Elsevier

Acta Materialia

Volume 61, Issue 1, January 2013, Pages 174-182
Acta Materialia

Evidence for the formation of distorted nanodomains involved in the phase transformation of stabilized zirconia by coupling convergent beam electron diffraction and in situ TEM nanoindentation

https://doi.org/10.1016/j.actamat.2012.09.047Get rights and content

Abstract

The transformation of zirconia from its tetragonal to its monoclinic phase is an important feature of the zirconia system. First found to be an advantage due to its important toughening effect, it can also be very detrimental when it occurs in the framework of low-temperature degradation, particularly in the case of biomaterial applications. One way to avoid or to control this phase transformation is to understand how it initiates and more particularly the stress states that can trigger it. A new technique available inside a transmission electron microscope seems to be particularly well suited for that type of study: convergent beam electron diffraction, a well-known technique to reveal stresses, was coupled to in situ transmission electron microscopy mechanical nanoindentation. The experiments reveal the presence of sheared nanoregions at grain boundaries. These could act as embryos for tetragonal-to-monoclinic phase transformations. This is an important first step in the understanding of the earliest stage of zirconia phase transformation.

Introduction

Ceramic materials demonstrate interesting properties and are used in a wide range of applications. One of their main weakness, however, is their propensity to brittle failure. Zirconia was found to be very interesting because of a toughening mechanism reducing crack-propagation velocities. This mechanism is based on a tetragonal-to-monoclinic (t–m) phase transformation at the tip of cracks which leads to an ∼4 vol.% expansion of the zirconia cell volume. Cracks are thus stopped or slowed down by the resulting compressive stress field. This mechanism was first proposed by Garvie et al. [1] as an advantage for zirconia, and provides zirconia with the best mechanical properties of oxide ceramics. Nevertheless, the t–m transformation also has an important drawback: it occurs at the surface of zirconia components in the presence of water or moisture, and the volume expansion leads to an increase in roughness, microcracking and possible delayed failure. This mechanism is called low-temperature degradation [2], [3] and is not yet fully understood. It is well known that the t–m phase transformation is martensitic: this was evidenced by numerous transmission electron microscopy studies and observations highlighting a twinning phenomenon in transformed grains [4], [5], [6], [7], [8]. However, the initiation of the transformation is not well characterized. An important step to better understand its mechanism is the study of local deformations (at the nanometer scale) either governing the transformation or resulting from it.

In this paper, the onset of phase transformation and consecutive deformation at the nanometer scale are identified. Convergent beam electron diffraction (CBED) analyses on stabilized zirconias under strain have been performed during in situ nanoindentation inside a transmission electron microscope. This innovative tool has already been used successfully to follow grain rotations in ceramic composite thin foil [9], plasticity and dislocation movements in metals [10], [11] or nanoparticle behaviors [12], [13], [14], [15] during nanoindentation, but to our knowledge never in CBED mode.

In this paper, the possibility of using CBED analysis on zirconia thin foils without nanoindentation is first considered. Then CBED is applied to analyze zirconia during in situ nanoindentation inside the transmission electron microscope. From these results, suggestions on the very first step of nucleation and propagation of the phase transformation are proposed.

Section snippets

Experimental methods

The stable pure zirconia phase at room temperature is monoclinic, but stabilized zirconia tetragonal phases are obtained by the addition of elements such as yttrium or cerium. In order to delay the phase transformation from tetragonal into monoclinic, two sintered, stabilized zirconia samples have been prepared: 3Y-TZP and 12Ce-TZP. The average grain sizes are 0.3 and 1 μm for 3Y-TZP and 12Ce-TZP, respectively.

3Y-TZP has been stabilized in the tetragonal phase by the introduction of 3 mol.% of Y2O

Results

Fig. 1 presents a conventional bright-field image observed on the 12Ce-TZP (Fig. 1a). The indexation of the obtained diffraction pattern (Fig. 1b) reveals that the grain is wholly in the tetragonal phase. This validates the sample preparation technique used, which minimizes the phase transformation of zirconia (no polishing before FIB milling). The zone axis in Fig. 1b is too simple to obtain the sharpest lines in CBED condition, which is why the pattern in Fig. 1c was obtained under a more

Discussion

In a CBED pattern, lines represent the traces of lattice planes. Therefore, line shifts and tilts reveal distortions of the unit cell while intensity differences are assigned to shifts of atomic position in the cell.

The monoclinic cell distortion, highlighted by the line shifts in the CBED pattern of 3Y-TZP, is induced by local strains. These can be induced by residual stresses related to the anisotropy of the coefficients of thermal expansion, which is more significant in the 3Y-TZP than in

Conclusion

By using CBED, LACBED and in situ TEM nanoindentation techniques, the very early beginning of t–m phase transformation in zirconia was investigated at the nanometer scale. We have determined the following:

  • The phase transformation is initiated preferentially at the grain boundaries, which are distorted to accommodate the increase in volume, but there are still specific crystallographic relationships between the initial phases and the new formed phase.

  • There is a zone of lattice plane distortion

Acknowledgements

The authors would like to acknowledge the CLYM (Centre Lyonnais de Microscopie, http://www.clym.fr) for the access to the JEOL 2010F and A. Schertel (Carl Zeiss NTS company GMBH) for the preparation of the thin foil by the “lift-out” technique. Financial support was provided by the Region Rhone-Alpes (Cluster MACODEV) and the Institut Universitaire de France.

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