Methods based on ultrasound and optics for the non- destructive inspection of thermally sprayed coatings

Abstract

Thermally sprayed coating are usually tested destructively, i.e. metallo-graphically or by applying various strength tests. In most cases the results of non-destructive testing NDT procedures, which are of major importance for the inspection and eventual repair of structural components, do not allow satisfactory decisions to be made on component safety. Much work has been carried out recently to develop NDT procedures based on ultrasound and optics which promise to be more suitable for testing thermally sprayed coatings. The ultrasonic techniques include the application of specific wave modes, such as polarized transverse waves, surface waves and Lamb waves which are useful for the improved testing of coatings and interfaces. Moreover, interference in sprayed coatings can be exploited for NDT purpose by using special transducer equipment. The use of controlled acoustic signals (the CS technique) is recommended for the examination of coating structures and defects, particularly in describing their type, dimensions and position, as it allows a frequency-dependent determination of these quantities to be made within wide limits. Evaluation methods such as tomography or the optical visualization of the acoustic field offer further improvements in the detection and description of defects. The limits of the applicability of this technique are demonstrated by means of test results from flame-arc- and plasma-sprayed coatings. As well as active test procedures such as ultrasonic methods, passive methods such as optical holography and acoustic emission analysis are also suitable for sprayed coatings. Excitation of the test sample, e.g. by mechanical or thermal loading, is required in these passive methods. Optical holography can be used to obtain information on defects by visualizing the deformation of the coating. Applications of these test methods to sprayed steel coatings have given encouraging results. However, both active and passive NDT methods have some physical limitations which are discussed in this paper.