Micro-arc oxidation (MAO), also called plasma electrolytic oxidation (PEO), belongs to a plasma-chemical and electrochemical process. The process combines electrochemical oxidation and a high-voltage spark treatment in an alkaline electrolyte. This produces a physically protective oxide coating on the metal surface, improving wear and corrosion resistance, thermal and chemical stability, and extending the component’s lifetime. Micro-arc oxidation is particularly well suited for surface oxidation and pigmentation of aluminum, titanium, niobium, zirconium, magnesium, and their alloys. Micro-arc oxidation parts can be employed in the building, mechanics, transportation, and energy industries.
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What is Micro-Arc Oxidation (Plasma Electrolytic Oxidation)?
Micro-arc oxidation is an electrochemical surface treatment process used to create oxide coatings on metals. It is similar to anodizing in that it uses higher potentials to cause discharges and the accompanying plasma to modify the structure of the oxide layer.
This process may be applied to create thick (tens or hundreds of micrometers) crystalline oxide coatings on metals, including aluminum, magnesium, and titanium. Due to their high hardness and ability to form a continuous barrier, these coatings can provide electrical insulation and protection against wear, corrosion, and heat. Surface coatings produced through plasma electrolytic oxidation are two to four times harder than hard anodizing or steel.
Generally speaking, the coating is formed by the chemical conversion of the substrate metal into its oxide, and it grows both inwards and outwards from the original metal surface. It is precisely because it develops both inwards and outwards that it has good adherence to the substrate metal. The coating may be applied to various substrate alloys, such as all wrought aluminum and most cast alloys, though excessive amounts of silicon may decrease coating quality.
How does the Micro-Arc Oxidation (MAO) Process Work?
The oxide film generated through the micro-arc oxidation process exhibits favorable characteristics such as good resistance to corrosion, superior hardness, minimal surface defects, and a generally dense structure. Below is the working principle of this process.
Step 1: Substrate Oxidation
MAO occurs in three distinct stages. The initial step involves immersing the base material into an electrolyte bath. The bath’s composition varies based on the desired characteristics, but it normally consists of a proprietary dilute aqueous solution. The electrolyte bath is typically charged with a high-voltage current, approximately 200V. If a thicker coating is needed, the voltage value can be increased. This is known as substrate oxidation.
Step 2: Plasma Modification
The high-voltage current generates high temperatures, causing a plasma discharge, moving from the base material’s surface into the electrolyte bath. The generated plasma flux produces the necessary physio-chemical conditions of high temperature and pressure on the metal surface to initiate oxidation. This brings the second stage of MAO called plasma modification. The contents of the thick and even oxidation layer are determined by the base metal or alloy subjected to the MAO process.
You are accurate if the description of MAO sounds suspiciously similar to anodizing. However, beginning with step 2, MAO outperforms anodizing. MAO ensures a uniform coating due to the plasma flux. MAO offers several other characteristics besides uniformity in the coating, such as chemical passivity, low stiffness, and thermal stability.
Step 3: Incorporation of Electrolyte Elements
The last stage is referred to as the incorporation of electrolyte elements. Plasma flux opens pores on the surface of the base material, enabling enhancing elements in the electrolyte solution to penetrate the base material. Engineers can change the electrolyte solution because of the bonding mechanics offered by the plasma flux, which means enhancers that would not be used with other techniques are accessible for usage.
Key Process Parameters that Influence MAO Coating Quality
The flexibility of the micro-arc oxidation process allows operators to tailor the coating characteristics to meet functional performance requirements. This is accomplished by adjusting key electrical parameters and electrolyte composition:
Voltage & Current Density – Increasing voltage/current density accelerates coating growth rate. Too little and arc initiation is difficult; too much causes excessive sparking.
Frequency – Higher frequencies favor finer, more uniform coatings. Lower frequencies generate coarser coatings.
Duty Cycle – The percentage of time the current is on during each cycle. Higher duty cycles create more turbulent reactions.
Electrolyte Composition – Determines coating composition and properties. Silicates favor thicker, harder coatings, while phosphates improve corrosion resistance. Other minerals, oxides & salts can modify characteristics.
Additives – Including pigments (for color), particles (for abrasion resistance), and other additives allow tailoring of coating functionality.
pH – Acidic or alkaline bath conditions significantly impact the coating deposition dynamics and ensuing microstructure.
Careful optimization of these parameters allows micro-arc oxidation to be effective on various metals and applications.
Coating Properties of Micro-Arc Oxidation
PEO’s unique and flexible process produces highly protective layers that can be enhanced with excellent properties designed for specific applications.
MAO coatings generally provide better component coverage than line-of-sight processes such as painting, powder coating, and plasma or flame spray techniques. Furthermore, the coating’s insulating properties provide excellent uniformity at corners and edges. Paints tend to thin at the corners, whereas electroplating techniques tend to thicken at the corners.
Wear Resistance and Hardness
The use of a high-sliding, wear-resistant MAO coating increases hardness. This extremely high hardness results from the combination of crystallization (of the oxides) and co-deposition of elements from electrolytes in the ceramic layer. Aluminum coatings are often harder than steel (1600HV vs 500HV), but the components can be as much as 66% lighter. MAO coatings’ enhancing properties have allowed light alloys, like magnesium, to be used in high-performance aerospace and automotive applications.
Enhancing corrosion resistance can be achieved by applying MAO coatings with chemically inert ceramic-like properties. In addition, we can modify the MAO process to thicken the oxide layers to the required level of corrosion resistance. Generally, MAO functions most effectively as a pretreatment for subsequent sealants, paints, and other polymers, ensuring optimal corrosion resistance.
High Strain Tolerance
High hardness does not always imply extensive wear resistance capabilities. Compliance is also a highly significant feature since it allows for some component deformation due to deflection or thermal expansion without putting undue stress on the coating-metal surface.
The combination of hardness and proper degrees of compliance increases a substrate’s wear resistance. Similarly, MAO’s distinctive microstructure gives the material exceptional fracture toughness, limiting the possibility of cracking under stress, implying that ceramic surface coatings function well in tribology applications.
Anti-microbial qualities are introduced into the MAO electrolyte by salting it with zinc and sodium/tungsten oxide, rendering light metal parts appropriate for medical, healthcare, and dental uses.
Dielectric Breakdown Strength
The insulating dielectric properties may be accomplished by precisely regulating the thickness of the MAO coating, which can be achieved by customizing the deposition process. The thickness of the oxide layer on the light metal can be increased to get dielectric strengths ranging from 59 to 79 kV/mm.
MAO is an environment-friendly choice. Electrolytic baths are generally low-concentration, chemically benign, aqueous solutions. After pH adjustment, process waste streams are normally discharged straight to drain.
Photocatalytic Surface Coatings
Titanium dioxide is widely recognized for its photocatalytic characteristics, which might be used to purify water. Photocatalysis often depends on ultraviolet (UV) lighting. Recent studies with MAO have demonstrated that adding nanoparticles, such as Ag, to an MAO coating on a titanium substrate significantly broadens the effective light spectrum that may effectively achieve photocatalysis.
Thermal Shock Resistance
MAO coatings adhere very well to the substrate and have better thermal shock resistance than traditional ceramic coatings. They can withstand rapid heating and cooling cycles.
The coatings have some level of porosity (10-20%), which allows lubricants, dyes, polymers etc to penetrate the coating. Porosity also promotes bone ingrowth in medical implants.
What Materials Can be Used in Micro-arc Oxidation?
Micro-arc oxidation, also known as plasma electrolytic oxidation (PEO), is an electrochemical surface treatment process that can be used to produce ceramic coatings on metals such as aluminum, magnesium, titanium, zirconium, tantalum, niobium, and their alloys. Some of the commonly used materials and electrolytes for micro-arc oxidation include:
Aluminum Alloys – It’s alloyed with elements like silicon, copper, zinc, and magnesium and used with alkaline silicate or phosphate-based electrolytes.
Magnesium Alloys – It’s alloyed with aluminum, zinc, and rare earths. This material is often used with alkaline silicate or salt solutions.
Titanium Alloys – Alloyed with aluminum, vanadium, molybdenum, and zirconium and used with silicate, phosphate, and acetate-based electrolytes.
Zirconium Alloys – Alloyed with tin, niobium, and oxygen. Used with phosphate and silicate electrolytes.
Tantalum – Often anodized in hot phosphoric or sulfuric acid electrolytes.
Niobium – Anodized in hot phosphoric/sulfuric acid electrolytes.
The electrolytes supply the oxygen to form the ceramic oxide coatings. Their composition can be tailored to incorporate other elements like silicon, magnesium, and phosphorus into the coatings. The resulting ceramic layers provide excellent corrosion and wear protection.
The Advantage of Micro-arc Oxidation
MAO coatings provide excellent corrosion protection, electrical insulation, thermal barriers, etc. The process is ideal for enhancing aluminum, magnesium, or titanium alloys in demanding environments. Here are some of the main advantages of micro-arc oxidation (MAO):
- Since the voltages exceed the breakdown voltage of the formed layer, open channels are not required to sustain the process; thus, thick, dense, nonporous layers can be produced.
- Forms a ceramic-like coating on light metals like aluminum, magnesium, and titanium that is very hard, durable, and corrosion/wear resistant. Coatings have good thermal insulation properties and electrical insulation.
- The primary advantage of MAO is the flexibility it provides. Engineers may tailor the electrolytic bath to improve practically any physio-chemical feature.
- The pretreatment process is also considerably easier than other surface coating methods. Most base materials do not necessitate the intensive degreasing typically applied before other conventional processes.
- Another advantage of MAO is that no heavy metals or toxic chemicals are used in manufacturing. As a result, MAO is a more environmentally friendly process than other coating technologies.
- Due to how the enhancing material bonds with the surface, MAO gives a longer-lasting coating.
- Various colors and aesthetic finishes are possible through the inclusion of dye pigments into the coating.
- No high-pressure or heat treatment post-processing is required.
- It generates an integrated ceramic-metal transition zone of up to 60% of the coating thickness for outstanding adhesion.
- Because of the non-line-of-sight process, it is possible to handle complicated shapes such as inner bores of tubes.
The Disadvantages of Micro-arc Oxidation
Although MAO has many benefits, there are still some disadvantages:
- MAO remains relatively costly in comparison to alternative coating methods. This is due in part to the high voltage and temperature requirements. The process requires substantial power consumption to achieve the necessary conditions.
- MAO needs to replace the electrolytes in the solution. The bath’s reusability is significantly restricted due to the high levels of heat and bonding mechanics.
- Requires more complex equipment and control compared to some coating processes.
- It only applies to certain conductive metals, primarily aluminum, magnesium, and titanium alloys.
- Coating properties are dependent on electrolyte formulation and process parameters.
Application of Micro-arc Oxidation
Thanks to its versatility, micro-arc oxidation has been adopted across various industries where enhanced surface durability, protection, electrical insulation, or aesthetics are needed. Some common application areas include:
Automotive – MAO coatings improve the wear and corrosion resistance of engine components like piston rings, cylinder bores, valves, etc., and drivetrain components.
Aerospace – MAO provides corrosion and wear protection for aerospace components like landing gear, hydraulics, fasteners, actuators, etc. Operating in harsh environments makes MAO suitable.
Medical Implants – Bioinert and biocompatible MAO coatings enhance osseointegration of orthopedic and dental implants. The porous surface allows bone cell integration.
Oil and Gas – MAO coatings protect against wear, galling, and corrosion in drilling equipment and components subject to abrasive fluids in downhole conditions.
Textiles – Applying MAO coatings to fabrics improves wear-resistance properties in protective clothing, upholstery, and awnings.
Construction – Improved wear, corrosion, and fire protection for aluminum extrusions and panels.
Food/Beverage – MAO coats components subject to corrosion from acids and cleaning chemicals to prevent premature wear and leaks. They are used on tanks, valves, pipes, and bottling/packaging machines.
Decorative – The wide range of colors/textures possible with MAO makes it suitable for decorative finishes on metal products used in architecture, art, jewelry, etc.
Compared with normal anodizing, micro-arc oxidation is adaptable to more types of metal materials and emphasizes film function more. As a result, this process may be employed in designing parts that require higher surface qualities, such as wear-resistant, heat-resistant, higher strength parts, and items that require customization or special treatment. Micro arc oxidation can also be employed for decorative purposes, such as achieving color matching. When employing metal materials, consider the surface condition, electrolyte composition, voltage and current density, temperature and time, and other parameters.
Do you need micro-arc oxidation service? Don’t hesitate to contact LEADRP today.
Plasma electrolytic oxidation – From Wikipedia
Micro-arc oxidation works effectively on light metals and alloys like aluminum, magnesium, titanium, zirconium, tantalum, niobium, and various silicate minerals. Nearly any conductive metallic material can undergo micro-arc treatment with the right electrolytic parameters.
A wide range of colors, from neutral grays and tans to vivid golds, greens, and blues, can be achieved by incorporating dying agents into the coatings. Different textures, ranging from smooth polished finishes to textured matte finishes, are also possible.
Micro-arc oxidation generally outperforms anodizing and powder coating regarding corrosion resistance thanks to the integrated cross-linked structure between coating and substrate. It matches or exceeds the performance of chrome-based coatings without environmental issues.