Impact of Chromium Nitride (CrN) Coating Deposited by PVD Technique on the Fatigue Behavior of Case Carburized SAE8620 Grade SteelPages 378-387
Kumar Sushil Abstract:
The fatigue performance of case-carburized SAE8620 steel with and without a Physical Vapour Deposition (PVD) chromium nitride (CrN) coating was investigated under four-point rotating bending. Cylindrical specimens were carburized, oil quenched, tempered and subsequently polished to obtain a surface hardness of 60-62 HRC. Half of the specimens were then coated with an approximately 2 μm thick CrN layer using an ion-sputtering PVD process. Fatigue tests were carried out at different stress amplitudes in accordance with ASTM E739 (2015), and the resulting stress-life (S-N) behaviour was compared using fitted regression curves because the coated and uncoated series were tested at different stress levels. The mean endurance limits de-termined using the Dixon-Mood staircase method were 770 MPa for the uncoated specimens and 657.5 MPa for the coated specimens. Fractographic examination showed coating-associated crack origins in the coated condition and isolated surface discontinuities in the uncoated condition. The observed fatigue-strength reduction may be associated with coating defects, interfacial effects and elastic mismatch; residual stress and adhesion were not measured directly. Within the experimentally investigated life range, a Basquin-based coating knockdown factor, K(N) ≈ 1.33N-0.032, indicates an increasing strength penalty with cycle number. Benchmarking against a companion CrN-coated 20MnCr5 dataset further indicates that the magnitude of the fatigue penalty is substrate-dependent. These results show that the fatigue consequences of CrN coating should be assessed separately from its established surface-engineering benefits.
Keywords:
Fatigue,
SAE8620,
Chromium Nitride,
PVD coating,
S-N curve,
Endurance limit
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