Preparation And Properties Of Alumina-based Fiber/Alumina Composite

Ceramic matrix composites are a kind of high-temperature-resistant materials with great potential for aerospace field,which have the characteristics of high strength,heat and corrosion resistance.Alumina ceramics have excellent thermal shock resistance and oxidation resistance,which makes them perform well in high temperature environment.However,the disadvantages of high brittleness and poor toughness of alumina ceramics seriously limit its wide application.Now,the continuous fiber reinforced alumina ceramic matrix composites can solve the fatal problem of brittle fracture of alumina ceramics and have a broad application prospect in high-temperature engineering and thermal protection field,such as the aerospace field.In this thesis,the preparation technology of alumina precursor sol has been studied,especially the effect of the amount of AlCl₃·6H₂O,H₂O and EtOH on the physical and chemical properties of the sol.The optimum proportion of precursor sol has been determined,which is m（EtOH）:m（H2O）:m（AlCl3·6H2O）=5:6:7.The viscosity of precursor sol is 13.2mPa·s,the solid content is 23.8%,and the ceramic yield is 13.5%.The phase analysis of the sol after heat treatment at 900,1000,1100 and 1200^oC shows that the precursor sol after heat treatment at 900 and 1000^oC is amorphous,and the precursor sol after heat treatment at 1100^oC and above is completely transformed intoα-Al₂O₃ phase,with prominent diffraction peak,obvious characteristic peak and good crystallinity.The preparation technology of alumina ceramic slurry has been studied,especially the effect of Al₂O₃ powder,H2O and PAM on the physical and chemical properties of the slurry.The optimum proportion of alumina ceramic slurry has been determined,which is m（Al₂O₃）:m（H2O）:m（PAM）=500:150:4.The viscosity of the prepared ceramic slurry is 310mPa·s,the mass fraction is 60%,the pH value is 10.6,and the alumina powder is well dispersed and suspended.After 12 hours,no obvious delamination occurs.Therefore,it is an ideal alumina ceramic slurry for improving the impregnation efficiency in the early stage.The influence of two kinds of alumina based ceramic fiber preforms on the properties of alumina composite has been studied.The bending strength of Mu_f/Al₂O₃ reinforced by needled mullite fiber preforms with the same density and Al-Si_f/Al₂O₃ reinforced by three-dimensional and four-direction aluminum silicon fiber preforms is 110.57 MPa and 98.13MPa respectively,and the fracture toughness is 2.21 MPa·m^1/2and 1.96 MPa·m^1/2respectively.The result shows that the mechanical properties of Muf/Al₂O₃ composites are better than that of Al-Sif/Al₂O₃ composites.From the study of densification process of the composites,it is found that the densification and strength of composites raise obviously with the increase of impregnation times,and the impregnation efficiency of composite decreases gradually with the increase of cyclic impregnation times,but it reaches the upper limit when the impregnation times reach 25-28.At the same time,due to the low ceramic yield of the sol,the preparation period of the composite is longer,which needs to go through more than 20cycles of impregnation.Due to the great chemical compatibility of alumina based fibers and alumina ceramic matrix,they are prone to strong bonding phenomenon at high temperature,resulting in brittle fracture phenomenon in the process of high-temperature application.In order to solve this problem,the preparation process of alumina based fibers or alumina composites is optimized.PyC and LaPO4 are selected as the fiber interfaces to form a weak bond between the fibers and the matrix,so that the fibers play a role of toughening and strengthening.The results show that the improvement of mechanical properties of the composite by the PyC interface is more obvious than that by the LaPO4 interface.The density of the PyC-Muf/Al₂O₃ composite is 2.19 g/cm³,the bending strength is 123.86 MPa,the compression strength is 170.51 MPa,and the fracture toughness is 6.8 MPa·m^1/2.At the same time,brittle fracture behavior becomes ductile fracture behavior,and the toughening effect is obvious.The bending strength,compressive strength and fracture toughness of LaPO4-Muf/Al₂O₃ composites are 102.56 MPa,140.43 MPa and 5.3 MPa·m^1/2,respectively.The toughening range of LaPO₄-Mu_f/Al₂O₃composites is slightly lower than that of PyC-Muf/Al₂O₃ composites.Although the oxidation resistance of LaPO4 interface is better,the performance of PyC interface is more excellent in the aspect of fiber coverage and mechanical property improvement.After densification,the matrix can also play a role of air insulation on the carbon interface,reducing the impact of oxygen on the PyC interface.