In recent years there has been growing interest in using lighter weight materials across a broad range of applications. This is particularly true of the aerospace and automotive industries, where there is increasing environmental pressure being brought to bear on manufacturers to reduce emissions. But, in fact, many other applications, including portable devices, can benefit from a switch to lighter materials given the appropriate surface characteristics.

Light alloys such as aluminum and magnesium present attractive alternatives to heavy steel or ceramic in many environments, but their use has traditionally been limited to the less demanding applications where corrosion and wear are not an issue.   However, many design engineers are starting to realize that innovative surface treatment technologies can create new possibilities for using light alloys in a variety of industries and applications.

Fig. 1. Illustration of the Keronite plasma electrolytic oxidation process.

One such treatment is the Keronite Plasma Electrolytic Oxidation (PEO) process that allows designers to use light alloys in places not previously possible in the past with conventional surface treatment systems. With hardness measuring up to 2000 HV (Vickers), Keronite on aluminum is harder and considerably more wear resistant than steel. Likewise, Keronite on AZ91D magnesium alloy is able to resist corrosion in a salt spray environment for as much as 2,000 hours, making magnesium a much more attractive material than ever before.

The process

Enlarge this picture Fig. 2. Scanning electron microscope image of a Keronite surface.

The patented, chrome-free Keronite PEO process transforms the surface of light alloys such as aluminum and magnesium into a complex ceramic composite by anodic conversion under plasma discharge conditions in a non-toxic electrolyte solution. (See Fig. 1.) The Keronite process resembles anodizing in that electricity is passed through a bath of electrolyte, but the technology is significantly different and produces very different results. It also has advantages over conventional ceramic coatings in that the Keronite layer is atomically bonded to the substrate alloy and, therefore, has outstanding adhesion and little tendency to crack or peel away.

Because Keronite is an immersion process, it is capable of coating complex shapes and even the internal surfaces of cavities. This gives it an advantage over line-of-sight processes like conventional plasma spray. During the process itself, a layer of oxide grows following the creation of the plasma discharge as the oxide film is fused and subsequently re-crystallized. Any metal dissolution is at microscopic levels. Although the Keronite PEO process is quite aggressive in terms of the extensive plasma discharge activity, and the oxide eruptions at very high local pressures, these processes are occurring on a minute scale, and there is no distortion or other adverse effect on the component being treated.

The process temperature and the component itself typically remain in a temperature range of 12 DegC to 30 DegC. The resulting oxidation of the alloy surface will include some elemental co-deposition from the electrolyte, producing a ceramic layer comprising both crystalline and amorphous phases.

Unlike conventional surface treatments, the proprietary electrolyte solutions used in the Keronite PEO process contain no heavy metals, ammonia, or other toxic chemicals. The low concentration alkaline solutions (pH 10 to pH 12) are non-hazardous. They need no special treatments before disposal, nor do they present any health or safety risk to the operator.

Keronite is a conversion coating that grows into the surface of the substrate in a uniform, controlled manner, forming a clean interface with the base metal. As a result, the Keronite layer has much better adhesion to the surface than most deposited coatings such as plasma sprayed ceramics.

Enlarge this picture Fig. 3. Comparative hardness of different surfaces measured in HV (Vickers).

In addition to this inward growth, the Keronite layer grows out from the original surface in a very consistent way. The extent of the outward growth depends upon the alloy in question, but it is typically between 10 percent and 40 percent of the overall thickness of the Keronite ceramic layer. In some applications, this may be desirable, but if not, it is possible to polish the surface back to the original dimensions of the part.

The natural Keronite surface has surface roughness (Ra) of approximately 10 percent of the thickness of the applied layer, but can be polished back to a very smooth finish if required. Under a scanning electron microscope, three distinct layers can be detected. (See Fig. 2.) Beginning with the bottom, the process yields:

• A thin intermediate layer, providing the strong molecular bond with the substrate.
• A functional, hard ceramic layer that protects against wear and corrosion. On aluminium, this layer has a micro-hardness of 500-2000 HV, depending upon the alloy, and a fine scale porosity of 2 percent to 10 percent. This functional layer comprises hard, crystalline phases in a matrix of softer oxide phases. This complex structure gives Keronite its unique combination of high hardness and wear resistance.
• An outer layer with up to 20 percent porosity, but on such a fine scale that the Keronite layer can be considered relatively dense. This very fine-scale pore structure is ideal for the retention of lubricants such as PTFE, producing a tough, non-stick or low-friction composite, or for impregnation with paints or lacquers.

Properties of Keronite

Enlarge this picture Fig 4. Comparison of corrosion resistance as measured by salt spray endurance in hours in accordance with ASTM B117.

Depending upon the alloy selected and the thickness of the ceramic layer generated by the process, the hardness of Keronite surfaces ranges from 500 HV to as much as 2000 HV — making it three times harder than hard anodizing and even harder than hardened tool steel, glass or many silicon-containing compounds. Even on magnesium, the hardness can reach 600 HV. (See Fig. 3.)

It is the unique structure of the Keronite ceramic layer that gives it such remarkable performance characteristics. Despite this extreme hardness, the fine scale porosity of Keronite gives it a degree of compliance which, combined with the hardness of the surface, makes for exceptional wear resistance — surfaces up to seven times more wear resistant than hard anodized equivalents on aluminum and far less prone to cracking around corners than hard anodizing. It also out-performs electroless nickel in ball-on-disk tests.

On magnesium, traditionally quite a soft material, just 10 microns of Keronite will withstand 10,000 cycles of a CS17 abrasion wheel without any sign of damage. Research at the University of Cambridge has demonstrated that the stiffness of Keronite can be as little as 30 GPa, making it more strain-tolerant than most ceramics.  When impregnated with PTFE, the wear resistance of Keronite surfaces can be even further enhanced.

Corrosion has always been the limiting factor in the use of magnesium, but Keronite on AZ91D magnesium alloy withstands 1,000 hours in salt spray (in accordance with ASTM B117) without needing to seal the surface. In one test, it even reached 2,000 hours with no detrimental effect. (See Fig. 4.)

Keronite surfaces have other functional characteristics including thermo-optical and dielectric properties. As an insulator, it can act as an effective thermal barrier and has excellent thermal cycling properties. It has a low friction co-efficient, and is used as a pre-treatment for topcoats such as paints or for other metals or ceramics to form composite coatings.

On magnesium

The increased hardness and wear resistance that Keronite provides is useful in wear parts, such as this ball valve.

Magnesium is currently going through a period of great change with world demand increasing rapidly, and prices falling at a similar pace as China takes on a more dominant role. World production of magnesium has increased by over 50 percent from 479,000 tonnes in 2000 to 726,000 tonnes in 2006. China supplied only 4 percent of world demand in 1995, and now the figure has reached a staggering 72 percent. With increasing capacity and falling prices, more and more design engineers are turning to magnesium as an alternative to plastics, composites, zinc, aluminium, or even steel.

New magnesium alloys and processing techniques have combined to overcome some of the traditional concerns about the properties of magnesium. There is now a range of alloys with excellent casting qualities in terms of flow properties and melt characteristics. The low density and small draft angles of magnesium make for cheap and rapid machining of high-precision parts. Magnesium can easily be molded into complex shapes using technologies such as thixomolding, similar to plastics molding techniques. Magnesium has the highest strength-to-weight ratio of any of the commonly used metals. It is lighter even than aluminum and has none of the problems associated with the processing of titanium.

There are many examples of applications for magnesium that have been enabled by Keronite. It has recently been specified as a replacement for toxic hexavalent chrome to treat magnesium door-frame inners for a leading automotive OEM. In this application, the Keronite layer not only provides the necessary corrosion prevention, but also acts as an ideal surface for adhesive bonding. In another example, on wing mirrors for a Japanese manufacturer, Keronite provides resistance to both corrosion and wear. Also in the automotive industry, magnesium treated using the Keronite process has been used on engine covers by an OEM in Germany since 2005.

Keronite is also being used in a variety of consumer products to provide corrosion resistance and a good base for topcoats of paints or lacquers on hand-held electronics such as mobile phone covers and laptop cases. Other examples include hand tools, where a hard surface and thermal stability are the key requirements, and bicycle components, where the inherent damping qualities of magnesium can bring benefits, but an alternative to chromate was sought as a means of preventing corrosion and providing good adhesion for decorative top coats.

On aluminum

The addition hardness and corrosion resistance provided by Keronite makes it more feasible to use light weight alloys in portable applications such as this power tool housing.

The aluminum market is also in good shape, with world production growing by 9 percent last year, and a further 8 percent to 9 percent expected this year, largely as a result of growing demand from China. As a result of widespread investment in new, low-energy capacity, particularly in Russia and the Middle East, prices are remaining attractive.

As an example of how Keronite can improve aluminum wear resistance in demanding environments, the process is widely used by the automotive industry on components such as piston crowns and top ring grooves. The Keronite surfaces are resistant to detonation damage, enabling higher specific power and leaner burn while, at the same time, preventing any peeling or cracking of the surface. Keronite is also widely used in the tooling and molding industry to enable a switch from steel to aluminum tools, while at the same time, extending the tool life and improving the surface properties through better adhesion of release agents such as PTFE.

There is growing interest in the use of aluminum as a lightweight alternative to steel in the oil and gas industry, for example in down-hole drilling applications where abrasive mud causes erosion and wear, or in sub-sea applications where corrosion is a real issue.

The process can also be used to create decorative surfaces to provide more design options. One example is the use of Keronite by architects to provide an unusual decorative surface for aluminium panels used to clad buildings. Keronite has also introduced a new, black ceramic surface for aluminum with extremely low optical reflectivity (<0.1 percent). In addition to providing a new design option, the matte charcoal finish is suitable for a variety of optical, aerospace and defense applications.

New applications for the Keronite technology continue to arise across a wide spectrum of industries. The functional and decorative properties of the surface treatment, along with its scalability, gives design engineers new, and previously unimagined options for using lightweight alloys.