Microstructural Tailoring And High Temperature Self-Healing Behavior Of Al₂O₃-GdAlO₃ Composites Reinforced With SiC_p Or Ti₃AlC_2p

Al₂O₃-based eutectic composites are promising candidates in engineering ceramics for high temperature structural applications such as advanced turbo-engines for aerospace and stationary power generation.Many efforts have been devoted to the development of a new family of Al₂O₃-based eutectic ceramics and to the improvement of its thermo-mechanical properties through novel preparation methods aiming to monitor their microstructure.In the present work,a cost effective chemical co-precipitation method has been successfully employed for the synthesis of nanoscale Al₂O₃-GdAlO₃ and Al₂O₃-ZrO₂-GdAlO₃ powders with eutectic composition by tailoring the reaction parameters and the calcination temperature.The effect of calcination temperature on the phase and morphology of the synthesized nanoscale Al₂O₃-GdAlO₃ and Al₂O₃-ZrO₂-GdAlO₃ powders with eutectic composition via chemical co-precipitation method was studied.Thus,?-Al₂O₃,GdAlO₃ and t-ZrO₂ are the main stable phases in the as-synthesized powders,while the intermediate phases of?-Al₂O₃ and Gd₃Al₅O₁₂ undergoes a phase transformation from?-Al₂O₃ to?-Al₂O₃ and a thermal decomposition of Gd₃Al₅O₁₂?GdAG?to GdAlO₃ and?-Al₂O₃ in sequence,respectively.The ceramic powders exhibit good dispersion,a high chemical homogeneity and a fairly good particle size distribution with an average size less than 100 nm.The optimization of the sintering parameters was conducted and the hot-pressing temperature was optimized to be 1500°C.The mechanical and thermo-physical properties of the as-sintered Al₂O₃-GdAlO₃ and Al₂O₃-ZrO₂-GdAlO₃ bulk ceramics are investigated.Al₂O₃-GdAlO₃ ceramic exhibits excellent mechanical properties,such as a fracture toughness of 7.5MPa m^1/2,a room-temperature flexural strength of 556MPa and a high-temperature flexural strength of 515MPa tested at 1000°C.Al₂O₃-ZrO₂-GdAlO₃ ceramic has a fracture toughness of 6.5MPa·m^1/2,a room-temperature flexural strength of 485MPa and a high-temperature flexural strength of only 335MPa at 1000°C.The as-sintered Al₂O₃-ZrO₂-GdAlO₃ ceramic has a low thermal conductivity of 1.9 W?m^-1?K^-1 at1200°C and relatively high thermal expansion coefficient of 9.49?10^-6K^-1 at1100°C.The microstructural evolution during solidification of Al₂O₃-GdAlO₃ eutectic ceramics was investigated.Hot-pressed Al₂O₃-GdAlO₃ ceramics exhibit a fine microstructure of?-Al₂O₃ and GdAlO₃ phases with random grain orientations.Moreover,some spherical-shaped GdAlO₃ nanograins with an average grain size of118 nm are dispersed in?-Al₂O₃ matrix.All nanoscale GdAlO₃ intragranular precipitates have the orientation relationship of[1?11]_GdAlO3//[0001]_Al2O3.The microstructure evolution in Al₂O₃-GdAlO₃ eutectic ceramic depends strongly upon the processing temperature.No melting occurs at 1700°C,while the eutectic microstructure is well established after increasing the processing temperature from1700°C to 1770°C.A partially-melted Al₂O₃-GdAlO₃ ceramic is obtained after heating at 1720°C.The structure in the melted zone consists of a three-dimensional continuously interpenetrated network?TDI?with fine eutectic colonies of a rod-like eutectic spacing of?_R=1.11?m and large GdAlO₃ grains at the boundaries.A very fine eutectic microstructure is established after melting at 1770°C.The colony structure is rapidly refined in the eutectic melt of Al₂O₃-GdAlO₃ ceramics,which is mainly composed of regular rod-like eutectic with an interphase spacing of?_R=400nm at the center and TDI at the boundaries.The predominant orientation in the colony structure is identified as[001]_GdAlO3//[011?1?]_Al2O3.Moreover,a two-step melting method leads to a complete eutectic microstructure.After melting at 1720^oC followed by isothermal solidification at 1620°C,the eutectic exhibits large colonies together with microstructural unhomogeneity and wide boundaries formed by TDI and large Al₂O₃ grains and rod-like eutectic spacing of?_R=2.08?m,while a fine eutectic structure forms after isothermal solidification at 1680°C with a relatively fine colony and homogeneity with?_R=1.15?m.The boundaries are mainly formed by TDI structure and large GdAlO₃ grains.Typical eutectic transition from TDI structure to lamellar/rod-like structure is found in Al₂O₃-GdAlO₃ eutectics.The complex irregular TDI structure coexists together with regular non-faceted GdAlO₃lamellar phase and rod-like GdAlO₃ phase.SiC particles and Ti₃AlC₂ MAX phase with various volume fractions were incorporated into the oxide powders of Al₂O₃-GdAlO₃,respectively.The effects of?-SiC nanoscale particles and Ti₃AlC₂ MAX phase addition on microstructure evolution,mechanical properties and strengthening/toughening mechanisms are investigated as a function of the particles volume fraction and sintering temperatures.Therefore,the as-sintered Al₂O₃-GdAlO₃-SiC ceramics consist mainly of?-Al₂O₃,GdAlO₃ and?-SiC phases,and exhibit a fine and uniform microstructure.SiC particles are dispersed uniformly within the interior of GdAlO₃grains or at the interfaces between the entangled?-Al₂O₃ and GdAlO₃ grains.While,?-Al₂O₃,GdAlO₃ and Ti₃AlC₂ are the main stable phases in the case of Al₂O₃-GdAlO₃-Ti₃AlC₂ ceramic with a fine homogenous microstructure,numerous intragranular GdAlO₃ nanograins dispersed in?-Al₂O₃ and layered Ti₃AlC₂ with few micrometers in size.The room-temperature flexural strength of Al₂O₃-GdAlO₃-SiC ceramics decreases subsequently with increasing the SiC content.The Al₂O₃-GdAlO₃-10vol.%SiC ceramic sintered at 1500°C has a flexural strength of 520 MPa.However,by increasing the SiC content from 10 to 20 vol.%,the high temperature flexural strength of Al₂O₃-GdAlO₃-SiC ceramics increases from 567MPa to 616MPa.While,the room-temperature flexural strength Al₂O₃-GdAlO₃-Ti₃AlC₂ ceramic fabricated at a sintering temperature of 1400°C is relatively higher than that of unmodified Al₂O₃-GdAlO₃ ceramic.Al₂O₃-GdAlO₃-15vol.%Ti₃AlC₂ sintered at 1400°C exhibits the highest value of flexural strength of 734MPa at room temperature and 757MPa after testing at a high temperature of 1000°C.The addition of SiC and Ti₃AlC₂ particles is beneficial to the improvement of fracture toughness.The unmodified Al₂O₃-GdAlO₃ ceramic has a maximum fracture toughness of 7.5MPa?m^1/2,however,the Al₂O₃-GdAlO₃-SiC ceramics sintered at1550°C exhibit a fracture toughness of 8.20MPa?m^1/2 at 10 vol.%SiC and9.28MPa.m^1/2 at 20 Vol.%SiC,respectively.Moreover,the fracture toughness of Al₂O₃-GdAlO₃-Ti₃AlC₂ ceramics increases with increasing the Ti₃AlC₂ content from 7.7 MPa·m^1/2 at 10 vol.%Ti₃AlC₂ to 8.1MPa·m^1/2 at 20 vol.%Ti₃AlC₂,when the sintering temperature was increased from 1350°C to 1400°C.Thus,the toughening mechanisms are mainly attributed to crack deflection,pinning,bridging and bifurcation.The oxidation resistance of Al₂O₃-GdAlO₃ composites after incorporation of?-SiC particles and Ti₃AlC₂ MAX phase into the oxide matrix was explored.The effects of oxidation temperature and time were examined to evaluate the oxidation mechanisms of these composites.The oxidation of Al₂O₃-GdAlO₃-10vol.%SiC composite in the present study follows a classical linear behavior with the activation energy of 222kJ/mol.Moreover,the oxidation of Al₂O₃-GdAlO₃-15vol.%Ti₃AlC₂composite generally obey a parabolic rate law with the activation energy of271.08kJ/mol in a temperature range of 600 to 800°C.The dense and adherent scales formed on the surface of Al₂O₃-GdAlO₃-10vol.%SiC at temperatures below1200°C are composed of SiO₂ and Al₂SiO₅ phases.With increasing the oxidation temperature,SiO₂ starts to react with GdAlO₃ to form Gd₂Si₂O₇.While,in the case of Al₂O₃-GdAlO₃-15vol.%Ti₃AlC₂ ceramic,the oxidized products consist mostly of TiO₂ and?-Al₂O₃.The formed oxide film has improved the flexural strength of Al₂O₃-GdAlO₃-10vol.%SiC,after 1h from the start of oxidation,the flexural strength increases from 673MPa to 773MPa when the oxidation temperature increases from 1000°C to 1300°C.The enhancement in the flexural strength of Al₂O₃-GdAlO₃-15vol.%Ti₃AlC₂ composite is from 765MPa to 781MPa,when the oxidation time is prolonged from 0.5h to 8h,which is mainly contributed to self-healing of small cracks and flaws filled with the oxidized products.The crack-healing behavior of Al₂O₃-GdAlO₃-SiC and Al₂O₃-GdAlO₃-Ti₃AlC₂composites by heat treatment of ceramic specimens pre-cracked by indentations at the center was examined.Surface morphologies of the crack-healed specimens were identified and the crack healing mechanism is established.The addition of SiC and Ti₃AlC₂ into Al₂O₃-GdAlO₃ ceramics provides a high-temperature crack healing characteristic.Al₂O₃-GdAlO₃-10vol.%SiC composite has a high-temperature crack self-healing ability in which a crack of 240?m in length and 1?m in width is completely healed after heat treatment at 1100°C for 10h.Likewise,Al₂O₃-GdAlO₃-15vol.%Ti₃AlC₂ composite has a good intermediate-temperature crack self-healing capability after heat treatment at 700°C for 10h to heal completely a crack of 240?m in length and 1?m in width.The degree of strength recovery increases with prolonging the annealing time after heat treatment at1100°C and 700°C,respectively,for Al₂O₃-GdAlO₃-10vol.%SiC and Al₂O₃-GdAlO₃-15vol.%Ti₃AlC₂ composites.Therefore,a full strength recovery is obtained after heat treatment at the above-mentioned temperatures for 10h to reach a value of 518MPa and 730MPa for Al₂O₃-GdAlO₃-10vol.%SiC and Al₂O₃-GdAlO₃-15vol.%Ti₃AlC₂ composites,respectively.