The tribological properties of hydrogen-free amorphous carbon (a-C) films are affected by the test environment and load condition. The wear mechanisms of a-C films sliding against Si3N4 ball under inert atmosphere exhibits different mode during different sliding cycles. The results show that the tribological behaviors for a-C films exhibit adhesive wear and it did not wear out even after sliding test 40,000 cycles when the normal load is 1 N. As the normal load increases, the wear mechanisms mainly show fatigue wear and three-body wear induced by fatigue wears due to the increasing of alternating stress. The emergence of three-body wear will drastically change the local stress distribution of a-C film. The position obtaining maximum Von-Mises stress shifts from the subsurface of substrate to subsurface of a-C film, and the maximum Von-Mises stress and Hertzian contact pressure increases sharply. The structures of in-situ formation graphene originated from the transformation of amorphous carbonaceous structure are detected on the wear scars. The formation of carbonaceous transfer film is beneficial to facilitate the graphitization transition with increasing Hertz pressure from 0.65 GPa to 1.35 GPa. On the other hand, the transition temperature of graphitization is sharply reduced to the flash temperature of contact zone due to the emergence of three-body wear obtaining the instantaneous Hertz pressure reached to 29.5 GPa. These conclusions possibly provide valuable references for designing film thicknesses, reveal tribological mechanism for a-C films under dry inertness atmosphere and broaden industrial application.