Fracture Toughness And Transformation Toughening Of Zirconia Ceramics Based On Single-edge V-notched Beam(SEVNB) Testing Method

Accurate measurement of fracture toughness is a major requisite for understanding the fracture behavior in ceramics.The fracture toughness determination of zirconia always been a controversial subject because a wide range of methods are developed to determine the KIc values,which greatly affected the study of the transformation toughening of zirconia.In this dissertation,we used a femtosecond laser to introduce an ultra-sharp V-notch(? ? 0.25 ?m)on three different yttria concentrations zirconia(3%,5% and 8%)ceramics for actual fracture toughness testing by SEVNB method.As obtained KIc values and the volume fraction of mZrO2 on fractured surface are compared with that of SENB method.And then a comprehensive study of the dependence of the KIc value and the transformation toughening of zirconia upon the phase compositions and the grain sizes of zirconia,the loading rates is carried out in this work.In order to investigate the influence of the yttria concentrations on the fracture toughness and transformation toughening of zirconia,three zirconia samples with different phase compositions were prepared: 3Y-TZP(?100 vol% t-ZrO2),5Y-PSZ(64 vol% t-ZrO2 + 36 vol% t-Zr O2)and 8Y-FSZ(?100 vol% c-ZrO2).The testing results show that the testing process of SEVNB method does not require the energy of crack initiation,and the actual KIc values for three zirconia samples measured by this method are 4.4,2.8 and 1.6 MPa?m1/2 for 3Y-TZP,5YPSZ and 8Y-FSZ,respectively.The volume fraction of m-ZrO2 on fractured surface of 3Y-TZP sample is 13 vol% for SENB method and 10 vol% for SEVNB method,however,for the 5YPSZ and 8Y-FSZ samples,both of values are 0 vol%.This can be considered that the cubic zirconia can't transform into the monoclinic phase and the fracture load during the fracture toughness testing is too small to make the tetragonal phase transform.To study the effects of loading rates on fracture toughness and the transformed amounts of zirconia,three loading rates of 0.05 mm/min,0.03 mm/min and 0.01 mm/min were applied by SENB and SEVNB testing for 3Y-TZP and 8Y-FSZ samples,respectively.The results show that only the load-deflection curves of 3Y-TZP samples measured by SEVNB method can be exhibit a fracture behavior similar to plastic deformation,resulting from the crack stable propagation.In addition,the fracture toughness and the transformed amount of t-ZrO2 on the fractured surface measured by SEVNB method are slightly decreased for 3Y-TZP samples,whereas for 8Y-FSZ samples those values remain stable as the loading rate decreasing.Finally,In order to study the effects of grain sizes on KIc value and the transformation behavior of 3Y-TZP,four group 3Y-TZP samples with the grain size of 374 nm,655 nm,738 nm and 1062 nm,were prepared by controlling the sintering schedules.The SENB and SEVNB testing results show the KIc values of 3Y-TZP samples are increasing with increasing the grain size.The transformed amount of t-Zr O2 was quantified by Raman microscopy and the results indicate that the larger t-ZrO2 grains in 3Y-TZP sample are easier transform into m-ZrO2,and a similarly parabolic relationship between the transformed amounts and the grain sizes of zirconia can be established and explained.In this dissertation,an accurate testing methond was used to determine the fracture toughness of three different yttria concentrations zirconia ceramics.On the basis of a series of tests and discussions,a basic understanding of fracture toughness and transformation toughening dependence of yttria concentrations,loading rates and grain sizes of zirconia has been achieved.