study the fatigue performance, both in air and in a 3 wt.% NaCl solution, of a quenched and tempered (Q&T) SAE 4340 steel substrate coated with a TiCN film of ∼4 μm in thickness deposited by plasma assisted physical vapor deposition (PAPVD). The results obtained show that the presence of the coating provides a significant increase, in the range of ∼140–180%, in fatigue life, in comparison with the uncoated substrate, when the material is tested in air under rotating bending conditions at maximum alternating stresses in the range of ∼550–700 MPa. It is also shown that when the material is tested in the NaCl solution only a marginal increase (25% at the most) in the number of cycles to failure is observed, particularly at maximum alternating stresses above ∼570 MPa, which suggests that the TiCN film is susceptible to the corrosive effect of the solution under the combined action of cyclic loading. The increase in the number of cycles to failure in air is explained in terms of the high mechanical strength of the film, its compressive residual stress state and excellent adhesion to the steel substrate. It is shown that fatigue cracks nucleate at the surface of the coating and propagate normal to the substrate–coating interface without bifurcation along the latter, after going through the entire coating thickness. Under the assumption of the validity of a simple linear law of mixtures, the tensile strength of the film is estimated in ∼34 GPa, very close to its absolute hardness of ∼33 GPa, whereas its fatigue limit is found to be in the range of ∼12.6 GPa.