Research On Tribology Performance And Friction Layer Of Ni₃Al Matrix Self-lubricating Composites With Graphene

Solid self-lubricating composite materials have attracted scientific interest to fulfill the demands for anti-friction and wear resistance of engineering machinery under the extreme conditions such as high-temperature,high-sliding velocity and high-load.However,the study of anti-wear and friction-reduction mechanisms of solid self-lubricating composite materials still exists many deficiencies over the world.By adding graphene into Ni3Al matrix,a new type of Ni3Al matrix self-lubricating composites?NMCs?is fabricated successfully in this paper.The relationship between the macroscopic tribological behavior and the micro-structure of NMCs is investigated.The state of existence and acting mechanism of graphene are studied.The formation and acting mechanisms of friction layer are also discussed in this paper.The concrete results are as following:Working conditions have great effect on the tribology performance of NMCs during the process of dry sliding friction.The effects of counterface balls,loads and sliding velocities on the tribology performance of NMCs are studied in this paper.When the hardness of counterface balls is close to that of NMCs,NMCs have the better tribology performance.Within the scope of working conditions?2-14N,0.05-0.8m/s?in this paper:NMCs have larger friction coefficients and the lowest wear rates under the conditions of lower loads and sliding velocities;NMCs have lower friction coefficients and larger wear rates under the conditions of lower loads and larger sliding velocities;NMCs have lower friction coefficients and the lowest wear rates under the conditions of larger loads and lower sliding velocities;NMCs have the lowest friction coefficients and lower wear rates under the conditions of larger loads and sliding velocities.Further research shows that there is a close relation between the macroscopic tribological behavior and the subsurface micro/nano-structural evolution of NMCs during the process of dry sliding friction.When NMCs have better tribology performance,the subsurfaces of NMCs show the similar structure.The stable friction layer can be defined as two layers:the lubricating layer and the friction-induced layer.Of which,the lubricating layer can be easily sheared,resulting in lower friction coefficient.The friction-induced layer can help to withstand the loads and protect the sample,resulting in the lower wear rate.The state of existence and acting mechanism of graphene are studied during the process of dry sliding friction in this paper.The graphene near by the wear scar is parallel to the worn surface.The results show that obvious changes of the structure of graphene have taken place.The effecting mechanism of graphene is different from that of traditional solid lubricants such as graphite and MoS₂.The graphene,which has particular two-dimension sheet structure and high fracture roughness,tiles on the worn surface of NMCs.On one hand,the graphene can prevent the hard protrusions of GCr15 steel from being pressed into the substrate,leading to the lower wear rate.On the other hand,the graphene can reduce the shear force of contact interface,resulting in the lower friction coefficient.In order to further reveal the cause of improvement of tribology performance,the formation and effecting mechanisms of friction layer are analyzed with the friction coefficient curves and the cross-sections of worn scars of NMCs.The results show that the main forming factors of friction layer is work hardening.The formation of friction layer can be defined as three processes:the process of work hardening;the formation of friction-induced layer;and the formation of friction layer,including the accumulation of friction-induced layer and the formation of lubricating layer under the protection of friction-induced layer.The results of this paper can be used to investigate the graphene which is a new solid lubricant with excellent tribology performance,to analyze the friction reduction and anti-wear mechanisms of NMCs,and to provide a theoretical support to industrial applications of NMCs.