Ceramic-Base Surface Treatment Technology for Light-Metal Alloys
An innovative surface modification method, called Keronite surface treatment technology, is finding in-creasing interest with coatings companies and OEMs seeking alternatives to some currently used traditional coating technologies that are less environmentally friendly.
Keronite surface treatment technology has its origins in Russia, where research scientists were given the task of developing a superhard, corrosion resistant coating for light-metal alloys. This work eventually led to the discovery of plasma electrolytic oxidation (PEO) technology.
In the 1990s, a company in the UK worked with the Russian developers to transfer the technology know-how and the associated intellectual property to the West. The process was developed further and introduced under the name Keronite. Following successful testing and evaluation of the coating, a company of the same name was established in 2000.
Keronite Ltd. recently formalized its partnership with KECO Engineered Coatings (Indianapolis, Ind.; www.kecocoatings.com) to form Keronite Indianapolis LLC. The partnership combines the technical expertise of Keronite and KECO's extensive knowledge and reputation in the U.S. market. KECO Engineered Coatings is a provider, developer and applicator of industrial coatings to diverse markets including aerospace, automotive, lighting, performance racing, packaging, food and glass. The company also provides a wide range of custom services such as research support, prototype development and distinct production. Included in the company's products are non-stick liquids and powders; dry film lubes; and metallic, ceramic and corrosion resistant coatings.
The Keronite process uses a patented bath-based process known as plasma electrolytic oxidation (PEO), specifically designed for the treatment of light-metal alloys. In the process, a pulsed voltage is passed through a bath of electrolyte solution and applied to the substrate (Fig. 1). The resulting plasma discharge rapidly produces a hard, fused ceramic layer on the surface of the component. The Keronite layer becomes self-regulating, and a uniform thickness is achieved automatically, even along the edges of components. This is a major advantage over conventional dip processes, which often have a tendency to produce points of weakness along critical edges.
Keronite surface treatment technology is a very green process. The proprietary electrolyte solution contains no chromium, no ammonia and no other toxic chemicals. In addition, the nonhazardous liquid requires no special treatment prior to disposal and presents no danger to those handling it.
The process produces a very dense coating having a nanoscale microstructure (Fig. 2), giving Keronite on aluminum the performance characteristics of alumina. The coating has very good wear and corrosion resistance, and also functions as an effective thermal barrier in a variety of applications.
The ceramic coating consists of two distinct layers. The fused layer closest to the substrate is very hard and dense, providing excellent protection against corrosion and wear. Depending upon the alloy used and the thickness of the coating applied, the hardness of Keronite on aluminum can reach 2,000 HV, at least three times harder than hard anodizing. Independent tests have demonstrated that the coating is seven times more wear resistant and far less prone to cracking than hard anodizing, and that it easily outperforms electroless nickel in ball-on-disk tests.
In addition to this dense, protective layer, the coating has a thinner, porous outer layer, which provides an ideal base for impregnation with decorative topcoats such as paints and lacquers. It also can be used to form composite coatings with PTFE (polytetrafluoroethylene), adhesives, lubricants or other metals. Together, the two layers have the qualities required for the best functional and decorative coatings.
As an immersion process, Keronite has much greater throwing power than plasma-sprayed ceramic coating systems and is, therefore, ideal for applying a protective coating to the inner surfaces of even the most complex shapes. Because the ceramic layer is produced by converting the substrate itself, it is attached to the substrate alloy by means of a strong atomic bond. As a result, the adhesion properties of the Keronite layer are similar to the fracture strength of aluminum itself and the risk of delamination is reduced considerably.
Keronite Ltd. provides unique surface solutions that enable designers to use lightweight metal alloys in the most demanding environments. Applications range from automotive and aerospace components to eyewear, architectural cladding and consumer electronics.
Keronite PEO coating properties allow its use as a thermal barrier. Its thermomechanical characteristics are of particular interest in those applications where components are likely to be exposed to high temperatures or repeated thermal cycling. Such environmental conditions often give rise to adhesion problems and residual stress, but PEO coatings offer an ideal solution due to their good adhesion and low residual stresses.
These coating properties are due in part to the way in which the coatings are formed and by the resulting microstructure. The coatings clearly benefit from the way in which the substrate is subjected to high temperatures, high pressure and high rates of cooling during the production process, but they also benefit from localized stress relief and annealing due to subsequent events.
Keronite on aluminum reduces the thermal conductivity significantly to approximately 1.6 +/-0.3 W/m/K, which is less than 10% of that of compressed alumina ceramic and around 1% that of the parent metal. In the case of hard anodizing, the thermal conductivity remains around 10% that of aluminum at 10 W/m/K. By reducing the thermal conductivity, the coatings function as excellent thermal insulators, reducing the surface temperature of the underlying aluminum alloy by around 30°C (54°F) during transient heating conditions.
Researchers at University of Cambridge have demonstrated the thermal stability of the coatings on aluminum at temperatures to 1400°C (2550°F); there is no significant change in the coating after heating to 1400°C and cooling to room temperature.
Tests conducted at TWI (The Welding Institute, Cambridge, UK; www.twi.co.uk) demonstrated that Keronite ceramic can also be used to improve the shock resistance of an aluminum component. For example, a 60-µm thick coating on 6082 aluminium alloy was subjected to alternate immersion in boiling water (100°C (212°F) and liquid nitrogen (-196°C, or -320°F) for 30 seconds in each bath, and this operation was repeated 50 times. There was no sign of delamination or cracking, even around the edges of the coating. Similar tests were carried out using AA 2219 rolled plate aluminum alloy and showed that Keronite ceramic retains its integrity and original microstructures with good adhesion and cohesion and high hardness even after exposure to extreme thermal shock.
Resistance to thermal shock is largely determined by the difference between the thermal expansion properties of the coating and the substrate. In the case of Keronite, the thermal expansion coefficient can be almost ten times lower than that of the substrate. By comparison, hard anodized coatings have a thermal expansion that is only around five times lower than that of aluminum.
Piston crowns for gasoline and diesel engines. A 40-µm thick Keronite coating on the piston crown (Fig. 3) reduces the temperature of the body of an aluminum piston by as much as 30°C (54°F), thereby preventing engine overheating. This is critical in terms of engine performance, as aluminum starts to lose its properties at 250-300°C (480-570°F).
In the case of gasoline engines, the coating protects the piston crown against the impact of spontaneous detonation and the engine can be run closer to the detonation region of its operating range. This means that the engine can achieve higher specific power or significant fuel economies and reduced emissions, depending upon how the engine is tuned.
Domestic gas burners. Aluminum gas burners, such as those used on a cooking range, are protected from the effects of heat using a layer of black Keronite ceramic (Fig.4), which makes them more durable and more corrosion resistant than the conventional hard anodized burners.
Automotive engine bay components. A ceramic coating on magnesium can be used to produce a number of automotive parts where a thermal barrier or heat shield is required. As well as being very lightweight, magnesium has excellent thermal properties. In the bulkhead, for example, magnesium is being more widely used to reduce vehicle weight. The Keronite layer helps to insulate the passenger compartment from the heat of components such as engines, exhaust manifolds and catalytic converters.
Gearbox casings. Keronite is being used successfully on magnesium gearbox casings, not only to protect against corrosion, but also to protect the oil in the gearbox from the heat of the exhaust.
Rocket venturi. Keronite on aluminum can withstand short exposures to temperatures as high as 2000°C (3630°F), making it an ideal solution for this type of extreme environment.
Electrical components. The thermal stability and insulating properties of the ceramic coatings make them an ideal surface on which to mount electrical components to protect delicate parts and to coat materials used to carry electronic windings and circuitry.
Keronite Indianapolis is now fully operational under the leadership of Ross Brown, U.S. Manager and Director. Processing equipment has been installed in a new facility close to KECO's headquarters in Indianapolis including a 250-kW Keronite machine with an 1,800 liter (475 gal.) bath and complementary PTFE and powdercoat facilities on site.
One of the first jobs for Keronite Indianapolis is the treatment of wheels for the Indy Race League (IRL). Following extensive testing in 2004, the IRL approved Keronite surface treatment technology as an effective means of preventing corrosion on magnesium race wheels (Fig. 5). According to Mike Land, Director of Keronite Indianapolis, all the teams in the IndyCar race series now use magnesium wheels because of the obvious weight advantages, but corrosion fatigue has always been a safety concern and the main cause of failure. Keronite coatings have demonstrated outstanding corrosion protection, and Keronite Indianapolis plans to be the primary supplier of coatings for the 2005 race season. IH