Tribological analysis of TiN and DLC coated contacts by 3D FEM stress modeling and fracture toughness determination

Surface coatings, like titanium nitride (TiN) and diamond-like carbon (DLC) coatings offer high wear resistance and low friction performance for a wide range of applications.By using new techniques such as modeling and simulation, the coating performance under load can be estimated and thus provide valuable information for the coating design and for the use of coatings in different applications.A three-dimensional Finite Element Method (3D FEM) model has been developed for calculating both the first principal stress distribution and the true stress components in the scratch test contact as the spherical diamond tip moves with increased load on a titanium nitride (TiN) and diamond-like carbon (DLC) coated steel surface.The three dimensional model is comprehensive in the sense that it considers elastic, plastic and fracture behavior of the contacting surfaces.Three main regions of stress concentration during the scratching action has been identified and analyzed. The first cracks to appear on the surface of a high-speed steel sample coated with a 2 µm TiN coating or 1 µm DLC coating are angular cracks on the edge of the scratch channel as the spherical diamond tip slides on the coated surface. This corresponds to the area of high two-directional stresses occurring at the side of the scratch channel.By identifying the location of the first crack and the crack density along the scratch channel and by using this as input data the fracture toughness of the coating can be evaluated.The influence of the coating thickness and elastic modulus on the stresses and strains generated during the sliding process is demonstrated.