The Tribological Performances of the Boron Carbide Films Tested under Wet Air and Wet N2 Conditions

In the previous literatures, the friction reduction of boron carbide (B4C) with the increasing relative humidity was mainly attributed to the formation of boric acid during the friction tests. In the present investigation, however, low friction coefficient was obtained under wet air (85% RH) without the formation of boric acid, suggesting that there is another low friction mechanism. Furthermore, low friction coefficient was also achieved under wet N-2 (85% RH, without O-2) without the formation of boric acid, implying that the core factor for the friction reduction is water vapor rather than oxygen. Therefore, we infer that the adsorption of H2O on the passivated boron carbide surface is the main reason for the reduction of friction coefficient. It should be noted that friction coefficients under wet air were lower than that under wet N-2, which demonstrates that O-2 plays a secondary role to reduce friction coefficient.