Structure and hardness evolution of the scale of a Zr-based metallic glass during oxidation

Hardness evolution of the oxidation scale of Zr58Cu16Ni13Al10Nb3 bulk metallic glass (BMG) is investigated by the controlled oxidation in the glassy state and in the supercooled liquid region at a short term stage (1.5 h) and a long-term stage (18 h). Their hardness evolutions under different oxidization conditions are characterized by nano/micro indentation. The hardness in the scale presents nose-like decreasing with the increasing of load influenced by the substrate. The continuity and the consistency between the oxidation scale and the substrate are observed by scanning electron microscope (SEM). Tetragonal ZrO2 (t-ZrO2) nanocrystalline favors the preservation of higher hardness and higher toughness more than monoclinic ZrO2(m-ZrO2) nanocrystaline. The transformation of t-ZrO2 into m-ZrO2 induces a lower hardness related to the crystallization of metallic glass. In addition, the element diffusion mechanism of Zr58Cu16Ni13Al10Nb3 BMG in the glassy state during the oxidation scale formation is investigated with an energy dispersive X-ray spectrometer (EDS) as further evidences to elucidate the hardness evolution.