Improvement of the dynamic strength of thermally sprayed coatings by means of machining surface conditioning

• Applicants: Professor Dr.-Ing. Dirk Biermann; Professor Dr.-Ing. Wolfgang Tillmann
• Subject Area: Metal-Cutting and Abrasive Manufacturing Engineering ; Coating and Surface Technology
• Term: from 2016 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 287021006

activation of the surface by blasting improves the interfacial binding between the substrate and coating by means of mechanical clamping of the coating particles. The roughening of the surface topography induces residual compressive stresses, which normally lead to an increase of the dynamic loading capacity in the transition one of the substrate. Recent studies on the fatigue behavior of thermally sprayed composite materials, however, verify the potential of substituting the grit blasting process with a fine machining process. Within the scope of this project, specific adjustments of the surface topography as well as the residual stress state in the transition zone of the substrate material are supposed to help determine the functional dependency between the interfacial binding of the coating and the machining process of the substrate. The residual stresses are induced by means of a turning process and vary, depending on the hardness of the material and the parameters of the turning process. The transition zone of the workpiece, resulting from the turning process, is removed by a subsequent honing process and a surface topography with varia-ble material ratio is generated in dependency of the parameters of the honing process. The stock removal to be removed can be influenced during the honing process and thus, the residual com-pressive stress states deeper in the workpiece can be found on the surface after fine machining. Additionally, further possibilities to adjust residual stresses induced by the thermal spraying pro-cess will be scrutinized. This involves a specific adaptation of the residual stresses by varying the gas composition during High Velocity Oxygen Fuel spraying as well as the handling parameters to control the spray gun. Furthermore, the effect of these altered residual stresses on the coating properties, especially on the adhesion onto the substrate will be closely investigated. The potential of an alternative surface activation will be scientifically evaluated towards the end of the project with the combined adjustment of the quality of the transition zone and the surface topography in rotation bending tests. The subsequent coating treatment will be analyzed by means of a honing process as well. Structured surfaces exhibit a higher wear resistance under tribological loads than non-structured surfaces. The specific insertion of profile grooves creates voids, which serve as storages for lubricants. However, under dynamic load, these grooves can lead to notch effects, which can negatively affect the dynamic strength. Therefore, the functional dependence between a specifically introduced structuring of the surface topography and the dynamic fatigue behavior needs to be analyzed as well.

• DFG Programme: Research Grants