Residual Stress Analysis in Thermally Sprayed Layer Composites, Using the Microhole Milling and Drilling Method

Abstract

Abstract Mechanical and tribological properties of layer composites like hardness, bonding strength, friction and wear coefficients as well as operation performance, component reliability and finally the system lifetime are mainly influenced by residual stresses after the thermal spraying and machining process. The residual stress situation in thermally spray coated composites is superimposed by different stress mechanisms occurring during the manufacturing process based on heat and mass transfer during the coating deposition. Using the microhole milling and drilling method, residual stress fields can be measured in a quasi non destructive way over the drilling depth with appropriate resolution. In several drilling and micromilling operations a circular, cylindrically shaped microhole is brought step by step into the component surface. The residual stresses are locally relieved due to material removal, deform the surface around the drilled microhole and are measured by high resolution measurement tools, e.g. strain gauges (DMS), for every drilling step in form of relaxed surface strains. Using calibration curves and material data (E, μ) the measured surface strains are converted into nominal strains at the bottom of the drilled hole for every drilling step. Out of the differentiated strains, in-plane stress fields can be incrementally determined by Hooke's law. This study describes residual stress measurement features, the FEM-calculation and idealization of calibration curves as well as results of exemplary stress measurements.