Friction can be reduced by heavier hydrogen

November 14th, 2007 - 8:25 am ICT by admin  
The research, done by the University of Pennsylvania, found a significant difference in friction exhibited by diamond surfaces that had been coated with different isotopes of hydrogen and then rubbed against a small carbon-coated tip.

Scientists do not have a comprehensive model of friction on the nanoscale and have only be able to grasp its atomic level causes. These range from local chemical reactions to electronic interactions to phononic, or vibrational qualities.

To investigate the vibrational aspect of friction, Anirudha Samant and his colleagues from Argonne used single-crystal diamond surfaces coated with layers of either atomic hydrogen or deuterium, a hydrogen atom with an extra neutron.

The team observed that the deuterium-terminated diamonds had lower friction forces because of their lower vibrational frequencies, which can be attributed to that isotope’s larger mass. They also observed the same trend on a silicon substrate, which is structurally similar to that of diamond.

All previous attempts to make hydrogen-terminated diamond surfaces have relied on the use of plasmas, which tended to etch the material.

“When you’re looking at such a small isotopic effect, an objectively tiny change in the mass, you have to be absolutely sure that there are no other complicating effects caused by chemical or electronic interferences or by small topographic variations,” said Sumant.

Such factors compelled the research team to look at a number of other ways to avoid etching. They even soaked the films in olive oil before applying the hydrogen layers. According to Sumant, “No method had provided a smooth, defect-free hydrogen layer with good coverage that would avoid generating background noise.”

But the team soon developed a system for depositing diamond thin films.

The technique, called hot filament chemical vapor deposition, involves the heating of a tungsten filament (like those found in luminous light bulbs) to over 2000 degrees Celsius.

Using this method, the diamond film is exposed to a flow of molecular hydrogen while sitting within a centimetre of the hot filament. The heat causes the molecular hydrogen to break down into atomic hydrogen, which reacts with the film’s surface to create a perfectly smooth layer. Since this method does not require the use of plasma, there is no danger of ion-induced etching.

“We’ve proved that this is a gentler method of terminating a diamond surface,” Sumant said.

The research team hopes to use the knowledge gained from the experiment to eventually discover a way to manipulate the friction of surfaces on the atomic level. Such a result would prove immensely valuable to the development of nanoelectromechanical systems, or NEMS, based on diamonds. (ANI)

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