L. M. Berger, D. Schneider, T. Großer, Fraunhofer Institute for Material and Beam Technology, Dresden, Germany
The steadily increasing requirements to the thermal spray coatings lead to the development of new characterization tools, in particular those of non-destructive testing. Laser induced surface acoustic waves have the potential to play an important role for non-destructive testing of thermal spray coatings. From these measurements primarily the Young's modulus of the coatings is obtained. The Young’s modulus represents the stiffness of the material and correlates with the strength of the inter-atomic bonds. Pores and micro-defects represent missing and broken bonds. This is the reason that they reduce Young's modulus distinctly even for dimensions of single defects below the detection limit of optical microscopy, if the defect density is sufficiently high. Therefore, measuring the Young’s modulus using a variety of techniques is a common method to evaluate the porosity and defect density in bulk ceramic materials. For coatings and near surface regions, the ultra-sonic technique of surface acoustic waves is promising due to their limited penetration depth in the test component that can be controlled by varying the frequency.
In this work advantage has been taken from this phenomenon, applying LAWave ®-device developed at the Fraunhofer Institute for Material and Beam Technology to evaluate thermal spray coatings. Systematic testing was done with hardmetal (WC-based) and ceramic coatings. Alternative mechanical test methods, as indention tests with different indenter geometry, were applied to draw conclusion from the effect of the different loading regime of the high-frequent ultrasonic technique and the quasi-static indentation test. Coatings were also studied by metallographic cross-sections and XRD. Links between coating composition, coating microstructure and the Young's modulus are discussed.
Possible conclusions on the porosity and defects in the coating microstructure in production on-line measurements represent the potential of this method.
Summary: Laser induced surface acoustic waves have the potential to play an important role for non-destructive testing of thermal spray coatings. From these measurements primarily the Young's modulus of the coatings is obtained. Systematic testing was done with hardmetal (WC-based) and ceramic coatings. Alternative mechanical test methods, as indention tests with different indenter geometry, were applied to draw conclusion from the effect of the different loading regime of the high-frequent ultrasonic technique and the quasi-static indentation test. Coatings were also studied by metallographic cross-sections and XRD. Links between coating composition, coating microstructure and the Young's modulus are discussed.
