Design, Preparation And Performance Of Oxidation Resistant Coatings Based On Glass-ceramics For C_f/SiC Composites

Carbon fiber reinforced silicon carbide matrix (C_f/SiC) composite is an attractive candidate for thermal structural components. Meanwhile, it is easy to be oxidized above 400℃in oxidizing atmosphere, which is a key obstacle for its application. In this dissertation, based on the study of oxidation kinetics for C_f/SiC composite fabricated by precursor infiltration and pyrolysis (PIP) process, four basic requirements for oxidation resistant coating were proposed. They are high density, appropriate coefficient of thermal expansion (CTE), proper viscosity at working temperatures and excellent binding with C_f/SiC composite. Three kinds of materials, which were MgO-Al₂O₃-SiO₂(MAS)glass-ceramic, BaO-Al₂O₃-SiO(2BAS)glass-ceramic and yttrium silicate, were chosen to prepare oxidation resistant coating. Some properties of them, such as CTE, self-sealing temperatures and sintering density, were characterized. Based on the properties, five effective coating systems used at different temperatures were designed and prepared. Microwave was adopted to firstly sinter coatings on the surface of C_f/SiC composite. Using glass as staring materials, glass-ceramic coatings were prepared by in situ sintering method. The performance of different coated C_f/SiC composites was tested in static state or under thermal shocks at 1000～1500℃in air.Kinetics of isothermal oxidation and non-isothermal oxidation of C_f/SiC composite was characterized by thermogravimetry at 600～1500℃. The results show that the oxditaion of C_f/SiC composite can be divided into linear stage and non-linear stage by the transition ratio of 70%. The apparent activation energies, at linear and non-linear oxidation stages at lower temperatures (600～800℃), are 135.08kJ/mol and 164.95kJ/mol, respectively. Whereas at medium temperatures (800～1100℃), those greatly decrease to 36.64kJ/mol and 58.79kJ/mol, respectively. At lower temperatures, the maximal weight losses (Xmax) of C_f/SiC composites are closer to each other at different temperatures, while Xmax decreases with the increasing of temperature at higher temperatures (1100～1500℃). The residual strength of C_f/SiC composite after oxidation debases with the increasing of oxidation temperature at temperatures above 1100℃. Model-free method was used to calculate isothermal oxidation kinetic of C_f/SiC composite. The results show that Friedman-Reich-Levi is an appropriate method for C_f/SiC composite. With the progress of oxidation, the oxidation temperature increases gradually and the activation energy of C_f/SiC composite decreases sharply.By disproportionation reaction in molten salt, titanium coating with 6～10μm in thickness can be fabricated on the surface of C_f/SiC composite. The titanium coating, binding tightly with the composite, can seal the defects on the surface of the composite and prevent the diffusion of carbon. Therefore, titanium coated composite can be protected from oxidation for certain duration at 1000℃. Using titanium coating as a transition layer, Ti/MAS1 coating was designed and prepared to prevent C_f/SiC composite from being oxidized at 1000～1200℃. The weight loss of the coated sample was 0.95% after oxidation at 1000℃for 720min, and the residual strength ratio was 99.3%. The coated sample was effective for long time at 1200℃in oxidizing atmosphere, while it receded above 1200℃for the low viscosity of MAS1 glass.BAS₂/BAS1 coating with good self-sealing ability at 1350℃was designed and prepared. The CTE of BAS glass is higher than that of C_f/SiC composite. Consequently, great thermal stress is induced in BAS coating during cooling process and the compatibility between the coating and the composite is not very good. However, a graded transition layer, formed by the elements diffusion of BAS glass into the composite at high temperatures, can enhance the binding strength between the coating and the composite, and improve the compatibility between them. The crystallinity of BAS₂ glass-ceramic coating sintered at optimized sintering temperature of 1050℃is the highest. The CTE of BAS₂/BAS1 coating decreases gradually from the outer layer to the inner layer, and the outer layer has better sealing ability. As a result, the weight loss of the BAS₂/BAS1 coated sample was 5.04% and little micro-cracks appeared after oxidition at 1350℃for 960 min and thermal shocks for 96 times.By adding Y₂O₃ to BAS₂ glass, a novel BAS₂-Y₂O₃ coating was fabricated. The reaction between Y₂O₃ and BAS₂ fusion and the influences on oxidation resistant capability were studied. The results show that yttrium silicate with higher melting point and lower CTE than BAS₂ glass-ceramic is produced in BAS₂-Y₂O₃ at 1327℃, then hexacelsian crystallizes from BAS fusion during cooling process. Consequently, BAS₂-Y₂O₃ coating with lower CTE can be used at higher temperatures. The densification of BAS₂-Y₂O₃ coating can be improved by microwave sintering. The diffusion capability of coating elements is enhanced in Ar-H2 atmosphere; thereby the binding between the coating and the composite is improved. For BAS₂-Y₂O₃ coating with Si:Y =1:1, the optimized technological conditions are: sintering at 1500℃for 30min and furnace cooling. After oxidation at 1400℃for 300min, the weight loss of BAS₂-Y₂O₃ coated composite was 1.08%. Because of higher CTE of the coating materials than that of the composite, the thermal shock resistance of the coating can not meet the requirements well.BAS₂-Y₂O₃/SiO2-Y₂O₃ double layered coating was designed and prepared. After sintering, the main phases of the inner layer are hexacelsian and Y2SiO5, and the main phase of the outer layer is Y2Si2O7. Little yttrium silicate is obtained in SiO2-Y₂O₃ sintered at 1500℃by the traditional method. Meanwhile, Y2Si2O7 was produced in the coating with Y₂O₃:SiO2=1:2 sintered by microwave. After thermal shocks of 15 times and oxidation for 150min at 1400℃, the weight loss of BAS₂-Y₂O₃/SiO2-Y₂O₃ coated composite was 1.22%. The coating became denser and crack-free during thermal shocks, which indicated that the coating should have good thermal shock resistance. Using BAS₂ as sintering addition, a graded B-50/B-10/B-0 yttrium silicate coating was prepared, which was effective at 1400～1500℃. The reaction temperature of Y₂O₃ and SiO2 is decreased to 1350℃because of BAS₂ glass, and the densification is also improved. During sintering, BAS glass diffuses from the inner layer to the outer layer, and B-50/B-10/B-0 coating is softened from the inner layer to the outer layer, thereby the coating becomes denser. The main phases of the sintered coating are Y2Si2O7 and BAS glass with some hexacelsian crystals. The thermal shock and oxidation resistance of B-50/B-10/B-0 coating is excellent. With the increase of oxidation time, the weight loss of the coated sample increased in linear. Thermal shock slightly influences the coating. After thermal shocks of 11 times and oxidation for 110min at 1400℃, the weight loss of coated sample was only 0.87%, while that was 2.45% at 1500℃. The residual strength of both samples was higher than 70%.Glass-ceramic is a promising candidate for oxidation resistant coating for C_f/SiC composite. Crystals with high melting points and low CTE keep the integrity of coating at high temperatures and the thermal matching between the coating and the composite. Glass softened at given temperatures has good sealing capability. On one hand, glass can seal the defects in the surface of the composite and improve the bond between them. On the other hand, the density of the coating is ensured by the softened glass at high temperatures. These two phases act together to endow good oxidation resistance for glass-ceramic coating.