Dry sliding tests were performed under various sliding speeds and loads in air for AISI H13 steel with different hardness values. Through investigating morphologies, compositions and phases of worn surfaces, the wear behaviors and mechanisms of AISI H13 steel as a function of sliding speed and hardness were explored, and especially, the effects of friction-oxide layers and their stability were disclosed. Sliding speed and the hardness of the steel significantly affected the wear behavior and mechanism due to the evolution of friction-oxide layers. With an increase of sliding speed, more oxides were produced by the process of friction oxidation. The stability of friction-oxide layers became a key factor in determining wear rate, which was closely related with the hardness of the steel. Those friction-oxide layers formed on the quenched and tempered steel with lower hardness remained stable, providing more protection from wear. Three types of wear mechanisms were found to prevail. Adhesive and abrasive wear were dominant accompanied with oxidation mild wear at relatively low sliding speeds, where the wear resistance was proportional to the hardness of the steel. As sliding speed increased, oxidation mild wear became dominating, where the wear resistance was not related to the hardness of the steel. As the sliding speed further increased, the wear fell in oxidation mild-to-severe wear transition region, in which the wear resistance was inversely proportional to the hardness of the steel.
