Preparation of Plasma-polymerized SiOx-like Thin Films From a Mixture

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Preparation of plasma-polymerized SiO x-like thin films from a mixture

of hexamethyldisiloxane and oxygen to improve the corrosion behaviour

E. Vassalloa,*, A. Cremonaa , L. Laguardiaa , E. Mesto b

a Istituto di Fisica del Plasma-CNR, Milano, Italy b Dipartimento di Geomineralogia, Università degli Studi di Bari, Italy

Received 27 July 2004; accepted in revised form 1 November 2004

Available online 2 December 2004

Abstract

Chemical and electrochemical properties of plasma polymerized SiO x-like thin films have been studied to search for an alternative to the

anticorrosive coatings produced by chromate conversion treatment. The films were deposited on steel substrates by means of

hexamethydisiloxane/oxygen-fed plasmas ignited in an RF capacitive coupled parallel plates reactor. Their surface chemical characterization

was carried out by means of X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FT-IR). The protective abilities of SiO x-like

films as a function of RF power and gas feed composition in aerated 1 M Na 2SO4 solution were examined by electrochemical methods.

Alone and in combination, these techniques help us to understand the relationship between the deposition parameters and the properties of the

deposited films.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Corrosion; PECVD; Plasma polymerization

1. Introduction

Corrosion is one of the most serious problems for

metallic materials utilised for many application. From many

years, zinc particles have been used as a metal coating on

steel to reduce corrosion. However, when materials covered

with zinc are exposed to the atmosphere or aqueous

solutions a corrosion product, Zn(OH)2, is rapidly gener-

ated. Therefore, chromate chemical conversion (CCC) films

have been widely used for many engineering materials to

provide improved corrosion resistance. Chromium chemicalanalysis has shown that the corrosion resistance of the CCC

films is due to the inhibiting effect resulting from the

dissolution of Cr(VI) ions [1]. Unfortunately, Cr(VI) ions

that remain in the establishment of the chromium plating are

remarkably toxic and carcinogenic for humans. According

to Appendix II of the European Directive 2000/53/CE, as

from 1st July 2007 coatings containing hexavalent chro-

mium shall no longer be used for corrosion prevention. In

the EU Directive on Waste Electrical and Electronic

Equipment, the use of hexavalent chromium is also

forbidden as from 1st January 2006. The dates quoted in

the legislation require immediate action by manufacturers of

coating materials, by coating plants, by fastener manufac-

turers and by vehicle manufacturers; consequently, an

alternative surface treatment is necessary [2].

Vapour phase deposition of thin films from a plasma

environment is a flexible strategy to modify the surface of materials [3]. Several surface chemical treatments are

possible and a wide range of substrate materials can be

coated. The application of these coating includes corrosion

protection, biomaterials, vapours barrier and area where

surface modification can enhance performance. In the

plasma deposition process, a gaseous or volatile compound

is introduced into a reaction chamber, fragmented and/or

ionized in a glow discharge plasma, and reassembled on the

surface of the sample. The variables that can be readily

controlled during this ionization–deposition process are

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

doi:10.1016/j.surfcoat.2004.11.001

* Corresponding author. Tel.: +39 2 66173245; fax: +39 2 66173203.

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

Surface & Coatings Technology 200 (2006) 3035– 3040

www.elsevier.com/locate/surfcoat


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power density, precursor gas flow rate, pressure, reactor

geometry and plasma frequency. The ionized species formed

in the process acquire energies typically in the range of 0–2

eV, while electrons and metastables can achieve energies up

to 20 eV [4]. The processes that simultaneously occur in the

reaction chamber are ionization, neutralisation, recombina-

tion, etching and polymerisation. For these reasons, plasma-enhanced chemical vapour deposition (PECVD) appears to

be an important technology for the surface modification of

metallic materials in order to increase their corrosion

resistance. The deposited films are usually branched, highly

crosslinked, insoluble, pinhole-free and adhere well to the

substrates. Organosilicon precursors can be utilised to

deposit SiO x thin films, which are a good candidate for

protective coating on metallic and other substrates [5–9].

This monomer has been chosen for the flexibility of the Si–

O–Si bond in the backbone [7] and also for the resistance to

water permeation characteristics of polisiloxane films [10].

In order to provide an adequate corrosion protection, thecoating must be uniform and well adhered on the substrate;

it has been demonstrated that Fe–O–Si oxane bonds, created

at the interface between the plasma-polymer and the steel

surface, enhance the adhesion of the film [11]. In any case

preview study demonstrated that hydrogen pre-treatment

removes surface contamination layer and improves the

coating-substrate performances. In fact, hydrogen removes

carbon from the surface and creates on the surface of the

substrate catalytic active points, so that the surface reactions

between the adsorbed precursor fragments and the substrate

are facilitated [12], also at low temperatures. The goal of

this paper is to evaluate the effect of several deposition

parameters on the electrochemical behaviour of the depos-

ited films.

2. Experimental

Plasma polymerization has been carried out in a stainless

steel reactor with a parallel plate configuration and a 5 cm

inter-electrode gap. The upper electrode (diameter=15 cm)

is RF driven, the lower ground steel electrode (diameter 20

cm) holds the substrate to be processed. The reactor was

pumped by a turbomolecular and a rotary pump. The flow

rates of the feed gases and of the HMDSO vapour werecontrolled by means of MKS mass flow meters, and the

pressure was monitored by an MKS capacitive gauge. SiO xfilms were deposited on iron dishes. The experiments were

performed varying the HMDSO–O2 flow rate ratio and the

input power, keeping constant the pressure at 100 mTorr.

After cleaning the sample using acetone, the substrates were

in situ pre-treated with hydrogen plasma at 50 W and 500

mTorr to give an oxygen-free surface and a buffer layer to

enhance the film adhesion.

SiO x films were also deposited onto polished Si

substrates for ex situ FTIR chemical characterization

performed by means of a Perkin Elmer Spectrum One IR

Fourier spectrometer under N2 purging and in the presence

of a desiccant reducing water adsorption on the samples. In

order to gather information about the chemical composition

of deposited SiO x films, X-ray photoelectron spectroscopy

(XPS) analysis was performed by a Leybold LHS10

spectrometer using an unmonochromatised Al K a source.

Base vacuum was 109 mbar or better. The energy scale of the spectrometer was calibrated using Cu and Au by the

binding energies Cu2p3/2=932.6 eV and Au4f 7/2=84.0 eV.

Survey spectra were acquired in fixed retarding ratio mode

(retarding ratio=3) and high resolution spectra were

recorded in fixed analyser transmission mode, with a pass

energy of 30 eV. Calibration of binding energy scales was

performed by taking the alkyl component of the C1s

photoelectron peak (BE=284.8 eV). Hardware and software

for spectra acquisition and processing are described in

Desimoni and Malitesta (1986) and Malitesta et al. (1989).

In particular background subtraction is performed by the

Shirley’s method (Shirley, 1972). As far as the fitting procedure is concerned, a non-linear least squares fitting

program was employed to fit as many peaks as the guessed

chemical species to experimental spectrum. In this process,

peak parameters (position, height, width, etc.) are optimised

starting from initial estimates. Quantitative analysis was

performed by using high-resolution spectra. Atomic ratios

were obtained by dividing each peak area by empirically

derived sensitivity factor. Polarization measurements of the

electrodes, both bare and covered with SiO x film, were

carried out potentiodynamically in aerated 1 M Na2SO4solution at pH 5.3 and at the laboratory temperature of

25F1 8C. After the immersion of the electrode in the

solution for the time necessary to achieve the stability of the

open-circuit potential (generally 3 h), the potential of the

electrode was swept at a rate of 0.166 mV/s from the initial

potential of 250 mV vs. E corr to the final potential of 1000

mV vs. E corr . A three-electrode electrochemical configu-

ration was used with a clamp on electrolyte cell attached by

a rubber O-ring. The exposure area of the samples was 1

cm2. A Silver–Silver chlorine (Ag–AgCl) electrode as

reference and a titanium counter electrode were employed.

3. Results and discussion

3.1. SiO x film characterization

3.1.1. Infrared spectroscopy

In Fig. 1, the infrared spectra of the deposited films

(about 1200 nm thick) are shown as a function of the O2/

monomer ratio in the plasma feed gas. The characteristic

features of the Si–O–Si group [13–20] are evident:

asymmetric stretching and bending mode at around 1070

and 800 cm1, respectively. The adsorption bands at 2970

and 1260 cm1, relative to the stretching of CH x and the

bending modes of methyl groups in Si(CH3) x, mark the

presence of organic components in the film.

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Another important feature is t he absorption of the

characteristic bands of Si–OH bonds [13–23], OH stretching

in H-bonded silanol at around 3500 cm

1

and the bendingmode at 930 cm1. It can be observed that, in low dilution

conditions, the film infrared spectra show the characteristic

features of carbon-containing moieties (in particular, Si–C

bonds). This is in agreement with the kinetics described by

Lamendola et al. [24], which states that under low dilution

conditions the deposition precursors are mainly SiC xH y and

CH x radicals, and the film stoichiometry can be expressed as

SiC xH y O z . As the O2/monomer ratio increases, the

enhanced oxidation efficiency produces more SiO radicals

and CO2volatile molecules than Si and CH, leading to a

marked inorganic character of the film (Si–O–Si functional

groups prevail over Si(CH3) x), thus the film chemistry

approaches the SiO2 one. Unfortunately, this can result in acontemporaneous increase of the silanol concentration [25–

27], reducing the chemical stability of the coating and

therefore its corrosion resistance. This inconvenience can be

solved rising the input power, in order to reduce the SiOH

concentration as well as the residual organic groups. The

effect of the power has been investigated at a constant O2/

HMDSO ratio of 6,6. In Fig. 2, the infrared adsorption

spectra of the plasma-deposited films are shown at different

power conditions.

It is interesting to observe that the presence of carbon

within the film is detected only for a power value of 50 W.The SiOH-related bands decrease when power increases.

This trend can be explained with a higher input energy,

leading to a higher ion bombardment of the growing film,

which can enhance the condensation of vicinal silanol

groups to form Si–O–Si bridges.

3.1.2. X-ray photoelectron spectroscopy

The chemical composition of the deposited SiO x films

has been investigated by XPS. In particular, it was studied

the intensity of the peaks Si2p, Si2s, C1s, and O1s. This

analysis, as well as infrared, permits to follow the evolution

of the concentration of diverse elements as a function of

preparation conditions. Fig. 3 shows the XPS spectrum of afilm obtained by an RF plasma of HMDSO mixed with

oxygen (oxygen percentage 58.8%).

X-ray photoelectron spectroscopy has shown that oxy-

gen content rises from 43% to 65%, while the silicon

atomic concentration remains almost constant at 29%, when

the O2/HMDSO ratio varies in the range from 1.4 to 13.3

(see Fig. 4).

Fig. 2. Effect of RF power on the FTIR spectra obtained at 100 mTorr and

O2/HMDSO ratio 6.6.

Fig. 3. XPS spectra of films deposited from HMDSO and 58.8% O 2, at 200

W and 5101 mbar.

Fig. 4. Surface composition as a function of the O2/HMDSO ratio for

samples obtained at 100 W and 5102 mbar.

Fig. 1. Effect of feed gas composition on FTIR spectra obtained at 100 W

and 100 mTorr (the O2/HMDSO ratios are marked on the spectra).

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Moreover, the carbon content drops to a nearly negligible

value, indicating the decay of the organic moieties as the

oxygen concentration in the plasma increases. The carbon/

silicon (C/Si) and oxygen/silicon (O/Si) atomic ratio trends

are plotted in Fig. 5a against the discharge power.

Considering that the C/Si and O/Si ratio are res pectively 3

and 0.5 for the monomer and 2 and 1 for PDMS [28], the C/ Si trend indicates that when power increases an abstraction

of the methyl groups, substantial loss of carbon on deposited

film occurs, while as Si–O bonds did not seem to undergo

scission to an appreciable extent. On the other hand, the O/

Si trend suggests that slight oxidation process takes place

when the power rises.

The experimental determined O1s binding energy of

532.2 eV is consistent with the O1s signal in siloxane (Si–

O–Si) by comparison with earlier studies (Gengenbach and

Griesser, 1999 and references therein). The carbon peaks

have been decomposed in two component, a high contribu-

tion, attributed to carbon bound to hydrogen (C–H) and tocarbon bound to silicium (C–Si), both considered at a

binding energy of 284.8 eV, and a minor component at

higher binding energy, consistent with carbon singly bound

to oxygen such as ether, hydroxyl or epoxy carbon

functionary at a binding energy of 286.3 eV. The inves-

tigation of the Si2p signal results in a peak shift towards

higher binding energy when power increases. The high

value of its FWHM indicates several environments of

silicon atoms. We assumed that this signal is a compound

line with unresolved contributions from chemically different

species and assuming that all the silicon atoms in the deposit

have a valence of four. The major factor in determining theSi2p binding energy is the number of oxygen atom. On the

other hand, the secondary effect upon the position of Si2p

signal due to various combination of carbon and hydrogen

atoms fulfilling the remaining bonding requirements of the

silicon has been assumed negligible in comparison to the

shift caused by an oxygen. The spectra data has been fitted

by four component peak within the Si2p envelope: SiOC3(BE=101.5 eV), SiO2C2 (BE=102.1 eV), SiO3C (BE=102.8

eV) and SiO4 (BE=103.4 eV). Each component has been

fitted as symmetrical peak. The Gaussian to Lorentzian ratio

is kept constant at 85%. The fitting of Si2 p spectra from

HDMSO/O2 deposits is illustrated in Fig. 5 b.The effect of power in glow discharges on the

distribution of the four silicon environments is reported in

Fig. 5c. When power increases, the proportions of SiOC3and SiO2C2 decrease in favour of the more highly oxidized

SiO3C and SiO4 species. This is indicative of the role of the

Fig. 5. (a) C/Si and O/Si trend as function of the power from O2/HDMSO=1.4 plasma deposits. (b) Curve fit of the Si2p peak of a deposit formed from O2/

HDMSO=1.4 plasma at 100 mTorr 50 W (a) and 100 W (b). (c) Functional composition of the deposited films determined by fitting of Si2p peak plotted

against the glow discharge power.



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discharge power in the process. An increase of power results

in the formation of CO x gaseous species and the formation

of silicon–oxygen moieties in the deposit.

3.2. Corrosion properties of iron with SiO x-deposited films

The potentiodynamic polarization behaviour of the

coated and uncoated iron af ter 1 h exposure in a 1 M

Na2SO4 solution is shown in Fig. 6. The corrosion current

measured of the uncoated iron is 2.0102 AA.

In the potential range examined, the polarization curve

generally has three distinct regions: the active dissolution

region (Tafel region), the active–passive transition region

and the limiting current region. Especially, a linear E /log i

relation with a Tafel slope of approximately 60 mV/decade

was obtained. This slope value indicates that the dissolution

process is determined by the rate of diffusion of soluble

species for the electrode surface into the bulk solution.

Fig. 7 shows the variation of the corrosion properties of an iron disk covered with a 1150 nm thick SiO x-like films

deposited by plasma with different O2/HMDSO ratios, after

1 h of immersion in 1 M Na2SO4 solution. The process was

performed at 100 W of input power after hydrogen plasma

pre-treatment.

In comparison with the bare metal, a significant decreaseof the current of corrosion is observed, especially for film

deposited by an oxygen-rich plasma. In these conditions,

inorganic SiO x-like coatings are obtained, as infrared

spectroscopy reveals. The best approach to obtain inorganic

deposits from organosilicon precursors is to increase the

oxygen content of the feed gas and the power ( Fig. 8).

4. Conclusions

XPS, IR and electrochemical methods have been used to

characterize the plasma deposition of SiO x films for iron

coatings with corrosive resistance capabilities. Increasing of

power or oxygen flow has an appreciable effect on the film

composition and on its properties. In particular, the

increasing of power in the glow discharge results in the

formation of CO x gaseous species and of higher oxidised

silicon species in the film. On the other hand, a major

oxygen content in plasma phase produces more inorganic,

compact and cross-linked films, with good anticorrosive

properties. Nevertheless, our results have demonstrated that

this is true only when the O2/HMDSO ratio is lower than

13.3, while for values greater than 13.3, when power

increases the current corrosion increases. Probably in these

conditions the coating-metal adhesion is worse, the coatingis stressed and the film defectiveness increases. As a matter

of fact, samples coated with SiO x films obtained in plasma

fed with 5 O2/HMDSO ratio at 200 W show a significant

improvement of corrosion resistance.

In conclusion, our corrosion measurements, in 1 M

Na2SO4 with pH 5.3, showed that PECVD appears to be a

promising technique in enhancing the corrosion properties of

materials. Oxygen and power play an important role in

determining the electrochemical behaviour of deposited film.

However, other experiments at pH 10 and pH 2 revealed that

SiO x thin films, obtained by means of the PECVD

technology, exhibit also considerable corrosion protection

Fig. 6. Potentiodynamic curve of uncoated iron.

Fig. 7. Variation of the corrosion behaviour of the iron disks covered with

SiO x-like films.

Fig. 8. Effect of the RF power on the corrosion properties of iron coated

with SiO x films.

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in mild basic and acid media [29]. To make an in-depth study

of the effect of SiO x films on corrosion behaviour,

impedance measurements for the coated and uncoated iron

are in progress. Moreover, electrochemical measurements

will be supported by scanning electron microscopy (SEM),

FTIR and XPS before and after corrosion test.

Acknowledgements

The authors of this work would like to thank Prof. C.

Malitesta for XPS measurements and for technical assistance.

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Preparation of Plasma-polymerized SiOx-like Thin Films From a Mixture

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