Open Access Research Article Just Accepted
A CMAS-phobic and infiltration-inhibiting protective layer material for thermal barrier coatings
Journal of Advanced Ceramics
Available online: 28 June 2024
Abstract PDF (2.6 MB) Collect

Calcium-magnesium-alumina-silicate (CMAS) corrosion has attracted special attention in the thermal barrier coating (TBC) field. At high temperatures when CMAS is melted, it is adhered to the coating surface and penetrates the interior severely destroying TBCs. In this study, a promising CMAS-phobic and infiltration-inhibiting material of GdPO4 has been proposed, on which molten CMAS is hard to wet and penetrate. These desirable attributes are explained based on analyzing the material characteristics of GdPO4 and its interface reaction with CMAS. GdPO4 is demonstrated to have low surface energy, making molten CMAS hard to wet and adhere to the surface. When contacted with molten CMAS, a double-layer structured reaction layer consisting of an acicular upper sublayer and a compact lower sublayer is formed on GdPO4 surface, which can impede molten CMAS spreading and penetration effectively. First-principles calculation results reveal that the reaction layer has low surface energy and low adhesion work with CMAS, which are favorable for molten CMAS-phobicity. Additionally, the formation of the reaction layer increases the viscosity of molten CMAS, which can drag the melt wetting and penetration. Hence, GdPO4, which exhibits excellent CMAS-phobicity and infiltration-inhibiting ability, is a promising protective layer material for TBCs against CMAS adhesion and attack.

Open Access Research Article Issue
Corrosion behavior of Gd2Zr2O7 thermal barrier coatings under Fe-containing environmental sediment attack
Journal of Advanced Ceramics 2024, 13 (4): 447-462
Published: 10 April 2024
Abstract PDF (13.8 MB) Collect

Environmental sediments mainly consisting of CaO–MgO–Al2O3–SiO2 (CMAS) corrosion are a serious threat to thermal barrier coatings (TBCs), in which Fe element is usually ignored. Gd2Zr2O7 TBCs are famous for their excellent CMAS resistance. In this study, the characteristics of Fe-containing environmental sediments (CMAS-Fe) and their corrosiveness to Gd2Zr2O7 coatings were investigated. Four types of CMAS-Fe glass with different Fe contents were fabricated. Their melting points were measured to be 1322–1344 , and the high-temperature viscosity showed a decreasing trend with increasing Fe contents. The corrosion behavior of four types of CMAS-Fe to Gd2Zr2O7 coatings at 1350 was investigated. At the initial corrosion stage (0.1 h), anorthite was precipitated in CMAS-Fe with a high Ca : Si ratio, while Fe-garnet was formed in the melt with the highest Fe content. Prolonging the corrosion time resulted in the formation of a reaction layer, which exhibited an interpenetrating network composed of Gd-oxyapatite, ZrO2, and residual CMAS-Fe. Some spinel was precipitated within the reaction layer. After 1 h or even longer time, the reaction layers tended to be stable and compact, which had comparable hardness and fracture toughness to those of Gd2Zr2O7 coatings. Under the cyclic CMAS-Fe attack, the residual CMAS-Fe in the interpenetrating network provided a pathway for the redeposited CMAS-Fe infiltration, resulting in the continuous growth of the reaction layer. As a result, the Gd2Zr2O7 coatings had a large consumption in the thickness, degrading the coating performance. Therefore, the Gd2Zr2O7 coatings exhibit unsatisfactory corrosion resistance to CMAS-Fe attack.

Open Access Research Article Issue
Design of Ti2AlC/YSZ TBCs for more efficient in resisting CMAS attack
Journal of Advanced Ceramics 2023, 12 (9): 1712-1730
Published: 01 September 2023
Abstract PDF (2 MB) Collect

Ti2AlC has been demonstrated as the promising protective layer material for thermal barrier coatings (TBCs) against calcium–magnesium–alumina–silicate (CMAS) attack. In this study, the reliability of Ti2AlC coatings against the CMAS corrosion was explored, and new Ti2AlC/YSZ TBCs more efficiently resistant to CMAS were designed. The fabricated Ti2AlC coatings inevitably contain some impurity phases (TiC and Al2Ti3), the contents of which were minimized by optimizing the spraying distance. Corrosion tests revealed that Ti2AlC/YSZ TBCs yielded higher resistance to the CMAS attack than YSZ TBCs, but with long-term exposure to CMAS, the Ti2AlC protective coating exhibited microstructure degradation due to the presence of the impurity phases, which caused the formation of a layer mixed with Al2O3 and TiO2 rather than a continuous compact Al2O3 layer on the surface. Pre-oxidation schemes were designed in air or with a controlled oxygen partial pressure, which revealed that the pre-oxidation at an oxygen partial pressure of ~630 Pa could promote a continuous Al2O3 layer formed on the Ti2AlC protective coating surface. Furthermore, a vacuum heat treatment at 867 ℃ for 10 h before pre-oxidation was beneficial for the formation of the compact Al2O3 layer. Through the above scheme design, new Ti2AlC/YSZ TBCs were obtained, which had reduced impurity phase contents and a pre-oxide layer with an ideal structure on the surface. New TBCs exhibit higher microstructure stability exposed to CMAS and more efficient CMAS resistance.

Open Access Research Article Issue
Effects of pellet surface roughness and pre-oxidation temperature on CMAS corrosion behavior of Ti2AlC
Journal of Advanced Ceramics 2022, 11 (6): 945-960
Published: 11 May 2022
Abstract PDF (15 MB) Collect

Calcium-magnesium-alumina-silicate (CMAS) corrosion is a serious threat to thermal barrier coatings (TBCs). Ti2AlC has been proven to be a potential protection layer material for TBCs to resist CMAS corrosion. In this study, the effects of the pellet surface roughness and temperature on the microstructure of the pre-oxidation layer and CMAS corrosion behavior of Ti2AlC were investigated. The results revealed that pre-oxidation produced inner Al2O3 layer and outer TiO2 clusters on the pellet surfaces. The content of TiO2 decreased with decreasing pellet surface roughness and increased along with the pre-oxidation temperature. The thickness of Al2O3 layer is also positively related to the pre-oxidation temperature. The Ti2AlC pellets pre-oxidized at 1050 ℃ could effectively resist CMAS corrosion by promoting the crystallization of anorthite (CaAl2Si2O8) from the CMAS melt rapidly, and the resistance effectiveness increased with the pellet surface roughness. Additionally, the CMAS layer mainly spalled off at the interface of CaAl2Si2O8/Al2O3 layer after thermal cycling tests coupled with CMAS corrosion. The Al2O3 layer grown on the rough interface could combine with the pellets tightly during thermal cycling tests, which was attributed to obstruction of the rough interface to crack propagation.

Open Access Research Article Issue
Composition optimization, high-temperature stability, and thermal cycling performance of Sc-doped Gd2Zr2O7 thermal barrier coatings: Theoretical and experimental studies
Journal of Advanced Ceramics 2022, 11 (3): 454-469
Published: 19 January 2022
Abstract PDF (3.5 MB) Collect

Sc was doped into Gd2Zr2O7 for expanding the potential for thermal barrier coating (TBC) applications. The solid solution mechanism of Sc in the Gd2Zr2O7 lattice, and the mechanical and thermophysical properties of the doped Gd2Zr2O7 were systematically studied by the first-principles method, based on which the Sc doping content was optimized. Additionally, Sc-doped Gd2Zr2O7 TBCs with the optimized composition were prepared by air plasma spraying using YSZ as a bottom ceramic coating (Gd-Sc/YSZ TBCs), and their sintering behavior and thermal cycling performance were examined. Results revealed that at low Sc doping levels, Sc has a large tendency to occupy the lattice interstitial sites, and when the doping content is above 11.11 at%, Sc substituting for Gd in the lattice becomes dominant. Among the doped Gd2Zr2O7, the composition with 16.67 at% Sc content has the lowest Pugh’s indicator (G/B) and the highest Poisson ratio (σ) indicative of the highest toughness, and the decreasing trends of Debye temperature and thermal conductivity slow down at this composition. By considering the mechanical and thermophysical properties comprehensively, the Sc doping content was optimized to be 16.67 at%. The fabricated Gd-Sc coatings remain phase and structural stability after sintering at 1400 ℃ for 100 h. Gd-Sc/YSZ TBCs exhibit excellent thermal shock resistance, which is related to the good thermal match between Gd-Sc and YSZ coatings, and the buffering effect of the YSZ coating during thermal cycling. These results revealed that Sc-doped Gd2Zr2O7 has a high potential for TBC applications, especially for the composition with 16.67 at% Sc content.

Open Access Research Article Issue
Effects of surface roughness on CMAS corrosion behavior for thermal barrier coating applications
Journal of Advanced Ceramics 2021, 10 (3): 472-481
Published: 01 March 2021
Abstract PDF (15.1 MB) Collect

Calcium–magnesium–alumina–silicate (CMAS) corrosion is an important cause for thermal barrier coating (TBC) failure, which has attracted increased attentions. In this study, some thermal barrier coating (TBC) materials including YSZ (yttria partially stabilized zirconia), GdPO4, and LaPO4 were prepared into bulks, and the effects of their surface roughness on wettability and spreading characteristics of molten CMAS were investigated. As-fabricated and polished bulks with different surface roughness were exposed to CMAS corrosion at 1250 ℃ for 1 and 4 h, following by macro and micro observations. Results revealed that compared with the as-fabricated bulks, molten CMAS on the polished samples had lower wettability and a smaller spreading area, mainly attributable to the reduced capillary force to drive the melt spreading. Meanwhile, GdPO4 and LaPO4 bulks exhibited lower CMAS wettability than YSZ bulk. It is thus considered that reducing the surface roughness is beneficial to CMAS corrosion resistance of TBCs.

Open Access Research Article Issue
Microstructure modification of Y2O3 stabilized ZrO2 thermal barrier coatings by laser glazing and the effects on the hot corrosion resistance
Journal of Advanced Ceramics 2020, 9 (2): 232-242
Published: 11 March 2020
Abstract PDF (14.8 MB) Collect

Y2O3 stabilized ZrO2 (YSZ) thermal barrier coatings (TBCs) are prone to hot corrosion by molten salts. In this study, the microstructure of atmospheric plasma spraying YSZ TBCs is modified by laser glazing in order to improve the corrosion resistance. By optimizing the laser parameters, a ~18 μm smooth glazed layer with some vertical cracks was produced on the coating surfaces. The as-sprayed and modified coatings were both exposed to hot corrosion tests at 700 and 1000 ℃ for 4 h in V2O5 molten salt, and the results revealed that the modified one had improved corrosion resistance. After hot corrosion, the glazed layer kept structural integrity, with little evidence of dissolution. However, the vertical cracks in the glazed layer acted as the paths for molten salt penetration, accelerating the corrosion of the non-modified coating. Further optimization of the glazed layer is needed in the future work.

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