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Nanostructured composite (i.e., “Nanocomposite”) coatings are usually formed from ternary or higher order systems and comprise at least two immiscible phases: two nanocrystalline phases or, more commonly, an amorphous phase surrounding nanocrystallites of a secondary phase. The most interesting and extensively investigated nanocomposite coatings are ternary, quaternary or even more complex systems, with nanocrystalline (nc-) grains of hard transition metal-nitrides (e.g. TiN, CrN, AlN, BN, ZrN, etc.), carbides (e.g. TiC, VC, WC, ZrC, etc.), borides (e.g. TiB2, CrB2, VB2, WB, ZrB2, etc.), oxides (e.g. Al2O3, TiO2, SiO2, MgO, TiO2, Y2O3, ZrO2, etc.), or silicides (e.g. TiSi2, CrSi2, ZrSi2, etc.) surrounded by amorphous (a-) matrices (e.g. Si3N4, BN, C, etc.). The synthesis of such nanocomposite (nc-/a-) coatings critically depends on the ability to co-deposit both the nanocrystalline and amorphous phases, such as Ti–Si–N (nc-TiN/nc-and a-TiSi2/a-Si3N4, Ti-Al-Si-N (nc-TiAlN/a-Si3N4, W-Si-N (nc-W2N/a-Si3N4), Cr-Si-N (nc-CrN/a-Si3N4), Ti-B-C-N (nc-TiB2 and TiC/a-BN), TiC/DLC (nc-TiC/a-C), WC/DLC (nc-WC/a-C), etc. as schematically presented in Figure 1(a). A variety of hard compounds can be used as the nanocrystalline phases, including nitrides, carbides, borides, oxides, and silicides. Veprek et al. suggested that the nano-crystalline grains must be 3-10 nm in size and separated by 1-2 monolayers of an amorphous phase as shown in Figure 1(a). For example, Ti-B-N nanocomposite, which consists of nanocrystalline TiN (-5 nm in size) in an amorphous BN matrix, has been synthesized and observed by Lu, as shown in Figure 1(b).
Figure 1. (a) Schematic diagram of a nanostructured nanocomposite coating proposed by Veprek et al.(1998) and (b) HRTEM image and selected area diffraction pattern (SADP) of nanocomposite Ti-B-N (nc-TiN/a-BN) [Lu et al., 2005].
This design or “Architecture” leads to ultra-hard (hardness above 80 GPa) coatings as reported by Veprek and co-authors most recently. The nanocrystalline phase may be selected from nitrides, carbides, borides, and oxides, while the amorphous phase may also include ceramics, metals and diamond-like carbon (DLC). The initial model proposed by Veprek to explain hardness in nanocomposites is that dislocation operation is suppressed in small grains (3-5 nm) and that the narrow space between them (1 nm separation) induces incoherence strains. The incoherence strain is likely increased, when grain orientations are close enough to provide interaction between matched but slightly misoriented atomic planes. In the absence of dislocation activity, Griffith’s equation, for crack opening was proposed as a simple description of the nanocomposite strength. This equation suggests that strength can be increased by increasing elastic modulus and surface energy of the combined phases, and by decreasing the crystalline grain sizes. It is noted that elastic modulus is inversely dependent on grain sizes that are in the nm size range due to lattice incoherence strains and the high volume of grain boundaries. In practice, grain boundary defects always exist, and a 3 nm grain size was found to be close to the minimum limit. Below this limit, a reverse Hall–Petch effect has been observed and the strengthening effect disappears because grain boundaries and grains become indistinguishable and the stability of the nanocrystalline phase is greatly reduced.
Figure 2 shows a high resolution TEM image obtained from the Ti-B-C-N-Si(5 at.%) film and the corresponding inverse fast Fourier transform (IFFT) image, which clearly show that the Ti-B-C-N-Si film has a three dimensional nanocomposite structure in which the crystallites exhibited regular and spherical shapes with sizes ranging from 2 to 3 nm in an amorphous matrix.
Figure 2 (a) a high resolution TEM image obtained from the Ti–B–C–N–Si(5 at.%) film and (b) corresponding inverse fast Fourier transform (IFFT) image calculated by Micrograph TM Gartan software.[Park et. al, ACSEL, CSM, 2009]
Ti-B-C-N-Si(5 at.%) nanocomposite films were deposited on AISI 304 stainless steel substrates using an unbalanced DC magnetron co-sputtering technique with 80mol%TiB2-20mol%TiC composite target and pure silicon target in N2/(Ar+N2) ratio of 1%. XRD, XPS, TEM analyses revealed that the synthesized Ti-B-C-N-Si(5 at.%) film was composed of a nanocomposite consisting of solid-solution (Ti,C,N)B2, Ti(C,N) and TiSi2 nanocrystals (2-3 nm in size) embedded in an amorphous SiOx/BNx/SiBx/BOx/TiOx/CNx/carbon matrix, as schematically shown in Figure 3.
Figure 3. Schematic illustration of the Ti-B-C-N-Si [nc-(Ti,C,N)B2/nc-Ti(C,N)/nc-TiSi2/a-BNx/a-SiOx/a-BOx/a-TiOx/a-SiC/etc] nanocomposites based on XRD, XPS, TEM analyses. [Park et. al, ACSEL, CSM, 2009]