Abstract
As a high-temperature protective coating, thermal barrier coatings can not only improve the heating temperature of the parts, but also can prolong their service life. It has a broad application prospects in the aerospace industry. However, oxidation of bond coat has been identified as one of the important modes of system degradation. Therefore, how to protect the bond coat against oxidation is the key way to improve the service life of the TBC.
In this work, we prepared four kinds of self-healing TBCs: NiAl +TAZ1+ YSZ、NiAl+TAZ2+ YSZ、 NiAl+TAZ3+ YSZ by atmosperific plasma spraying. And the microstructures, phase composition and properties (thermal shock resistance and high temperature oxidation resistance) of self-healing thermal barrier coatings were systematically investigated compared to conventional double layer thermal barrier coatings (NiAl +YSZ).
The results show that the surface of the four coatings is a stable cubic phase. The macroscopic surface is rough and dense, and microscopic consists of some micro cracks,holes and unmelted particles. YSZ ceramic layer and self-healing middle layer mainly consists of dense fully melted region, but there still exist a few small pores. NiAl bond layer is denser than YSZ layer on the whole.
The porosity of self –healing TBC decreased as prolonging the heating time at 600 °C,and the whole coating became denser. Oxygen passed through the pores in the YSZ coatings, and the TiC in the coating oxidizes at high temperature and the oxidation production fills and seals the cracks and pores inside the coating, thus the coating is self healed and prohibit the bond coat from oxidation.
Thermal shock tests of the four kinds of composite coatings are carried out at 800℃、900℃、1000℃, respectively. The results indicate that the number of thermal shock cycles of the coatings decrease with the increase of the temperature. The self-healing coating of NiAl +TAZ1+ YSZ has the best thermal shock resistance. At 1000°C, the thermal shock resistance of this self-healing TBC improved more than 50% in comparison with the conventional TBC. After thermal shock testing, the surface of the four coatings is still a stable cubic phase. The failure of the coatings are all spallation of YSZ, while the middle coat of the self-healing TBC was tightly combined with the substrate and pores did not exist at the middle coat/bond coat interface. With increasing the test temperature, the content of the oxide changed very little. For conventional double layer thermal barrier coatings, the higher the test temperature, the more oxides are found in the bond coat.Thermal oxidation resistance tests of the four kinds of thermal barrier coatings are carried out. The results show that the self-healing thermal barrier coatings can reach constant weight after oxidation in short time under high temperature. Moreover, the oxygen content of the bond coat of self-healing thermal barrier coatings is much lower than the conventional thermal barrier coatings after oxidation under the same conditions.All of these results show that the self-healing thermal barrier coatings are superior than the conventional thermal barrier coatings in respects of low porosity, thermal shock resistance and thermal oxidation resistance. This work provides an efficient technical reserve and theoretical basis for developing high-temperature protective coatings.
Keywords: thermal barrier coatings, plasma spraying, self-healing coatings, self-healing heat treatment
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