Experimental and numerical evaluation of the contact fatigue resistance of AlCrN, FexN and AlCrN/FexN coatings on AISI 4140 steel

Abstract

An experimental-numerical study of the contact fatigue resistance of AlCrN coatings formed on an AISI 4140 steel was performed. Three different coatings were formed: a coating of aluminum chrome nitrides (AlCrN) by a physical vapor deposition process (PVD) with a thickness of 2.2 ± 0.3 μm, a layer of iron nitrides (Fe x N) by a gas nitriding process with a compound layer of 10 ± 1 and an underlying diffusion zone of 65 ± 4 μm and finally a duplex system of AlCrN/Fe x N with a thickness of 3.0 ± 0.2 μm of AlCrN, a compound layer with a thickness of 2.5 ± 0.3 μm and 16 ± 3 μm of the diffusion zone. Contact fatigue tests were performed on a MTS Acumen electrodynamic test system, by applying cyclic loading to each of the coatings using a sphere to generate Hertzian contact stresses. The contact fatigue test methodology consisted of two main stages. First, the critical load of each formed system was obtained by means of the application of monotonic loads, where the monotonic load value in which the first circular cracks on the coating surface were observed was set as the critical load. The critical loads of the three coatings were: Fe x N: 800 N, AlCrN: 1000 N and AlCrN/ Fe x N: 1100 N. Second, fatigue conditions were performed up to 100,000 cycles using subcritical loads with a frequency of 5 Hz. Cohesive damage was observed in the samples containing iron nitrides, whilst delamination was caused on the AlCrN single layer coating. A numerical model based on the finite element method was developed to evaluate the stress field generated in the systems by cyclic contact loading. The thinner thickness and the abrupt transition of mechanical properties from the coating to substrate caused the maximum principal stress to be located within the contact zone in the AlCrN single layer coating. Results showed better resistance to contact fatigue in the AlCrN/Fe x N duplex system because of the presence of the intermediate layer of iron nitrides. • AlCrN/Fe x N coatings were evaluated by standing contact fatigue. • Influence of nitride layer in the duplex system under contact fatigue was analyzed. • Failure mechanism was less severe in the duplex system under contact loading. • Contact stresses were related with failure mechanisms observed in the coatings. • Nitride layer in the duplex system avoid the adhesive failure of coating.