Non-destructive quality control for more sustainable coating processes: Potentials and limits of surface acoustic wave spectroscopy

Fast and non-destructive material testing carried out directly on components saves time and money. Moreover, it contributes significantly to reducing CO2 emissions during production, as no components have to be destroyed for cross section imaging, reference samples can be avoided and failures can be detected early in the process, even for large test volumes. A promising method for coating characterization is the laser-induced surface acoustic wave spectroscopy (LiSAWS), enabling access to the effective mechanical behavior of coatings. The mechanical weakening due to cracks, pores and delamination can precisely be evaluated. In the recent years, several applications have been studied with LiSAWS, based on thermal spraying (including cold spraying), laser cladding and powder bed generation. Furthermore, methodical progress has been made with measurement at elevated temperatures and automation. A new understanding of processes can also be achieved by mapping of material properties, especially for local deposition methods. This contribution reviews the current state of the art for this wide range of testing tasks. From the perspective of quality assurance in production, the possibilities of the method, prerequisites, current limitations, and possibilities for further development are critically discussed. Application examples from own research and literature are presented for this purpose.