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Omar A. Elshrief, Robert Coward, Babak Anasori, and Michel W. Barsouma) Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104 (Received 11 April 2011; accepted 26 September 2011)

Herein rutile (TiO2) single-crystal surfaces, with (001) and (100) orientations, were indented with hemispherical indenters with radii of 13.5, 5, and 1.4 lm. By converting the load–displacement data to nanoindentation (NI) stress–strain curves, together with microscopic post-indentation observations, we conclude that in the (001) orientation, plastic deformation occurs by the activation of all four {101},101. slip systems. In the (100) orientation, only two of the four {101},101 . slip systems, along with {100},010. slip, are activated. Because the four {101},101. slip systems in the (001) orientation intersect, the surface is harder and exhibits higher hardening rates after the nucleation of dislocations. The latter are manifested by pop-ins, some of which are large. The pop-in stresses are adequately described by Weibull statistics and were significantly higher for the (001) orientation. The elastic moduli, determined from spherical NI stiffness versus contact radii plots, were 349 6 5 and 229 6 4 GPa for (001) and (100) orientations, respectively. Fully spontaneous reversible, stress–strain hysteretic curves—only manifest in the (100) orientation—are attributed to the to-and-fro motion of dislocations comprising incipient kink bands in the {100},010. slip system.

I. INTRODUCTION

Single-crystal rutile (TiO2), a wide band gap semiconductor, is one of the most suitable materials for spectral prisms and polarizing devices such as, optical isolators and beam displacers because of its large birefringence. Rutile is popular among experimentalists because of the ease of fabrication in single crystal, thin film as well as polycrystalline form.1 It has also been used for a wide range of important technological applications; for example, as a photocatalyst in solar cells,2 as a gas sensor,3 as optical coatings,4 as biocompatible bone implants,5 and as a gate insulator for MOSFET devices6 among others. In all these applications, a detailed knowledge of its mechanical deformation behavior is of great importance for better design and stability of the devices. Using an etch-pit technique, Hirthe and Brittain7 were the first to report on dislocations on the {101} and {110} planes in rutile single crystals. They also showed evidence for {100}, 0 10. slip in rutile. Later, most of the work on the deformation of TiO2 single crystals was performed by bulk mechanical tests at high temperatures, typically above 850 °C.8,9 Ashbee and Smallman8 reported that only the {101},101. slip system gets activated during a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.337 J. Mater. Res., Vol. 27, No. 1, Jan 14, 2012

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compression along the [001] and [100] directions. However, when single crystals were compressed 4