Smart coatings

Doch dünne Schichten können mehr: Schneidwerkzeue intelligent machen oder Folien zum Leuchten bringen.

Thin coatings are already part of our everyday life. They suppress glare, protect tools and components from wear and friction, enhance the appearance of buildings, make plastic as hard as steel and prevent food from sticking to the frying pan. Thermal-insulation coatings on window glazing help to save energy, and impermeable films on food packaging preserve freshness by keeping out oxygen and light. Surface engineering not only enhances many materials but also creates the basis for new products: compact discs, DVDs, flat screens, hard disks and high-speed processor chips have one thing in common: Without high-performance thin-film and surface technology, these technical marvels would not exist.
New applications for thin-film coatings regularly emerge, such as the force-sensor coatings which provide a warning of wear and can thus add a `smart¿ dimension to tools, components and machines. We have all at some time inadvertently touched something straight from the oven, such as a hot baking tray, and experienced the same reflex action: In a fraction of a second our hand pulls back in order to avoid a serious burn. ¿Machines and tools unfortunately do not have such reflexes. If extremely high temperatures arise during operation, the machine does not react but continues running,¿ explains Holger Lüthje from the Fraunhofer Institute for Surface Engineering and Thin Films IST. This can result in serious damage to the machine. As a precaution, tools are therefore usually replaced before they really need to be ¿ causing downtime and additional costs. To avoid having to replace tools until they are worn, research workers at the IST have developed smart coatings. Some of the intelligent systems consist of tribologically optimized carbon films with an integrated sensor function. The new, multifunctional surfaces are referred to as `smart coatings¿. They provide a warning when a tool needs to be replaced because of wear or indicate when a screw is no longer tight.

Thin films make components `smart¿
Lüthje explains the functioning of the piezo-resistive coating: ¿The hard film contains tiny nanoparticles and has been modified in such a way that it is not only resistant to wear and friction but also acts as a force sensor. If a force impacts on the hard film, the electrical resistance changes. This makes it possible to check machine components by means of simple electrical resistance measurements.¿
The smart coatings have a wide range of applications. One example: The `intelligent¿ washer. In order to ensure correct fitting and continuous monitoring of safety-relevant bolt connections in future it will only be necessary to apply a force-sensor film of hard material to the washer. ¿At present many bolts are tightened using a torque wrench. But this only measures the frictional force,¿ explains Lüthje. The `intelligent¿ washer offers a different approach. Here the electrical resistance only changes if a force is applied to the washer ¿ that is, the bolt is tightened. But the smart coatings do more than make the initial bolt connections safer. They also permit continuous active monitoring of the bolt connection.

The outlook is bright
Modern surface engineering can even endow plastic film with light-emitting properties. The Bavarian company Cool-Light produces highly flexible films just one millimeter thick which emit monochromatic cold light when a voltage is applied. The light is pleasant on the eyes and is more clearly visible than any other light source, even in dusty, smoky or foggy conditions.
The just one millimeter thick rollable film has a sandwich structure comprising several layers. On the substrate film of highly transparent polyester, a likewise transparent, electrically conductive layer is applied. Then in the middle follows a thin, active light-emitting layer which is separated from the electrode coating by an insulating layer.
¿The key part of the film is the front electrode. The layer of silver which has a thickness of just two nanometers conducts electricity but still lets through up to 80 percent of the light,¿ states Matthias Fahland from the Fraunhofer Institute for Electron Beam and Plasma Technology FEP in Dresden, emphasizing the high transparency of the coating. For comparison: the non-coated film lets through 88 percent of the light.
Light-emitting plastic film opens up amazing opportunities for advertising, signs and lighting. Different parts of the film can be activated in alternating sequences. The new lighting technology paves the way for a broad range of applications ¿ wall coverings that light up, ceilings that can glow in different colors, and animated advertising billboards.

Transparent and conductive coatings
Transparent, conductive coatings have a wide range of applications. Solar cells, flat screens, optoelectronic components and thermally insulating architectural glass require systems capable of producing large sheets of coated material possessing optical transparency and metallic conductivity. ¿Especially for front contacts in solar cells and for flat screens the production of large sheets of transparent conductor material is rapidly gaining in importance,¿ explains Dr. Bernd Szyzska from the IST. Another application is the development of a transparent car window heating system. Thanks to modern coating technology, iced-up car windows could soon be a thing of the past.
Transparent conductors, smart coatings and antireflex solar panels are just a few examples of what thin-film coatings have to offer. Over recent decades, surface engineering has evolved as a key cross-sectional technology. In the years to come, thin-film coatings will continue to drive forward advances in technology and change our world.
Birgit Niesing (Fraunhofer Magazine/engine)

glare - grelles Licht
conductivity - Leitfähigkeit
indexable insert - Wendeschneidplatte
coating - Beschichtung
enhance, to - verbessern
emit, to - ausstrahlen, abgeben
suppress, to - unterdrücken
wear - Verschleiß
friction - Reibung
frying pan - Bratpfanne
window glazing - Verglasung, Fensterglas
impermeable - undurchlässig, undurchdringlich
marvel - Wunder
emerge, to - auftauchen
inadvertently - aus Versehen
baking tray - Backblech
arise, to - auftreten
precaution - Vorkehrung, Schutzmaßnahme
replace, to - ersetzen
downtime - Ausfallzeit
tribologically - tribologisch
carbon - Kohlenstoff
be referred, to - werden genannt
piezo-resistive - piezoresistiv
impact, to - einwirken
range - Spektrum
washer - Unterlegscheibe
safety-relevant - sicherheitsrelevant
bolt - Schraube
tighten, to - anziehen
torque wrench - Drehmomentschlüssel
frictional force - Reibungskraft
approach - Methode, Ansatz
initial - anfänglich
endow, to - ausstatten
property - Eigenschaften
apply, to - anwenden
comprising - bestehend
layer - Schicht
conductive - leiftfähig
insulating - isolierend
comparison - Vergleich
amazing - toll, unglaublich
pave the way, to - den Weg ebnen
sheet - Fläche
decade - Jahrzehnt
evolve, to - entwickeln

Links: http://www.engine-magazin.de