Flexural Behavior of Glass Fiber Reinforced Silica Composites via Multiple Inltration Processing

Two-dimensional glass bers reinforced silica matrix composites (GFS) were fabricated by silica sol inltration (SSI) method by varying the number of inltration cycles. A uniform weight gain was observed after each inltration up to 9 th inltration suggesting the uniform loading of nano silica within GFS composites. The relationship between the inltration cycle and the physical and exural behavior of composites was measured by means of a density and 3-point exural test along with supporting evidences via microstructural characterization. The results of mechanical testing indicated an increase in the exural properties by up to 25%, after each inltration cycle, due to increase in density. Finally, microstructural study revealed the presence of various toughening mechanisms during fracture, under exural loading in the GFS composites.


Introduction
Advanced ceramics are an important class of engineering materials which offer numerous enhancements in performance, durability, reliability, hardness, high mechanical strength at high temperature, stiffness, low density,, electrical, and thermal insulation, radiation resistance, and so on. [1,2]. Among various ceramics materials, silica exhibits a unique combination of properties such as high melting point with high fracture toughness, which makes it suitable for various technological applications [3]. However, direct application of monolithic silica for structural components is not appropriate due to its inferior exural strength (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30), lower compression strength (60 MPa) and extremely low fracture toughness (0.62 MPa m½) [4]. Thus, it is necessary to improve mechanical properties of monolithic silica to make it acceptable for certain structural applications.
One of the means of achieving advanced ceramic materials with improved mechanical properties is by using either particulate or networks of continuous bers as reinforcements with silica as a matrix material. This leads to newer structural materials, known as ber-reinforced ceramic-matrix composites [4,5]. It is expected that a combination of glass ber as a reinforcement and silica as a matrix has the potential to overcome the drawbacks of brittleness with enhanced damage tolerance.
Literature shows that silica matrix based composites can produce near net shape products with improved fracture toughness and impact resistance at minimal cost than monolithic silica [6]. However, in comparison to particulate based silica composites, continuous silica ber reinforced silica matrix composites has gained more attention. Such composites can be developed at relatively higher temperatures through chemical vapor in ltration and hot slurry impregnation methods, however the risk of ber degradation at high temperatures limits the effectiveness and utilization of such methods. In addition, high temperature treatment in oxidizing environments introduces a limitation to thermomechanical and thermo-chemical compatibilities [7][8][9]. On the other hand, a relatively newer processing method to fabricate continuous ber reinforced silica matrix composites in the form of sol in ltration (SI) technique has emerged as a promising alternative to high temperature processing routes.
The SI method involves low densi cation temperatures with minimal shrinkage and reduced drying stresses. The utility of SI method as an effective silica matrix composites fabrication technique has been presented in recent studies conducted by numerous researchers [5,10,11]. These researchers have incorporated continuous silica bers with silica matrix through silica sol in ltration (SSI) technique to obtain a silica matrix composite with enhanced mechanical properties [4,5,12,13]. Prasad et al. studied the elastic behavior of silica-silica ber-reinforced ceramic matrix composites that were fabricated through silica sol in ltration sintering method (SIS) [14]. Similarly, Kim et al. also studied the mechanical properties of 2-D silica-silica continuous ber reinforced ceramic matrix composites [15]. Also, Liu et al. incorporated 2.5D silica bers into silica matrix through SIS process to achieve a toughening effect in composites [16]. Additionally, Li et al. prepared 3D seven directional braided silica composites through SIS approach [13]. In another study Liu et al. performed mechanical testing of 2.5D silica reinforced composites prepared through SIS approach. The achieved exural strength for shallow bend joint was 50.3 MPa while for shallow straight joint the strength was 48.4 MPa [16]. These researchers concluded that the incorporation of continuous silica bers into silica matrix provided the composites with a kind of pseudo ductility by preventing catastrophic crack growth by such mechanisms as ber debonding, matrix cracking, ber-pull out and bridging effect.
In the present study, a systematic yet relatively a fresh approach has been de ned to develop glass bers reinforced silica matrix (GFS) composites through SSI method. This method was selected due to its effectiveness in relatively low densi cation temperature, low shrinkage and reduced drying stresses. The approach involves multiple in ltrations of silica sol into fabric preform under vacuum followed by multiple drying cycles. The multiple in ltrations, aid in homogenizing the distribution of nano silica in the preform and vacuum assistance ensure maximum voids removal. To study the behavior of GFS composites, microscopic characterization of the brous preform was performed after 3rd, 5th, 7th and 9th in ltration cycles. The coating of nano silica on preform after selected in ltration cycles were investigated under a eld emission scanning electron microscope. Afterwards, mechanical testing was performed on nal sintered GFS composites to assess the strength and exural behavior of GFS composites. Finally, the results were compared with the available data in terms of relative improvement in mechanical properties.

Materials
A 2/2 twill weave E glass fabric was procured from China (Fig. 1a). The average diameter of single ber was ~ 7 µm. Colloidal silica sol having 30 vol. % SiO 2 content was obtained from Sigma Aldrich (Fig. 1b).
The diameter of nano silica was 10-12 nm and purity level was > 90%.

Silica Sol In ltration
Vacuum assisted in ltration process was opted to fabricate GFS composites. Multiple in ltrations were employed to improve the quality of GFS composites. Fourteen layers of 2D woven dry glass fabric were cut into equal dimensions of 200 mm x 200 mm and placed over an aluminum plate to prepare the setup for infusion. Afterwards, a polyester peel ply, a distribution mesh and resin in-out pipes were attached and adjusted accordingly. An airtight nylon vacuum bag was placed over the setup and a vacuum pump was attached to generate a constant vacuum pressure of 0.9 bar for 01 hour. The in ltration of sol was kept at a slow rate in order to achieve complete wetting of brous preform. Subsequently, the pump was switched off while keeping the setup under vacuum.

Drying Cycle
The wet preform setup was then placed in an oven at 80 °C for 1 hour followed by drying at 110 °C for another hour as shown in Fig. 2. This heating removed the coupling agent and bound water present in the sol. The same drying process was repeated after each in ltration cycle.

Sintering of composites
Finally, the GFS composite panel was sintered at 550 °C for 2 hours. The samples of required dimensions were cut from the composite panels for microstructural and mechanical property characterization.

Testing and Characterization
The densities of sintered composites were measured using a densimeter (AU-900S, Dong Guan Hong Tuo Instrument Company, Guangdong, China) with an accuracy of 10 − 3 g.
The exural strength and exural behavior of the composites were determined by a three-point bend test following ASTM standard C1341 [17]. The test was performed on a universal testing machine (WDW-30, Jinan Testing Equipment IE Corporation, Jinan, China). The strain rate used for the testing of specimens of dimensions 35 × 5 × 3.5 mm was 0.1 mm/min, while at least ve tests of each composite were performed. The samples were cut using a low speed cutter (Buehler Isomet, ITW Company, Lake Bluff, Illinois).
Scanning Electron Microscopy (SEM) of the composites was performed on a eld emission microscope (MIRA-III, FEG-SEM, Tescan Orsay Holding, Brno, Czech Republic). Images were acquired in the secondary electron mode at an operating voltage of 5 kV. The samples were placed on an aluminum stub with carbon tape. The fracture surfaces of GFS composite specimens were also studied via SEM to investigate the fracture morphology.

Weight Gain
The weight of GFS composite was measured before and after in ltration as shown in Fig. 3. With each in ltration the weight gain increased, however after the 7th in ltration a steady weight gain was observed. The initial increase in weight gain could be attributed to voids and open spaces that were lled with silica sol. The relative change in weight gain decreased with successive in ltration cycles, apparent from the slope (tangent) of the curves in Fig. 3. However, the weight gain reached a maximum at the 9th in ltration cycle. Figure 4 shows the variation of density vs. number of in ltration cycles. The density shows an increasing trend. The reason for the initially low density is the presence of voids due to absence of enough silica and lower compaction of the GFS composites. However, with increase in in ltration cycles, the voids or empty spaces get lled with silica matrix, resulting in a more compact composite structure with enhanced density. As evident from Fig. 4 (a-d) the density after the 3rd cycle increased from 1.56 g/cm 3 to 1.84 g/cm 3 after the 9th in ltration cycle. This trend in increase in density is similar to previously reported work of Han et al [18] & Liu et al [4,16]. After the silica reaches saturation; there is hardly any change in the density, with further in ltration cycles. The highest density achieved in this research work was 1.84 g/cm 3 after the 9th in ltration cycles which is about 4% higher than previously reported by Liu et al [4,16].

Microscopy of silica coated glass bers
SEM was performed to observe the extent of silica deposition on GFs as shown in Fig. 5. The gradual increase in silica content can be observed. Figure 5 (a) con rms the presence of silica as matrix around bare bers. Similarly, Fig. 5 (b) and (c) shows (at higher magni cation) the presence of brittle matrix around stiff bers. Figures 5 (d-f) shows increase in matrix, with the gap between ber and the matrix being a manifestation of the weak interfacial bonding. However, Figs. 5 (g-i) depicts the increase in silica quantity as the pores are increasingly lled. The gap between ber and matrix has reduced with matrix covering more than half of the bers. Accordingly, it can also be seen from Figs. 5 (j-k) an almost complete coverage of bers with the silica matrix. Surface roughness has also increased indicating enhanced interfacial bonding between bers and the matrix. The matrix around bers shows absence of gaps.

Flexural Strength
The exural properties of GFS composites are shown in Fig. 6. All the samples were tested under same conditions by keeping the ber volume constant. The change in properties was studied as function of silica content (in ltration cycles). It is evident that with increasing silica content, the overall exural properties of GFS composite improves. The exural strength of GFS-3 composite (after three in ltration cycles) was found to be 18.96 ± 0.01 MPa which is signi cantly low due to high levels of porosity.
Whereas from GFS-5 onwards, the silica matrix showed strengthening effect. The average exural strength of the composite after ve in ltrations was 24 ± 0.02 MPa which was 20% more than the value of GFS-3 composite. Similarly, GFS-7 composite showed an improvement of 18% with an average strength of 28.18 ± 0.025 MPa. Finally, after nine in ltration cycles, GFS-9 composite showed an improvement of 27% as compared to GFS-7 composite with an average value of 39.10 ± 0.03 MPa. In a comparable study on the mechanical behavior of 2D woven silica bers reinforced ceramics matrix composites, Kim et al. has reported a maximum exural strength of 23.2 MPa after 09 in ltration cycles [15]. Figure 7 shows the stress-strain curves for GFS-5 to GFS-9 composites. The graphs indicate that initially matrix cracking occurs in the composite as represented by point "a" in each curve, followed by F/M debonding shown as point "b" and then the failure induced by ber pull-out and/or crack de ection mechanisms shown as point "c". The composites exhibits time delayed fracture producing strain via ber pull-out. The areas under the stress-strain graph is indicative of the amount of energy absorbed by the composite during failure and the fracture toughness could be estimated via the area size [19]. However, the variation in the degree of nonlinearity and strain values might cause disparity in toughness values of these composites, as previously reported by Liu et al [20].

Flexural Behavior
The data in Fig. 7 (a-c) reveals that with successive increasing in ltration, the strain decreased and strength increased. The silica concentration in uenced the ber-matrix (F/M) interfacial bonding and hence the behavior of the GFS composites. Similar behavior has been reported previously [21]. With successive in ltrations, the pores among glass bers were lled with silica matrix; that established a strong F/M bonding which hindered crack propagation. In case of GFS-5 ( Fig. 7-a) the F/M interface bonding is relatively weak, the crack de ects at various planes instead of penetrating through the bers causing ber pullout. In case of GFS-7 ( Fig. 7-b) the ber pullout is low, fracture therefore occurs at low strain. A similar behavior was observed and reported by Li et al. [16] and Dassios et al. [22] in their studies on continuous ber reinforced silica matrix composites. In GFS-9 ( Fig. 7-c), the matrix concentration being much higher than the previous cycles, the matrix cracks at a high stress and low strain. The low strain indicates that F/M debonding was di cult, therefore fracture occurs sooner in comparison to previous cycles.

Fractography
Fractured surfaces of failed samples are shown in Fig. 8. The feature in the micrographs supports the failure mechanism proposed in the previous section. The failure is a combination of matrix cracking, F/M debonding, crack de ection and ber pullout. The elongation in GFS composites mainly depends on de ection of the crack at F/M interface and the magnitude of pullout length. With successive in ltration cycles the fracture toughness of GFS composite decreased although the strength increased, consistent with the study reported by Liu et al [20] The GFS-3 fractograph as shown in Fig. 8-a manifests a multistep broom like failure with extensive ber pullout; an indication of weak F/M interface where debonding was relatively easy due to low matrix concentration and compaction. The GFS-5 fractograph in Fig. 8 (b) exhibits a large amount of matrix along the bers indicating the adherence of the matrix to the bers, making that F/M interface bonding relatively stronger than GFS-3 composite. Similarly, the GFS-7 fractograph as shown in Fig. 8 (c) depicts multiple crack de ection with minimal ber pullout as compared to GFS-5 composite. Finally, the GFS-9 in Fig. 8 (d) illustrates the ber fracture at various planes with almost negligible magnitude of ber pullout. Additionally, the ber surface was ragged, indicative of ber cracking along a cleavage plane. The ber failure rather than pullout indicates that failure is occurring in the same plane, simultaneously in the matrix and the ber. All GFS composites fractured in translaminar fashion. Because the fracture strain of ber is higher, therefore when load was increased, the composite deformed elastically with small strain, until micro cracks developed, and coalesced in the brittle matrix.

Conclusions
Glass ber reinforced silica (GFS) composites were successfully developed through vacuum assisted silica sol in ltration (SSI) technique. The matrix was silica sol while the reinforcing material was glass ber. The study shows that variation in matrix concentration has a strong in uence on the physical and mechanical properties of GFS composites. As the porosity decreased, the density of the GFS composites increased from ~ 1.54 g/cm 3 to ~ 1.84 g/cm 3   Supplementary Files This is a list of supplementary les associated with this preprint. Click to download.