Deposition and properties of carbon-based amorphous protective coatings

Surfaceand CoatingsTechnology80 (1996)121-125

Deposition and properties of carbon-based amorphous protective coatings C-P. Klages, A. Dietz, T. Hiiing, R. Thyen, A. Weber, P. Willich Frnunhofer-Institutfiir

Schicht- und OberjWhentechnik,

D-38108 Braunschweig,

Geuruny

Abstract

Using a combination of temperature-humidity testing in D,O vapour with secondary ion mass spectrometry (SIMS) depth profiling of deuterium, it has been demonstrated that moisture-tight coatings can be grown from acetylene, tetramethylsilane (TMS) and hexamethyldisiloxane (HMDSO) in an r.f. plasma deposition process, if a sufficient flux of energetic ions to the substrate surface is provided by the self-bias voltage at the substrate electrode. Using electrochemicalimpedancespectroscopic (EIS) investigationson films depositedon steelsubstrates,significant differencesin protection of the substratesagainstcorrosion in aeratedelectrolyte solution werefound for films grown from TMS and HMDSO, respectively.Films depositedfrom TMS were usually stablefor a period of time up to 300h beforelocalisedcorrosion began.One samplewascompletelystableover the whole measuringtime of 460h. Films depositedfrom HMDSO, however,immediatelyformed ionic current paths,asrevealedby changes in EIS after only 24 h, and the characteristicappearanceof the 0 h spectra. Theseresultsshow that very good corrosion protection can be achievedby properly depositedamorphoushydrogenatedsilicon carbon (a-SiC:H) films, if film defectsare carefully avoided. Key\oo&: Plasmapolymers;Plasmadeposition;Moisture penetration; Corrosion protection; Barrier coatings

1. Introduction The formation of polymeric materials from organic

precursors by means of plasma activation has become a versatile method to deposit thin films with a considerable application potential. Among other applications, plasma polymer thin films are being used to provide corrosion protection to metallic substrates. The atmospheric corrosion of a metal under a coating generally requires: (a) the presence of ions to provide electrolytic conductivity; (b) the presence of liquid water at the coating/metal interface; (c) oxygen access [l]. Owing to their amorphous nature, characterised by the presence of a free volume, organic polymers and organic coatings conventionally used for corrosion protection are permeable to water, oxygen and ions. Using quartz microbalance and Kelvin microprobe measurements, Feser and Stratmann determined oxygen and water diffusion coefficients in alkyd coatings of about 2 x 10Bs and 3 x 10mg cm2 s-l [l]. A 20-pm thick coating of this kind will saturate with water and oxygen within 10 min. For many classes of coatings, water fluxes through free films of l-10 mg cm-’ per day are typical 0257-8972/96/$15.00 0 1996ElsevierScience S.A.All

rights

reserved

for 100-pm film thickness [2]. Therefore, the corrosion protection offered by these coatings is not the result of a barrier action against water or oxygen, but of inhibition of electrochemical reactions in the absence of liquid water at the intact coating/metal interface. In microelectronic circuits and passive electronic components corrosion of metallic conductors is an important potential failure mechanism. In the presence of an electrical potential difference between neighbouring conductors, the formation of an electrolytic bridge can lead to anodic dissolution of one conductor, while water or protons are reduced to hydrogen at the cathodic conductor [ 31. Again, protective coatings prevent corrosion damage by inhibiting the formation of such an electrolyte bridge between anode and cathode. Suitable plasma polymers are known to be able to passivate metals; the best known example is probably the corrosion protection of evaporated Al mirrors by PP-HMDSO (plasma polymerised hexamethyldisiloxane) [4]: a film only 25 nm thick is able to prolong the lifetime of a reflector in a climate test (40 “C, 100% relative humidity r.h.) from a few hours to 1000 h. Also, moisture-sensitive NiCr resistors can be protected by 100 nm PP-HMDSN (hexamethyldisilazane) against very severe testing conditions (1000 h, 95 “C, 100% r.h., 12 V d.c. bias on), resulting in zero failure and minimal drift [S].

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The investigations presented here are related to the authors’ work on plasma polymers as corrosion protection for bulk metals as well as for thin conductors on insulating substrates, as are found in passive electronic components. They were planned to elucidate the protection mechanism of plasma deposited amorphous carbonbased films and to answer the question of whether the passivation is due to a hermetic sealing of the substrate from water vapour or, as in conventional polymers, to their tight bonding to the substrate surface, inhibiting the formation of the liquid water film necessary for corrosive attack. By hermetic we mean the complete prevention of the water diffusion front from reaching the substrate within a time span of about ten years (~3 x 10’ s). A diffusion coefficient of less than lo-l6 cm2 s-l is required for a lo-pm thick coating to be hermetically tight according to this definition. Conventional polymers are not able to provide sufficient barrier action to seal a substrate completely against water. It was, however, pointed out by Angus and Jansen [6], that the argon content to be detected in diamondlike amorphous hydrogenated carbon films (a-C:H), even several years after their deposition, implies that the argon diffusion coefficient must be < lo-l8 cm2 s-l, if a homogeneous flrn structure is assumed. Simultaneously the authors note, that this experimental finding does not necessarily mean that a-C:H is an impermeable barrier, because a continuously connected floppy phase, providing a diffusion path, could separate constrained regions entrapping the argon. In fact, permeability of a-C:H to oxygen and probably other gases was recently inferred from the EPR linewidth broadening observed in a-C:H samples upon exposure to oxygen gas [7]. However, a-C:H coatings on TbFe magnetooptic films were claimed to be an effective diffusion barrier to atmospheric oxygen or water, as concluded from the nearly complete prevention of changes in Kerr rotation upon exposure to wet air (1600 h, 7O”C, 95% r.h.) [S]. The moisture penetration is restricted to the upper roughness region of less than 5 nm in thickness as shown by ellipsometric investigations applied to a-C:H [9]. While highly crosslinked and structurally overconstrained diamondlike carbon, a-C:H, usually grown under pronounced ion assistance, can be assumed to present the upper end of the range of permeation resistances to be achieved by plasma deposited carbon-based films, typical plasma polymers, deposited under low bombardment of energetic species, are expected to provide significantly lower permeation barriers. According to Yasuda [lo] a large reduction in the permeation of small molecules such as water or oxygen by ultrathin (mO.1 pm) layers of typical plasma polymers cannot be expected. This statement agrees, for example, with water diffusion coefficients between lo-* and lo-’ cm2 s-l

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measured for PP-HMDSO films grown by microwave plasma deposition [ 111. Aside from a-C:H, the investigations presented here were focussed on films deposited from tetramethylsilane (TMS) and hexamethyldisiloxane (HMDSO) employing r.f. plasma deposition under conditions of energetic ion contributions. The obtained a-SiC:H and a-SiCO:H films are of interest as wear resistant low-friction coatings with lower intrinsic stress and better adherence to steel substrates than a-C:H [12] and as low-surface energy coatings [13]. In the first part of the present paper, the incorporation of deuterium into the film from liquid or gaseous D,O will be used to investigate the question of whether moisture-tight films can be grown from acetylene, HMDSO and TMS in a parallel plate r.f. plasma deposition process. In the second part, the results of preliminary electrochemical impedance spectroscopic investigations on plasma-deposited amorphous carbonbased films on steel substrates will be reported.

2. Experimental Table 1 summarises deposition parameters of a number of samples which are referred to in this study. Polished steel (lOOCr6) and silicon wafers were used as substrates. Samples with numbers consisting of one, two or four figures were deposited in a single plate r.f. (13.56 MHz) plasma deposition apparatus. The deposition of films with three-figure numbers was performed in a parallel plate r.f. (13.56 MHz) plasma deposition reactor providing the possibility of feeding both electrodes simultaneously and independently, as shown schematically in Fig. 1. The diameters of the lower (substrate) electrode and the upper electrode were 200 and 150 mm, respectively. In contrast to the upper electrode, the lower one had no dark space shield. The interelectrode distance was 65 mm. In the course of deposition experiments performed for this study it turned out that the total r-f. power delivered to the reactor was not an unambiguous function of the d.c. bias voltages measured at the electrodes. Instead the electrical status of the reactor was found to change owing to deposits formed on the electrode plates and side walls; a thorough cleaning procedure of the major surfaces in the chamber led to a significantly lower power requirement to achieve the desired combination of d.c. bias voltages. Samples with numbers 7xy were deposited after this cleaning. The elemental composition of the deposits was determined using a CAMECA SX50 electron microprobe (EPMA). In order to investigate the water diffusion into the films, they were exposed to steady state temperaturehumidity test conditions using an atmosphere containing air and D,O vapour (160 hours, 85 “C, 80%~85% r.h.). Samples were then analysed with respect to the concen-

C.-P. Klages et al./Surface and Coatings Technology 80 (1996) 121-128

123

Table 1 Deposition parameters of samples referred to in this study” Sample no.

63; 633 631 121 4 7 8 9 10 12 17 550 582 584 614 628 629 632 1300 3615 516 716 729 731 134 736 741 742

Monomer

Substrate

- V, SIC (V)

P W)

PIlU0 (Pa)

PAr

d

O-core.

Pa)

(wd

(at.%)b 0.5/0.6 0.6 n.a. 0.3 n.a.

D-depth b-4

acetylene acetylene acetylene acetylene acetylene

Si Si Si Si Si

600/O 100/600 1000/0 5Ojl50 10/500

250’ 50 500 40 50

1.5 0.4 0.4 0.4 0.4

0 1.0 1.0 1.0 1.0

1.9 0.2 0.12 0.4 0.15

TMS TMS TMS TMS TMS TMS TMS TMS TMS TMS TMS TIMS TMS TMS TMS TMS

Steel Steel Steel Steel Steel Steel Steel Si Si Si Si Si Si Si Si Si

1000 1000 1000 1000 700 700 800 500/o 100/600 50/l 50 100/600 100/600 100/600 600/O 1000 600

120 146 164 190 82 113 650d 100 200 30 50 80 50 110 100 50

1.5 1.5 3.0 3.0 1.5 3.0 1.5 1.0 0.4 0.4 1.4 0.7 1.0 1.4 1.5 1.5

0 0 0 0 0 0 0 1.0 1.0 1.0 0 0.7 0.4 0 0 0

10.0 2.1 2.9 2.5 2.3 4.0 6.0 1.4 0.16 0.5 0.6 0.35 0.5 0.55 3.0 3.0

?

< 0.04 0.02
HMDSO HMDSO HMDSO HMDSO HMDSO HMDSO HMDSO HMDSO

Si Si Si, Steel Si, Steel Si Si, Steel Si Si

100/600 50/500 o/500 50/500 100/500 500/o 501750 100/690

200 40 40 50 60 80 150 150

0.4 0.5 0.5 0.5 0.5 0.5 0.5 0.5

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

0.5 0.35 1.4 3.2 3.6 2.1 0.3 0.4

10.0/10.1 18.9 16.6 19.4 23.9 13.0 19.2 19.0

0.05 0.3 (=d) >0.5 >0.12 > 0.04 >O.Ol 0.3 (= d) 0.4 ( = d)

0.3/0.4 12.2115.4 2.2 6.4 1.1

0.05 0.02 0.04 0.04 0.15 (=d)

a V,, substrate bias voltage or bias voltage at substrate and counter electrode (S/C); P, power deposited in the plasma; pi, partial pressures of monomer and argon; d, film thickness; 0-cont., oxygen concentration as determined with EPMA; D-depth, film depth in which deuterium incorporation was detected after exposure to D20 vapour. n.a., not available. b If two figures are given, the latter corresponds to a sample exposed to the temperature-humidity test. ’ 40 W reflected power. d High reflected power.

tration depth proses of deuterium, hydrogen, and other elements using secondary ion mass spectrometry (SIMS). These investigations were carried out in a CAMECA IMS 5f ion microscope using a 5.5 keV Cs+ primary ion beam (lo-50 nA), scanning across areas of 100 x 100-250 x 250 pm2. Negative secondary ions were accepted from a circular area on the sample limited to a diameter of 60 btm. Depth calibration was derived from film thicknesses determined by measuring the height of a step (profilometer). The measured intensity ratio ID/I, of samples not treated with D,O, about l-3 x 10W3, was always larger than the natural D/H ratio of about 1.5 x 10e4, owing to an interference with H, molecular ions under the conditions of low mass resolution (M/AM ~400) used. 1,/I, ratios as high as 0.1 measured on some D,O-treated samples correspond to a D concentration of roughly 2 at.% when a typical H concentration of 20 at.% and similar ionisation probabilities for D and H are assumed.

Electrochemical impedance spectrometry (EIS) was applied to coatings deposited on polished low-alloyed steel (lOOCr6) discs in aerated aqueous sodium chloride solutions (3 wt%) buffered to pH= 5 by a commercially available citrate buffer. An Ag/AgCl/Clelectrode in saturated NaCl solution was used as a reference electrode and platinum wire grid as counter electrode. The exposed film area was 1 cm’. Measurements were performed between 1 mHz and 100 kHz at the open circuit potential using a ZAHNER IM5d EIS system applying 5mV a.c. amplitude voltage.

3. Results and discussion 3.1. D,O penetration

In a preliminary test, the uptake of D,O was compared for pieces of sample no. 1300 (3.0 pm PP-TMS, a-SiC:H)

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and Coatings

7-J-l

COUNTER

ELECTRODE PLASMA GAS INLET RING SUBSTRATE ELECTRODE ARGON

RF z

HMDSO TMS

LE.99

r

ETHIN PUMP Fig. 1. Schematic view of the two-electrode r.f. (13.56 MHz) plasmadeposition reactor used in this study.

which were either boiled in DzO under atmospheric pressure for 200 h or exposed to the temperature-humidity test described above. Using SIMS, D was detected in the outer 0.3-0.4 pm after the temperature-humidity test but only in a 0.2 pm thick film region after 200 h boiling. The temperature-humidity test was therefore, as the more severe test, adopted for all other coatings. As mentioned in the introduction, amorphous hydrogenated carbon, a-C:H, can be expected to yield a very tight random network of atoms, due to the high coordination numbers and small atomic radii of its constituents, sp2 and sp3 coordinated carbon atoms. In fact, films deposited from acetylene/argon mixtures at negative substrate bias voltages of at least 50 V turned out to show no significant increase of the D ( + H,) underground SIMS signal after applying the temperature-humidity tests. In the SIMS depth profile shown in Fig. 2 (left spectrum), there is no significant increase of the D/H ratio in sample no. 637, in another measurement on the same sample an increase of this ratio was restricted to the outer 40 nm (see the column D-depth in Table 1). In the sample no. 727 (only - 1OV substrate bias), however, an increase of the D/H intensity ratio to about 0.02 after exposure to D,O was found virtually through the whole film thickness. A counterargumentation, that deuterated water might well penetrate the film under test conditions, but would not lead to a D/H exchange owing to a lack of exchangeable hydrogen in the film, can be refuted by considering the SIMS spectra near the am/substrate interface: the native silicon oxide on the Si substrate (note the increased

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oxygen signal in Fig. 2, left spectrum), would, in the presence of D20, lead to a significantly enlarged D/H ratio in this region (see below); the lack of an enrichment of D in the oxide can be considered as a proof of the hermeticity of the films against water vapour. Using a sufficient substrate bias voltage, moisture tight a-C:H films can be deposited without adding argon to the reactive gas, as demonstrated by sample no. 2. The role of the argon/monomer partial pressure ratio was investigated in some more detail with TMS. With -100 V substrate bias and -600 V counter electrode bias, a film which did not exhibit uptake of any deuterium could only be grown with pAr/pmono =2.5 (no. 582); deposited at a pressure ratio of 0 (no. 614), 0.4 (no. 629), or 1 (no. 628) the film was completely penetrated by D,O in the humidity-temperature test. Electron probe microanalyses of the latter two films (several weeks after deposition, but before exposure to the humidity-temperature treatment) showed also an increased oxygen content, compared, e.g. to sample no. 632, probably due to a post-deposition hydrolytic or oxidative uptake from the atmosphere. In order to refute providently the counter-argumentation given above, sample no. 550 was grown with a starting and an intermediate OH-containing layer, each obtained by adding ethanol for 5 min to the source gas. IR spectra of this sample were taken to confirm the presence of hydroxyl groups which are known to show a rapid H/D exchange if deuterium ions are present. SIMS spectra after humidity treatment with D20 (Fig. 2, righthand spectrum) clearly show a transitory increase of the oxygen content at half film thickness, but no simultaneous increase of the D/H ratio in this region. Interestingly the spectrum measured in the vicinity of a delaminated region (distance= 100 pm) exhibits a large increase of deuterium at the interface, owing, most probably, to a high lateral mobility of water at the film/substrate interface. Far from the delamination no increase of the D concentration was found at the interface. A comparison of samples no. 632 and no. 3615, grown in different reactor types, demonstrates that a high bias voltage alone is not a guarantee for good hermeticity of the deposit; other factors like power density and residence times of species have to be taken into account, too, in order to understand the prerequisites for high-quality film growth. Interestingly, these samples differ considerably also with respect to their mechanical properties: Using a nanoindentor, hardnesses (Young’s moduli) of 15 GPa (125 GPa) and 6 GPa (40 GPa) were measured for sample nos. 632 and 3615, respectively. Results of measurements on films deposited from HMDSO are also summarised in Table 1; the D/H intensity ratios for two samples, nos. 716 and 736 are plotted in Fig. 3 as a function of depth before and after treatment with D20. No uptake of deuterium is found in the latter sample, grown under -500 V substrate

C.-P. Klages

et

al./Surface and Coatings

cc/s1

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125

[c/s1 7

10 6

1

0.4 pm a-C:H

1.40 pm a-SiC:H

#637

#550

10

1 10 0 10

3

-1 10

I I

-2

-

10

I'

I

'

/

I'

I

t

r

3x0

1030

Spdt.TirZ~[s] Fig. 2. SIMS spectra of samples nos. 637 and 550 (with an oxygen-enriched intermediate and transient layer, respectively) after 160 h of humiditytemperature treatment in D,O vapour (85 “C, SO%-85% r.h.). The D signal contains contributions from H, ions. Measurement on no. 550 was performed about 100 pm from a delaminated region.

bias, while the permeation of D,O is clearly visible for no. 716, note the enrichment of D (D/H ratiozO.l) in the native silicon oxide. The corresponding D/H intensity peak is missing in the a-SiC:H film z 629, compared to a-SiC:Hz582 in Fig. 3, as the D/H intensity ratio is about 0.1 throughout the whole film. As in the case of a-SiC:H coatings, the oxygen content of films deposited from HMDSO is indicative of the degree of crosslinking; good moisture tightness was only found for films with 13 at% oxygen or less. The uptake of atmospheric oxygen during aging after deposition was recently also demonstrated for HMDSO derived coatings prepared at bias voltages between -800 and - 1000 V (80-250 W) in the absence of argon admixtures [14]. The hermeticity found for film no. 576 could not be reproduced after the thorough reactor cleaning mentioned above (cp. no. 742, grown under similar conditions, but completely penetrated by D,O under test conditions), confirming the above mentioned role of the substrate bias and other parameters. However, using proper deposition conditions, growth of moisture tight films from HMDSO was again possible.

4. Electrochemical

investigations

Electrochemical impedance spectroscopic (EIS) measurements were performed on a range of samples from Table 1 as an indicator for the usability of these plasmadeposited coatings as corrosion protection, EIS has found widespread use for mechanistic studies of corrosion phenomena, corrosion monitoring and the assessment of coatings on metals (see ref. [15] and literature cited there). It was applied to plasma-deposited coatings by Doblhofer and coworkers [16,17]. In order to support the understanding of the measurements performed in the present investigation, the electric properties of the tilms were also studied using sputtered circular gold contacts of 0.015 cm2 area. Owing to the high resistivities of the films investigated, Bode plots (log (limpedancel) vs. log(frequency)) of data measured with gold contacts were dominated by capacitive behaviour down to frequencies below 10 Hz. Below 100 mHz, impedances of films deposited from HMDSO were generally too high to be measurable, measurements on films from TMS ended with plateaus in the 0.5 to 3 GR region, corre-

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and Coatings

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80 (1996)

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l.OOE+OO f

l.OOE-01 I

a-SiC:H

#582

8 =

a-SiC:H

#629

l.OOE-02 r

8 k

0.05

0.1

0

0.1

l.OOE+OO

1

1

a-SiCO:H

I

I

t

0.3

0.4

0.5

Depth[pm1

Depth[pm1

l.OOE-01

I 0.2

#736

a-SiCO:H

#716

0.2

0.3

I

@ tz

l.OOE-02

t

G 1.OOE-03

l.OOE-04 i 0

0.5

1

1.5

Depth [pm1

2

0

0.1

0.4

Depth[pm1

Fig. 3. Ratio of intensities of D (+ HJ and H in SIMS depth profiles measured for two samples deposited from TMS (a-SiC:H) and HMDSO (a-SiCO:H), respectively, before (weak lines) and after (bold lines) 160 h of humidity-temperature treatment in D,O vapour (85 “C, 80%-85% r.h.).

sponding to through-resistances between 8 and 45 MQ cm’. (Owing to unknown contributions from surface conduction these figures have to be considered with caution, however.) Upon immersion in the electrolyte solution, impedance spectra change significantly: Samples nos. 4, 7, 8 and 9, deposited from TMS at - 1000 V bias exhibit 0 h spectra characterised by two mainly capacitive regions with phase angles (absolute values) near 90” on the high frequency side and above 60” on the low frequency end of the spectra, separated by a more or lessflat plateau region with phase angles below 20” in between (Fig. 4(a): no. 9). Although the impedance in the plateau region (l-30 MR) is generally smaller than the low-frequency ohmic impedance as measured with a Au contact, it appears reasonable to relate this plateau to the electrical conductivity of the film. A similar decrease in film resistance Rf upon replacing a metal contact by an electrolyte contact was also observed by Vouagner et al. [18] in their work on

a-C:H films, however accompanied by a simultaneous increase of the film capacity Cr, keeping the product RfCf constant, different from what is observed here. The capacity in the order of nF which can be deduced from the high frequency region can be related to the coating capacity, on the low frequency end the impedances are presumably determined by the capacity of the electrochemical double layer at the electrolyte/coating interface, about 1-3 I.~F(in Ref. [ 181 a value of 1.8 PF was found). Upon prolonged contact with the electrolyte, the EIS spectra of these sampleswere stable for a certain time, up to 160h in caseof no. 9, but eventually all coatings failed, forming a d.c.current passwith decreasingimpedance through the film. EIS spectraof samplesnos. 10and 12are characterised by Bode plots with a plateau at about 50 ML?, extending from 1 to 50 mHz, and an intermediate region with a negative slope smaller than 1 (phase angles 40”-60”) in the 1 Hz to 100Hz region, The spectra are stable for

C.-P. Klages et aLlSurface and Coatings Technology 80 (1996) 121-128 Impedance

Imoedanca -.r---

Phase'

@)

--

127

/O

Phns;P'

IOOM 80 IOM IM

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IOK IK

4

IOOM

Impedance ,

IOOm Frequency

10 (Hz)

IK -+

/Q

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10

IK /

IOOK

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Impedance/R

Phase'

IOOK IOK

4

IOOm 10 Frequency/

IK

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IOOm Frequency

10

IK /

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Fig. 4. EIS spectry of a-SiC:H films (nos. 9, 10, 17) and a SiCO:H film (no. 729) in aerated NaCl solution at rest potential. Exposed sample area 1 cm’; results of the 0.015 cm2 Au contact measurement of #17 are scaled to 1 cm2 contact area.

96 h (no. 10, Fig. 4(b)) and 300 h (> 12 days) (no. 12) before corrosion begins. Microscopic investigations after prolonged exposure to the electrolyte usually exhibits localised corrosion pits with a density of typically l-10 cm-‘. In contrast to these electrochemical measurements, no deviations from pure capacitive behaviour were found with Au contacts on nos. 10 and 12 down to below 10 Hz. The highest stability against corrosive attack on the substrate measured so far was found for sample #17 with a 6.0 pm thick coating: EIS spectra were unchanged over the whole investigation time period of 460 hours (Fig. 4(c). Three coatings derived from HMDSO were investigated electrochemically; nos. 729, 731 and 736. EIS spectra of these samples are dominated by the film capacity at the high-frequency end and ohmic behaviour (phase angles < 10”) at low frequencies. In the intervening region, in case of nos. 729 (Fig. 4(d)) and 736 clearly visible in the Bode plots already below 50 kHz, a negative slope < 1 (phase angles 40”-60”) prevails. In neither case, a measurement after 24 h yielded the same impedance spectrum as at 0 h: A continuous decrease of the low frequency impedance as well as the impedance in the 100 mHz to 10 kHz range right from the beginning of the series of measurements was found in all samples, irrespective whether the impedance at 1 mHz at 0 h was

1 ML2 (no. 729) or 100 ML2 (no. 731). The electrochemical behaviour of no. 736, which was found to be water tight even after 160 h at 85 “C, was not significantly different from that of the other two samples. A microscopic inspection of the samples nos. 729 and 731 after EIS measurements revealed extended regions of coating blistering and delamination, up to several 100 ,um in size and often filiform in shape. The pronounced deviation from capacitive behaviour below 50 kHz of samples nos. 729 and 736 is not reflected in the dry measurements on these samples which yielded strictly linear Bode plots with slopes of - 1 down to at least 10 Hz. Interestingly the qualitative appearance of these EIS spectra resembles closely to that of steel samples coated with polybutadiene after a phosphatising pretreatment [15]. The authors mentioned possible reasons for the appearance of an impedance varying with frequency to a power < 1, diffusion-limited processes or a distribution of pore resistances, without further investigating the question. In any case, an immediate formation of ionic current paths through the films deposited from HMDSO upon immersion into the test solution appears evident. Remarkably, an increase of the Cl- ion signal in SIMS by about one order of magnitude, compared with an untreated sample, was found for sample no. 741 after exposing it to the liquid D,O/NaCl phase (85 “C, 160 h). This experiment suggests, that the coating is not an

128

C.-P. Klages et al. JSurface and Coatings Technology 80 (1996) 121-128

ideal ion barrier. Further experiments on the dielectric and electrochemical behaviour on these and other plasma-deposited films are however necessary to interprete in more detail the characteristic EIS spectra shown here.

4. Summary Using a combination of temperature-humidity testing in DzO vapour (85 “C, 80%-85% relative humidity) with SIMS depth profiling of deuterium, it has been demonstrated, that moisture tight coatings can be grown from acetylene, tetramethyl-silane and hexamethyldisiloxane in a r.f. plasma-deposition process, if a sufficient flux of energetic ions to the substrate surface is provided by the self-bias voltage at the substrate electrode. Using argon admixtures to the source gas, the bias voltage required to deposit moisture tight a-SiC:H coatings from TMS can be decreased significantly. A quantitative evaluation of the results in terms of a diffusion coefficient of DzO is rendered difficult by the coupling of diffusion and H/D exchange reactions. If however the typical square-root scaling is assumed for the time dependence of the penetration depth, a tirn thickness of 2 pm can be estimated to be sufficient to prevent access of water vapour to a substrate within a time span of ten years, provided the penetration depth is below 50 nm in a 160 h test, as found for several of the samples investigated. Using electrochemical impedance spectroscopic investigations on films deposited on steel substrates, significant differences in protection against corrosion of the substrates in aerated electrolyte solution were found for films from TMS and HMDSO, respectively. The a-SiC:H films deposited from TMS were usually stable for a certain period of time, up to 300 h, before localised corrosion began, and one sample was completely stable over the whole measuring time of 460 h. Films deposited from HMDSO, however, immediately formed ionic current paths, as to be concluded from the changes in EIS after only 24 h and the characteristic appearance of the 0 h spectra. These results show that very good corrosion protec-

tion can be achieved by properly deposited a-SiC:H films, if film defects and flaws are carefully avoided.

Acknowledgements The authors are indebted to Mrs. Ulrike Schmidt and Mr. Reinhold Bethke for EPMA and SIMS analytical measurements and to Mr. Kirsten Schiffmann for AFM investigations. Part of the work was funded by the Bundesministerium fiir Bildung, Wissenschaft, Forschung und Technologie, grant no. 03M2738B.

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