Study On Fabrication Of MgO·nAl₂O₃?n=1,1.44? Transparent Ceramics By A Multi-step Sintering

As a structural and functional integration material,magnesium aluminate spinel transparent ceramics?Mg-Al TC?show immerse demand and potential application value in military and civil fields.Compared to sapphire and aluminum oxynitride,Mg-Al TC has excellent optical properties,such as a higher cut-off edge in the mid-infrared region,but inferior mechanical properties.By controlling the microstructure,the Mg-Al TC with both excellent mechanical and optical properties could been obtained,which request that grain size is reduced less than a few microns and the residual porosity is below than 0.01%.However,it it will be a great challenge to the art of Mg-Al TC fabrication.In this study,an innovative strategy called multi-step sintering process was proposed and implemented,which including Spark plasma sintering?SPS?,Pressureless sintering?PS?and Hot isostatic pressing?HIP?.The arrangement of above sintering technologies and design of their sintering parameters based on the idea that the microstructure of the green body could affect the sintering performance of the ceramic during the subsequent sintering process.Firstly,SPS was adopted for partial densification to obtain a porous body with uniform pores and fine grains.The special microstructure provides high sintering activity for the porous body,such as fast densification and lower densification temperature exhibited during the subsequent pressureless sintering.Finally,the Mg-Al TCs were fabricated by HIP sintering.In MgAl₂O₄,the activation energy between densification and grain growth are comparable,which means that the densification process and the grain growth process are difficult to be separated.By suppressing the grain growth in the early stage of sintering,we promote subsequent densification and successfully avoide the separation of pores from grain boundaries during the final stage of sintering.The optimized SPS conditions were at temperature of 1425°C under a pressure of 20 MPa for 1 min.The obtained porous body had a relative density of 88%,grain and pore size of 230 nm and28 nm,respectively.In contrast,the grain and pore sizes were 730 nm and 77 nm,respectively,in the sample densificated by the PS when the the same density was reached.In the subsequent PS process,the SPSed sample exhibited a higher densification rate than that of the PSed sample,which could be explained by adoptting the intermediate-stage sintering of Coble's model.In addition,the exponent and activation energy of the grain growth shown that the grain growth of MgAl₂O₄ is pore displacement controlled by lattice diffusion?oxygen vacancy?.The sintering trajectory combined with the microstructure observation showed that the SPS sample exhibited a flattening sintering path and a higher critical density for open pores closure,which effectively inhibited the pore-grain boundary separation at the final stage of the pressureless sintering.The optimized SPS condition for the porous ceramic of MgO·1.44Al₂O₃ was at a temperature of 1400°C under a pressure of 30 MPa for 1 min.The obtained porous body had a relative density of 85%with grain size and pore size of 225 nm and 29 nm,respectively.The apparent densification and grain growth activation energy of MgO·1.44Al2O3 are 642±84 kJ/mol and 943±49 kJ/mol,respectively,which indicate the effect of microstructure on sintering process clearly.Potentially,the densification of MgO·1.44Al2O3 is related to the formation or diffusion of oxygen vacancy.Meanwhile,the increase of grain growth activation energy could attributted to the fine and homogeinous pore network in the SPSed porous body which hinders grain boundary movement.Finally,the fine-grained MgO·nAl₂O₃?n=1,1.44?transparent ceramics were prepared by HIP at tempreture of 1500°C under a pressure of 180 MPa for 5 h.The grain size of the obtained MgAl₂O₄ and MgO·1.44Al₂O₃ were 1.9?m and 1.8?m,respectively.And the Vickers hardness?HV1?was about 13.9 GPa.The the in-line transmittance of MgAl₂O₄ and MgO·1.44Al₂O₃ samples maintained 80%?thickness2.0-2.1 mm?range over 1-5?m,and reached 62%and 57%at wavelength of 550 nm,respectively.The fine-grained Mg-Al TC with different compositions have be prepared by the multi-step sintering process in this study,which has high hardness and transmittance.Besides,partial densification by SPS could used to obtain porous body with fine and homogeneous microstructure wich promote the pressureless sintering process.We believe that multi-step sintering process could also applied to the other materials where fine-grained and fully dense microstructure are desired.