A Comparative Study of CrNxcoatings Synthesized by Dc and Pulsed Dc

7/28/2019 A Comparative Study of CrNxcoatings Synthesized by Dc and Pulsed Dc

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A comparative study of CrNx coatings Synthesized by dc and pulsed dcmagnetron sputtering

J. Lin a,, Z.L. Wu a,b, X.H. Zhang a,c, B. Mishra a, J.J. Moore a, W.D. Sproul d

a Advanced Coatings and Surface Engineering Laboratory (ACSEL), Colorado School of Mines, Golden, Colorado, USAb Surface Engineering Laboratory, School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, Chinac Department of Material Science and Engineering, South East University, Nanjing 210096, Chinad Reactive Sputtering, INC, 2152 Goya Place, San Marcos, California, USA

a b s t r a c ta r t i c l e i n f o

Article history:

Received 15 July 2008

Received in revised form 24 September 2008

Accepted 25 September 2008

Available online 7 October 2008

Keywords:

Chromium nitride (CrN)

Pulsed magnetron sputtering (PMS)

Hard coatings

Ion energy distribution (IED)

Plasma diagnostic

Wear

Chromium nitride (CrNx

) coatings were prepared by reactively sputtering chromium metal target with

various nitrogen flow rate percentages (fN2) using a closed field unbalanced magnetron sputtering system

operated in dc and middle frequency pulsed condition (100 kHz and 50% duty cycle). In this study,

plasma examination proved that a large amount of ions with a wide range of ion energies (up to 65 eV and

mainly from 1030 eV region) was identified in the pulsed plasma compared to the low ion flux and energy

(010 eV) in a dc discharged plasma. The results showed that the phase structure of CrN x coatings was

changed from nitrogen doped Cr(N) to pure -Cr2N, and to a mixture of-Cr2N and c-CrN and then to pure c-

CrN phases with an increase in the fN2 in both dc and pulsed conditions. However, the pulsed CrNx coatings

exhibit lower N concentrations than dc CrNx coatings prepared under the same fN2, which leads to the

existing of-Cr2N phase within a wide range offN2 (3050%). In comparison with the typical large columnar

structure in the dc sputtered coatings, the pulsed CrNx coatings exhibit dramatic microstructure

improvements which benefited from the improved plasma density and ion bombardment from the pulsed

plasma, where the super dense and nearly equi-axial structures were observed in a wide range of fN2. The

microstructure improvements contributed to the enhancements in the hardness and wear resistance of

pulsed CrNx coatings. In the pulsed CrNx coatings, the hardness values were above 30 GPa when the fN2 is inthe range of 3040%, which is related to the formation of the -Cr2N phase. With the formation of a mixture

of-Cr2N and c-CrN phases in the coatings deposited with 4050% fN2, a low COF of 0.36 and wear rate of

1.66106 mm3 N1 m1 can be achieved.

Published by Elsevier B.V.

1. Introduction

Chromium nitride (CrN) coating has attracted scientific attention

for many years due to their high hardness, good wear and oxidation

resistance [113]. It also exhibits superior corrosion resistance than

titanium based nitride. These characteristics make CrN an excellent

protective candidate in forming tools, die casting dies, and wear pro-

tection applications as a replacement for chromium electroplating.

Reactive magnetron sputtering is an effective technique to syn-

thesize CrNx coatings by sputtering metal chromium target in the

nitrogen reactive atmosphere. In recent years, the pulsed magnetron

sputtering (PMS) technique has been widely used for producing non-

conducting materials in a near arc free process with desired adhesion

[1416]. Compared to low plasma density in the conventional dc

magnetron sputtering, increased ion flux with a wide ion energy

distribution (up to hundreds of eV) has been well documented within

the pulsed plasma in PMS [1719]. The increased ion energy and ion

flux within a pulsed plasma could have critical effects on the compo-

sition, texture, microstructure and properties of the coatings [2023].

It was also recognized that the compositions, phase structure,

texture,grainsize andproperties of CrNcoatingsare strongly influenced

by the reactive nitrogen gas content, the applied substrate bias and the

substrate temperature [39]. Depending on the nitrogen content, the

CrN coatings may consist of Cr, -Cr2N, c-CrN and/or mixtures of these

phases. It was also found that thenitrogen gas content in the sputtering

atmospherewill change the discharged plasmaion energy distributions,

for example, a decrease in the maximum ionenergy with an increase in

the reactive nitrogen gas percentage was found in pulsed closed field

magnetron sputtering of CrAlN coatings [17].

Accordingly, CrNx coatings were synthesized by dc and pulsed re-

active magnetronsputtering at various nitrogenflow rate percentages in

a closed field unbalanced magnetron sputtering (CFUBMS) system. The

ion energy distributions (IED) in the discharged plasma were character-

ized usingan electrostatic quadrupole plasmamassspectrometer (EQP).

Comparative studies of the compositions, microstructure, mechanical

and wear resistance properties between dc and pulsed CrNx coatings

were carried out using grazing incident X-ray diffraction (GIXRD), field

Thin Solid Films 517 (2009) 18871894

Corresponding author. Tel.: +1 303 273 3178; fax: +1 303 273 3795.

E-mail address: [email protected] (J. Lin).

0040-6090/$ see front matter. Published by Elsevier B.V.

doi:10.1016/j.tsf.2008.09.093

Contents lists available at ScienceDirect

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emission scanning electron microscopy (FESEM), transmission electron

microscopy (TEM), nanoindentation and microtribometry.

2. Experimental details

CrNx coatings were synthesized in a CFUBMS system by sputtering

a metal Cr target (100 mm 280 mm) in a gas mixture of high purity

(99.999%) Ar and N2. The deposition system is a cylinder chamber

equipped with four rectangular unbalanced magnetrons installedvertically around the chamber wall with a 90 degree interval to form a

closed magnetic field. The Cr target was powered using an Advanced

Energy Pinnacle Plus Power supply which can be operated in both dc

and middle frequency pulses conditions. The pulsed parameter was

set at 100 kHz and 50% duty cycle (hereafter refer to 100/5.0). The

pulsed target voltage waveform is in asymmetric pulse shape. During

the positive pulse period, the target voltage is reversed to 10% of the

nominal negative sputtering voltage.

Mirror polished AISI 304 stainless steel coupon with the surface

roughness of Ra=30 nm and silicon (100) wafer were used as the

substrates. The substrates were ultrasonically cleaned in acetone and

denatured alcohol for 20 min respectively and then placed about

150 mm away from the Cr target within the chamber. After the

vacuum system was evacuated below a base pressure of 1.2104 Pa,

the substrates were sputter etched using Ar plasma for 20 min with a

pulsed bias voltage of 400 V (100 kHz and 90% duty cycle). A

chromium adhesion layer (about 100 nm) was firstly deposited onto

the substrates and then the CrNx coatings were deposited for 70

80 min using dc and pulsed dc sputtering with a target power density

of 5.7 W/cm2 (calculated using the effective sputtering area within the

sputter track, which is about 180 cm2), deposition temperatures of

250280 C and a dc substrate bias of50 V. During coating deposi-

tions, the total gas flow rate was maintained constantly at 221 sccm

together with a controlled pumping speed to achieve a constant

working pressure of 0.27 Pa. The flow rate of the nitrogen was varied

from 10 to 70% of the totalflow rate with simultaneous changes in the

Ar flow rate, as both controlled by the MKS 146C vacuum gauge

measurement system together with separate MKS 100 sccmmass flow

controllers. The nitrogen flow rate percentage will be expressed as fN2hereafter.

A Hiden Analytical Ltd EQP was used to characterize the IED in the

discharged plasma. The ion energies measured by the EQP are the

plasma potential relative to the ground potential. During the plasma

examination, the EQP orifice with a diameter of 100 m was posi-

tioned at the same location where the substrate was located during

the normal deposition processes. The same tuning parameters were

applied for all plasma measurements in an effort to keep a consistent

comparison for the IED.

The chemical compositions of the coatings were analyzed using

an energy dispersive X-ray analysis (EDAX) attached to the FESEM.

The EDS measurements were performed on the flat coating surface

(without tilt) on the coated Si substrate with the following constant

parameters for all measurements: a 10 kV accelerating voltage, a 10 Aprobe current and a fixed 10 mm working distance. The crystal struc-

ture of the coatings was characterized using monochromatic Cu-Kradiation on a Siemens X-ray diffractometer (Model KRISTALLOFLEX-

810) operatedat 30kV and 20mA in the GIXRD modeusing a 3-degree

incident angle to minimize the substrate effect. The microstructure of

the coatings was characterized on the fractured cross-sections of the

coatings deposited on the silicon wafer using a JSM-7000F FESEM

operated at 5 kV and a Philips/FEI CM200 TEM operated at 200 kV.

Cross-sectional TEM specimens were prepared by standard cutting

and mechanical grinding method followed by the Ar+ ion milling

process (Gatan Duo-mill).

The nanoindentation hardness and Young's modulus of the coatings

were evaluated using a nanoindenter (NanoIndenter XPTM, MTS Sys-

tems Corporation) equipped with a diamond Berkovich tip. Indentation

hardness (H) and Young's modulus (E) were calculated based on the

model of Oliver and Pharr [24] from the loaddisplacement curves

which were obtained from a constant indentation depth of 300 nm. For

each sample, at least sixteen effective measurements separated by a

distance of 200 m were made to obtain the statistical results.

The wear properties of the coatings were evaluated by a ball on

disk microtribometer (Center for Tribology, Inc) under lubricant free

sliding conditions at constant room temperature of 222 C. The

relative humidity was found to be in the range of 20

25% for all tests.The wear tests were carried out along a circular track of 12 mm

diameter under a load of 3 N and at a constant sliding speed of 25mm/

s, for the duration upto 4800 cycles. A WC6 wt.%Co ball of a diameter

of 1 mmwas selected as thecounterpart. After thewear tests, thewear

tracks were examined using a Daktek surface profilometer to measure

the wear volume by taking average measurements along the wear

track. The wear rate of the coatings, which is defined as the wear

volume of the coating divided by the applied load and the sliding

length (mm3 N1 m1), can be subsequently calculated.

3. Results and discussion

3.1. The discharged plasma diagnostic

Fig. 1 shows the IED of the 52Cr+ species in the dc and 100/5.0

pulsed plasma with a 60% fN2 measured using the EQP at the same

Fig. 1. The ion energy distributions of 52Cr+ species measured from: (a) dc magnetron

discharge and (b) pulsed discharge at 100 kHz and 50% duty cycle. (The fN2 is 60% and

the applied target power density is 5.7 W/cm2

).

1888 J. Lin et al. / Thin Solid Films 517 (2009) 18871894


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target power density (5.7 W/cm2) and working pressure (0.27 Pa). It

can be seen that the IED of52Cr+ ions from the dc discharge shows a

peak at energy of 7 eV and has low ion energy values less than 10 eV

(Fig.1a). In contrast, the 52Cr+ ions exhibit a wide range of ion energies

up to 65 eV in the 100/5.0 pulsed plasma (Fig. 1b). It can be seen that

the presence of the pulsed ion species is mainly from 717 eV and 22

32 eV two ion energy regions which can be correlated to the energy

gain from the asymmetric positive target voltage during the reversed

pulse period [20]. A very small fraction of 50

65 eV high ion energyregion can also be detected in the pulsed plasma. Since a 50 V dc

substrate bias was applied during the depositions, the maximum ion

energy in the dc discharged plasma was about 60 eV, while the

maximum ion energy reached 115 eV in the 100/5.0 pulsed plasma. In

addition, a significant increase in the amount of positive ions (corre-

lated to the areas under the IED curves) was revealed in the 100/5.0

pulsed plasma as compared to the dc discharged plasma, suggesting a

higher ion flux and plasma density. The 36Ar+ and 29N2+ ion species

exhibit similar IED spectra except for the relatively higher maximum

ion energies compared to the 52Cr+ ions due to their mass difference,

and will not be shown here.

3.2. Coating compositions and crystal structure

The chemical compositions of the as-deposited coatings are shown

in Table 1. The composition analyses indicate that the nitrogen

concentration in the coatings increased as the fN2 was increased in the

chamber with the simultaneously decrease in the Cr content. The

oxygen concentrations in all coatings are below 4 at.%. The incor-

poration of Ar atoms was identified for both dc and pulsed CrNxcoatings. Nevertheless, the coatings synthesized in the pulsed con-

ditions exhibit relatively higher Ar incorporation, indicating a higher

ion bombardment in the pulsed plasma. It was also found that the

pulsed CrNx coatings exhibit lower N contents in comparison with

those of the dc coatings synthesized at the same fN2. By increasing

the fN2 to above 50% will lead to near stoichiometric compositions

(N/CrN0.96) in CrNx coatings deposited in the dc condition, whereas

a higher fN2 of 60% is necessary for obtaining similar N/Cr ratio (0.95)

in the coatings prepared at 100/5.0 pulsed conditions.The GIXRD patterns of the coatings deposited at dc and pulsed

conditions are presented in Fig. 2a and b, respectively. In the dc

sputtering conditions (Fig. 2a), the coating deposited at 10% fN2 exhi-

bits a bcc-Cr structure doped with small amount of N atoms, in that

the Cr (110) peak at standard 44.4 (JCPDF 06-0694) was shifted to a

smaller angle indicating a distortion of the Cr lattice due to the

increase of the N interstitials in the bcc-Cr sites [8]. The diffrac-

tion pattern of the dc coating deposited at 20% fN2 indicates the

formation of -Cr2N phase with (111), (112) and (300) reflections

(JCPDF35-0803). Theasymmetry peak at 42.6 indicatesthat there is a

possible weak Cr (110) reflection near 44.4, which canbe attributed to

the coexistence of a small amount of Cr(N) phase in the coating since

the N/Cr ratio here is 0.45 (less than 0.5), as shown in Table 1. When

Table 1

The chemical compositions of CrNx coatings synthesized at various fN2 under dc and

pulsed magnetron sputtering conditions (100 kHz and 50% duty cycle)

DC condition Pulsed at 100 kHz and 50% duty

cycle

fN2[%]

N2 flow

[sccm]

Cr

[at.%]

N

[at.%]

O

[at.%]

Ar

[at.%]

Cr

[at.%]

N

[at.%]

O

[at.%]

Ar

[at.%]

10 2.25 79.23 15.34 3.25 2.18 84.81 7.68 2.74 4.77

20 4.44 65.5 29.61 2.48 2.41 76.07 17.27 2.31 4.35

30 6.85 59.89 35.53 2.62 1.96 63.91 29.21 2.86 4.02

40 9.00 52.27 42.54 3.15 2.04 59.67 34.24 2.47 3.62

50 11.1 48.44 46.41 3.60 1.55 53.67 40.37 2.18 3.78

60 13.2 48.19 48.04 2.47 1.30 48.27 45.70 2.67 3.36

70 15.6 47.21 48.14 2.98 1.67 47.18 46.33 2.94 3.55

Fig. 2. GIXRD patterns of CrNx coatings deposited at various fN2 under (a) dc magnetron

sputtering and (b) pulsed magnetron sputtering (100 kHz and 50% duty cycle).

1889J. Lin et al. / Thin Solid Films 517 (2009) 18871894


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the fN2 was increased to 30%, the dc coating exhibits a polycrystalline

structure consisting of a mixture of-Cr2N andc-CrNphases as shown

in Fig. 2a. Further increasing the fN2 up to 40% in the atmosphere leads

to the formation of near stoichiometric c-CrN phase (JCPDF 77-2494)

in the dc coatings.

For CrNx coatings deposited at 100/5.0 pulsed conditions, similar

crystal phase evolution from Cr(N) to -Cr2N and to a mixture of-

Cr2N and c-CrN, and then to the pure c-CrN with an increase in the fN2

was observed (Fig. 2b). However, it is also recognized that higherfN2 isneeded to achieve these crystal phase changes in the pulsed condition

in comparison with in the dc sputtering. As shown in Fig. 2b, pulsed

coatings exhibit a bcc-Cr structure doped with small amount of N

atoms at both 10% and 20% fN2. The formation of near stoichiometric-Cr2N phases was found at 30% fN2 in the pulsed conditions. When

the fN2 was increased to 40% and 50%, the pulsed coatings contain a

mixture of-Cr2N and c-CrN phases. With further increasing the fN2up to 60%, pure c-CrN phase was observed in thepulsed coating where

the N/Cr ratio reaches 0.95. In general, the crystal structure observa-

tions in the XRD patterns are in good agreement with the coating

composition change.

In the current studies, it is evident that the -Cr2N phase can

only be formed within a very narrow range of fN2 (2030%) in the dc

magnetron sputtering condition, as also reported in several references

[35]. However, it was also found that the -Cr2N phase exists in a

wider range of fN2 (3050%) in the 100/5.0 pulsed condition. In

addition, according to the composition and XRD analysis, higher fN2 is

needed for forming -Cr2N (30% fN2) and c-CrN (5060% fN2) phases in

the pulsed conditions compared to 20% and 40% respectively in the dc

sputtering, suggesting that the efficiency of the nitrogenincorporation

in the CrNx coatings is quite different in the dc and pulsed sputtering

approaches. This phenomenon is possibly related to the enhanced ion

bombardment from the higher ion energies and ion fluxes in the

pulsed plasma, which may induce extensive re-sputtering of the N

atoms near the substrate region due to its smaller atomic mass and

size compared to the Cr atoms, therefore decreasing the fraction of N

atoms and the reactivepossibility between Cr andN atoms arriving on

the substrate. The energetic ion bombardment induced composition

and phase differences in CrN coatings have also been observed fromthe previous reports on the effect of the substrate bias [25,26]. Vyas et

al., found that as the substrate bias was increased from 40 to 140 V,

the structure of the CrN coating completely changes from CrN to Cr2N

phase, indicating a decreased N incorporation efficiency at higher ion

bombardment.

Another observation from the XRD patterns is that the coatings

containing -Cr2N phases exhibit broad diffraction peaks which is

possibly related to the smaller grain sizes, e.g. 20% fN2 in the dc mode

and 3040% fN2 in the 100/5.0 pulsed condition. A tendency of

increase in the grain size can be seen in both pulsed and dc conditions

when the near stoichiometric c-CrN phase was formed at higher fN2.

3.3. The deposition rate and microstructure of CrNx coatings

Fig. 3 shows the deposition rate of the coatings as a function of the

fN2 in the chamber. The deposition rate dropped as the nitrogen was

increased in both dc and pulsed conditions at the same target power

density and the substrate to target distance. This phenomenon can

be explained by the poor N2 sputtering capability compared to Ar

(reduced Ar in the chamber) and also the target poisoning effect

(nitride formationon thetarget surface) when the N2 was increased in

the system. It also can be seen that the deposition rate exhibits lower

values in the pulsed conditions compared to those in the dc sputtering

conditions at the same fN2, which is due to the fact that the pulsed

discharge contains less effective sputtering period than in the dc

mode.

The cross-sectional SEM micrographs of CrNx coatings deposited

with different fN2 at dc and 100/5.0 pulsed conditions are shown in

Fig. 4. All dc sputtered coatings exhibit columnar-type structure. The

dc coating deposited at 10% fN2 which contains the bcc-Cr phase with

small amount of N interstitials exhibits a columnar-type structure

where the column grain size is above 100 nm ( Fig. 4a). By increasing

the fN2 to 20%, a denser microstructure with nearly equi-axial grains

was revealed (Fig. 4b). This structure change is related to the

formation of the -Cr2N phase with broad diffraction peaks as

revealed in the XRD patterns (Fig. 2a). When the fN2 was increased

to 30%, the dc CrNx coating exhibits a dense columnar structure

consisting of short columnar grains with possible different grain sizes

(Fig. 4c). As suggested in the XRD results, this coating contains a

mixture of -Cr2N and c-CrN phases, which possibly comprises

different sizes of the grains. Further increase of the fN2 to above 40% in

the chamber will result in the formation of a large amount of c-CrN

phase in the dc conditions, which normally exhibits typical Zone T

columnar structure as shown in Fig. 4dg. The structural evolution in

the dc sputtered CrNx coatings when the fN2 was increased is tightly

connected with the phase structure and the grain size changesrevealed in the XRD studies (Fig. 2a).

On the other hand, the coatings deposited at 100 kHz and 50% duty

cycle pulsed condition exhibit significant structural improvements in

comparison with the dc sputtered coatings (Fig. 4hn). As shown in

Fig. 4h, the pulsed coating deposit with 10% fN2 exhibits a fine

columnar structure which is similar to the coating deposited in the DC

condition (Fig. 4a). Nevertheless, higher density and finer grain size

have been achieved in the pulsed coating. With the gradual formation

of the -Cr2N and a small volume fraction of c-CrN phases by

increasing the fN2 in the system from 20 to 50% in the pulsed

conditions, super denser microstructure and nearly equi-axial grains

can be observed, as shown in Fig. 4il. When the fN2 was further

increased to above 60% with the formation of large volume fraction of

c-CrN phases (as shown in Fig. 2b), the pulsed coatings exhibit anincrease in the grain size with the formation of short columnar grains,

as shown in Fig. 4m and n. The short columnar grain structure

indicates that the renucleation during the local columnar grain

growth as a result of the increased ion bombardment in the PMS

played an important role for the structural change.

As compared to the dc sputtered CrNx coatings, the structural

changes in the pulsed CrNx coatings suggest a significant decrease in

the grain size and the densification of the microstructure. This

microstructure difference could be explained by two aspects. Firstly,

based on the SEM and XRD studies, the CrNx coatings consisting of the-Cr2N phase exhibit a densermicrostructure andfinergrainsize than

the coatings containing c-CrN phase which typically exhibit large

columnar structure. The microstructure change is consistent with

several previous reports [6,26]. Therefore, the possibility of formation

Fig. 3. The deposition rate of CrNx coatings as a function of the fN2 for dc and pulsed

conditions (100 kHz and 50% duty cycle).



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of the -Cr2N phase within a wide range of fN2 (3050%) in PMS as

observed in the XRD patterns (Fig. 2b) may contribute to the structure

improvement.

In another more important aspect, the beneficial effects of high

ionization degree and density plasma with properly controlled

energies from PMS for improving the structure and properties of the

coatings are needed to be considered [20]. In the current study, by

applying 50 V dc substrate bias on the substrate, a large amount of

ionswithion energiesin the range of 60 to80 eVtogether witha small

fraction of ions with ion energies about 115 eV (including 50 V from

the substrate bias) will be attracted towards the substrate bombarding

the growing films in the 100/5.0 pulsed conditions. This energetic

bombardment can effectively transfer the energies to the adatoms,

increase the adatom mobility and nucleation sites, thereby decreasing

the grain size, sealing the porosities between columnar grains and

densifying the coatings. In addition, renucleation on the growing

Fig. 4. Cross-sectional SEM micrographs of CrNx coatings deposited at various fN2 (1070%) in dc magnetron sputtering (a, b, c, d, e, f, g) and 100 kHz and 50% duty cycle pulsed

conditions (h, i, j, k, l, m, n).



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grains due to the ion bombardment may also account for the den-

sification of the film and a fine grain size in the structure [27]. In

contrast, in the dc magnetron sputtering, only a small amount of ions

with the energies of 50 to 60 eV (including 50 V from the substrate

bias) will contribute to the effective ion bombardments, which is

obviously notenough to break down thelargecolumn growth, seal the

porosities and achieve high densities in the coatings.

This second effect is further proven in the TEM study. Cross-

sectional TEM micrographs and insert SAED (selected area electrondiffraction) patterns of CrN coatings deposited at 60% fN2 in dc and

100/5.0 pulsed conditions are presented in Fig. 5a and b respectively.

The structure of the coating deposited by continuous dc is character-

ized as a clearcolumnar structure with average columnwidth of about

2030 nm. The voids between the columnar grains (indicated by the

dark strips) are clearly identified (Fig. 5a). The discontinued diffraction

rings in the SAED pattern confirm the presence of relatively large

grains of CrN B1 NaCl reflections. In contrast, the pulsed CrN coating

exhibits a less pronounced columnar structure (Fig. 5b). The voids

between the column grains become less remarkable, suggesting a

denser microstructure was obtained. In addition, the SAED pattern

shows more continuous rings without sharp spots compared to the dc

coating, indicatingfinergrains (b10 nm from the micrograph observa-

tion) in the pulsed coating.

Besides the positive effect of the energetic deposition in PMS, it is

also recognized that excessive ion bombardment will induce the

accumulation of the residual stress and point/line defects in the

growing coatings, which is detrimental to the coating toughness and

adhesion [5,20]. The plasma analysis in the current study indicated

that only a very small fraction of ions with high energies in the range

of 100115 eV were presented in the 100/5.0 pulsed plasma, while a

large fraction of ions come from 5080 eV (adding 50 V from the

substrate bias), suggesting a possible small incorporation of stress anddefects in the growing coatings.

3.4. Mechanical and tribological properties

The nanoindentation hardness values of CrNx coatings deposited

by dc and PMS as a function of fN2 are presented in Fig. 6. As can be

seen, the hardness of the coatings with an increase in the fN2 exhibits

very similar trend for both dc and pulsed conditions. With the

incorporation of a small fraction of N (fN2=10%), a rapid increase in the

hardness from 1011 GPa in the pure Cr coatings to 2022 GPa in

the Cr(N) coatings were observed in both conditions, which is due to

theformation of thecovalent CrN bonds. The highest hardnessvalues

of25 GPa and 31 GPa werefoundat 20% and 30%fN2 in the dc and 100/

5.0 pulsed conditions respectively, which correspond to the formation

Fig. 4 (continued ).



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of a large volume fraction of -Cr2N phases as identified in the

XRD pattern. This is probably because the coatings consisting of pure

-Cr2N phase exhibit higherdensityandfinergrainsize than theother

mixture phase or pure c-CrN coatings, as shown in the SEM ob-

servations (Fig. 4). It was also well recognized that the -Cr2N phase is

more covalent than the c-CrN phase leading to higher intrinsic

hardness than that of the cubic phases [28]. After that, a slightly

decrease in the hardness can be seen in both conditions when the

coatings consist a mixture of-Cr2N and c-CrN phases when the fN2 is

in the range of 3040% in the dc condition, and 4050% in the pulsed

condition. In both cases, the hardness values of CrNx coatings were

slightly increased again after 40% and 60% fN2 respectively ac-

companied with the formation of near stoichiometric c-CrN phase in

the coatings. The slightly drop of the hardness when mixture phases

were presented in the CrNx coatings is consistent with the previous

reports [6].

Furthermore, the hardness results confirm that when the fN2 ex-

ceeded 20%, the coatings deposited in the pulsed conditions exhibitsuperior hardness values than the coatings produced by dc sputtering.

The enhancement of hardness in the pulsed coatings was probably

attributed to their higher density, finer grain size and also possible

higher residual stress generated from higher ion bombardment.

Fig. 5. Cross-sectional TEM micrographs of CrNx coatings deposited at 60% fN2 in (a) dc

magnetron sputtering and (b) pulsed magnetron sputtering (100 kHz and 50% duty

cycle).

Fig. 6. The hardness of CrNx coatings as a function offN2 produced under dc and pulsed

magnetron conditions (100 kHz and 50% duty cycle).

Fig.7. (a)The coefficientof friction and(b) thewearrateof CrNx coatings as a function of

fN2 prepared in dc and pulsed magnetron conditions (100 kHz and 50% duty cycle).



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Thecoefficient of friction (COF) and the wear rates of CrNx coatings

deposited under dc and pulsed conditions sliding against a WC6%Co

ball in the lubricant free condition at a normal load of 3 N are plotted

in Fig. 7a and b respectively. All COF values were measured from the

steady sliding state between the counter part and the coatings. The

COF was found to firstly decrease with an increase in the fN2 and

reached the lowest values of 0.47 at 40% fN2 and 0.36 at 50% fN2 in the

dc and pulsed coatings respectively, and then increase again with a

further increase in the fN2 (Fig. 7a).By examining the wear rate after the sliding tests, it was found

from Fig. 7b that the wear rate evolution as a function of the fN2followed the trend of the COF. As the fN2 is higher than 20%, the

wear rate of CrNx coatings decreased rapidly and exhibited low

values of about 3 106 mm3 N1 m1 in the dc CrN coatings and 1.66

to2 106 mm3 N1 m1 in the pulsed coatings at 4050% fN2 (Fig. 7b).

The XRD patterns shown in Fig. 2 indicate the presence of the mix-

ture of -Cr2N and c-CrN phases at 30% fN2 in the dc sputtered

coatings and at 3050% fN2 in the pulsed coatings. Therefore, the

decrease in the COF and wear rate is possibly associated with the

presence of-Cr2N and c-CrN mixturein the coatings. However, with

the formation of single c-CrN phase at higher fN2 percentages, the

COF and the wear rate of the coatings increased again in both

conditions.

It was also evident that the COF and the wear rate of all pulsed

coatings were lower than those of the dc coatings prepared at the same

fN2. This behavior emphasizes that the denser structure,finer grain size,

and higher hardness, observed in the pulsed CrNx coatings will result

in the improved wear resistance compared to the dc sputtered coatings.

4. Conclusions

Chromium nitride (CrNx) coatings were prepared using a CFUBMS

system under dc and middle frequency pulsed condition (100 kHz and

50% duty cycle) with different nitrogen flow rate percentages (fN2) in

the system. It was found that the crystal phases changed from bcc-Cr

structure doped with small amount of N atoms to pure -Cr2N, and to

a mixture of-Cr2N and CrN phases, and then to the pure c-CrN with

an increase in the fN2 for both dc and pulsed conditions. However, itis also recognized that the N2 incorporation efficiency in the PMS is

lower than that in the dc condition, therefore higher fN2 is needed in

the pulsed sputtering for the corresponding crystal structure changes

observed in the dc sputtering. The CrNx coatings consisting of near

stoichiometric -Cr2N phase exhibit the highest hardness in both dc

and pulsed conditions, while the low coefficient of friction was found

in the coatings containing a mixture of-Cr2N and c-CrN phases.

Plasma examination showed that a large amount of ions with a

wide range of ion energies (mainly from 1030 eV) were identified in

the 100 kHz and 50% duty cycle pulsed plasma compared to lower ion

flux and energy (10 eV) in a dc discharged plasma. By applying 50 V

dc substrate bias, the increased ion fluxes and energies in the pulsed

plasma could be accelerated towards the substrate to enhanced the

ion bombardment, which can be utilized to increase the adatom mo-

bility, increase the nucleation sites, and seal the void columnar grain

boundaries, thereby resulting in denser structure and finer grain size

in the pulsed coatings. The improved microstructure contributed to

the improvements in the hardness and wear resistance of pulsed CrN

coatings, where the high hardness values above 30 GPa were obtained

when the fN2 is in the range of 3040% and a low COF of 0.36 and a

wear rate of 1.66 to 2106 mm3 N1 m1 were found in the coating

deposited with 40

50% fN2.

Acknowledgements

The authors are grateful for the financial support of this research

program from DOE-OIT, ATI, and the North American Die Casting

Association (NADCA).

References

[1] P.H. Mayrhofer, G. Tischler, C. Mitterer, Surf. Coat. Technol. 142/144 (2001) 78.[2] P.H. Mayrhofer, H. Willmann, C. Mitterer, Surf. Coat. Technol. 146/147 (2001) 222.[3] A.P. Ehiasarian, P. Eh. Hovsepian, L. Hultman, U. Helmersson, Thin Solid Films

457 (2004) 270.[4] G.A.Zhang, P.X. Yan, P. Wang, Y.M. Chen, J.Y. Zhang, Mater. Sci.Eng. A 460/461 (2007)

301.

[5] E. Fornis, R. EscobarGalindo, O. Snchez,J.M. Albella,Surf.Coat. Technol. 200 (2006)6047.

[6] Zenghu Han, Jiawan Tian, Qianxi Lai, Xiaojiang Yu, Geyang Li, Surf. Coat. Technol.162 (2003) 189.

[7] Z.G. Zhang, O. Rapaud, N. Bonasso, D. Mercs, C. Dong, C. Coddet, Vacuum 82 (2008)501.

[8] G. Wei, T.W. Scharf, J.N. Zhou, F. Huang, M.L. Weaver, J.A. Barnard, Surf. Coat.Technol. 146/147 (2001) 357.

[9] N. Schell, J.H. Petersen, J. Bttiger, A. Mcklich, J. Chevallier, K.P. Andreasen,F. Eichhorn, Thin Solid Films 426 (2003) 100.

[10] Z.B. Zhao, Z.U. Rek, S.M. Yalisove, J.C. Bilello, Thin Solid Films 472 (2005) 96.[11] L. Cunha, M. Andritschky, Surf. Coat. Technol. 111 (1999) 158.[12] T. Polcar, N.M.G. Parreira, R. Novk, Surf. Coat. Technol. 201 (2007) 5228.[13] J.W. Seok, N.M. Jadeed, R.Y. Lin, Surf. Coat. Technol. 138 (2001) 14.[14] R.D. Arnell, P.J. Kelly, J.W. Bradley, Surf. Coat. Technol. 188/189 (2004) 158.[15] P.J. Kelly, O.A. Abu-Zeid, R.D. Arnell, J. Tong, Surf. Coat. Technol. 86/87 (1996) 28.[16] J. O'Brien, P.J. Kelly, Surf. Coat. Technol. 142/144 (2001) 621.[17] J. Lin, J.J. Moore, B. Mishra, W.D. Sproul, J.A. Rees, Surf. Coat. Technol. 201 (2007)

4640.[18] J.W. Bradley, H. Bcker, Y. Aranda-Gonzalvo, P.J. Kelly, R.D. Arnell, Plasma Sources

Sci. Technol. 11 (2002) 165.[19] J.W. Bradley, H. Bcker, P.J. Kelly, R.D. Arnell, Surf. Coat. Technol. 142/144 (2001)

337.[20] J. Lin, J.J. Moore, B. Mishra, M. Pinks, W.D. Sproul, J.A. Rees, Surf. Coat. Technol.

202 (2008) 1418.[21] P.J. Kelly, T.vom Braucke, Z. Liu,R.D. Arnell, E.D.Doyle,Surf.Coat. Technol. 202(2007)

774.[22] P.J. Kelly, C.F.Beevers, P.S. Henderson, R.D. Arnell, J.W. Bradley,H. Bcker,Surf. Coat.

Technol. 174/175 (2003) 795.[23] P.S. Henderson, P.J. Kelly, R.D. Arnell, H. Bcker, J.W. Bradley, Surf. Coat. Technol.

174/175 (2003) 779.[24] W.C. Oliver, G.M. Pharr, J. Mater Res. 7 (1992) 1564.[25] A. Vyas, Y.G. Shen, Z.F. Zhou, K.Y. Li, Compos. Sci. Technol. 68 (2008) 2922.[26] T. Hurkmans, D.B. Lewis, J.S. Brook, W.D. Munz, Surf. Coat. Technol. 86/87 (1996)

192.[27] I. Petrov, P.B.Barna, L. Hultman, J.E. Greene, J.Vac. Sci. Technol.A 21(5) (2003)S117.[28] R. Sanjins, P. Hones, F. Lvy, Thin Solid Films 332 (1998) 225.


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