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℃.