Exploring tribological behaviour of diamond film by hot-filament chemical vapour deposition on tungsten carbide for lunar exploration

To explore the future application of diamond films coated by hot-filament chemical vapour deposition on tungsten carbide for lunar exploration, a comparative study of tribological behaviour of the film sliding in air and in vacuum under four different temperatures was conducted. Results showed that the coefficient of friction (COF) in air basically decreased with elevated temperature and, in vacuum, increased initially followed by a decrease with elevated temperature. The diamond film surface in air was observed to be smooth at 50 °C and to form a passivation layer that reduces friction at 250 °C. Its counterpart in vacuum was observed: (i) to adhere with debris at 50 °C, (ii) to form a transfer layer by the agglomerated debris at 250 °C, which reduced the friction and increased the level of agglomeration with the elevated temperature, (iii) to crack at 450 °C which subsequently increased the sliding friction. Surface graphitization of diamond was favourable in reducing the COF and detected for the sliding both in air and vacuum when the temperature was approaching 650 °C.