Thermal barrier coatings (TBCs) are a fundamental technology used in high-temperature applications to protect superalloy substrate components. However, extreme high-temperature environments present many challenges for TBCs, such as the degradation of their thermal and mechanical properties. Hence, highly insulating, long-life TBCs must be developed to meet higher industrial efficiency. This paper reviews the main factors influencing the thermal insulation performance of TBCs, such as material, coating thickness, and structure. The heat transfer mechanism of the coating is summarized, and the degradation mechanism of the thermal insulation is analyzed from the perspective of the coating structure. Finally, the recent advances in improving the thermal insulation and lifetime of coatings are reviewed in terms of advanced materials and structural design, which will benefit advanced TBCs in future engineering applications and provide guidance for the next generation of high thermal insulating TBCs.

Thermal barrier coatings (TBCs) have been developed to protect superalloys against high-temperature heat fluxes, which are required for the development of high-performance gas turbines. TBCs have porous structures, which are densified by sintering. The resulting stiffening is a major cause of TBC failure in service. Therefore, there is a need to reduce the negative sintering effect on the life span of TBCs. In this study, the sintering mechanism and the dominant factors causing changes in stiffening and mechanical properties were revealed experimentally. The experimental results show that the multiscale undulation of the originally smooth two-dimensional (2D) pore inner surface triggers multipoint contact between the upper and lower inner surfaces, resulting in pore healing during thermal exposure. The healing of 2D pores is the main structural characteristic change in TBCs after thermal exposure and the main reason for the stiffening and changes in mechanical properties. Then, the sintering effect on TBCs with vertically cracked structures was designed and simulated. We found that implanting vertical cracks in the topcoat can reduce the sintering effect and driving force for cracking by 87.9% and 79.9%, respectively. The degree of reduction depends on the space between vertical cracks. Finally, the mechanism responsible for the sintering-resistant TBCs was analyzed and discussed. Vertically cracked structures exhibited scale-sensitive stiffening, indicating that macroscopic stiffening is much lower than microscopic stiffening. In other words, the macroscopic sintering effect was lowered, and the TBCs remained highly resistant to global strain during thermal exposure. The resulting strain energy release rates are much lower than those of conventional TBCs. The results of this study contribute to the long-life thermal protection of superalloy-based components used in advanced gas turbines.

The large-arc discharge problem occurs in the processing of magnesium alloy by micro-arc oxidation. It will cause local ablation of ceramic coating and even magnesium alloy matrix material. Thus, clarifying the mechanism and influencing factors is a necessary approach to avoid the large-arc discharge problem. In this study, a new “cooling time” parameter based on frequency and duty cycle is proposed to study the large-arc discharge trend and understand the relationship between large-arc discharge and process parameters. Then, the mechanism of the large-arc discharge phenomenon and the action of the cooling time were investigated by the theory of local-arc discharge mechanism and load characteristics of micro-arc oxidation. The results show that the mechanism of large-arc discharge is that the arc discharge is continuous in situ, and the aggregation or merger of arc discharge occurrence. Therefore, the large-arc problem can be resolved effectively through discretizing the arc discharge by adjusting the cooling time and regulating the pulse frequency and duty cycle. The cooling time made the location area of arc discharge and oxides fully cooled, and provided time for the sufficient discharge of the load capacitance, to effectively inhibit the problem of large-arc discharge.

In this study, Inconel 625 coatings were deposited onto the surface of 2Cr13 stainless steel via laser cladding technology to ensure their corrosion resistance and mechanical properties. The microstructure and characteristics of coatings were adjusted by varying laser power (1200, 1500, and 1800 W), scanning speed (10, 15, and 25 mm/s), and overlap rate (40%, 50%, and 70%). The results showed that the impact resistance of blades was improved by 23% to 30% compared to the substrate, whereas the self-corrosion current density was reduced by 94%–98%, which indicated the outstanding resistance of specimens to damage and corrosion. At the same time, the appropriate processing parameters enabled the surface hardness of the 2Cr13 substrate to be improved. This study provides practical technical guidance for the repair of 2Cr13 blades and a comprehensive enhancement of their corrosion resistance and mechanical properties through parameter optimization.

To determine the relationship between the deposition characteristics of semi-molten particles and the porosity of porous materials prepared by flame spraying, metal powders of Mo and 316L were used and the melting degree of deposition particles was controlled. Based on the experimental observations, a three-dimensional, random-stacking model was established by introducing the slipping characteristics of semi-molten particles, which could help predict the porosity of the generated porous metal materials. The results show that the maximum porosity of porous materials deposited by semi-molten particles was about 82%. A one-to-one relationship was observed between the melting degree of particles and the porosity of deposited samples. The three-dimensional random model could successfully predict the porosity of porous materials by combining the melting degree and slipping characteristics of spray particles, and the predicted values were consistent with the experimental results. The results of this study are useful to control the pore structure of porous materials deposited by semi-molten spraying particles.

To address the issue of depositing thermal barrier coatings (TBCs) on rotating curved surfaces, atmospheric plasma spraying (APS) was employed to prepare yttria partially stabilized zirconia (YSZ) coatings on a rotating curved substrate. Three standoff distances of 80 mm, 100 mm and 120 mm were selected. The microstructure, microhardness, elastic modulus and fracture toughness of three YSZ coatings were tested. The results indicate that as the standoff distance increased from 80 mm to 120 mm, porosity increased from 11.27% to 13.29%, microhardness decreased from 760.8 HV_0.3 to 713.2 HV_0.3, elastic modulus decreased from 24.0 GPa to 22.6 GPa, and fracture toughness decreased from 1.14 MPa·m^1/2 to 1.04 MPa·m^1/2. The properties of the YSZ coating in the case, such as elastic modulus and fracture toughness, were significantly lower than those of the YSZ coating deposited on stationary planar substrates. Solidification of the molten particles impacted on rotating curved substrates was accelerated and splat spreading was constrained because of the coupled effect of centrifugal force and elevated cooling rate. Therefore, under identical spraying parameters, the process parameters optimized for planar substrates cannot be directly transferred to rotating curved components.