What is Micro Arc Oxidation and How Does it Work？

Demand for lightweight and corrosion resistant materials like aluminum and titanium alloys is constantly rising within aerospace, automotive, and electronics industries. However, the inherent softness and poor corrosion resistance often limit their performance in harsh environments. To overcome these limitations, advanced surface modification techniques are essential. This article will discuss micro arc oxidation, a process that enhances the surface properties of these crucial light metals.

Overview of Micro Arc Oxidation

Micro-Arc Oxidation (MAO) is an advanced electrochemical surface treatment process. It is used to form a robust ceramic film of a specific thickness on non-ferrous metals and their alloys, such as aluminum, magnesium, and titanium. This is achieved by allowing the metal to react with solutes in an electrolyte under suitable electrical parameters.

When applied to aluminum and its alloys, the technology grows a ceramic film primarily composed of Al2O3. This coating exhibits excellent wear resistance and corrosion resistance. Furthermore, its performance properties and color can be tailored by adjusting the electrical parameters and the chemical composition of the electrolyte.

Characteristics

The application of micro arc oxidation for the surface hardening of aluminum and its alloys offers significant industrial advantages, including a simple process, small footprint, strong processing capacity, high production efficiency, and suitability for large-scale industrial manufacturing.

From an environmental standpoint, MAO electrolytes do not contain toxic substances or heavy metal elements. The electrolytes have high resistance to contamination and high recovery/reuse rates, resulting in minimal environmental pollution. This aligns with the requirements for high-quality, clean production and the strategies for sustainable development.

The ceramic film formed on the aluminum substrate after MAO treatment offers superior performance, such as:

• High Hardness: Up to 1200 HV (Vickers Hardness).
• Strong Corrosion Resistance: Demonstrated by CASS (Copper-Accelerated Acetic Acid Salt Spray) test results exceeding 480 hours.
• Excellent Insulation: Film resistance greater than 100 mΩ.
• Strong Adhesion: Excellent bond strength between the film and the base metal.
• Good Performance: Exhibits good wear resistance and thermal shock resistance.

The table below clearly illustrates the superior performance of Micro arc oxidation compared to conventional anodizing:

	Micro Arc Oxidation	Conventional Anodizing
Max Thickness (μm)	300 to 400	50 to 80
Microhardness (MPa)	15,000 to 30,000	3,000 to 5,000
Breakdown Voltage (V)	2,000	Low
Porosity (%)	0 to 40	10 to 40
Toughness	Good	Brittle
Corrosion Resistance	Excellent	Moderate
Wear Resistance	Excellent	Poor
Roughness	Low	Moderate

Micro arc oxidation offers strong process capability and versatility. Different film characteristics can be achieved by adjusting the process parameters to meet diverse requirements. Moreover, the coating can be given specific characteristics or different colors by changing or regulating the electrolyte’s composition. It is also possible to subject the same workpiece to multiple MAO treatments using different electrolytes to produce multi-layered ceramic oxide films with distinct properties.

Application of Micro Arc Oxidation

Due to the superior advantages and characteristics outlined above, MAO technology has an extremely wide range of application prospects across industrial sectors such as machinery, textiles, electronics, aerospace, and construction/civil engineering.

Its primary appliactions include the surface hardening of aluminum-based components that demand high performance in wear resistance, corrosion resistance, thermal shock resistance and high electrical insulation.

Micro arc oxidation is also highly valuable for the surface treatment of aluminum-based materials used in the construction and civil industries, where decorative appeal, wear resistance, and resistance to different types of corrosion are critical. Furthermore, it can be used to strengthen special aluminum-based alloys that cannot be effectively treated by conventional anodizing.

Examples of Applications:

• Automotive: Aluminum-based pistons, piston seats, cylinder blocks, and other components.
• Machinery & Chemical Industry: Various aluminum molds, and the inner walls of aluminum tanks/vessels.
• Aerospace: Various aluminum components, such as cargo hold floors, rollers, and guide rails.
• Consumer Industry: Various aluminum-based hardware products, fitness equipment, etc.

Mechanism of Micro Arc Oxidation

Micro arc oxidation evolved from the conventional anodizing process, and its mechanism is considerably complex.

Traditional anodizing operates in the Faraday region. As the potential on the metal anode increases, the current also rises. Once the voltage is sufficiently high, the process enters the electrical spark discharge region, where phenomena like corona, glow, and spark discharge appear on the metal’s surface. While glow discharge is a lower-temperature process that minimally affects the oxide film structure, the spark discharge region involves micro-area, high-temperature, high-pressure plasma discharge. This extremely high temperature not only changes the structure of the oxide film but does so without causing destructive damage to the aluminum alloy substrate.

Taking aluminum as an example, the anodic oxide film initially consists mainly of amorphous Al2O3, Al2O3, and AlOOH. Under the influence of micro-area, high-temperature, high-pressure plasma discharge, the aluminum anodic oxide film undergoes a phase transformation and crystallization process, converting into γ – Al2O3 and α – Al2O3. This results in the formation of a micro-arc ceramic oxide film with high hardness (up to 2000 HV and above) and excellent corrosion resistance.

Micro Arc Oxidation Process

The Micro arc oxidation process for aluminum and its alloys primarily involves 3 stages: pre-treatment of the aluminum substrate, the micro-arc oxidation process itself, and post-treatment.

The general process flow is as follows:

Pre-treatment

Aluminum alloys easily oxidize when exposed to air for extended periods, forming a thin, passive film (several micrometers thick). Additionally, the surface readily absorbs impurities from the surrounding environment, creating a surface adsorption layer. To ensure the final ceramic film achieves optimal mechanical properties, a degreasing pre-treatment of the sample surface is essential before the MAO process.

Micro-Arc Oxidation

The basic equipment configuration for MAO is largely similar to conventional anodizing, consisting of an oxidation tank, a power supply, and a solution cooling and stirring system.

• Oxidation Tank: Typically welded from stainless steel and externally wrapped with plastic plates.
• Power Supply: Unlike the typical DC, AC, or pulsed power supplies used in conventional anodizing, MAO requires a higher voltage (400-600V). A dedicated AC power supply, typically with a specific positive-to-negative ratio, is often used. Because the forming film exhibits diode characteristics, the resistance difference between the forward and reverse directions is significant. Applying the same positive and negative voltage could result in an excessively large negative current, potentially damaging the power supply and hindering the formation of the Al2O3 film. Therefore, dedicated MAO power supplies often utilize two independent sets of power sources for the positive and negative directions, which increases the complexity of the power unit design.
• Cooling and Stirring: The cooling and stirring system for the electrolyte solution is critical. The high voltage and large current (400–600V) generate significant heat due to arc discharge and thermal decomposition of water. If this heat is not removed promptly, the bath temperature will rise, negatively affecting the performance of the MAO film. This is typically managed using an external circulation system with a heat exchanger.

Micro arc oxidation electrolyte compositions are relatively simple and can be broadly classified based on their acidity:

• Acidic Electrolytes: Historically, concentrated H2SO4 has been used at around 500V DC to produce ceramic films. Phosphoric acid electrolytes followed by chromate sealing can also yield thicker films. Adding fluorine-containing salts to these electrolytes can produce alumina ceramic coatings with moderate strength and hardness, but excellent adhesion, corrosion resistance, electrical insulation, and thermal conductivity.
• Alkaline Electrolytes: Alkaline oxidation methods are generally more environmentally friendly, as the metal ions generated at the anode can often be reused. Furthermore, the incorporation of other metal ions from the electrolyte into the film allows for the adjustment of the film’s microstructure, granting it new properties.

Factors Affecting Micro Arc Oxidation

While MAO is relatively tolerant to the pre-treatment state of the workpiece, key operating parameters significantly impact the final film quality: voltage, current density, electrolyte concentration, and electrolyte temperature.

Electrolyte Composition

The composition of the MAO electrolyte is the technical key to obtaining a quality film. Different electrolyte compositions and process parameters yield coatings with different properties. MAO electrolytes typically consist of alkaline salt solutions containing specific metal or non-metal oxides (such as silicates, phosphates, or borates), ideally present in a colloidal state.

Micro arc oxidation can be performed in both acidic and alkaline electrolytes, but weakly alkaline electrolytes (pH 9-13) are commonly used today.

• Concentration: At the same MAO voltage, a higher electrolyte concentration generally leads to a faster film formation rate and a slower rise in film temperature. Conversely, a lower concentration slows down film formation and causes the solution temperature to rise faster.
• Composition: The type and composition of the electrolyte critically affect the MAO ceramic layer. The growth rate, structure, composition, and elemental distribution of the ceramic layer all differ depending on the electrolyte used.

Current Density and Oxidation Voltage

Current density and operating voltage significantly affect the growth and performance of the MAO ceramic layer, influencing its thickness, hardness, and protective capabilities.

• Current Density: This is one of the critical parameters affecting the luminosity, growth rate, and performance of the MAO ceramic layer. Within a certain range, increasing the current density typically increases the ceramic layer thickness and linearly increases its hardness. However, there is a maximum limit; exceeding this value can cause burn damage during the growth of the ceramic layer, resulting in a rough surface and a rapid increase in energy consumption.
• Oxidation Voltage: Precise control of the oxidation voltage is equally vital for obtaining a quality film. Different aluminum substrates and electrolytes have different dielectric breakdown voltages for micro-arc discharge. The MAO voltage is generally controlled to be tens to hundreds of volts higher than the breakdown voltage. Varying the oxidation voltage changes the resulting ceramic film’s performance, surface morphology, and thickness. Depending on the required film properties and process conditions, the MAO voltage can range from 200 to 600V.

Temperature and Stirring

Unlike conventional aluminum anodizing, MAO permits a wider range of electrolyte temperatures, from 10 to 90 °C. A higher temperature accelerates film formation due to more vigorous water vaporization at the workpiece/solution interface, but it also increases surface roughness and electrolyte evaporation. Therefore, the MAO electrolyte temperature is typically maintained within the range of 20 to 60 °C.

Since most of the energy in the MAO process is released as heat, the temperature of the oxidizing solution rises faster than in conventional anodizing. Consequently, a large-capacity heat exchange/refrigeration system is necessary to control the bath temperature during the MAO process.

Oxidation Time

The oxidation time is generally controlled between 10 and 60 minutes.

• Impact of Time: Longer oxidation times generally lead to better film compactness but also greater surface roughness.
• Evolution Over Time: As the oxidation time increases, the average pore size on the film surface gradually increases, the number of micropores decreases, the film growth rate slows down, and the film thickness increases. The average surface hardness first increases and then decreases. While the friction coefficient of the MAO ceramic layer is largely unaffected, the wear life is significantly improved. The corrosion potential gradually increases, and the corrosion current first decreases and then increases.

About Getzshape

When your aluminum or titanium project demands enhanced surfce hardening treatment, Micro arc oxidation will be a good choice. Getzshape delivers expert micro arc oxidation and type II & III anodizing services for complex aluminum parts. Contact us to discuss the your next high-precison project.