Sizings on Alkali-Resistant Glass Fibers: Environmental Effects on

2496

Langmuir 2003, 19, 2496-2506

Sizings on Alkali-Resistant Glass Fibers: Environmental Effects on Mechanical Properties§ Shang-Lin Gao,† Edith Ma¨der,*,† Anwar Abdkader,‡ and Peter Offermann‡ Institute of Polymer Research Dresden, Hohe Strasse 6, 01069 Dresden, Germany, and Institute of Textile and Clothing Technology, Dresden University of Technology, Dresden, Germany Received September 11, 2002. In Final Form: December 5, 2002 The alkali-resistant glass fibers (ARG) are coated with special sizings to provide superior alkali resistance and are designed to reinforce cementitious and other alkali matrixes. The assessment of changes in the fiber surface nanomechanical properties is essential for understanding the fiber bulk mechanical fracture behavior. Here we present examples demonstrating sensitive surface stiffness and dissipated energy with regard to the depth profile at the nanoscale, using atomic force microscopy with nanoindentation. A nondimensional energy index is proposed to estimate inhomogeneous surface properties and account for the substrate effect. Additionally, the variations in surface chemistry were evaluated by Fourier transform infrared attenuated total reflection spectroscopy and thermal gravimetric analysis. Among three types of aqueous environments evaluated, an alkaline solution is the most aggressive to the fiber surface. Subsequently, we describe the effect of surface property variability on the fiber tensile performance using a modified bimodal Weibull statistical distribution analysis. A new effective surface thickness factor κ(d) is given to reveal the influences of various surface treatments on the extrinsic failure. The sizing is shown to significantly affect both the population and size of flaws on the fiber surface by healing effects. Finally, we correlate the tensile strength and surface roughness with Griffith fracture predictions. The maximum height roughness of ARG fibers follows very closely the line predicted by the Griffith fracture criterion. It highlights the importance of the sizing to the environmental resistance of the alkali-resistant glass fibers.

Introduction The multifunctional sizings on the glass fiber surface play an important role in the manufacture and performance of the fibers such as surface protection, abrasion resistance, strength maintenance, and interphase formation of their composites.1,2 The alkali-resistant glass (ARG) fiber was designed to reinforce cementitious and other alkaline matrixes which have been widely used in construction and civil engineering with a great deal of success since the late 1960s.3 The alkali resistance of ARG fibers is enhanced mainly by a high percentage of zirconia (ZrO2 > 15 wt %) content in the glass.4 There has been concern, however, that the tensile strength and the impact strength of glass-fiber-reinforced cement products decrease with age due to possible corrosive reactions between the fiber surface and the matrix, even though the fibers are alkali resistant.5,6 The ultimate concern of the practicing civil engineer generally focuses on the impact of the degradation mechanisms on parameters of interest for design. Surface sizing technology has made significant progress recently with respect to solving problems related to fiber wetting, adhesion, and aging. To improve the long* To whom correspondence should be addressed. E-mail: [email protected]. Phone: 49-351-4658-305. Fax: 49-351-4658284. † Institute of Polymer Research Dresden. ‡ Dresden University of Technology. § This work is dedicated to Prof. Dr. K. Lunkwitz on his 65th birthday. (1) Ma¨der, E.; Mai, K.; Pisanova, E. Compos. Interfaces 2000, 7, 133. (2) Zinck, P.; Ma¨der, E.; Gerard, J. F. J. Mater. Sci. 2001, 36, 1. (3) Majumdar, A. J.; Ryder, J. F. Glass Technol. 1968, 9, 78. (4) Loewenstein, K. L. In Manufacturing technology of continuous glass fibers; Elsevier Science: Amsterdam, 1993. (5) Majumdar, A. J.; Nurse, R. W. Mater. Sci. Eng. 1974, 15, 107. (6) Al-Dulaijan, S. U. Ph.D. Thesis, The Pennsylvania State University, University Park, PA, 1996; p 319.

term performance of glass-fiber-reinforced cement products, it is thus a crucial point to examine how the sizings modify the ARG fiber surface properties and protect the fiber from environmental influences such as alkali, moisture, and temperature. Furthermore, how the sizings interact with the surrounding matrix is very important for the performance of the composites. The thin sizing mixture, which is typically a few tens of nanometers in average thickness, basically consists of components of a organo-silane coupling agent, a polymer film former, and a lubricant, leading to variable properties and great difficulty in both process control and in situ characterization. The mechanical properties of sizing must be adequate to guarantee mechanical stability and integrity for their period of use. However, there is a lack of information regarding the mechanical properties and morphology of the sizing layer obtained from industrial processing, especially for its Young’s modulus, hardness, plastic behavior, interfacial adhesion, and fracture mechanics. Moreover, little has been solidly established about the exact corrosion mechanisms of the thin sizings and/or fiber surface layer in various environmental conditions. The technical topics7 rated as having the highest need for attention are as follows: (i) what are the nanoscale fiber surface features and glass/sizing interactions; (ii) how sizing affects the origin of defects leading to fiber strength reduction; (iii) the necessity of developing techniques to measure and control sizing properties locally and reliably. One of the major problems with glass fibers is that the measured tensile strength is significantly lower than its theoretical value. This is evidently due to preexisting defects which show multiple populations with varying gauge length.8 The surface flaws providing extra stress at (7) Allendorf, M. D. Thin Solid Films 2001, 392, 155. (8) Schmitz, G. K.; Metcalfe, A. G. Mater. Res. Stand. 1967, 7, 146.

10.1021/la020778t CCC: $25.00 © 2003 American Chemical Society Published on Web 02/19/2003

Sizings on Alkali-Resistant Glass Fibers

the tip of the cracks can lead to stress-corrosion cracking at a very low stress level. Surface sizing treatment is believed to protect the fiber surface efficiently against moisture attack and improve its mechanical properties through a three-dimensional graded network on the glass surface.9 This healing effect was viewed as a disappearance of the severe surface flaws because of an increase of the crack tip radius, the flaw filled by sizing being elliptical rather than sharp.2 Gomez et al.10 reported an effort to characterize the influence of the chemical structure of coupling agents on the glass fiber tensile strength. An increase in the functionality of the alkoxysilane results in an increase in the tensile strength and better protection of the glass surface from the most severe surface flaws, which is attributed to better bonding to the glass surface and facilitation of the siloxane network formation. Recently, statistics studies were used as an indirect observation technique of the effect of a sizing layer on fiber tensile properties.11,12 E-glass fiber strength was found to be well described by either a single two-parameter Weibull cumulative distribution function (CDF) for an elastomer sizing or a bimodal Weibull CDF for water-based and silane-based sizing systems. However, it was suggested that the sizings influence neither the internal flaws directly nor the amount of surface defects. Without consideration of the sizing properties, a few early studies have been devoted to the influences of the environment on the glass fiber strength. Proctor et al.13 evaluated the kinetics of glass fiber strength reduction in cementitious environments and validated an Arrhenius type model for glass strength as a function of time and temperature. To predict the long-term behavior of ARG fibers in concrete in a short term, accelerated aging methods that entail raising alkalinity, temperature, and/ or chloride content of the test environment are typically employed. Litherland et al.14 demonstrated and modeled the accelerated strength degradation of glass fibers in moist cementitious materials using water immersion at an elevated temperature. Clearly, the diversity and complexity of mechanisms that affect the durability of ARG fibers in concrete are associated with surface sizing. However, both the nanoscale fiber surface features and their effect on fiber strength retention have remained unclear, particularly in a corrosive environment involving moisture, alkaline solutions, and acidic solutions. This is primarily due to the experimental difficulties involved and the lack of appropriate techniques required for accurate measurements of the surface properties. Progress has recently been made in this area, and several novel techniques have been devised to study the fiber surface properties with varying degrees of success. They include secondary neutral mass spectrometry (SNMS), X-ray photoelectron spectroscopy (XPS), the surface acoustic wave technique (SAW), atomic force microscopy (AFM), and so forth.15,16 Although not concerned directly with fiber surface sizing, Frischat and coworkers15 used SNMS to study the in-depth concentration profiles of the glass fiber surface layer subjected to an (9) Eckstein, Y. J. Adhes. Sci. Technol. 1988, 2, 339. (10) Gomez, J. A.; Kilgour, J. A. In Technical Note; Osi Specialties, Inc.: Greenwich, CT, 1993. (11) Ahlstrom, C.; Gerard, J. F. Polym. Compos. 1995, 12, 305. (12) Zinck, P.; Pays, M. F.; Rezakhanlou, R.; Gerard, J. F. J. Mater. Sci. 1999, 34, 2121. (13) Proctor, B. A.; Oakley, D. R.; Litherland, K. L. Composites 1982, 13, 173. (14) Litherland, K. L.; Oakley, D. R.; Proctor, B. A. Cem. Concr. Res. 1981, 11, 455. (15) Frischat, G. H.; Priller, S. Mikrochim. Acta 2000, 133, 23. (16) Flannery, C. M.; Murray, C.; Streiter, I.; Schulz, S. E. Thin Solid Films 2001, 388, 1.

Langmuir, Vol. 19, No. 6, 2003 2497

attack in strongly basic KOH solutions. For short times, leach analysis displayed a relatively steep increase in the amount of leached Na. The modified fiber was less attacked and showed a strong Na2O depletion near the surface. A close investigation of the fiber surface properties is needed to understand the bulk mechanical performance of fibers under aggressive environmental conditions. In addition to its conventional topographical imaging, recent developments have led to the utilization of the atomic force microscope as a nanoindentation device.17,18 It is an effective and relatively straightforward method to assess the local properties of complex material systems on the nanometer scale by conducting simultaneous measurements of the indentation normal force and depth with an extremely small diamond tip. For sizing and thin films, nanoindentation is often the only method available to infer their Young’s modulus, stiffness, hardness, and yield stress.19,20 Previous attempts at quantitative descriptions of the coating systems, that is, hardness, involved various law-of-mixture models,21-23 which are very difficult to apply to the fiber surface since the real sizing thickness and contact area remain undetermined on the nanometer scale. To the best of our knowledge, to date experimental information obtained with the use of AFM nanoindentation for fiber surfaces has been rarely reported in the literature. For these reasons, we measured directly the elastic and plastic nature of the fiber surface layer associated with environmental variables. It is also necessary and interesting to compare the energy-absorbing capacities of the fiber surface under indentation. The overall objective of this paper is to quantify both the micro-level surface properties and bulk single-fiber tensile performance after aggressive environmental conditions. Two kinds of ARG fibers with different percentages of zirconia, namely, NEG and CemFIL, are subjected to high-temperature pyrolysis and aqueous solutions with different pH values, temperatures, and times. A combination of nanoindentation, AFM, Fourier transform infrared attenuated total reflection (FTIR-ATR) spectroscopy, and thermal gravimetric analysis (TGA) is used in order to provide additional useful structural information on the ARG fiber surface. Both the elastic and the inelastic behavior of the fiber surface are examined. The novel aspect of the present work is to correlate results from the fiber surface nanoindentation with results from the singlefiber tensile tests along with a bimodal Weibull statistical analysis. Special emphasis is placed on the estimation of surface property degradation by two nondimensional parameters (energy index and effective surface thickness factor) and Griffith fracture predictions correlated with surface roughness variability. Experimental Section ARG Fibers and Treatments. The control samples of the NEG and CemFIL ARG fibers have been supplied by Nippon Electric Glass Co., Ltd., and CemFIL International, Vetrotex Espana S A, respectively. The major constituents determined by X-ray fluorescence spectroscopy and other general properties are given in Table 1. The sizings can be partially and even totally dissolved in most organic solvents and start to decompose at 200 °C. The most commonly selected temperature for accelerated (17) Sheiko, S. S. Adv. Polym. Sci. 1999, 151, 61. (18) VanLandingham, M. R.; Dagastine, R. R.; Eduljee, R. F.; McCullough, R. L.; Gillespie, J. W., Jr. Composites, Part A 1999, 30, 75. (19) van der Varst, P. G. Th.; de With, G. Thin Solid Films 2001, 384, 85. (20) Taylor, J. A. J. Vac. Sci. Technol. 1991, A9, 2464. (21) Jonsson, B.; Hogmark, S. Thin Solid Films 1984, 114, 257. (22) Burnett, P. J.; Rickerby, D. S. Thin Solid Films 1987, 148, 41. (23) Bull, S. J.; Rickerby, D. S. Surf. Coat. Technol. 1990, 42, 149.

2498

Langmuir, Vol. 19, No. 6, 2003

Gao et al.

Table 1. Main Constituents and General Properties of the Control ARG Fibers According to Experimental Results constituent [wt %]

property

SiO2 Al2O3 ZrO2 Na2O K2O TiO2 filament diameter [µm] yarn fineness [tex] industrial strand tensile strength [GPa] elastic modulus [GPa] strain at break [%] density [g cm-3] sizing content [%]

NEG

CemFIL

58.3 0.2 21.2 13.0 2.8 2.8 13 620 1.4

60.6 0.2 18.1 14.1

74 2 2.7 0.5

72 2.4 2.6 1.4

0.1 14 2400 1.7

testing of reinforcement fibers for infrastructure applications is lower than 100 °C. In this investigation, 100 °C was chosen for acceleration and determination of the sizing weight reduction. The environmental conditioning regime entails exposure of fibers to the following treatments: (I) extraction in distilled water at 23 °C for (a) 28, (b) 90, and (c) 180 days; (II) extraction in a mixture of 0.84 wt % sodium hydroxide (NaOH), 3.29 wt % potassium hydroxide (KOH), and 0.46 wt % calcium hydroxide (Ca(OH)2) aqueous solution, which has a pH of 12.5, at 23 °C for (a) 28, (b) 90, and (c) 180 days; (III) extraction in hexane (CH3(CH2)4CH3) and ethanol (CH3CH2OH) for 5 h, respectively; (IV) extraction in distilled water at 100 °C for 3 h; (V) extraction in 5 wt % NaOH aqueous solution at 23 °C for (a) 90 and (b) 180 days; (VI) pyrolysis at temperatures of 600 °C for 1 h to determine the sizing content. After conditioning, the fibers were allowed to dry out in an ambient environment (50% relative humidity (RH), 25 °C) for at least 2 days before they were prepared as test specimens. Fiber Surface Characterization. To gain insight into what mechanisms are responsible for tensile strength variation after exposure to various environmental attacks, investigation of the surface morphology, chemical composition, and mechanical elastic/plastic property of the ARG fiber was conducted. An atomic force microscope (a Digital Instruments D3100, Digital Instruments, Santa Barbara, CA) was used as both a surface imaging tool and a nanoindentation device to evaluate the fiber samples. The topography of samples was studied in tapping mode. A piezo stack excites a silicon cantilever’s substrate vertically, causing the tip to bounce up and down at its resonant frequency of 162 kHz with a drive amplitude of 200 mV. The cantilever (Ultrasharp NSC16/50, MikroMasch, Estonia) has a normal spring constant of 35 N/m, a tip cone angle of 20°, a radius of 5-10 nm, and a modulus of 160 GPa to ensure good imaging resolution and nanometer scale indents. Specimens were prepared by fixing separate short fiber filaments on a steel plate, with a thin layer of precoated epoxy at the bottom side of the fiber. For all samples, several images were recorded at different locations to verify the reproducibility of the observed features. Within the box cursor (1 µm2), the image mean roughness (Ra) is the arithmetic average of the absolute values of the surface height deviations measured from the mean plane and the maximum height roughness (Rmax) is the difference in height between the highest and lowest points on the cross-sectional profile relative to the center line over the length of the surface profile, which are calculated based upon ASME B46.1 (Surface Roughness, Waviness and Lay) available from the American Society of Mechanical Engineers. The mechanical properties of a thin layer on the fiber surface were determined by the nanoindentation, which allows a precise continuous measurement of the load F and penetration depth h, with a displacement resolution of 0.16 nm. Located by the AFM imaging, the tip was forced perpendicularly into the fiber surface along the fiber diameter line at a frequency of 1 Hz and to a selected maximum force range from 0.03 to 2 µN. To estimate the substrate and tip geometry effect, indentation was also performed comparatively on films of low-density polyethylene (LDPE), polyamide 6 (PA) (Bayer AG), and poly(phenylene

sulfide) (PPS) (Union Carbide) with approximate thicknesses of 100 µm. The normal indent load was determined by multiplying the cantilever spring constant by the cantilever deflection sensitivity (the ratio of the change in photodiode voltage to nanometers of tip deflection). The cantilever deflection was calibrated using an “infinitely stiff” sapphire for which vertical piezo motion is directly translated into vertical tip deflection. FTIR-ATR spectroscopic measurements were performed to monitor the chemical changes in the fiber surfaces using a Bruker spectrometer IFS 66v/s with a “Golden Gate” ATR unit (Ge crystal, penetration depth of 166-1106 nm at 4000-600 cm-1). In addition, the TGA (Perkin-Elmer) technique was used to study the surface sizing’s thermal stability and content. The TGA was calibrated for mass and temperature before use. A 5 mg sample of the fibers was spread out on a Pt pan, and a nonisothermal test was carried out at a heating rate of 10 K min-1 and temperatures between 100 and 620 °C under a nitrogen atmosphere. Fiber Tensile Tests. The single-fiber tensile measurement was conducted under 50% RH and ambient conditions using the Fafegraph ME testing device (Fa. Textechno) equipped with a 10 N force cell. The test cross velocity is 0.5 mm/min, and the gauge length is 20 mm in accordance with EN ISO 5079. On the basis of a vibration approach, the fineness of each selected fiber was determined by using a Vibromat ME (Fa. Textechno) in accordance with EN ISO 53812 and ASTM D 1577. The tensile strength was computed as the quotient of the fracture load divided by the actual cross section of each fiber, and each point represents an average of 50 individual measurements. The tensile modulus was evaluated at an elongation between 0.5 and 1%, although the force-displacement curves reveal a Hookean behavior. Special care was taken during testing to avoid producing additional flaws.

Results and Discussion Surface Properties for ARG Fibers. We first investigated the fiber surface elastic/plastic properties together with surface morphological and chemical characteristics. During the nanoindentation on the fiber surface, various phenomena were observed by means of the typical indentation force-displacement curves, as shown in Figure 1 along with the corresponding AFM topographical images. Specifically, a nonuniform sizing layer, produced during the coating process, is found on the control fiber surface with the highest surface roughness values (Ra ∼ 25 nm, Rmax ∼ 300 nm) among all the systems studied. We observed relatively neat and smooth surfaces (Ra ∼ 4 nm, Rmax ∼ 130 nm) on NEG fibers upon treatments with either chemical etching by mixed NaOH/KOH/Ca(OH)2 aqueous solution (condition II) or thermal pyrolysis (condition VI). These data indicate that the surface sizing was almost fully removed. However, it is unclear whether the polymer layer plays an important role in preventing the ingress of aggressive solutions to the bulk of the glass fiber. In the case of the fiber etched by NaOH aqueous solution (condition V), interestingly, a close look revealed a microrough porous surface morphology with intermediate roughness (Ra ∼ 14 nm, Rmax ∼ 170 nm) between the above cases. It seems that not only is most of the sizing removed but also the topmost layer of glass is chemically corroded, which is discussed further in the following. Our nanoindentation tests at a range of indentation forces were made to provide an indication of the variability of the local stiffness/energy characteristics. As shown in Figure 1, a typical variety of indentation force-penetration depth responses was measured at a constant maximum force of 120 nN. The loading curve is a result of the tip being compressively penetrated into the specimen, inducing both elastic and plastic strains, while the unloading curve results from the tip being elastically recovered and pulled out from the sample surface. For the control NEG fiber, the significantly different loading-unloading path


Langmuir, Vol. 19, No. 6, 2003 2499

Figure 1. AFM topography images and typical indentation force-penetration depth curves defining key experimental quantities for the control and aggressive environment treated NEG fibers. The unloading stiffness, S, can be calculated using the slope of the initial part of the unloading force curve.

reveals the dominant plastic deformation arising from the polymer sizing layer. In sharp contrast, both the mixed NaOH/KOH/Ca(OH)2 aqueous solution (condition II) and the pyrolysis (condition VI) specimens exhibited almost identical loading-unloading paths, suggesting the tipsample interaction is dominated by significant elastic recovery rather than inelastic deformation, which is clearly consistent with the above imaging observations of the bare glass surfaces. Again, the fiber etched by NaOH aqueous solution shows intermediate behavior between these cases, in which there is a modest plastic deformation. We assume that the chemical corrosion causes a reduction of the intrinsic glass surface properties. This is also supported by the above imaging observation of the microrough porous surface without sizing. To assess the elastic deformation in detail, the unloading stiffness, S, at a wide range of penetration depths for differently treated glass fiber surfaces in comparison with pure polymer films is presented in Figure 2. It can be observed that the unloading stiffness greatly depends on the penetration depth, h, reflecting its complicated dependence on the geometry of the tip, specimen and substrate elastic deformation. Based on Sneddon’s model,24 the relationship between the stiffness and Young’s modulus for a parabolic shape AFM tip with radius R gives

S)

dF 2ExRh ) dh 1-ν2

(1)

s

where E and νs are the modulus and Poisson’s ratio of the specimen, respectively. The contribution from the inden-

tation tip is neglected because the modulus Eindentor ∼ 160 GPa . Epolymer ∼ 3 GPa. By considering a semi-infinite sample geometry for the polymer films (thickness . indentation depth), a Poisson’s ratio of ∼0.42, and the tip radius value of ∼10 nm, one can obtain a crude estimation of the modulus values for comparison purposes. According to the fit lines of the experimental data (Figure 2a), the calculated results are 0.5, 3.5, and 5.1 GPa for LDPE, PA, and PPS, respectively, which are reasonably comparable to their tensile moduli, that is, ∼0.3, 3.2, and 4 GPa, respectively. However, the estimate from the above equation can result in a substantial problem in extracting moduli of inhomogeneous materials; in this study we choose to investigate stiffness-depth profiles on the fiber surface. In a comparison of the apparent stiffness profiles of the fiber surface as shown in parts b and c of Figure 2, a few observations can be summarized. First, both NEG and CemFIL fibers show overall similar stiffness curve shift behavior after environmental attack; specifically, the stiffness values increase at a fixed depth in the following order: pyrolysis (condition VI) g the mixed NaOH/KOH/ Ca(OH)2 (condition II) > NaOH (condition V) > control ARG. Second, fibers after both condition II and VI treatments show quite similar curves with a sharp increase of stiffness as a function of penetration depth. In addition, their stiffness values are much higher than those of the control and the condition V specimen at a very small penetration depth. This is clearly attributed to the sizing layer being almost fully removed and to the intrinsic (24) Sneddon, I. N. Int. J. Eng. Sci. 1965, 3, 47.

2500


Gao et al.

Figure 3. Dissipated energy versus indentation force for the surface on NEG fibers.

To reveal the extent of inelastic deformation in detail, we used a simple energy approach to estimate inhomogeneous surface properties (see Figure 3). By use of depth sensing indentation, the work of indentation can conveniently be calculated from the loading/unloading curve as illustrated in Figure 1. The area under the loading curve gives the total work, w, done by the indenter during indentation:

w ) wp + we

Figure 2. Variation of contact stiffness as a function of penetration depth on the surfaces of (a) polymer films, (b) NEG fibers, and (c) CemFIL fibers.

properties of the glass surface, possibly being free of corrosion, which is further confirmed by the FTIR-ATR results shown in the following section. Third, there is a translation occurring on the curve of the control specimen giving evidence of the substrate effect. This can be understood by noting that the average sizing thickness values of NEG and CemFIL fibers, according to their sizing contents of 0.5 and 1.4 wt % (Table 1), are about 30 and 100 nm, respectively. Thus, at a low indentation depth of less than 20 nm, the stiffness data are in the same range as those of the aforementioned pure thick polymer film, indicating the polymer nature of the sizing. When the indentation depth is higher than a specific value (say 20 nm for the NEG fiber), the steep increase of the stiffness reflects the dominant substrate glass elastic recovery. With an increase of sizing thickness for the CemFIL fiber, it should not surprise that the stiffness curve might become a gradual translation as shown in Figure 2c. Finally, NaOH-treated fibers have comparable and even lower surface stiffness than that of the control, for which a large amount of scatter appears at a small indentation depth, further emphasizing a heterogeneous and porous surface structure. It is believed that the constituents of SiO2, ZrO2, TiO2, K2O, and so forth on the glass surface enable different resistance to corrosion by alkali solutions.

(2)

where the dissipated energy, wp, related to the energy creating damage in the form of plastic/viscous deformation, surface energy, heat, sizing/fiber interface debonding, and so forth can be deduced from the area between loading and unloading curves. To study these inelastic mechanisms in detail is actually a challenging task. The rebounded elastic energy, we, which is stored elastically in the specimen and transferred back to the indenter reversibly, is obtained from the area under the unloading curve. Figure 3 displays the variations in dissipated energy versus indentation force. As expected, the energies for all samples increase as the force increases, identified as indicative of additional damage. It can be clearly seen that the “desized” specimen (conditions II and VI) has consumed very small quantities of energy because of the dominated elastic nature of glass, whereas the sized one (control) has an overall significantly higher value. Specifically, the value of the sized sample increases sharply by a few orders of magnitude at the initial stage, reflecting the translation from elastic to plastic responses, where it is relatively constant with change in force up to about 500 nN, that is, the indentation depth close to or higher than the thickness of the sizing. The values of the NaOH-treated sample vary by a few orders of magnitude, and such a significant local inelastic property difference provides additional strong evidence for the heterogeneous components on the fiber surface. By comprehensively considering both elastic and inelastic behavior, we propose a nondimensional energy index parameter (EI), determined by the ratio between the dissipated work and the reversible elastic work. Physically, EI can be interpreted as indicative of the extent of damage, and its value, ranging from zero for no damage (elastic-perfect interaction) to infinity for extensive damage (plastic-perfect indentation), can therefore be applied to “sense” the relative variation between elastic and inelastic behavior of the material. Figure 4 shows a


Langmuir, Vol. 19, No. 6, 2003 2501

Figure 4. Energy index versus indentation force for the surface on NEG fibers.

typical plot of the EI as a function of force. Interestingly, it is observed that all data of the desized samples from either thermal pyrolysis or aqueous extraction (condition II) are quite small and overlapped, which points directly toward the occurrence of less extensive surface damage and their structural similarity, respectively. In other words, it implies no detectable degradation in the glass surface upon the mixed NaOH/KOH/Ca(OH)2 aqueous extraction. Within the load range, the limited extent of increase in EI is associated with a marginal increase in indentation depth. On the other hand, the overall data from the control specimen are considerably high, indicating that the sizing layer plays a significant role of inelastic energy absorption capacity. Most notable is that its EI values are strongly dependent on the applied load and the penetration depth. We observed that after an increase of the EI with increasing load to a certain level, the EI starts to decrease and eventually tends to the same values as those of desized surfaces. This variation reveals that the inelastic energy capacity is reduced more and more due to the constraint by the substrate glass when the yielded zone under the tip expands toward the fiber surface at increasing load. Subsequently, the substrate glass elastic deformation is dominated and the relative contribution from sizings can be negative, resulting in an apparent reduction of EI. This approach, based on analyzing the complete set of experimental data on energy ratio under different loads, overcoming the complexities of involved fracture detail, offers a method to describe the transition between the sizing-dominated response and the substrate (fiber) dominated response. However, the dissipated energy very much depends on the dynamic response of the system, that is, the time-dependent properties of the sample, indentation speed, and tip geometrical configurations. Potential work along this line will provide information about surface viscoelasticity, chain relaxation, and diffusion to establish connections between nanoscale surface structure and time-dependent properties. In addition, we tested the chemical variations in the surface layer using FTIR-ATR with a detecting depth of only a few microns, which is suited to study the outermost layer of the fiber consisting of both components from the sizing and the bare glass. As shown in Figure 5 along with the frequencies, the position and the shape of the characteristic absorption bands do change significantly after treatments. Here, no attempt is made to discuss the full sizing package, since the sizings are multicomponent (patented) mixtures in which polymer film former, silane

Figure 5. ATR spectra of the carbonyl and the C-H stretching at selected (a) low-frequency and (b) high-frequency regions for the surface on NEG fibers.

coupling agent, surfactant, and lubricant are mainly included. The control fiber with the sizing layer displays clear bands. It is generally known that the ν(CdO) stretch of the functional group typically lies around 1720 cm-1 while the 1510 cm-1 peak is assigned to the parasubstituted benzene rings found in the commercially available sizing film former backbone.25 The absorption peaks in the range of 3050-2850 cm-1 correspond to the C-H bonds. Very broad bands in the carboxylate region at ∼1568 cm-1 are probably due to a variety of carbonate species on zirconia.26,27 In the case of the pyrolyzed specimen, the above characteristic bands are fully absent, suggesting that the organic layer was completely removed. This directly correlates with the TGA analysis (Figure 6) indicating that extensive chemical decomposition (weight loss) of the sizing polymers on both NEG and CemFIL fibers occurs at about 350-400 °C during pyrolysis. However, a different situation was observed for the surface upon extracting with NaOH solution. Two ν(C-H) bands arise at 2851 and 2919 cm-1, but most other bands disappear except for the aforementioned broad bands at 1568 cm-1 with a little decrease of the intensity. The chemical corrosion in both polymer sizing and glass surface is primarily responsible for the differences. They may also be partly due to zirconia gain on the fiber surface free of (25) Gorowara, R. L.; Kosik, W. E.; McKnight, S. H.; McCullough, R. L. Composites, Part A 2001, 32, 323. (26) Pawsey, S.; Yach, K.; Reven, L. Langmuir 2002, 18, 5205. (27) Bachiller-Baeza, B.; Rodriguez-Ramos, L.; Guerrero-Ruiz, A. Langmuir 1998, 14, 3556.

2502


Gao et al.

Figure 6. TGA plot of the control NEG and CemFIL fibers in nitrogen.

corrosion. Overall, the surface chemical structure information is strongly consistent with the morphology and micromechanical characterization. Tensile Properties for ARG Fibers. The influence of surface properties on the tensile characteristics of both NEG and CemFIL glass fibers was investigated. The effects of various types of environmental attacks (water, mixed NaOH/KOH/Ca(OH)2, hexane/ethanol, NaOH, and pyrolysis) on the average strength and modulus are compared roughly in Figure 7. The first observation to be made is that the NEG fibers (Figure 7a) exhibit a very similar response to the CemFIL fibers’ (Figure 7b) behavior, only shifted toward slightly higher values in both strength and modulus. This shift may be attributed to different constituent materials of the fibers as shown in Table 1 and also surface sizing natures, which will be further confirmed by the more detailed Weibull statistical analysis. For both kinds of fibers, the Young’s modulus exhibits a very similar value except the case of pyrolysis (condition VI), reflecting that the elastic modulus of the fiber is insensitive to various surface treatments, that is, it is dominated by the bulk material property. With regard to the failure load, it is evident that both water (condition I) and the mixed NaOH/KOH/Ca(OH)2 aqueous solution (condition II) would not yield an evident influence on the tensile strength regardless of the soaking time. Considerable strength reduction upon other treatments occurred, which is in the following order: control > hexane/ethanol (condition III) > hot water (condition IV) > NaOH (condition V) > pyrolysis (condition VI). The pyrolysis treatment results in the smallest strength but the highest modulus among all of the experimental conditions reported here. We attribute this mechanical performance to the high pyrolysis temperature which results not only in degradation of surface coatings but also in a variation of bulk microstructure and thermal residual stresses of the fiber. We make three additional observations regarding Figure 7. First, it reveals that in general an increase of soaking time resulted in a slight further reduction in the strength. Second, a significant trend in the case of water treatments (conditions I and IV) is that the strength values are influenced only by temperature rather than by the soaking time. We attribute the strength reduction in boiling water to a “stress-enhanced” reaction of water with the glass lattice at a crack tip, which results in a lengthening of the crack. However, this reduction does not occur at ambient temperature, perhaps due to the surface sizing preventing the water from direct contact with glass. Third, as aforementioned, the mixed NaOH/

Figure 7. Effect of the surface treatment on the average tensile strength and modulus with standard deviation of (a) NEG and (b) CemFIL fibers at a gauge length of 20 mm.

KOH/Ca(OH)2 (condition II) which was used to simulate the cement environment has marginal adverse and almost no measurable effects to the strength of NEG and CemFIL fibers, respectively. Interestingly, however, there are remarkable different effects between the mixed NaOH/ KOH/Ca(OH)2 and the NaOH (condition V) aqueous solutions, although the solutions have a similar concentration of NaOH and pH value. This result is essentially consistent with the surface mechanical properties as mentioned previously in this paper. One possible other explanation for this behavior is that the big alkali ions in the mixed NaOH/KOH/Ca(OH)2 solution may diffuse into the glass surface, replacing smaller ions, which produces surface compression causing the increase of the ultimate tensile stress required for crack propagation. Subsequently, the strength improvement of the surface-ionexchange process could compensate partly for the reduction of glass strength due to removal of the surface sizing. Behind the aforementioned diverse and complex phenomena, the data would suggest that different surface corrosion mechanisms exist which are associated with surface sizing and in turn strongly affect both fiber surface and bulk mechanical properties. Weibull Statistical Distributions of ARG Fiber Strength. To verify the effect of surface properties on the statistical distribution of fiber tensile strength, the failure


Langmuir, Vol. 19, No. 6, 2003 2503

probabilities were fitted through the least-squares method. Basically, the well-known single two-parameter Weibull model28 assumes that the strength of brittle materials is controlled by homogeneously distributed flaws with a single population. In reality, this is inappropriate since the classification of fracture can be based on two modes: (i) intrinsic failure characterized by fiber internal defects and surface defects which are not influenced by either the sizing or surface treatments; (ii) extrinsic failure characterized by surface flaws which are strongly surface property dependent. Therefore, a statistical bimodal for fitting competing distributions is applied. The cumulative probabilities of failure for the single and bimodal Weibull distribution functions are given by the following analytical formulas,28 respectively:

[()] σ s0

m0

single modal:

P ) 1 - exp -

bimodal:

P ) fiPi + (1 - fi)Pe

(3) (4)

where P is the cumulative probability of failure (i/(n + 1)) at the tensile stress σ. The parameters m0 and s0 are the Weibull shape factor (or modulus, slope of the distribution) and the scale factor (or characteristic strength) of fractured fibers, respectively. According to the range of our experimental results, the Weibull shape factor is representative of the separation, that is, the homogeneity of the defects, whereas the scale factor is related to the severity of the distribution. The fraction of intrinsic failure is ×a6i, and its probability density is described by the following equation:

[()] σ si

Pi ) 1 - exp -

mi

(5)

To reveal the extrinsic failure of a fiber with fixed length, we can assume that the probability of some surface layer with thickness d0, where the biggest or so-called strengthcontrolling surface defects are located, failing at stress σ is Re(σ):

Re(σ) )

() σ se

me

(6)

Figure 8. Comparison of Weibull plots of fracture probability as a function of tensile strength for (a) NEG and (b) CemFIL fibers.

Then the probability of a layer with thickness ∆d failing at a given stress is R(σ) ∆d/d0. In other words, the probability of ∆d surviving is 1 - R(σ) ∆d/d0. Thus, we write the probability that a layer of thickness d survives at this stress as Pe(d):

[

Pe(d + ∆d) ) Pe(d) 1 -

]

R(σ) ∆d d0

(7)

Let the thickness increment become infinitesimal and κ(d) ) ∆d/d0; then

[ ()]

Pe ) 1 - exp -κ(d)

σ se

me

(8)

where κ(d) is a coefficient dependent on the effective surface thickness. In the above description, it is assumed that (i) no interaction occurs between intrinsic and extrinsic failures and (ii) the volume of fiber containing the internal defects is not influenced by surface treatments because of the small thickness variation at the surface layer in comparison with the diameter of the fiber. (28) Weibull, W. J. Appl. Mech. 1951, 18, 293.

Figure 9. Comparison of Weibull plots of fracture probability as a function of fiber tensile strength after pyrolysis (condition VI) treatments.

A detailed description of the experimental data by both the single two-parameter Weibull CDF and the bimodal Weibull CDF is shown in Figures 8 and 9. From the single Weibull best-fit lines, the boiling water and NaOH surface treatments result in apparent lower scale but higher shape

2504


Gao et al.

Table 2. Values of the Shape (mi, me), Scale (si, se), Mixing (fi), and Effective Surface Thickness (K(d)) Parameters of the Bimodal Weibull Two-Parameter Cumulative Distribution Function of ARG Fibers treatment condition

mi

control I (a) II (c) III IV V (a) V (b) VI

15.99 6.26 6.26 20.58 2.57 8.83 6.77 15.99

control I (a) II (c) III IV V (a) V (b) VI

6.30 3.46 6.08 14.57 30.94 30.48

si

me

se

κ(d)

fi

NEG Fiber 1.42 9.18 1.26 11.16 1.36 8.36 1.56 5.93 1.35 14.29 1.40 35.38 1.36 34.77 0.41 6.13

2.80 2.96 2.42 2.31 2.12 1.68 1.48 0.59

1.06 0.99 0.88 1.39 1.41 1.84 1.81 0.96

0.03 0.07 0.10 0.03 0.07 0.40 0.44 0.19

CemFIL Fiber 1.89 13.54 1.81 7.64 1.85 9.38 1.80 6.84 1.76 11.70 17.61 12.70 0.38 4.76

2.51 2.64 2.45 2.27 2.07 1.33 1.29 0.48

1.05 0.92 1.00 1.48 1.74 1.60 1.31 1.07

0.26 0.19 0.39 0.26 0.19 0 0 0.06

parameters than that of the control, indicating more severe flaws but a lower heterogeneity. This suggests that the treatments provide additional surface flaws with similar severity and therefore flaw homogeneity relative to the control fibers, since the nonhomogeneously distributed sizing on the control fiber could lead to surface flaws being partly healed, resulting in different flaw severities. The fact that the plots show different slopes indicates that the failure is governed by different types of flaws. As expected, the bimodal curves are in general more suitable for the description of the nonlinear probability plots than the single Weibull lines. A summary of the bimodal Weibull results is given in Table 2. It clearly demonstrates that two populations of defects are present on both NEG and CemFIL fibers for most treatment conditions. Several additional observations can be made. First, the intrinsic Weibull scale parameter si remains almost constant and is not influenced by surface treatment with the exception of the high-temperature pyrolysis. However, the extrinsic parameter se has an apparent correlation with surface treatment. Second, the proportion of the extrinsic failure (1 - fi) attributed to severe surface flaws is higher than 70% in most cases. It suggests that the fiber failure strength is strongly surface flaw dependent, in particular for the NEG fibers. In comparison with NEG fibers, the two main differences for CemFIL fibers are both the intrinsic failure distribution and its proportion, which are shifted to higher values. In fact, the variation of proportion value is controlled by the difference between the se and si parameters. It appears that the se values are in general higher than the si values for most conditions studied in this work, indicating that the defects related to intrinsic type failure are more severe than those of extrinsic failure. In the extreme case of the NaOH-treated CemFIL fibers, its se value (∼1.3) is smaller than the si value (assumed to be same as those of other surface treatments for CemFIL fibers, ∼1.8). Its proportion of extrinsic failure (1 - fi) approaches 100%, implying that the fit profile is approximately independent of the intrinsic parameters (ignored in Table 2), which in turn suggests the dominant unimodal extrinsic type fracture. Overall, only two unimodal distributions can be found (Figures 8b and 9), in which the Weibull plots are almost linear and the two plots are nearly parallel to each other. Either the dominated internal defect severity because of the high temperature (se ≈ si values of pyrolysis < si values of other

treatments) or the aforementioned extensive surface flaw severity due to alkali corrosion (se < si) is primarily responsible for the unimodal distribution. Finally and interestingly, the factor κ(d) has values from around 1 to 1.8 at most. When κ(d) is around 1, the surface treatments, for instance, fiber extraction in water (condition I) and mixed NaOH/KOH/Ca(OH)2 aqueous solution (condition II), have little influence on the mechanical tensile properties of the fibers. In contrast, an increasing κ(d) value upon other extractions is associated with the large drop in tensile strength and the reduction of the se value. Behind this phenomenon, it seems that κ(d) is sensitive to variation of surface properties although it represents the effect related to effective surface thickness deduced from statistical analysis, which correlated with the above nanoindentation results and will be further discussed in the next section. Fracture Mechanical Analysis of ARG Fibers. Our previous fractographic observations29 using AFM phase imaging suggest that the significant change in fracture behavior associated with the degradation of the fiber strength was major due to the variations of surface properties. We can go further by considering the correlation of surface roughness and Griffith fracture criterion to provide a better understanding of the mechanisms and controlling factors affecting the performance and durability of ARG fibers. According to the well-known linear elastic fracture mechanics (LEFM),31 the condition for brittle failure in loading mode I is that the value of the stress intensity factor KI reaches its critical stress intensity factor KIc value. The former factor determines the stress state in the crack tip vicinity and is dependent on loading conditions and the flaw size; the latter one represents a fundamental material property that is unique for a particular material and is also a function of many other factors such as temperature, strain rate, and microstructure. As a first approximation, one can consider that the experimentally measured critical stress intensity factor in loading mode I, KIc, of the glass fiber is near 1 MN m-3/2.30 Here we treat the fractures as the result of a competition between external surface and internal flaws as a simple interpretation of the complicated statistical phenomenon. The former case can be modeled as an externally circumferentially cracked rod in tension where the crack length is assumed to be far smaller than the diameter of the rod; the latter one can be simulated as a penny-shaped flaw in an infinite body under tension. Thus, the strength of a brittle glass fiber, σ, is given by

σ ) minimum(σe, σi) σe )

KI 1.12xπa

σi )

πKI 2xπb

(9) (10)

(11)

where the most severe surface flaws have crack length of a and internal flaws have crack lengths of 2b. The variation of the tensile strength with the defect size is shown in Figure 10, together with the experimental tensile and maximum height surface roughness (Rmax) results. (29) Gao, S. L.; Ma¨der, E.; Abdkader, A.; Offermann, A. J. NonCryst. Solids, submitted. (30) Dıáz, G.; Kittl, P.; Martıńez, V. J. Mater. Sci. 1996, 31, 3675. (31) Matthews, W. T. In Plan strain fracture toughness data handbook for metals; AMMRC MS73-6; U.S. Army Materials and Mechanics Research Center: Watertown, MA, 1973.


Langmuir, Vol. 19, No. 6, 2003 2505

Figure 10. Comparison of the measured maximum height surface roughness with the Griffith strength prediction for both NEG and CemFIL fibers. The range of the maximum internal flaw size resulting in the intrinsic failure mode remains constant with the exception of the apparent increase of flaw size during high-temperature pyrolysis, while the maximum surface flaw size shifts because of different treatment conditions.

The surface roughness is determined mostly by the fiber surface topography along with the remaining sizing layer after surface treatments. According to the above results of AFM characterization, great variation in Rmax was found between the various treatments. One interesting observation of this investigation is that the maximum height roughness followed very closely the line predicted by the Griffith fracture criterion, except for the cases of the control and condition II treated NEG fibers and pyrolysis-treated fibers of both kinds. No correlation was found between Ra values and predicated strength-limiting flaw size. Indeed, the higher the maximum height roughness, the bigger the most severe flaw that will be present in the surface layer and, consequently, the lower the expected value of the ultimate tensile strength. This also provides additional supporting evidence and a physical basis for the variation of the effective surface thickness parameter (κ(d)) given in the previous section. As to the exceptions, our interpretation is that the control surface sizings cover the real surface flaw sizes. In the case of the pyrolysis, the fracture is dominated by the probably enlarged internal flaws from the high temperature, resulting in a σi value that is far less than the σe value, which is consistent with the above observation of the unimodal failure distribution. Based on both the prediction line of internal flaws and the bimodal Weibull distribution parameter (si), one implication is that the size of internal flaws seems to range within approximately a few hundred nanometers, which appears to be the same order of magnitude as the observed grain diameter (∼500 nm) in the fracture surface of the fiber.29 Therefore, the grain boundary could be a key region affecting the fiber fracture behavior, particularly in the case (σe > σi) when the most severe surface flaws are relatively much smaller and filled by the sizings. On the other hand, the surface flaw size causing fiber failure is only 1/6 of the internal flaw size causing fiber fracture, which highlights the strong sensitivity of tensile strength to the surface flaws. On the basis of the local mechanical property and morphology characterization, suitable chemical-resistant surface sizings preventing the glass surface corrosion are assigned to be an area worthy of further

thorough investigation in obtaining reliable and durable high strength in fibers and fiber-reinforced composites. Summary This paper provides important new insights into both surface and bulk mechanical behavior of ARG fibers subjected to various environmental attacks. The variability of the fiber surface local stiffness/energy characteristics was revealed using nanoindentation tests at a range of forces. Taking into consideration comprehensively elastic and inelastic properties on the fiber surface, a nondimensional energy index parameter (EI) is found to be an excellent measurement for characterizing the extent of damage and a sensitive parameter to describe the transition between the sizing-dominated response and the substrate-dominated response. The surface chemical structure information regarding the degradation behavior is clearly consistent with the AFM morphology observation and nanomechanical characterization. In view of a competition between intrinsic and extrinsic responses, the effect of surface properties on the tensile strengths was confirmed based on both a modified statistical analysis and Griffith fracture mechanics. The effective surface thickness factor κ(d) reveals the influences of various surface treatments on the extrinsic failure; that is, the fiber failure strength is strongly surface flaw dependent. The great influence of the sizing is shown to affect both the population and size of flaws on the fiber surface by healing effects. The bimodal Weibull curves are more suitable for the description of the nonlinear probability plots than the single Weibull lines, which demonstrates that two populations of defects are present for most treatment conditions. Unimodal distributions can only be found for the case which is dominated by either internal defect severity or surface flaw severity. The maximum height roughness followed very closely the line predicted by the Griffith fracture criterion. The importance of the sizing on environmental resistance of the alkali-resistant glass fibers is highlighted. Nanoindentation and a modified bimodal Weibull statis-

2506


Gao et al.

tical distribution analysis, with their capability of directly or indirectly determining surface properties, clearly have enormous potential for detailed surface mechanical research on the fiber itself and related mechanisms in fiberreinforced materials.

Collaborative Research Center “Textile Reinforcement for Structural Strengthening and Retrofitting (SFB528)”. We are grateful to Dr. D. Fischer and Mrs. G. Adam for their assistance at FTIR-ATR measurements and Mrs. L. Haüβler for TG analysis.

Acknowledgment. This work was supported in part by the German Research Foundation (DFG) in the

LA020778T

Sizings on Alkali-Resistant Glass Fibers: Environmental Effects on

Recommend Documents