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This study investigates the hot corrosion behaviours of La 1- x Gd x MgAl 11 O 19 (x = 0, 0.2, 0.4, 0.6, 0.8) bulk ceramics and ceramic coatings. The hot corrosion resistance of the ceramic coating was inferior to that of the bulk ceramic, owing to the formation of an amorphous phase during the plasma spraying process. The smaller size of Gd3+ in comparison with La3+ facilitated the ready combination of Gd3+ with V O 4 3 − . LaVO 4 was obtained as the corrosion product in the LaMgAl 11 O 19 (LMA) sample. Upon doping the LMA with Gd 2 O 3 , the LaVO 4 was almost entirely replaced by GdVO 4. Therefore, the sacrificial action of GdMgAl 11 O 19 protected the LMA from hot corrosion, while also providing a new method for modifying the corrosion resistance of molten slag. [ABSTRACT FROM AUTHOR]

Abstract La 2 Ce 2 O 7 (LC) is a new promising thermal barrier coating (TBC) material for high-temperature applications. However, the sudden decrease of thermal expansion coefficient (TEC) at ∼623 K limits its application. In this study, the plasma-sprayed La 2 Ce 1.7 Ta 0.3 O 7.15 (LCT) coating was developed by partial substitution of Ce4+ in LC with Ta5+. LCT coating shows lower thermal conductivity between 298 K and 1273 K (0.54–0.71 W/(m·K)) than LC coating (0.65–0.85 W/(m·K)) and the traditional yttria partially stabilized zirconia (YSZ) coating (1.53–1.72 W/(m·K)). It also exhibits excellent thermal stability at least up to 1573 K for 1000 h. What is more, the sudden TEC drop is suppressed owing to the reduced oxygen vacancy concentration governed by Ta5+-substitution content. As a result, LCT TBC shows an improved thermal cycling lifetime in an air furnace as compared to LC TBC. [ABSTRACT FROM AUTHOR]

In this study, LaMgAl 11 O 19 (LMA) thermal barrier coatings (TBCs) were deposited by atmospheric plasma spraying with different spraying powers. The effects of spraying power on the amorphous phase content, microstructure and porosity of the coatings were investigated using X-ray diffraction and scanning electron microscopy. The thermal expansion behavior, microhardness and bonding strength of the coatings were also studied. Results indicated that the amorphous phase content of the coatings increased with the spraying power increasing, while the porosity of the coatings exhibited an opposite trend. Coating with high amorphous phase content suffered large volume shrinkage and exhibited low thermal expansion coefficient (TEC). Bonding strength and microhardness of the coatings increased with the spraying power increasing from 22 to 32 kW and then decreased from 32 to 42 kW. Thermal cycling behavior of LMA coatings at 1100 °C was also investigated. Spallation of the TBCs was mainly correlated with the formation of thermally grown oxides and mismatch of TECs between the topcoat and the substrate, which were influenced by the microstructure and the amorphous phase content of the coatings. The longest lifetimes were achieved on LMA coating deposited at 32 kW, owing to its relatively low amorphous phase content and the highest bonding strength. [ABSTRACT FROM AUTHOR]

Amorphous phase is commonly found in LaMgAl 11 O 19 (LMA) coating prepared by atmospheric plasma spraying. In order to improve their performance, the as-sprayed LMA thermal barrier coatings on a nickel-based DZ125 substrate were heat-treated at 900, 1000 and 1100 °C for 5–20 h, respectively. The effect of heat treatment on the microstructure, porosity, phase composition, and crystallization behavior of the LMA coatings was investigated using scanning electron microscopy, X-ray diffraction, differential scanning calorimetry analysis. Samples before and after heat treatment for 20 h were thermally cycled at 1127 °C. Results indicated that the annealing of the as-sprayed LMA coating prior to thermal cycling is preferable to promote the crystallization of amorphous phase. Variations in microstructure, phase composition and thermal cycling lifetime of LMA coatings have been correlated to the heat treatment. Heat treatment at 900 °C improved the lifetime of LMA coatings, whereas their thermal cycling performance did not significantly improved when the aging temperature increased to 1000 or 1100 °C. The corresponding mechanism had been explored by tracing the microstructure and thermo-physical properties evolutions during thermal annealing process. [ABSTRACT FROM AUTHOR]

To obtain hydroxyapatite (HA) coatings with high crystallinity which have long-term stability in clinical applications, coarse powders were usually injected to less energetic plasma. However, the HA coatings accumulated by partly melted particles usually have high porosity and poor mechanical properties, especially poor bonding strength. In this work, by profiting its quenching and mechanical impact, dry-ice blasting was in-situ employed during plasma spray process to improve the microstructure characterization and bonding strength of HA coatings. In addition, the influence of in-situ dry-ice blasting on the phase composition and crystallinity of plasma-sprayed HA coatings was investigated. The results show that a significant reduction of porosity and an apparent increase in bonding strength are revealed in plasma-sprayed HA coatings due to the cleaning effect of dry-ice blasting on the convex unmelted particles and splashing fragments. HA coatings prepared by the combination process of plasma spraying and dry-ice blasting have a compromise structure with minimum globular pores but with pronounced microcracks. The disappearance of CaO phase and the increase in crystallinity also derive from the application of dry-ice blasting. [ABSTRACT FROM AUTHOR]

• The crystallization of amorphous LMA occurred at 900 and 1170 °C, respectively. • A novel crystallization mechanism on the amorphous LMA was proposed. • Crystallization kinetic of the amorphous LMA was studied. Amorphous phase is often detected in the plasma-sprayed LaMgAl 11 O 19 (LMA) coating, the crystallization of which can cause the volume reduction of the coating, resulting in the failure of the coating. However, it is still not clear for the crystallization mechanism of the amorphous phase. In this paper, the coating samples were deposited using atmospheric plasma spraying. Adequate evidence showed that the crystallization of amorphous LMA occurred in two stages at about 900 and 1170 °C, respectively, by studying the phase transition of Al 2 O 3 and the synthesis process of LMA. A novel crystallization mechanism on the amorphous LMA coating was also proposed. Results showed that highly charged La3+ in LMA magnetoplumbite structure controls the crystallization rate. The higher the La content in the composition the harder the crystallization occurs. [ABSTRACT FROM AUTHOR]

Double ceramic layer thermal barrier coatings (DCL-TBCs) of LaMgAl 11 O 19 /YSZ (LMA/YSZ) were prepared by atmospheric plasma spraying (APS). Composition, microstructure and phase stability of coatings were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and differential scanning calorimetry (DSC). The amorphous phase content of LMA coating increased with increasing the plasma gun current from 350 A to 650 A. The DCL-TBCs of LMA/YSZ whose LMA coating was prepared with a plasma gun current of 600 A (600LMA/YSZ) had the lower amorphous phase content, an excellent thermal insulation performance and the longest thermal cycling lifetime, indicating that properties of LMA coating could be largely improved by adjusting the APS conditions. [ABSTRACT FROM AUTHOR]

Abstract Forsterite-type Mg 2 SiO 4 was investigated systematically for thermal barrier coating (TBC) applications. Results showed that Mg 2 SiO 4 synthesized by solid-state reaction possessed the good phase stability up to 1573 K. The thermal conductivity of Mg 2 SiO 4 at 1273 K was lower ˜20% than that of yttria stabilized zirconia (8YSZ). Mg 2 SiO 4 also presented moderate thermal expansion coefficients, which increased from 8.6 × 10−6 K−1 to 11.3 × 10−6 K−1 (473˜1623 K). Mechanical properties including hardness, fracture toughness, and Young's modulus of Mg 2 SiO 4 were comparable to those of 8YSZ. The sintering results indicated a promising low-sintering activity of Mg 2 SiO 4. Mg 2 SiO 4 samples were subjected to water quenching test at 1573 K and showed a superior thermal shock resistance compared to 8YSZ. Mg 2 SiO 4 coating with stoichiometric composition was produced by atmospheric plasma spraying. The thermal cycling test result showed that Mg 2 SiO 4 coating had a lifetime more than 830 cycles at 1273 K, which is desirable for TBC applications. [ABSTRACT FROM AUTHOR]

In this study, 16 mol% CeO 2 –2 mol% Sc 2 O 3 co-stabilized ZrO 2 (4S16CZ) coating was prepared by atmospheric plasma spraying (APS) technology. Parameters related to the application of 4S16CZ as a thermal barrier coatings (TBCs) were studied, including high-temperature stability, thermal insulation performance, coefficient of thermal expansion (CTE), thermal shock behavior and CaO-MgO-Al 2 O 3 -SiO 2 (CMAS) resistance. After heat treatment at 1400 °C for 100 h, the 4S16CZ coating maintained a single non-transformable tetragonal phase (t'), exhibiting excellent high-temperature stability. Additionally, at the test temperatures (25 °C–1000 °C), the thermal conductivity of the 4S16CZ coating ranged from 0.79 W/(m·K) to 1.02 W/(m·K), lower than that of the ZrO 2 coating stabilized by 7.6 wt% Y 2 O 3 (YSZ, 0.88–1.13 W/(m·K)), The CTE of the 4S16CZ coating was from 8.3 × 10−6 K−1 to 11.2 × 10−6 K−1, which was comparable to the YSZ. Moreover, the thermal shock behaviors of 4S16CZ TBCs and 4S16CZ/YSZ TBCs were evaluated at 1050 °C. The thermal durability of the 4S16CZ/YSZ TBCs was higher than that of 4S16CZ TBCs due to the buffering effect of the YSZ layer. The main reasons for the failure of both TBCs were the oxidation of the bond coat, thermal mismatch stress, low fracture toughness and sintering effects. In CMAS isothermal corrosion tests, CMAS exhibited low wettability on the 4S16CZ coating. Compared with the YSZ coating, the 4S16CZ coating showed better resistance to CMAS attacks. • The 4S16CZ coating exhibited excellent high-temperature stability, low thermal conductivity, and high CTE. • 4S16CZ coating showed better phase stability and higher resistance to CMAS corrosion than the YSZ coating. • The poor thermal cycling life was related to the TGO growth, thermal mismatch stress, low fracture toughness and sintering effect. [ABSTRACT FROM AUTHOR]

The wear resistance of chromium oxide (Cr 2 O 3 ) coatings could be improved by doping modification and changing the structural scale, etc. In this study, micrometric Cr 2 O 3 coatings were doped with different additives, CeO 2 and Nb 2 O 5 . Moreover, Cr 2 O 3 coatings were deposited from nanostructured feedstock by the combination process of plasma spraying and dry-ice blasting. The correlation between the microstructure, chemical components and tribological properties of plasma-sprayed Cr 2 O 3 -based coatings was discussed based on the investigation of their porosity, hardness and friction behaviors. The results showed that the composite coatings doped with additives exhibited a higher microhardness, corresponding to a lower porosity than pure Cr 2 O 3 coating under the identical plasma-spray condition. CeO 2 constituent was found to improve the wear resistance of Cr 2 O 3 coating while Nb 2 O 5 incorporation corresponds to a steep rise in the friction coefficient. The mismatch of coefficient of thermal expansion (CTE) between Cr 2 O 3 and Nb 2 O 5 lamellae facilitated the origin of fatigue cracks and the formation of microfracture pits. Although the combination process promotes a porosity reduction, the nanostructured Cr 2 O 3 (n-Cr 2 O 3 ) coatings present a lower microhardness than micrometric coatings, due to their loosen microstructure from insufficient plasma power compared to microscaled coatings. The wear mechanisms of both the micro- and nanometric Cr 2 O 3 coatings are fatigue cracks and material transfer. [ABSTRACT FROM AUTHOR]

• Dense columnar LaMgAl 11 O 19 (LaMA) sealing layer is in-situ created by laser-glazing. • Dense columnar LaMA layer shows good interface compatibility and thermal stability. • In-situ generated dense LaMA layer exhibits much better CMAS resistance at 1350 °C. • La, Mg and Al from the dense LaMA layer arrest the melted CMAS very effectively. High-temperature degradation of the plasma sprayed LaMA coating before and after in-situ laser-glazing under CMAS attacks at 1350 °C for 1 h, 12 h and 48 h was comparatively investigated. Results indicate that a dense laser-glazed top layer was successfully created in-situ for a ～1 mm thick LaMA coating. It behaved as a sealing layer and reservoir of Al3+, Mg2+ and La3+. Melted CMAS penetration was accordingly effectively arrested accompanied by crystallizations of the CaAl 2 Si 2 O 8 anorthite, MgAl 2 O 4 spinel and Ca 2 La 8 (SiO 4) 6 O 2 apatite. CMAS-induced degradation mainly occurred near the surface of the laser-glazed dense LaMA top layer. While significant penetration of the melted CMAS along the entire thickness occurred to the as-sprayed LaMA coating without laser-glazing treatments after 48 h degradation. Ca 2 La 8 (SiO 4) 6 O 2 apatite phase did not crystallize for the as-sprayed LaMA due to the much reduced local La3+ concentration in melted CMAS resulted from its porous microstructure. Effects of laser-glazing on the CMAS-induced degradation of the plasma sprayed LaMA was carefully analyzed and discussed in terms of the coating's chemical composition, microstructure and surface physical chemistry characteristics base on XRD, SEM/EDS and TEM results. [ABSTRACT FROM AUTHOR]