部分安定化ジルコニアの相転移とクラックの伸展

抄録

In order to clarify the dependence of crack growth on phase transformation and the effect of stress on phase transformation in partially stabilized zirconia, the hardness and fracture toughness of tetragonal zirconia polycrystalline specimens containing 3mol% Y₂O₃ (the average grain size of 0.3-0.4μm) were measured as a function of indenting load by using the Vickers indentation method. The fracture energy was obtained by the single edge notched beam method at various strain rates and by the work of fracture method. The same measurements were performed also on the specimens heat treated at different temperatures in which a part of tetragonal phase transformed to the monoclinic phase. Furthermore, the degree of phase transformation at 300°C was determined as a function of time and bending stress. The results obtained are summarized as follows:
(1) All the specimens with or without heat treatment had almost the same hardness of about 12GPa, although a slight decreasing tendency with increasing load was observed. They also showed a similar tendency in fracture toughness except that the untreated specimen showed very high toughness when indented at low loads of less than 200N.
(2) The K_Ic obtained by SENB method increased with increasing stress rate in the range above 10^-2mm/min, but remained almost constant at the stress rate lower than 10^-2mm/min. This seems to indicate that some threshold stress exists to start slow crack growth in tetragonal zirconia.
(3) The fracture energy obtained by SENB method was several times larger than that of WOF method. Since the former reflects the energy needed to start the fracture rather than the energy needed to make a new fracture surface, a very large energy is spent to start crack growth, probably due to the phase transformation.
(4) The phase transformation was enhanced by the residual stress around the Vickers indent when heat-treated at 300°C. This stress induced transformation becomes apparent when the tensile stress exceeds 450MPa.