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Environment-dependent friction, Raman and µ-RBS study of ta-C coatings deposited by filtered Laser arc

Krause, M.; Härtwig, F.; Munnik, F.; Garcia Valenzuela, A.; Makowski, S.; Lorenz, L.

Tetrahedral, hydrogen-free amorphous carbon (ta-C) is characterized by the highest sp3-carbon content and the highest hardness of HIT > 40 GPa of the group of diamond-like carbon (DLC) coatings [1]. The formation of sp3-C bonds in this material requires impinging atoms or ions with energies of around 100 eV, provided by cathodic vacuum arc or laser arc deposition [2]. Plasma filter techniques implemented in the recent years enable the production of ta-C coatings with significantly improved surface quality.
In this study, friction and wear properties of ta-C coatings deposited by filtered Laser arc are studied as a function of the environment: in humid and dry air, in nitrogen as well as in vacuum from 1 mbar to 10-7 mbar. Low friction with coefficients of friction (COF) < 0.1 is found for the friction pair ta-C/ ta-C at normal pressure, nearly independently on the relative humidity. Likewise, the wear rates are not dependent on whether dry or humid conditions are established. In vacuum, the COF and wear rates increased by a factor of approx. ten and three, respectively. Raman studies reveal a complex structure evolution of ta-C in the wear track and on the contact area of the counter body with decreasing pressure. In the wear tracks at least two types of carbon are found. One of them shows an almost unchanged G line position and an unmeasurable ID/IG ratio as the initial coating, while the second one has a by 20 cm-1 lowered G line position and an ID/IG ratio of approx. 0.4. This structure has similar Raman signatures as the counter body contact area, indicating the formation of an identical tribolayer on both friction partners. Laterally and depth resolved atomic insight in the transfer layer formation is obtained by micro-beam Rutherford backscattering spectrometry for ta-C coatings in contact with steel, brass, alumina, and silicon carbide.
Financial support by the DFG, grant No. 415726702, project TRIGUS, is gratefully acknowledged.
[1] J. Robertson, Diamond-like amorphous carbon. Materials Science & Engineering R-Reports 37, 129-281, doi:10.1016/s0927-796x(02)00005-0 (2002).
[2] F. Kaulfuss, et al. Effect of Energy and Temperature on Tetrahedral Amorphous Carbon Coatings Deposited by Filtered Laser-Arc. Materials 14, 13, doi:10.3390/ma14092176 (2021).

Keywords: ta-C; diamond-like coatings; wear; friction; µ-RBS; Raman spectroscopy

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