Deformation and fracture mechanics of compounds with nitride hard coatings for tooling applications

The functional performance of tools plays an important role for productivity improvement of forming and cutting processes. With the development of high strength workpiece materials, the stress collectives on the tools are increasing. Nitride hard coatings, deposited with physical vapor deposition (PVD), can increase the damage resistance of tools against abrasion, adhesion, oxidation and diffusion. However, to avoid tool failure by plastic deformation and crack growth, the compound of coating and tool substrate material needs to withstand the mechanical or thermomechanical loads in challenging forming and cutting operations. For this purpose, knowledge-based coating designs to increase the functional capability of PVD coated tools are required. Hence, this dissertation focuses on fundamental investigation of deformation and fracture mechanics of compounds in correlation with the coating thickness, residual stress state or coating architecture under indentation loadings. The investigated compounds include CrAlN coated high-speed steel HS6-5-2C and TiAlCrSiN coated cemented carbide (WC-Co). Instrumented indentation testing is used to understand the deformation behavior of coatings as well as the compounds. Moreover, coating deformation and crack growth mechanisms under indentation loadings are studied with high resolution electron microscopy. For CrAlN/HS6-5-2C compounds, increasing the coating thickness from s ≈ 1.7 µm to s ≈ 3.5 µm reduces plastic deformation of compounds under indentation loading. Moreover, coupling of s ≈ 1.7 µm and moderate compressive residual stresses σr ≈ (2 – 3) GPa increases impact fatigue resistance of compounds. For monotonic indentation loading of the TiAlCrSiN/WC-Co compounds, higher coating thickness of s ≈ 3.6 µm compared to s ≈ 2.0 µm simultaneously increases the plastic deformation and crack resistance of compounds at room as well as at higher temperatures until T = 600 °C. Despite the lower crack resistance of oxynitride top layer, the additional interface in the bilayer TiAlCrSiN/TiAlCrSiON architecture leads to comparable temperature dependent compound deformation and surface crack resistance with the monolayer TiAlCrSiN architecture. The utilization of indentation test data is extended to understand the correlations between temperature dependent compound deformation behavior, mechanisms of coating deformation and the fracture mechanics. Moreover, a data driven modeling approach is developed to predict the temperature dependent compound deformation and surface cracking behavior by combining indentation test data with the coating characteristics using machine learning algorithms.