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MICROSTRUCTURE ANALYSIS OF NANOCRYSTALLINE MATERIALS AND NANOCOMPOSITES USING THE COMBINATION OF X-RAY DIFFRACTION AND TRANSMISSION ELECTRON MICROSCOPY D.

Rafaja1 , V. Klemm1 , C. Wüstefeld1 , M. Motylenko1 , M. Dopita1,2

1 Institute of Materials Science, TU Bergakademie Freiberg, Gustav-Zeuner-Str. 5, D-09599 Freiberg, Germany

2 Department of Condensed Matter Physics, Charles University Prague, Ke Karlovu 5, CZ-121

16 Prague 2, Czech Republic Corresponding author: [email protected] Keywords: nanocomposites, partial coherence of crystallites, X-ray diffraction, line profile analysis, transmission electron mi- croscopy Abstract The capability of the combination of the X-ray diffraction and the transmission electron microscopy for the microstructure investigations on thin film and bulk nanocomposites are illustrated on three experimental ex- amples: two Cr-Al-Si-N coatings with different chemical compositions and one BN bulk nanocomposite. Using a modified kinematical diffraction theory that describes and explains the phenomenon of the partial crystallographic coherence of crystallites, we could show that the analysis of the X-ray diffraction line broadening is able to reveal nanocrystalline domains organised in semi-coherent clus- ters, to determine the size of the nanocrystalline domains and the clusters, and to quantify the mutual orientation of the partially coherent crystallites within these clusters. Introduction The knowledge of the microstructure of functional materi- als is inevitable for both explanation and modification of their properties. Thus, the microstructure analysis became an obligatory experimental method in the materials design in the last decades. One possibility for tailoring of the mate- rials properties is the production of nanocrystalline materi- als or nanocomposites. Typical application fields of these materials are the catalytic converters, in which the ex- tremely small size of the particles enlarges their active sur- face [1], the self-cleaning surfaces based on the TiO2 thin films, in which the small crystallite size improves their photo-catalytic activity [2], or the magnetic materials, in which the magnetic behaviour can be modified by uncom- pensated magnetic moments in the near-surface region and thus by the ratio between the surface and the volume of crystallites [3, 4]. The experimental examples shown in this contribution illustrate the microstructure development in ultra-hard nanocomposites, in which the small crystallite size is employed to improve their mechanical properties, particularly their hardness [5]. The most important microstructure feature that improves the hardness in the ul- tra-hard nanocomposites is a high density of the crystallites boundaries, which hinder the movement of dislocations and some other microstructure defects. The increase of the hardness with decreasing crystallite size is described by the well-known Hall-Petch relationship [6, 7]. The optimum crystallite size in the ultra-hard nanocomposites is about

3 nm [8-10], which also agrees with the optimum thickness of individual layers in ultra-hard multilayers [11, 12]. If the crystallite size in the ultra-hard nanocomposites or the indi- vidual layer thickness in the ultra-hard multilayers are smaller than the optimum ones, their hardness decreases. In our Cr-Al-Si-N nanocomposite coatings, the maximum hardness reached

45 GPa. An additional experimental ex- ample illustrates the development of microstructure in bulk boron nitride nanocomposites, which hardness approached

100 GPa [13]. Concerning the role of the crystallite boundaries, it is anticipated that the mechanical properties of the ultra-hard nanocomposites are strongly influenced not only by their density, i.e. by the crystallite size, but also by their mor- phology and atomic structure. Therefore, besides the tradi- tional tasks of the microstructure analysis, i.e. the phase analysis, the texture analysis, the analysis of the crystallite size, the analysis of the residual stresses and the mi- cro-strains, also the analysis of the more subtle local microstructure features, like the atomic ordering at the crystallites boundaries, and the prediction of the intrinsic residual stresses are required. A very important approach for the local microstructure analysis using X-ray diffrac- tion (XRD) is the line profile analysis and its modification that employs the phenomenon of the partial coherence of crystallites to the X-ray scattering [14]. The partial coher- ence of crystallites for X-rays is observed in nano- crystalline materials and in nanocomposites with the crystallite size below approximately