水熱処理によるイットリア系ジルコニア（Y-TZP）の低温劣化

抄録

Y-TZP is used in the field of dental implants mainly for abutment, and Y-TZP fixtures have already been marketed in Europe and the U.S. However, it is that Y-TZP suffers low-temperature degradation, and so shows decreased strength under hydrothermal treatment. Therefore, effects of hydrothermal processes on the low-temperature degradation of a Y-TZP were examined in this study.
Hydrothermal treatment were conducted using a special autoclave under four conditions: at 134℃ and 0.2 MPa for 5 hours (h) or 100 h; or at 180℃ and 1.0 MPa for 5 h or 20 h. Five test specimens for each condition, including a test without hydrothermal process, were prepared. Measurement was conducted using the IF method under conditions of indentation fracture load of 294 N and indentation time of 15 s. Fracture toughness values were calculated from the obtained measurement values. The test specimens after the measurements were observed under a scanning electron microscope (SEM) . Furthermore, X-ray diffractometry (XRD) was conducted to measure phase transformation caused by hydrothermal treatment. In addition, to observe differences of crystal structure, mirror polished test specimens were baked again as a thermal etching process, and then observed under SEM.
For conventional Y-TZP, surface fracture occurred under conditions of 134℃/0.2 MPa/100 h, 180℃/1.0 MPa/5 h, and 180℃/1.0 MPa/20 h; therefore, measurement could not be conducted under these conditions. The fracture toughness values of the untreated specimen and hydrothermal specimen treated at 134℃/0.2 MPa/5 h were 7.36 MPa/m^1/2 and 7.02 MPa/m^1/2, respectively. There was no significant difference between these two conditions. For the novel Y-TZP, measurements were conducted under all conditions, and fracture toughness values were between 14.98 and 15.60 MPa/m^1/2. No significant differences were observed among the conditions in the results of multiple comparisons. Results of XRD showed a notable increase of monoclinic crystals and decrease of tetragonal crystals at 134℃/100 h, 180℃/5 h, and 180℃/120 h by the conventional Y-TZP. The novel Y-TZP indicated a smaller increase of monoclinic crystals and minimal decrease of tetragonal crystals. In the novel Y-TZP, zirconia grains and sparse alumina grains were observed by SEM observation.
The results of this study indicated that the novel Y-TZP has a higher fracture toughness value and not lowtemperature degradation than conventional Y-TZP.