抄録
TiO2, has been widely used as one of functional ceramics, however, it has a fault of decreasing strength in the case when its polycrystalline texture is consisted of large grains. This phenomenon has been considered to be due to its thermal expansion anisotropy, i. e., cracks produced by thermal stress in the quenching process after sintering, propagate in the texture and decrease the strength of TiO2. The aim of this study is to determine experimentally the statistical character of the effect of grain size on mechanical properties of TiO2, and clarify its correspondence with microfracture model.
The specimens having a grain size range 10-60μm were prepared by various annealing temperatures and times after sintering. Young's modulus and bending strength decreased remarkably when the grain size (abbreviated as d) was larger than approximately 20μm, and indicated almost constant values at d>35μm.Stress intensity factor (Kc') showed a constant value (2.2MNm-3/2) throughout the all grain sizes tested, when the notch radius in SENB method was taken into consideration.This must imply the plane strain fracture toughness of TiO2.The relation between bending strength（σ) and grain size (d) was σ=-87.1+829.7d-1/2 The crack length of fracture origin calculated from bending strength and stress intensity factor was almost constant (1000-1400μm) for low-strength specimens. This resulted in the fact that the bending strength was independent of grain size at d>20μm. The crack length agreed also with the observation results of fracture surface.
Critical grain size for crack propagation into grain was calculated from the arrangement of Evans' model in which polycrystals with 2-dimensional hexagonal grains was adopted. The calculated value was 22.2μm and coincided well with the experimental result.
Weibull's statistical analysis was used to analize the effect of grain size on fracture diagnostics in the results of bending strength.
