The origin of superhardness in TiN/Si₃N₄ nanocomposites: the role of the interfacial monolayer

Abstract

Recent progress in the understanding of the origin of hardness enhancement in superhard nc-TiN/a-Si₃N₄ nanocomposites (‘nc-’ stands for ‘nanocrystalline’, ‘a-’ for ‘X-Ray amorphous’) is summarized. The optimum nanostructure with one monolayer of interfacial Si ₃N ₄ was confirmed by both experiments with the deposition of TiN/Si₃N₄ heterostructures by Söderberg et al. and by first principle density functional theory calculations of Hao et al. These calculations show that the decohe-sion energy of the TiN/Si ₃N ₄/TiN sandwich is enhanced as compared with bulk Si ₃N ₄. A new, non-linear constitutive materials model, which accounts for the pressure enhancement of elastic moduli and of flow stress has been developed and tested by means of advanced FEM calculations. Significant deviation of the mechanical behaviour of super- and ultrahard materials from the prediction of linear mechanics with a constant yield stress has been demonstrated.

Keywords:

• Superhard nanocomposites
• Origin of superhardness
• Interfacial monolayer
• TiN Si ₃N ₄

Acknowledgements

The work reported in this paper was financially supported by the European Commission within the 6th Framework Programme under Contract No. AST 3-CT-2003-502741—‘MACHERENA’, and Contract No. CA 505549-1—‘DESHNAF’. I would like to thank all my coworkers who participated in the research work on superhard nanocomposites, and Professor Magnus Odén, Professor Lars Hultman, Dr. Shiqiang Hao and Professor Catherine Stampfl for providing us with their results prior to publication.

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Stan Veprek

Email: [email protected]