Abstract
The effects of temperature difference and interlayer thickness on the thermal fatigue life of S45C/Cu/Si3N4 composites with brazing filler metals of AgCu and AgCuTi are described. First, specimens of S45C/Cu/Si3N4 systems with interlayer thicknesses of 150 and 200 μm were prepared by brazing. Next, thermal fatigue tests were conducted under cyclic heating conditions with temperature differences of 25-300 and 25-600℃. Further, fatigue fracture surfaces due to the cyclic thermal loadings were observed using an optical microscope. The results revealed that cyclic thermal loading(25-300℃)of the composites caused fatigue failure to the brazing filler metal or Cu/Si3N4 interface. In contrast, cyclic thermal loading with the relatively large temperature difference caused fatigue failure to not only Cu/Si3N4 interface, but also S45C/Cu. These results indicated that the fatigue damage of the composites depended on the thermal stress amplitudes around the interfaces. The thermal fatigue life decreased logarithmically with an increase in temperature difference, and was found to obey Manson-Coffin's law. The increase in the thickness of interlayer contributed to the extension of thermal fatigue life. In addition, a model was proposed for evaluating the thermal fatigue life of the composites, and the effects of temperature difference and interlayer thickness on the thermal fatigue life were discussed using the model.
