Preparation And Performance Of Continuous Alumina Fiber Reinforced Alumina Matrix Composites

The requirements for thermal-structure materials in gas turbines for power generation and aerospace propulsion is increasingly urgent.Owing to excellent oxide resistance and mechanical properties,continuous oxide fiber-reinforced oxide matrix composites?oxide/oxide composites?have a broad application prospect in the field of aerospace.As one of oxide/oxide composites,the continuous alumina fiber-reinforced alumina matrix composite was chosen as the high-temperature structural materials.Based on the material fabrication,characterization of micro-mechancial properties,construction of the relationship between micro-mechanical properties and macro-mechanical properties,the Al₂O₃/Al₂O₃ composites with excellent excellent oxide resistance and mechanical properties were developed successfully.This study would establish the foundation for research and further engineering application of oxide/oxide composites.The attentions were firstly put on the effect of heat-treatment temperature on the microstructure and mechanical properties of alumina fibers?F2 and N610?,then the F2/Al₂O₃ composites were fabricated though the sol-gel process,and the N610/Al₂O₃composites were fabricated though the slurry infiltration.The influences of fabrication parameter on the micro-mechanical properties of components and macro-mechanical properties of composites were mainly investigated.The relationship between micro-mechanical properties of components and macro-mechanical properties of composites was emphatically expounded.Finally,mechanical properties of alumina composites at high-temperature and after thermal aging were studied.Firstly,the effects of heat-treatment on the microstructure and mechanical properties of F2 and N610 fiber were investigated.The F2 fiber?Nitivy,Japan?was composed of amorphous SiO₂ and?-Al₂O₃,and its tensile strength was 1.80 GPa.The N610 fiber?3M,USA?was composed of?-Al₂O₃,and its tensile strength was 3.10 GPa.After heat-treated at 1200oC,the mullite phase was formed in F2 fiber,while the phase of N610 fiber kept stable.Both the grain was grown gradually,which lead to coarse microstructure.The tensile strength of F2 and N610 fiber were reduced to 1.03 GPa and2.17 GPa,respectively.Then,the Al₂O₃ sols and Al₂O₃ powder were chosen as raw material for matrix.The Al₂O₃ gel was composed of boehmit.After heat-treatment,a series of phase transition occurred:boehmite??-Al₂O₃??-Al₂O₃??-Al₂O₃??-Al₂O₃.The Al₂O₃ slurry had several advantages,such as high solid content,moderate viscosity and good stability.The density of 1300oC fabricated Al₂O₃ ceramic was 3.76 g/cm³,which is 96.4%of theoretical density,showing excellent sintering property.To gain alumina composite with good mechanical property and low cost,the F2/Al₂O₃ composites were fabricated though the sol-gel process.As the most important processing parameter,the fabricating temperature has a marked impact on the micro-and macro-mechanical properties of composite.The fabricating temperature was600-1200oC according to the stability of F2 fiber.The micro-mechanical properties of in-situ fiber,matrix and interface were quantified by nanoindentation and fiber-push in.The elastic modulus and hardness of the Al₂O₃ matrix were enhanced,while those of the F2 fiber were kept stable,the interfacial reaction was triggered at 1200oC,leading to strong interfacial bonding.The mechanical properties of the F2/Al₂O₃ composites increased firstly and then decreased.The 800oC fabricated composite had the much stronger mechanical properties,which the flexural strength,interlaminar shear strength and fracture toughness were 106.56±6.78 MPa?8.9±1.3 MPa and 4.6±0.2 MPa·m^1/2,respectively.The decreased property of fiber and enhanced interfacial bonding resulted in the degradation of composite.The mechanical properties of this composite could maintain stable at 800oC and 1000oC,while degraded at 1200oC.The sol-gel has long preparing period,which lead to the thermal damage of F2 fiber and low strength of F2/Al₂O₃ composite.To further enhance the mechanical property of composite,the N610/Al₂O₃ composites were fabricated by slurry infiltration.The effect of fabricating temperature on the microstructure and mechanical properties of the N610/Al₂O₃ composites was firstly studied.The fabricating temperature was 1100-1300oC according to the stability of N610 fiber,and the solid content of slurry was 45 vol%.The elastic modulus and hardness of the Al₂O₃ matrix were increased,while those of the N610 fiber were kept stable,the interfacial bonding strength was enhanced.The mechanical properties of the N610/Al₂O₃ composites increased firstly and then decreased.The 1200oC fabricated N610/Al₂O₃ composite had the much stronger mechanical properties,which the flexural strength,flexural modulus and interlaminar shear strength were 307.07±11.61 MPa?103.18±2.48 GPa and 17.26±0.08 MPa,respectively.The decreased property of fiber and enhanced interfacial bonding resulted in the degradation of composite.The effect of solid content of slurry on the microstructure and mechanical properties of the N610/Al₂O₃ composite was also studied.The solid content was 45-60 vol%according to the charateristics of slurry and feasibility of preparation process.As the solid loading increased,the elastic modulus and hardness of the Al₂O₃ matrix were increased,while those of the N610 fiber were kept stable,the interfacial bonding strength was enhanced.The mechanical properties of the N610/Al₂O₃ composites increased firstly and then decreased.The 55 vol%slurry fabricated N610/Al₂O₃ composite had the much stronger mechanical properties,which the flexural strength,flexural modulus and interlaminar shear strength were393.39±19.60 MPa?108.32±8.44 GPa and 26.94±0.35 MPa,respectively.The viscosity of 60 vol%slurry was too high,which increased the difficulty of shaping composite and decreased the volume fraction of fiber,leading to the degradation of composite.The N610/Al₂O₃ composite was fabricated by the optimal process parameters by slurry infiltration process,the flexural strength,flexural modulus,interlaminar shear strength and the fracture toughness was 386.46±18.21 MPa?107.52±13.39 GPa?26.87±1.88 MPa and 14.05±0.88 MPa·m^1/2,respectively.The mechanical properties of this composite are better than those oxide/oxide composites reported in literatures.The mechanical properties of this composite could maintain stable at temperature not higher than 1050oC.After heat-treatment at 700oC and 900oC for 300 h and at 1100oC for 30h,the strength retention of this composite was exceeded than 100%.The coefficients of thermal conductivity?CTC?and thermal expansion?CTE?of N610/Al₂O₃ composites at25oC were 0.28 W/?m·K?and 2.42×10-6 K^-1.As temperature increased,the CTC decreased firstly and then increased,while the CTE increased.