Improved Performances of Sibcn Powders Modified Phenolic Resins-Carbon Fiber Composites

processes

Article Improved Performances of SiBCN Powders Modiﬁed Phenolic Resins-Carbon Fiber Composites

Wenjie Yuan 1,2, Yang Wang 1,2, Zhenhua Luo 1,2, Fenghua Chen 1,2,*, Hao Li 1,2,* and Tong Zhao 1,2,*

1 Key Laboratory of Science and Technology on High-tech Polymer Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China; [email protected] (W.Y.); [email protected] (Y.W.); [email protected] (Z.L.) 2 School of Chemical Science, University of Chinese Academy of Sciences, Beijing 100049, China * Correspondence: [email protected] (F.C.); [email protected] (H.L.); [email protected] (T.Z.)

Abstract: The effect of SiBCN powder on properties of phenolic resins and composites was analyzed. Compared with phenolic resins, the thermal stability of SiBCN powder modified phenolic resins (the SiBCN phenolic resins) by characterization of thermogravimetric analysis (TGA) improved clearly. It was found by X-ray photoelectron spectroscopy (XPS) that reactions between SiBCN powder and the pyrolysis product of phenolic resins were the main factor of the increased residual weight. TGA and static ablation of a muffle furnace were used to illustrate the roles of SiBCN powder on increasing oxidation resistance of SiBCN powder-modified phenolic resin–carbon fiber composites (SiBCN–phenolic/C composites), and the oxidative product was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). For SiBCN–phenolic/C composites, the occurrence of oxidation reaction and the formation of protective crust contributed to improving oxidative resistance. The result of the oxygen-acetylene test showed that the linear ablation rate (LAR) and mass ablation rate (MAR) of phenolic resin–carbon fiber Citation: Yuan, W.; Wang, Y.; Luo, Z.; composites reduced from 0.052 ± 0.005 mm/s to 0.038 ± 0.004 mm/s and from 0.050 ± 0.004 g/s to Chen, F.; Li, H.; Zhao, T. Improved 0.043 ± 0.001 g/s by introducing SiBCN powder, respectively. The mechanism of ablation resistance Performances of SiBCN Powders after the introduction of SiBCN powder was investigated. The high melt-viscosity of SiBCN powder Modified Phenolic Resins-Carbon caused SiBCN powder to remain on the surface of composites and protect the internal resins and Fiber Composites. Processes 2021, 9, carbon fibers. The oxidation of SiBCN powder and volatilization of oxide can consume energy and 955. https://doi.org/10.3390/ oxygen, thus the ablation resistance of SiBCN–Ph composite was improved. pr9060955

Keywords: SiBCN powder; phenolic resin-carbon ﬁber composites; thermal stability; oxidation Academic Editor: Mainul Islam resistance; ablative resistance

Received: 1 April 2021 Accepted: 7 May 2021 Published: 28 May 2021 1. Introduction

Publisher’s Note: MDPI stays neutral Phenolic resin–carbon fiber composites with excellent thermal stability, dimensional with regard to jurisdictional claims in stability, mechanical properties, lightweight, and ablation properties are regarded as effec- published maps and institutional affil- tive thermal protection material to protect space vehicles in the high temperature aerobic iations. environment [1]. With the rapid development of aerospace craft, the applied environment becomes harsh and a large area of the craft must face a high temperature (>1000 ◦C) aerobic environment for a long time [2]. However, phenolic resins with poor oxidation resistance difficultly meet the requirements of long-term application under high temperature

Copyright: © 2021 by the authors. and aerobic conditions [3,4]. Thus, it is urgent to improve the performance of phenolic Licensee MDPI, Basel, Switzerland. resin–carbon ﬁber composites [5]. This article is an open access article In recent years, different kinds of inorganic powder [6,7] had been used to improve the distributed under the terms and ablation resistance, thermal stability and other properties of phenolic resins. L Asaro [8] and conditions of the Creative Commons his co-workers studied the influence of the introduction of mesoporous silica particles on the Attribution (CC BY) license (https:// ablative properties of phenolic/carbon fiber composites. The composite with 20 wt % silica creativecommons.org/licenses/by/ particle loadings had the lowest mass erosion rate, because the melted silica can remain 4.0/).

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above the carbon fibers to protect the surface of the material during ablation. Kandasubra- manian Balasubramanian group [9] introduced boron nitride to reinforce phenolic-carbon fiber composites, and composites with 3 wt % BN loadings showed maximum improvement of ablation performance. Performance enhancement resulted from phase transformation of h-BN at higher temperatures and typical floral assimilation which can lead to significant energy consumption. Zahra Amirsardari [10] and his coworkers found that the introduction of nanosized ZrB2 powder could improve the ablative properties of phenolic/carbon fiber composites in terms of mass loss and erosion rate. Jie Ding [11,12] provided an effective way to modify phenolic resins by the addition of ZrSi2, which prominently improved the ablation performance of phenolic-carbon fiber composites by the formation of ZrO2 and SiO2. Zhaofeng Chen [1] etc. thought that SiC-modified phenolic resins/carbon fiber composites are a kind of great potential thermal structural material in aeronautics and astronautics with low specific weight, good thermal stability, oxidation resistance and excellent ablation resistance. In general, the introduction of inorganic powder was frequently utilized to improve the performance of phenolic-carbon fiber composites [13]. Polymer-derived silicon-boron-carbon-nitrogen (SiBCN) quaternary system ceramic (PDCs) have an amorphous structure and steady three-dimensional network structure formed by a strong covalent bond, which draws widespread addition [14,15]. Amorphous SiBCN contains three main chemical structures: (1) amorphous carbon clusters (“free” or sp2-carbon) in nanoscale, (2) BN(C) phase, and (3) an amorphous Si-C-N matrix consisted of SiC, Si3N4 and SiCxNy [15,16]. The special structure endowed SiBCN ceramics with extremely high thermal structural stability [14,17]. SiBCN ceramics still have excellent creep resistance in the range of 1400~1500 ◦C. The viscosity coefficient of SiBCN ceramics is about 1015 Pa·s at 1550 ◦C, which is an order of magnitude higher than that of fused silica [18]. SiBCN ceramics have better oxidation resistance at temperatures up to 1800 ◦C, ◦ ◦ compared with traditional SiC (>1600 C) and Si3N4 (>1500 C) ceramics [14,16]. Moreover, the density of SiBCN powder was 1.87 g/cm3 (same as carbon fibers) and lower than most ceramics, which contributed to alleviate the weight of the composites [17]. Therefore, SiBCN ceramic powder could be used to modify the performance of phenolic resins-carbon fiber (Carbon/phenolic) composites as suitable inorganic powder. Although SiBCN powder has excellent properties, SiBCN powder never is introduced into resin matrix composites to improve comprehensive performance. In this paper, we prepared SiBCN phenolic resins and SiBCN–phenolic/C composites. The residue constituents of SiBCN phenolic resins after pyrolysis were also investigated, and the effects of the introduction of SiBCN powder on the thermal stability and degradation behavior of SiBCN phenolic resins were analyzed. The roles of SiBCN particles on oxidation resistance and ablation resistance of SiBCN–phenolic/C composites were studied. Changes in chemical structure and microstructure of the ablated specimens were researched to discuss the oxidative and ablative mechanisms.

2. Experimental 2.1. Materials Resole-type phenolic resins, the matrix of composites, were synthesized in our laboratory according to the literature [3]. Ethanol (Sinopharm Chemical Reagent Co., Beijing, China) was used as the solvent. Plain weave carbon ﬁber cloth (T300, Japan Toray Com- pany, Tokyo, Japan) was used as reinforcement of composites. SiBCN powder with the amorphous structure was prepared in our laboratory according to the literature [16,19].

2.2. Sample Preparation 2.2.1. Fabrication Process of SiBCN-Modiﬁed Phenolic Resins (the SiBCN Phenolic Resins) Phenolic resins (Ph) were mixed with SiBCN powder in the mass proportion of 1:0.1 by mechanical stirring and ultrasonic dispersion to prepare SiBCN modiﬁed phenolic resin (the SiBCN phenolic resins). The SiBCN phenolic resins were cured at 80 ◦C for 4 h, Processes 2021, 9, 955 3 of 15

followed by 120 ◦C for 2 h, and then 180 ◦C for 4 h, all in an oven. Finally, the cured SiBCN phenolic resins were obtained.

2.2.2. Fabrication Process of SiBCN-Modified Phenolic Resin-Carbon Fiber Composites (SiBCN–Phenolic/C Composites) SiBCN powder was added to the phenolic resin/ethanol solution (50 wt %) by mechanical stirring for 2 h, and the SiBCN-phenolic ethanol mixture solution was obtained. The mass proportion of phenolic resins, ethanol and SiBCN powder was 1:1:0.1 in the solution. Plain weave carbon fiber cloth was coated with the above mixture, the weight ratio of carbon fiber and hybrid resins was 1:1. Then, the impregnated clothes were put in the fume hood at ambient temperature for 12 h to evaporate the solvent and put in the oven at 80 ◦C for 60 min. Then the prepared prepregs were cut into 100 × 100 mm pieces, put in the mold and pressed on the hot plate under the pressure of 30 kg·cm−2 at 120 ◦C for 2 h and 180 ◦C for 4 h, and the laminated board was obtained. Finally, the SiBCN–phenolic/C composites were fabricated into cylindrical samples with sizes of Φ 30 × 10 mm (a diameter of 30 mm and a thickness of 10 mm) for the oxygen-acetylene ablation testing. The fabrication process of phenolic/C composites was the same as that of SiBCN–phenolic/C composites.

2.3. Characterization and Testing Fourier transform infrared spectroscopy (FTIR) measurements were performed on a Tensor-27 spectrometer at room temperature. Samples were ground, mixed with KBr (the weight ratio of sample and KBr was 1:100) and pressed into small flakes for testing. The number of sample scans was 16 and the resolution was 4. Thermogravimetric Analysis (TGA) was carried out from ambient temperature to 1200 ◦C on a Netzsch STA409PC at a heating rate of 10 ◦C·min−1 in nitrogen and air atmosphere, respectively. The form of the sample was powder (particle size: 300~450 µm), mass of the samples was 6~8 mg, flow rate of carrier gas was 50 mL/min and flow rate of purge gas was 15 mL/min. The cured resins were pyrolyzed 2 h in a tube furnace at 1200 ◦C and volume shrinkage was defined as the volume ratio of the cured resins before and after pyrolysis. The number of replicates for each sample was 3. The cured phenolic resins and SiBCN phenolic resins were quenched and cracked in liquid nitrogen, and the fracture surface of the hybrids was observed on a Hitachi S-8020 scanning electron microscope (SEM) at an accelerating voltage of 10 kV. Energy-dispersive X-ray spectroscopy (EDS) was performed on Oxford INCAx-sight 7593 system attaching to the SEM apparatus. A high-temperature oxidation test was performed in a muffle furnace at the desired temperature with different times. The number of replicates for each sample was 3. The internal gas in a muffle was air and no gas entered after the start of the test. The oxyacetylene flame test was carried out to evaluate the mass ablative rate of the composite according to GB323A-96. At least three specimens with a dimension of approximately Φ 30 mm (diameter) × 10 mm (thickness) were prepared for each test, and the exposure time for each ablation test under the torch flame was 30 s. The back temperature of the samples was measured with a K-type thermocouple. The average value was obtained for each sample. The chemical composition of the oxidized SiBCN phenolic resins was determined by X-ray photoelectron spectroscopy (XPS) measurement on an ESCALAB250XI instrument (Thermo Fisher Scientific, Waltham, MA, USA). One sample was tested for this characterization. The diameter of beam spot was 400 µm and constant analyzer energy was 100.0, 1.00 eV. The obtained data were calibrated by C 1 s standard peak and analyzed by PEAK XPS software. The crystal structure of the SiBCN ceramics powder was characterized by X-ray diffraction (XRD, Bruker D8A A25, Karlsruhe, Germany) with Cu Kα radiation at 40 kV and 40 mA. The diffraction patterns were scanned from 10◦ to 80◦ of 2θ in a step-scan mode at a step of 0.26◦ and a counting time of 0.02 s/step. Particle size distribution was measured with a laser particle size analyzer (Mastersizer 3000, Malvern Instruments Ltd., Malvern, UK). Processes 2021, 9, x 4 of 16

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3. Results and Discussion 3.1.3. ResultsMorphology and of Discussion SiBCN Powder on the SiBCN Phenolic Resins and the SiBCN–Phenolic/C Composites3.1. Morphology of SiBCN Powder on the SiBCN Phenolic Resins and the SiBCN–Phenolic/CAs shown in Figure Composites 1a,b, SiBCN powder have a broad distribution of particle size (D10 = 2.45 Asμm, shown D50 = in 21.7 Figure μm1 anda,b, SiBCND90 = 50.8 powder μm) haveand irregular a broad distribution shape. For the of particle cured SiBCN size phenolic(D10 = 2.45resinsµm, (Figure D50 = 1c), 21.7 powderµm and was D90 em =bedded 50.8 µm) in and the irregularresin matrix shape. and For tightly the curedbound withSiBCN resin. phenolic From Figure resins (Figure1c, the 1dispersionc), powder of was SiBCN embedded powder in was the resinhomogeneous. matrix and In tightly Figure 1d,e,bound SiBCN with powder resin. From with Figure large1 c,particle the dispersion sizes accumulated of SiBCN powder between was the homogeneous. layers of carbon In fiberFigure fabric1d,e, and SiBCN SiBCN powder powder with with large small particle particle sizes sizes accumulated can penetrate between carbon the fiber layers fabric of aftercarbon the fiberpreparation fabric and of SiBCNSiBCN–phenolic/C powder with smallcomposites. particle In sizes addition, can penetrate since the carbon density fiber of SiBCNfabric afterpowder the preparationwas almost ofthe SiBCN–phenolic/C same as carbon fibers composites. and lower In addition, than normal since the metal/non- density metalof SiBCN oxide powder and carbide, was almost the density the same of as the carbon SiBCN–phenolic/C fibers and lower composite than normal (1.456 metal/non- g/mm3) metal oxide and carbide, the density of the SiBCN–phenolic/C composite (1.456 g/mm3) was not significantly higher than that of the phenolic composite (1.451 g/mm3). was not significantly higher than that of the phenolic composite (1.451 g/mm3).

Figure 1. Particle size distribution (a) and scanning electron microscopy (SEM) image (b) of SiBCN Figure 1. Particle size distribution (a) and scanning electron microscopy (SEM) image (b) of SiBCN powder,powder, SEM SEM images images of of the the cu curedred SiBCN SiBCN phenolic phenolic resins resins (c (c)) and and the the SiBCN–phenolic/C SiBCN–phenolic/C compo- compos- sitesites ( (dd,,ee).).

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3.2. Thermal Stability of the Cured SiBCN Phenolic Resins 3.2. Thermal Stability of the Cured SiBCN Phenolic Resins As shown in Figure2, thermal stability of the cured phenolic resins and the cured As shown in Figure 2, thermal stability of the cured phenolic resins and the cured SiBCNSiBCN phenolic phenolic resins were resins studied were studiedby thermal by gravimetric thermal gravimetric analysis. The analysis. thermal Thestability thermal stability of phenolicof phenolic resins resinswas improved was improved by the intr byoduction the introduction of SiBCN powder. of SiBCN At powder.the first stage At the ﬁrst stage ◦ (<300(<300 °C), accordingC), according to reported to reported papers, th papers,e release the of releasewater and of waterother small and othermolecule small molecule productsproducts was the was main the cause main of cause weight of loss weight [20]. lossIt was [20 found]. It was from found Table from1 that TableT5% and1 that T 5% and R300 increasedR300 increased slightly, slightly,which indicated which that indicated the introduction that the of introduction SiBCN powder of made SiBCN phe- powder made nolicphenolic resins gain resins a small gain improvement a small improvementin thermal stability in thermalbecause of stability the thermal because stability of the thermal of SiBCNstability powder. of SiBCN At the powder.second stage At the(300~800 second °C), stage rapid (300~800 weight loss◦C), occurred rapid weightwith theloss occurred releasewith of low the molecular release of weight low molecular phenolic and weight aromatic phenolic compounds and aromatic and other compounds low molec- and other low ular weight hydrocarbons (CO, CO2, CH4, H2O, H2, CH4, C2H2, C2H6) produced by pyrol- molecular weight hydrocarbons (CO, CO2, CH4,H2O, H2, CH4,C2H2,C2H6) produced ysis [11]. Almost the same Tmax showed that the introduction of powder did not change by pyrolysis [11]. Almost the same Tmax showed that the introduction of powder did not the thermal pyrolysis behavior of resins. However, the difference in the residual weight change the thermal pyrolysis behavior of resins. However, the difference in the residual between the cured SiBCN phenolic resins and the cured phenolic resins was growing in weight between the cured SiBCN phenolic resins and the cured phenolic resins was growing this stage. The residual weight difference increased from 1.07% at 300 °C to 5.15% at 800◦ °C. Forin the this third stage. stage The (>800 residual °C), the weightweight loss difference of phenolic increased resins slowed from down 1.07% at at 800~900 300 C to 5.15% at ◦ ◦ °C, and800 thenC. the For weight the third loss stageaccelerated. (>800 TheC), cured the weight SiBCN phenolic loss of phenolic resins had resins the same slowed down at ◦ trend,800~900 but its durationC, and of then the thestage weight with slow loss weight accelerated. loss rate The was cured longer SiBCN than phenolic phenolic resins had resins.the Assuming same trend, no interactions but its duration between of the the powder stage withand the slow resin, weight theoretical loss rateresidual was longer than weightphenolic of the cured resins. SiBCN Assuming phenolic no resins interactions could be betweencalculated the according powder to and the residual the resin, theoretical weightresidual of phenolic weight resins of and the curedSiBCN SiBCNpowder phenolicduring the resins thermal could pyrolysis be calculated process. The- according to the oreticalresidual residual weight weight of of phenolic the cured resins SiBCN and phenolic SiBCN resins powder and the during actual the residual thermal weight pyrolysis process. wereTheoretical almost the same residual at 300 weight°C. However, of the th curedeoretical SiBCN residual phenolic weight of resins the cured and SiBCN the actual residual phenolicweight resins were at 1200 almost °C should the same theoretically at 300 ◦C. be However, 59.70%, but theoretical the actual residualresidual weight of the cured of that was 62.98%. The 3.28% promotion may have been caused by the reaction between SiBCN phenolic resins at 1200 ◦C should theoretically be 59.70%, but the actual residual SiBCN powder and phenolic resin during the pyrolysis process. weight of that was 62.98%. The 3.28% promotion may have been caused by the reaction between SiBCN powder and phenolic resin during the pyrolysis process.

Figure 2. Thermogravimetric (TG) curves of the cured phenolic resins and the cured SiBCN phenolic Figure 2. Thermogravimetric (TG) curves of the cured phenolic resins and the cured SiBCN phe- resins in N2 atmosphere. nolic resins in N2 atmosphere.

Table 1. Thermogravimetric analysis (TGA) results under N2 atmosphere of the cured phenolic resins and the cured SiBCN

phenolic resins.

◦ a ◦ b c d e f Samples T5% ( C) Tmax ( C) R300 (%) R800 (%) R1000 (%) R1200 (%) The cured phenolic resins 288.67 541.07 94.64 62.62 59.89 55.87 SiBCN powder \\ 98.59 98.24 97.98 98.01 The cured SiBCN–phenolic resins 329.18 545.65 95.71 67.77 66.02 62.98 Theoretical residual weight of \\ 95.00 65.86 63.35 59.70 SiBCN phenolic resins a The temperature of the 5% weight loss. b The temperature of the maximum degradation rate. c Residue yield at 300 ◦C. d Residue yield at 800 ◦C. e Residue yield at 1000 ◦C. f Residue yield at 1200 ◦C. Processes 2021, 9, 955 6 of 15

XPS was used to analyze the chemical compositions of the cured SiBCN phenolic resins after pyrolyzed 2 h (ensure complete pyrolysis) at 1200 ◦C in Ar (consistent with the thermogravimetric environment). As can be seen in Figure3a, two peaks were displayed on the Si element spectrum of SiBCN powder: (1) 101.80 eV: Si3N4 (Si 2p3/2)[21]; (2) 102.60 eV: SiC(SiC/BN) (Si 2p3/2)[22], which indicated that amorphous Si-C-N matrix contained the Si3N4 phase and SiC phase. However, four peaks were shown in the Figure3b: (1) 101.50 eV: Si3N4 (Si 2p3/2)[23]; (2) 102.40 eV: SiC(SiC/BN) (Si 2p3/2)[24]; (3) 100.10 eV: SiC (Si 2p) [22]; (4) 103.20 eV: SiO2/SiC (Si 2p3/2)[25]. The appearance of peak 103.20 eV indicated that SiO2 produced in pyrolysis progress by the reaction [1,15,21]: Si3N4 (s) + 3CO2 (g) → 3SiO2 (s) + 3C (s) + 2N2 (g) Si3N4 (s) + 6CO (g) → 3SiO2 (s) + 6C (s) + 2N2 (g) Si3N4 (s) + 6H2O (g) → 3SiO2 (s) + 3H2 (g)↑ + 2N2 (g) SiC (s) + 2H2O (g) → SiO2 (s) + CH4 (g) SiC (s) + 3CO2 (g) → SiO2 (s) + 4CO (g) In Figure3c, two peaks were displayed on the B element spectrum of SiBCN: (1) 190.60 eV: BN (B 1 s) [26]; (2) 192.40 eV: BCN (B 1 s) [27], which meant that BN(C) phase consisted of BN phase and BCN phase. However, the peak of 192.40 eV had disappeared and the peak of 192.70 eV (BNO (B 1 s)) [28] appeared in Figure3d, which indicated that the boride of SiBCN was oxidized in the pyrolysis progress [9]. 2BN (s) + CO2 (g) → 2BNO (s) + C (s) 2BCN (s) + CO2 (g) → 2BNO (s) + 3C (s) Reactions of boride and silicide during pyrolysis processes made carbon and oxygen Processes 2021, 9, xelements of volatiles remain in the matrix with the formation of the BNO, SiO2 and C. 7 of 16 The occurrence of these reactions increased the residual weight and enhanced the thermal stability of SiBCN phenolic resins at high temperature.

Figure 3. Si spectra of SiBCN powder (a) and the pyrolyzed SiBCN phenolic resins at 1200 ◦C in Figure 3. Si 2p spectra 2pof SiBCN powder (a) and the pyrolyzed SiBCN phenolic resins at 1200 °C in Ar atmosphere(b), B 1s spectra of ArSiBCN atmosphere powder ( (cb) ),and B 1s thespectra pyrolyzed of SiBCN SiBCN powder phenolic (c) resi andns the at 1200 pyrolyzed °C in Ar SiBCN atmosphere phenolic (d). resins at 1200 ◦C in Ar atmosphere (d). After 2 h pyrolysis at 1200 °C in an Aar atmosphere, the cured SiBCN phenolic resins shrunk but did not crack. Compared to the volume shrinkage ratio of phenolic resins (31.11%), that of cured SiBCN phenolic resins was 24.31%, which indicated that the introduction of SiBCN powder contributed to enhancing the dimensional stability. Because SiBCN ceramic with excellent dimensional stability can restrain shrinkage of phenolic resins at pyrolysis process. As shown in Figure 4, the fracture morphology of the pyrolyzed SiBCN phenolic resins (1200 °C, 2 h) was observed by SEM and the element content of SiBCN powder surface was analyzed by EDS. The result illustrated that SiBCN powder reacted with the pyrolysis atmosphere of phenolic resin to generate oxides on the surface of the powder.

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After 2 h pyrolysis at 1200 ◦C in an Aar atmosphere, the cured SiBCN phenolic resins shrunk but did not crack. Compared to the volume shrinkage ratio of phenolic resins (31.11%), that of cured SiBCN phenolic resins was 24.31%, which indicated that the introduction of SiBCN powder contributed to enhancing the dimensional stability. Because SiBCN ceramic with excellent dimensional stability can restrain shrinkage of phenolic resins at pyrolysis process. As shown in Figure4, the fracture morphology of the pyrolyzed SiBCN phenolic resins (1200 ◦C, 2 h) was observed by SEM and the element content of SiBCN powder surface was analyzed by EDS. The result illustrated that SiBCN powder Processes 2021, 9, x 8 of 16 reacted with the pyrolysis atmosphere of phenolic resin to generate oxides on the surface of the powder.

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FigureFigure 4. 4.SEM SEM image image of the of SiBCNthe SiBCN phenolic phenolic resins afterresins pyrolyzed after pyrolyzed 2 h at 1200 2◦ Ch inat Ar1200 atmosphere. °C in Ar Figure 4. SEM image of the SiBCN phenolic resins after pyrolyzed 2 h at 1200 °C in Ar atmosphere. atmosphere.3.3. Oxidation Resistance of the Cured SiBCN Phenolic Resins 3.3.3.3. Oxidation OxidationAs shown Resistance Resistance in of Figure the Cured5 ofand SiBCN the Table CuredPhenolic2, TGSiBCNResins curves Phenolic of the Resins cured phenolic resins and the cured SiBCNAs shown phenolic in Figure resins 5 and in Table air atmosphere2, TG curves of demonstratedthe cured phenolic that resins at and the the temperature of 5% weight cured SiBCNAs shownphenolic resinsin Figure in air atmosphe 5 and reTable demonstrated 2, TG thatcurv at esthe oftemperature the cured of phenolic resins and the loss, the maximum degradation rate and the temperature of final decomposition of the cured 5%cured weight SiBCNloss, the maximum phenolic degradation resins in rate air and atmosphe the temperaturere demonstrated of final decomposi- that at the temperature of tionSiBCN of the cured phenolic SiBCN resins phenolic were resins both were improved both improved by by the the introduction introduction of ofSiBCN. SiBCN. The residual weight 5% weight loss, the maximum degradation rate and the temperature of final decomposi- Theof residual the cured weight phenolic of the cured resins phenolic and re SiBCNsins and powder SiBCN powder at 800 at◦ 800C was°C was 0.76% 0.76%and 97.43%, respectively. andIftion phenolic97.43%, of the respectively. resinscured areSiBCN If phenolic oxidized phenolic resins completely, are resins oxidized were thecompletely, residual both theimproved weightresidual weight of by the the cured introduction SiBCN phenolic of SiBCN. ofThe the cured residual SiBCN weightphenolic resinsof the should cured be phenolic8.86% (the amount resins of and introduced SiBCN SiBCN powder at 800 °C was 0.76% powderresins was should 9.09%). beHowever, 8.86% the (the residual amount weight ofof SiBCN introduced phenolic SiBCNresins at 800 powder °C was was 9.09%). However, 13.31%.theand residual It97.43%, was obvious weight respectively. that ofthe SiBCN introduction If phenolicphenolic of SiBCN resins resins powder at arecan 800 enhanceoxidized◦C was the 13.31%.oxidation completely, It was the obvious residual that weight the resistance.introductionof the cured of SiBCN SiBCN phenolic powder can resins enhance should the oxidationbe 8.86% resistance.(the amount of introduced SiBCN powder was 9.09%). However, the residual weight of SiBCN phenolic resins at 800 °C was 13.31%. It was obvious that the introduction of SiBCN powder can enhance the oxidation resistance.

FigureFigure 5. TG 5. TGcurves curves of the cured of the pheno curedlic resins phenolic and the resins curedand SiBCN the phenolic cured resins SiBCN in air phenolic at- resins in air atmosphere. mosphere.

Figure 5. TG curves of the cured phenolic resins and the cured SiBCN phenolic resins in air atmosphere.

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Table 2. TGA results under air atmosphere of the cured phenolic resins and the cured SiBCN modified phenolic resins.

5% a max b end c 800 d Processes 2021, 9, 955 Samples T (°C) T (°C) T (°C) R8 of 15 (%) The cured 291.7 591.4 652.9 0.76 phenolic resins SiBCNTable 2. powderTGA results under air\ atmosphere of the cured\ phenolic resins and the\ cured SiBCN modiﬁed97.43 Thephenolic cured resins.SiBCN 328.0 647.1 774.3 13.31 phenolic resins ◦ a ◦ b ◦ c d Samples T5% ( C) Tmax ( C) Tend ( C) R800 (%) a The temperature of the 5% weight loss. b The temperature of the maximum degradation The cured phenolic resins 291.7 591.4 652.9 0.76 rate. c The temperatureSiBCN powder of final decomposition\\\ d Residue yield at 800 °C. 97.43 The cured SiBCN phenolic resins 328.0 647.1 774.3 13.31 a TheSubsequently, temperature of the the 5% weight chemical loss. b The structure temperature of of residual the maximum SiBCN degradation phenolic rate. c The resins temperature after being d ◦ oxidizedof ﬁnal decomposition for 2 h (to ensureResidue yieldcomplete at 800 C. oxidation) at 1000 °C in air (consistent with the thermogravimetric environment) was analyzed by XRD (Figure 6a) and Fourier transform in- Subsequently, the chemical structure of residual SiBCN phenolic resins after being fraredoxidized (FTIR) for spectroscopy 2 h (to ensure complete (Figure oxidation) 6b). The atpatterns 1000 ◦C of in airXRD (consistent shown within Figure the thermo- 6a illustrated SiBCNgravimetric powder environment) was slightly was oxidized analyzed by to XRD generate (Figure B6a)2O and3 in Fourier the oxidation transform process infrared [29]. Be- sides,(FTIR) as shown spectroscopy in the (Figure FTIR6 b).spectrum The patterns (Figure of XRD 6b), shown the absorption in Figure6a illustratedbands appearing SiBCN at 1085 cmpowder−1 and 1476 was slightlycm−1 were oxidized attributed to generate to Si-O-Si B2O3 in and the B-O- oxidationB. The process appearance [29]. Besides, of these as signals −1 indicatedshown inthat the SiBCN FTIR spectrum powder (Figure was oxidized.6b), the absorption Two signals bands located appearing at 884 at 1085 cm− cm1 and 677 cm−1 and 1476 cm−1 were attributed to Si-O-Si and B-O-B. The appearance of these signals that belonged to Si-O-B illustrated that borosilicate might be generated during oxidation indicated that SiBCN powder was oxidized. Two signals located at 884 cm−1 and 677 cm−1 −1 −1 progress.that belonged Peaks toat Si-O-B around illustrated 918 cm that and borosilicate 787 cm might were be ascribed generated to during Si–N oxidationand Si–C, which meantprogress. that the Peaks powder at around was 918 not cm oxidized−1 and 787 completely. cm−1 were ascribed to Si–N and Si–C, which meant that the powder was not oxidized completely.

Figure 6. X-ray diffraction (XRD) pattern (a) and the Fourier transform infrared (FTIR) spectrum (b) of the SiBCN phenolic Figureresins 6. X-ray after oxidizeddiffraction in the (XRD) muffle pattern furnace at (a 1000) and◦C. the Fourier transform infrared (FTIR) spectrum (b) of the SiBCN phenolic resins after oxidized in the muffle furnace at 1000 °C. 3.4. Oxidation Resistance of SiBCN–Phenolic/C Composites 3.4. OxidationA static Resistance ablation test of in SiBCN–Phenolic/C the muffle furnace wasComposites used to further study the influence of theA SiBCN static powderablation on test the oxidationin the muffle resistance furnace of composites. was used Figure to further7a showed study the residualthe influence of weight of phenolic composites and the SiBCN–phenolic/C composites after oxidization at thedifferent SiBCN temperaturespowder on the for 1oxidation h. In Figure resistance7a, the residual of composites. weight of phenolic/C Figure 7a composites showed the resid- ualwas weight almost of equal phenolic to that composites of SiBCN–phenolic/C and the compositesSiBCN–phenolic/C after oxidized composites at 600 ◦C. Itafter was oxidiza- tionindicated at different that the temperatures oxidation reaction for 1 ofh. phenolic In Figure resin 7a, was the the residual main result weight of weight of phenolic/C loss at com- posites600 ◦ C.was The almost residual equal weight to that of the of SiBCN–phenolic/C SiBCN–phenolic/C composites composites was only after 3.09% oxidized higher at 600 °C. than that of phenolic composites after being oxidized for 1 h at 800 ◦C. When composites It was indicated that the◦ oxidation reaction of phenolic resin was the main result of weight losswere at 600 tested °C. 1 hThe at1000 residualC, the weight residual of weight the SiBCN–phenolic/C of SiBCN–phenolic/C composites composites increased was only 3.09% 31.42% (phenolic composites) to 40.67%. With increasing oxidation temperature, the higherdifference than that of residual of phenolic weight composites between phenolic/C after being composites oxidized and for SiBCN–phenolic/C 1 h at 800 °C. When composites were tested 1 h at 1000 °C, the residual weight of SiBCN–phenolic/C composites

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increased 31.42% (phenolic composites) to 40.67%. With increasing oxidation temperature, the difference of residual weight between phenolic/C composites and SiBCN–phe- Processes 2021, 9, x 10 of 16 nolic/C composites became larger. Moreover, as shown in Figure 7b, the residual weight of SiBCN–phenolic/C composites after being oxidized for 2 h at 800 °C increased from 30.76% (phenolic composites) to 46.19%. However, the residual weight of phenolic/C com- positesincreased and SiBCN–phenolic/C 31.42% (phenolic composites)composites wasto 40.67%. 53.92% With and increasing57.01% after oxidation being oxidatedtempera- Processes 2021, 9, 955 for 1ture, h at the800 difference °C, respectively. of residual With weight the longbetweener oxidation phenolic/C time, composites the residual and9 of weight 15SiBCN–phe- gap nolic/C composites became larger. Moreover, as shown in Figure 7b, the residual weight between phenolic/C composites and SiBCN–phenolic/C composites was more obvious. of SiBCN–phenolic/C composites after being oxidized for 2 h at 800 °C increased from This result indicated that the introduction of SiBCN powder contributed to improving the 30.76% (phenolic composites) to 46.19%. However, the residual weight of phenolic/C com- oxidationcomposites resistance became of larger. composites. Moreover, as shown in Figure7b, the residual weight of SiBCN– positesphenolic/C and compositesSiBCN–phenolic/C after being composites oxidized for was 2 h at53.92% 800 ◦C and increased 57.01% from after 30.76% being oxidated for(phenolic 1 h at composites) 800 °C, respectively. to 46.19%. However, With the the long residualer oxidation weight of phenolic/C time, the compositesresidual weight gap betweenand SiBCN–phenolic/C phenolic/C composites composites wasand 53.92% SiBCN–ph and 57.01%enolic/C after composites being oxidated was for more 1 h obvious. at 800 ◦C, respectively. With the longer oxidation time, the residual weight gap between This result indicated that the introduction of SiBCN powder contributed to improving the phenolic/C composites and SiBCN–phenolic/C composites was more obvious. This result oxidationindicated that resistance the introduction of composites. of SiBCN powder contributed to improving the oxidation resistance of composites.

Figure 7. Residual weight of phenolic/C composites and SiBCN–phenolic/C composites at different temperatures for 1 h (a); residual weight of phenolic/C composites and SiBCN–phenolic/C composites at 800 °C for different times (b).

Figure 7. Residual weight of phenolic/C composites and SiBCN–phenolic/C composites at differ- FigureWhenent temperatures 7.the Residual SiBCN–phenolic/C for 1weight h (a); residualof phenolic/C weightcomposites ofcomposites phenolic/C and andphenolic composites SiBCN–phenolic/C composites and SiBCN–phenolic/C compositeswere oxidized at differ- 2 h at 1000entcomposites temperatures°C in a at muffle 800 ◦ Cfor for oven,1 differenth (a); phenolic residual times (b weight). compos of phenolic/Cites had no composites residue, andbut SiBCN–phenolic/Cthe residual weight composites at 800 °C for different times (b). of SiBCN–phenolic/CWhen the SiBCN–phenolic/C composites was composites still 7.1% and. phenolicAs shown composites in Figure were 8a,b, oxidized some carbon fibers2 remained h at 1000 ◦ Cat in the a muffle bottom oven, of SiBCN–phen phenolic compositesolic/C had composites no residue, and but the residualsectional funda- When the SiBCN–phenolic/C composites and phenolic composites were oxidized 2 h mentalweight framework of SiBCN–phenolic/C of SiBCN–phenolic/C composites wascomposites still 7.1%. was As shown maintained in Figure after8a,b, being some oxidized. at 1000 °C in a muffle oven, phenolic composites had no residue, but the residual weight To furthercarbon explore fibers remained the reasons at the bottomfor this of phen SiBCN–phenolic/Comenon, microstructure composites and of the SiBCN–phenolic/C sectional offundamental SiBCN–phenolic/C framework composites of SiBCN–phenolic/C was still composites7.1%. As shown was maintained in Figure after 8a,b, being some carbon composites surface was analyzed by SEM (Figure 8c). It can be seen that the residual fibersoxidized. remained To further at explorethe bottom the reasons of SiBCN–phen for this phenomenon,olic/C composites microstructure and ofthe SiBCN– sectional funda- SiBCNmentalphenolic/C powder framework compositesand their of SiBCN–phenolic/Coxide surface are was connected analyzed composites by to SEMform (Figure a was protective 8maintainedc). It can carapace be after seen thatbeingthat can oxidized. slow downTothe oxygen further residual exploreerosion SiBCN powder theof thereasons andinternal their for this oxide matrix phen are connectedinomenon, the oxidation tomicrostructure form a process. protective of The carapaceSiBCN–phenolic/C oxidation re- that can slow down oxygen erosion of the internal matrix in the oxidation process. The sistancecomposites of SiBCN–phenolic/C surface was analyzed composites by SEM can be(Figur attributede 8c). It tocan two be aspects. seen that First, the theresidual oxi- dationoxidation reaction resistance of SiBCN of SiBCN–phenolic/C powder and vola compositestilization can of be oxide attributed can toconsume two aspects. oxygen and SiBCNFirst, the powder oxidation and reaction their oxide of SiBCN are powderconnected and to volatilization form a protective of oxide cancarapace consume that can slow heat down[10].oxygen Second, oxygen and heat theerosion [10 residual]. Second, of the SiBCN the internal residual powder matrix SiBCN and in powder thetheir oxidation oxide and their can process. oxide form can a The formprotective oxidation a cara- re- pacesistance protectiveto prevent of carapace SiBCN–phenolic/C oxygen to preventfrom eroding oxygen composites fromthe internal eroding can thebe matrix internalattributed and matrix carbonto andtwo carbonaspects.fibers. fibers. First, the oxidation reaction of SiBCN powder and volatilization of oxide can consume oxygen and heat [10]. Second, the residual SiBCN powder and their oxide can form a protective carapace to prevent oxygen from eroding the internal matrix and carbon fibers.

Figure 8. Figurethe picture 8. The pictureof the ofSiBCN–phenolic/C the SiBCN–phenolic/C composites composites oxidized 2 2 h ath 1000at 1000◦C( a°C,b )( anda,b) SEM and image SEM ofimage the surface of the of surface of ◦ the SiBCN–phenolic/Cthe SiBCN–phenolic/C composites composites oxidized oxidized 2 h 2at h at1000 1000 °CC( (cc).).

Figure 8. the picture of the SiBCN–phenolic/C composites oxidized 2 h at 1000 °C (a,b) and SEM image of the surface of the SiBCN–phenolic/C composites oxidized 2 h at 1000 °C (c).

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3.5. Ablation Resistance of the SiBCN–Phenolic/C Composites As depicted in Table3, ablation properties were reﬂected by linear ablation rate (LAR) and mass ablation rate (MAR). The average LAR and MAR of phenolic/C composites were 0.052 ± 0.005 mm/s and 0.050 ± 0.004 g/s, respectively. However, the introduction of SiBCN powder helped to decrease the average LAR and MAR of SiBCN–phenolic/C composites to 0.038 ± 0.004 mm/s and 0.043 ± 0.001 g/s, respectively. The LAR of SiBCN– phenolic/C composites reduced by 27% compared with that of phenolic/C composites, the MAR of SiBCN–phenolic/C composites reduced by 14%. It was found that the ablation resistance of composites can be signiﬁcantly improved by introducing SiBCN powder.

Table 3. Ablation properties of phenolic/C composites and SiBCN–phenolic/C composites.

Samples Linear Ablation Rate (mm/s) Mass Ablation Rate (g/s) Phenolic/C composites 0.052 ± 0.005 0.050 ± 0.004 SiBCN–phenolic/C composites 0.038 ± 0.004 0.043 ± 0.001

To further study the mechanism of ablation performance improvement by the introduction of SiBCN powder, SEM was used to observe the microstructure of the ablated Ph and SiBCN–phenolic/C composites. During the ablation process, the center temperature of the ablation surface was higher than 2000 ◦C[2]. As Figure9a,b shows, phenolic resins were oxidized and stripped out completely by oxygen-acetylene steam and only carbon fibers remained on the surface of phenolic/C composites, the carbon fibers were easily oxidized to CO and CO2 to form a needle-like structure. Therefore, carbon fibers without the protection of resins matrix were easily eroded and destroyed by the high- temperature flame flow. In Figure9e, because phenolic resins could be pyrolyzed at aerobic high-temperature conditions, many pores appeared in the ablation layer and the originally dense composite material becomes loose after ablation. However, owing to high viscosity at high temperature, SiBCN powder on the ablation surface could remain to protect carbon fibers under the scouring of high-temperature flame flow (Figure9c,d). Moreover, according to the result of energy-dispersive X-ray spectroscopy (EDX), the appearance of Si, C, O and B elements on the residual resin matrix can indicate that SiBCN powder could be oxidized to SiO2 and B2O3. Meanwhile, the chemical structure of residue left on the surface of the SiBCN–phenolic/C composites was further characterized by XPS. Comparing to Figure3a,c, four peaks were shown in the Figure 10b: (1) 101.20 eV: Si 3N4 (Si 2p3/2)[21]; (2) 102.60 eV: SiC(SiC/BN) (Si 2p3/2)[22]; (3) 100.20 eV: SiC (Si 2p) [22]; (4) 103.70 eV: SiO2/(SiC+ SiO2) (Si 2p3/2)[30]. Three peaks were shown in the Figure9a: (1) 190.30 eV: BN (B 1 s) [21]; (2) 192.20 eV: BCN (B 1 s) [28]; (3) 193.20: B2O3 (B 1 s) [31]. This result further proved the oxidation of SiBCN on the surface of SiBCN–phenolic/C composites. The occurrence of oxidation reaction of SiBCN powder consumed oxygen to slow down oxyacetylene flame erosion of phenolic resins and carbon fibers. The volatilization process of SiO2 and B2O3 was endothermic, and the volatilized outward airflow will reduce the erosive strength of the oxyacetylene heat flow [6,11]. Moreover, as shown in Figure9f, compared with phenolic composites during the ablation process, the ablation layer of SiBCN–phenolic/C composites had the tighter links between closely woven carbon fibers and fewer large cavities. Three reasons mentioned above could improve the ablation resistance of SiBCN–phenolic/C composites. Processes 2021, 9, x 12 of 16

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Figure 9. SEM images of the surface (a,b) and the section (e) for the ablated phenolic/C compo- Figure 9. SEM images of the surface (a,b) and the section (e) for the ablated phenolic/C composites, sites, SEM images of the surface (c,d) and the section (f) for the SiBCN–phenolic/C composites. FigureSEM images 9. SEM of the images surface of (c the,d) andsurface the section (a,b) (andf) for the the section SiBCN–phenolic/C (e) for the composites.ablated phenolic/C composites, SEM images of the surface (c,d) and the section (f) for the SiBCN–phenolic/C composites.

Figure 10. B 1s spectra (a) and Si 2p spectra (b) of products on the surface of the ablated SiBCN–phenolic/C composites. Figure 10. B 1s spectra (a) and Si 2p spectra (b) of products on the surface of the ablated SiBCN–phenolic/C composites. Figure 10. B 1s spectra (a) and SiTemperature 2p spectra (b curves) of products of the back on surfacethe surface of the of SiBCN–phenolic/C the ablated SiBCN–phenolic/C composites and composites. phe- Temperaturenolic/C composites curves were of shown the inback Figure surface 11. During of the the ablationSiBCN–phenolic/C process, the temperature composites and phenolic/CincreasingTemperature composites rate of the SiBCN–phenolic/Ccurveswere shown of the in back Figure composites surface 11. wasDuring of slowerthe theSiBCN–phenolic/C than ablation that of the process, phenolic/C composites the temper- and phenolic/Ccomposites. Thecomposites different results were betweenshown in SiBCN–phenolic/C Figure 11. During composites the ablation and phenolic/C process, the temperature increasing rate of the SiBCN–phenolic/C composites was slower than that of the aturecomposites increasing were listed rate below. of the Firstly, SiBCN–phenolic/C the thermal insulation composites of SiBCN was was slower high because than that of the phenolic/C composites. The different results between SiBCN–phenolic/C composites and phenolic/Cof the amorphous composites. structure, The which different can hinder result heats between spread [32 SiBCN–phenolic/C]. Secondly, the oxidation composites and phenolic/Creaction composites of SiBCN powder were and listed volatilization below. ofFi oxiderstly, couldthe thermal consume theinsulation quantity ofof heat,SiBCN was phenolic/C composites were listed below. Firstly, the thermal insulation of SiBCN was high whichbecause can of reduce the theamorphous diffusion of structure, heat flow into wh theich matrix. can hinder heat spread [32]. Secondly, high because of the amorphous structure, which can hinder heat spread [32]. Secondly, the oxidationTo illustrate reaction the of ablation SiBCN resistance powder of phenolicand volatilization and SiBCN–phenolic/C of oxide could composites, consume the thea simple oxidation schematic reaction diagram of SiBCN (Figure 12powder) was composed and volatilization on basis of of the oxide above could analysis. consume the quantity of heat, which can reduce the diffusion of heat flow into the matrix. quantityFor SiBCN–phenolic/C of heat, which composites, can reduce SiBCN the powder diffusion with of large heat particle flow into sizes the accumulated matrix. between the layers of carbon fiber fabric and the small particle size powder embedded in carbon fiber after preparation. In the ablation process, phenolic resins on the surface were pyrolyzed easily and SiBCN powder was reserved under the scour of oxyacetylene flame flow because of their high melt-viscosity. With the erosion of oxyacetylene, the reserved SiBCN powder was oxidized to B2O3 and SiO2. The formation and volatilization of B2O3 and SiO2 can consume heat and slow down the erosion rate of flame flow, which can

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alleviate the erosion of oxyacetylene flame. The oxide can adhere to the surface of SiBCN powder, preventing further oxidation. Moreover, reserved SiBCN powder and its oxides can form a protective domain to hinder the entry of oxyacetylene flame flow, so carbon fibers and resins matrix covered by SiBCN powder and their oxides can remain. Finally, the interior SiBCN powder also played an important role in preventing the oxygen from eroding further. However, without the protection of SiBCN powder, the surface of phenolic Processes 2021, 9, x 13 of 16 Figurecomposites 11. Back temperature was be eroded of SiBCN–pheno seriously and oxygenlic/C and can phenolic/C enter to internal composites. deeper position, which caused high MAR and LAR. To illustrate the ablation resistance of phenolic and SiBCN–phenolic/C composites, a simple schematic diagram (Figure 12) was composed on basis of the above analysis. For SiBCN–phenolic/C composites, SiBCN powder with large particle sizes accumulated between the layers of carbon fiber fabric and the small particle size powder embedded in carbon fiber after preparation. In the ablation process, phenolic resins on the surface were pyrolyzed easily and SiBCN powder was reserved under the scour of oxyacetylene flame flow because of their high melt-viscosity. With the erosion of oxyacetylene, the reserved SiBCN powder was oxidized to B2O3 and SiO2. The formation and volatilization of B2O3 and SiO2 can consume heat and slow down the erosion rate of flame flow, which can alleviate the erosion of oxyacetylene flame. The oxide can adhere to the surface of SiBCN powder, preventing further oxidation. Moreover, reserved SiBCN powder and its oxides can form a protective domain to hinder the entry of oxyacetylene flame flow, so carbon fibers and resins matrix covered by SiBCN powder and their oxides can remain. Finally, the interior SiBCN powder also played an important role in preventing the oxygen from eroding further. However, without the protection of SiBCN powder, the surface of phenolic composites was be eroded seriously and oxygen can enter to internal deeper posi-

tion,Figure Figurewhich 11. 11. Back causedBack temperature temperature high of MARSiBCN–pheno of SiBCN–phenolic/C andlic/C LAR. and phenolic/C and phenolic/Ccomposites. composites.

To illustrate the ablation resistance of phenolic and SiBCN–phenolic/C composites, a simple schematic diagram (Figure 12) was composed on basis of the above analysis. For SiBCN–phenolic/C composites, SiBCN powder with large particle sizes accumulated between the layers of carbon fiber fabric and the small particle size powder embedded in carbon fiber after preparation. In the ablation process, phenolic resins on the surface were pyrolyzed easily and SiBCN powder was reserved under the scour of oxyacetylene flame flow because of their high melt-viscosity. With the erosion of oxyacetylene, the reserved SiBCN powder was oxidized to B2O3 and SiO2. The formation and volatilization of B2O3 and SiO2 can consume heat and slow down the erosion rate of flame flow, which can alleviate the erosion of oxyacetylene flame. The oxide can adhere to the surface of SiBCN powder, preventing further oxidation. Moreover, reserved SiBCN powder and its oxides can form a protective domain to hinder the entry of oxyacetylene flame flow, so carbon fibers and resins matrix covered by SiBCN powder and their oxides can remain. Finally, the interior SiBCN powder also played an important role in preventing the oxygen from eroding further. However, without the protection of SiBCN powder, the surface of phenolic composites was be eroded seriously and oxygen can enter to internal deeper posi- Figure 12. Diagram of the ablationtion, which resistance caused mechanism high MAR of and the LAR. phenolic/C composites and the SiBCN–phenolic/C composites.

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Figure 12. Diagram of the ablation resistance mechanism of the phenolic/C composites and the SiBCN–phenolic/C composites.

3.6. Mechanical Properties of the SiBCN–Phenolic/C Composites Figure 13a showed the interlaminar shear strengths (ILSS) for phenolic/C composites and SiBCN–phenolic/C composites. Based on the results of the interlaminar shear test, it revealed that SiBCN–phenolic/C composites had poorer ILSS properties as compared with phenolic/C composites. A decrease in the ILSS strength of SiBCN–phenolic/C composites can be attributed to that the effective interfacial area and fiber–resin adhesion between Processes 2021, 9, 955 13 of 15 resins and fibers were reduced. Therefore, the load transfer capacity of the matrix de- creases, which can result in lower ILSS properties [33]. As shown in Figure 13b, bending strengths of SiBCN–phenolic/C composites were higher than that of phenolic/C compo- increasesites. The of increase bending of strengths bending of strengths SiBCN–phenolic/C of SiBCN–phenolic/C composites composites might result might from result that SiBCNfrom that powder SiBCN prevented powder crack prevented growth crack and absorbedgrowth and energy absorbed [34]. However, energy [34]. the differenceHowever, ofthe ILSS difference properties of ILSS and properties bending strengths and bendin betweeng strengths phenolic/C between composites phenolic/C and composites SiBCN– phenolic/Cand SiBCN–phenolic/C composites was composites small, so was the introductionsmall, so the of introduction SiBCN powder of SiBCN had little powder effect had on thelittle mechanical effect on the properties mechanical of composites. properties of composites.

FigureFigure 13.13.Interlaminar interlaminar shearshear strength (a) and bending strength strength ( (bb)) of of the the phenolic/C phenolic/C composites composites andand thethe SiBCN–phenolic/CSiBCN–phenolic/C composites.

4.4. ConclusionsConclusions InIn thisthis paper,paper, SiBCNSiBCN powder with with low low density density was was introduced introduced to to phenolic phenolic resins resins to toobtain obtain the the SiBCN–phenolic/C SiBCN–phenolic/C composites composites with with excellent excellent properties. properties. Due Due to the to theintroduc- intro- duction of SiBCN powder, the thermal stability of the cured SiBCN phenolic resins was tion of SiBCN powder, the thermal stability of the cured SiBCN phenolic resins was im- improved. The residual constituents of SiBCN phenolic resins after carbonization were proved. The residual constituents of SiBCN phenolic resins after carbonization were also also investigated by XPS, and it was illustrated that the occurrence of reactions between investigated by XPS, and it was illustrated that the occurrence of reactions between SiBCN SiBCN powder and pyrolysis product of phenolic resins was the main reason. The roles powder and pyrolysis product of phenolic resins was the main reason. The roles of SiBCN of SiBCN particles in improving the oxidation resistance of SiBCN phenolic resins and particles in improving the oxidation resistance of SiBCN phenolic resins and SiBCN–phe- SiBCN–phenolic/C composites were studied. For SiBCN–phenolic/C composites, the nolic/C composites were studied. For SiBCN–phenolic/C composites, the occurrence of occurrence of the oxidation reaction and the formation of protective crust contributed to the oxidation reaction and the formation of protective crust contributed to the improve- the improvement of oxidative resistance. The result of the oxygen-acetylene test illustrated ment of oxidative resistance. The result of the oxygen-acetylene test illustrated that the that the LAR and MAR of phenolic/C composites reduced by 27% and 14% after the LAR and MAR of phenolic/C composites reduced by 27% and 14% after the introduction introduction of SiBCN powder, respectively. The improvement of the ablative property was attributedof SiBCN topowder, the high respectively. viscosity of The SiBCN improvemen at high temperature,t of the ablative which property caused SiBCN was attributed powder canto the remain high onviscosity the surface of SiBCN of SiBCN–phenolic/C at high temperature, composites which caused to protect SiBCN the internalpowder matrix.can re- Inmain addition, on the the surface oxidation of SiBCN–phenolic/C of SiBCN powder and comp volatilizationosites to protect of oxide the absorbed internal thematrix. energy In andaddition, oxygen. the Therefore, oxidation the of introductionSiBCN powder of SiBCNand volatilization powder can of enhance oxide theabsorbed thermal the stability, energy oxidationand oxygen. resistance Therefore, and ablationthe introduction resistance of of SiBCN composites. powder In can addition, enhance the the introduction thermal sta- of SiBCNbility, oxidation powder had resistance little effect and on ablation the mechanical resistance properties of composites. of the composites.In addition, the introduction of SiBCN powder had little effect on the mechanical properties of the composites Author Contributions: Data curation, W.Y.; Formal analysis, W.Y.; Investigation, W.Y. and Y.W.; Methodology, T.Z.; Project administration, H.L.; Resources, Z.L. and T.Z.; Software, H.L.; Supervision, H.L. and T.Z.; Writing—original draft, W.Y.; Writing—review & editing, F.C. and H.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China: 51873215. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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