Mechanical properties of nanocrystalline materials

Nanocomposites, a high performance material exhibit unusual property combinations and unique design possibilities. With an estimated annual growth rate of about 25% and fastest demand to be in engineering plastics and elastomers, their potential is so striking that they are useful in several areas ranging from packaging to biomedical applications. In this unified overview the three types of matrix nanocomposites are presented underlining the need for these materials, their processing methods and some recent results on structure, properties and potential applications, perspectives including need for such materials in future space mission and other interesting applications together with market and safety aspects. Possible uses of natural materials such as clay based minerals, chrysotile and lignocellulosic fibers are highlighted. Being environmentally friendly, applications of nanocomposites offer new technology and business opportunities for several sectors of the aerospace, automotive, electronics and biotechnology industries.

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Nanocomposites: synthesis, structure, properties and new application opportunities

In recent years, near-nano (submicron) and nanostructured materials have attracted increasingly more attention from the materials community. Nanocrystalline materials are characterized by a microstructural length or grain size of up to about 100 nm. Materials having grain size of ∼0.1 to 0.3 μm are classified as submicron materials. Nanocrystalline materials exhibit various shapes or forms, and possess unique chemical, physical or mechanical properties. When the grain size is below a critical value (∼10–20 nm), more than 50 vol.% of atoms is associated with grain boundaries or interfacial boundaries. In this respect, dislocation pile-ups cannot form, and the Hall–Petch relationship for conventional coarse-grained materials is no longer valid. Therefore, grain boundaries play a major role in the deformation of nanocrystalline materials. Nanocrystalline materials exhibit creep and super plasticity at lower temperatures than conventional micro-grained counterparts. Similarly, plastic deformation of nanocrystalline coatings is considered to be associated with grain boundary sliding assisted by grain boundary diffusion or rotation. In this review paper, current developments in fabrication, microstructure, physical and mechanical properties of nanocrystalline materials and coatings will be addressed. Particular attention is paid to the properties of transition metal nitride nanocrystalline films formed by ion beam assisted deposition process.

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Nanocrystalline materials and coatings

Microstructural design of hard coatings

Hardness can be defined microscopically as the combined resistance of chemical bonds in a material to indentation. The current review presents three most popular microscopic models based on distinct scaling schemes of this resistance, namely the bond resistance, bond strength, and electronegativity models, with key points during employing these microscopic models addressed. These models can be used to estimate the hardness of known crystals. More importantly, hardness prediction based on the designed crystal structures becomes feasible with these models. Consequently, a straightforward and powerful criterion for novel superhard materials is provided. The current focuses of research on potential superhard materials are also discussed.

Microscopic theory of hardness and design of novel superhard crystals

Different approaches to superhard coatings and nanocomposites

The original finding of Veprek et al. that in nc-TiN/a-Si 3 N 4 and in nc-TiN/a-Si 3 N 4 /TiSi 2 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 Si 3 N 4 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-SiN x -TiN sandwich is higher than that of bulk SiN x . 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 SiN x 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.

Superhard nanocomposites: Origin of hardness enhancement, properties and applications

We present a comparative study of the preparation and properties of superhard ‘Ti–B–N’ coatings deposited by plasma CVD and by Vacuum Arc Evaporation (PVD) of Titanium combined with Plasma CVD of TiB2 and BN. Using high frequency plasma CVD at a total pressure of several mbar with TiCl4, BCl3, N2 and H2 as reactants, superhard (HV≈40–50 GPa) nanocomposite coatings were successfully and reproducibly deposited and characterized in terms of mechanical properties (indentation and scanning electron microscope (SEM)), phase composition (X-ray photoelectron spectroscopy and elastic recoil detection) and nanostructure (X-ray diffraction, SEM). Using reactive sputtering, several authors reported about the preparation of superhard TiN/TiB2 coatings with only a small fraction of BN. Efforts to increase the fraction of the BN phase resulted in soft films. In contrast, plasma CVD yields superhard nc-TiN/a-BN and nc-TiN/a-TiB2/a-BN coatings in a wide range of the fractions of BN and TiB2 phases. This is attributed to the high chemical activity of nitrogen under the conditions of plasma CVD. Industrial scale vacuum arc evaporation PVD in combination with plasma CVD is used for preparation of ‘Ti–B–N’ coatings. In the system nc-TiN1−x/a-TiB2 with a minor fraction of the a-BN phase superhard coatings with a very low fraction of microparticles and resultant low surface roughness of Rm≈0.15 μm were successfully prepared and those are intended for testing under a variety of cutting conditions.

Superhard nc-TiN/a-BN and nc-TiN/a-TiBx/a-BN coatings prepared by plasma CVD and PVD: a comparative study of their properties

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.

The formation and role of interfaces in superhard nc-MenN/a-Si3N4 nanocomposites

Using high frequency plasma chemical vapor deposition (P CVD) at a total pressure of several mbar with TiCl 4 , BCl 3 , N 2 and H 2 as reactants, superhard nanocomposite nc-TiN/a-BN and nc-TiN/a-BN/a-TiB 2 coatings with hardness of 40–50 GPa were reproducibly deposited and characterized in terms of their phase composition, nanostructure, thermal stability, oxidation resistance and mechanical properties. By analogy with earlier studied systems nc-M n N/a-Si 3 N 4 (M=Ti, W, V) and nc-TiN/a-Si 3 N 4 /a- and nc-TiSi 2 , the maximum hardness of the nc-TiN/a-BN and nc-TiN/a-BN/a-TiB 2 coatings was obtained at the percolation threshold when there is about one monolayer of thin continuous tissue of a-BN between the TiN nanocrystals. It was shown that the thermal stability and oxidation resistance of these coatings is fairly high but lower than that of the nc-TiN/a-Si 3 N 4 and nc-TiN/a-Si 3 N 4 /a-TiSi 2 coatings.

Maritza G. J. Veprek-Heijman

Papers

Different approaches to superhard coatings and nanocomposites

Superhard nanocomposites: Origin of hardness enhancement, properties and applications

Superhard nc-TiN/a-BN and nc-TiN/a-TiBx/a-BN coatings prepared by plasma CVD and PVD: a comparative study of their properties

The formation and role of interfaces in superhard nc-MenN/a-Si3N4 nanocomposites

Properties of superhard nc-TiN/a-BN and nc-TiN/a-BN/a-TiB2 nanocomposite coatings prepared by plasma induced chemical vapor deposition