Evaluation of Internal Cohesion of Plasma-Sprayed Coatings by Cavitation Erosion Test

Thermally sprayed components have applications in aggressive environments where wear resistance is required. Coatings are often susceptible to microscale wear and most of the failure mechanisms belong to the brittle-behaviour of ceramics and the interaction between all inherent microstructural features, such cracks, pores, splats or lamellae boundaries. Therefore, the study of microstructure becomes the key. The analysis of defects into it, of micro-cracks as well as the evaluation of the internal cohesion are the first steps toward a complete characterization of wear behaviour of materials. In this work, the feasibility of indirect cavitation test with vibratory apparatus was evaluated as a measure of the internal cohesion. The installation of the testing machinery followed the standard ASTM G32 (adapted) and its continuous upgrading permitted a major control of the most important parameters, such as temperature of the test liquid. Several plasma-sprayed coatings, corresponding to two categories of materials (alumina and TBC) having distinctive microstructures have been tested. For alumina class: (i) conventional Al_2 O_3 coating from dry powder; (ii) porous Al_2 O_3 coating from solution; and (iii) dense Al_2 O_3 coating from ethanol-base suspension. For TBC class: (iv) YSZ coating from ethanol-based suspension; (v) hybrid co-sprayed coating with Al_2 O_3 from dry powder and fine YSZ from suspension; (vi) conventional YSZ coating from dry powder; and (vii) NiCrAlY bond coat from dry powder.