ABSTRACT
Micro Arc Oxidation (MAO) is an electrochemical surface treatment process for generating oxide coatings on metals. Here, the substrate metal is chemically converted into its oxide. The MAO coating grows both inward and outward from the original metal surface. It is a conversion coating. Therefore, unlike a typically deposited coating, e.g., by plasma spraying, it almost obviously has excellent adhesion to the substrate metal. It is similar to anodizing, but it employs much higher potentials of 200 V or so. As a result, discharges occur and the plasma forms. This plasma modifies the structure of the oxide layer. A wide range of substrate alloys can be coated. For example, it can include all wrought aluminum alloys and most cast alloys, although the high levels of silicon can reduce coating quality. On metals such as aluminum, magnesium, and titanium, this MAO process can be used to grow largely crystalline, oxide coatings that are tens or hundreds of micrometers thick. Because the MAO coatings generally possess high hardness and a continuous barrier, they are capable of offering protection against wear, corrosion, or heat as well as electrical insulation. Despite the wealth of literature on Al and Ti alloys [1-4], researchers have very seldom addressed [5] the nanohardness and elastic modulus of MAO coatings on AZ31B Mg alloys (≈3% Al, 1% Zn, and 96% Mg) evaluated at the microstructural length scale by the nanoindentation technique. The as-prepared silicate-based MAO (S-MAO) samples are designated as SU-MAO, while the sealed silicate-based MAO samples are designated as SS-MAO. At least 20 indents were made at five different locations of the sample, as the nature of the surface of the coating was porous and heterogeneous. The measurements were taken well within 10% of the coating thickness, such that there would not be any influence of the substrate’s mechanical properties on the measured data.
