Corrosion Behavior of Ti6Al4V Coated with SiOx by PECVD Technology

V. Togan1, G. Ioniţã1, I. Antoniac2

1 University Valahia of Târgovişte, Romania,

2 University Politechinica of Bucharest Romania

Keywords: PECVD, silicon oxide, biomaterials, EIS, SEM analyse.

Abstract: This paper aims to present the mechanism of PECVD SiOx coating thin layer technology

on biomaterials, such as Ti and its alloys, with reference to the available literature. The thin

organosilan coating surface will be analysed by SEM technique and the corrosion behaviour by EIS

technique. The study shows that plasma-assisted fabrication allows us to prepare dense,

homogeneous and high adherent coatings and thereby, will improve the corrosion resistance.

Introduction

Ti and Ti alloys offer many attractive properties, which lead to an increasing interest in using

them in different areas such as medical devices and automotive parts. Regarding the medical

research field, the permanent metallic prostheses or dental implants, are usually fabricated from

light metal or alloys which exhibit superior mechanical properties such as tensile strength,

toughness and fatigue resistance. Mainly, their excellent specific strength and outstanding corrosion

resistance have made them very important [1].

In the last years, the research field regarding corrosion behavior protection was focused on

organic and inorganic thin films. Depending on the nature of the gas/vapor feed, thin film of organic

or inorganic composition can be deposited practically on all solid substrates. This paper will

highlight the deposition of SiOx assisted by Plasma Enhanced Chemical Vapor Deposition

(PECVD), used on metallic biomaterials substrate as titanium and its alloys. These biomaterials are

widely used as artificial hip joints, bone plates and dental implants due to their excellent mechanical

properties and endurance [2].

A great progress of research on the deposition of silicon oxide films has been reported using

various techniques. Some techniques available for SiO2 coating are: evaporation [3], ion beam

sputtering (IBS) of SiO2, thermal chemical vapor deposition (CVD) [4], metal–organic CVD

(MOCVD) [5, 6], plasma-enhanced CVD (PECVD) [7,8], etc.

From the specified techniques, the most used are the thermal chemical vapor deposition (CVD)

and the plasma enhanced chemical vapor deposition (PECVD. PECVD is an exceptionally versatile

technique for depositing high-quality, well adhered films. The main advantage of PECVD over

thermal CVD is the ability to prepare good quality silicon dioxide films at relatively low substrate

temperatures (typically less than 300°C). Here it can be seen the reason why PECVD is preferred in

many applications.

The requirements of the low temperature deposition increased in the microelectronics industry

due to their usefulness in a large scale of applications. Moreover, low temperature silicon oxide is a

suitable material for transparent hard coating on plastics, e.g. polymethylmethacrylate or

polycarbonate.

Thereby, this paper will be focused on the corrosion behavior SiOx coatings performed by

PECVD technology on the Ti and its alloy substrates.

Key Engineering Materials Vol. 583 (2014) pp 22-27Online available since 2013/Sep/10 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.583.22

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 141.85.36.98, Polytechnic University of Bucharest, Bucharest, Romania-06/10/14,10:54:22)

Figure 1.Organosilanes molecular structures

Materials and methods

When a gas, or a vapor, is exposed to an electric field able to fragment molecules in active

species (atoms, radicals, ions, electrons), plasma is formed, ionized gas, the so called 4th state of the

matter.

The molecular structures of the most used organosilanes (HMDSO – hexamethyldisiloxane,

HMDSN – hexamethyldisilazane and TEOS – tetraethoxysilane) are presented bellow in Figure 1.

PECVD is suitable for growing silicon oxide at low temperatures which is surely an advantage,

as the number of applications with low temperature requirements is increasing [9-12].

Thin silicon oxide films are used widely in microelectronics, optoelectronics and optics for their

outstanding ability to protect metals and polymers. These coatings have several advantages: they are

transparent, chemically inert and have sufficient hardness. Silicon oxide is used as a protective layer

in the reflectors of automobiles or mobile phones and can be used as a scratch resistant layer on

transparent polymers [12-15].

Therefore, in the recent years, a special attention has been paid to organosilicons. Their

compounds are non-toxic and non-explosive and hence, no special safety installation is required as

in the case of silane. Tetraethoxysiloxane (TEOS) mixed with oxygen is used in PECVD of silicon

oxide films for more than 30 years.

TEOS-SiO2 films exhibit good step coverage even when deposited at a relatively low

temperature, but it is widely known that there are several problems such as high hygroscopicity and

abundance of carbon and water related impurities which affect the electrical properties of the films.

Surface modification techniques based on plasma processes, allow to improve the surface

composition, morphology and properties of materials in several applications

In order to achieve a complete understanding of the plasma process, first we should understand

that “plasma” and “glow discharge” have different meanings.

The glow discharge doesn’t have a uniform change density as plasma and in some areas the

electrical neutrality is not respected. Another difference between plasmas and glow discharge is the

fact that the latter is always in non-thermodynamic equilibrium (the degrees of freedom have

different mean energies) while plasmas can be characterized also by situations of thermodynamic

equilibrium.

Silicon dioxide is one of the most widely used thin film coatings due to its exceptional electrical,

chemical and mechanical properties. Depending on the nature of the gas/vapor feed, thin film of

organic or inorganic composition can be deposited practically on all solid substrates.

Electrons play a fundamental role in determining plasma properties and therefore two parameters

are generally used to characterize glow discharges: the plasma density “Ne” (numbers of

electrons/cm3) and the electron temperature “Te”, which is related to the mean electron energy and

is defined according to relation (1). This relation comes by the assumption that plasma electrons

follow the statical Maxwell-Boltzmann distribution: the higher is “Te”, higher is the mean electron

energy.

HMDSO TEOS HMDSN

Key Engineering Materials Vol. 583 23

Figure 2. SiOx deposition mechanism

ee KTE

2

3= (1) i

kii

e

kee

m

Ev

m

Ev

2;

2 == (2) 2en

KT

e

o

eD

ελ = (3)

where: K is the Boltzmann constant.

ε0 is the dielectric constant ;

e is the elementary charge.

Due to their low mass (me), the electrons can easily follow the electric field oscillation and

increase their kinetic energy (Eke) much more than the heavy ions (Eki). In Figure 2 it is possible to

observe a general sketch mechanism deposition of SiOx on substrate such as Ti or Ti alloys, in case

that the plasma chamber is alimented with organosilicon compounds. The electron velocity (ve), on

the other hand, is very high since it depends on the inverse of the square root of the mass (2).

In condition of non-thermodynamic equilibrium, most of the energy, in fact, remains as kinetic

energy of electrons, whose temperature can be as high as 10eV (110.000 K), since the transitional

gas temperature, Tg, remains low (500 K).

Another very important parameter which must be clarified is the Debye length (λD) which

indicates the distance from an electrical perturbation (charged particle) where the electric field is

reduced to 1/e(0.37) of its initial value : within a distance of 2-3 λD the electrical potential becomes

negligible. The distance of λD increases with electron temperature and decreases with plasma

density according to relation (4) [16]:

The oxidation of Si takes place in three steps: transport of the oxygen to the surface, diffusion of

the oxygen through the already grown oxide and finally the reaction of the oxygen with the silicon

at the interface between silicon and silicon oxide. With growing oxide thickness, the growing rate

slows down because the time of the diffusion through the oxide depends on its thickness, and

therefore becomes relevant for the oxidation rate. Very thin oxides can also grow at reduced

pressure or in RTP systems (rapid thermal anneal). By the oxidation of the silicon, the silicon is

consumed and the interface moves into the substrate [17]. The possibilities for producing films of

various materials and for tailoring their properties by manipulation of reactant gases or vapours and

glow-discharge parameters are very extensive. Plasma deposition processes are used widely to

produce films at lower substrate temperatures and in more energy-efficient fashion than can be

produced by other techniques. For example, they are widely used to form secondary-passivation

films of plasma silicon nitride on semiconductor devices, and to deposit hydrogenated, amorphous

silicon layers for thin-film solar cells.

Reactor process chamber

PECVD equipment

24 BiomMedD V

Thus, the architecture of RF plasma is influenced by many process parameters. In fact, to

achieve a complete understanding and control of plasma process is one of the goals of this paper.

Results and discussion

In this paper will be presented the homogeneity and the adhesion of the oxide made by EIS,

which is one of the best methods for evaluating the layers deposited on the surface. The purpose of

depositing silicon oxide layer is to increase the corrosion resistance of the substrate after its

submission in a corrosive environment. We will use the Nyquist and Bode diagrams to assess the

silicon oxide layer on the surface of titanium alloy.

The substrate that will be used for silicon oxide deposition is Ti6Al4V, which is one of the most

commonly used titanium alloys is an alpha-beta alloy containing 6% Al and 4% V. This alloy,

usually referred to as Ti6Al4V, exhibits an excellent combination of corrosion resistance, strength

and toughness. In the case of medical applications, stringent user specifications require controlled

microstructures and freedom from melt imperfections. The interstitial elements of iron and oxygen

are carefully controlled to improve ductility and fracture toughness. Chemical composition, in

wt.%, of the Ti and the Ti–6Al–4V alloy is presented in table 1.

In order to perform the silicon oxide deposition by PECVD technology, a group of Ti6Al4V

samples were mechanically polished on SiC paper to 2400 grit and ultrasonically cleaned, in a

solution containing distilled water. Silicon oxide layer is obtained after combination of: Ar

=38sccm + O2=1sccm + TEOS=1sccm (sccm- Standard Cubic Centimeters per Minute), the

deposition time was 30 min with 50W input power. The equipment used is CVD600 PECVD –

PVD (physical vapour deposition) which is running by Lookout software manufactured specially in

order to be controlled by an external PC. The equipment is presented in 3D in Fig. 2. The system is

equipped with several sensors to check pressure and temperature inside the chamber. The

equipment is provided with process chamber (atmospheric to 5x10-9

mbar.) and a turbomolecular

pump (5x10-4

mbar. – this is the pressure used during the deposition of silicon oxide).

Table 1. Chemical and physical properties of Ti6Al4V

Chemistry Physical

Avg. Wt. % Properties

Nitrogen max. 0.011 Density 0.160 lbs/in3

Carbon, max. 0.015 Modulus of Elasticity 16.5 psi x 106

Hydrogen, max. 0.0058

Iron, max. 0.12 Thermal Conductivity 6.6-6.8 w/mK

Oxygen, max. 0.11 Electrical Resistivity 1.71 µohms-m

Aluminum 6.06

Vanadium 3.97

Titanium Balance

Regarding the corrosion assets, the samples were immersed in a weak electrolyte, a solution of

NaCl dissolved in distilled water, molar concentration of 0.1 M (5.8g NaCl in 1000ml distilled

water).To evaluate the corrosion resistance was used EIS impedance spectroscopic results which are

illustrated by Nyquist and Bode diagram. The impedance of coated metals has been very heavily

studied. The Interpretation of impedance from coatings behaviour can be very complicated. The

electrochemical cell used in this case includes the working electrode (WE - samples of Ti6Al4V),

the reference electrode (RE - type Calomel) and the counter electrode (CE –Platinum electrode).

The technique where the cell or electrode impedance is plotted vs. frequency is called

electrochemical impedance spectroscopy (EIS). Electrochemical measurements are now widely

used in most fields of corrosion. To assess the corrosion behaviour of the Ti6Al4V were used

Nyquist and Bode complex diagrams. The samples were measured after five min., 1h, 24h, 48h and

72h.

Key Engineering Materials Vol. 583 25

0.00E+00

2.00E+05

4.00E+05

6.00E+05

8.00E+05

1.00E+06

1.20E+06

0.00E+00 1.00E+05 2.00E+05 3.00E+05 4.00E+05 5.00E+05 6.00E+05 7.00E+05 8.00E+05

1st measure after 1h After 24h

After 48h After 72h

Figure 3. Nyquist diagram Figure 4. Bode Diagram – Impedance values

In Figure 4 and 5 it is possible to see that Ti alloy presents a high impedance value at 1st measure

and after 1h due to the protective SiOx layer.

Figure 5 Bode Diagram – Phase angle Figure 6. Electrochemical equivalent circuit

where: Cdl – capacitance double layer Rp – polarization rezistance;

Rs – solution resistance; Csi – silicon oxide capacitance;

Cp – capacitance polarization; Rsi – silicon oxide resistance;

Rdl – double layer resistance;

Figure 7. SEM image of the Ti6AL4V after 1h Figure 8. SEM image of the Ti6Al4V after 72h

-Im

Re

0

1

2

3

4

5

6

7

-3 -2 -1 0 1 2 3 4 5 6

log

Z ,

oh

m .

cm2

Hz

1st measure After 1h After 24h

After 48h After 72h

-80

-70

-60

-50

-40

-30

-20

-10

0

-3 -2 -1 0 1 2 3 4 5 6

Ph

ase

an

gle

Hz

1st measure After 1h After 24h

SiOx layer started to be destroyed SiOx layer Ti6Al4V surface without SiOx

26 BiomMedD V

Conclusions

In this paper is presented the mechanism of silicon oxide deposition by PECVD and the

corrosion test by means of EIS. Titanium alloys primarily stand out due to two properties: high

specific strength and excellent corrosion resistance. This also explains their preferential use in the

aerospace sector, in the chemical industry, medical engineering, and in the leisure sector.

In order to obtain a compact protective organosilicon layer, was used the PECVD technology.

The 3D equipment of the PECVD was designed in Unigraphics NX5. The presence and the

efficiency of the layer was assessed by the EIS analyse (Nyquist and Bode diagram). In the Figure

7 and 8 are shown SEM images after 1h and 72h of Ti6Al4V immersed in sodium chloride. After

1h of immersion seems that the silicon oxide is stable. After 72h of immersion, due to the

interaction of the SiOx with the sodium chloride electrolyte, the layer is starting to be damaged.

Thereby, the silicon oxide layer offers a protective corrosion interface between the metal and the

electrolyte.

According to EIS spectrum, the silicon oxide doesn’t have a high adherence due to the fact that

the plasma composition was executed with a small rate of oxygen (O2 = 1sccm). The input power

and the quantity of the oxygen added in the reactor chamber interact with TEOS and influence the

thickness and the properties of the layer that starts to be deposited on the substrate surface.

References

[1] C. Leyens, M. Peters, “Titanium and Titanium Alloys”, Köln, Ed. Wiley, 2003, pp.1-55.

[2] M. Roy, A. Bandyopadhyay, S. Bose, “Surface Coating Technology” No. 205, 2011.

[3] P. Markschla¨ger, G. Kampschult, M. Eckel, O. Morlok, “Surface Coating Technology”,

No.838, 1995.

[4] K.H.A. Bogart, N.F. Dalleska, E.R. Fisher, “Science Technology”, No. 476, 1995.

[5] M.G.J. Veprek-Heijman, D. Boutard, J. “Journal of the Electrochem Society”No.2042, 1991.

[6] F. Fracassi, R. d’Agostino, P. Favia,”Electrochem Society” No.139, 1992.

[7] F. Rouzhebi, B. Catoire, M. Goldmann, J. Amouroux, “Revue internationale des hautes

temperatures et des refractaires” No. 211, 1986.

[8] K.S. Chen, N. Inagaki, K. Katsuura, “Journal appl.polymer science” No. 27, 1982.

[9] W.J. Patrick, G.C. Schwartz, J.D. Chapple-Sokol, R. Carruthers, K. Olsen, “Journal

Electrochem. Society” Vol. 139, No. 2604, 1992.

[11] S.C. Deshmukh, E.S. Aydil, “Application physic letter” Vol. 65, No. 3185, 1994.

[12] Y. Inoue, O. Takai, “Thin Solid Films”, Vol.316, No. 79 (1998).

[13] H. Nakashima, K. Furukawa, Y.C. Liu, D.W. Gao, Y. Kashiwazaki, K. Muraoka, K. Shibata,

T. Tsurushima “Journal vac. science technology” Vol. 15, No. 1951, 1997.

[14] B.L. Chin, E.P. van de Ven, “Solid State Techn.”, No. 119, 1988.

[15] F. Gaillard, P. Brault, P. Brouquet, “Journal vac. science technology”, No. 2478, 1997.

[16] Riccardo d’Agostino, Pietro Favia, Francesco Fracassi “Plasma processing of polymers”

Kluwer Academic Publishers, London, pp 48-49, 1997.

[17] Fonash, S. J., “solid state technology”, p. 201, April 1985, and Pang, S. W., “Journal

electrochem. society”, 133:784, 1986.

Key Engineering Materials Vol. 583 27

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