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Baranov, Oleg O., Fang, Jinghua, Rider, Amanda E., Kumar, Shailesh, &Ostrikov, Kostya (2013) Effect of ion current density on the properties ofvacuum arc-deposited TiN coatings. IEEE Transactions on Plasma Sci-ence, 41(12), pp. 3640-3644.

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1

Submitted to IEEE Transactions on Plasma Science.

Effect of ion current density on the properties of

vacuum arc-deposited TiN coatings

Oleg Baranov,1 Jinghua Fang,2,3* Amanda E. Rider, 2,4 Shailesh Kumar,2

and Kostya (Ken) Ostrikov2,4

1Plasma Laboratory, National Aerospace University “KhAI,” Kharkov 61070, Ukraine 2CSIRO Materials Science and Engineering, P.O. Box 218, Lindfield, NSW 2070, Australia

3School of Physics, University of Melbourne, Parkville, VIC 3010, Australia 4Complex Systems, School of Physics, The University of Sydney, Sydney, NSW 2006, Australia

The influence of ion current density on the thickness of coatings deposited in a vacuum

arc setup has been investigated to optimize the coating porosity. A planar probe was used

to measure the ion current density distribution across plasma flux. A current density from

20 to 50 A/m2 was obtained, depending on the probe position relative to the substrate

center. TiN coatings were deposited onto cutting inserts placed at different locations on

the substrate, and SEM was used to characterize the surfaces of the coatings. It was

found that low-density coatings were formed at the decreased ion current density. A

quantitative dependence of the coating thickness on the ion current density in the range

of 20 to 50 A/m2 were obtained for the films deposited at substrate bias of 200 V and

nitrogen pressure 0.1 Pa, and the coating porosity was calculated. The coated cutting

inserts were tested by lathe machining of the martensitic stainless steel AISI 431. The

results may be useful for controlling ion flux distribution over large industrial-scale

substrates.

DC discharges; ion-assisted deposition; process control; thin films

* Corresponding author. Email: jinghua.fang@csiro.au

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I. Introduction

Plasma reactors and systems [1] have been widely used for etching, deposition and modification of

thin films and other nanostructures [2,3,4,5]. During the plasma treatment process, the structure and

properties of films grown at low ion energy and strong ion fluxes differ from those of the films

deposited at high ion energy [6,7] but low ion flux [8,9,10]. The film growth temperature is also

influenced by the ion current and in turn, affects the film properties [11,12,13]. Therefore, the ion

current density is the key parameter to be controlled either to ensure uniformity of the plasma

treatment from the center of the substrate to the edge [14,15], or to tailor the characteristics of the

growing film by varying the current density in particular areas of the substrate [12,16,17]. Whilst

structure zone diagrams (SZD) may be used to qualitatively describe the influence of ion current

density on the properties of a processed surface, they are not suitable to completely explain a

material’s behaviour. This is because any combination of substrate, film material, and deposition

conditions constitues a unique system that is not adequately described by SZD and must be treated

experimentally.

Recently a method involving the control of ion current density to regulate the ion flux extracted

from a range of plasma reactors was published [18,19,20,21]. This method allows the plasma flow

to be focussed to a particular area of the substrate to obtain high-density ion fluxes, as well as de-

focussed to uniformly process large-area substrates. Defocusing ensures a rise in the productivity by

increasing the number of pieces being processed in a single cycle. However, an average ion current

density over the substrate remains low. As a result, the coating performance will decrease due to the

formation of the pores in the coating. Thus, it is very important to optimize the ion current density

control to decrease the chance of the porous coating formation and to develop more effective and

efficient plasma coating technology applications.

3

In this paper we investigate the influence of the ion current density on the formation of dense

coatings on the substrates in the vacuum arc deposition system. The motivation of the work is to

determine the limits for the ion current density that are acceptable for the formation of TiN coatings

on a surface of cutting tool inserts.

II. Experimental setup and procedure

A schematic of the experimental setup is shown in Fig. 1. It includes a vacuum arc plasma source

and a planar probe for measuring the radial distribution of an axial component of the ion flux. The

plasma source was fitted with a water-cooled truncated cone-shaped titanium cathode and a tubular

water-cooled anode. The cathode cone was 60 mm long, with a 50 mm diameter upper surface and a

base diameter of 60 mm. The anode had a 210 mm inner diameter and a length of 200 mm. A

guiding coil was mounted on the anode which was used as a plasma duct.

The plasma source was mounted on a flange of 500 mm diameter, 500 mm long cylindrical

vacuum chamber. The dc arc current, Ia = 100 A, was applied between the cathode and the anode,

which was grounded. Focusing and guiding coils generated an axial magnetic field in the plasma

source. The focusing magnetic field Bf (0.03 T at the center of the focusing coil) was used to retain

cathode spots on the cathode face. The guiding magnetic field Bg (0.016 T at the center of the

guiding coil) was used to guide the plasma beam towards the substrate.

A disk-shaped substrate made of non-magnetic stainless steel was installed in the vacuum

chamber at a distance of 250 mm from the plasma duct exit in such a way that the substrate and

plasma duct (anode) axes of symmetry coincided. The substrate diameter and thickness were

400 mm and 8 mm, respectively. The substrate was under a negative potential of -200 V relative to

the grounded walls of the vacuum chamber. An automated gas supply system maintained a nitrogen

4

pressure of 0.1 Pa in the chamber. The pressure was measured with the help of a thermocouple

vacuum gauge and ionization gauge.

A planar probe was used to measure the ion current density distribution in the axial plasma flux

as a function of the probe position over the substrate (r axis in Fig. 1). The probe was a

50×48×0.5 mm current-collecting plate made of polished non-magnetic stainless steel with a high-

temperature insulator on one side. The probe was connected to the power supply via a separate

ammeter. When the vacuum arc plasma source was on, the voltage drop between the probe and the

anode was 200 V, so the ion saturation current was collected [ 22 ]. The duration of each

experimental run was 2 s.

The cutting tool inserts (16×16×5 mm, 89 % WC + 15 %( Ti+Ta)C + 6 % Co) were placed on

the substrate at points r = 0, 40, 70, 100, 130, and 160 mm to deposit TiN coating on the cutting

surfaces. Before deposition, the inserts were cleaned and heated by the ion flux at a bias potential of

1.5 kV for 8 minutes. The deposition time was 30 minutes. After deposition, the side surfaces of the

cutting inserts were polished, and SEM images of the coating were made to determine the coating

thickness and morphology.

The cutting inserts were tested by lathe machining of the martensitic stainless steel AISI 431

which is well-known as a material possessing very high hardness and strength combined with

excellent toughness [23]. This steel is successfully used in a variety of aircraft and general industrial

applications including fasteners, bolts, valve components, and chemical equipment. The cutting

mode was as follows: cutting speed 230 m×min-1, line feed rate of 0.21 mm per rev., and depth of

cut 1.0 mm. As a criterion of the critical wear of the insert, 0.4 mm of flank wear was assumed.

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3. Experimental results and discussion

The experimental data were approximated using the Gaussian distribution function, and the

following expression for the ion current density is obtained:

( )

−+−=2

652102426089

.

rexp..rJ i , (A/m2) (1)

where r is the coordinate along the substrate surface, mm.

The dependence of the ion current density distribution on radius r (see Fig. 1) was examined

during stable arc operation. Fig. 2(a) shows the results of the approximation of the experimental

data points, which were obtained by averaging 10 measurements when using the planar probe. As is

seen, the radial distribution of the current density is strongly non-uniform, it reaches a maximum of

50 A/m2 at the substrate center (r = 0) and decreases to 18 A/m2 near the substrate edge

(r = 180 mm).

The distribution of the experimentally determined thickness of the coating deposited on the

cutting inserts is shown in Fig. 2(b). The thickness data obtained were also approximated using a

Gaussian distribution function as follows:

( )

−+=2

193945420

rexp..rhc . (µm) (2)

The distribution of the coating thickness is non-uniform varying from 5.01 µm at the substrate

center to 2.74 µm near the substrate edge (r = 160 mm). It can be noted that decreasing the ion

current density in 2.1 times (Ji(160) = 24 A/m2) leads to a decrease in the coating thickness by a

factor of 1.8 times only. The maximum tool life Tc max = 33 minutes was obtained in the lathe

machining test for the cutting insert located at r = 40 mm. A plot of a relative tool life Tc(r)/Tc max as

a function of location r of the cutting inserts during the coating deposition is shown in Fig. 2(c). It

6

can be seen that a sharp drop of the tool life is observed for the cutting inserts located at

r > 120 mm.

Relative ion current distribution and coating thickness were also calculated, and are presented in

Fig. 2(d). These results confirm a gradual divergence between the dependencies of the ion current

density and coating thickness on radius r. Thus, the non-linear behavior of the coating thickness on

the ion current density and formation of the porous coatings is present.

Scanning electron microscopy (SEM) images of the coatings on the cutting inserts at different

locations along the substrate surface are shown in Fig. 3. As one can see, the coatings in images (a)-

(d) are not damaged by the polishing, which can be regarded as an evidence of the dense coating.

The coating in Fig. 3(e) demonstrates sighs of porosity, since the polished section is not even and

the coating is damaged by the polishing near its surface. The fracture and peeling the coating off the

insert surface shown in Fig. 3(f) can be due to inadequate mechanical properties of the coating. The

fracture reveals the columnar morphology of the coating with the column diameter of about 200 to

400 nm.

To calculate the porosity, we use the property of an equation of a tangent line derived for a point

of a function, to predict the function in the linear approximation. In our case, the tangent line

equation derived for the dependence of the coating thickness on the ion current density for the ion

current density at which the dense coating is formed, allows the value of the coating thickness to be

predicted, assuming dense coating formation only. Any deviation of the real dependence from the

tangent line can be calculated and considered as the porosity-related effect.

The thickness of the coating is related to the ion current density through equations (1) and (2):

7

( )1841

2460

89945420

.

iic .

.J..Jh

++= . (µm) (3)

Assuming that the non-linearity of the above dependence results in the formation of a low-dense

(porous) coating, the influence of the ion current on the coating structure may be estimated as

follows: The dependence of the thickness of the dense coating on the ion current density is

approximated by the equation of the tangent line to the curve (3) drawn through the point of the

maximum ion current density, where the most dense coating is formed (i.e. 50 A/m2, according to

Fig. 2(a)):

( ) ( ) 10255009700 .J.Jh iic +−= . (µm) (4)

A relative volume of the coating can be estimated by the expression ( ) ( )( )30 icic JhJh which is

plotted in Fig. 4, together with values hc(Ji) and hc0(Ji). The coating porosity is calculated as

( ) ( )( ) 130 −= icicCoat JhJhε , (5)

and hence, an increase of the porosity with decreasing ion current density can be determined. This

increase may be attributed to the lower substrate temperature at the deposition site.

For the cutting inserts, a test by lathe machining of the stainless steel has proven the workability

of the coating obtained at the ion current density not less then 30-32 A/m2, which corresponds to the

coating porosity of about 6 %. However, the acceptable level of the porosity and hence the

minimum value of the ion current density can be varied for other applications such as decorative

coatings.

It should be noted that the influence of nitrogen pressure on the ion current density distribution

over substrate surface was investigated in our previous work [21]. According to the results obtained,

the nitrogen pressure weakly influences the ion current density in the pressure range of 0.01 to

0.1 Pa, with the density decreasing when pressure approaches 0.1 Pa. The minimum current density

of about 25 A/m2 was obtained for the pressure of 1 Pa. In this work, the coatings deposited at the

minimum current density have demonstrated a poor performance. However, larger vacuum arc

8

plasma sources can provide higher ion current densities, and thus the formation of coatings with

better performances can be expected for the ion current density above 50 A/m2. Thus, it can be

supposed that the dependence of the TiN coating thickness on the ion current density obtained in

this paper will be sound for the nitrogen pressure in the range of 0.01 to 1 Pa.

4. Conclusion

The investigation of the influence of the ion current density on the formation of dense coatings

on the substrates in the vacuum arc deposition setup demonstrated:

• A non-linear dependence of the coating thickness on the ion current density over the

substrate immersed into the plasma flux extracted from the vacuum arc plasma source.

• Decreasing ion current density from the substrate center toward its edges results in

deposition of more porous coating, which is consistent with the trends of the structure

zone diagram.

In our experiments, the ion current density decreased by 2.1 times (r = 160 mm) while the coating

thickness decreased by 1.8 times only. From these data, the quantitative dependence of the

thickness of TiN coating on the ion current density ranging from 20 to 50 A/m2 was obtained for

coatings deposited at substrate bias of 200 V and nitrogen pressure 0.1 Pa.

The results of this work can be used in the technological vacuum arc plasma setups with the

controlled ion current density over the substrate. The results allow selection of the minimum value

of the ion current density, and hence, maximum deposition area, to obtain uniform TiN coatings

over large substrates for different coating applications (wear-resistant, corrosion-resistant,

decorative etc.).

9

References

[1] K. Ostrikov. “Sustainable nanoscience for a sustainable future,” IEEE Trans. Plasma Sci., vol.

41, no. 4, pp. 716–724, April 2013.

[2] K. Ostrikov, E.C. Neyts, and M. Meyyappan, “Plasma nanoscience: from nano-solids in

plasmas to nano-plasmas in solids.” Adv. Phys., vol. 62, no. 2, pp. 113–224, Jun. 2013.

[3] V. N. Zhitomirsky, R. L. Boxman, and S. Goldsmith, “Transport of a vacuum arc plasma

beam through the aperture of an annular anode,” IEEE Trans. Plasma Sci., vol. 33, no. 5, pp.

1631–1635, Sep. 2005.

[4] U. Cvelbar, K. Ostrikov, and M. Mozetic, “Reactive oxygen plasma-enabled synthesis of

nanostructured CdO: tailoring nanostructures through plasma–surface interactions,”

Nanotechnology, vol. 19, no. 40, 405605, Oct. 2008.

[5] P. K. Chu, “Control of surface degradation on biodegradable magnesium alloys by plasma-

based technology,” IEEE Trans. Plasma Sci., vol. 41, no. 4, pp. 725–730, Apr. 2013.

[6] M. G. Kong B. N. Ganguly, and R. F. Hicks, “Plasma jets and and plasma bullets,” Plasma

Sources Sci. Technol., vol. 21, no. 3, 030201, Jun. 2012.

[7] X. P. Lu, Z. H. Jiang, Q. Xiong, Z. Y. Tang, and Y. Pan, “A single electrode room-

temperature plasma jet device for biomedical applications,” Appl. Phys. Lett., vol. 92, no. 15,

151504, Apr. 2008.

[8] B. K. Tay, Z. W. Zhao, and C. Q. Sun, “Effects of substrate bias and growth temperature on

properties of aluminium oxide thin films by using filtered cathodic vacuum arc,” Surf. Coat.

Technol., vol. 198, no. 1, pp. 94–97, Aug. 2005.

[9] C. S. Shin, D. Gall, Y. W. Kim, N. Hellgren, I. Petrov, and J. E. Greene, “Development of

preferred orientation in polycrystalline NaCl-structure d-TaN layers grown by reactive

magnetron sputtering: Role of low-energy ion surface interactions,” J. Appl. Phys., vol. 92, no.

5, pp. 5084–5096, Aug. 2002.

[10] I. Denysenko and N. A. Azarenkov, “Formation of vertically aligned carbon nanostructures in

plasmas: numerical modelling of growth and energy exchange,” J. Phys. D.-Appl. Phys.,

vol. 44, no. 17, 174031, May 2011.

[11] L. M. Yang, A. K. Tieu, D. P. Dunne, S. W. Huang, H. J. Li, D. Wexler, and Z. Y. Jiang,

“Cavitation erosion resistance of NiTi thin films produced by Filtered arc deposition,” Wear,

vol. 267, no. 5, pp. 233–242, Oct. 2009.

10

[12] X. X. Zhong, W. Zhou, X. C. Wu, L. Q. Yuan, Q. W. Shu, W. Li, Y. X. Xia, “Low

temperature deposition of nanocrystalline TiO2 films: enhancement of nanocrystal formation

by energetic particle bombardment,” J. Phys. D.-Appl. Phys., vol. 40, no. 1, pp. 219–226, Jan.

2007.

[13] K. Ostrikov, U. Cvelbar, and A. B. Murphy, “Plasma nanoscience: setting directions, tackling

grand challenges,” J. Phys. D: Appl. Phys., vol. 44, no. 17, 174001, April 2011.

[14] F. F. Chen and J. P. Chang, Lecture Notes on Principles of Plasma Processing (New York:

Plenum: 2002).

[15] I. Levchenko, M. Korobov, M. Romanov, and M. Keidar, “Ion current distribution on a

substrate during nanostructure formation,” J. Phys. D: Appl. Phys., vol. 37, no. 12, pp. 1690–

1695, June 2004.

[16] Q. Cheng, S. Xu,, and S. Huang, “Inductively coupled plasma-assisted RF magnetron

sputtering deposition of highly uniform SiC nanoislanded films,” IEEE Trans. Plasma Sci.,

vol. 36, no. 4, pp. 870–871, Aug. 2008.

[17] D. Manova, J. W. Gerlach, and S. Mandl, “Thin Film Deposition Using Energetic Ions,”

Materials, vol. 3, no. 8, pp. 4109–4141, July 2010.

[18] O. Baranov, M. Romanov, J. Fang, U. Cvelbar, and K. Ostrikov, “Control of ion density

distribution by magnetic traps for plasma electrons,” J. Appl. Phys., vol. 112, no. 7, 073302,

Oct. 2012.

[19] O. Baranov, M. Romanov, and K. Ostrikov, “Effective control of ion fluxes over large areas

by magnetic fields: From narrow beams to highly uniform fluxes,” Phys. Plasmas, vol. 16, no.

5, 053505, May 2009.

[20] O. Baranov and M. Romanov, “Process intensification in vacuum arc deposition setups,”

Plasma Proc. Polym., vol. 6, no. 2, pp. 95–100, Dec. 2009.

[21] O. Baranov and M. Romanov, “Current distribution on the substrate in a vacuum arc

deposition setup,” Plasma Proc. Polym., vol. 5, no. 3, pp. 256–262, Mar. 2008.

[22] O. Zarchin, V.N. Zhitomirsky, S. Goldsmith, and R.L. Boxman, “Interaction of a vacuum arc

plasma beam with an obstacle positioned normal to the plasma flow,” J. Phys. D.: Appl. Phys.,

vol. 36, no. 5, pp. 2262–2268, Sep. 2003.

[23] E. Isakov, Cutting data for turning of steel (Industrial Press Inc.: 2009).

Figures

Fig. 1. (a) Experimental setup and (b) configuration of the magnetic field

installed at the top of the vacuum chamber.

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and (b) configuration of the magnetic field. The arc plasma source i

vacuum chamber.

. The arc plasma source is

12

Fig. 2. Distribution of ion current and coating parameters as a function of coordinate r along the

substrate surface. (a) Ion current density distribution. (b) Coating thickness. (c) Relative tool life

Tc(r)/Tc max (Tc max = Tc(40) = 33 minutes). (d) Relative ion current density and coating thickness.

13

Fig. 3. SEM images of the coatings deposited on the cutting inserts at different locations r along the

substrate surface for deposition time t = 30 minutes. (a) r = 0 mm; (b) r = 40 mm; (c) r = 70 mm;

(d) r = 100 mm; (e) r = 130 mm; (f) r = 160 mm.

14

Fig. 4. Relative volume of coating as a function of ion current density and dependence of the

coating thickness in linear and non-linear approximation.

15

Biosketches and photos of authors

Oleg O. Baranov received the Ph.D. degree in the aircraft technology and materials science from the National Aerospace University, Kharkov, in 2000.

Since 2000, he has been at the faculty of the Aircraft Engines, National Aerospace University, Kharkov. His research interests are in the plasma physics and nanotechnologies including PVD coating processes, magnetron and vacuum-arc deposition, plasma control and diagnostics in the deposition setups, mechanical properties of materials and thin films, and surface plasma processing.

Kostya (Ken) Ostrikov received the Ph.D. degree in 1992 and the D.Sc. degree in 1996. He is currently a CEO Science Leader, Australian Future Fellow, and Chief Research Scientist with CSIRO Materials Science and Engineering, Australia. He is also an Honorary Professor with the University of Sydney, the University of Wollongong, and the University of Technology Sydney, Australia, having ten full professor-level appointments in six countries in total.

Prof. Ostrikov was the recipient of two prestigious medals from national academies of sciences, the Walter Boas Medal of the Australian Institute of Physics 2010, and six highly-competitive international fellowships, in addition to the recent Building Future Award 2012. He has authored more than 400 refereed journal papers and three research monographs. He has held more than 100 plenary, keynote, and invited talks at international conferences. He has supervised research training of nearly 30 researchers with Ph.D. and more than 60 research students, has about 150 collaborators in last six years and also secured more than $10 million in competitive research funding. His main research program on nanoscale control of energy and matter in plasma-surface interactions contributes to the solution of the grand and as-yet-unresolved challenge of directing energy and matter at the nanoscale, a challenge that is critical for the development of renewable energy and energy-efficient technologies for a sustainable future.

16

Shailesh Kumar received his BS degree in Physics from Banaras Hindu University, India (2003), an MS degree from Indian Institute of Technology Bombay (2005) and PhD from Nanotechnology and Integrated Bioengineering Centre at the University of Ulster, UK (2009). Currently, he is a holder of a prestigious CEO Science Leader Research Fellowship with CSIRO Materials Science and Engineering, Australia. His research work is focused on the plasma-based nanofabrication of nanomaterials, including their applications in renewable energy devices, clean water technologies and environmental monitoring systems.

Jinghua Fang has the BS and MS degrees from Yunnan University in 2000 and 2003. After finishing her master program, she joined the teaching and research faculty of Modern Analysis and Testing Centre, Yunnan University. Her research interests focused on AFM and nanodiamond fabrication and applications. In 2012, she received her Ph.D. degree from the School of Physics, University of Melbourne. She is currently an OCE postdoctoral fellow at CSIRO Materials Science and Engineering, Australia. Her major interest is material science and engineering, focusing on one-dimensional nanostructure fabrication, characterizations and applications.

Dr Amanda Rider received her BSc (Adv, Hons I) in 2007 and her PhD (Physics) in 2011, both from the University of Sydney. She is an OCE Postdoctoral Fellow at CSIRO Materials Science and Engineering, Lindfield, NSW, Australia and an Honorary Associate at the School of Physics, The University of Sydney. Her research focus is plasmonic devices for sensing applications and plasma-aided nanoscale synthesis.