High-temperature phase transformation and low friction behaviour in highly disordered turbostratic graphite

Microstructure of turbostratic graphite consists of a large amount of pores and voids which eventually convert to plates and flakes constituting layered microfolding upon high-temperature heat treatment. This treatment results in apparent phase transformation from 3D to 2D structured graphite where stacking order between adjacent layers is absent. Interestingly, low friction coefficients 0.08 and 0.06 were measured in 2D graphite in ambient atmosphere. This is explained in terms of passivation of carbon dangling bonds by the formation of C-O and C-OH where both physisorbed and chemisorbed oxygen species are present. In contrast, these values were as high as 0.17 and 0.19 in 3D graphite. At significantly less humid atmosphere, progressive increase of friction coefficient (up to the value of 0.8) with sliding distance is observed in both 3D and 2D graphites. This consistent increase in friction coefficient is ascribed to gradual loss of residual passivating chemical species such as oxygen and H2O molecules due to tribochemical reaction. Interestingly, the structure of graphite remains similar while Raman spectra obtained from the locations of wear track, where ultra-low and high friction coefficient is measured.