Antioxidant modification of C/C composites by in situ hydrothermally synthesized 4ZnO·B₂O₃·H₂O

2.1 Antioxidant modification

A total of 1.5 mmol of Zn(NO₃)₂·6H₂O (99.5% purity) and 4.5 mmol of H₃BO₃ (99.5% purity) were dissolved in 15 mL of deionized water. The pH value of the solution was adjusted to 9 by adding 5 mol/L of NaOH (99.5% purity) to obtain the hydrothermal precursor solution.

C/C composites with a density of 1.50 g/cm³ were cut and polished into 4×4×4-cm³ bulks. After ultrasonic cleaning with absolute alcohol for 30 min and vacuum drying at 40°C for 2 h, each C/C sample was put into a hydrothermal vessel with the precursor solution and the volume filling ratio was set to 2:3. Then the vessel was put into an electric thermostatic drying oven. After 4 h, the reaction vessel was taken out to cool it down naturally to room temperature. The as-modified samples were washed several times with deionized water and absolute alcohol, respectively, with a later drying at 40°C for 2 h in a vacuum drying oven to obtain the as-modified C/C specimens.

2.2 Characterization

The surface patterns of products were obtained using an X-ray diffractometer (D/max 2200PC, Rigaku, Akishima, Japan) operating with Cu Kα radiation (λ=0.15406 nm) to determine the phase composition. Surface images of the as-prepared samples were observed using a scanning electron microscope operating at 20 kV (SEM 525-M, Philips, Eindhoven, Netherlands). The isothermal oxidation test was used to determine the oxidation resistance of the as-modified C/C composites, and the average weight loss of a group of samples (five samples per group) was measured and calculated every 1 h. The average weight loss ΔW (g/cm³) was calculated according to Equation 1, where m₀ is the initial mass of the as-modified C/C composites, m is their mass after the isothermal oxidation test and ν₀ is the initial volume of the as-modified C/C composites. Concurrently, a thermogravimetry/differential scanning calorimetry (TG-DSC) apparatus was used to test the thermal stabilities of the as-prepared 4ZnO·B₂O₃·H₂O crystallites in air at 30–600°C with a heating rate of 10°C/min.

(1)ΔW=(m0-m)/ν0 (1)

3.1 Surface XRD analysis

Figure 1 shows the surface X-ray diffraction (XRD) patterns of the modified and non-modified C/C composites. The surface XRD pattern of the non-modified C/C composite shows that this kind of carbon fiber-reinforced carbon composite is composed of a carbon phase and a graphite phase. All the surface XRD patterns from the modified C/C composites can be indexed to the mono-phase of the monoclinic crystal structure of 4ZnO·B₂O₃·H₂O without the exposure of C/C composite XRD patterns, indicating that this 4ZnO·B₂O₃·H₂O crystal layer is dense. Obviously, 4ZnO·B₂O₃·H₂O can be obtained at the temperature range of 100–180°C. In addition, it can be seen that all the XRD diffraction peaks slightly shifted to the low 2θ angle, which implies that the interplanar distance expands to some degree with the increase in hydrothermal temperature according to the Bragg equation (2d sinθ=nλ). It indicates that higher hydrothermal temperature may lead to tensile stress in lattice anisotropy during the crystallization, which may result in the crystalline arrangement on the surface of C/C composites.

Figure 1

Surface XRD patterns of the C/C composites modified at different hydrothermal temperatures (pH 9, t=4 h) and the non-modified C/C composite.

3.2 Surface SEM analysis

The surface SEM images of the as-modified C/C composites treated at different hydrothermal temperatures are shown in Figure 2. Clearly, the coatings show greatly different morphology after modification at different hydrothermal temperatures, in correspondence to the XRD analysis. From the details, using an enlarged scale, it can be seen that the grain changes may indicate that higher hydrothermal temperature leads to tensile stress in lattice anisotropy during the crystallization and may result in the crystalline arrangement on the surface of C/C composites. A smooth and crack-free 4ZnO·B₂O₃·H₂O coating was achieved when the hydrothermal temperature reached 140°C. When the hydrothermal temperature reached 160–180°C, microcracks started to appear, which may have resulted from the increase in coating thickness and a mismatch in thermal expansion coefficient between the zinc borate (8.0×10^-6 K^-1) ²⁴ coating and the C/C composite (1–2×10^-6 K^-1) ²⁵. Because the surface XRD analysis showed that the layer is composed only of 4ZnO·B₂O₃·H₂O crystals, and because no other elements (except for Zn, B and O) are observed in Figure 2G, the “white deposits” in Figure 2E are larger 4ZnO·B₂O₃·H₂O crystal particles formed from the secondary crystallization during the 180°C hydrothermal process.

Figure 2

Surface SEM images of the C/C composites modified at different hydrothermal temperatures (pH 9, t=4 h) and the non-modified C/C composite: (A) T=100°C; (B) T=120°C; (C) T=140°C; (D) T=160°C; (E) T=180°C; (F) non-modified C/C composite; (G) energy dispersive spectra of the white deposits.

3.3 Isothermal oxidation test

The isothermal oxidation curves of the modified C/C composites are shown in Figure 3. It can be seen that the modified composites showed better oxidation resistance when compared with the non-modified composites. The C/C composite modified at 140°C exhibited the best oxidation resistance, which may have resulted from the smoother and crack-free morphology of the 4ZnO·B₂O₃·H₂O layer. After oxidation in air at 600°C for 15 h, the weight loss of the modified composites was only 1.93 g/cm³, while that of the non-modified composite was 5.20 g/cm³ when oxidized in air at 600°C for 10 h. The improvement in oxidation resistance is obvious.

Figure 3

Isothermal oxidation curves of the C/C composites modified at different hydrothermal temperatures in air at 600°C.

At the temperature range of 100–180°C, higher hydrothermal temperature will accelerate the crystallization of 4ZnO·B₂O₃·H₂O and it will also speed up the diffusion rate of ions, which is conductive to fill the defects of the C/C matrix, which may improve the antioxidant property of the coating for the C/C matrix. However, too high of a temperature (160–180°C) will result in the increase in coating thickness and decrease in oxidation resistance.

Figure 4 shows the surface SEM images of the as-modified C/C composite samples after oxidation in air at 600°C wherein agglomeration of grains can be observed. The microholes observed may be generated by agglomeration, and oxygen could come in contact with the carbon through them, which accelerates the oxidation. It can be concluded that if grain agglomeration could be prevented in further studies, the oxidation resistance of this 4ZnO·B₂O₃·H₂O coating for C/C composites may be improved.

Figure 4

Surface SEM images of the as-modified C/C composites at different hydrothermal temperatures after oxidation in air at 600°C (pH 9, t=4 h): (A) T=100°C; (B) T=120°C; (C) T=140°C; (D) T=160°C; (E) T=180°C.

Figure 5 shows the cross-section SEM images of the as-modified C/C composite samples after oxidation in air at 600°C. It can be seen that, at the temperature range of 100–180°C, the coating thickness increases (from 100 to 200 μm) that enlarged the diffusion distance of oxygen to the C/C substrate may help improve the antioxidant protection of the coating for C/C composites. The formation of microcracks may have resulted from the quick cooling down of the samples from 600°C to room temperature during the isothermal oxidation test. In addition, during the said test, oxygen would diffuse into the interface of the coating and C/C matrix through the microcracks when the samples were out of the furnace, which led to the oxidation of the matrix. The escape of CO and CO₂ led to the formation microholes in the surface layer. Therefore, the failure of the coating here was attributed to the thermal shock of the samples when going from high temperature to room temperature, although this observation needs further research.

Figure 5

Cross-section SEM images of the as-modified C/C composites at different hydrothermal temperatures after oxidation in air at 600°C (pH 9, t=4 h): (A) T=100°C; (B) T=120°C; (C) T=140°C; (D) T=160°C; (E) T=180°C.

Undoubtedly, the improvement in oxidation resistance of C/C composites comes from the 4ZnO·B₂O₃·H₂O coating. By TG-DSC analysis (Figure 6), it was found that the as-prepared 4ZnO·B₂O₃·H₂O crystallite coating showed good thermal stability at temperatures up to 600°C. A small weight loss was detected, which was attributed to the loss of one molecular crystal water of 4ZnO·B₂O₃·H₂O at about 520°C. The dehydration temperature is different from that in Ref. [19] (415°C). The affecting factor may be the different combinations of H₂O in the 4ZnO·B₂O₃·H₂O crystal. But further study is needed. This coating prevents the interaction between oxygen and C/C composite at the initial stage of oxidation. The coating itself contributes to preventing the diffusion of oxygen into the C/C composite. And the loss of crystal water also helps absorb heat to slow down the oxidation of C/C composites. These results indicate that 4ZnO·B₂O₃·H₂O can be used to improve the oxidation protection of C/C composites.

Figure 6

TG-DSC curves of the as-prepared 4ZnO·B₂O₃·H₂O crystallite coating synthesized by the hydrothermal process (pH 9, T=140°C, t=4 h).