Influence of Pyrogenic Particles on the Micromechanical Behavior of

ARTICLE pubs.acs.org/Langmuir

Influence of Pyrogenic Particles on the Micromechanical Behavior of Thin Sol
Gel Layers B. Sch€onstedt,* G. Garnweitner, N. Barth, A. M€uhlmeister, and A. Kwade Institute for Particle Technology, TU Braunschweig, Volkmaroder Str. 5, 38104 Braunschweig, Germany

bS Supporting Information ABSTRACT:

Coatings based on sol
gel technology with different types of nanoparticles embedded into the sol
gel matrix were fabricated, and the resulting properties were investigated. Pyrogenic silica nanoparticles were added to the sol before coating. The silica particles varied in primary particle size and agglomerate size, and in their surface modification. The particles were wetted in ethanol and dispersed to certain finenesses. The difference in agglomerate size was partly caused by varying particle types, but also by the dispersing processes that were applied to the particles. The resulting coatings were examined by visual appearance and SEM microscopy. Furthermore, their micromechanical properties were determined by nanoindentation. The results show an important influence from the added nanoparticles and their properties on the visual appearance as well as the micromechanical behavior of the sol
gel coatings. It is shown that, in fact, the particle size distribution can have a major impact on the coating properties as well as the surface modification.

’ INTRODUCTION Thin films represent one of the most important fields of application for nanosized structures. Ultrathin hard materials, the creation of superhydrophobic surfaces, or nanocomposite materials are promising applications.1
3 Various processes are established for the manufacturing of thin and ultrathin films. Besides gas phase based procedures like vapor deposition and sputtering, solvent based processes are being increasingly used, also for industrial applications.4 The sol
gel technology represents one of the most important solvent based coating procedures. Sol
gel films are used in various applications, ranging from corrosion prevention to antiadhesive or antireflective coatings. Although sol
gel based coatings are widespread and already established for a host of industrial applications, topical research focuses on the improvement of the mechanical behavior of sol
gel coatings, especially for heat sensitive substrates where high temperature hardening can not be applied. From the fields of automotive coatings and polymer nanocomposites it is known that embedded nanoparticles can significantly enhance the performance of coatings in practical applications, e.g., the scratch resistance.5 Other effects like UV absorbance and corrosion protection are also realized by the addition of nanoparticle fillers.6 Although nanoparticles are already embedded into commercial r 2011 American Chemical Society

coatings, the effect of nanoparticles on the micromechanical behavior of nanocomposite coatings is in the focus of research. On the one hand it is known that the addition of certain nanofillers to polymers leads to increased Young’s modulus and hardness which results in an improvement of wear and scratch resistance.7,8 On the other hand, it was shown, especially for automotive coatings, that scratch resistance can not only be achieved by hardness but rather by elastic behavior, avoiding sharp edges in the penetrated surfaces and thus leading to a decreased loss of gloss after stressing.9 Besides the change of chemical structure,5 the addition of nanofiller particles was proved to be appropriate for the enhancement of scratch resistance of coatings.5,10
12 However, known investigations on the effect of filler particles typically involved the use of only one type of particle of a certain material. Furthermore, only little attention has been paid to the dispersing process of the nanofillers, even though pyrogenic nanoparticles, which are used as such fillers in the majority of cases, commonly exist as strong aggregates and agglomerates. For use in a coating, the particles are desired as Received: February 27, 2011 Revised: May 9, 2011 Published: June 01, 2011 8396

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Table 1. Particle Types Used As Fillers in the Coating BET surface Aerosil

primary particle size surface modification in gas

(m2/g)

(nm)

50 ( 16

40

200

200 ( 25

12

300

300 ( 30

7

R 812 S

220 ( 25

7

Ox 50

phase

TMS

suspended in the respective matrix and dispersed to primary particles or to a certain aggregate size. Commonly, the agglomerates are difficult to disperse, and high stabilization of the particles in the solvent is here a necessary but not sufficient condition. The required stress intensity for fragmentation strongly depends on the binding strength within the respective aggregates and agglomerates. Mechanical stress has to be applied on the agglomerates using dispersing devices like dissolver, stirred media mill, three roller mill, or high pressure homogenizer. These devices differ in construction and functionality, leading to different stress mechanisms and frequencies,13,14 which in turn result in different particle size distributions of the suspension. Systematic studies of the influence of the particle size distribution on the micromechanical behavior of thin coatings have not been published so far. In the study presented here, a novel approach is followed to fabricate sol
gel based coatings with embedded fumed silica nanoparticles from ethanolic sols by means of a dip coater. In order to investigate the influence of the particle properties on the micromechanical behavior of the resulting films, the particle type embedded into the layer was varied, meaning that the particles differed in primary particle size, in surface modification, and in the type of dispersion process that was applied after wetting. Additionally, the particle concentration in the layer was varied. The purpose was not the optimization of the dispersion for a certain application of the coating but rather the improvement of a detailed knowledge of the influence of different nanoparticle types on the resulting properties of the coating with a special focus on the micromechanical properties. To avoid any influence of other parameters and organic components, a prehydrolyzed TEOS sol was chosen as a a simple and well understood standard system.

’ EXPERIMENTAL SETUP Sol
Gel Coating. For fabrication of the sol
gel coatings, tetraethyl orthosilicate (TEOS, Si(OC2H5)4) was used as precursor (Fluka, >98% (GC) purity). Analogously to previous works15,16 on the formation of thin composite sol
gel films, a prehydrolyzed stock solution was set up by mixing the following constituents at 60 °C: 1 mol TEOS, 3.8 mol ethanol, 1 mol deionized H2O, and 5  10-5 mol 0.07 M hydrochloric acid. For the preparation of the final coating sol, ethanol based dispersions of pyrogenic SiO2 particles were added to the stock solution. The volume ratio of the final coating sol was 1 mL TEOS stock solution/ 2 mL silica dispersion/0.04 mL deionized H2O. The coatings were fabricated by dip-coating (performed on an Id Lab Coater, model 4) of stainless steel substrates, performed with a residence time of the substrate in the coating sol of 60 s and a withdrawal velocity of 500 mm/min under ambient conditions. Finally, a heat treatment at 120 °C in ambient atmosphere was performed for 3 h to dry the coatings. Dispersions. Pyrogenic SiO2 nanoparticles (Aerosil, Evonik Degussa GmbH, Germany) were added to the coating sols after wetting

and dispersing in ethanol. Different commercial particle types were used as shown in Table 1 (supplier’s specification). The materials were chosen to investigate the effect of primary particle size in a rather broad range. As an additional particle type, Aerosil R812S was used, which was obtained from the supplier as gas phase surface-modified pyrogenic SiO2.17 Trimethylsilyl (TMS) groups are attached to the surface of these particles as result of the surface modification process.17 The primary particle sizes listed in Table 1 do not correlate with the measured diameters (e.g., by dynamic light scattering), due to the formation of aggregates and agglomerates (see SEM image in Figure SI 1). In order to achieve a certain product fineness, deagglomeration with dispersing machines is necessary. Due to the presence of silanol groups, the silica nanoparticles possess a hydrophilic surface, except for the TMS modified type R812S which shows hydrophobic behavior. Silanes were used for surface modification of the silica particles prior to the dispersion process. Therefore, nitric acid and deionized water as well as 3-glycidyloxypropyltrimethoxysilane (Glymo) (Fluka, g 98% purity), 3-glycidyloxypropyltriethoxysilane (Glyeo) (Aldrich, g 98% purity), or hexamethyldisilazane (HMDS) (Fluka, 98% purity) were added to the silica dispersions in ethanol (1 wt % of the particle mass) prior to the dispersing process. All ligands were added to the dispersion at room temperature under stirring with a common laboratory agitator. Dispersing Devices. A high pressure homogenizer and a stirred media mill were used for the deagglomeration of the silica suspensions. High pressure homogenizing was performed with two passages in a Lab 60-15 TBS (APV Gaulin) with a pressure drop of Δp = 600 bar. A PML 2 from B€uhler was employed for the deagglomeration in stirred media mill. It was equipped with a 0.9 L grinding chamber, a disk agitator, and 0.5 mm polystyrene grinding media. The energy input required to disperse agglomerates to certain size is typically measured as mass related specific energy Em.18 For stirred media mills Em is defined as Z ðP
P0 Þ dt ð1Þ Em ¼ mP Here P is the power consumption of the mill in the dispersing process, P0 the idle power consumption, and mP the mass of the solid content. For high pressure homogenizers, eq 2 is used to calculate Em, taking the pressure drop Δp, the solid content cm, and the density of the suspension Fsusp into account: Em ¼

Δp cm Fsusp

ð2Þ

Characterization. A Zetasizer Nano S from Malvern Instruments was used for particle size analysis, applying the principle of dynamic light scattering (scattering angle: 173°). For selected coating sols, the wetting angle on steel substrates was measured, using a contact angle measurement system G10 (Kr€uss). This system enables us to apply a single fluid droplet on the substrate under investigation and to measure the contact angle via integrated image based software. For a more detailed examination of the fabricated film surfaces, SEM microscopy was performed (Zeiss EVO MA25). An atomic force microscope (AFM) (Park Systems, model XE100) was used to determine the film thickness. The thicknesses of the coatings with embedded particles were determined up to about 4 μm. Therefore, edges of the coatings were scanned in lateral direction, and the variation of level is registered giving an image of the film thickness. The AFM was equipped with a tipless cantilever (NanoWorld Arrow TL1) with a glass bead that was glued to the tip, assuring smooth stress of the film during lateral scanning. For the characterization of the micromechanical properties of the sol
gel layers, nanoindentation measurements were conducted using a 8397

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Table 2. Dispersing Results Achieved with High Pressure Homogenizer (HP) and Stirred Media Mill (SMM) surface

specific

solid modification energy agglomerate agglomerate in fluid

Em (kJ/

size d50

size d90

wt %

phase

kg)

(nm)

(nm)

dispersing content Aerosil machine Ox50

HP

15

Glymo

447 551

260

331

200

HP

15

Glymo

447 551

149

318

300

HP

15

Glymo

447 551

142

296

300

HP

15

Glyeo

447 551

141

305

R 812 S HP

5

HMDS

447 551

132

241

R 812 S HP

10

HMDS

447 551

126

230

R 812 S HP R 812 S HP

15 18

HMDS HMDS

447 551 447 551

120 117

279 226

Ox50

15

Glymo

5305

1334

2330

SMM

Ox50

SMM

15

5404

2197

4300

300

SMM

15

Glymo

5706

136

411

R 812 S SMM

15

HMDS

6289

71

364

TriboIndenter TI 900 (Hysitron Inc.), equipped with a Berkovich tip. This tip geometry has a pyramidal shape with 3 surfaces and an angle of 65.3° between central axis and surface, assuring a constant ratio of contact area to indentation depth. While indenting the material with normal displacement, a load
displacement graph was recorded. The measurements were conducted in displacement-controlled mode; i.e., a certain indentation depth was set, and the required indentation force was measured as function of indentation depth. The maximum displacement was set in the range from 50 to 1000 nm. Similar to previous works19 on the nanoindentation of thin solid films, a constant segment time was set. Each indentation was accomplished within 10 s; thus, for example, for 50 nm maximum displacement the loading rate was set to 5 nm/s. In order to achieve statistically relevant results, multiple measurements were conducted (each depth is measured 5
40 times). Therefore, a pattern of indentation points was set on the respective layer, assuring that each single measurement was performed at a point not penetrated before. This way, effects of former indents on the measurement are avoided.

’ RESULTS Several types of pyrogenic silica were used to produce the dispersions of the nanoparticle fillers (Table 1). Since the agglomerate size of the silica may vary between some hundred nanometers and some micrometers, they have to be dispersed. The dispersion processes were accomplished in a high pressure homogenizer and a stirred media mill. Moreover, different kinds of stabilizing ligands were added in order to achieve accurate stability and to determine the effect of different ligands on the resulting coatings. All ligands are assumed to fully bind to the particle surface; as for all cases, the viscosities of the dispersions decreased significantly after addition of the ligands.20 For example, the viscosity of the Aerosil 300 dispersion at a shear rate of 200 s
1 decreased from 250 to 45 mPa s. An exeption was the addition of glymo and glyeo to R812S particles, which was assumed not to work, since no decrease of viscosity was observed in this case. Additionally, the solid content was varied for the silica R812S system. Table 2 summarizes the dispersing results that could be achieved with the high pressure homogenizer and the stirred media mill.

Figure 1. Particle size distributions of the R812S (15 wt %) dispersions processed with stirred media mill and high pressure homogenizer.

The dispersing results in Table 2 show the median particle size d50 as measured by dynamic light scattering. However, as will be shown later, it is important to consider that the different dispersing devices vary in their construction, stress mechanism, and stress frequency and, thus, lead to different appearances of the respective particle size distributions. As an example, in Figure 1 the size distributions of the 15 wt % R812S dispersions produced with stirred media mill and high pressure homogenizer are compared. Although the stirred media mill produces smaller particles and the d50 is significantly smaller (71 nm) than that of the high pressure homogenizer (120 nm), the width of the particle size distribution is much higher. From the TEOS stock solution and the dispersions described previously, coatings were fabricated using a dip coater. Here, 2
5 coatings were made from each dispersion. For coatings based on the same dispersion, the variation in optical appearance, coating thickness, and micromechanical properties was inferior to the respective measurement accuracy. As can be seen in the Supporting Information (Figure SI 2), the film thickness is not constant over the whole coated area. The film thickness at the bottom part of the sample is much larger than at the top.21 Also, on the peripheral edges of the substrate areas of low film thickness are observed within 1
5 mm to the edges. Thus, the following mechanical characterization was performed in the center of the coated area in order to ensure reproducible measurements. For all coatings fabricated, the edge effects as described above are visible, though to a varying degree (Supporting Information, Figure SI 3). Furthermore, the optical quality of the coatings shows huge differences. Whereas the coating based on the Ox50 dispersion (dispersed in SMM) has an inhomogeneous appearance also for the inner zone, the one based on R812S (SMM) is homogeneous. The coatings based on Aerosil 200 and Aerosil 300, stabilized with Glymo, show an optical quality in between these two extremes (Supporting Information, Figure SI 3). For a more detailed examination of the coatings, SEM images of selected coatings were recorded. As shown exemplarily for coatings based on the Aerosil Ox50 and R812S samples in Figure 2, the homogeneity of the coatings differs strongly also on the microscale, depending on the type of used particles. Generally, a good agreement between macro- and microscale appearance could be found, meaning that coatings that were classified as macroscopically homogeneous also showed a smooth 8398

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Figure 2. SEM images of coatings based on Ox50 and R812S.

Table 3. Wetting Angle of Coating Solutions on the Steel Substrates surface

standard

d50 (nm)

σ (deg)

deviation (deg)

HP

120

78

18

SMM

121

67

8

Glymo Glymo

HP HP

149 142

27 15

13 9

A 300

Glymo

SMM

136

18

9

A 300

Glyeo

HP

141

20

11

Ox50

HP

Glymo

260

a

a

Ox50

Glymo

SMM

1334

a

a

SMM

2197

a

a

device

R 812 S

HMDS

R 812 S

HMDS

A 200 A 300

Ox50 a

dispersing particle size wetting angle

Aerosil modification

Not measurable, instantaneous spreading.

surface in SEM pictures. An explanation of this effect might be the different wetting behavior of certain coating solutions on the substrate. Therefore, the wetting angle on the steel substrates was measured for various coating sols (Table 3). Important differences in the wetting behavior were found. Whereas the coating solutions containing R812S particles showed a rather large wetting angle (78° and 67°), the wetting angles of the Ox50 based coatings were not even measurable because the solutions spread immediately on the steel substrate, indicating a very low wetting angle. The wetting angles detected for coating solutions based on Aerosil 200 and 300, stabilized with Glymo and Glyeo, are in between these extremes. The values range here from 15° (Aerosil 300, HP) to 27° (Aerosil 200, HP). The most considerable influence on the wetting angle seems to be the surface modification of the used particles. Whereas the TMS groups from the R812S lead to an increasing wetting angle, the epoxy tails of Glymo and Glyeo result in low wetting angles of the Aerosil 200, 300 and Ox50 solutions. The particle size seems to have minor influence; regarding the differences between Aerosil 200 and 300 on the one hand and Ox50 on the other hand, however, a certain influence is most likely. As indicated in Table 3, the observed wetting angles showed considerable standard deviations. To ensure statistically relevant results, each coating solution was measured 5 times each on two separate steel substrates. The observed results are rather surprising. Generally, a high wetting angle is attributed to poor wettability and therefore to unfavorable properties in coating processes. However, for the investigated system, the homogeneous coatings (based on R812S) were obtained from solutions with a high wetting angle, whereas the Ox50 based coating solutions with a minuscule wetting angle led to very poor homogeneity. The Aerosil 200 and 300 based

coating solutions are in between these extremes with respect to both homogeneity of the coatings and wetting angles. Another possible explanation for the varying homogeneity of the coatings might be the particle size of the respective silica dispersion: The HMDS stabilized silica could be dispersed to the smallest particle size, which might also be responsible for the homogeneous surface of the coatings. The effect of the particle size was investigated by using nondispersed R812S agglomerates with a d50 value of 220 nm. The coating containing nondispersed particles shows substantially higher turbidity (see Supporting Information, Figure SI 4). However, although the particle size is significantly larger than for Aerosil 200 and 300, the surface of the R812S coatings has a higher homogeneity. Thus, the higher homogeneity must be caused by the used ligand: Either the wetting behavior of the coating sol on the substrate or differences in embedding of the particles into the sol
gel matrix are responsible for the changes in quality of the coatings. Here it seems to be most likely that the varying homogeneity is in fact caused by both effects. Since the surface modification of the particles was shown to possess great impact on the visual appearance of the coatings, the need to quantify the influence of the particle type on other properties, i.e., the mechanical behavior, arises. For the presented sol
gel-based thin films an investigation of the micromechanical properties by nanoindentation is presented as an appropriate method. Figure 3 shows an example of a load
displacement graph observed for the investigated samples. The curve shows typical hysteresis behavior due to a certain plasticity. Integration of the load function of the nanoindentation (Figure 3) delivers the total deformation energy Etotal. The integral of the relieving curve is synonymous with elastic deformation energy Eelastic. Z x ¼ xmax Etotal ¼ Fload dx ð3Þ x¼0

Z Eelastic ¼

x ¼ xmax

x¼0

Frelieving dx

ð4Þ

Subtracting the integral of the relieving curve from the integral of the load curve delivers the plastic deformation energy Eplastic (eq 5).22,23 Dividing Eplastic by Eelastic delivers the ratio R of plastic to elastic deformation energy (eq 6). Eplastic ¼ Etotal
Eelastic

ð5Þ

R ¼ Eplastic =Eelastic

ð6Þ

Commonly, the aim of depth sensing indentation is the determination of the material parameters elastic modulus, E, 8399

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Figure 3. Typical load
displacement indentation graph of the sol
gel layer (here with a set indentation depth of 500 nm).

and hardness, H.24 Therefore, a set of models exists to ensure accurate determination of these material parameters depending on the character of the elasticity.24,25 Moreover, for the determination of E and H of viscoelastic materials, different testing methods, like dynamic measurements, have to be applied.25 However, the ratio of plastic to elastic deformation work (see Figure 3) measured for the different coatings varied strongly from coating to coating depending on the particle type used as will be shown later. Because of the high differences in plasticity of the coatings, no general testing method for the determination of E and H can be chosen. Furthermore, the primary objective of this study was not the quantification of elasticity modulus and hardness. As stated initially, the setup of scratch resistant clear coatings might require high flexibility whereas other strategies aim to harden the materials with nanoparticles. Therefore, the purpose was the accurate determination of the influence of different modifications and dispersing methods of the particles on the micromechanical properties of the sol
gel coatings. For this reason, the differences in the micromechanical properties of the layers were determined by calculating the median value of all maximum indentation forces of each indentation depth for the respective layer. For indentation of thin films, the possible interaction of the examined film with the substrate has to be considered.25,26 In our case, a relatively soft film was deposited on a hard substrate. Thus, the film is thinned between substrate and indenter,25 which might lead to an influence of the substrate on the measured properties for deep indentations. It has been shown in previous publications that the eminent influence of the substrate can be determined from the load
displacement curve by an abrupt change of the slope.25,27 For this reason, special attention was paid to the appearance of the load
displacement curves. The load curve in Figure 3 shows a relatively constant slope, which would not be the case for indentations with deeper penetration. With careful evaluation of the load
displacement curves it was possible to accomplish measurements up to a displacement of 1 μm showing no serious influence of the substrate. Layers that were fabricated without addition of particles were significantly thinner than those with particles; hence, it was not possible to conduct nanoindenter measurements without visible influence of the substrate (see Supporting Information Figure SI 6). Figure 4 shows the mean maximum indentation forces determined for coatings containing particles dispersed with the high pressure homogenizer. The indentation forces are plotted as function of the indentation depth which was varied from 100 to

Figure 4. Mean maximum indentation forces as function of the indentation depth for coatings containing varying types of fumed silica particles dispersed by high pressure homogenization. The d50 values of the respective particle types are the following: Ox50, Glymo, 260 nm; 300, Glyeo, 141 nm; 300, Glymo, 142 nm; 200, Glymo, 149 nm; R812S, HMDS, 120 nm.

1000 nm. For comparison, also the force-displacement graph of the uncoated steel substrate is plotted. Here, even for smallest indentation depths, forces superior to 1000 μN are necessary. The figure shows that the force required for indention of a certain depth varies in a range of 1 power of 10, depending on the properties of the added silica particles. The most pronounced difference is found between the coatings containing Aerosil 200 (Glymo modified) and R812S (HMDS modified). For an indentation depth of 650 nm, a moderate indentation force of 15 μN was required in the case of the R812S coating, whereas for an indentation depth of 650 nm, for the Aerosil 200 coating a force of 228 μN was applied. Comparing the results plotted in Figure 4 to the agglomerate sizes listed in Table 1, a general trend can be stated: The lower the agglomerate size, the lower the indentation force required to reach a certain indentation depth. This trend is however not reflected by the coating containing Ox50 particles (d50 = 260 nm). The force displacement curve of this particle type is for moderate penetrations located marginally below that of Aerosil 200 (d50 = 149 nm). Moreover, the curve of maximal load
displacements is for Ox50 particles only plotted up to a displacement of 800 nm. This is due to the effect that the load
displacement curves, on which the mean values of these particles in Figure 4 are based, showed a “pop-in” effect. This means that for an indentation depth of around 600 nm, the displacement increases without a significant increase in force. For deeper penetrations, the required forces even decrease. This behavior indicates a porous structure of the film. The difference between the R812S and the Aerosil 300 coatings is quite large, as was seen by the difference in visual appearance of the respective coatings (Supporting Information Figure SI 3). Notably, the visual appearance of the Aerosil 200 and 300 coatings was very similar, while that of R812S coatings was significantly different. However, in the micromechanical analysis, also the properties of Aerosil 200 and Aerosil 300 coatings deviate significantly. Since for both particle types Glymo was used as stabilizing ligand and the agglomerate size differs only to about 15 nm, the difference in mechanical properties is probably caused by the different primary particle sizes. Interestingly, coatings prepared from dispersions processed with a stirred media mill show very different micromechanical 8400

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Figure 5. Mean maximum indentation forces of the coatings as function of the indentation depth for sol
gel layers containing varying types of fumed silica particles processed with a stirred media mill. The d50 values of the respective particle types are the following: Ox50, 2197 nm; Ox50, Glymo, 1334 nm; 300, Glymo, 136 nm; R812S, HMDS, 71 nm.

properties (Figure 5). The required forces for indentation are significantly higher than those plotted in Figure 4. However, a similar influence of the particle type as for the high pressure homogenized samples is visible. The HMDS-modified R812S and the Glymo modified Aerosil 300 coatings show comparable results, although the forces required to indent the R812S containing coating are in this case slightly higher than those required for the Aerosil 300 coating. Again, the indentation forces of the Ox50 (Glymo modified) coating are almost 1 power of 10 higher than those containing Aerosil 300 and R812S. As a further system, coatings with unmodified Ox50 particles were characterized (Figure 5). The force required for indentation is far greater than for all coatings with modified particles. The reason for this behavior might be found in the particle size, which due to absent stabilization is about 2.2 μm, significantly larger than for the modified particles. A comparison of the coatings containing high pressure homogenized (Figure 4) and stirred media milled silica particles (Figure 5) shows that the properties of the coatings differ, although the same types of particles with roughly similar mean diameters were used. The indentation forces for the coatings with the high pressure homogenizer dispersed agglomerates are significantly below the respective forces for coatings based on stirred media milling. For an explanation, the particle size distributions should be taken into account (see Figure 1 and Table 2). Although the median d50 value of the stirred media mill dispersion is smaller than that of the high pressure homogenized dispersion, the particle size distribution is significantly wider. The particle size distributions of the Ox50 and the Aerosil 200 dispersions show analogous appearances, which lead to larger d90 values of all dispersions processed in the stirred media mill. Comparing the d90 values in Table 2 with the respective graphs in Figures 4 and 5, it can be stated that a high d90 value of the dispersion tends to lead to high required indentation forces. As mentioned initially, the appearance of the load
unload curve of the nanoindention gives valuable information about elasticity and plasticity of the indented material. Therefore, the ratio R of plastic to elastic deformation energy was determined according to eq 6. The ratios for selected layers are shown in the Supporting Information (see Figure SI 5). For all layers, the ratio

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increases with increasing indentation depth. Furthermore, the plot shows that there is a variation not only in ratio of displacement to force depending on the added fumed silica particles (Figure 4 and Figure 5), but also in the elasticity of the layers. While the ratio of the coating with R812S, dispersed in the high pressure homogenizer, does not exceed a value of 2, the ratio R of Ox50 (not modified) becomes steady at a value of about 11. Thus, the span of R is within a factor of 5, depending on the particle type and dispersing method. This shows that the choice of particle type has an effect not only on the resistance of the coating against deformation but also on the plasticity. With a comparison of the ratios of plastic to elastic deformation work (Figure SI 5) with the results presented in Figures 4 and 5, it seems that a high resistance against deformation goes along with high plasticity, whereas weak resistance is typical for more elastic films. Conclusively, it can be stated that the agglomerate size of fumed silica particles embedded in sol
gel coatings has a great impact on the resulting micromechanical behavior. However, it is important not only to consider the mean particle size but also to take the whole particle size distribution into account and to be aware that the use of different dispersing machines, due to different stress mechanisms, can lead to different particle size distributions. For the chosen formulations, it was found that the influence of the particle size on the micromechanical properties had at least the same importance as the choice of ligands. Since it could be shown that the type of particles embedded into the sol
gel-matrix has a fundamental impact on the micromechanical properties, the influence of the content of silica particles in the coatings was investigated. Further samples were fabricated, using dispersions containing 5, 10, 15, and 18 wt % of Aerosil R812S (functionalized with HMDS, dispersed by high pressure homogenization). The ratio of TEOS-based stock solution to particle dispersion was not varied. In the resulting coatings, the ratio of SiTEOS to SiDispersion was in the range 2.85 (coatings with 5 wt % dispersion), 1.25 (10 wt % dispersion), 0.77 (15 wt % dispersion), and 0.61 (18 wt % dispersion). The observed indentation forces of the resulting coatings are plotted in Figure 6. Additionally, the respective mean particle sizes d50 are added to the caption. The difference in particle sizes is due to the deagglomeration process. All dispersions were processed under equal conditions (HP, 600 bar, 2 passages). However, the varying solid content caused different viscosities of the dispersions, which led to a variation of efficiency of the dispersing process. It is clearly visible in Figure 6 that there is a strong influence on the solid content of the dispersions on the mechanical properties of the resulting coatings. For an indentation depth of 1000 nm, differences over a magnitude of 102 were observed. Although the particle sizes vary slightly, this is rather unlikely to cause the large differences. Thus, according to the graph, an increasing solid content leads to a decrease of force required for a certain indentation depth. However, this relation is not linear: Whereas the decrease of required force from 10 to 15 wt % is significant, the differences observed between 5 wt % and 10 wt % on the one side and 10 wt % and 18wt % on the other side are rather narrow. This indicates that, first, a certain solid content is required to achieve a significant influence of the particles. Second, a kind of saturation can be observed in this influence, with 15 wt % being close to this optimum. However, the decrease in force with increasing particle content was originally not expected. Since the coatings were dried at moderate temperature, a rather flexible behavior of the sol
gel matrix was assumed, whereas the addition of pyrogenic particles 8401

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Figure 6. Mean maximum indention forces as function of the indention depth for coatings fabricated with suspensions of varying solid contents of Aerosil R812S, all stabilized with HMDS and dispersed with high pressure homogenizer.

was assumed to harden the coating. Nevertheless, the embedding of the R812S particles apparently enhances the flexibility of the coating. Considering the ratio of 0.61 and 0.77 of SiTEOS to SiDispersion for 15 wt % and 18 wt % samples, the structure of the dried coatings can be assumed as a close packing of agglomerates, which are bonded together by the sol
gel matrix. The distances bridged by the matrix are relatively short. With a comparision of the results achieved with dispersions with a solid concentration of 15 and 18 wt % with those of 5 and 10 wt %, it becomes obvious that this structure with short matrix bridges seems to be most favorable for high flexibility on the microscale. This might be explained by the fact that the embedded particles, due to their surface modification, are relatively inert, which in turn inhibits the bonding to the sol
gel matrix. In this way, the development of an inflexible matrix is constricted. Furthermore, the behavior of the coatings with different particles types can be explained by considering this proposed coating structure. A structure that consists of agglomerates with narrow variation in diameter (HP dispersed suspensions) favors short and regular sol
gel bridges, which result in high flexibility. By contrast, dispersions with a wide particle size distribution or huge agglomerates required higher indentation forces for a certain penetration depth. One reason might be that an assembly of particles with a great variation of particle sizes leads to irregular distribution of sol
gel bridges. In this irregular structure also sol
gel matrices with wider expansions occur, which would, considering the proposed model, lead to lower flexibility. Moreover, in structures with small particles a single particle can only interact mechanically with a limited part of the surrounding matrix resulting in relatively small resistance forces if the coating is externally deformed. However, in a structure with huge agglomerates in between smaller ones, the coarser particles interact with a bigger part of the structure resulting in a higher resistance against deformation.

’ CONCLUSION Sol
gel based coatings with embedded pyrogenic silica nanoparticles were fabricated by dip coating. Fumed silica particle systems with different particle size distribution and

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surface modification were utilized. The coatings were subsequently evaluated with respect to their optical appearance and micromechanical properties. For the optical appearance, an eminent influence of the surface modification was determined. The mechanical properties were determined by means of nanoindentation. A strong influence of the surface modification on the coating properties could be verified. However, the agglomerate size of the utilized dispersions has at least the same amplitude of influence. In general, a small mean particle size led to a decreased indentation load for a certain indentation depth, meaning that the coatings had a higher flexibility. In more detailed investigations it could be shown that also the width of the particle size distribution has a great influence on the micromechanical behavior. The differences in particle size distribution were partly due to the pyrogenic production process and partly due to the different dispersing processes applied. As a possible explanation for the higher flexibility of certain coatings, the interruption of the sol
gel matrix by the particles was found. Moreover, smaller particles with a narrow width of the particle size distribution are assumed to have less mechanical interaction with the surrounding coating due to the smaller special dilatation. Thus, they can pass only a small amount of inducted deformation, which leads again to higher flexibility. Conclusively, the influence of pyrogenic particles on the micromechanical behavior can be not only explained by their chemical composition, but also with consideration of the primary particle size and agglomerate size. Moreover, the required indentation force of coatings which differ only in the method used to disperse the particles varied up to a factor of 10. Hence, besides a correct selection of adequate particles and stabilization, the fabrication of coatings with optimized mechanical properties requires a careful and knowledge based choice of dispersing process.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional figures and discussion of observed properties of TEOS coatings without particles. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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