Microstructural investigation of thermally sprayed ceramic coatings by laser acoustic surface waves

Young's modulus is not only one of the important mech. properties of thermally sprayed coatings but also a sensitive indicator of the coating's microstructural defects. The ceramic coatings studied in this work were based on Al2O3, TiO2 and Cr2O3 and prepd. by APS (Atm. Plasma Spraying), HVOF (High Velocity Oxy Fuel spraying) and suspension-HVOF spraying. The Young's modulus was systematically studied by laser acoustic surface waves, which is a non-destructive, fast and reliable technique, using two different devices: a tabletop tester for samples and small components (LAwave) and a hand-held portable device. For oxide coatings, it was obsd. that the results distinctly vary depending on the spray technol. A comparison with the values derived from the instrumented indentation test sho ws that for most investigated ceramic coatings lower values were measured by laser acoustics. This is due to the influence of the pores and defects in the coatings. The LAwave results are expected to be close to the effective modulus of the material, due to the larger material vol. evaluated during the test, which takes the coating defects into consideration.