Fatigue Threshold R-Curve Behavior of Grain Bridging Ceramics: Role of Grain Size and Grain-Boundary Adhesion

摘要

To better understand the role of grain size and grain-boundary adhesion on the fatigue threshold R-curve behavior of grain bridging ceramics, a study was conducted on the fatigue threshold behavior of 99.5% pure polycrystalline alumina with two different microstructures (fine and coarse) and in two different environments (moist air and dry N_2). The fine-grained micro-structure showed higher fatigue thresholds at short crack sizes, while the coarse-grained microstructure demonstrated higher fatigue thresholds at long crack sizes. The former effect lead to slightly higher calculated fatigue strengths and was attributed to the crack stalling process that leads to earlier elastic bridge formation in that microstructure. The latter effect is attributed to toughening that is dominated by frictional and mechanical interlocking bridges at longer crack sizes where the larger grains are able to give more bridging. By testing the coarse micro-structure in a dry environment, a higher K_0 was achieved for the glassy grain boundaries giving a higher R-curve at short crack sizes and higher calculated fatigue strengths.