Optimization of wear-resistant coating architectures using finite element analysis

Abstract

The goal of this work was to develop an approach for the design of multilayer coatings with enhanced toughness to fracture and improved adhesion for wear-resistant applications. Finite element analysis (FEA) was utilized to investigate the distribution of stress in single layer, bilayer and multilayer films under combined normal and tangential loads. Two-dimensional models were created for single layer CrN and Cr2N films and for a number of multilayer combinations of Cr, CrN and Cr2N layers. The FEA results were validated for uncoated solids through comparison with analytical solutions for Hertzian contacts, with and without frictional effects. A good fit was observed in all cases. Then the influence of film architecture and substrate material on the mechanical stress within the films was studied. It was determined that metallic layers had significantly lower σxx stress than ceramic layers, while both σyy and τxy stress within the layers was largely independent of the layer material. Simulation data was then compared to actual data obtained from several chromium nitride based thin films with architectures corresponding to those of the FEA models. Coatings were deposited by unbalanced reactive magnetron sputtering on A2 steel and 2024 aluminum substrates. Residual stress in the films was determined from sin2 Ψ x-ray measurements. When used in multilayer architectures CrN films had high compressive stress on both steel and aluminum substrates, while Cr films had lower compressive stress on steel substrates and tensile stress on aluminum substrates. The residual stress in Cr2N layers could not be determined due to overlap of the x-ray peak. Coating adhesion was measured by a scratch test and wear rates were measured using a pin-on-disk testing apparatus under normal loads of 4 and 10 N for the aluminum and A2 steel substrates, respectively. The strongest correlations were found between the σxx stress at the film–substrate interface and coating adhesion and wear resistance. The stress at the interface between ceramic layers and metallic substrates was very high, resulting in an overall reduction in performance of single layer Cr2N and multilayer CrN/Cr2N films. The highest adhesion and the lowest wear rates were observed in the films that consisted of alternating CrN layers separated by thin Cr layers. Increasing the thickness of the Cr layers degrades the performance of the films due to a decrease in the overall stiffness of the coating.