With the increasing inlet temperature of gas turbines, the development of TBCs is extremely important to enable modern gas-turbines to operate well at gas temperatures above the melting point of the superalloy. Nowadays, the aim is to prolong the lifetime, increase the inlet temperature and also reduce the cost of TBCs. Therefore, a better oxidation resistance of bond coats at high temperature, a lower thermal conductivity as well as a higher strain tolerance to spallation of YSZ top coat are the key problems to be solved in this thesis. TBCs were prepared, where NiCrAlY type coatings and ZrO2 stabilized by 8wt%Y2O3 with hollow spherical powder (HSP-YSZ) coating by detonation gun spraying (D-gun) were used for bond coats and top coats, respectively. The high-temperature oxidation properties of bond coats, thermal conductivity of HSP-YSZ and failure mechanism of thermal barrier coatings have been analyzed and discussed. Based on D-gun TBCs, a TBC system with a better thermal shock resistance was successfully obtained using AIP/D-gun two step method.
The results indicated that NiCrAlY coatings obtained by D-gun spraying have good isothermal oxidation resistance at 1100˚C owing to the formation of -Al2O3 and Cr2O3 protective oxide scales on the surface. The oxidation kinetics followed the parabolic law. For D-gun NiCrAlY coatings after vacuum annealing, the formation of TiO2 due to the diffusion of Ti from the substrate alloy and NiO impaired the continuity of the protective scale, accelerating the oxidation and leading to failure.
Size distributions of powders in a narrow range were beneficial to obtaining coatings with uniform porosity, microcracks and good adherence, thus the corresponding HSP-YSZ top coats exhibited an enhanced thermal shock resistance. D-gun sprayed HSP-YSZ coatings showed a dense, uniform and fine lamellar microstructure with some vertical and horizontal cracks which produced 10% porosity. HSP-YSZ coating was composed of the tetragonal and cubic phases, which is favorable to the stabilization of the coating at high temperature than the monoclinic phase found within the sprayed powders. D-gun HSP-YSZ coatings exhibited a low thermal conductivity of 0.8-1.2 W/m-K in the range from 200 to 1200˚C，resulting in a good thermal insulation to the substrate alloy.
The failure mechanism of the D-gun sprayed TBCs was determined by means of thermal shock cycling, in other words, the TBCs were heated at 1100˚C and then quenched in water. The results showed the primary failure mechanism of D-gun TBCs involves the depletion of aluminum in the bond coat and formation of Ni/Cr non-alumina oxides within TGO layer at the bond coat/ceramic top coat interface. Undulation development induced micro-cracking at the YSZ/TGO interface and the formation of voids decrease the toughness between the TGO and YSZ. As a result of aluminum depletion in the bond coat, extensive cracking of the rapidly formed Ni/Cr oxides due to strain accumulation during cooling in turn aids to delamination of the outer ceramic layer and expected failure at the regions where chemical failure has occurred. Failure of the D-gun sprayed TBC starts with crack initiation along the splats boundary in the ceramic top coat and around the Ni/Cr oxides. The cracks propagate and coalesce with increasing thermal cycles which can induce wedge-shaped spalling areas in the ceramic coat.
Compared with the oxidation of D-gun bond coats, bond coats deposited by AIP had an improved oxidation resistance at high temperature. The results also indicated that Al content increased from 8% to 10% can improve the oxidation properties. Therefore, Ni-32Co-20Cr-10Al-0.5Y-1Si-0.03B coating showed enhanced oxidation resistance compared with Ni-32Co-20Cr-8Al-0.5Y-1Si-0.03B coatings.
Different from the failure mechanism of D-gun TBCs, the lifetime of AIP/D-gun TBCs is controlled by the initiation of a sub-critical interfacial crack. The AIP/D-gun TBCs failed at the weak interface of TGO/bond coat with some TGO adhered to YSZ top coat. Interfacial separations along the interface led to the lamellar spallation of TBC when cooling to room temperature.
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