织构化氧化铝陶瓷的烧结机制OA

Introduction Textured ceramics have attracted much attention in functional and structural applications due to their intrinsic anisotropy and superior physical and chemical properties.Among them,textured transparent alumina ceramics are particularly promising for optical windows,high-power lasers,and harsh-environment engineering components because of their superior mechanical strength,chemical stability,and light transmittance.The templated grain growth(TGG)method emerges as a powerful route to fabricate textured ceramics,as it exploits the preferential orientation of seed crystals to induce large-scale alignment in polycrystalline matrices.However,despite its application in producing highly oriented alumina ceramics with remarkable transparency,the fundamental sintering mechanisms underlying texture development remain incompletely unclear.Conventional TGG research is hindered by two intrinsic challenges,i.e.,the high number of template particles leads to frequent collisions,masking the true dynamic growth behavior;and the concurrent coarsening of templates and matrix grains at high temperatures diminishes size advantages,thereby weakening the thermodynamic driving force for template-guided growth.Consequently,the existing models,which largely attribute orientation development to the Ostwald ripening and localized grain boundary migration,cannot adequately explain the experimental observations reported,such as the persistence of abnormal texture alignment even beyond direct template-matrix contact regions.In this work,we systematically investigated the sintering mechanism of textured alumina ceramics via introducing large sapphire single crystals with controlled orientations(i.e.,A-(110),C-(001),M-(100),and R-(012))into high-purity polycrystalline Al2O3 matrices.This sapphire template-ceramic composite strategy could enable a direct observation of template-matrix interactions,while avoiding the kinetic distortions induced by template collisions in conventional TGG.In addition,we also analyzed a dual mechanistic framework that could integrate localized template coalescence and long-range stress-induced orientation via employing advanced electron backscatter diffraction(EBSD)and microstructural analysis,thus providing theoretical insights into the texture formation of alumina ceramics. Methods High-purity α-Al2O3 powders(purity>99.99%,D50 ≈ 200 nm)were used as a matrix material,and MgO was added as a sintering aid in selected experiments.Sapphire single-crystal plates of four orientations(i.e.,A,C,M,and R)with the dimensions of 10.0 mm×10.0 mm×0.5 mm were embedded into compacted Al2O3 powder beds.Composite green bodies were prepared via uniaxial pressing,followed by cold isostatic pressing at 200 MPa.After a binder was burnout at 800℃,the samples were sintered in vacuum at 1300-1860℃for 6-12 h.The microstructures were examined by scanning electron microscopy(SEM),and the crystallographic orientations were characterized by electron backscattered diffraction(EBSD).The sapphire-ceramic composites and dual-crystal systems(A-ceramic-C combinations)both were analyzed to evaluate the microstructural evolution,grain growth distances,and orientation gradients with and without MgO additives. Results and discussion The results reveal that the embedding of sapphire single crystals induces a distinct microstructural evolution in surrounding alumina grains.For undoped systems,the radial growth distances of A-and C-oriented templates are comparable,reaching approximately 200 μm at 1800℃,with microstructures dominated by template-driven consumption of neighboring grains.However,the introduction of MgO significantly reduces the coalescence distances to～118 μm,even at elevated temperatures of 1860℃,and effectively suppresses an abnormal grain growth,highlighting its role in grain boundary pinning and microstructural homogenization.The EBSD mapping unveils a striking divergence in orientation fields around different template planes.C-oriented templates generate orientation gradients extending up to 400 μm,which is far beyond their physical coalescence range.In contrast,A-oriented templates induce alignment only within～200 μm.This discrepancy originates from anisotropic thermal expansion.In detail,α-Al2O3 exhibits the largest thermal expansion coefficient along the C-axis,producing residual stresses during sintering that drive surrounding matrix grains to rotate and align toward the template orientation.As a result,template-induced texture formation encompasses two complementary mechanisms,i.e.,localized template-matrix coalescence driven by grain boundary energy minimization,and long-range orientation induction mediated by anisotropic thermal stresses.The dual-template experiments further substantiates this framework.In A-ceramic-C composite systems without MgO,matrix grains between the templates are fully consumed,adopting the orientation of the A-plane due to its higher surface energy and faster growth rate.In contrast,for MgO doping,abnormal grain growth is suppressed,and matrix grains between the templates display a similar preference for A/C-plane orientation.These results reconcile inconsistencies in earlier TGG studies and explain previously puzzling observations,such as the progressive increase in grains with small misorientation angles relative to the plane(006)during high-temperature sintering.Collectively,texture evolution cannot be solely attributed to the Ostwald ripening and template impingement,but must also consider stress-driven long-range orientation effects. Conclusions This work elucidated the sintering mechanism of textured alumina ceramics through a sapphire template-ceramic composite strategy.We established a dual mechanistic framework for texture development via systematically analyzing the microstructural evolution at different template orientations,sintering temperatures,and MgO doping conditions.The localized coalescence was obtained via grain boundary energy minimization and long-range orientation induction mediated by anisotropic thermal expansion stresses.The synergistic mechanism of template coalescence and non-contact orientation induction could inherent to the classical TGG.Among different orientations,the C-plane template demonstrated a superior capacity for long-range texture control,extending its influence up to 400 μm,compared to～200 μm for the A-plane.Besides,MgO additives could play a critical role in suppressing abnormal grain growth and reducing template coalescence distances,thereby enabling a finer microstructural control.