Hard-Coating Removal with an Abrasive Waterjet
Coating removal with high-pressure waterjets has been reviewed in previous newsletters. This short article will review a hard-coating removal method with an abrasive waterjet, which was described by Ruusuvuori et al.* in their award-winning paper on the 2005 WJTA conference.
Hard-coatings are increasingly being used to enhance the wear resistance of critical and expensive components, especially in the aviation industry. Thermal spraying methods, such as HVOF (High-Velocity-Oxygen-Fuel), are used to apply hard metal coatings, such as WC-Co and Cr3C2-NiCr, onto these components. These coatings are very dense and have very high adhesion. As good as they are in protecting the components, they are difficult to remove when the components need repair and recoating. Traditionally, these coatings are removed with electro-chemical baths or mechanical grinding. These methods are environmentally unfriendly and time consuming.
Abrasive waterjet can effectively remove these coatings with an erosion process. The impact of process parameters to coating removal speed follows trends similar to those in cutting processes. Higher water pressure and abrasive flow rate, as well as slower traverse speed, will result in higher removal speed. Abrasive size should be optimized for a specific coating. In their tests, aluminum oxide abrasive (instead of garnet, which is popular in cutting applications) was used because the aviation industry generally does not use iron-containing abrasives like garnet. Unfortunately, selected values of other process parameters were not disclosed in the paper.
However, using abrasive waterjet for hard coating removal has a risk of cutting into the softer base material once the coating has been penetrated. The authors have conducted an in-depth research study of different methods for accurately measuring the thickness of the coating as a way of controlled removal. With low carbon steel as the substrate material, an ordinary eddy-current-low-frequency coating-thickness gauge was found to give the best results.
This new hard coating stripping technique has gained acceptance or attention from some large corporations in the aviation industry, like KLM and HT Lasertekniikka.
* Ruusuvuori, K, Lahhenpera, K, Oksa, M., Turunen, E., Kauppila, J., Wonderen, M. van, “Controlled HVOF Hard Coatings Removal Method,” Proceedings of the 2005 WJTA American Waterjet Conference, Houston, Texas, August 21-23, 2005, Paper 5B-2.
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