y 201 (2006) 347–352www.elsevier.com/locate/surfcoat

Surface & Coatings Technolog

Analysis of the corrosion protective ability of PACVD silica-based coatingsdeposited on steel

David Pech a, Philippe Steyer a,⁎, Anne-Sophie Loir b, Juan Carlos Sánchez-López b,Jean-Pierre Millet a

a Laboratoire de Physico-Chimie Industrielle, INSA de Lyon, Bât. L. de Vinci, 69621 Villeurbanne Cedex, Franceb Instituto de Ciencia de Materiales de Sevilla (Centro Mixto CSIC, Universidad de Sevilla), 41092 Sevilla, Spain

Received 29 September 2005; accepted in revised form 23 November 2005Available online 28 December 2005

Abstract

Silica-based coatings are well known for their high performance in terms of mechanical, tribological, and electrochemical properties. The aimof this paper is to analyze the protective character of SiOx coatings deposited on M2 tool steel. Coatings were deposited by r.f. plasma assistedchemical vapour deposition using tetraethoxysilane as precursor. Surface chemical and morphological characterization of coatings was carried outby X-ray Photoelectron Spectroscopy, Atomic Force and Transmission Electron Microscopies, while emerging porosity was evaluatedelectrochemically. Intrinsic electrical behaviour of the film was determined by electrochemical measurements performed in Hg-pool. Thecorrosion protective properties of deposited films were assessed by means of Electrochemical Impedance Spectroscopy in aerated NaCl solution.

Amorphous SiOx films provide excellent corrosion protection of steel, which is explained, on the one hand, by a tiny porosity rate, and on theother hand, by intrinsic insulating properties of the coating. This barrier effect avoids any galvanic coupling deleterious to the uncoated metallicsurface, and is correlated with the SiO2-like character of the film.© 2005 Elsevier B.V. All rights reserved.

Keywords: Corrosion; PACVD; TEOS; Silica-based coating; Open porosity; EIS

1. Introduction

Nowadays, new coatings have to be more and moremultifunctional in order to meet industrial needs. They haveto fulfil specific characteristics and, in addition, an efficientcorrosion protection of the substrate is now also required.Chromate conversion layers, as they combine decorativeaspects with good wear properties, have been extensivelyused for corrosion protection in a wide variety of applications.However, the European Union decided to totally banish the useof Cr(VI) after July 2007, in order to avoid environmentalcontamination either during the surface treatment or during therecycling process of coated sheets [1]. As a result, extensiveresearch has been made in the last years to find a chromatesubstitute for protective coatings. Several processes such as new

⁎ Corresponding author. Tel.: +33 4 72 43 81 69; fax: +33 4 72 43 87 15.E-mail address: philippe.steyer@insa-lyon.fr (P. Steyer).

0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2005.11.130

conversion layers free from Cr(VI) [2] and organic coatings [3]can be proposed, but these solutions suffer from poormechanical characteristics. Another well investigated solutionto produce functional layers without affecting the environmentconcerns deposition processes performed under a controlledatmosphere such as chemical or physical vapour deposition(CVD, PVD) [4,5]. Among these layers, amorphous silica-based coatings appear to be very promising. They presentinteresting physical properties such as optical transparency [6],high scratch resistance [7] and temperature resistance [8].Elaborated by the sol–gel process, silica layers have also shownsome interesting anti-corrosive properties [9]. Unfortunately,such elaboration process induces chemical baths requiring theirpost decontamination. The use of PACVD (Plasma AssistedChemical Vapour Deposition) of silicon-containing organiccompounds is interesting as it is a highly versatile technique thatleads to the production of materials with a wide range ofproperties [10]. This process displays several advantages as, forexample, high density films, good adherence, with a high

348 D. Pech et al. / Surface & Coatings Technology 201 (2006) 347–352

deposition rate. Furthermore, the low deposition temperatureallows the coating of plastic substrates [11]. Some encouragingresults have also been reported on the protective effectiveness ofsilica-based thin films deposited via plasma. Although PACVDfrom organosilicon precursors appears particularly promisingfor employment in the field of corrosion protection of metallicmaterials, in the literature there are few studies on this subject.The precursor most often used is hexamethyldisiloxane(HMDSO), and the high corrosion resistance is then mainlycorrelated with the density of the film and with its inorganic/polymeric nature [12–14].

This paper deals with the corrosion behaviour of an M2 toolsteel protected by a silica-based coating elaborated in atetraethoxysilane (TEOS)-containing plasma at low tempera-ture. The nature of the film is determined by TEM and XPSexperiments, while the electrochemical behaviour is deducedfrom impedance measurements performed in saline solution andmercury. The aim of this paper is to identify the protectionmechanism afforded by the film in relation to its intrinsicphysical properties.

2. Experimental

2.1. Coating deposition

Films were deposited on a mirror-polished M2 high speedsteel and on silicon wafers. Coated steel allows characterizingthe electrochemical behaviour of the part, while coated wafersare used as thin foil for TEM observations. Prior to deposition,substrates were pre-treated in a hydrogen-containing plasma toremove surface contamination layers and create catalytic activepoints improving the coating–substrate adhesion [15]. SiOx

coatings were prepared by r.f. PACVD using tetraethoxysilaneas precursor. A capacitive r.f. electrical discharge is used in thePACVD chamber. The r.f. driven electrode is connected to a13.56 MHz r.f. power supply, whereas the counter electrode isconnected to the ground. The temperature measured by athermocouple close to the sample in the ground electrode doesnot exceed 100 °C. The reduction of the C and H content isachieved by adding oxygen to the plasma process due to theformation of volatile CO2 and H2O molecules leavingtransparent, hard inorganic films. The experimental conditionsused during the process are summarized in Table 1, and moredetails can be found in Ref. [13]. Thickness of the filmsdeposited on steel was estimated to be around 600 nm by theball-crater method.

Table 1Deposition parameters for M2 steel substrates

Hydrogen pre-treatment SiOx deposition

Pressure (mTorr) 1000 100Power (W) 16 250TEOS flow rate (sccm) – 2O2 flow rate (sccm) – 45Ar flow rate (sccm) – 22H2 flow rate (sccm) 20 –Treatment time (min) 30 90

2.2. Characterization

XPS (X-ray Photoelectron Spectroscopy) measurementswere carried out using a Leybold-Heraeus spectrometercombined with Auger and XPS system and equipped with anEA-200 hemispherical electron multichannel analyzer. Sampleswere ultrasonically cleaned in acetone, ethanol and deionisedwater. Ex situ XPS and X-AES (X-ray Auger ElectronSpectroscopy) operating with a non-monochromated 120 W,30 mA MgKα X-ray source (1253.6 eV) and the hemisphericalelectron energy analyzer were used to determine the chemicalcomposition and type of bonding of the film. All theexperiments were conducted at ambient temperature. Thedepth profile analyses of the near-surface regions were carriedout by sputtering with Ar+ ions using a current intensity of 6 mAand an acceleration voltage of 3.5 kV (ion current, 8 μA) at anargon pressure of 2 ·10−5 mTorr during 0, 120 and 240 s beforeexperiments. Spectra of Si 2p, O 1s and C 1s were recorded tomeasure concentration profiles. Spectra of silicon Augertransitions were also analyzed in order to calculate the modifiedAuger parameter. The C 1s peak with a binding energy (BE) of284.6 eV was used as reference for the correction of surfacecharging. Transmission Electron Microscopy (TEM) analysiswas carried out in a Philips CM200 microscope working at200 kV with a LaB6 filament. TEM cross sections specimenswere prepared on samples grown on Si (100) substrates using aGatan ion beam thinner polishing system. TEM was used for adescription of the interface substrate/film at the nanometricscale.

Surface roughness was measured by means of atomic forcemicroscopy (Digital Instrument 3100) in intermittent contactmode. Enhanced resolution scanning images of the surface wasperformed at a scan rate of 0.5 Hz using SuperSharpSilicon™tips (NanoSensors) with radii typically as low as 2 nm. Thescanned area was 1 μm×1 μm at a resolution of 512×512, i.e. astep size of 2 nm.

Before all electrochemical measurements, a delay of1 h without polarization is imposed to specimen to reach a steadystate. The exposed area is 0.5 cm2. The corrosion behaviour ofsamples in aerated 3% NaCl solution was assessed by means ofpolarization curves obtained with a scan rate of 10 mV/min froman initial potential of −150 mV below the corrosion potential(Ecorr) to a final potential of 200 mV beyond Ecorr. Electro-chemical Impedance Spectroscopy (EIS) measurements werecarried out at open circuit potential by applying a sinusoidalsignal of 10 mVamplitude and frequencies ranging from 105 Hzto 10−1 Hz using a PARSTAT 2273 potentiostat. EIS data inBode form were fitted using equivalent circuits. In oursimulation, the capacitance values were represented by aconstant phase element (CPE) that accounts for deviation fromthe ideal dielectric behaviour. The significance of the CPE hasbeen widely discussed [16–19] and would be mainly related tosurface inhomogeneities. We have applied the equationproposed by Hsu and Mansfeld [20] for correction of capacityto its real value.

To characterize the intrinsic electrical properties of thecoating, further impedance measurements were also performed

0 60 120 180 240

0

10

20

30

40

50

60

70

80

90

100

Ato

mic

per

cen

tag

e (%

)

Sputtering duration (s)

Si O C

Fig. 2. Chemical composition of SiOx film versus the abrasion durationdetermined by XPS analysis.

120 sSi 2p

Inte

nsi

ty (

A.U

.)

349D. Pech et al. / Surface & Coatings Technology 201 (2006) 347–352

in Hg-pool instead of NaCl solution. The low wettability of Hgallows determining the electrical properties of the overallcoating thickness without taking into account the influence ofthe coating defects. This technique has been already success-fully used to characterize dielectric properties of polymercoatings [21,22]. In order to enhance the electrochemicalresponse, an ac perturbing signal of 50 mV from 105 Hz to10−1 Hz was chosen for these measurements.

3. Results and discussion

3.1. Microstructural and chemical characterizations

Fig. 1 displays a TEM cross-sectional view of an SiOx

sample in the vicinity of the substrate/film interface. Themicrostructure of the coating appears very dense and amor-phous. No evidences of crystalline features, defects or porescould be seen. The interface between the substrate and thecoating appears very sharp and well defined. The plasma pre-treatment is demonstrated to reduce the carbon adventitiouslayer, and the film defectiveness [23], what results in animprovement of film adhesion and corrosion resistance.

The chemical composition of the coatings was estimated byXPS depth profiling using Ar+ ions sputtering at 0, 120 and240 s. According to the sputtering conditions, the depth of theremoved film is estimated to be thinner than 10 nm. As it can benoticed in Fig. 2, the elemental atomic concentrations are almostconstant after removal of the hydrocarbon contaminated toplayer, leading to the next composition: O (∼73%), Si (∼25%),C (∼2%). These atomic percentages give an SiOx stoichiometryhigher than 2. Previous publications of plasma-enhancedchemical vapour deposition of SiOx have also reported an O/Si ratio slightly higher than 2 for this type of films. This isusually observed in the case of inorganic PECVD coatingsdeposited at low temperature [24]. In order to obtain furtherinformation concerning the chemical bonding in the coating, theSi photoelectron peaks and Si KLL Auger transitions weremeasured for each condition. They were found very similarexcept for the initial state owing to the presence of the

5 nm

Fig. 1. TEMmicrograph of SiOx film deposited with an H2 plasma pre-treatmenton silicon wafer: a sharp and well-defined substrate/film interface is observed.

adventitious contamination layer. Fig. 3 exhibits the Si 2p andthe Si KLL spectra for a sample after 120 s Ar+ ions sputtering.The binding energy of the Si 2p photoelectron peaks was foundto be around 103.7 eV. This value is in the range of thosereported for Si–O bonds [25–30]. Particularly we have studiedthe modified Auger parameter α′ which is defined by the sum ofthe kinetic energy of the Si KLL Auger transition and the Si 2pphotoelectron peak. This parameter has the advantage that itsvalue is independent of photon energy and charge effects. Theobtained value for the studied sample is 1711.8 eV which is inagreement with SiO2-like layers as previously reported [25,31].

Open porosities and pinholes in coatings can form directpaths between the corrosive environment and the substrate. It

1596 1600 1604 1608 1612 1616 1620

Si KLL 120 s

Kinetic Energy(eV)

Inte

nsi

ty (

A.U

.)

109 108 107 106 105 104 103 102 101 100 99Binding Energy (eV)

Fig. 3. Si2p and Si KLL spectra of the film after 120 s Ar+ ions sputtering.

Fig. 5. AFM image of an as-received coated sample.

350 D. Pech et al. / Surface & Coatings Technology 201 (2006) 347–352

can lead to a rapid localised galvanic attack of the steel [32,33].The aim of this part is to quantitatively estimate the emergingporosity rate, and then to determine if a possible galvanic effectmay be involved between the coating and the substrate. Openporosity is evaluated using the polarization curves cross-checking method [34]. This method is based on the ratio ofthe corrosion current density ig of the coated part over the baresubstrate one at the mixed potential ia(Em). The porosity rate pdefined as the exposed surface in contact with the electrolyte is:

p ð%Þ ¼ ig=iaðEmÞ � 100 ð1ÞFig. 4 presents the polarization curves of both coated and

uncoated steels. No significant potential shift is observedbetween coated and bare substrate, so that the current density ofthe bare substrate at the mixed potential ia(Em) corresponds toits simple corrosion current density. The drop of several decadesof the current density of coated steel with respect to the substrateshows the beneficial role of the silica-based coating on thedegradation rate. The measured current density reaches areproducible value of 50 pA/cm2. The corresponding calculatedequivalent open porosity is so low (∼10−4%) that it can beconsidered as negligible. The surface of the film was observedat the AFM scale to determine the morphology of the defects(Fig. 5). Only small protuberances correlated to small filmoutgrowth are observed. The roughness of the coating Ra ismeasured to be about 0.029 μm. No holes are detected evenwith a 2 nm radius tip at a low scan rate. It can be concluded to avery small size of the pores in the area investigated, which isconsistent with the tiny porosity rate measured byelectrochemistry.

3.2. Electrochemical behaviour of coated steel

Fig. 6 shows a representative Bode plot for coated steel after100 h immersion in NaCl solution. We clearly observe two timeconstants at the high and medium frequency ranges. The firsthigh frequency loop is related to the film properties, while thesecond loop, at lower frequencies, is linked to the substratesurface. Interpretation of the experimental EIS results was

-700 -650 -600 -550 -500 -450 -400 -3501p

10p

100p

1n

10n

100n

1μμ

10μ

100μ

1m

10m

Coated Sample

i (A

/ cm

2 )

E (mV / SCE)

Bare Substrate

Fig. 4. Polarization curves of bare M2 substrate and coated M2 sample.

performed on the basis of the equivalent circuit presented inFig. 7. This circuit is commonly employed for simulation ofthe coating/substrate systems [35–38]. It consists of thefollowing elements: Re is the electrolyte resistance; Rcoat andCcoat are the resistance and capacitance of the coating; Rpore isthe resistance of the electrolyte in the pores; Rct is the chargetransfer resistance and Cdl is the double layer capacitance of thesubstrate/coating interface. The influence of Rcoat is oftenneglected since its value is extremely high. In order to considerseparately the contribution of each material with immersiontime, Figs. 8 and 9 present the electrochemical responseinvolved at the substrate and at the coating, respectively. Thedifferent parameters are deduced from EIS spectra according tothe equivalent circuit proposed previously.

Upon initial exposure, almost no electrolyte has penetratedthe coating yet to create a path to the underlying metal, whichexplains the extremely high value of Rct and Rpore. Then, thedecrease of Rct simultaneously with the increase of Cdl is mainlyrelated to the increase of the substrate area in contact withelectrolyte. The charge transfer resistance Rct rapidly reaches aplateau at 1.5 MΩ cm2, with no visible traces of corrosion. Insuch environment, this high value after 100 h immersionindicates the high efficiency of the coating to protect thesubstrate against corrosion. The capacitance value is low(b0.15 μF cm2) as corrosion reaction occurs only on a verysmall fraction of the total coated steel area.

10-1 100 101 102 103 104 1050

10

20

30

40

50

60

70

80

χ2= 2.16 10-3

Pha

se (-

ϕϕ°)

Z m

od

ule (Ω

.cm2)

Frequency (Hz)

Exp Fit

102

103

104

105

106

107

Fig. 6. Bode plot of a steel SiOx-coated sample after 100 h of immersion to 3%NaCl solution.

0

50

100

150

200

250

300

350

Rpo

re (

kΩ.c

m2 ) C

coat (n

F.cm

-2)

0

5

10

15

20

0 20 40 60 80 100Immersion time (h)

Fig. 9. Pore resistance (Rpore) and coating capacitance (Ccoat) versus immersiontime in 3% NaCl solution.

Fig. 7. Schematic cross-section of a coated conductive substrate showing a pore,and corresponding electrical equivalent circuit used to model the system.

351D. Pech et al. / Surface & Coatings Technology 201 (2006) 347–352

In the high frequency range related to the film properties, theimpedance is dominated by the capacity with a phase close to−90° (Fig. 6). Film capacitance remains constant throughoutimmersion at an average value of 5.5 nF/cm2 reflecting anexcellent chemical inertness of the coating (Fig. 9). This lowvalue can be correlated with the insulating nature of the coating.

The evolution of the resistance of the pores with immersiontime gives information about the distribution of the pores.Usually, the resistance of the pore corresponds to the resistanceof the electrolyte inside channels. In this case, the resistancevalue is usually lower than 104 Ω cm2 [22].

In our case, Rpore is very high (104–105 Ω cm2) revealing the

poor communicating nature of such pores. Such high valueswould be thus associated to pores not absolutely open [35].Their diameters are so limited that only some specific ions ormolecules can gradually migrate to the substrate surface. Thedecrease in the value of Rpore with immersion time illustratesthis penetration within the confined nanopores to the steelsurface.

3.3. Intrinsic electrical properties of the coating

Impedance measurements were also carried out in Hg-poolto determine the intrinsic electrical behaviour of the film. The

0 20 40 60 80 1000

5

10

15

55606570

Immersion time (h)

Rct

(M

Ω.c

m2 )

0,00

0,05

0,10

0,15

Cdl (μF

.cm-2)

Fig. 8. Time dependence of charge transfer resistance (Rct) and double layercapacitance (Cdl) during immersion in 3% NaCl solution.

Bode plot of the coated steel obtained in Hg-pool shows only asingle time constant (Fig. 10), representative of the filmcharacteristics. Owing to its high surface tensile, mercurydoes not penetrate into the pores and we have direct access tothe resistance of the coating Rcoat. Considering a coating of600 nm in thickness, a highly insulating behaviour with anapproximate resistivity of 100 GΩ cm can be deduced. Thisvalue is lower than the theoretical value for silica (1016 Ω cm),and can be related to impurity atoms as shown in Fig. 2. Suchhigh value prevents nevertheless any galvanic current betweenthe coating and the substrate through potential defects. SiOx

coating acts thus as a barrier layer. The measured capacity of thefilm is 4.1 nF/cm2, which is consistent with the values deducedfrom EIS spectra in NaCl solution. The small differencebetween capacitance values measured in both mercury andsaline solution is attributed to a smaller exposed area formercury which does not wet the overall surface. Thecorresponding dielectric constant (ε∼2.8) is in agreementwith values reported elsewhere [39].

4. Conclusion

Elaborated by PACVD using TEOS as precursor, silica-based films are amorphous and very dense. XPS analysis has

Hg

10-1 100 101 102 103 104 1050

10

20

30

40

50

60

70

80

90

Pha

se (-

ϕϕ°)

Z m

od

ule (Ω

.cm2)

Frequency (Hz)

102

103

104

105

106

107

Fig. 10. Impedance measurements in Bode form of SiOx coated steel in Hg.

352 D. Pech et al. / Surface & Coatings Technology 201 (2006) 347–352

identified the SiO2-like chemical nature of the coating, thiscomposition being homogeneous throughout the whole thick-ness. The coating porosity is so low that it can be considered asnegligible.

Silica-based layers are very promising since they affordexcellent corrosion resistance in saline media. The highinsulating properties of SiOx coatings explain their outstandingperformance in terms of corrosion protection. No galvanicattack of the substrate can thus occur, and the coating acts as abarrier layer. Moreover, electrochemical measurements, carriedout on a laboratory scale, on extended immersion test underlinethat such high protective efficiency is stable and durable.

Acknowledgments

The authors acknowledge the European Commission (No.Project STREP 505928-1) for its financial support. The authorsare also grateful to R. d'Agostino, F. Fracassi, F. Palumbo andS. Laera, from the Dipartimento di Chimica, Universita di Bari,for the elaboration and delivery of coated samples.

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