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Surface & Coatings Technology

Synthesis and mechanical properties of Cr–Si–N coatings deposited

by a hybrid system of arc ion plating and sputtering techniques

Jong Hyun Park, Won Sub Chung, Young-Rae Cho, Kwang Ho Kim*

School of Materials Science and Engineering, Pusan National University, Busan 609-735, Korea

Available online 11 September 2004

Abstract

Ternary Cr–Si–N coatings, in which Si was incorporated into CrN, were synthesized onto steel substrates (SKD 11) using a hybrid system

of arc ion plating (AIP) and sputtering techniques. In the hybrid coating system for Cr–Si–N coatings, the CrN coating process was

performed substantially by a cathodic AIP technique using Cr target, and Si could be added by sputtering Si target during CrN deposition. In

this work, comparative studies on microstructure and mechanical properties between CrN and Cr–Si–N coatings were conducted. The

hardness of the Cr–Si–N coatings increased from ~22 GPa for CrN, and reached a maximum value of approximately ~34 GPa at the Si

content of 9.3 at.%, and then decreased again with further increase of Si content. The high hardness of Cr–Si–N coatings was related to the

composite microstructure consisting of fine CrN crystallites and amorphous Si3N4. The average friction coefficient of Cr–Si–N coatings

gradually decreased with increase of Si content in CrN coatings.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Cr–Si–N; Hybrid coating system; Microhardness; Friction coefficient

1. Introduction

CrN coatings are popularly used for applications of

tribological forming and casting [1,2], because the coatings

have superior wear resistance due to low friction coefficient

as well as fairly high hardness [3]. Besides, CrN coatings

show excellent corrosion resistance under severe environ-

mental condition [4] and superior oxidation resistance up to

800 8C [5]. Thus, CrN coatings produced by various

physical vapor deposition (PVD) techniques such as

sputtering, cathodic arc evaporation, ion beam sputtering,

etc. have been intensively studied [6–11]. Recently, multi-

component Cr–X–N coatings, where X is the alloying

elements such as Ti [12,13], Al [14,15], B [16], C [17,18],

Ta [19,20], Nb [21], and Ni [22], have been explored in

order to improve the properties of CrN coatings. Never-

theless, Cr–Si–N coatings and the effect of Si addition were

scarcely studied [23]. On the other hand, the effect of Si

0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2004.08.045

* Corresponding author. Tel.: +82 51 510 2391; fax: +82 51 510 3660.

E-mail address: kwhokim@pusan.ac.kr (K.H. Kim).

addition into TiN coatings have been reported to improve

both mechanical properties [24] and oxidation resistance up

to 800 8C [25] due to its composite microstructure

comprising fine TiN crystallites and amorphous silicon

nitride [26,27].

In this paper, ternary Cr–Si–N coatings were synthesized

on steel substrates using a hybrid system of arc ion plating

(AIP) and DC magnetron sputtering techniques. The effect

of Si addition on the microstructural modification and

mechanical properties of CrN coatings were systematically

investigated.

2. Experimental

2.1. Deposition

The Cr–Si–N coatings were deposited on steel substrates

by a hybrid coating system, AIP method combined with a

magnetron sputtering technique. An arc cathode gun for the

Cr source and a DC sputter gun for the Si source were

installed inside the chamber depicted in previous works

188–189 (2004) 425–430

Table 1

Typical deposition conditions for Cr–Si–N coatings by a hybrid coating

system

Base pressure 2.7�10�3 Pa

Working pressure 4.0 Pa

Working gas ratio N2/Ar=5:2

Arc target source Cr (99.99%)

Sputter target source Si (99.99%)

Arc current 50 A

Sputter currents 0–2.2 A

Substrate temperature 300 8CSubstrate bias voltage 0 V

Rotational velocity of substrate 25 rpm

Substrate to target distance 300 mm

Typical film thickness ~3 Am

Fig. 1. X-ray diffraction patterns of Cr–Si–N coatings with various Si

contents.

J.H. Park et al. / Surface & Coatings Technology 188–189 (2004) 425–430426

[24]. Purities of Cr and Si targets were 99.99% and 99.99%,

respectively. A rotating substrate holder was located on a

straight line between the two sources. The rotational speed

of the substrate was 25 rpm. The steel (SKD 11) substrates,

which had been machined to disc types of 20 mm in

diameter and 3 mm in thickness, were cleaned in ultrasonic

bath cleaner using an acetone and alcohol, and cleaned

again by ion bombardment using a bias voltage of �600 V

under Ar atmosphere of 32 Pa for 12 min immediately

before deposition. The substrates were heated by a

resistance heater set inside the chamber. The base pressure

of the vacuum chamber was lower than 2.7�10�3 Pa. The

working pressure was kept at 4.0 Pa. The deposition

temperature was fixed at 300 8C. Typical deposition

conditions for Cr–Si–N coatings by a hybrid coating system

are summarized in Table 1.

2.2. Characterization

The coating thickness was measured using scanning

electron microscopy (SEM, Hitach, S-4200) and a stylus

(a-STEP) instrument. Compositional analyses of the coat-

ings to determine the contents of Cr, Si, and N were

carried out by electron probe micro-analysis (EPMA,

Shimadzu, EPMA 1600). The crystallinity of the Cr–Si–

N coatings was analyzed with low-angle X-ray diffraction

(XRD, PHILIPS, X’Pert-MPD System) using CuKa

radiation. X-ray photoelectron spectroscopy (XPS, VG

Scientifics, ESCALAB 250) was also performed to

observe the bonding status in the Cr–Si–N coatings.

Structural information on the coatings was obtained from

the high-resolution transmission electron microscopy

(HRTEM) using a field emission transmission microscope

(FE-TEM, JEM-2010F, JEOL) with a 200 kV acceleration

voltage.

Computer-controlled nanoindentation (MTS, Nanoinden-

tation XPII) equipped with Berkovich diamond indenter was

used to measure the hardness of Cr–Si–N coatings. The

indenter was operated at a constant displacement rate of 0.2

nm/s until a depth of approximately 100 nm was reached, in

order to avoid the influence of substrate effect.

The average 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

Hv0.2) was used as a counterpart material. The sliding tests

were conducted with a sliding speed of 0.2 m/s under a load

of 1 N at ambient temperature (around 25 8C) and relative

humidity (25–30% RH) condition. Scanning electron micro-

scopy was employed to observe the morphology of the wear

track after each sliding experiment. Energy dispersive

spectroscopy (EDS) was used to reveal the compositions

of wear debris formed during the wear experiment.

3. Results and discussion

Fig. 1 shows the X-ray diffraction patterns of Cr–Si–N

coatings with various Si contents. The diffraction patterns

exhibited a pattern of crystalline CrN coatings with random

orientations of (111), (200), (220), and (311) crystal planes.

As the Si content in the CrN coating increased, the

diffraction patterns of Cr–Si–N coatings showed a weak

(200) preferred orientation, and the diffraction peak position

was a little shifted to the higher angle. However, the

diffraction peak intensity was reduced and a peak broad-

ening phenomenon took place largely from at a Si content of

9.3 at.%. Any XRD peaks corresponding to Cr2N, Cr,

Si3N4, and cromium silicide, such as CrSi2, were not

observed in Fig. 1. Fig. 2 shows the interplanar distance,

d200, of CrN (200) crystal plane as a function of Si content.

The d200 value continually decreased with increasing Si

content, and then exhibited a minimum value at the Si

content of approximately 11.0 at.%. This result can reflect

that the added Si is dissolved into CrN lattice by substitu-

tional replacement of smaller Si atoms for Cr sites. Cr–Si–N

system was scarcely reported, and the solubility data does

not appear in the literature. Although the solubility limit of

Si in the CrN phase is supposed to be very small considering

Fig. 2. Interplaner distance of (200) crystal plane as a function of Si

contents.

J.H. Park et al. / Surface & Coatings Technology 188–189 (2004) 425–430 427

the crystal structure and formation enthalpy of Si3N4 and

CrN, Si can be solved to some extent into CrN crystal

because the vapor depositions, which are proceeded under

high non-equilibrium status are not often related with

expectations from thermodynamics. In Ti–Al–N system,

the phase diagram data indicates that TiN and AlN cannot

have a solubility into each phase [28]. Nevertheless, it is

well reported that Al is dissolved into TiN up to more than

60 at.% [29]. An evidence for the solubility appears with the

change of lattice parameter.

In order to investigate the microstructural change of Cr–

Si–N coatings with addition of Si, TEM observations were

performed on two different coatings. Fig. 3 shows the cross-

sectional HRTEM images, electron diffraction patterns, and

Fig. 3. Cross-sectional HRTEM images, electron diffraction patterns, and dark-

dark-field TEM images for CrN and Cr–Si(9.3 at.%)–N

coatings. From the HRTEM image, electron diffraction

pattern, and dark-field TEM image of the CrN coatings (Fig.

3a and b), the coating layer was found to have a well-grown

crystalline phase having a columnar structure. From the

TEM works (Fig. 3c and d) for the Cr–Si(9.3 at.%)–N

coatings, it was observed that the coating layer was a

composite microstructure consisting of crystallites and an

amorphous phase. In addition, it was also found that large

columnar microstructure of CrN was modified into the

refined grains with relatively random orientation with Si

addition. The electron diffraction patterns of Fig. 3a and c

clearly show this change.

Fig. 4 shows XPS spectra near the binding energies of

Si 2p and Cr 2p for the Cr–Si–N coatings with various Si

contents. The peak corresponding to 101.8 eV, which is in

good agreement with that of Si3N4 compound, was clearly

observed as the Si content increased above 9.3 at.% (Fig.

4a). On the other hand, the peak corresponding to 575.8

eV was found to be that of stoichiometric CrN [30], and

this peak intensity decreased with increase of Si content

(Fig. 4b). The decrease of CrN peak intensity above 9.3

at.% Si content is believed to be attributed to the

appearance of Si3N4 phase. From our XPS results it was

found that the coatings were synthesized as a composite

comprising CrN and Si3N4 as the Si content increased

above 9.3 at.%.

It was concluded from our instrumental analyses of

XRD, TEM, and XPS that the coatings with Si contents

above 9.3 at.% must be a composite consisting of fine CrN

crystallites and amorphous Si3N4.

field TEM images for CrN (a, b) and Cr–Si(9.3 at.%)–N (c, d) coatings.

Fig. 4. XPS spectra near the binding energies of Si 2p (a) and Cr 2p (b) for

the Cr–Si–N coatings with various Si contents.

J.H. Park et al. / Surface & Coatings Technology 188–189 (2004) 425–430428

Fig. 5 shows the microhardness of Cr–Si–N coatings as a

function of Si content. As the Si content increased, the

hardness of the Cr–Si–N coatings gradually increased from

~22 GPa for CrN, and reached maximum values of

approximately 34 GPa at the Si content of 9.3 at.%, and

then steeply decreased again with further increase of Si

Fig. 5. Microhardness of Cr–Si–N coatings as a function of Si content.

content. The hardness value (~34 GPa) of Cr–Si–N coatings

having a Si content of 9.3 at.% was significantly increased

compared with that (~22 GPa) of CrN coatings. The

hardness change of CrN coatings in Fig. 5 results from

the microstructural change with Si addition. The hardness

increase of Cr–Si–N coatings must be strongly related with

grain size refinement of CrN and fine composite micro-

structure. The coatings with 9.3 at.% Si were already proved

by our instrumental analyses to be a fine composite

comprising fine CrN crystallites and amorphous Si3N4. As

seen in Fig. 3, the crystallite size was largely reduced with

codeposition of amorphous silicon nitride. Thus, the grain

boundary hardening described by Hall–Petch relationship

would take place. Furthermore, the codeposition of amor-

phous Si3N4 phase among CrN crystallites would enhance

cohesive energy of the interphase boundaries [30]. On the

other hand, the hardness reduction with further increase of

Si content after maximum hardness, as indicated in Fig. 5,

was suggested to be due to the increase of volume fraction

of amorphous Si3N4 phase. It was reported that the increase

in volume fraction of amorphous Si3N4 phase resulted in the

hardness reduction [24,27]. The Si effect on the micro-

structural evolution and the hardness has been reported

similarly in other systems such as Ti–Si–N [24,26] and Ti–

Al–Si–N [31] systems. Recently, Martinez et al. [23] have

synthesized Cr–Si–N coatings using a sputtering method

and reported that the hardness of Cr–Si–N coatings

increased from ~17 GPa for CrN to the maximum value

of ~23 GPa at Si content of 3 at.%, and then dramatically

decreased. They explained the hardness increase with Si

content due to a solid solution of Si in the CrN lattice, and

the hardness drop beyond the maximum was attributed to

the formation of Si3N4. The differences in the maximum

hardness and Si content between our study and that of

Martinez et al. are considered to be due partly to the

difference of the coating process.

Fig. 6 shows the friction coefficients of the CrN, Cr–

Si(9.3 at.%)–N, and Cr–Si(12.5 at.%)–N coatings against a

Fig. 6. Friction coefficients of the CrN, Cr–Si(9.3 at.%)–N, and Cr–Si(12.5

at.%)–N coatings against a steel ball.

Fig. 7. SEM morphologies of wear track and composition analyses for the wear debris after sliding test: (a) CrN, (b) Cr–Si(9.3 at.%)–N, and (c) Cr–Si(12.5

at.%)–N coatings.

J.H. Park et al. / Surface & Coatings Technology 188–189 (2004) 425–430 429

steel ball. The average friction coefficient of coatings

largely decreased from 0.5 for CrN up to 0.2 with increasing

Si content. Fig. 7 shows SEM morphologies of wear track

and composition analyses for the wear debris after the

sliding test with different Si contents. As the Si content

increased, the wear track morphology became smooth as

shown in Fig. 7. From EDS analyses of Fig. 7, the Fe

element in wear debris decreased with increasing Si

contents. On the other hand, Si and O elements increased

with addition of Si. It is considered that the wear behavior

between the coatings and the steel ball would be adhesive

wear, which was generally caused by hard material and

relatively soft steel ball [32]: the wear tracks were torn off

and the Fe elements from counter material were transferred

into the CrN coating as shown in Fig. 7. The decrease in

friction coefficient with the increase in Si contents would be

caused by a smoother surface due to codeposition of the

amorphous phase and by a tribo-chemical reaction, which

often takes place in many ceramics, e.g., Si3N4 reacts with

H2O to produce SiO2 or Si(OH)2 tribo-layer. These products

of SiO2 and Si(OH)2 are known to a function as a self-

lubricating layer [33]. The formation of tribo-layer would be

more promoted with increasing Si content. The increase of

the self-lubricating layer resulted in decreasing quantity of

transferred Fe from the steel ball. These results correspond

well with the EDS results.

4. Conclusions

Cr–Si–N coatings were deposited on steel substrates by

the hybrid coating system, where the AIP method was

combined with a magnetron sputtering technique. The Cr–

Si–N coatings with the Si contents above 9.3 at.% were a

composite consisting of fine CrN crystallites and amor-

phous Si3N4. The hardness of the Cr–Si–N coatings

exhibited a maximum value of ~34 GPa at a Si content

of 9.3 at.% due to the microstructural change to a fine

composite microstructure and the refinement of CrN

crystallites. The average friction coefficient of the Cr–Si–

N films largely decreased with an increase in Si content.

This behavior can be attributed to the tribo-chemical

reaction between Si and ambient humidity, which enables

to formation of SiO2 or Si(OH)2 tribo-layer, playing a role

as self-lubricant.

Acknowledgement

This work was performed through National Research

Laboratory (NRL) project supported by Ministry of Science

and Technology of Korea (MOST). Authors also thank Dr.

Won of Korea Basic Science Institute for obtaining and

discussing XPS results.

J.H. Park et al. / Surface & Coatings Technology 188–189 (2004) 425–430430

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