Mechanical and thermal properties of double-layer and triple-layer thermal barrier coatings with different ceramic top coats onto polyimide matrix composite

Abstract

Abstract The present study has discussed the deposited thermal barrier coatings of Cu-Sn/mullite and Cu-Sn/NiCrAlY/YSZ onto a carbon-fiber reinforced bismaleimide (BMI) matrix composite. The thicknesses of the metallic bonding layers and ceramic top coats were 100 µm and 300 µm, respectively. The microstructural, mechanical and thermal properties of the TBC systems were evaluated through x-ray diffraction (XRD), scanning electron microscopy (SEM), tensile adhesion strength testing (TAT), micro-hardness tests, roughness measurement, thermal shock testing and laser flash analysis (LFA). A comparison of the microstructure of the ceramic top coats showed that the YSZ had a homogeneous structure with high crystallinity while the mullite exhibited a multi-phase structure containing mullite, alumina (γ-Al2O3) and amorphous phases. The mullite layer recorded higher residual stress than that the YSZ layer because of its higher Young's modulus and the transformation of the mullite phase to alumina and amorphous phases during deposition. The adhesion strength of the sample with a triple-layer coating was greater than that with a double-layer coating because it exhibited a milder residual stress gradient across the coating. The thermal shock resistance of the triple-layer coated sample was 1.5-fold greater than that of the double-layer coated sample. This could be due to the existence of an oxidation-resistant layer of NiCrAlY and smaller differences in the coefficients of thermal expansion between coating layers in the triple-layer coating system. Thermal conductivity of the triple-layer coating sample was much less than that of the double-layer coating which can increase the operating temperature of the polymeric substrate to a higher value.