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Hofmann, J., & Veprek, S. (1998). Ultra thin 3C-SiC pseudomorphic films on Si (100) prepared by organometallic CVD with methyltrichlorosilane. Thin Solid Films, 318(1–2), 18–21.
Abstract: The large lattice mismatch and different thermal dilatation make the pseudomorphic growth of 3C-SiC on Si to appear impossible. Therefore very thick films (≥20 μm) have to be grown in order to relax the interface defects and obtain good quality heteroepitaxial films [H. Morkoc, S. Strite, G.B. Gao, M.E. Lin, B. Svertlov, M. Burns, J. Appl. Phys., 76 (1994) 1363; S. Veprek, Th. Kunstmann, D. Volm, B. Meyer, J. Vac. Sci. Technol. A, 15 (1997) 10 (and references therein)]. In the course of the study of the initial stages of the growth using a UHV-compatible CVD apparatus and in-situ low energy electron diffraction (LEED) we find that up to several nm thin films provide LEED patterns corresponding to a 3×2 reconstructed surface (as documented in literature for 3C-SiC surfaces) and appear relatively homogeneous. Increasing the thickness upon longer deposition time leads to the disappearance of this LEED pattern and to pronounced surface roughening. These results indicate that the novel organometallic CVD technique [Veprek et al.] may open a way towards the preparation of a pseudomorphic 3C-SiC films.
Keywords: 3C-SiC; Heteroepitaxial film; Organometallic CVD
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Veprek, S., Männling, H. - D., Karvankova, P., & Prochazka, J. (2006). The issue of the reproducibility of deposition of superhard nanocomposites with hardness of ≥50 GPa. Surface and Coatings Technology, 200(12–13), 3876–3885.
Abstract: It is shown that the lack of the reproducibility of the data of Veprek et al. on the deposition of superhard nanocomposites with hardness reaching and even exceeding 50 GPa, that can be found in many recent papers of other groups, is due to either the choice of inappropriate deposition conditions, such as too low deposition temperature and pressure of nitrogen, or impurities. The problem of impurities is particularly difficult to solve when reactive sputtering is used. It is shown that using a sufficiently high deposition temperature of >500 °C, high deposition rates and nitrogen pressure of ≥0.002 mbar, hardness of ≥50 GPa is achieved in the binary nc-TiN/a-Si3N4 system, provided the oxygen impurities are below about 0.2 at.%.
Keywords: Nanocomposite coatings; Superhard coatings; Reproducibility; Impurities
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Veprek, S., & Veprek-Heijman, M. G. J. (2007). The formation and role of interfaces in superhard nc-MenN/a-Si3N4 nanocomposites. The International Conference on Superhard Coatings The International Conference on Superhard Coating, 201(13), 6064–6070.
Abstract: The early finding of Veprek and Reiprich that the maximum hardness in the nc-TiN/Si3N4 nanocomposites is achieved at about one monolayer of the interfacial Si3N4 has been confirmed for a number of different nc-MenN/XxNm systems (Me = Ti, W, V, (TiAl)N,…; X = Si, B). More recently, this has been confirmed experimentally for TiN–Si3N4 heterostructures and by first principle density functional theory calculations. We present a consistent understanding of the formation of the nanocomposites with one monolayer Si3N4 interface by spinodal phase segregation. This interface is energetically stabilized as compared to bulk Si3N4, which results in an enhanced bond strength and corresponding cohesion energy. Such an enhancement appears to be of a general validity in nano-sized solids with well-ordered interfaces. The paper concludes a brief summary of the recent progress of the understanding of the mechanical properties of these nanocomposites.
Keywords: Superhard nanocomposites; Interfaces; Enhanced cohesive energy; Non-linear FEM
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Veprek, S., Zhang, R. F., Veprek-Heijman, M. G. J., Sheng, S. H., & Argon, A. S. (2010). Superhard nanocomposites: Origin of hardness enhancement, properties and applications. Proceedings of the European Materials Research Socierty (E-MRS)Spring Meeting 2009 Symposium P, 204(12–13), 1898–1906.
Abstract: The original finding of Veprek et al. that in nc-TiN/a-Si3N4 and in nc-TiN/a-Si3N4/TiSi2 nanocomposites, deposited under conditions which allow complete phase segregation by spinodal mechanism, the maximum hardness of ≥ 45 and > 100 GPa, respectively, is achieved when the thickness of the interfacial Si3N4 is about 1 monolayer, has been recently confirmed by both experiments and theory. First principle calculations explain why the decohesion and shear strength of a TiN–SiNx–TiN sandwich is higher than that of bulk SiNx. Combined ab initio DFT calculations of shear resistance of the interfaces, their averaging according to Sachs for randomly oriented polycrystalline material to obtain tensile yield strength, Tabor's criterion, Hertzian analysis and pressure-enhanced flow stress explain in a simple way the experimentally achieved high values of hardness of > 100 GPa, in excess of diamond. Friedel oscillations of the valence charge density, originating from negative charge transfer to the strengthened SiNx interface, cause decohesion and ideal shear to occur between Ti–N bonds near that interface. The extraordinary mechanical properties of these and related quasi-binary superhard nanocomposites can be understood in terms of nearly flaw-free strong materials with no need to invoke any new mechanism of strengthening. We shall present selected examples of industrial applications of the superhard nanocomposite coatings.
Keywords: Superhard nanocomposites; First principle calculations; TiSiN; TiAlN; CrAlN
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Veprek, S., Veprek-Heijman, M. G. J., Karvankova, P., & Prochazka, J. (2005). Different approaches to superhard coatings and nanocomposites. Thin Solid Films, 476(1), 1–29.
Abstract: Different approaches to the preparation of superhard coatings such as intrinsically superhard materials, coatings whose hardness is enhanced by energetic ion bombardment during deposition, and nanostructured superhard materials are discussed with the emphasis on the question of how to distinguish between the different mechanisms of hardness enhancement in thin coatings. We compare the thermal and long-term stability in air and some further properties of such coatings. The lack of success of some workers to reproduce the high value of hardness reported by Veprek et al. is explained in terms of inappropriate choice of the deposition conditions and/or impurities.
Keywords: Superhard coatings; Superhard nanocomposites; Impurities; Preparation; Properties
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