High-Silica Fiber Felt/Ti₃SiC₂ Reinforced Phenolic Aerogel Composites for High-Temperature Thermal and Mechanical Performance

Abstract

To address the critical limitation of insufficient high-temperature structural stability in traditional formaldehyde-resorcinol aerogels for thermal protection applications, this study designed and fabricated a high-silica fiber felt-reinforced phenolic aerogel composite capable of in situ ceramization. The thermal insulation performance, structural stability, mechanical properties, and oxidation resistance mechanism after heat treatment at 1000 °C for 600 s were systematically investigated. Results demonstrated tunable density (0.398–0.629 g·cm⁻³), low room-temperature thermal conductivity (0.0414 W·m⁻¹·K⁻¹), and a stabilized back temperature of 408.6 °C during butane torch flame testing. After high-temperature treatment, the composite series exhibited a minimum volume shrinkage of 13.9% and a maximum mass retention of 77.6%. Specifically, the compressive strength and specific strength of the HS/C-75 sample reached 4.39 and 1.96 times those of the HS/C-0 sample, respectively. Further analysis revealed that the synergistic effect between the skeletal support of high-silica fibers and the in situ-formed ceramic phase effectively suppressed thermal shrinkage and improved oxidation resistance, achieving an optimized balance between thermal insulation and mechanical integrity. This work provides a theoretical foundation and viable technical pathway for developing advanced thermal protection materials with enhanced stability and reliability.

1. Introduction

With the rapid advancement of high-speed aerospace technology, particularly in the hypersonic domain, the continuously increasing Mach numbers pose significant challenges to thermal protection systems (TPS) during service [1,2,3]. Hypersonic vehicles, defined as those operating at Mach numbers exceeding 5, represent one of the critical frontiers in 21st-century aerospace research and development [4,5]. As flight velocity increases, aerodynamic heating intensifies. For instance, when the flight speed rises from Mach 3 to Mach 6, the stagnation point temperature at the vehicle’s nose can increase by a factor of 4.49 [6], leading to a rapid surge in surface temperatures beyond 2000 °C within a short duration. Due to high heat flux density and dynamic pressure [7], critical TPS components such as nose cones, leading edges, scramjet combustion chambers, and nozzles are subjected to extreme aerodynamic heating. Under such severe thermal conditions, the thermal shock resistance, high-temperature mechanical properties, and oxidation resistance of existing TPS materials are increasingly inadequate to meet operational demands [8,9]. Therefore, further research and exploration are essential to develop thermal protection materials that combine excellent thermal shock resistance, oxidation resistance, and high-temperature mechanical performance.

Thermal protection materials are broadly categorized into ablative and non-ablative types. In non-ablative systems, thermal insulation materials serve as the primary barrier against heat flux, demanding ultralow thermal conductivity, minimal density, and exceptional thermal stability. Aerogels, particularly organic variants, exhibit compelling potential in this regard due to their nanoscale porous architecture and tunable composition [10,11,12,13,14,15]. However, conventional resorcinol-formaldehyde (RF) aerogels suffer from severe structural collapse, obvious shrinkage (>30%), and rapid oxidative degradation above 600 °C [16,17], critically limiting their applicability in sustained extreme-temperature environments. This instability necessitates structural reinforcement and high-temperature functionalization.

To address these limitations, two primary modification pathways have emerged. Fiber reinforcement (e.g., silica, carbon fibers) enhances mechanical properties while effectively suppressing macroscopic deformation [18,19], yet weak fiber-matrix interfaces often lead to debonding under thermal environments. Ceramic-phase integration—such as boron-modified resins [20] or catalytic pore-structure optimization [21,22]—improves thermal resistance but typically sacrifices flexibility or fails to establish a continuous protective network. Existing strategies typically optimize individual properties in isolation, overlooking the synergy between structural reinforcement and high-temperature functionalization. Without an in situ ceramic barrier, the organic matrix remains vulnerable to oxidation during prolonged heating—a critical bottleneck in aerogel thermal protection design.

MAX phases (M_n+1AX_n, where M is an early transition metal, A is an A-group element, and X is C/N) are a family of layered hexagonal ceramics combining metallic and ceramic characteristics. Their nanolaminated structure (strong M–X bonds interleaved with relatively weak M–A bonds) endows them with high thermal stability, damage tolerance, and good thermal/electrical transport, making them attractive for high-temperature structural and functional applications [23,24]. At elevated temperatures in oxidizing environments, many MAX phases form adherent oxide scales (typically Al₂O₃, SiO₂, or mixed oxides depending on the A element), which can act as diffusion barriers and improve oxidation resistance. These structure–property relationships motivate the incorporation of MAX phases as “reactive/ceramizable” fillers in polymer-derived ceramic systems to tailor microstructure evolution and high-temperature durability [25]. Building on MAX-phase and polymer-derived ceramic concepts, this study employs a composite strategy for phenolic aerogels: high-silica fiber felt provides structural support to suppress shrinkage, while Ti₃SiC₂ (a representative MAX-phase material) acts as the ceramic functional phase [26]. Upon pyrolysis above 800 °C, Ti₃SiC₂ decomposes to generate a continuous multi-phase ceramic layer (SiO₂/TiO₂-rich) that (i) consumes ambient oxygen via oxidation reactions, (ii) forms a dense barrier inhibiting further oxidative diffusion, and (iii) enables microcrack self-healing through viscous flow of silica phases. Critically, the in situ ceramizable system bonds with the high-silica fiber skeleton, synergistically enhancing mechanical properties, thermal insulation, and oxidation resistance. Accordingly, a series of high-silica fiber felt-reinforced, Ti₃SiC₂-containing phenolic aerogel composites (HS/FRF-x) were fabricated via sol–gel polymerization and ambient-pressure drying. This study systematically investigates (i) room-temperature thermal insulation performance; (ii) dimensional stability, mass retention, and mechanical properties after 1000 °C heat treatment; and (iii) the evolution of the ceramic phase and oxidation-resistant mechanism through multi-technique characterization (SEM, EDS, XRD, XPS, and Raman). This study provides a practical strategy for developing aerogel composites suited to extreme thermal environments, offering a reference for thermal protection material design in aerospace and high-temperature industrial applications.

2. Materials and Methods

2.1. Materials

Formaldehyde (37wt%, AR), resorcinol (≥99%, AR), and furfural (≥99%, AR) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Deionized water was supplied by the laboratory. Trifluoroacetic acid (CF₃COOH, ≥99%, AR) was purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). and used as a catalyst. Ti₃SiC₂ filler (200 mesh, 98%) was provided by Forsman Technology (Beijing, China) Co., Ltd. (Beijing, China). High-silica fiber felt was purchased from Jiangsu Tianniao High-tech Co., Ltd. (Yixing, China). and was used as the reinforcement, with a density of approximately 0.12 g·cm⁻³ and a thickness of 15 mm. All chemicals were used as received without further purification.

2.2. Preparation of the Composite Materials

Formaldehyde, resorcinol, furfural, and deionized water were mixed in varying proportions through physical blending. Different amounts (calculated relative to a solid content of 1 for the precursor solution) of the ceramic-forming functional component Ti₃SiC₂ were added to the prepared precursor solution, followed by mixing at 30 °C using a magnetic stirrer for 30 min. The solution was then poured into a mold containing high-silica fiber felt using an impregnation method. After the suspension underwent gelation and aging at room temperature, the resulting hydrogel was dried in a forced-air drying oven (using a drying program of 50 °C for 24 h, 70 °C for 24 h, and 90 °C for 24 h) to obtain high-silica fiber felt-reinforced in situ ceramizable aerogel composites. The specific formulations are detailed in

Table 1. Formulation of in situ ceramizable phenolic aerogel composites reinforced by high-silica fiber felt.

Samples	Weight
	Precursor Solution (%)	High-Silica Oxygen Fiber Felt (%)	Ti3SiC2 (wt%)
HS/FRF-0	100	30	0
HS/FRF-25	100	30	25
HS/FRF-50	100	30	50
HS/FRF-75	100	30	75

, and the preparation process is illustrated in Figure 1. The high-silica fiber felt-reinforced in situ ceramizable aerogel composites were subjected to high-temperature heat treatment in a muffle furnace at 1000 °C for 10 min. The corresponding samples after heat treatment are designated as HS/C-x, where x represents the addition amount of the ceramic-forming functional component Ti₃SiC₂.

2.3. Characterization

The density of samples of high-silica fiber felt reinforced in situ ceramizable aerogel composites (before and after heat treatment) is calculated using Formula (1).

$$\rho ={\displaystyle \frac{m}{a\times b\times h}}$$

(1)

In the formula, a—length of sample (mm); b—width of composite (mm); h—height of composite (mm); and m—weight of composite (g).

The density of high-silica fiber felt reinforced in situ ceramizable phenolic aerogel composites, and the samples after high-temperature heat treatment were measured by the geometric method, and the volume shrinkage of carbonation was calculated according to Formula (2).

$$S_V={\displaystyle \frac{(V-V\mathrm{’})}{V}}\times 100\%$$

(2)

In the formula, S_V—carbonization shrinkage rate (%); V—volume of composite material (mm³) before high-temperature treatment; and V′—volume of composite material after high-temperature treatment (mm³).

A comprehensive thermal analyzer (STA449F3, NETZSCH, Waldkraiburg, Germany) was used to test the thermal stability of aerogel composites. A butane torch was employed as the heat source. Samples of high-silica fiber felt-reinforced in situ ceramicizable phenolic aerogel composite, measuring 80 mm × 80 mm, were tested with the nozzle positioned 10 cm away. The test duration was set at 10 min. A THERMAL CAMERA (H21Pro+ HIKMICRO, Hangzhou, China) was placed 0.5 m from the rear surface of the sample to record the temperature distribution, measure the steady-state backside temperature, and evaluate the thermal insulation performance.

The morphology and microstructure of the samples were characterized by field emission scanning electron microscopy (SEM) coupled with an X-Max N80 energy spectrometer (ZEISS, Jena, Germany). The thermal conductivity was determined at 25 °C by the transient plate heat source method using the thermal constant analyzer TPS 2500 S (Hot Disk AB, Goteborg, Sweden). A comprehensive thermal analyzer (STA449F3) was used to test the thermal stability of aerogel composites. Characterization of the chemical bond energies within the samples was conducted using a laser confocal micro-Raman spectrometer (Lab RAM Odyssey, HORIBA, Palaiseau, France). The phase composition of the ceramic component in high-silica fiber felt-reinforced in situ ceramized aerogel composite samples before and after high-temperature heat treatment was determined by X-ray diffraction using a rotating anode X-ray diffractometer (D8 Advance, Bruker, Karlsruhe, Germany) over a 2θ scanning range of 5° to 80°. X-ray photoelectron spectroscopy (XPS) analysis was performed on the ceramic component of the high-silica fiber felt-reinforced in situ ceramized aerogel composite after high-temperature heat treatment using an X-ray photoelectron spectrometer (AXIS Supra+, Kratos Analytical, Manchester, UK) to investigate the chemical changes occurring during the oxidation process. The compressive strength of the sample was tested by an electronic universal testing machine. The sample was a structured cuboid of 20 mm (a) × 20 mm (b) × 10 mm (h), and the test speed was 2.0 mm·min⁻¹.

3. Results and Discussion

3.1. Density and Mechanical Properties

Table 2. The density of high-silica fiber felt reinforced in situ ceramizable phenolic aerogel composites.

Samples	HS/FRF-0	HS/FRF-25	HS/FRF-50	HS/FRF-75
Density (g·cm−3)	0.398	0.484	0.538	0.629

presents the density of the high-silica fiber felt-reinforced in situ ceramizable phenolic aerogel composites. The density of the composites shows a positive correlation with the content of the ceramic functional component Ti₃SiC₂, although the overall density remains relatively low. The density of HS/FRF-0 is 0.398 g·cm⁻³, and with increasing Ti₃SiC₂ addition, the density reaches a maximum of 0.629 g·cm⁻³, corresponding to an increase of 30.9%. The rise in density is primarily attributed to the high intrinsic density of Ti₃SiC₂ (4.52 g·cm⁻³). Additionally, Ti₃SiC₂ can occupy micropores formed by the evaporation of water and small molecules during heating.

Figure 2 presents the stress–strain curves (a) and the compressive strength and specific compressive strength plots (b) of the high-silica fiber felt-reinforced in situ ceramizable phenolic aerogel composites. As shown in Figure 2a, the stress–strain curve of HS/FRF exhibits a serrated pattern with abrupt changes at high strain stages. In the low-strain region, the high-silica fiber felt primarily functions as the reinforcing skeleton to bear compressive loads. As strain increases, the fiber felt becomes compacted, and its mechanical contribution diminishes. Consequently, internal pores within the material gradually compress, transferring the load to the aerogel matrix and the ceramic-forming functional component. This leads to significant stress fluctuations at certain stages, resulting in the serrated appearance of the curve. The incorporation of solid ceramic-forming functional components provides structural support during high-strain stages, effectively bearing loads. As a result, the stress–strain curve progressively rises and becomes more regularized with increasing content of the ceramic-forming functional component.

According to Figure 2b, the compressive strength of HS/FRF increases positively with the rising content of the ceramic-forming functional component. The compressive strength of HS/FRF-0 is 1.330 MPa, while that of HS/FRF-75 reaches 2.747 MPa, marking an improvement of 106.3%. This significant enhancement is largely attributed to the reinforcing support and stress-bearing capacity provided by the rigid ceramic-forming functional components. However, the specific compressive strength peaks at HS/FRF-25, reaching 4.670 N·m/g, as the influence of density on specific strength becomes more pronounced when the content of the ceramic-forming functional component is higher. The peak specific strength of HS/FRF-25 stems from an optimized compositional balance between the ceramic phase and the high-silica fiber felt framework. At this formulation, the ceramic component delivers sufficient rigidity and reinforcement to enhance compressive strength, while the skeletal framework of the high-silica fiber felt remains fully effective in load distribution. Critically, the density increase induced by the ceramic content remains moderate, thereby avoiding the specific strength reduction typically associated with excessive densification. This synergy achieves an ideal trade-off between structural reinforcement and mass efficiency. Beyond this threshold, the accelerating density growth outweighs the marginal strength improvement, thereby reducing specific strength.

3.2. Thermal Stability and Insulation

Table 3. Thermal conductivity of high-silica fiber felt reinforced in situ ceramizable phenolic aerogel composites.

Samples	HS/FRF-0	HS/FRF-25	HS/FRF-50	HS/FRF-75
Thermal conductivity (W·m−1·K−1)	0.0414	0.0644	0.0938	0.1017

presents the thermal conductivity of the high-silica fiber felt-reinforced in situ ceramizable phenolic aerogel composites, while Figure 3 illustrates the thermal conduction mechanism within these composites. The thermal conductivity of HS/FRF-0 is as low as 0.0414 W·m⁻¹·K⁻¹. With an increase in the content of the ceramic-forming functional component, the thermal conductivity of the composites progressively rises to 0.0644 W·m⁻¹·K⁻¹, 0.0938 W·m⁻¹·K⁻¹, and 0.1017 W·m⁻¹·K⁻¹, representing increases of 55.8%, 126.7%, and 145.9%, respectively, compared to the sample without the ceramic-forming functional component.

As depicted in Figure 3, heat transfer in the high-silica fiber felt-reinforced in situ ceramizable phenolic aerogel composites primarily comprises three pathways: solid-phase conduction (through the aerogel skeleton, ceramic-forming functional component, and high-silica fiber felt, indicated by red dashed arrows), gas-phase conduction (via heat transfer between gas molecules, shown by blue arrows), and radiation (thermal radiation, represented by purple wavy arrows). Solid-phase conduction dominates the overall thermal conductivity. This solid-phase conduction is influenced by the structure of the aerogel skeleton (red circles), as well as the presence of the ceramic-forming functional component (yellow circles) and the high-silica fiber felt (white lines).

The increasing trend in thermal conductivity is primarily attributed to the high thermal conductivity of the ceramic-forming functional component Ti₃SiC₂ and the additional solid-phase conduction pathways it introduces. Nevertheless, the overall thermal conductivity of the composites remains relatively low. This can be attributed to two key factors: first, the uniform distribution of high-silica fiber felt within the aerogel provides structural support, reduces density concentration, and minimizes solid-phase conduction. Simultaneously, it helps prevent the collapse of the aerogel’s porous structure upon the addition of the ceramic-forming functional component, thereby enhancing the contribution of gas-phase conduction. Second, the microporous framework formed by the fibers creates minute air interlayers, which significantly impede heat transfer [27] and help maintain a low thermal conductivity.

The high-temperature thermal insulation performance of the high-silica fiber felt-reinforced in situ ceramizable phenolic aerogel composites was evaluated using a butane torch, with thermal imaging conducted via a THERMAL CAMERA (H21Pro+, HIKMICRO, Hangzhou, China).

Figure 4 presents the steady-state backside temperature images captured by the THERMAL CAMERA (H21Pro+, HIKMICRO, Hangzhou, China). For HS/FRF-0, the pyrolysis of the aerogel matrix on the tested surface resulted in structural damage. The residual pyrolytic carbon partially retained thermal insulation properties, yielding a steady-state backside temperature of 408.6 °C. Upon the incorporation of the ceramic-forming functional component Ti₃SiC₂, its relatively high thermal conductivity enhanced heat transfer efficiency, significantly influencing the time and temperature required to reach a steady-state backside temperature. On one hand, the oxidation of Ti₃SiC₂ under heating impedes inward oxygen diffusion, while the resulting oxidation products form a protective layer on the surface. This layer shields the internal matrix and pyrolytic carbon, thereby improving thermal insulation and reducing the steady-state backside temperature. On the other hand, the high thermal conductivity of Ti₃SiC₂ and its oxidation products increases the overall thermal conductivity, which tends to elevate the steady-state backside temperature. In HS/FRF-25 and HS/FRF-50, the effect of increased thermal conductivity due to Ti₃SiC₂ and its oxidation products dominated, leading to steady-state backside temperatures of 421.2 °C and 474.0 °C, respectively. In HS/FRF-75, the extensive protective layer formed by the oxidation products of Ti₃SiC₂ effectively mitigated inward oxygen diffusion and protected the internal matrix, resulting in a reduced steady-state backside temperature of 457.5 °C.

3.3. High-Temperature Resistance and Phase Analysis

The high-silica fiber felt-reinforced in situ ceramizable phenolic aerogel composites were subjected to high-temperature heat treatment at 1000 °C for 10 min. The microstructure and mechanical properties of the composites after heat treatment were investigated, along with an exploration of their oxidation resistance mechanism. Figure 5 shows physical images of the composites before and after heat treatment, with the top row displaying the original samples and the bottom row showing the samples after heat treatment.

In the original samples in Figure 5, the surfaces show no exposed fibers or visible ceramic particles, indicating that the in situ ceramizable phenolic aerogel uniformly coats the high-silica fiber felt. In the heat-treated samples in Figure 6, the composite without the ceramic-forming functional component exhibits noticeable fiber exposure on the surface. This suggests that the aerogel matrix undergoes pyrolysis and oxidation in the high-temperature oxidative environment, converting into gaseous products that escape.

In contrast, the surfaces of composites containing the ceramic-forming functional component show no significant fiber exposure. Instead, ceramic particles are observable on the surface. This occurs because the particles generated from the reaction of the ceramic-forming functional component encapsulate both the aerogel matrix and the fiber surfaces. This protective layer shields the aerogel matrix and fibers, limiting them to thermal pyrolysis that produces pyrolytic carbon. This process achieves carbon fixation and preserves the cross-linked structure that bonds the fibers together.

The properties of the samples after heat treatment are partially summarized in

Table 4. Partial properties of the sample after heat treatment.

Samples	Volume Shrinkage Rate (%)	Residual Weight Rate (%)	Density (g/cm3)
HS/C-0	32.2	52.9	0.312
HS/C-25	25.8	62.2	0.407
HS/C-50	19.9	69.4	0.467
HS/C-75	13.9	77.6	0.567

. Following high-temperature heat treatment, the high-silica fiber felt-reinforced in situ ceramizable phenolic aerogel composites exhibit volumetric shrinkage. HS/C-0 shows a volumetric shrinkage of 32.2%, whereas HS/C-75 demonstrates a significantly lower shrinkage of only 13.9%, representing a reduction of 56.8%. This difference is attributed to the degradation of the aerogel skeleton under high-temperature oxidative conditions, which diminishes its structural support. However, the presence of the high-silica fiber felt provides mechanical reinforcement to the overall composite structure, resulting in generally lower volumetric shrinkage after heat treatment.

The residual weight ratio of the composites is positively correlated with the content of the ceramic-forming functional component. HS/C-0 exhibits a residual weight ratio of 52.9%, while HS/C-75 achieves 77.6%, marking an increase of 46.6%. This substantial improvement can be attributed to two main factors: first, the reaction of the ceramic-forming functional component upon heating is mass-gaining, contributing directly to weight retention; second, the ceramic particles generated from the oxidation of the functional component adhere to the surfaces of the aerogel matrix and high-silica fibers, forming a protective layer that inhibits further internal oxidation.

After high-temperature heat treatment, the density of the samples remains positively correlated with the addition amount of the ceramic-forming functional component. This is because the increase in residual weight ratio is more pronounced. Although volumetric shrinkage is mitigated, its effect is less significant compared to the change in mass.

Figure 6 presents the stress–strain curves (a) and the compressive strength and specific compressive strength plots (b) of HS/C after high-temperature heat treatment. According to Figure 6a, when the content of the ceramic-forming functional component is zero or low, the stress–strain curves approximate a linear relationship. This behavior is attributed to the pyrolysis and oxidation of the aerogel matrix, which diminishes its load-bearing capacity. Under these conditions, the fiber felt can withstand a certain degree of deformation while maintaining its fundamental mechanical properties, resulting in a nearly linear stress–strain response.

In contrast, when the ceramic functional component content is high (as in HS/C-75), the ceramic particles formed from the reaction of the functional component also contribute to load-bearing. This leads to a significantly elevated and non-linear stress–strain curve for HS/C-75.

Overall, the mechanical properties of the composites after high-temperature heat treatment are substantially lower compared to those before heat treatment. For instance, the compressive strength and specific strength of HS/C-0 are 0.180 MPa and 0.579 N·m/g, respectively, representing only 13.6% and 17.3% of their pre-treatment values. However, as the content of the ceramic-forming functional component increases, both compressive strength and specific strength improve. HS/C-75 achieves values 4.39 times and 1.96 times higher than those of HS/C-0 in compressive strength and specific strength, respectively. This enhancement is due to the combined effects of the fiber felt, which supports the overall structure, and the ceramic oxidation products, which not only protect the internal matrix but also provide additional mechanical reinforcement, thereby improving compressive strength. In addition, we have systematically calculated and compiled the compressive strength data of both the original samples and those subjected to heat treatment at 1000 °C for 10 min. The results are presented in a supplementary table (Supporting Information, Table S1), which clearly quantitatively characterizes the evolution of structural stability of the materials under high-temperature conditions.

The microstructure after high-temperature heat treatment is shown in Figure 7. SEM images reveal severe matrix loss on the fiber surface of HS/C-0 after heat treatment, indicating that the pyrolytic carbon from the phenolic aerogel decomposition at high temperatures was almost completely oxidized and escaped as gaseous oxides, leading to a significant reduction in adhesion between fibers. In contrast, composites containing the ceramic-forming functional component exhibit markedly less fiber exposure. This is attributed to the chemical reactions of the ceramic particles attached to the fibers, which consume oxygen in situ. The in situ generated ceramic phases, such as TiO₂ from the functional component, encapsulate the internal phenolic aerogel matrix, thereby isolating it from oxygen. This encapsulation limits the matrix to thermal pyrolysis, producing pyrolytic carbon without further oxidation. This mechanism achieves carbon fixation and preserves the cross-linked structure that bonds the fibers together.

Energy-dispersive X-ray spectroscopy (EDS) was employed to analyze the distribution of ceramic elements. The specific elemental compositions are presented in Figure 8. Based on the elemental distribution, the exposed fiber regions in HS/C-25 are primarily composed of carbon. This is attributed to the pyrolysis of the aerogel matrix at high temperatures, which generates pyrolytic carbon adhering to the fiber surface. However, this carbon layer provides limited protection and support to the fibers and the internal aerogel matrix.

In HS/C-25, distinct distributions of silicon (Si) and titanium (Ti) elements are observed coating the high-silica fibers, which can be attributed to the oxidation of the ceramic-forming functional component Ti₃SiC₂ under high-temperature oxidative conditions. As described in Equations (3)–(8), Ti₃SiC₂ undergoes a series of oxidation and redox reactions in oxygen- or water vapor-containing atmospheres, producing oxide phases such as SiO₂, TiO₂, Ti₂O₃, and TiO, accompanied by the release of CO₂ and H₂. These reaction products preferentially form and deposit in situ on the fiber and matrix surfaces, leading to the development of a continuous ceramic phase structure. The resulting ceramic-phase products not only isolate the internal matrix from oxygen but also act as binders for the pyrolytic carbon. Moreover, as the content of the ceramic-forming functional component increases, its protective effect on both the fibers and the matrix becomes more pronounced.

Ti₃SiC₂ (s) + 4.5O₂(g) = 3TiO (s) + SiO₂ (s) + 2CO₂(g)

(3)

Ti₃SiC₂ (s) + 6O₂(g) = 3TiO₂ (s) + SiO₂ (s) + 2CO₂(g)

(4)

Ti₃SiC₂ (s) + 5.25O₂(g) = 1.5Ti₂O₃ + SiO₂ + 2CO₂(g)

(5)

Ti₃SiC₂ (s) + 9H₂O(g) = 3TiO (s) + SiO₂ (s) + 2CO₂(g) + 9H₂(g)

(6)

Ti₃SiC₂ (s) + 12H₂O(g) = 3TiO₂ (s) + SiO₂ (s) + 2CO₂(g) + 12H₂(g)

(7)

Ti₃SiC₂ (s) + 10.5H₂O(g) = 1.5Ti₂O₃ (s) + SiO₂ (s) + 2CO₂(g) + 10.5H₂(g)

(8)

To investigate the oxidation resistance mechanism of the composites, X-ray diffraction (XRD) analysis was conducted on the composites before and after high-temperature heat treatment, in conjunction with EDS spectra. Figure 9a presents the XRD patterns of HS/FRF-50 and HS/C-50. Based on the XRD results, the primary crystalline phase in HS/FRF-50 before heat treatment is Ti₃SiC₂, with only peaks corresponding to Ti₃SiC₂ detected in the sample.

After heat treatment, the Ti₃SiC₂ peaks in HS/C-50 are significantly weakened, indicating consumption of the ceramic-forming functional component under oxidative conditions and a consequent reduction in its content. The appearance of diffraction peaks corresponding to TiO₂ and TiC suggests that reactions primarily forming these phases occurred at high temperatures. Possible reactions leading to TiC formation include Equations (9)–(11).

2TiO + 3C = 2TiC + CO₂(g)

(9)

Ti₂O₃ + 3.5C = 2TiC + 1.5CO₂(g)

(10)

TiO₂ + 2C = TiC + CO₂(g)

(11)

Raman spectroscopy was employed to analyze the pyrolytic carbon in HS/C-50, with the corresponding spectrum presented in Figure 9b. The results indicate that the pyrolytic carbon generated from the reaction is of low purity, suggesting incomplete conversion of the phenolic matrix into pyrolytic carbon during thermal pyrolysis. Peak fitting of the Raman spectrum reveals the presence of a D band (associated with disordered graphitic structures, centered at 1359.20 cm⁻¹) and a G band (corresponding to graphitic carbon, centered at 1603.75 cm⁻¹). The intensity ratio of the D band to the G band (I_D/I_G) is commonly used to assess the degree of graphitization. A low I_D/I_G ratio indicates fewer defects and a higher proportion of ordered sp² structures, whereas a high ratio suggests a greater concentration of defects, structural disorder, or a higher content of amorphous carbon. For the HS/C-50 sample, the I_D/I_G ratio is approximately 1.03, indicating that the porous pyrolytic carbon is predominantly amorphous and has not undergone a high degree of graphitization.

X-ray photoelectron spectroscopy (XPS) analysis was performed on the heat-treated composite HS/C-50, with the results shown in Figure 10. In addition, a weak F1s signal was observed in the survey spectrum. This peak is attributed to trace fluorine-containing residues originating from the CF₃COOH (trifluoroacetic acid) catalyst used during precursor preparation. Despite the 1000 °C/10 min heat treatment, XPS is highly surface-sensitive; therefore, trace-level surface species may still be detectable. Through peak deconvolution and fitting of the C1s spectrum, the carbon chemical states were identified as adsorbed carbon (284.80 eV), C–C (284.00 eV), C–O (286.19 eV), C=O (287.78 eV), and π–π* transition (290.00 eV). For the Ti 2p spectrum, peak fitting revealed a high proportion of Ti(IV) states (458.99 eV and 464.68 eV), indicating the presence of TiO₂. Additionally, a small amount of Ti(III) (457.71 eV and 462.71 eV) was detected. The fitting analysis reveals that the relative content ratio of Ti(III) to Ti(IV) is 0.185 (i.e., Ti(III) accounts for approximately 15.6% of the total Ti content, while Ti(IV) accounts for approximately 84.4%). This quantitative result further confirms that under the heat treatment condition of 1000 °C, Ti₃SiC₂ has undergone significant oxidation, with the majority of titanium existing as Ti(IV) in the form of TiO₂ and TiC, and only a small amount of Ti(III) remaining (which may correspond to incompletely oxidized Ti₃SiC₂ or intermediate oxidation states). Peak fitting of the Si2p spectrum confirmed the presence of a significant amount of SiO₂. However, this phase was not observed in the XRD pattern, indicating that the SiO₂ did not form a crystalline phase.

4. Conclusions

This work developed a high-silica fiber felt-reinforced, in situ ceramizable phenolic aerogel composite using high-silica fiber felt as the reinforcing phase and Ti₃SiC₂ as the ceramic functional phase. The thermal insulation properties, mechanical behavior, and thermal stability of the composites were systematically evaluated at room temperature and after muffle furnace treatment at 1000 °C, with concurrent elucidation of the synergistic interaction between the high-silica fiber felt-reinforced phase and the Ti₃SiC₂ ceramic functional phase. The key findings are summarized as follows: Comprehensive evaluation under room temperature conditions and after heat treatment revealed tunable density (0.398–0.629 g·cm⁻³), ultralow thermal conductivity (0.0414 W·m⁻¹·K⁻¹), and stable back temperature of 408.6 °C during butane torch exposure. After high-temperature treatment, incorporating Ti₃SiC₂ in the composites markedly suppressed volume shrinkage (32.2% for HS/C-0, 13.9% for HS/C-75), with the compressive strength and specific strength of HS/C-75 reaching 4.39 and 1.96 times those of HS/C-0, respectively. Multi-technique characterization (SEM, EDS, XRD, XPS, Raman) confirmed that the in situ formed multi-phase ceramic network establishes a dense protective barrier, substantially enhancing oxidation and ablation resistance. The aerogel composites provide high-performance support for extreme thermal environment protection through its synergistic advantages in multiple properties.