Process analysis of cracking a-C:H/CNP/a-C:H sandwich films under stress using nanoindentation*

Stress reduction has been a topic of great interest for applications of hydrogenated amorphous carbon (a-C:H), which play an important material for protective coating. However, increasing mass density and thickness of films resulted in cracking the films due to their stress. So far, we have succeeded in reducing the film stress by fabricating a sandwich structure of a-C:H/carbon nanoparticle(CNP)/a-C:H [1]. The stress for 8.9 % in the CNP coverage decreased 35.8 % of that for the films without CNP insertion. Here, we studied the cracking process of the films by performing nanoindentation on sandwiched a-C:H films to elucidate the mechanism of the stress reduction by CNP insertion. We performed nanoindentation to apply the external stress to our sandwiched films. We found that a displacement jump occurs at a minimum of 5 mN for coverage of 4.4 % and 10 mN for coverage of 8.9 %. The displacement at the displacement jump was approximately equal to the thickness of the second layer of the a-C:H film. These results suggested that the adhesion of the film is increased by the CNPs layer, and cracking occurs at the interface between two the a-C:H layers.

 

[1] S. H. Hwang et al., Jpn. J. Appl Phys. 59, 100906 (2020).