RESUMEN
MCrAlY (M = Ni and/or Co) metallic coatings are essential for the protection of hot-end components against thermal and corrosion damage. Increasing the Al content is considered a feasible solution to improve the high-temperature performance of MCrAlY coatings. In this paper, the effects of high Al contents (12-20 wt.%) on the phase constituents and cast microstructures in MCrAlY alloys were studied by high-energy X-ray diffraction and electron microscopy techniques combined with phase equilibria calculations. High Al content improved the stability of ß, σ, and α phases. Meanwhile, an evolution of the cast microstructure morphology from a dendrite structure to an equiaxed grain structure was observed. The thermal properties were analyzed, which were closely related to the phase constituents and solid-to-solid phase transitions at evaluated temperatures. This work is instructive for developing high-Al-content MCrAlY coatings for next-generation thermal barrier applications.
RESUMEN
Thermal barrier coatings (TBCs) are widely used to protect high-temperature components against harsh environments, such as extremely high temperatures. In this work, a second generation Ni-based single crystal superalloy (DD6) was treated in two ways: (1) via simple surface sandblasting under different pressures with no additional coating, and (2) through simple surface sandblasting under different pressures and then by applying NiCoCrAlYHf (HY5) coatings. The effects of pre-treatment (sandblasting) and the HY5 coating on the surface recrystallization of the alloy were thoroughly investigated. According to the results, both sandblasting pressure and the presence or absence of a coating significantly influence surface recrystallization. In particular, the critical sandblasting pressure for recrystallization increased the maximum recrystallization depth in both the coated and uncoated samples. Meanwhile, the recrystallization depth of the alloy with a coating was reduced compared to that without a coating. In addition, the number of recrystallized cells in the coated alloy was decreased, which indicated that the HY5 coating effectively reduced the degree of recrystallization.
RESUMEN
La2Zr2O7 coatings are promising candidates to substitute YSZ coatings in advanced gas turbine engines. In this work, Sm-doped La2Zr2O7 coatings were deposited by physical vapor deposition. This work focuses on the crystal structure, thermal conductivity, thermal expansion coefficient, morphology, composition, and thermal durability of LaSmZrO coatings. The LaSmZrO ceramics exhibit low thermal conductivity (1.69 W/mK at 800°C) and high thermal expansion coefficient (9.72∗10-6 K-1 at 1500°C) compared with La2Zr2O7. The LaSmZrO/YSZ coatings with feathery microstructure show relatively good thermal durability (8183 cycles or 856 h) under high temperature. The broken regions are observed at the ceramic coating/bond coating interface. The failure behaviors are relevant with crack evolution and thermally grown oxide growth. This work might guide the investigation of advanced coatings under high temperature.
RESUMEN
The controllable synthesis of high-quality and large-area graphene by chemical vapor deposition (CVD) remains a challenge nowadays. The massive grain boundaries in graphene grown on polycrystalline Cu by CVD significantly reduce its carrier mobility, limiting its application in high-performance electronic devices. Here, we confirm that the synergetic pretreatment of Cu with electropolishing and surface oxidation is a more efficient way to further suppress the graphene nucleation density (GND) and to accelerate the growth rate of the graphene domain by CVD. With increasing the growth time, we found that the increasing amount of GND and growth rate of the graphene domain were both decreasing during the whole CVD process when the Cu surface was not oxidized. By contrast, they kept growing over time when the Cu surface was pre-oxidized, which suggested that the change trends of the effects on the GND and growth rate between the Cu surface morphology and oxygen were opposite in the CVD process. In addition, not only the domain shape, but the number of graphene domain layers were impacted as well, and a large number of irregular ellipse graphene wafers with dendritic multilayer emerged when the Cu surface was oxidized.