Preparation And High-temperature Performance Of Vacuum Bag Molding 2D-Al₂O₃/Al₂O₃ Composites

Continuous alumina fiber reinforced alumina matrix composites（Al₂O₃/Al₂O₃composites）with excellent high-temperature mechanical properties and oxidation resistance,are important alternative materials for high-temperature components of aeroengines in the future.At present,Al₂O₃/Al₂O₃composites in the form of two-dimensional laminates（2D-Al₂O₃/Al₂O₃）occupy the mainstream position in the research and application of such materials,while the interlaminar shear strength of which is relatively low.In order to improve the mechanical properties,especially the interlaminar shear properties,overcome the problem of fiber damage in the process of PIP-post densification,enrich the research on the micro mechanical parameters of oxide composites and establish the relationship between the micro and macro mechanical properties of composites,the preparation and micro//macro mechanical properties of2D-Al₂O₃/Al₂O₃composites are carried out in this paper.In this paper,2D-Al₂O₃/Al₂O₃composites were prepared by slurry brush coating combined with vacuum bag molding,and followed by process of post densification.Before the preparation of composites,it is necessary to prepare Al₂O₃slurry with low viscosity,high stability and high solid content.The viscosity and zeta potential can be effectively controlled by changing the p H of the slurry.As the slurry p H increased from3.3 to 3.7,the viscosity firstly decreased and then increased,and the zeta potential firstly increased and then decreased.Therefore,p H=3.5 was appropriate.At this time,Al₂O₃slurry with solid content up to 53 vol%owned low viscosity（～700 m Pa·s）and high stability（zeta potential～60 m V）.The processing temperature and solid content of molding slurry are the key process parameters to control the micro/macro mechanical properties of 2D-Al₂O₃/Al₂O₃composites.As the processing temperature increased from 900°C to 1200°C,the Al₂O₃matrix modulus and fiber matrix interface bonding strength increased,leading that the bending strength of the composite increased from 125.1 MPa to 298.1 MPa,and the interlaminar shear strength increased from 7.7 MPa to 22.2 MPa.However,when the preparation temperature exceeded 1100°C,the matrix modulus and interfacial bonding strength increased excessively,and the change of crack propagation mode led to the obvious decrease of fracture toughness.1100°C and 53 vol%were determined as the best parameters through process optimization.At this time,the mechanical properties of the composites were as follows:bending strength 276.7±26.2 MPa,interlaminar shear strength 17.2±2.0 MPa,fracture toughness 13.9±1.0 MPa·m^1/2.To further improve the mechanical properties,especially the interlaminar shear properties of 2D-Al₂O₃/Al₂O₃composites,the post-densification treatment of low solid content slurry"impregnation-drying-heat treatment"cycle was used to densify the composites.The results showed that with the progress of post-densification,the modulus of Al₂O₃matrix and the bonding strength of fiber matrix interface increased,so did the load transfer efficiency,resulting in the continuous increase of bending strength and interlaminar shear strength of the composites;However,the fiber/matrix modulus mismatch decreased,resulting in the continuous decline of fracture toughness.It is found that when N=4,both strength and toughness can be considered.At this time,the mechanical properties of the composites are as follows:flexural strength 393.3±44.1MPa,interlaminar shear strength 21.8±1.2 MPa and fracture toughness 12.6±1.2MPa·m^1/2.Based on the work above,the high-temperature mechanical properties of2D-Al₂O₃/Al₂O₃composites were further studied.The results showed that the tensile strength,flexural strength,interlaminar shear strength and fracture toughness of the composites at 1000°C are 363.7±3.5 MPa,364.4±27.5 MPa,22.6±1.4 MPa and16.8±1.5 MPa·m^1/2respectively.The composites own good quasi-static mechanical properties,which are basically equivalent to those at room temperature.The tensile fatigue performance is good at 1000°C.When the fatigue life exceeded 10⁵cycles,the fatigue ultimate strength exceeded 250.0 MPa（～70%UTS）,which is mainly due to the porous characteristics of Al₂O₃matrix and the natural excellent oxidation resistance of components in the composite.In the thermal-aging assessment,the sintering degree of Al₂O₃matrix increaseed with the increase of temperature and test time.At this time,the increase of matrix modulus significantly increased the interlaminar shear strength of the composite,but the excessive increase of interfacial bonding strength led to the weakening of fiber-pullout characteristics and the decrease of fracture toughness of the composite;At the same time,the properties of n610 fiber degraded and the bearing capacity decreased,resulting in the obvious decrease of the bending strength of the composite.