About Microarc Oxidation

A. The microarc oxidation is a Russian technology called also plasma electrolytic oxidation (PEO). It is a method to produce hard and corrosion resistant coatings on Al, Mg, Ti and other light metals.

A. The microarc oxidation is still an expensive coating compared to traditional treatments on aluminium. The short life of the electrolyte and the great amount of power are limits. It cannot satisfy a big market but in some cases I have found that the qualities of this treatment are very interesting. In the textile field there are many pieces that could take advantage of MAO coating instead of the plasma spray ceramic coatings or anodizing. About the reproducibility, it requires proper position of the pieces in a well-formulated electrolyte.

A. Hello,

Let me introduce the technology of micro-arc oxidation.

MICRO-ARC OXIDATION

SUBJECT: Technology and equipment for wear-resistant, corrosion-resistant, electroinsulative and decorative coatings by the method of micro-arc oxidation on the parts made of aluminium and titanium alloys.

The method of micro-arc oxidation (MAO method) is very effective for protective and decorative coatings on aluminium profiles and other constructive elements, what is especially important for vehicles and constructions (aircraft, boats, window frames, siding of buildings, roofs, etc.).

The method of micro-arc oxidation possesses high output, possibility of automation of the process and variation of coating properties within a wide range. The method provides for a deposit of oxide films with unique properties on the parts of complicate configuration and large dimensions. The process is environmentally safe.

MAO method uses the effect of forming micro-arc plasma discharges on the surface of the unit treated in water electrolytic solution; because of influence of these discharges on the articles, a multi-function protective coating forms.

Alkaline solutions of low concentrations (2-3%) are used in electrolytes, what ensure the environmentally safe process.

The method is useful for treating the passivating class metals, such as Aluminium, Titanium, Zirconium, etc., as well as their alloys.

The unique of the oxide coating made by MAO method is explained by the possibility of forming the different micro-structure and phase composition coating, also the fused amorphous oxide films.

The articles of Aluminium alloy with coating :

• have the increased corrosion resistance, also in the humid tropical climate (the test of samples in the 5% sea fog within 2000 hours did not show any trace of corrosion);
• have the increased wear resistance (wear speed of MAO-coating is lower by more than an order of magnitude as compared to those of mullite articles tested at the same conditions, microhardness of wear resistant coating is 15000 - 20000 MPa instead of 4000-6000 MPa on the anodised coatings;
• the breakdown voltage on the electroinsulating MAO coatings amounts to 5000 V (5 times as high as under anodising).

General information about the technology proposed.

For application of the coating, the conventional baths are used, which are normally used for galvanic process of anodising, but the quantity of them are considerably reduced (2 instead of 8-10); low concentration alkaline environment-friendly solutions are used as electrolytes.

The process is carried out under higher voltage (more than 150-200 V) than those by anodising one. Depending on the application of the coating, the power consumption varies between 0,80-27,0 kWatt-hour/dm2.

The dimensions of the bath depend on the dimensions of the parts to be coated; correlation of area of the part surface to the area of internal surface of the bath should be 1:10.

Cost of 1 m2 of corrosion resistant coating is $3.00.

A. There is at least one company in the West doing micro-arc oxidation. They have ties to MIT. They claim that they have got the process time down to 15 minutes - apparently the original Soviet technique took hours, which is probably where the charges of energy-inefficiency originated as well.

A. Several companies in Europe and more recently Asia and the USA are using it for wear resistance and corrosion resistance on Al, Mg and Ti alloys. There is a good deal of technical information available on the internet.

A. Microarc Oxidation

Microarc oxidation (MAO) technology allows obtaining coatings with a unique complex of physical and mechanical properties on metals of valve group - aluminum,zinc, Magnesium titanium and their alloys.
Compared to conventional anodising techniques the MAO process is able to produce ceramic coatings with higher performance, which particularly, have superior microhardness, abrasion resistance and corrosion resistance.

Advantages of the MAO process:
can be used to anodise alloys difficult to anodise with traditional anodising processes, e.g. high silicon content die castings or high copper content aluminum alloys
capability of producing high-thickness coatings (up to 300 microns)
pre- and post-treatments of the surface are not necessary
usage of environmentally friendly and inexpensive electrolytes
decorative attractiveness of colors of the coatings

The typical properties of the coatings are:
high hardness - up to 24000 MPa
low coefficient of friction f=0,005-0,01
high wear resistance ip=10-12
heat resistance (working temperatures - up to 800-1200 C)
piezoelectric properties
corrosion resistance in various mediums
dielectric durability 10-20 V/micron
heat conductivity factor 5-10 W/(m c)
surface finish Rz~1-40 microns (without post-processing) and Ra~0,04-0,08 (after polishing)
microstructure - 1-10 microns.

The MAO process exploits microarc discharges to form coatings on the metal surface. In fact, MAO is a surface modification process because the oxides layer grows into the part depth due to the oxidation of the metal. The microarc anodizing and oxidizing and the coating formation occur when high (up to 1000 V) asymmetric pulsed voltage is applied between a metal part and an electrolyte bath. The process parameters (current and voltage modes, electrolyte composition and concentration) are optimised to achieve a desired combination of the coating properties.
For each material the process parameters are determined experimentally.

The following table shows the comparative characteristics of MAO technology and anodising:

Parameter
Anodizing MAO technology
Potential, V 10 - 30 100 - 300 and more
Current density, A/dm2 up to 10 20 - 100
Hardness, kg/dm2 up to 500 - 600 up to 2000 -250
Coating type amorphous crystalline
Electrolyte type acid alkaline
Microarc charges no yes

Equipment

The equipment for the MAO process includes a special power supply able to generate positive and negative pulses, an electrolytic bath, a control unit, an electrolyte conditioning system, an exhaust ventilation system. The main characteristics of the equipment are the current density (ranged from 80 to 160 A/dm2) and the productivity (m2 per hour).
The MAO equipment varies from small capacity units to large industrial plants. Below are given examples of the MAO production equipment . . .

A. Just because any process doesn't seem to have any kind of limitations, you cannot dismiss it. I have tested MAO alumina coatings for biological applications in my lab and they have shown very low wear and friction as compared to many other processes. They have their limitations, very high temperature and pressure are required to deposit these coatings, which makes these coatings not very industry friendly.

 Hi, Juhi,
We've printed two long, glowing postings testifying to the benefits of MAO coatings, plus your testimonial and a couple of others. You say "they have their limitations" -- and that is exactly the point we keep asking about! This is a technical forum, and we are looking to technically educate ourselves and our readers on any limitations as well as the benefits.

Master's theses & doctoral dissertations present things very differently from the way a sales manager does; we keep requesting information in the former format, yet we are receiving nothing but the latter :-)

Every choice in the world has advantages & disadvantages, strengths & limitations, so I continue to ask what kind of applications are best suited to MAO, what properties do the deposits have that might be considered disadvantageous in certain applications, and so on. I ask, not to rain on anyone's parade, but to help readers apply processes on the right parts rather than on the wrong ones, and to keep this site about technical education not spam.

Is it never better to use a PVD coating or Electroless Nickel plating? There is no longer any instance where DOW 17 or HAE might be better? Are there no situations where magnesium should not be used even with a PEO coating? Are defective parts ever produced, and if so, how do we recognize them rather than putting them out in the field? Are they easily stripped and reworked? If parts are stripped do they return to original dimension like plated parts, or are they undersize like stripped anodized parts? All those installations out there and nobody has ever even encountered a hiccup? C'mon. Does the same process line accommodate aluminum, titanium, and magnesium components. What of assemblies? Does anything untoward happen if steel or stainless steel pems are already inserted into the parts before processing?

We want to understand this process rather than be buried in sales pap about it.

Thanks!

Regards,

A. As far as I know one of the major limitations of PEO is that it is not well understood due to its complexity and also that the results vary between components due to it not being fully understood.

A. Answering Robert Gordon:
1) Oxide layer depends on oxidation materials in MAO-PEO process. As an example: it is possible to achieve up to 300µ of Alumina on Aluminum (the only thing to be taken into account is that 100% dense Alumina layer will be around 150µ, the rest will be porous). So, now will 150µ be enough for 2000 V dielectric strength?!
You need to have a powerful power supply for MAO/PEO process.
2) No

A. Hi Benedikt. Absolutely. The "Modified Ollard Test" is used to test the adhesion of hard chrome coatings on steel. Please see letter 18002, or search our site for that term and you will find a sketch of the setup. Replace the word "nickel" with "PEO coating", and it looks to me like this would be completely applicable for giving an exact value for your situation. Good luck.

Regards,

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Ed. note: For additional discussions please see also letter 44269, "Micro-arc oxidation (MAO) vs. plasma electrolytic oxidation (PEO)"

A. Found this [Sept. 2014] thesis from the University of Western Ontario, ["Surface Modification of Aluminum Alloys by Plasma Electrolytic Oxidation" by Vahid Dehnavi], for a realistic look at plasma electrolytic oxidation, specifically of 6061 aluminum:

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.682.593&rep=rep1&type=pdf

It is obviously energy-intensive compared to conventional anodizing, by a couple orders of magnitude, for both the current/voltage to the bath, and for the associated cooling.