Residual stress analysis of ceramic coatings

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Coatings are commonly applied in order to ensure functional properties (such as corrosion or wear protection, sensors) or decorative features on the surface of an object. But in many cases, layers (coatings) possess manufacturing-related residual stresses that create undesirable effects (e.g. cracking and spalling of layers), and therefore, it is essential to know the residual stresses and their effects in the composite in order to appropriately adapt the coating process. In the 1st project phase, a laser-optical method for contactless and minimally invasive measurement of surface residual stresses, based on a laser ablation process with coupled digital holographic interferometry, has been developed and validated. Based on the results of the 1st project phase, the objective of the project (2nd project phase) now consists of the real-time in-situ determination of residual stresses (i.e. measurement of temporary stresses during the ongoing process) by contactless online measurement. Thereby, new knowledge of the coating process is to be achieved, as well as the possibility to create an in-line process control to ensure the desired coating properties. With the information gained and by comparison with data from process simulation, the manufacturing of coatings with a defined level of residual stresses will be possible by the control of torch trajectory (kinematics), simultaneous cooling and variation of the coating process parameters. The long-term aim of method development is the process control for selective and reproductive generation of residual stress states in layers, which were generated by means of thermal spraying processes. Moreover, the systematics developed for the determination of residual stresses by laser ablation should be extended to comparatively small objects (surface boundary influence) and cylindrical or freeform surfaces.