Strong dependency of the tribological behavior of CuZr-based bulk metallic glasses on relative humidity in ambient air

Thanks to their outstanding mechanical properties, Bulk Metallic Glasses (BMGs) are new alternatives to traditional crystalline metals for mechanical and micromechanical applications including power transmission. However, the tribological properties of BMGs are still poorly understood, mostly because their amorphous nature induces counter intuitive responses to friction and wear. In the present study, four different BMGs (Cu₄₇Zr₄₆Al₇, Zr₄₆Cu₄₅Al₇Nb₂, Zr₆₀Cu₂₈Al₁₂, and Zr₆₁Cu₂₅Al₁₂Ti₂) underwent ball-on-disc friction tests against 100Cr6 steel balls (American Iron and Steel Institute (AISI) 52100) at different relative humidities (RHs) ranging from 20% to 80%. Controlling humidity enabled to observe a high repeatability of the friction and wear responses of the BMG. Interestingly, the friction coefficient decreased by a factor of 2 when the humidity was increased, and the wear rate of BMGs was particularly low thanks to a 3rd-body tribolayer that forms on the BMG surface, composed of oxidized wear particles originating from the ball. The morphology of this tribolayer is highly correlated to humidity. The study also identifies how the tribolayer is built up from the initial contact until the steady state is achieved.