Study On Sintering Process And Hydrothermal Aging Properties Of Ternary Rare Earth Co-stabilized Zirconia Based Ceramics

Zirconia ceramics have the advantages of high strength,high hardness,low thermal conductivity,high corrosion resistance and excellent biocompatibility,etc.They are widely used in medical instruments,auto parts,aerospace and other fields.Among them,the medical scalpel is a new field of application.However,the preparation of the medical scalple by using zirconia ceramics has the following two problems:（1）The characteristics of zirconia ceramics determine that they lack the plastic deformation ability similar to metals and have poor fracture toughness;（2）Zirconia ceramics have the phenomenon of low-temperature aging,and the high temperature steam sterilization process involved in medical scalpel and the application environment of hot and humid require that zirconia ceramics have good hydrothermal aging resistance.These two problems limit the application of zirconia ceramics in the field of scalpel.In order to improve the phase transformation toughening effect,rare earth stabilizers are often added to zirconia ceramics,whose function is to stabilize the medium and high temperature tetragonal（t-Zr O₂）to the room temperature.Compared with single rare earth stabilized zirconia based ceramics,multiple rare earth co-stabilized zirconia ceramics can not only give play to the synergistic toughening advantages of multiple rare earth elements,but also make up for the deficiency of single rare earth element doping.However,the traditional co-precipitation method to prepare multielement rare earth co-stabilized zirconia powders has some problems,such as complicated process and agglomeration.Therefore,a novel coating method was used in this work to prepare totally stable zirconia powders with Y-Ce-La.Then the parameters of pressureless sintering process was optimized,and the factors influencing the hydrothermal aging performance of zirconia ceramics with Y-Ce-La were explored.The aim is to provide a technical reference for the production of ceramic scalpels with excellent comprehensive mechanics while taking into account good anti-hydrothermal aging performance.By using rare earth yttrium nitrate,cerium nitrate,lanthanum nitrate and nano monoclinic zirconia powders as raw materials,the novel coating method is used to prepare 1.5mol.%Y₂O₃-5.5mol.%Ce O₂-0.3mol.%La₂O₃ to stabilize the zirconia powders（1.5Y5.5Ce0.3La-Zr O₂）.Combined with cold isostatic pressing and pressureless sintering process,the densification and grain growth during the sintering process was systematically discussed.The effects of sintering temperature,holding time and heating rate on Vickers hardness,fracture toughness and flexural strength of Y-Ce-La co-doped zirconia-based ceramics were studied.The results show that the stability degree of t-Zr O₂ increases gradually with the increase of sintering temperature,and the densification process mainly occurs between 1200℃and 1550℃.When the sintering temperature is 1550℃,the relative density of the sample reaches more than 98.80%,and the fracture toughness is 10.81 MPa.m^1/2.As the holding time prolongs,the relative density and mechanical properties of the samples increase firstly and then decrease,and the best comprehensive mechanical properties are obtained when holding time is 1.5 h.Shortening the holding time reduces the relative density of ceramics.Prolonging the holding time makes the grains of ceramics grow,and reduces the mechanical properties of zirconia ceramics.The optimal sintering process parameters are as follows:sintering temperature 1550℃,holding time 1.5 h,and heating rate 1.0℃/min.The effects of the content and type of rare earth stabilizer,hydrothermal aging temperature,aging time and two-step sintering holding time on the hydrothermal aging properties of zirconia based ceramics were studied.3Y-Zr O₂,Y-Ce and Y-Ce-La co-stabilized zirconia ceramics were prepared by the coating method.The results show that 3Y-Zr O₂ has the worst resistance to hydrothermal aging.After hydrothermal aging,there are more large grains.A lot of tetragonal phases transform into monoclinic phases,and the mechanical properties of 3Y-Zr O₂ decline remarkably.The anti-aging property of 1.5Y5.5Ce-Zr O₂ is improved to some extent.After hydrothermal aging,the grain size of zirconia does not coarsen.The monoclinic phase content increasing rate slows down,and the comprehensive mechanical properties of 1.5Y5.5Ce-Zr O₂are better than 3Y-Zr O₂.1.5Y5.5Ce0.3La-Zr O₂ has the best hydrothermal aging performance.The addition of a tiny amount of La₂O₃ can obviously refine the grain size,thus the anti-aging performance can be significantly improved.The prolonging aging time and the higher hydrothermal aging temperature have different effects on hydrothermal aging properties of1.5Y5.5Ce0.3La-Zr O₂,especially the increase of hydrothermal aging temperature accelerates the aging process remarkably.Although the mechanical properties of 1.5Y5.5Ce0.3La-Zr O₂prepared by the two-step sintering process（TSS）are slightly lower than those of the same material prepared by the single-step sintering process（SSS）,they are basically not affected by hydrothermal aging even at a high temperature（200℃）.Prolonging the two-step sintering holding time is beneficial to the improvement of hydrothermal aging resistance.