Skin-deep functionality01 October 2005

Beauty may only be skin deep, but protective coatings, which tend to be even thinner, determine corrosion and wear resistance, as well as the product's outward appearance.

Although coatings usually represent only a fraction of the cost of the item, they can dramatically affect a product's working life and ability to function, so it's not surprising that a great deal of research and development work goes into them.

Paints, lacquers and electroplating still have their place and advances are constantly being made. A recent DTI Global Watch Mission to Italy, for example, found that Italian extrusion makers were making and selling twice as many extrusions in value terms per head of population as their British competitors. British window frames come only in white, whereas those made by their Italian counterparts can come in various wood-effect powder coat finishes, which are more attractive. And 'Effecta', as the new coating is called, adheres and protects against corrosion better than conventional powder coatings, according to a report produced by the Bureau of Chemical, Materials and ROW Materials Engineering and Metallurgy at the University of Rome. The manufacturing company is TSM Italia, based in Meolo, and, while it is well represented in continental Europe, it doesn't yet appear to have a UK agent.

Not decorative, but immensely hard are the 'Keronite' coatings for aluminium, magnesium and titanium alloys produced by Plasma Electrolytic Oxidation, the brainchild of Dr A Shatrov, a Russian researcher originally working in a small academic research spin-off company in Moscow. The process consists of applying an alternating 50Hz mains frequency voltage of more than 250V to components suspended in an alkaline solution. Unlike conventional anodising, current flows in both directions and the high voltage generates a spark discharge at the interface between the component and the solution.

The process was originally revealed to the UK in Eureka magazine in June 1998, and is now available from Keronite Ltd of Great Abingdon, near Cambridge. Temperatures during deposition can reach 1,0000 to 10,0000 C, with instantaneous pressures of up to 100 MPa. Processes in the plasma zone lead to the formation of crystallites and high-temperature modifications of oxides and spinels, instead of the amorphous oxides found in conventional anodised coatings. The high electrical resistance of the oxides directs electric current to those areas where the coating is thinnest. The process is thus self levelling and produces coatings of even thickness.

It is possible to control the properties of the coating layer by choosing both process conditions and the underlying alloy composition. The hardest coatings made so far are on aluminium-copper-magnesium alloys and they also offer the highest wear resistances. Keronite is said to offer higher surface hardness (2000 HV) than anodising on aluminium alloys and better wear resistance. Conventional hard anodising of aluminium can reduce the fatigue strength of aluminium by about 50%, whereas trials with Keronite give a reduction of about 15-20%. Set against thermally sprayed coatings, Keronite offers a higher adhesion and retains the dimensions of the original component. Compared with electroplated nickel silicon carbide coatings, Keronite delivers higher surface micro hardness and doubled or quadrupled wear resistance. Tribological (friction) properties match those of nickel silicon carbide plating. Keronite also achieves better adhesion and retention of adhesion after thermal cycling.

Keronite coatings are said to have enabled manufacturers of bicycle frames and components to switch to magnesium, with all the benefits that brings, in terms of weight reduction, damping of vibrations and resistance to impact.

Associated with a layer of PTFE, the coatings have been used successfully in the textile industry, enabling aluminium to be used in place of steel, but also improving the overall performance of the machinery. In high-performance engines, the coatings provide an effective thermal barrier, reducing the temperature of the piston crown and improving overall engine performance. One customer is said to have turned to Keronite coatings to enable him to use aluminium engine pulleys. And sunglasses coated with Keronite can be given a high-quality, scratch-resistant finish, and are well protected against the corrosive combination of cosmetics, sun creams and sweat.

Even harder are the diamond like carbon (DLC) and other carbon-based coatings available from Tecvac in Swavesey, also near Cambridge. Tecvac has pooled its knowledge with ICC and IonBond company, based near Paris. Two coatings are already available: Diamolith, a DLC coating with a surface performance similar to that of natural diamond, with extreme hardness up to 4500 HV plus high lubricity, and Nitron MC. The latter is a carbon-based coating with a lubricity better than rubber on ice, while still providing a hardness better than chrome plate. Diamolith is used in F1 motor sport components and medical implants, and can be applied to both metals and ceramics.

Looking to the future, the Institute of New Materials in Saarbruecken in Germany has pioneered the development of coatings based on nano structures, whose component elements are less than a single micron across. One of the developments is a paint coating for cars only 10 microns thick, but containing several thousand 15 nanometre aluminium oxide particles per cm2.

The institute offers coating technologies for aluminium, magnesium, copper, brass, silver and steel. The coatings are transparent and also adhere to glass, ceramics and polymers. They can be applied on dipping, spraying and flow coating, plus thermal and/or photochemical curing methods. Thicknesses can be down to 5 microns.

Colouration and pigmentation is possible, plus a special product is available with anti-fingerprint properties. When we met up with the institute at this year's Hanover Fair, it told Plant Engineer that one of their main licensees was a spectacle lens manufacturer, but they are eager to find others.

The other German research institution expert on nano-structured coatings is the Fraunhofer Gesellschaft, a family of research institutes. Nanotechnology is a declared priority area with the emphasis on coatings.

"Surface coating and modification techniques are the front runner in the advancement of nanotechnology, serving as an enabling technology that leads to many innovations in other areas of science," says Professor Günter Bräuer, director of the Fraunhofer Institutes for Thin films and Surface Engineering in Braunschweig and for Electron Beam and Plasma Technology FEP in Dresden.

The Institute in Braunschweig is working hard on atmospheric-pressure plasma coatings for surface coating and treatment. The plasma is generated by dielectric barrier discharges, ignited in the gas gap between two electrodes to which an AC voltage has been applied. The micro-sparks measure no more than 0.1mm across and extinguish in nanoseconds. Barrier discharges have been used commercially for some time as a means of preparing plastic surfaces for printing, coating and adhesive bonding. "If gas phase monomers are injected into the discharge zone, it is possible to deposit layers on a conducting or insulating substance," says Professor Claus-Peter Klages. "Atmospheric pressure plasma processes not only dramatically reduce the cost of applying coatings, but also open the way to new applications."

SOE

This material is protected by MA Business copyright
See Terms and Conditions.
One-off usage is permitted but bulk copying is not.
For multiple copies contact the sales team.