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Fractal polyzirconosiloxane cluster coatings
Thin Solid bTIms, 216 (1992) 249 258
249
Fractal polyzirconosiloxane cluster coatings T. Sugama,
N. Carciello and M. Miura
Energy E[ficien~3' and Conservation Division, Department ~[" Applied Science, Brookhaven National Laboratory, Upton, N Y 11973 (USA)
(Received October 29, 1991; accepted March 23, 1992)
Abstract Fractal polyzirconosiloxane (PZS) cluster films were prepared through the hydrolysis polycondensation pyrolysis synthesis of two-step HC1 acid NaOH base catalyzed sol precursors consisting of N-[3-(triethoxysilyl)propyl]-4,5dihydroimidazole, Zr(OC3H7) 4, methanol, and water. When amorphous PZSs were applied to aluminum as protective coatings against NaCl-induced corrosion, the effective film was that derived from the sol having a pH of 8.6. The following four factors played an important role in assembling the PZS coating films: (1) a proper rate of condensation, (2) a moderate ratio of Si O Si to S i - O - Z r linkages formed in the PZS network, (3) hydrophobic characteristics, and (4) a specific microstructural geometry, in which large fractal clusters were linked together.
I. Introduction We have described the charcteristics of an inorganic polymetallosiloxane (PMS) film, formed by the pyrolysis of sol precursor-induced organopolymetallosiloxane xerogels, intended for use as corrosion-protective coatings on aluminum substrates [1, 2]. The chemical components of the precursor solution consisted of monomeric organosilanes, metal alkoxides (M(OC3H7)4, M = Zr, Ti, and Ge), methanol, water, and hydrochloric acid. Our findings suggested that the following chemical factors play an important role in ensuring that the PMS coatings afford adequate protection: (1) the addition of HCI used as a hydrolysis accelerator for the organosilanes, and M(OC3H7)4, which produces a clear sol solution, thereby aiding in the formation of smooth, uniform coating layers, (2) the organosilane to M(OC3 H7)4 ratios are critical in achieving spreadability of the sol solution on aluminum surfaces, and in leaving a minimum amount of organic and crystalline byproducts in the organopolymetallosiloxane xerogel formed at a sintering temperature of about 150 °C, (3) moderate densifications of the S i - O - M linkages in amorphous PMS networks derived from the 300 °C pyrolytic conversion of xerogel are needed to minimize the development of stress cracks in the films, and (4) the formation of covalent oxane bonds at the interfaces between the PMS and aluminum substrate increase the likelihood of strong adhesion forces. All the present information on the PMS coatings was obtained from experiments with the sol precursor solution in the presence of acid catalysts. As described by several investigators [3 5], the microstructure of the film can be altered by varying the rate of polymeriza-
0040-6090/92/$5.00
tion of sol particles; namely, the extent of growth of the polymeric sol in an aqueous medium depends primarily on the pH of the precursor solution. When acid-type catalysts were added to the solution, the sol consisted of entangled linear polymers. By contrast, a highly condensed sol consisting of randomly branched chains was prepared by incorporating base-type catalysts. Xerogel films derived from the acidic sol precursor system had a continuous, dense microstructure, while the base-type system resulted in the formation of globular structures consisting of aggregations of randomly grown individual clusters. On the basis of this information, the objective of the present study was to investigate the characteristics of polyzirconosiloxane (PZS) coating films derived from two-step, acid-base-catalyzed precursors consisting of N - [3-(triethoxysilyl) propyl] -4,5- dihydroimidazole (TSPI), Zr(OC3H7) 4, methanol, and water, over a broad pH range from 1.0 to 13.0. To understand the characteristics of the sol-derived coating films, our work focused on thermal behavior (changes in molecular structure with thermal treatment at temperatures up to 300 °C), microstructural images, elemental compositions, water wettability of the xerogel, and pyrolyzed xerogel films. These data were integrated and correlated directly with the effectiveness of PZS films as protective coatings for aluminum.
2. Experimental methods 2. I. M a t e r i a l s
TSPI, supplied by Petrarch Systems Ltd., and zirconium(IV) isopropoxides (Zr(OC3H7)4), obtained from
~': 1992
ElsevierSequoia. All rights reserved
T. Sugama et al. / Polyzirconosiloxane cluster coatings
250
TABLE I. Compositions of sol precursor solulions in the pH range from 1.1 to 12.8 pH
TSPI (wt.%,)
Zr(OC3HvI 4 (wtJ!A,)
CH~OH (wt.%)
Water (wt.%,)
HCI:(TSPI + Zr(OC3HT)4) (wt.%)
NaOH:(TSPI + Zr(OC~HT)4) (wt.%)
1.1 2.6 7.1 8.6 11.5 12.8
7.7 7.7 7.7 7.7 7.7 7.7
5.1 5.1 5.1 5.1 5.1 5.1
7.7 7.7 7.7 7.7 7.7 7.7
79.5 79.5 79.5 79.5 79.5 79.5
50.0 50.0 50.0 50.0 50.0 50.0
0.1) 10.4 14.0 18.0 26.0 39.0
Alfa Products, were used as the network-forming monomeric material. The film-forming mother liquor, which served as the precursor solution, was prepared by incorporating the T S P I - Z r ( O C 3 H 7 ) 4 mixture into a methyl alcohol water mixture containing HCI as a hydrolysis accelerator. HCl-catalyzed hydrolysis of the alkoxy groups in the TSPI and Zr(OC3H7) 4 provided a clear precursor solution. To increase the pH of the acid solution, N a O H as a condensation promoter was added to the mother liquors. Table 1 shows the compositions of the precursor solutions prepared through such a two-step acid base-catalyzed method, as a function of pH. The metal substrate was 6061-T6 aluminum sheet, containing the following chemical constituents: 96.3 wt.% AI, 0.6 wt.% Si, 0.7 wt.% Fe, 0.3 wt.% Cu, 0.2 wt.% Mn, 1.0 wt.% Mg, 0.2 wt.% Cr, 0.3 wt.% Zn, 0.2 wt.% Ti, and 0.2 wtY/,, other. The aluminum surfaces were coated by PZS xerogels in the following sequence. The aluminum surfaces were wiped with acetone-soaked tissues to remove any gross surface contaminants. The aluminum substrates were then dipped into the colloidal precursor solution at ambient temperature. After dipping, the substrates were withdrawn slowly from the soaking bath. The precursor-wetted substrate surfaces were preheated in an oven for 30 min at 100 ~C to yield a xerogel coating film. The xerogel films were subsequently pyrolyzed for 30 min in air at temperatures of 250-300 ~'C.
2.2. Measurements The several important physicochemical factors, such as thermal decomposition, microstructure, elemental components, and chemical conformations and profiles, were determined for the original xerogel and pyrolyzed xerogel films, using the combined techniques of thermogravimetric analysis (TGA), differential thermal analysis (DTA), specular reflectance in the IR, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray ( E D X ) analysis, and Auger electron spectroscopy (AES). The magnitudes of susceptibility to moisture of film surfaces formed at temperatures up to 300 cC were
obtained by estimation of the wettability of the film surfaces which was determined by measuring the contact angle within the first 30 s after dropping water onto the surfaces. Electrochemical tests of corrosion were made with an E G & G Princeton Applied Research model 362-1 corrosion measurement system. The electrolyte was 0.5 M sodium chloride, made from distilled water and reagent-grade salt. The specimen was mounted in a holder and then inserted into an E G & G model K47 electrochemical cell. A specimen with a surface area of 1.0 cm 2 was exposed to an aerated 0.5 M NaC1 solution at 25 °C. The polarization curves containing the cathodic and anodic regions were measured at a scan rate of 0.5 mV s ~ in the corrosion potential range from - 1 . 2 to - 0 . 3 V .
3. Results and discussion
3.1. Xerogel coating films The xerogel films were deposited on the aluminum substrate surfaces by a dipping method. The thickness of films dried at 100 ~'C, measured with a surface profile system, ranged from 0.6 to 1.2 ~tm. Figure 1 shows the IR absorption spectra over the wavenumber region from 1350 to 700 cm t for such films from sols with p H 1.1, 2.6, 8.6, 11.5, and 12.8. The spectrum of the p H 1.1 film (spectrum a) had six pronounced bands. As described in our previous paper [ 1], HCl-catalyzed hydrolysis of TSPI promotes the cleavage of the N CH2 linkage; breakage of this bond then leads to the formation of the imidazoline derivative, and the organosilanol compounds containing chlorine-substituted end groups. Thus, the band at 1300cm -~ is due to the Zr O - C linkages formed by the dehydrochlorinating reaction between the chlorine-substituted end groups in the organosilanol and the hydroxy groups in the hydrated zirconium compounds. Since the band in the frequency range from 1000 to 900 cm 1 is attributable to the vibrational mode involving the oxygen-bridging S i - O - m e t a l linkages [6, 7], we believe that the peak at 930cm -1 is due to the formation of the Si O Zr
77 Sugama et al./ Polyzirconosiloxane cluster coatings 100°C a
770
1300
uJ L~ Z
z
J
I
I
1300
1100
900
WAVENUMBER,
i
700
c m -1
Fig. 1. IR absorption spectra for the xerogels sintered at 100 cC at pH 1.1 (spectrum a), 2.6 (spectrum b), 8.6 (spectrum c), 11.5 (spectrum d), and 12.8 (spectrum e).
linkages. The other four bands are assigned to the following absorbing species: the Si C stretch in the silicon-joined propyl groups at l l 9 0 c m -~ [8], the SiO - C stretch in the silicon ethoxide at l l 0 0 c m -~ [9], and the asymmetric and symmetric stretches of Si O Si linkages at 1030cm t and 770cm ~ respectively [10, 11]. The S i - O - S i linkages reflect the formation of polymeric organosiloxane, induced by a dehydrating polycondensation reaction between the neighboring silanol functions. As reported by some investigators [12-14], the dehydrating condensation reactions between the hydrous zirconias ( Z r - O H ) containing nonbridging hydroxo groups allow the formation of Z r - O - Z r linkages. The absorption band pertinent to this linkage occurs at 710 cm 1. However, no such Z r - O - Z r band was found in this spectrum. When N a O H was added to the acid-catalyzed sol at 10.4% (by weight) of total amount of TSPI and Z r ( O C 3 H 7 ) 4 , striking changes occurred in the spectral features (Fig. 1, spectrum b), compared with the spectrum at pH 1.1: (1) there was a decrease in the intensity of frequencies at 1300, 1190, 1100 and 770cm -~, and (2) there was a shift in the S i - O - S i absorption band at 1030cm -~ and the S i - O - Z r band at 930cm ~ to 1010cm l a n d 960cm -~ respectively. Referring to the first change, a further decrease in the intensity of these frequencies was observed as the pH of sol was increased. The disappearance of Z r - O - C groups at 1300 c m - J can be seen in spectra d and e of films made at pH 11.5 and 12.8. Spectrum c of the pH 8.6 film showed that the S i - O - Z r band at 960cm ~ shifts
251
further to high frequency sites. Special attention was paid to the changes in intensity of the shifted S i - O - S i and S i - O - Z r bands as a function of the pH value of the sol. The S i - O - Z r band becomes stronger than that of S i - O - S i , while the intensity of both absorptions markedly grows with increasing pH values. These IR results strongly suggested that the progressive condensations of zirconium-incorporated organosilane, brought about by adding a large amount of N a O H catalyst, preferentially promote the formation of S i - O - Z r linkages rather than S i - O - S i linkages, in the organopolyzirconosiloxane structures. The SEM surface images of films from sols with pH 1.1, 2.6, 8.6, and 11.5 are shown in Fig. 2. The SEM micrograph (Fig. 2(a)) of the pH 1.1 sol-derived xerogel displays the continuous features of a smooth transparent film, suggesting that a strong acid-catalyzed precursor leads to the formation of a transparent xerogel. As seen in Fig. 2(b), an increase in the pH from 1.1 to 2.6 resulted in a change in the morphology of film, which then showed two different cluster species, a randomly distributed branch-type polymer, and a transparent-type polymer. With a sol of pH 8.6, the microtexture of the film (Fig. 2(c)) disclosed a compacted version of highly branched clusters, which completely covered the aluminum substrates. There were no transparent-type clusters present. This change implies that, under basic conditions, the condensation reaction is much faster, and quickly consumes newly generated monomeric species. Such a progression of condensation oh the peripheral sites of growing branches leads to the formation of strong interclusters, in terms of the fractal polymeric clusters. Thus, use of the combination of acid and base catalysts promotes decoupling between hydrolysis and condensation. Although the incorporation of an excessive amount of N a O H resulted in a high rate of condensation, the morphology of the film deposited on aluminum surfaces expressed the formation of an entirely different xerogel. At pH 11.5, the SEM image (Fig. 2(d)) showed that the film had a large number of microcracks. The reason for the development of microcracks may be the chemical reaction between a high pH precursor and an aluminum substrate; namely, the OH ions in the precursor favorably react with the aluminum ion in the aluminum oxide lattice to promote the pitting-type corrosion of aluminum alloys [15, 16]. Such an interfacial reaction may lead to bond breakage and re-formation of the primary building polymer conformation. Before surveying the properties of the pyrolyzed PZS films deposited on the aluminum substrates, we investigated the thermal behavior of zirconium-incorporated organosilanes derived from sols with different pHs, using T G A - D T A . Figure 3 illustrates the T G A and DTA curves which reveal the decomposition characteristics
252
7". Sugama et al. / Polyzirconosilo.vane cluster coatings
m Fig. 2. SEM micrographs for pH (a) 1.1, (b) 2.6, (c) 8.6, and (d) 11,5 sol-derived xerogel films deposited on the aluminum surfaces.
253
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Fig. 3. TGA and DTA curves for the pyrolysis of xerogels heated in air at a rate of 10 'C rain ~. during pyrolysis of samples of xerogel powders preheated at 150 ~'C. All these samples display a slight weight loss in the initial temperature range from 30 to 100 c'C. This weight loss, which is associated with the endothermic peak at 70 °C, may reflect the liberation of moisture chemisorbed onto the dried xerogels. The curves indicate that the amount of liberated moisture depends on the p H of the sol; namely, the uptake of moisture decreases with increasing pH. This suggests that the xerogels formed by adding a N a O H catalyst are not susceptible to moisture attack. The T G A curve for the sol-derived xerogel samples at p H 1.1 indicates that there are three decomposition states: the first begins at the onset temperature Td~ near 250 ~'C, the second occurs between about 380 °C and about 470 °C, and the third from about 500 °C to about 610 °C. The first and second decompositions, reflecting the D T A endothermic peaks at 320 °C and 420 "~'C respectively, are due to the removal of hydrocarbon species from the xerogels. The third, corresponding to the D T A peak at 550 cC, may be caused by the elimination of excess carbon species. Beyond pyrolyzing temperatures of about 610 '~C, there is little weight loss with temperature. Compared with the T G A curve of the p H 1.1 samples, the shape of the curve changes in the N a O H incorporated samples. There are three notable differences: (1) an increase in the p H of the sol lowers the temperature of onset of thermal decomposition in the third stage, (2) the intensity of the D T A peak at 420 °C, corresponding to the second decomposition, significantly decreases, and (3) the total weight loss of samples at 610 °C decreases with increasing pH. These
L
I
I
1100
900
WAVENUMBER.cm
n
700
-1
Fig. 4. IR spectra for the xerogels pyrolyzed at 300 C at pH l.l (spectrum a), 2.6 (spectrum b), 8.6 (spectrum c), ll.5 (spectrum d), and 12.8 (spectrum e). findings imply that NaOH-catalyzed condensation plays an important role in forming the xerogel containing a low concentration of removable carbonaceous groups and moisture, and in promoting the conversion of the organic-inorganic mixed phase into the inorganic phase at a relatively low temperature. 3.2. Pyrolyzed xerogel films Figure 4 shows the changes in the IR spectral features of xerogel films pyrolyzed at 300 °C. The changes in the spectral structure of the p H 1.1 film (Fig. 4, spectrum a), compared with that of a sample preheated at 100 °C, can be described as follows: (1) the elimination of the peaks at frequencies at 1300cm J and l l 9 0 c m i originating from the Z r - O - C and S i - C bands respectively, (2) a shift in the absorption of the S i - O - C band, while its peak intensity becomes much weaker, and (3) a shift in the S i - O - Z r band from 930 to 970 c m - ~. The first two results are indicative of the removal of many carbonaceous groups from the polymeric organozirconosiloxane networks. Such pyrolytic phenomena will lead to the third result; the PZS network structure at 300 c'C has a high degree of densification of S i - O - Z r linkages. For all the N a O H catalyst-induced films, spectral features resembling those recorded on the films dried at 100 °C can be seen in the frequency ranges from 1100 to 9 0 0 c m ~. The peak absorbance of the S i - O - Z r band at 970 cm n grows conspicuously with the increase in concentration of N a O H , suggesting that the extent of densification of S i - O Zr linkages in the formed PZS films is related strongly to the p H of the sol. On the contrary, the
254
f. Subarea et al. / Polvzirconosih~.vane cluster coatinj,,s
Fig. 5. SEM micrographs for xerogels pyrolyzed al 300 C at pH (a) 1.1, (b) 2.6, (c) 7.1, (d) 8.6, and (e) 12.8.
signal intensity of the S i - O - S i band at 1010cm declines with a rise in pH. Figure 5 shows the SEM micrographs of films sintered at 1 0 0 ' C at p H 1.1, 2.6, 7.1, 8.6, and 12.8 after pyrolysis for 30 min at 300 ~'C. Although uniform and continuous coating films formed on the pH 1.1 film sintered at 100 ~C (see Fig. 1, spectrum a), pyrolysis at 300 ":C led to the development and propagation of microcracks and local separation of the film from the substrate. The major cause for this failure was assumed to be the pyrolytic liberation of a relatively large amount of carbonaceous species from the sintered films, which induces large stress cracks caused by a high degree of shrinkage in the film layers. Surface microstructures similar to those of films dried at 100 ~C were observed in films from p H values ranging from 2.6 to 8.6. In considering the ability of these coating films to protect the metals against corrosion, the dense microstructure of fractal clusters linked together (Figs. 5(c) and 5(d)) that was seen in films derived from sols
at p H 7.1 and 8.6 is of particular interest. There was no evidence of film damage, or of local separation of the film from the substrate. In addition, there was no evidence of porosity created by the packing of these fractal clusters, which suggests that they are space filling. In contrast, an undesirable porous film, which had many crater-like pits, was fabricated from the sol at pH 12.8 (see Fig. 5(e)), because of etching of the interracial aluminum surfaces caused by the reaction of high pH precursor with aluminum substrates. In connection with the IR results, the degree of densification of the Si O Si and S i - O - Z r linkages in the xerogel structures seemed to have a strong influence on the film-forming performances of the pyrolyzed PZS xerogels. We found that the proper proportions of S i - O Si and S i - O - Z r are important factors in achieving a good film-forming performance; a good film structure can be produced using p H 7.1 and 8.6 sols. Figure 6 gives the EDX spectra for film surfaces pyrolyzed at 300 ' C at pH 1.1, 2.6 and 8.6. Spectrum a,
T. Sugama et al. / Polyzirconosiloxane cluster coatings
255
0.282nm
Si
0.1gOnm
N
'
2o
3'0
4b
5'o
8'o
Cu Ka 2e
Fig. 7. X R D patterns for xerogels pyrolyzed at 300 '~C at pH 1.l (spectrum a), 2.6 (spectrum b), and 8.6 (spectrum c).
0
1
2
3
4
ENERGY IN KeV Fig. 6. EDX spectra for xerogel film surfaces pyrolyzed at 300 C at pH 1.1 (spectrum a), 2.6 (spectrum b), and 8.6 (spectrum c).
of the pH 1. l film, in the absence of an N a O H catalyst, has three prominent lines, corresponding to elemental silicon, zirconium, and chlorine. The silicon and zirconium elements virtually belong to the PZS, while the presence of chlorine is associated with the HC1 used as the hydrolysis promoter of the alkoxy groups in the TSPI and Z r ( O C 3 H 7 ) 4. When the pH of this HCI catalyst-induced sol was adjusted to 2.6 by adding NaOH, the spectral structure spectrum (b) is characterized by a remarkable increase in the signal intensity of chlorine, and a signal of sodium emerges. A further increase in N a O H in the sol system results in the specific features of spectrum c, involving a significantly intensified chlorine signal and a growing intensification of the sodium line. Thus, the acceleration of the rate of the condensation reaction by the N a O H catalyst not only contributes to an increase in densification of Si O - Z r linkages, but also promotes the formation of sodium-related chlorine compounds as byproducts in the PZS layers. To identify the phase of such a byproduct, the same samples used in the EDX inspections were examined by X R D tracings over the diffraction ranges from 0.509 to 0.154 nm. Figure 7 shows that there were no lines for the pH 1.1 film in X R D pattern a, suggesting that the PZS was essentially amorphous. In contrast, reflections at 0.326, 0.282, 0.199, and 0.163 nm were observed in the film from the pH 2.6 sol (Fig. 7, spectrum b). These spacings clearly reveal the formation of halite (crystalline NaCI) [ 17]. A further increase in the pH to 8.6 (spectrum c), resulted in an increase in the line intensities of these spacings; therefore, the addition of N a O H
to the HCl-catalyzed sol systems leads to the generation of well-crystallized halite particles in the amorphous PZS layers. Since NaC1 is one of the electrolyte species which promotes the corrosion of metals, the presence of NaCI in the films may be undesirable considering that the films are intended as a protection against corrosion. Fortunately, such an NaC1 byproduct can readily be eliminated by rinsing the film with water. In fact, after rinsing the pH 8.6 film with tap water, the EDX spectrum coupled with the SEM micrograph reveal no sodium and only a weak line of chlorine; at the same time, there were no specific changes in the morphology and topography of film surfaces (Fig. 8). To demonstrate further that the NaC1 byproduct had been removed from the PZS layers, we inspected the elemental composition depth profile in the water-rinsed pH 8.6 film layers and in the PZS-A1 interfacial regions, using AES in conjunction with argon ion sputter etching. The sputter rate for the depth profiling was about 30 nm min- ~. Figure 9 depicts the changes in Auger peak heights of representative elements such as oxygen, silicon, zirconium, chlorine, and aluminum v s . sputter time for the pH 8.6 film deposited on aluminum samples. The peak heights of three major elements, oxygen, silicon, and zirconium, reflecting the PZS film, were gradually reduced as the sputtering time was increased. After 27 min, the silicon- and zirconium-related Auger peaks had completely disappeared, while the height of the oxygen signal levelled off. The signal of aluminum, which is associated with the aluminum substrate, emerged at a depth of about 630 nm from the surface, and this signal intensity rapidly rose with elapsed sputtering time until the peak height stabilized after 27 min. The stabilization of the oxygen signal and the elimination of silicon and zirconium atoms suggest that the presence of aluminum and oxygen elements at depths beyond 810 nm is due mainly to the aluminum oxide phases at the outermost surface site of the aluminum
T. Sugama et al. / Po(vzirconosiloxane cluster coatings
256
Si Zr
CI
0
1 ENERGY
i
I
!
2
3
4
IN
KeY
Fig. 8. SEM EDX inspection of a pH 8.6 film surface pyrolyzed at 300 C after rinsing with water.
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Q) -1-
Ox
~x
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A~~ 6
9
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15
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27
30
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SputterTime,min. Fig. 9. AES depth profile of pyrolyzed PZS-coated aluminum after rinsing with water.
substrate. Thus, the depths ranging from about 630 to about 810 nm seem to represent a P Z S - A I 2 0 3 mixed layer in the interfacial areas. The chlorine at the surface of the film (zero sputter time) had the lowest peak, corresponding to a minor atomic presence. The height of the peak decreased with increasing sputter time and then disappeared at a depth of about 540 nm. This
finding implies that there is no chlorine in the PZS layers adjacent to the aluminum substrate. In making water-impermeable xerogel films, the magnitude of the wettability of the coating's surfaces by water is among the most important factors governing good protective performance. Therefore, the degree of water wettability of the xerogel film surface was estimated from the average value of the advancing contact angle on this surface. Figure 10 is a plot of 0 (deg) as a function of the film-treating temperatures, for the pH 8.6 film. Since a high contact angle correlates with a low rate of wetting, the resultant 0-temperature curve showed that film surfaces treated with a high temperature are less wettable. As the film is exposed to high temperatures the original hydrophilic xerogel surfaces are converted to hydrophobic surfaces. A 0 value similar to that of the 250 C film was obtained from the film treated at 300 '~C, suggesting that PZS films having a minimum water-wetting behavior can be prepared by pyrolysis at temperatures of 250 ~'C or more. All the above data were correlated with the corrosion protection provided by the pyrogenic PZS cluster coatings on aluminum. The corrosion data were obtained from the polarization curves for PZS-coated aluminum
T. Sugama et al. / Poly:irconosiloxane cluster coatings
257
101
40
30 < 10 C
t~ 0 0 t-
20
<
*6 0
10
,
10-1 IO0
I
I
-200
300
Treatment Temperature, °C 50
I
I
I
I
I
100
150
200
250
300
Treatment Temperature, °C
Fig. 11. C h a n g e s in the corrosion current l~,,r r of PZS xerogel-coated a l u m i n u m substrates as a function of temperatures of treatment of the coatings for pH 1. I ( ~ ) , 2.6 ( ± ), 7.1 ( [] ), 8.6 ( • ) , and 12.8 ( x ).
Fig. 10. Effect of temperature on the reduction of water wettabi]ity of xerogel film surfaces.
samples exposed to an aerated 0.5 M sodium chloride solution at 25 °C. The typical cathodic anodic polarization curves exhibited a short Tafel region in the cathodic polarization, but no Tafel region was found at the anodic sites. Since O H - ions are generated by cathodic reaction of aluminum with atmospheric reactants such as 02 and H 2 0 , 2 H 2 0 + O 2 + 4 e , 4 O H - [18], the corrosion current I~o, determined on the cathodic curve for PZS-coated aluminum samples can be evaluated to estimate the rate of O H adsorption into aluminum oxides; a decrease in Ico.-r corresponds to a low rate of adsorption of O H - ions. In other words, good coverage of PZS coating films over aluminum is shown by a decrease in I~.... because of the low penetration rate of 02 and H 2 0 reactants which pass through the PZS film. Thus, the corrosion current lcor~ values were measured by extrapolation of the cathodic Tafel slope. Figure 11 illustrates the variation in I~or~ value (~tA) of aluminum substrates as a function of the treating temperatures for p H 1.1, 2.6, 7.1, 8.6, and 12.8 solderived PZS coating films. The protective ability of the coatings depends primarily on the p H value of the sol precursors and the treatment temperature of the films. The I~orr value for the coatings preheated at 100 :'C ranged from 1.2 to 1.9 laA. Such a high I~o~r represents p o o r protection. The major reason for the poor protection is the hydrophilic nature of the coatings, which have a high susceptibility to water wettability. In fact, all the films were dissolved after immersion for about 10 h in water at 25 ~'C. A reduction in Icorr to a low current density was measured for all films treated at 200 °C, except for the pH 12.8 coating. As discussed in SEM observations of film surfaces, the films formed in sol
with a high p H ( 11.5 or more) develop numerous stress cracks. Thus, the I~orr value for the p H 12.8 coatings remained essentially constant, within the range from 100 to 300~'C, thereby eliminating their protective effect. For the other coatings, increasing the temperature of treatment to 250 °C acted to reduce l~orr further; beyond this temperature, there was only a slight decrease in /corr. Therefore, films treated at temperatures of 250 ~'C or more have the m a x i m u m ability to protect aluminum substrates. In contrast, the Icorr value of pH 1.1 film treated at 200 '=C tends to increase with increased temperature. This phenomenon can be associated with failure of the film, induced by high shrinkage that is related to the liberation of m a n y carbonaceous groups at elevated temperatures, thereby resulting in a poor protection performance. The lowest I~orr value of 0.5 x 10 -I gA, corresponding to the best protection, was measured on the pH 8.6 films treated at 300 °C. This value was approximately one order of magnitude less than for the film treated at 300 ~'C at pH 1.1. The ability of films treated at 300 °C to protect aluminum was rated in the following p H order: 8.6 > 7.1 > 2.6 > 12.8 > 1.1. This finding strongly suggested that a dense PZS fractal cluster coating containing a proper proportion of S i - O - S i to S i - O - Z r linkages provides an effective barrier to the corrosion of aluminum substrates and minimizes the corrosion rates of substrates.
4. Conclusions
F r o m our study we can draw the following generalized conclusions. The microstructure and thermal behaviors of xerogel films depend primarily on the p H of sol precursor solution. The xerogel film derived from a
258
T. Sugama et al. / Polyzirconosiloxane cluster eoathl~s
pH 1.1 sol without N a O H showed the formation of a transparent organopolyzirconosiloxane network structure. When the pH of the sol was increased by adding an appropriate amount of NaOH, the morphology of film derived from this sol particle was characterized by fractal cluster species having highly branched microfeatures. These clusters eventually link together to build ideal xerogel films with particle-packing geometries. A further increase in pH to 11.5 (by adding more NaOH) resulted in the microstructure of a porous xerogel film, suggesting that the interaction between high pH precursor and aluminum substrate leads to bond breakages and re-formation of the primary polymer structure. The rates of condensation, reflecting the changes in pH of the sol, not only govern the changes in the microstructure of xerogels but also have a great influence on the degree of densification of Si O Zr linkages in the polymer conformations, and the yields of species volatilizable at 100 C such as alcohol, water, and halite (NaCI) as byproducts. The molecular structure of xerogel made from the sol at pH 8.6 exhibited a moderate densification of S i - O - Z r linkages, reflecting the deposition of ideal fractal cluster films on the aluminum substrates. On the contrary, highly progressed condensation led to an increase in amount of volatile species at 100 C , thereby resulting in a low rate of weight loss during pyrolytic treatments at 250-300 ' C of xerogels sintered at 150 ' C which are significantly susceptible to hydrolysis. However, the pyrolysis of pH 1.1 xerogel, having a low rate of condensation, produced numerous microcracks caused by the pyrolytic liberation of a large amount of carbonaceous species from the sintered film. Although the combination of HCI and N a O H catalysts promotes decoupling between hydrolysis and condensation, the reaction between the acid and base catalysts led to the formation of crystalline NaC1 particles in the pyrolyzed xerogel films. However, the NaC1 byproduct was readily removed by rinsing the films with water. Accordingly, we conclude that the hydro-
phobic PZS fractal cluster coating, which is reliable in protecting the aluminum substrate against NaCl-induced corrosion, can be prepared by the pyrolysis at 250-300 "C of the xerogel film from sol at pH 8.6.
Acknowledgments This work was performed under the auspices of the US Department of Energy, Washington, DC, under Contract DE-AC02-76CH00016, and supported by the US Army Research Office Program MIPR-ARO-103-90 and the Physical Sciences Department of the Gas Research Institute, under Contract 5090-260-1948.
References 1 T. Sugama. L. E. Kukacka and N. Carciello, Prog. Org. Coat., II (1990) 173. 2 T. Sugama, N. Carciello and C. Taylor, J. Non-Cryst. Solids, 134 (1991) 58. 3 C. J. Brinker and G. W. Scherer, J. Non-Crvst. Solids. 70 (1985) 301. 4 J. Livage, M. Henry and C. Sanchez, Prog. Solid State ('hem., 18 (1988) 259. 5 J. D. LeMay, R. W. Hopper, L. W. Hrubesh and R. W. Pekala, M R S Bull., 15 (1990) 30. 6 M. F. Best and R. A. Condrate, Sr., J. Mater. Sci. Lett., 4 (1985) 994. 7 S. W. Lee and R. A. Condrate, Sr., J. Mater. Sci., 23 (1988) 2951. 8 A. L. Smith, Speetrochim. Acta, 16 (1960) 87. 9 L. J. Bellamy, The lr~li~a-red Spectra o1" Complex Molecules, Chapman and Hall. London, 1975. 10 R. Hanna, J. Am. Ceram. Soe., 48 (1965) 595. 11 N. P. Bansal, J. Am. Ceram. Soe., 71 (1988) 666. 12 K. Haberko. Ceramurgia h~t., 5(1979) 148. 13 K. Kendell, N. M. Alford and J. D. Birchall, Proc. Br. Ceram. Sot.. 37 (1986) 255. 14 S. L. Johes and C. J. Norman, J. Am. (~,ram. Sot., 71 (1988) c-190. 15 L. Liepina and V. Kadek, Corros. Sci., 6 (1966) 177. 16 R. T. Foley, Corrosion, 42 (1986) 277. 17 Powder Dif/i'action File, ASTM, Philadelphia, PA, Card 5-628. 18 W. A. Bell and H. C. Campbell, Br. ('orros, J., I (1965) 72.
249
Fractal polyzirconosiloxane cluster coatings T. Sugama,
N. Carciello and M. Miura
Energy E[ficien~3' and Conservation Division, Department ~[" Applied Science, Brookhaven National Laboratory, Upton, N Y 11973 (USA)
(Received October 29, 1991; accepted March 23, 1992)
Abstract Fractal polyzirconosiloxane (PZS) cluster films were prepared through the hydrolysis polycondensation pyrolysis synthesis of two-step HC1 acid NaOH base catalyzed sol precursors consisting of N-[3-(triethoxysilyl)propyl]-4,5dihydroimidazole, Zr(OC3H7) 4, methanol, and water. When amorphous PZSs were applied to aluminum as protective coatings against NaCl-induced corrosion, the effective film was that derived from the sol having a pH of 8.6. The following four factors played an important role in assembling the PZS coating films: (1) a proper rate of condensation, (2) a moderate ratio of Si O Si to S i - O - Z r linkages formed in the PZS network, (3) hydrophobic characteristics, and (4) a specific microstructural geometry, in which large fractal clusters were linked together.
I. Introduction We have described the charcteristics of an inorganic polymetallosiloxane (PMS) film, formed by the pyrolysis of sol precursor-induced organopolymetallosiloxane xerogels, intended for use as corrosion-protective coatings on aluminum substrates [1, 2]. The chemical components of the precursor solution consisted of monomeric organosilanes, metal alkoxides (M(OC3H7)4, M = Zr, Ti, and Ge), methanol, water, and hydrochloric acid. Our findings suggested that the following chemical factors play an important role in ensuring that the PMS coatings afford adequate protection: (1) the addition of HCI used as a hydrolysis accelerator for the organosilanes, and M(OC3H7)4, which produces a clear sol solution, thereby aiding in the formation of smooth, uniform coating layers, (2) the organosilane to M(OC3 H7)4 ratios are critical in achieving spreadability of the sol solution on aluminum surfaces, and in leaving a minimum amount of organic and crystalline byproducts in the organopolymetallosiloxane xerogel formed at a sintering temperature of about 150 °C, (3) moderate densifications of the S i - O - M linkages in amorphous PMS networks derived from the 300 °C pyrolytic conversion of xerogel are needed to minimize the development of stress cracks in the films, and (4) the formation of covalent oxane bonds at the interfaces between the PMS and aluminum substrate increase the likelihood of strong adhesion forces. All the present information on the PMS coatings was obtained from experiments with the sol precursor solution in the presence of acid catalysts. As described by several investigators [3 5], the microstructure of the film can be altered by varying the rate of polymeriza-
0040-6090/92/$5.00
tion of sol particles; namely, the extent of growth of the polymeric sol in an aqueous medium depends primarily on the pH of the precursor solution. When acid-type catalysts were added to the solution, the sol consisted of entangled linear polymers. By contrast, a highly condensed sol consisting of randomly branched chains was prepared by incorporating base-type catalysts. Xerogel films derived from the acidic sol precursor system had a continuous, dense microstructure, while the base-type system resulted in the formation of globular structures consisting of aggregations of randomly grown individual clusters. On the basis of this information, the objective of the present study was to investigate the characteristics of polyzirconosiloxane (PZS) coating films derived from two-step, acid-base-catalyzed precursors consisting of N - [3-(triethoxysilyl) propyl] -4,5- dihydroimidazole (TSPI), Zr(OC3H7) 4, methanol, and water, over a broad pH range from 1.0 to 13.0. To understand the characteristics of the sol-derived coating films, our work focused on thermal behavior (changes in molecular structure with thermal treatment at temperatures up to 300 °C), microstructural images, elemental compositions, water wettability of the xerogel, and pyrolyzed xerogel films. These data were integrated and correlated directly with the effectiveness of PZS films as protective coatings for aluminum.
2. Experimental methods 2. I. M a t e r i a l s
TSPI, supplied by Petrarch Systems Ltd., and zirconium(IV) isopropoxides (Zr(OC3H7)4), obtained from
~': 1992
ElsevierSequoia. All rights reserved
T. Sugama et al. / Polyzirconosiloxane cluster coatings
250
TABLE I. Compositions of sol precursor solulions in the pH range from 1.1 to 12.8 pH
TSPI (wt.%,)
Zr(OC3HvI 4 (wtJ!A,)
CH~OH (wt.%)
Water (wt.%,)
HCI:(TSPI + Zr(OC3HT)4) (wt.%)
NaOH:(TSPI + Zr(OC~HT)4) (wt.%)
1.1 2.6 7.1 8.6 11.5 12.8
7.7 7.7 7.7 7.7 7.7 7.7
5.1 5.1 5.1 5.1 5.1 5.1
7.7 7.7 7.7 7.7 7.7 7.7
79.5 79.5 79.5 79.5 79.5 79.5
50.0 50.0 50.0 50.0 50.0 50.0
0.1) 10.4 14.0 18.0 26.0 39.0
Alfa Products, were used as the network-forming monomeric material. The film-forming mother liquor, which served as the precursor solution, was prepared by incorporating the T S P I - Z r ( O C 3 H 7 ) 4 mixture into a methyl alcohol water mixture containing HCI as a hydrolysis accelerator. HCl-catalyzed hydrolysis of the alkoxy groups in the TSPI and Zr(OC3H7) 4 provided a clear precursor solution. To increase the pH of the acid solution, N a O H as a condensation promoter was added to the mother liquors. Table 1 shows the compositions of the precursor solutions prepared through such a two-step acid base-catalyzed method, as a function of pH. The metal substrate was 6061-T6 aluminum sheet, containing the following chemical constituents: 96.3 wt.% AI, 0.6 wt.% Si, 0.7 wt.% Fe, 0.3 wt.% Cu, 0.2 wt.% Mn, 1.0 wt.% Mg, 0.2 wt.% Cr, 0.3 wt.% Zn, 0.2 wt.% Ti, and 0.2 wtY/,, other. The aluminum surfaces were coated by PZS xerogels in the following sequence. The aluminum surfaces were wiped with acetone-soaked tissues to remove any gross surface contaminants. The aluminum substrates were then dipped into the colloidal precursor solution at ambient temperature. After dipping, the substrates were withdrawn slowly from the soaking bath. The precursor-wetted substrate surfaces were preheated in an oven for 30 min at 100 ~C to yield a xerogel coating film. The xerogel films were subsequently pyrolyzed for 30 min in air at temperatures of 250-300 ~'C.
2.2. Measurements The several important physicochemical factors, such as thermal decomposition, microstructure, elemental components, and chemical conformations and profiles, were determined for the original xerogel and pyrolyzed xerogel films, using the combined techniques of thermogravimetric analysis (TGA), differential thermal analysis (DTA), specular reflectance in the IR, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray ( E D X ) analysis, and Auger electron spectroscopy (AES). The magnitudes of susceptibility to moisture of film surfaces formed at temperatures up to 300 cC were
obtained by estimation of the wettability of the film surfaces which was determined by measuring the contact angle within the first 30 s after dropping water onto the surfaces. Electrochemical tests of corrosion were made with an E G & G Princeton Applied Research model 362-1 corrosion measurement system. The electrolyte was 0.5 M sodium chloride, made from distilled water and reagent-grade salt. The specimen was mounted in a holder and then inserted into an E G & G model K47 electrochemical cell. A specimen with a surface area of 1.0 cm 2 was exposed to an aerated 0.5 M NaC1 solution at 25 °C. The polarization curves containing the cathodic and anodic regions were measured at a scan rate of 0.5 mV s ~ in the corrosion potential range from - 1 . 2 to - 0 . 3 V .
3. Results and discussion
3.1. Xerogel coating films The xerogel films were deposited on the aluminum substrate surfaces by a dipping method. The thickness of films dried at 100 ~'C, measured with a surface profile system, ranged from 0.6 to 1.2 ~tm. Figure 1 shows the IR absorption spectra over the wavenumber region from 1350 to 700 cm t for such films from sols with p H 1.1, 2.6, 8.6, 11.5, and 12.8. The spectrum of the p H 1.1 film (spectrum a) had six pronounced bands. As described in our previous paper [ 1], HCl-catalyzed hydrolysis of TSPI promotes the cleavage of the N CH2 linkage; breakage of this bond then leads to the formation of the imidazoline derivative, and the organosilanol compounds containing chlorine-substituted end groups. Thus, the band at 1300cm -~ is due to the Zr O - C linkages formed by the dehydrochlorinating reaction between the chlorine-substituted end groups in the organosilanol and the hydroxy groups in the hydrated zirconium compounds. Since the band in the frequency range from 1000 to 900 cm 1 is attributable to the vibrational mode involving the oxygen-bridging S i - O - m e t a l linkages [6, 7], we believe that the peak at 930cm -1 is due to the formation of the Si O Zr
77 Sugama et al./ Polyzirconosiloxane cluster coatings 100°C a
770
1300
uJ L~ Z
z
J
I
I
1300
1100
900
WAVENUMBER,
i
700
c m -1
Fig. 1. IR absorption spectra for the xerogels sintered at 100 cC at pH 1.1 (spectrum a), 2.6 (spectrum b), 8.6 (spectrum c), 11.5 (spectrum d), and 12.8 (spectrum e).
linkages. The other four bands are assigned to the following absorbing species: the Si C stretch in the silicon-joined propyl groups at l l 9 0 c m -~ [8], the SiO - C stretch in the silicon ethoxide at l l 0 0 c m -~ [9], and the asymmetric and symmetric stretches of Si O Si linkages at 1030cm t and 770cm ~ respectively [10, 11]. The S i - O - S i linkages reflect the formation of polymeric organosiloxane, induced by a dehydrating polycondensation reaction between the neighboring silanol functions. As reported by some investigators [12-14], the dehydrating condensation reactions between the hydrous zirconias ( Z r - O H ) containing nonbridging hydroxo groups allow the formation of Z r - O - Z r linkages. The absorption band pertinent to this linkage occurs at 710 cm 1. However, no such Z r - O - Z r band was found in this spectrum. When N a O H was added to the acid-catalyzed sol at 10.4% (by weight) of total amount of TSPI and Z r ( O C 3 H 7 ) 4 , striking changes occurred in the spectral features (Fig. 1, spectrum b), compared with the spectrum at pH 1.1: (1) there was a decrease in the intensity of frequencies at 1300, 1190, 1100 and 770cm -~, and (2) there was a shift in the S i - O - S i absorption band at 1030cm -~ and the S i - O - Z r band at 930cm ~ to 1010cm l a n d 960cm -~ respectively. Referring to the first change, a further decrease in the intensity of these frequencies was observed as the pH of sol was increased. The disappearance of Z r - O - C groups at 1300 c m - J can be seen in spectra d and e of films made at pH 11.5 and 12.8. Spectrum c of the pH 8.6 film showed that the S i - O - Z r band at 960cm ~ shifts
251
further to high frequency sites. Special attention was paid to the changes in intensity of the shifted S i - O - S i and S i - O - Z r bands as a function of the pH value of the sol. The S i - O - Z r band becomes stronger than that of S i - O - S i , while the intensity of both absorptions markedly grows with increasing pH values. These IR results strongly suggested that the progressive condensations of zirconium-incorporated organosilane, brought about by adding a large amount of N a O H catalyst, preferentially promote the formation of S i - O - Z r linkages rather than S i - O - S i linkages, in the organopolyzirconosiloxane structures. The SEM surface images of films from sols with pH 1.1, 2.6, 8.6, and 11.5 are shown in Fig. 2. The SEM micrograph (Fig. 2(a)) of the pH 1.1 sol-derived xerogel displays the continuous features of a smooth transparent film, suggesting that a strong acid-catalyzed precursor leads to the formation of a transparent xerogel. As seen in Fig. 2(b), an increase in the pH from 1.1 to 2.6 resulted in a change in the morphology of film, which then showed two different cluster species, a randomly distributed branch-type polymer, and a transparent-type polymer. With a sol of pH 8.6, the microtexture of the film (Fig. 2(c)) disclosed a compacted version of highly branched clusters, which completely covered the aluminum substrates. There were no transparent-type clusters present. This change implies that, under basic conditions, the condensation reaction is much faster, and quickly consumes newly generated monomeric species. Such a progression of condensation oh the peripheral sites of growing branches leads to the formation of strong interclusters, in terms of the fractal polymeric clusters. Thus, use of the combination of acid and base catalysts promotes decoupling between hydrolysis and condensation. Although the incorporation of an excessive amount of N a O H resulted in a high rate of condensation, the morphology of the film deposited on aluminum surfaces expressed the formation of an entirely different xerogel. At pH 11.5, the SEM image (Fig. 2(d)) showed that the film had a large number of microcracks. The reason for the development of microcracks may be the chemical reaction between a high pH precursor and an aluminum substrate; namely, the OH ions in the precursor favorably react with the aluminum ion in the aluminum oxide lattice to promote the pitting-type corrosion of aluminum alloys [15, 16]. Such an interfacial reaction may lead to bond breakage and re-formation of the primary building polymer conformation. Before surveying the properties of the pyrolyzed PZS films deposited on the aluminum substrates, we investigated the thermal behavior of zirconium-incorporated organosilanes derived from sols with different pHs, using T G A - D T A . Figure 3 illustrates the T G A and DTA curves which reveal the decomposition characteristics
252
7". Sugama et al. / Polyzirconosilo.vane cluster coatings
m Fig. 2. SEM micrographs for pH (a) 1.1, (b) 2.6, (c) 8.6, and (d) 11,5 sol-derived xerogel films deposited on the aluminum surfaces.
253
T. Sugama et al. / Polyzireonosiloxane cluster coatings o
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.....
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Ox
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100
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Temperature.
600
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Fig. 3. TGA and DTA curves for the pyrolysis of xerogels heated in air at a rate of 10 'C rain ~. during pyrolysis of samples of xerogel powders preheated at 150 ~'C. All these samples display a slight weight loss in the initial temperature range from 30 to 100 c'C. This weight loss, which is associated with the endothermic peak at 70 °C, may reflect the liberation of moisture chemisorbed onto the dried xerogels. The curves indicate that the amount of liberated moisture depends on the p H of the sol; namely, the uptake of moisture decreases with increasing pH. This suggests that the xerogels formed by adding a N a O H catalyst are not susceptible to moisture attack. The T G A curve for the sol-derived xerogel samples at p H 1.1 indicates that there are three decomposition states: the first begins at the onset temperature Td~ near 250 ~'C, the second occurs between about 380 °C and about 470 °C, and the third from about 500 °C to about 610 °C. The first and second decompositions, reflecting the D T A endothermic peaks at 320 °C and 420 "~'C respectively, are due to the removal of hydrocarbon species from the xerogels. The third, corresponding to the D T A peak at 550 cC, may be caused by the elimination of excess carbon species. Beyond pyrolyzing temperatures of about 610 '~C, there is little weight loss with temperature. Compared with the T G A curve of the p H 1.1 samples, the shape of the curve changes in the N a O H incorporated samples. There are three notable differences: (1) an increase in the p H of the sol lowers the temperature of onset of thermal decomposition in the third stage, (2) the intensity of the D T A peak at 420 °C, corresponding to the second decomposition, significantly decreases, and (3) the total weight loss of samples at 610 °C decreases with increasing pH. These
L
I
I
1100
900
WAVENUMBER.cm
n
700
-1
Fig. 4. IR spectra for the xerogels pyrolyzed at 300 C at pH l.l (spectrum a), 2.6 (spectrum b), 8.6 (spectrum c), ll.5 (spectrum d), and 12.8 (spectrum e). findings imply that NaOH-catalyzed condensation plays an important role in forming the xerogel containing a low concentration of removable carbonaceous groups and moisture, and in promoting the conversion of the organic-inorganic mixed phase into the inorganic phase at a relatively low temperature. 3.2. Pyrolyzed xerogel films Figure 4 shows the changes in the IR spectral features of xerogel films pyrolyzed at 300 °C. The changes in the spectral structure of the p H 1.1 film (Fig. 4, spectrum a), compared with that of a sample preheated at 100 °C, can be described as follows: (1) the elimination of the peaks at frequencies at 1300cm J and l l 9 0 c m i originating from the Z r - O - C and S i - C bands respectively, (2) a shift in the absorption of the S i - O - C band, while its peak intensity becomes much weaker, and (3) a shift in the S i - O - Z r band from 930 to 970 c m - ~. The first two results are indicative of the removal of many carbonaceous groups from the polymeric organozirconosiloxane networks. Such pyrolytic phenomena will lead to the third result; the PZS network structure at 300 c'C has a high degree of densification of S i - O - Z r linkages. For all the N a O H catalyst-induced films, spectral features resembling those recorded on the films dried at 100 °C can be seen in the frequency ranges from 1100 to 9 0 0 c m ~. The peak absorbance of the S i - O - Z r band at 970 cm n grows conspicuously with the increase in concentration of N a O H , suggesting that the extent of densification of S i - O Zr linkages in the formed PZS films is related strongly to the p H of the sol. On the contrary, the
254
f. Subarea et al. / Polvzirconosih~.vane cluster coatinj,,s
Fig. 5. SEM micrographs for xerogels pyrolyzed al 300 C at pH (a) 1.1, (b) 2.6, (c) 7.1, (d) 8.6, and (e) 12.8.
signal intensity of the S i - O - S i band at 1010cm declines with a rise in pH. Figure 5 shows the SEM micrographs of films sintered at 1 0 0 ' C at p H 1.1, 2.6, 7.1, 8.6, and 12.8 after pyrolysis for 30 min at 300 ~'C. Although uniform and continuous coating films formed on the pH 1.1 film sintered at 100 ~C (see Fig. 1, spectrum a), pyrolysis at 300 ":C led to the development and propagation of microcracks and local separation of the film from the substrate. The major cause for this failure was assumed to be the pyrolytic liberation of a relatively large amount of carbonaceous species from the sintered films, which induces large stress cracks caused by a high degree of shrinkage in the film layers. Surface microstructures similar to those of films dried at 100 ~C were observed in films from p H values ranging from 2.6 to 8.6. In considering the ability of these coating films to protect the metals against corrosion, the dense microstructure of fractal clusters linked together (Figs. 5(c) and 5(d)) that was seen in films derived from sols
at p H 7.1 and 8.6 is of particular interest. There was no evidence of film damage, or of local separation of the film from the substrate. In addition, there was no evidence of porosity created by the packing of these fractal clusters, which suggests that they are space filling. In contrast, an undesirable porous film, which had many crater-like pits, was fabricated from the sol at pH 12.8 (see Fig. 5(e)), because of etching of the interracial aluminum surfaces caused by the reaction of high pH precursor with aluminum substrates. In connection with the IR results, the degree of densification of the Si O Si and S i - O - Z r linkages in the xerogel structures seemed to have a strong influence on the film-forming performances of the pyrolyzed PZS xerogels. We found that the proper proportions of S i - O Si and S i - O - Z r are important factors in achieving a good film-forming performance; a good film structure can be produced using p H 7.1 and 8.6 sols. Figure 6 gives the EDX spectra for film surfaces pyrolyzed at 300 ' C at pH 1.1, 2.6 and 8.6. Spectrum a,
T. Sugama et al. / Polyzirconosiloxane cluster coatings
255
0.282nm
Si
0.1gOnm
N
'
2o
3'0
4b
5'o
8'o
Cu Ka 2e
Fig. 7. X R D patterns for xerogels pyrolyzed at 300 '~C at pH 1.l (spectrum a), 2.6 (spectrum b), and 8.6 (spectrum c).
0
1
2
3
4
ENERGY IN KeV Fig. 6. EDX spectra for xerogel film surfaces pyrolyzed at 300 C at pH 1.1 (spectrum a), 2.6 (spectrum b), and 8.6 (spectrum c).
of the pH 1. l film, in the absence of an N a O H catalyst, has three prominent lines, corresponding to elemental silicon, zirconium, and chlorine. The silicon and zirconium elements virtually belong to the PZS, while the presence of chlorine is associated with the HC1 used as the hydrolysis promoter of the alkoxy groups in the TSPI and Z r ( O C 3 H 7 ) 4. When the pH of this HCI catalyst-induced sol was adjusted to 2.6 by adding NaOH, the spectral structure spectrum (b) is characterized by a remarkable increase in the signal intensity of chlorine, and a signal of sodium emerges. A further increase in N a O H in the sol system results in the specific features of spectrum c, involving a significantly intensified chlorine signal and a growing intensification of the sodium line. Thus, the acceleration of the rate of the condensation reaction by the N a O H catalyst not only contributes to an increase in densification of Si O - Z r linkages, but also promotes the formation of sodium-related chlorine compounds as byproducts in the PZS layers. To identify the phase of such a byproduct, the same samples used in the EDX inspections were examined by X R D tracings over the diffraction ranges from 0.509 to 0.154 nm. Figure 7 shows that there were no lines for the pH 1.1 film in X R D pattern a, suggesting that the PZS was essentially amorphous. In contrast, reflections at 0.326, 0.282, 0.199, and 0.163 nm were observed in the film from the pH 2.6 sol (Fig. 7, spectrum b). These spacings clearly reveal the formation of halite (crystalline NaCI) [ 17]. A further increase in the pH to 8.6 (spectrum c), resulted in an increase in the line intensities of these spacings; therefore, the addition of N a O H
to the HCl-catalyzed sol systems leads to the generation of well-crystallized halite particles in the amorphous PZS layers. Since NaC1 is one of the electrolyte species which promotes the corrosion of metals, the presence of NaCI in the films may be undesirable considering that the films are intended as a protection against corrosion. Fortunately, such an NaC1 byproduct can readily be eliminated by rinsing the film with water. In fact, after rinsing the pH 8.6 film with tap water, the EDX spectrum coupled with the SEM micrograph reveal no sodium and only a weak line of chlorine; at the same time, there were no specific changes in the morphology and topography of film surfaces (Fig. 8). To demonstrate further that the NaC1 byproduct had been removed from the PZS layers, we inspected the elemental composition depth profile in the water-rinsed pH 8.6 film layers and in the PZS-A1 interfacial regions, using AES in conjunction with argon ion sputter etching. The sputter rate for the depth profiling was about 30 nm min- ~. Figure 9 depicts the changes in Auger peak heights of representative elements such as oxygen, silicon, zirconium, chlorine, and aluminum v s . sputter time for the pH 8.6 film deposited on aluminum samples. The peak heights of three major elements, oxygen, silicon, and zirconium, reflecting the PZS film, were gradually reduced as the sputtering time was increased. After 27 min, the silicon- and zirconium-related Auger peaks had completely disappeared, while the height of the oxygen signal levelled off. The signal of aluminum, which is associated with the aluminum substrate, emerged at a depth of about 630 nm from the surface, and this signal intensity rapidly rose with elapsed sputtering time until the peak height stabilized after 27 min. The stabilization of the oxygen signal and the elimination of silicon and zirconium atoms suggest that the presence of aluminum and oxygen elements at depths beyond 810 nm is due mainly to the aluminum oxide phases at the outermost surface site of the aluminum
T. Sugama et al. / Po(vzirconosiloxane cluster coatings
256
Si Zr
CI
0
1 ENERGY
i
I
!
2
3
4
IN
KeY
Fig. 8. SEM EDX inspection of a pH 8.6 film surface pyrolyzed at 300 C after rinsing with water.
~180nm-~ Q) 0
Q) -1-
Ox
~x
18
21
o 3
A~~ 6
9
12
15
24
27
30
33
SputterTime,min. Fig. 9. AES depth profile of pyrolyzed PZS-coated aluminum after rinsing with water.
substrate. Thus, the depths ranging from about 630 to about 810 nm seem to represent a P Z S - A I 2 0 3 mixed layer in the interfacial areas. The chlorine at the surface of the film (zero sputter time) had the lowest peak, corresponding to a minor atomic presence. The height of the peak decreased with increasing sputter time and then disappeared at a depth of about 540 nm. This
finding implies that there is no chlorine in the PZS layers adjacent to the aluminum substrate. In making water-impermeable xerogel films, the magnitude of the wettability of the coating's surfaces by water is among the most important factors governing good protective performance. Therefore, the degree of water wettability of the xerogel film surface was estimated from the average value of the advancing contact angle on this surface. Figure 10 is a plot of 0 (deg) as a function of the film-treating temperatures, for the pH 8.6 film. Since a high contact angle correlates with a low rate of wetting, the resultant 0-temperature curve showed that film surfaces treated with a high temperature are less wettable. As the film is exposed to high temperatures the original hydrophilic xerogel surfaces are converted to hydrophobic surfaces. A 0 value similar to that of the 250 C film was obtained from the film treated at 300 '~C, suggesting that PZS films having a minimum water-wetting behavior can be prepared by pyrolysis at temperatures of 250 ~'C or more. All the above data were correlated with the corrosion protection provided by the pyrogenic PZS cluster coatings on aluminum. The corrosion data were obtained from the polarization curves for PZS-coated aluminum
T. Sugama et al. / Poly:irconosiloxane cluster coatings
257
101
40
30 < 10 C
t~ 0 0 t-
20
<
*6 0
10
,
10-1 IO0
I
I
-200
300
Treatment Temperature, °C 50
I
I
I
I
I
100
150
200
250
300
Treatment Temperature, °C
Fig. 11. C h a n g e s in the corrosion current l~,,r r of PZS xerogel-coated a l u m i n u m substrates as a function of temperatures of treatment of the coatings for pH 1. I ( ~ ) , 2.6 ( ± ), 7.1 ( [] ), 8.6 ( • ) , and 12.8 ( x ).
Fig. 10. Effect of temperature on the reduction of water wettabi]ity of xerogel film surfaces.
samples exposed to an aerated 0.5 M sodium chloride solution at 25 °C. The typical cathodic anodic polarization curves exhibited a short Tafel region in the cathodic polarization, but no Tafel region was found at the anodic sites. Since O H - ions are generated by cathodic reaction of aluminum with atmospheric reactants such as 02 and H 2 0 , 2 H 2 0 + O 2 + 4 e , 4 O H - [18], the corrosion current I~o, determined on the cathodic curve for PZS-coated aluminum samples can be evaluated to estimate the rate of O H adsorption into aluminum oxides; a decrease in Ico.-r corresponds to a low rate of adsorption of O H - ions. In other words, good coverage of PZS coating films over aluminum is shown by a decrease in I~.... because of the low penetration rate of 02 and H 2 0 reactants which pass through the PZS film. Thus, the corrosion current lcor~ values were measured by extrapolation of the cathodic Tafel slope. Figure 11 illustrates the variation in I~or~ value (~tA) of aluminum substrates as a function of the treating temperatures for p H 1.1, 2.6, 7.1, 8.6, and 12.8 solderived PZS coating films. The protective ability of the coatings depends primarily on the p H value of the sol precursors and the treatment temperature of the films. The I~orr value for the coatings preheated at 100 :'C ranged from 1.2 to 1.9 laA. Such a high I~o~r represents p o o r protection. The major reason for the poor protection is the hydrophilic nature of the coatings, which have a high susceptibility to water wettability. In fact, all the films were dissolved after immersion for about 10 h in water at 25 ~'C. A reduction in Icorr to a low current density was measured for all films treated at 200 °C, except for the pH 12.8 coating. As discussed in SEM observations of film surfaces, the films formed in sol
with a high p H ( 11.5 or more) develop numerous stress cracks. Thus, the I~orr value for the p H 12.8 coatings remained essentially constant, within the range from 100 to 300~'C, thereby eliminating their protective effect. For the other coatings, increasing the temperature of treatment to 250 °C acted to reduce l~orr further; beyond this temperature, there was only a slight decrease in /corr. Therefore, films treated at temperatures of 250 ~'C or more have the m a x i m u m ability to protect aluminum substrates. In contrast, the Icorr value of pH 1.1 film treated at 200 '=C tends to increase with increased temperature. This phenomenon can be associated with failure of the film, induced by high shrinkage that is related to the liberation of m a n y carbonaceous groups at elevated temperatures, thereby resulting in a poor protection performance. The lowest I~orr value of 0.5 x 10 -I gA, corresponding to the best protection, was measured on the pH 8.6 films treated at 300 °C. This value was approximately one order of magnitude less than for the film treated at 300 ~'C at pH 1.1. The ability of films treated at 300 °C to protect aluminum was rated in the following p H order: 8.6 > 7.1 > 2.6 > 12.8 > 1.1. This finding strongly suggested that a dense PZS fractal cluster coating containing a proper proportion of S i - O - S i to S i - O - Z r linkages provides an effective barrier to the corrosion of aluminum substrates and minimizes the corrosion rates of substrates.
4. Conclusions
F r o m our study we can draw the following generalized conclusions. The microstructure and thermal behaviors of xerogel films depend primarily on the p H of sol precursor solution. The xerogel film derived from a
258
T. Sugama et al. / Polyzirconosiloxane cluster eoathl~s
pH 1.1 sol without N a O H showed the formation of a transparent organopolyzirconosiloxane network structure. When the pH of the sol was increased by adding an appropriate amount of NaOH, the morphology of film derived from this sol particle was characterized by fractal cluster species having highly branched microfeatures. These clusters eventually link together to build ideal xerogel films with particle-packing geometries. A further increase in pH to 11.5 (by adding more NaOH) resulted in the microstructure of a porous xerogel film, suggesting that the interaction between high pH precursor and aluminum substrate leads to bond breakages and re-formation of the primary polymer structure. The rates of condensation, reflecting the changes in pH of the sol, not only govern the changes in the microstructure of xerogels but also have a great influence on the degree of densification of Si O Zr linkages in the polymer conformations, and the yields of species volatilizable at 100 C such as alcohol, water, and halite (NaCI) as byproducts. The molecular structure of xerogel made from the sol at pH 8.6 exhibited a moderate densification of S i - O - Z r linkages, reflecting the deposition of ideal fractal cluster films on the aluminum substrates. On the contrary, highly progressed condensation led to an increase in amount of volatile species at 100 C , thereby resulting in a low rate of weight loss during pyrolytic treatments at 250-300 ' C of xerogels sintered at 150 ' C which are significantly susceptible to hydrolysis. However, the pyrolysis of pH 1.1 xerogel, having a low rate of condensation, produced numerous microcracks caused by the pyrolytic liberation of a large amount of carbonaceous species from the sintered film. Although the combination of HCI and N a O H catalysts promotes decoupling between hydrolysis and condensation, the reaction between the acid and base catalysts led to the formation of crystalline NaC1 particles in the pyrolyzed xerogel films. However, the NaC1 byproduct was readily removed by rinsing the films with water. Accordingly, we conclude that the hydro-
phobic PZS fractal cluster coating, which is reliable in protecting the aluminum substrate against NaCl-induced corrosion, can be prepared by the pyrolysis at 250-300 "C of the xerogel film from sol at pH 8.6.
Acknowledgments This work was performed under the auspices of the US Department of Energy, Washington, DC, under Contract DE-AC02-76CH00016, and supported by the US Army Research Office Program MIPR-ARO-103-90 and the Physical Sciences Department of the Gas Research Institute, under Contract 5090-260-1948.
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