Oxidation behavior of Si3N4 fibers derived from polycarbosilane

Corrosion Science 136 (2018) 9–17

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Oxidation behavior of Si3N4 fibers derived from polycarbosilane a,b,⁎

a

a

a

a

a

T a

Siwei Li , Yongcai Li , Hongrui Xiao , Xiangdong Li , Qi Wang , Ming Tang , Huibin Tu , Jinqiu Huanga, Lifu Chena, Zhiyang Yuc a b c

College of Materials, Fujian Key Laboratory of Advanced Materials, Xiamen University, China Key Laboratory of High Performance Ceramic Fibers (Xiamen University), Ministry of Education, Xiamen, 361005, China School of Materials Science and Engineering, Xiamen University of Technology, Xiamen, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Silicon nitride fibers Oxidation kinetics Microstructure Polymer derived ceramics

Si3N4 fibers prepared from polycarbosliane are exposed in dry and wet air respectively from 800 to 1300 °C for up to 1 h. After dry air oxidation, an uniform SiO2 coating doped with nitrogen is formed on fiber surface. However, the coating obtained in wet air is much thicker with more complex microstructure containing sublayers of N-doped SiO2, near stoichiometric Si3N4 and graphite-like nanoribbons. Activation energy for oxidation in wet and dry air is determined as 108 and 152 kJ mol−1, respectively. Both of the values are lower than the known Si3N4 materials with Ea of 259–485 kJ mol−1.

1. Introduction Continuous silicon nitride (Si3N4) thin fiber is a family of polymerderived ceramic fibers composed of Si and N, and is commonly doped with different amount of C and O according to the precursor structures [1–4] and preparation techniques [5–7]. The non-stoichiometric nature of the precursor results in uniformly disordered structures in the fiber after pyrolysis. Consequently, most of the polymer derived Si3N4 fibers are amorphous up to 1400 °C and retain high tensile strength [8–11]. With additional characters of high electrical resistivity, low thermal conductivity and low dielectric constant, the continuous Si3N4 fibers are promising reinforcements for thermal insulation and microwavetransparent composites [12–14]. In recent years, one of the expected applications of Si3N4 fibers has been indicated as reinforcement in the high temperature microwave-transparent composites as replacement of the SiO2 fiber, which losses most of its original strength when serving at above 900 °C, owing to the severe grain coarsening [15–17]. Nevertheless, also the Si3N4 fibers face challenges during service in air, because of passive oxidation of the fiber surface and microcracking at the fiber/oxidation coating interface. To date, studies on the oxidation behavior of Si3N4 were conducted mainly on powders and bulks [18–25]. Results indicated that the oxidation rates of Si3N4 depended upon the purity of the material. The lowest oxidation rates were observed for high-purity chemical vapor deposited (CVD) Si3N4 [25]. However, these works contributed little to knowing the oxidation behavior of the continuous Si3N4 fibers, which was not only because of the variations in composition between bulks

⁎

and fibers, but also due to the particularity of the fibers with respected to heat transfer process and the resultant structural characters. Studies regarding the Si3N4 fibers mainly dealt with the crystalline behavior of the fibers at high temperatures in inert atmospheres. Matsuo et al. studied the structural evolution of perhydropolysilazane (PHPS)-derived Si3N4 fiber after heating at 1400–1500 °C in N2. α-Si3N4 particles were only detected on the fiber surface, which seemed to be proceeded by a vapor phase growth mechanism [26]. Gilkes et al. explored the microstructure of hydridopolysilazane (HPZ)-derived Si3N4 fibers by using neutron scattering and nuclear magnetic resonance [27]. The HPZ-Si3N4 fibers were found to contain excess crystalline silicon and disordered carbon, and the fibers annealed in N2 at 1400 °C for 100 h contained a cristobalite phase. Similar results evidenced that these polymer-derived Si3N4 fibers had excellent thermal stability [28]. Cinibulk et al. investigated the oxidized microstructure of a Si-B-C-Nbased fiber derived from an N-methylpolyborosilazane precursor. With presence of B, complicated coating structure formed on the fiber surface upon heating in stagnant laboratory air at 1500 °C for 2 h. Although the fiber core retained its amorphous nature, the fiber suffered significant strength degradation after oxidation [29]. In the present work, Si3N4 fiber derived from polycarbosilane (PCS) are exposed to wet and dry air respectively in a temperature range from 800 to 1300 °C for up to 1 h. Microstructure, mechanical property and oxidation kinetics of the oxidized Si3N4 fiber are explored. The effect of reactive gas species on oxidation behavior of the Si3N4 fiber is here discussed.

Corresponding author at: College of Materials, Fujian Key Laboratory of Advanced Materials, Xiamen University, China. E-mail address: [email protected] (S. Li).

https://doi.org/10.1016/j.corsci.2018.02.032 Received 14 July 2017; Received in revised form 26 January 2018; Accepted 16 February 2018 Available online 21 February 2018 0010-938X/ © 2018 Elsevier Ltd. All rights reserved.

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Table 1 Chemical composition of the as prepared Si3N4 fibers. Element

Si

N

O

C

Concentration (wt.%)

71.06

27.47

0.57

0.90

2. Experimental procedure The PCS precursor is synthesized following the Yajima’s route [30,31]. The average molecular weight (M) is 1190, and the softening point is 189 °C. The obtained PCS is used as precursor of the Si3N4 fiber and treated through the route developed by Japan atomic energy research institute [7]. Table 1 shows the chemical composition of the asprepared Si3N4 fibers. Average tensile strength of the fiber is 936 ± 87 MPa. For oxidation experiment, green fibers are placed in alumina crucible and pushed into alumina tube furnace with MoSi2 as heating element. Wet air is obtained by blowing compressed dry air (Linde Gas, 99.99%, China) into deionized water in an airtight container, which was connected directly to the furnace tube. Partial pressure of the water vapor can be controlled by changing the dry air pressure as well as the water temperature according to Ideal Gas Law. In this work, the partial pressure of water vapor is 2.4 KPa. The heating rate of the oxidation is 5 °C min−1. The holding times at each target temperature, 800, 1000 and 1300 °C, are 0.5 h and 1 h respectively. Tensile strength of the single fiber is tested at room temperature using an electronic single fiber strength tester (SLBL-10N, SHIMADZU, Japan). 25 specimens are used in each test. The gauge length is 25 mm and the loading rate is 1 mm min−1. The phases of the fibers are identified using an X-ray diffractometer (D8 Advance, Bruker-AXS, Germany). The microstructure is characterized using SEM of 30 keV (SU-70, Hitachi, Japan) and by TEM of 200keV(JEM 2100, JEOL, Japan and Talos F200S, FEI, USA). The TEM specimens are prepared by a special embedding method reported elsewhere [32]. The embedded specimens are thinned by cutting, mechanical grinding, dimpling and ion milling.

3. Result and discussion 3.1. Microstructural evolution of the Si3N4 fiber after oxidation

Fig. 2. X-ray diffraction patterns of the Si3N4 fibers after oxidized in (a) wet air and (b) dry air.

Fig. 1 shows the weight changes of the Si3N4 fibers after oxidation. All of the treated fibers gain weight. The weight gain in wet air is significantly higher than that in dry air, indicating that the oxidation reaction is promoted by wet air condition. Similar results were also obtained in the oxidation of crystalline Si3N4 and SiC materials

[33–35]. Whether in wet or dry air, the weight of the oxidized fibers increases with increasing temperature. As the Si3N4 fibers are oxidized at 800 °C, the weight gain in isothermal processes changes almost linearly with the soaking time, indicating the oxidation process is controlled by surface reaction. However, above 1000 °C, the weight gain in last half hour becomes lower than that in first half hour. This indicates that the oxidation is retarded at high temperatures, since the inward diffusion of oxidizing gases is restricted and becomes a rate-controlling process as the thickness of the oxidated coating increases. Fig. 2 shows the XRD pattern of the treated Si3N4 fibers. All the Si3N4 fibers remain amorphous even after oxidation at 1300 °C for 1 h. The sharp peak at 21.8° after heating at 1300 °C belongs to lattice plane (004) of tridymite, which results from the crystallization of the silica coating. Based on the Scherrer formula, grain size of the tridymite coatings obtained after different treatments can be estimiated by the change of the FWHM (Full width at half maximum of peak intensity). As compared with the dry air oxidation, the Si3N4 fiber oxidized in wet air at 1300 °C/1 h has a (004) peak of higher intensity with narrower FWHM (∼70% width of the FWHM obtained in dry air), indicating a larger grain size of the tridymite and a higher crytallinity degree of the coating. Results suggest that water vapor promotes not only the oxidation reaction, but also grain growth of SiO2.

Fig. 1. Weight gain of Si3N4 fibers after oxidized in (a) wet air and (b) dry air.

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Fig. 3. Cross-section morphology of the Si3N4 fibers oxidized in (a) wet air and (b) dry air.

Fracture surface of the oxidized Si3N4 fibers is shown in Fig. 3. For the fibers before oxidation, the fracture initiates mainly from the fabrication-induced defects, which distribute randomly in the fibers. After oxidation, the fiber failure starts instead from the interface of fiber/ coating. When the fibers are oxidized below 1000 °C, amorphous brittle

fracture characters with mirror zone, mist zone and hackle zone are clearly identified (Fig. 3a), showing the traces of crack propagation from stable state to the final unstable state. After oxidation at 1300 °C, with the increase of the coating thickness, more interfacial microcracks are induced upon fiber cooling. When the oxidized fibers are loaded 11

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with stress, these interfacial microcracks are activated almost simultaneously, connecting to each other before unstable crack propagation thus leaving a very smooth surface. Additionally, for the fibers oxidized at 1300 °C in dry air, many pores are observed at the fiber/coating interface, as shown in Fig. 3b. Oxidation of the Si3N4 fiber in dry air can be described as a generally accepted reaction [25,36]: Si3N4(s) + 3O2(g) = 3SiO2(s,l) + 2N2(g)

(1)

As the increase of the silica thickness, the gas products hardly escape out of the fiber and are finally trapped as pores at the interfacial region. It should be noted that formation of the N2 may have the possibility of reducing the reaction rate by counter-diffusion or blocking of diffusion paths of the oxidizing gases. Du et al. reported the dependence of oxidation scale growth kinetics on the ambient N2 partial pressure for the oxidation of CVD Si3N4 [20]. Results indicated that the changes in the N2 partial pressure had little effect on the calculated parabolic rate constants and activation energies for oxidation of Si3N4. They concluded that the N2 is not actively involved in the rate limiting process during the oxidation of Si3N4. However, after a systematic calculation, Luthra [37] offered different results and claimed that the rate-limiting step of the oxidation of Si3N4 is influenced by the gradients in partials pressures of the gas species. If the O2 partial pressure at the substrate/ silica interface is the lowest possible while the N2 partial pressure is the same as that at the coating surface, the oxidation is controlled by O2 diffusion. On the other hand, if the oxygen partial pressure at the Si3N4/ SiO2 interfaces is the same as that in the gas phase while the corresponding N2 partial pressure is very high, the rate of oxidation is limited by the N2 diffusion. As this time, gas bubbles will form to prevent the N2 partial pressure from rising significantly over the gas pressure. In the current work, formation of the gas bubbles at the fiber/coating interface also indicating a high pressure of the N2. It should be a sign that the oxidation kinetics is turned to be influenced by the outward diffusion of N2 or counter-diffusion of O2/N2. The Si3N4 fibers oxidized in wet air do not display interfacial pores in SEM images, indicating that the resultant gases escape easily during the wet air oxidation. Former studies pointed out that water is tend to dissolve in the silica by forming SieOH bonds [38], which leads to formation of additional defects in the microstructure and accelerates ionic diffusion as well as the gas release. The conclusion is supported by current results. The microstructure of the oxidized fiber is investigated by TEM and is shown in Fig. 4. The as-prepared fiber shows amorphous structure wherein brighter regions of 2–5 nm in diameter are embedded (Fig. 4a). In a high angle annular dark field (HAADF) images (Fig. 4b) with totally Z contrast, the nano-regions show the dark contrast just like the vacuum space in the same image (Fig. 4b), confirming the features of mesopores. However, BET measurement on the as-prepared fibers shows no available absorption signals, indicating that these mesopores are closed. In the preparation of the current Si3N4 fiber, one of the key processes is aminolysis of the crosslinked PCS to displace C as N (< 800 °C). After that, hydrogen and methane are released as gas products. Based on the TEM observations, it can be speculated that release of gas products during aminolysis produce a network of nanochannels throughout the ceramic fibers. During subsequent pyrolysis at elevated temperatures (800–1300 °C), these nanochannels are enclosed as mesopores with the fiber shrinkage under a driving force of reduction of surface free energy [39]. After oxidation in dry air at 1300 °C for 1 h, an oxidation coating of about 300 nm thick is formed on fiber surface, as shown in Fig. 5a. Selected area electron diffraction (SAED) pattern identifies that the coating is monocrystalline tridymite, consisting well with the XRD results. However, the tridymite coating is quite unstable under the electron irradiation in TEM mode. In 1–2 min, the irradiated region totally turns into amorphous structure, while the unirradiated region remains crystalline, as shown in Fig. 5b. High resolution image (HRTEM) of Fig. 5c shows that the fiber/coating interface is not quite smooth which

Fig. 4. TEM images of the Si3N4 fibers before oxidation. (a) HRTEM image and (b) HAADF image.

preserves part of the surface shape of the mesopores within fiber matrix, corroborating a relatively large contacting area for the fiber oxidation. Previous studies showed that the SiO2 polymorphs of quartz, tridymite or even cristobalite have high sensitivity for electron irradiation damage (EID) under the TEM [40–42]. The EID-sensitivity depends on the degree of the polymerization (“connectivity”) of SiO4tetrahedra, which is not only related to the total irradiation dose and the resultant temperature rise, but also influenced by the existence of impurity element substitution [42]. The elemental distribution of the irradiated fiber coating is explored by EDS mapping under STEM mode with beam energy of 200 keV (Fig. 5d). Apparently, the tridymite coating is composed by not only Si and O, but also N in lower concentration as compared to the fiber matrix. The doping content of N in the tridymite coatign is determined as ∼8 wt.% according to the EDS quantitative examination. Based on the previous study [42], the doping of N should contribute to increasing the EID sensitivity of the oxidized coating. When the heating is conducted in wet air at 1300 °C for 1 h, a fourlayer coating with complicated structure is formed on the fiber (Fig. 6). The thickness of the whole coating exceeds 500 nm, much larger than the thickness growth in dry air. Rodríguez Viejo et al. [43]

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Fig. 5. TEM images of the oxidation film formed in dry air at 1300 °C for 1 h. (a) Bright filed image before and (b) after irradiation for 2 min, (c) HRTEM image of the fiber/film interface, (d) EDS mapping results obtained in STEM mode.

in layer 4 is clearly higher than in layer 3 and even higher than the fiber matrix. Additionally, quantitative EDS results show that the C concentration in layer 4 is lower than both of the fiber matrix and the layer 3 (C signal for layer 3 is induced by the presence of epoxy resin aside the TEM specimen). It is reasonably speculated that only C is depleted by O2 in the layer 4, leaving the Si and N to reach higher contents as compared with the fiber matrix, but more close to stoichiometric Si3N4. Formation of layer 4 should be attributed from a significant reduction of the partial pressure of O2 as it diffuses through a certain thickness from layer 1–3. A threshold value of the O2 partial pressure should exist for oxidizing the SieN bond. Below this pressure, the O2 could only be consumed by oxidizing the C in the fiber [46]. Presence of the gradient of the O2 partial pressure supports the conclusion that oxidation of the PCS-Si3N4 fiber is controlled mainly by O2 inward diffusion rather than by N2 outward diffusion. However, trapping of the gases in layer 3 (Fig. 6d and e) should be a sign that the outward diffusion of gas products starts to become ignorable to influence the oxidation rate of the Si3N4 fiber. It seems that the TEM images capture the transient state of the oxidation from O2 diffusion-controlled state to a counter diffusioncontrolled state. It is also interesting that the highest concentration of N is identified in layer 2. Additionally, C of high concentration with about 50 nm depth is also detected in this layer. Combining the HRTEM and EDS results, the ordered nano domains in layer 2 may be graphite ribbons, which is induced by decomposition of CO under high temperature and certain positive pore pressure [47,48]. Formation of CO is resulted from oxidation of the C in the fiber. The high concentration of N in the layer 2 might refer to the gathering of N2 bubbles. It should be noted that the crystalline phase in layer 2 also has the possibility of being graphite like C3N4 (g-C3N4), which is one allotrope type of C3N4

demonstrated that oxidation of Si-based ceramics should be retarded after the crystallization of silica, since the diffusivity of oxygen in crystalline silica is much lower than that in vitreous silica [44,45]. However, present results indicates the oxidation of Si3N4 fiber is not evidently retarded in wet air after crystallization of silica, but accelerates with the dissolution of the water vapor in the coating. The multilayers are labeled from 1 to 4, as shown in Fig. 6a. Single crystalline layers of 1 and 3 are separated by layer 2, which is mainly amorphous but doped with highly ordered graphite-like structures. The crystalline layer 3 and the fiber surface are separated by layer 4, which is totally amorphous with 50 nm in thickness (Fig. 6b). Amorphization of the coatings inevitably happens also after electron irradiation for 1–2 min under TEM mode. The irradiated layer 1 and 3 totally turns into amorphous structure thus showing a uniform contrast, while layer 2 remains unchanged, as shown in Fig. 6c. By enlarging the region near layer 2, traces of gas release can be identified (Fig. 6d). Pores of about 10–50 nm in diameter are embedded in layer 3 closed to layer 2. Dark field image of Fig. 6e reveals that the depth of gas trapping region in layer 3 is about 100 nm. It proves that the gas products are partially trapped when diffusing outward the tridymite layer 3 and notably blocked as they reach layer 1.The gas trapping results in the formation of porous layer 2. The graphite-like structures, with brightest contrast in Fig. 6e, is located not only in layer 2, but also in some porous regions in layer 3, suggesting formation of these ordered domains associated with the gas trapping factors. The composition of the multi4-layered coating is also explored by EDS mapping under STEM mode. Layer 1 and Layer 3 have similar elemental composition containing Si, N and O, confirming formation of the N doped tridymite layer. It is worth noting that the N concentration 13

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Fig. 6. TEM images of the oxidized layers formed in wet air at 1300 °C/1 h. (a) Bright field image of the coating. (b) Enlarged image of the region near layer 4. (c) Bright field image of the coating after irradiation for 2 min. (d) Enlarged image of the irradiated coating near layer 2. (e) Dark field image of the irradiated coating. (f) EDS mapping results obtained in STEM mode. In the above figures, “1” and “3”refer to N doped tridymite layers, “2” refer to the porous layer containing graphite-like structures, “4” refers to the layer of near stoichiometric Si3N4.

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and has similar structure as graphite [49]. However, it is difficult to separate g-C3N4 from graphite by HRTEM image since the (002) spacing of the two phases are very close and ranging from 0.32 nm to 0.34 nm [50,51]. The chemical composition of the graphite-like structure remains unclear and will be studied further. The above mentioned microstructure reveals a complex oxidation process of the present Si3N4 fiber containing Si, N, C and O. The quaternary composition of the fiber makes it combine the oxidation features of SiC-Si3N4 and results in simultaneous diffusion of CO and N2 through the oxide layer outwards. It also involves some oxygen initially present in the fiber and possibly some traces of hydrogen [46]. Beside, mesopores in the fiber also promote oxidation thus increasing the complexity of the oxidation structure. Neglecting the presence of hydrogen, the overall reaction of the fiber oxidation could be described by the following equation [52,53]. SiNxCyO(1-x-y)(s) + δO2(g) → SiN(x-2γ)O(2δ-1+x)(s) + yCO(g) + γN2(g) (2)

Fig. 8. Evolution of strength retained ratio of the Si3N4 fibers after oxidation in dry or wet air.

Fig. 7 gives an overview on the structural evolution of the PCS-derived Si3N4 fiber. Apparently, oxidation behavior of the Si3N4 fiber in wet air is much more complex than in dry air. The diffusion of the gas species across the oxidized coating is accelerated in wet air with the assistance of water vapor. As a result, the coating thickens in a faster speed with higher crystallinity. However, both the thickening of the coating and enhancement of its crystallinity restrict the inner/outer diffusion of the gas species. The competition among gas diffusion/ convection, coating thickening and crystallinity enhancement leads to the separation of the coating as multilayers

with increasing temperatures and prolonged treating time. It is also seen that the residual strength of fiber oxidized in wet air is evidently higher than that of the fiber oxidized in dry air. For example, after oxidation at 1300 °C for 1 h in wet air, the Si3N4 fiber retains 52% of its original strength. The ratio reduces to 40% after the Si3N4 fibers are oxidized in dry air at same heating parameter. Strength decay is related to the formation of large interfacial defects during dry air oxidation. It is thus deduced that the instinct strength of the as-prepared Si3N4 fibers is mainly influenced by the size and distribution of the fabrication-induced defects, while the strength loss of the oxidized fiber is controlled by the interfacial defects between the fiber and the coating. Table 2 summarizes the thicknesses of the oxidized layers. The kinetics constant KT is calculated according to Eq. (3).

3.2. Mechanical property and oxidation kinetics Fig. 8 shows that the tensile strength of the Si3N4 fibers decreases

Fig. 7. Illustration of the oxidation behaviors of the PCS-derived Si3N4 fiber in dry or wet air.

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4. Conclusions

Table 2 Average coating thickness and kinetic constant, KT. Temperature (°C)

800 1000 1150 1300

Oxidation behavior of the PCS-Si3N4 fibers is explored. The main findings are summarized as follows.

KT(nm2 s−1)

Average coating thickness after oxidation at T in wet air for 0.5/1.0 h (nm)

Average coating thickness after oxidation at T in dry air for 0.5/1.0 h (nm)

Dry air

NA/114 199/231 293/345 405/496

NA 95/110 152/178 251/308

NA 1.7 5.6 17.6

Wet air

(1) The as-prepared Si3N4 fiber shows amorphous structure with mesopores. The weight changes of the fiber oxidized in wet and dry air show similar trend with different rates. Oxidation kinetics of the fiber is determined as reaction-controlled at 800 °C and changed into diffusion-controlled at above 1000 °C. The Ea of the present Si3N4 fiber for oxidation in wet and dry air are 108 kJ mol−1 and 152 kJ mol−1, respectively. (2) TEM evidences that microstructure of the oxidized coating obtained in wet air is more complicated than the coating formed in dry air. For the wet air oxidation, competition among gas diffusion/convection, coating thickening and crystallinity enhancement is more vigorous, leading to the formation of the sub-layers of N-doped tridymite, near stoichiometric Si3N4 and graphite-like nanoribbons.

NA 10.3 18.6 45.6

Fundings This work was supported by the National Natural Science Foundation of China (NSFC) under Grants No: 51302234 and by the Creative Research Foundation of Science and Technology on Thermostructural Composite Materials Laboratory (China) under Grants No: 6142911040113. References [1] D. Mocaer, G. Chollon, R. Pailler, L. Filipuzzi, R. Naslain, Si-C-N ceramics with a high microstructural stability elaborated from the pyrolysis of new polycarbosilazane precursors. 5. Oxidation-kinetics of model filaments, J. Mater. Sci. 28 (1993) 3059–3068. [2] B.G. Penn, J.G. Daniels, F.E. Ledbetter, J.M. Clemons, Preparation of silicon carbide-silicon nitride fibers by the pyrolysis of polycarbosilazane precursors – a review, Polym. Eng. Sci. 26 (1986) 1191–1194. [3] M. Arai, O. Funayama, H. Nishii, T. Isoda, High-purity Silicon Nitride Fibers, (1989) US. [4] K. Okamura, M. Sato, Y. Hasegawa, Silicon-nitride fibers and silicon oxynitride fibers obtained by the nitridation of polycarbosilane, Ceram. Int. 13 (1987) 55–61. [5] T. Vaahs, M. Bruck, W.D.G. Bocker, Polymer-derived silicon-nitride and silicon carbonitride fibers, Adv. Mater. 4 (1992) 224–226. [6] S. Kamimura, T. Seguchi, K. Okamura, Development of silicon nitride fiber from Sicontaining polymer by radiation curing and its application, Radiat. Phys. Chem. 54 (1999) 575–581. [7] 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. [8] Y. Li, J.C. Gao, Preparation of silicon nitride ceramic fibers from polycarbosilane fibers by gamma-ray irradiation curing, Mater. Lett. 110 (2013) 102–104. [9] H.T.N.K. Aoki, T.T.N.K. Suzuki, T.T.N.K. Katahata, M.T.N.K. Haino, G.T.N. Nishimura, H.T.N.K. Kaya, K.T.N.K. Tamura, T.T.N.K. Isoda, Amorphous Silicon Nitride-based Fibers Composite Material Reinforced with the Fibers and Processes for Production Thereof, (1989) EP. [10] G. Chollon, T. Vogt, K. Berroth, Thermal stability of a silicon nitride basedfiber, Key Eng. Mater. 127–131 (1997) 185–192. [11] A. Vital, U. Vogt, Amorphous silicon-oxynitride submicron fibres, Ceram. Eng. Sci. Proc. 23 (2008) 355–360. [12] D. Li, High temperature wave-transparent silicon nitride materials, Aerospace Mater. Technol. 41 (2011) 4–9. [13] Y. Li, J.C. Gao, Y.S. Xu, Preparation of high strength silicon nitride ceramic fibers from polycarbosilane fibers by γ-ray irradiation and thermal cross-linking, HeHuaxue yu Fangshe Huaxue/J. Nucl. Radiochem. 36 (2014) 272–276. [14] C.R. Zou, C.R. Zhang, Y.D. Xiao, L.I. Bin, S.Q. Wang, F. Cao, K. Liu, Progress and prospect of high performance wave-transparent ceramic fibers, Bull. Chin. Ceram. Soc. 32 (2) (2013) 274–279. [15] N.E. Prasad, D. Loidl, M. Vijayakumar, K. Kromp, Elastic properties of silica–silica continuous fibre-reinforced, ceramic matrix composites, Scripta Mater. 50 (2004) 1121–1126. [16] N.E. Prasad, S. Kumari, S.V. Kamat, M. Vijayakumar, G. Malakondaiah, Fracture behaviour of 2D-weaved silica–silica continuous fibre-reinforced, ceramic-matrix composites (CFCCs), Eng. Fract. Mech. 71 (2004) 2589–2605. [17] B. Li, C.R. Zhang, F. Cao, S.Q. Wang, J.S. Li, B. Chen, Effects of curing atmosphere pressure on properties of silica fibre reinforced silicon?boron nitride matrix composites derived from precursor infiltration and pyrolysis, Mater. Technol. 22 (2013) 81–84. [18] P. Colombo, G. Mera, R. Riedel, G.D. Soraru, Polymer-derived ceramics: 40 years of

Fig. 9. Comparison of the kinetics constants KT for the oxidation of Si-based materials at total pressure of 1 atm. Details of the different plots are as follows: (-●-) PCS-derived Si3N4 fiber oxidized in wet air (Ea = 108 kJ mol−1); (-■-) PCS-derived Si3N4 fiber oxidized in dry air (Ea = 152 kJ mol−1); (-〇-) Silicon oxidized in dry O2 (Ea = 119 kJ mol−1)[56] (-▼-) Hot-pressed SiC oxidized in dry O2 (Ea = 155 kJ mol−1)[58]; (-¡ó-) PCS derived SiC fiber (Hi Nicalon) oxidized in dry O2 (Ea = 107 kJ mol−1)[59]; (-¡ø-) CVD Si3N4 oxidized in dry O2 (Ea = 330 kJ mol−1)[22]; (-¡ô-) CVD Si3N4 oxidized in steam (Ea = 259 kJ mol−1)[22].

e2t − e20 = KTt

(3)

where e\0 is the layer thickness as the fibers are heated to the target temperature T before isothermal process, and et is the layer thickness as the oxidation soaking time is t [54,55]. Fig. 9 shows the change of KT with oxidation temperature of the PCS-Si3N4 fibers. KT of the related Sibased materials is also plotted in for comparison. Oxidation activation energies (Ea) are calculated according to Arrhenius equation [55]. For the present Si3N4 fibers, Ea are determined as 108 kJ mol−1 and 152 kJ mol−1 for the wet air and the dry air condition, respectively. The values are very close to the Ea of some Si-based materials, such as Si (111) of 119 kJ mol−1 (in pure O2) [56], hot-pressed SiC of 155 kJ mol−1 (in pure O2) [57,58], and PCS-SiC fiber (Hi-Nicalon) of 107 kJ mol−1 (in pure O2) [59]. However, the Ea obtained for the present Si3N4 fiber are much lower than that of the stoichiometric Si3N4 ceramics(Ea = 259–485 kJ mol−1) [24]. It is known that as compared with the oxidation of Si and SiC, formation of suboxide buffer of Si2N2O or SiNxOy at the SiO2/Si3N4 interface during the oxidation of stoichiometric Si3N4 was the key factor to reduce the diffusivity of the oxidizing gases and to higher the Ea [53]. The absence of the buffer layer in the current fiber coating should be responsible for the low Ea of the PCSSi3N4 fiber. It is also suggested that the diffusivity of oxidizing gases in the current fiber coating with low N concentration (∼8 wt.%) has no evident differences as compared in pure SiO2 coatings. Presence of the mesopores in the whole fiber should accelerate the oxidation rate since the surface area of the oxidation is evidently enlarged. Moreover, existence of the trace impurities of C and O from precursor also might have contributed to enhancing the oxidation activity. 16

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[19] [20] [21] [22] [23]

[24] [25] [26]

[27] [28]

[29] [30] [31]

[32]

[33]

[34] [35] [36] [37] [38]

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