3090 S Deville et al. /Journal of the European Ceramic Sociery 25(2005)3089-3096 rials. For a review on the subject, see the work of Green increasing stabilizer content, i.e. 10, 12 and 16 mol% Ceo2 Hannink et al. 17 Grain size(measured by the linear intercept method on ther Several reinforcing effects might account for an increase mally etched samples)and fracture toughness( measured by of material toughness. The critical stress intensity factor can double torsion experiments)are given in Table 1. This shows be described by the combination of the matrix intrinsic tough- that the grain size is the same for all the samples, the only ness and the addition of crack-shielding mechanisms, among difference lying in the alloying content. It is widely docu- which transformation toughening and crack bridging arise in mented from the literature2 that the larger the CeO, content the particular case of ceria-doped zirconia. The prediction of the lower the toughnes the toughness can be achieved by the prediction and quan- tification of these different crack-shielding mechanisms In 2.2. Double torsion tests particular, the development of a reliable theory of transforma- The double torsion test was used to induce stress-assisted and strain field distribution in the crack tip surrounding zone. phase transformation in the surrounding of the propagat crystallography(PTMC) 8 9 to ogical theory of martensitic The relevance of the phenomen ng crack and to assess quantitatively transformation tough describe the strain field ening effects. The details of the method may be found now recognized A contribution to transformation toughening elsewhere.20), 2I No guiding groove was machined in the spec by transformation-induced plasticity results from the forma- imen in order to avoid any residual stress intensity factor.A tion of martensitic variant pairs with large associated shear notch was machined with a diamond saw and an indentation strain, absorbing some energy in the formation of these vari- was performed at low load (10 kg)to initiate a small crack, ants, energy that would otherwise be available for crack prop- as seen in Fig. 1. Crack rates versus KI curves were used agation, increasing thus the toughness of the material Using to determine the fracture toughness values of the materials the PTMC to describe transformation toughening is very ap. These curves will be discussed in another paper pealing indeed. However, even if the theory can predict pre cisely the local strain distribution, achieving the comparison 2.3. Atomic force microscopy and optical observations of theoretical calculations and experimental results has not yet been possible, as a result of the observational difficulties AFM experiments were carried out with a D3100 at the scale at which the transformation is occurring(a few nanoscope from Digital Instruments Inc, using oxide sharp- nanometers). Fortunately, the development of atomic force ened silicon nitride probes in contact mode, with an average microscopy provides a tool for investigating local relief of scanning speed of 10 ums. Since the t-m phase transfor few nanometers height. The potentiality to observe autoclave mation is accompanied by large strains(4% volume and 16% ageing induced martensitic relief in yttria stabilized zirconia shear), surface relief is modified by the formation of mon- with great has been demonstrated recently. 0 The aclinic phase. The lateral(2 nm)and vertical (0. I nm)res- aim of this study is to show that further insights can be gained olution of AFM makes it possible to follow very precisely from AFM experiments in the description and subsequent un- the transformation induced relief at the surface. The transfor derstanding of transformation toughening in zirconia mation zones were also photographed with an optical micro- scope using the Normarsky interference contrast technique (Zeiss Axiophot, Germany) 2. Materials and methods 2.1. Processing 3. Results Ceria-stabilized zirconia(CeO2-TZP)materials were pro- 3.1. Transformation bands cessed by a classical processing route, using Zirconia Sales Ltd. powders, with uniaxial pressing, cold isostatic pressing The surface of double torsion samples after partial crack and sintering at 1550C for two hours. Residual porosity was propagation observed by optical microscopy in Normarsky negligible. Different compositions have been processed, with contrast is shown in Fig. 1. Great differences in behavior are Table I Materials of the study Material Ceria content Fracture toughness ( mol% (MPam-I2 3 16Ce-TZP All the samples exhibit a similar grain size. The only variable is the stabilizer content. Fracture toughness values were provided by double torsion relaxation3090 S. Deville et al. / Journal of the European Ceramic Society 25 (2005) 3089–3096 rials. For a review on the subject, see the work of Green2 or Hannink et al.17 Several reinforcing effects might account for an increase of material toughness. The critical stress intensity factor can be described by the combination of the matrix intrinsic tough￾ness and the addition of crack-shielding mechanisms, among which transformation toughening and crack bridging arise in the particular case of ceria-doped zirconia. The prediction of the toughness can be achieved by the prediction and quan￾tification of these different crack-shielding mechanisms. In particular, the development of a reliable theory of transforma￾tion toughening requires a deep understanding of the stress and strain field distribution in the crack tip surrounding zone. The relevance of the phenomenological theory of martensitic crystallography (PTMC)18,19 to describe the strain field is now recognized. A contribution to transformation toughening by transformation-induced plasticity results from the forma￾tion of martensitic variant pairs with large associated shear strain, absorbing some energy in the formation of these vari￾ants, energy that would otherwise be available for crack prop￾agation, increasing thus the toughness of the material. Using the PTMC to describe transformation toughening is very ap￾pealing indeed. However, even if the theory can predict pre￾cisely the local strain distribution, achieving the comparison of theoretical calculations and experimental results has not yet been possible, as a result of the observational difficulties at the scale at which the transformation is occurring (a few nanometers). Fortunately, the development of atomic force microscopy provides a tool for investigating local relief of a few nanometers height. The potentiality to observe autoclave ageing induced martensitic relief in yttria stabilized zirconia with great precision has been demonstrated recently.10 The aim of this study is to show that further insights can be gained from AFM experiments in the description and subsequent un￾derstanding of transformation toughening in zirconia. 2. Materials and methods 2.1. Processing Ceria-stabilized zirconia (CeO2–TZP) materials were pro￾cessed by a classical processing route, using Zirconia Sales Ltd. powders, with uniaxial pressing, cold isostatic pressing and sintering at 1550 ◦C for two hours. Residual porosity was negligible. Different compositions have been processed, with Table 1 Materials of the study Material Ceria content (mol%) Sintering temperature (◦C) Grain size (linear intercept) (
m) Fracture toughness (MPa m−1/2) 10Ce–TZP 10 1550 3.7 18 12Ce–TZP 12 1550 3.5 8.1 16Ce–TZP 16 1550 3.4 4.3 All the samples exhibit a similar grain size. The only variable is the stabilizer content. Fracture toughness values were provided by double torsion relaxation experiments. increasing stabilizer content, i.e. 10, 12 and 16 mol% CeO2. Grain size (measured by the linear intercept method on ther￾mally etched samples) and fracture toughness (measured by double torsion experiments) are given in Table 1. This shows that the grain size is the same for all the samples, the only difference lying in the alloying content. It is widely docu￾mented from the literature2 that the larger the CeO2 content, the lower the toughness. 2.2. Double torsion tests The double torsion test was used to induce stress-assisted phase transformation in the surrounding of the propagat￾ing crack and to assess quantitatively transformation tough￾ening effects. The details of the method may be found elsewhere.20,21 No guiding groove was machined in the spec￾imen in order to avoid any residual stress intensity factor. A notch was machined with a diamond saw and an indentation was performed at low load (10 kg) to initiate a small crack, as seen in Fig. 1. Crack rates versus KI curves were used to determine the fracture toughness values of the materials. These curves will be discussed in another paper. 2.3. Atomic force microscopy and optical observations AFM experiments were carried out with a D3100 nanoscope from Digital Instruments Inc., using oxide sharp￾ened silicon nitride probes in contact mode, with an average scanning speed of 10
m s−1. Since the t–m phase transfor￾mation is accompanied by large strains (4% volume and 16% shear), surface relief is modified by the formation of mon￾oclinic phase. The lateral (2 nm) and vertical (0.1 nm) res￾olution of AFM makes it possible to follow very precisely the transformation induced relief at the surface. The transfor￾mation zones were also photographed with an optical micro￾scope using the Normarsky interference contrast technique (Zeiss Axiophot, Germany). 3. Results 3.1. Transformation bands The surface of double torsion samples after partial crack propagation observed by optical microscopy in Normarsky contrast is shown in Fig. 1. Great differences in behavior are