Nanostructure Evaluation

The characterization of nanostructure is an important stage in nanomaterials processing in order to synthesize nanocomposites with the required properties. Owing to the very small sizes of the structural elements (grains, layers), there are peculiarities in the determination of the nanostructure. The commonly used method of light microscopy (LM) does not work in the case of nanostructured materials because the LM resolution is limited to approximately 1 ¡xm. As has been noted [274], direct imaging of a nanostructure with transmission electron microscopy (TEM) and indirect evaluation of nanograin (or nanolayer) size with x-ray diffraction (XRD) are the two basic experimental methods to estimate the nanosizes of grains/layers.

XRD is a standard structural method that has been widely used [690]. There are a number of in-depth publications about X-ray diffraction [733-737]. Here, attention is payed to methods specifically employed for nanostructure characterization of superhard nanocomposites. XRD is utilized for measurement of both the bilayer repeat period in nanolay-ered composites and the average nanograin size in nanocrys-talline composites. The main peculiarity of nanostructure measurement is that nanostructured materials may contain some phases whose crystal structure may be different from that of coarse-grained materials due to the inherent properties of the nanocrystalline state [690, 738]. In the case of nanocomposites, X-ray diffraction allows measurement of the average nanograin size using Scherrer's formula.

TEM [739] is able to image the local nanostructure of thin layers, but the investigated volume of a sample is relatively very small. High-resolution TEM [740] can especially resolve the structure on an atomic level (0.1-0.2 nm), but sample preparation and image analysis are nonroutine tasks.

For the determination of the bilayer repeat period, any method that represents a function of any physical property correlating with the bilayer repeat period, for example, the dependence of the Rayleigh wave velocities on the bilayer repeat period [741], can generally be used for this aim.

Transmission Electron Microscopy Owing to the nanoscale sizes of the grains, investigations of the nanostruc-ture of superhard nanocomposites have some peculiarities. In this case, TEM becomes an irreplaceable tool for nano-structure analysis. TEM is a well-established technique as a direct method for nanostructure studies [742-744]. A recent comprehensive review of the microscopy methods has been published elsewhere [739]. The resolution of modern transmission electron microscopes can reach 0.1-0.2 nm, which allows easy imaging of nanostructures. High-resolution TEM can be realized [740] that resolves separate atoms located in a crystal lattice.

The following features of nanocomposites may be noted from TEM investigations:

• Orientation of nanograins

• Nanograin (or nanolayer) sizes

• Atomic structure of boundaries

• Atomic structure of interphase boundaries

TEM methods were used for the investigation of nanolay-ered composite coatings such as TiN/VN, TiN/NbN, and TiN/TaN, nanocrystalline composite coatings such as nc-TiN/a-Si3N4, nc-TiC/a-C: H, and nc-W3C/nc-carbynes.

Small-Angle X-Ray Diffraction Small-angle X-ray diffraction is a common method employed for the definition of the bilayer repeat period (A) of nanolayered composite coatings [397, 432, 745-747]. Theoretical prediction of the formation of satellite peaks around the Bragg reflection, when the diffracting crystal planes are stacked with nanoscale distance between them, has been presented elsewhere [748, 749]. Measurements rely on the direct scattering with the difference in X-ray refractive index of the two nanolayers. The Bragg reflexes are flanked by superlattice satellites, which are caused by long-range periodicity in the nanolayered composites [279]. The thickness ratio within the bilayer repeat period and the effect of interfacial diffusion as well as roughness at the interfaces can be determined from small-angle X-ray diffraction [279], which agrees to within 5% in the common nanoscale thickness range [750].

The bilayer repeat period could be calculated from the angular position of the positive (6+) and of the negative (6_) satellite X-ray reflexes relative to the central Bragg reflection 6b. For these aims, the following equation can be used [218, 235, 751, 752]:

where À is the X-ray wavelength, m is the order of the satellite reflections, and A is the bilayer repeat period.

A typical low-angle X-ray diffraction that scans around the Bragg reflection from the TiN/NbN nanolayered composite coatings is depicted in Figure 16. The figure displays a low-angle X-ray diffraction for the 90-nm bilayered repeat period of the nanolayered composite coating in which seven diffraction maxima were observed. The weak reflections of the bilayered repeat period are assigned as +1, +2,..., +7 satellites.

The strength of small-angle X-ray scattering is that the nanolayer thickness can be evaluated even if the nanolay-ered composite coatings represent amorphous-amorphous or crystalline-amorphous nanolayered structures [218].

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Figure 16. Typical low-angle X-ray diffraction of the TiN/NbN nanolayered composite coating. Reprinted with permission from [432], X. T. Zeng, Surf. Coat. Technol. 113, 75 (1999). © 1999, Elsevier Science.

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Figure 16. Typical low-angle X-ray diffraction of the TiN/NbN nanolayered composite coating. Reprinted with permission from [432], X. T. Zeng, Surf. Coat. Technol. 113, 75 (1999). © 1999, Elsevier Science.

In one report [753] on the estimation of the interface roughness of an island-like structure with a one-monolayer thickness and a lateral size greater than 30 nm, the analysis was performed by broadening the luminescence line widths.

Small-angle X-ray diffraction has been employed for the evaluation of the bilayer repeat period of nanolayered composite coatings such as TiN/NbN [436], ZrN/TixAl1-xN, NbN/TaN, CNx/TiN, and c-AlN/TiN.

Scherrer's Formula Scherrer's equation is the most frequently used in XRD analysis to determine the average nanocrystalline size and lattice strains in nanocomposites. The nanosizes of the grains result in a broadening of the X-ray peaks in XRD patterns [274, 690, 735, 736, 744]. All three causes, such as nanosize of grains, stress, and distortion, may be simultaneously present, although the broadening of diffraction peaks may cause only one of them. It is possible to separate a nanocrystalline-sized broadening from distortion broadening. The Warren-Averbach method [754, 755], which is based on Fourier analysis and requires accurate approximation of line profiles, allows separating both the particle size and the distortion effects [756].

Scherrer's equation displays the relationship between the average nanocrystalline diameter (D) and the X-ray wavelength (A), Bragg angle (6), and peak width at full width at half-maximum after correcting for instrumental broadening (6). It is calculated as follows [690, 735, 736, 744, 757]:

where e is the lattice strain. The Scherrer's formula allows estimation of the average nanocrystal size if the inhomoge-neous strains are absent and the nanograin size distribution is relatively narrow. To separate crystalline-sized broadening from distortion broadening, a simple method which is based on the distinct dependence of the line breadth and expressed as a function of the reciprocal space variable (2 sin 6)/A, has been described elsewhere [735, 744].

Scherrer's formula was used for measurement of the average sizes of nanocrystals for nanocrystalline composite coatings such as nc-Ti„N/a-Si3N4, nc-Ti1-xAlxN/a-AlN, and nc-TiN/a-WNx, nc-ZrN/a-Ni, nc-ZrN/a-Y, and nc-W3C/nc-carbines.

Scanning Tunneling and Atomic Force Microscopy

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) are novel methods, which are starting to be used for investigation of nanostructured composites [200, 398, 541, 758-762]. Cross sections of the coatings can be imaged using STM. Fourier analysis of the STM images allows evaluation of the bilayer repeat period [760], which has been found to be in good agreement with X-ray diffraction measurements [278]. As has been noted [278], STM is the most useful tool for measuring a bilayer repeat period that is beyond the resolution of X-ray diffraction, especially when the bilayer repeat period is too large; scanning electron microscopy is appropriate when the bilayer repeat period is too small.

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