Structure refinement and hardness enhancement of titanium nitride films by addition of copper

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Abstract

Titanium nitride films with a low added copper content were synthesized by ion beam-assisted sputtering deposition. The role of copper was examined with regard to the structure, hardness and elastic/plastic deformation capacity of the composite films produced. It was found that copper had significant modifying effects on the structure and property of titanium nitride films. When a very low content of Cu (<2 at.%) was added, the film showed apparent hardness enhancement. Films with low content of Cu also displayed highly elastic characteristics (91% elastic recovery under nanoindentation). On the contrary, films with >2 at.% copper were comparatively soft, up to 53% of the deformation under indentation could be plastic in this case. In addition, copper additions could also be used to tune the grain size of TiN in the range of 25–5 nm, at the same time causing a transition from a strong (111) preferred orientation to random polycrystalline morphology.

Introduction

Titanium nitride is one of the most thoroughly studied hard coating materials for wear-protective purposes. Despite the wide-range applications in which TiN is now used, compared with the many other hard/superhard materials (like other transition metal nitrides, carbides, DLC, etc.), titanium nitride has only modest tribological properties such as hardness, wear resistance and friction coefficient. Consequently, the recent trend in studying titanium nitride-based thin films is to add some other elements or materials by various film deposition techniques. The successful studies include homogenous/composite films like Ti(CN), Ti(BN), (TiAl)N, (TiCr)N, etc. [1], [2], [3], [4], and multilayered architectures like TiN/TiC, TiN/DLC, TiN/TaN, TiN/TiO, TiN/Si3N4, etc. [5], [6], [7], [8]. While significant improvement in tribological performance has been achieved, the film synthesis is often accompanied by a sophistication of the experimental procedure, especially for the multilayered structures. Moreover, the optimized experimental parameters usually lead to a substrate temperature of several hundred degrees Centigrade which is compatible with most tooling applications but unacceptable for some specific substrates.

Meanwhile, a concept of incorporating immiscible materials into bi-phase nanocomposites has been proposed by Veprek et al. [9], [10]. The crux is to form isolated nanocrystals in a thin amorphous matrix so that dislocation movement and grain boundary sliding can be suppressed. This concept appears to be very effective and extremely high hardness (approx. 100 GPa) are claimed [11]. However, according to this concept, both the two component materials should be hard materials. A different approach was reported by Musil et al. [12] who combined soft and hard materials to successfully synthesize superhard nanocomposites. Due to the larger variety of possible material combinations, the hardness enhancement in hard/soft material systems is interesting. However, unfortunately, less attention has been paid to this phenomenon.

In this work, we synthesized titanium nitride films with the addition of elemental copper by low energy ion beam-assisted sputtering deposition (IBAD); the purpose being to check the concept of coating hardness enhancement by use of nanosized hard/soft material combinations. The results confirmed that the hardness of TiN could be significantly improved by addition of the soft metal copper, provided that an appropriate (small) amount was selected. What is more interesting is that the addition of copper also refined the polycrystalline structure and tuned the internal compressive stress.

Section snippets

Experimental

Thin films were prepared in an IBAD system which was equipped with two Kaufman ion sources used for sputter deposition (8 cm focusing, argon) and bombardment assistance (15 cm defocusing, nitrogen), respectively. Both ion sources could be operated in the range of 50 eV∼1 keV with the beam current varying from 30 to 500 mA. The maximum beam density was approximately 1 mA/cm2. The main chamber was evacuated by a turbo-molecular pump supported by a booster-rotary pump combination. A Polycold®

Results and discussion

The composition of pure titanium nitride film was found to be nearly stoichiometric TiN. X-Ray diffraction (XRD) indicated that the film had fcc structure and very strong (111) orientation. For those films with copper, the copper content was determined in the range of 1∼12 at.%.

Fig. 2 shows the film hardness as a function of the copper content. The zero point represents the pure titanium nitride. It is noteworthy that some films with copper displayed higher hardness than pure TiN. The film with

Conclusion

In summary, copper was found to have significant effects on the structure and properties of titanium nitride films. When very low content of Cu (<2 at.%) was added into TiN, the film showed apparent hardness enhancement. Films with low content of Cu also displayed highly elastic characteristic (91% elastic recovery under nanoindentation). On the contrary, films with >2 at.% copper were soft, up to 53% of the deformation under indentation could be plastic in this case.

The hardness behavior of

Acknowledgements

This work was supported by the Japanese Society for Promotion of Science. The authors gratefully thank Dr Nogi and Dr Zhou in Osaka University for their great help in the nanoindentation measurements.

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