Investigations into the Phase Segregation, Formation of nanostructure with a high thermal stability and origin of the hardness enhancement in Superhard Nanocomposites consisting of a stable, hard, transition metal nitride and Si3N4 or BN as interfacial phase by means of Thermodynamic and ab initio DFT Calculations
Final Report Abstract
In summary, we achieved the goals originally planned in our proposal and conducted several additional studies which broadened up the output of our work and contributed to a more general understanding of the origin of hardness enhancement in superhard nanocomposites and heterostructures. The focus of the first part of our work has been the combined ab initio DFT calculation and thermodynamic modelling in order to study the possibility of spinodal decomposition and formation of nanocomposites with high thermal stability. Besides of the Ti-Si-N and Ti-B-N systems which have been published, we studied also several other nitride and oxide systems. These results are summarized in the Ph. D. Thesis of S.H. Sheng and in preparation for publications. Using ab initio DFT we conducted in depth studies of the mechanical properties and of the mechanism of tensile de-cohesion and shear deformation for heterostructures consisting of few nm thick TiN slabs with one monolayer of interfacial SiN. It has been shown that, although the bulk fcc-SiN is unstable, the I ML SiN layer is strongly heteroepitaxially stabilized. The weakest links are the Ti-N bonds adjacent to that interface where the de-cohesion and shear occur. The heterostructures (and nanocomposites) with I ML SiN are stronger than those with 2 ML because of increasing weakening of the Ti-N bonds in the latter case. Moreover, we have clarified why ReB2 cannot be superhard, and that the recently reported high hardness of c-BCs is not an intrinsic property but a result of its nanosize nature. These results are very important in the general discussion of how to design strong materials.
Publications
- Stability of Ti-B-N solid solutions and the formation of nc-TiN/a-BN nanocomposites studied by combined ab initio and thermodynamic calculations. Acta Materialia, 56, 4440 (2008)
R. F. Zhang, S. H. Sheng and S. Veprek
- Electronic structure, stability, and mechanism of the de-cohesion and shear of interfaces in superhard nanocomposites and heterostructures, Phys. Rev B 79, 245426 (2009)
R. F. Zhang, A. S. Argon and S. Veprek
- Friedel oscillations are limiting the strength of superhard nanocomposites and heterostructures, Phys. Rev. Lett. 102, 015503 (2009)
R. F. Zhang, A. S. Argon and S. Veprek
- Understanding why the thinnest SiNx interface in transition metal nitrides is stronger than ideal, bulk crystal, Phys. Rev B 81, 245418 (2010)
R. F. Zhang, A. S. Argon and S. Veprek
