Multi-layered thermal barrier coatings on BMI polyimide matrix composite

Al/YSZ and (Cu-6Sn)/YSZ as the double layer thermal barrier coatings (TBCs) and Cu-6Sn/NiCrAlY/YSZ as the triple layer TBC with 100 μm thick for bonding layers and ceramic top coat were deposited on bismaleimide (BMI) matrix composite substrates. The results showed the coherent and clean substrate/coating interface in the triple layer coating system. On the other hand, significant pores, cavities and cracks were appeared throughout substrate/bond coat interfaces when double layer coating systems were created on the polymeric substrate. Fourier Transform Infra-Red (FTIR) spectroscopy showed that decomposition of polymeric substrate took place for double layer coating system in spite of that for the triple layer coating system. The results of laser flash analysis showed that lower thermal diffusivity value of Cu-6Sn/NiCrAlY bonding layer in the triple layer coating system than the two other bonding layers in double layer coating systems may result in lower heat flux transmitted to the substrate during ceramic top coat deposition. NiCrAlY intermediate layer reduced the differences of microhardness and coefficient of thermal expansion (CTE) values between YSZ top coat and metallic inner layer resulting in reduced mismatch in the thermal and mechanical properties at the two interfaces of the triple layer coating system as compared to double layer coating systems. It can be inferred that the triple layer coating system with 300 μm thick YSZ might increase the working temperature of polymeric substrate due to significant reduction in its thermal conductivity value. Thermal gradient cycling test at the coating surface temperature between 300 °C to 450 °C showed that the increase of surface temperature is followed by the significant decrease of TBC lifetime. The failure mode was the crack formation and delamination at the bond coat/substrate interface.