A case study on fatigue damage in PVD coated tool steel under cyclic bending load

Several investigations on physical vapor deposition (PVD) coated steel substrates under cyclic bending load have reported an increase in fatigue strength of the coated compound. The compressive residual stress state of the coating can delay the initiation and propagation of surface or subsurface fatigue cracks. There are also investigations reporting no significant effect or a negative effect of PVD coatings on fatigue behavior of steels. The argument here is that the coating cracks during deformation and may lead to an earlier crack initiation in the substrate. Therefore, the effect of PVD coatings on fatigue behavior needs to be studied independently for different substrate materials. Hereby, the investigations on fatigue behavior of PVD coated tool steels are rather limited. Moreover, the fatigue damage mechanism has been analyzed ex situ after the sample failure mostly for low stress amplitudes. The presented work is aimed at investigating the effect of CrAlN coating on the fatigue behavior of HS6–5-2C tool steel along with the analysis of fatigue cracks initiation and propagation in coated compound. For this purpose, the fatigue behavior of uncoated and coated tool steel was studied at high as well low bending stress amplitudes. Scanning electron microscopy (SEM) was used to analyze the fractured samples. Moreover, the fatigue damage mechanism in coated compound was studied in situ by carrying out a cyclic bending test inside large chamber SEM. It was shown that the fatigue damage in PVD coated high speed tool steel compounds under cyclic bending load is largely initiated in the substrate at low stress amplitudes. Hereby, the large carbide particles, carbide clusters or nonmetallic inclusions at the substrate surface or subsurface act as fatigue crack initiation sites. Depending on bending stress amplitude and residual stress state of the PVD coating, the coating may resist the propagation of fatigue cracks in certain cases to positively influence the fatigue behavior of tool steel substrate. The investigation contributes to fundamental understanding of fatigue damage in PVD coated tool steel compounds.