Another promising approach is to modify C/C composites with UHTCs or Si-based ceramics. Instead of fabricating the protective coating on the surface of C/C, the aim of matrix modification is to introduce the ceramics into the C/C matrix, which could couple the excellent oxidation/ablation resistance of UHTCs or Si-based ceramics with the good fracture toughness of C/C [24, 25]. Processing methods have been carried out on the fabrication of C/C-UHTCs, such as chemical liquid-vapor deposition , chemical vapor infiltration , precursor infiltration and pyrolysis , carbo-thermal reduction reaction , slurry infiltration [24, 29, 30], hot pressing , microwave hydrothermal  and reactive melt infiltration . In addition, two or more methods (see above) are often combined to facilitate the preparation of the modified C/C composites. Among these preparation methods, chemical vapor infiltration, precursor infiltration and pyrolysis, reactive melt infiltration and slurry infiltration are often used, which are described as follow.
(1) Chemical vapor infiltration (CVI)
This process involves complex physico-chemical phenomena, such as the transport of precursors, carriers, and by-product gases in the reactor . The prepared composite possesses well-controlled composition and microstructure, good mechanical and anti-ablation properties. By using TaCl5-CH4-H2-Ar, HfCl4-TaCl5-CH4-H2-Ar and HfCl4-TaCl4-CH4-H2-Ar as the gas sources, C/HfC, C/TaC and C/HfC/TaC composites were fabricated . Although this method could prepare high-quality materials, it is expensive and time-consuming.
(2) Precursor infiltration and pyrolysis
Precursor infiltration and pyrolysis (PIP) is the method used in the preparation of ceramic matrix composites, which comprises an infiltration of a low viscosity polymer into the fiber preforms followed by pyrolysis: heating the polymer precursor in the absence of oxygen when it decomposes and converts into a ceramic. It could introduce variable kinds of ceramics and benefit for achieving near net shape manufacturing. Based on this method, C/C-HfC , C/C-ZrB2-SiC  and C/C-ZrC-SiC  were successfully prepared. The major disadvantage of PIP is time-consuming, and that matrix is easy to shrink during pyrolysis, which may result in formation of cracks and pores.
(3) Reactive melt infiltration
Reactive melt infiltration (RMI) is used to introduce carbide or boride ceramics into composites through the reaction between molten metal mixtures and C/C composites. With respect to CVI and PIP, it is a cost-effective and time-saving method. For example, Wang et al. used Si0.87Zr0.13 alloy as the raw material. When heat-treated at high temperatures, the molten Si0.87Zr0.13 alloy was infiltrated into the C/C to form C/C–SiC–ZrC composites . But the inevitable reactions between the molten mixture and carbon fibers resulted in the mechanical property degradation.
(4) Slurry infiltration
In slurry infiltration (SI), the precursor is a slurry which could infiltrate into fiber preforms. It is also convenient to introduce different ceramics into C/C according to requirements. Sun et al. used this method to introduce ZrSiO4 into C/C . However, the agglomeration of ceramics particles usually blocks the `pores of fiber preforms and then makes it more difficult for the subsequent densification.
Purpose of this study
As discussed above, coating technology and matrix modification are the two efficient methods to enhance the oxidation and ablation performances of C/C. They are also the focuses of our work. The novelty of our study is described as follow and compared to the reported literature.
For the coating technology, up to now, many different types of protective coatings, such as Si-Mo-Cr , mullite/SiC , AlPO4/SiC , BN/SiC/Si3N4-ZrO2-SiO2 , ZrB2-SiC/SiC , have been developed. However, the CTE (coefficient of thermal expansion) mismatch (see Table 1-1) between the coating and C/C substrate easily results in the generation of cracks during high-low temperature cycles, which would provide the entrance channels for oxygen . Evans et al.  have discussed the interface degradation mechanism, indicating that the large CTE difference will lead to the increase of internal stress, resulting in the degradation. As a result, the CTE mismatch is the key factor that results in the coating performance degradation.
To relieve the CTE mismatch between the coating and substrate, many attempts have been performed. The introduction of a second phase into the coating is the most widely adopted strategy to tackle this problem so as to improve the toughness of the coating considerably, where carbon nanotubes (CNTs) and SiC nanowires have been used [46, 47]. The introduction of a second phase proves to be ineffective for the improvement of the coating/substrate interface. Feng et al.  use low-density C/C composites (1.2 g/cm3) to make use of their porous structures, which could provide the diffusion paths to the coating raw materials and then contribute to the increase coating/substrate interfaces. These interfaces could promote the oxidation performance of the produced coating. However, to ensure the thermal-structural components with favorable mechanical performances, C/C composites with enough density (>1.7 g/cm3) and low porosity are required. Thus, this method is difficult to adopt in practical applications. In light of this problem, as an alternative method, pre-oxidation treatment of C/C was developed to construct a preferential interlocking transition coating/substrate interface . The results showed that the oxidation treated region of C/C composites was difficult to control, and the treatment time was relatively longer, resulting in the inevitable damage of the mechanical performance of C/C .
As a result, finding a simpler, faster and more efficient way to construct an interlocking coating/substrate interface on high-density C/C (in the premise of minimum mechanical property loss) is becoming particularly important. So, in this study, blasting treatment of C/C is proposed. In addition, in actual environment, a non-destructive method is very important for the coating structure characterization and its service reliability evaluation. So in this thesis, Kossel interferences of proton-induced X-ray emission combined with X-ray reflectivity measurement, as a novel characterization method, is developed.
For the matrix modification, based on the above fabrication methods (discussed in section 1.4), a series of composites, such as C/C-HfC , C/C-ZrC-ZrB2-SiC , C/C-ZrC-SiC [51-53], and C/C-ZrB2-SiC , have been successively developed. It is evident that although a considerable amount of studies have been performed on the development of C/C-UHTCs composites, much of them are focused on HfC and Zr-based ceramics, with little consideration of HfB2 . HfB2 as one of the family of UHTCs, also possesses high melting point (3250°C) and good oxidation/ablation resistance . When the boron oxide layer (the oxidation product of HfB2) is sufficiently fluid, it could also cover the surface and act as an efficient barrier to restrict the inward diffusion of oxygen. HfB2 is thus a promising candidate to improve the ablation properties of C/C composites.
A. Paul et al. fabricated C/C-HfB2 by slurry infiltration [24, 29, 30]. However, the agglomeration of the HfB2 particles easily blocked the pores in the outer layer of the C/C preforms and then made it difficult for the successive densification. To ensure the service reliability of C/C-HfB2 composites, it is of great significance to resolve the problem of agglomeration and find out the valid dispersion method of HfB2. So in this study, PIP is adopted to introduce HfB2 into C/C matrix. Compared with slurry infiltration, this method possesses a larger infiltration depth, which can be expected to improve the uniformity of HfB2 and realize the net shape manufacturing.
According to the discussion above, the main contents of the PhD work are listed as follow:
(1) Effect of blasting treatment of the C/C composites on microstructure, adhesive strength, oxidation/ablation performance of the ceramic coating (chapter 3).
(2) C/C blasting treatment combined with a second phase introduction to enhance the oxidation/ablation performances of the ceramic coating (chapters 3 and 4).
(3) Feasibility of Kossel interferences of proton-induced X-ray emissions combined with X-ray reflectivity, as a novel non-destructive characterization method, where X-ray planar waveguides are designed (chapter 5).
(4) Preparation of HfB2 and HfB2-SiC modified C/C composites. Effect of HfB2 and HfB2-SiC on the oxidation/ablation performances of C/C composites are studied (chapter 6).
In this chapter, we will give a detailed description of the raw materials, fabrication apparatus and characterization methods used in this thesis.
2D carbon fiber felt was used as the reinforcement for C/C composites, as shown in Figure 2-1. The preform is made up of 90° weftless ply, short-cut fibre web and 0° weftless ply (see Figure 2-1 (a)). They are alternatively stacked with a needle punching technique (Figure 2-1 (b-c)). The fiber volume content of the preform is about 20–25%. C/C composites are prepared through the densification of the carbon fiber preform by thermal gradient chemical vapor infiltration (TCVI). During this process, CH4 is used as the precursor with a flow rate of 4-8 L/min. The deposition time, temperature and pressure were 100-120 h, 1050-1150oC and 5-10 kPa, respectively. The final density of the C/C composites was about 1.69-1.75g/cm3. Morphology and phase composition of the C/C are shown in Figure 2-2. They consist of a single carbon element (Figure 2-2 (b)), and carbon fiber is surrounded by pyrolytic carbon matrix (Figure 2-2 (a)).
Figure 2-1. Schematic illustration of needled integral felts including a needling process: (a) position of weftless ply and short-cut fibre web; (b) detailed view of a Z-fibre bundle generated during the needling process; and (c) needling process .
(1) High-temperature heat treated furnace: the maximum temperature is 2300 °C.
(2) Cutting machine: MODEL150.
(3) Ball mill: PM-4L, rotation speed <600 rpm.
(4) Supersonic cleaner: KQ-100A.
(5) Drying oven: 101A-2, the maximum temperature is 200 °C.
(6) Analytical balance: TG328B, accuracy ±0.0001 g.
(7) Universal mechanical test machine: CMT5304-30 kN.
(8) Thermal expansion test: DIL402C and DIL402E Dilatometer.
(9) Infrared radiation thermometer: Raytek MR1SCSF, accuracy of 0.75%.
(10) X-ray diffraction (XRD): Philips X’Pert MPD diffractometer.
(11) Thermal analysis: Mettler Toledo Star TGA/SDTA 851.
(12) Confocal laser scanning microscope : C130, Lasertec Corp., Yokohama, Japan.
(13) Scanning electron microscopy (SEM): VEGA 755136XM.
(14) Thermal cycling and ablation test: OA-Ш oxyacetylene ablation machine.
(15) Film deposition: magnetron sputtering.
(16) Système d’Analyse par Faisceaux d’Ions Rapides (analysis system with high speed ion beams) platform in Sorbonne University.
(17) ANDOR iKon-M energy dispersive CCD camera
Blasting treatment of C/C composites to construct a porous surface
Cylinder specimens (Ø10 mm × 10 mm) used as substrates were cut from the prepared C/C composites (described in section 2.1). They were hand-abraded with 100 and 400 grit SiC papers in turn, then cleaned ultrasonically with ethanol and dried at about 80°C for 2 h. A porous surface of C/C was achieved rapidly through blasting treatment using oxyacetylene torch. The flame was vertical to the C/C, as shown in Figure 2-3. In our study, C/C was treated in three different heat fluxes (2.38, 3.2 and 4.18 MW/m2) . The parameters are listed in Table 2-1. During blasting treatment, the gas pressures of O2 and C2H2 were kept constant, and the heat flux of the oxyacetylene torch was adjusted through changing the flow ratio of O2 and C2H2. The treatment time was 20-30 s.
To investigate the effect of blasting treatment, SiC coating was prepared on the treated C/C (obtained in section 2.3.1) by pressure-less reaction sintering. Powder compositions were as follows: 65-80 wt.% Si (300 mesh), 10-25 wt.% graphite (325 mesh) and 5-15 wt.% Al2O3 (300 mesh). The role of Al2O3 was used to increase the rate of diffusing reaction at high temperature. The powders were mixed together in an agate vial and stirred for 8 h in a ball mill (PM-4L) with the speed of 500 rpm. Then the mixed powders and the treated C/C specimens were put in a graphite crucible and heated to 1750-2100°C and held at that temperature for 1-3 h in argon atmosphere.
(2) Si-Mo-Cr coating
Figure 2-4 shows the process of blasting treatment and preparation of Si-Mo-Cr coating. Firstly, blasting treatment of C/C composites was conducted (described in section 2.3.1). Then, Si-Mo-Cr coating was prepared by pressure-less reactive sintering. Powder compositions were as follows: 45-60 wt.% Si (300 mesh), 25-30 wt.% MoSi2 (200 mesh), 5-15 wt.% Cr (200 mesh) and 8-15 wt.% graphite (200 mesh). The powder mixtures and C/C specimens were mixed together and in turn put in a graphite crucible, and then heat treated at 1750-2100°C for 1-3 h in argon atmosphere.
Figure 2-4. Schematic illustration of blasting treatment of C/C and preparation of the Si-Mo-Cr coating.
(3) SiC nanowires toughened Si-Mo-Cr coating
Figure 2-5 shows the schematic illustration of blasting treatment of C/C, in situ grown SiC nanowires and preparation of Si-Mo-Cr coating. It was divided into three steps. The first step was to construct a porous surface on C/C via blasting treatment (details are in section 2.3.1). In the second step, in situ growth of SiC nanowires was prepared on the surface of the treated C/C composites by chemical vapor deposition (CVD). Mixed powders of SiO2, Si and graphite were placed on the bottom of a crucible. Then, the treated C/C composites were placed above the mixed powders and heat treated in an argon atmosphere to form the SiC nanowires. Finally, Si-Mo-Cr coating was prepared by pressure-less reactive sintering (see in Figure 2-4).
Figure 2-5. Schematic illustration of C/C blasting treatment, in situ grown SiC nanowires and preparation of Si-Mo-Cr coating.
(4) HfC-SiC coating
The procedure of constructing the porous surface on C/C composites and the preparation of HfC-SiC coating is shown in Figure 2-6.
Figure 2-6. Schematic illustration of C/C blasting treatment and the preparation of HfC-SiC coating.
In the first step, a porous surface was constructed on C/C. Then the HfC-SiC coating was prepared on the pre-treated C/C composites by pressure-less reactive sintering. Powder compositions for pressure-less reactive sintering were as follows: 45-65 wt.% Si (300 mesh), 10-15 wt.% HfC (400 mesh) and 8-30 wt.% graphite (300 mesh). The powders were mixed uniformly in an agate vial after being stirred for 12 h in PM-4L ball mill with a speed of 500 rpm. Then, the obtained powder mixtures and the pre-treated C/C specimens were put in a graphite crucible, which were heated to 1800-2300oC and held for 1-4 h in an argon atmosphere.
(5) ZrB2-SiC coating
The procedure of constructing the porous surface on C/C and the preparation of ZrB2-SiC coating is shown in Figure 2-7. The process is the same as that of HfC-SiC coating. ZrB2-SiC coating was prepared on the pre-treated C/C composites by pressure-less reactive sintering. Powder composition was as follow: 45-65 wt.% Si (300 mesh), 10-15 wt.% ZrB2 (300 mesh) and 8-30 wt.% graphite (300 mesh).
Precursor infiltration and pyrolysis (PIP) is a common method to introduce ceramics into C/C preforms. It usually consists of two processes: infiltration of a low viscosity precursor and pyrolysis at high temperature. In our study, C/C-HfB2 was prepared by PIP, as shown in Figure 2-10. In the first step, the carbon fiber felt (described in section 2.1) was densified to 1.0-1.1 g/cm3 by isothermal chemical vapor infiltration (ICVI) process. The ICVI temperature and time were set at 1000-1150°C and 30-40 h, respectively. Then HfB2 was introduced into C/C composites by PIP. Details of PIP are given in Figure 2-11. A solution of organic hafnium boride polymer and xylene was used as the precursor. The obtained low-density C/C composites in the first step were put in an airtight container. The container was evacuated (pressure lower than 6 kPa). Because of the pressure difference inside and outside of the container, the liquid precursor was inhaled and immersed the prepared C/C samples for 1-2 h. Then the samples were dried at 90 -100°C in air for 24 h. After that, the samples were put in a graphite crucible and held at 1500-1800oC for 1-4 h in argon atmosphere (shown in Figure 2-10). The above process was repeated until the density increased to 1.3-1.4 g/cm3. After that, the obtained samples were further densified by pyrolysis carbon through TCVI. During this process, CH4 was used as the carbon source. Finally, the prepared composites were graphitized at 2100-2300oC for 2 h in argon atmosphere. The density of the prepared C/C.
(2) Preparation of C/C-HfB2-SiC composites
The preparation process of C/C-HfB2-SiC composites is similar to that of C/C-HfB2. The main difference is the sample shape and the PIP precursor. In the first step, the carbon fiber felt (described in section 2.1) was densified to 1.0-1.1 g/cm3 by ICVI. Then the composites were machined into nose shape to simulate the thermal-structural components used in actual environment, as is shown in Figure 2-12 (a-b). The nose-shaped sample is made up of two parts: a hemisphere (the radius is 8 mm) and a cylinder (the diameter is 12 mm and the height is 6 mm). Then a mixed solution of organic hafnium boride polymer and polycarbosilane was prepared, which was dispersed in dimethylbenzene with a weight ratio of 1:1. The solution was used as the precursor to introduce HfB2-SiC into C/C. The final density of the composites was 1.94-2.03 g/cm3 after ten PIP cycles.
Figure 2-12. Schematic diagram of the prepared nose-shaped C/C-HfB2-SiC composite (a) and the corresponding dimensions (b).
(1) Single ablation test
Ablation behavior was investigated using oxyacetylene torch. The experiment setup consisted of oxygen and acetylene tanks, an oxyacetylene gun, a control cabinet and a sample stage. The inner diameter of the oxyacetylene gun tip was 2 mm, and the corresponding distance to the sample was 10 mm. Two different heat fluxes (2.38 and 4.18 MW/m2) were used. Detailed parameters of the heat flux were described in section 2.3.1. During ablation, surface temperature was measured by a two-color pyrometer (Raytek MR1SCSF, accuracy of 0.75%). The linear and mass ablation rates of the specimens were obtained according to the formula (2-1) and (2-2).
where Rl is the linear ablation rate; d1, d2 are the thickness of the sample center before and after ablation; Rm is the mass ablation rate; m1, m2 are the sample mass before and after ablation; t is the ablation time. The final ablation value is the average of the measurements over three specimens.
(2) Cyclic ablation test
Cyclic ablation was conducted using vertical and parallel oxyacetylene torch, as shown in Figures 2-13 and 2-14. Surface temperature of the sample was measured by the Raytek MR1SCSF thermometer. In our study, two cyclic ablation tests (1600oC to room temperature and 1750oC to room temperature) were performed. Gas fluxes of O2 and C2H2 were 0.88 and 0.65 m3/h. The ablation temperature was controlled through the adjustment of the distance between the oxyacetylene torch and the sample.
Table of contents :
Chapter 1: Introduction
1.1 Carbon/carbon composites
1.2 Application of carbon/carbon composites
1.3 Coating technology
1.4 Matrix modification
1.5 Purpose of this study
Chapter 2: Experimental methods
2.1 Carbon/carbon composites
2.2 Experimental apparatus
2.3 Sample fabrication
2.3.1 Blasting treatment of C/C composites to construct a porous surface
2.3.2 Coating preparation on blasting-treated C/C composites
2.3.3 Thin film waveguides preparation on Si substrate
2.3.4 HfB2 and HfB2-SiC modified C/C composites
2.4.1 Ablation test
2.4.2 Adhesive strength test
2.4.3 PIXE-Kossel experiment
2.4.4 X-ray reflectivity measurement
2.4.5 Microstructure and phase composition
2.4.6 Thermophysical analysis
Chapter 3: The effects of C/C blasting treatment
3.2 Surface modification of C/C via blasting treatment
3.3 Blasting treatment on the cyclic ablation performance of Si-Mo-Cr coating
3.4 Blasting treatment combined with SiC nanowires to enhance the cyclic ablation performance of Si-Mo-Cr coating
3.4.1 Microstructure and cyclic ablation test
3.4.2 Ablation mechanism
Chapter 4: The effects of C/C blasting treatment and modifying SiC coating with SiC/HfC (ZrB2)
4.2 Blasting treatment and modifying SiC coating with SiC/HfC additive
4.2.1 Microstructure and cyclic ablation test
4.2.2 Ablation mechanism
4.3 Blasting treatment and modifying SiC coating with SiC/ZrB2 additive
Chapter 5: Develop a non-destructive characterization method for multilayer ceramic coating
5.2 Particle-induced X-ray emissions (PIXE)
5.3 Kossel interferences
5.4 X-ray waveguides
5.5 PIXE-Kossel to study thin film waveguides
5.5.1 Design of the thin film waveguides
5.5.2 Results and discussion
5.6 Design and simulation of the HfC/SiC/HfC multilayers
Chapter 6: HfB2 and HfB2-SiC modified C/C composites
6.1 HfB2 modified C/C composites
6.1.2 Microstructure and ablation test
6.1.3 Oxidation and ablation mechanism
6.2 HfB2-SiC modified C/C composites
6.2.2 Microstructure and ablation test
6.2.3 Ablation mechanism
Chapter 7: Conclusion and Perspectives