Acoustic in-situ monitoring of coating formationprocesses during thermal spraying by takinghigh-frequency impulse measurements

In order to ensure the protective function of thermally sprayed coatings, it is basically necessary to avoid any cracks which weaken the cohesion and adhesion of the coating and permit attacks by corrosive media. Cracks in thermally sprayed coatings are mostly caused by high residual tensile stresses. The most frequent crack detection method is the metallographic investigation of transverse sections or the dye penetrant test. Both procedures can only be carried out after the spraying process has been concluded. Recent developments in the field of crack detection are striving for the in-situ monitoring of the cracking by analysing the acoustic emissions. This article gives an overview of the in-situ crack detection method which consists of taking high-frequency impulse measurements and was elaborated within the framework of the research project entitled “Non-destructive in-situ monitoring in order to optimise the coating morphology in the case of processes based on plasma, arc wire and HVOF spraying (OptiMorph)”. Using the example of twin-wire arc spraying (TWAS) with the FeCrBSi material, the course of action for developing this method is described as an example.