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Plasma-polymerized coatings of trimethylsilane deposited on cold-rolled steel substrates Part 2. Effect of deposition conditions on corrosion performance
PROGRESS Progress
ELSEVIER
in Organic
Coatings
25 (1995) 319-337
Plasma-polymerized coatings of trimethylsilane deposited on cold-rolled steel substrates Part 2. Effect of deposition conditions on corrosion performance W.J. van Ooij a~*, D. Surman b, H.K. Yasuda ’ aDepartmen! of MaterialsScience,
Universi~
ofCincinnati,
Cincinnari, OH 45221,
’ Kratos Analytical Inc., Ramsey, NJ 07446,
’ University of Missouri-Columbia, Columbia, MO 65221. Received
2 August
1993; in revised
form
USA
USA
10 August
USA 1994
Abstract Trimethyl silane (TMS) plasma-polymerized films were deposited on cold-rolled steel (CRS) under different conditions. The ‘films were characterized by angular-dependent Xray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and time-offlight secondary ion mass spectrometry (TOFSIMS). The complementary information generated by these surface techniques gave detailed information on the film composition. The corrosion performance of the plasma films was estimated by electrochemical impedance spectroscopy (EIS) and by exposure in a humidity test. All films were Si-based and their composition was a function of the deposition conditions and the plasma cleaning of substrate prior to deposition. A reducing plasma for metal surface treatment resulted in a film with the highest impedance. The plasma film surfaces were highly oxidized. The contact angle was the lowest for plasma films deposited from a mixture of TMS and oxygen and their corrosion performance was the poorest. Keywordst
Trimethyl
silane
plasma
polymerized
films; Deposition
conditions;
Corrosion
performance
1. Introduction Plasma treatment of materials is an attractive way for surface modification of polymers or for polymerization of monomers to form dense and highly crosslinked films [l]. A glow discharge is formed by exposing a gaseous monomer at low pressure (< 10 torr) to an electric field. Plasma polymers are then deposited in * Corresponding 030@9440/95/$09.50
author. 0
1995 Elsevier
SSDI 0300-9440(94)00548-F
Science
S.A.
All rights reserved
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W.J. van Ooij et al. / Progress in manic
Coatings 25 (1995) 319-337
the form of a thin film or powder on surfaces contacting the glow discharge. Applications of plasma processes include deposition of fiims on metals or polymers and modification of surface properties for improvement of metal-polymer adhesion or corrosion performance as has recently been suggested [2,3]. The plasma film composition and properties are a function of the plasma process parameters and the type of monomer used. There is a considerable amount of literature on the effect of the plasma process parameters on the deposited film properties [1,3,4,5]. A widely used parameter to characterize the deposition process is the W/FM ratio, where W is the discharge power, F is the flow rate of the monomer and M is the molecular weight of the monomer. This parameter permits one to distinguish between the extremes of ‘monomer rich/power lean’ and ‘monomer lean/power rich’ deposition regimes in a given apparatus and to compare results obtained in different laboratories. In the latter situation the film structure is markedly different from the monomer structure and tends to become more ‘inorganic’ [ 11. Plasma films from monomers such as hydrocarbons, fluorocarbons and organosilicones have been widely studied. Plasma-deposited films of organosilicon compounds are complex because the starting monomers contain at least three different elements (Si, C and H) and in some cases a fourth (such as 0 or N). Some of the organosilicon monomers are gases or are sufficiently volatile at near ambient temperatures, so that they can be used in normal plasma deposition procedures. Also, the chemical affinity between pure single-crystalline silicon and organosilicon plasma polymers has motivated research into organosilicon plasma films for applications in the area of semiconductor technology [5]. Recently, plasma-polymerized organosilicon films have been applied as protective coatings on metal substrates [3]. The emphasis here was to use plasma coatings as interface modifiers between metals and paints. The important parameters in adhesion performance were the surface cleaning and monomer deposition conditions. Characterization of plasma-polymerized coatings was performed using numerous techniques. Common among them is infrared spectroscopy, used in the reflection-absorption mode. This technique gives useful information on the bulk composition of the films. Other techniques that have been used successfully are X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and nuclear magnetic resonance (NMR). Both XPS and AES have been used as a tool to determine surface elemental composition and concentration depth profiles. NMR has been used to determine the bulk film structure. Surface analytical techniques such as XPS and AES provide information on the outermost 10-75 8, layer of the film. XPS in particular can give information from different depths by varying the take-off angle. Chemical shift data in XPS can also identify the types of bonds present in the plasma film. However, detailed structure determination of an organosilicon polymer by XPS is difficult, since the Si 2p peak shifts are limited. One way to get around this problem is to use the Auger parameter which is independent of surface charge and the shifts in the Si Auger lines are often higher than those of the photoelectron lines [6]. ABS
W.J. van Ooij et al. / Progress in Organic Coatings 25 (1995) 3X9-337
321
with its depth profiling capability has been shown to be very useful for studying the bulk composition and the plasma polymer-metal interface [2]. The profiles of the trimethyl silane plasma films clearly showed at least three different regions in the films, a surface layer, bulk region and the interfacial region. It is necessary to characterize the surface region of the plasma polymer as any further processing will be a function of the surface composition and energy. It should be noted that PP films are very thin and do not find applications as coatings per se. They can only be used as primer treatments. Time-of-flight secondary ion mass spectrometry (TOFSIMS) has recently been used effectively for characterizing plasma film surfaces [2]. This technique consists of bombarding the surface of materials with energetic ions, such as Ar+, and identifying the secondary ions with a mass spectrometer. The ion dose is considerably less than 1013 ions/cm’, which is the limit for static conditions where the analysis conditions can be considered non-destructive. The static SIMS technique produces a fingerprint spectrum of the outermost 5 8, of a solid material. Most of the instruments used thus far were equipped with quadrupole mass analyzers. With the development of time-of-flight mass analyzers the technique is now capable of very high mass resolution (for example, M/AM up to 8000 at mass 27 amu) and unlimited mass range. TOFSIMS is therefore increasingly becoming a popular technique for surface and interface studies of polymers and other materials [7,8].. This paper is a continuation of Part I in which several surface characterization techniques were used to study trimethylsilane films on cold-rolled steel [2]. Some of the major conclusions of that study were that (1) plasma-polymerized films have a surface composition different from that of the bulk of the film and; (2) the coating-metal interface characteristics are a function of the surface pretreatment. In the current paper the quality of plasma-polymerized coatings from the same trimethyl silane monomer on CRS were deposited under different conditions and their corrosion and protection properties evaluated using electrochemical impedance spectroscopy (EIS). This technique has never been used before to study plasma-polymerized films. By this technique the barrier properties of the films and the corrosion at the interface can be monitored. The EIS results were compared with the corrosion performance observed when the panels were exposed to a high-humidity environment. Contact angle measurements were also done to understand the relationship between contact angle and surface composition. The films were characterized using AES to obtain the film composition with depth. Angular-dependent XPS and TOFSIMS were used for chemical and structural characterization of the film surfaces. The quality of the films in terms of corrosion performance judged by EIS and humidity exposure will be discussed in light of the composition of the coating and its deposition conditions. 2. Experimental 2.1. Materials The monomer gas (CH,),SiH (trimethyl silane, TMS) was obtained from Htils America, Inc. The purity of the material was 98% (2% N2). The substrates used
322
W.J. van Ooij et al. I Progress in Organic Coatings 25 (1995) 319-337
were cold-rolled, low carbon steel type 1010 of automotive by Armco Steel Company (Middletown, Ohio).
grade, manufactured
2.2. Plasma film deposition Plasma polymerization was carried out in a bell-jar type glass reactor. A d.c. source with a d.c. plasma generator MDX-1K Magnetron power supply was used. Magnets were placed behind the anodes to confine the plasma. The details of the reactor and the procedure for film deposition have been described in Part I and other places [2,3,9]. The treatments given to the substrates, prior to film deposition in this study are summarized in Table 1. 2.3. Analytical techniques XPS analysis of plasma films was carried out using a Kratos XSAM 800i instrument. Spectra were recorded at both 90” (surface normal) and lo” take-off angles. MgKa X-rays (14 kV, 20 mA) were used. Photoelectrons were collected from a 700 pm diameter spot. The initial survey spectra were recorded at 40 eV pass energy, all other spectral regions were recorded with a 10 eV pass energy. Curve fitting was carried out on the Kratos VISION data system using a Gaussian line shape with 10% Lorentzian tails for both the carbon and silicon peaks. TOFSIMS analyses were performed on a Kratos PRISM instrument. It was equipped with a reflectron-type time-of-flight mass analyzer and a pulsed 25 kV liquid metal ion source of monoisotopic 69Ga’ ions with a minimum beam size of 500 A. Positive and negative spectra were obtained at a primary ion energy of 25 keV, a pulse width of 5-50 ns and a total integrated ion dose of not more Table 1 Substrate Sample
pretreatment
film deposition
Pretreatment Gas
Film 1
and plasma
conditions Deposition
Pressure (m torr)
Power (watt)
Time (min)
No treatment 2
Monomer
Pressure (m tori-)
Power (watt)
Time (min)
TMS
50
80
3
TMS
50
80
2
TMS
50
80
2
Film 2
O2 HZ
50 50
20 40
L
Film 3
O2 HZ
50 50
20 40
2 60
Film 4
0,
50
20
2
TMS+O,
50
80
2
Film 5
O2
50 flushing
20
2
TMS
50
80
2
HZ Film 6
Ar+H,
50
40
12
TMS
50
80
2
Film 7
Ar+H,
50
40
12
TMs+oz
50
80
2
W.J. van Ooij et al. / Progress in Organic Coatings 25 (1995) 319-337
323
than 10” ions/cm2. The mass resolution at 50 amu varied from M/hM= 1000 at 50 ns pulse width to about 3500 at G 10 ns pulse width. AES analysis was performed on a Perkin Elmer 590A scanning Auger spectrometer using an 8 keV electron beam of 2000 A minimum diameter. Depth profiles were recorded by sputtering with a beam of 1 keV Ar’ ions. The etch rate was calibrated for a film of Ta,O, of known thickness. EIS measurements were done using a PAR model 5301 Lock-in Amplifier with a model 5315 two-channel preamplifier over the range of 0.01 Hz to 100 kHz. A 3.5% NaCl solution was used for the EIS measurements. The measurements were made in a standard corrosion cell at the open circuit potential at 25 “C with air gently bubbling through the solution. The exposed area of the samples in the EIS experiments was 10 cm2. The EIS data were modeled using an equivalent circuit software program [lo]. Equilibrium sessile drop contact angles were determined with water using a Kruss ACAMSautomatic contact angle measuring system.
3. Results The seven types of plasma films as described in Table 1 were characterized using the techniques described in the previous section. They were analyzed in the as-received condition and again following a 30 s ultrasonic treatment in methanol to remove low molecular weight materials. 3.1. AES analyses Depth profiles of the films 2, 3 and 4 are shown in Fig. 1. All films show Si, 0, C and small amounts of N. The profiles of Si, C and Fe indicated a sharp film-metal interface. Films 1, 2 and 5 had very similar thickness and composition throughout the film. The average composition for these three films varied from SiC,H, to SiC30,.3H, based on these AES analyses. The films had two regions in the bulk with different C/Si concentration ratios. Table 2 summarizes the Ci Si ratios in these regions. Films 3 and 6 were also similar to each other. Their composition varied from SiC,H, to SiC,H, (note that the monomer has the formula of SiC,H,,). Films 4 and 7 (deposited from TMS+ 0, plasmas) had a very low CiSi ratio compared with the other plasma polymers. It is clear from these results that the type of surface treatment of the CRS substrate had an effect on the film composition and thickness. Mixtures of TMS with oxygen were used during the plasma deposition of films 4 and 7 only. These two films were 250 8, and 150 A thick, respectively, much thinner than all other films The approximate composition for the films in both cases was SiCO,,, which suggests that the film was more inorganic. The oxygen profiles in films 1, 2 and 5 were very similar. The surface concentration of oxygen in these films was high but below the surface region (top 50 A) it dropped to zero. No oxygen was seen in the bulk region of the films. However,
I5
a-
1.
-
Sputtering
50
II -
3..
f&l
Time (min.) @ SAlmin
CC)
(a)
for Ta,O,
0.
F
+
40 @ Wmin
50 for Ta,O,
80
Fig. 1. Auger depth profiles of plasma-polymerized trimethyl silane films on cold-rolled steel. Sputtering rate was 5 &min for Ta20s: (a) plasma film 2; (b) plasma film 3; (c) plasma film 4. For details on the films, see Table 1. The peak-to-peak heights plotted have been corrected for the elemental sensitivity differences. Three different regions have been indicated which are discussed in the text.
sputtering lime (min.)
20
(b)
325
W.J. van Ooij et al. I Progress in Organic Coatings 25 (1995) 319-337 Table 2 C/Si ratios Sample
from
AES
depth
profiles
Total thickness’
Top region
(A)
C/Si ratio
Interface Thickness
C/Si ratio
1 2 3 4 5 6 7
1000 1000 650 250 1000 600 150
~‘Bascd on a Ta,O, Table 3 C/Si ratios
6.0 5.6 4.1 1.0 6.0 4.8 1 .o
500 500 300 100 500 300 100
Thickness (A)
(‘Q Film Film Film Film Film Film Film
region
3.0 3.0 2.9 1.4 3.5 3.0
450 400 300 100 400 250
standard.
from XPS data
Sample
Top
Top 75 A -
15 A
C/Si
O/Si
C/Si
(>.‘Si
Film I Film 2 Film 3 Film 4
4.6 3.2 3.8 2.2
1.5 1.1 1.8 2.4
3.4 3.1 3.2 1.3
(I.8 (1.8 I .o
Film 5
4.0
1.9
3.2
_ -..J ^s _.
Film 6 Film 7
4.1 2.0
1.8 2.9
3.0 1.4
z .h
I .i)
in the interface region a sharp oxygen peak was observed at the metal-plasma interface. The sharp oxygen peak at the interface was from the oxide on the metal which is probably also the source of some of the oxygen in the film. The oxide peak at the interface was much smaller for films 3 and 6 and very little oxygen was observed in the interface region of the plasma film. This can be attributed to the use of a reducing plasma for the metal surface preparation. 3.2. XPS analyses All plasma films were also analyzed using XPS at lo” and 90” take-off angles after cleaning in methanol (Table 3). The sampling depth at these angle depths can be estimated to be approximately 10 A and 75 A, respectively. The carbon profile in all samples can be deconvoluted into four singlets. These may be interpreted as arising from C-C or C-Si, C-O (shift 1.2 eV), C=O (shift 2.6 eV) and O-C=0 (shift 4.5 eV) binding states [2]. The silicon peak in most cases can be deconvoluted into two peaks -Si-C and -Si-0 (shift 1.5 eV). When comparing the results of the various films, the following conclusions can be drawn.
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WJ. van Ooij et al. / Progress in Otganic Coatings 25 (1995) 319-337
(1) The XPS results for plasma film 1 show that the C-O and C=O peaks are unusually high in the top 15 A of the film as compared with any of the other films. (2) Deconvoluted Si 2p lines for film 6 at the two take-off angles are shown in Fig. 2. The presence of two distinctly different binding states was confirmed by the KLL Auger line shape shown in Fig. 3. The two peaks were from Si-0 groups with an Auger parameter at around 1711.5 eV and an -Si-C group at 1714.5 eV, respectively [6]. (3) Plasma films 4 and 7 contained silicon in the form of Si-0 only. This suggests a O/Si ratio of 1. The observed concentration of oxygen is twice the silicon concentration at the film surface because oxygen is also present in various other forms such as O-C, O=C, 02, H20. (4) When comparing the Si peaks at the two angles, it is found that at low angles the Si=O concentration was higher at the surface of the films 2, 3, 5 and 6 as compared with the bulk. The bulk of these films had more bonds of the Si-C or Si-Si type. 3.3. TOFSIMS analyses Previous work has shown that the trimethyl silane plasma surface may contain non-uniformly distributed low molecular weight materials SIMS has the potential 1400;
..90 deg
Si 2p
!
; 12001 2 1000
iii
108
107
105
1 9E
103 Bindlng Energy eV
z 96 Blnding Energy eV
Fig. 2. Si 2p XPS spectra of plasma of 90”; (b) take-off angle of 10”.
film 6 showing
Si-0
and Si-C
binding
states:
(a) take-off
angle
W.J. van Ooij et al. / Progress in Orgnnic
1,1600 ,
,
1602
,
1604
,
1606
,
1608
,
1610
Coatings
,
1612
, ~--------_I 1616 1618 1620
1614
14 o_ ,“: : ,Y :” ;: p: ‘: H
327
25 (1995) 319-337
1622
+lO deg
1201
1
100: (b) 80. 60.
1600 Fig. 3. Si Auger of 90”; (b) take-off
1602
1604
lines of plasma angle
1606
1608
1610
film 6 showing
1612 Si-0
1614
1616
and Si-C
1616 i6’26
binding
states:
(a) take-off
angle
of lo”.
to detect such materials. Therefore SIMS analyses were done before and after cleaning the sample in an appropriate solvent [2]. Figs. 4 and 5 show the positive SIMS spectra of plasma film 6 before and after solvent cleaning. The main feature in these spectra is the removal of low molecular weight materials with solvent cleaning as evidenced by the disappearance of many higher mass peaks characteristic of low molecular weight siloxanes [8]. After solvent cleaning the spectra of all materials were found to be reproducible. The following conclusion can be drawn when comparing the positive and negative SIMS spectra for the various films. (1) The major peaks in all the samples are silicon-containing peaks such as those at +28 (Si’), +43 (SiCH,+) and +73 amu (Si(CH3)3f). The presence of hydrocarbons can be concluded from peaks +27 (C,H,‘) and + 55 amu (C,H?+). (2) The hydrocarbon content in plasma films 4 and 7 is lower than the other films. The peak at +73 amu is higher for films 4, 6 and 7. High mass peaks such as + 147 (CH,),SiO(CH,),‘, + 207 (C,H,,Si,O,+) amu from low molecular weight siloxanes, have higher relative intensities for films 4, 6 and 7. (3) The oxygen content (O- at - 16 amu) is very high for films 4 and 7. Film 6 has lower oxygen peak intensity as compared to films 4 and 7, but the oxygen peak intensity is still higher than in the other films. (4) The hydrocarbon peaks - 13 (CH-) and - 25 (C,H-) also suggest that films 4 and 7 have less hydrocarbons than the other films.
328
W.J. van Ooij et al. I Progress in Organic Coatings 25 (1995) 319-337
35
“’
30 25
16
20 15
,
10
(b) 5
0> 0
Fig. 4. High mass resolution hexane rinsing: (a) positive
4 20
TOFSIMS spectrum;
spectrum of plasma film 6 on cold-rolled (b) negative spectrum.
steel substrate
before
(5) The peak at -60 amu (SiOZ-) is higher for films 4 and 7. This peak confirms that 0-Si species were present in these samples, as was also concluded from the XPS results. All samples which were analyzed after methanol cleaning were further cleaned with hexane. SIMS spectra were acquired after cleaning with each solvent. The purpose here was to see if additional cleaning removed or changed the surface. The most significant change observed is in the typical siloxane peaks such as +73, + 147 and + 207 amu. The relative intensities of all these peaks decreased further on cleaning, suggesting these to be low molecular weight materials. Another observation made was that the relative peak intensities changed with the time of analyses. The peaks from low molecular weight materials generally increased with aging.
W.J. van Ooij et al. / Progress in Organic Coatings 25 (1995) 319-337
Fig. 5. High mass resolution hexane rinsing: (a) positive
TOFSIMS spectrum of plasma film 6 on cold-rolled spectrum; (b) negative spectrum.
steel substrate
329
after
3.4. Powder formation
Powder formation during plasma polymerization has been studied and reported [5]. These powders have deleterious effect on the quality and properties of the film, especially their corrosion properties. SEM was used to examine all the samples to see if the plasma conditions used here caused the formation of powders. No powders were observed in the present set of samples studied here. 3.5. Contact angle measurements
Table 4 summarizes the water contact angles measured for the seven plasma films. Lower contact angles were observed for films 4 and 7. This observation suggests that the surface energy of these two films is somewhat higher, which is consistent with a higher concentration of Si-0 bonds observed by XPS.
330
W.J. van Ooij et al. t Progress in Organic Coatings 25 (1995) 319-337
Table 4 Water contact Film Film Film Film Film Film Film
angles
for the plasma
1 2 3 4 5 6 7
“Average
films
65.6 73.5 75.4 60.3 74.4 88.3 49.0 of two readings
per sample.
Afier
-
,(jay
Afterlhour
_
_
ld Frequency (Hz)
Fig. 6. EIS spectra
3.6. Impedance
of Plasma
film 3 on steel before
and after
exposure
to 3.5 wt.%
NaCl
solution.
measurements
A good correlation was found between the contact angle data and the impedance measurements. In general, samples with low contact angles showed poorer corrosion resistance compared with samples with higher contact angle. Fig. 6 shows the Bode plots of plasma film 3 after exposure to 3.5 wt.% NaCl solution for 1 h and for 1 d. Modeling the measured EIS data with an equivalent circuit program [lo] showed that the spectra of plasma films had two time constants. The parameters used to construct the equivalent circuit were: C,, the film capacitance; Rpo, the pore resistance of the film; Cd,, the double layer capacitance, and R,, the polarization resistance. Table 5 summarizes the values of these parameters for the seven films. Also, an additional parameter R1 is reported which is calculated as follows: R, = loglG&‘,,,t
331
W.J. van Ooij et al. I Progress in Organic Coatings 25 (199.5) 319-337 Table 5 Impedance parameters in corrosion test *
of the plasma
films after
exposure
to 3.5% NaCl solution
for 1 h and ranking
Sample
CP (F)
R, P)
G, (F)
RP (at
R,
Ranking
Film 1 Film 2 Film 3 Film 4 Film 5 Film 6 Film 7 untreated
1.4 x 10-I 1 x 1o-4 4.7 x 1om5 4.9x 10-a 1.1 x 1o-4 8x1o-5 5.8X 1o-4
15 60 187 41 28 85 49
1.4x1o-4 lxlo-5 3.4x10-” 1.1 x10-4 3.6x10-’ 1 x 1o-5 8.8x10-5 1.6 x 10P9
176 750 2017 201 445 830 124 360
0.17 0.80 1.12 0.46 0.58 0.98 0.46
7 3
a In decreasing
Table 6 Impedance Sample
Film 1 Film 2 Film 3 Film 4 Film 5 Film 6 Film 7 uncoated
order
parameters
of performance
of the plasma
after
24 h at 65 “C and 80% relative
films after
exposure
CP (F)
Rw (a)
8.7X 10-x 1.7x lo-’ 2x10-3 4.5 x 10-7 1.0x 10-7 1.2x 10-I 6.3x 1O-7
35 27 46 22 19 43 21
to 3.5% NaCl
1 5 4 2 6 8
humidity
solution
G (F)
completely
*
1 x 10-3 9x Lo-4 4x Lo-4 1.6:< IO-’ 5 x 1OY 2.5 x lo-’ l.l>< 10-J rusted
for 24 h fG (01 130 132 245 76 128 165 89
where the impedance, Zi, is measured at the frequency fi (f= 100 and 10000 Hz). This ratio has been suggested to be a measure of the extent of degradation of an organic coating [ll]. The R, values suggest that plasma films 3 and 6 have degraded the least, whereas plasma films 1, 4 and 7 have degraded the most. The film capacitance values also confirm this conclusion. All substrates under the film have some tendency for corrosion. The double layer capacitance and the polarization resistance values measure this corrosion activity. Here, too, plasma film 3 had the highest polarization resistance; thus, very little corrosion was seen in this sample. The table indicates that all parameters correlate fairly well with the ranking of the samples observed in the humidity exposure test. The R, parameter appears to have the best correlation. EIS measurements for the 7 films were also taken for samples after exposure to the NaCl solution for 24 h. Table 6 summarizes these results. All spectra indicate a high corrosion activity of the steel substrate. Plasma film 3 which shows the highest resistance to corrosion, still has the highest R,, and R, values after the NaCl exposure, but the differences between the films are largely wiped out.
332
W.J. van Ooij et al. I Progms
in organic
Coatings 25 (1995) 319-337
The results presented in this section were reproducible. panel were tested and the results shown in Tables 5 Although in absolute sense the numbers are rather low, the films here have thicknesses which are 2-3 orders paint coatings, being hardly thicker than oxide films.
Several samples of each and 6 were typical data. it should be realized that less than that of regular
4. Discussion In this section the polymerization process will be described followed by a discussion of the film composition and structure. The corrosion performance will be discussed and compared with the deposition conditions and the film composition. 4.1. Plasma polymerization process In the plasma polymerization of trimethyl silane, the monomer is considered to be multifunctional, because by electron impact dissociation of Si-C and C-H bonds takes place. Unlike conventional polymerization, the rate of initiation is considered to be much higher during plasma polymerization, leading to a much higher concentration of precursors. The polymerization process also results in monomer fragmentation of trimethylsilane forming methyl groups, radicals and ions. It has been postulated that the propagation step in general involves the chain growth of silene units which, because of their diradical nature, polymerize by a radical growth mechanism [12]: I
n(Si=CH2) +
I
I
I
&H;,(Si--CH2),_,4i*
I
I
Owing to the high concentration of different types of radicals the termination step dominates over the propagation step [5]. This is possible as continuous reinitiation and propagation take place in the plasma [13]. The composition of the plasma polymer is a function of the process parameters. For methylsilanes a high pressure and high concentration of monomer results in silicon-containing ions in the plasma. However, in the present work the pressure and power were kept constant in the deposition of all films. The variables studied were the surface treatment and the monomer mixture. The final film formed was found to have three distinct regions: (i) surface region; (ii) bulk region, and (iii) interface region. These regions have been marked in Fig. 1. The composition of the films will now be discussed. 4.2. Bulk film characteristics The major difference between the deposition conditions of the seven films was pretreatment of metal substrates. The film composition (from the AES depth profiles) varied between Si&H, and SiC,O,,H, for plasma films 1, 2 and 5. The film thicknesses were around 1000 A. As the deposition starts on the bare cold-
W.J. van Ooij et al. / Progress in Organic Coatings 25 (1995) 319-337
333
rolled steel, the surface treatment of the substrate has an effect on the film composition. The metal substrate was only cleaned by a solvent before the deposition of film 1. Films 2 and 5 were deposited on metal surfaces etched with an oxygen plasma for 2 min. Plasma film 2 was deposited after an additional etching with hydrogen for 1 min. All three films had an oxide film as indicated by the oxygen peak at the metal-film interface and oxygen is also seen in the film close to the interface. As the plasma treatment is a d.c. process, with a voltage of around 600 V applied, this results in the bombardment of the sample by energetic positive ions. These ions sputter-etch the surface, introducing oxygen-containing species in the plasma. The C/Si ratio in the interface region is the same as in the monomer. The C/Si ratio, however, increases to 6 in the bulk region (region II in Fig. l(a)). The film composition apparently becomes more organic and has an average formula of SiC6H,. A possible explanation for the lower C/Si ratio near the interface is that the oxygen in the plasma acts as a radical scavenger, so when the film grows, the oxygen from the plasma is consumed and the concentration of certain radicals increases. Plasma films 3 and 6 were deposited after surface cleaning with a hydrogencontaining plasma. The film composition varied between SiC,H, to SIGH,. The etching was a reducing atmosphere and it removed a portion of the oxide film from the steel surface in the case of plasma film 3. Film 6, however, still showed the presence of the oxygen at the interface. It is possible that iron oxide was reduced to its lower valence state as a result of the reducing atmosphere. The thickness of both films 3 and 6 was around 600 A. Film 3 had a C/Si ratio of 4.1 in the top 300 8, of the film, a ratio lower than in films 1, 2, 5 and 6. Plasma films 4 and 7 were deposited from a mixture of TMS and oxygen. The average film composition was SiCOZH,. The deposition rate was much reduced tompared to the other films. The final thickness of the coating was around 200 A. The low thickness probably is related to oxygen species present in the plasma. The incorporation of oxygen in the plasma has two effects: (1) it enhances the elimination of organic groups from the polymer film as it acts as a radical scavenger and promotes the formation of 0 bridges between Si atoms resulting in a film which is more inorganic; and (2) the increase in the concentration of ions leads to plasma etching effects, which compete with the polymerization process. To summarize the bulk film characteristics, it can be said that the films have a heterogeneous structure because their composition (C/Si) changes with thickness. Three types of films were produced here. Films 1, 2 and 5 were 1000 8, thick and the C/Si ratio was high for these films. Films 3 and 6 were deposited after extensive surface cleaning of the substrate which gave a 600 A thick film with intermediate C/Si ratio. Plasma films 4 and 7 were produced from a TMS/oxygen mixture. These produced 200 %, thick films with very low C/Si ratio and high 0 content. 4.3. Correlation
of film
composition
to substrate pretreatment
Even though the deposition conditions for the plasma films 1, 2, 3, 5 and 6 were similar, the compositions of the films were different (C/Si ratios). These
334
W.J. van Ooij et al. i Progressin Organic Coatings 25 (1995) 319-337
effects can probably be explained by the formation of a plasma sheath near the surface [14,15]. This plasma sheath is dependent on the surface characteristics of the substrate material. Even when a deposit of sufficient thickness has formed to block any interaction between the plasma and the original substrate surface, the sheath effects are known to have an influence on the characteristics of the growing film [5]. The different surface pretreatments therefore create different plasma gas compositions. Further work needs to be done to understand these differences as a result of oxidizing plasma pretreatment versus reducing plasma pretreatment. 4.4. Sugace film characteristics The surface composition of the films is determined by the bulk film characteristics and by post plasma aging upon exposure to ambient conditions and is necessary because subsequent treatments, such as painting, depend on the surface properties. It has been reported that aging processes in plasma-polymerized coatings frequently result in an increase in weight by several percent and a rapid decrease in the concentration of trapped free radicals with increasing duration of exposure to ambient conditions [l&17]. The film aging process is known to include various reactions of alkyl and silyl radicals with atmospheric water and oxygen which lead to the formation of Si-0-Si and Si-O-C linkages. Surface analysis of the films confirms the presence of some of these groupings. Here the film composition changed significantly from the top couple of monolayers to around 75 A. The extreme surface had higher concentration of C and 0. TOFSIMS and XPS analyses confirmed Si-C, Si-0-Si, C=O, C=C and -OH groups to be present in all film surfaces. IR analysis of plasma-polymerized methylsilicones by other investigators also confirms these findings [17]. Films 1, 2, 3, 5 and 6 have a surface composition which is around SiC,0,,8H,. The AES depth profiles suggest that the aging process is diffusioncontrolled, as the oxygen seems to have diffused into the film to a depth of about 50 A. It should be noted that oxygen does not diffuse throughout the entire plasma coating. Plasma films 4 and 7 clearly have lower carbon and higher oxygen concentrations at the surface, compared with the other films. These films were deposited from a mixture of TMS and oxygen which explains the higher oxygen concentration. An oxygen-rich surface (SiCO,,SH, from XPS) would have a high surface energy which, as a result of post-plasma reactions with hydrocarbons, gives a surface composition of Si&O,.,H,. The origin of the hydrocarbon could be long-lived alkyl radicals or hydrocarbons in the reactor which are immediately adsorbed at the end of the deposition. 4.5. Low molecular weight material When plasma film 6 was analyzed after 6 months of aging at ambient, peaks from the low molecular weight material were found to have increased in intensity.
W.J. van Ooij
et al. i Progress in Organic
Coafings
25 (1995)
319-337
335
Low molecular weight materials (LMWM) can be formed either as a result of post plasma reaction processes or by the oligomerization process [S]. However, further work needs to be done to determine if this oligomerization process plays an important role in the TMS plasma films studied here. 4.6. Corrosion
and sugace
properties
of plasma
films
The corrosion performance results indicate that there are three types of samples: (1) samples where the film provides relatively good corrosion protection to the substrate; (2) mediocre plasma films where the films are thicker than those in group 1, but their corrosion performance is not as good; and (3) poor performance. The performance here is apparently not a function of the film thickness but more related to the film composition and the plasma metal pretreatment used. These aspects will be discussed in detail in this section. The open circuit potential of all the samples clearly showed that the films do not provide good barrier protection but the corrosion protection is more likely the result of the passivation of the substrate, as we have recently also demonstrated for TMS and HMDS (hexamethoxydisiloxane) films deposited from RF plasmas [l&19]. The plasma film quality was measured by EIS and more specifically estimated from the capacitance (C,) and pore resistance values (R,,). Plasma film 3 has the highest pore resistance and lowest coating capacitance. The relative corrosion performance from the impedance results can thus be written in t he following decreasing order: Film 3 > Film 6 > Film 2 > Film 5 > Film 4 > Film 7 > Film 1 The same qualitative order of performance was observed when the samples were exposed to 85% relative humidity at 60 “C for 24 h. Film 1 was completely rusted whereas film 3 showed not more than 10% rust coverage. Corrosion under conventional organic coatings such as paint has been studied by many researchers [lO,ll]. The plasma films here are extremely thin (< 0.1 pm) and therefore the mechanism of protection can be expected to be different. The following variables we feel are important in the corrosion performance of the plasma films. 4.6.1. Coverage of the substrate On visual examination the film shows uniform coverage (color) of the substrate. However, the substrate surface has a certain microroughness whose effects on film coverage are unknown. If we assume that the plasma film grows from certain nucleating sites then the number of these nucleating sites would be crucial in determining the film coverage due to the low film thickness. The rather low values of the pore resistance for most coatings indicate that there must be conducting pathways through the film. Film 3 has the highest pore resistance value which can therefore be interpreted as having the lowest pore density. 4.6.2. Film composition The detail analysis of the films has clearly shown that the pretreatment has an effect on the film composition. The capacitancevalues from the EIS measurements
336
W.J. van Ooij et al. I Progress in Organic Coatings 25 (1995) 319-337
confirm this. It can be postulated that films 3 and 6 have a higher crosslink density which may manifest itself as a lower C/Si ratio [2]. Also, the contact angle data suggest that films 3 and 6 have poor wettability and thus the initiation of corrosion will be delayed in these samples. 4.6.3. Interface characteristics The double layer capacitance and the polarization resistance show the corrosion activity under the film at the interface. It should be noted that an uncoated steel substrate when exposed to the same experimental conditions gave C,, = 1.6 x lo3 F and R,= 360 R. These numbers suggest that the steel substrate under plasma film 3 has been passivated effectively or alternatively these films adhere very well to the substrate. The latter characteristic has been confirmed by painting similar substrates with a primer after the plasma cleaning and deposition steps (unpublished work). To summarize, the higher impedance of the films 3 and 6 suggests that metal surface pretreatment with hydrogen creates the optimum surface characteristics for the formation of a film with good passivating properties. As discussed earlier, the resulting composition of the film is a function of the substrate surface. Further work is needed to improve our understanding of the mechanism behind the superior corrosion performance for films deposited on substrates pretreated with a hydrogen plasma.
5. Conclusions
In summary, the following conclusions can be drawn from the results presented here. It should be noted that these results were obtained with a magnetronenhanced d.c. plasma reactor so that these conclusions may not necessarily apply to coatings made in different types of reactors. Plasma films of trimethylsilane on cold-rolled steel substrates have different surface compositions as compared to the bulk of the film. The surface is an oxygen-rich silicon-based polymer formed as a result of post-deposition interactions between the plasma film and the atmosphere. All films studied here have low molecular weight siloxane-like components at the surface. There was no oxygen diffusion through the plasma films. The plasma film-metal interface characteristics and the bulk film composition are a function of the surface pretreatment. An oxygen plasma surface treatment leaves an oxide film which is partly sputtered away during the plasma deposition, resulting in incorporation of some oxygen into the plasma film. A hydrogen plasma surface treatment reduces the oxide film and the resulting film deposited as lower C/Si ratio in the bulk of the film as compared to films formed after other surface treatments. Electrochemical impedance spectroscopy is a useful technique to evaluate corrosion properties of plasma films. EIS measurements showed that plasma film 3 of Table 1 has the best corrosion protection properties and shows very little
W.J. van 004
et al. I Progress in Organic Coatings 25 (199-F) 319-337
337
interfacial corrosion as compared to the other films. The type of metal surface pretreatment strongly affects the corrosion performance of the plasma film coated substrate and the ranking of films observed in EIS is the same as the ranking observed after exposure to a corrosive environment. The values of the R, parameter correlate best with the observed ranking in the humidity test. Plasma-polymerized films from a mixture of trimethylsilane and oxygen gases are oxygen-rich at the surface and contain 0 throughout the bulk of the film. These films can be considered to be largely inorganic and they have very poor corrosion properties as determined by EIS and exposure to humidity.
Acknowledgement The authors would like to thank Mr Frank Wang for preparation plasma films in this study.
of the
References
111 H.K. Yasuda, Plasma 121 A. Sabata, W.J. van [31 [41 [51 [61 [71 PI [91 [lOI El11 [I21 [I31 P41 I151
1191
Polymerization, Academic Press, Orlando, FL, 1985. Ooij and H.K. Yasuda, Su$ Interface Anal., 20 (1993) 845. T.J. Lin, B.H. Chun, H.K. Yasuda, D.J. Yasuda, D.J. Yang and J.A. Antonelli, J. Adhesion Sci. Technol., 5 (1991) 893. 0. Auciello and D.L. Flamm (eds.), Plasma Diagnostics, Vol. 1, Discharge Parameters and Chemistry, Vol. 2, Surface Analysis and Interactions, Academic Press, San Diego, CA, 1989. R. d’Agostino (ed.), Plasma Deposition, Treatment and Etching of Polymers, Academic Press, Boston, 1990. D. Briggs and M.P. Seah (eds.), Practical Surface Analysis,Vol. 1, Auger and X-Ray Photoelectron Spectroscopy, Wiley, Chichester, UK, 2nd edn., 1992. A. Sabata, W.J. van Ooij and H.K. Yasuda, Proc. VIIIth Int. Conf: Secondary Ion Mass Spectroscopy (SIMS VIII), Amsterdam, Sept. 16-20, 1991, p. 189. D. Briggs, A. Brown and J.C. Vickerman, Handbook of Static Secondary Ion Mass Spectrometp (SIMS), Wiley, Chichester, UK 1989. H.K. Yasuda, T.J. Lin, D.J. Yang and J.A. Antonelli (E.I. du Porn de Nemours Kr C.), US Patent No. 4 980 I96 (1990). B.A. Boukamp, Solid State Ionics, 20 (1986) 31. S. Haruyama, M. Asari and T. Tsuru, in M.W. Kendig and H. Leidheiser, Jr. (eds.), Corrosion Protection by Organic Coatings, Proc. The Electrochemical Society, Vol. 87-2, 1987, p. 197. A.G. Alexander and O.P. Strausz, J. Phys. Chem., 80 (1987) 2531. A.M. Wrobel, M.R. Wertheimer, J. Dib and H.P. Schreiber, J. Macromol. Sci.-Chem. A, 14 (1980) 321. L.F.*Thompson and G. Smolinsky, J. Appl. Polym. Sci., 16 (1972) 11’79. K.W. Gerstenberg and W. Beyer, Proc. E-MRS Confereme, StrasbouK, France, 1987, Vol. 17, p. 481. H.K. Yasuda and T. Hsu, J. Polym. Sci., PO&m. Chem. Ed., 25 (1977) 81. A.M. Wr6be1, J. Macromol. Sci., Chem., 22 (1985) 1089. W.J. van Ooij, A. Sabata and Ih-Houng Ioh, Modifications of Passive Films, Eur. Fed. Corrosion Publications No. 12, The Institute of Materials, London, 1994, p. 253. K.D. Conners, W.J. van Ooij, S.J. Clarson and A. Sabata, J. Appl. Pal. Sci., 54 (1994) 167.
ELSEVIER
in Organic
Coatings
25 (1995) 319-337
Plasma-polymerized coatings of trimethylsilane deposited on cold-rolled steel substrates Part 2. Effect of deposition conditions on corrosion performance W.J. van Ooij a~*, D. Surman b, H.K. Yasuda ’ aDepartmen! of MaterialsScience,
Universi~
ofCincinnati,
Cincinnari, OH 45221,
’ Kratos Analytical Inc., Ramsey, NJ 07446,
’ University of Missouri-Columbia, Columbia, MO 65221. Received
2 August
1993; in revised
form
USA
USA
10 August
USA 1994
Abstract Trimethyl silane (TMS) plasma-polymerized films were deposited on cold-rolled steel (CRS) under different conditions. The ‘films were characterized by angular-dependent Xray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and time-offlight secondary ion mass spectrometry (TOFSIMS). The complementary information generated by these surface techniques gave detailed information on the film composition. The corrosion performance of the plasma films was estimated by electrochemical impedance spectroscopy (EIS) and by exposure in a humidity test. All films were Si-based and their composition was a function of the deposition conditions and the plasma cleaning of substrate prior to deposition. A reducing plasma for metal surface treatment resulted in a film with the highest impedance. The plasma film surfaces were highly oxidized. The contact angle was the lowest for plasma films deposited from a mixture of TMS and oxygen and their corrosion performance was the poorest. Keywordst
Trimethyl
silane
plasma
polymerized
films; Deposition
conditions;
Corrosion
performance
1. Introduction Plasma treatment of materials is an attractive way for surface modification of polymers or for polymerization of monomers to form dense and highly crosslinked films [l]. A glow discharge is formed by exposing a gaseous monomer at low pressure (< 10 torr) to an electric field. Plasma polymers are then deposited in * Corresponding 030@9440/95/$09.50
author. 0
1995 Elsevier
SSDI 0300-9440(94)00548-F
Science
S.A.
All rights reserved
320
W.J. van Ooij et al. / Progress in manic
Coatings 25 (1995) 319-337
the form of a thin film or powder on surfaces contacting the glow discharge. Applications of plasma processes include deposition of fiims on metals or polymers and modification of surface properties for improvement of metal-polymer adhesion or corrosion performance as has recently been suggested [2,3]. The plasma film composition and properties are a function of the plasma process parameters and the type of monomer used. There is a considerable amount of literature on the effect of the plasma process parameters on the deposited film properties [1,3,4,5]. A widely used parameter to characterize the deposition process is the W/FM ratio, where W is the discharge power, F is the flow rate of the monomer and M is the molecular weight of the monomer. This parameter permits one to distinguish between the extremes of ‘monomer rich/power lean’ and ‘monomer lean/power rich’ deposition regimes in a given apparatus and to compare results obtained in different laboratories. In the latter situation the film structure is markedly different from the monomer structure and tends to become more ‘inorganic’ [ 11. Plasma films from monomers such as hydrocarbons, fluorocarbons and organosilicones have been widely studied. Plasma-deposited films of organosilicon compounds are complex because the starting monomers contain at least three different elements (Si, C and H) and in some cases a fourth (such as 0 or N). Some of the organosilicon monomers are gases or are sufficiently volatile at near ambient temperatures, so that they can be used in normal plasma deposition procedures. Also, the chemical affinity between pure single-crystalline silicon and organosilicon plasma polymers has motivated research into organosilicon plasma films for applications in the area of semiconductor technology [5]. Recently, plasma-polymerized organosilicon films have been applied as protective coatings on metal substrates [3]. The emphasis here was to use plasma coatings as interface modifiers between metals and paints. The important parameters in adhesion performance were the surface cleaning and monomer deposition conditions. Characterization of plasma-polymerized coatings was performed using numerous techniques. Common among them is infrared spectroscopy, used in the reflection-absorption mode. This technique gives useful information on the bulk composition of the films. Other techniques that have been used successfully are X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and nuclear magnetic resonance (NMR). Both XPS and AES have been used as a tool to determine surface elemental composition and concentration depth profiles. NMR has been used to determine the bulk film structure. Surface analytical techniques such as XPS and AES provide information on the outermost 10-75 8, layer of the film. XPS in particular can give information from different depths by varying the take-off angle. Chemical shift data in XPS can also identify the types of bonds present in the plasma film. However, detailed structure determination of an organosilicon polymer by XPS is difficult, since the Si 2p peak shifts are limited. One way to get around this problem is to use the Auger parameter which is independent of surface charge and the shifts in the Si Auger lines are often higher than those of the photoelectron lines [6]. ABS
W.J. van Ooij et al. / Progress in Organic Coatings 25 (1995) 3X9-337
321
with its depth profiling capability has been shown to be very useful for studying the bulk composition and the plasma polymer-metal interface [2]. The profiles of the trimethyl silane plasma films clearly showed at least three different regions in the films, a surface layer, bulk region and the interfacial region. It is necessary to characterize the surface region of the plasma polymer as any further processing will be a function of the surface composition and energy. It should be noted that PP films are very thin and do not find applications as coatings per se. They can only be used as primer treatments. Time-of-flight secondary ion mass spectrometry (TOFSIMS) has recently been used effectively for characterizing plasma film surfaces [2]. This technique consists of bombarding the surface of materials with energetic ions, such as Ar+, and identifying the secondary ions with a mass spectrometer. The ion dose is considerably less than 1013 ions/cm’, which is the limit for static conditions where the analysis conditions can be considered non-destructive. The static SIMS technique produces a fingerprint spectrum of the outermost 5 8, of a solid material. Most of the instruments used thus far were equipped with quadrupole mass analyzers. With the development of time-of-flight mass analyzers the technique is now capable of very high mass resolution (for example, M/AM up to 8000 at mass 27 amu) and unlimited mass range. TOFSIMS is therefore increasingly becoming a popular technique for surface and interface studies of polymers and other materials [7,8].. This paper is a continuation of Part I in which several surface characterization techniques were used to study trimethylsilane films on cold-rolled steel [2]. Some of the major conclusions of that study were that (1) plasma-polymerized films have a surface composition different from that of the bulk of the film and; (2) the coating-metal interface characteristics are a function of the surface pretreatment. In the current paper the quality of plasma-polymerized coatings from the same trimethyl silane monomer on CRS were deposited under different conditions and their corrosion and protection properties evaluated using electrochemical impedance spectroscopy (EIS). This technique has never been used before to study plasma-polymerized films. By this technique the barrier properties of the films and the corrosion at the interface can be monitored. The EIS results were compared with the corrosion performance observed when the panels were exposed to a high-humidity environment. Contact angle measurements were also done to understand the relationship between contact angle and surface composition. The films were characterized using AES to obtain the film composition with depth. Angular-dependent XPS and TOFSIMS were used for chemical and structural characterization of the film surfaces. The quality of the films in terms of corrosion performance judged by EIS and humidity exposure will be discussed in light of the composition of the coating and its deposition conditions. 2. Experimental 2.1. Materials The monomer gas (CH,),SiH (trimethyl silane, TMS) was obtained from Htils America, Inc. The purity of the material was 98% (2% N2). The substrates used
322
W.J. van Ooij et al. I Progress in Organic Coatings 25 (1995) 319-337
were cold-rolled, low carbon steel type 1010 of automotive by Armco Steel Company (Middletown, Ohio).
grade, manufactured
2.2. Plasma film deposition Plasma polymerization was carried out in a bell-jar type glass reactor. A d.c. source with a d.c. plasma generator MDX-1K Magnetron power supply was used. Magnets were placed behind the anodes to confine the plasma. The details of the reactor and the procedure for film deposition have been described in Part I and other places [2,3,9]. The treatments given to the substrates, prior to film deposition in this study are summarized in Table 1. 2.3. Analytical techniques XPS analysis of plasma films was carried out using a Kratos XSAM 800i instrument. Spectra were recorded at both 90” (surface normal) and lo” take-off angles. MgKa X-rays (14 kV, 20 mA) were used. Photoelectrons were collected from a 700 pm diameter spot. The initial survey spectra were recorded at 40 eV pass energy, all other spectral regions were recorded with a 10 eV pass energy. Curve fitting was carried out on the Kratos VISION data system using a Gaussian line shape with 10% Lorentzian tails for both the carbon and silicon peaks. TOFSIMS analyses were performed on a Kratos PRISM instrument. It was equipped with a reflectron-type time-of-flight mass analyzer and a pulsed 25 kV liquid metal ion source of monoisotopic 69Ga’ ions with a minimum beam size of 500 A. Positive and negative spectra were obtained at a primary ion energy of 25 keV, a pulse width of 5-50 ns and a total integrated ion dose of not more Table 1 Substrate Sample
pretreatment
film deposition
Pretreatment Gas
Film 1
and plasma
conditions Deposition
Pressure (m torr)
Power (watt)
Time (min)
No treatment 2
Monomer
Pressure (m tori-)
Power (watt)
Time (min)
TMS
50
80
3
TMS
50
80
2
TMS
50
80
2
Film 2
O2 HZ
50 50
20 40
L
Film 3
O2 HZ
50 50
20 40
2 60
Film 4
0,
50
20
2
TMS+O,
50
80
2
Film 5
O2
50 flushing
20
2
TMS
50
80
2
HZ Film 6
Ar+H,
50
40
12
TMS
50
80
2
Film 7
Ar+H,
50
40
12
TMs+oz
50
80
2
W.J. van Ooij et al. / Progress in Organic Coatings 25 (1995) 319-337
323
than 10” ions/cm2. The mass resolution at 50 amu varied from M/hM= 1000 at 50 ns pulse width to about 3500 at G 10 ns pulse width. AES analysis was performed on a Perkin Elmer 590A scanning Auger spectrometer using an 8 keV electron beam of 2000 A minimum diameter. Depth profiles were recorded by sputtering with a beam of 1 keV Ar’ ions. The etch rate was calibrated for a film of Ta,O, of known thickness. EIS measurements were done using a PAR model 5301 Lock-in Amplifier with a model 5315 two-channel preamplifier over the range of 0.01 Hz to 100 kHz. A 3.5% NaCl solution was used for the EIS measurements. The measurements were made in a standard corrosion cell at the open circuit potential at 25 “C with air gently bubbling through the solution. The exposed area of the samples in the EIS experiments was 10 cm2. The EIS data were modeled using an equivalent circuit software program [lo]. Equilibrium sessile drop contact angles were determined with water using a Kruss ACAMSautomatic contact angle measuring system.
3. Results The seven types of plasma films as described in Table 1 were characterized using the techniques described in the previous section. They were analyzed in the as-received condition and again following a 30 s ultrasonic treatment in methanol to remove low molecular weight materials. 3.1. AES analyses Depth profiles of the films 2, 3 and 4 are shown in Fig. 1. All films show Si, 0, C and small amounts of N. The profiles of Si, C and Fe indicated a sharp film-metal interface. Films 1, 2 and 5 had very similar thickness and composition throughout the film. The average composition for these three films varied from SiC,H, to SiC30,.3H, based on these AES analyses. The films had two regions in the bulk with different C/Si concentration ratios. Table 2 summarizes the Ci Si ratios in these regions. Films 3 and 6 were also similar to each other. Their composition varied from SiC,H, to SiC,H, (note that the monomer has the formula of SiC,H,,). Films 4 and 7 (deposited from TMS+ 0, plasmas) had a very low CiSi ratio compared with the other plasma polymers. It is clear from these results that the type of surface treatment of the CRS substrate had an effect on the film composition and thickness. Mixtures of TMS with oxygen were used during the plasma deposition of films 4 and 7 only. These two films were 250 8, and 150 A thick, respectively, much thinner than all other films The approximate composition for the films in both cases was SiCO,,, which suggests that the film was more inorganic. The oxygen profiles in films 1, 2 and 5 were very similar. The surface concentration of oxygen in these films was high but below the surface region (top 50 A) it dropped to zero. No oxygen was seen in the bulk region of the films. However,
I5
a-
1.
-
Sputtering
50
II -
3..
f&l
Time (min.) @ SAlmin
CC)
(a)
for Ta,O,
0.
F
+
40 @ Wmin
50 for Ta,O,
80
Fig. 1. Auger depth profiles of plasma-polymerized trimethyl silane films on cold-rolled steel. Sputtering rate was 5 &min for Ta20s: (a) plasma film 2; (b) plasma film 3; (c) plasma film 4. For details on the films, see Table 1. The peak-to-peak heights plotted have been corrected for the elemental sensitivity differences. Three different regions have been indicated which are discussed in the text.
sputtering lime (min.)
20
(b)
325
W.J. van Ooij et al. I Progress in Organic Coatings 25 (1995) 319-337 Table 2 C/Si ratios Sample
from
AES
depth
profiles
Total thickness’
Top region
(A)
C/Si ratio
Interface Thickness
C/Si ratio
1 2 3 4 5 6 7
1000 1000 650 250 1000 600 150
~‘Bascd on a Ta,O, Table 3 C/Si ratios
6.0 5.6 4.1 1.0 6.0 4.8 1 .o
500 500 300 100 500 300 100
Thickness (A)
(‘Q Film Film Film Film Film Film Film
region
3.0 3.0 2.9 1.4 3.5 3.0
450 400 300 100 400 250
standard.
from XPS data
Sample
Top
Top 75 A -
15 A
C/Si
O/Si
C/Si
(>.‘Si
Film I Film 2 Film 3 Film 4
4.6 3.2 3.8 2.2
1.5 1.1 1.8 2.4
3.4 3.1 3.2 1.3
(I.8 (1.8 I .o
Film 5
4.0
1.9
3.2
_ -..J ^s _.
Film 6 Film 7
4.1 2.0
1.8 2.9
3.0 1.4
z .h
I .i)
in the interface region a sharp oxygen peak was observed at the metal-plasma interface. The sharp oxygen peak at the interface was from the oxide on the metal which is probably also the source of some of the oxygen in the film. The oxide peak at the interface was much smaller for films 3 and 6 and very little oxygen was observed in the interface region of the plasma film. This can be attributed to the use of a reducing plasma for the metal surface preparation. 3.2. XPS analyses All plasma films were also analyzed using XPS at lo” and 90” take-off angles after cleaning in methanol (Table 3). The sampling depth at these angle depths can be estimated to be approximately 10 A and 75 A, respectively. The carbon profile in all samples can be deconvoluted into four singlets. These may be interpreted as arising from C-C or C-Si, C-O (shift 1.2 eV), C=O (shift 2.6 eV) and O-C=0 (shift 4.5 eV) binding states [2]. The silicon peak in most cases can be deconvoluted into two peaks -Si-C and -Si-0 (shift 1.5 eV). When comparing the results of the various films, the following conclusions can be drawn.
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WJ. van Ooij et al. / Progress in Otganic Coatings 25 (1995) 319-337
(1) The XPS results for plasma film 1 show that the C-O and C=O peaks are unusually high in the top 15 A of the film as compared with any of the other films. (2) Deconvoluted Si 2p lines for film 6 at the two take-off angles are shown in Fig. 2. The presence of two distinctly different binding states was confirmed by the KLL Auger line shape shown in Fig. 3. The two peaks were from Si-0 groups with an Auger parameter at around 1711.5 eV and an -Si-C group at 1714.5 eV, respectively [6]. (3) Plasma films 4 and 7 contained silicon in the form of Si-0 only. This suggests a O/Si ratio of 1. The observed concentration of oxygen is twice the silicon concentration at the film surface because oxygen is also present in various other forms such as O-C, O=C, 02, H20. (4) When comparing the Si peaks at the two angles, it is found that at low angles the Si=O concentration was higher at the surface of the films 2, 3, 5 and 6 as compared with the bulk. The bulk of these films had more bonds of the Si-C or Si-Si type. 3.3. TOFSIMS analyses Previous work has shown that the trimethyl silane plasma surface may contain non-uniformly distributed low molecular weight materials SIMS has the potential 1400;
..90 deg
Si 2p
!
; 12001 2 1000
iii
108
107
105
1 9E
103 Bindlng Energy eV
z 96 Blnding Energy eV
Fig. 2. Si 2p XPS spectra of plasma of 90”; (b) take-off angle of 10”.
film 6 showing
Si-0
and Si-C
binding
states:
(a) take-off
angle
W.J. van Ooij et al. / Progress in Orgnnic
1,1600 ,
,
1602
,
1604
,
1606
,
1608
,
1610
Coatings
,
1612
, ~--------_I 1616 1618 1620
1614
14 o_ ,“: : ,Y :” ;: p: ‘: H
327
25 (1995) 319-337
1622
+lO deg
1201
1
100: (b) 80. 60.
1600 Fig. 3. Si Auger of 90”; (b) take-off
1602
1604
lines of plasma angle
1606
1608
1610
film 6 showing
1612 Si-0
1614
1616
and Si-C
1616 i6’26
binding
states:
(a) take-off
angle
of lo”.
to detect such materials. Therefore SIMS analyses were done before and after cleaning the sample in an appropriate solvent [2]. Figs. 4 and 5 show the positive SIMS spectra of plasma film 6 before and after solvent cleaning. The main feature in these spectra is the removal of low molecular weight materials with solvent cleaning as evidenced by the disappearance of many higher mass peaks characteristic of low molecular weight siloxanes [8]. After solvent cleaning the spectra of all materials were found to be reproducible. The following conclusion can be drawn when comparing the positive and negative SIMS spectra for the various films. (1) The major peaks in all the samples are silicon-containing peaks such as those at +28 (Si’), +43 (SiCH,+) and +73 amu (Si(CH3)3f). The presence of hydrocarbons can be concluded from peaks +27 (C,H,‘) and + 55 amu (C,H?+). (2) The hydrocarbon content in plasma films 4 and 7 is lower than the other films. The peak at +73 amu is higher for films 4, 6 and 7. High mass peaks such as + 147 (CH,),SiO(CH,),‘, + 207 (C,H,,Si,O,+) amu from low molecular weight siloxanes, have higher relative intensities for films 4, 6 and 7. (3) The oxygen content (O- at - 16 amu) is very high for films 4 and 7. Film 6 has lower oxygen peak intensity as compared to films 4 and 7, but the oxygen peak intensity is still higher than in the other films. (4) The hydrocarbon peaks - 13 (CH-) and - 25 (C,H-) also suggest that films 4 and 7 have less hydrocarbons than the other films.
328
W.J. van Ooij et al. I Progress in Organic Coatings 25 (1995) 319-337
35
“’
30 25
16
20 15
,
10
(b) 5
0> 0
Fig. 4. High mass resolution hexane rinsing: (a) positive
4 20
TOFSIMS spectrum;
spectrum of plasma film 6 on cold-rolled (b) negative spectrum.
steel substrate
before
(5) The peak at -60 amu (SiOZ-) is higher for films 4 and 7. This peak confirms that 0-Si species were present in these samples, as was also concluded from the XPS results. All samples which were analyzed after methanol cleaning were further cleaned with hexane. SIMS spectra were acquired after cleaning with each solvent. The purpose here was to see if additional cleaning removed or changed the surface. The most significant change observed is in the typical siloxane peaks such as +73, + 147 and + 207 amu. The relative intensities of all these peaks decreased further on cleaning, suggesting these to be low molecular weight materials. Another observation made was that the relative peak intensities changed with the time of analyses. The peaks from low molecular weight materials generally increased with aging.
W.J. van Ooij et al. / Progress in Organic Coatings 25 (1995) 319-337
Fig. 5. High mass resolution hexane rinsing: (a) positive
TOFSIMS spectrum of plasma film 6 on cold-rolled spectrum; (b) negative spectrum.
steel substrate
329
after
3.4. Powder formation
Powder formation during plasma polymerization has been studied and reported [5]. These powders have deleterious effect on the quality and properties of the film, especially their corrosion properties. SEM was used to examine all the samples to see if the plasma conditions used here caused the formation of powders. No powders were observed in the present set of samples studied here. 3.5. Contact angle measurements
Table 4 summarizes the water contact angles measured for the seven plasma films. Lower contact angles were observed for films 4 and 7. This observation suggests that the surface energy of these two films is somewhat higher, which is consistent with a higher concentration of Si-0 bonds observed by XPS.
330
W.J. van Ooij et al. t Progress in Organic Coatings 25 (1995) 319-337
Table 4 Water contact Film Film Film Film Film Film Film
angles
for the plasma
1 2 3 4 5 6 7
“Average
films
65.6 73.5 75.4 60.3 74.4 88.3 49.0 of two readings
per sample.
Afier
-
,(jay
Afterlhour
_
_
ld Frequency (Hz)
Fig. 6. EIS spectra
3.6. Impedance
of Plasma
film 3 on steel before
and after
exposure
to 3.5 wt.%
NaCl
solution.
measurements
A good correlation was found between the contact angle data and the impedance measurements. In general, samples with low contact angles showed poorer corrosion resistance compared with samples with higher contact angle. Fig. 6 shows the Bode plots of plasma film 3 after exposure to 3.5 wt.% NaCl solution for 1 h and for 1 d. Modeling the measured EIS data with an equivalent circuit program [lo] showed that the spectra of plasma films had two time constants. The parameters used to construct the equivalent circuit were: C,, the film capacitance; Rpo, the pore resistance of the film; Cd,, the double layer capacitance, and R,, the polarization resistance. Table 5 summarizes the values of these parameters for the seven films. Also, an additional parameter R1 is reported which is calculated as follows: R, = loglG&‘,,,t
331
W.J. van Ooij et al. I Progress in Organic Coatings 25 (199.5) 319-337 Table 5 Impedance parameters in corrosion test *
of the plasma
films after
exposure
to 3.5% NaCl solution
for 1 h and ranking
Sample
CP (F)
R, P)
G, (F)
RP (at
R,
Ranking
Film 1 Film 2 Film 3 Film 4 Film 5 Film 6 Film 7 untreated
1.4 x 10-I 1 x 1o-4 4.7 x 1om5 4.9x 10-a 1.1 x 1o-4 8x1o-5 5.8X 1o-4
15 60 187 41 28 85 49
1.4x1o-4 lxlo-5 3.4x10-” 1.1 x10-4 3.6x10-’ 1 x 1o-5 8.8x10-5 1.6 x 10P9
176 750 2017 201 445 830 124 360
0.17 0.80 1.12 0.46 0.58 0.98 0.46
7 3
a In decreasing
Table 6 Impedance Sample
Film 1 Film 2 Film 3 Film 4 Film 5 Film 6 Film 7 uncoated
order
parameters
of performance
of the plasma
after
24 h at 65 “C and 80% relative
films after
exposure
CP (F)
Rw (a)
8.7X 10-x 1.7x lo-’ 2x10-3 4.5 x 10-7 1.0x 10-7 1.2x 10-I 6.3x 1O-7
35 27 46 22 19 43 21
to 3.5% NaCl
1 5 4 2 6 8
humidity
solution
G (F)
completely
*
1 x 10-3 9x Lo-4 4x Lo-4 1.6:< IO-’ 5 x 1OY 2.5 x lo-’ l.l>< 10-J rusted
for 24 h fG (01 130 132 245 76 128 165 89
where the impedance, Zi, is measured at the frequency fi (f= 100 and 10000 Hz). This ratio has been suggested to be a measure of the extent of degradation of an organic coating [ll]. The R, values suggest that plasma films 3 and 6 have degraded the least, whereas plasma films 1, 4 and 7 have degraded the most. The film capacitance values also confirm this conclusion. All substrates under the film have some tendency for corrosion. The double layer capacitance and the polarization resistance values measure this corrosion activity. Here, too, plasma film 3 had the highest polarization resistance; thus, very little corrosion was seen in this sample. The table indicates that all parameters correlate fairly well with the ranking of the samples observed in the humidity exposure test. The R, parameter appears to have the best correlation. EIS measurements for the 7 films were also taken for samples after exposure to the NaCl solution for 24 h. Table 6 summarizes these results. All spectra indicate a high corrosion activity of the steel substrate. Plasma film 3 which shows the highest resistance to corrosion, still has the highest R,, and R, values after the NaCl exposure, but the differences between the films are largely wiped out.
332
W.J. van Ooij et al. I Progms
in organic
Coatings 25 (1995) 319-337
The results presented in this section were reproducible. panel were tested and the results shown in Tables 5 Although in absolute sense the numbers are rather low, the films here have thicknesses which are 2-3 orders paint coatings, being hardly thicker than oxide films.
Several samples of each and 6 were typical data. it should be realized that less than that of regular
4. Discussion In this section the polymerization process will be described followed by a discussion of the film composition and structure. The corrosion performance will be discussed and compared with the deposition conditions and the film composition. 4.1. Plasma polymerization process In the plasma polymerization of trimethyl silane, the monomer is considered to be multifunctional, because by electron impact dissociation of Si-C and C-H bonds takes place. Unlike conventional polymerization, the rate of initiation is considered to be much higher during plasma polymerization, leading to a much higher concentration of precursors. The polymerization process also results in monomer fragmentation of trimethylsilane forming methyl groups, radicals and ions. It has been postulated that the propagation step in general involves the chain growth of silene units which, because of their diradical nature, polymerize by a radical growth mechanism [12]: I
n(Si=CH2) +
I
I
I
&H;,(Si--CH2),_,4i*
I
I
Owing to the high concentration of different types of radicals the termination step dominates over the propagation step [5]. This is possible as continuous reinitiation and propagation take place in the plasma [13]. The composition of the plasma polymer is a function of the process parameters. For methylsilanes a high pressure and high concentration of monomer results in silicon-containing ions in the plasma. However, in the present work the pressure and power were kept constant in the deposition of all films. The variables studied were the surface treatment and the monomer mixture. The final film formed was found to have three distinct regions: (i) surface region; (ii) bulk region, and (iii) interface region. These regions have been marked in Fig. 1. The composition of the films will now be discussed. 4.2. Bulk film characteristics The major difference between the deposition conditions of the seven films was pretreatment of metal substrates. The film composition (from the AES depth profiles) varied between Si&H, and SiC,O,,H, for plasma films 1, 2 and 5. The film thicknesses were around 1000 A. As the deposition starts on the bare cold-
W.J. van Ooij et al. / Progress in Organic Coatings 25 (1995) 319-337
333
rolled steel, the surface treatment of the substrate has an effect on the film composition. The metal substrate was only cleaned by a solvent before the deposition of film 1. Films 2 and 5 were deposited on metal surfaces etched with an oxygen plasma for 2 min. Plasma film 2 was deposited after an additional etching with hydrogen for 1 min. All three films had an oxide film as indicated by the oxygen peak at the metal-film interface and oxygen is also seen in the film close to the interface. As the plasma treatment is a d.c. process, with a voltage of around 600 V applied, this results in the bombardment of the sample by energetic positive ions. These ions sputter-etch the surface, introducing oxygen-containing species in the plasma. The C/Si ratio in the interface region is the same as in the monomer. The C/Si ratio, however, increases to 6 in the bulk region (region II in Fig. l(a)). The film composition apparently becomes more organic and has an average formula of SiC6H,. A possible explanation for the lower C/Si ratio near the interface is that the oxygen in the plasma acts as a radical scavenger, so when the film grows, the oxygen from the plasma is consumed and the concentration of certain radicals increases. Plasma films 3 and 6 were deposited after surface cleaning with a hydrogencontaining plasma. The film composition varied between SiC,H, to SIGH,. The etching was a reducing atmosphere and it removed a portion of the oxide film from the steel surface in the case of plasma film 3. Film 6, however, still showed the presence of the oxygen at the interface. It is possible that iron oxide was reduced to its lower valence state as a result of the reducing atmosphere. The thickness of both films 3 and 6 was around 600 A. Film 3 had a C/Si ratio of 4.1 in the top 300 8, of the film, a ratio lower than in films 1, 2, 5 and 6. Plasma films 4 and 7 were deposited from a mixture of TMS and oxygen. The average film composition was SiCOZH,. The deposition rate was much reduced tompared to the other films. The final thickness of the coating was around 200 A. The low thickness probably is related to oxygen species present in the plasma. The incorporation of oxygen in the plasma has two effects: (1) it enhances the elimination of organic groups from the polymer film as it acts as a radical scavenger and promotes the formation of 0 bridges between Si atoms resulting in a film which is more inorganic; and (2) the increase in the concentration of ions leads to plasma etching effects, which compete with the polymerization process. To summarize the bulk film characteristics, it can be said that the films have a heterogeneous structure because their composition (C/Si) changes with thickness. Three types of films were produced here. Films 1, 2 and 5 were 1000 8, thick and the C/Si ratio was high for these films. Films 3 and 6 were deposited after extensive surface cleaning of the substrate which gave a 600 A thick film with intermediate C/Si ratio. Plasma films 4 and 7 were produced from a TMS/oxygen mixture. These produced 200 %, thick films with very low C/Si ratio and high 0 content. 4.3. Correlation
of film
composition
to substrate pretreatment
Even though the deposition conditions for the plasma films 1, 2, 3, 5 and 6 were similar, the compositions of the films were different (C/Si ratios). These
334
W.J. van Ooij et al. i Progressin Organic Coatings 25 (1995) 319-337
effects can probably be explained by the formation of a plasma sheath near the surface [14,15]. This plasma sheath is dependent on the surface characteristics of the substrate material. Even when a deposit of sufficient thickness has formed to block any interaction between the plasma and the original substrate surface, the sheath effects are known to have an influence on the characteristics of the growing film [5]. The different surface pretreatments therefore create different plasma gas compositions. Further work needs to be done to understand these differences as a result of oxidizing plasma pretreatment versus reducing plasma pretreatment. 4.4. Sugace film characteristics The surface composition of the films is determined by the bulk film characteristics and by post plasma aging upon exposure to ambient conditions and is necessary because subsequent treatments, such as painting, depend on the surface properties. It has been reported that aging processes in plasma-polymerized coatings frequently result in an increase in weight by several percent and a rapid decrease in the concentration of trapped free radicals with increasing duration of exposure to ambient conditions [l&17]. The film aging process is known to include various reactions of alkyl and silyl radicals with atmospheric water and oxygen which lead to the formation of Si-0-Si and Si-O-C linkages. Surface analysis of the films confirms the presence of some of these groupings. Here the film composition changed significantly from the top couple of monolayers to around 75 A. The extreme surface had higher concentration of C and 0. TOFSIMS and XPS analyses confirmed Si-C, Si-0-Si, C=O, C=C and -OH groups to be present in all film surfaces. IR analysis of plasma-polymerized methylsilicones by other investigators also confirms these findings [17]. Films 1, 2, 3, 5 and 6 have a surface composition which is around SiC,0,,8H,. The AES depth profiles suggest that the aging process is diffusioncontrolled, as the oxygen seems to have diffused into the film to a depth of about 50 A. It should be noted that oxygen does not diffuse throughout the entire plasma coating. Plasma films 4 and 7 clearly have lower carbon and higher oxygen concentrations at the surface, compared with the other films. These films were deposited from a mixture of TMS and oxygen which explains the higher oxygen concentration. An oxygen-rich surface (SiCO,,SH, from XPS) would have a high surface energy which, as a result of post-plasma reactions with hydrocarbons, gives a surface composition of Si&O,.,H,. The origin of the hydrocarbon could be long-lived alkyl radicals or hydrocarbons in the reactor which are immediately adsorbed at the end of the deposition. 4.5. Low molecular weight material When plasma film 6 was analyzed after 6 months of aging at ambient, peaks from the low molecular weight material were found to have increased in intensity.
W.J. van Ooij
et al. i Progress in Organic
Coafings
25 (1995)
319-337
335
Low molecular weight materials (LMWM) can be formed either as a result of post plasma reaction processes or by the oligomerization process [S]. However, further work needs to be done to determine if this oligomerization process plays an important role in the TMS plasma films studied here. 4.6. Corrosion
and sugace
properties
of plasma
films
The corrosion performance results indicate that there are three types of samples: (1) samples where the film provides relatively good corrosion protection to the substrate; (2) mediocre plasma films where the films are thicker than those in group 1, but their corrosion performance is not as good; and (3) poor performance. The performance here is apparently not a function of the film thickness but more related to the film composition and the plasma metal pretreatment used. These aspects will be discussed in detail in this section. The open circuit potential of all the samples clearly showed that the films do not provide good barrier protection but the corrosion protection is more likely the result of the passivation of the substrate, as we have recently also demonstrated for TMS and HMDS (hexamethoxydisiloxane) films deposited from RF plasmas [l&19]. The plasma film quality was measured by EIS and more specifically estimated from the capacitance (C,) and pore resistance values (R,,). Plasma film 3 has the highest pore resistance and lowest coating capacitance. The relative corrosion performance from the impedance results can thus be written in t he following decreasing order: Film 3 > Film 6 > Film 2 > Film 5 > Film 4 > Film 7 > Film 1 The same qualitative order of performance was observed when the samples were exposed to 85% relative humidity at 60 “C for 24 h. Film 1 was completely rusted whereas film 3 showed not more than 10% rust coverage. Corrosion under conventional organic coatings such as paint has been studied by many researchers [lO,ll]. The plasma films here are extremely thin (< 0.1 pm) and therefore the mechanism of protection can be expected to be different. The following variables we feel are important in the corrosion performance of the plasma films. 4.6.1. Coverage of the substrate On visual examination the film shows uniform coverage (color) of the substrate. However, the substrate surface has a certain microroughness whose effects on film coverage are unknown. If we assume that the plasma film grows from certain nucleating sites then the number of these nucleating sites would be crucial in determining the film coverage due to the low film thickness. The rather low values of the pore resistance for most coatings indicate that there must be conducting pathways through the film. Film 3 has the highest pore resistance value which can therefore be interpreted as having the lowest pore density. 4.6.2. Film composition The detail analysis of the films has clearly shown that the pretreatment has an effect on the film composition. The capacitancevalues from the EIS measurements
336
W.J. van Ooij et al. I Progress in Organic Coatings 25 (1995) 319-337
confirm this. It can be postulated that films 3 and 6 have a higher crosslink density which may manifest itself as a lower C/Si ratio [2]. Also, the contact angle data suggest that films 3 and 6 have poor wettability and thus the initiation of corrosion will be delayed in these samples. 4.6.3. Interface characteristics The double layer capacitance and the polarization resistance show the corrosion activity under the film at the interface. It should be noted that an uncoated steel substrate when exposed to the same experimental conditions gave C,, = 1.6 x lo3 F and R,= 360 R. These numbers suggest that the steel substrate under plasma film 3 has been passivated effectively or alternatively these films adhere very well to the substrate. The latter characteristic has been confirmed by painting similar substrates with a primer after the plasma cleaning and deposition steps (unpublished work). To summarize, the higher impedance of the films 3 and 6 suggests that metal surface pretreatment with hydrogen creates the optimum surface characteristics for the formation of a film with good passivating properties. As discussed earlier, the resulting composition of the film is a function of the substrate surface. Further work is needed to improve our understanding of the mechanism behind the superior corrosion performance for films deposited on substrates pretreated with a hydrogen plasma.
5. Conclusions
In summary, the following conclusions can be drawn from the results presented here. It should be noted that these results were obtained with a magnetronenhanced d.c. plasma reactor so that these conclusions may not necessarily apply to coatings made in different types of reactors. Plasma films of trimethylsilane on cold-rolled steel substrates have different surface compositions as compared to the bulk of the film. The surface is an oxygen-rich silicon-based polymer formed as a result of post-deposition interactions between the plasma film and the atmosphere. All films studied here have low molecular weight siloxane-like components at the surface. There was no oxygen diffusion through the plasma films. The plasma film-metal interface characteristics and the bulk film composition are a function of the surface pretreatment. An oxygen plasma surface treatment leaves an oxide film which is partly sputtered away during the plasma deposition, resulting in incorporation of some oxygen into the plasma film. A hydrogen plasma surface treatment reduces the oxide film and the resulting film deposited as lower C/Si ratio in the bulk of the film as compared to films formed after other surface treatments. Electrochemical impedance spectroscopy is a useful technique to evaluate corrosion properties of plasma films. EIS measurements showed that plasma film 3 of Table 1 has the best corrosion protection properties and shows very little
W.J. van 004
et al. I Progress in Organic Coatings 25 (199-F) 319-337
337
interfacial corrosion as compared to the other films. The type of metal surface pretreatment strongly affects the corrosion performance of the plasma film coated substrate and the ranking of films observed in EIS is the same as the ranking observed after exposure to a corrosive environment. The values of the R, parameter correlate best with the observed ranking in the humidity test. Plasma-polymerized films from a mixture of trimethylsilane and oxygen gases are oxygen-rich at the surface and contain 0 throughout the bulk of the film. These films can be considered to be largely inorganic and they have very poor corrosion properties as determined by EIS and exposure to humidity.
Acknowledgement The authors would like to thank Mr Frank Wang for preparation plasma films in this study.
of the
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