Microstructure and indentation fracture of dysprosium niobate
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The high-temperature indentation fracture and microstructures of dysprosium niobate (DyNbO4) were investigated by optical, scanning, and transmission electron microscopy (OM, SEM, and TEM). Polycrystalline samples were sintered at 1350 °C for 3 h and cut into 3 mm disks for TEM. The disks were indented in a Nikon QM (Tokyo, Japan) hot hardness indenter at room temperature up to 1000 °C. Many lamellar twins having different widths were observed by TEM as well as intergranular microcracks. The room temperature hardness was relatively low at 5.64 GPa and decreased with elevated temperatures. Crack lengths were short, showing a typical micro-cracking effect. In the sample indented at 1000 °C, dislocations in periodic arrays were evident, and their density increased markedly due to heavy plastic deformation.
INTRODUCTION
Transformation toughening has been accepted as a remarkable toughening mechanism in structural ceramics for improving their reliability. It is basically due to the relaxation of crack propagation energy by the volume change accompanying the martensitic transformation, which is stress-induced when a crack is propagated into a body.1–3 Prime examples are zirconia (ZrO2) ceramics stabilized in the tetragonal phase by yttria (Y2O3), magnesia (MgO), calcia (CaO) or ceria (CeO2). Some volume fraction of monoclinic zirconia (m-ZrO2) has well been used as a toughening agent in alumina (Al2O3)4 or silicon nitride (Si3N4) composites5,6 leading to such toughening mechanisms as microcrack toughening, crack deflection, and plastic deformation. Indentations of single crystal monoclinic zirconia were made by First and Heuer7 at room temperature to 800 °C. A maximum Knoop hardness (HK) of ∼8 GPa along the [001] direction was reported. Deformation twinning on {110} and also (100) occurred from room temperature upwards, together with significant hardness anisotropy on (100) and {110}. More recently, single crystals of mullite (3Al2O3⭈2SiO2) have been indented on (001) and (010) orthorhombic faces as a function of temperature up to 1400 °C.8 In comparison, the mean microhardness at room temperature was ∼16 GPa, which decreased to a HV ∼ 13 GPa at 300 °C. Above 300 °C, the microhardness was only slightly reduced with increasing
II. EXPERIMENTAL
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0202 1422
temperature to HV ∼ 10 GPa at 1000 °C. However, LaNbO4 has not received as much interest in the structural ceramic community due to its comparatively low transformation temperature. There have been several reports9–13 of crystalline lanthanum niobate (LaNbO4), which transforms from tetragonal to monoclinic symmetry at a temperature in the range of 490–525 °C and which is similar to the transformation in ZrO2. Grains contain many twins11 and exhibit shape memory effects in a compressive stress field.13 Recent studies in our laboratory have focused on dysprosium niobate (DyNbO4) due to its tetragonal to monoclinic transformation at ∼800 °C and the occurrence of twinning similar
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