Sliding friction properties of carbon nanotube coatings deposited by microwave plasma chemical vapor deposition

Sliding friction properties of carbon nanotube coatings catalytically deposited by microwave plasma CVD from a gaseous mixture of methane and hydrogen have been investigated by means of ball-on-disk type friction testing. Carbon nanotubes with different crystallinity were deposited by changing microwave power and bias voltage applied to a silicon substrate. Nickel was used as a catalyst and coated on the substrate by DC sputtering before deposition. SEM and Raman spectroscopy were employed to examine carbon nanotubes. In friction testing, a steel ball slid on the substrate coated with carbon nanotubes or covered with vapor grown carbon fibers used as a reference carbon material. Friction force was measured in air containing water vapor and a vacuum. Consequently, friction was reduced when carbon nanotube coatings are less defective and, in particular, in a water-free atmosphere such as a vacuum. Furthermore, the improvement of adhesion of carbon nanotube coatings on cemented carbide and silicon nitride substrates has led to lower friction coefficients, below 0.1, depending on the density of the carbon nanotubes that remain on the sliding surfaces.