Diamond coating in engines has promise for efficiency improvement

Researchers at Germany’s Fraunhofer Institute have developed a laser arc method for coating engine components they claim could save “billions of litres of fuel worldwide every year”.

Scientists already coat components with diamond-like carbon to minimize friction. But now Fraunhofer researchers have developed a laser arc method that allow layers of carbon almost as hard as diamond to be applied on an industrial scale, at high coating rates and high thicknesses.

By applying coatings to engine components such as piston rings and pins, fuel consumption can be reduced, the engineers said. “Systematic application of our new method could save more than 100 billion litres of fuel each year over the next ten years,” said Professor Andreas Leson of the Fraunhofer Institute for Material and Beam Technology IWS, which is based in Dresden. A study suggesting this was published in the journal Tribology International in 2012.

The scientists said that coating engine components with hard carbon reduces friction to almost zero – a development that could save fuel. Carbon-based coatings are already used in volume production. Now the team of IWS researchers, led by Professor Leson, Dr Hans-Joachim Scheibe and Dr Volker Weihnacht, has succeeded in producing hydrogen-free Ta-C coatings on an industrial scale at a consistent level of quality.

Tantalum carbides – Ta-Cs – form a family of binary chemical compounds of tantalum and carbon. They are extremely hard ceramic materials with metallic electrical conductivity. The tetrahedral amorphous carbon coatings are significantly harder and more resistant to wear than conventional diamond-like coatings, the researchers claimed.

“Unfortunately, you can’t just scrape off diamond dust and press it onto the component. So we had to look for a different method,” said Dr Scheibe.

Similar to old-fashioned film projectors, the laser arc method generates an arc between an anode and a cathode – the carbon – in a vacuum. The arc is initiated by a laser pulse on the carbon target. This produces a plasma consisting of carbon ions, which is deposited as a coating on the workpiece in the vacuum.

On an industrial scale, a pulsed laser is vertically scanned across a rotating graphite cylinder as a means of controlling the arc. The cylinder is converted evenly into plasma thanks to the scanning motion and rotation. To ensure a consistently smooth coating, a magnetic field guides the plasma and filters out any particles of dirt.

BMW is said to be working intensively on the industrial-scale implementation of Ta-C engine components in its various vehicle models with the aim of reducing their fuel consumption.

The laser arc method can be used to deposit very thick Ta-C coatings of up to 20 micrometres at high coating rates.