From the paper "Origin of ultralow friction and wear in ultrananocrystalline diamond" by AR Konicek, DS Grierson, P Gilbert, WG Sawyer, AV Sumant, RW Carpick (Physical review letters, 100, 235502 (2008)):
“The mechanism of low friction supported by this spectroscopy is passivation of dangling bonds [1,3,4,35]. This is the first definitive spectroscopic evidence supporting the passivation hypothesis. This mechanism hinges on a balance between the processes of bond breaking during each sliding pass (which depends on contact stress, sliding rate, and temperature) and passivation by dissociative adsorption of gaseous species, in particular, water vapor (which depends on RH and reciprocation frequency [36,37]).”