Reliable Evaluation Of Fracture Toughness Of Ceramics Tested By SEVNB Method Based On Sub-micrometer V-notch

Accurately evaluating the fracture toughness of ceramics has always been of great concern to material researchers.However,conventional fracture toughness testing methods have their own disadvantages.They either cannot accurately determine the actual fracture toughness of ceramic materials or cannot be widely used due to the challenge of sample preparation.In this thesis,femtosecond lasers were successfully used to induce ultra-sharp V-notch with a tip radius of less than 0.5?m on ceramic samples.The conventional SEVNB method was improved to measure the actual fracture toughness of ceramic materials quickly,accurately,and reliably.This modified method provides a basis for rational design,selection and wider application of ceramic materials.In order to study the influence of the femtosecond laser thermal effect on the fracture toughness determined by the modified SEVNB method,ultra-sharp V-notches were cut on the sintered ceramic specimen and the ceramic green body using a femtosecond laser.For the sintered ceramic specimen,no microcrack zone can be found in front of the V-notch tip,but a molten layer with thick of 2?m can be observed on the surface of the V-notch.In addition,after the green body with the V-notch was sintered,the sharpness of the V-notch tip did not change.Meanwhile,the microstructure and crystalline structure of the V-notch surface is the same as that of the matrix.By comparing the fracture toughness of the 3Y-TZP,Al₂O₃,and 8Y-FSZ samples with the V-notches cut by the two different preparation methods measured by the modified SEVNB method,it is found that the influence of femtosecond laser thermal effect on the fracture toughness test can be ignored.In order to study the effect of notch depth on the fracture toughness tested by the modified SEVNB method,U-grooves with different depths were processed on the1.5Y/3Ca/6.5Al/1.5Si-PSZ,2.5Y-TZP,3Y-TZP,Al₂O₃ and 8Y-FSZ sample by diamond blades,and an ultra-sharp V-notch was induced at the bottom of the U-groove using a femtosecond laser.The fracture toughness of all the samples was tested by the modified SEVNB method.The results reveal that the actual fracture toughness test requires a deep notch for ceramic sample with a steep R resistance curve.A shallow notch is available to test the actual fracture toughness of ceramic samples without R resistance or with weak R resistance curves.In addition,the notch depth a/w?a:notch depth,w:sample thickness?of 0.56-0.72 can be used for the actual fracture toughness test of yttrium stabilized Zr O₂ ceramics.In order to analyze the load-displacement curves of fracture toughness testing,the fracture toughness of Si₃N₄,3Y-TZP,SiC and 8Y-FSZ ceramic samples was measured by the modified SEVNB method.The results show that the crack experiences a stable propagation stage in the ceramic materials with R resistance curve.However,for ceramic materials without R resistance curve,specimens will encounter brittle fracturing.The fracture toughness of 8Y-FSZ ceramic specimens was also tested under different loading rates and test environment.The results show that the load-displacement curves are affected by the test environment and loading rates in brittle ceramic samples.Low loading rate should be avoided to measure the fracture toughness,and moisture corrosion at the crack tip should be prevented when testing fracture toughness by the modified SEVNB method.Based on the research of fracture toughness of 3Y-TZP ceramics in our previous work,we evaluated the fracture toughness of ZrO₂ ceramics with different phase compositions.Firstly,the 2Y-TZP and 2.5Y-TZP ceramic samples were prepared by the co-precipitation method.Compared with the fracture toughness of commercial3Y-TZP ceramics(4.4?0.1 MPa.m^1/2),the fracture toughness of 2Y-TZP and2.5Y-TZP ceramic samples increased 45%and 20%,respectively.Secondly,for pressureless sintered ZrO₂ ceramic materials,we established the relationship between fracture toughness measured by SENB method and actual fracture toughness measured by SEVNB method(K_SENB=1.07(K_Ic)^1.37).Using this formula,the fracture toughness value measured by SENB method can be used to estimate the actual fracture toughness of yttria-stabilized zirconia ceramic samples.Finally,for the yttria-stabilized zirconia ceramics prepared by the co-precipitation method,the fracture toughness of the yttria-stabilized zirconia ceramics was evaluated by the IM method.The results show that,for the brittle zirconia ceramics,the values tested by IM method are closer to their actual values.On the contrary,for the toughed zirconia ceramics,the values tested by IM method have large difference with their actual values.In order to improve the fracture toughness of Y-TZP ceramics,we also prepared TZP ceramics with inhomogeneous yttrium distribution by a mixture method.The fracture toughness was measured by the modified SEVNB method.The results show that although the yttrium distribution of Y-TZP prepared by the mixture method is not homogeneous with that prepared by co-precipitation method,the inhomogeneous yttrium distribution leads to a deeper phase transformation.Therefore,it is feasible to prepare Y-TZP ceramic by the mixture method.When the content of yttrium oxide is2.5 mol%,the highest fracture toughness can be obtained.In this thesis,the reliability of the fracture toughness of ceramics tested by the modified SEVNB method was studied,and the influence of thermal effect induced by femtosecond laser and notch depth on the fracture toughness test was analyzed.Therefore,this work provides guarantee for widely using the modified SEVNB method to test the actual fracture toughness of ceramics.Secondly,the load-displacement curves of fracture toughness measured by the modified SEVNB method are analyzed,which is of great significance for understanding the fracture behavior of ceramic samples.Finally,Y-TZP ceramics with different phases were prepared by co-precipitation method and mixture method,and their actual fracture toughness was evaluated,which will provid an important reference for further enlaging the application of zirconia ceramics.