Effect Of Temperature And Pattern On Surface Friction Of A Zr-based Bulk Metallic Glass

Reduction of surface friction coefficient is not only beneficial to enhance the thermoplastic formability of bulk metallic glasses?BMGs?,but opens a new route towards functional applications as antifriction surface.However,friction laws and mechanisms are not clear owing to the lack of systematic research.In this study,the above problems were investigated systematically and profoundly by means of surface friction experiment,finite element simulation and theoretical analysis.The main contents and results were as follow:Firstly,a Zr₃₅Ti₃₀Be_26.75Cu_8.25?at.%?BMG system was considered for this study by arc-melting,followed by copper mould casting.Micro-honeycomb structures with various pitches?P?between adjacent cells were hot-embossed on BMGs surface.The effect of pitch geometry on the frictional behavior was systematically investigated by pin-on-flat reciprocating tests on a Bruker UMT-3 Tribometer.The results revealed that all textured metallic glass surfaces show a reduction in friction coefficient???compared to smooth surface under dry or wet friction condition.When pitch increased to 75.5?m,for instance,?decreased from 1.08±0.08 to 0.43±0.08,which could achieve up to approximately 60%reduction under dry condition.More intriguingly,a U-or V-like shape can be observed,namely the friction coefficient first decreased and then increased gradually with increasing pitches.The similar phenomena of wear rates also could be observed.Theoretical analysis showed that on the basis of the classical friction theory,friction force was proportional to the real contact area that decreased with the increase of pitch,which evidently resulted in the reduction of friction coefficient,while the crucial role of surface micro-patterns acted as oil pockets to reduce friction and the assistance in tripping off wear debris and the improvement of the load carrying capacity due to hydrodynamic effect.However,the increasing pitch induced the reduction of real contact area raised the contact stress significantly.According to the reasonable analysis and derivation of adhesive friction theory,the calculated data fitted well with the experimental data.Then,the surface friction and wear properties of Zr-based BMGs at various temperature from room temperature to supercooled liquid region?SCLR?were investigated.Results indicated that a U-or V-like shape can be observed,namely?decreased first and then increased with increasing temperature.In despite of the reduction of friction coefficient with the increase of sliding velocity,however,the influence of temperature on friction coefficient was similar at different sliding speeds,wherein the minimum value obtained at the glass transition temperature.Analysis illustrated that friction coefficient decreased with the increase of temperature resulted in the decrease of the shear force of BMGs surface;however,the real contact area increased rapidly while surface oxidation occurred in SCLR,thus plowing effect played a dominant role,which also led to an increase in friction coefficient and wear rate at elevated temperature.In addition,the increase of temperature resulted in the transition of wear mechanism from abrasive wear and slightly plastic deformation to severe plastic deformation,accompanied with oxidation wear and microcracks.The above experimental results revealed the mechanisms of the effect of temperature and micro-patterns on the tribological properties of BMGs surfaces,which was of great significance for the development of BMGs in the field of micro-or nano-sized features and antifriction surface.