High-performance Alumina-based Ceramics And Densification Study Based DLP Stereolithography 3D Printing

Fabricating alumina-based ceramics through DLP（Digital Light Processing）stereolithography 3D printing technology has the advantages of high precision,high surface finish,and high efficiency,which can meet the requirements of forming complex-shaped and fine ceramic components in the field of aerospace,automobile manufacturing,medical health,and others.However,the low solid loadings of UV（Ultraviolet）-curable ceramic suspension and the easy occurrence of defects in the debinding process lead to poor mechanical properties of prepared alumina-based ceramics,which greatly limit the application and promotion of this technology.This paper closedly focuses on the preparation of UV-curable alumina-based ceramic suspensions and sintering densification of alumina-based ceramics.Scientific researches in four aspects were systematacially carried out:1)the rheological properties of UV-curable Al₂O₃ suspensions were regulated,and the dispersion mechanism was revealed;2)the relationship between UV-curing and adhesion properties of suspensions during the printing process were studied,and the thermal debinding mechanism and defect generation behavior of green body under the different atmosphere were studied;3)the alumina ceramic parts with excellent mechanical properties was fabricated by optimizing the size and morphology of Al₂O₃ powdes via DLP stereolithography;4)Zirconia-toughened alumina（ZTA）ceramics were fabricated by DLP stereolithography to further optimize mechanical properties,and the application of complex-shaped ZTA ceramic components in the field of water treatment was investigated.The above researches provide new ideas for the preparation of high-performance suspensions,also providing solutions for dropping of unaccomplished ceramic parts in the process of printing and generation of defects（e.g.,cracks,delaminations）in th process of debinding.This work provides technical support and theoretical basis for the process optimization of developing high-performance ceramic parts through stereolithography.The main research conclusions are summarized as follows:（1）From the perspective of the mechanism of inter-particle interactions,the rheological performance and suspended state of suspensions dispersed using the different dispersants were studied through the rheometer,particle size analyzer,and FTIR,the dispersion mechanism of Al₂O₃ powder in UV-curable resin was revealed,and the ceramic suspension with excellent rheological properties was also prepared,which laid the foundation of fabricating high-performance ceramic parts through DLP stereolithography.Results showed that the molecular weight and hydrophobic groups were two major determinants for the dispersing effect of a dispersant.This study revealed the two critical mechanisms for regulating the network microstructure and rheological properties of UV-curable suspension systems,namely,steric repulsion and particle wetting.The polymer copolymer dispersant BYK111exhibited the best dispersion.The viscosity of prepared suspension was only 1.04 Pa·s at a shear rate of 30 s^-1when the solid loading was 55 vol%,also showing the excellent printing self-leveling performance.（2）On the basis of（1）,this study overcomed the two common problems of dropping of unaccomplished structural parts during printing and defects generation during debinding.The relationship between the adhesion and UV-curing performances of the suspensions were analyzed by FTIR and adhesion test.Results showed that the dropping of unaccomplished structural parts was caused by the large shrinkage of suspension and hindered shrinkage in the direction of printing platform during the printing process.In this paper,by adjusting the functionality and structure of UV-curable monomers,the shrinkage was reduced and the adhesion of the suspension was improved to solve the dropping of unaccomplished parts.The degradation mechanism of organics in green-body and generation behavior of defects during debinding were studied using TG/DSC,TG-FTIR,and industrial CT.Results indicateed that oxidative degradation was the main mechanism for debinding under air atmosphere,which followed the unreacted core model of the gas-solid reaction system and was affected by the diffusion of oxygen.And thermal degradation was the main mechanism of debinding under argon atmosphere.Compared with oxidative degradation,the green-body released less gases such as CO₂,CO,H₂O,and small molecular organic gas during the thermal degradation process,and the thermal degradation process on the surface and inside of the green-body was uniform,which effectively inhibited the generation of cracks.（3）On the basis of optimizing the properties of suspension and the printing and debinding process,the densification technology of 3D-printed alumina ceramics through DLP stereolithography were studied by SEM and AFM.The effects of size and morphology of Al₂O₃powders on the entire DLP stereolithography 3D printing process and the mechanical properties of sintered alumina ceramic parts were explored.Results showed that the approximate spherical Al₂O₃powders with moderate particle size could be selected to prepare suspension exhibiting the best rheological and UV curing properties,and the mechanical properties of alumina ceramics were also best with a relative density of 99.2%and a flexural strength of396.5 MPa.Due to the"layer-by-layer"molding characteristics of DLP stereolithography,the flexural strength of the prepared alumina ceramics was still anisotropic,which limits the improvement of the overall mechanical properties of the structural parts.（4）On the basis of（3）,ZTA ceramics were fabricated by adding ZrO₂powder to the UV-curable Al₂O₃suspension to further improve its mechanical properties.The densification technology and strengthening and toughening mechanisms of ZTA ceramics were also studied by SEM,XRD,and TEM.The results showed that the YSZ-15 sample sintered at 1600℃exhibited best mechanical properties,the flexural strength was 522.6 MPa,and the fracture toughness was increased to 7.4MPa·m^1/2.The t-ZrO₂in ZTA ceramics refines grain size ofα-Al₂O₃.Hence,the surface roughness of the prepared ZTA ceramics was lower than that of pure alumina ceramics,and the printing accuracy was also improved.The t-ZrO₂grains also strengthened the bonding ofα-Al₂O₃grains between adjacent layers to improve the anisotropy of flexural strength.At last,this study further fabricated complex-shaped ZTA honeycomb ceramics and ceramic scaffolds for water treatment with excellent mechanical properties.By in-situ synthesis of Al₄B₂O₉whisker structures on the surface of ZTA structural parts,a ceramic filter with superhydrophilic and underwater superoleophobic properties was successfully fabricated,which could achieve efficient oil-water separation of 93.7%.This study showed that DLP stereolithography 3D printing technology can fabricate ceramic products with complex shapes and precise dimensions without molds,which has wide application prospects.