Plasma electrolytic oxidation of metals and alloys

Abstract

Porous oxide layers formed on metals and alloys via Plasma Electrolytic Oxidation (PEO) have been developed and used for decades in medicine and for technical purposes. The following metals—Ti, Ta, Nb, Zr, Al, and Mg—are now in focus for PEO treatment, which can help improve their biomedical and mechanical properties. Apart from single metals, selected two-, three-, and four-component titanium alloys, such as Ti-xMo, Ti-xZr-xNb, and Ti-xTa-xNb-xZr, are currently undergoing PEO processing. Differing from Ti-6Al-4V alloy, which has been routinely used in medicine, these new and advanced titanium alloys do not include vanadium. They are characterized by low Young’s modulus and biocompatible alloy elements, and after PEO treatment may be used for medical applications, such as promising metallic implants to bone [1–6]. Lightweight designs in many industries, transportation, and aerospace require the application of light metals (Al, Mg) and alloys, enhanced by special surface treatments. The requirements, therefore, not only concern mechanical strength, but also significant protection of the materials against corrosion and wear resistance. Here, plasma electrolytic oxidation (PEO) comes into play to obtain hard and wear-resistant oxide coatings [7,8]. In comparison to the PEO treatments mentioned earlier that last 3 to 5 min, plasma electrolytic oxidation of aluminum and its alloys take one to three hours to obtain strong coatings with proper thickness and structure [7]. Magnesium (Mg) and its alloys are classified as biodegradable materials and are promising for medical implant engineering due to their biocompatibility, non-toxicity, mechanical properties, and biodegradation behavior. The corrosion products of Mg and its alloys are physiologically beneficial to humans. Unfortunately, their uncontrolled rate of degradation is a problem. Surface modification can improve the corrosion resistance of Mg alloys and decrease their degradation rate. The developed CaP coatings can be applied in the manufacturing of medical biodegradable implants and devices (fixators, screws, plates, scaffolds) from Mg alloys for orthopaedic and cardiology applications.