August 2009 Advanced Ceramic Composite for Hip Prosthesis 182l (3) Spectroscopic Proof for Invariance of Bulk Toughness Table II. Optical Parameters for Different Component Upon Aging in Autoclave Phases In environmentally assisted phase-transformation prod polymorphic changes are maximized at the sample surfac eand ry quite steeply with tion along the in-depth axis. On the Alumina +tetragonal I mixture 1545 other hand. the raman is of a finite diameter and de Monoclinic zirconia 0.03 nd thus the collected at each location represents an average(and weighted) Raman emission over the entire prob olume. The main consequence of the finite size of the prol s that, even if the phase-fraction data are collected with a highly parameters from Table II in Eq (1), an iterative computational confocal iguration, they need to be treated by a routine was run using commercially available software to find mathematical deconvolution procedure to retrieve the actual in the parameters A, B, C, m, and n, which best fit the experi depth transformation profile. The profiles of monoclinic phas VMzo) values, as retrieved from the in-depth confocal line scan. fraction, VMzo), which were collected with an in-depth confocal The results of this mathematical deconvolution procedure are scanning along the subsurface of samples in the as-received con- plotted in Fig. 6. The deconvoluted monoclinic fraction profile. dition and autoclaved for 300 h, are shown in Fig. 6. In this plot. VMz), is also compared in Fig. 6 with the monoclinic fraction corresponds to the in-depth abscissa, which locates the probe profile typically found in the as-received material. This latter profile was almost unaltered after 10-h exposure in moist envi positive zo value refers here to a translation of the focal plan ronment. On the other hand, after exposures in autoclave a below the free surface of the sample. The plot, VM=VM(zo) long as 300 h, the thickness of material affected by an enhanced was translated into a plot, VM= VM(z), with z being the abscissa amount of monoclinic phase increased. This spectroscopic as- of the actual phase fraction distribution along the in-depth di- sessment may be invoked to explain the reason why the bulk rection. defined similarly to zo. In other words, the experimental toughness value was almost independent of autoclaving expo- profile of phase fraction was cleaned up from the averaging sure time. In fact, we notice that, whatever steep the variance in tion confined to the level of a few tens of micrometers even after very long-term exposure in autoclave. The thickness of the transfo M(20) exp(-2x(4+n2)M(2)P(4+n(2 tion layer is important because it might affect the residual exp(-2(4+n2)P(4+n(2,=0d stress fields developed on the bearing surface and, thus, the wear behavior of the material. It has been reported that a compres- sive residual stress on the bearing surface contributes to reduce P(=,z0 (2) both amounts of wear and friction in alumina materials.In +(2-20) Pezzotti et al.,we have quantitatively measured the residual stress in the alumina phase on the surface of the composite be- where the para 16. the subscripts in brackets locate whether ameters P, P, and Pe are the material /probe inter- e to moist environment. a trend w found showing an initial moderate decrease to reach a magni- the parameter refers to an alumina/ tetragonal zirconia mixture mixture tude of few tens of MPa in compression after about 20-h expo- r to the monoclinic zirconia phase(A+T and M, respectively) sure(transformation thickness of about 6 um) and then a Solving the integral Eq (1) for VM(z)involves the knowledge of gradual increase of the residual stress to reach zero magnitude the mathematical character of the in-depth profile function after about 45 h and toward the tensile side(with Based on previous Raman assessments from the sample cross 45 h). This may suggest that the alumina matrix in the pre tion to be structured as follows 6amics, we assume this func- sections in monolithic zirconia ce omposite could be more wear resistant than monolithic al VM()e Aexp(-Bx)+ In this study we hay acture where A, B, C, m, and n are constants, and Vo is the(initial) mechanics and confocal Raman microp volume fraction of monoclinic phase in the bulk of the as- characterize both bulk and surface toughness of an alumina/zir eceived material. After substituting for Eq. (3)and the probe conia composite used in advanced joint prostheses (e.g,BIO- LOX delta femoral heads). In a previous paper, we showed that transformation occurs in the material as a consequence of an environmentally assisted process. The autoclave testing p As-recerved cedure adopted in that study was quite severe and corresponded to exposures in human body of several lifetimes. The effect of an Aged for 10 h exposure of 10 h in autoclave at 134C was also examined 805050 hich, according to the activation energy for environmental re- sistance of the present composite, corresponds to a lifetime pectation of 39 years in vivo. Based on these results, the main 点A outcomes of the present fracture mechanics assessments can be ummarized as follows: (i) the crack-tip toughness level mea- ured in the as-received material is comprehensive of a tangible contribution by transformation toughening: (ii) even after the material is environmentally aged for periods of the order of 20 hundreds of hours and its surface toughness is reduced by about In-depth abscissa, z, or z (um) one-third the measured toughness of the composite measured Fig. 6. Profiles of monoclinic phase fraction, VAEo), as collected with y any method is at least the same as that of monolithic al- an in-depth confocal along the subsurface of the sample au- a:(ii) the observed decrease of fracture toughness by about best-fit convoluted curves and the cor. one-third is limited to the very surface of the material (i. e, to a responding deconvolut (calculated according to Eqs. (1H3)) layer of the order of a few tens of micrometers) and does not are also shown affect the bulk fracture toughness of the composite, as shown by(3) Spectroscopic Proof for Invariance of Bulk Toughness Upon Aging in Autoclave In environmentally assisted phase-transformation processes, polymorphic changes are maximized at the sample surface and vary quite steeply with position along the in-depth axis. On the other hand, the Raman probe is of a finite diameter and depth, and thus the spectrum collected at each location represents an average (and weighted) Raman emission over the entire probe volume.14,15 The main consequence of the finite size of the probe is that, even if the phase-fraction data are collected with a highly confocal microprobe configuration, they need to be treated by a mathematical deconvolution procedure to retrieve the actual in￾depth transformation profile. The profiles of monoclinic phase fraction, VM(z0), which were collected with an in-depth confocal scanning along the subsurface of samples in the as-received con￾dition and autoclaved for 300 h, are shown in Fig. 6. In this plot, z0 corresponds to the in-depth abscissa, which locates the probe focal plane, with origin at the material surface (cf. Fig. 1). A positive z0 value refers here to a translation of the focal plane below the free surface of the sample. The plot, VM 5 VM(z0), was translated into a plot, VM 5 VM(z), with z being the abscissa of the actual phase fraction distribution along the in-depth di￾rection, defined similarly to z0. In other words, the experimental profile of phase fraction was cleaned up from the averaging effect of the probe upon solving the following integral equa￾tion16: VMðz0Þ ¼ R þ1 0 expð2að Þ AþT zÞVMðzÞPð Þ AþT ðz; z0Þdz R þ1 0 expð2að Þ AþT zÞPð Þ AþT ðz; z0Þdz (1) Pðz; z0Þ ¼ p2 c p2 1 þ p2  p2 c p2 c þ ðz  z0Þ 2 ! p2 p2 þ ðz  z0Þ 2 (2) where the parameters P, p, and pc are the material/probe inter￾action parameters16; the subscripts in brackets locate whether the parameter refers to an alumina/tetragonal zirconia mixture or to the monoclinic zirconia phase (A1T and M, respectively). Solving the integral Eq. (1) for VM(z) involves the knowledge of the mathematical character of the in-depth profile function. Based on previous Raman assessments from the sample cross sections in monolithic zirconia ceramics, we assume this func￾tion to be structured as follows16: VMð Þffi z A expð Þþ Bz Ctn ð Þ z þ 1 m þ V0 (3) where A, B, C, m, and n are constants, and V0 is the (initial) volume fraction of monoclinic phase in the bulk of the as￾received material. After substituting for Eq. (3) and the probe parameters from Table II in Eq. (1), an iterative computational routine was run using commercially available software24 to find the parameters A, B, C, m, and n, which best fit the experimental VM(z0) values, as retrieved from the in-depth confocal line scan. The results of this mathematical deconvolution procedure are plotted in Fig. 6. The deconvoluted monoclinic fraction profile, VM(z), is also compared in Fig. 6 with the monoclinic fraction profile typically found in the as-received material. This latter profile was almost unaltered after 10-h exposure in moist envi￾ronment. On the other hand, after exposures in autoclave as long as 300 h, the thickness of material affected by an enhanced amount of monoclinic phase increased. This spectroscopic as￾sessment may be invoked to explain the reason why the bulk toughness value was almost independent of autoclaving expo￾sure time. In fact, we notice that, whatever steep the variance in the transformed near-surface portion of material, it remained confined to the level of a few tens of micrometers even after very long-term exposure in autoclave. The thickness of the transfor￾mation layer is important because it might affect the residual stress fields developed on the bearing surface and, thus, the wear behavior of the material. It has been reported30 that a compres￾sive residual stress on the bearing surface contributes to reduce both amounts of wear and friction in alumina materials. In Pezzotti et al.,15 we have quantitatively measured the residual stress in the alumina phase on the surface of the composite be￾fore and after exposure to moist environment. A trend was found showing an initial moderate decrease to reach a magni￾tude of few tens of MPa in compression after about 20-h expo￾sure (transformation thickness of about 6 mm) and then a gradual increase of the residual stress to reach zero magnitude after about 45 h and toward the tensile side (with aging time 445 h). This may suggest that the alumina matrix in the present composite could be more wear resistant than monolithic al￾umina. V. Conclusion In this study, we have applied combined techniques of fracture mechanics and confocal Raman microprobe spectroscopy to characterize both bulk and surface toughness of an alumina/zir￾conia composite used in advanced joint prostheses (e.g., BIO￾LOXs delta femoral heads). In a previous paper,15 we showed that transformation occurs in the material as a consequence of an environmentally assisted process. The autoclave testing pro￾cedure adopted in that study was quite severe and corresponded to exposures in human body of several lifetimes. The effect of an exposure of 10 h in autoclave at 1341C was also examined which, according to the activation energy for environmental re￾sistance of the present composite, corresponds to a lifetime ex￾pectation of 39 years in vivo. Based on these results, the main outcomes of the present fracture mechanics assessments can be summarized as follows: (i) the crack-tip toughness level mea￾sured in the as-received material is comprehensive of a tangible contribution by transformation toughening; (ii) even after the material is environmentally aged for periods of the order of hundreds of hours and its surface toughness is reduced by about one-third, the measured toughness of the composite measured by any method is at least the same as that of monolithic al￾umina; (iii) the observed decrease of fracture toughness by about one-third is limited to the very surface of the material (i.e., to a layer of the order of a few tens of micrometers) and does not affect the bulk fracture toughness of the composite, as shown by Fig. 6. Profiles of monoclinic phase fraction, VM(z0), as collected with an in-depth confocal scanning along the subsurface of the sample au￾toclaved for 300 h at 1211C. The best-fit convoluted curves and the cor￾responding deconvoluted profiles (calculated according to Eqs. (1)–(3)) are also shown. Table II. Optical Parameters for Different Component Phases Crystal phase p (mm1 ) a (mm1 ) Alumina1tetragonal mixture 15.45 0.12 Monoclinic zirconia 4 0.03 August 2009 Advanced Ceramic Composite for Hip Prosthesis 1821