Ceramic coatings for fiber matrix composites: Titania thin films on ...

Ceramic coatings for fiber matrix composites: Titania thin films on bismaleimide-glass fiber composites Anna Razgon and Chaim N. Sukenika) Department of Chemistry, Bar Ilan University, Ramat Gan, 52900 Israel (Received 30 March 2005; accepted 7 June 2005)

Bismaleimide polymer composites were coated with adherent, uniform, crack-free, titania films. Selective deposition of either crystalline (anatase) or amorphous films was achieved as a function of deposition conditions. Films are deposited under near ambient conditions from aqueous solution and thus can be adapted to any sample geometry and still provide complete coating of the polymer composite substrate. They can be obtained crack-free in thicknesses up to 0.5 ␮m by using drying procedures that combine temperature and humidity control.

I. INTRODUCTION

Polymer matrix composite materials are increasingly important structural materials in various weight-sensitive applications as lightweight, easily fabricated replacements for metal components. Bismaleimide (BMI) resins are a class of high-temperature thermosetting additiontype polyimides widely used as reinforced matrix resins for advanced composites in the electronic and aerospace industries. BMI-based materials provide a unique combination of high service temperature, the ability to be processed, and good mechanical properties.1 BMI composites also have significant performance limitations. Polymer–matrix composites evidence significant degradation when exposed to oxidizing atmospheres at temperatures >300 °C. Surface oxidation leads to weight loss and mechanical failure. An approach to increasing the resistance of these materials to high temperatures and oxidizing atmospheres is the use of inorganic coatings that physically block the interaction of oxygen and oxy-radicals with the polymer. Progress in the application of ceramic thin-film coatings has included advances in film deposition technology involving chemical vapor deposition, sputtering, laser ablation, and evaporation.2 These techniques, however, have significant shortcomings. Capital equipment costs can be prohibitively high. The most common techniques still involve line-of-sight deposition, making them applicable only to simple surfaces and shapes. Most importantly, elevated temperature is usually required to convert the as-deposited material into crystalline films. Sol-gel

a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0318 2544

J. Mater. Res., Vol. 20, No. 9, Sep 2005

techniques, which have also undergone extensive development in the last three decades, often have similar hightemperature requirements. These significantly limit existing film-synthesis technologies, especially for polymeric substrates.3 A recent example of such a coating is the formation of silica on BMI using sol-gel technology.4 An alternative strategy for the preparation of ceramic films is liquid phase deposition (LPD). LPD is an aqueous technique for depositing oxide films, which has been widely used for SiO25 but is being used increasingly to form films of other oxides such as TiO2.6,7 The distinguishing characteristic of LPD is the use of a solution of metal-fluoride complexes whose hydrolysis in water is modulated by adding boric acid (H3BO3) or aluminum metal. The fluoride ligands provide for a slower and more controllable hydrolysis, while the boric acid or the aluminum function as F− scavengers.8 Most of the LPD work has been done on silanol-bearing surfaces (silicon wafers and glass). LPD from aqueous solution, under mild conditions of temperature (艋55 °C) and pH (2.88–3.88), can produce thin (0.1–1.0 ␮m), adherent titania films. LPD titania films deposited on silicon wafers with and without functionalized monolayer coatings have been studied in our laboratory.9 Variations in film crystallinity and in its adherence to differently functionalized surfaces (with or without surface sulfonic acid groups) as a function of the specific LPD recipe were reported. High-temperature polyimide resin (PMR-15) surfaces have been successfully coated with adherent titania films using different oxide deposition recipes.10 LPD deposition of ceramic films on polymeric substrates has also been reported by other groups. For example, TiO2 thin films were deposited on polystyrene by liquid phase deposition,11,12 polyamide microcapsules were coated with TiO2,13 and silica films were deposited from © 2005 Materials Research Society

A. Razgon et al.: Ceramic coatings for fiber matrix composites: Titania thin films on bismaleimide-glass fiber composites

hexafluoro-silicic acid solution onto polycarbonate, polymethylmethacrylate, polyethersulfone, and cycloolefin polymers.14 It has also been shown that one of the promising approaches to maintaining a hydrophilic surface on a hydrophobic polymer substrate is by forming a surface layer of silicon oxide.15 The objective of the study reported herein was to create ceramic thin films as barrier layers on BMI polymer matrix composites with an eye toward improving its abrasion resistance and thermo-oxidative stability. A previously studied example of this strategy is the deposition of a thin, continuous SiO2 film on BMI resin substrates using a spin-coating process.4 The potential advantage of bath-deposition methodologies suggested possible benefits for such an approach. To create an effective barrier layer, it was important that we produce a conformal, crack-free, ceramic coating of controlled thickness. Given our aqueous processing conditions, the issue of drying the sample (removing both water trapped in the oxide film and water absorbed into the polymer matrix) while not leaving drying cracks (“mud-cracks”) was also crucial. Generally, cracking is less of a problem in thinner films than in thicker ones in as much as lateral shrinkage of the films is less severe and the amount of trapped solvent is proportionately less. However, cracks often cannot be avoided when a critical film thickness is reached. Alternative ways to address this problem have been reported. Lead zirconate titanate Pb(Zr,Ti)O3 (PZT) thin films were deposited by a sol-gel spin-coating method on an Al/SiN/Si (100) substrate. The method for preparation of PZT thin films caused the appearance of cracking caused by internal stress during the heat treatment used for the decomposition of gel solutions and formation of ceramic films. In that case, lowering the concentration of the sol-gel solution solved the cracking problem.16 RuO2–TiO2 coatings were prepared using an electrodeposition process on Pt and Ti substrates. The cracking problem was addressed by using multiple deposition cycles because cracking is less of an issue in the deposition of successive thin layers.17 We report herein coating procedures for attaching thin titania layers using LPD chemistry. We have examined different solution conditions and different surface priming strategies (including various etching procedures and using an initial silica layer to enhance titania adhesion). We have learned how to control the thickness of the oxide film using different deposition times, both for procedures that have been previously demonstrated to attach to silanol bearing surfaces (method #1, below) and for procedures that have been shown to anchor to surfaces bearing sulfonic and/or carboxylic acid groups (method #2, below). We have also learned to minimize cracks by a careful interplay of temperature and humidity in the drying of the coated BMI. While we have yet to fully

assess the performance of such coatings, we have used thermogravimetric analysis (TGA) to provide a preliminary indication of the effect of the ceramic coatings on the various decomposition processes that can undermine the integrity of the BMI composite. II. EXPERIMENTAL A. Chemicals and equipment

All chemicals were obtained from Sigma Aldrich (St. Louis, MO). Ultraviolet (UV) ozone cleaning of substrates was done using a UVOCS Model 10-X10/OES/E (UVOCS, Colmar, PA). Solution pH was measured using a Metrohm model 691 pH meter (Herisau, Switzerland). Wetting properties were assessed by water-contact-angle measurements using a Rame Hart Model 100 Contact Angle Goniometer (Mountain Lakes, NJ). X-ray photoelectron spectroscopy (XPS) analyses were done on a Kratos AXIS-HX spectrometer (Kanagawa, Japan) with a monochromatic Al x-ray source. Optical microscopy was done using an Olympus Model BX-60 microscope (Tokyo, Japan) equipped with a U-PO polarizer, a U-AN360 analyzer, and a U-DICR Nomarsky lens connected to a Nikon CoolPix 990 digital camera (Nikon, Tokyo, Japan). Cross-sectional scanning electron microscopy (SEM) and energy-dispersive x-ray analyses (EDAX) were performed on a JEOL 840 microscope (Tokyo, Japan). EDAX data acquisition and surface analysis were performed using Link ISIS (Oxford, UK) software (Oxford, England). SEM analysis of the drying experiments was performed on a JEOL JSM-B40 microscope. “Scion” image analysis software was used to quantitate the degree of film cracking. Thermogravimetry (TGA) analyses were done using a Mettler Toledo TGA/ SDTA851 e thermogravimetric analyzer (Columbus, OH). External reflection Fourier transform infrared (FTIR) microscopy spectra were collected on a Bruker Model Vector 22 FTIR spectrometer (Ettlingen, Germany) using the A691 microscope reflection accessory with an internal A690/3-R MCT detector (Ettlingen, Germany). B. Substrate preparation and activation

Glass BMI prepreg coupons F-650/120 were received from Israel Aircraft Industries (Lod, Israel). They were cut to the desired size and rinsed with acetone and ethanol and dried under a filtered nitrogen flow. Given the heterogeneity of the composite sample (a mix of glass fibers and polymer matrix), it was necessary to focus on the polymeric portions of the BMI composite to obtain consistent analyses. To activate the surface, samples were treated for 20 min in a Ultraviolet Ozone Cleaning System (UVOCS) cleaner and used immediately upon removal. Samples treated in this way were substantially more hydrophilic than untreated BMI.


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An alternative method of priming the BMI surface to promote LPD titania attachment was to first deposit a layer of SiO2 using the following modified sol gel procedure. A cleaned BMI sample was dipped vertically into a solution of 1 ml (EtO)4Si, 1 ml ethanol, 10 ␮l 1M HCl, and 145 ␮l H2O and withdrawn from the solution followed by drying at room temperature. The resulting silica-coated samples were treated in the UVOCS cleaner prior to titania LPD processing. C. Titania deposition procedures

was 80%. When the drying was done at 70 °C, the initial RH of the chamber was 70%. Samples were dried by slowly reducing the humidity in steps from 70/80% to 60% to 40% to 20% and holding the RH at each new humidity for 20 h. Finally, samples were cooled to 25 °C over 3 h and then removed into the ambient environment. The overall time of this drying procedure was 90 h, including the times needed to achieve each new set of conditions. The drying time could be shortened to 50 h by reducing the time interval at each RH to 10 h.

1. LPD using titanium fluoride pH 3.88 (method #1)

3. Drying method #3

A sample was immersed (vertically) into a solution of 0.3 M H3BO3 and 0.1 M (NH4)2TiF6 at an initial pH of 3.88 and at room temperature. Substrates were left in the solution for 22–44 h and then washed with water and dried using one of the procedures detailed below.

After water rinsing, samples were treated in the same oven used above, but using a different approach to changing the humidity in the chamber. Samples were introduced into the chamber at either 40 °C with 80% RH or 70 °C with 70% RH and the RH was reduced in a steady ramp down to 20%. At the end of the drying procedure, samples were cooled to 25 °C over 3 h and then removed into the ambient environment. The overall time of this drying procedure was either 50 or 90 h (by varying the rate of the steady decrease in RH). Figure 1 summarizes the 90 h step and ramp drying procedures.

2. LPD using titanium fluoride complex pH 2.88 (method #2)

A sample was immersed (vertically) into 0.15 M H3BO3 and 0.05 M (NH4)2TiF6 solution. Concentrated HCl was added to adjust the pH to 2.88 and the solution was kept at 50 °C. Substrates were left in the solution for 3–20 h and then washed with water and dried using one of the procedures detailed below. D. Drying of TiO2 films

Since the titania depositions are done from aqueous solution, controlling the removal of the water from the samples is an important factor in determining the quality of the films obtained. Drying samples by simply allowing the water to evaporate (air-dried samples) always produced cracking that was readily visible in an optical microscope. Drying samples by changing the humidity (while maintaining a constant temperature) was done in 3 ways. 1. Drying method #1

After water rinsing, samples were placed in a closed chamber whose relative humidity was controlled by equilibration over various salt solutions. The chamber was kept at 25 °C, and the solution in the base of the chamber was changed every 24 h in the following sequence: water (100% humidity), saturated NaCl (75% humidity), saturated Mg(NO3)2 (49% humidity), and saturated LiCl (11% humidity). After 4 days, samples were removed from the chamber into the ambient environment and characterized.

E. Characterization and testing of TiO2-coated BMI

Characterization of TiO2-coated BMI samples involved any or all of the following techniques. External reflection FTIR microscopy was used to compare the infrared (IR) spectrum of coated and uncoated BMI and to look for changes in the structure of the polymer that had been exposed to titania deposition conditions. Contact angle goniometry revealed changes in surface wetting. XPS and energy-dispersive x-ray analysis (EDAX) identified the titania on the polymer surface. Crosssectional SEM (after fixation of the coated BMI coupons in epoxy) was used to determine the thickness of the titania coating. Optical microscopy and/or SEM were used to estimate the degree of film cracking. Such analyses were always done on 5 separate points on each coated coupon and were quantified using “Scion” image analysis software.

2. Drying method #2

After rinsing with water, samples were placed in an oven with programmable control over both temperature and humidity. When the oven was kept at a temperature of either 40 or 60 °C, the initial relative humidity (RH) 2546

FIG. 1. Schematic illustration of the 90 h step and ramp drying programs.



The thermo-oxidative behavior of the TiO2-coated BMI was assessed by TGA using samples of ∼10 mg which were heated at a rate of 2 °C/min in air or N2. The sample weight loss was measured as a function of temperature. These experiments were each repeated twice. III. RESULTS A. Surface priming of BMI

Cleaning and activation of the surface of the BMI coupons were done by first rinsing in organic solvents and then exposing the BMI surface to UVOCS activation for 20 min. This pre-treatment was essential for good TiO2 film adhesion. External reflection FTIR microscopy showed that no damage was caused to the polymer by the UVOCS exposure. Unless explicitly indicated, all samples described in this work were primed by UVOCS activation. While most of the titania depositions described herein were done directly on UVOCS activated BMI samples, in some cases a silica layer made by a modified sol-gel procedure preceded the titania deposition. Rather than heating these samples to complete the sol-gel preparation, after coating with the sol-gel silica precursor, the sample was exposed to UVOCS for an additional 20 min to provide the silica coating. External reflection FTIR showed the expected Si–O–Si signal at 1113 cm−1 and Si–O–C signals (from residual alkoxysilane) at 975 cm−1. Figure 2 shows how we could use FTIR mapping of the BMI surface to differentiate between the glass fibers in the BMI composite and a silica overlayer. Figure 2(a) shows an optical micrograph highlighting the fibers in the BMI matrix. Figures 2(b) and 2(c) show an FTIR map of a comparable section of the BMI composite, before and after coating with silica. Mapping the intensity of the silica IR signals (covering the entire region of 1155– 900 cm−1) readily distinguishes between the fiber regions and the overall surface of the silica-coated sample. In Fig. 2(b) the glass fibers can be seen, while in Fig. 2(c) the entire surface shows these silica signals. B. Titania deposition and charaterization

LPD methods were applied to deposit TiO2 on surfaceactivated BMI composite samples. BMI is a moderately hydrophobic material with an advancing water contact angle of 80°. Upon titania film coating, the sample surface became hydrophilic and the advancing water contact angle dropped to 10°. The stability of the titania coatings was assessed by a combination of tape tests and sonication. All of the TiO2-on-BMI coatings reported herein were found to be stable to sonication in water and were sufficiently adherent so that they could not be removed by a standard tape test.

FIG. 2. (a) Optical microscope image of clean BMI and (b) external reflection FTIR microscope mapping images of clean BMI, and (c) BMI coated with SiO2.

Limited attempts to create films of zirconia on BMI composites were unsuccessful. Published procedures were used to deposit ZrO2 both by LPD18 and by sol-gel methods.19 These experiments gave rise to small ZrO2 crystallites on the BMI. They could be observed by optical microscopy but were not adherent and were easily removed by tape test or sonication. Cross-sectional SEM (on samples prepared by fixing coated BMI coupons in an epoxy polymer) was used to determine titania film thickness. EDAX verified the identity of the titania layer and its thicknesses could be directly determined from the SEM images. Figure 3 shows samples ranging in thicknesses from 200 to 900 nm. The two thicker films [Figs. 3(b) and 3(c)] were comparable (±20%) to those reported for sulfonate-monolayerfunctionalized silicon substrates (400 and 900 nm, respectively, as measured by ellipsometry).9 The thinner film [Fig. 3(a)] was somewhat thicker than the reported result on a clean silicon wafer (200 nm versus 80 nm). However, the significance of this difference, particularly for such thin films is questionable due to the variability in initiation times for the onset of solution deposition


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FIG. 4. FTIR external reflection spectra (collected using IR microscope) of BMI composite: (a) coated for 22 h by method #1; (b) coated for 48 h by method #1; (c) untreated BMI exposed to 48 h water; and (d) untreated BMI.

FIG. 3. Cross-sectional SEM images of BMI coated with TiO2: (a) method #1, 6 h coating time; (b) method #2, 6 h coating time; and (c) method #2, 20 h coating time.

leading to variations in film thicknesses achieved at shorter deposition times. Earlier work on titania-coated PMR-15 polymer composites10 had shown that the polymer surface can undergo some hydrolysis when exposed to the conditions of 2548

FIG. 5. (a) SEM microscope image of cracked titania film on BMI coated by method #2, 20 h coating time and (b) Scion analysis software image of cracked titania film on BMI coated by method #2, 20 h coating time.



LPD titania deposition. The same behavior can be seen by following the FTIR spectra of BMI samples as a function of exposure time to the TiO2 deposition solutions. Figure 4 shows the external reflection FTIR spectra of BMI exposed to method #1 LPD solution for 6, 22, and 44 h and also compares the IR spectrum of uncoated BMI. In untreated BMI, the imide carbonyl adsorption appears at 1713 cm−1. After exposure to the titania deposition solution for 6 h the carbonyl peak appears to be unaffected (as was the case for a control sample kept for 48 h in water). However, after exposure to the method #1 LPD solution for 22 or 44 h, the carbonyl peak is significantly broadened, allowing for the possible formation of amide (1650 cm−1), imide (1713 cm−1), and/or acid (1700 cm−1). The broad absorption in the 600–1200 cm−1 region is due to the TiO2 film. C. Drying in controlled humidity environment to minimize cracking

Forming a good barrier layer requires the creation of a dense, crack-free, ceramic film on the surface of the polymeric substrate. Thin films that are prepared by wet chemical methods (like LPD) suffer from cracking problems during drying, particularly as film thicknesses increase. An effective strategy for drying LPD titania films is to slowly reduce the humidity around the coated samples. Using a salt solution chamber (at room temperature) to provide controlled humidity drying gave some improvement but still showed cracks that could be easily seen under an optical microscope. Using an oven with controlled temperature and variously programmed changes in relative humidity was much more effective. Specifically, we evaluated the effect of such programmed drying both on films prepared by the LPD method #2 (known to be oriented anatase) and on the films prepared by LPD method #1 (known to be amorphous). Drying in a climatic chamber by reducing the relative humidity in a controlled fashion had a measurable effect on cracking. Samples treated by such variation in humidity were analyzed by SEM and the degree of film cracking was calculated using image analysis software (Scion). Figure 5 shows an SEM microscope image of a

cracked titania film on BMI (coated by method #2 for 20 h) and also shows the Scion analysis image of the same sample. The crack percentage was calculated from the ratio of white to black pixels. It was shown that amorphous TiO2 films (made by method #1) could be made with almost no detectable cracks by controlled humidity drying at either 60 or 70 °C. The crystalline films (made by method #2) needed to be dried at 70 °C. It was interesting to note that while controlled humidity drying of the amorphous films resulted in a clear reduction in the number of cracks as the drying temperature was increased. In the case of the crystalline films, controlled humidity drying at progressively higher temperatures primarily reduced the width of the cracks but not necessarily their numbers. In either case, films of 300–500 nm thickness could readily be made crack-free and films of up to a micron in thickness were brought to a level of less than 1.5% cracks. The results are summarized in Table I and SEM micrographs of representative samples are shown in Fig. 6. D. TGA analysis of the coated BMI

TGA was used to provide insight into the effect of the titania coating on composite stability. The degree of dynamic weight loss, either in air (thermo-oxidative degradation) or in an inert environment (thermal degradation), provides useful indicators of thermal stability. Thus, the TGA behavior of variously coated samples was measured over the range of 30–600 °C. First, the TGA thermograms of clean BMI in air and in nitrogen were compared (Fig. 7). Two weight loss steps (at ∼400 and ∼500 °C) are seen when the sample is heated in air, as opposed to only one (at ∼400 °C) when it is heated under nitrogen. This suggests that the degradation mechanism in air includes a lower temperature, purely thermal, degradation process, and a higher temperature degradation of a thermo-oxidative nature. This is consistent with the behavior reported in the literature for carbon fiber bismaleimide composites.20 TGA in air was used to assess the thermo-oxidative stability of variously coated BMI. The TGA data for each

TABLE I. Crack percentage of the TiO2 film. Method #2 TiO2 coating method Drying procedure Ramp program Step program

Temp.: Temp.: Temp.: Temp.:

40 70 40 60

°C; °C; °C; °C;

RH: RH: RH: RH:

from from from from

80% 70% 80% 80%

to to to to

20% 20% 20% 20%

(90 (50 (90 (90

h) h) h) h) (replicate runs)

Temp.: 70 °C; RH: from 70% to 20% (90 h) Temp.: 70 °C; RH: from 70% to 20% (50 h)

Method #1 TiO2 coating method

3h

6h

20 h

22 h

44 h

4% 1% 1.4% 1.2% (1.8%) 0.6% 1.3%

3.7% 1.2% ⭈⭈⭈ 2.8% (2.5%) 2.1% 1.4%

4.5% 1.4% 7.1% 3% (3%) 1.8% 2.8%

1.4% 0% 4.2% 0.6% (0%) 0.2% 0.2%

⭈⭈⭈ 0.6% ⭈⭈⭈ 1.5%


⭈⭈⭈ ⭈⭈⭈

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FIG. 6. The SEM pictures of titania dried by humidity step program: (a) method #1, 90 h, 40 °C; (b) method #1, 90 h, 60 °C; (c) method #1, 90 h, 70 °C; (d) method #2, 90 h, 40 °C; (e) method #2, 90 h, 60 °C; and (f) method #2, 90 h, 70 °C.

sample was converted to a first derivative curve. In Fig. 7, the weight loss curve in nitrogen (e.g. Fig. 8) shows only a single drop while that in air shows two distinctly different regions of weight loss. The weight loss in nitrogen (by first derivative determined to be at 406 °C) is comparable to the first weight loss in air (408.5 °C). The titania coatings (different thicknesses, made by each of the two methods) were compared based on this first derivative analysis. The results are summarized in Table II. IV. DISCUSSION

The success achieved in creating adherent thin films of titania on the surface of a BMI substrate demonstrates the promise of such coatings as performance-enhancing barrier layers on polymeric substrates. Each potential 2550

application must be examined in terms of the effect of the deposition chemistry on the mechanical and thermal properties of the polymer substrate, the requisite thickness of the ceramic layer, and optimization of postprocessing steps. Nevertheless, the low-cost and convenience of LPD coatings and their lack of line-of-site limitations strongly recommend their being considered as a general strategy. Moreover, the relatively mild conditions for ceramic film formation using LPD methodology makes them prime candidates for application on polymer substrates. It is interesting that variations in the LPD recipe that give rise to very different rates of ceramic film formation (method #2 > method #1) and that can be controlled to provide either crystalline (method #2) or amorphous (method #1) titania are both equally effective in producing adherent conformal thin film coatings on BMI. This



TABLE II. Two weight loss points of 1st derivative graph of variously coated BMI. TGA (heating rate 2 °C/min)

FIG. 7. TGA curves of clean BMI treated (a) in air and (b) in N2.

FIG. 8. (a) TGA curve for clean BMI sample and (b) first derivative of TGA curve.

is in contrast to what is seen for either silanol-bearing surfaces (good for method #1 only) or sulfonated surfaces (good for method #2 only). BMI substrates accommodate both titania preparations. This is likely due to partial hydrolysis of the polymer surface providing a mix of carboxylic acid and amide sites that anchor the titania by a combination of coulombic and chelation-based effects, as has been reported for titania deposition on PMR15.10 The work presented herein is the first attempt to systematically address the post-processing concerns of filmcracking when applying aqueous-based LPD coatings to polymer composite surfaces. The desirability of aqueous

TiO2 coating method

1st minimum point (°C)

2nd minimum point (°C)

Clean BMI Method #1, coated for 22 h Method #2 coated for 3 h SiO2 + Method #1 (22 h) Method #2 (3 h) + Method #1 (22 h)

408.5 405.6 408.6 406.8 411.9

524.3 529.4 531.1 533.1 530.8

reaction media as a part of a general trend towards “green” chemistry is clear. However, the presence of absorbed water both in the ceramic coating and in the polymer substrate must be considered. The cracks caused by removal of this water can significantly reduce the usefulness of the barrier layers formed. Drying protocols based on gradual changes in temperature and humidity seem to be suitable for any aqueous based deposition system. While their application to the thicker films that are likely to be needed for certain barrier applications (e.g., scratch resistance) is likely to present important challenges, scaling of such a process for large-area applications should be possible and the relative simplicity of such an approach strongly recommends its consideration. The trends observed in our work (e.g., the benefits of drying at higher temperatures while carefully controlling changes in relative humidity) are likely to be found in other water-based systems, though assessment of the influence of prolonged exposure to hot humid conditions is likely to be an important variable in subsequent polymer/composite performance. Finally, the use of TGA analysis for an initial assessment of the effect of such coatings on polymer stability and the attempts to differentiate thermo-oxidative from purely thermal degradation processes is also a good model for future work. While extended aging studies with accompanying assessment of weight loss and mechanical integrity are the real measures of success, the use of such TGA-based rapid assessment tools provides an important initial basis for comparing various coating protocols. The possibility that combinations of ceramic layers (e.g., TiO2 by method #1 on an initial ad-layer of SiO2) may be convenient routes to effective barrier layers is also a potentially general strategy that will be the subject of future research. ACKNOWLEDGMENTS

The authors thank Israel Aircraft Industries for providing BMI samples. Dr. Thomas Bayer’s assistance with FTIR microscope measurements and the financial support of the Office of the Chief Scientist of the Ministry of Commerce are gratefully acknowledged.


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