Preparation And Mechanical Properties Of ZrO₂ And CaHPO₄ Ceramics By Cold Sintering

Sintering has been extensively studied in modern ceramic science for more than 50 years and many new techniques have been developed.In the case of conventional hot sintering of ceramic materials,the sintering temperature needs to be 50%～70%of the melting point,sintering temperature is often higher than 1000℃.Cold sintering technology（CSP）has attracted the attention of many scientists since it was developed in 2016.So far,cold sintering technology has the lowest temperature of all sintering technology,sintering temperature<350℃.Different from the traditional high-temperature thermal activation mechanism,cold sintering relies on high pressure and chemical mechanism.In the high pressure state,the dynamic chemical interaction between liquid solvent and inorganic material surface drives dissolution and causes particle growth.Zr O₂ceramics and Ca HPO₄ceramics have excellent biocompatibility and are widely used in the field of biomedicine.However,there are few reports on cold sintering of these two ceramic materials.In this paper,Zr O₂（pure Zr O₂and 3YSZ）ceramics and Ca HPO₄ ceramics are mainly taken as cold sintering objects to explore the possibility of cold sintering preparation and analyze the mechanical properties.The research contents are as follows:（1）Pure Zr O₂powder and 3YSZ powder were prepared in sheets using graphene oxide as template and trihydroxymethyl aminomethane as precipitator,and the cold sintering process of two zirconia materials was explored.The results show that under the condition of cold sintering at 300℃,500 MPa,holding temperature for 20 min,liquid phase content of 20 wt%and HCl as sintering additive,the relative density of pure Zr O₂ ceramics and 3YSZ ceramics can be obtained by about70%and 61%,respectively.The addition of HCl promoted the phase transformation,particle rearrangement and dissolution-precipitation of Zr O₂,which was conducive to the increase of cold sintering density of Zr O₂.（2）Flake Ca HPO₄·2H₂O powder was synthesized by precipitation method,Ca HPO₄ ceramics were prepared by the powder,and its cold sintering process was explored.The results show that the relative density of Ca HPO₄ceramics reaches 95%and the grain growth is obvious under the cold sintering process conditions of 250℃,450 MPa,holding temperature for 15 min,liquid content of15 wt%and deionized water as sintering additive.However,the relative density of Ca HPO₄ceramics is only 78%by traditional sintering method.The results show that the crystallization water and the added deionized water have a significant effect on promoting sintering.In addition,the effects of different pressures on mechanical properties of Ca HPO₄ ceramics were analyzed experimentally.The results show that the mechanical properties of ceramics under 450 MPa are the best,the flexural strength is 49 MPa,Young’s modulus is 59 GPa and Vickers hardness is 1.72 GPa.（3）Affected by the properties of Ca HPO₄ itself,it is difficult to improve the mechanical properties of compact Ca HPO₄ ceramics.Although it is difficult to sintered in one step at low temperature,sheet zirconia can be added to Ca HPO₄ as an enhanced toughening phase to make up for the low mechanical properties of Ca HPO₄ material itself.In particular,3YSZ containing tetragonical phase also has a phase transition toughening mechanism.Based on this idea,Ca HPO₄·2H₂O/3YSZ powder was obtained by in-situ growth of Ca HPO₄·2H₂O.At 250℃,450MPa,deionized water as sintering additive and liquid content of 15 wt%,Ca HPO₄/3YSZ（40 vol%）multiphase ceramics with a relative density of 85%were obtained.According to the mechanical properties test,the flexural strength,Vickers hardness and fracture toughness of the ceramics are up to 83 MPa,2.76 GPa and 1.73 MPa·m^1/2,respectively,which are 69%,61.4%and 140.3%higher than that of Ca HPO₄ ceramics,indicating that 3YSZ sheets has a significant strengthening and toughening effect.