Friction of Alloys at High Temperature

A brief review is given about the friction and wear properties of high temperature alloys. Above a critical temperature, if the oxide becomes ductile, it will flow over the surface and prevent metal-to-metat contact. In order to study the tribology of oxide lubrication. a series of tests were carried out using Cu(ReO4)2 as a lubricant. The effects of time. Surface finish. substrates. load and temperature were investigated. A mechanism of lubrication is proposed in which the surface slip predominates along with mechanical attachment of oxide to the surface.

Mechanical properties and friction-wear characteristics of VN/Ag multilayer coatings with heterogeneous and transition interfaces

The heterogeneous multilayer interface of VN/Ag coatings and transition multilayer interface of VN/Ag coatings were prepared on Inconel 781 and Si(100),and the microstructures,mechanical and tribological properties were investigated from 25 to 700℃.The results showed that the surface roughness and average grain size of VN/Ag coatings with transition multilayer interface are obviously larger than those of VN/Ag coatings with heterogeneous multilayer interface.The coatings with transition multilayer interface have higher adhesion force and hardness than the coatings with heterogeneous multilayer interface,and both coatings can effectively restrict the initiation and propagation of microcracks.Both coatings have excellent self-adaptive lubricating properties with a decrease of friction coefficient as the temperature increases,but their wear rates reveal a drastic increase.The phase composition of the worn area of both coatings was investigated,which indicates that a smooth Ag,Magnéli phase(V2O5)and bimetallic oxides(Ag3VO4 and AgVO3)can be responsible to the excellent lubricity of both coatings.To sum up,the coatings with transition multilayer interface have excellent adaptive lubricating properties and can properly control the diffusion rate and release rate of the lubricating phase,indicating that they have great potential in solving the problem of friction and wear of mechanical parts.

Insight into the high-temperature tribological mechanism of VAlTiCrW high entropy alloy film:AlV_(3)O_(9)from tribochemistry

High-entropy alloys have made significant progress in high mechanical properties,wear resistance,and corrosion resistance properties.Excellent tribological properties,especially high-temperature lubrication,have become another sought performance.In this work,VAlTiCrW high-entropy alloy film with body-centered cubic(BCC)structure was prepared on superalloy substrate by magnetron sputtering.It is found that the VAlTiCrW film shows very low friction coefficient of 0.15 and a low wear rate of 10^(-5) orders of magnitude at 800℃.After 800℃oxidation,the film can still obtain a friction coefficient of no more than 0.2 at 700℃.XRD and TEM revealed the formation of ternary oxide AlV_(3)O_(9)with preferred orientation of(002)crystal plane with large spacing of 0.71 nm on the wear surface of the film,a high-temperature lubricating phase that has not been reported,realizes the low friction coefficient.This AlV_(3)O_(9)can be formed by tribochemical reaction under the thermal-mechanical action at 700℃,but pre-oxidation at 800℃is the prerequisite in order to form the precursors of V-rich and Al-rich oxide layer.

Insights on high temperature friction mechanism of multilayer ta-C films

In this work,the high temperature friction mechanism of the tetrahedral amorphous carbon(ta-C)film was elucidated.The multilayer ta-C film with alternating hard and soft sub-layers exhibited a low friction coefficient of 0.14 at 400℃ before a sudden failure occurred at 4600 cycles.The wear failure was attributed to the gradual consumption of the ta-C film at the contact region.The design of a hard or soft top layer effectively regulated the high temperature friction properties of the multilayer ta-C.The addition of a hard top layer contributed to a low friction coefficient(0.11)and a minor wear rate(4.0×10^(-7)mm^(3)/(N m)),while a soft top layer deteriorated the lubrication effect.It was proposed that the passivation of dangling bonds at the sliding interface dominated the low-friction mechanism of the ta-C film at high temperature,while the friction induced graphitization and the formation of sp^(2)-rich carbonaceous transfer layer triggered C-C inter-film bonding,resulting in serious adhesion force and lubrication failure.Moreover,the multilayer ta-C film with hard top layer obtained excellent friction performance within 500℃,while the high temperature induced oxidation and volatilization of carbon atoms led to the wear failure at 600℃.

Influence of Test Temperature on the Tensile Properties along the Thickness in a Friction Stir Welded Aluminum Alloy

The aim of this study was to evaluate microstructures and the influence of test temperature on the tensile properties, strain hardening behavior and fracture characteristics of friction stir welded （FSWed） 2219- T62 aluminum alloy thick plate joints. A fine and equiaxed recrystallized grain structure had no significant change in grains at the top of weld nugget zone （WNZ） at a rotational rate of 500 r/min compared with 300 r/min, but the grains and second-phase particles at the middle of WNZ exhibited obvious coarsen- ing. The yield strength, ultimate tensile strength and joint efficiency were observed to decrease with increasing test temperatures. However, the elongation presented a contrast trend. Compared with the middle and bottom slices, the top slice （216 and 342 MPa） had a higher strength and a lower elongation （8.5%） at different test temperatures. Hardening capacity and strain hardening exponent of bottom slices were higher than those of the top and middle slices. Both of them at room temperature （RT） were bigger than those at higher temperature （HT） and lower temperature （LT）. The FSWed joints basically failed in the border area between the thermo-mechanical affected zone （TMAZ） and heat-affected zone （HAZ） of the top slice, and in the HAZ of the middle or bottom slices, while the fracture surfaces exhibited dimple fracture characteristics at different test temperatures.

Tribological response of an epoxy matrix filled with graphite and/or carbon nanotubes

Reinforced polymer-matrix composites are widely used under sliding contact conditions in various boating and automotive applications.In this paper,the friction and wear of bulk epoxy and carbon filler reinforced epoxy composites have been investigated using a pin-on-disc tribometer.The effect of different fillers on the tribological behavior of an epoxy has been studied using treated and untreated carbon nanotubes,graphite,and a mixture of graphite and carbon nanotubes.Filler addition greatly enhances the tribological properties of the epoxy resin,by reducing the friction coefficient and the wear rate.In addition,it was found that the treated carbon nanotubes/epoxy composites have the best tribological behavior.Moreover,a correlation between contact temperature and friction coefficient is reported.Finally the wear mechanisms were determined by scanning electronic microscopy.

Comparative study on local and global mechanical properties of bobbin tool and conventional friction stir welded 7085-T7452 aluminum thick plate

7085-T7452 plates with a thickness of 12 mm were welded by conventional single side and bobbin tool friction stir welding （SS-FSW and BB-FSW, respectively） at different welding parameters. The temperature distribution, microstructure evolution and mechanical properties of joints along the thickness direction were investigated, and digital image correlation （DIC） was utilized to evaluate quantitatively the deformation of different zones during tensile tests. The results indicated that heat-affected zone （HAZ）, the local softening region, was responsible for the early plastic deformation and also the fracture location for SS-FSW samples, while a rapid fracture was observed in weld nugget zone （WNZ） before yield behavior for all BB-FSW specimens. The ultimate tensile strength （UTS） of SS-FSW joints presented the highest value of 410 MPa, 82% of the base material, at a rotational speed of 300 rpm and welding speed of 60 mm/min, much higher than that of BB-FSW joints, with a joint efficiency of only 47%. This should be attributed to the Lazy S defect produced by a larger extent of heat input during the BB-FSW process, The whole joint exhibited a much higher elongation than the slices. Scanning electron microscopic （SEM） analysis of the fracture morphologies showed that joints failed through ductile fracture for SS-FSW and brittle fracture for BB-FSW.