Degradation of bending strength occurred by corrosion of sintered silicon nitride in aqueous acidic solutions

摘要

Three silicon-nitride (Si_(3)N_(4)) ceramic specimens (differing in terms of sintering additive and bending strength) were corroded and tested in 3-mol/L-HCl aqueous solutions at 80°C for 1500 h. The corrosion resistance of each specimen was evaluated by measuring weight loss and bending strength of the specimens before and after the immersion tests. The corroded and fractured surfaces of the specimens were observed by SEM. When the specimens were soaked in the aqueous HCl solutions, weight loss and bending strength decreased with immersion time. Moreover, after immersion, the color of a layer in the fractured section (called “discolored layer” hereafter) changed, and the layer became thicker with immersion time. The discolored layer included a corroded layer at the point of contact with the corrosion solution (where grain boundaries were eluted). The correlation between corroded-layer thickness and weight loss and that between bending strength and weight loss were both found to be linear. However, the gradients of those correlations for each test specimen were found to differ, so it is difficult to summarize these correlations with one linear mathematical expression. Measured bending strength and bending strength predicted using fracture toughness (K_(IC)) of the Si_(3)N_(4)-ceramic specimens and crack length were found to be closely related. In this prediction, the corroded-layer thickness of the specimens immersed in HCl solutions was considered to be equivalent to the diameter of semi-circular surface cracks.