FLUG REVUE May 2004: Nanotechnology enters aerospace

NANOTECHNOLOGY - VENTURING INTO THE MICROWORLD

By Matthias Gründer

The ancient Greeks used the word “micro” to describe something very small. Today, engineers are feeling their way forward into the world of atoms and uncovering amazing possibilities for tomorrow's technology.

The philosophers of ancient Greece already suspected that our material world is made up of tiny particles, which they called “atomos”, meaning indivisible. The atomistic theory was developed by Leucippus and Democritus, who postulated that atoms are the smallest constituents of matter – indivisible, impenetrable, indestructible and similar in quality, yet each distinct in shape and size. For centuries these atoms remained nothing more than a theoretical concept because nobody could prove their existence.

With the development of atomic physics as a separate branch of science, and thanks to the invention of high-precision scanning electron microscopes last century, it finally became possible to make atoms visible. What the scientists were able to see was astonishing at the time: under the microscope, apparently smooth surfaces turned out to look more like rugged lunar landscapes! This is where engineers came in, reasoning that if it is possible to see something, it must also be possible to influence it.

Interdisciplinary collaboration in research and the practical application of its results in engineering led to the development of a new discipline, nanotechnology

THE LOTUS EFFECT – LEARNING FROM NATURE

Biologists discovered that the leaves of tropical lotus plants do not take in any water or dirt because nothing adheres to their surface; everything simply drips or falls off. It is not yet possible to emulate all the miracles of nature artificially – not by a long way – but in this particular case, considerable progress has been made.

Although it is not possible to create the complex natural surface of the lotus leaf artificially (or at least not yet), it has been possible to treat a wide variety of materials chemically to achieve the same effect. Thanks to new nanocoatings, the rugged surfaces of metals or ceramic glazes are made much smoother, so that foreign bodies can no longer adhere to them. The constituents of these coatings are so small that they simply cover up any fissures or craters in the material.

It comes as no surprise that industry is extremely interested in these coatings, with the automotive and aerospace industries making the running. The ultimate benefits of the mass use of nanocoatings can still only be imagined.

NO CHANCE OF CORROSION

If water simply runs off a surface, there is no chance of it corroding. In a study carried out at the Technical University of Dresden, researchers examined the extraordinary anti-corrosion properties of nanocoating. Samples of material were subjected to a salt water corrosion test. The test was terminated after 3000 hours because no damage of any kind could be detected, whereas comparison samples showed signs of damage after 500 hours. Nanocoated materials thus offer unprecedented longevity. Windscreens coated with transparent nanocoating no longer require windscreen wipers because the water just runs off them, leaving nothing behind. Nor is there any need for heated windscreens – where there is no water, no ice can form.

This makes these coatings highly attractive to the aerospace industry. If it were possible to do without the time-consuming and expensive de-icing of aircraft on the ground and complex, heavy de-icing systems in the aircraft, huge savings could be made. Anti-ice coatings like this were tested at the end of March by Nano Tech Coatings GmbH in Tholey under natural environmental conditions.

The results of the tests are still being analysed, but Dr.-Ing. Georg Wagner, the chief executive of the Saarland-based company, is confident. After all, the laboratory tests carried out in advance delivered excellent results. A coating of only five micrometres was found to have excellent water-repelling properties not just on natural glass panes but also on aluminium and magnesium compounds.

This makes these coatings feasible for combating ice and corrosion on wings and tail planes as well as cockpit windscreens. The weight savings of these microscopically thin coatings are a further benefit; less weight means lower fuel consumption and thus extends the range of an aircraft.

Finally, there is one further property that makes nanocoatings an interesting option for the aerospace industry. To save weight, a great deal of aluminium is used in aircraft construction. But though aluminium is light and strong, its surface is difficult to treat.

CHROMIC SALTS TO BE BE REPLACED

Unlike steel, for example, aluminium cannot be galvanised to protect it against corrosion. Instead, it has to be treated with chromic salts, and the resulting chromatised coating not only offers protection but also provides the coating with something to adhere to. But chromic salts are toxic and carcinogenic, which is why they are to be banned as of 2008 by an EU regulation. Nanocoatings seem tailor-made for this situation.

Dr. Wagner's Clearcoat U-Sil is available in two forms. One is used as a primer, which is then painted over, while the other is mixed with colour pigments and then applied immediately. This cuts down on the number of steps involved and dramatically reduces the use of solvents. The hard, dense polymeric layer does not allow aggressive chemical substances or oxygen to get past, which is why it is also suitable as underbody protection on cars and for sealing the external skin of fighter planes on aircraft carriers.

Tests have already been carried out on Royal Navy Harrier jets, while Eurocopter Deutschland is more interested in protection against dirt. Here, too, highly promising results have been achieved with selected, pretreated parts. However, tests also revealed one problem with nanocoating that requires more research: even minor damage to the coating cannot be repaired because the hardened and incredibly smooth underlayer will simply not accept the repair coating.

There are also a number of other problems to be sorted out before mass production can begin. These include finding a suitable technology for coating entire aircraft, which are often very large, the behaviour of the coatings under constantly changing environmental conditions, such as the large differences between temperatures on the ground and at flying altitudes, and health issues. Before the coating can be approved it must be shown not to be harmful to health.

In the past, industrial dusts, for example, have almost always turned out to be pathogenic. However, the dust particles involved are like great boulders of rock compared to the constituents of the new coatings that can be released through evaporation during the drying process. Not until all these reservations can be dealt with – and possibly others as well – will the triumphal march of these amazing coatings begin.

From page 128 of FLUG REVUE 5/2004

Last updated 15 April 2004

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