Superlow friction of ta-C lubricated by glycerol: An electron energy loss spectroscopy study

Abstract

Energy-filtering transmission electron microscopy (EFTEM) analysis coupled with the technique of samples preparation, focused ion beam, was used to study physical, chemical, and mechanical properties of diamond-like carbon coatings (DLCs). Two different coatings (ta-C and a-C:H) were investigated, presenting different tribological behaviors in a boundary lubrication regime with glycerol. Electron energy loss spectroscopy appears to be a very powerful technique to characterize such DLC coatings. Special attention was paid to the maximum energy of the plasmon peak, which was used to evaluate some physical and mechanical properties of DLC coatings (density, sp3∕sp2 ratio, hardness). For ta-C superlubric coating, EFTEM results show a rearrangement of the DLC bulk structure under the friction process. Typically, the transformation of sp3 carbon into sp2 carbon was clearly observed and permits a self-adaptation of the coating, allowing it to support shearing without any delamination in spite of important compressive residual stresses in the coating. Moreover, the formation of graphitic carbon is evidenced at the top surface. This graphitic layer certainly helps the lubrication by glycerol in forming OH termination.

Energy filtering transmission electron microscopy and atomistic simulations of tribo-induced hybridization change of nanocrystalline diamond coating

“…It can be seen that the thickness of the sp 2 -rich carbon layer at the top surface has increased from 10 to 15 nm to about 40 nm after the lubricated test. The sp 2 hybridization change of sliding diamond-like carbon surfaces under ultralow friction was also observed for amorphous ta-C coatings but with a larger thickness of 50 nm [13].…”

“…Nevertheless, strong structural transformations of the bulk of the diamond coating under friction cannot be excluded. Previous studies using TEM imaging coupled with electron energy loss spectroscopy (EELS) have evidenced major structural changes for hard amorphous ta-C films, consisting in a clear carbon hybridization change from sp 3 to sp 2 , after ultralow friction in the presence of glycerol [13]. The low-density sp 2 rich amorphous phase is a manifestation of a tribochemically transformed phase of matter that is found at the sliding interface in many triboystems [14].…”

Influence of surface passivation on the friction and wear behavior of ultrananocrystalline diamond and tetrahedral amorphous carbon thin films

“…For both UNCD and ta-C, the work performed [12][13][14][15][16][17][18][19][20] consistently shows that tribochemical reactions -surface chemical modifications induced by sliding contact -have a strong influence on wear. This is true not only for the flat surface of a tribological test, but also for the counterface.…”

Impact of laser power density on tribological properties of Pulsed Laser Deposited DLC films

“…The short range order in DLC film allows a relaxation of the selection rules for transitions and the energy levels are smeared. 54 In Fig. 6, the energy loss relationship of π -π * and σ -σ * states are calculated and quantitative results of sp 2 fractions are given in the Table I.…”