Abstract
Functionally Gradient Material (FGM) promise to be used for heat insulation structures in advanced engine hot sections including gas turbines and supersonic/hypersonic propulsion systems. An application-capable damage mode modeling, however, is needed to realize the full potential of the thermal barrier performance and the durability benefits in practical service environments. The work reported herein describes the results of experimental and numerical considerations on the thermomechanical response of the FGM in transient thermal cycle environment. The maximum thermal shock resistant temperature of the stabilized zirconia/nickel chrome FGM was significantly dependent on the compositional distribution. Spallation was observed in very early periods after initiate heating when exposed in higher heat flux exceeding the heat resistance limit. Photomicrographical evidences indicated that the spallation is caused by the in-plain microcracks linking-up to the vertical cracks associated with the inelasticity of the zirconia ceramics. FEM analysis revealed both type of these cracks to be initiated as a result of the large in-plain compressive stress during the heat loaded cycle. Ceramics with adequate coefficient of thermal expansion are estimated to reduce the in-plain stress, improving the heat resistance limit of the FGM. Further thermal cycle test was conducted on stabilized zirconia-30 vol% cordierite/nickel-chrome FGM to illustrate the effect of the high temperature side thermal expansion on the in-plain stress reduction. The optimized FGM was 120 K higher in heat resistant surface temperature than the stabilized zirconia based FGM.
