Plasma Electrolytic Oxidation: The Future of Lightweight Designs in Aerospace and Defense

August 1, 2023

Aerospace and defense (A&D) components face a range of extreme conditions for prolonged periods. Their lifespan quickly becomes diminished as a result and become susceptible to critical faults. Because of this, vital components have traditionally consisted of heavy alloys due to their tensile/yield strength, high thermal resistance and corrosive protection. Moving into next generation designs, the light-weighting of aircraft design has become a priority to improve fuel efficiency, enhance speed, and provide more available carry weight for vehicles in action.

To support the advancement of using lighter metals in A&D designs, metal coating technologies have been developed to protect and extend the lifespan of critical components; especially those facing strenuous conditions. That said, traditional coatings can often fail to provide suitable protection to replace heavy metals, and also produce waste materials that are harmful to the environment.

Plasma Electrolytic Oxidation (PEO) is a unique surface coating technology that specializes in light metals, aluminum, magnesium and titanium specifically. It enables the lightweighting of components that face extreme environments from a reliable and effective coating process. Moreover, it provides full customizability of coating characteristics to maximize component lifespan in specific conditions. So much so that it was used to coat sensitive equipment on the James Webb telescope in the extreme cryogenic conditions of space.

In order to understand the true impact PEO can have on the A&D industry, one first needs to understand the challenges of using these light metals in designs, and the metal coating process itself.

Light Metals: Features and Challenges

An image of a hard anodized coating at a sharp corner.

Many conventional surface coatings technologies require thorough pre-treatment to ensure a high-quality result. PEO, on the other hand, only needs a simple degreasing of the metal’s surface before coating for a high-quality result.

Aluminum (Al) production and development has paralleled that of the modern aircraft, serving as a structural aspect to both individual designs and to the industry itself. Early engineers incorporated the metal to create lighter components to improve fuel efficiency, increase carry weight and optimize the capabilities of their designs. Being a third of the weight of steel yet exhibiting an impressive strength-to-weight ratio and yield/tensile strength (30-500 MPa) has made Al alloys a vital material in aircraft manufacturing.

From the latter half of the 20th and into the 21st century, titanium (Ti) was integrated as another pivotal metal in aerospace designs. Thanks to its strength (240-1260 MPa), corrosive resistance and ability to retain reliable structural and mechanical performance at high service temperatures, Ti alloys have become commonplace for structural elements, hydraulic systems and engine components. So much so that 80 percent of all Ti alloys are produced commercially for A&D industries. Additionally, various R&D has been performed on magnesium (Mg) as another possible light metal solution for specific components; cast Mg alloys exhibit tensile strengths of up to 280 MPa whilst being the lightest structural metal on earth, making them an extremely desirable material for weight optimization.

However, with the widespread application, use and research performed on these light metals, there is still a prominent issue that plagues A&D engineers to this day; the lifespan of components that consist of light metals as a result of their various weaknesses. AI exhibits a high thermal conductivity (244 W/mK) that extremely diminishes its mechanical performance at high temperatures. It is also fairly reactive due to its chemical affinity for oxygen; its protective outer layer of aluminum oxide is negligible in alkaline and acidic environments. Moreover, Al is vulnerable to friction-initiated wear, limiting component lifespan in abrasive environments. Ti plates and components often run into the complication of cracking. They are sensitive to fatigue, especially in the cases of the notch sensitivity effect; areas of geometric discontinuity have a detrimental effect on the fatigue strength of the metal, causing it to crack. Mg is an extremely reactive metal. It is a group II element, has low ionization energy and contains only two electrons in its outermost shell. Because of this, magnesium will lose these electrons and corrode even in ambient environments, let alone in extreme conditions.

A solution to the material challenges of these light metals is surface coating technology. A&D industries have long utilized anodizing and chromate conversion coatings to provide additional hardness and corrosive protection to light metal components. Anodizing increases the thickness of a metal’s natural oxide layer to provide abrasive resistance and hardness, whereas chromate conversion consists of a chemical or electrochemical treatment to produce an amorphous and corrosion resistant coating.

However, these technologies provide limited protective qualities when it comes to components facing a range of extreme conditions, and also exhibit an array of harmful chemicals and heavy metals as waste products. A key example is the through-thickness cracks that appear from anodized coatings on complex geometries, causing areas of lowered fatigue strength as a result.

PEO is an advanced surface coating solution for light metals with bespoke capabilities. It provides a range of resistances that can be tailored to meet the requirements of the component without sacrificing the quality or durability of the coating itself. It is also far cleaner than alternative coating technologies, free of harmful chemicals or heavy metals in the process.

PEO utilizes an electrolyte bath that consists of a proprietary dilute aqueous solution, free of heavy metals such as chromium as well as harmful chemicals. The leftover byproduct is no more hazardous than run-off water from a domestic washing machine. The electrolyte solution also contains chosen reagents to provide specified coating characteristics at the end of the process.

A high and controlled voltage (typically 200V or greater) is passed through the solution, with possible variations depending on the coating characteristics required of the component. High temperatures, pressure and voltage creates plasma discharges on the metal’s substrate, forming a crystalline oxide outer layer. Plasma modifies the microstructure of the coating during the process, creating a consistent and insulated layer even for complex geometries. The irregularly shaped microcrystals help to provide greater protection on corners that would otherwise exhibit weakening through thickness cracks with conventional coatings.

For sealants and reagents, the resultant outer layer at the final stage of PEO is a porous ceramic composition as a result of the plasma that modifies the microstructure of the coating during the coating process.

At the final stage of PEO, the resultant outer layer is a porous, ceramic composition as a result of the plasma. This porosity allows for the introduction of specified reagents or sealants that are placed into the electrolyte bath at the beginning of the process. The reagents adhere to the porous outer layer, providing further ways to customize the protective capabilities of the coating; a silicate-rich electrolyte reduces the thermal conductivity of aluminum to 0.5 W/mK, and polyester helps enhance corrosion resistance.

PEO Applications

The Near Infrared Spectrograph (NIR-Spec) onboard the revolutionary James Webb telescope provides generation-defining capabilities to detect infrared signatures in deep space. A result of this precision is extremely sensitive components that require sufficient protection from the extremities of space. These include cold welding, cryogenic conditions and extreme fretting wear.

The ceramic structure of PEO’s coating exceeded upwards of 2,000 hours in salt spray testing without corrosion, against traditional anodizing lasting just 200 hours.

A specialized PEO composite was developed for use on the complex structural parts of the NIRSpec. The PEO process provided the components with a consistent and reliable coating across their complex geometries, eliminating the risk of through-thickness cracks. The coating itself provided a hard and durable composite with low sticking characteristics to protect the NIRSpec from wear, extremely low temperatures and the risk of cold welding.

Weight optimization is a crucial element of electric vertical takeoff and landing (eVTOLs) design. A kilogram of petrol holds 13 KWh, whereas the same of lithium-ion batteries holds 0.3 KWh. Because of this, energy efficiency is a priority to maximize the operational range of these aircraft; an aspect which is directly affected by weight. Light metals and alloys enable the reduction of weight to both improve efficiency and maximize payloads but face extreme conditions as a result.

Mg alloys became a tested material for eVTOLs due to its strength-to-weight ratio that optimizes the propulsion power of electrical systems. That said, Mg faces two extremities that are commonplace in aircraft; corrosion from extreme weather conditions at high altitudes and abrasive wear from moving parts such as the electric multirotor. PEO not only provides sufficient hardness to protect Mg alloy components from these factors but provides the characteristics to increase their operational lifespan.

Weight optimization is a foundational engineering element of aerospace and defense. Light metals and alloys have developed in parallel, pushing the boundaries of what is considered the limit of design. Advanced material science and surface coating technologies are protagonists in pushing these boundaries, with scientists realizing bespoke and customizable solutions to a range of challenges. PEO acts as the trailblazer of modern material science; a fully customizable surface coating solution that provides engineers and designers with the specific characteristics their components need to maximize their lifespan. With its use already adopted by NASA and Boeing, it is clear that PEO is a technology that can drive the modern designs of A&D forward.

This article is written by Robin Francis, Chief Technology Officer at Keronite, a Curtiss-Wright Surface Technology (CWST) business. For more information, visit here .