Reciprocating wear mechanisms in a Zr-based bulk metallic glass

The dry sliding friction coefficient μ and the wear volume loss W, in a zirconium-based bulk metallic glass (BMG) under high-frequency (50 Hz) reciprocating conditions, were investigated with the objective of assessing the influence of free volume and crystallization on the wear behavior of amorphous metals. The BMG samples were annealed either below the glass transition temperature T_g to induce structural relaxation and hence reduce the free volume that controls plasticity through shear-band formation or above T_g to crystallize the amorphous BMG prior to wear testing. Results show that the wear behavior of both the as-cast and relaxed glasses was dominated by the oxidation of the surface layers. A sharp transition in the contact electrical resistance complemented by a marked increase in μ was noted. This was attributed to the formation of a thick tribo film with high oxygen concentration and its subsequent delamination. The μ values, before as well as after the transition, in the relaxed glasses were similar to those for the as-cast alloy. However, a gradual decrease in W with annealing temperature was observed. A good correlation between W and nanohardness was noted, implying that the intrinsic hardness in the BMGs controlled the wear rate.