A B S T R A C TThe crack-healing behaviour of machining cracks in Si 3 N 4 /20 wt% SiC composite was investigated. The machining cracks were introduced by a heavy machining process, during the creation of a semicircular groove. The machined specimens were healed at various temperatures and times in air. The optimized crack-healing condition of the machined specimens was found to be a temperature of 1673 K and a time of 10 h. The specimens healed by this condition exhibited almost the same strength as the smooth specimens healed. Moreover, the bending strengths and the fatigue limits of the machined specimens healed were systematically investigated at temperatures from room temperature to 1673 K. The machined specimens healed at the optimized condition exhibited an almost constant bending strength (∼700 MPa) up to 1673 K. Also, the specimens exhibited considerably high cyclic and static fatigue limits at temperatures from 1073 to 1573 K. These results demonstrated that the crack-healing could be an effective method for improving the structural integrity and reducing machining costs of the Si 3 N 4 /SiC composite ceramic.
as a sintering additive was investigated. The machining cracks were introduced by a wheel grinding process, which is the most common method for finishing ceramic components. A semicircular groove was made at the center of small bending specimens by the machining. The machined specimens were healed at various temperatures and times in air. The optimized crack-healing condition of the machined specimen was found to be a temperature of 13001C and a time of 1 h. The specimens healed by this condition exhibited almost the same strength as the smooth specimens that underwent the healing process. Moreover, the bending strength and fatigue limit of the machined and healed specimens were systematically investigated at temperatures ranging from room temperature to 13001C. The heat-resistance temperature has been determined to be approximately 10001C. Also, the specimens exhibited high static and cyclic fatigue limits at temperatures of 8001 and 10001C. These results demonstrate that crack healing could be an effective method for improving the structural integrity and reducing the manufacturing costs of a Si 3 N 4 /SiC composite ceramic.
Si3N4/SiC composite containing Y2O3 as sintering additive was hot-pressed to examine their crack-healing behavior as a function of oxygen partial pressure (pO2 = 50−21,000Pa). A semi-elliptical surface crack with length of about 100μm was introduced on the center of tensile surface by the indentation method. These specimens were healed at various temperatures and times using tube furnace. After the crack-healing process, the bending strength of each specimen was measured at room temperature and at high temperatures of 800−1,400. Even under pO2 = 50Pa, the cracks were completely healed by heat treatment at 1,300 for 10h. Bending strengths of the crack-healed specimens exhibited almost the same strength as the smooth specimens healed. Moreover, the specimens exhibited an almost constant bending strength (~800MPa) up to 1,400 .
We fabricated synthetic ferrimagnetic layers (SyFL) based on NiFe/Ru/NiFe or NiFe/V/NiFe for the free electrode of magnetic tunnel junctions (MTJ) and characterized their spin flopping, coercive and saturation fields. NiFe/Ru/NiFe SyFL exhibited a coercive field of 10 Oe, which would produce a coercive field of less than 50 Oe when incorporated into sub-micrometer sized MTJ. In comparison, NiFe/V/NiFe SyFL showed extremely low exchange coupling; only when the thickness of the V layer was ~0.8 nm, appreciable level of exchange coupling was observed. The coercive field of the NiFe/V/NiFe SyFL, however, did not have a large dependence on the thickness of V and remained at ~2 Oe. The coercive field of the V-SyFL is expected to maintain below 10 Oe even after patterned into a tunnel junction. Both NiFe/Ru/NiFe and NiFe/V/NiFe SyFLs were stable up to 525 K without showing any gross changes in microstructure and magnetic properties when thermally annealed.
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