Abstract
Functionally graded materials (FGM), composed of zirconia and nickel system, were sintered from laminated mixtures by the pulse discharge sintering method (PDS). The partially stabilized zirconia (Y2O3-3mol%, size; 0.8μm) and nickel (size; 5μm) were mixed at 90, 80, 60, 40vol%PSZ in a ceramic ball mill for 2 hours with ethanol. These mixtures and two pure materials were laminated in a mold made of graphite. The designed thickness of each layer varies from 0.2mm to 2mm. The different thickness FGM specimens were sintered. These were evaluated by the ultrasonic imaging, and also observed by the optical microscope at the cross section. By the microscope, it was found that the matrix of the FGM specimens consisted of zirconia and very fine nickel particles dispersed in the matrix. In some FGM, ultrasonic images showed no clear defect in all interfaces at just after sintering, however after about 2 weeks, slight defect images were recognized, then the defect changed to the crack image after about 2 months. To know the mechanism of the change, 3 kinds of 2 layers sintered samples (90/80, 80/60, 90/60vol%PSZ) were fabricated, these samples were examined by ultrasonic imaging after each thermal treatment. As the results, it was observed that some of defects gradually propagated to inside, then finally changed to large cracks or a delamination.
