Coatings can add muscle to a lightweight

Poeton R and D Director Keith Stevens provides an update on the development of surface coatings to improve the performance of aluminium.

With new surface coating technologies to harden its surface and expand its use into ever widening engineering markets, production of refined aluminium is rising fast - up 5% in 2002.

As an engineering material, aluminium has many limitations: it is soft, it wears and galls, and it corrodes.

Although this may not seem like a very promising start, the situation can be recovered by applying an appropriate surface treatment.

The most common of these is anodising, where parts are oxidised by an electrolytic process, producing a hard aluminium oxide coating that grows out of the parent metal.

Techniques vary: room temperature processes generate a relatively thin coating (typically 15um) that provides some corrosion resistance and a base for colouring, whereas anodising carried out at 0C results in a much harder and thicker (450Hv, 60um) coating that provides greatly improved wear and corrosion resistance.

A major feature of the anodising process is the fine columnar structure produced in the coating, which provides excellent opportunities for sealing or polymer infiltration.

The latest composite anodising processes make good use of this structure, reducing friction and increasing the ability to withstand environments such as salt or high humidity.

This provides wider scope for designers when selecting construction material and extends service life for end users.

With such attractive properties, hard anodised aluminium is now finding increasing applications in the aerospace, packaging, moulding, medical and automotive industries.

Although the processing plant itself is relatively straightforward - consisting mainly of a bath and a power supply - the holding of parts by specially designed tooling is a highly skilled procedure that distinguishes the best coating companies from the rest.

Alloys respond to anodising in different ways, with considerable variation.

Silicon and copper both create problems.

For example, the copper phase dissolves during the process, leaving porosity in the coating and reducing the corrosion protection.

For best wear and corrosion results, designers should specify 6000 series alloys.

The latest advance in anodising has now moved to plasma-based processes.

Plasma electrolytic oxidation (PEO) is able to generate oxide coatings with much greater hardness and thickness.

Layers of over 100um are possible, as is a hardness of 1400Hv - bringing exceptional wear resistance.

It means that coatings can resist abrasive wear in a whole range of process industries such as textiles and paper, threatening the dominance of plasma sprayed ceramics.

And, when used with a 6000 series alloy, a PEO coating can push the salt mist endurance to 5kh without any post-sealing.

The PEO process certainly has attractions, producing a distinctive glow discharge around the part, and providing coatings without adverse environmental concerns.

The electrolyte is benign, there is no need to pretreat in hostile chemicals, and there are no emissions.

And now process innovators are looking even further, combining the anodising and PEO layers with further coatings such as polymers and lacquers to produce novel hybrid layers that promise a whole new range of properties, including nonstick, electrical insulation and controlled light reflectance.

For example, Poeton has recently completed an 8-month cyclic corrosion test for a major automotive industry component supplier that has, using a specially developed anodic/composite coating, provided a component with a 400% increase in corrosion protection.

The component, which is manufactured from an aluminium alloy, was specified with an anodic coating to provide 1kh of corrosion protection.

Unfortunately, none of the supplier's existing anodisers was able to produce more than 500h.

At this point the company approached Poeton with the problem.

Following a series of meetings with staff at our Technical Centre we were able to convince the customer that we could develop and test a dedicated coating, using our own in-house laboratory.

The customer was sufficiently impressed to fund a coating and test program and we produced Apticote 300SP, an improved anodic treatment that not only met the 1kh requirement but went much further, providing an unheard off 2kh.

Following this successful programme, a number of global automotive manufacturers have been convinced by the additional benefits of the Apticote 300SP coated component; and a major vehicle contract has been secured as a result of the customer using the new coating.

Anticipating growth from this and other similar applications, Poeton has invested in a new, world class, state-of-the-art hard anodising line for its Apticote 300 aluminium coating process.

The high-capacity line offers 24 hours a day, seven days a week operation with a standard of consistency that has previously been impossible to achieve, and enables Poeton to provide a fast response on a wide range of high quality, low-cost, hard anodising options to suit manufacturing industry's requirements.

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