A NUMERICAL STUDY ON THE FATIGUE RESISTANCE OF A COATED TOOL STEEL

摘要

The aim of this study is to clarify how the fatigue resistance of hot work tool steels is influenced by thin hard coatings. Chromium nitride (CrN) coatings were deposited by physical vapour deposition. The residual stress state of the coating was adjusted by variation of the substrate bias during deposition. Stress controlled fatigue experiments with a load ratio of R = 0.1 were performed to determine the fatigue strength of the coated tool steel. In addition, in-situ 3-point bending experiments were conducted in a scanning electron microscope to examine the initiation and growth of cracks in the coating under monotonic loading. 3D finite element (FE) computations of the in-situ experiments were performed to resolve the local stress and strain conditions when the first cracks origin in the coating. The crack driving force was determined in a subsequent 2D FE analysis where the crack length in the beam was varied between 30% and 160% of the coating thickness. The crack driving force was calculated with the configurational force concept which enables to take into account the locally varying material properties as well as the locally varying residual stress state.