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Interest in applications of stabilized zirconia (PSZ) as dental material has recently increased. In a corrosive environment such as a living body, PSZ is subjected to a cyclic load and needs higher fatigue strength. In our previous paper, appreciable degradation occurred in the fatigue lifetime of yttria-doped tetragonal zirconia polycrystals (TZP). In this study, PSZ specimens were annealed for 100 h at temperatures of 200, 400 and 600°C in a low vacuum atmosphere to improve the fatigue resistance. Both static and cyclic fatigue tests were conducted on the annealed specimens under static four-point bending and under cyclic reversed plane bending, respectively. As a comparison, specimens without annealing were tested in the same way. The test results indicated that the lifetime appreciably increases in cycling fatigue under a low applied load in specimens annealed at the low temperatures of 200 and 400°C from that of specimens without annealing. It was supposed that the occurrence of fine microcracks accompanied by the phase transformation from tetragonal to monoclinic caused enlargement of the process zone in the wake field of a crack in cyclic loading and contributed to the increase of crack resistance.

It has been known that the high fracture toughness of partially stabilized zirconia (PSZ) is due to a process zone caused by stress induced transformation at a crack tip. The process zone will give an effect not only on fracture toughness but also on fatigue crack growth behavior. In this paper, the R-curve and fatigue crack growth behavior were investigated by some methods using indentation crack specimens. The toughening exponent obtained by the indentation-strength-in-bending method, where beam strength measurements were made using the specimens with indented loads in the range of 48-294 N, is almost zero, that is, a flat R-curve behavior. On the other hand, KI-V curve obtained by constant stress rate tests using indented beam specimens indicated that there was much subcritical crack growth, i.e. a small crack propagation parameter of n=7.4. Moreover, the static and cyclic fatigue tests conducted under static four-point bending and under cyclic reversed plane bending respectively, indicated that there was an appreciable degradation in fatigue lifetime due to cyclic loading, and the crack propagation parameter (n=9.3) obtained from cyclic fatigue test was obviously smaller than that (n=23.9) obtained from static fatigue tests. X-ray diffraction patterns measured in fracture surfaces showed a sign that a part of tetragonal phase is transformed to the monoclinic phase by static and cyclic loading. As the results, it is considered that the high fracture toughness is caused by transformation toughening in process zone, and the degradation of lifetime in cyclic fatigue is due to micro cracks induced by cyclic loading in the process zone.