Elsevier

Applied Surface Science

Volume 252, Issue 23, 30 September 2006, Pages 8319-8325
Applied Surface Science

Difference in high-temperature oxidation resistance of amorphous Zr–Si–N and W–Si–N films with a high Si content

https://doi.org/10.1016/j.apsusc.2005.11.038Get rights and content

Abstract

The high-temperature oxidation resistance of amorphous Zr–Si–N and W–Si–N films with a high Si content (≥20 at.%) deposited by reactive dc magnetron sputtering at different partial pressures of nitrogen was systematically investigated by means of a symmetrical high-resolution thermogravimetry in a flowing air up to an annealing temperature of 1300 °C (a temperature limit for Si(1 0 0) substrate). Additional analyses including X-ray diffraction (XRD), light optical microscopy (LOM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and microhardness measurement were carried out as well. The obtained results showed (i) an excellent high-temperature oxidation resistance of the Zr–Si–N films up to 1300 °C, (ii) a considerably lower oxidation resistance of the W–Si–N films. The W–Si–N films are completely oxidized at 800 °C with a subsequent volatilization of unstable WOx oxides. On the other hand, the Zr–Si–N films are oxidized only very slightly on the surface, where a stable oxide barrier layer preventing further inward oxygen diffusion is formed. The thickness of the oxide layer is only about of 3% of the total film thickness. The phase composition, thermal stability of individual phases and amorphous structure were found to be key factors to achieve a high oxidation resistance.

Introduction

A crucial factor of many major industrial sectors, such as material processing, chemical engineering, power generation or aerospace, is an ability to operate at high temperatures. For this reason new advanced high-temperature materials with heat-resistant capabilities need to be developed and tested. The most essential capability is the resistance to the mechanical and corrosive conditions imposed by the operating environment.

Oxidation is one of the most commonly proceeding reactions at elevated temperature in many applications. The protection of a base material from the reaction with ambient oxygen is usually ensured either by alloying with relevant elements or by deposition of a protective coating on the substrate surface. The well-known elements inhibiting oxidation are Al and Cr [1], [2]. Recently, the role of Si in the enhancement of the oxidation resistance has been also demonstrated. The majority of the studies has been focused on the oxidation resistance of Ti–Si–N films with a low Si content (<20 at.%) at annealing temperatures not exceeding 1000 °C [3], [4], [5], [6], [7], [8], [9]. Depending on the deposition method and the Si content reported the anti-oxidation behavior can be generally improved by 100 or 200 °C compared to pure TiN films. The similar trend has been also reported for Zr–Si–N [10], [11], Cr–Si–N [12], W–Si–N films [13] with a low Si content. Except of these ternary systems, coatings of quaternary systems, such as Ti–Al–Si–N [14], [15], [16], [17] or Cr–Al–Si–N [18], based on a combined anti-oxidation action of Al/Cr and Si are of special interest as well. While Ti–Si–N films exhibit a comparable oxidation resistance to Ti–Al–N films, the oxidation resistance of Ti–Al–Si–N films is even better. Tanaka et al. reported a greatly improved oxidation resistance of Al–Ti–Si–N films with a few atomic percentages of Si up to 1100 °C in air [16].

Poor attention has been, however, devoted to the characterization of the oxidation behavior of films with a relatively high amount of Si (≥20 at.%). Except of our previous studies on Ta–Si–N [19], [20], Zr–Si–N [21], W–Si–N [22] and Mo–Si–N systems [23], only Hirvonen et al. [24], Louro and Cavaleiro [25] and Choi et al. [26] have reported some results on the oxidation resistance of Mo–Si–N, W–Si–N and Ti–Si–N films with ≥20 at.% of Si, respectively. Choi et al. obtained the best results and observed no TiO2 peaks after annealing in air up to 900 °C/150 min for 27 at.% of Si.

The reason of a small interest in hard films with a high Si content lies in their lower hardness (∼30 GPa) compared to that of superhard films (>40 GPa). However, the excellent oxidation resistance of these films is more important than superhardness for the most applications.

The main aim of this paper is a comparison of the high-temperature oxidation resistance of reactively dc magnetron sputtered amorphous Zr–Si–N and W–Si–N films with a high Si content (≥20 at.%) in air at annealing temperatures up to 1300 °C.

Section snippets

Experimental

Zr–Si–N and W–Si–N films were reactively sputter deposited in Ar + N2 by an unbalanced dc magnetron equipped with a ceramic ZrSi2 and WSi2 target of diameter 100 mm, respectively. The effect of the nitrogen content in the film on the high-temperature oxidation resistance of the Zr–Si–N and W–Si–N films was systematically studied by a variation of the partial nitrogen pressure pN2 in the range of 0.05–0.50 Pa. Other deposition parameters were selected as constant: the discharge current Id = 1 A, the

Results and discussion

The properties of the amorphous Zr–Si–N and W–Si–N films in the as-deposited state prepared at raising nitrogen partial pressure pN2 are summarized in Table 1, Table 2. For both types of the films the nitrogen content increases with increasing pN2 and the Si content remains over 20 at.% in all the films. The X-ray structure of all the films is amorphous, which is due to a high content of the amorphous Si3N4 phase. The Si3N4 content increases with increasing pN2 and saturates about 56 and 60 vol.%

Conclusions

High-temperature annealing of sputtered amorphous Zr–Si–N and W–Si–N films with a high Si content (≥20 at.%) in a flowing air up to 1300 °C was carried out.

It was found that the Zr–Si–N films deposited on the Si substrate exhibit an excellent oxidation resistance up to a 1300 °C due to the formation of very thin (only 115 nm) oxide surface layer. On the contrary, the W–Si–N films exhibit much lower oxidation resistance. Above 800 °C the W–Si–N films are completely oxidized with a subsequent

Acknowledgements

The authors would like to thank to RNDr. J. Kasl, CSc. and RNDr. D. Jandova, Ph.D. of Skoda research for measurements of the elemental composition of films using EDXS, Mgr. J. Matejkova of Institute of Scientific Instruments, Academy of Sciences of the Czech Republic for SEM characterization of films, and Doc. RNDr. J. Pavlik, CSc. and Mgr. Z. Stryhal, Ph.D. of J.E. Purkyne University for AFM measurements of films. This work was supported in part by the Ministry of Education of the Czech

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