Some properties of (Ti,Mg)N thin films deposited by reactive dc magnetron sputtering

doi:10.1016/j.surfcoat.2005.01.075

Surface and Coatings Technology

Volume 200, Issues 1–4, 1 October 2005, Pages 227-231

https://doi.org/10.1016/j.surfcoat.2005.01.075 Get rights and content

Abstract

(Ti,Mg)N coatings were deposited onto high speed steel (HSS) and glass substrates using reactive dc magnetron sputtering. Because of the addition of Mg to TiN the colour changed from golden to violet and metallic blue as measured by spectrophotometry. The chemical composition, morphology and structure of the coatings was investigated by energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The addition of Mg to TiN leads to a shift of the TiN peaks in the XRD patterns. Probably, this is due to the substitution of titanium atoms by the bigger magnesium atoms. The indentation hardness and the open-circuit-potential (in NaCl solution) of the (Ti,Mg)N coatings is decreasing with increasing Mg contents. The oxidation resistance of (Ti,Mg)N coatings could be drastically improved with increasing Mg content.

Introduction

Titanium nitride is one of the best investigated and commercially long-established hard coatings deposited by CVD (chemical vapour deposition) or PVD (physical vapour deposition) methods. The application of this coating ranges from tool coatings, diffusion barriers in the semiconductor industry to decorative coating of numerous consumer goods based on its golden colour. One disadvantage of TiN is its low oxidation resistance [1]. But this can be improved by alloying TiN with a third element, e.g. Al, Cr, Si etc. [1], [2], [3], [4], [5], [6]. Simultaneously, the alloying elements influence the colour of TiN [7]. By means of a variation of the concentration of the alloying element and the nitrogen content the colour and oxidation resistance of TiN can be directly influenced. The discovery of new colours for hard coatings is commercially interesting because of the limited colour palette available with respect to bulk colours. The aim of the present study is to investigate the change of some properties (e.g. colour, oxidation resistance, hardness) of TiN through the alloying with magnesium systematically.

Section snippets

Experimental details

All depositions were carried out in a Leybold L560 UV vacuum system. Two magnetron sputter cathodes and a rotating substrate holder were used for the (Ti,Mg)N deposition. The purity of the Ti and the Mg target was 99.9 %. The rotation speed of the substrate holder was fixed at 135 rpm, so that no complete atomic layer could grow on the substrate per rotation. This led to a complete mixture of the metallic components in the coating. Further details about the deposition parameters can be found in

Results and discussion

The chemical composition of the (Ti_1−xMg_x)N coatings was measured by EDX. The Mg content of the coatings was in the range of x = 0 to x = 0.67 (0–35 at.−%) when varying the power on the Mg cathode between 0 and 100 W (see Table 2). At the same time, the nitrogen content decreased a little bit with increasing Mg content (from 57 at.−% to 48 at.−%). The morphology of the coatings was columnar (e.g. Fig. 1) and changed to non-columnar only for the highest Mg content (Fig. 2). The crystallographic

Conclusions

(Ti_1−xMg_x)N coatings could be successfully deposited onto high speed steel (HSS) and glass substrates using reactive dc magnetron sputtering. The following conclusions can be drawn from the investigations:

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The morphology of (Ti_1−xMg_x)N coatings is columnar up to a Mg content of 24 at.−%. No columnar structure could be observed in coatings with 35 at.−% of Mg.
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XRD peaks of the coatings are shifted to lower 2θ values with increasing Mg content due to a change in the lattice parameters. It is

Acknowledgements

This work was supported by the German Research Foundation under grant no. FE 613/1-1. The authors would like to thank R. Bretzler, L. Schmalz and K. Petrikowski (FEM) for performing different characterisation tests.

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Based on the findings of this study, it can be concluded that the excellent corrosion performance along with the mechanical properties are supported by the Gd addition to the coatings. The TiMgN coatings show a change in chemical composition, microstructure as well as in hardness after corrosive exposure, which cannot be observed on TiMgGdN coatings. Consequently, alloying TiMgN with Gd maintains the stability of the coating during corrosive exposure. The analytical results show that the excellent corrosion performance and stability of the TiMgGdN coatings are due to a stable and stronger passivation layer by the formation of Mg(OH)₂ and Gd₂O₃ on the TiMgGdN surface. Furthermore, the Gd₂O₃ contributes to a strong hydrophobic effect. Regarding the microstructure, TiMgGdN coatings are seemingly denser compared to the TiMgN coatings, which positively influences the corrosion resistance of this coating.
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Ti_1−xMg_xN(001) layers with 0.00 ≤ x ≤ 0.49 are deposited on MgO(001) by reactive magnetron co-sputtering from titanium and magnesium targets in 5 mTorr pure N₂ at 600 °C. X-ray diffraction ω-2θ scans, ω-rocking curves, φ-scans, and high resolution reciprocal space maps show that the Ti_1−xMg_xN layers are rock-salt structure single crystals with a cube-on-cube epitaxial relationship with the substrates: (001)_TiMgN║(001)_MgO and [100]_TiMgN║[100]_MgO. Layers with thickness d = 35–58 nm are fully strained, with an in-plane lattice parameter a_|| = 4.212 ±0.001 Å matching that of the MgO substrate, while the out-of-plane lattice parameter a_⊥ increases with x from 4.251 Å for TiN(001) to 4.289 Å for Ti_0.51Mg_0.49N(001). This yields a relaxed lattice parameter for Ti_1−xMg_xN(001) of a_o = (1-x)a_TiN + xa_MgN – bx(1-x), where a_TiN = 4.239 Å, a_MgN = 4.345 Å, and the bowing parameter b = 0.113 Å. In contrast, thicker Ti_1−xMg_xN(001) layers with d = 110–275 nm are partially (pure TiN) or fully (x = 0.37 and 0.49) relaxed, indicating a critical thickness for relaxation of 50–100 nm. The in-plane x-ray coherence length is large (100–400 nm) for fully strained layers with 0.00 ≤ x ≤ 0.45 but drops by an order of magnitude for x = 0.49 as the composition approaches the phase stability limit. It is also an order of magnitude smaller for thicker (d ≥ 110 nm) layers, which is attributed to strain relaxation through the nucleation and growth of misfit dislocations facilitated by glide of threading dislocations.
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By analyzing the NSS test results it was found that the corrosion behavior of the coated samples did not only depend strongly on the Mg content, but also on the sample individual defect concentrations. Therefore this subject is extensively discussed.
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This necessitates the tuning of Mg in the coatings and for this, the coatings were deposited on titanium substrates by cathodic arc physical vapor deposition technique and HA formation was achieved in simulated body fluids (1× SBF, 5× SBF) at 37°C. A few studies [41,42] exist on magnesium containing TiN coatings and these studies were mainly focused on optical [41] and high temperature oxidation behaviors [42]. In our early study, the role of magnesium released from (Ti,Mg)N coatings on nucleation, growth and characteristics of HA deposited in SBF was investigated.
TiN and (Ti,Mg)N thin film coatings were deposited on titanium substrates by using cathodic arc physical vapor deposition (arc-PVD) technique with magnesium contents of 0, 4.24 at% (low Mg) and 10.42 at% (high Mg). The presence of magnesium on both normal (hFOB) and cancer (SaOS-2) osteoblast cell behavior was investigated in (Ti,Mg)N surfaces with or without prior hydroxyapatite (HA) deposition (in simulated body fluid, SBF). Mg incorporation on TiN films was found to have no apparent effect on the cell proliferation in bare surfaces but cell spreading was better on low Mg content surface for hFOB cells. SaOS-2 cells, on the other hand, showed an increased extra cellular matrix (ECM) deposition on low Mg surfaces but ECM deposition almost disappeared when Mg content was increased above 10 at%. HA deposited surfaces with high Mg content was shown to cause a significant decrease in cell viability. While the cells were flattened, elongated and spread over the surface in contact with each other via cellular extensions on unmodified and low Mg doped surfaces, unhealthy morphologies of cells with round shape with a limited number of extended arms was visualized on high Mg containing samples. In summary, Mg incorporation into the TiN coatings by arc-PVD technique and successive HA deposition led to promising cell responses on low Mg content surfaces for a better osteointegration performance.
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Some properties of (Ti,Mg)N thin films deposited by reactive dc magnetron sputtering

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Conclusions

Acknowledgements

Thin Solid Films

Surf. Coat. Technol.

Thin Solid Films

J. Vac. Sci. Technol., A

J. Vac. Sci., Technol. A