Structural disordering in WC thin films induced by SiC additions
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I. INTRODUCTION
THE influence of microstructure on the mechanical properties of thin ceramic films is currently a subject of active research and investigation. While the choice of deposition parameters used to deposit thin films can alter film microstructure,[1,2] a number of researchers have pursued the addition of silicon to transition metal nitride and carbide films as an alternate route for microstructural modification.[3–16] Veprek et al.[3,4,5] investigated Ti-Si-N films and observed microstructures consisting of nanocrystalline (nc) TiN embedded in an amorphous silicon nitride phase. For samples in which the TiN crystallite size was 3 to 5 nm and the amorphous silicon nitride interlayer between the crystallites was 1 to 2 nm, hardness levels well above that for bulk TiN were reported.[5] It was proposed that maximum hardness results when the percolation threshold for the silicon nitride interlayer is reached. Further optimization of the structure was obtained by adjusting the film composition to include the nc-TiSi2 phase,[6] and even higher hardness levels were reported (100 GPa). Additional studies by other investigators on the effects of Si on TiN film hardness produced mixed results; in some cases,[7–10] hardness increases were obtained (but not to the level reported in Reference 6), while in another case, in which lower deposition temperatures were used,[11] the Si had little effect. JAMES E. KRZANOWSKI and JONATHAN WORMWOOD are with the Mechanical Engineering Department and Materials Science Program, University of New Hampshire, Durham, NH 03824. Contact e-mail: jamesk@ cisunx.unh.edu. Manuscript submitted December 19, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
The effect of silicon on the structure and properties of transition metal carbides has also been examined in a number of studies.[12,13,14] Additions of SiC to sputter-deposited TiC films[12,13] revealed that, while the grain size did not change significantly, the formation of a “mottled” structure within the grains was observed, which was attributed to phase separation between the TiC and SiC (two immiscible compounds). The hardness was found to increase with the SiC content, but it still only reached 22 GPa. A study comparing pulsed-laser-deposited TiC with Ti-Si-C films[14] showed that, while the crystallinity of the Ti-Si-C films increased with the substrate temperature, films deposited at 25 °C were amorphous; at 600 °C, however, the films were still only 50 pct crystalline. These results demonstrated the strong disordering effect of Si in transition metal carbide films deposited at low-to-moderate temperatures. The hardness of the films increased with the fraction of the crystalline phase, to a maximum of 32 GPa. Studies on the effects of carbon stoichiometry on TiC films reported that carbon also had a strong disordering effect that resulted in nanocrystalline or nearly amorphous films.[15,16] The resulting hardness was in one case above,[15] but in the other case below,[16] that of crystalline bulk TiC. These studies illustra
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