Takuji Okabe

Observation and Removal of Atmospheric Micro-Contaminants on SUS304 Steel

Micro-contaminants on SUS304 stainless steel were observed and confirmed by atomic force microscope, and the micro-contaminant removal was carried out by ultraviolet (UV) illumination. The amount of micro-contaminants on the specimen surface decreased with an increase in the UV illumination time, with extensive removal of the organic substance in the contaminants but leaving part of the contained water in the contaminants. The surface for macro-droplets after the UV illumination became hydrophilic, while no large change of the wettability for micro-droplets on the same surface can be observed.

Influence of Microstructure on Fatigue Fracture Characteristics of Al2O3.

In order to improve the fatigue fracture characteristics of Al2O3, the fracture behavior of two kinds of Al2O3, whose microstructure was controlled by sintering additives, was investigated under static and cyclic loading. It was observed that there was a significant difference in fracture behavior between the fine equiaxial grained Al2O3 (Material M) and the coarse columnar grained one (Material SC). The examination of fatigue lifetime showed that the fatigue strength of Material SC was higher than that of Material M, but simultaneously a significant decrease by cyclic loading. The crack propagation rate da/dt of Material M made no difference between in air and in water, but that of Material SC accelerated in water. According to the results of an energy dispersive X-ray spectroscopy (EDX) analysis on the fracture surface of Material SC, it was found that the sintering additive components of Si and Ca decreased markedly in SCG region under cyclic loading in water, and it was expected that hydration reaction on the glass phase of grain boundary occurred actively by water. Consequently, the reason why the fatigue strength of Material SC decreased under cyclic loading in water was considered to the significant promotion of stress corrosion cracking at the crack tip which is caused by marked decrease of particle bridging effect (i.e., increase in KItip).

Crack Propagation Behavior of Si3N4 Ceramic Under Cyclic Mixed-Mode Loading.

Fracture behavior of Si3N4 ceramic was investigated under cyclic mode I/mode II mixed-mode loading in various environments. The crack propagation rate was higher than that for cyclic mode I loading, and it was higher in water than in nitrogen gas. The fretting and hydration reaction seemed more active than those for cyclic mode I loading, and therfore even if the stress shielding effects by particle bridging and wedging exist, those effects were easily decreased, and the effective stress intensity factor range ΔKtip was increased. When the debris was accumlated by the fretting between the crack surfaces, the substantial effective stress intensity factor for mode II was increased. Therefore, it was suggested that in the case that KI was decreased by the particle bridging and KII was increased by the fretting the fracture morphology became transgranular fracture rather than interglanular fracture.

Fracture Behavior of Ceramics under Cyclic Mode I/Mode II Mixed-Mode Loading.

Fracture behavior of Al2O3, PSZ and Si3N4 ceramics was investigated under cyclic mode I/mode II mixed-mode loading in high purity water at room temperature. The time to fracture was shorter than that for cyclic mode I loading, and it decreased markedly with increasing the mode ratio ΔKII/ΔKI. The crack propagation rate was higher than that for cyclic mode I loading, and it increased markedly with increasing the mode ratio ΔKII/ΔKI. The time to fracture was much shorter than the predicted based on cyclic mode I loading. The difference between them became larger in the order, Al2O3, Si3N4, PSZ. In the case of PSZ, the phase transformation from tetragonal to monoclinic took place under the cyclic mixed-mode and mode I loading tests, and there was almost no difference in the quantity. It was observed that the morphology of fracture surface was different between them, and that the fracture surface in the region of SCG for the mixed-mode loading developed a flat surface which is considered to occur owing to the addition of ΔKII.