Solid-solution directionally solidified eutectic oxide composites: Part I. Eutectic growth and characterization
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G. Dhalenne and M. Vela´zquez Laboratoire de Physico-Chimie de l’Etat Solide, Universite´ de Paris-Sud, Orsay, France (Received 20 July 2001; accepted 10 January 2002)
In this work, the solid solution series directionally solidified eutectic, Co1−xNixO/ZrO2(CaO), 0 艋 x 艋 1, was examined as a possible model system for interfacial fracture studies. These materials were grown by the optical floating zone method in an image furnace under controlled atmospheres to prevent the formation of Co3O4. For all compositions, lamellar microstructures were produced. The interface plane orientation relationship {111} Co1−xNixO㛳{100} ZrO2(CaO) was maintained throughout the solid solution series. The phase compositions were close to nominal values with some solubility of CoO in ZrO2(CaO). Preliminary indentation measurements demonstrate the onset of interfacial delamination for compositions of x < 0.2.
I. INTRODUCTION
Directionally solidified eutectic (DSE) oxides have been previously investigated as ultrahigh-temperature (>1500 °C) composites and as model systems for fundamental materials science studies.1,2 Materials such as YAG/Al2O3 (YAE)3–8 and Al2O3/ZrO2 (AZE)9–11 have shown promising creep resistance and high-temperature strength, both in fiber form and as monolithic materials. However, it is the low fracture toughness of these materials (4–5 MPa m1/2) at room temperature that has hindered their development as reliable structural materials.12 The low fracture toughness of oxide DSEs is attributed primarily to the strong interfacial bonding between phases, which does not permit dissipation of crack energy by interfacial processes such as delamination, deflection, microcracking, etc. Fracture studies on AZE and YAE have shown that cracks propagate through interfaces virtually unhindered, indicating greater interfacial adhesion strength than cohesive strength for either phase.12 These materials can be classified as micro-laminates because their microstructures are composed of lamellar plates approximately 1–5 m in thickness. The micro-laminate nature of these composites means that the material has a high interfacial density. Consequently, the material’s fracture performance can either be enhanced or degraded, depending upon the interfacial fracture characteristics of the eutectic. To better understand the interfacial fracture properties of oxide DSE systems, a number of model systems have 760
J. Mater. Res., Vol. 17, No. 4, Apr 2002
been explored with the aim of establishing fundamental structure–property correlations at the micron and atomic length scales. Lamellar oxide DSEs have so far been grown and characterized in binary systems such as NiO– CaO, NiO–Y2O3, NiO–Gd2O3, CoO–ZrO2(monoclinic), and CaZrO3–ZrO2(CaO).13–17,18 Eutectic microstructures have also been grown for pseudo-binary systems such as NiO–ZrO2(CaO)-cubic, NiO–ZrO2(Y2O3)-cubic, and CoO–ZrO2(CaO).18–21 The interfacial structure and bonding of NiO–ZrO2(CaO)-cubic have been studied extensively at the atomic scale.22–25 All of these systems have lamellar micro
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