Effects of substrate bias on nanocrystal-(Ti, Al)N x /amorphous-SiN y composite films
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Nanocrystal-(Ti,Al)Nx /amorphous-SiNy composite films were prepared in a codeposition process under different substrate bias voltages. The effects of substrate bias voltage on the deposition rate, composition, microstructure, and mechanical properties of nanocomposite films were investigated. Results indicated that the films with bias voltages caused resputtering due to the bombardment of high-energy ions on film surface. The resputtering effect had substantial influence on deposition rate, surface morphology, and composition of films. The films with (220) preferred orientation were also observed as the applied substrate bias voltages exceeded 50 V. As the substrate bias voltage increased, the nanocrystallite size increased, lattice strain raised, and the hardness decreased.
I. INTRODUCTION
Microstructural design is one of the best ways to enhance the strength of the film material. A new superhard coating of nanocrystalline/amorphous (nc/a) structure has shown a considerable strength enhancement as compared with both elemental materials.1–4 The results indicated that the strengthening mechanism of nanocrystalline/amorphous composite films was dislocation pinning. Dislocations formed in the crystallites under a higher applied stress; they could not freely move through the amorphous matrix, and therefore the strength of the material was enhanced.1–4 The strength of materials could be decreased by the formation and movement of dislocations.5 Crystals several nanometers in grain size were reported to be free of dislocations.2 Randomly oriented nanocrystals embedded in a thin amorphous matrix also supply a good coherence at the grain boundaries and could be beneficial for hard coatings on tribological application than purely polycrystalline composites.4 Novel cutting tool materials with nanocrystal-(Ti,Al)Nx / amorphous-SiNy [nc-(Ti,Al)Nx /a-SiNy] composite films were studied previously. 6 Results indicated that nanocomposite films significantly enhanced the hardness by 44% and 80% as compared to the monolithic (Ti,Al)Nx and a-SiNy films, respectively. To further investigate the
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 8, Aug 2003
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effects of deposition parameters on nanocomposite films, the substrate negative bias voltage was applied during the codeposition process. The films were bombarded by noble-gas ions in a plasma form, and the energy of the arriving ions was influenced by the substrate bias voltage.7 The kinetic energy of bombarding ions had substantial effects on the nucleation and growth of films, and the crystallographic, physical, and mechanical properties.7 In the present study, nc-(Ti,Al)Nx /a-SiNy composite films were deposited on high-speed steel. The effects of substrate bias on the deposition rate, composition, microstructure, and mechanical properties of nc-(Ti,Al)Nx / a-SiNy composite films were investigated. II. EXPERIMENTAL A. Sample preparation
The nc-(Ti,Al)Nx /a-SiNy com
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