Structure and mechanical property of boundary lubricating film formed in belt-type CVT oil

摘要

Cross-sectional transmission electron microscopy (TEM) was used to study the crystal structure of boundary lubricating films formed during the sliding test in belt-type CVT oil using vane-on-disc type tester. The boundary lubricating films formed at various normal loads were from 10 to 80nm thick. It was observed that every film consisted of one inhomogeneous layer containing P, O, S, Ca, Zn and Fe. HRTEM images and electron diffraction indicated that the layer was a mixture of fine (5-10nm) crystalline particles and amorphous matrix at lower normal loads, while at higher normal loads only fine crystalline grains were observed. Nano-indentation technique was also used to evaluate the mechanical properties of the boundary lubricating film formed at the normal load of 300N. The hardness of the film was compared with that of soft metals such as Ag, Zn, Sn and In. It was found that the hardness of that boundary lubricating film was located in between that of Ag and In. It was concluded that there is a strong correlation between the crystalline state of the boundary lubricating film and the friction coefficient.