Extensive deformation behavior of an all-oxide Al 2 O 3 -TiO 2 nanostructured multilayer ceramic at room temperature

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Yu Fu Liu Research Center for Advanced Science and Technology, The University of Tokyo, Meguro-ku, Tokyo 153-8904, Japan

Yutaka Kagawaa) Research Center for Advanced Science and Technology, The University of Tokyo, Meguro-ku, Tokyo 153-8904, Japan; and National Institute for Materials Science, Tsukuba-shi, Ibaraki 305-0047, Japan (Received 6 July 2009; accepted 3 September 2009)

An all-oxide Al2O3-TiO2 ceramic multilayer composed of 10–100 nm thick alternating layers was fabricated using the reactive magnetron sputtering process. Microindentation tests were carried out on the multilayer ceramic followed by microstructural observations of the cross-sections of the indented sites to characterize the indentation response of the system. During the observations, it was noted that an extensive room temperature “deformation” occurred in the multilayer ceramic material. The material shows a thickness reduction of as much as 40% under a conical indenter at 300 mN of load without microcracking and dislocation-assisted deformation. The room temperature deformation mechanism is governed by the relative movement and rearrangement of the anisotropic nanoscale columnar grains along the intergranular boundaries containing elongated voids. The relative sliding along the intergranular boundaries, and the subsequent granular rotation under indentation were well captured by finite element simulation.

I. INTRODUCTION

Ceramics exhibit unique and attractive property combinations, such as high Young’s modulus, hardness, and strength, as well as low density and high temperature durability.1,2 However, due to their high sensitivity to inherent flaws, ceramics generally fail catastrophically. This brittle behavior is strongly attributed to a lack of ductility, emphasizing the absence of mechanisms to reduce mechanical energy at stress concentration sites.3 To overcome the brittle nature of ceramics dictated by their highly covalent and/or ionic bonding, the incorporation of microstructural heterogeneity and cumulative microfracture behavior into their structure is essential.4,5 In homogeneous ceramics, the incorporation of a microfracture process into the structure is limited, as there is no barrier to halt unstable crack growth. It is known that a mixture of different sorts of ceramics promotes unique deformation and fracture behaviors. Therefore, heterogeneous structures with thermal and/or elastic property misa)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0418 J. Mater. Res., Vol. 24, No. 11, Nov 2009

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matches allow the operation of microscale fracture and crack-interface interaction, the extent of which is usually controlled by the scale of the microstructure, ultimately leading to crack propagation resistance. The incorporation of such mechanisms and their successful operation was effectively applied in ceramics with heterogeneous microstructures,6,7 in particle-dispersed ceramic-matrix composites,8,9 and in laminated ce

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