Study On Dry Sliding Wear Behavior Of TiB₂/Al Composites

Wear is a main cause for the failure and energy loss of the moving mechanical components.Therefore,TiB₂/Al composites with high specific strength,stiffness and self-lubricating behavior have been paid more attention.However,the wear mechanism of the TiB₂/Al composites has not been fully understood.Moreover,it is very challenge to achieve self-lubricating property in low content TiB₂/Al composites.Therefore,in the present work,the friction behavior of the 10%,30%and 50%TiB₂/Al composites has been analyzed by universal friction and wear testing machine?UMT-3?,laser scanning confocal microscopy?LSCM?,focused ion beam?FIB?,scanning electron microscopy?SEM?,energy dispersive spectrometer?EDS?,transmission electron microscopy?TEM?,X-ray photoelectron spectroscopy?XPS?,and the effect of the endopathic?volume fraction?and exopathic?load,sliding speed,etc.?factors on the wear properties has been investigated.The microstructure evolution of the Al matrix and TiB₂particles has been discussed and the wear mechanism map of the TiB₂/Al composites has been obtained.Furthermore,based on the wear mechanism map,the effect of the h-BN particles on the wear properties of the TiB₂/Al composites has been studied.The wear behavior of the TiB₂/Al composites is significantly affected by the applied load and sliding speed.The friction coefficient of TiB₂/Al composites increases while the wear rate decreases with the increase of the applied load and sliding speed,respectively.Moreover,the wear resistance of the composites is increased of the amount of TiB₂ particles.Based on the wear rate result of the different volume fraction TiB₂/Al composites,the wear behavior has been divided into mild wear ?,mild wear ? and severe wear stages while the friction coefficient changes in order of magnitude when the wear transition occur.The microstructure of the wear surface is significantly affected by the volume fraction,applied load and sliding speed.The wear surface layers are mainly composed by the oxide layer,mechanical mixed layer,tribolayer and compacted layer.The wear surface is changed from continuous oxide layer to non-continuous oxide layer with the increase of volume fraction during the mild wear I stage.However,mechanical mixed layer is observed in the composite with 30 vol.%TiB₂particles while tribolayers are found in the composite with 10 and 50 vol.%TiB₂particles during the mild wear II stage,respectively.During the severe wear stage,the thickness and continuity of the tribolayer are increased in the 10%and 50%TiB₂/Al composites while compacted layer forms in the 30%TiB₂/Al composites due to the insufficient mechanical mixing effect.It has been found that the oxidation,plastic deformation and fracture behavior of the sub-surface are the main factors that determined the microstructure of the sub-surface.The oxide layer is mainly composed by the oxide of aluminum with amorphous structure and the refined Al grains.The friction layer is mainly composed by the TiB₂,Si₃N₄ particles,aluminum oxide formed by the friction and the severe refined Al grains.However,a mechanical mixed layer is composed by the oxide of aluminum,nano-TiB₂ and Si₃N₄ particles when the sufficient mechanical mixing occurred,and the formation of the mechanical mixed layer could delay serious wear.Furthermore,the compacted layer with high density pores and cracks form due to the compaction of the debris,which contains a lot of holes and cracks.The contact stress and temperature of wear surface show that the wear mechanism has been changed from oxidative wear+abrasive wear to adhesive wear+delaminated wear when the fracture occurs in the sub-surface of the composites.Moreover,the dislocation climbing and dynamic recrystallization mechanism in Al matrix are the main mechanisms for the transformation from the mild wear to severe wear in 10 and 50 vol.%TiB₂/Al composites.However,the high contact stress in 30 vol.%TiB₂/Al composites leads to the fracture of the compaction layer,resulting the transformation from mild wear to severe wear,while the corresponding wear mechanism is also changed to be three body abrasive wear.According to the Hertzian contact theory and Mises yield criterion,the volume fraction of?TiB₂+h-BN?/Al composite with self-lubricating property is determined to be 20 vol.%TiB₂ and 10 vol.%h-BN.The self-lubricating properties of TiB₂/Al composites are improved significantly by the addition of the h-BN particles.Compared with the 30%TiB₂/Al composites,the self-lubricating conditions of?TiB₂+h-BN?/Al composites are significantly widened,and the self-lubricating properties at low speed?0.2⁰.8m/s?are realized.