Low-cycle fatigue strength under step loading of a Si3N4+SiCnanocomposite at 1350 degrees C

The low-cycle fatigue behaviour of a hot pressed silicon nitride/silicon ca rbide nanocomposite and a reference monolithic Si3N4 have been investigated in 4-point bending at 1350 degreesC in air using stepwise loading. The nan ocomposite was prepared using 20% of SiCN amorphous powder as an additive, together with 5% yttria, to crystalline alpha -silicon nitride powder. Two types of specimen have been tested, with and without a sharp notch (notch t ip radius similar to 10 mum) at applied loads from 50 N with steps of 25 N and from 50 N with steps of 50 N, respectively. Five cycles have been perfo rmed at all applied load levels with an amplitude of 50 N for both types of specimen. The deflection of the specimens has been recorded up to specimen failure. The failure load of the unnotched nanocomposite was significantly higher than that of the monolithic material whereas for the notched specim ens only a small difference has been found between the failure loads of the monolithic and the composite. Notched specimens of both materials exhibite d a similar size of the slow crack growth area at catastrophic fracture, wh ereas for unnotched specimens the size of the slow crack growth area was si gnificantly larger for the monolithic ceramic. The nanocomposite exhibits h igher fatigue strength due to its higher resistance against stress corrosio n damage and stress corrosion crack growth. (C) 2001 Kluwer Academic Publis hers.