ICMCTF2010 Session B6-2: Hard and Multifunctional Nano-Structured Coatings

Superhard nanocomposites of -TiN/a-Si₃N₄ can reach hardness of more than 100 GPa when prepared under conditions which allow the formation of fully segregated phases with about 1 monolayer (ML) thick Si₃N₄-like interface, free of impurities, which is significally strengthened by valence charge transfer from surounding TiN nanocrystals [1,2]. In the present paper we shall show, that such strengthening results in weakening of the neigbour TiN by Friedel oscillations of the valence charge density [3]. The 1 ML SiN_x interface is the strongest configuration because when its thicknes increases, the Friedel oscillations cause further weakening of the neighbor TiN. The Ti-Si-N system will then be compared with several others consisting of different transition metal and covalent nitrides in order to elucidate the possibility of finding a better one with reduced amplitude of the weakening Friedel oscillations.

It has been shown earlier, that impurities, in particular oxygen, strongly degrade the mechanical properties of these materials by embrittling the SiN_x interfaces [4]. We shall briefly discuss the recent progress in the understanding of this detrimental effect on the atomic scale, and compare it with several examples from other disciplines to show, that this effect is not uncommon. The final part of our paper deals with the control of the impurities in industrial-size coating applications.We shall show how an impurity content of ≤ 0.1 at.% is achieved today in industrial production, and outline possibilities of its further decrease down to a few 100 ppm.

[1] S. Veprek et al., Phil. Mag. Lett. 87(2007)955. [2] M. Veprek-Heijman et al., Surf. Coat. Technol. 203(2009)3385. [3] R. F. Zhang et al., Phys. Rev. Lett. 102(2009)015503; Phys. Rev. B 79(2009)245426. [4] S. Veprek et al., J. Vac. Sci. Technol. B 22(2004)L5; B 23(2005)L17

The recent developments in sputtering processes have allowed the achievement of complex ceramic hard coatings. The motivation of such research efforts is to provide multifunctional coatings in order to meet industrial needs, for instance in severe machining applications.

In this study, we are focusing on ternary nanocomposite CrSiN coatings deposited by arc evaporation PVD, process which is increasingly used in industry. Our objective is to improve CrN-based coatings durability by modifying, on the one hand, their chemistry (addition from 0 to 20 at.% Si) and, on the other hand, their architecture (single- and multi-layered films). Special attention will be paid to mechanisms involved in the formation of the nanocomposite CrSiN structure, and to their consequences on oxidation and wear behaviours. Microstructure was investigated by XRD analyses, SEM and TEM observations. Mechanical properties were measured by nanoindentation and SEM in situ tensile tests. Silicon enrichment leads to a drastic improvement of the oxidation resistance, whereas it appears to be detrimental to the wear resistance in the case of single-layered films. Nevertheless, the use of a multi-layered architecture leads to the best functional properties. These results will be discussed in light of films microstructure, especially regarding grain boundaries and interfaces.

The PVD synthesis of wear and oxidation resistant aluminum oxide and derivative coatings is currently attracting large scientific and technical interest. Ternary Al-Cr-O thin films with mechanical properties comparable or superior to binary Al-O thin films can be deposited at moderate deposition temperatures. New coatings from the quaternary Al-Cr-O-N system could even offer increased strength, hardness and toughness. A combinatorial approach to the growth of Al-Cr-O-N thin films by means of reactive r.f. magnetron sputtering will be presented. For specific deposition conditions well adherent, nanocrystalline Al-Cr-O-N thin films with high Vickers hardness and elastic modulus values were grown at non-equilibrium conditions on cemented carbide and silicon substrates. Detailed results on the coatings composition, constitution, microstructure and properties will be presented and discussed in comparison to ternary Al-Cr-O thin films deposited under identical conditions.

In this talk, we present and summarize the effect of the coating properties by adding other elements such as Cr and (or) Si to this conventional TiAlN coating deposited by arc ion plating system, especially because of its tendency to form nanocomposite structure.

Quaternary TiCrAlSiN coatings with varying (Al+Si) content formed a nanocomposite structure and showed superior properties with high hardness, good oxidation resistance and thermal stability [1, 2]. The addition of Cr and Si to TiAlN resulted in a mixed-phase coating at some compositions, which degraded the coating properties. Therefore, a ternary TiAlSiN coating was introduced to clarify the effect of Si addition and to inquire a new hard and tough coating. The structural transformation easily occurs by adding other elements and the choice of the right composition is an essential factor. The addition of Y to TiAlN can induce the growth of protective oxide scales at high temperatures, which prompts the good oxidation resistance [3]. Another trend in hard coatings for wear-protection is the development of multilayer coatings with small multilayer periods (nanometer thick layers). The multilayer coating show peculiar characteristic of hardness at small periods and new multilayer coating can be designed not only to increase the hardness, but also to compensate the degrading properties of each layer. The TiAlSiN coating with low Al content was layered with high Al content CrAlN coating to develop a new multilayer coating that could have a high Al content with a single cubic structure [4]. This multilayer coating showed a superior property with good oxidation resistance compared to monolayer coatings.

[1] K. Ichijo, H. Hasegawa, T. Suzuki, Surf. Coat. Technol., 201, (2007) 5477.

[2] H. Ezura, K. Ichijo, H. Hasegawa, K. Yamamoto, A. Hotta, T. Suzuki, Vacuum, 82, (2008) 476.

[3] T. Urakawa, N. Fukumoto, T. Suzuki, PSE 2009 in press.

[4] N. Fukumoto, H. Ezura, T. Suzuki, Surf. Coat. Technol., (in press).

Earlier studies have shown that magnetron sputtered nc-TiC/a-C nanocomposite coatings have excellent electrical contact properties [1]. Consequently, this type of material has a potential use in, e.g., sliding contacts where low contact resistance and high wear resistance combined with low friction are required. We propose that the performance of the nanocomposite coating can be further improved by exchanging Ti towards another carbide-forming transition metal. In this study we have investigated sputtered nanocomposite films in the Nb-C system for multifunctional applications with a focus on electrical and mechanical properties. Thin films of nc-NbC/a-C were deposited by nonreactive, unbalanced dc-magnetron sputtering from two separate Nb and C targets. Samples with relative carbon content between 49 and 64 at. % were deposited through tuning of magnetron currents. The structure of the films was characterized with X-ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM). X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to characterize the relative composition and chemical bonding. The mechanical properties of the films was determined with nano-indentation and pin-on-disc measurements. Contact resistance measurements were carried out in a custom-built set-up with crossed cylinder geometry.

The results show that both the grain size of the NbC nanocrystallites as well as the relative amount of carbide and amorphous carbon matrix can be controlled by the process parameters. The electrical and mechanical properties are strongly dependent on the amount of matrix and carbide grain size. Optimum conditions are found with amorphous tissue phase between the carbide grains. The contact resistance for nc-NbC/a-C was about 50-90 µΩ at 100 N compared to 60-110 µΩ for nc-TiC/a-C at similar conditions. Possible mechanisms for this behaviour will be discussed.

[1] E. Lewin, O. Wilhelmsson, U. Jansson, Journal of Applied Physics 100, 054303 (2006)