Residual stress analysis of ceramic coatings
Final Report Abstract
Ceramic coatings are commonly applied on technical components for improving wear or heat resistance, provide electrically non-conductive surfaces or decorative features. In many cases, coatings possess manufacturing-related residual stresses that create undesirable effects (e.g. cracking and spalling of layers). Therefore, it is essential to know the residual stresses and their effects in the composite in order to appropriately adapt the coating process. The standard method used for the assessment of residual stresses inside coatings is based on hole drilling and measurement of the released stresses by strain gauges. This quasi-non-destructive method has the disadvantage that requires mechanical drilling; furthermore, strain gauges can only be applied on flat and relatively smooth surfaces. The method is not suitable for investigations during the coating process where the surface has high temperature. In the 1st project phase, a method for contactless and minimally invasive measurement of residual stresses has been developed. The stresses are released by a laser ablation process and the resulting deformation of the coated surface are measured by digital holographic interferometry. During the 2nd project phase, the technique was systematically further investigated and a compact system for the in-situ determination of residual stresses by contactless online measurement was built and tested. It was shown that the system allows measurements of planar and non-planar (cylinders having diameters between 8 and 12 mm were investigated) surfaces under industrial conditions (strong vibrations dust and electromagnetic perturbations). Some measurements were carried out at high temperature (360°C) just after the coating process. It was shown that at high temperature there are almost no residual stresses in the coating and these appear during the cooling process. Surface displacements (stress relaxations) on the coating due to incrementally deeper laser-ablated notches were calculated by Finite Element method simulations. Those calculated displacements were used to compute calibration coefficients for establishing a correspondence between optically measured displacements and residual stresses at different depths below the surface of the coating. In the frame of this project, new knowledge of the coating process was 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.
Publications
- “Evaluation of Residual Stress Determinations Conducted with Laser Ablation and Optical Displacement Measurement”, Residual Stresses 2016: ICRS-10, 2, 323- 328, (2017)
P. Weidmann, G. Pedrini, V. Martinez-Garcia S. M. Wenzelburger, A. Killinger, S. ,Schmauder, R. Gadow, W. Osten
(See online at https://doi.org/10.21741/9781945291173-55) - Optisches Verfahren und Anordnung zur Eigenspannungsmessung, insbesondere an beschichteten Objekten, DE 10 2015 006 697 B4 2018.08.02
W. Osten, G. Pedrini, R. Gadow, K. Körner
- Optical method and arrangement for measuring residual stresses, in particular in coated objects. US000010481020B2
Erfinder: W. Osten, G. Pedrini, R. Gadow, K. Körner
- “Non-contact residual stress analysis method with displacement measurements in the nanometric range by laser made material removal and SLM based beam conditioning on ceramic coatings”, Surface & Coatings Technology, 371, 14–19, (2019)
V. Martínez-García, G. Pedrini, P. Weidmann, A. Killinger, R. Gadow, W. Osten, S. Schmauder
(See online at https://doi.org/10.1016/j.surfcoat.2018.12.123) - “Residual Stress Evaluation in Ceramic Coating Under Industrial Conditions by Digital Holography”, IEEE Transactions on Industrial Informatics, 16, 1102– 1110, (2019)
I. Alekseenko, G. Pedrini, V. Martínez-García, A. Mora, A. Killinger, A. Kozhevnikova, S. Schmauder, R.Gadow, W. Osten
(See online at https://doi.org/10.1109/TII.2019.2939972)
