Nanoindentation and nanowear tests on amorphous carbon films

Abstract

To clarify the physical and tribological properties of pure amorphous carbon films, nanoindentation and scanning-scratched wear tests were conducted on pure amorphous carbon films, diamond and graphite, by using an atomic force microscope with a diamond tip. Two types of pure amorphous carbon films (amorphous carbon 1 and amorphous carbon 2) were deposited on silicon substrates. To evaluate the tribological characteristics of the surface layers of the films, the thickness of the films was set at about 50nm to eliminate the substrate effect. Deposition was performed by electron cyclotron resonance plasma sputtering. The internal stress of amorphous carbon film was compressive and it was about 0·4 GPa for the amorphous carbon films and 1·4 GPa for the amorphous carbon 2 films. Raman spectra of the amorphous carbon films showed an amorphous graphite-like structure. Auger electron spectroscopy, secondary-ion mass spectroscopy and Rutherford back-scattering spectroscopy showed that the deposited films contained argon and several atomic percentage of hydrogen. Nanoindentation tests showed that the order of hardness was diamond > amorphous carbon 2 > amorphous carbon 1 ≫ graphite. The ratio of the residual indentation depth at 20 μN was about 1:3:6 for amorphous carbon 2: amorphous carbon 1: graphite (the residual indentation depth of diamond was zero). Scanning-scratched wear tests (2 cycles) showed that the wear of diamond and amorphous carbon 2 was shallow and that of amorphous carbon 1 was several times deeper than that of amorphous carbon 2. The order of wear resistance was diamond > amorphous carbon 2 > amorphous carbon 1 ≫ graphite. The ratio of the wear depth at 40–80 μN loads was about 1:2:5 for diamond: amorphous carbon 2:amorphous carbon 1. The wear of graphite was extremely deep. Scanning-scratched wear tests (repeated cycles) showed that amorphous carbon 2 was more wear resistant than amorphous carbon 1. The wear depth of amorphous carbon 1 at a load over 5μN increased as the number of scanning-scratch cycles increased, but the wear at 1μN remained very shallow within 50 cycles. On the other hand, the wear of amorphous carbon 2 at loads below 10 μN remained very shallow within 50 cycles, but the wear depth at 20μN increased with increasing number of cycles. There was an explicit correlation between the indentation hardness and the wear resistance for the amorphous carbon films. Furthemore, we presume that hard and wear-resistant characteristics of the amorphous carbon films resulted from randomly assembled graphite cluster structures.