Study On Self-lubrication Mechanism For TiB₂/Al Composite

Self-lubricating material is generally used for the solid lubricant as lubricating phase,the metal material as the carrier body, usually a large amount of the solid lubricant is neededto satisfy the lubricating requirment. But the mechanical properties and self-lubrication cannot be satisfied at the same time, therefore one kind of material, which has good mechanicalstrength and self-lubrication is needed.Zhao min has found TiB₂/Al composites can meet the above requirements, but did notanalysis its self-lubricating mechanism in depth. The purpose of this thesis is to investigatethe self-lubricating mechanism of TiB₂/Al composite in detail.The reinforcement TiB₂particles are pre-oxidized, and the pre-oxidation temperature isoptimized, and then TiB₂/Al composites are fabricated by the pressure infiltration method.Microstructure is analyzed by using scanning electron microscopy （SEM）, transmissionelectron microscopy （TEM）, high-resolution transmission electron microscopy （HRTEM）.Tribological properties were tested using CJS111A and WTM-2E type pin-on-disc weartester, and tribological mechanism was discussed.For tribology, there are system dependence and time dependence. The tribologicalsystem is divided into internal and external factors for TiB₂/Al-GCr15system. The internalfactors, including the composite interface and volume fraction; external factors, includingload, sliding speed, surface roughness, vacuum and argon atmosphere. Both factors areinvestigated in detail. Finally, through the study of time-dependence of coefficient offriction, the self-lubricating mechanism of TiB₂/Al composite is deduced..Microstructure observation showed that TiB₂particles after pre-heating were coated byone layer （0.02-0.03μm） of mixed oxide composed of TiO2and B₂O₃. Furthermore, B₂O₃reacted with H2O from the air to generate H₃BO₃, which has good lubrication. This wasthe intrinsic self lubricating factor for TiB₂/Al composites.The investigation of volume fraction showed that there were critical values:non-adhesion wear volume fraction （24.3%） and self lubricating volume fraction （31.6%）.And theoretical calculation showed that the critical volume fraction of non-adhesive wearand self-lubricating,24.3%and31.6%, respectively. Volume fraction of less than24.3%donot have self-lubricating properties; higher than31.6%, self-lubrication is expected;between the two values is the transition zone, self-lubricating phenomenon is affected bythe test conditions.0.49,0.98,1.96,3.92N were selected to investigate the effect of load on tribologicalproperties. Test results show that when load>0.49N, self-lubricating characteristics do notappear, load=0.49N, the material showed self-lubrication. It can be deduced: Duringsliding, the lubricating film of the surface is thin, it can not reduce friction effectively. The microstructure observed after pre-oxidation of the surface oxide layer showed a thickness of20-30μm, which can not withstand a greater load.0.2,0.5,0.8,1.0,1.5,2.0m/s were selected as the experimental conditions toinvestigate the effect of speed on tribological properties. The results show that1) As thespeed increases, the coefficient of friction is from0.5to0.2. Wear mechanism was fromadhesive wear to oxidation wear.2) material has lubricant by itself, introduced bypre-oxidation of reinforcement, but still do not have self-lubricating properties at thelow-speed, which means the maintaining of self-lubricating properties needs a large numberof lubricant, the supply of pre-oxidation of TiB₂oxidized is not enough.3) there wasself-lubricating film in the wear surface, local distributed.Effect of surface roughness of GCr15steel. There was one optimal surface roughnessfor self-lubrication, and the value was related to the thickness of oxide coated TiB₂. SetTiB₂surface oxide thickness as h, surface roughness was Ra. Then, the size of optimalsurface roughness is in the same magnitude of TiO2and B₂O₃layer thickness, equivalent to0.02-0.03μm, i.e. Ra⁰.020-0.030μm.1) Ra>> h, corresponding to Ra0.606μm andRa0.372μm. Oxide layer can not fill the asperity, resulting in a large number of furrows.2)Ra^h, corresponding to Ra0.023μm. The oxide layer just fill asperities on the disc gap, toprevent the adhesion of the disc with the matrix alloy, reducing the friction.3) Ra <<h,corresponding to Ra0.005μm. The oxide layer can fill the disc asperity gap, but the surfaceis too smooth, so that the molecular mechanism is dominant, and the actual contact area isgreatly increased, therefore increased friction.The introduction of pre-oxidation of the oxide layer is embodied in the curve of thecoefficient of friction. Real-time changes in the Coefficient of friction curve, reducingor increasing was an external performance for different factors influencing friction.Coefficient of friction curves in different load, sliding speed, surface roughness wereanalyzed, results show that the pre-oxidation of TiB₂particles influence the coefficientof friction in the initial stage of the friction curve. In a stable stage, the frictional heataccumulation enhanced TiB₂oxidation, providing a steady source of lubricant to reducefriction. For TiB₂/Al-GCr15system, during self-lubricating period, there are twosources of lubricant: pre-oxidation of TiB₂and oxidation of the reinforcement TiB₂during sliding.