Hard face for soft alloys

Anne Wilde
Keronite International Ltd.
Cambridge, U.K.

Magnesium racing wheels are being coated using the proprietary plasma electrolytic oxidation process at Keronite International's largest facility in Bicester, near Oxford in the U.K.

Thomas Sheldrake, Keronite engineer, examining a section of Keronite under the microscope to measure the surface roughness.

Keronite plasma electrolytic oxidation coatings for light metals can be spec'd by automotive designers to reduce vehicle weight. That in turns helps improve fuel economy and decreases emissions.

As an immersion process, Keronite can be used to treat the inner surfaces of complex shapes. The ceramic layers can be adjusted for optimal performance.

The increasingly dire reports of global warming coupled with gas prices topping three bucks a gallon this past summer have heightened pressure on automotive designers to further reduce CO2 emissions and improve fuel economy. One obvious solution is to drop vehicle weight: With every 10% decrease comes a 7% reduction in fuel consumption and associated emissions, government sources say.

Shaving 10 kg (22 lb) of steel from an automobile and replacing it with only 4 kg (8 lb) of magnesium, for example, would drop greenhouse gas emissions by 100 kg (220 lb) over the lifetime of the vehicle. That equates to 4 million metric tons (4.4 million short U.S. tons) annually if such a substitution was made in every passenger car produced.

But America's increasing appetite for more onboard bells and whistles along with its continued love affair with heavy-duty trucks and SUVs are major counterbalances that designers must over-come. Peugeot, for example, estimates that adding creature comforts such as entertainment and navigation systems along with advanced safety gear has already boosted vehicle weight on average 80 kg (176 lb).

An aggressive materials substitution program can improve environmental performance without compromising consumer demands. In addition to the host of advanced thermoplastics and carbon-fiber-reinforced composites available, designers increasingly consider lighter-weight aluminum and magnesium alloys to replace steel.

Use of lightweight alloys is often constrained, however, by their susceptibility to corrosion and abrasion. Protection of such effects has been previously provided by the application of durable anodic or conversion coatings. Anodic coatings are tough, hard, and have excellent wear properties but their cost is often too high for all applications.

Chromate-based conversion coatings are cheaper and have been the most widely used defense against corrosion for aluminum, zinc, magnesium, and galvanized metals. But they are heavily regulated because of their toxic nature. As the hexavalent-chromium (Cr ⁺⁶) content leaches out of the coating it contaminates the environment and leaves less inhibitor available to protect the metal down the road.

A HARDER DEFENSE
An alternative surface treatment that uses Plasma Electrolytic Oxidation (PEO) converts surfaces of light metals into an extremely hard, dense ceramic with a nanoscale microstructure. Independent tests have shown that the Keronite surface treatment on aluminum is up to four times more wear resistant and far less prone to cracking on corners than hard anodizing. It also outperforms electroless nickel in ball-on-disk tests.

But unlike conventional surface treatments, the proprietary electrolyte solutions used in the Keronite PEO process contain no heavy metals (Cr, lead, mercury, cadmium, arsenic, thallium, and mercury), ammonia, or other toxic chemicals. The ecofriendly liquids need no special treatments before disposal nor do they present a threat to workers handling them.

On aluminum, the alpha and gamma phases of alumina (Al2O3) are formed. The Keronite PEO process transforms the metal surfaces creating a tight atomic bond with the substrate. This ensures much stronger adhesion than is possible with externally applied processes, such as plasma-spray coating. The Keronite layer grows both above and below the surface. Under a scanning electron microscope, three distinct layers can be seen. A thin intermediate layer provides a strong molecular bond between the base metal and the ceramic upper layers.

The functional hard, ceramic layer is up to 100- m (0.0039-in.) thick and protects against wear and corrosion. On aluminum it has a micro-hardness of 900 to 2,000 HV depending on alloy and a fine scale porosity of 2 to 20%. The ceramic consists of hard (up to 2,000-HV) crystalline phases dislocated in a matrix of softer (800 to 1,200-HV) oxide phases. The complex structure gives Keronite a combination of high hardness and wear resistance along with up to three times less impact on fatigue strength than hard anodizing.

The outer layer makes up about 10 to 20% of the coating's total thickness. It features a micro-hardness of 500 to 1,000 HV and up to 20% porosity. The porous surface lets designers add scratch-resistant, decorative topcoats including metal coatings, paints, and lacquers as well as nonstick PTFEs or adhesives.

KERONITE ON ALUMINUM AND MAGNESIUM
Depending on the alloy and coating thickness, the hardness of Keronite can improve the surface hardness of aluminum 2XXX Series alloys (with high copper content) up to 2,000 HV. That is at least three times harder than hard anodizing.

The dense nanoscale microstructure of the ceramic layer also makes treated aluminum parts candidates for applications where components are exposed to large-scale thermal shocks (of less than 2-sec duration) at temperatures to 2,000°C (3,632°F).

Keronite on aluminum reduces the thermal conductivity significantly to approximately 1.6 W/m/K, which is less than 10% of that of compressed alumina ceramic and around 1% that of the aluminum substrate. Hard anodizing, on the other hand, has a thermal conductivity of about 10% that of aluminum at 10 W/m/K. Lowering thermal conductivity lets coatings function as thermal insulators.

Tests at the University of Cambridge have also shown that there is no significant change in the coating properties after repeated heating to 1,400°C (2,550°F) and cooling to room temperature.

Likewise, tests by The Welding Institute (TWI) in Cambridge, U.K., showed Keronite also improves thermal-shock resistance of aluminum components. A 60- m-thick layer on 6082 aluminum alloy, for example, was subjected to alternate 3-sec immersions in boiling water (100°C, 212°F) and liquid nitrogen (-196°C, -320°F) baths. After 50 submersions in alternating baths, there were no signs of delamination or cracking, even around the coating edges.

Similar thermal shock tests on AA 2219 rolled plate aluminum alloys showed that Keronite retains its integrity and original microstructures with no degradation to adhesion and cohesion or hardness. This performance is mainly due to the low Young's Modulus of the coating.

In the case of magnesium, the Keronite PEO treatment imparts a surface hardness on the order of 400 to 700 HV. This is due to the fact that the process fuses the coating into MgAl2O (spinel).

This lets magnesium compete with hard-anodized aluminum. In two-body abrasive wear scenarios, the coating reduces the wear rate by a factor of 20 compared to uncoated magnesium alloys, and is similar to that of case-hardened steel. Keronite also helps eliminate high friction and galling typically associated with magnesium. Once polished it has a friction coefficient of less than 0.15 against steel. Treated magnesium parts also withstand brief exposure to temperatures up to 1,000°C (1,832°F).

The Keronite PEO surface treatment also counters the other Achilles' heel of light alloys, i.e., being prone to corrosion and staining in wet and salt-laden environments. The fused ceramic layers are inert to most chemicals and corrosive conditions. Unsealed Keronite on aluminum, for example, was tested in a salt/fog test chamber. The coating survived 1,000 hr and received an ASTM Standard B117 rating of nine. This is a fourfold performance improvement over electroless nickel and twice that of sealed hard anodizing.

Magnesium is known for its poor resistance to atmospheric and galvanic corrosion. Tests done at TWI have also shown that magnesium alloy AZ91D with 35 m of Keronite can survive a month-long immersion in a saline solution and 1,000 hr under salt-spray conditions with little visual evidence of corrosion attack.

The University of Cambridge also tested various magnesium alloys and rated the results in accordance to ASTM D1654. Tests showed that 10 m of Keronite on AZ91D magnesium alloy together with an e-coat (McDermid Electro-lac High Build XD4434 and BASF Corp.'s GV82/9438) or a powder coat (H. B. Fuller P4M5229 poly-ester) consistently received top ratings of 10 after 750 hr in salt spray per ASTM B117.

In tests to determine resistance to galvanic corrosion, the Cambridge researchers tested AZ91 cast magnesium with a 10- m-thick layer of Keronite that was powder coated. A galvanic cell was introduced by tightening a zinc-plated-steel bolt to the coupon with a torque of 5 Nm (3.7 lb-ft). The combination survived 2,000 hr of salt-spray exposure and received an ASTM D1654 rating of 10.

Keronite also significantly improves wear resistance of strong WE 54 magnesium alloys. This widens the range of applications in engines, loaded elements of pumps, fuel and pneumatic drives, as well as sliding bearings. The ability to produce magnesium pistons, for example, that won't wear out and will be 23% lighter than aluminum could significantly impact how con rods are designed.

With the lighter-weight pistons, the con rods needn't be as strong because there is less inertia. Consequently, crankshafts can be redesigned with a smaller footprint. Gearboxes can also glean weight reductions with Keronite-coated aluminum. However, in terms of gear sets, aluminum likely won't have enough bulk strength.

AUTOMOTIVE APPLICATIONS
According to a specialized automotive OEM, Keronite has shown success as a low-cost aluminum coating process for pistons, exhaust valves, and valve seats. And when applied to diesel combustion bowls, there's a direct efficiency improvement thanks to less heat lost to walls. Although this is not a large benefit, it also lets components tolerate higher temperatures during combustion and expansion. This, in turn, improves performance and can lead to downsizing.

Reducing diesel-engine footprints can boost fuel economy by 10%, contends the OEM. The application of Keronite on diesel piston top-ring grooves, for example, will cut cost and weight compared to their steel counterparts. This has positive knock-on effects in other areas of engine design providing benefits above those specifically associated with the pistons.

The same OEM concludes that the application of PEO surface treatments to gasoline engine parts is a key enabler to downsizing. It is common in high-specific-output gasoline engines to have cylinder pressure approaching that of current production diesel engines. In this case it is normal to hard-anodize the top-ring groove to provide a suitable level of durability. The Keronite PEO process has environmental advantages over hard anodizing as well as performance benefits (increased wear protection) on this application.

Another automotive OEM is using the Keronite treatment on aluminum and magnesium in areas previously outside these materials' technical capability.

A prime example, says the OEM, is the development of rotary-valve-head systems that will become the standard technology fitted to all piston engines over the coming years. Keronite in the valve bearings allows them to run dry by design. This gives obvious environmental benefits through reduced emissions.

Keronite also provides high-temperature insulation for the aluminum valves that replace the current steel camshafts and all of the steel valves and springs in conventional poppet valve heads. "This development will allow the ongoing use of the internal combustion engine for many more decades, thanks to the doubling of power for a given engine size while at the same time reducing fuel consumption and emissions," states the OEM.

Keronite PEO at a glance
Keronite coating is based on Plasma-Electrolytic Oxidation (PEO). It resembles anodizing in that it uses an electric power supply and an electrolyte bath. However, it differs significantly from anodizing in that it produces a different structure of alumina that is comparatively harder and is more wear resistant while using environmentally less harmful alkali electrolytes and a specially modulated ac voltage. The process works on light alloys including aluminum, magnesium, titanium, and other intermetallides. But it cannot be used to coat steel, copper, zinc, stainless steel, or other ferrous metals.

The proprietary PEO process transforms surfaces of light metals into a complex ceramic matrix by passing a pulsed, bipolar electrical current in a specific wave formation through a bath of low concentration of alkaline solution. This creates a plasma discharge on the outer surface of the substrate, transforming it into a hard, dense ceramic oxide, without subjecting the substrate itself to damaging thermal stresses. Acoustic vibration in the tank works in synergy with the complex electrical pulses to ensure that the ceramic layer is as smooth, hard, and compact as possible.

Because of the nature of the PEO process, the ceramic layer is self-regulating and forms a highly uniform thickness, even along the edges and inner surfaces of complex parts. This is an advantage over conventional dip processes that can result in points of weakness around critical edges.

A bath with a capacity of 200 liters (53 gallons) and a power rating of 80 kW can coat a substrate with a surface area of 1,500 to 2,000 cm ²(1.6 ft ²) at a rate of 1 to 2 m/min (0.00003 to 0.00008 in./min). In applications needing wear resistance, Keronite can be no more than 150 m (0.006 in.) on aluminum alloys and about 25 m (0.001 in.) on magnesium alloys. Alternatively, layers 200 to 600 m (0.008 to 0.024 in.) in thickness can be produced for applications where electrical insulation is the main requirement.

The PEO process also involves fewer stages than hard anodizing. This makes the process much shorter than conventional surface treatments and, therefore, drops component cost. It is also a room-temperature process. This makes it easier to use and decreases the cost of cooling. Additionally, compared to traditional processes Keronite PEO cuts in half the volume of water needed for rinsing parts which also adds to cost savings and environmental appeal.

Keronite on AlBeMet
Aluminum-beryllium alloys are attractive for aerospace applications because of their combination of high strength and low density. However, their use is limited because beryllium powder is a potential health hazard. Experiments conducted in cooperation with Poeton Industries Ltd., in the U.K., prove that Keronite can be applied to any grade of AlBeMet for lightweight, high-strength applications, including cast parts. Further salt/fog tests (to ASTM G 75) have shown that AlBeMet 162 coated with 100 m of Keronite is unaffected by corrosion after more than 1,000-hr exposure. Likewise, taber-abrasion tests have shown that the wear resistance of Keronite on AlBeMet 162 is competitive with hard chrome plate.

MAKE CONTACT
Keronite International Ltd., +44 (0) 1223 893 222, www.keronite.com
Poeton Industries Ltd., +44 (0) 1452 300 500, www.poeton.co.uk