Elsevier

Surface and Coatings Technology

Volume 201, Issue 6, 4 December 2006, Pages 2835-2843
Surface and Coatings Technology

Microstructure development in Cr–Al–Si–N nanocomposites deposited by cathodic arc evaporation

https://doi.org/10.1016/j.surfcoat.2006.05.033Get rights and content

Abstract

Phase and texture analysis using X-ray diffraction, analysis of the diffraction line broadening, analysis of the lattice parameters and high-resolution transmission electron microscopy were employed to characterize the microstructure development in the Cr–Al–Si–N thin film nanocomposites with a variable [Cr] / ([Al] + [Si]) ratio deposited by cathodic arc evaporation. At the highest chromium contents, a single face centered cubic phase formed in the coatings. Below [Cr] / ([Cr] + [Al] + [Si])  0.52, a second crystalline phase developed that was identified as hexagonal AlN. The size of the fcc crystallites decreased with increasing aluminum and silicon contents until it reached 5 nm in the sample with the overall chemical composition Cr0.40Al0.52Si0.08N. The small crystallite size and the presence of two crystalline phases were found to be responsible for a high hardness of the Cr–Al–Si–N nanocomposites. Analysis of the lattice parameters revealed strong crystal anisotropy of the elastic constants in the cubic phase that decreased with increasing aluminum and silicon contents.

Introduction

Chromium nitride coatings are regarded as an alternative to the ultra-hard coatings based on titanium nitride. The industrial applications of the CrN-based coatings exploit excellent wear and hardness properties of CrN, which are accompanied by its very good corrosion resistance and thermal stability [1], [2], [3], [4], [5]. For some applications, it is advantageous that CrN possesses low residual stress, thus relatively thick coatings can be deposited [6]. Because of their properties, CrN-based coatings are primarily used for special tools like hobs for automotive industry, sliding parts or molding dies [7]. Various physical vapor deposition (PVD) processes can be used for the deposition of the CrN coatings as summarized in Ref. [8]. The cathodic arc evaporation (CAE) [9], [10], [11] is one of them. During the last years, the technical importance of the thin films nanocomposites proposed in Ref. [12] increased rapidly. This trend is also evident for nanocomposites based on chromium nitride [13], [14], [15], [16]. In the Ti–Al–N and Ti–Al–Si–N systems, the formation of nano-sized domains was explained by a spinodal decomposition process [17], [18], [19], [20], [21], [22], [23], [24] producing a face-centered cubic (fcc) phase of Ti1−xAlxN and a hexagonal phase of AlN. The third phase in the coatings containing silicon is amorphous Si3N4 [25]. Regarding an analogy between the Ti–Al–Si–N and Cr–Al–Si–N systems, the phase stability of the Cr–Al–Si–N coatings should be one of the parameters controlling their microstructure and properties like for the Ti–Al–Si–N coatings. The fcc-Cr1−xAlxN phase having the NaCl-type crystal structure was found to be stable up to the stoichiometry ratio x = 0.67–0.80 [26], [27]; the critical stoichiometry ratio did depend on the nature of the deposition process.

Recently, we have shown on the example of the Ti–Al–N and Ti–Al–Si–N coatings that the dependence of the stress-free lattice parameter in the fcc phase on the overall chemical composition of the coatings can be used to recognize the decomposition of these systems into two crystalline phases, fcc-Ti1−xAlxN and h-AlN [28]. Although only the Poisson ratio is needed for calculation of the stress-free lattice parameters in cubic thin films [29], the calculation of the stress-free lattice parameters in Cr–Al–Si–N nanocomposites from the X-ray diffraction (XRD) data is not straightforward, because the dependence of the Poisson ratio on the chemical composition is not known. Besides, CrN belongs to materials with a strong crystal anisotropy of the elastic constants [8], [30], [31], [32], [33], which complicates the calculation of the stress-free lattice parameters in the Cr–Al–Si–N nanocomposites additionally. In anisotropic materials, the complete set of the X-ray elastic constants (XECs) must be known to be able to calculate the stress-free lattice parameters. XECs can be calculated from the single-crystalline elastic constants using the famous models [34], [35], [36], [37], [38] if the single-crystalline elastic constants are known, which is not true for the Cr–Al–Si–N system. Other ways of determining the XECs in coatings were proposed in Refs. [8], [30], [33]. However, these experimental techniques work only for coatings containing a single phase, not for composites.

In Ti–Al–Si–N nanocomposites, nanocrystallites with a very small mutual disorientation, i.e. with a high degree of the local preferred orientation, and thus with a high degree of the partial crystallographic coherence were found [28], [39]. The partial crystallographic coherence was recognized from the dependence of the XRD line broadening on the size of the diffraction vector as it reduces the diffraction line broadening at small diffraction vectors [39]. In Ti–Al–Si–N coatings, the partial crystallographic coherence of nanocrystalline domains supported the development of intrinsic residual stresses [28], which improved the hardness of the coatings. The high local preferred orientation of crystallites was accompanied by a three-dimensional global texture, i.e. the preferred orientation of crystallites in the plane and normal to the plane of the coatings. The preferred orientation of crystallites reported for the CrN thin films in Ref. [40] is very similar to the texture observed in the Ti–Al–N and Ti–Al–Si–N coatings deposited using CAE [41].

Section snippets

Experimental details

The Cr–Al–Si–N coatings were deposited using CAE in nitrogen atmosphere at the working pressure of 1.3 Pa using two laterally rotating arc cathodes (π-80 from PLATIT) [42]. One cathode was made of chromium, the second one from aluminum containing 11 at.% Si. The ion current on the Cr cathode was 80 A, on the Al–Si cathode 120 A. The bias voltage was − 75 V. Polished plates of cemented carbide were used as substrates. The base pressure was 5 × 10 3 Pa; the deposition temperature was approximately

Phase composition of the coatings and preferred orientation of crystallites

XRD phase analysis has shown that the samples up to the overall chemical composition Cr0.52Al0.43Si0.05N contain only one fcc phase. At higher aluminum and silicon contents, hexagonal AlN with the wurtzite type structure was detected as the second crystalline phase. Its amount increased with increasing aluminum contents as shown in Fig. 1. Still, the fcc phase prevailed up to the overall chemical composition Cr0.24Al0.65Si0.10N that is comparable with the maximum aluminum contents in the fcc-Cr

Conclusions

It was confirmed that the formation of the nanocomposites is responsible for high hardness of the Cr–Al–Si–N coatings deposited using cathodic arc evaporation. Formation of an amorphous phase surrounding fcc nanocrystallites of Cr–Al(Si)–N was concluded from the combination of GAXRD and HRTEM for [Cr] / ([Cr] + [Al] + [Si]) < 0.7. In this concentration range, GAXRD revealed a higher stress-free lattice parameter than expected for the amount of aluminum and silicon obtained from the EPMA/WDS analysis.

Acknowledgements

The financial support of this work due to the DFG (German Research Foundation) under project # RA-1050/9 is highly appreciated. The HRTEM JEM 2010 FEF was financed through DFG in the frame of the Priority program # 1062. This work is a part of the research program MSM 0021620834 financed by the Ministry of Education of the Czech Republic.

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