Preparation And Performance Of Alumina Fiber Reinforced Alumina Matrix Composites

In recent years,with the rapid development of aeroengine,the requirements of hot end parts materials are more and more strict,and the traditional alloy materials cannot meet their needs.As an important branch of continuous fiber reinforced ceramic matrix composites,continuous oxide fiber reinforced oxide matrix composites have excellent oxidation resistance,can work for a long time under high temperature and oxygen environment,while the mechanical properties and component structure can be relatively stable.As the continuous oxide fiber reinforced oxide matrix composite has high strength,high modulus,oxidation resistance,thermal shock resistance,corrosion resistance and other excellent properties,it has become a new candidate material for hot end components in the field of aerospace.In this paper,the preparation and performance of Al₂O₃/Al₂O₃ composites were studied comprehensively for the purpose of developing high-performance oxide/oxide composites.The brand of Al₂O₃ fiber is Nextel 610（3m,USA）,which is a high-purity Al₂O₃fiber containing 99%α-Al₂O₃.The fiber composition is relatively stable at high temperature,and there is no phase transition reaction.However,the grain size of Al₂O₃fiber will gradually increase with the increase of heat treatment temperature,and the excessive grain size of Al₂O₃ fiber will lead to the serious degradation of the mechanical properties of the fiber.1200℃is the key temperature point for the change of structure and properties of Al₂O₃ fiber.After heat treatment below 1200℃,the internal grain size of the fiber changes little,and the strength retention rate of the fiber is high;after heat treatment above 1200℃,the grain size of the fiber increases obviously,and the strength retention rate of the fiber is low.After heat treatment at 1400℃for 2 hours,the fiber strength retention rate is less than 50%.In addition,according to the statistical results of Weibull distribution,with the increase of heat treatment temperature,the shape parameter（m）decreases gradually,indicating that the tensile strength of the fiber is more and more discrete.Al₂O₃ sol has high solid content（20%）,moderate particle size（10～15nm）,low viscosity（8Mpa·s）,high ceramic yield（～20%）and other excellent qualities.The phase transformation of Al₂O₃ sol fromγ-Al O（OH）→γ-Al₂O₃→α-Al₂O₃ occurs in the process of inorganic synthesis.The sintering degree of Al₂O₃ ceramics increases with the increase of temperature.When sintered at 1400℃,the density of Al₂O₃ ceramics reached the maximum of 2.23g/cm³,and the porosity of Al₂O₃ ceramics reached the minimum of 12.58%.In addition,with the increase of sintering temperature,the elastic modulus and hardness of Al₂O₃ ceramics also increase.After sintering at 1400℃,the elastic modulus and hardness of Al₂O₃ ceramics reached the maximum,which were63.6GPa and 1.87GPa,respectively.The relationship between the preparation temperature and the properties of Al₂O₃/Al₂O₃ composite was studied.The densification degree of Al₂O₃/Al₂O₃composite increases with the increase of preparation temperature.The density of the composite prepared at 1400℃is the highest,reaching 2.70g/cm³,and the porosity is the lowest,which is 12.33%.With the increase of preparation temperature,the flexural strength of the composite increases first and then decreases.Among them,the mechanical properties of Al₂O₃/Al₂O₃ composites prepared at 1100℃are the best,the flexural strength is 147.63MPa,and the elastic modulus is 50.48GPa.The results show that the optimum temperature for preparing Al₂O₃/Al₂O₃ composites is 1100℃in sol-gel process.The properties of Al₂O₃/Al₂O₃ composites prepared at 1100℃were characterized.At room temperature,the specific heat capacity,thermal diffusivity and thermal conductivity of the composites are 0.78J/（g·K）,0.77m²/s and 1.44w/（m·K）,respectively,and the thermal expansion coefficient of the composites is 5.46×10^-6K^-1.At room temperature,the average dielectric constant and loss tangent of Al₂O₃/Al₂O₃ composites are 4.33 and 0.021 respectively.After heat treatment at different temperatures,the strength of the composites decreases to different degrees.After treatment at 1500℃for one hour,the flexural strength of the composites is less than 50%.In high temperature environment,the mechanical properties of the composites are greatly reduced.At1300℃,the tensile strength retention rate of the composite materials is less than 50%,only 39.12MPa;while the flexual strength retention rate of the composite materials is only 30%,only 42.52MPa.Al₂O₃/Al₂O₃ composites has excellent thermal shock resistance.After 50 thermal shock tests at 1300℃,the flexural strength of the composites is 124.34MPa,and the retention rate of the flexural strength is high,close to85%.The ablation resistance of Al₂O₃/Al₂O₃ composites is good.After 120s test at2200℃,the linear ablation rate is only 0.00321mm/s,and the mass ablation rate is only0.00156g/s.After ablation,there are no obvious cracks,ablation holes or holes in the samples,and the original plate shape is maintained.The composites have certain structural integrity and reliability.The Py C interfacial phase were prepared to weaken the interfacial bonding strength between fiber and matrix.The results showed that the composites with Py C interfacial phase had a higher strength,up to 80.62mpa,and a strength retention rate of nearly 65%after heat treatment at 1500℃for one hour.In addition,the fiber at the fracture was pulled out obviously and showed ductile fracture characteristic.Zr O₂ and Y₂O₃environment barrier coatings were prepared to improve the ablative performance of the composites.The results showed that the composites with coating was less affected after ablation tests,and the environment barrier coating,as sacrificial barrier layer,better protected the structural integrity of the composite material.After ablation tests,no obvious debonding and layering effect occurred in the two coatings,and the coatings were still closely combined with the surface of the composites.