Non-destructive testing of coatings by surface acoustic waves

The steadily increasing requirements to the properties of thermal spray coatings have to lead to the development of new characterization tools, in particular for non-destructive testing. Laser acoustic surface waves relate to the most promising methods for cost-effective non-destructive testing. In this work seven HVOF-sprayed WC-based coatings were systematically studied by laser acoustic surface waves using the LAWave device. Due to short measurement and calculation times the coating can be easily multiply tested. Young's modulus and densities of the coating were obtained by this method. The values of the Young's modulus were compared with those derived from a micro-indentation method using Vickers indents and were found to be in a good agreement. Moreover, Vickers hardness values of the coatings obtained by using different loads were compared and the phase composition was studied by X-ray diffraction. The coating porosity was determined by image analysis of optical micrographs of metallographic cross-sections. It is proposed that in the case of WC-based coatings changes in the theoretical density of the material composition due to phase transformations induced by the spray process (formation of W2C and solid solutions on their base) prevent a direct access to the porosity values. Entnommen aus <a_href="http://www.fiz-technik.de/db/b_tema.htm" target="_blank">TEMA</a>