Correlating nano-tribological behavior with the free volume of Zr-based bulk metallic glasses via their fictive temperature

Abstract

Understanding the structure-property relationship is important for guiding the design of metallic glasses with advanced properties. In this work, we employ nanoscratching experiments to establish a correlation between structure and nano-tribological behavior in Zr-based metallic glasses where the structural state of the material is characterized by its fictive temperature, Tf. The results indicate that independent of the applied load, the scratching depth and residual depth of the scratch track increase with an increase in Tf, which is ascribed to the increase in free volume that metallic glasses experience when they are prepared with higher Tf. Furthermore, it was found that the effect of Tf on the friction coefficient varied significantly with the applied load: At low loads, adhesion-induced friction dominates so that low-Tf samples display higher friction coefficients, which we correlate to their denser packed structure affecting interfacial shear. At high loads, however, plowing becomes dominant, which causes the friction coefficient of low-Tf samples to become lower than the ones of high-Tf samples and is ultimately a consequence the higher elastic recovery capability for the samples with lower Tf. Meanwhile, sudden sink-in events of scratching depth are observed when ramping the load during nanoscratching in the indenter face forward direction, revealing the occurrence of scratch-induced yielding. Thereby, the higher degree to which free volume is available in high-Tf samples causes them to show higher plasticity, which in turn is responsible for the higher critical loads at which the sink-in event occurs. This finding implies the possibility of using ramping load scratching to characterize the ductile-to-brittle transition of metallic glasses as an alternative to carrying out time-consuming fracture tests.