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To reduce the formation of detrimental complex oxides, bond coatings in the thermal barrier coatings for gas turbines are typically fabricated using vacuum plasma spraying (VPS) or the high-velocity oxygen fuel (HVOF) process. Herein, VPS and HVOF processes were applied using NiCoCrAlY + HfSi-based powder to assess the oxidation behavior of the bond coatings for both coating processes. Each coated sample was subjected to 50 cyclic heat treatments at 950 °C for 23 h and cooling for 1 h at 20 °C with nitrogen gas, and the weight change during the heat treatment was measured to evaluate the oxidation behavior. After the oxidation test, the coating layer was analyzed with X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The VPS coating exhibited faster weight gain than the HVOF coating because the alumina particles generated during the initial formation of the HVOF coating inhibited oxidation and diffusion. The VPS coating formed a dense and thick thermal growth oxide (TGO) layer until the middle of the oxidation test and remained stable until the end of the evaluation. However, the HVOF coating demonstrated rapid weight loss during the final 20 cycles. Alumina within the bond coat suppressed the diffusion of internal elements and prevented the Al from being supplied to the surface. The isolation of the Al accelerated the growth of spinel TGO due to the oxidation of Ni, Co, and Cr near the surface. The as-coated VPS coating showed higher hardness and lower interfacial bonding strength than the HVOF did. Diffusion induced by heat treatment after the furnace cyclic test (FCT) led to a similar internal hardness and bonding strengths in both coating layers. To improve the quality of the HVOF process, the densification of the coating layer, suppression of internal oxide formation, and formation of a dense and uniform alumina layer on the surface must be additionally implemented. [ABSTRACT FROM AUTHOR]

The thermal durability of thermal barrier coatings (TBCs) obtained using feedstock powders with different purity and phase content was investigated by cyclic thermal testing, including the effects on the sintering and phase transformation behaviors. Three kinds of 8 wt.% yttria-stabilized zirconia, namely regular purity (8YSZ), high purity (HP), and no monoclinic phase (nMP), were employed to prepare top coats by atmospheric plasma spraying on a NiCoCrAlY bond coat using a high-velocity oxy-fuel system. Use of 8YSZ, HP, and nMP for plasma spraying affected the microstructure and lifetime of the TBC in furnace cyclic testing (FCT) at 1100 °C and the sintering rate during annealing at 1400 °C for 50, 100, 200, and 400 h. In FCT, the TBC formed from nMP showed the longest durability, while that formed from HP showed lifetime performance similar to that obtained with regular-purity 8YSZ. The TBC obtained with nMP also exhibited the lowest monoclinic phase transition rate, followed by those obtained using HP and 8YSZ. [ABSTRACT FROM AUTHOR]

In this study, a high-entropy approach was employed to design a new single-phase A 2 B 2 O 7 oxide for thermal insulation applications. Multicomponent high-entropy oxides, containing up to seven different cations, were successfully synthesized in a single defective fluorite structure. We observed that the incorporation of the functional cation, Zn2+, effectively blocked the heat radiation phenomenon by reducing the photon mean free path, and further reduced the high-temperature thermal conductivity owing to the larger free carrier concentration originated from the large number of oxygen vacancies. Although a large concentration of Zn2+ caused a slight reduction in thermal expansion, this study suggests that functional cations can be easily incorporated; thus, expanding the material diversity beyond the typical doping levels to develop new thermal barrier materials. [ABSTRACT FROM AUTHOR]

In this study, we designed a thermal barrier coating (TBC) system—Yb 2 O 3 –Gd 2 O 3 –Y 2 O 3 co-stabilized ZrO 2 (YGYZ) coating—with a Lewis neutral B–SiO 2 layer applied on its surface via a sol–gel-based glassification process and investigated its long-term corrosion resistance under corrosive environments. The corrosion behavior of the B–SiO 2 -covered YGYZ coating was assessed after 20, 40, 80, 160, and 320 h exposure to a hot corrosive environment of Na 2 SO 4 + V 2 O 5 at 1100 °C. Results indicated that the B–SiO 2 layer effectively prevented the penetration of corrosive salts and formation of m -ZrO 2 phase in the coating layer as well as generated vertical cracks that could release the residual stress. Therefore, it is concluded that depositing a Lewis neutral B-SiO 2 layer on YGYZ coatings is highly beneficial for extending the lifetime of the TBCs. • Lewis neutral B–SiO 2 layer was coated on Yb 2 O 3 –Gd 2 O 3 –Y 2 O 3 co-stabilized ZrO 2. • B–SiO 2 -covered YGYZ coatings effectively blocked the penetration of corrosive salts. • With increasing exposure time, B–SiO 2 -covered YGYZ coating generate vertical cracks. • B–SiO 2 -covered YGYZ coatings have better thermal durability than existing systems. [ABSTRACT FROM AUTHOR]

Hot corrosion and CaO–MgO–Al 2 O 3 –SiO 2 (CMAS) resistance of thermal barrier coatings (TBCs) are required in an industrial gas turbine employed with impure fuels and high turbine inlet temperatures. Therefore, various TBC systems with different structural combinations were investigated in thermochemical aspects of hot corrosion and CMAS causing the degradation of TBCs. Four types of 8 wt% yttria-stabilized zirconia (8YSZ), namely dense microstructure (8YSZ), porous microstructure (C–8YSZ), no monoclinic phase (NoM–8YSZ), and low thermal conductivity (low-k), were employed to prepare topcoats using an atmospheric plasma spray on the two bondcoats coated by a vacuum plasma spray with feedstocks of NiCoCrAlY and NiCoCrAlY+Hf + Si. Hot corrosion components penetrated through the whole topcoat layer at 1000 °C however did not infiltrate to the bondcoat layers because of the thermally grown oxide layer formed between the topcoat and the bondcoat. In addition, the elements Hf and Si contained in the bondcoat composition suppressed diffusion of β phase aluminum. In the visual inspection after the hot corrosion at 1000 °C and CMAS test at 1250 °C, the TBC system with the NoM–YSZ topcoat layer showed a sound condition without failure unlike other TBCs, and no monoclinic phase after hot corrosion and the CMAS test, indicating that the phase stability plays a critical role in TBCs' lifetime in the hot corrosion and CMAS environments. • Superiority of Hf and Si elements in MCrAlY bondcoat was observed through hot corrosion test. • Conventional and advanced TBCs failed with a similar trend independent of chemicals and porosity. • low-k TBCs showed a negative dependency on both hot corrosion and CMAS • Phase stability of topcoat layer can also be proposed to have more life time under the environment of hot corrosion and CMAS. [ABSTRACT FROM AUTHOR]