Microstructural Tailoring and High Temperature Self-Healing Behavior of Al2O3-GdAlO3 Composites Reinforced with SiCp or Ti3AlC2p


Supervisor:ou yang jia hu


Degree Year:2018





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,α-Al2O3,GdAlO3 and t-ZrO2 are the main stable phases in the as-synthesized powders,while the intermediate phases ofθ-Al2O3 and Gd3Al5O12 undergoes a phase transformation fromθ-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 m1/2,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·m1/2,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 ofα-Al2O3 and GdAlO3 phases with random grain orientations.Moreover,some spherical-shaped GdAlO3 nanograins with an average grain size of118 nm are dispersed inα-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 ofλR=1.11μm 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 ofλR=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 1720oC 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 ofλR=2.08μm,while a fine eutectic structure forms after isothermal solidification at 1680°C with a relatively fine colony and homogeneity withλR=1.15μm.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 ofα-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 ofα-Al2O3,GdAlO3 andα-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,α-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 inα-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?m1/2,however,the Al2O3-GdAlO3-SiC ceramics sintered at1550°C exhibit a fracture toughness of 8.20MPa?m1/2 at 10 vol.%SiC and9.28MPa.m1/2 at 20 Vol.%SiC,respectively.Moreover,the fracture toughness of Al2O3-GdAlO3-Ti3AlC2 ceramics increases with increasing the Ti3AlC2 content from 7.7 MPa·m1/2 at 10 vol.%Ti3AlC2 to 8.1MPa·m1/2 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 ofα-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 andα-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 240μm in length and 1μm 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 240μm in length and 1μm 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.