Preparation And Properties Of Continuous Carbon Fiber Reinforced Alumina Matrix Composites

Owing to chemical and thermal stability,relatively good strength and low density characteristics,alumina?Al₂O₃?has a promising prospect in the field of high-temperature structural and thermal protection,armor and friction materials,combined availability in abundance and low cost of the material resources.However,the fracture toughness of alumina is not sufficient which limits the application seriously.In this paper,three-dimensional braided carbon fiber reinforced alumina matrix composites were fabricated through“sol infiltration-drying-heat treatment”?SIH?route.In sol infiltration process,Al₂O₃ sol with high solid content was used as raw materials,and its properties were studied firstly.Then,in order to improve the composite performance,seeking for optimum SIH conditions which was more suitable for C/Al₂O₃ composites was carried out.Finally,for different applications,properties and failure behaviors in simulative environment of the optimal C/Al₂O₃ composite was researched.The inherent stability and particles homogeneity of Al₂O₃ sol were very high at room temperature?RT?.Al₂O₃ sol had a solid content of 34.3wt.%at 200?and a ceramic yield of 17.3wt.%at 1200?.After calcined at 1200?,the amorphous Al₂O₃was irreversibly crystallized into steady?-Al₂O₃.The linear shrinkage of pressed Al₂O₃gel powders was equably increased as the heat treatment temperature increased from800?to 1400?,Whereas it was improved remarkably after sintered above 1600?,and the corresponding value was 32.2%.Base on exploring the effect of heat treatment temperature and infiltration cycles on fabrication and mechanical properties,C/Al₂O₃ composites was prepared through 28cycles at 1400?,showed an apparent density of 2.68g/cm³,a total porosity of 16.6%and an open porosity of 8.7%.In addition,the flexural strength and the elastic modulus were 208.5±27.3MPa?44.3±5.0GPa,respectively.Compared with monolithic alumina ceramics,the fracture toughness of composites was remarkable for being nearly triple,which reached to 8.1±1.1MPa·m^1/2.High-temperature mechanical properties of C/Al₂O₃ composites under inert and static air were studied.After soaked for 1h,the composites had good high-temperature resistance under inert atmosphere at 1200?and 1400?,which the flexural retention ratios were 84.4%and 73.3%,respectively.However,after oxidation for 30min at1200?and 1400?,the composites showed undesirable oxidation resistance,and the corresponding flexural retention ratios were 66.7%and 51.9%,respectively.The oxidation began at the cracks or holes of surface,and propagated gradually to the inner,in the ultimate,led to the failure of the carbon fiber.Through introducing s ilica sol in the infiltration process,the flexural retention ratios reached to 94.1%and 84.0%,which were improved remarkably.The viscous flow sintering of SiO₂ at high-temperature was responsible for the outstanding anti-oxidation property.Thermal physical properties of C/Al₂O₃ composites were researched from RT to1400?.The results showed the characteristics of composites were low thermal conductivity and low thermal expansion.At RT,the coefficient of thermal expansion?CET?was 4.03×10^-6K^-1,then decreased and remained stable at above RT,with a minimum of 1.70×10^-6K^-1 at 600?.The thermal diffusivity???was 3.45mm²/s and the specific heat capacities?C_p?was 0.77 J/?g·K?.From RT to 1400?,?decreased gradually,while C_p increased gradually.The thermal conductivity was decreased first and then increased,and reached a minimum of 4.55W/?m·K?at 1200?.Slected Al₂O₃ particle with the diameter of 75?m as erosion solid to test the solid particle erosion resistance of C/Al₂O₃ composites,the effect of particle feeding rate,impact angle,and temperature was investigated.At RT,as the particle feeding rate and impact angle increased,the higher erosion rate resulted in the maximum of 0.166mm³/g with conditions of 10g/min and 90°?The erosion rate of composites at high temperature was higher than at low temperature due to the desquamation of ceramic layer and the oxidation of carbon fiber.When erosion temperature reached to 1000?,the maximal erosion rate was 0.816mm³/g at 75°impact angle with 10g/min.Static and dynamic compressive strength were measured to characterize the impact resistance of C/Al₂O₃ composites.The static compressive strength in X direction and Z direction were 330.3MPa and 422.7MPa,respectively.Under dynamic loading in the Z direction,the dynamic compressive strength added with the strain rate increasing,and the composites exhibited strain rate hardening effect.The dynamic compressive strength stabilized to 500MPa-650MPa when the strain rate ranged from 800s^-1 to 1800s^-1.However,it was distinct decreased when the strain rate reached to 2700s^-1.The C/Al₂O₃ composites exhibited excellent tribological properties such as stable friction performance and low wear resistance.Under the premise to fully meet with GB5763-2008,the friction coefficient of the composites was stable in the range of 0.34to 0.54,and the maximum wear rate was only 0.21×10^-7cm³·?N·m?^-1 which attribute to lubricant effect of the oxidation film and carbon debris.The wear mechanism of the composites was adherence wear.