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Study On Friction And Wear Properties Of CNTs-Ag-G Composites With Electrical Current

Posted on:2010-03-13Degree:DoctorType:Dissertation
Country:ChinaCandidate:J WangFull Text:PDF
GTID:1101360275977804Subject:Materials science
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CNTs are thought as perfect enhancements for composites for their attractive mechanical property and good electrical and thermal conductivity. In this paper, the CNT was covered with a layer of silver to enhance the strength of interface between CNT and Ag matrix and improve the distribution of CNT in composites. Because of small size and big curvature, CNT was apt to twist togther and very difficult to coat a layer of silver on the surface of CNT. It was found that, the activation of CNT would be enhanced after grinding, oxidization, sensitization and activation.The CNTs-Ag-G composites were fabricated by means of powder metallurgy method. The process included mixing powder, pressing at 200MPa, sintering for 1h in pure H2 protective atmosphere at 700℃and repressing at 400MPa. Adding graphite and CNTs could guarantee the self-lubricating property and good strength and hardness of composites.The effects of graphite and CNTs content on the microstructure and performances of composites were studied. Firstly, keeping CNTs mass fraction as 1%, the composites were added graphite with mass fraction of 8%, 10%, 13%, 15% and 18%, respectively. The results showed that the relative density and Brinell hardness decreased while the electrical resistivity increased with the increase of graphite content. And the performances of composites after repressing were much better than composites before repressing. Secondly,remain silver volume fraction as 65%, using carbon nanotubes replaced graphite partially and the CNTs volume fraction were 0%, 2%, 4%, 6% and 8%, respectively. The resultes showed that when having a little CNTs, the relative density and hardness was increased with the CNTs volume fraction while the electrical resistivity continuously decreased. While exceeding 6%, aggregation of CNTs brought about the decrease of relative density and Brinell hardness.The influences of elecrrical polarity, current density, content of graphite and CNTs on contact voltage drop were discussed. The results showed that, during electrical wear process, the contact voltage drop of both positive and negative brushes had the same trend: be stable after first increase, and the contact voltage of negative brush was always bigger than positive brush. Contact voltage drop had a slight increase (non-linear relationship) with the increase of current density which meet the requirements of electric power brush. The contact voltage drop first increased then tended to be stable with the increasing of graphite mass fraction. Thirdly, the influence of CNTs content on contact voltage drop of composite was studied. The results showed that, the contact voltage drop increased with the increasing of CNTs content, and it would decreased with continuous increasing of CNTs content.The effect of graphite and CNTs content and current density on the friction coefficient of composites was studied. The results showed that, with the increasing of graphite content, the friction coefficient persisted decrease and the friction coefficient in electrical wear was bigger than that in mechanical wear. This because that, with the increasing of graphite mass fraction, the lubricating film beame more integrity, and this layer of film was easier to be ruptured in electrical wear. With the increasing of CNT content, the friction coefficient of composites was decrease after first increase and the maximum value was obtained when adding 6vol% CNTs. The electrical friction coefficient was mutiple bigger than that of mechanical friction coefficient. With the increase of current density the friction coefficient had a slight incrase and got a bigger value in the initial stage of friction process and then decrease until stabile.The wear mass loss of composites influenced by content of graphite and CNTs, electrical polarity and current density was also studied. The results showed that, wear mass loss of compostie was increase after first decrease with the increasing of graphite mass fraction, and the minimum value was obtained when composite having about 13wt% graphite. This was caused by the transition of wear mechanism with increasing of graphite content. The wear resistance was improved when adding a little CNTs (less than 6vol%) and the wear mass loss decreased rapidly. While the content of CNTs bigger than 6vol%, wear mass loss had a tendency to increase because of the appearance of tangled CNTs in matrix and weakness of CNTs reinforcement. Furthermore,the wear mass loss increased with the increasing of current density. Polar differences were existence in electrical wear, that is, the wear mass loss of positive brush was always greater than the negative brush and with increasing of current density the mass loss of positive brush increased multiply while the mass loss of negative brush had a little relationship with current density. And this kind of polar difference became more apparent with the current density increases.
Keywords/Search Tags:CNTs-Ag-G composite, electrical polarity, current density, graphite content, CNTs content, contact voltage drop, friction coefficient, wear mass loss
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