Elsevier

Applied Surface Science

Volume 363, 15 February 2016, Pages 217-224
Applied Surface Science

Comparison in mechanical and tribological properties of CrTiAlMoN and CrTiAlN nano-multilayer coatings deposited by magnetron sputtering

https://doi.org/10.1016/j.apsusc.2015.12.005Get rights and content

Highlights

  • Well-defined layer interfaces were observed by TEM.

  • CrTiAlMoN coatings exhibit higher hardness, lower COF and wear rate.

  • The hardness of these coatings are determined by the type of layer interfaces.

  • The COF of these coatings are related to Mo content and their oxidation behaviors.

Abstract

CrTiAlN and CrTiAlMoN nano-multilayer coatings were deposited by closed field unbalanced magnetron sputtering. TiMoN and CrTiMoN nano-multilayer coatings with same Mo2N layer thickness were also prepared for comparison. The structure of these coatings is investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The mechanical and tribological properties were characterized and compared by nano-indentation and ball-on-disc test. It was found that these coatings were structured by fcc metal nitride phases (including CrN, TiN, AlN and Mo2N) and the preferred orientation changed from (1 1 1) to (2 0 0) with the increase of Mo content. The TEM results showed that the coatings exhibited typical columnar structure and nano-multilayer structure with modulation periods ranged from 3.2 nm to 7.6 nm. Among these coatings, CrTiAlMoN coatings presented the highest hardness, lowest coefficient of friction (COF) and wear rate. The hardness of these nano-multilayer coatings were determined by layer interfaces: TiN/Mo2N and AlN/Mo2N layer interface showed benefit on hardness enhancement while CrN/Mo2N layer interface led to a great hardness decrement. In comparison with the other as-deposited coatings, the low COF of CrTiAlMoN coatings was not only affected by Mo addition but also related to its oxidation behaviors.

Graphical abstract

CrTiAlMoN coatings exhibit much lower coefficient of friction and higher hardness than that of CrTiAlN and other Mo containing nitride coating.

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Introduction

Transition metal nitride coatings have been used as a protection of cutting and forming tools for several decades. To extend the lifetimes of these protective nitride coatings, numerous efforts have been made on enhancing hardness and oxidation resistance by incorporating elements such as Cr, Al, Si etc into TiN to form multilayer coatings [1], [2], [3], [4]. By forming multilayer structures, tribological properties of nitride coatings could possibly be improved by increasing the coating/substrate adhesion and load support, by reducing the surface stressed and by improving the resistance for crack propagation [5]. However, it couldn’t fulfill the increasing demands of machining processes like high-speed cutting and dry cutting because of the abrasion and tribo-oxidation caused by the rising mechanical and thermal loads [6]. Reducing the coefficient of friction (COF) is supposed to be helpful in improving their performance in such kinds of applications. However, although nitride coatings with intrinsic solid lubricant layers, such as TiN/MoS2 [7] multilayer coatings exhibits low COF below 0.15, it often failed in their tribological effectiveness because of its poor oxidation resistance [8], [9]. Therefore, integrating high temperature lubricants into the nitride coating could be a possible way to solve these problems.

Thermo- and tribo-oxidatively formed Magnéli oxide phase, which show easy moveable crystallographic shear structure and oxidation stability, are supposed to be ideal high temperature solid lubricants [10], [11]. Molybdenum nitrides are known to form such oxygen-deficient Magnéli phase at elevated temperatures [12], [13]. Nevertheless, monolithic Mosingle bondN coatings would also not be suitable for practical applications because of its easy oxidation characteristic, but it would be a potential candidate for developing nitride coatings with low COF.

Until now researches about niride/molybdenum nitride multilayer coatings are mainly limited on TiMoN [14], [15], [16], CrMoN [17] etc binary multilayers. It has been demonstrated that nano-multilayered CrTiAlN coatings exhibit a quite low wear rate, a high oxidation resistance and good thermal stability, which are important factors for application on dry cutting [18]. In addition, in most of the nitride/nitride multilayer systems, the maximum hardness values show at modulation period around 2–8 nm [19]. Therefore, in this work, with the aim to explore the effects of Mo addition to CrTiAlN coatings on structural and tribological properties, we fabricated CrTiAlN and CrTiAlMoN nano-multilayer coatings. A TiMoN and a CrTiMoN nano-multilayer coating were also prepared for investigating the effect of Mo2N/MeN(Me: Cr, Ti, Al) layer interface on mechanical and tribological properties.

Section snippets

Experimental

TiMoN, CrTiMoN, CrTiAlMoN and CrTiAlN coatings were deposited on (1 0 0) silicon wafers and M2 high speed steel substrates by closed field unbalanced magnetron sputtering ion plating system. Schematic diagram of the system and arrangement of rectangular (330 mm × 133 mm) Cr, Ti, Al and Mo elemental targets are presented in Fig. 1. Silicon wafers and mechanically polished steel substrates was firstly ultrasonically cleaned in acetone and then placed on a rotating sample holder. The substrate to target

XPS analysis

The typical Ti 2p, Cr 2p, Al 2p, Mo 3d, N 1s and O 1s XPS spectra of CrTiAlMoN coatings are presented in Fig. 2. For the Ti 2p spectra (Fig. 2a), two characteristic peaks corresponding to the 2p3/2 and 2p1/2 electrons have been systematically deconvoluted into four chemical binding states, which could be assigned to the Tisingle bondN bond (binding energy (BE) = 454.9 eV, 460.7 eV) in TiN and Tisingle bondNsingle bondO bond (BE = 456.9 eV, 462.2 eV) in TiNxOy. The Cr 2p spectra (Fig. 2b) was also deconvoluted into four binding states,

Conclusions

CrTiAlMoN, CrTiMoN, TiMoN and CrTiAlN coatings were deposited by closed field unbalanced magnetron sputtering ion plating system. All of the coatings exhibit a face center cubic structure similar to B1-NaCl. The CrTiAlN coating exhibits (1 1 1) texture, while with the increment of relative Mo2N layer thickness, the preferred orientation of these Mo-contained coatings gradually changed from (1 1 1) to (2 0 0). Both of the CrTiAlMoN and CrTiAlN coatings exhibit columnar growth multilayer structures.

Acknowledgments

This subject was supported by the National Natural Science Foundation of China ( Grant Nos. 51171149 and 51371141), the National Science Technology Supporting Program of China (Grant No. 2012BAE06B02) and the Science Technology Co-ordination Innovation Project of Shaanxi Province (Grant No. 2013KTZB01-03-04).

References (37)

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