Plasma Electrolytic Oxidation (PEO) Coatings on a Mg Alloy from Particle Containing Electrolytes
Plasma electrolytic oxidation (PEO) processing for Mg alloy is known for decades and has been established as a well-known industrial surface treatment offering a reasonable wear and corrosion protection. However the long-term protection is often limited by the intrinsic porosity and limited phase compositions in the PEO layer. A novel optimization approach is to introduce particles to the PEO electrolyte, aiming at their in-situ incorporation into PEO coatings during growth. The idea is that with the help of particles the defects can be sealed, and the composition range and the functionalities of produced coatings can be enhanced. The thesis reports the influence of particle addition on PEO processing, how the particle up-take can be controlled and how the morphology, microstructure, phase composition, and properties of PEO coating on Mg alloy are influenced. The mechanisms of uptake and incorporation of particles into PEO layers as well as the coating growth mechanisms are also discussed. It was found that addition of particles cannot avoid/seal fully the high porosity of PEO coatings. Moreover, the growth rate of the coatings is reduced in the presence of particles. The nature of particle itself, together with electrical and electrolyte parameters during the process determine the way and efficiency of particle uptake and incorporation into PEO coatings. The final incorporation of the particles into the coating can range from inert to reactive, which can also be controlled by modifying the processing parameters. Although the corrosion resistance of the coating cannot be significantly improved by the particles, it is feasible to control and modify the biodegradability and compatibility of the coating via addition of particles. Furthermore, multifunctional coatings with anti-wear and photocatalytic properties were produced and it was demonstrated that particle properties can be transferred directly to the coatings.