Tribological analysis of TiN and DLC coated contacts by 3D FEM modelling and stress simulation

Abstract

Surface coatings, like titanium nitride (TiN) and diamond-like carbon (DLC) coatings, offer high wear resistance and good friction performance for a wide range of applications. With novel advanced techniques like modelling and simulation, the performance of these coatings can be predicted under a wide range of loading conditions. This provides valuable information for the coating design and for the use of coatings in different applications. A previously developed three-dimensional finite element method (FEM) model was used for calculating the first principal stress distribution in a scratch test contact as a spherical diamond tip is moving with increased load on DLC and TiN coated high speed steel surfaces containing no residual stresses. The used three-dimensional model is comprehensive in the sense that it considers elastic and plastic behaviour of the contacting surfaces. The first cracks to appear on the surface are angular cracks on the edge of the scratch groove. This corresponds to the region of high two directional tensile stresses occurring in FEM stress simulations at the edge of the scratch groove. The coating/substrate stiffness ratio influences considerably on the coating behaviour. The TiN coating that had higher Young's modulus compared to the substrate material experienced high tensile stresses when loaded by the diamond stylus. The DLC coating that had lower stiffness compared to substrate material experienced comparatively low tensile stresses. The DLC coatings had maximum tensile stresses in the range of 700–900 MPa. The TiN coatings had tensile stresses in the range 2200–3000 MPa. The coating thickness had only minor effect on the maximum tensile stress level for the 1 and 2 μm thick coatings. The location of the experimentally observed first crack in the surface corresponds to the location of maximum tensile stress concentrations in the stress simulations and the direction of the observed crack corresponds with the stress components in the calculated stress field. The role of residual stresses is discussed.