Microstructural regulation, oxidation resistance, and mechanical properties of Cf/SiC/SiHfBOC composites prepared by chemical vapor infiltration with precursor infiltration pyrolysis

To further improve the oxidation resistance of polymer derived ceramic (PDC) composites in harsh environments, Cf/SiC/SiHfBOC composites were prepared by chemical vapor infiltration (CVI) and precursor impregnation pyrolysis (PIP) methods. The weight retention change, mechanical properties, and microstructure of Cf/SiC/SiHfBOC before and after oxidation in air were studied in details. Microscopic analyses showed that only the interface between the ceramics and fibers was oxidized to some extent, and hafnium had been enriched on the composite surface after oxidizing at different temperature. The main oxidation products of Cf/SiC/SiHfBOC composites were HfO2 and HfSiO4 after oxidation at 1500 °C for 60 min. Moreover, the weight retention ratio and compressive strength of the Cf/SiC/SiHfBOC composites are 83.97% and 23.88±3.11 MPa, respectively. It indicates that the Cf/SiC/SiHfBOC composites should be promising to be used for a short time in the oxidation environment at 1500 °C.


1.Introduction
With the emergence of hypersonic spacecraft, the requirements of thermal protection materials such as lightweight, strengthening-toughening, and oxidation resistance of thermal protection materials are increasing in the combustion chamber and tail nozzle of aerospace vehicles [1][2][3]. The thermal protection materials represented by ceramic thermal insulation tiles are difficult to meet the requirements of increasing the service environment temperature of hypersonic spacecraft [4][5][6][7].
In addition, ceramic matrix composites have attracted much attention due to their excellent thermal stability and high compressive strength [8]. However, the intrinsic brittleness and poor thermal shock resistance of ultra-high temperature seriously restrict the wide application of ceramic thermal protection materials [9,10]. The high-performance silicon-based precursor ceramics have excellent thermodynamic stability. SiOC based precursor ceramics have been applied in the hot end components of aerospace vehicles. However, the strong carbothermal reduction ccurs in the environment above 1300 ℃, which seriously restricts the development of silicon-based precursor ceramics. It is found that the high-temperature stability of SiOC ceramics can be effectively improved by doping boron, nitrogen, zirconium, hafnium, and other elements. At present, a variety of precursor ceramics have been synthesized, such as SiBOC [11]、SiZrOC [12]、SiHfOC [12]、 SiAlOC [13]、SiBCN [14]、SiHfBOC [3]、SiBCNZr [14], etc.
Continuous carbon fiber (or carbon fiber perform) reinforced ceramic matrix composites have excellent properties, such as low density [15], high toughness [16], high strength, and high reusability [17], especially the toughening property [18], which helps to solve the inherent brittleness of ceramic materials [19]. It not only improves the thermal shock resistance of ceramic materials but also maintains the inherent high-temperature stability and low thermal expansion coefficient of ceramic materials [8]. Therefore, continuous carbon fiber reinforced ceramic matrix composites has been widely studied in recent years.
However, due to the low reaction activity of carbon fiber and the damage of carbon fiber in a hightemperature oxidation environment, the interface bonding performance between continuous fiberreinforced phase and the ceramic matrix will be reduced, and the excellent characteristics of continuous carbon fiber cannot be brought into full play. The coating will reduce the area of carbon fiber exposed to oxygen atmosphere, thus improving the oxidation resistance of the composites Therefore, Chemical vapor deposition (CVD), chemical vapor infiltration (CVI), and hydrothermal methods can be used to prepare coatings on carbon fiber surface [1,17,20]. The types of coatings can include C coating [21], BN coating [22], SiC coating [23], etc. These coatings can not only change the surface roughness of carbon fiber but also improve the interface bonding performance between ceramic matrix and carbon fiber reinforcement. In the process of ceramic pyrolysis, the coating can also protect carbon fibers by inhibiting the surface damage of carbon fiber.
The precursor solution of SiHfBOC was prepared in the previous work. Therefore, in this paper, SiC coating was prepared on the surface of carbon fiber by CVI, and then SiHfBOC precursor sol was ultrasonically impregnated into the framework of carbon fiber preform coated with SiC coating by PIP. After solvothermal reaction and high-temperature pyrolysis, Cf/SiC/SiHfBOC ceramic matrix composites were prepared. Their micromorphology and phase evolution were analyzed.
Additionally, the strengthening-toughening mechanism and oxidation resistance of Cf/SiC/SiHfBOC ceramic matrix composites were also evaluated.

Materials synthesis and processing
In our previous study, the preparation method of the SiHfBOC precursor solution was reported [3].
At the same time, SiC coating was prepared on the surface of carbon fiber by CVI method to protect the carbon fiber and improve its bonding strength with the SiHfBOC precursor solution. After degumming, the Cf preform (64 mm × 64 mm × 25 mm) was placed in chemical vapor infiltration (CVI) equipment. The MTS mass fraction of the precursor gas mixture was 40 wt.%, the gas flow rate was 40 ml/min, and the deposition temperature was 1000 ℃. Afterward, the SiHfBOC precursor solution was ultrasonically impregnated into the Cf preform coated with SiC coating. The precursor solution of SiHfBOC was impregnated into SiC/Cf preform by ultrasonic and then put into the reactor. After a solvothermal reaction at 120 ℃ for 720 min, the sample was further pyrolyzed in a tubular furnace. Finally, Cf/SiC/SiHfBOC composites were prepared by multiple PIP cycle times.

Characterization
This experiment used an X-ray diffractometer (XRD) of the type Empyrean Sharp (Panalytical, Netherlands) equipped with monochromatic Mo Kα radiation at a scan speed of 10 °/min in the 2theta range of 10-90 °. It was used for crystal identification, phase identification, and quantitative analysis. It has high reception efficiency, sensitivity, and precision. The surface morphology Cf, Cf/SiC, and Cf/SiC/SiHfBOC composites were analyzed by SEM.Scanning electron microscopy

Static oxidation tests and mechanics performance testing
The static oxidation resistance test was carried out in a muffle furnace. The Cf/SiC/SiHfBOC composites with the dimension of 10 mm × 10 mm × 10 mm were placed in a corundum crucible, and then the crucible was placed in a muffle furnace at the target temperature. After oxidation at the set oxidation temperature for a period, the oxidized samples were taken out and cooled naturally at room temperature. The weight of samples before and after the oxidation test was recorded by analytical balance. The calculation formula of oxidation weight retention ratio (W%) was shown in Where W% is oxidation weight retention ratio of sample；m0, m1 is the weight of the sample before and after oxidation, respectively. The oxidation weight retention ratio is the average of three samples. Finally, the static oxidation resistance of Cf/SiC/SiHfBOC composites was analyzed according to formula (1) and SEM, EDS, and XRD results. Where Pf and Pc are maximum loads during the test；L is span；h is sample height；W is sample width; σf, σc are flexural and compressive strength, respectively. The flexural and compressive strength is the average of three samples. Then the strength retention ratio was calculated by the ratio of compressive strength after and before the static oxidation test. And the strengthening and toughening mechanism of Cf/SiC/SiHfBOC composites were analyzed based on the fracture images observed by SEM.

Microstructure regulation of Cf/SiC/SiHfBOC composites
The XRD patterns and microstructure of Cf and SiC/Cf are shown in Fig. 2. The amorphous diffraction peak of C was detected near 2θ=25.5 ° in untreated Cf samples. In addition to the amorphous diffraction peaks of C, the diffraction peaks of SiC at 35.6 °, 60.1 °, and 71.9 ° were also found in the XRD patterns of SiC/Cf samples [23,25,26], which further indicated the existence of SiC coating. It can be seen from Figs. 2 (b)-2 (d) that the carbon fiber bundles are evenly arranged. The diameter of carbon fiber monofilament is about 6-7 μm. SiC coating uniformly covers carbon fiber, and the thickness of the SiC coating is about 300 nm. Archimedes drainage method was used to test the bulk density and porosity of Cf/SiC/SiHfBOC composites under different PIP cycle times. The specific results are shown in Table 1. With the increase of PIP cycle times, the density of Cf/SiC/SiHfBOC composites increase gradually, while the porosity decreases gradually. The density of untreated Cf preform is 0.35 g/cm 3 , and its porosity is 85.73 %. The density of Cf/SiC preform is 0.50 g/cm 3 , and its porosity is 74.86 %. After the third PIP cycle times, the density of Cf/SiC/SiHfBOC composite was 1.02 g/cm 3 , and its porosity was 47.98 %.
The density of Cf/SiC/SiHfBOC composites increased by about 0.12 g/cm 3 after the third to fifth PIP cycle times, and its density growth rate gradually slowed down. After the seventh PIP cycle times, the density of Cf/SiC/SiHfBOC composites increased only by 0.03 g/cm 3 , so the density of Cf/SiC/SiHfBOC composites has reached the upper limit.   Table 1, the density of the composite increases with the increase of PIP cycle times. In Fig.   3 (a), the Cf/SiC/SiHfBOC-4 was prepared after four PIP cycle times, with a density of 1.15 g/cm 3 and a porosity of 41.35 %. Therefore, a great number of pores can be observed in the fiber preform body, and many carbon fibers have not been filled by SiHfBOC ceramics. The Cf/SiC/SiHfBOC-5 was prepared after five PIP cycle times, with a density of 1.27 g/cm 3 and a porosity of 36.84 % ( Fig. 3 (b)). However, there are still many holes in the composite, and it is observed that the carbon fiber not coated by SiHfBOC ceramic has decreased significantly. The Cf/SiC/SiHfBOC-6 was prepared after six PIP cycle times. Its density is increased to 1.37 g/cm 3 , and the porosity is 33.44 % (Fig. 3(c)). It is observed that the pores in the composite have been further reduced, and most of the carbon fibers are covered by SiHfBOC ceramics, and the internal filling is relatively complete. The Cf/SiC/SiHfBOC-7 was prepared after seven PIP cycle times as in Fig. 3 (d), and the surface of the carbon fiber in the composite material is completely wrapped by SiHfBOC ceramic. The density of Cf/SiC/SiHfBOC composites increased was not obvious, which was 1.40 g/cm 3 , and its porosity was 28.87 %. SiHf-BOC ceramics covered most of the carbon fibers, that was, the composites were filled with SiHfBOC ceramic matrix. There are only a few holes in the composite, and the maximum pore width is about 3 μm. With the increase of PIP cycle times, the spacing of fibers is uniform, and the surface pores are gradually reduced. It is found that the microstructure and density of the composites can be controlled by the different PIP cycle times, and the macro and micropores in the composites are greatly reduced after seven times of impregnation and pyrolysis.

Study and analysis of oxidation resistance of Cf/SiC/SiHfBOC composites
The oxidation behavior of Cf/SiC/SiHfBOC composites at different PIP cycle times are evaluated by XRD and SEM. As shown in Fig. 4, the XRD phase diagram of Cf/SiC/SiHfBOC-7 composite is oxidized at 1100 ℃ for 10 min. It indicates that not only the diffraction peaks of SiC and HfO2 are detected [27], but also the diffraction peaks of HfSiO4 are detected at 20.0 °, 27.1 °, 44.0 °, and 53.7 ° after oxidation test [28].  Table 1, it can be found that the pore of the Cf/SiC/SiHfBOC composites surfaces gradually decreases with the increase of PIP cycle times. At the same time, the contact area between carbon fiber and oxygen is gradually decreasing, so the oxidation resistance of the composite material is also improved. After oxidation at 1500 ℃ for 10 min, a dense oxide film has been formed on the surface of the sample.  Fig. 8(a)). When the oxidation temperature is 1200 ℃, the m-HfO2 in the sample is the main crystalline phase. As the oxidation temperature increases, it is found that the diffraction peak of m-HfO2 gradually decreases, and the diffraction peak of HfSiO4 gradually increases.
This indicates that m-HfO2 and SiO2 in Cf/SiC/SiHfBOC composites are more likely to react to form HfSiO4 in an oxidizing environment with higher temperatures. The weight retention ratio of the composite material decreases from 96.05 % to 94.85 % ( Fig. 8 (b)), when the oxidation temperature increases from 1200 °C to 1500 °C. With the increase of oxidation temperature, the weight retention ratio of Cf/SiC/SiHfBOC composites gradually decreases, but the decreasing range is small. It shows that as the oxidation temperature increases, the surface dense oxide film of the sample reacts with oxygen gradually becomes severe, so the oxidation resistance of the Cf/SiC/SiHfBOC composites at high-temperature also decreases. The micromorphology of the Cf/SiC/SiHfBOC-7 composites after static oxidation treatment at 1500 ℃ in a muffle furnace for 30min, 60min, 90min, and 120min is shown in Fig. 9. As the oxida-  composed of HfSiO4 、 m-HfO2 、 SiO2 and SiC. In Fig. 10 (b), the weight retention of Cf/SiC/SiHfBOC-7 composites first increase, then decreases, and then increases with the extension of oxidation time, but it generally shows a decreasing trend. On the whole, the weight reduction of Cf/SiC/SiHfBOC-7 composites is very small. According to the analysis in Fig. 9, the Cf/SiC/SiHfBOC-7 composites is seriously oxidized with the prolongation of oxidation time, hightemperature oxidation resistance is gradually weakened. It also indicates that Cf/SiC/SiHfBOC-7 composites still have a high weight retention ratio after long time oxidation at 1500 ℃. However, the weight retention ratio of Cf/SiC/SiHfBOC-7 composites after oxidation at 1500 ℃ for 120 min is roughly the same as that after oxidation for 90 min, indicating that the weight change of the composites after oxidation for 90 min reaches a dynamic equilibrium. The main phases of the Cf/SiC/SiHfBOC composites prepared in this paper mainly include Cf, SiC, HfO2, BCxO3-x, B(SiO)3 and SiOxCy, according to the above data and reference [2,14,15,[28][29][30][31][32].
Therefore, the high-temperature oxidation mechanism of Cf/SiC/SiHfBOC composites are analyzed, and the possible chemical reaction formulas under high-temperature oxidation conditions are as follows: Comparing the Gibbs free energy of each phase composition in Cf/SiC/SiHfBOC composites, it is found that the Gibbs free energy of the reaction between SiC and O2 is the lowest [33,34]. Combined with the morphology, XRD phase diagram, and the above chemical reaction formula under different oxidation conditions, the oxidation behavior can be divided into three parts: 1. The oxidation temperature rises from room temperature to 1100 °C. Due to the difference in thermal expansion coefficients of each phase's composition and the volatilization of gaseous components, it can be seen from the SEM that there are many cracks and pores on the surface of the composites. Although the generated B2O3 has a certain fluidity in this temperature range, it can protect the composites to a certain extent. However, due to the low B element content in the material, the defects of the surface of the composites cannot be healed. When the temperature continues to rise, the generated B2O3 gradually begins to volatilize. The process continues from low temperature to high-temperature, while a small amount of HfSiO4 is generated.
2. The oxidation temperature rises from 1200 °C to 1500 °C. The SiO2 in the composites begin to melt, so the fluidity of the surface of the composites is greatly improved. The cracks on the surface of the composites are gradually healed, leaving only a few holes. At this time, the oxygen channel is gradually blocked, so that oxygen atoms can no longer enter the interior of the composites to react. Therefore, the thickness of the oxide layer decreases, and the generation of HfSiO4 gradually increases with increasing temperature.
3. After oxidation at 1500 ℃ for 30 min, there are many pores on the surface of the composites.
After 60 minutes of oxidation, the amorphous SiO2 is oxidized to crystalline SiO2. However, with the extension of the oxidation time, the peak intensity of crystalline SiO2 gradually weakens, while the content of HfSiO4 gradually increased. After five PIP cycle times, the compressive strength of Cf/SiC/SiHfBOC-5 was 71.97 ± 8.97 MPa in the x/y direction, and 23.92 ± 4.11 MPa in the z-direction. The density of Cf/SiC/SiHfBOC-7 was the highest after seven PIP cycle times, and the compressive strength of Cf/SiC/SiHfBOC-7 increased slightly in the x/y direction and z-direction to 77.56 ± 8.56 MPa and 40.03 ± 5.48 MPa, respectively.

Cf/SiC/SiHfBOC composites
The increase of compressive strength decreases with the increase of PIP cycle times. As shown in Fig.   11, the compressive strength of Cf/SiC/SiHfBOC composites in x/y direction and z-direction is positively correlated with PIP cycle times of the sample. Besides, the compressive property of the samples with the same PIP cycle times in the x/y direction is higher than that in the z-direction. The main reason is that the fiber arrangement mode of the three-dimensional carbon fiber preform is that the twodimensional carbon fiber cloth is arranged along the z-direction, and the z-direction is the needling process.  The compressive strength-strain curve in the x/y direction is obvious changed (Fig. 12 (a)), with the increase of PIP cycle times of Cf/SiC/SiHfBOC composites. It can be seen from the first elastic strain stage that the compressive strength reaches the maximum value when the PIP cycle times is 6, and the composite itself has a large deformation. There are only two stages in the compressive strength-strain curve of Cf/SiC/SiHfBOC composites in the z-direction ( Fig. 12 (b)), namely, elastic stage and yield stage. The reason is that only part of the fiber bundle breaks in the z-direction of the sample under the external load, while the fiber cloth layers perpendicular to the z-direction start to stack without being damaged, so the stress-strain curve of the composite rises continuously.
As shown in Fig. 13 (a)  which is mismatched with the ceramic matrix and begins to debond [35]. In Fig. 14 (b), as the load continues to increase, the crack interface between continuous carbon fiber and ceramic matrix begins to loosen, resulting in stress relaxation. The ceramic matrix is damaged and then spalling occurs. The continuous carbon fiber is exposed, which is called the fiber pull-out phenomenon. When the carbon fiber is pulled out, it will continue to absorb external energy, and absorb the most energy. The continuous carbon fiber begins to fracture (Fig. 14 (c)). In the process of carbon fiber fracture, when the crack extends to the surface of carbon fiber, the carbon fiber will continue to absorb energy and deform, until the critical point of carbon fiber fracture, it cannot continue to absorb energy, and finally fracture. After the fracture, the carbon fiber will recover to the original state, and then release the energy absorbed before. It is indicated that the strengthening and toughening mechanism of Cf/SiC/SiHfBOC composites are mainly composed of fiber debonding, fiber pulling out, and fiber fracture. The debonding and pullout of continuous carbon fibers play an important role in the failure of Cf/SiC/SiHfBOCceramic matrix composites during flexural fracture experiments. The compressive strength in the x/y direction of Cf/SiC/SiHfBOC-7 composites after oxidation at 1100 ℃, 1200 ℃, 1300 ℃, 1400 ℃ and 1500 ℃ for 10 min is 75. 16 (Fig. 15 (a)), respectively. The results show that the compressive strength of Cf/SiC/SiHfBOC composites decreases with the increase of oxidation temperature. The compressive strength of Cf/SiC/SiHfBOC composites decreases with the increase of oxidation temperature. The compressive strength decreased to 96.90 % after oxidation at 1100 ℃ and 51.03 % after oxidation at 1500 ℃., but it still has high mechanical properties after a short time of high-temperature oxidation. Three stages of the stress-strain curve in the x/y direction can be observed ( Fig. 15 (b)). In the first elastic stage of all composites, the compressive strength is the largest and the deformation is the smallest, when the oxidation temperature is 1100 ℃.  Fig. 16 (b) shows the stress-strain curves of Cf/SiC/SiHfBOC-7 composites after oxidation at 1500 ℃ for a different time.
Three stages of the stress-strain curve in the x/y direction can be seen ( Fig. 16 (b)), with the increase of oxidation time. From the first elastic stage, when the oxidation time is 30 min, the compressive strength reaches the maximum value, and the sample itself has a large deformation. Moreover, the strain range of yield point in the first stage is between 2 % and 7 %. After reaching the yield point, stress yield begins to occur.

Conclusion
In this paper, the on-demand preparation of Cf/SiC/SiHfBOC composites is realized, to control the microstructure and properties of the composites. When the PIP cycle times increased from 3 to 7, the density and porosity of the composites increased from 1.02 g/cm 3 and 52.53 % to 1.40g/cm 3 and 28.87 %, respectively. And the compressive strength in the x/y and z directions increased from 31.32 ± 6.12 MPa and 9.31 ± 1.23 MPa to 77.56 ± 8.56 MPa and 40.03 ± 5.48 MPa, respectively. The strengthening and toughening methods mainly include fiber debonding, fiber pulling out, and fiber breaking. The products of Cf/SiC/SiHfBOC composites after the static oxidation test mainly include CO2, CO, B2O3, SiO2, HfSiO4, etc. According to the analysis of compressive stress-strain curve, oxidation weight retention rate, and oxidation surface SEM, it was found that the weight retention and compressive strength of Cf/SiC/SiHfBOC-7 sample were 83.97 % and 23.88 ± 3.11 MPa respectively after being oxidized at 1500 ℃ for 60 min, which further proved that the Cf/SiC/SiHfBOC composites could be used in oxidation environment at 1500 ℃ for a long time.