The Influence of SiO2 + SiC + Al (H2PO4)3 Coating on Mechanical and Dielectric Properties for SiCf/MWCNTS/AlPO4 Composites

SiC fiber-reinforced AlPO4 matrix (SiCf/MWCNTs/AlPO4) composites were fabricated using a hot laminating process with multi-walled carbon nanotubes (MWCNTs) as the absorber. A coating prepared from SiO2 + SiC + Al (H2PO4)3 was applied to the surface of the SiCf/MWCNTs/AlPO4 composites prior to an anti-oxidation test at 1273 K in air for 40 h. The anti-oxidation effect was verified by a three-point bending test, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and a dielectric property test. Anti-oxidation mechanism investigations revealed that the coating effectiveness could be attributed to three substances, i.e., SiO2, SiP2O7, and SiO2 + AlPO4 solid solution from the reactions of SiC + O2→SiO2 + CO, SiO2 + P2O5→SiP2O7 and SiO2 + AlPO4→solid solution, respectively.


Introduction
Electromagnetic wave-absorbing materials, designed to decrease reflected electromagnetic radiation by absorbing electromagnetic waves and transforming it into other energy, is a topic of extensive interest in the aerospace and military fields [1][2][3][4][5][6]. Currently, the research for applications in high-temperature environments is the main topic for electromagnetic wave absorbing materials.
Continuous fiber-reinforced ceramic matrix composites (SiC f /SiC [7], SiC f /C [8], C f /C [9], C f /SiC [10], and SiC f /AlPO 4 [11]), showing excellent fracture toughness, good thermal stability, and environmental durability, have been evaluated and modified for use as structural electromagnetic wave-absorbing materials. Especially, continuous SiC fiber-reinforced AlPO 4 (SiC f /AlPO 4 ) composites have demonstrated good potential [12][13][14]. Their low dielectric constants provide the opportunity to tailor the dielectric properties and wave absorbing abilities by the addition of conductive fillers (carbon black, carbon nanotube, and graphene). However, the conductive fillers, SiC fibers, and C fibers are easily oxidized in oxidation environments, which limits the high-temperature application for ceramic matrix composites. Therefore, it is very important to form high-temperature antioxidant coatings on the surfaces of ceramic matrix composites to improve their oxidation resistance.
Currently, many efforts have been made regarding antioxidant coatings [15][16][17]. SiC ceramic coatings are usually used as bonding layers in environmental barrier coating (EBC) for C/SiC composite coatings owing to their good chemical and physical compatibilities with C/SiC composites [18,19]. However, micro-cracks develop owing to the difference in the thermal expansion coefficient between the SiC coating and C/SiC composites during the long oxidation process. To avoid the above problems, top coatings should be prepared Materials 2022, 15, 5178 2 of 10 on the surface of the SiC coating to heal the micro-cracks, such as mullite, MoSi 2 , ZrSiO 4 , Y 2 SiO 5 , CrSi 2 , and c-AlPO 4 top coatings. AlPO 4 ceramic, with many excellent properties such as a high melting point (above 1773 K), strong self-healing ability, low Young's modulus, and low oxygen permeability is usually an ideal high-temperature anti-oxidation coating material for many ceramic composites [20,21]. With the lowest low oxygen permeability in various oxide ceramics, SiO 2 ceramic also shows its advantage in the field of anti-oxidation coating [22,23].
Based on these results, a multi-composition coating, including SiC, SiO 2 , and AlPO 4 was prepared on the surface of SiC f /MWCNTs/AlPO 4 composites in this paper. The anti-oxidation efficiency of the coating was proved and examined in detail.

Materials
The SiC fiber was provided by the National University of Defense Technology (Changsha, China). The 2D SiC fiber cloths with a 40% fiber volume fraction were fabricated by Nanjing Glass Fiber Institute (Nanjing, China). MWCNTs used as conductive filler were supplied by the Shenzhen Nanotech port Co. Ltd., Shenzhen, China. The diameter of MWCNTs ranged from 20 to 80 nm, and length was 5-15 µm, and the purity was 95%. Figure 1a,b show the SEM image of 50 vol.% 2D SiC cloths and the TEM image of MWCNTs, respectively. The diameters of SiO 2 , Al 2 O 3 , and β-SiC powders are in the range of 1-5 µm.
Materials 2022, 15, 5178 2 of 10 bilities with C/SiC composites [18,19]. However, micro-cracks develop owing to the difference in the thermal expansion coefficient between the SiC coating and C/SiC composites during the long oxidation process. To avoid the above problems, top coatings should be prepared on the surface of the SiC coating to heal the micro-cracks, such as mullite, MoSi2, ZrSiO4, Y2SiO5, CrSi2, and c-AlPO4 top coatings. AlPO4 ceramic, with many excellent properties such as a high melting point (above 1773 K), strong self-healing ability, low Young's modulus, and low oxygen permeability is usually an ideal high-temperature antioxidation coating material for many ceramic composites [20,21]. With the lowest low oxygen permeability in various oxide ceramics, SiO2 ceramic also shows its advantage in the field of anti-oxidation coating [22,23]. Based on these results, a multi-composition coating, including SiC, SiO2, and AlPO4 was prepared on the surface of SiCf/MWCNTs/AlPO4 composites in this paper. The antioxidation efficiency of the coating was proved and examined in detail.

Materials
The SiC fiber was provided by the National University of Defense Technology (Changsha, China). The 2D SiC fiber cloths with a 40% fiber volume fraction were fabricated by Nanjing Glass Fiber Institute (Nanjing, China). MWCNTs used as conductive filler were supplied by the Shenzhen Nanotech port Co. Ltd., China. The diameter of MWCNTs ranged from 20 to 80 nm, and length was 5-15 μm, and the purity was 95%. Figure 1a,b show the SEM image of 50 vol.% 2D SiC cloths and the TEM image of MWCNTs, respectively. The diameters of SiO2, Al2O3, and β-SiC powders are in the range of 1-5 μm.

Preparation of the Composites
The Al(H2PO4)3 solution, which is a precursor of AlPO4, was synthesized from aluminum hydroxide (Al(OH)3) and orthophosphoric acid (H3PO4, 85%). Al(OH)3 at 1 mol was dispersed in deionized water, and H3PO4 (85%) at 3 mol was added into the suspension liquid to maintain the theoretical Al/P atomic ratio of 1:3. The mixed solution was then allowed to react at 90 °C for several hours, and the viscous Al(H2PO4)3 solution was obtained. The MWCNTs and Al2O3 powders were uniformly mixed with as-received Al(H2PO4)3 solution by ball milling for 4 h to obtain the slurry. The SiC fiber cloths were impregnated in the slurry. After air drying for 24 h, the 10 sheets of cloths obtained were laminated and hot pressed in a steel die at 100 and 200 °C for 1 h in turn. A pressure of 3 MPa was applied when the temperature reached 100 °C , and such pressure was maintained until the end of hot pressing. Then, these samples were heated at a rate of 5 °C /min

Preparation of the Composites
The Al(H 2 PO 4 ) 3 solution, which is a precursor of AlPO 4 , was synthesized from aluminum hydroxide (Al(OH) 3 ) and orthophosphoric acid (H 3 PO 4 , 85%). Al(OH) 3 at 1 mol was dispersed in deionized water, and H 3 PO 4 (85%) at 3 mol was added into the suspension liquid to maintain the theoretical Al/P atomic ratio of 1:3. The mixed solution was then allowed to react at 90 • C for several hours, and the viscous Al(H 2 PO 4 ) 3 solution was obtained. The MWCNTs and Al 2 O 3 powders were uniformly mixed with as-received Al(H 2 PO 4 ) 3 solution by ball milling for 4 h to obtain the slurry. The SiC fiber cloths were impregnated in the slurry. After air drying for 24 h, the 10 sheets of cloths obtained were laminated and hot pressed in a steel die at 100 and 200 • C for 1 h in turn. A pressure of 3 MPa was applied when the temperature reached 100 • C, and such pressure was maintained until the end of hot pressing. Then, these samples were heated at a rate of 5 • C/min in a vacuum furnace to 500 • C for 1 h, and SiC f /MWCNTs/AlPO 4 composites were obtained.

Preparation of the Coating
Al(H 2 PO 4 ) 3 solution was mixed with the SiO 2 and SiC powders in the ratio 5:3:2 (w/w) Al(H 2 PO 4 ) 3 :SiO 2 :SiC. After ball milling for 3 h, the obtained mixture was brushed onto the surface of SiC f /AlPO 4 composites and dried at 373 K for 1 h prior to annealing at 1473 K for 3 h at a heating rate of 283 K/min in vacuum atmosphere. After cooling at ambient temperature, the sample was given two infiltration-drying-annealing cycles to yield the coated SiC f /AlPO 4 composites. Uncoated and coated SiC f /AlPO 4 composites were heated to 1273 K in a muffle furnace. Treated samples were cooled to room temperature under ambient conditions.
The flexural strength of composites at room temperature was obtained by the three-point bending test, with a crosshead rate of 0.5 mm/min and an outer support span of 30 mm. The test was conducted following the general guidelines of ASTM standard C1341.
The complex permittivity values for the composites were measured based on the measurements of the reflection and transmission module between 8.2 and 12.4 GHz. The method was performed in the fundamental wave-guide mode TE10 using rectangular samples (10.16 mm × 22.86 mm × 3.00 mm). After calibration using an intermediate of a short circuit and blank holder, the reflection and transmission coefficients were obtained using an automated measuring system (E8362Bnetworkanalyzer). For dielectric materials (µ0 = 1, µ = 0), the relative error varied between 1% (pure dielectric) and 10% (highly conductive material). The schematic diagram is shown in Figure 2. tained.

Preparation of the Coating
Al(H2PO4)3 solution was mixed with the SiO2 and SiC powders in the ratio 5:3:2 (w/w) Al(H2PO4)3:SiO2:SiC. After ball milling for 3 h, the obtained mixture was brushed onto the surface of SiCf/AlPO4 composites and dried at 373 K for 1 h prior to annealing at 1473 K for 3 h at a heating rate of 283 K/min in vacuum atmosphere. After cooling at ambient temperature, the sample was given two infiltration-drying-annealing cycles to yield the coated SiCf/AlPO4 composites. Uncoated and coated SiCf/AlPO4 composites were heated to 1273 K in a muffle furnace. Treated samples were cooled to room temperature under ambient conditions.
The flexural strength of composites at room temperature was obtained by the threepoint bending test, with a crosshead rate of 0.5 mm/min and an outer support span of 30 mm. The test was conducted following the general guidelines of ASTM standard C1341.
The complex permittivity values for the composites were measured based on the measurements of the reflection and transmission module between 8.2 and 12.4 GHz. The method was performed in the fundamental wave-guide mode TE10 using rectangular samples (10.16 mm × 22.86 mm × 3.00 mm). After calibration using an intermediate of a short circuit and blank holder, the reflection and transmission coefficients were obtained using an automated measuring system (E8362Bnetworkanalyzer). For dielectric materials (μ0 = 1, μ″ = 0), the relative error varied between 1% (pure dielectric) and 10% (highly conductive material). The schematic diagram is shown in Figure 2.

Investigation of Bending Strength
The bending strengths for SiCf/MWCNTs/AlPO4 composites obtained by the threepoint bending test are reflected in Figure 3.

Investigation of Bending Strength
The bending strengths for SiC f /MWCNTs/AlPO 4 composites obtained by the three-point bending test are reflected in Figure 3.  The three specimens initially showed an elastic response with increasing d ment. After reaching the maximum strength, the bending strength of the as-receive imen displayed an inelastic decrease before reducing abruptly. This differed fr curves of the oxidized specimens (with and without the coating), which showed The three specimens initially showed an elastic response with increasing displacement. After reaching the maximum strength, the bending strength of the as-received specimen displayed an inelastic decrease before reducing abruptly. This differed from the curves of the oxidized specimens (with and without the coating), which showed a direct reduction in bending strength at maximum strength. The brittle fracture for the three curves could be attributed to the absence of an interface, which led to the loss of toughening mechanisms including fiber pull-out and debonding, and crack deflection. After oxidizing for 40 h, the bending strength of the coated specimen decreased from 205 to 190 MPa, and the displacement was reduced to 0.38 mm. These effects were due to the influence of high temperature on the SiC fibers. The specimen without the coating attained a bending strength of 60 MPa and displacement of 0.14 mm.
The corresponding fracture surface morphologies of the specimens (with and without the coating) subjected to oxidizing conditions are given in Figure 4. The fracture surface of the coated specimen was smooth and little fiber pull-out was observed (Figure 4a). The cross sections of SiC fibers were complete and clearly visible. Figure 4b,c show the SEM and TEM pictures of uncoated SiC f /MWCNTs/AlPO 4 composites undergoing 40 h oxidation. Obviously, a strong bond occurred and a reaction zone was formed between the AlPO 4 matrix and SiC fiber, which could be attributed to the reaction of the AlPO 4 matrix and SiO 2 , produced from the oxidation of SiC. ment. After reaching the maximum strength, the bending strength of th imen displayed an inelastic decrease before reducing abruptly. This curves of the oxidized specimens (with and without the coating), whic reduction in bending strength at maximum strength. The brittle frac curves could be attributed to the absence of an interface, which led to th ing mechanisms including fiber pull-out and debonding, and crack de dizing for 40 h, the bending strength of the coated specimen decrease MPa, and the displacement was reduced to 0.38 mm. These effects wer ence of high temperature on the SiC fibers. The specimen without the bending strength of 60 MPa and displacement of 0.14 mm.
The corresponding fracture surface morphologies of the specimen out the coating) subjected to oxidizing conditions are given in Figure 4 face of the coated specimen was smooth and little fiber pull-out was ob The cross sections of SiC fibers were complete and clearly visible. Fig  SEM and TEM pictures of uncoated SiCf/MWCNTs/AlPO4 composite oxidation. Obviously, a strong bond occurred and a reaction zone wa the AlPO4 matrix and SiC fiber, which could be attributed to the rea matrix and SiO2, produced from the oxidation of SiC.

Investigation of Dielectric Property
The real part (ε ) and imaginary part (ε ) of the complex permittivity for the coated SiC f /AlPO 4 composites are shown in Figure 5. tivity of the composites increase by free electrons shifting and hopping, which explains the increase in ε″.
After 40 h oxidization, the values of ε′ and ε″ for the coated SiCf/AlPO4 composites with 1.5 wt.% MWCNTs showed little change compared with the values before oxidation. These results showed that the MWCNTs were still present and functional. These findings demonstrated that the anti-oxidation effect of the coating was effective for the SiCf/MWCNTs/AlPO4 composites in an oxidizing environment at 1273 K.  Figure 6a shows the fracture surface image of coated SiCf/MWCNTs/AlPO4 composites before oxidization. The coating showed a strong bond with the AlPO4 matrix, and no obvious boundary was distinguished (indicated by the black arrows). At the same time, fiber pull-out was observed in the image, which proved that SiC fibers had no reaction with the AlPO4 matrix and the coating was effective. Figure 6b shows the surface image of the coating after preparation. It was observed from Figure 6b that the coating was dense and smooth, which showed a glassy state. No holes and cracks existed.

Investigation of the Coating
According to the phase diagram of AlPO4-SiO2 [23], some phases of solid solution (C-AlPO4 solid solution, T-AlPO4 solid solution, Cr-SiO2 solid solution, and Tr-SiO2 solid solution) might be formed when the preparation temperature of the coating was maintained at 1273K, and these were dependent on the content of AlPO4 and SiO2 in the mixture. Consequently, this result was theoretically responsible for the strong bond between the AlPO4 matrix and the coating.  The complex permittivity values of pure SiC f /AlPO 4 composites (no coating and MWCNTs) have been discussed in [14]. The value of ε was in the range of 3.6-4.1 and the value of ε was in the range of 0.1-0.2. The values of ε and ε were small due to the insulated AlPO 4 matrix, which was observed in Table 1. After the introduction of the coating, the ε and ε values for the SiC f /AlPO 4 composites were in the range of 4.2-4.5 and 0.2-0.5 within the entire X-band, respectively. Compared to the result of Sample 1, the values of ε and ε showed little change, which proved that the introduction of the coating had little influence on the dielectric property. This was ascribed to the low dielectric constants of the coating substances, reflected in Table 1. With the introduction of 1.5 wt.% MWCNTs, the ε and ε for the coated SiC f /MWCNTS/AlPO 4 composites ranges increased from 4.2-4.5 to 5.0-6.3 and 0.2-0.5 to 1.8-3.6, respectively. The main reasons can be given next. Complex permittivity is expressed by the following equation: ε = ε − jε . ε is an expression of the polarization ability of a material. ε is an expression of the capacity of dielectric losses, which comprise polarization loss and electric conductance loss. The complex permittivity affects the absorbing wave property. When the value of ε is too high, the electromagnetic wave cannot enter the composites, leading to a poor absorbing wave effect. When the value of ε is too small, the electromagnetic wave cannot be consumed, leading to a poor absorbing wave effect. So, a suit value of ε /ε is needed to satisfy the impedance matching rule.
According to the Debye theory of the dielectric, ε and ε of the composites can be calculated as follows: where ε s is the static permittivity, ε ∞ is the permittivity at the high-frequency limit, ω is the angular frequency, τ is the relaxation time, σ(T) is the temperature-dependence electrical conductivity, and ε 0 is the dielectric constant in a vacuum. As described in Formulas (1) and (2), ε was determined by the relaxation time (τ). ε was determined by both the relaxation time (τ) and electrical conductivity of the composites (σ(T)). The possible polarization mechanisms at the microwave frequency included electronic, atomic, relaxation, and space charge polarizations. The contribution of atomic and electronic polarizations to permittivity was small and negligible. The effect of space charge polarization on the GHz range was lost because a long duration of time was required to establish polarization. So, the increase in ε could be attributed to the electronic relaxation polarization enhanced by the MWCNTs. The introduction of MWCNTs not only brought the electronic relaxation polarization, but also made the electrical conductivity of the composites increase by free electrons shifting and hopping, which explains the increase in ε .
After 40 h oxidization, the values of ε and ε for the coated SiC f /AlPO 4 composites with 1.5 wt.% MWCNTs showed little change compared with the values before oxidation. These results showed that the MWCNTs were still present and functional. These findings demonstrated that the anti-oxidation effect of the coating was effective for the SiC f /MWCNTs/AlPO 4 composites in an oxidizing environment at 1273 K. Figure 6a shows the fracture surface image of coated SiC f /MWCNTs/AlPO 4 composites before oxidization. The coating showed a strong bond with the AlPO 4 matrix, and no obvious boundary was distinguished (indicated by the black arrows). At the same time, fiber pull-out was observed in the image, which proved that SiC fibers had no reaction with the AlPO 4 matrix and the coating was effective. Figure 6b shows the surface image of the coating after preparation. It was observed from Figure 6b that the coating was dense and smooth, which showed a glassy state. No holes and cracks existed.  Figure 6a shows the fracture surface image of coated SiCf/MWCNTs/AlP sites before oxidization. The coating showed a strong bond with the AlPO4 ma obvious boundary was distinguished (indicated by the black arrows). At the fiber pull-out was observed in the image, which proved that SiC fibers had with the AlPO4 matrix and the coating was effective. Figure 6b shows the sur of the coating after preparation. It was observed from Figure 6b that the coating and smooth, which showed a glassy state. No holes and cracks existed.

Investigation of the Coating
According to the phase diagram of AlPO4-SiO2 [23], some phases of solid s AlPO4 solid solution, T-AlPO4 solid solution, Cr-SiO2 solid solution, and Tr solution) might be formed when the preparation temperature of the coating tained at 1273K, and these were dependent on the content of AlPO4 and SiO2 ture. Consequently, this result was theoretically responsible for the strong bon the AlPO4 matrix and the coating.  According to the phase diagram of AlPO 4 -SiO 2 [23], some phases of solid solution (C-AlPO 4 solid solution, T-AlPO 4 solid solution, Cr-SiO 2 solid solution, and Tr-SiO 2 solid solution) might be formed when the preparation temperature of the coating was maintained at 1273 K, and these were dependent on the content of AlPO 4 and SiO 2 in the mixture. Consequently, this result was theoretically responsible for the strong bond between the AlPO 4 matrix and the coating.
The XRD spectrum of reaction products derived from the Al(H 2 PO 4 ) 3 solution and SiO 2 is reflected in Figure 7. Four major peaks around 2θ values of 20.4 • , 24.2 • , 25.9 • , and 30.4 • (at 10 wt.% SiO 2 ) were homologous with the crystal phase of Al(PO 3 ) 3 and decreased in intensity with the increasing content of SiO 2 and were absent at 40 wt.% SiO 2 . However, the intensity of the two peaks around 2θ values of 20.8 • and 26.5 • increased. Supposing these peaks were ensured to be SiO 2 , the disappearance of Al(PO 3 ) 3 peaks could not be accepted; if they are ensured to be AlPO 4 (i.e., decomposition products of Al(PO 3 ) 3 ), the disappearance of SiO 2 peaks could not be accepted. Hence, it was concluded that the SiO 2 reacted with AlPO 4 to form a solid solution, which was responsible for the diffraction peaks in the XRD spectrum of 40 wt.% SiO 2 . The continuous decomposition and final exhaustion for Al(PO 3 ) 3 was attributed to the consumption of AlPO 4 from an abundance of SiO 2 . Hence the results given in Figures 4b and 6b experimentally confirm that the SiO 2 -AlPO 4 solid solution was tightly correlated with the strong bond between the AlPO 4 matrix and the coating.
SiO2 is reflected in Figure 7. Four major peaks around 2θ values of 20.4°, 24 30.4° (at 10 wt.% SiO2) were homologous with the crystal phase of Al(PO3)3 a in intensity with the increasing content of SiO2 and were absent at 40 wt.% S the intensity of the two peaks around 2θ values of 20.8° and 26.5° increase these peaks were ensured to be SiO2, the disappearance of Al(PO3)3 peaks accepted; if they are ensured to be AlPO4 (i.e., decomposition products of disappearance of SiO2 peaks could not be accepted. Hence, it was concluded reacted with AlPO4 to form a solid solution, which was responsible for t peaks in the XRD spectrum of 40 wt.% SiO2. The continuous decomposition haustion for Al(PO3)3 was attributed to the consumption of AlPO4 from an SiO2. Hence the results given in Figures 4b and 6b experimentally confirm AlPO4 solid solution was tightly correlated with the strong bond between t trix and the coating. The presence of low melting point SiP2O7, derived from the reaction of (Al(PO3)3→AlPO4+ P2O5), also strengthened the bonding within the coating holes and cracks in the coating to make it be a smooth, dense, and glassy sta be confirmed by the phase diagram of SiO2-P2O5 [14]. These two chemical tributed to the formation of a dense coating. Figure 8a shows the fracture surface image of coated SiCf/MWCNTs/A sites after 40 h oxidization. The reaction of the AlPO4 matrix and the SiO2 f ened the bond between the matrix and coating. Figure 8b shows the surfac coating after 40 h oxidization. It was observed from Figure 8 that the coa dense and smooth, which showed a glassier state. No holes and cracks exis fill of low melting point SiP2O7. On one hand, the routes of oxygen diffusi due to the absence of holes and cracks; on the other hand, the efficiency of sion was decreased due to the SiO2, which was composed of the raw mate reaction SiO2 from the oxidization of SiC. These results proved that the coat tive in preventing composite oxidization. The presence of low melting point SiP 2 O 7 , derived from the reaction of SiO 2 and P 2 O 5 (Al(PO 3 ) 3 →AlPO 4 + P 2 O 5 ), also strengthened the bonding within the coating and filled the holes and cracks in the coating to make it be a smooth, dense, and glassy state. This could be confirmed by the phase diagram of SiO 2 -P 2 O 5 [14]. These two chemical reactions contributed to the formation of a dense coating. Figure 8a shows the fracture surface image of coated SiC f /MWCNTs/AlPO 4 composites after 40 h oxidization. The reaction of the AlPO 4 matrix and the SiO 2 filler strengthened the bond between the matrix and coating. Figure 8b shows the surface image of the coating after 40 h oxidization. It was observed from Figure 8 that the coating was still dense and smooth, which showed a glassier state. No holes and cracks existed due to the fill of low melting point SiP 2 O 7 . On one hand, the routes of oxygen diffusion were filled due to the absence of holes and cracks; on the other hand, the efficiency of oxygen diffusion was decreased due to the SiO 2 , which was composed of the raw materials SiO 2 and reaction SiO 2 from the oxidization of SiC. These results proved that the coating was effective in preventing composite oxidization.  Table 2 shows the results of bend strength, dielectric constants. Obviously, it can be found that the coating is effective in protecting the SiCf/MWCNTS/AlPO4 composites from being oxidized.  Figure 9 shows the schematic diagram of the anti-oxidation mechanism for the coating. During the preparation of the coating, P2O5 (g) is readily released while the formation of SiP2O7 slows. However, the relatively long oxidation time was enough for P2O5 to react with SiO2 and form SiP2O7. Figure 9 gives a summary schematic representation of the mechanism of anti-oxidation based on the results from this study. During the preparation of the coating, the AlPO4 matrix reacts with SiO2 to form a SiO2 -AlPO4 solid solution lead-  Table 2 shows the results of bend strength, dielectric constants. Obviously, it can be found that the coating is effective in protecting the SiC f /MWCNTS/AlPO 4 composites from being oxidized.  Figure 9 shows the schematic diagram of the anti-oxidation mechanism for the coating. During the preparation of the coating, P 2 O 5 (g) is readily released while the formation of SiP 2 O 7 slows. However, the relatively long oxidation time was enough for P 2 O 5 to react with SiO 2 and form SiP 2 O 7 . Figure 9 gives a summary schematic representation of the mechanism of anti-oxidation based on the results from this study. During the preparation of the coating, the AlPO 4 matrix reacts with SiO 2 to form a SiO 2 -AlPO 4 solid solution leading to a strong chemical bond between SiC f /AlPO 4 composites and the coating. The formation of SiP 2 O 7 and SiO 2 -AlPO 4 solid solution facilitates the bonding of the particles in the coating, which contributes to the formation of a dense coating. Under oxidizing conditions, the SiC in the coating is partially transformed into SiO 2 as it consumes the incoming oxygen gas, and the decomposition of Al(PO 3 ) 3 increases with the increasing time. The reactions of SiO 2 -AlPO 4 and SiO 2 -P 2 O 5 occur throughout the coating, linking particles to form a dense coating of low oxygen permeability.
ing to a strong chemical bond between SiCf/AlPO4 composites and the coating. The formation of SiP2O7 and SiO2 -AlPO4 solid solution facilitates the bonding of the particles in the coating, which contributes to the formation of a dense coating. Under oxidizing conditions, the SiC in the coating is partially transformed into SiO2 as it consumes the incoming oxygen gas, and the decomposition of Al(PO3)3 increases with the increasing time. The reactions of SiO2 -AlPO4 and SiO2 -P2O5 occur throughout the coating, linking particles to form a dense coating of low oxygen permeability.  Table 3 shows the calculated Gibbs free energy (∆G) for the oxidation of SiC at 1273 K. The reaction shown by Code (4) was favored by its minimal value of ∆G. As oxygen gas is introduced, SiC particles are oxidized to SiO2, which then reacts with P2O5 and AlPO4. The integration of SiO2, SiP2O7, and the SiO2 -AlPO4 solid solution into the coating is effective in preventing the oxygen gas from further diffusion into the SiCf/MWCNTs/AlPO4 composites.

Conclusions
This study presents a detailed investigation of the anti-oxidation mechanism of the SiO2 + SiC + Al(H2PO4)3 coating. The anti-oxidation effect of the SiCf/MWCNTs/AlPO4 composites in an oxidizing environment (1273 K, 40 h) was confirmed by a three-point bending test, microstructure characterization, and dielectric property. SiCf/MWCNTs/AlPO4 composites were chemically bonded with the coating. Oxygen gas in the environment was consumed by SiC particles to form SiO2, which subsequently reacted with P2O5 and AlPO4 to form SiP2O7 and SiO2 -AlPO4 solid solution, respectively. The integration of SiO2, SiP2O7, and SiO2 -AlPO4 solid solution into the coating was effective in preventing the oxygen gas from consuming MWCNTs. The coating gives SiCf/MWCNTs/AlPO4 composites the potential to be applied as high-temperature structural wave absorbing materials.   Table 3 shows the calculated Gibbs free energy (∆G) for the oxidation of SiC at 1273 K. The reaction shown by Code (4) was favored by its minimal value of ∆G. As oxygen gas is introduced, SiC particles are oxidized to SiO 2 , which then reacts with P 2 O 5 and AlPO 4 . The integration of SiO 2 , SiP 2 O 7 , and the SiO 2 -AlPO 4 solid solution into the coating is effective in preventing the oxygen gas from further diffusion into the SiC f /MWCNTs/AlPO 4 composites.

Conclusions
This study presents a detailed investigation of the anti-oxidation mechanism of the SiO 2 + SiC + Al(H 2 PO 4 ) 3 coating. The anti-oxidation effect of the SiC f /MWCNTs/AlPO 4 composites in an oxidizing environment (1273 K, 40 h) was confirmed by a three-point bending test, microstructure characterization, and dielectric property. SiC f /MWCNTs/AlPO 4 composites were chemically bonded with the coating. Oxygen gas in the environment was consumed by SiC particles to form SiO 2 , which subsequently reacted with P 2 O 5 and AlPO 4 to form SiP 2 O 7 and SiO 2 -AlPO 4 solid solution, respectively. The integration of SiO 2 , SiP 2 O 7 , and SiO 2 -AlPO 4 solid solution into the coating was effective in preventing the oxygen gas from consuming MWCNTs. The coating gives SiC f /MWCNTs/AlPO 4 composites the potential to be applied as high-temperature structural wave absorbing materials.

Conflicts of Interest:
The authors declare no conflict of interest.