| With the development of hypersonic vehicles,the high temperature spring is urgently needed in high temperature seal and damping installation. Due to the poor behavior of metal spring at high temperature, carbon fiber reinforced silicon carbide (1D C/SiC) composite spring was proposed. The preparation of C/SiC spring included two steps: shaping the spring preform via three method (winding and shaping fiber tows around the graphite mould, turning and dewaxing moulding), and densifying the spring by precursor infiltration and pyrolysis (PIP) process. The preparation process of C/SiC spring was researched and optimized. The influence of structure parameter, preform structure and PIP cycles on the properties of C/SiC spring was studied. The room and high temperature mechanical properties of C/SiC spring were compared with those of high temperature alloy (Inconel X-750) spring. SiC coating was deposited on surface of C/SiC spring by chemical vapor deposition (CVD) process, which greatly improved the oxidation resistance of C/SiC spring.The structure, dimension and preform of C/SiC spring were designed. The typical parameters of C/SiC spring were determined as follows: 2mm line diameter, 16mm mean diameter, 5 active coils, 7 total coils, 5mm pitch distance, and 28mm free height. According to the force analysis of spring, fiber tows were twisted to form the spring preform along the opposite direction of spring coils.1D C/SiC composite with fiber content of 4 tows/mm2 was prepared, and its tensile strength, tensile modulus, poisson ratio, shear modulus, double shear strength, and fracture toughness were 307MPa, 92.2GPa, 0.35, 34.1GPa, 89MPa and 14.6 MPa?m1/2, respectively. According to these performance parameters, the constant and maximal deflection of C/SiC spring were calculated as 3.33N/mm and 4.5mm, respectively.The preparation process of spring preform was investigated and optimized. The method of winding and shaping fiber tows around the graphite mould was an easy process, but it was difficult to control the dimensions and shape of spring coils, and the surface morphology was rough, leading to a spring constant of only 1.87N/mm. The turning and dewaxing moulding could control spring dimensions and surface state much better. When the deflection was less than 5mm, the constant of the spring by turning was 3.74N/mm, which was a little higher than that by dewaxing moulding (3.36 N/mm). The spring by turning could load more, but its reliability was poor. When the deflection was more than 5mm, the constant of the spring by turning decreased to 1.58N/mm and its spring line was damaged, while that by dewaxing moulding showed no change. Therefore, the method of dewaxing moulding was a better choice to shape spring preform.The influence of PIP cycles on the properties of C/SiC spring was studied. When the number of PIP cycles increased from 7 to 13, the spring constant increased from 1.10N/mm to 1.75N/mm, and more PIP cycles made little contribution to spring constant.The influence of spring dimensions and preform on the properties of C/SiC spring was studied, too. As a result, the spring constant decreased from 2.84N/mm to 0.54N/mm as the spring index increased from 6.4 to 10.7, and when the helix angle increased from 5.7? to 12.3?, the spring constant decreased from 1.75N/mm to 0.54N/mm. Besides, twisting the fiber tows could avoid fiber tows delamination and make much contribution to the spring constant. When the number of twist increased from 0 twist/m to 50 twist/m, the spring constant increased from 1.00N/mm to 3.01N/mm, and the best range of twist number was 35～50 twist/m.The mechanical properties of C/SiC spring and Inconel X-750 spring at room and high temperature were compared. At room temperature, the constants of C/SiC spring and Inconel X-750 spring were 3.36N/mm and 5.61N/mm respectively. As the temperature increased, the constant of Inconel X-750 spring decreased gradually, showing that working temperature was lower than 600°C. However, the compression restoration rate of C/SiC spring could reach 100% at 1000℃, but its spring constant decreased at the speed of 0.078N/mm?min with the time prolonged. When C/SiC spring was held at 1000℃for 10 minutes, the retention ratio of the spring constant was about 69.2%.SiC coating was deposited on surface of C/SiC spring by chemical vapor deposition (CVD) process, and the influence of the coating on the properties of C/SiC spring was studied. SiC coating with 48.6?m in thickness greatly improved the spring constant of C/SiC spring. At room temperature, the retention ratio of the spring constant was about 132.1% after SiC coating was deposited. At 1000℃, the constant of C/SiC spring increased with the holding time increase at first, then reached the maximum of 3.97N/mm at holding time of 25 minutes, after that decreased gradually because of the crack extension in the SiC coating and the oxidation of carbon fibers. |
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