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Coating of poly(carborane-carbosilane-phenylacetylene) on carbon fibers with excellent oxidation protection
Accepted Manuscript Coating of poly(carborane-carbosilane-phenylacetylene) carbon fibers with excellent oxidation protection
on
Dejin Tong, Haipeng Wang, Lei Wang, Lei Chen, Zhanxiong Li PII: DOI: Reference:
S0257-8972(17)30359-6 doi: 10.1016/j.surfcoat.2017.04.014 SCT 22257
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
27 January 2017 7 April 2017 7 April 2017
Please cite this article as: Dejin Tong, Haipeng Wang, Lei Wang, Lei Chen, Zhanxiong Li , Coating of poly(carborane-carbosilane-phenylacetylene) on carbon fibers with excellent oxidation protection. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi: 10.1016/j.surfcoat.2017.04.014
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ACCEPTED MANUSCRIPT
Coating of poly(carborane-carbosilane-phenylacetylene) on carbon fibers with excellent oxidation protection Dejin Tonga, Haipeng Wang a, Lei Wang a, Lei Chena, Zhanxiong Lia,b, a
College of Textile and Clothing Engineering, Soochow University, Suzhou 215021, China. National Engineering Laboratory for Modern Silk, Suzhou 215123, China
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b
Abstract: Linear carborane-carbosilane-phenylacetylene co-polymer has been
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synthesized as precursor for thermosets and ceramics for the protection of carbon
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fibers from oxidation in an oxidizing environment. The novel linear co-polymers can be processed conveniently and converted into thermoset or ceramics since they are
organic
solvents.
Treatment
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either liquids or low melting solids at room temperature and are soluble in most of
carbon
fibers
with
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poly(carborane-carbosilane-phenylacetylene) by precursor infiltration and pyrolysis (PIP) process can provide a protective barrier at elevated temperatures. Tensile
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strength measurement revealed that the coated carbon fiber maintained 81.39% of its
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original strength. It was found that the novel co-polymer is highly efficient in protecting the carbon fibers from oxidation breakdown when used as a matrix material (ceramic). Boron and -C≡C- group appear to be the key to the unique oxidative
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stability of the composite compositions. The derived ceramic coatings on carbon fibers were characterized by scanning electron microscopy (SEM) and energy
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dispersive spectroscopy (EDS). Anti-oxidation studies were performed by thermogravimetric analyses (TGA). The results showed that the oxidation resistance of carbon fibers has been promoted obviously by the ceramic coatings. Keywords: Oxidation resistance; Coating; Micro-structure; Carbon fibers
1. Introduction Materials used in the advanced aero and space applications, such as turbine
Corresponding author at: College of Textile and Clothing Engineering, Soochow University, No. 199 Renai Road, Industry Park, Suzhou 215021, China. Tel: +86-512-67061190; Fax: +86-512-67246786. E-mail address: [email protected] (Z. X. Li).
ACCEPTED MANUSCRIPT engine components, hypersonic flight vehicles, and spacecraft reentry thermal protection systems, are required to be capable of withstanding degradation and retain satisfactory mechanical properties under high temperature, high pressure or even in aerobic environment. Carbon fibers, which possess high strength-to-weight ratio, excellent thermal shock resistance, low expansion coefficient and relative flexibility,
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are considered to be one of the most promising high temperature structure materials [1-5]. In the presence of an inert atmosphere or vacuum, carbon sublimes at
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temperatures in excess of 3500 ℃. However, carbon fiber will start oxidation at low temperature (400 ℃) in oxygen atmosphere. Coating process, which is one of the most
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efficient surface modification approaches, can enhance the oxidation resistance of carbon fiber. A lot of coating systems have been invented by now. Among these
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coatings, silicon carbide (SiC) coating, which has excellent mechanical properties, relatively good oxidation resistant and good compatibility with C/C composites, has
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been widely applied to protect C/C composites from oxidation [6-8]. What′s more, SiC is oxidized to SiO2 in aerobic environment which may fill the emerging
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micro-cracks in the refractory coating and improve the diffusion barrier efficiency
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[9-10].
There are three common ways to fabricate the SiC ceramic coating: chemical vapor deposition (CVD) [11-14], precursor infiltration and pyrolysis (PIP) [15-18]
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and sol-gel method [19-22]. PIP process has been widely employed for fabrication of C/SiC composites as demonstrated because of its advantages in evident reduction in
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costs, large-scale productions, controllable ceramic composition and micro-structure. R. Gadow and F. Kern [15-17] had demonstrated the liquid phase solution coatings with subsequent cross-linking as thin solid film and final ceramization was engineered to pilot plant size. However, the gas evolution and the different thermal expansion coefficient between the coating and carbon fiber substrate, which will result in forming pores and cracks in the coating during the pyrolysis of a pre-ceramic polymer and further influence the C/SiC composites’ performance, is still a challenge need to solve. Effective consolidation is a key technical issue to yield minimum amount of pores and cracks [23]. Aim to obtain a high density and performance C/SiC
ACCEPTED MANUSCRIPT composites through PIP process, it is essential to use a new pre-ceramic polymer. Carboslianes and carbosilazanes were the first generation of precursors employed [13,18]. Riedel et al. and Motz et al. developed Boron and Nitrogen containing ternary and quaternary Si-ceramic phases with high oxidation resistance because of its unique chemical and physical properties [24-27]. Z. Luo investigated the liquid
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polyvinylcarbosilane with active -Si-H and -CH=CH2 groups as the SiC matrix precursor due to these active groups can initiate cross-linking reaction when curing at
mechanical properties were efficiently fabricated [28].
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300 ℃. As a result, the composites with superior density and high-temperature
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Our research efforts have been devoted to the synthesis and development of inorganic-organic hybrid linear polymers containing carborane, carbosilane, and
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phenylacetylene units in the backbone. These linear polymers exhibit satisfactory thermal and oxidative stability to at least 1000 ℃ [29-34] and are being developed as
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precursors to high temperature thermosets and ceramics. The carborane provides high temperature and oxidative stability, the carbosilane shows thermal stability and chain
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flexibility, and the phenylacetylene provides the site for cross-linking and conversion
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to a three-dimensional infusible structure. Since the precursor linear polymers are either liquid or low melting solids at room temperature and are soluble in common organic solvents, carborane-carbosilane-phenylacetylene co-polymers have the
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advantage of being extremely easy to process into thermosets or ceramics, to use as coatings, and to fabricate composite components.
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In this paper, a linear co-polymer 3, poly(carborane-carbosilane-phenylacetylene), under development as a high-temperature matrix material was synthesized and deposited as a film onto carbon fibers and converted by thermal means to a ceramic coating to protect the fibers against oxidation at elevated temperatures. The micro-structure and the oxidation resistant properties for the carbon fiber with and without coating in static air were investigated.
2. Experimental 2.1. Oxidation of carbon fibers
ACCEPTED MANUSCRIPT The fibers used in the current work were all high strength polyacrylonitrile (PAN) based fibers (T-700, 12K). Those fiber was treated at 400 ℃ for 2h under inert conditions aimed to remove the epoxy resin glue on the surface of fibers, Heat treatment of the fibers to 1000 ℃ at 10 ℃/min under a flow of air (20 cc/min) resulted in a catastrophic decomposition of the fibers between 600 and 860 ℃. The carbon
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fibers were completely consumed.
2.2. Formation of anti-oxidation coatings by heating slowly
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After resin removal, the carbon fibers were placed in a beaker containing
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concentrated HNO3 and were heated on water bath maintaining temperature of reactant at 80-85 ℃ for 120 min. After reaction, the fibers were removed from the acid
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and rinsed in distilled water several times to remove all the remaining nitrates on its surfaces. Thereafter, the carbon fibers were impregnated into a solution containing
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0.12/ 0.24/ 0.36 g of poly(carborane-carbosilane-phenylacetylene) and methylene chloride (1.0 ml). After air drying, the fibers were placed into a furnance preheated at 250/ 350/ 450/ 550 ℃ and cured for 1 h under flowing nitrogen to convert the film of
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poly(carborane-carbosilane-phenylacetylene) to a ceramic coating. This procedure
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was repeated to deposit several thin layers of ceramic onto the fibers. Subsequently, the coated fibers were placed into a TGA aluminum oxide pan and heated to 1000 ℃
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in a flow of air (20 cc/min) to test the anti-oxidation performance.
2.3. Oxidative aging of carbon fibers with or without the coatings
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The activated fibers with or without ceramic coatings were used for oxidation aging studies by heating sequentially in a flow of air (20 cc/min) at various temperatures for 2 h.
2.4. Characterization Thermal analyses were performed with a DiaMond 5700 thermal analyzer equipped with a thermogravimetric analyzer (TGA) at a gas flow rate of 20 cc/min. Thermal and oxidative studies were achieved in air emphatically, The TGA studies were performed on melts and films of the polymer 3 and carbon fibers. The oxidation resistance properties of carbon fibers with or without the coating were researched by
ACCEPTED MANUSCRIPT isothermal oxidation, all pyrolysis studies were performed under atmospheric conditions and all aging studies were accomplished at the milligram scale in a TGA chamber. The surface morphologies of carbon fibers were investigated using a scanning electron microscopy (SEM, S-4800, Japan Hitachi Company) equipped with an energy dispersive spectroscopy (EDS) detector. The thickness of the coating was
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examined with SEM (SEM, S-4800, Japan Hitachi Company) by measuring the diameter of the fiber before and after coating. Multiple measurements of the coating
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thickness have been conducted. Tensile strength of the carbon fiber with and without coating was measured at room temperature by Instron 3365. Single fibers extracted
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from a tow were fixed on paper frames using a hard acrylic resin. The gauge length was 25mm, and the loading speed was 0.3 mm/min. At least 25 fibers were
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mechanically tested for each treatment condition for a single data point.
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3. Results and discussion 3.1. Curing of precursor polymer C
C
C
C
MgBr
Cl
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BrMg
CH3
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CH3
Si
C
C
CH3
B10 H10
Si
Ph
C
C B10 H10
Si
CH3 C
Ph
C B10 H10
Si
Cl
CH3
2
Ph
AC
*
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1
CH3
Si
Ph
CH3 C
C B10 H10
Si
*
n
CH3
3
Thermoset
Ceramic
4
5
Scheme. 1. Structure and Synthesis of poly(carborane-carbosilane-phenylacetylene) (3).
Poly(carborane-carbosilane-phenylacetylene) 3 was synthesized [35-38] by a
ACCEPTED MANUSCRIPT one-pot, two-stage reaction via the condensation polymerization of 1 and 2 according to Grignard reaction mechanism (Scheme. 1). The product is a liquid at room temperature and can be heated to at least 160 ℃ for an extended period without any detection in a viscosity increase. The liquid polymer 3 is readily converted to a thermoset 4 by thermal means. Further heat treatments at temperatures in excess of 550 ℃ result in the conversion to a ceramic material 5 with the shrinkage of the
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volume. Thermal treatment on 3 under inert conditions at 800 ℃ formed a residual,
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which was named as ceramic 5 with 86.78% yield (Fig. 1a). When 5 was cooled back to room temperature and re-heated to 1000 ℃ under an oxidizing environment, the
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product actually gained some weight (2.69%) due to surface oxidation (Fig. 1b). Surface analysis studies showed that a borosilicate has formed on the surface, which
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can prevent the interior from further oxidation.
(a) Treatment of polymer 3 under inert condition
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(b) Treatment of Ceramic 5 under oxidation environment
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Fig. 1 Thermal treatment of Polymer 3 and Ceramic 5 under different environment.
Fig. 2. IR spectrum of the Polymer 3 and Ceramic 5.
The IR spectrum of the Polymer 3 and Ceramic 5 were showed in Fig. 2. Polymer
ACCEPTED MANUSCRIPT 3 showed absorption peaks at 3295.15 cm-1 (stretching vibration of ≡C-H), 2154.06 cm-1 (stretching vibration of -C≡C-), 3061.75 cm-1 (stretching vibration of -C-H in benzene), 2956.3 cm-1 and 2865.56 cm-1 (stretching vibration of -CH3). These characteristic peaks were disappeared after cured in ceramic 5, which indicated that a thermally induced cross-linking process involving internal ethynylene polymerization has developed. In addition, distortion vibration of -B-H at 2580.02 cm-1 of Polymer 3
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was also disappeared after treatment while a new peak formed at 1376.83 cm-1 in the
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curve of ceramic 5, which is ascribed to the distortion vibration of -B-O. A film can be deposited on carbon fibers by using a dilute methylene chloride
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solution of 3 and then converted to a ceramic coating through heat pre-treatment. This
the function of oxidation protection.
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procedure can be performed several times to obtain multi-layers coatings and enhance
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3.2. Effect of coating conditions on the oxidation resistance of carbon fibers
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3.2.1. Effect of pre-treatment temperature
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Tab. 1 coating at different cured temperature Samples
The pristine
at 250 ℃
at 350 ℃
600.02
326.74
374.50
467.27
528.89
97.99
87.10
92.71
93.70
95.99
0.28
2.39
2.48
2.52
18.01
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fibers
Pyrolysis temperature/℃
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Residual mass at 600 ℃/% Residue/%
Pretreated samples with different temperature at 450 ℃
at 550 ℃
The dynamic TGA analysis was used to find the appropriate temperature which could convert the film of poly(carborane-carbosilane-phenylacetylene) to a ceramic coating. The result was listed in Table. 1. The pyrolysis for pristine carbon fiber (with 2% weight lost) started at around 600 ℃. When the temperature reached about 860 ℃, the fiber nearly burnt out completely. For the samples coated with ceramic, which were dipped in precursor solution with the concentration of 0.12 g/ml and heated in a furnace at 250 ℃, 350 ℃, 450 ℃ and 550 ℃ respectively, for 1 h under flowing
ACCEPTED MANUSCRIPT nitrogen. It could be obviously observed from Table. 1 that the higher the pretreatment temperature was used, the higher initial pyrolysis temperature of coated fiber will be. The same rule was watched on the residual mass at 600 ℃ which ranged from 87.10% to 95.99%. More importantly, the residue reached 18.01% when the temperature of the pre-treatment was raised to 550 ℃. The thermal oxidation weight-loss curves of fibers were displayed in Fig. 3. The fiber without coating decomposed rapidly and
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consumed up at about 860 ℃. As compared to the pyrolysis behavior of the pristine
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fibers without coating, the coated fiber was tested to possess about 20% mass residual when the temperature reached to 820 ℃ and after that, the sample maintained a
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constant residual. The result showed that, before met the suitable pre-treatment temperature, the film did not completely convert to a ceramic coating and thus caused
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the weight-loss itself when heating at 600 ℃. After 600 ℃, the coated fiber started to lose weight because of the substrate pyrolysis. The optimal condition of pre-treatment
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temperature was found to be 550 ℃.
Fig. 3. Effect of pre-treatment on the formation of the ceramic coating.
3.2.2. Solution concentration and immension-pyrolysis cycles The thermal property of the ceramic coating was studied by TGA at a heating rate
ACCEPTED MANUSCRIPT of 10 ℃/min under air atmosphere from room temperature to 1000 ℃. The result showed that the thermo-oxidative stability of the coated carbon fibers improved as increase the coating thickness by raising the solution concentration of precursor and immension-pyrolysis cycles. The influence of the processing parameters with pre-treating temperature of 550 ℃ on the oxidation resistant property of the coated
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carbon fibers was reported in Fig. 4.
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Fig. 4. Influence of the processing parameters (pre-treating at 550 ℃) on the oxidation resistant property of the coated carbon fibers. (a) with different solution concentration, (b) with different immension-pyrolysis cycles.
In Fig. 4(a), The coated carbon fibers dipped in dilute solution of
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poly(carborane-carbosilane-phenylacetylene) (0.12 g/ml) were heated at 10 ℃/min to
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1000 ℃ under an oxidizing conditions resulting in a 81% weight loss. In fact, the sample was relatively stable with a slight weight loss of 4.01% before the temperature
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reached 600 ℃. The rapid decomposition started from 600 ℃ to 820 ℃ with 77.98% weight loss. After then, the weight of the sample almost remains stable between
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820 ℃ to 1000 ℃. When double the concentration of the solution to 0.24 g/ml, the anti-oxidation performance improved, though the oxidation started at about 600 ℃ as well with a 29% (weight) residual at 860 ℃. Further increase the concentration of precursor to 0.36 g/ml, a thicker film of poly(carborane-carbosilane-phenylacetylene) deposited on the carbon fibers. The resulted fibers were tested by heating at 10 ℃/min in a flow of air (20 cc/min) to 600 ℃ with only 2.59% weight loss. However, the sample was observed to possess a rapid weight loss of 87%, which was substantially more than that of the concentration of 0.12 g/ml and 0.24 g/ml. As the heating continued, the weight was constant as well.
ACCEPTED MANUSCRIPT Apparently, the efficiency of oxidation proof could be promoted by increasing the concentration of precursor and the thicker of coating films. When 0.24 g/ml concentration of precursor was conducted with the thickness of coating film of 200 nm (Fig. 5), the oxidation proof efficiency was found to be inferior to that of 0.36 g/ml concentration of precursor. This could be attributed to the spallation of the
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coating layer at the relatively high concentration of treating agent. The shrinkage of the coating during the heat treatment and the disparity of thermal expansion
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coefficients between the ceramic coating and the carbon fiber led to the exfoliation of the coating from carbon fiber when the coating thickness exceeded a certain value. To
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prevent the coating film from cracking, the dipping and heating pre-treatment process was suggested to carry out for cycles.
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In Fig. 4(b), the carbon fibers were all treated at the same precursor solution (0.24 g/ml) with immension-pyrolysis cycling. Rising immension-pyrolysis cycles increase
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the thickness of the ceramic coating, and promote the antioxidant performance of treated fibers. The carbon fibers with three cycling treatment provide the residual of
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up to 74.46%. When heating the coated fibers from room temperature to 1000 ℃,
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some micro-cracking occurred in coating film and led to the oxidation of the substrate fibers. The boron components in the coating film were oxidized followed by volume increase, which can repair the failure sites of the coating film and provide the
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continuous anti-oxidation function of protecting the carbon fibers [39]. The coating was too thin at 0.12 g/ml concentration to protect carbon fiber, while it was too thick
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at 0.36 g/ml concentration to form a stable coating film on surface of carbon fiber. Increment of the immension-pyrolysis cycles at dilute concentration of precursor can solve this problem. The most optimal parameters were found to be the solution concentration of precursor 0.24 g/ml with three immension-pyrolysis cycles.
3.3. Micro-structure of the coating At 550 ℃, the film of poly(carborane-carbosilane-phenylacetylene) is converted to a ceramic coating, this transformation is profited from a thermally induced
ACCEPTED MANUSCRIPT cross-linking process involving internal ethynylene polymerization, led to the
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formation of a protective barrier on the surface of carbon fibers.
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Fig. 5. SEM images of carbon fibers: (a) the pristine carbon fiber, (b) the carbon fiber infiltrate in concentration of 0.12 g/ml, 0.24 g/ml (c) and 0.36 g/ml (d) precursor with one immension-pyrolysis cycle.
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SEM images of the ceramic coated carbon fibers treated at 550 ℃ are displayed in Fig. 5. Fig. 5(a) shows the surface images of the uncoated carbon fibers with a smooth Fig.
5(b)
shows
the
coating
prepared
by
dilute
solution
of
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surface.
poly(carborane-carbosilane-phenylacetylene) (0.12 g/ml) with the cover layer to be incomplete. In Fig. 5(c), the coating was relatively smooth and uniform on the surface of fibers in spite of some micro-pits, while the coating formed in a concentrated solution (0.36 g/ml) cracked and spalled (Fig. 5(d)) due to the thermal expansion coefficient between carbon fiber and the coating did not match. In addition, the cross-section SEM of coated carbon fiber with (0.24 g/ml) concentration of precursor was obtained in fig. 6(b). The coating combined well with fiber and covered the surface completely with the layer thickness of 50 nm to 200 nm.
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Fig. 6. The cross-section images of carbon fibers with (b) and without (a) coating.
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In Fig. 7. Fig. 7(a) to (c) corresponding the surface images of carbon fibers treated from one to three immension-pyrolysis cycles, respectively. It can be seen that
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the ceramic coating formed on carbon fibers was not dense and smooth at one or two immension-pyrolysis cycles. Only when the cycles raise to three times, the coatings
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on the surface of carbon fibers formed to be complete and uniform with the thickness of about 150-200 nm. Meanwhile, it is consistent with the conclusion draw from Fig. that
the
oxidation
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immension-pyrolysis cycles.
resistant
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4(b),
property
was
promoted
with
rising
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Fig. 7. SEM images of the ceramic coated fibers by different immension-pyrolysis cycles from (a) to (c), and the overall perspective of coated fiber (concentration of 0.24 g/ml, 3 immension-pyrolysis cycles) (d).
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In this study, coating obtained at the concentration of 0.24 g/ml precursor was identified the C, Si and B elements and the distributions in Fig. 8 by EDS element analysis. The Si and B elements distributed uniformly and evenly in the film on the
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surface of fibers treating in 0.24 g/ml precursor, which proved that the ceramic
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coatings on the surface of carbon fiber and excellent oxidation resistance.
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Fig. 8. EDS element analysis result corresponding to spectrum 1 in Fig. 7(d) for (a) B, (b) C, (c) O, and (d) Si.
3.4. Mechanism of anti-oxidation for the coating For the carbon-carbon composites, the oxidation weight loss was proportional to the reaction time when weight loss was below 70% [40](Eq.(1)):
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(1)
Where m0 is the initial mass of the sample, m is that of at time t, and k is a reaction rate constant at a fixed temperature. The correlation of reaction rate constant with temperature followed the Arrhenius
Ea 1 R T
(2)
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ln k ln A
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equation (Eq.(2)):
Where A is a pre-exponential factor, Ea is the apparent activation energy of fiber
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oxidation (kJ mol-1), T is the absolute temperature (K), and R is the gas constant (J mol-1 k-1)
m0 m E 1 (ln A ln t ) a m0 R T
（3）
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ln
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When Eqs. (1) and (2) are combined, one obtains:
The fitted line of Arrhenius was obtained by plotting ln((m0-m)/m0) versus 1/T
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(the data of m0, m, and T were obtained from Fig. 4), with Ea being the slope of this
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line. (shown in Fig. 9) The Arrhenius plot which possesses two different slope lines exhibited good linearity with the apparent activation energy, Ea, to be 142.83 kJ/mol and 31.58 kJ/mol, respectively. This means that there are two different oxidative
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reaction mechanisms during the decomposition around 600 ℃, before and after that
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the Ea values changed which directly reflected the oxidation reaction rate changing.
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Fig. 9. The fitted line of Arrhenius of ceramic coated carbon fibers (concentration of 0.24 g/ml, 3 immension-pyrolysis cycles)
Fig. 10. DTG curves of ceramic coated carbon fiber (concentration of 0.24 g/ml, 3 immension-pyrolysis cycles)
From DTG curve (Fig. 10), the oxidation rate (da/dT) of coated fibers reached the
ACCEPTED MANUSCRIPT maximum at 26.14% weight loss. This was characterized as self-catalytic reaction according to Ref. [41]. For the initial period of decomposition of pristine carbon fiber, the number of active carbon atoms (ACAs) on the fiber surface was small with the lowest oxidation rate. The extended 2-D chain structure of fiber was broken off by oxidation to form
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several smaller species, thereby increasing the ACAs [41]. Therefore, with the increasing oxidation-weight loss, the amount of ACAs increased. What’s more, the
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reaction rate increased as well. Thus, the self-catalytic characteristic was displayed with a critical point (66.64%) arising [42].
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As for the coated fiber samples at the concentration of 0.24 g/ml with 3 immension-pyrolysis cycles, as the coating was densely and well bonded to the fiber
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(Fig. 7), the oxidation decomposition started at 561.32 ℃. During this stage, there is only a small quantity of ACAs on the surface and the resulted diffusion rate of oxygen
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was low. As the oxidation process was mainly dominated by diffusion of oxygen, the decomposing Ea Value was high. However, when the temperature reached higher than
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the critical temperature 673.93 ℃ , there was crack formed in the refractory coating,
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which was caused by the coefficient of thermal expansion mis-match of the ceramic coatings and the fiber [43]. Oxygen could enter inside through the cracks on the coating and directly react with the carbon fiber, the diffusion rate of oxygen in cracks
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was higher than that in the ceramic coating, the oxidative reaction interface moved inside gradually. At this stage, the oxidation process was mainly controlled by
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chemical reaction and gas diffusion, which resulted in the low Ea value. The oxidation rate of fibers reached the maximum when the carbon fiber was 26.14% burnt off.
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Fig. 11. DTA traces of carbon fibers with and without coatings in air.
Fig. 11 shows the DTA traces ( in air ) of the carbon fibers before and after coating. The DTA curve of the original carbon fibers displays a strong broad
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exothermic peak centered at about 834 ℃, while at 783 ℃ for the coated fibers
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(concentration of 0.24 g/ml, 3 immension-pyrolysis cycles). The exothermic enthalpy indicated the catastrophic decomposition of the fibers between 600 and
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860 ℃. For comparison, the exothermic enthalpy of coated fibers is relatively low with a narrow exothermic area, which demonstrated the excellent oxidation
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proofing performance.
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3.5. Oxidation behavior of the coating
Fig. 12. Isothermal oxidation of the ceramic coated carbon fibers heated at different temperatures for 2 h.
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Oxidation aging test was performed on ceramic coated carbon fibers to determine
fibers
with
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the durability of protecting function. Fig. 12 shows the isothermal oxidation of carbon triple-layers
coating
(concentration
of
0.24
g/ml
with
3
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immension-pyrolysis cycles) at 600 ℃, 700 ℃, 800 ℃ or 900 ℃ in a flow of air (20 cc/min) for 120 min, respectively. In Fig. 12, the weight loss of the samples decreased linearly, and the slope of isothermal oxidation curves increased with the increase of
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the oxidation temperature. After be oxidized in static air for 120 min at 600 ℃ and 700 ℃, the mass of the sample decreased by 19.67% and 31.02%, respectively. The weight loss of the coated carbon fiber was less than that of uncoated fiber, which burnt out after being oxidation for 63 min at 700 ℃. When it came to 800 ℃, the weight loss of coated carbon fibers declined rapidly after being oxidized for 61 min in static air, and maintained constant after then. Moreover, the coated carbon fiber left a char residual of about 99% after heating at 900 ℃. The result showed that the carbon fiber might have been pyrolyzed completely at 800 ℃ for 61 min, while the coating
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preserved and formed a hollow structure. This phenomenon was confirmed in Fig. 13.
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Fig. 13. Isothermal oxidation of the ceramic coated carbon fibers oxidized at 700 ℃ (a) and 900 ℃ (b) for 2 h.
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3.6. Mechanical properties of the coated fibers
Fig. 14 exhibits the tensile strength of carbon fibers processed under different
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conditions. The tensile strength of pristine fiber decreased after heating at 400 ℃ for 2h in N2 atmosphere, which can remove the epoxy resin adhesive on the surface of
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carbon fiber and reduce surface area by eliminating the surface pores [44]. However,
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the strength of the samples heating followed by activing treatment with HNO3 decreased from 4.42 GPa to 3.74 GPa. Because the activating agent HNO3 impregnated into carbon fiber then etched out carbon and developed a much richer
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carbon content materials with porosity during the chemical activation process [45-46], the pores’ formation decreased the strength of the fiber. After coated with ceramic
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coating (concentration of 0.24g/ml), the strength of activated carbon fiber increased significantly because the porosity decreased and thus the cracks was filled effectively. Unfortunately, the tensile strength of 4277 MPa was tested for coated activated carbon fibers, which decreased by 12.7% compared with that of original fiber of 4900 MPa. In addition, because the mismatch of modules and coefficient of thermal expansion [47] between the coating and substrate, the residual stress gradually increased with the increase of coating thickness, which instead of improving strength, the tensile strength of coated carbon fibers decreased.
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4. Conclusion
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Fig. 14. Tensile strength for carbon fiber samples: (a) pristine carbon fiber, (b) carbon fiber heating at 400 ℃ for 2 h, (c) carbon fiber chemically activated by HNO3 for 2 h after step (b), (d) to (f) corresponding the activated fiber with ceramic coating from one to three immension-pyrolysis cycles.
In this study, a novel polymer named poly(carborane-carbosilane-phenylacetylene)
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was synthesized as precursor for ceramic coating on carbon fibers via PIP method. After that, a thin film of the precursor was deposited on carbon fibers by dipped in
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methylene chloride solution and then converted to a ceramic by heating. The dipping-heating procedure was performed cycling to enhance the complete coating on fiber and the oxidation resistance of treated fibers. The optimal procedure was found to dip the fiber in 0.24 g/ml of precursor with three immension-pyrolysis cycles and heating pre-treatment at 550 ℃. After the treatment, the tensile strength was tested to be 3988 MPa with a decrement of 18.61% compared with that of original fiber of 4900 MPa, while the treated carbon fibers possessed excellent anti-oxidation property with the mass residual of up to 74.46% heating at 800 ℃.
ACCEPTED MANUSCRIPT Acknowledgements We gratefully acknowledge the financial supports by the National Natural Science Foundation of China (51673137 and 51273140), and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education
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Institutions.
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Graphical abstract
ACCEPTED MANUSCRIPT Highlights
Poly(carborane-carbosilane-phenylacelene)
was
synthesized
according
to
Grignard reaction mechanism. The precursor polymer can provide a protective barrier via PIP process.
Heating pre-treatment has a great effect on the formation of the ceramic coatings.
The mechanism of thermo-oxidative stability was analyzed.
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Dejin Tong, Haipeng Wang, Lei Wang, Lei Chen, Zhanxiong Li PII: DOI: Reference:
S0257-8972(17)30359-6 doi: 10.1016/j.surfcoat.2017.04.014 SCT 22257
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
27 January 2017 7 April 2017 7 April 2017
Please cite this article as: Dejin Tong, Haipeng Wang, Lei Wang, Lei Chen, Zhanxiong Li , Coating of poly(carborane-carbosilane-phenylacetylene) on carbon fibers with excellent oxidation protection. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi: 10.1016/j.surfcoat.2017.04.014
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Coating of poly(carborane-carbosilane-phenylacetylene) on carbon fibers with excellent oxidation protection Dejin Tonga, Haipeng Wang a, Lei Wang a, Lei Chena, Zhanxiong Lia,b, a
College of Textile and Clothing Engineering, Soochow University, Suzhou 215021, China. National Engineering Laboratory for Modern Silk, Suzhou 215123, China
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b
Abstract: Linear carborane-carbosilane-phenylacetylene co-polymer has been
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synthesized as precursor for thermosets and ceramics for the protection of carbon
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fibers from oxidation in an oxidizing environment. The novel linear co-polymers can be processed conveniently and converted into thermoset or ceramics since they are
organic
solvents.
Treatment
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either liquids or low melting solids at room temperature and are soluble in most of
carbon
fibers
with
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poly(carborane-carbosilane-phenylacetylene) by precursor infiltration and pyrolysis (PIP) process can provide a protective barrier at elevated temperatures. Tensile
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strength measurement revealed that the coated carbon fiber maintained 81.39% of its
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original strength. It was found that the novel co-polymer is highly efficient in protecting the carbon fibers from oxidation breakdown when used as a matrix material (ceramic). Boron and -C≡C- group appear to be the key to the unique oxidative
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stability of the composite compositions. The derived ceramic coatings on carbon fibers were characterized by scanning electron microscopy (SEM) and energy
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dispersive spectroscopy (EDS). Anti-oxidation studies were performed by thermogravimetric analyses (TGA). The results showed that the oxidation resistance of carbon fibers has been promoted obviously by the ceramic coatings. Keywords: Oxidation resistance; Coating; Micro-structure; Carbon fibers
1. Introduction Materials used in the advanced aero and space applications, such as turbine
Corresponding author at: College of Textile and Clothing Engineering, Soochow University, No. 199 Renai Road, Industry Park, Suzhou 215021, China. Tel: +86-512-67061190; Fax: +86-512-67246786. E-mail address: [email protected] (Z. X. Li).
ACCEPTED MANUSCRIPT engine components, hypersonic flight vehicles, and spacecraft reentry thermal protection systems, are required to be capable of withstanding degradation and retain satisfactory mechanical properties under high temperature, high pressure or even in aerobic environment. Carbon fibers, which possess high strength-to-weight ratio, excellent thermal shock resistance, low expansion coefficient and relative flexibility,
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are considered to be one of the most promising high temperature structure materials [1-5]. In the presence of an inert atmosphere or vacuum, carbon sublimes at
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temperatures in excess of 3500 ℃. However, carbon fiber will start oxidation at low temperature (400 ℃) in oxygen atmosphere. Coating process, which is one of the most
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efficient surface modification approaches, can enhance the oxidation resistance of carbon fiber. A lot of coating systems have been invented by now. Among these
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coatings, silicon carbide (SiC) coating, which has excellent mechanical properties, relatively good oxidation resistant and good compatibility with C/C composites, has
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been widely applied to protect C/C composites from oxidation [6-8]. What′s more, SiC is oxidized to SiO2 in aerobic environment which may fill the emerging
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micro-cracks in the refractory coating and improve the diffusion barrier efficiency
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[9-10].
There are three common ways to fabricate the SiC ceramic coating: chemical vapor deposition (CVD) [11-14], precursor infiltration and pyrolysis (PIP) [15-18]
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and sol-gel method [19-22]. PIP process has been widely employed for fabrication of C/SiC composites as demonstrated because of its advantages in evident reduction in
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costs, large-scale productions, controllable ceramic composition and micro-structure. R. Gadow and F. Kern [15-17] had demonstrated the liquid phase solution coatings with subsequent cross-linking as thin solid film and final ceramization was engineered to pilot plant size. However, the gas evolution and the different thermal expansion coefficient between the coating and carbon fiber substrate, which will result in forming pores and cracks in the coating during the pyrolysis of a pre-ceramic polymer and further influence the C/SiC composites’ performance, is still a challenge need to solve. Effective consolidation is a key technical issue to yield minimum amount of pores and cracks [23]. Aim to obtain a high density and performance C/SiC
ACCEPTED MANUSCRIPT composites through PIP process, it is essential to use a new pre-ceramic polymer. Carboslianes and carbosilazanes were the first generation of precursors employed [13,18]. Riedel et al. and Motz et al. developed Boron and Nitrogen containing ternary and quaternary Si-ceramic phases with high oxidation resistance because of its unique chemical and physical properties [24-27]. Z. Luo investigated the liquid
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polyvinylcarbosilane with active -Si-H and -CH=CH2 groups as the SiC matrix precursor due to these active groups can initiate cross-linking reaction when curing at
mechanical properties were efficiently fabricated [28].
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300 ℃. As a result, the composites with superior density and high-temperature
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Our research efforts have been devoted to the synthesis and development of inorganic-organic hybrid linear polymers containing carborane, carbosilane, and
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phenylacetylene units in the backbone. These linear polymers exhibit satisfactory thermal and oxidative stability to at least 1000 ℃ [29-34] and are being developed as
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precursors to high temperature thermosets and ceramics. The carborane provides high temperature and oxidative stability, the carbosilane shows thermal stability and chain
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flexibility, and the phenylacetylene provides the site for cross-linking and conversion
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to a three-dimensional infusible structure. Since the precursor linear polymers are either liquid or low melting solids at room temperature and are soluble in common organic solvents, carborane-carbosilane-phenylacetylene co-polymers have the
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advantage of being extremely easy to process into thermosets or ceramics, to use as coatings, and to fabricate composite components.
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In this paper, a linear co-polymer 3, poly(carborane-carbosilane-phenylacetylene), under development as a high-temperature matrix material was synthesized and deposited as a film onto carbon fibers and converted by thermal means to a ceramic coating to protect the fibers against oxidation at elevated temperatures. The micro-structure and the oxidation resistant properties for the carbon fiber with and without coating in static air were investigated.
2. Experimental 2.1. Oxidation of carbon fibers
ACCEPTED MANUSCRIPT The fibers used in the current work were all high strength polyacrylonitrile (PAN) based fibers (T-700, 12K). Those fiber was treated at 400 ℃ for 2h under inert conditions aimed to remove the epoxy resin glue on the surface of fibers, Heat treatment of the fibers to 1000 ℃ at 10 ℃/min under a flow of air (20 cc/min) resulted in a catastrophic decomposition of the fibers between 600 and 860 ℃. The carbon
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fibers were completely consumed.
2.2. Formation of anti-oxidation coatings by heating slowly
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After resin removal, the carbon fibers were placed in a beaker containing
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concentrated HNO3 and were heated on water bath maintaining temperature of reactant at 80-85 ℃ for 120 min. After reaction, the fibers were removed from the acid
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and rinsed in distilled water several times to remove all the remaining nitrates on its surfaces. Thereafter, the carbon fibers were impregnated into a solution containing
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0.12/ 0.24/ 0.36 g of poly(carborane-carbosilane-phenylacetylene) and methylene chloride (1.0 ml). After air drying, the fibers were placed into a furnance preheated at 250/ 350/ 450/ 550 ℃ and cured for 1 h under flowing nitrogen to convert the film of
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poly(carborane-carbosilane-phenylacetylene) to a ceramic coating. This procedure
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was repeated to deposit several thin layers of ceramic onto the fibers. Subsequently, the coated fibers were placed into a TGA aluminum oxide pan and heated to 1000 ℃
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in a flow of air (20 cc/min) to test the anti-oxidation performance.
2.3. Oxidative aging of carbon fibers with or without the coatings
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The activated fibers with or without ceramic coatings were used for oxidation aging studies by heating sequentially in a flow of air (20 cc/min) at various temperatures for 2 h.
2.4. Characterization Thermal analyses were performed with a DiaMond 5700 thermal analyzer equipped with a thermogravimetric analyzer (TGA) at a gas flow rate of 20 cc/min. Thermal and oxidative studies were achieved in air emphatically, The TGA studies were performed on melts and films of the polymer 3 and carbon fibers. The oxidation resistance properties of carbon fibers with or without the coating were researched by
ACCEPTED MANUSCRIPT isothermal oxidation, all pyrolysis studies were performed under atmospheric conditions and all aging studies were accomplished at the milligram scale in a TGA chamber. The surface morphologies of carbon fibers were investigated using a scanning electron microscopy (SEM, S-4800, Japan Hitachi Company) equipped with an energy dispersive spectroscopy (EDS) detector. The thickness of the coating was
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examined with SEM (SEM, S-4800, Japan Hitachi Company) by measuring the diameter of the fiber before and after coating. Multiple measurements of the coating
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thickness have been conducted. Tensile strength of the carbon fiber with and without coating was measured at room temperature by Instron 3365. Single fibers extracted
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from a tow were fixed on paper frames using a hard acrylic resin. The gauge length was 25mm, and the loading speed was 0.3 mm/min. At least 25 fibers were
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mechanically tested for each treatment condition for a single data point.
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3. Results and discussion 3.1. Curing of precursor polymer C
C
C
C
MgBr
Cl
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BrMg
CH3
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CH3
Si
C
C
CH3
B10 H10
Si
Ph
C
C B10 H10
Si
CH3 C
Ph
C B10 H10
Si
Cl
CH3
2
Ph
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*
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1
CH3
Si
Ph
CH3 C
C B10 H10
Si
*
n
CH3
3
Thermoset
Ceramic
4
5
Scheme. 1. Structure and Synthesis of poly(carborane-carbosilane-phenylacetylene) (3).
Poly(carborane-carbosilane-phenylacetylene) 3 was synthesized [35-38] by a
ACCEPTED MANUSCRIPT one-pot, two-stage reaction via the condensation polymerization of 1 and 2 according to Grignard reaction mechanism (Scheme. 1). The product is a liquid at room temperature and can be heated to at least 160 ℃ for an extended period without any detection in a viscosity increase. The liquid polymer 3 is readily converted to a thermoset 4 by thermal means. Further heat treatments at temperatures in excess of 550 ℃ result in the conversion to a ceramic material 5 with the shrinkage of the
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volume. Thermal treatment on 3 under inert conditions at 800 ℃ formed a residual,
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which was named as ceramic 5 with 86.78% yield (Fig. 1a). When 5 was cooled back to room temperature and re-heated to 1000 ℃ under an oxidizing environment, the
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product actually gained some weight (2.69%) due to surface oxidation (Fig. 1b). Surface analysis studies showed that a borosilicate has formed on the surface, which
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can prevent the interior from further oxidation.
(a) Treatment of polymer 3 under inert condition
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(b) Treatment of Ceramic 5 under oxidation environment
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Fig. 1 Thermal treatment of Polymer 3 and Ceramic 5 under different environment.
Fig. 2. IR spectrum of the Polymer 3 and Ceramic 5.
The IR spectrum of the Polymer 3 and Ceramic 5 were showed in Fig. 2. Polymer
ACCEPTED MANUSCRIPT 3 showed absorption peaks at 3295.15 cm-1 (stretching vibration of ≡C-H), 2154.06 cm-1 (stretching vibration of -C≡C-), 3061.75 cm-1 (stretching vibration of -C-H in benzene), 2956.3 cm-1 and 2865.56 cm-1 (stretching vibration of -CH3). These characteristic peaks were disappeared after cured in ceramic 5, which indicated that a thermally induced cross-linking process involving internal ethynylene polymerization has developed. In addition, distortion vibration of -B-H at 2580.02 cm-1 of Polymer 3
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was also disappeared after treatment while a new peak formed at 1376.83 cm-1 in the
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curve of ceramic 5, which is ascribed to the distortion vibration of -B-O. A film can be deposited on carbon fibers by using a dilute methylene chloride
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solution of 3 and then converted to a ceramic coating through heat pre-treatment. This
the function of oxidation protection.
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procedure can be performed several times to obtain multi-layers coatings and enhance
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3.2. Effect of coating conditions on the oxidation resistance of carbon fibers
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3.2.1. Effect of pre-treatment temperature
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Tab. 1 coating at different cured temperature Samples
The pristine
at 250 ℃
at 350 ℃
600.02
326.74
374.50
467.27
528.89
97.99
87.10
92.71
93.70
95.99
0.28
2.39
2.48
2.52
18.01
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fibers
Pyrolysis temperature/℃
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Residual mass at 600 ℃/% Residue/%
Pretreated samples with different temperature at 450 ℃
at 550 ℃
The dynamic TGA analysis was used to find the appropriate temperature which could convert the film of poly(carborane-carbosilane-phenylacetylene) to a ceramic coating. The result was listed in Table. 1. The pyrolysis for pristine carbon fiber (with 2% weight lost) started at around 600 ℃. When the temperature reached about 860 ℃, the fiber nearly burnt out completely. For the samples coated with ceramic, which were dipped in precursor solution with the concentration of 0.12 g/ml and heated in a furnace at 250 ℃, 350 ℃, 450 ℃ and 550 ℃ respectively, for 1 h under flowing
ACCEPTED MANUSCRIPT nitrogen. It could be obviously observed from Table. 1 that the higher the pretreatment temperature was used, the higher initial pyrolysis temperature of coated fiber will be. The same rule was watched on the residual mass at 600 ℃ which ranged from 87.10% to 95.99%. More importantly, the residue reached 18.01% when the temperature of the pre-treatment was raised to 550 ℃. The thermal oxidation weight-loss curves of fibers were displayed in Fig. 3. The fiber without coating decomposed rapidly and
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consumed up at about 860 ℃. As compared to the pyrolysis behavior of the pristine
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fibers without coating, the coated fiber was tested to possess about 20% mass residual when the temperature reached to 820 ℃ and after that, the sample maintained a
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constant residual. The result showed that, before met the suitable pre-treatment temperature, the film did not completely convert to a ceramic coating and thus caused
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the weight-loss itself when heating at 600 ℃. After 600 ℃, the coated fiber started to lose weight because of the substrate pyrolysis. The optimal condition of pre-treatment
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temperature was found to be 550 ℃.
Fig. 3. Effect of pre-treatment on the formation of the ceramic coating.
3.2.2. Solution concentration and immension-pyrolysis cycles The thermal property of the ceramic coating was studied by TGA at a heating rate
ACCEPTED MANUSCRIPT of 10 ℃/min under air atmosphere from room temperature to 1000 ℃. The result showed that the thermo-oxidative stability of the coated carbon fibers improved as increase the coating thickness by raising the solution concentration of precursor and immension-pyrolysis cycles. The influence of the processing parameters with pre-treating temperature of 550 ℃ on the oxidation resistant property of the coated
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carbon fibers was reported in Fig. 4.
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Fig. 4. Influence of the processing parameters (pre-treating at 550 ℃) on the oxidation resistant property of the coated carbon fibers. (a) with different solution concentration, (b) with different immension-pyrolysis cycles.
In Fig. 4(a), The coated carbon fibers dipped in dilute solution of
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poly(carborane-carbosilane-phenylacetylene) (0.12 g/ml) were heated at 10 ℃/min to
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1000 ℃ under an oxidizing conditions resulting in a 81% weight loss. In fact, the sample was relatively stable with a slight weight loss of 4.01% before the temperature
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reached 600 ℃. The rapid decomposition started from 600 ℃ to 820 ℃ with 77.98% weight loss. After then, the weight of the sample almost remains stable between
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820 ℃ to 1000 ℃. When double the concentration of the solution to 0.24 g/ml, the anti-oxidation performance improved, though the oxidation started at about 600 ℃ as well with a 29% (weight) residual at 860 ℃. Further increase the concentration of precursor to 0.36 g/ml, a thicker film of poly(carborane-carbosilane-phenylacetylene) deposited on the carbon fibers. The resulted fibers were tested by heating at 10 ℃/min in a flow of air (20 cc/min) to 600 ℃ with only 2.59% weight loss. However, the sample was observed to possess a rapid weight loss of 87%, which was substantially more than that of the concentration of 0.12 g/ml and 0.24 g/ml. As the heating continued, the weight was constant as well.
ACCEPTED MANUSCRIPT Apparently, the efficiency of oxidation proof could be promoted by increasing the concentration of precursor and the thicker of coating films. When 0.24 g/ml concentration of precursor was conducted with the thickness of coating film of 200 nm (Fig. 5), the oxidation proof efficiency was found to be inferior to that of 0.36 g/ml concentration of precursor. This could be attributed to the spallation of the
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coating layer at the relatively high concentration of treating agent. The shrinkage of the coating during the heat treatment and the disparity of thermal expansion
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coefficients between the ceramic coating and the carbon fiber led to the exfoliation of the coating from carbon fiber when the coating thickness exceeded a certain value. To
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prevent the coating film from cracking, the dipping and heating pre-treatment process was suggested to carry out for cycles.
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In Fig. 4(b), the carbon fibers were all treated at the same precursor solution (0.24 g/ml) with immension-pyrolysis cycling. Rising immension-pyrolysis cycles increase
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the thickness of the ceramic coating, and promote the antioxidant performance of treated fibers. The carbon fibers with three cycling treatment provide the residual of
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up to 74.46%. When heating the coated fibers from room temperature to 1000 ℃,
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some micro-cracking occurred in coating film and led to the oxidation of the substrate fibers. The boron components in the coating film were oxidized followed by volume increase, which can repair the failure sites of the coating film and provide the
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continuous anti-oxidation function of protecting the carbon fibers [39]. The coating was too thin at 0.12 g/ml concentration to protect carbon fiber, while it was too thick
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at 0.36 g/ml concentration to form a stable coating film on surface of carbon fiber. Increment of the immension-pyrolysis cycles at dilute concentration of precursor can solve this problem. The most optimal parameters were found to be the solution concentration of precursor 0.24 g/ml with three immension-pyrolysis cycles.
3.3. Micro-structure of the coating At 550 ℃, the film of poly(carborane-carbosilane-phenylacetylene) is converted to a ceramic coating, this transformation is profited from a thermally induced
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formation of a protective barrier on the surface of carbon fibers.
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Fig. 5. SEM images of carbon fibers: (a) the pristine carbon fiber, (b) the carbon fiber infiltrate in concentration of 0.12 g/ml, 0.24 g/ml (c) and 0.36 g/ml (d) precursor with one immension-pyrolysis cycle.
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SEM images of the ceramic coated carbon fibers treated at 550 ℃ are displayed in Fig. 5. Fig. 5(a) shows the surface images of the uncoated carbon fibers with a smooth Fig.
5(b)
shows
the
coating
prepared
by
dilute
solution
of
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surface.
poly(carborane-carbosilane-phenylacetylene) (0.12 g/ml) with the cover layer to be incomplete. In Fig. 5(c), the coating was relatively smooth and uniform on the surface of fibers in spite of some micro-pits, while the coating formed in a concentrated solution (0.36 g/ml) cracked and spalled (Fig. 5(d)) due to the thermal expansion coefficient between carbon fiber and the coating did not match. In addition, the cross-section SEM of coated carbon fiber with (0.24 g/ml) concentration of precursor was obtained in fig. 6(b). The coating combined well with fiber and covered the surface completely with the layer thickness of 50 nm to 200 nm.
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Fig. 6. The cross-section images of carbon fibers with (b) and without (a) coating.
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In Fig. 7. Fig. 7(a) to (c) corresponding the surface images of carbon fibers treated from one to three immension-pyrolysis cycles, respectively. It can be seen that
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the ceramic coating formed on carbon fibers was not dense and smooth at one or two immension-pyrolysis cycles. Only when the cycles raise to three times, the coatings
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on the surface of carbon fibers formed to be complete and uniform with the thickness of about 150-200 nm. Meanwhile, it is consistent with the conclusion draw from Fig. that
the
oxidation
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immension-pyrolysis cycles.
resistant
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4(b),
property
was
promoted
with
rising
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Fig. 7. SEM images of the ceramic coated fibers by different immension-pyrolysis cycles from (a) to (c), and the overall perspective of coated fiber (concentration of 0.24 g/ml, 3 immension-pyrolysis cycles) (d).
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In this study, coating obtained at the concentration of 0.24 g/ml precursor was identified the C, Si and B elements and the distributions in Fig. 8 by EDS element analysis. The Si and B elements distributed uniformly and evenly in the film on the
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surface of fibers treating in 0.24 g/ml precursor, which proved that the ceramic
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coatings on the surface of carbon fiber and excellent oxidation resistance.
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Fig. 8. EDS element analysis result corresponding to spectrum 1 in Fig. 7(d) for (a) B, (b) C, (c) O, and (d) Si.
3.4. Mechanism of anti-oxidation for the coating For the carbon-carbon composites, the oxidation weight loss was proportional to the reaction time when weight loss was below 70% [40](Eq.(1)):
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(1)
Where m0 is the initial mass of the sample, m is that of at time t, and k is a reaction rate constant at a fixed temperature. The correlation of reaction rate constant with temperature followed the Arrhenius
Ea 1 R T
(2)
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ln k ln A
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equation (Eq.(2)):
Where A is a pre-exponential factor, Ea is the apparent activation energy of fiber
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oxidation (kJ mol-1), T is the absolute temperature (K), and R is the gas constant (J mol-1 k-1)
m0 m E 1 (ln A ln t ) a m0 R T
（3）
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ln
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When Eqs. (1) and (2) are combined, one obtains:
The fitted line of Arrhenius was obtained by plotting ln((m0-m)/m0) versus 1/T
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(the data of m0, m, and T were obtained from Fig. 4), with Ea being the slope of this
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line. (shown in Fig. 9) The Arrhenius plot which possesses two different slope lines exhibited good linearity with the apparent activation energy, Ea, to be 142.83 kJ/mol and 31.58 kJ/mol, respectively. This means that there are two different oxidative
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reaction mechanisms during the decomposition around 600 ℃, before and after that
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the Ea values changed which directly reflected the oxidation reaction rate changing.
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Fig. 9. The fitted line of Arrhenius of ceramic coated carbon fibers (concentration of 0.24 g/ml, 3 immension-pyrolysis cycles)
Fig. 10. DTG curves of ceramic coated carbon fiber (concentration of 0.24 g/ml, 3 immension-pyrolysis cycles)
From DTG curve (Fig. 10), the oxidation rate (da/dT) of coated fibers reached the
ACCEPTED MANUSCRIPT maximum at 26.14% weight loss. This was characterized as self-catalytic reaction according to Ref. [41]. For the initial period of decomposition of pristine carbon fiber, the number of active carbon atoms (ACAs) on the fiber surface was small with the lowest oxidation rate. The extended 2-D chain structure of fiber was broken off by oxidation to form
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several smaller species, thereby increasing the ACAs [41]. Therefore, with the increasing oxidation-weight loss, the amount of ACAs increased. What’s more, the
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reaction rate increased as well. Thus, the self-catalytic characteristic was displayed with a critical point (66.64%) arising [42].
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As for the coated fiber samples at the concentration of 0.24 g/ml with 3 immension-pyrolysis cycles, as the coating was densely and well bonded to the fiber
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(Fig. 7), the oxidation decomposition started at 561.32 ℃. During this stage, there is only a small quantity of ACAs on the surface and the resulted diffusion rate of oxygen
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was low. As the oxidation process was mainly dominated by diffusion of oxygen, the decomposing Ea Value was high. However, when the temperature reached higher than
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the critical temperature 673.93 ℃ , there was crack formed in the refractory coating,
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which was caused by the coefficient of thermal expansion mis-match of the ceramic coatings and the fiber [43]. Oxygen could enter inside through the cracks on the coating and directly react with the carbon fiber, the diffusion rate of oxygen in cracks
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was higher than that in the ceramic coating, the oxidative reaction interface moved inside gradually. At this stage, the oxidation process was mainly controlled by
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chemical reaction and gas diffusion, which resulted in the low Ea value. The oxidation rate of fibers reached the maximum when the carbon fiber was 26.14% burnt off.
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Fig. 11. DTA traces of carbon fibers with and without coatings in air.
Fig. 11 shows the DTA traces ( in air ) of the carbon fibers before and after coating. The DTA curve of the original carbon fibers displays a strong broad
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exothermic peak centered at about 834 ℃, while at 783 ℃ for the coated fibers
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(concentration of 0.24 g/ml, 3 immension-pyrolysis cycles). The exothermic enthalpy indicated the catastrophic decomposition of the fibers between 600 and
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860 ℃. For comparison, the exothermic enthalpy of coated fibers is relatively low with a narrow exothermic area, which demonstrated the excellent oxidation
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proofing performance.
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3.5. Oxidation behavior of the coating
Fig. 12. Isothermal oxidation of the ceramic coated carbon fibers heated at different temperatures for 2 h.
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Oxidation aging test was performed on ceramic coated carbon fibers to determine
fibers
with
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the durability of protecting function. Fig. 12 shows the isothermal oxidation of carbon triple-layers
coating
(concentration
of
0.24
g/ml
with
3
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immension-pyrolysis cycles) at 600 ℃, 700 ℃, 800 ℃ or 900 ℃ in a flow of air (20 cc/min) for 120 min, respectively. In Fig. 12, the weight loss of the samples decreased linearly, and the slope of isothermal oxidation curves increased with the increase of
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the oxidation temperature. After be oxidized in static air for 120 min at 600 ℃ and 700 ℃, the mass of the sample decreased by 19.67% and 31.02%, respectively. The weight loss of the coated carbon fiber was less than that of uncoated fiber, which burnt out after being oxidation for 63 min at 700 ℃. When it came to 800 ℃, the weight loss of coated carbon fibers declined rapidly after being oxidized for 61 min in static air, and maintained constant after then. Moreover, the coated carbon fiber left a char residual of about 99% after heating at 900 ℃. The result showed that the carbon fiber might have been pyrolyzed completely at 800 ℃ for 61 min, while the coating
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preserved and formed a hollow structure. This phenomenon was confirmed in Fig. 13.
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Fig. 13. Isothermal oxidation of the ceramic coated carbon fibers oxidized at 700 ℃ (a) and 900 ℃ (b) for 2 h.
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3.6. Mechanical properties of the coated fibers
Fig. 14 exhibits the tensile strength of carbon fibers processed under different
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conditions. The tensile strength of pristine fiber decreased after heating at 400 ℃ for 2h in N2 atmosphere, which can remove the epoxy resin adhesive on the surface of
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carbon fiber and reduce surface area by eliminating the surface pores [44]. However,
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the strength of the samples heating followed by activing treatment with HNO3 decreased from 4.42 GPa to 3.74 GPa. Because the activating agent HNO3 impregnated into carbon fiber then etched out carbon and developed a much richer
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carbon content materials with porosity during the chemical activation process [45-46], the pores’ formation decreased the strength of the fiber. After coated with ceramic
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coating (concentration of 0.24g/ml), the strength of activated carbon fiber increased significantly because the porosity decreased and thus the cracks was filled effectively. Unfortunately, the tensile strength of 4277 MPa was tested for coated activated carbon fibers, which decreased by 12.7% compared with that of original fiber of 4900 MPa. In addition, because the mismatch of modules and coefficient of thermal expansion [47] between the coating and substrate, the residual stress gradually increased with the increase of coating thickness, which instead of improving strength, the tensile strength of coated carbon fibers decreased.
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4. Conclusion
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Fig. 14. Tensile strength for carbon fiber samples: (a) pristine carbon fiber, (b) carbon fiber heating at 400 ℃ for 2 h, (c) carbon fiber chemically activated by HNO3 for 2 h after step (b), (d) to (f) corresponding the activated fiber with ceramic coating from one to three immension-pyrolysis cycles.
In this study, a novel polymer named poly(carborane-carbosilane-phenylacetylene)
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was synthesized as precursor for ceramic coating on carbon fibers via PIP method. After that, a thin film of the precursor was deposited on carbon fibers by dipped in
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methylene chloride solution and then converted to a ceramic by heating. The dipping-heating procedure was performed cycling to enhance the complete coating on fiber and the oxidation resistance of treated fibers. The optimal procedure was found to dip the fiber in 0.24 g/ml of precursor with three immension-pyrolysis cycles and heating pre-treatment at 550 ℃. After the treatment, the tensile strength was tested to be 3988 MPa with a decrement of 18.61% compared with that of original fiber of 4900 MPa, while the treated carbon fibers possessed excellent anti-oxidation property with the mass residual of up to 74.46% heating at 800 ℃.
ACCEPTED MANUSCRIPT Acknowledgements We gratefully acknowledge the financial supports by the National Natural Science Foundation of China (51673137 and 51273140), and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education
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Institutions.
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Graphical abstract
ACCEPTED MANUSCRIPT Highlights
Poly(carborane-carbosilane-phenylacelene)
was
synthesized
according
to
Grignard reaction mechanism. The precursor polymer can provide a protective barrier via PIP process.
Heating pre-treatment has a great effect on the formation of the ceramic coatings.
The mechanism of thermo-oxidative stability was analyzed.
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