The modern gas turbines are constantly pursuing the development trend of high heat insulation and long service life, making the operating temperature of turbine blade continuously rising. Thermal barrier coatings (TBCs) have high temperature resistance and high heat insulation to protect metal alloy substrates. The ceramic topcoat layer is typically composed of yttria-stabilized zirconia (YSZ) material. However, the spallation failure of YSZ is easy to occur during high-temperature service. Gd₂Zr₂O₇(GZO) is one of the candidate materials for the new generation of thermal barrier coatings, due to its low thermal conductivity and phase stability at high temperatures. The porous structure of ceramic coatings inevitably undergoes sintering and densification during high-temperature service, the hardness and elastic modulus of coating were decreased.

Adjusting the spraying distance influence both the temperature and velocity of particles, thereby influencing the microstructure of the porous coating. In this work, three kinds of Gd₂Zr₂O₇ (GZO) coatings deposited at different spraying distances were prepared by atmospheric plasma spraying (APS). The as-deposited coatings were thermal exposure to 1 300 ℃ for different hours. The phase structure, microstructure and mechanical properties of the obtained coatings were characterized. The relationships of pore structure and mechanical properties of GZO coatings were revealed during high temperature thermal exposure.

Atmospheric plasma spraying equipment was used to deposit GZO coatings with a thickness of about 300 μm at different spraying distances. The GZO coatings were placed into a Muffle furnace to carry the thermal exposure tests at 1 300 ℃ for 10, 20, 50, 100 h and 200 h, respectively. The microstructure of the as-prepared GZO coating was observed by SEM. The porosity of the coating was statistically obtained using Image-J software. The phase structure was analyzed by D8-Advance X-ray diffractometer (XRD) using Cu Kα radiation with an operating voltage and current of 40 kV and 40 mA, respectively. Vickers hardness tester (American Micromet-5104 microscopy) was used to test the hardness on the polished section of the coating, with a load of 300 g for 30 s. The elastic modulus of the coating was measured by Knoop indentation method, and parameters were similar with that of Vickers hardness.

Results show that the as-deposited GZO coatings exhibited a typical layered structure. After thermal exposure, the inter-splat cracks, intra-splat cracks and porosity of GZO coatings decreased, while the elastic modulus and hardness increased.

With the increase of thermal exposure time, the apparent porosity of GZO coatings decreased sharply in the early stage (0-20 h) then decreased slowly in the middle stage (20-100 h), and remained stable in the late stage (100-200 h). In the early stage, the crack tip and narrow cracks were preferentially healed by multi-point connection, which hindered the quickly decrease of apparent porosity in the followed two stages, and after 200 h thermal exposure, the apparent porosity of GZO coatings were maintained at 7%-9%. In addition, with the increase of thermal exposure time, varieties of hardness and elastic modulus of GZO coating also presented a multi-stage change rule, similar to that of apparent porosity. The apparent porosity of GZO coatings was increased with the spraying distance, while the hardness and elastic modulus were decreased. The phase structure of GZO coating remained the same before and after thermal exposure. Namely, the mechanical behavior of GZO coatings under high temperature thermal exposure process was mainly affected by its microstructure.

When thermal exposure at 1300 ℃, compositions of GZO coating were defective fluorite (F phase), in consistent with that of the as-sprayed coatings. When thermal exposure for different time, values of apparent porosity, elastic modulus and hardness of coatings all present multi-stage changes, varieties of these parameters mainly occurred in the early stage of thermal exposure (0-20 h). After thermal exposure at 1 300 ℃ for 200 h, the porosity of coatings was remained at about 8%. The hardness and elastic modulus of coating prepared at a spray distance of 120 mm were about (7104.5±658.2) MPa, and (40.1±1.5) GPa.