Structural transformation and friction behavior in turbostratic graphite sliding against Si3N4, SiC and Al2O3 balls

Friction behavior of turbostratic graphite was investigated in ambient atmospheric condition while sliding against Si3N4, SiC and Al2O3 spherical balls. Low friction coefficients of ~ 0.02 and ~ 0.1 were measured for tests with Si3N4 and SiC balls, respectively. In contrast, a high friction coefficient of 0.22 was observed in same graphite sample when Al2O3 ball was used for sliding counterbody. In all the three wear tracks formed by Si3N4, SiC and Al2O3 balls, structural transformation of 2D turbostratic to 3D graphite was observed. This transformation is driven by kinetic energy released during sliding. Structural transformation was quite similar in all the three wear tracks of turbostratic graphite. However, the value of friction coefficient differs greatly from each other. Therefore, friction behavior cannot be explained through structural transformation alone. In the light of above facts, carbon transferlayer and chemical interaction between the sliding interfaces were considered to explain friction mechanism. Absence of carbon transferlayer on Si3N4 ball scar leads to less dangling bonds, which are efficiently passivated by the oxygen molecules and water vapor present in the ambient atmosphere. Such surface passivation results in ultra-low-friction coefficient. In contrast, the high friction coefficient in Al2O3 ball against graphite is due to the formation of graphitic transferlayer on the ball scar. Transferlayer assists formation of C-C strong bond between the sliding members leading to chemical locking.