Study On The Preparation And Phase Transformation Behavior Of Zirconia-based Ceramics At High Temperature And High Pressure

In recent years,with the rapid development of the ceramics industry,China's ceramic industry has made great progress;however,China still has a long way to go before entering the upper echelon of high-end ceramic manufacturing.With continued research on the properties of ceramic materials,the most fundamental solution is to break through the technical barriers existing in today's ceramic materials.Zirconia(ZrO₂)is a very important ceramic material with high fracture toughness,wear resistance,impact resistance and thermal shock resistance.These characteristics of zirconia cause it to be used in many applications such as cutting tools,refractories,thermal barrier coatings,electrolytes of solid oxide fuel cells,bearings,and so on.The internal phase transformation behavior of ZrO₂ceramics is the main reason for its excellent properties.In high-strength and ultra-fine complex working environments,it is necessary to ensure its excellent mechanical properties and stability.A thorough understanding of the phase transition behavior of ZrO₂ceramics is necessary to ensure stable performance while maintaining excellent properties.As a new sintering technology,the high pressure and high temperature(HPHT)method is very suitable for ceramic applications.Therefore,it is advantageous to sinter ZrO₂ceramics at HPHT and analyze their phase transformation behavior and properties.The main research contents of the study are:First,we used HPHT technology to sinter partially stabilized Mg O-ZrO₂ceramics(Mg-PSZ).A series of Mg-PSZ materials were prepared at different synthesis temperatures(1370–1610°C)at a synthesis pressure of2.5 GPa.The phase composition,microstructure,density,and Vickers hardness of Mg-PSZ were characterized.The sintering behavior of Mg-PSZ under high pressure and the influence of synthesis temperature on the phase transformation behavior and mechanical properties of Mg-PSZ prepared under HPHT were discussed.Secondly,a series of Mg-PSZ materials were prepared at different synthesis pressures(2.5,3.7,and5.0 GPa)and temperatures(1370–1930°C).Then,the phase composition,microstructure,density,and Vickers hardness of Mg-PSZ were characterized.The effects of synthesis pressure on the phase transformation behavior and mechanical properties of Mg-PSZ prepared under HPHT were studied.Third,it is very important to study the phase transition of pure ZrO₂under high pressure.We used the HPHT method to sinter pure monoclinic ZrO₂at different synthesis pressures(3.0,4.0,and 5.0GPa)and temperatures(200–800°C).The phase compositions of all samples at high pressure was investigated with a particular interest trying to explain the translucency phenomenon at 5.0 GPa and 300°C.Fourth,with advanced developments in science and technology,the sintering temperature of ZrO₂-based ceramics can be greatly reduced by adopting novel sintering methods and nano-scale raw materials.Adding liquid phase additives to the sintering of Zr O_2-based ceramics can significantly reduce the sintering temperature.Pure monoclinic ZrO₂-B₂O₃ceramics were sintered at HPHT with B₂O₃as the sintering aid.Low temperature sintering of monoclinic ZrO₂-based ceramics was realized under the combined action of the liquid phase sintering aid,B₂O₃,and high pressure.In this paper,the synthesis and phase transition behavior of zirconia-based ceramics under HPHT were systematically studied,and the main research results are as follows:1.10 mol%Mg-PSZ ceramics were prepared with micron-sized ZrO₂as the raw material at different synthesis temperatures(1370–1610°C)at 2.5 GPa using HPHT synthesis technology,and the influence of synthesis temperature on the phase transformation behavior and mechanical properties of 10 mol%Mg-PSZ prepared under HPHT was analyzed.We found that high pressure increased the sintering driving force,shortened the sintering time,and lowered the sintering temperature.The sintering time of all samples was only 60 min,and a maximum density was obtained at a synthesis temperature of 1530°C,which is 140°C lower than atmospheric sintering.SEM and Vickers hardness analysis show that when the synthesis temperature is lower than the sintering temperature,the high pressure helps in grain refinement.However,when the synthesis temperature is greater than the sintering temperature,the grains tend to grow,and the hardness of the sample first increases and then decreases.When the synthesis temperature is 1370–1450°C,the grains do not grow(1–3?m).When the synthesis temperature reaches 1530°C,the grain size rapidly increases to 24?m,and the maximum hardness of the sample is 14.9 GPa.When the synthesis temperature reaches 1610°C,the porosity increases and the grains coarsen,and the hardness of the sample decreases to 9.6 GPa.XRD and TEM results show the phase transition behavior of 10 mol%Mg-PSZ under HPHT.With increasing synthesis temperature,monoclinic phase content gradually decreases.When the temperature reaches 1530°C,the monoclinic phase disappears,and the sample consists of a cubic phase and tetragonal phase.2.Experiments were run to understand the effect of pressure on the sintering process.10 mol%Mg-PSZ was prepared at different synthesis pressures(2.5,3.7,and5.0 GPa)and different synthesis temperatures(1370–1930°C),and the influence of synthesis pressure on the phase transformation behavior and mechanical properties was analyzed.We found that with increasing synthesis pressure,the transformation of the ZrO₂monoclinic phase into the tetragonal and cubic phases was inhibited,and the phase transition temperature increased.At a synthesis pressure of 5.0 GPa,the temperature at which 10 mol%Mg-PSZ completely transforms into the(t+c)phase(1930°C)is 400°C higher than that at 2.5 GPa.Then,with increasing synthetic pressure,the maximum hardness gradually increases.The maximum hardness of the sample synthesized at 2.5GPa and 1530°C is 14.9 GPa.The maximum hardness of the sample synthesized at 3.7GPa and 1690°C is 15.3 GPa.The maximum hardness of the sample synthesized at 5.0GPa and 1850°C is 15.8 GPa.With increasing synthetic pressure,the crack length at the four corners of the diamond indentation becomes longer and the fracture toughness decreases.Therefore,we used HRTEM and metallographic microscopy to further observe the microstructure of the samples prepared under a synthesis pressure of 5.0GPa and temperature of 1850°C.It was found that there was lattice distortion,dislocation,and other defects in the samples and a great deal of stress at grain boundaries.The helps to explain why the fracture toughness decreases with increasing synthesis pressure.3.Using HPHT synthesis technology,the phase transition behavior of pure monoclinic ZrO₂at 50 nm was studied under different synthesis pressures(3.0,4.0,and5.0 GPa)and lower temperatures(200–800°C).XRD results show that when the synthesis pressure is 4.0–5.0 GPa and the synthesis temperature is 400–600°C,we used HPHT technology to intercept part of the high-voltage orthogonal I phase of ZrO₂.At the same time,the translucent phenomenon of pure monoclinic ZrO₂was discovered.We found that as the synthesis pressure increased,the transmittance of the sample improves,which indicates that the pressure can cause the grains to closely move together,accelerate grain rearrangement,reduce porosity,and increase density.Translucent ZrO₂with a densification of 92%can be prepared at 5.0 GPa in only 30min.We also found that with increasing synthesis temperature at 5.0 GPa,the transmittance of semi-transparent ZrO₂gradually increased in the visible wavelength range and reached a maximum at 800 nm.The light transmittance values under different synthesis temperatures(150,200,250 and 300°C)were 4.1%,13.1%,35.6%,and40.1%,respectively.When the synthesis pressure is 5.0 GPa and the synthesis temperature is greater than 400°C and due to the existence of the high pressure orthogonal phase and increasing number of pores,the refractive index and reflectivity near the grain boundary increase and the translucency of ZrO₂disappears.4.Pure monoclinic ZrO₂-B₂O₃ceramics were prepared by HPHT synthesis technology with 2 mol%B₂O₃as the liquid phase sintering aid at different synthesis pressures(3.0,4.0,and 5.0 GPa)and different synthesis temperatures(700–900°C).We found that with increasing synthesis pressure,the density of ZrO₂ceramics increases and the temperature at which it reaches a maximum approximate density decreases.When the synthesis pressure is 5.0 GPa and the synthesis temperature is750°C,the maximum approximate density is 97.6%,which is 100°C lower than than that at 2.5 GPa.At a synthesis pressure of 3.0 GPa,XRD and TEM results show that the addition of B₂O₃inhibits the formation of the high pressure orthogonal phase and tetragonal phase,and all samples are composed of a monoclinic phase.At 5.0 GPa and varying temperatures,we found that the grain size gradually increased with increasing temperature,while hardness first increased and then decreased.At a synthesis temperature of 750°C,B₂O₃was partially but evenly distributed in the ceramic sintered body,the maximum hardness of the sample was 9.1 GPa,and the grain size was(130±28)nm.Under the combined action of 5.0 GPa high pressure and B₂O₃liquid phase sintering aid,the sintering driving force increased,and the low temperature sintering of pure m-ZrO₂was realized,which provides a new method for researchers to study the sintering and properties of pure m-ZrO₂ceramics.In summary,we used HPHT technology to prepare ZrO₂-based ceramic materials at different temperatures and pressures,and analyzed their phase transformation behavior and mechanical properties,which provided a new research idea to prepare high-performance ZrO₂-based ceramic materials.