August 2009 Advanced Ceramic Composite for Hip Prosthesis 1819 E=E/(1-v) is the plane strain elastic modulus; x, a, and b are an abscissa along the crack path( behind the crack tip and with 3.1 MPam origin at the tip). the distance between the crack tip and K-22 MP. mn the geometrical center of the indentation print, and the half diagonal of the indentation print, respectively. F(a, x)is a a Monolithic al weight function for the median surface crack configuration. as 且 Ritchie en by Fett et al. 2 and recently applied by Kruzic and 80.15 K"3.2 MPa Craclgure 2(a)shows FEG-SEM images of the main indentation crack path in a zone of relatively high COD values, while Fig 2(b) shows a microscopic branched area in the very neigh- Kx=22 MPam borhood of the crack tip. Both these figures refer to the material in the as-received state. Similar FEG-SEM pictures were used 0600 throughout the investigation for COD assessments in the as- F(a, x) received and autoclave-aged material. In Fig. 3, cod plots are the composite in the as-received state, after 10 and 300 h aging in cracks as collected in the as-received material and in the material au- moist atmosphere. From the best-fit sl f the plots, KI oclaved for 300 h at 121C. The slopes of the plots represent the crack values equal to 3. 2, 3. 1, and 2.2 MPa m/ were found for the tip toughness of the materials. as-received, 10-h, and the 300-h autoclaved material, respec- tively. The COd assessments show that no effect occurs on showed a surface toughness value similar to that of the com- toughness after 10-h aging, but also suggest that, ultimately, a posite after very long-term exposure in moist environment reduction in toughness by about one-third will occur after a very long-term exposure in autoclave. Note that the reduction in oughness observed in this experiment may also represent the actual contribution of phase transformation to toughening in (2) Bulk Toughness Versus Aging in Moist Atmosphere the as-received material. as discussed in a later section. Table I The results of fracture mechanics tests performed as a function nows a comparison between Kic values obtained from COD of stable crack extension, a, from a sharp notch root are shown measurements and those calculated according to the standard able I In such low cross-head speed fracture experi dentation method (namely, from indentation crack-length he critical stress intensity factor for crack initiation from the asurements). From a comparison between the two meth- otch root was KIc=3.0±0.24,2.9±0.20,and28±0.18 ds, it is clear that Kic values obtained from indentation MPa. 2. for the as-received sample and the samples aged in crack-length measurements were systematically larger than autoclave for 10 and 300 h, respectively. a detectably higher hose obtained from COD measurements. An explanation of fracture resistance was noticed when notched samples were frac this discrepancy will be given in Section IV(1). However, it is tured at high cross-head speed(cf. data listed in Table I); ho nportant to note here that, despite the observed discrepancy ever, again no significant difference in the toughness value could n the absolute values of toughness, a similar reduction in be found before and after autoclaving the samples. In other toughness upon autoclaving was found after both COd and in- ords, even long-term autoclaving does not appear to signifi- dentation crack-length measurement methods. In Table I,a antly affect the bulk toughness of the composite. In fracturing arison is also given with monolithic alumina, which notched bars at relatively fast cross-head speed, the observed Kic value was almost twice that measured at low cross-head speed a qualitative explanatio difference will be given the next section. Here, it is important to note that, as far as the bulk toughness value is concerned, the composite remains tougher than monolithic alumina even after very long-term ex- posures in moist environment (cf. Table D). In the present com- posite, a conspicuous toughening contribution is expected to irise from the residual stress fields associated with the tetrago- nal-to-monoclinic polymorphic transformation of zirconia - COD dispersoids in the neighborhood of the crack path. This phe- nomenon is likely to give a primary contribution to the tough- ening behavior In Section IV(2), we shall show a set of Raman also be attempted in Section IV(3) to provide a rationale for the bserved invariance of bulk toughness data upon aging in moist IV. Discussion (1) Rationale for the Discrepancy in Tou Measured by Di∥ erent In the examination of toughness data collected on the present 1 um composite material by various testing methods(as listed in Table D), large differences among absolute values obtained by different Fig. 2. Field-emission-gun scanning electron microscope images of the methods and or testing conditions can be noticed While this finding may stress the importance of a clear specification of the testing procedure adopted before comparing toughness data (COD)values;(b)the bridging zone in the neighborhood of the crack from different materials, it also calls for a clarification of the tip. Arrows here indicate COD displacements in(a) and bridging loca physical reasons behind the observed discrepancies in measuring tions in(b). toughness values. An attempt to interpret the reasons behind theE0 5 E/(1n2 ) is the plane strain elastic modulus; x, a, and b are an abscissa along the crack path (behind the crack tip and with origin at the tip), the distance between the crack tip and the geometrical center of the indentation print, and the half diagonal of the indentation print, respectively. F(a, x) is a weight function for the median surface crack configuration, as first given by Fett et al.,23 and recently applied by Kruzic and Ritchie.24 Figure 2(a) shows FEG-SEM images of the main indentation crack path in a zone of relatively high COD values, while Fig. 2(b) shows a microscopic branched area in the very neigh￾borhood of the crack tip. Both these figures refer to the material in the as-received state. Similar FEG-SEM pictures were used throughout the investigation for COD assessments in the as￾received and autoclave-aged material. In Fig. 3, COD plots are shown as collected on typical indentation cracks introduced in the composite in the as-received state, after 10 and 300 h aging in moist atmosphere. From the best-fit slopes of the plots, KIC values equal to 3.2, 3.1, and 2.2 MPa  m1/2 were found for the as-received, 10-h, and the 300-h autoclaved material, respec￾tively. The COD assessments show that no effect occurs on toughness after 10-h aging, but also suggest that, ultimately, a reduction in toughness by about one-third will occur after a very long-term exposure in autoclave. Note that the reduction in toughness observed in this experiment may also represent the actual contribution of phase transformation to toughening in the as-received material, as discussed in a later section. Table I shows a comparison between KIC values obtained from COD measurements and those calculated according to the standard indentation method (namely, from indentation crack-length measurements18). From a comparison between the two meth￾ods, it is clear that KIC values obtained from indentation crack-length measurements were systematically larger than those obtained from COD measurements. An explanation of this discrepancy will be given in Section IV(1). However, it is important to note here that, despite the observed discrepancy in the absolute values of toughness, a similar reduction in toughness upon autoclaving was found after both COD and in￾dentation crack-length measurement methods. In Table I, a comparison is also given with monolithic alumina, which showed a surface toughness value similar to that of the com￾posite after very long-term exposure in moist environment. (2) Bulk Toughness Versus Aging in Moist Atmosphere The results of fracture mechanics tests performed as a function of stable crack extension, a, from a sharp notch root are shown in Table I. In such low cross-head speed fracture experiments, the critical stress intensity factor for crack initiation from the notch root was KIC 5 3.070.24, 2.970.20, and 2.870.18 MPa  m1/2, for the as-received sample and the samples aged in autoclave for 10 and 300 h, respectively. A detectably higher fracture resistance was noticed when notched samples were frac￾tured at high cross-head speed (cf. data listed in Table I); how￾ever, again no significant difference in the toughness value could be found before and after autoclaving the samples. In other words, even long-term autoclaving does not appear to signifi- cantly affect the bulk toughness of the composite. In fracturing notched bars at relatively fast cross-head speed, the observed KIC value was almost twice that measured at low cross-head speed. A qualitative explanation for this difference will be given in the next section. Here, it is important to note that, as far as the bulk toughness value is concerned, the composite remains tougher than monolithic alumina even after very long-term ex￾posures in moist environment (cf. Table I). In the present com￾posite, a conspicuous toughening contribution is expected to arise from the residual stress fields associated with the tetrago￾nal-to-monoclinic polymorphic transformation of zirconia dispersoids in the neighborhood of the crack path. This phe￾nomenon is likely to give a primary contribution to the tough￾ening behavior. In Section IV(2), we shall show a set of Raman spectroscopic results supporting the above argument. In addi￾tion, in-depth Raman assessments with a confocal probe will also be attempted in Section IV(3) to provide a rationale for the observed invariance of bulk toughness data upon aging in moist environment. IV. Discussion (1) Rationale for the Discrepancy in Toughness Values Measured by Different Methods In the examination of toughness data collected on the present composite material by various testing methods (as listed in Table I), large differences among absolute values obtained by different methods and/or testing conditions can be noticed. While this finding may stress the importance of a clear specification of the testing procedure adopted before comparing toughness data from different materials, it also calls for a clarification of the physical reasons behind the observed discrepancies in measuring toughness values. An attempt to interpret the reasons behind the Fig. 2. Field-emission-gun scanning electron microscope images of the profile of a crack propagated in the alumina/zirconia composite: (a) a main zone of the crack with relatively high crack-opening displacement (COD) values; (b) the bridging zone in the neighborhood of the crack tip. Arrows here indicate COD displacements in (a) and bridging loca￾tions in (b). Fig. 3. Crack-opening displacement (COD) plots for indentation cracks as collected in the as-received material and in the material au￾toclaved for 300 h at 1211C. The slopes of the plots represent the crack￾tip toughness of the materials. August 2009 Advanced Ceramic Composite for Hip Prosthesis 1819