Research On The Properties Of Porous Ceramics And Their Composites Fabricated By Binder-Jet 3D Printing

Binder-jet 3D printing（Binder jetting）is a rapid prototyping technology that a liquid binder is precisely jetted onto a layer of powder and then bonded together to achieve the complete shape construction.In principle,it integrates printing technology,powder deposition,dynamic binder/powder interaction and post-processing technologies.Binder jetting can overcome the difficulties in preparing complex-shaped ceramic parts and the large sintering shrinkage rate,making the additive manufacturing of complex ceramic parts possible.However,the development of equipment,the synthesis of binder and the method of post-processing that maximize the performance of the binder jetted parts remain a challenge.This paper introduces the principle,procedure and application of binder jetted porous ceramic.Four key factors（powder,printing parameter,binder and post-treatment）for binder jetted porous ceramic are studied.Accordingly,the effect of powder（powder composition design）,printing parameters（particle size distribution,layer thickness,binder saturation）,binder（bonding methods,droplet-formation mechanism,droplet-infiltration kinetics）and the post-processing（curing,sintering）on density,roughness,strength and accuracy are analyzed and discussed.Finally,the influences of printing parameters on the microscopic mechanism of powder adhesion are analyzed in detail,giving guidance on the treatment of raw material and the selection of printing parameters for achieving desired performance.The research work is concluded as follows:In terms of the effect of printing parameters on the accuracy and the strength of binder jetted porous ceramic,a new type of modified silicate binder was applied to the printing process in order to investigate the effects of process parameters on surface quality and the relative density of binder jetted ceramic parts.The binder was composed of butyl acrylate,polyacrylamide,Na₂O·3·2SiO₂,silane coupling agent and water.The effect of binder composition on the bond strength and viscosity were studied.At the optimal concentration of0.6 wt.%silane coupling agent,5 wt.%modified silicate binder had the best printing effect.The influences of layer thickness and binder saturation on the relative density,surface roughness,linear shrinkage and bending strength of the binder jetted ceramic were also analyzed.The results showed that the powder bed density reached the highest when the mixing ratio of coarse powder and fine powder was 4:1.The orthogonal experiment had shown that a layer thickness of 0.1 mm and a binder saturation of 100%was the optimized parameters.The top and side surface roughness values were 27.1±1.2μm and 35.0±2.0μm,and the green density value was 40.1%under the optimized conditions.After sintering,the dumbbell-shaped part printed at optimum printing condition produced a denser and more uniform morphology.In the application of binder jetting technology to prepare Al₂O₃ ceramic core,a decomposable binder was put forward by dispersing different contents of zirconium basic carbonate（ZBC）into a modified silicate binder.After curing and sintering,the micro-ZrO₂particles were deposited in the pores of alumina ceramic powder.By increasing the content of ZBC particles and sintering temperature,the density and mechanical properties of the thin alumina ceramic core were improved.The effects of different ZBC contents on the printability of the binder and the performance characteristics of the ceramic cores by binder-jetting were investigated.The results show that the surface tension of the binder decreases with the increasing of ZBC contents,indicating that the addition of ZBC particles perturbs the interaction between water molecules.The presence of newly generated ZrO₂ particles decomposed by ZBC demonstrated a significant effect on the mechanical properties of the ceramic cores.The sintered density increased by about 44%,the bending strength improved from 60 to 79 MPa,and linear shrinkage decreased from 20 to 13%after sintering at 1500°C as the ZBC content was increased from 0 to 35 wt.%.Micron-sized ZrO₂particles can not only deposit among Al₂O₃particles,but also effectively inhibit the growth of Al₂O₃ grains during sintering to make the microstructure more uniform and smoother.Regarding the selection of binder for manufacturing porous ceramic parts,the viscosity and surface tension of phenolic resin,polymer（PVP）,inorganic binder and ceramic suspension binder（ZBC binder）were compared.The results showed that phenolic resin and inorganic binder were not suitable for printing ceramic parts.Compared with PVP binder,ZBC binder did not break wires in the printing process,which increased the bonding strength of each layer of powder and produced better printing quality.Moreover,the density of ceramic parts printed by ZBC binder was less dependent on sintering temperature.The surface prepared by ZBC binder was more uniform and smoother than that of PVP binder.The pores formed by PVP binder were mainly large pores caused by uneven powder accumulation and incomplete infiltration.While,the pores formed by ZBC binder were mainly small pores due to inherent particle spacing.At the same sintering temperature and binder saturation,the bending strength of ZBC binder was higher than that of PVP binder,and the shrinkage was lower.In the process of preparing porous ceramic structure by binder jetting,nano-HA powder was put forward to fill in the YSZ powder to solve the problem of structural stability and mechanical properties caused by different powder composition.The effects of nano-HA content on the pore formation,surface quality,phase stability,microstructure and mechanical properties of YSZ/nHA structures were investigated.The results showed that with the increase of nano-HA from 0 to 25wt.%,the roughness of the top,side and bottom decreased,and the relative density increased from 39.3 to 48.8%.?After sintering at 1450°C,the YSZ/nHA porous structure showed a reduce in shrinkage（18.1%to 11.5%）and absorption（4.4%to 2.3%）,an increase in compressive strength（30.99MPa to 41.25MPa）.However,when the content of nano-HA increased to 25 wt.%,the overall strength of YSZ/nHA porous structure was weakened by the reaction of nano-HA with zirconia.The sintered phase of YSZ/nHA ceramic presented a new microstructure evolution process,with t-Zr O₂ and Ca₃（PO₄）₂ as main crystalline phase,containing trace amount of Ca_0.15Zr_0.85O_1.85 cubic solid solution.The relative content of Ca₃（PO₄）₂and HA showed a trend of as one falls,another rises.Different size of Ca₃（PO₄）₂ particles were agglomerated and embedded in zirconia particles.With the increase of nano-HA content,the size of pore and grain decreased,while the number and size of bright particles（Ca₃（PO₄）₂）increased.