LUBRICANT-ENHANCED FATIGUE-CRACK GROWTH IN SILICON-NITRIDE

Effect of a fluorinated lubricant on the subcritical crack growth beha vior of a Si3N4 ceramic was examined under cyclic loading condition. T he fatigue lifetime of the specimens tested in the oil was shorter, pa rticularly in the low stress regime, compared to that in air. The grow th rate of the surface crack was measured in the oil and compared with that in humid air. Crack growth rate in the oil runs twice that in th e air. XPS analysis of the fatigue fracture surfaces revealed that F h ad reacted with yttria-containing grain boundary phase, while the frac tographical examination indicated that the oil had caused the dissolut ion of the grain boundary phase. The dissolution was shown to reduce t he extent of crack bridging behind the crack tip, resulting in a highe r fatigue crack growth rate in the oil.