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Carbon/carbon composites
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)).
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) [58]. 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.
Coating preparation on blasting-treated C/C composites
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.
Thin film waveguides preparation on Si substrate
Three thin films waveguides were designed: Pt(4 nm)/Fe(6 nm)/Pt(10 nm)/Si, Pt(4 nm)/Fe(8 nm)/Pt(10 nm)/Si and Ta(4 nm)/Cr(10 nm)/Pt(12 nm)/Si, as shown in Figure 2-8. The designed thin films were deposited at room temperature on a Si (100) substrate by magnetron sputtering, as shown in Figure 2-9. The power applied on the sputtering targets was 10 W, the base pressure was 10-8 mbar, and the sputtering gas was argon at the working pressure of 5×10-2 mbar. The sputtering rates of Pt, Fe, Cr and Ta were 0.066, 0.024, 0.024 and 0.035 nm/s, respectively, as measured by a quartz microbalance.
HfB2 and HfB2-SiC modified C/C composites
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- HfB2 composites was about 1.77-1.84 g/cm3.
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.
X-ray reflectivity measurement
To know the actual thickness and roughness of the deposited stacks, X-ray reflectivity (XRR) measurement is conducted. XRR is widely employed to study thin films and multilayers. In this study, the XRR measurements were performed at the 0.1542 nm wavelength on a Rigaku five-circle diffractometer. Then with the help of the IMD software [60] and the optical constants of the CXRO [61], the measured XRR curves were fitted to determine the thickness, roughness and density of the various layers.
Microstructure and phase composition
The microstructures and morphologies of the samples are analyzed by scanning electron microscopy, equipped with energy dispersive spectrometry (EDS). The phases are analyzed by X-ray diffraction with a Cu Kα radiation (λ=0.1542 nm) from an x-ray tube operating at 40 kV and 35 mA. With the help of a confocal laser scanning microscope, the average surface roughness (Ra) of C/C before and after blasting operation was measured.
Thermophysical analysis
Thermogravimetric analysis was carried out in air condition by thermal analysis apparatus. Coefficient of thermal expansion (CTE) was measured by a dilatometer (see section 2.2).
Table of contents :
Contents
Acknowledgements
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 Characterization
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
3.1 Introduction
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
3.5 Conclusions
Chapter 4: The effects of C/C blasting treatment and modifying SiC coating with SiC/HfC (ZrB2)
4.1 Introduction
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
4.4 Conclusions
Chapter 5: Develop a non-destructive characterization method for multilayer ceramic coating
5.1 Introduction
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
5.7 Conclusions
Chapter 6: HfB2 and HfB2-SiC modified C/C composites
6.1 HfB2 modified C/C composites
6.1.1 Introduction
6.1.2 Microstructure and ablation test
6.1.3 Oxidation and ablation mechanism
6.2 HfB2-SiC modified C/C composites
6.2.1 Introduction
6.2.2 Microstructure and ablation test
6.2.3 Ablation mechanism
6.3 Conclusions
Chapter 7: Conclusion and Perspectives
Annex
Bibliography
