Design-Synthesis-Evaluation of CrN-Based Nanocomposite Hard Coating

Kwang Ho Kim, Se-Hun Kwon and Ji Hwan Yun

Copyright AD-TECH; licensee AZoM.com Pty Ltd.
This is an AZo Open Access Rewards System (AZo-OARS) article distributed under the terms of the AZo-OARS https://www.azom.com/oars.asp which permits unrestricted use provided the original work is properly cited but is limited to non-commercial distribution and reproduction.
AZojomo (ISSN 1833-122X) Volume 6 November 2010

Topics Covered

Abstract

CrN, Cr-Al-N, Cr-Si-N, Cr-Al-Si-N multi-component coatings were deposited by a hybrid coating system combining the arc ion plating and sputtering technique. The microstructure of the nanocomposite coatings was evaluated by both XRD and TEM. The crystalline CrN coating layer was modified to a nanocomposite one by co-deposition of amorphous silicon nitride. The Cr-Al-Si-N coatings exhibited a high hardness of approximately 55 GPa, while those of CrN, Cr-Al-N, and Cr-Si-N coatings were 23, 25, and 34 GPa, respectively. The average friction coefficient of the Cr-Si (9.3at.%)-N and Cr-Al-Si(8.7at.%)-N coatings were largely decreased from 0.51 for CrN coatings and 0.84 for Cr-Al-N coating to 0.30 and 0.57, respectively, by Si addition of ~ 9at.%.

Keywords

CrN, Cr-Al-N, Cr-Si-N, Cr-Al-Si-N, nanocomposite, microstructure

Introduction

CrN coatings are widely used for tribological applications such as forming and casting [1,2] because of superior wear resistance due to low friction coefficient as well as high hardness [3]. Besides, CrN coatings show excellent corrosion resistance under severe environmental condition [4]. Therefore, CrN coatings by various physical vapor deposition (PVD) techniques such as sputtering, cathodic arc evaporation, ion beam sputtering, etc. have been intensively studied [5-7]. Recently, various alloying elements such Ti, Si, Al, B, and C have been added into the binary coatings in order to further improve various properties of the coatings [8]. Among these ternary systems, Cr-Al-N films showed much improved oxidation-resistance up to 900°C due to the formation of stable oxidation barrier of Al₂O₃ layer by migrated Al atoms to surface regions [9-10], and showed slightly higher hardness than that of CrN coatings. On the other hand, Cr-Si-N coatings showed largely increased hardness value and good tribological properties due to characteristics of nanocomposite where the nanosized CrN crystallites were embedded in an amorphous SiNx phase [11,12]. The effect of the addition of both Si and Al together into CrN crystal was, however, not fully investigated. Quaternary Cr-Al-Si-N coatings can have superior properties tailored through two ternary coatings of Cr-Al-N and Cr-Si-N.

In this study, the microstructural design, synthesis and evaluation for the quaternary Cr-Al-Si-N coatings were investigated. Also, the overall comparisons among CrN, Cr-Al-N, Cr-Si-N, and Cr-Al-Si-N coatings were conducted in terms of microstructure and mechanical properties of these coatings.

Experimental Procedure

Four kinds of coatings CrN, Cr-Al-N, Cr-Si (9.3at.%)-N, Cr-Al-Si (8.7at.%)-N were deposited on WC-Co substrates and Si wafer by a hybrid coatings system, where the AIP method was combined with a magnetron sputtering technique. An arc cathode gun for the Cr source and DC sputter guns for the Al and Si sources were installed on each side of the chamber wall. A rotational substrate holder was located among the sources. Ar (99.999 %) and N2 gas (99.999 %) was injected into the chamber. Puritie of Cr, Al and Si targets were 99.99 %. The WC-Co substrates of the disc type (20 mm in diameter and 3 mm in thickness) were cleaned in an ultrasonic cleaner using acetone and alcohol for 20 min.

The compositions of the deposited films were analyzed by using eletron probe microanalyzer (EPMA, Shimadzu). The crystallinity and phase formation of deposited films were investigated with X-ray diffraction (XRD, PHILIPS) using Cu-Ká radiation. The detailed microstructure and crystallite size in the coatings were determined from the direct observation by a field emission-transmission electron microscope (FE-TEM, FEOL, JEM-2010F) operating at 200kV.

Microhardness of the coatings was evaluated using a microhardness tester with Knoop indenter (Matsuzawa, MMT-7) under a load of 25g. The friction coefficient and wear behavior were evaluated through sliding tests using a conventional ball-on-disc wear apparatus. A steel ball (diameter 6.34 mm, 700 Hv_0.2) was used as a counterpart material. The sliding tests were conducted with a sliding speed of 0.157 m/s under a load of 1 N at ambient temperature (around 25°C) and relative humidity (25-30 % RH) condition.

Results and Discussion

Figures 1 (a), (b), (c) and (d) show the schematic microstructure of the CrN, Cr-Al-N, Cr-Si-N and Cr-Al-Si-N coatings, respectively. CrN coatings generally have crystalline microstructure with fairly large grain size at moderate deposition conditions. Cr-Al-N coatings have been known as a solid solution in which Al atoms were substituted for Cr site in CrN without largely affecting the grain size refinement. In Cr-Si-N coatings the amorphous phase between the CrN grains, forming a nanocomposite structure of CrN crystallites are embedded in a SiN_x phase [13]. The designed microstructure of quaternary Cr-Al-Si-N coatings can be expected as a nanocomposite as shown Fig. 1d.

Figure 1. Schematic diagram for the microstructure of the CrN, Cr-Al-N, Cr-Si-N and Cr-Al-Si-N coatings.

Four kinds of coatings (CrN, Cr-Al-N, Cr-Si(9.3at.%)-N, Cr-Al-Si(8.7at.%)-N coatings) were successfully deposited on WC-Co substrates and Si wafer by the hybrid coatings system as described above. Figure 2 shows the XRD patterns for the above coatings: CrN, Cr-Al-N, Cr-Si(9.3at.%)-N, and Cr-Al-Si(8.7at.%)-N coatings. The diffraction pattern of CrN coatings showed the B1 NaCl crystal type with multiple orientations of (111), (200), (220), and (311). As the addition of Al or Si atoms into CrN, the diffraction peak position was a little shifted to higher diffraction angle compared to pure CrN crystals. These peak shift phenomena reflect that the added Al and Si would be dissolved into CrN lattice by substitutional replacement of smaller Al or Si atoms for Cr sites. However, the diffraction peak intensities reduced and the peak shapes were broadened in case of Cr-Al-Si(8.7at.%)-N coatings compared to Cr-Al-N coatings. Such an XRD peak broadening, in general, is an indication of diminution of the grain size in the coating [14]. Any XRD peaks corresponding to Cr₂N, Cr, Si₃N₄, CrSi₂, and AlN were not observed in Fig.2

Figure 2. X-ray diffraction patterns of CrN, Cr-Al-N, Cr-Si-N, Cr-Al-Si-N coatings.

Figure 3 shows the cross-sectional high resolution TEM (HRTEM) images with selected area diffraction patterns (SADP) and dark-field TEM images of CrN, Cr-Al-N, Cr-Si(9.3at.%), and Cr-Al-Si(8.7at.%)-N coatings. As shown in Figs. 3 (a) and (b), HRTEM image and diffraction pattern indicate that the CrN, Cr-Al-N coatings was composed of relatively large columnar grains. However, the Cr-Si-N, Cr-Al-Si-N coatings was found to be composites of fine crystallites and amorphous phase (Figs. 3 (c) and (d)). From the dark field TEM images of Figs. 3 (e), (f), (g), and (h), the above mentioned microstrucutral changes can be more clearly observed. In addition, it was also found that microstruture of Cr-Al-Si(8.7 at.%)-N coatings turned into the refined grains with relatively random orientation by the percolation of Si₃N₄ amorphous phases compared to CrN, Cr-Al-N, and Cr-Si-N coatings.

Figure 3. Cross-sectional HRTEM images with selected area diffraction patterns (SADP) of (a) CrN, (b) Cr-Al-N, (c) Cr-Si (9.3 at.%)-N, and (d) Cr-Al-Si (8.7 at.%)-N coatings and the corresponding dark field TEM images of (e) CrN, (f) Cr-Al-N, (g) Cr-Si (9.3 at.%)-N, and (h) Cr-Al-Si (8.7 at.%)-N coatings.

Figure 4 shows the comparison of microhardness value of CrN, Cr-Al-N, Cr-Si-N, Cr-Al-Si-N coatings. The microhardness values of CrN and Cr-Al-N coatings showed 23 GPa and 25 GPa, respectively. In case of Si incorporation into CrN or Cr-Al-N coatings, however, the hardness value largely increased up to 34 GPa for Cr-Si-N coatings and 55GPa for Cr-Al-Si-N coatigs due to the formation of nanocomposite sutructure and solid-solution hardening.

Figure 4. Microhardness values of CrN, Cr-Al-N coefficients of Cr-Si-N, Cr-Al-Si-N coatings.

Figure 5 shows the friction coefficients of the CrN, Cr-Al-N, Cr-Si(9.3at.%), and Cr-Al-Si(8.7at.%) coatings against a steel ball. The average friction coefficient of the Cr-Si(9.3at.%)-N and Cr-Al-Si(8.7at.%)-N coatings were largely decreased from 0.51 for CrN coatings and 0.84 for Cr-Al-N coatings to 0.30 for Cr-Si-N coatings and 0.57 for Cr-Al-Si-N coatings by Si addition.

Figure 5. The average friction coefficients of CrN, Cr-Al-N, Cr-Si-N, and Cr-Al-Si-N coatings.

Conclusions

CrN, Cr-Al-N, Cr-Si(9.3at%)-N, Cr-Al-Si(8.7at%)-N coatings were deposited on WC-Co substrates using a hybrid coating system of arc ion plating (AIP) and sputtering techniques. From the XRD, HRTEM, and dark field TEM analyses, it was revealed that the synthesized Cr-Al-N coatings consisted of solid-solution of (Cr,Al)N crystallites, and the Cr-Si-N and Cr-Al-Si-N coatings with Si content of ~9at.% were nanocomposites consisting of nanosized solid-solution (Cr,Si)N or (Cr,Al,Si)N crystallites embedded in an amorphous SiN_x matrix. Cr-Al-Si-N coatings with Si content of 8.7at.% were observed to have a maximum hardness of 55Gpa due to the nanocomposite structure as well as solid-solution hardening. The average friction coefficient of the Cr-Si(9.3at%)-N and the Cr-Al-Si(8.7at%)-N coatings with Si content of about 9 at.% were largely decreased compared with those of CrN and Cr-Al-N coatings.

Acknowledgement

This work was supported by a grant from the National Core Research Center (NCRC) Program (R15-2006-022-01002-0) funded by KOSEF and MOST

References

Contact Details

Kwang Ho Kim, and Ji Hwan Yun
Division of Materials Science and Engineering, Pusan National University
Busan 609-735, South Korea

Se-Hun Kwon
Department of Materials Science and Engineering, KAIST
Daejon 305-701, South Korea

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 10[2] (2008) 85-88.