Plasticity4 nduced Fatigue Damage in Ceria-Stabil ized Te t ragonal Z i r con ia Pol y crys tal s

Current studies on the fatigue lifetime of ceramics are mostly focused on the relation between the stress amplitude (or maximum stress) and cycles to failure. For a more com-pliant and plastic ceramic which has a pronounced nonlin-ear stress-strain relation, the role of plastic strain in the fatigue damage is investigated for the first time in this study using a 12 mol % Ce-TZP. By testing at different tempera-tures, we were able to vary the amount of transformation plasticity with the same microstructure. The Coffin-Man-son relationship, which suggests that fatigue lifetime in the low cycle fatigue regime is best correlated with the plastic strain range, was confirmed for the tough ceramic. Fatigue damage is found to be a bulk process which continuously degrades flaw tolerance by microcracking. Evidence for the latter mechanism was also provided by uniaxial cyclic tension-compression stress-strain response and by TEM examination. Despite such damage, the possibility of plas-ticity-induced surface-crack nucleation in fatiguing ceram-ics, unlike in metals, appears unimportant. zirconia polycrystals (Ce-TZP). Plasticity in Ce-TZP has its ori-gin in phase transformation.*'.* * Lowering the test temperature without changing the microstructure increases the driving force for the phase transformation, which in turn results in a higher amount of transformation plasticity. Compared to the other two zirconia materials which were studied previously, yttria-stabilized zirconia (Y-TZP)2'~~*5 and magnesia-stabilized zirco-nia (Mg-PSZ),25 Ce-TZP is notable in that its plasticity can be much greater in magnitude. " This makes Ce-TZP an ideal ceramic material to investigate the plastic strain effect on fatigue. Inasmuch as plasticity-dominated fatigue behavior might be expected in Ce-TZP, it is also interesting to question the rele-vance of fatigue crack nucleation by deformation, especially in the near surface regions. Current understanding of fatigue behavior in metals and polymers states that the fatigue lifetime is a three-stage process including initiation, propagation, and final failure, and that a large plastic strain range facilitate