| Due to the high hardness,high wear resistance and good toughness of Polycrystalline diamond(PCD),it can be used for geological drilling,cutting and other fields.During the drilling process,it is difficult to ensure that the PCD is always in a liquid good lubrication condition.Howerer,the humidity atmosphere interface will seriously affect the drilling or cutting efficiency.What’ more,the tribochemistry will affect the wear of materials.So,this paper tested the tribological behaviors of polycrystalline diamond(PCD)the top materials of polycrystalline diamond compacts via high temperature/high pressure process sintered.The tribological measurements conducted in various relative humidity(RH)levels(5%-85% RH),10 N load,20 N,rotate speed 400 rpm and testing time 30 minutes conditions using silicon nitride(Si3N4)spheres as sliding counterbodies.After tribotests,the analysis of the effects of surface dangling bond passivation,carbon rehybridization,the formation of interface carbonaceous transfer films via the measurements of optical microscope,three-dimensional topography,Raman,AFM and SEM&EDS etc.In order to establish a relationship among real service environment factors,the surface-interface effects tribochemical effects of PCD and micro failure mechanism.The total study results is as follows.(1)Under different RH levels(5%-50%),the formation of carbonaceous transfer films on Si3N4 wear scars is the main role of influencing the steady-state friction coefficient.Under 5% RH,the longest run-in periods ~ 4000 cycles can ensrue the formation of the ordered,continues,and homogeneous transfer film,which can significantly decrease the adhensive force across the slidng interface.At last,the friction coefficient reach an ultrlow value ~ 0.04.Under 10%-50% RH levels conditions,there is enough water molecules concentration and the sliding interface can form –H,-OH and H2 O passivated surface.However,the formation of carbonaceous transfer film can be restrained by surface dangling passimation effect.Besidis,under dry atmosphere ~ 5% RH,PCD obtain the most serious wear rate ~ 9.1 × 10-11mm3/Nmm.(2)Under high RH levels(55%-85%),the high concentration of water molecule can be very easily absorbed to the sliding interface by physical absorption.A liquid bridge can be formed across the interface by capillary condensate effect.And then,the meniscus force increased at sliding interface.Therefore,the tribotest system reach an instable and high steady-state friction coefficient ~ 0.105.However,under water lubrication condition,the steady-state friction coefficient reach the lowest value~ 0.06.The effective water molecules lubreication film can form across the sliding interface.The lubrication film can effectively decrease the friction force and then the system reaches the lowest friction coefficient.For the wear of friction pairs,there are different diameters of wear scar formed.With the increasing of RH levels(55%-85%),the diameters of wear scar effectively increased.Under 85% RH level,the wear scar reached the largest one.However,under water lubrication,a minimum wear scar formed.(3)In conclusion,the dangling bonds passivation effects affect the covalent interaction at the sliding interface corresponding to the length of run-in period and the wear rates of PCD surface.Finally,the longer run-in period and higher wear loss of PCD assure the transfer film effectually form,and the real factor of showing low steady-state friction coefficient is the efficient and continuous transfer flms formation.What’s more,under RH levels(55%-85%)conditions,the the wear rates of Si3N4 can be controlled by various ratios of Si-N/Si-OH+Si-O and Si-OH/Si-O. |
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