High-temperature Evolutions Of The Compositions, Microstructures And Properties Of C_f/SiC Composites Fabricated Via The PIP Route

Recently, the carbon fiber reinforced silicon carbide (C_f/SiC) composites have been widely investigated due to the popular applications in the aeronautic, astronautic and military fields. As one kind method to fabricate C_f/SiC composites, the precursor infiltration and pyrolysis (PIP) route has attracted much more attentions, due to the advantages of simple requests for equipments, near net shape for complex components, and so on. However, the compositions, microstructures and properties of C_f/SiC composites will be changed, when the C_f/SiC composites are applied in high-temperature circumstances for several hours. Consequently, in order to offer technologies for applications of the C_f/SiC composites, the paper investigated the high-temperature evolutions of the compositions, microstructures and properties of C_f/SiC composites fabricated via the PIP route. Especially, the evolutions of SiC matrix, C fiber and interphase, as well as the damage mechanisms of mechanical properties of C_f/SiC composites, were highly focused.The composition and microstructure evolutions of SiC matrix derived from polycarbosilane (PCS) in the protective atmosphere were researched. It is found that in the SiC matrix derived from PCS at 1200℃(called as SC1200), there are a little of elementals O (1.99 wt%) and H except the Si and C; And the elemental O mainly exists as SiO₂, when the C mainly exists as both SiC and free carbon (nearly 11 wt%); Its microstructure is amorphous including part of free carbon dissolved in the SiC continuum. At 1500℃, the concentration of elemental O decreases due to the carbothermic reductions; At 1600℃, the free carbon dissolved in the SiC continuum deposits, and the free carbon phase becomes turbostratic from amorphous; At 1800℃, the SiC matrix is nanocrystalline with the turbostratic carbon surrounding the SiC nanocrystals (about 7 nm).Obviously, the thermal, oxidation and ablation characteristics of the SiC bulk ceramics derived from PCS are affected by the microstructures. Compared with the SC1200, the thermal expansion coefficient of the SiC ceramics after 1800℃annealing (called as SC1800) increases from 4.3×10^-6/℃to 4.8×10^-6/℃, when the thermal diffusivity becomes 0.086 cm²/s from 0.016 cm²/s; Due to the increase of crystalline and thermal diffusivity, and the better thermal stability of C phase than SiC phase in the poor oxidation atmosphere, the anti-ablation properties of the SC1800 also increases. The oxidation characteristics of the SiC ceramics are as follows: Although the free carbon phases all start to be oxidized at 600℃in the SiC ceramics annealed at various temperatures, the oxidation degrees are different. For the SC1200, the ratio of weight loss is only 2.24 wt% before the SiC phase starts to be oxidized at 731℃, because there is part of free carbon dissolved in the amorphous SiC continuum; For the SiC ceramics annealed above 1600℃, the oxidation degrees of the free carbon phases increase, resulting in the ratio of weight loss of the SiC matrix over 7.00 wt%, and the oxidation temperatures of the SiC phases are prolonged more than 100℃, because the free carbon dissolved in the SiC continuum deposits and the SiC nanocrystals are protected by the free carbon continuum.The high-temperature evolutions of C fibers under vacuum were also investigated. Below 1000℃, the fiber sizing of C fibers is eliminated; During 1000¹600℃, the ratio of weight loss, the graphite degree and the tensile strength of C fibers all gradually increase with the increasing temperature, and especially, at 1600℃, the texture of C fibers start to become the turbostratic carbon with the biggest tensile strength; Above 1600℃, the graphite degree of C fibers increases, whereas the tensile strength decreases with the increasing temperature. Especially, at 1800℃, there are nearly 4 nm wide straps of the graphite along the axes of C fibers, and the tensile strength is still high equal to the raw fiber.The microstructure evolutions of interphases in the C_f/SiC composites in the protective atmosphere were investigated. In the C_f/SiC composites fabricated at 1200℃(called as CSC1200), the interphase is an organized multilayer (C/SiO₂), and due to the differences of elastic characteristics between the amorphous SiO₂ layer and the turbostratic carbon layer, the matrix cracks extend between them when the composites bear loads, resulting in the good reinforcing effect of C fibers on the composites'strengths. After 1800℃annealing, the interface is the chemical bonding between the nanocrystalline SiC interphase and C fibers with the turbostratic carbon structure, and there are big SiC crystals (200 nm) on the interface.Effects of annealing conditions on the mechanical properties of C_f/SiC composites were researched systematically concerning temperature, holding time and pressure. When the annealing time is 1 h under Ar, the C_f/SiC composites annealed at 1600℃show the low strength and tough fracture behavior, and the mechanical properties of the annealed composites after the redensification rout recover well, which can reach above 84% of that of CSC1200; However, the C_f/SiC composites annealed above 1700℃show the low strength and brittle fracture behavior, and the mechanical properties of the annealed composites after the redensification rout are still poor and keep brittle. Consequently, the damage mechanism of mechanical properties of the C_f/SiC composites changes in the temperature range from 1600℃to 1700℃.When the C_f/SiC composites are annealed at 1600℃under Ar, the ratios of weight loss gradually increase and the mechanical properties decrease with the annealing time increasing. And the fracture behaviors of the C_f/SiC composites become brittle from tough, when the annealing time is longer than 5 h. Then, the effects of annealing time at 1600℃under vacuum on the C_f/SiC composites were compared. It is found that the mechanical properties gradually decrease while the ratios of weight loss hardly change with the increasing time, and the strengths of the C_f/SiC composites are better than that of the C_f/SiC composites annealed under Ar for the same annealing time. When the annealing time is longer than 10 h, the fracture behaviors of C_f/SiC composites become brittle from tough. Finally, after the redensification route, the mechanical properties of the C_f/SiC composites annealed under Ar in 5 h or annealed under vacuum in 10 h recover well, and become tough again. So the annealing times (5 h under Ar or 10 h under vacuum) are the critical points of the qualitative changes about the microstructures and properties of the C_f/SiC composites when the annealing temperature is 1600℃.The damage mechanisms of mechanical properties of the C_f/SiC composites annealed at various conditions were systematically investigated. The results show that the chemical damage is the major damage for the C fibers in the C_f/SiC composites during the high-temperature annealing, and the reinforcing effect of C fibers is affected by the thermo-mismatch stress at the same time. In the CSC1200, there is the elemental O in the matrix and interphase, even higher content in interphase due to a C/SiO₂ multilayer as the interphase. So during protective atmospheres, the carbothermic reductions happen above 1500℃. The product SiO gas accelerates the elemental Si diffusing to the C fibers, and then the new interphase SiC is formed at the surfaces of C fibers. At 1600℃under Ar, the original interphase (the C/SiO₂ multilayer) is gradully weakened with the annealing time increasing, while the chemical damage to C fibers is gradully enhanced due to the transplant of SiO gas, and the chemical bonding between the new interphase SiC and C fibers is also enhanced; When the annealing time is 5 h, the fracture behaviors of the C_f/SiC composites become brittle from tough, because the chemical damage to C fibers and the new interphase start to become dominant, and there are big thermo-mismatch stresses at the same time, so the fracture behaviors of the C_f/SiC composites recover tough after the low-temperature stress-releasing annealing; When the annealing time is above 5 h, the chemical damage to C fibers and the new interphase completely become dominant, so the fracture behaviors of the C_f/SiC composites become brittle from tough, and can not recover forever. At 1600℃under vacuum, the chemical damage to C fibers and the forming of the new interphase are decelerated by the releasing of SiO gas, so the critical point of the qualitative changes about the microstructures and properties of the C_f/SiC composites is prolonged to 10 h. Consequently, effects of the transplant and reaction of SiO gas in the C_f/SiC composites are confirmed by the result. When the annealing temperature is above 1700℃, the elemental Si diffusing, the chemical damage to C fibers and the forming of new interphase are deteriorated by the SiO₂ melt, because the melting point of SiO₂ is 1723℃. So the fracture behaviors of the C_f/SiC composites become brittle from tough in a short time (less than 1 h), and can not recover forever.After the carbon coating on the C fibers is fabricated by PIP process, the thermal stability of the C_f/SiC minicomposites is obviously improved. Based on the completely understanding about the thermal stability of the SiC matrix and C_f/SiC composites, the high-temperature evolutions of ablation characteristics were further researched. It is found that the anti-ablation properties of the C_f/SiC composites gradully become better after the higher temperature annealing, and the result is the same with the evolutions of anti-ablation properties of the SiC bulk ceramics, because the SiC matrix plays the key role to prevent the destructions by the circumstance, and protects the C fibers. Then, the Zr-doped C_f/SiC composites were fabricated via PIP route using the precursors of Zirconium n-butoxide, Divinylbenzene and PCS. The anti-ablation properties of the Zr-doped C_f/SiC composites are better than that of the C_f/SiC composites, because there are ultra high temperature ceramics in the Zr-doped C_f/SiC composites, which have the better anti-ablation properties than SiC and can form the oxide films on the surfaces of the ablation zones.