Nanostructure evolution and properties of two-phase nc-Ti(C, N)/a-(C, CN x ) nanocomposites by high-resolution transmiss
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Y.G. Shena) Department of Manufacturing Engineering & Engineering Management, City University of Hong Kong, Kowloon, Hong Kong (Received 21 January 2007; accepted 14 March 2007)
High-resolution transmission electron microscopy, x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy were used to study phase configuration and nanostructure evolutions of Ti–Cx–Ny thin films with different amounts of C incorporation. It was found that the atomic ratio of (C + N)/Ti played a crucial role in phase configuration and nanostructure evolutions as well as mechanical behaviors. When the ratio was less than one unit, a nanocrystalline (nc-) Ti(C, N) solid solution was formed by way of dissolution of C into TiN lattice. When this dissolution reached saturation, precipitation of a small amount of amorphous (a-) C phase along nc-Ti(C, N) grains was followed with more C incorporation. Further increase of C content (up to ∼19 at.% C) made the amorphous phase fully wet nanocrystallites, which resulted in the formation of two-phase nanocomposite thin films with microstructures comprising of ∼5 nm nc-Ti(C,N) crystallites separated by ∼0.5 nm a-(C, CNx) phase. Thicker amorphous walls and smaller sized grains were followed when the C content was further increased, accompanying with the formation of some disorders and defects in nc-grains and amorphous matrices. When the C content was increased to ∼48 at.%, 1–3 nm nanocrystallites with an average size of ∼2 nm were embedded into amorphous matrices. Both microhardness and residual compressive stress values were increased with increase of the atomic ratio in solid solution thin films when the atomic ratio value was less than one unit. Their maximums were obtained at stiochiometry nc-Ti(C,N) solid solution. Enhancement of hardness values was attributed to solid solution effect.
I. INTRODUCTION
In recent years, there has been considerable interest in fabricating two-phase nanocomposite thin films with microstructures composed of nanocrystalline phase in an amorphous matrix.1–6 Many studies show that the formation process of two-phase nanostructures may strongly influence the physical and mechanical properties of these newly developed engineering materials. For the best known example, namely, by adding small amounts of silicon into TiN, nanocrystalline (nc-) TiN/amorphous
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0304 2460 J. Mater. Res., Vol. 22, No. 9, Sep 2007 http://journals.cambridge.org Downloaded: 29 Mar 2015
(a-) Si3N4 composite films were formed.4 The general understanding is that the amorphous phase (a-Si3N4) surrounded nanocrystallites (nc-TiN) plays a barrier effect to block dislocation movement, and the plastic deformation is significantly suppressed. Thus the mechanical properties of nanocomposite films are improved.4,7 It is obvious that the existence of amorphous Si3N4 leads to a remarkable decrease in nc-TiN grains. Moreover, their hardness maximum is usually achieved when the flexible amorphous phase just wets
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