Microstructure, Performance And Irradiation Behavior Of PIP-SiC/SiC Composites

Fusion is the only known technology capable in principle of producing a large fraction of the world’s energy. In a fusion reactor, the blanket is the first structure surrounding the plasma, which must be capable to withstand extremely high neutron and thermal fluxes together with the primary mechanical loads. Continuous Si C fiber reinforced Si C matrix composites(Si C/Si C) have been considered as promising candidate materials for both structural and functional applications in fusion power reactors. In this thesis, different interfaces were synthesized on KD-I Si C fibers and Si C/Si C composites were fabricated by Polymer Infiltration and Pyrolysis(PIP) process with different precursors. The microstructure and properties of the composites were characterized. The effects of heat treatment on the microstructure and properties of the Si C/Si C composites were investigated. Electron irradiation experiments on the Si C/Si C composites were carried out. Tungsten coating was fabricated on the Si C/Si C composite substrates and the microstructure and properties of the coating were studied.The microstructural evolutions of the interfaces during the fabrication processes were observed. Pyrocarbon(Py C) coating and Carbon nanotubes(CNTs) interface were synthesized by chemical vapor deposition(CVD) process. The Py C coating was dense, smooth and exhibited turbostratic graphite structure. The coating was in fair preservation after the fabrication of the composites and acted as a mechanical fuse. The CNTs were dense and uniform. However they were entrapped in the matrix during the fabrication process of the composites, resulting in the direct bonding of the Si C matrix and the fibers, which was unsatisfactory for the crack deflection.The effects of heat treatment on the microstructure and properties of the polymer derived Si C ceramics were investigated. PCS derived Si C(PCS1100) consisted of microcrystalline β-Si C, free carbon and amorphous Si-C-O phases. The quantity and dimension of the Si C crystalline increased with increasing of heat treatment temperature. Stacking faults formed and the free carbon phase gradually got ordered at 1600 °C, a large quantity of turbostratic graphite formed at 1800 °C(PCS1800). LPVCS derived Si C(LPVCS1100) was amorphous, microcrystalline β-Si C formed after heat treatment at 1400 °C. The ceramic became more crystalline with elevation of heat treatment temperature, and the amorphous phase disappeared at 1800 °C(LPVCS1800).The mechanical properties of the Si C/Si C composites with different interfaces and precursors were studied. Si C/Si C composites fabricated with Py C as the fiber coating(PL1100 and PP1100) exhibited excellent mechanical properties. The bending strength and fracture toughness of PL1100 and PP1100 were 520.6 MPa, 24.8 MPa·m1/2 and 521.2 MPa and 22.9 MPa·m1/2. The mechanical properties of PP1100 were improved by the consolidation process due to the elimination of pores pores along the fiber segments and increase of fiber volume fraction by the pressurization. In situ SEM observation of crack growth in the Si C/Si C composites was carried out. In Si C/Si C composite fabricated with CNTs as the interface layer(CL1100), the crack initiated at the notch tip, and continued to grow through the fibers, the final failure was caused by the very crack. In PL1100 and PP1100, the cracks initiated near the notch and were deflected by the interfaces, besides the main crack, lots of secondary cracks can be found all over the specimen.Closed pores and damage development of the Si C/Si C composites were studied. Closed porosities of CL1100 and PP1100 were calculated after 3D visualization. Si C/Si C composites fabricated with LPVCS as the precursor exhibited low porosity and high densification efficiency. High-resolution synchrotron X-ray tomography was applied to PP1100 combined with Hertzian indentation followed by image-based modeling of the composite. Agreement between Digital Volume Correlation(DVC) analysis and Cellular Automata Finite Element(CAFE) simulation is good. CAFE method exhibited better sensitivity than conventional FEM method.The effects of heat treatment temperature on the microstructure and properties of the Si C/Si C composites were investigated. After heat treatment at 1400 °C, the mechanical properties of PP1100 were improved, the flexural strength was 576.7 MPa and the fracture toughness was 25.7 MPa·m1/2. The improvement of mechanical properties could be attributed to the undegraded fibers and the improved matrix because of crystallisation. The density and mechanical properties of PP1100 decreased noticeably when the heat treatment temperature exceeded 1600 °C. The density and mechanical properties of PL1100 decreased with increasing of heat treatment temperature. Nanoindentation results indicated that the mechanical properties of both the fibers and matrix decreased with elevation of heat treatment temperature. Moreover, the electrical conductivity of both composites increased as the heat treatment temperature increased.The effect of heat treatment duration on the properties of PP1100 at 1400°C for 0.5 to 3 hours was studied. The density and porosity changed slightly, the weight loss was only 1.2 % after 3 hours. The mechanical properties of the composites increased in heat treatment procedures with duration up to 1 hour. Higher heat treatment durations had a detrimental effect on the mechanical properties of the composites. However, the bending strength of the composites after heat treatment at 1400 °C for 3 hours was up to 462.7 MPa, indicating excellent thermal stability of the composites.Si C ceramics and Si C/Si C composites were irradiated using 1.8 Me V electron. No evidence for peak emission was found in the radioluminescence spectra. The lack of observable emission was due to the self absorption of the free carbon or Py C coating in the polymer derived Si C ceramics and Si C/Si C composites. Surface damages of LPVCS1100 were observed after the irradiation, the irradiated area exhibited rimose morphology. The elemental composition changed slightly, however amorphization of the crystal structure took place. The morphology, composition and structure of LPVCS1800 changed little after irradiation. Etching marks were observed in the irradiated area of PCS1100, however the composition and structure changed little. The irradiated area got darker after irradiation, a number of crater-like areas were observed where enrichment of carbon was found. Evident amorphization occurred and a lot of noncrystal area was observed. No obvious changes of morphology, composition and structure of PL1100 were observed, the morphology and composition of PP1400 changed slightly, yet amorphization occurred after irradiation.Tungsten coating was fabricated by plasma spray process on the Si C/Si C composite substrate. The density and porosity of the coating were 17.6 g/cm3 and 6.7 %, the microhardness and modulus of the coating were 19.5 GPa and 164.1 GPa, the adhesion strength between the coating and the substrate was 8.1 MPa. The thickness of the coating was ~200 μm, a typical lamellar microstructure of the thermal sprayed coating was observed. Cracks and pores existed mainly near the unmelted particles, oxygen distributed along the edge of the melted particles. The fracture surface of the coating exhibited intercrystalline cracking morphology, where columnar crystal structure was observed. The columnar crystals were generally several microns in height and 0.2~1 μm in length and width.