Study On Preparation Processes And Strengthening-toughening Mechanism Of Multielement Synergistically Toughened Zirconia-based Ceramics

Due to an ideal combination of strength and fracture toughness,good corrosion resistance and wear resistance,stress-induced tetragonal to monoclinic?t?m?phase transformation toughened 3Y-TZP ceramics are widely used in engineering applications.However,with a long-term heating or in a hydrothermal environment,the aging of 3Y-TZP greatly deteriorates its mechanical properties.In addition,3Y-TZP is not applicable at high temperatures because of the t?m transformation sensitivity to temperature.The co-stabilization of zirconia by Ce⁴⁺and Y³⁺can solve these problems efficiently,but the doping of Ce⁴⁺makes the grains coarser and weakens the mechanical properties.In this work,based on the 1.5 mol.%Y₂O₃+5.5 mol.%CeO₂ co-stabilized zirconia with good aging resistance and mechanical properties,0.1-0.3 mol.%of La₂O₃ was added to regulate the t?m phase transformation and reduce the grain size.Through the formation of a non-transformable metastable t'phase,synergistic toughening mechanisms,including ferroelastic domain switching and stress-induced phase transformation are introduced to achieve the strengthening and toughening of the ceramic while ensuring the high-temperature performance.In addition,on the basis of internal phase transformation toughening,external toughening of thermally reduced graphene was introduced,and the corresponding toughening mechanism was studied.3Y-TZP,Ce-Y and Ce-Y-La co-stabilized zirconia ceramics with different rare earth element distributions were prepared by coprecipitation,ball milling and coating methods,respectively.The effects of La₂O₃ dopant and distribution of rare earth stabilizers on the microstructure and mechanical properties of zirconia were investigated.At the same time,the properties of high-temperature phase stability,friction and wear characteristics were evaluated.The tetragonal phase stabilization temperature for the zirconia prepared by the coprecipitation method,the coating method,and the ball milling method respectively increases sequentially increased.The redox of ceria is induced by segregation of trivalent dopants at the grain boundary,and subsequent changings of oxygen vacancy concentration and grain size cause the difference in tetragonal stability of ZrO₂ with different La³⁺contents.The doping of a small amount of La₂O₃ can promote the high-temperature densification process of zirconia,thereby increasing the density of the ceramic.During the grain growth procedure,the pinning of the grain boundary by La³⁺leads to a significant grain refinement effect.After sintering at1600°C,the average grain size of G1.5Y5.5Ce0.3La is 1.05?m,which is 33.5%lower than G1.5Y5.5Ce,and only about 6%larger than G3Y-TZP.The average grain size of C1.5Y5.5Ce0.3La is reduced by about 16.2%compared to G1.5Y5.5Ce0.3La.Within the range of solubility,the addition of a small amount of La₂O₃ is beneficial to the improvement of mechanical properties.After sintering at 1600°C for 2 h,the hardness of G1.5Y5.5Ce0.3La,C1.5Y5.5Ce0.3La and Q1.5Y5.5Ce0.3La reaches 1202.7 HV₁₀,1091.7HV₁₀ and 1080 HV₁₀,respectively.The corresponding fracture toughness is 7.92 MPa·m^1/2,10.16 MPa·m^1/2 and 10.43 MPa·m^1/2 respectively,which is greatly improved compared with Ce-Y co-stabilized ZrO₂ prepared by the same process.For C1.5Y5.5Ce0.3La with a phase transformation rate similar to C3Y-TZP,the enhanced ferroelastic elastic toughening due to grain refinement gives it the higher fracture toughness.The fracture toughness of C1.5Y5.5Ce0.3La is 24.8%higher than that of C1.5Y5.5Ce.The values are 78.9%and 23.5%higher than those of G3Y-TZP and C3Y-TZP,respectively.Due to the non-uniform distribution of rare earth elements,the t?m transformation sensitivity is enhanced.The fracture toughness of zirconia prepared by coprecipitation,coating and ball milling respectively increases with the increase of elemental non-uniformity.The toughening mechanism changes from t?m transformation toughening of 3Y-TZP to synergistic toughening of phase transformation and ferroelastic switching.The flexural strength of G1.5Y5.5Ce0.3La,C1.5Y5.5Ce0.3La and Q1.5Y5.5Ce0.3La is 917.5 MPa,740.6 MPa and883.4 MPa,respectively.Compared to Ce-Y co-stabilized ZrO₂ prepared by the same process,the values are increased by 4%,23.1%,and 12.5%,respectively.Grain refinement strengthening is the main strengthening mechanism.The addition of a low level of La₂O₃ is beneficial to the improvement of the stability of the tetragonal phase at high temperatures.After heat treatment at 1482°C for 192 h,3Y-TZP prepared by different processes shows severe tetragonal phase instability,while G1.5Y5.5Ce0.3La and C1.5Y5.5Ce0.3La show good high-temperature tetragonal phase stability.Despite the high mechanical properties,zirconia prepared by ball milling is not suitable for high-temperature applications.The tribological properties show that the zirconia prepared by the coating method has better wear resistance than those by the other two processes.Plastic deformation and delamination are the main wear mechanisms of rare earth stabilized zirconia ceramics.In addition,GO/3Y-TZP mixed powders with uniform GO dispersion were prepared by a colloidal method.In-situ thermally reduced and pre-reduced graphene oxide toughened zirconia ceramics were prepared by SPS.Graphene oxide can be thermally reduced in situ during sintering,and the obtained IrGO has few layers and is uniformly dispersed.The fracture toughness of IGZ0.09 increases from 6.07 MPa·m^1/2 of 3Y-TZP to 10.64 MPa·m^1/2,while the fracture toughness of rGZ is about 7.12 MPa·m^1/2,which is much lower.Further increasing the GO content leads to an increase in the number of IrGO layers and a decrease in fracture toughness.Grain refinement results in a slight increase in hardness.Among them,the hardness of IGZ0.09 and rGZ0.09 ceramics are 1343.3 HV₁₀ and 1361.5 HV₁₀,respectively.In addition to the conventional toughening mechanism,the higher fracture toughness of IGZ compared to rGZ and 3Y-TZP is related to the C-O-Zr bond between the IrGO and zirconia matrix.The presence of a small amount of rGO or IrGO results in a slight increase in the friction coefficient of the 3Y-TZP ceramics,while the wear resistance is significantly enhanced.The wear resistance of IGZ0.5 increases by 80%compared to 3Y-TZP.In terms of improved wear resistance,in-situ thermally reduced graphene oxide is better than pre-reduced graphene oxide due to better interfacial bonding.