Nanoindentation of yttria-doped zirconia: Effect of crystallographic structure on deformation mechanisms
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s article presents a nanoindentation study of polycrystalline and single crystals of yttria-doped zirconia with both tetragonal and cubic phases. Analysis of the deformation mechanisms is performed by both atomic force microscopy (AFM) and micro-Raman spectroscopy. Phase transformation from tetragonal to monoclinic phase is clearly distinguished on tetragonal crystals, whereas in cubic crystals the plastic deformation seems to be controlled by dislocation nucleation and interactions. AFM observations in tetragonal zirconia grains have shown that both grain size and autocatalytic transformation strongly influence the size of the transformed zone. Furthermore, the martensitic phase transformation seems to be also strongly dependent of the indenter shape. Experimental results suggest that a critical contact pressure is necessary to induce the phase transformation.
I. INTRODUCTION
Zirconia ceramics with stabilizing oxides (like yttria, ceria, magnesia, or calcia) have been exhaustively studied because they present a large variety of deformation mechanisms, some of them leading to a substantial increase in fracture toughness. The strong interest around these alloys has conducted to an extensive characterization of their mechanical behavior under mechanical solicitations.1–3 Indeed, dislocation generation,1–3 twinning,4 domain switching,5 phase transformation,6 and cracking can be enumerated as the principal deformation mechanisms, which depend on the initial microstructure (poly or single crystal, phase nature, oxide content. . .). In particular, the martensitic phase transformation from tetragonal to monoclinic (t!m) crystal structures and the ferroelastic switching of domains have received considerable attention in the last decades, because they are the main mechanisms that confer to zirconia alloys a relatively high fracture toughness.6,7 In particular, when these two deformation mechanisms are activated by the stress field in front of the crack tip, crack closure compressive stresses are generated that decrease the effective stress intensity factor. This enhanced fracture toughness has allowed the use of zirconia ceramics in many structural applications where relatively high loads are present. In addition to a relative high toughness, zirconia ceramics show excellent surface mechanical properties common to other structural ceramics, such as hardness, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0091 J. Mater. Res., Vol. 24, No. 3, Mar 2009
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corrosion resistance, wear, and contact resistance, which makes this class of materials a preferred choice in applications where contact loadings are present, such as dental and hip replacements or tribological parts. It is then critical to know the mechanical response and the deformation mechanisms of zirconia ceramics at the surface; not only because in many applications the stress in the bulk is induced by contact loads acting on the surface, but also because the surfa
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