Transmission Electron Microscopy and X-ray Diffraction Investigation of the Microstructure of Nanoscale Multilayer TiAlN
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Cubic NaCl-B1 structured multilayer TiAlN/VN with a bi-layer thickness of approximately 3 nm and atomic ratios of (Ti+Al)/V ⳱ 0.98 to 1.15 and Ti/V ⳱ 0.55 to 0.61 were deposited by unbalanced magnetron sputtering at substrate bias voltages between −75 and −150 V. In this paper, detailed transmission electron microscopy and x-ray diffraction revealed pronounced microstructure changes depending on the bias. At the bias −75 V, TiAlN/VN followed a layer growth model led by a strong (110) texture to form a T-type structure in the Thornton structure model of thin films, which resulted in a rough growth front, dense columnar structure with inter-column voids, and low compressive stress of −3.8 GPa. At higher biases, the coatings showed a typical Type-II structure following the strain energy growth model, characterized by the columnar structure, void-free column boundaries, smooth surface, a predominant (111) texture, and high residual stresses between −8 and −11.5 GPa.
I. INTRODUCTION
Nanoscale multilayer films of TiN/VN and TiN/NbN, epitaxially grown on single-crystal MgO(100) substrate by unbalanced magnetron (UBM) sputtering,1 have shown super-high hardness values up to HV0.025 55 GPa. The super-hardness phenomenon was observed when the multilayer period falls into a specific range of a few nanometers. The micro-hardness, microstructure, and hardening mechanisms of the TiN/VN and TiN/NbN can be found in a series of publications, e.g., Refs. 2–5. The multilayers of TiN/NbN grown on a tool steel substrate achieved super-hardness values similar to single-crystal TiN/VN and TiN/NbN when the physical vapor deposition (PVD) conditions such as ion current, bias, and nitrogen partial pressure were optimized.6,7 However, the films grown on steel substrate exhibited polycrystalline microstructure, which changed with the variation of deposition parameters. In particular, the highest hardness values were obtained when the films were grown at high bias voltage, e.g., −150 V.6 Cross-sectional transmission electron microscopy (XTEM) of the TiN/NbN showed significant structure densification with increasing bias voltage.7 Following the pioneering work of the superhard TiN/ VN and TiN/NbN, polycrystalline multilayer coating a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0143 J. Mater. Res., Vol. 19, No. 4, Apr 2004
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TiAlN/VN with a TiAlN-VN bi-layer thickness 3.5 nm has been grown on tool steel, which showed hardness values of HK0.025 33 GPa and HP50mN of 39 GPa.8 Whereas TiAlN is known for having oxidation resistance superior to that of TiN,9 the incorporation of VN in the multilayer brought about a low friction coefficient (0.18– 0.4) and a low wear coefficient around 1.26 × 10−17 m3N−1m−1. This is due to the formation of a lubricious vanadium oxide tribo-film on the worn surface.10 Because of these properties, the TiAlN/VN has potential for applications as a wear-resistant coating, e.g., in cutting tools. However, the fundamental
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