Plasma Electrolytic Oxidation

Introduction

There are various surface treatment options available due to technological advances in production processes. Application of the correct method is crucial to achieving desired results. What property of which material needs to be improved is the key question.

If the primary intended result is to improve wear and corrosion resistance, plasma electrolytic oxidation, or PEO for short, is a novel surface coating method. In this article, we will take an in-depth look at the process of PEO and study its advantages and disadvantages.

How Does It Work?

PEO happens in 3 stages. The base material is first dipped into an electrolyte bath. The composition of the bath differs depending on the target characteristics however, usually, the bath consists of a proprietary dilute aqueous solution. The electrolyte bath is charged with a high-voltage current, usually around 200V. The voltage value may be greater if a thicker coating is desired. This process is called substrate oxidation.

High-temperature levels created by the high-voltage current trigger a plasma discharge, from the surface of the base material towards the electrolyte bath. Generated plasma flux creates the essential physio-chemical condition of high temperature and pressure on the metal surface to kickstart oxidation, concluding the second stage of PEO called plasma modification. The contents of the thick and uniform layer of oxidation depend on the base metal or alloy taken through the process of PEO.

If the description of PEO sounds extremely similar to anodizing, you are correct. However, starting from phase 2, PEO proves to be superior over anodizing. Due to the plasma flux, PEO ensures a homogenous coating. In addition to homogeneity in the coating, PEO provides other features such as chemical passivity, low stiffness, and thermal stability.

The final phase is called the incorporation of electrolyte elements. Plasma flux, opens up pores at the surface of the base material, allowing enhancing elements of the electrolyte solution to penetrate into the base material. Because of the bonding mechanics provided by the plasma flux, engineers have the flexibility to alter the electrolyte solution, meaning enhancers that would not be taken advantage of with other methods are available for use.

Improvable Properties and Various Types of Coating Available With Plasma Electrolytic Oxidation

• High strain tolerance
• Corrosion resistance
• Dielectric breakdown strength
• Thermal barrier coatings
• Photocatalytic coatings
• Anti-microbial coatings
• Hydrophobic properties
• Environmentally friendly coatings

Advantages of Plasma Electrolytic Oxidation

As mentioned earlier, the greatest benefit of PEO is the flexibility it offers. Engineers can custom-design the electrolytic bath to improve almost every physio-chemical property of their choosing.

The pretreatment process is much simpler than other methods of surface coatings as well. Most base materials do not require the intensive degreasing that they would normally require before other conventional processes.

Another benefit of PEO is the lack of heavy metals and harmful chemicals used in the process. This makes PEO an environmentally friendly alternative to other coating methods. In addition, the lack of harmful materials means a safer workplace.

Finally, the PEO provides longer-lasting results because of the way the enhancing material bonds with the surface.

Disadvantages of Plasma Electrolytic Oxidation

As with all novel methods, PEO is still quite expensive compared to the other methods of coating. The average market price is around $2-$3 to coat an area of 1 m2. This is partially because of the high temperature and voltage requirement. Immense amounts of power are consumed during the process to reach the necessary conditions.

Another disadvantage of PEO is the need of replacing the electrolytes in the solution. The reusability of the bath is quite limited due to the high levels of heat and the bonding mechanics.

Compatible Metals

PEO works specifically on valve metals due to their naturally occurring passivating oxide layer. The list of PEO-compatible metals is given down below:

• Aluminum
• Magnesium
• Titanium
• Zirconium
• Tantalum
• Niobium
• Hafnium
• Cobalt
• Most of their alloys

Application Product Examples

PEO has a wide range of uses because of the amazing characteristics it bestows to light metals. However, because of its expensive price, its employment has been a little bit constrained.

Its users include businesses in the aerospace, military, medical, and technology sectors.

For instance, PEO coating is used by aircraft firms to strengthen their lightweight metals. The fuel pumps and piston heads of Humvees were tested by the US Army. These components improved in terms of heat resistance and life span. Micro-arc oxidation is being used in the medical industry to create better bone replacement prosthesis. PEO is used by tech companies to coat the exterior of mobile phones.

PEO has been applied, among other things, to:

• Aerospace parts
• Military vehicle parts
• Bone replacement prosthetics
• Mobile phone shells

Conclusion

Anodizing is being replaced as the most popular technique for industrially improving lightweight metals by plasma electrolytic oxidation. The procedure has been improved after decades of research to increase the heat and corrosion resistance of low-cost metals.

PEO still costs a lot of money and calls for cutting edge technology. However, businesses and academic institutions are attempting to make it widely used.