Study On The Densification,Structure And Properties Of Sintering Structural Ceramics By High Pressure

In the conventional pressureless sintering process of ceramic materials,ceramic powder densification is realized by atomic diffusion,but grain growth is accelerated inevitably during the final stage of sintering,thus degrading the mechanical properties of the obtained ceramic materials.High-pressure sintering can not only solve the problems of incomplete densification,grain growth,and intracrystalline pores in the conventional pressureless sintering process,but also endow structural ceramics with unique microstructures and mechanical properties through plastic deformation densification mechanism.However,the densification mechanisms,microstructures or kinetics of the high-pressure sintering process of structural ceramics have not been well explored and the plastic deformation densification mechanism still need to be further experimentally verified.The study aims to clarify and verify the densification mechanism of structural ceramics sintered under high pressure.Firstly,with Al₂O₃ as the research target,the fully dense alumina ceramic materials with limited grain growth were prepared by MPa-scale high-pressure sintering at relatively low temperature.The microstructure evolution,grain boundary energy,and residual stress of high-pressure sintering were studied and compared with those of conventional sintering processes.Secondly,the mechanical properties and high-temperature plastic deformation behavior of rapid high-pressure sintered large-sized alumina ceramics were studied.Finally,structural Al₂O₃ and ZrB₂ ceramics with an ultra-high melting point were respectively prepared under GPa-scale pressure.The dominant densification mechanism of ceramics under extreme sintering conditions and its promoting effect on sintered ceramics were investigated.The internal microstructures,grain boundary,grain size,and comprehensive performance of ceramic materials prepared under ultra-high pressure were systematically characterized.Our team described the relationship between the structure and properties of ultra-high-pressure sintered ceramic materials from the perspective of micro-size scale and discussed the feasibility of ultra-high-pressure sintering of ceramic materials.The study will promote the development of the new ultra-high-pressure sintering technology in ceramic materials.The main contents and conclusions are summarized as follows:1)Two kinds of commercially pureα-Al₂O₃ powders,with average particle sizes of 220 nm and 3μm,were densified via spark plasma sintering at relatively low temperatures and under high pressures from 30 to 200 MPa.The densification temperature and the starting threshold temperature of grain growth(T_sg)were determined by the applied pressure and the surface energy relative to grain size,as they were both observed to increase with grain size and to decrease with applied pressure.The grain boundary energy,residual stress,and dislocation density of the ceramics sintered under high pressure and low temperature were higher than those of the samples sintered without additional pressure through the microstructure and morphology characterizations.Plastic deformation occurring at the contact area of the adjacent particles was proved to be the dominant mechanism for sintering under high pressure,and a mathematical model based on the plasticity mechanics,tribology and close packing of equal spheres was established.Based on the mathematical model,the predicted relative density of an Al₂O₃ compact can reach～80%via the plastic deformation mechanism,which fits well with experimental observations.The densification kinetics at the final stage were investigated from the sintering parameters,i.e.,the holding temperature,dwell time,and applied pressure.Diffusion,grain boundary sliding,and dislocation motion were assistant mechanisms in the final stage of sintering,as indicated by the stress exponent and the microstructural evolution.The deformation tends to increase defects and vacancies generation at the final stage,both of which accelerate lattice diffusion and thus enhance grain boundary immigration and dynamic grain growth.2)High-pressure sintering could be used to prepare alumina ceramics with smaller grain size and improve high-temperature plasticity by changing grain boundary structures.In our study,alumina ceramics sintered under the conditions of 80 MPa,1150℃and a heating rate of 100℃/min showed widened grain boundaries and high-temperature plasticity occurred at 1200℃,which was nearly 300℃lower than that of alumina ceramics obtained by pressureless sintering.The high-temperature plastic deformation mechanisms were mainly grain boundary sliding as well as pores,60°twin boundary and dislocation climbing along grain boundary.However,as for alumina ceramics obtained with a heating rate of 5℃/min,the high-temperature plastic deformation mechanism was mainly grain boundary sliding and grain boundary diffusion as well as 60°twin boundary.Therefore,the work hardening effect of alumina obtained under high-pressure sintering was attributed to twin boundaries and dislocation multiplication along grain boundaries.The shape of grain boundary became curved due to grain boundary sliding.Consequently,high-pressure sintering under a rapid heating rate provided an approach to improve the high-temperature plasticity of alumina ceramics so as to meet the requirements for high-temperature machinability.3)When the sintering pressure increased to the GPa scale,fully dense ceramics were obtained even at lower temperature and possessed unique microstructures and excellent mechanical properties.In our study,the densification mechanisms of alumina ceramics under GPa-scale pressure（1.5 GPa to 15 GPa）were discussed.Nano-sized alumina ceramics with high internal residual compressive stress,stable coherent grain boundaries and remarkable mechanical properties were successfully prepared at low temperature（600℃）under the ultra-high pressure（15 GPa）.The obtained alumina ceramics exhibited high hardness（26.0 GPa）and fracture toughness(3.18 MPa·m^1/2).The hardness and fracture toughness of the obtained alumina ceramics were respectively 36%and 51%higher than corresponding values of sapphire.The dominant densification mechanism of high-pressure sintering under 15 GPa was plastic deformation of grains and grain boundaries,which contributed to 96.3%of the relative density.With the increase in temperature,the compressive prestress gradually decreased and the unique grain boundary structure disappeared.The comparison results of the microstructure evolution,mechanical properties and residual stress of alumina samples sintered at various temperatures and after heat treatment indicated that the substructures near high-angle coherent grain boundaries could produce high residual compressive stress,which was the main strengthening mechanism of alumina ceramics.4)Though ZrB₂ structural ceramics is regarded as one of the most promising ultra-high melting point materials applied to extreme hard environments,the sintering of ZrB₂ ceramics is very difficult due to its low diffusion coefficient,which limits its industrial applications.In our study,fully dense and grain-refined ZrB₂ was successfully prepared under the ultra-high pressure of 15 GPa at low temperature of1450℃.The as-prepared ZrB₂ ceramics exhibited excellent mechanical and oxidation-resistant properties.The grain size of ZrB₂ ceramics was 60%smaller than that of raw powder.The hardness and fracture toughness of ZrB₂ ceramics sintered under the conditions of 15 GPa and 1450℃were respectively about 46%and 69%higher than corresponding values of ZrB₂ ceramics sintered under the conditions of 2000℃and200 MPa.The dislocation density was also 3 orders of magnitude higher,but the grain size was considerably decreased by 96%.The grain refining,substructures and high dislocation density which were introduced by the plastic deformation densification mechanism could enhance the overall mechanical properties based on the work-harden effect,Hall-petch and Taylor dislocation density forest effect.Additionally,due to the unique structure,its threshold oxidation temperature in air was 1100℃,which was about 250℃higher than that of high-temperature sintered ZrB₂.A developed densification mechanism of dislocation multiplication with grain refining is proposed and proved to dominate ultra-high pressure sintering,which is responsible for simultaneous improvements in mechanical and oxidation-resistant properties.