S. Deville et al. Acta Materialia 52(2004)5709-5721 was expected to be very different. Finally, microcracking ence ABCI was demonstrated. For this correspondence, was observed in the surroundings of transformed grains, it was shown the characteristics of the habit planes and and related to the volume and shear components of the transformation matrix allow a complete accommoda transformation. However, no direct observation and tion of transformation strains by a surface uplift outside clear interpretation of the microcracks formation has of the free surface. Minimal residual stresses should be been reported. The observation of martensite formed expected in that case. a particular interest of this anal during aging treatment is very appealing in regards of ysis relies in the interpretation of martensitic variants the lack of validation of the various theories, for several arrangement in the volume of the material from the reasons. The transformation is occurring at surface so experimental observations of relief features at the that it may be observed straightaway, and the influence surface of a free surface on the transformation can potentially On the other hand, if the martensitic phenomeno- be assessed. Moreover, the transformation is propagat- logical theory was successfully applied, it is worth ing step by step when performing several aging treat- remembering the theory does only describe the trans- ment in autoclave, so that intermediate stages of the formation in mathematical terms, and by no means ransformation should be observed. Then, the less stable in physical terms. No informations on the chemical parts(from an energetical point of view) of the surface mechanism of transformation are brought by the the- are the first one to transform: the transformation ory. If the spatial arrangement was well understood not triggered by the experimental observations, like in terms of transformation strain accommodation grains transforming under the beam during TEM obser the remaining questions were thus: how are these vations. Finally, the sample geometry has no effect on arrangements formed? How do they grow and how he transformation behavior, as opposed to thin foils fast? where the very low thickness induces some peculiar fea In this study, this approach is applied to investigate tures. It should therefore be possible determining some and analyze the nucleation and growth of martensite of the factors affecting the stability in ceria stabilized zirconia and subsequent consequences Considering the scale at which the transformation like microcracking. It is shown how the combination of occurring, very few qualitative and quantitative reports AFM observations and of the outputs of the phenome- of transformation induced relief can be found in the lit nological analysis can provide new insights on both the erature. Considering the dilatational(0.04)and shear physical and the chemical mechanisms of the transfor- (0.16)components of the transformation, this dearth mation, with particular attention being paid on the of experimental observations is not surprising; modifica- influence of the free surface on the variants growth tions of the surface relief occur below the micrometer modes. AFM observations were performed at different range, typically from 10 to 100 nm for a zirconia poly- steps of the aging treatment in order to follow locally crystal of typical grain size(0.5-3 um). In absence of the transformation at the surface of the sample with experimental validation of the relief features, the predic tions of transformation induced relief rely on the valid ity and relevance f the phenomenological theory [9, 10] of martensite transfor- 2. Experimental methods mation. Validation of these predictions requires precise quantitative measurements of martensite relief. The ef- Ceria stabilized zirconia (Ce-TZP) materials were fect of free surface on the transformation local charas processed by classical powder processing route, using teristics relies mostly so far on the outputs of Zirconia Sales Ltd powders, with uniaxial pressing and calculations [11-13]. Preliminary reports [14-16] have sintering at 1823 K for 2 h Residual porosity was neg- nonetheless drawn the attention of the potentialities of- ligible Samples were mirror polished with standard dia fered by atomic force microscopy(AFM), technique mond based products. offering a vertical resolution below the nanometer range The martensitic transformation was induced by a AFM allows straightforward observations of trans- thermal treatment in water vapor autoclave. This kind formed surfaces of bulk samples. The observation of of treatment is known to induce the tetragonal to mond partial transformation of the grains [16] already ques- clinic phase transformation in zirconia [3, 18, 19].Hence, tioned the existence of a critical size for the transforma these treatments were conducted in autoclave at 413K tion. The technique has just been applied [17] to in saturated water vapor atmosphere, with a 2 bar pres- investigate the martensitic relief at the end of the trans- sure, inducing phase transformation at the surface of the formation, and compared with the outputs of the crys- samples with time. Thermal treatment steps were tallographic theory of martensitic transformation. An bounded to the thermal activation of the transformation excellent quantitative agreement was found between and the technical limits of the autoclave. These steps he experimental observations and theoretical predic- could have been reduced by several decades if an higher tions. In particular, the peculiar behavior of correspond- treatment temperature had been chosen.was expected to be very different. Finally, microcracking was observed in the surroundings of transformed grains, and related to the volume and shear components of the transformation. However, no direct observation and clear interpretation of the microcracks formation has been reported. The observation of martensite formed during aging treatment is very appealing in regards of the lack of validation of the various theories, for several reasons. The transformation is occurring at surface so that it may be observed straightaway, and the influence of a free surface on the transformation can potentially be assessed. Moreover, the transformation is propagat￾ing step by step when performing several aging treat￾ment in autoclave, so that intermediate stages of the transformation should be observed. Then, the less stable parts (from an energetical point of view) of the surface are the first one to transform; the transformation is not triggered by the experimental observations, like grains transforming under the beam during TEM obser￾vations. Finally, the sample geometry has no effect on the transformation behavior, as opposed to thin foils where the very low thickness induces some peculiar fea￾tures. It should therefore be possible determining some of the factors affecting the stability. Considering the scale at which the transformation is occurring, very few qualitative and quantitative reports of transformation induced relief can be found in the lit￾erature. Considering the dilatational (0.04) and shear (0.16) components of the transformation, this dearth of experimental observations is not surprising; modifica￾tions of the surface relief occur below the micrometer range, typically from 10 to 100 nm for a zirconia poly￾crystal of typical grain size (0.5–3 lm). In absence of experimental validation of the relief features, the predic￾tions of transformation induced relief rely on the valid￾ity and relevance of the outputs of the phenomenological theory [9,10] of martensite transfor￾mation. Validation of these predictions requires precise quantitative measurements of martensite relief. The ef￾fect of free surface on the transformation local charac￾teristics relies mostly so far on the outputs of calculations [11–13]. Preliminary reports [14–16] have nonetheless drawn the attention of the potentialities of￾fered by atomic force microscopy (AFM), technique offering a vertical resolution below the nanometer range. AFM allows straightforward observations of trans￾formed surfaces of bulk samples. The observation of partial transformation of the grains [16] already ques￾tioned the existence of a critical size for the transforma￾tion. The technique has just been applied [17] to investigate the martensitic relief at the end of the trans￾formation, and compared with the outputs of the crys￾tallographic theory of martensitic transformation. An excellent quantitative agreement was found between the experimental observations and theoretical predic￾tions. In particular, the peculiar behavior of correspond￾ence ABC1 was demonstrated. For this correspondence, it was shown the characteristics of the habit planes and transformation matrix allow a complete accommoda￾tion of transformation strains by a surface uplift outside of the free surface. Minimal residual stresses should be expected in that case. A particular interest of this anal￾ysis relies in the interpretation of martensitic variants arrangement in the volume of the material from the experimental observations of relief features at the surface. On the other hand, if the martensitic phenomeno￾logical theory was successfully applied, it is worth remembering the theory does only describe the trans￾formation in mathematical terms, and by no means in physical terms. No informations on the chemical mechanism of transformation are brought by the the￾ory. If the spatial arrangement was well understood, in terms of transformation strain accommodation, the remaining questions were thus: how are these arrangements formed? How do they grow and how fast? In this study, this approach is applied to investigate and analyze the nucleation and growth of martensite in ceria stabilized zirconia and subsequent consequences like microcracking. It is shown how the combination of AFM observations and of the outputs of the phenome￾nological analysis can provide new insights on both the physical and the chemical mechanisms of the transfor￾mation, with particular attention being paid on the influence of the free surface on the variants growth modes. AFM observations were performed at different steps of the aging treatment in order to follow locally the transformation at the surface of the sample with time. 2. Experimental methods Ceria stabilized zirconia (Ce–TZP) materials were processed by classical powder processing route, using Zirconia Sales Ltd powders, with uniaxial pressing and sintering at 1823 K for 2 h. Residual porosity was neg￾ligible. Samples were mirror polished with standard dia￾mond based products. The martensitic transformation was induced by a thermal treatment in water vapor autoclave. This kind of treatment is known to induce the tetragonal to mono￾clinic phase transformation in zirconia [3,18,19]. Hence, these treatments were conducted in autoclave at 413 K, in saturated water vapor atmosphere, with a 2 bar pres￾sure, inducing phase transformation at the surface of the samples with time. Thermal treatment steps were bounded to the thermal activation of the transformation and the technical limits of the autoclave. These steps could have been reduced by several decades if an higher treatment temperature had been chosen. 5710 S. Deville et al. / Acta Materialia 52 (2004) 5709–5721