Polymer-Derived Carbon Matrix Composites with Boron Nitride Nanotube Reinforcement

Abstract

This study explored the use of boron nitride nanotubes (BNNTs) as reinforcing fillers to enhance the mechanical properties of polymer-derived carbon matrix composites. BNNT-reinforced carbon matrix composites containing 0.5–5 wt% BNNTs were fabricated with pyrolysis conducted at different temperatures. X-ray diffraction and Raman spectroscopy revealed enhanced crystallinity and reduced defects in carbon matrix composites with BNNT addition. At 1200 °C pyrolysis temperature, sample shrinkage decreased from 28% in the control sample without BNNT addition to 12% with 5 wt% BNNTs, demonstrating BNNTs’ significant influence on the matrix. The density increased by 20.1% with 5 wt% BNNTs. Mechanical testing demonstrated an enhancement in the failure strain from 0.7% to 0.8% and an 87.8% increase in the work of fracture with 5 wt% BNNTs. Furthermore, the flexural strength and modulus improved by 68.7% and 55.6%, respectively, at this BNNT concentration. Increasing the pyrolysis temperature to 1500 °C further boosted the mechanical properties, with the flexural strength increasing by 283.7% and the flexural modulus by 528.6% when comparing samples containing 5 wt% BNNTs to those without BNNT reinforcement. Samples processed at 1500 °C with 5 wt% BNNT composition exhibited optimal performance.

1. Introduction

Carbon matrix composites have emerged as advanced materials celebrated for their exceptional characteristics, including low density, high stiffness, superior strength, and excellent thermal properties, making them particularly suitable for extreme environment applications [1,2,3]. Their unique combination of high thermal conductivity and low thermal expansion coefficient, coupled with robust resistance to thermal shock and ablation, positions such composites as vital components in the aerospace and aeronautics sectors. Specifically, these materials are extensively utilized in high-energy applications such as aircraft braking systems, rocket nozzles, and spacecraft heat shields, particularly during re-entry [4,5]. The fabrication of carbon matrix composites primarily involves methods such as chemical vapor deposition (CVD) [6] and polymer infiltration and pyrolysis (PIP) [7]. The CVD technique deposits hydrocarbon gases onto carbon reinforcement preforms under strictly controlled pressure and temperature, while in the PIP process, a resin system infiltrates the preform fabric, followed by carbonization. However, the carbonization phase often results in weight reduction, shrinkage, and cracking, leading to a porous structure that significantly compromises mechanical properties [8]. This porosity decreases flexural strength and elastic modulus, as cracks are typically initiated and propagate through pores, further exacerbating interlaminar delamination during bending failures [9]. Consequently, multiple infiltration cycles are essential for matrix densification to enhance mechanical performance, but this process remains time-consuming, costly, and technically challenging, limiting the widespread application of carbon matrix composites [10].

To minimize multiple infiltration cycles and cost yet still retain the improved mechanical properties of carbon matrix composites, recent efforts have focused on incorporating various carbon fillers into carbon precursors. Micro- and meso-sized carbon beads [11], vapor-grown fibers [12], carbon black [13,14], and graphite powder [15,16] have been identified as effective fillers that reduce weight loss, shrinkage, and cracking during carbonization while enhancing the microstructure of carbon matrix composites to improve their mechanical properties. Similarly, integrating ceramic fillers into carbon matrix composites has proven effective in enhancing fiber/matrix bonding, as seen in ZrC-doped C/C composites, where pyrolytic carbon interphases have been shown to increase flexural strength by 67% [17]. Furthermore, the introduction of one-dimensional nanomaterials, such as carbon nanotubes (CNTs) and silicon carbide (SiC) nanowires and nanorods, has been used to improve matrix cohesion in composites by forming nanoscale mechanical interlocks between carbon fibers and pyrolytic carbon (PyC) matrices. This interlocking inhibits crack propagation and enhances load transfer efficiency at fiber/matrix interfaces, significantly boosting interlaminar shear strength [18]. Notably, incorporating CNTs into carbon matrix composites has been demonstrated to create a marked increase in tensile strength (30.5%), Young’s modulus (23.6%), and toughness (130%), leading to a transition in fracture mode from brittle to pseudo-plastic [19]. The application of CNTs as reinforcement materials has also proven to be an effective strategy for enhancing the mechanical properties of carbon fibers and nanofibers, originating from their templating graphitization effects [20,21]. While CNTs have shown great potential in the modification of carbon fiber/carbon matrix (C/C) composites, they can also be utilized as standalone reinforcements with carbon matrices (CNT/C), paving the way for the development of lighter and stronger CNT/C composites over C/C composites, due to the superior mechanical properties of CNTs [4]. For example, CNT/C composites have been shown to exhibit tensile modulus and strength of approximately 275 GPa and 1.5 GPa, respectively (for two infiltration cycles) [22], compared to C/C composite exhibiting tensile modulus and strength of 95 GPa and 900 MPa, respectively [23]. Therefore, the incorporation of carbon nanotubes (CNTs) as reinforcements into carbon matrices enhances the overall performance of the composite material. Similarly, boron nitride nanotubes (BNNTs), which are analogous to CNTs, also exhibit exceptional mechanical properties. These include an elastic modulus of up to 1.3 TPa and a tensile strength of 33 GPa, values comparable to those of CNTs [24]. Recent studies highlight the advantages of using BNNTs to reinforce polymers [25,26], ceramics [27,28], and metals [29,30], attributed to their polarized atomic structures, which may facilitate stronger binding interfaces with these engineering materials [31].

Despite their structural similarity to CNTs, BNNTs offer distinct multifunctional advantages, including thermal stability, electrical insulation, high neutron absorption, piezoelectricity, and optical transparency, which can enhance carbon composites [32]. For instance, BNNTs have been used to enhance the thermal properties of polymers, as they can withstand temperatures above 900 °C in oxidative environments, hence BNNTs are able to improve the thermal resilience of carbon composites in aerospace applications such as propulsion systems, heat shields, and hypersonic vehicles [33]. Due to their high boron content, BNNTs can provide neutron shielding and hence radiation resistance for spacecraft and deep-space mission structures which are mostly manufactured out of carbon composites. Also, unlike CNTs, BNNTs are electrical insulators (bandgap ~5.5 eV), making them ideal for EMI shielding, aerospace systems, and electronic applications requiring controlled conductivity. BNNTs maintain stability in oxidative and corrosive environments, resisting degradation from moisture, saltwater, and reactive gases [31,34]. This superior chemical durability can extend the lifespan and reliability of BNNT-reinforced carbon composites, making them highly suitable for extreme operational conditions. BNNTs are able to enhance energy harvesting in carbon composites for self-powered sensing, aerospace power systems, and structural health monitoring due to their strong piezoelectric properties [35]. Until now, studies evaluating BNNTs’ reinforcement performance on carbon matrices are scarce in the literature.

This study investigates the mechanical properties of BNNT-reinforced carbon matrix composites. PyC derived from phenolic resin was reinforced with various BNNT concentrations and pyrolyzed at 1200 °C and 1500 °C. Raman and X-ray diffraction (XRD) analyses were performed to assess the impact of BNNTs on the crystallinity of the carbon matrix composites. Scanning electron microscopy (SEM) imaging was used to examine how the microstructure influenced the composites’ mechanical properties. Additionally, measurements of linear shrinkage and density were conducted to explain the impact of BNNTs on the carbon matrix composites. Finally, mechanical testing was carried out, and the results were analyzed to understand the influence of BNNT reinforcement on the composites’ performances.

2. Materials and Methods

2.1. Boron Nitride Nanotube and Polymer

The carbon matrix composites utilized in this study were derived from phenolic resin purchased from the Plenco plastic Engineering company, Sheboygan, WI, USA. BNNTs (SP10-R) were purchased from BNNT, LLC, Newport News, VA, USA. Tetrahydrofuran (THF reagent > 99.9%) was purchased from Sigma-Adrich, St. Louis, MO, USA. All materials were used as received.

2.2. Nanocomposite Preparation

The composite samples were prepared according to the following procedure. A total of 1 mg of boron nitride nanotubes (BNNTs) was dispersed in 10 mL of tetrahydrofuran (THF), using a probe sonicator set to 30% amplitude, in an ice bath for 2 h, ensuring thorough dispersion. Concurrently, 200 mg of phenolic resin was dissolved in 5 mL of THF, allowing the solution to sit for 20 min under a fume hood to ensure complete dissolution, given the resin’s high viscosity. The BNNT/THF solution was subsequently combined with the phenolic/THF solution. To mitigate BNNT re-aggregation, the mixed solution was subjected to bath sonication in an ice bath while the solvent evaporated to half of its original amount. The resulting BNNT/phenolic solution was cast onto a polytetrafluoroethylene (PTFE) film and left under a fume hood overnight to allow for complete solvent evaporation. Following this, the BNNT/phenolic film on the PTFE was dried in an air oven at 150 °C for 1 h. After this initial drying phase, the film was removed from the PTFE substrate and further dried on a hot plate at 200 °C for an additional hour. The film underwent post-curing at 300 °C in a hot press under a force of 2 MPa for 2 h. Subsequently, the film was sintered at the desired temperatures with a heating and cooling rate of 20 °C/min for 1 h. Prior studies have indicated that carbonized phenolic resin exhibits optimal hardness and elastic modulus between 1000 °C [36] and 1500 °C [37], while exceeding 1500 °C increases fracture toughness at the expense of strength and stiffness. For this reason, in the present work, 1200 °C and 1500 °C were used to process the samples. BNNT compositions as low as 0.5 wt% have been used to significantly enhance the mechanical properties of polymers [25], while 5 to 10 wt% content of nanofillers have demonstrated optimal performance for different material properties. Hence BNNT contents of 0.5 wt%, 1 wt%, and 5 wt% were selected to evaluate their influence on the mechanical and structural properties of BNNT-reinforced carbon composites. Figure 1a–f presents photographs showing the different stages of the fabrication processes, as described above.

2.3. Materials Characterization

Pyrolysis was performed in a high-temperature graphite furnace (Thermal Technologies HP-4560-20, Blythewood, SC, USA). The furnace was first evacuated to a pressure of 10⁻⁵ torrs, then backfilled with high-purity nitrogen gas to atmospheric pressure. During the pyrolysis process, a continuous nitrogen gas flow of 2 SCFH was maintained. The density of each sample was determined using a densitometer based on Archimedes’ principle. The chemical structure of the samples was analyzed with a confocal Raman spectrometer (Invia, Renishaw, UK), which utilized an Argon ion laser operating at 633 nm and a 50× objective lens. All measurements were performed at room temperature and peak intensity ratios (I_D/I_G) were obtained using Origin software (2018). An X-ray diffractometer (D2-Phaser, Bruker, Camarillo, CA, USA) with a Cu Kα radiation (λ = 0.154178 nm) source was used for XRD analysis. A 3-point bending test was performed using a DMA Q800 (TA Instruments, Inc., New Castle, DE, USA) and stress–strain curves were obtained. The samples were 8 mm long, 5 mm wide, and 100 µm thick. Measurements were obtained with a controlled static force using a strain rate of 0.1 N/mm. Three test samples were measured and the average reported. SEM images were captured using a Phenom XL G2 (Thermo Fisher Scientific, Waltham, MA, USA) with a 15 kV accelerating voltage. The samples were coated with gold using a sputter coater. Transmission electron microscopy (TEM) images were collected using a JEM-ARM200cF (a Cs-corrected scanning/transmission electron microscope from JEOL) at 80 kV. A copper grid was used to support the TEM sample, which was prepared by a focused ion beam. Fourier transform infrared spectroscopy (FTIR) scans were performed to detect the different functional groups present in the material after pyrolysis. Sixty-four scans were performed of five different locations of 3 samples for each composite and the average spectra were reported.

3. Results and Discussion

3.1. Raman Spectroscopy

Figure 2a presents the Raman spectra for pristine carbon and BNNT-reinforced carbon samples heat-treated at 1200 °C, exhibiting distinct D and G bands. The D bands are indicative of structural disorder and defects within the carbon structures [38], while the G bands reflect the degree of graphitic order, representing the preferential arrangements of carbon atoms [39]. With BNNT incorporation, the relative intensity and width of the D bands decrease, signaling reductions in defects and disordered regions, whereas the G bands become narrower, and their intensities increases with higher BNNT content, suggesting enhanced graphitic ordering of carbon atoms. The increases in crystallinity can likely be attributed to BNNTs’ templating effect, as BNNTs have similar hexagonal structures to CNTs, which are known to facilitate stress-induced graphitization [40]. Accordingly, BNNTs may similarly promote graphitic ordering in adjacent carbon atoms, enhancing crystallinity and playing critical roles in the graphitization process. To understand how BNNTs enhance the crystallinity of the carbon matrix, the overlapping peaks of the G bands were each deconvoluted into two subpeaks. In the literature, Lorentzian, Guassian, and pseudo-Voigt fitting methods have commonly been used to deconvolute Raman spectra [41]. In this study, the Lorentzian fitting method was utilized to analyze the effect of BNNT incorporation on the carbon matrix. Specifically, the G peaks in each spectrum were deconvoluted into D3 and D2 sub-peaks to assess structural modifications. The D3 peaks, occurring within the 1500–1550 cm⁻¹ range, correspond to amorphous carbon, while the D2 peaks, located at approximately 1630 cm⁻¹, represent in-plane defects within the graphite layers, distinct from the D1 peaks, which are associated with edge defects [42]. The data in Figure 2b–d indicate that increasing BNNT content leads to reductions in in-plane defects, as evidenced by the gradual decreases in D2 peak intensities. Similar trends were observed for the D3 peaks, suggesting that BNNT incorporation enhances the crystallinity of the carbon matrix by mitigating disorder and improving structural order.

3.2. X-Ray Diffraction

Figure 3a presents the XRD patterns of pristine carbon, and BNNT-reinforced carbon composites within a 2θ range of 15° to 40°. The XRD pattern of pure BNNTs is presented in Figure 3b. The XRD pattern of pure BNNTs displays a sharp and intense peak at 2θ = 26.5°, corresponding to the (002) diffraction planes (Figure 3b). This peak indicates the ordered arrangement of boron and nitrogen atoms in a hexagonal stacked configuration [31]. In contrast, the pristine carbon and BNNT-reinforced carbon composites exhibit a broad peak ranging from 2θ~20° to 25.8°. This broad peak is indicative of a turbostratic carbon structure [43], reflecting disordered graphene layer stacking and the presence of edge defects [44]. The turbostratic peak of the pristine carbon is observed to be wider than those of the BNNT carbon matrix composites, indicating more defective and disorderly arrangements of graphene-stacked layers. A distinct narrow peak at 2θ = 26.56° is observed for pristine carbon, signifying the existence of an orderly arrangement of carbon atoms with an interlayer spacing (d₀₀₂) smaller than that found in graphite (2θ ≈ 26.5°) [45], as shown in

Table 1. Calculated parameters of XRD for BNNT/carbon heat-treated at 1200 °C.

BNNT	d002 (Å)	Lc (nm)	N
0%	3.312	1.15	3
1%	3.356	1.255	4
5%	3.355	3.304	10

. Upon incorporation of BNNTs into the carbon matrix, this peak shifts from 2θ = 26.56° to 26.5°, aligning with the peak observed for BNNTs. As the BNNT content increases, the intensity of the 2θ = 26.5° peak intensifies and narrows, while the broad turbostratic peak sharpens and shifts towards the 2θ = 26.5° peak. This behavior suggests that the hexagonal arrangements of BNNTs act as templates, promoting the orderly alignment of adjacent carbon atoms through stress-induced graphitization [46]. To further analyze these observations, Bragg and Scherrer equations (1 and 2, respectively) were applied to calculate the interlayer spacing (d₀₀₂), out-of-plane crystallite stacking heights (L_c), and number of graphene layers (N) present in the carbon matrix composites, where β₀₀₂ represents the FWHM of θ₀₀₂ and K is taken to be 0.9 for L_c [47]. The results are summarized in Table 1.

$$d_{002}={\displaystyle \frac{\lambda }{2sin{\theta }_{002}}}$$

(1)

$$L_c={\displaystyle \frac{K\lambda }{{\beta }_{002}cos{\theta }_{002}}}$$

(2)

As BNNTs are incorporated into the carbon matrix, the interlayer spacing approaches that of graphite (3.35 Å) [48], and the stacking height increases. Furthermore, the number of graphene layers increases from 3 to 10, indicating an enhancement in crystallinity with the addition of BNNTs. Papkov et al. [49] similarly reported improved crystallinity with the addition of 1.2 wt% double-walled carbon nanotubes (DWNTs) in phenolic-formaldehyde-derived glassy carbon, noting an accelerated graphitization process at lower carbonization temperatures (1000 °C) due to templated carbonization. The strong interfacial bonding between DWNT bundles and carbon matrices facilitated oriented graphitic growth, with nanotubes serving as orientation guides during carbonization. It is likely that a similar templating mechanism occurs in BNNT-reinforced carbon matrix composites due to the similar crystalline structures of CNTs and BNNTs [50].

3.3. SEM

Figure 4 shows SEM images revealing microstructural changes in the pristine and BNNT-reinforced carbon matrix composite samples. The BNNTs are well dispersed within the carbon matrix, with no visible agglomeration, indicating that the dispersion method was highly effective. EDS element analyses were conducted to further confirm the absence of any agglomeration within the matrices. Figure 5 shows EDS elemental color mapping showing homogenous distributions of boron (green), carbon (blue), and nitrogen (red) atoms at wide ranges over the fracture surfaces of the carbon composites with different BNNT wt%. It is therefore concluded that the dispersion method used in this work can effectively disperse BNNTs at up to 5 wt%. In the pristine carbon sample, extensive crack propagation is observed along the fracture surface, which is likely to be due to cleavage along crystal planes, suggesting a high degree of crystallinity. This high degree of crystallinity may contribute to a brittle fracture mode, characterized by crack propagation, as seen in the SEM micrograph insert, which highlights the anisotropic nature of the sample. The incorporation of 1 wt% BNNTs into a carbon matrix composite result in shorter crack lengths, indicative of reduced crack propagation (the red arrow), and at 5 wt% BNNTs, no visible cracks are observed. This suggests that BNNTs impart a toughening effect within the carbon matrix composites by minimizing crack propagation. Known toughening mechanisms of BNNTs in ceramics—such as crack deflection, fiber bridging, and pull-out—may similarly enhance the toughness of carbon matrix composites [27]. However, while BNNTs contribute to improved toughness, their incorporation also leads to an increase in porosity. The SEM images indicate that both pore size and overall porosity increase with BNNT content. At 1 wt% BNNTs, pore size and porosity are reduced so as to be almost impossible to notice, but as BNNT content increases to 5 wt%, both parameters significantly rise. The increase in porosity with higher BNNT content may be attributed to two primary factors: (1) the evolution of volatiles during carbonization, which BNNTs may enhance, thereby accelerating the carbonization process, and (2) the formation of gaps at the BNNT/matrix interface due to thermal mismatch resulting from differential linear shrinkage [51]. To further examine this phenomenon, we analyzed the impact of BNNTs on linear shrinkage during carbonization, which is discussed in subsequent sections. Meanwhile, the high-resolution TEM images in Figure 4e,f show that the multiwall BNNTs are embedded in the carbon matrix composites.

3.4. Physical Properties

3.4.1. Linear Shrinkage

Figure 6a shows a marked reduction in linear shrinkage of carbon material with the addition of BNNTs. Carbon materials typically experience substantial shrinkage during carbonization, primarily due to the release of volatiles and the reorganization of carbon atoms. The pristine carbon sample exhibited approximately 28% linear shrinkage, whereas BNNT incorporation progressively reduced shrinkage, reaching as low as 12% at 5 wt% BNNTs. These findings suggest that BNNT reinforcement improves the structural stability of the carbon matrix, which enhances the composites’ abilities to maintain their physical and mechanical integrity under various loading conditions. It is likely that this enhancement is attributable to the high thermal stability and mechanical strength of BNNTs. As volatiles evolve, linear shrinkage consistently decreases with increasing BNNT content. This indicates the ability of BNNTs to reduce linear shrinkage during the densification and crystallization of carbon matrix composites. The packing density of BNNTs may compensate for the voids left by evolved volatile molecules, thereby minimizing matrix shrinkage.

3.4.2. Density

Figure 6b shows the density measurements of pristine carbon and BNNT-reinforced carbon composites following heat treatment at 1200 °C. The pristine carbon samples exhibited a density of 1.39 g/cm³, aligning with previous studies [52]. Initially, BNNT incorporation reduced density at low BNNT concentrations, but density increased as BNNT content increased, reaching a maximum of 1.67 g/cm³ at 5 wt% BNNTs, corresponding to a 20.8% increase. The increase in density of phenolic resin has been attributed to chemical densification, which occurs due to chemical structural changes during carbonization [53]. Ko et al. [54] observed that the density increased to approximately 1.536 g/cm³ at 1000 °C, beyond which a decrease in density was noted. In the current study, the observed increase in density at 1200 °C indicates that BNNT further enhances chemical densification, thereby contributing to the higher density. Additionally, the higher concentration of BNNTs may also contribute to the observed increase in density.

3.5. Mechanical Properties

3.5.1. BNNT Carbon Composite Pyrolyzed at 1200 °C

Figure 7a illustrates the stress–strain behavior of pristine carbon and BNNT-reinforced carbon composites with 1200 °C pyrolysis. In BNNT-reinforced composites, the addition of BNNTs initially decreases the failure strain, and then raises it to 0.83% at 5 wt%. The pristine carbon sample exhibited a flexural modulus of 19.4 ± 2.7 GPa, consistent with previous studies [55]. With increasing BNNT content, the flexural modulus reached a peak of 30.2 ± 3 GPa at 5 wt% BNNTs, marking a 55.6% improvement over the pristine sample (Figure 7b). A similar trend was observed in the flexural strength of BNNT-reinforced carbon composites. Figure 7c shows the flexural strength of pristine carbon measured at 133.4 ± 57 MPa, which increased to 225.1 ± 28 MPa with the addition of 5 wt% BNNTs, representing a 68.7% increase compared to the pristine carbon. These enhancements in mechanical properties can be attributed to the exceptional strength and stiffness of BNNTs, underscoring their potential in reinforcing carbon matrices by providing greater load-bearing capacity. This stiffening behavior is comparable to the effect of nanotubes in polymer matrices, where nanotubes restrict polymer chain mobility, thereby increasing stiffness [25].

To estimate the work of fracture, the area under the stress–strain curves in Figure 7a was calculated using the Origin software (2018 version) and is presented in Figure 7d. The pristine carbon exhibited a work of fracture of approximately 493.6 ± 106 kJ/m³. With BNNT incorporation, this initially decreased, then increased significantly, reaching 892.8 ± 137 kJ/m³ at 5 wt% BNNTs, an 87.8% increase compared to pristine carbon. This increase is consistent with BNNTs’ ability to enhance toughness in carbon matrices, similar to its effects observed in metals and ceramics [29,56].

3.5.2. BNNT Carbon Composite Pyrolyzed at 1500 °C

Figure 8a illustrates that failure strain decreased with the incorporation of BNNTs into a carbon matrix composite at 1500 °C pyrolysis. As the BNNT content increased, the failure strain consistently decreased, reaching a minimum of approximately 0.35% at 5 wt% BNNTs. It is also noted that the flexural strength of the samples increased as the failure strain decreased. This trend suggests the formation of a strong interfacial bond between the BNNTs and the carbon matrix at elevated pyrolysis temperatures to facilitate more efficient load transfer between them. At elevated temperatures, the surfaces of BNNTs undergo activation, increasing their reactivity and facilitating enhanced chemical bonding with the carbon matrix.

This improved interfacial interaction contributed to the formation of stronger interfacial bonds, thereby significantly enhancing the mechanical properties of the composites. Consequently, the BNNT-reinforced carbon composite samples demonstrated significant enhancement in flexural modulus (Figure 8b) and flexural strength (Figure 8c), reaching approximately 127.7 ± 22 GPa and 456.1 ± 84 MPa, respectively, at 5 wt% BNNTs. These values correspond to improvements of 528.64% and 283.7% over the pristine carbon. This indicates that the reinforcing effect of BNNTs on the carbon matrix is further improved at higher pyrolysis temperatures. Notably, the BNNT-reinforced carbon composites exhibited steady increases in work of fracture with increasing BNNT content, peaking at 928.1 ± 150 kJ/m³ at 5 wt% BNNTs (Figure 8d), which represents a 165.1% improvement compared to the pristine carbon. This work of fracture is approximately twice that of the samples at 1200 °C pyrolysis.

3.5.3. BNNT Carbon Interfacial Interaction

Fourier transform infrared (FTIR) analysis was conducted on the pure carbon sample and the 5 wt% BNNTs composite pyrolyzed at 1200 °C and 1500 °C, and the transmittance spectrums are presented in Figure 9. The spectra of the BNNT composites revealed the presence of boron carbide peaks at 1119 cm⁻¹ for the 1200 °C sample, which shifted to lower wavelengths of 1039.4 cm⁻¹ and 1089.4 cm⁻¹ [57] with increased intensity in the 1500 °C-treated sample. Boron hydride peaks were also identified at 944.1 cm⁻¹ and 926.12 cm⁻¹ wavelengths [57] for the 1200 °C and 1500 °C samples, respectively, with significantly higher intensity in the latter. Notably, a carbon nitride peak at 2229.9 cm⁻¹ [58] emerged with high intensity in the 1500 °C sample but was absent in the 1200 °C sample. The observed formation of new bonds such as boron carbide, and boron hydride, indicates strong interactions between the carbon matrix composites and the BNNTs, suggesting interfacial bonding. The progressive shifts and increased intensities of these peaks at higher temperatures further support the development of stronger interfacial bonds between the matrices and the BNNTs. Moreover, the pronounced formation of carbon nitride in the 1500 °C sample suggests enhanced bonding, corroborating the results obtained from mechanical testing.

Studies of the reinforcement of carbon matrix composites with BNNTs remain scarce in the literature. However,

Table 2. Mechanical properties comparison of BNNT-carbon composite.

BNNTs	BNNT-Carbon Composite	Flexural Strength (MPa)	Flexural Modulus (GPa)	Tensile Strength (MPa)	Tensile Modulus (GPa)	Reference
50 wt% (Experiment)	Hybrid interlayer BNNT/CF	493.1	41.73			Reyes et al. [59]
5 wt% (Experiment)	BNNT/PAN fiber			764	17.2	Chang et al. [60]
6.8 vol% (MD simulation)	BNNT/phenolic resin				60.79	Kumar et al. [61]
5 wt%	BNNT/Phenolic derived carbon	456.6	127.7			Present work

presents a comparative analysis of the mechanical properties obtained in this study with those reported by other researchers. The results indicate that the current study achieved comparable flexural strengths and an exceptional flexural modulus, despite utilizing lower BNNT weight percentages. This suggests that the material developed in this study has the potential to deliver superior performance in harsh environments.

4. Conclusions

In conclusion this study examines the reinforcing effects of BNNTs on carbon matrix composites derived from phenolic resin, subjected to heat treatment at 1200 °C and 1500 °C, and with varying BNNT concentrations. XRD and Raman spectroscopy analyses confirmed significant refinements of the carbon matrices with the addition of BNNTs, with an increase in crystallinity as the BNNT content rose. At 1200 °C pyrolysis, the shrinkage of the carbon matrix decreased from 28% to 12% with the incorporation of 5 wt% BNNTs, highlighting the stabilizing role of BNNTs in the matrix structure. The density of the BNNT-reinforced carbon composite increased by 20.1% at 5 wt% BNNTs, which is attributed to BNNTs’ ability to further enhance chemical densification of the carbon matrix. At this concentration, both failure strain and work of fracture reached maximum values of 0.8% and an 87.8% increase, respectively, indicating a significant improvement in toughness. The flexural strength and modulus also increased by 68.7% and 55.6%, respectively, due to the excellent mechanical reinforcement effects of BNNTs. Increasing the pyrolysis temperature to 1500 °C led to a reduction in failure strain to approximately 0.35%, while the work of fracture showed a maximum increase of around 165.1% with increasing BNNT content. The pyrolysis of the samples at 1500 °C exhibited significant improvements in mechanical properties. The samples increased in capacity to withstand stress as the BNNT concentration increased, contributing to the observed enhancement in the work of fracture. The flexural strength and modulus increased by 283.7% and 528.6%, respectively, reaching maximum values of 456.1 MPa and 127.6 GPa at 5 wt% BNNTs. The enhanced mechanical properties at 1500 °C can be attributed to a stronger interfacial bond between the BNNTs and the carbon matrix, which facilitated more effective load transfer between the two.