Fabrication And Thermal Shock Resistance Of Ca-ZrO₂ Ceramics And FGMs With Porosity Gradient

ZrO₂ ceramic is one of the best candidate material for high temperature structural and ultra-high temperature shielding applications because of its excellent properties,such as high temperature resistance,low thermal conductivity,and low thermal expansion coefficient.However,the phase transformation and bad thermal shock properties of ZrO₂ ceramic limit its applications.In view of zirconia charateristics and advantages as well as the traditional thermal-shock theories,this paper studies mainly effects of the phase composition,the porosity,and the structural design with porosity gradient on mechanical properties,thermal properties and thermal shock resistance of Ca-ZrO₂(CSZ)ceramic materials.As the properties of ZrO₂ ceramic have a good correlation with its phase compositions,preparation of controlled phase compositions of ZrO₂ ceramic is the key issue that will be investigated in this study.Different amount of H₃BO₃ is added to 7wt.%CaO stabled ZrO₂ powder,after pressing,the samples are sintered at the sintering temperature from 1400～1700℃by presureless sintering.The effect of sintering temperature,H₃BO₃ content and heat treatmenton the phase compositions is studied,quantitative phase analysis is carried out by Rietveld method and the mechanism of phase compositions control by adjusting H₃BO₃ content is explored. The results suggest that within these samples after sintering at 1700℃and holding for 2h,tetragonal ZrO₂ phase content increases with increasing the H₃BO₃ content.E.g., 15 wt.%t-CSZ is formed when adding 0.5 wt.%H₃BO₃,whereas 49 wt.%t-CSZ is produced when the H₃BO₃ content is 2.0 wt.%.Thus,the tetragonal and cubic ZrO₂ phases can be well controlled by adjusting the H₃BO₃ content,because of the depletion of CaO,which has reacted with H₃BO₃.When the samples which contain t-CSZ and c-CSZ are heat treated at 1500℃for 1h,Ca precipitating is observed, suggesting the Ca migration from t-CSZ to c-CSZ occurs at a certain temperature.The mechanical properties,thermal properties and thermal shock resistance of the prepared ZrO₂ ceramic with different t-CSZ and c-CSZ content are characterized. The results suggest that the fracture toughness and bending strength increase with increasing the t-CSZ content,the highest value is 3.35 MPa·m^1/2and 267 MPa respectively,whereas elastic modulus shows nearly no variation.For thermal properties,when t-CSZ content increases,the thermal diffusion coefficient and thermal conductivity increase while the thermal capacity and thermal expansion coefficient decreases gradually.The residual strength is tested after quenching the samples for several times.The results show that with increasing the thermal shock times,the bending strength of samples composed of all c-CSZ decreases obviously, whereas the bending strength of samples composed of both t-CSZ and c-CSZ phases increases gradually.It is the added H₃BO₃ that introduces t-CSZ to the CSZ ceramic which apparently improves the thermal shock resistance of the samples.During the thermal shock process,Ca precipitating promotes the internal pressure,this is why the residual strength increases with increasing the thermal shock times.On the other hand,porous ZrO₂ ceramic with different porosities is prepared by adding the zirconia hollow sphere into ZrO₂ powder,the mechanical properties, thermal expansion coefficient and thermal shock resistance are investigated.The results suggest that the hollow sphere content shows ahnost linear variation against the porosity which greatly influences the bending strength of the samples.However, the bending strength shows little changes when the hollow sphere content is higher than 20 wt.%.For thermal expansion properties,samples with different porosities exhibit nearly the same characteristic with stabled ZrO₂ ceramic from room temperature to 1400℃,whereas samples doped with 1.5wt.%H₃BO₃ and hollow sphere show higher thermal expansion coefficient.The bending strength of samples without H₃BO₃ decreases with increasing the thermal shock times,but decreases slow with increasing the hollow sphere content.While the bending strength of samples with H₃BO₃ increases with increasing the thermal shock times.From the phases analysis and microstructure before and after thermal shock tests,it can be clearly seen that porosity and t-CSZ are considered to be two key issues for thermal shock improvement,and it is the t-CSZ that enhances the strength even after thermal shock test.Based on the thermal conductivity of the functionally graded material decreases from the surface to the inside while thermal expansion coefficient increases,it is significant to adjust the thermal conductivity and thermal expansion coefficient by adding t-CSZ and hollow sphere.CSZ ceramic with porosity gradient is prepared and the microstructure,phase compositions and bending strength before and after the thermal shock test are investigated.The results suggest that the microstructures show no change even after thermal shock for several times.The bending strength increases significantly against the thermal shock times,suggesting the prepared material has a good thermal stable property and thermal reliability.