| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 90 === The dynamic fatigue performance of gas-pressure-sintered β-phase silicon nitride containing 6wt.% ytterbium oxide and 2wt.% aluminum oxide as a sintering aid have been investigated in four-point flexure at 1000, 1200, 1300 and 1400℃ in ambient air using different loading rates from 0.01 to 1 mm/min.
The fatigue strength decreased with decreasing loading rate at all temperatures, and the fatigue resistance at 1200℃ was better, i.e. fatigue parameter (n) was higher than that at 1000℃.The superior fatigue resistance at 1200℃ was attributed to crystallization and refractoriness of grain boundary phases. Crack healing and crack blunting could also influence the crack propagation at high temperature.
This material was more susceptible to slow crack growth, which was reflected by the slopes of the flexure strength vs. loading rate curves, as the test temperature was increased to 1300 and 1400℃. Oxidation assisted slow crack growth was the dominant damage mechanism at 1300 and 1400℃, and creep damage may also affect the fatigue behavior in low loading rate.
In addition, all fracture surfaces possessed sweeping stress-oxidation damage zone characterizing subcritical crack growth at high temperature. The formation and growth of a stress-oxidation damage zone are primarily responsible for the strength and fatigue resistance degradation of silicon nitride ceramics at elevated temperature in air atmosphere.
|
|---|
