Influence Of Metal-coated Graphite Powders On Microstructure And Properties Of The Bronze-matrix/graphite Composites

Bronze-matrix graphite self-lubricating composite material is a composite material consisting of graphite particles and the matrix of copper alloy.It not only has the advantages of high strength,high conductivity,high thermal conductivity,arc erosion and other excellent properties of copper matrix,but also has good self-lubrication property of graphite.It has excellent comprehensive properties,and is an ideal material for the preparation of modern self-lubricating friction parts and electric contact conductive parts,has been widely used in transportation,military,aerospace,electronics and other fields.However,the bronze matrix and graphite particles are almost non wetting at high temperature and the effect of the interfacial bonding between the copper matrix and graphite particles is poor after high temperature sintering,it restricts the application and promotion of copper matrix graphite composites.In this paper,the bronze-matrix/graphite composites were fabricated by powder metallurgy route by using Cu-coated graphite,Ni-coated graphite and pure graphite respectively.The microstructure,mechanical properties and corrosive behaviors of bronze/Cu-coatedgraphite(BCG),bronze/Ni-coated-graphite(BNG)and bronze/pure-graphite(BPG)were characterized and investigated by using SEM,EDS,XRD and wear testing machine(MGW-02).The effects of graphite content on the density,micro hardness and friction and wear properties of copper matrix graphite composites were studied by changing the content of graphite(0%Gr,1%Gr,3%Gr,5%Gr).Meanwhile,the friction and wear properties of three kinds of graphite copper matrix composites under different wear pressure(10N,20 N,40N,60N)were analyzed.The results showed that:(1)It could be found that the Vickers microhardness of the BPG and BCG composites decreased as the graphite particulate content increased,which is mainly affected by the change of ultimate density.Although the ultimate density of BNG composite may decrease with the increase of graphite particulate content,the Vickers microhardness still increased when the graphite particulate content gradually increased from 0 wt.% to 5 wt.%.The existence of nickel and the intermetallic compounds in the BNG composite may improve the Vickers microhardness.(2)The less porosity content and better bonding between the bronze matrix and graphite particles in BCG composite could be attributed to the encapsulation of Cu-coated graphite,which leads to the formation of Cu/Cu contacts between two different particles during cold pressing and sintering behaved like the consolidation process of pure copper powder.The principle mentioned above also fitted for the BNG composite,and the formation of Ni17Sn3 and Cu9NiSn3 intermetallic compounds was beneficial for the combination between the bronze matrix and Ni-coated graphite during sintering and resintering.(3)It is obvious that the friction coefficient of the BPG composite(0.10~0.125)is lower than that of BCG and BNG composites(0.125~0.15)at the beginning of friction and wear.However,it can be found that the friction coefficient of the BCG and BNG composites was more stable and lower than BPG composite at the later of wear test.(4)The coefficient of friction decreased as the amount of graphite added into the composites BPG,BCG and BNG increased.The presence of smeared graphite layer on sliding surface acted as a solid lubricant and reduced the contact area between the pin and counterface.With the increase of graphite content,the graphite layer is easily to be formed in the contact area.The wear rate of the BCG and BNG composites stayed at a lower level than that of BPG under same graphite content.(5)Due to the good bonding strength between the nickel coated graphite and bronze matrix and the high microhardness of the specimen surface,the wear rate and the friction stability of the BNG can be maintained in the process of friction and wear,and BNG can bear greater load in the process of wear and tear,which can be used under the condition of higher load.(6)The electrochemical polarization curve test in 3.5% NaCl solution show that the corrosion potential(Ecorr)of BCG and BNG is shifted to-0.308 V and-0.279 V,which is positively moved about 24 mV and 29 mV compared with BPG,respectively.Meanwhile,the self-corrosion electrical current density of BNG is about two orders of magnitude lower than that of BPG and BCG,and the corrosion resistance is improved greatly.The improvement of corrosion resistance provides a broader prospect for the application of BNG.