Fracture Toughness And Transformation Toughening Of Ce-ZTA Based On SEVNB Testing Method

Stress-induced phase transformation toughening and micro-crack toughening are two main toughening mechanisms of ZTA composite ceramics.For ZTA materials without stabilizers,micro-crack toughening is the main mechanism to improve the toughness of materials.For ZTA composite ceramics with stabilizers,stress-induced phase transformation toughening is the main mechanism to improve the toughness of materials,and both of them can be found in one composite to toughen the matrix.Therefore,the effect of these two toughening mechanisms on improving the toughness of ZTA ceramics has been widely concerned by researchers.Accurate and reliable fracture toughness value is an important basis for objective analysis of the toughening mechanisms and toughening effect of ZrO2 in ZTA ceramics.Current methods for toughness testing have many shortcomings:too-high or unstable test results and troublesome procedure of sample-preparation.In this thesis,we used femtosecond laser to process an ultra-sharp V-notch with a radius of less than 0.5 ?m at the tip of the U-groove,and applied this ultra-sharp V-notch in the modified SEVNB method to test the ture fracture toughness of Ce-ZTA composite ceramics.Based on the test results,we evaluated the effects of grain size and contents of CeO2 and ZrO2 on the true fracture toughness of Ce-ZTA samples,and further study the phase transformation toughening effect of zirconia on the true fracture toughness of Ce-ZTA samples.The contribution of fracture toughness improvement of Ce-ZTA composite ceramics is discussed.The differences of error values of SENB method between different materials are summarized and compared.The reasons and rules of the changing of error value are studied.In order to study the effect of CeO2 and ZrO2 contents on the volume fraction of transformed t-ZrO2,its phase transformation toughening and the ture toughness of Ce-ZTA specimens,six ceria-stabilized zirconia toughened alumina ceramic specimens with different ceria and zirconia contents were prepared.The test results show that the volume fraction of transformed t-ZrO2 in specimens 8D-80,10D-80,12D-80,8D-85,10D-85,12D-85 are 12.3%,5.0%,3.8%,8.0%,4.0%,2.9%,and their true fracture toughness are 5.210.1 MPa·m1/2,4.9±0.1 MPa m1/2,4.3±0.1 MPa m1/2,5.2±0.1 MPa m1/2,5.7±0.1 MPa m1/2,5.7±0.1 MPa m1/2,respectively.The experimental results indicate that the phase transformation toughening and microcrack toughening of zirconia in Ce-ZTA specimens are combined to improve the fracture toughness of composite Ce-ZTA materials.The fracture toughness of 10D-85 specimens is 45%higher than that of pure alumina ceramics(3.94±0.13 MPa·m1/2).In order to study the effect of grain size on phase transformation toughening of zirconia and fracture toughness of Ce-ZTA samples,two other samples were prepared by changing the material compositions of samples and the raw materials of alumina matrix:8D-90 and 8S-80.The average grain sizes of ZrO2 in 8D-80,8S-80 and 8D-90 are 0.37 ?m,0.57 ?m,and 0.42?m,and the average grain sizes of Al2O3 are 0.69 ?m,1.28 ?m,and 1.01 ?m,respectively.The volume fraction of transformed t-ZrO2 in 8D-80,8S-80 and 8D-90 are 12.3%,3.7%,9.8%,and the true fracture toughness are 5.2±0.1 MPa·m1/2,3.8±0.1 MPa·m1/2 and 4.9±0.1 MPa m1/2,respectively.The testing results of fracture toughness by the modified SEVNB and SENB methods show that the volume fraction of transformed t-ZrO2 of fractured surfaces decreases with the increase of grain size,and the true fracture toughness increases mainly with the decrease of average grain size of ZrO2,but is less affected by grain size of Al2O3.Finally,in order to explore the reasons for the difference of the error value of fracture toughness measured by SENB method between different materials,the volume fraction of transformed t-ZrO2 of fractured surfaces of the modified SEVNB method and SENB method and the error value of various ceramic specimens are statistically analyzed.Then the reason and rule of the changing of error value are summarized.The experimental conclusions are drawn as follows:the error value of SENB method is related to the difference value of volume fraction of transformed t-ZrO2 of fractured surfaces of samples by SENB method and the modified SEVNB method.In this experiment,the true fracture toughness of Ce-ZTA materials is characterized by the modified SEVNB method,which can accurately measure the fracture toughness of fine-grained ceramics.The effects of grain size and contents of CeO2 and ZrO2 on the true fracture toughness of Ce-ZTA samples are studied.The experimental results are of great significance to understand the effect of phase transformation toughening of zirconia in composite Ce-ZTA samples.Secondly,the rule and reason of the changing of error value in SENB method between different materials are summarized and analyzed,which is meaningful for further understanding the reliability of the modified SEVNB method for testing the fracture toughness of ceramic materials.