Crack propagation and deformation behavior of Al 2 O 3 -24 vol. % ZrO 2 composite studied by transmission electron micro
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Crack propagation and deformation behavior of a pressureless-sintered A12O3 24 vol. % ZrO 2 composite have been studied by transmission electron microscopy on Vickers-indented specimens from room temperature to 1200 °C. Hardness of the composite gradually decreases with increasing temperature, whereas the ratio of indent to crack lengths, which corresponds to the apparent toughness of materials, decreases up to about 1000 °C and then quickly increases with increasing temperature. In the samples indented at room temperature and 1000 °C, most of the cracks propagate along Al 2 O 3 /ZrO 2 interfaces and A12O3 grain boundaries, but a few monoclinic ZrO 2 grains are transgranularly fractured. These fractured grains are heavily deformed and produce a marked reduction of the driving force for propagation of cracks at room temperature. In the sample indented at 1200 °C, cracks are hardly observed, but on the other hand, formation of subgrain boundaries, elongation of grains, and grain boundary sliding are observed both in the A12O3 and ZrO 2 grains located around the indentation site.
I. INTRODUCTION Recently, the micro-indentation fracture (MIF) technique has been widely used not only for measuring hardness and fracture toughness,1"3 but also for characterizing deformation and fracture behavior in brittle materials.4'5 Particularly, using the combination of the high-temperature Vickers indentation and transmission electron microscopy (TEM), detailed study on the relationship between mechanical behavior and microstructure of ceramics at various temperatures has become possible. ZrO2-toughened A12O3 composites, which are considered to be subject to a phase-transformation toughening mechanism,6 are candidate materials for wear parts and tool materials because of their improved mechanical properties, particularly fracture strength and toughness. Especially, the recent availability of high purity and fine-grained A12O3 and ZrO 2 powders has made it possible to produce composites with superior mechanical properties. Recently, it was found that Al 2 O 3 -24 vol. % ZrO 2 composites, which consist of submicron-size grains, can be densified by pressurelesssintering at a relatively low temperature of 1450 °C.7 In a previous paper,8 the micro-indentation fracture behavior of pressureless-sintered Al 2 O 3 -ZrO 2 (Y 2 O 3 ) composites containing different Y 2 O 3 contents was studied by TEM at room temperature. The purpose of the present work is to study crack propagation and deformation behavior of an Al 2 O 3 -ZrO 2 composite as a function of temperature.
wet ball milling in a plastic jar. The mixture was dried in vacuum, preformed by uniaxial pressing with 10 kg/mm 2 , and sintered in air for 2 h at 1450 °C. The sintered body was cut into thin slices using a diamond cutter. Samples for hardness indentation were 2000 o 1 kg (high temp.) 1800x 1 kg (room temp.) 16001400120010008006004002000 0
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600 800 1000 Temperature (°C)
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1400
II. EXPERIMENTAL PROCEDURES Starting powders with a nominal composition of 76 vol. % A1
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