A method for increasing the adhesive strength of plasma electrolytic coatings on titanium alloys

The aim of the work is to develop a method for forming an MAO coating on a titanium alloy, which helps to increase the adhesive strength of the coating with the substrate and to establish the effect of the processing mode on the thickness and roughness of the coating. A method for using ultrasonic vibrations (US) during the microarc oxidation (MAO) process is proposed and studied. The described method allows obtaining a multifunctional coating by the microarc oxidation method using ultrasonic vibrations on the OT-4 titanium alloy with increased adhesive strength to the base metal. And also a uniform thickness and roughness of the coating over the entire surface of the workpiece is achieved. The positive effect of US on the adhesive strength of the coating is confirmed. A mode for processing titanium alloys by the MAO method under US and a current pulse repetition frequency of 90 Hz is proposed, which helps to increase the adhesive strength of the MAO coating by more than 60 %. Presumably, the mechanism of coating growth is that under the influence of ultrasound, hydrogen bubbles formed in the process break into smaller bubbles and intensively approach the surface of the substrate. As they grow, the bubbles burst under the influence of ultrasound and release a large amount of heat and energy, which accelerates the formation of the film. Subsequently, the cavitation effect of ultrasound decreases due to the coating and then the ultrasonic waves, due to reflection from the coating, form a standing wave at the surface, which leads to a stable distribution and compression of bubbles at the boundary of the solution and the coating. This contributes to the uniform distribution of microarcs and energy over the surface of the coating being formed. As a result, a coating of uniform thickness with less roughness and firmly bonded to the substrate material is formed.