MAO-coating protective ceramic coating.
Heat-resistant, insulating, decorative, corrosion-resistant and protects against fretting corrosion in particular, wear-resistant protective ceramic coating, and the coating, which is a sublayer for deposition of polymeric materials
The MAO coating is a ceramics of a complex composition (protective ceramic coating).
MAO-coating protective ceramic coating formed in the micro-arc oxidation process due to the surface oxidation of the metal, while the metal surface are formed of oxide and hydroxide forms of the metal. On the other hand, the coating grows at the expense of inclusion in its composition of elements from the electrolyte. Electrolyte elements are included into the coating in the form of salts, oxides and hydroxides of complex composition. If necessary, the technology allows MDO to enter into the coating any desired chemical element. The more time to handle the details, the more elements from the electrolyte are accumulated in the surface layer. The bottom layer of the coating adjacent to the metal-based, it consists mainly of oxide compounds.
Microarc oxidation allows to obtain coatings for various purposes: heat-resistant, insulating, decorative, corrosion-resistant and protects against fretting corrosion in particular, wear-resistant as well as being a sublayer for deposition of polymeric materials.
The thickness of the coatings is determined by several main factors. It is the nature of the electrolyte, material of alloy metal, the processing mode and process time. MDO allows to obtain coatings with thickness from fractions to hundreds of micrometers. The required thickness of the coating depends on the purpose and operating conditions. For applying a sublayer for painting simply 5-10 microns, to impart insulating properties and high wear resistance must be 50-100 microns. Decorative properties and anticorrosive properties under atmospheric conditions provide 20-40 microns of coating.
– high temperature resistance. MAO coatings are highly resistant to thermal and thermal-cycle loads. Coating can operate at temperatures from -40 to +600 °C. With the gradual heating, the temperature resistance of the MAO coating is limited by the melting point of the metal of the workpiece, since these values for metals obviously lower than for ceramics. Tests showed that the coatings can withstand up to 280 cycles 310 – 15 °C and 25 cycles 500 – 15 °C. During these tests, the coated sample is heated to a predetermined temperature in a furnace and then thrown into cold water.,
– high corrosion resistance. The MAO method allows to obtain protective ceramic coating, resistant to atmospheric conditions and in different corrosive environments chemically aggressive fluids, vapours, sea water, etc. as MDO coating is a ceramics of a complex composition, the corrosion resistance of the coating material is sufficiently large. Protection against corrosion of the base metal to provide the coating thickness and by controlling the amount and structure of pores. Additional protection is provided by impregnation of the pores with an inert material (usually PTFE). Tests of aluminium samples with a polymer-ceramic MAO coating showed that the obtained coatings can be used as corrosion resistant for 15 years
– dielectric properties. The voltage at which breakdown occurs of the coating as the corrosion resistance depends on the coating thickness, the type and size of the pores. Also this value can be significantly increased by application of the material filling the pores. The average breakdown voltage of the coating is 600 V. the breakdown Voltage of the coating to the filling of pores up to 2500 V,
– high adhesion. MAO coatings have excellent adhesion with the metal base, which is provided by a transition layer at the metal surface. The transition layer is formed both inside the metal and outside, and also has a profile with many curves. As a result, the adhesion of the coating to the metal is greater than the hardness of the coating and at loading there is no separation of the coating along the interface metal-coating. Calculated from the results of Scrach-test values for adhesion reach 350 MPa,
– light reflectance and light absorption. The reflectivity of the MAO coatings is about 80 %. For black MAO coatings the absorption coefficient reaches 90 %. To get the highest optical performance apply alloys with a minimum content of impurities,
– high hardness. The hardness of the MDO-coatings up to 21 GPA.