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Surface defect healing effect of silica coatings on silicon nitride fibers
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Surface defect healing effect of silica coatings on silicon nitride fibers ⁎
Xuan Hu, Changwei Shao , Jun Wang
⁎
Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, National University of Defense Technology, Changsha 410073, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Silicon nitride fibers Silica coating Strength improvement Surface flaws
In this study, silica coatings with different thickness were prepared on silicon nitride fibers by a continuous dipcoating method. The effects of the coatings on the mechanical properties of the silicon nitride fibers were investigated. The SiO2 coatings with uniform thickness were prepared from a sol solution with a concentration of 0.75 wt% and then heat-treated at 400 °C, and the strength of the fibers was improved by the treated coating. The tensile strength of a coated fiber was approximately 26% higher than that of an uncoated fiber because the thin coating healed the surface defects. Our study also confirmed that the size of sol particles must match that of the flaws on the fiber surface before these flaws could be effectively repaired. Finally, a probable mechanism will be proposed here to explain this effect. The present results demonstrate that the strength of silicon nitride fibers can be enhanced by coating them through the sol–gel process, and the findings are expected to provide guidelines for repairing strength-limiting flaws in other fibers.
1. Introduction Silicon nitride fibers are widely used as reinforcing components in composite materials because of their excellent properties such as high specific strength and modulus, low dielectric constant, and high electrical resistivity, being intensively utilized in wave-transparent antenna radomes, hypersonic vehicles, and other aerospace applications [1]. Near-stoichiometric silicon nitride fibers have been prepared by nitridation of cured polycarbosilane fibers, as previously reported by our group [2]. Structural defects such as pores are usually generated in the fibers owing to the release of gases and backbone breakage during the de-carbonitriding process [3], and they negatively affect the strength of silicon nitride fibers fabricated by this route. Therefore, it is very important to develop a way to repair the defects. Very few institutions fabricate coatings on silicon nitride fibers, with a rare example being Tonen Corp., which used chemical vapor deposition (CVD) to coat silicon nitride fibers with a C–B–Si layer [4,5]. Two types of coatings improved the strength of the raw fibers, and the retention ratios of these two coated fibers were 106% and 121%, respectively. However, this phenomenon was not specifically analyzed. The healing effect of the coatings can be observed in other fibers. For example, Naganuma et al. deposited polyimide coatings on T1000GB carbon fibers via dip-coating and high-temperature vapor deposition polymerization (VDP) [6]. All these polyimide coatings led to an increase in the filament strength of the flaw-prone carbon fibers; in addition, substances less than a nanometer in size were found to be more effective in completely penetrating the nanoscale flaws on the ⁎
fiber surfaces and creating an interlayer with relatively high density. Gao et al. [7] described a process used to apply a nanometer-scale hybrid coating layer based on a styrene–butadiene copolymer with single- or multi-walled carbon nanotubes to glass fibers; after a coating with a low fraction of nanotubes was applied, the traditional glass fibers showed significantly improved mechanical properties. Cao et al. [8] reported that a carbon coating with appropriate thickness deposited by CVD using a C3H6–N2 gas system considerably increased the mechanical properties of SiC fibers; they explained that the thin carbon coatings provided compressive stress on SiC fibers, which helped to seal the defects on the fiber surface. In another study, BN was coated on SiC fibers via a modified dip-coating method by introducing the sol–gel method to the dipping process, using boric acid and urea as the BN precursors and acrylamide as the gel-casting monomer [9]; the fibers that were dipped into the sol twice exhibited the highest tensile strength, which was 8% higher than that of the original fibers, but the strength-improving mechanism was not presented. Based on the above studies, it can be concluded that healing surface flaws with a coating is an effective way to enhance the filaments’ tensile properties. Compared with other methods, the sol–gel process is a simple and uninterrupted way to form coatings on continuous fibers. As silicon nitride fibers have good dielectric properties, the dielectric constant and tan δ of the coating should match those of the bare fibers to avoid affecting their wave-transparent applications. The silica sol happens to be the proper raw material for this type of coating, in addition to being easily accessible, economical, and highly transparent. Moreover, the size of colloids in commercial silica sols can be customized to the
Corresponding authors. E-mail addresses: [email protected] (C. Shao), [email protected] (J. Wang).
http://dx.doi.org/10.1016/j.ceramint.2017.09.061 Received 27 July 2017; Received in revised form 2 September 2017; Accepted 9 September 2017 0272-8842/ © 2017 Published by Elsevier Ltd.
Please cite this article as: Hu, X., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.09.061
Ceramics International xxx (xxxx) xxx–xxx
X. Hu et al.
2.3. Characterization
nanometer scale, which can help us understand the determining factors of fiber strength. It is also worth noting that silica coatings have already been successfully applied on other fibers. For example, Katzman et al. [10] prepared silica-coated carbon fibers through the sol–gel process, and the coated fibers were confirmed to show enhanced wettability when they were immersed in molten magnesium. Therefore, to understand the healing effect of coatings, silica-coated silicon nitride fibers were prepared by the continuous dip-coating method. The effects of the coating process on the morphology and thickness of the fiber coatings were investigated, and the uniformity of film thickness was mainly controlled by the solid content of the sol solution. Based on these experiments, a surface-healing mechanism will be proposed here to explain the defect-repairing effect of silica coatings on fibers with minor flaws.
The tensile strength and tensile modulus of the fibers were measured using an Instron-type test machine (Micro-350, Testometrix), with a gauge length of 25 mm and a crosshead speed of 5 mm min−1. The average tensile strength was obtained from the measured results of 24 filaments. The microstructure was observed by scanning electron microscopy (SEM, S-4800, Hitachi). Quantitative analysis of oxygen was carried out using a N/O analyzer (EMGA-820, Horiba). Atomic force microscopy (AFM) experiments were performed on a Bruker Dimension Fastscan system with a silicon probe in non-contact mode. Transmission electron microscopy (TEM, Tecnai F20, FEI) was performed with an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher) coupled with Ar+ etching was used for the analysis of the relative atomic percentage of elements in depth profile on the coated fibers. Small-angle X-ray scattering (SAXS) experiments were carried out on the SAXS station at beamline 1W2A of Beijing Synchrotron Radiation Facility (BSRF). The storage ring was operated at 2.5 GeV with current about 180 mA. The incident X-ray wavelength was selected to be 0.154 nm by triangular bending Si(111) crystal monochromator. The SAXS data were collected with Mar165 CCD (charge coupled device) and the sample-to-detector distance was set at a constant value of 1578.87 mm.
2. Experiment 2.1. Materials Bundles of continuous silicon nitride fibers without sizing agents were directly used as samples for dip coating in this study. Silica sols (Xinyu Chemicals Co. Ltd., China; Peak-Tech New Material Co. Ltd., China) were used as the precursors of SiO2. Ethanol (AR, Hengxing Co. Ltd., China) and distilled water (home-made) were used as the dispersants. Solutions of different concentration were prepared as follows: a mixture containing an appropriate ratio of the commercial sol and ethanol was ultrasonically treated for 30 min and then stirred for over 12 h. The protective gas in the furnace was N2 (99.9999%, Xiangfeng Co. Ltd., China).
3. Results and discussion 3.1. Effects of heat-treatment temperature on mechanical properties of silicon nitride fibers As shown in Fig. 2, the optimal heat-treatment temperature was 400 °C. When the temperature was above 600 °C, the coatings did not increase the tensile strength of the fibers. Based on this finding, the heat treatment of the sol in all other experiments was carried out at 400 °C to obtain the most effective enhancement of the mechanical properties. Furthermore, the elastic modulus fluctuated to a small extent, and the discrepancies can be considered negligible.
2.2. Preparation of the coatings The silica coatings were formed using the laboratory-designed rollto-roll apparatus illustrated in Fig. 1; it consisted of a coating vessel, a drying oven, a heating furnace, and two strand winders. The drying oven was set at 120 °C to evaporate the solvent in the sols. The furnace temperature was set to a value in the range of 300–800 °C. A strand of fibers was dipped into the sol solution to be coated, and then it was dried in the drying oven to form a gel. After that, the fibers were heated under a nitrogen atmosphere in the furnace. The strand was fed from the original bobbin to the final bobbin at 40–120 cm min−1, and different sol concentrations were used to obtain coatings with varying thickness. Distilled water was first used as the dispersant of the sol. However, when the impregnated bundles were dried at 120 °C for several seconds, the dispersant did not evaporate, introducing water to the heating furnace as a result. Therefore, we replaced the distilled water with ethanol in the following experiments. According to the analysis result of an earlier orthogonal test, the reel speed had less impact on the strength than other factors, and 80 cm min−1 was the optimal reel speed in the range of 40–120 cm min−1 for producing fibers with better mechanical properties. The reel speed was thus set at 80 cm min−1 for subsequent experiments.
3.2. Relationships between coating thickness and sol concentration When the sol concentration was above 3%, the thick SiO2 layer caused adhesion between individual filaments of the bundle. The thick coatings on the fibers and the easy formation of cracks can be easily seen in Fig. 3a and b. In contrast, when the concentration of the sol solution was below 1.5%, the relatively thinner layers were not noticeable in the SEM images. The density of the original silicon nitride fibers was measured in ethanol by the Archimedes method to be 2.15 g cm−3, and the average diameter of the original silicon nitride fibers was 12.5 µm. In addition, the density of the silica coating was approximately 2.2 g cm−3, which is very close to the value of the uncoated fibers. Based on the difference between the oxygen content of the uncoated and coated fibers, the assumed thickness of the coating can be estimated using the following equation:
y2 =
y1 πr12lρ1 + y0 π [r22 − r12] lρ0 , πr12lρ1 + π [r22 − r12] lρ0
(1)
where y1, y2, and y0 are the oxygen contents of the uncoated fiber, coated fiber, and amorphous SiO2 layer, respectively; r1 and r2 are the radii of the uncoated fiber and coated fiber respectively; ρ1 and ρ0 are the densities of the uncoated fiber and amorphous SiO2 layer, respectively; and l is the fiber length. According to this equation, y1 and r1 of the original silicon nitride fibers were 0.95 wt% and 6.25 µm, respectively; y0 = 0.53(•) wt%; and ρ1 ≈ ρ0. Using Eq. (1), which assumes that the coating was smooth and uniform, completely shielding the fiber, one can also derive r2 from the parameter y2. The thickness d of
Fig. 1. Schematic drawing of the apparatus for the fiber coating.
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Fig. 2. Mechanical properties of coated fibers that were prepared from sols with solid content of (a) 1.5% and (b) 0.75% and then heat-treated at different temperatures.
Fig. 3. Surface and cross-section morphologies of coated fibers fabricated from sols with different solid contents: (a, f) 6%; (b, g) 3%; (c, h) 1.5%; (d, i) 0.75%; (e, j) 0.3%.
the coating is given by d = r2–r1, and the values of d for different coated fibers are listed in Table 1. To verify the accuracy of the calculation, the thickness of the coatings was measured by the SEM examination of the cross sections of coated fibers, as shown in Fig. 3f–j. The measured thickness was basically in accordance with the calculated thickness, especially for the 1.5%- and 0.75%-sol-coated fibers, confirming that Eq. (1) can be used to derive the thickness of the coating. As shown in Table 1, the elastic modulus remained almost unchanged for coatings of different thickness, which means a single fiber's resistance against elastic deformation remained stable. This indicates that the elastic modulus was mainly affected by the microstructure of the original material as a whole, and it was not significantly influenced by the surface state.
Table 1 Characterization of the original and coated silicon nitride fibers. Sol concentration by weight
Elastic modulus (GPa)
Oxygen content (wt%)
Assumed thickness of the coating (μm)
– 6% 3% 1.5% 0.75% 0.3%
152 151 152 152 145 149
0.95 4.55 2.88 1.92 1.49 1.20
– 0.228 0.119 0.059 0.033 0.024
Fig. 4. XPS depth profile of coated silicon nitride fibers prepared from sols with solid content of (a) 1.5% and (b) 0.75% (sputtering rate: 3 nm min−1).
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single filaments of the uncoated and coated fibers. Because the silicon nitride fibers exhibited typical features of brittle fracture and a significant range of values for the tensile strength, the two-parameter Weibull model, which is based on the theory of the weakest link and widely used to describe tensile strength of brittle materials [11], was adopted for evaluation of the strength distribution. In this model, the failure probability P is estimated by Eq. (2):
P = 1 − exp[−(σ / σ0)m],
(2)
where σ is the fracture stress, m is the Weibull modulus parameter, and σ0 is the scale parameter. By taking the natural logarithm of Eq. (2) and rearranging the terms, we obtained Eq. (3),
ln ln[1/(1 − P )] = m ln σ –m ln σ0,
where the slope is equal to m, and the y intercept is equal to –mlnσ0. The σ values are the experimental tensile stresses, and P can be defined as
Fig. 5. Weibull plots for fibers with and without coatings.
Pi = i/(n + 1), Table 2 Weibull parameters for uncoated and silica-coated fibers. Type
Average tensile strength (GPa)
Weibull modulus, m
Scale parameter,σ0 (GPa)
Related coefficient, R2
No coating 6% 3% 1.5% 0.75% 0.3%
1.3085 0.9000 1.0834 1.5367 1.6456 1.3384
3.2278 9.1388 4.2480 5.5393 6.0244 3.8044
1.4549 0.9513 1.1879 1.6678 1.7762 1.4818
0.90489 0.97308 0.97196 0.95554 0.97452 0.97769
(3)
(4)
where n is the number of data points, and i is the rank of the ith data point. The Weibull plots for the uncoated fibers and fibers coated with different concentration of sol are represented in Fig. 5, and the corresponding results of the Weibull analysis are given in Table 2. The m values of the coated fibers were remarkably higher than that of the uncoated fibers, indicating a narrower distribution of the tensile strength among the silica-coated fibers [8]. The surface morphology of the uncoated and coated fibers was analyzed using AFM. Both images in Fig. 6 are shown on the same scale to facilitate direct comparison. As seen in Fig. 6a, the original fibers had a rather rough surface with pits and bulges. However, it was difficult to observe these minor defects solely by SEM. In comparison, the coated fibers had a smoother surface with fewer defects, which confirmed the flaw-repairing effect of the glassy SiO2 phase. However, it did not go unnoticed that the tensile strength of the fibers decreased with increasing sol concentration (from 0.75% to 6%). This deterioration occurred because the silica coatings also had unfavorable effects on the fiber strength, which included the increased cross-sectional area and stress concentration induced by minor cracks of amorphous SiO2, as confirmed by the SEM images in Fig. 3. For the 1.5%- and 0.75%-sol-coated fibers, whose assumed coating thickness was 59 and 33 nm, respectively, the surface-healing effect was dominant, as indicated by their higher strength when compared to that of the uncoated fibers. For the 6%- and 3%-sol-coated fibers whose assumed coating thickness was 228 and 119 nm, respectively, the strength loss resulting from the sticky and cracking nature of the thick coatings
The XPS depth profiles of the coated fibers are shown in Fig. 4. The spectra show that substantial amounts of Si and O were present in the surface layer, confirming the successful coating of the fibers. The thickness of the coating on the 1.5%-sol-coated fibers was apparently higher than that of the 0.75%-sol-coated fibers. The composition was in accordance with the bulk atomic ratio of the coatings, except for the carbon-rich and nitrogen-rich layer originating from contamination in the surface region. 3.3. Effects of silica coatings on mechanical properties of silicon nitride fibers To evaluate the effects of silica coatings on the mechanical properties of the fibers, we focused on investigating the tensile strength of
Fig. 6. AFM images (3D representation of height) of (a) uncoated fibers and (b) 0.75%-sol-coated fibers.
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Fig. 7. TEM micrographs of SiO2 sols containing colloids of different sizes.
Fig. 8. SEM micrographs of the 0.75%-sol-coated fibers fabricated from sols consisting of colloids measuring (a) 30–40 nm and (b) 70–80 nm.
Fig. 9. AFM images of the 0.75%-sol-coated fibers prepared from sols containing colloids of different sizes: (a) 30–40 nm; (b) 70–80 nm.
3.4. Effects of particle size of the sol on mechanical properties of silicon nitride fibers
started to counteract the surface-healing effect; consequently, the strength of the coated fibers was much lower than that of the uncoated fibers. However, the strength the 0.3%-sol-coated fibers was not affected, probably because the sol was too dilute.
Fig. 7a shows the colloid size of the silica sol (type I) we used in prior studies, which was 2–8 nm for most particles. To determine whether the colloid size would influence the repair of surface flaws, we purchased two silica sols with different particle sizes, referred to as types II and III and shown in Fig. 7b and c. The sizes of types II and III 5
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Fig. 10. (a) 2D SAXS pattern and (b) the corresponding micropore size distribution of the original fibers.
3.5. Mechanism of the surface-healing effect
Table 3 Scatterer size distribution by cascade tangent rule. Radius of gyration without assumption (nm)
Diameter of assumed globular scatterer (nm)
Normalized scatterer volume fraction
0.80 1.44 2.69
2.07 3.72 6.95
0.568 0.302 0.130
The two-dimensional SAXS pattern, shown in Fig. 10a, of the original silicon nitride fibers was collected with the Mar165 CCD and converted into digital data by the Fit2D software package [12]. The SAXS curves of intensity versus scattering-vector were obtained by rectangular integration, and the data analysis was performed by using the program S.exe, written in Intel Visual FORTRAN for SAXS data processing and analysis [13]. To obtain the size distribution of the scatterers, we used the method first described by Jellinek and Fankuchen in 1946 [14,15]. Using this method, one can derive the radius of gyration Rg. The radius r of a perfect sphere that is assumed to represent an X-ray-scattering particle is obtained from the equation Rg = (3/ 5)0.5r [16], and the results are listed in Table 3. Therefore, the average radius of gyration (without assumptions of the scatterer shape) was 1.24 nm, and the average diameter of a globular scatterer was 3.20 nm. Moreover, the scatterers could be flaws on the fiber surface or pores inside the fibers, both of which could cause non-uniformity in the electron density.
were approximately 30–40 nm and 70–80 nm, respectively, which are much higher than the size of type I colloids. It turned out that after the same optimal coating process, the type-II and type-III coatings did not increase the tensile strength of silicon nitride fibers. As shown in Fig. 8, the fibers coated with type-II and type-III silica exhibited rougher surfaces with visible particles, as suggested by the AFM images in Fig. 9.
Fig. 11. (a) Failed healing effect of thick silica coatings. (b) Model of the mechanism of the surface defect healing effect. (c) Failed healing effect of large colloid particles.
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Acknowledgements
Sawyer et al. [17] reported that surface defects play a more important role than internal ones in determining the tensile strength of ceramic fibers. The flaw–strength relationship of SiC fibers in their study confirmed that the low tensile strengths they obtained were caused by surface flaws and particulate defects, and most of the strength limitations occurred because of surface damage. Hence, as discussed above, the strength of silica-coated fibers with the appropriate thickness was higher than that of uncoated fibers because the thin coatings healed the surface defects on the fibers. It should be noted that only coatings prepared from sols with the appropriate colloid size had this kind of strength-improving effect because the particle size should match the surface defect size to repair the flaws. On the other hand, silica alkoxide molecules that dip-coated the fiber surface combined to form a Si–O–Si network along the fiber axis during polycondensation, probably helping to increase the strength. Based on the above analysis, the simple model in Fig. 11 can be used to describe the mechanism of surface defect healing by the coating. Two failed mechanisms are also depicted in the figure.
This work is supported by the National Natural Science Foundation of China (No. 51203184). References [1] Z. Krstic, V.D. Krstic, Silicon nitride: the engineering material of the future, J. Mater. Sci. 47 (2012) 535–552. [2] X. Hu, C. Shao, J. Wang, H. Wang, Characterization and high-temperature degradation mechanism of continuous silicon nitride fibers, J. Mater. Sci. 52 (2017) 7555–7566. [3] X. Hu, C. Shao, J. Wang, H. Wang, J. Cheng, Effects of residual radicals on compositional and structural stability of silicon nitride fibers, J. Eur. Ceram. Soc. 37 (2017) 4497–4503. [4] Y. Yokoyama, T. Nanba, I. Yasui, H. Kaya, T. Maeshima, T. Isoda, X-ray diffraction study of the structure of silicon nitride fiber made from perhydropolysilazane, J. Am. Ceram. Soc. 74 (1991) 654–657. [5] K. Sato, H. Morozumi, O. Funayama, H. Kaya, T. Isoda, Developing interfacial carbon-boron-silicon coatings for silicon nitride-fiber-reinforced composites for improved oxidation resistance, J. Am. Ceram. Soc. 85 (2002) 1815–1822. [6] T. Naganuma, K. Naito, J. Yang, High-temperature vapor deposition polymerization polyimide coating for elimination of surface nano-flaws in high-strength carbon fibers, Carbon 49 (2011) 3881–3890. [7] S.L. Gao, E. Mader, R. Plonka, Nanocomposite coatings for healing surface defects of glass fibers and improving interfacial adhesion, Compos. Sci. Technol. 68 (2008) 2892–2901. [8] S.Y. Cao, J. Wang, H. Wang, Effect of CVD carbon coatings on properties of SiC fibres, Surf. Eng. 33 (2017) 573–577. [9] J. Liu, S. Wang, P. Li, M. Feng, X. Yang, A modified dip-coating method to prepare BN coating on SiC fiber by introducing the sol–gel process, Surf. Coat. Technol. 286 (2016) 57–63. [10] H.A. Katzman, Fibre coatings for the fabrication of graphite-reinforced magnesium composites, J. Mater. Sci. 22 (1987) 144–148. [11] Y. Paramonov, J. Andersons, A family of weakest link models for fiber strength distribution, Composites: Part A 38 (2007) 1227–1233. [12] 〈www.esrf.eu/computing/scientific/FIT2D/〉. [13] Z.H. Li, A program for SAXS data processing and analysis, Chin. Phys. C 37 (2013) 108002. [14] M.H. Jellinek, E. Solomon, I. Fankuchen, Measurement and analysis of small-angle X-ray scattering, Ind. Eng. Chem. 18 (1946) 172–175. [15] W. Wang, X. Chen, Q. Cai, G. Mo, L.S. Jiang, K. Zhang, Z.J. Chen, Z.H. Wu, W. Pan, In situ SAXS study on size changes of platinum nanoparticles with temperature, Eur. Phys. J. B 65 (2008) 57–64. [16] A. Guinier, G. Fournet, C.B. Walker, Small-angle Scattering of X-rays, J. Wiley & Sons, New York, 1955. [17] L.C. Sawyer, R. Arons, F. Haimbach, M. Jaffe, K.D. Rappaport, Characterization of Nicalon®: strength, structure and fractography, in: Proceedings of the 9th Annual Conference on Composites and Advanced Ceramic Materials: Ceramic Engineering and Science Proceedings, 20–23 January, ACS, Columbus, Cocoa Beach, 1985.
4. Conclusion Silica coatings were prepared on silicon nitride fibers by the sol–gel process. Using the modified dip-coating method, a continuous and uniform gel film was formed on the silicon nitride fibers. After heat treatment, the gel film turned into a relatively compact silica coating. When the optimal sol concentration, colloid size, heat-treatment temperature, and reel speed were used, the tensile strength of the coated fibers was approximately 26% higher than that of the uncoated fibers owing to the surface flaw healing effect of the coatings and the Si–O–Si network imparted by polycondensation of the sol. In contrast, the silicacoated fibers prepared from sols with concentrations of 3% and higher exhibited lower strength than that of the uncoated fibers, probably because of the sticky and cracking nature of the thick coatings. The proposed mechanism of the surface defect healing effect is mainly determined by the particle size in the sol and the thickness of coatings, and it can be described by a simple model. Conflicts of interest None.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Surface defect healing effect of silica coatings on silicon nitride fibers ⁎
Xuan Hu, Changwei Shao , Jun Wang
⁎
Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, National University of Defense Technology, Changsha 410073, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Silicon nitride fibers Silica coating Strength improvement Surface flaws
In this study, silica coatings with different thickness were prepared on silicon nitride fibers by a continuous dipcoating method. The effects of the coatings on the mechanical properties of the silicon nitride fibers were investigated. The SiO2 coatings with uniform thickness were prepared from a sol solution with a concentration of 0.75 wt% and then heat-treated at 400 °C, and the strength of the fibers was improved by the treated coating. The tensile strength of a coated fiber was approximately 26% higher than that of an uncoated fiber because the thin coating healed the surface defects. Our study also confirmed that the size of sol particles must match that of the flaws on the fiber surface before these flaws could be effectively repaired. Finally, a probable mechanism will be proposed here to explain this effect. The present results demonstrate that the strength of silicon nitride fibers can be enhanced by coating them through the sol–gel process, and the findings are expected to provide guidelines for repairing strength-limiting flaws in other fibers.
1. Introduction Silicon nitride fibers are widely used as reinforcing components in composite materials because of their excellent properties such as high specific strength and modulus, low dielectric constant, and high electrical resistivity, being intensively utilized in wave-transparent antenna radomes, hypersonic vehicles, and other aerospace applications [1]. Near-stoichiometric silicon nitride fibers have been prepared by nitridation of cured polycarbosilane fibers, as previously reported by our group [2]. Structural defects such as pores are usually generated in the fibers owing to the release of gases and backbone breakage during the de-carbonitriding process [3], and they negatively affect the strength of silicon nitride fibers fabricated by this route. Therefore, it is very important to develop a way to repair the defects. Very few institutions fabricate coatings on silicon nitride fibers, with a rare example being Tonen Corp., which used chemical vapor deposition (CVD) to coat silicon nitride fibers with a C–B–Si layer [4,5]. Two types of coatings improved the strength of the raw fibers, and the retention ratios of these two coated fibers were 106% and 121%, respectively. However, this phenomenon was not specifically analyzed. The healing effect of the coatings can be observed in other fibers. For example, Naganuma et al. deposited polyimide coatings on T1000GB carbon fibers via dip-coating and high-temperature vapor deposition polymerization (VDP) [6]. All these polyimide coatings led to an increase in the filament strength of the flaw-prone carbon fibers; in addition, substances less than a nanometer in size were found to be more effective in completely penetrating the nanoscale flaws on the ⁎
fiber surfaces and creating an interlayer with relatively high density. Gao et al. [7] described a process used to apply a nanometer-scale hybrid coating layer based on a styrene–butadiene copolymer with single- or multi-walled carbon nanotubes to glass fibers; after a coating with a low fraction of nanotubes was applied, the traditional glass fibers showed significantly improved mechanical properties. Cao et al. [8] reported that a carbon coating with appropriate thickness deposited by CVD using a C3H6–N2 gas system considerably increased the mechanical properties of SiC fibers; they explained that the thin carbon coatings provided compressive stress on SiC fibers, which helped to seal the defects on the fiber surface. In another study, BN was coated on SiC fibers via a modified dip-coating method by introducing the sol–gel method to the dipping process, using boric acid and urea as the BN precursors and acrylamide as the gel-casting monomer [9]; the fibers that were dipped into the sol twice exhibited the highest tensile strength, which was 8% higher than that of the original fibers, but the strength-improving mechanism was not presented. Based on the above studies, it can be concluded that healing surface flaws with a coating is an effective way to enhance the filaments’ tensile properties. Compared with other methods, the sol–gel process is a simple and uninterrupted way to form coatings on continuous fibers. As silicon nitride fibers have good dielectric properties, the dielectric constant and tan δ of the coating should match those of the bare fibers to avoid affecting their wave-transparent applications. The silica sol happens to be the proper raw material for this type of coating, in addition to being easily accessible, economical, and highly transparent. Moreover, the size of colloids in commercial silica sols can be customized to the
Corresponding authors. E-mail addresses: [email protected] (C. Shao), [email protected] (J. Wang).
http://dx.doi.org/10.1016/j.ceramint.2017.09.061 Received 27 July 2017; Received in revised form 2 September 2017; Accepted 9 September 2017 0272-8842/ © 2017 Published by Elsevier Ltd.
Please cite this article as: Hu, X., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.09.061
Ceramics International xxx (xxxx) xxx–xxx
X. Hu et al.
2.3. Characterization
nanometer scale, which can help us understand the determining factors of fiber strength. It is also worth noting that silica coatings have already been successfully applied on other fibers. For example, Katzman et al. [10] prepared silica-coated carbon fibers through the sol–gel process, and the coated fibers were confirmed to show enhanced wettability when they were immersed in molten magnesium. Therefore, to understand the healing effect of coatings, silica-coated silicon nitride fibers were prepared by the continuous dip-coating method. The effects of the coating process on the morphology and thickness of the fiber coatings were investigated, and the uniformity of film thickness was mainly controlled by the solid content of the sol solution. Based on these experiments, a surface-healing mechanism will be proposed here to explain the defect-repairing effect of silica coatings on fibers with minor flaws.
The tensile strength and tensile modulus of the fibers were measured using an Instron-type test machine (Micro-350, Testometrix), with a gauge length of 25 mm and a crosshead speed of 5 mm min−1. The average tensile strength was obtained from the measured results of 24 filaments. The microstructure was observed by scanning electron microscopy (SEM, S-4800, Hitachi). Quantitative analysis of oxygen was carried out using a N/O analyzer (EMGA-820, Horiba). Atomic force microscopy (AFM) experiments were performed on a Bruker Dimension Fastscan system with a silicon probe in non-contact mode. Transmission electron microscopy (TEM, Tecnai F20, FEI) was performed with an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher) coupled with Ar+ etching was used for the analysis of the relative atomic percentage of elements in depth profile on the coated fibers. Small-angle X-ray scattering (SAXS) experiments were carried out on the SAXS station at beamline 1W2A of Beijing Synchrotron Radiation Facility (BSRF). The storage ring was operated at 2.5 GeV with current about 180 mA. The incident X-ray wavelength was selected to be 0.154 nm by triangular bending Si(111) crystal monochromator. The SAXS data were collected with Mar165 CCD (charge coupled device) and the sample-to-detector distance was set at a constant value of 1578.87 mm.
2. Experiment 2.1. Materials Bundles of continuous silicon nitride fibers without sizing agents were directly used as samples for dip coating in this study. Silica sols (Xinyu Chemicals Co. Ltd., China; Peak-Tech New Material Co. Ltd., China) were used as the precursors of SiO2. Ethanol (AR, Hengxing Co. Ltd., China) and distilled water (home-made) were used as the dispersants. Solutions of different concentration were prepared as follows: a mixture containing an appropriate ratio of the commercial sol and ethanol was ultrasonically treated for 30 min and then stirred for over 12 h. The protective gas in the furnace was N2 (99.9999%, Xiangfeng Co. Ltd., China).
3. Results and discussion 3.1. Effects of heat-treatment temperature on mechanical properties of silicon nitride fibers As shown in Fig. 2, the optimal heat-treatment temperature was 400 °C. When the temperature was above 600 °C, the coatings did not increase the tensile strength of the fibers. Based on this finding, the heat treatment of the sol in all other experiments was carried out at 400 °C to obtain the most effective enhancement of the mechanical properties. Furthermore, the elastic modulus fluctuated to a small extent, and the discrepancies can be considered negligible.
2.2. Preparation of the coatings The silica coatings were formed using the laboratory-designed rollto-roll apparatus illustrated in Fig. 1; it consisted of a coating vessel, a drying oven, a heating furnace, and two strand winders. The drying oven was set at 120 °C to evaporate the solvent in the sols. The furnace temperature was set to a value in the range of 300–800 °C. A strand of fibers was dipped into the sol solution to be coated, and then it was dried in the drying oven to form a gel. After that, the fibers were heated under a nitrogen atmosphere in the furnace. The strand was fed from the original bobbin to the final bobbin at 40–120 cm min−1, and different sol concentrations were used to obtain coatings with varying thickness. Distilled water was first used as the dispersant of the sol. However, when the impregnated bundles were dried at 120 °C for several seconds, the dispersant did not evaporate, introducing water to the heating furnace as a result. Therefore, we replaced the distilled water with ethanol in the following experiments. According to the analysis result of an earlier orthogonal test, the reel speed had less impact on the strength than other factors, and 80 cm min−1 was the optimal reel speed in the range of 40–120 cm min−1 for producing fibers with better mechanical properties. The reel speed was thus set at 80 cm min−1 for subsequent experiments.
3.2. Relationships between coating thickness and sol concentration When the sol concentration was above 3%, the thick SiO2 layer caused adhesion between individual filaments of the bundle. The thick coatings on the fibers and the easy formation of cracks can be easily seen in Fig. 3a and b. In contrast, when the concentration of the sol solution was below 1.5%, the relatively thinner layers were not noticeable in the SEM images. The density of the original silicon nitride fibers was measured in ethanol by the Archimedes method to be 2.15 g cm−3, and the average diameter of the original silicon nitride fibers was 12.5 µm. In addition, the density of the silica coating was approximately 2.2 g cm−3, which is very close to the value of the uncoated fibers. Based on the difference between the oxygen content of the uncoated and coated fibers, the assumed thickness of the coating can be estimated using the following equation:
y2 =
y1 πr12lρ1 + y0 π [r22 − r12] lρ0 , πr12lρ1 + π [r22 − r12] lρ0
(1)
where y1, y2, and y0 are the oxygen contents of the uncoated fiber, coated fiber, and amorphous SiO2 layer, respectively; r1 and r2 are the radii of the uncoated fiber and coated fiber respectively; ρ1 and ρ0 are the densities of the uncoated fiber and amorphous SiO2 layer, respectively; and l is the fiber length. According to this equation, y1 and r1 of the original silicon nitride fibers were 0.95 wt% and 6.25 µm, respectively; y0 = 0.53(•) wt%; and ρ1 ≈ ρ0. Using Eq. (1), which assumes that the coating was smooth and uniform, completely shielding the fiber, one can also derive r2 from the parameter y2. The thickness d of
Fig. 1. Schematic drawing of the apparatus for the fiber coating.
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Fig. 2. Mechanical properties of coated fibers that were prepared from sols with solid content of (a) 1.5% and (b) 0.75% and then heat-treated at different temperatures.
Fig. 3. Surface and cross-section morphologies of coated fibers fabricated from sols with different solid contents: (a, f) 6%; (b, g) 3%; (c, h) 1.5%; (d, i) 0.75%; (e, j) 0.3%.
the coating is given by d = r2–r1, and the values of d for different coated fibers are listed in Table 1. To verify the accuracy of the calculation, the thickness of the coatings was measured by the SEM examination of the cross sections of coated fibers, as shown in Fig. 3f–j. The measured thickness was basically in accordance with the calculated thickness, especially for the 1.5%- and 0.75%-sol-coated fibers, confirming that Eq. (1) can be used to derive the thickness of the coating. As shown in Table 1, the elastic modulus remained almost unchanged for coatings of different thickness, which means a single fiber's resistance against elastic deformation remained stable. This indicates that the elastic modulus was mainly affected by the microstructure of the original material as a whole, and it was not significantly influenced by the surface state.
Table 1 Characterization of the original and coated silicon nitride fibers. Sol concentration by weight
Elastic modulus (GPa)
Oxygen content (wt%)
Assumed thickness of the coating (μm)
– 6% 3% 1.5% 0.75% 0.3%
152 151 152 152 145 149
0.95 4.55 2.88 1.92 1.49 1.20
– 0.228 0.119 0.059 0.033 0.024
Fig. 4. XPS depth profile of coated silicon nitride fibers prepared from sols with solid content of (a) 1.5% and (b) 0.75% (sputtering rate: 3 nm min−1).
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single filaments of the uncoated and coated fibers. Because the silicon nitride fibers exhibited typical features of brittle fracture and a significant range of values for the tensile strength, the two-parameter Weibull model, which is based on the theory of the weakest link and widely used to describe tensile strength of brittle materials [11], was adopted for evaluation of the strength distribution. In this model, the failure probability P is estimated by Eq. (2):
P = 1 − exp[−(σ / σ0)m],
(2)
where σ is the fracture stress, m is the Weibull modulus parameter, and σ0 is the scale parameter. By taking the natural logarithm of Eq. (2) and rearranging the terms, we obtained Eq. (3),
ln ln[1/(1 − P )] = m ln σ –m ln σ0,
where the slope is equal to m, and the y intercept is equal to –mlnσ0. The σ values are the experimental tensile stresses, and P can be defined as
Fig. 5. Weibull plots for fibers with and without coatings.
Pi = i/(n + 1), Table 2 Weibull parameters for uncoated and silica-coated fibers. Type
Average tensile strength (GPa)
Weibull modulus, m
Scale parameter,σ0 (GPa)
Related coefficient, R2
No coating 6% 3% 1.5% 0.75% 0.3%
1.3085 0.9000 1.0834 1.5367 1.6456 1.3384
3.2278 9.1388 4.2480 5.5393 6.0244 3.8044
1.4549 0.9513 1.1879 1.6678 1.7762 1.4818
0.90489 0.97308 0.97196 0.95554 0.97452 0.97769
(3)
(4)
where n is the number of data points, and i is the rank of the ith data point. The Weibull plots for the uncoated fibers and fibers coated with different concentration of sol are represented in Fig. 5, and the corresponding results of the Weibull analysis are given in Table 2. The m values of the coated fibers were remarkably higher than that of the uncoated fibers, indicating a narrower distribution of the tensile strength among the silica-coated fibers [8]. The surface morphology of the uncoated and coated fibers was analyzed using AFM. Both images in Fig. 6 are shown on the same scale to facilitate direct comparison. As seen in Fig. 6a, the original fibers had a rather rough surface with pits and bulges. However, it was difficult to observe these minor defects solely by SEM. In comparison, the coated fibers had a smoother surface with fewer defects, which confirmed the flaw-repairing effect of the glassy SiO2 phase. However, it did not go unnoticed that the tensile strength of the fibers decreased with increasing sol concentration (from 0.75% to 6%). This deterioration occurred because the silica coatings also had unfavorable effects on the fiber strength, which included the increased cross-sectional area and stress concentration induced by minor cracks of amorphous SiO2, as confirmed by the SEM images in Fig. 3. For the 1.5%- and 0.75%-sol-coated fibers, whose assumed coating thickness was 59 and 33 nm, respectively, the surface-healing effect was dominant, as indicated by their higher strength when compared to that of the uncoated fibers. For the 6%- and 3%-sol-coated fibers whose assumed coating thickness was 228 and 119 nm, respectively, the strength loss resulting from the sticky and cracking nature of the thick coatings
The XPS depth profiles of the coated fibers are shown in Fig. 4. The spectra show that substantial amounts of Si and O were present in the surface layer, confirming the successful coating of the fibers. The thickness of the coating on the 1.5%-sol-coated fibers was apparently higher than that of the 0.75%-sol-coated fibers. The composition was in accordance with the bulk atomic ratio of the coatings, except for the carbon-rich and nitrogen-rich layer originating from contamination in the surface region. 3.3. Effects of silica coatings on mechanical properties of silicon nitride fibers To evaluate the effects of silica coatings on the mechanical properties of the fibers, we focused on investigating the tensile strength of
Fig. 6. AFM images (3D representation of height) of (a) uncoated fibers and (b) 0.75%-sol-coated fibers.
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Fig. 7. TEM micrographs of SiO2 sols containing colloids of different sizes.
Fig. 8. SEM micrographs of the 0.75%-sol-coated fibers fabricated from sols consisting of colloids measuring (a) 30–40 nm and (b) 70–80 nm.
Fig. 9. AFM images of the 0.75%-sol-coated fibers prepared from sols containing colloids of different sizes: (a) 30–40 nm; (b) 70–80 nm.
3.4. Effects of particle size of the sol on mechanical properties of silicon nitride fibers
started to counteract the surface-healing effect; consequently, the strength of the coated fibers was much lower than that of the uncoated fibers. However, the strength the 0.3%-sol-coated fibers was not affected, probably because the sol was too dilute.
Fig. 7a shows the colloid size of the silica sol (type I) we used in prior studies, which was 2–8 nm for most particles. To determine whether the colloid size would influence the repair of surface flaws, we purchased two silica sols with different particle sizes, referred to as types II and III and shown in Fig. 7b and c. The sizes of types II and III 5
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Fig. 10. (a) 2D SAXS pattern and (b) the corresponding micropore size distribution of the original fibers.
3.5. Mechanism of the surface-healing effect
Table 3 Scatterer size distribution by cascade tangent rule. Radius of gyration without assumption (nm)
Diameter of assumed globular scatterer (nm)
Normalized scatterer volume fraction
0.80 1.44 2.69
2.07 3.72 6.95
0.568 0.302 0.130
The two-dimensional SAXS pattern, shown in Fig. 10a, of the original silicon nitride fibers was collected with the Mar165 CCD and converted into digital data by the Fit2D software package [12]. The SAXS curves of intensity versus scattering-vector were obtained by rectangular integration, and the data analysis was performed by using the program S.exe, written in Intel Visual FORTRAN for SAXS data processing and analysis [13]. To obtain the size distribution of the scatterers, we used the method first described by Jellinek and Fankuchen in 1946 [14,15]. Using this method, one can derive the radius of gyration Rg. The radius r of a perfect sphere that is assumed to represent an X-ray-scattering particle is obtained from the equation Rg = (3/ 5)0.5r [16], and the results are listed in Table 3. Therefore, the average radius of gyration (without assumptions of the scatterer shape) was 1.24 nm, and the average diameter of a globular scatterer was 3.20 nm. Moreover, the scatterers could be flaws on the fiber surface or pores inside the fibers, both of which could cause non-uniformity in the electron density.
were approximately 30–40 nm and 70–80 nm, respectively, which are much higher than the size of type I colloids. It turned out that after the same optimal coating process, the type-II and type-III coatings did not increase the tensile strength of silicon nitride fibers. As shown in Fig. 8, the fibers coated with type-II and type-III silica exhibited rougher surfaces with visible particles, as suggested by the AFM images in Fig. 9.
Fig. 11. (a) Failed healing effect of thick silica coatings. (b) Model of the mechanism of the surface defect healing effect. (c) Failed healing effect of large colloid particles.
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Acknowledgements
Sawyer et al. [17] reported that surface defects play a more important role than internal ones in determining the tensile strength of ceramic fibers. The flaw–strength relationship of SiC fibers in their study confirmed that the low tensile strengths they obtained were caused by surface flaws and particulate defects, and most of the strength limitations occurred because of surface damage. Hence, as discussed above, the strength of silica-coated fibers with the appropriate thickness was higher than that of uncoated fibers because the thin coatings healed the surface defects on the fibers. It should be noted that only coatings prepared from sols with the appropriate colloid size had this kind of strength-improving effect because the particle size should match the surface defect size to repair the flaws. On the other hand, silica alkoxide molecules that dip-coated the fiber surface combined to form a Si–O–Si network along the fiber axis during polycondensation, probably helping to increase the strength. Based on the above analysis, the simple model in Fig. 11 can be used to describe the mechanism of surface defect healing by the coating. Two failed mechanisms are also depicted in the figure.
This work is supported by the National Natural Science Foundation of China (No. 51203184). References [1] Z. Krstic, V.D. Krstic, Silicon nitride: the engineering material of the future, J. Mater. Sci. 47 (2012) 535–552. [2] X. Hu, C. Shao, J. Wang, H. Wang, Characterization and high-temperature degradation mechanism of continuous silicon nitride fibers, J. Mater. Sci. 52 (2017) 7555–7566. [3] X. Hu, C. Shao, J. Wang, H. Wang, J. Cheng, Effects of residual radicals on compositional and structural stability of silicon nitride fibers, J. Eur. Ceram. Soc. 37 (2017) 4497–4503. [4] Y. Yokoyama, T. Nanba, I. Yasui, H. Kaya, T. Maeshima, T. Isoda, X-ray diffraction study of the structure of silicon nitride fiber made from perhydropolysilazane, J. Am. Ceram. Soc. 74 (1991) 654–657. [5] K. Sato, H. Morozumi, O. Funayama, H. Kaya, T. Isoda, Developing interfacial carbon-boron-silicon coatings for silicon nitride-fiber-reinforced composites for improved oxidation resistance, J. Am. Ceram. Soc. 85 (2002) 1815–1822. [6] T. Naganuma, K. Naito, J. Yang, High-temperature vapor deposition polymerization polyimide coating for elimination of surface nano-flaws in high-strength carbon fibers, Carbon 49 (2011) 3881–3890. [7] S.L. Gao, E. Mader, R. Plonka, Nanocomposite coatings for healing surface defects of glass fibers and improving interfacial adhesion, Compos. Sci. Technol. 68 (2008) 2892–2901. [8] S.Y. Cao, J. Wang, H. Wang, Effect of CVD carbon coatings on properties of SiC fibres, Surf. Eng. 33 (2017) 573–577. [9] J. Liu, S. Wang, P. Li, M. Feng, X. Yang, A modified dip-coating method to prepare BN coating on SiC fiber by introducing the sol–gel process, Surf. Coat. Technol. 286 (2016) 57–63. [10] H.A. Katzman, Fibre coatings for the fabrication of graphite-reinforced magnesium composites, J. Mater. Sci. 22 (1987) 144–148. [11] Y. Paramonov, J. Andersons, A family of weakest link models for fiber strength distribution, Composites: Part A 38 (2007) 1227–1233. [12] 〈www.esrf.eu/computing/scientific/FIT2D/〉. [13] Z.H. Li, A program for SAXS data processing and analysis, Chin. Phys. C 37 (2013) 108002. [14] M.H. Jellinek, E. Solomon, I. Fankuchen, Measurement and analysis of small-angle X-ray scattering, Ind. Eng. Chem. 18 (1946) 172–175. [15] W. Wang, X. Chen, Q. Cai, G. Mo, L.S. Jiang, K. Zhang, Z.J. Chen, Z.H. Wu, W. Pan, In situ SAXS study on size changes of platinum nanoparticles with temperature, Eur. Phys. J. B 65 (2008) 57–64. [16] A. Guinier, G. Fournet, C.B. Walker, Small-angle Scattering of X-rays, J. Wiley & Sons, New York, 1955. [17] L.C. Sawyer, R. Arons, F. Haimbach, M. Jaffe, K.D. Rappaport, Characterization of Nicalon®: strength, structure and fractography, in: Proceedings of the 9th Annual Conference on Composites and Advanced Ceramic Materials: Ceramic Engineering and Science Proceedings, 20–23 January, ACS, Columbus, Cocoa Beach, 1985.
4. Conclusion Silica coatings were prepared on silicon nitride fibers by the sol–gel process. Using the modified dip-coating method, a continuous and uniform gel film was formed on the silicon nitride fibers. After heat treatment, the gel film turned into a relatively compact silica coating. When the optimal sol concentration, colloid size, heat-treatment temperature, and reel speed were used, the tensile strength of the coated fibers was approximately 26% higher than that of the uncoated fibers owing to the surface flaw healing effect of the coatings and the Si–O–Si network imparted by polycondensation of the sol. In contrast, the silicacoated fibers prepared from sols with concentrations of 3% and higher exhibited lower strength than that of the uncoated fibers, probably because of the sticky and cracking nature of the thick coatings. The proposed mechanism of the surface defect healing effect is mainly determined by the particle size in the sol and the thickness of coatings, and it can be described by a simple model. Conflicts of interest None.
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