Failure mechanisms of a tungsten-modified hydrogenated amorphous carbon coating in load-scanning tests

Titelangaben

Hetzner, Harald ; Schaufler, Jens ; Tremmel, Stephan ; Durst, Karsten ; Wartzack, Sandro:
Failure mechanisms of a tungsten-modified hydrogenated amorphous carbon coating in load-scanning tests.
In: Surface and Coatings Technology. Bd. 212 (2012) . - S. 46-54.
ISSN 1879-3347
DOI: https://doi.org/10.1016/j.surfcoat.2012.09.012

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Abstract

Polished cylindrical specimens of a hardened and tempered cold work steel were coated with a tungsten-modified hydrogenated amorphous carbon coating (a-C:H:W). On a load-scanning test rig, tribological-mechanical tests under dry conditions with coated specimens sliding against identical, but uncoated specimens were performed. Additionally, comparative tests with a-C:H:W coated specimens sliding against a-C:H:W coated specimens were carried out. During each test cycle, the normal load was gradually increased from 13 to 350. N (Hertzian contact pressure of 1.0 to 3.0. GPa). The coefficient of friction was monitored as a function of the normal load, with a significant increase in friction indicating failure of the coating. These tests were repeated and stopped at different total numbers of load cycles. After the tests, a FIB-assisted microscopical analysis in terms of wear and damage of the coating was performed, revealing the (subsurface) failure mechanisms. The results obtained show that at Hertzian pressures of more than 2. GPa and after about eight load cycles, failure of the a-C:H:W coating is initiated by locally accumulated adhesion of steel flakes, repeatedly occurring at random contact pressures in this load range. The adherence of steel goes along with a local increase in friction, inducing the formation of small shear cracks in the coating top layer. Continued crack formation finally results in partial delamination of coating material and disruption of the surface, in affected areas only. Reference tests with the pairing a-C:H:W/a-C:H:W underline that, very much unlike pure hydrogenated amorphous carbon (a-C:H), a-C:H:W is a rather ductile material with the ability to stick to itself. A discussion of this study's results against the background of a similarly conducted previous study of an a-C:H-based coating system reveals some similarities, but also significant differences in the tribological and mechanical behaviour of both types of amorphous carbon.