| The preparation of polycrystalline bulk ceramics mostly uses nanopowders as the starting material.Generally,high-performance and high-densitify nano-polycrystalline bulk ceramics are synthesized by traditional sintering methods at high-temperature for a long holding time.However,the high surface energy of nanopowders usually leads to the inevitable phenomenons of agglomeration and adsorption,which induces grain growth during high-temperature dominated sintering process that consequently causes the disappearance of fine grains?nanostructures and submicron structures?and the degradation of mechanical properties.In addition,it is difficult to prepare the uniformly dispersed nanopowders.At present,how to suppress the grain growth and maintain the characteristics of nanostructures during high-temperature sintering process is a technical problem in preparing high-densitify and high-performance polycrystalline bulk ceramics.In this study,the high-pressure phase transformation of microcrystalline powder overcome the technical problem,and obtained high-performance and high-densitify polycrystalline bulk ceramic materials.Most ceramic materials have the phenomenon of homogeneity and polymorphism.It is possible to occur phase transformation from one phase structure to another phase structure under appropriate thermodynamic conditions.Phase transformation is a process in which the old phase disappears and the new phase appears.Regardless of the grain size of the starting material,the formation of new phase must undergo a process of nucleation and growth.Direct transformation of microcrystalline starting material under high pressure and high temperature?HPHT?has two advantages:the use of micro-sized powder guarantees the formation of a number of clean grain boundaries,which can enhance the bonding strength between adjacent new grains.HPHT conditions can facilitate the phase transformation,increase the nucleation rate,and suppress the grain growth.These two factors ensure that densely bonded fine grains are obtained,and the grain size of new phase is smaller than that of the starting powder.In this study,high-performance submicron-sized polycrystalline?-Si3N4 bulk materials was synthesized using micron-sized?-Si3N4 powder?2?m?as the starting material under the condition of 5.5 GPa and 1500-1900°C.The research results indicate:1.The full phase transformation from?-Si3N4 to?-Si3N4 was incompleted at 5.5 GPa and 1500-1700°C.But full transformation was observed under 5,5 GPa,1800°C for no less than 5 min and 1900°C for no less than 3 min.And the mean grain size of fully transformed submicron-sized polycrystalline?-Si3N4 bulk samples increases as the holding time extends.2.The well-prepared polycrystalline?-Si3N4 bulk sample obtained at 5.5 GPa,1900°C for 3 min has a mean grain size of 263±68 nm.The measured Vickers hardness HV of the sample could reach up to about 20 GPa,approaching the upper limit of the single crystal?-Si3N4.The measured fracture toughness KIC of the sample achieved 4.1 MPa m1/2,which is36%tougher than the?-Si3N4 compact(KIC?3.0 MPa m1/2)obtained by HIP.3.In this study,the transformation mechanism from?-Si3N4 to?-Si3N4 is that the molten liquid Si3N4 phase could be formed when the temperature reached or approached the melting point of?-Si3N4,and then the?-Si3N4 crystals will be formed by the nucleation-growth mechanism of liquid-solid phase transformation.Therefore,the fully transformed submicron-sized polycrystalline?-Si3N4 bulk samples could be obtained.4.Reducing the grain size can not only enhance the hardness but also increase the fracture toughness for the fully transformed submicron-sized polycrystalline?-Si3N4 bulks,which break through the old paradigm?the higher the fracture toughness of a material,the lower the hardness.The hardness and fracture toughness of the submicron-sized polycrystalline?-Si3N4 bulks increase simultaneously as the grain size decreases,which comply with the Hall-Petch effect.5.The high-performance submicron-sized polycrystalline?-Si3N4 bulk materials could be synthesized using the micron-sized?-Si3N4 powder as the starting materail by high-pressure transformation,which overcomes the grain-growth problem of polycrystalline?-Si3N4 synthesized by conventional methods.This synthetic technical provides an efficient method to prepare good quality submicron-sized or nanosized polycrystalline bulk ceramics by high-pressure phase transformation of microcrystalline powder in the field of ceramic materials. |
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