Controlled HVOF hard coating removal method

There is an increasing demand for recoating of the expensive coated components like turbine blades, rollers and pumps. For economical recoating an efficient coating removal (stripping) method is required. This paper presents the development of the adaptive Abrasive Water Jet (AWJ) stripping method. The method was applied to remove the WC-CoCr and Cr3C2-NiCr HVOF coatings in a controlled manner. In the test run the coating thickness was reduced (stripped) down to 10-20 µm without damaging the base material. Aluminum oxide powder (Al2O3) was applied as an abrasive. With this novel method the use of the environmentally problematic chemicals can be significantly reduced or entirely avoided. A fast measuring system capable to measure the coating thickness non-destructively with adequate accuracy was integrated to the AWJ removal cell. The recorded coating thickness values were applied to control the power of stripping and to stop the stripping before the coating is completely removed thus preventing the unwanted base material erosion. The applied measuring technique depends on the coating and base materials. The eddy current or X-ray fluorescence methods can be applied to measure the coating thickness. The measurement results of samples with magnetic substrate are described. Two industrial applications of the AWJ method are given, one focusing on the method validation and the future prospects in the aircraft industry (KLM) and another on the coating removal of the components applied in the process industry (HT Lasertekniikka).