MS-3-P-2424 Physicochemical issues of hardening mechanisms in Me-N – based nancomposite coatings

The investigated material is a ZrN-SiN thin film with different amount of the SiN phase. This type of structure ensures high hardness of up to 60 GPa. The addition of elements such as X = Al, Cr or O causing an improvement of the oxidation resistance. The aim of this study is to determine the effect of the structure of interfaces between the crystalline ZrXN grains and the amorphous SiN matrix at the atomic level as well as of the chemical composition on the hardness of the deposited coatings. The nanocomposite is a 3D structure, thus even in a TEM lamellas as thin as 50 nm several ZrXN grains will overlap. Thus unambiguous imaging and interpretation of the interface structure between the crystalline grains and the amorphous matrix is impossible. Therefore we proposed to grow a model multilayer system consisting of single-crystalline layers of ZrXN separated by amorphous layer of SiN with thickness corresponding to 1,…,9 monolayers, Fig 1. These multilayers allowed to visualize the atomic structure of the interfaces between the ZrXN and the SiN phases. It was also possible to determine from which thickness the SiN layer really grows amorphous. This 2-D system was chosen to test the premises for different models of superhardening effects observed in nanocomposite systems. The high hardness of MeN-XN layers is related to their nanocomposite structure. A review of the literature is not clear about the effect of the interfaces between the crystalline grains and the amorphous matrix on the hardness. There are two contradictory theories explaining the high hardness. The first one states that the XN matrix around the crystalline MeN grains should be amorphous starting from the first monolayer [1]. The second theory explains the high hardness by forming epitaxial crystallized XN matrix having a thickness of 1-2 monolayers around crystal grains of MeN phase [2]. The obtained structural results for the multilayer model system will be extrapolated to the 3D structure of nanocomposites.

Multilayers coatings were deposited by magnetron sputtering on single crystalline substrates. The identification of the average grain size for different contents of XN phase in nanocomposites was made by XRD. The microstructural investigation was performed using transmission electron microscopy and scanning transmission electron microscopy on a Tecnai Osiris. The chemical analysis was accomplished by energy dispersive X-ray, and the TEM samples, were prepared by mechanical grinding and as a final step the samples were ion milled.

References

[1] S. Veprek and M G J Veprek-Heijman. Surf. & Coat. Technol. 201(13) p. 6064 (2007).

[2] L. Hultman, J Bareno, A Flink, H Soederberg, K Larsson, V Petrova, M Oden, J E Greene and I Petrov, Phys. Rev. B, Vol. 75, No. 15, p. 155437 (2007).