Fretting Damage Behavior Of Cu/Mo Gradient Multilayers

Metallic multilayers are one of the most promising coatings for fretting damage protection due to their high hardness and good toughness.However,peak strength/hardness can be obtained when the thickness of the metal multilayer film reaches the range of several nanometers to several tens of nanometers,but this results in constrained dislocation activity and storage,significantly reduced strain hardening capacity,resulting in a significant loss of plastic deformation capacity and a tendency to brittle fracture during deformation.This strong plasticity disorder severely limits the effectiveness of fretting damage protection of metal multilayers.In order to improve the toughness of metal multilayers without reducing their peak strength/hardness,a series of Cu/Mo gradient multilayers were prepared in this paper by designing a gradient variation of the monolayer thickness of the multilayers.The influence law and mechanism of structural gradient on its strength and toughness were analyzed,and the fretting wear and fretting fatigue behaviors were studied.In particular,the hardness and toughness of Cu/Mo gradient multilayers were significantly higher than those of Cu/Mo nanomultilayers M10(with a constant thickness of about 10 nm)in a certain range of structural gradients.Meanwhile,the back stress of Cu/Mo gradient multilayers showed a trend of increasing,then decreasing and then increasing with the increase of the structural gradient.Combining the strain gradient theory with the study of nanoindentation deformation behavior,it is known that the gradient multilayer film can effectively suppress the strain localization.The gradient structure design can increase the nonuniformity during deformation,which makes the gradient multilayer film have higher back stress reinforcement effect than M10.In the mixed slip zone with a displacement amplitude of 15 μm and a normal load of 80 N,the fretting wear behavior of the Cu/Mo gradient multilayer film is more complex,and there is no obvious correlation pattern with the structural gradient,but the wear rates are significantly lower than those of M10;the gradient multilayer film is dominated by abrasive wear accompanied by fatigue wear,while M10 is dominated by fatigue wear.In the complete slip region with a displacement amplitude of 30 μm,with the increase of the structural gradient,the fretting wear mechanism of the gradient multilayers changed from fatigue wear to abrasive wear dominated,in which the wear rate of the Cu/Mo gradient multilayers with both high hardness and high toughness was lower.Under the same experimental conditions,the conventional fatigue and microfatigue lives of the preferred Cu/Mo gradient multilayer specimens were improved by 28.5%and 15.5%,respectively,compared with those of the Cu/Mo nanomultilayer M10.This is mainly due to the high hardness and good toughness of the Cu/Mo gradient multilayer film,which has better coordinated deformation and resistance to fatigue crack expansion than the nano-multilayer film M10.