Humidity-dependent friction mechanism in an ultrananocrystalline diamond film

Friction behaviour of an ultrananocrystalline diamond film deposited by the microwave plasma-enhanced chemical vapour deposition technique is studied in a controlled humid atmosphere. The value of friction coefficient consistently decreases while increasing the humidity level during the tribology test. This value is 0.13 under 10% relative humidity conditions, which is significantly decreased to 0.004 under 80% relative humidity conditions. Such a reduction in friction coefficient is ascribed to passivation of dangling covalent bonds of carbon atoms, which occurs due to the formation of chemical species of hydroxyl and carboxylic groups such as C?COO, CH3COH and CH2?O bonding states.