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℃.

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.

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.