Research On High Quality Structural Ceramics Fabricated By Oscillatory Pressure Sintering And Corresponding Mechanisms

High-quality structural ceramics have attracted great attention in the fields of energy and environmental protection,mechanical engineering,aerospace and medical care due to their excellent performance.However,with the advancement in technology,great improvements of relative performance in ceramic materials are needed.However,it is still difficult to obtain structural ceramic materials with high density and ultra-high strength by current sintering techniques.To solve the above issues,on the basis of the constant pressure assisted sintering,a novel idea on dynamic oscillatory pressure applied in the sintering process of ceramic powders has been proposed by our group.Based on the new sintering idea,a new generation of dynamic oscillatory pressure sintering equipment was researched and developed,after that,several typical structural ceramics were fabricated by oscillatory pressure sintering（OPS）.Combining with density,grain size,microstructure,and fracture morphology,the influence of oscillatory pressure on the densification process and subsequent performance of ceramic materials were deeply discussed.In addition,a comparative investigation of conventional pressureless sintering（PS）and hot-pressing sintering（HP）was performed to explore the sintering and fracture mechanisms of oscillatory pressure sintering process in ceramics.Studies have shown that the densification temperature of zirconia ceramics is reduced by 150°C compared to PS and HP processes aided by the oscillating pressure.Compared with the conventional sintering methods,the OPS process promotes particle rearrangement and the removal of pores as well as grain boundary sliding and plastic deformation during the intermediate and final stages of sintering.With the sintering process changing from PS and HP to OPS,the bending strength of zirconia ceramics increased from 856MPa,1305MPa to 1455MPa,accompanied with the Weibull modulus increased from 10.71 to 11.75 to 17.24.Considering the various fracture modes and elastic modulus,the oscillatory pressure strengthens the grain and grain boundary of the zirconia ceramic.The densification and mechanical properties of alumina ceramics in the OPS process were all affected by the sintering temperature,soaking time,amplitude and frequency of oscillatory pressure.Significantly,the application of oscillatory pressure has an active effect on the densification process and inhibits grain growth.Furthermore,the difference in grains sizes was reduced and approximately equiaxed alumina with fine grains was obtained.According to the variation of the lattice parameters and the elastic modulus of alumina,the oscillatory pressure helps to strengthen the grain strength,promote the removal of the pore defects,and thus improve the mechanical properties.With the aid of oscillatory pressure,the silicon nitride ceramics reveal a complete conversion fromαphase toβphase,and the sintering driving force is significantly improved.With the analysis of the crack paths,extra crack deflection and crack branch toughening mechanisms were observed in the OPS specimens compared to the HP specimens,and the fracture toughness was further improved.For Si₃N₄-SiC_w composite ceramics,the introduction of oscillatory pressure increases the grain length and aspect ratio,and the interface bonding strength between the whisker and the matrix is also enhanced.The Weibull modulus increases from 17.60 to 22.11 with the change of sintering process from HP to OPS.Combining the IOP value and the microscopic morphology,the driving force introduced by the oscillatory pressure effectively promote the directional growth and alignment of the h-BN grains compared with the constant pressure.The high proportion of parallel platelets in the OPS sample indicates high degree of densification,and the fracture reliability and thermal conductivity are further improved.