Low friction behaviour of boron carbide coatings (B4C) sliding against Ti-6Al-4V

Boron carbide (B4C) is a potential tool coating for machining of titanium alloys due to its high hardness and high temperature stability. In addition, B4C coatings annealed at 600 °C, form a layer that acts as solid lubricant leading to a low coefficient of friction, COF, against titanium. This paper shows that a transfer layer consisting of graphite could be established on B4C surface when sliding against Ti-6Al-4V and passivation of graphitized carbon would reduce COF. Ball-on-disk (B4C) type sliding tests were performed in dry air, argon and nitrogen atmospheres (< 0.5% RH), and in air with humidity levels varying between 25% and 85% RH. B4C samples were also tested while they were immersed in water, ethanol (C2H5OH) and iso-propyl alcohol (C₃H₇OH). The B4C coatings exhibited high COF values of 0.59-0.65 in dry atmospheres. Sliding under an air atmosphere containing 70% RH reduced the steady state COF to 0.25 and a further reduction of COF to 0.20 was recorded in air with 85% RH. A COF of 0.15 was measured when the tests were carried out in ethanol and 0.07 in iso-propyl alcohol. Low COF values of B4C were attributed to H and OH passivation of the graphitized transfer layers observed by the Raman, Fourier transform and X-ray photoelectron spectroscopy techniques.