Collection of Thermal Barrier and Enviromental Barrier Coating(202312)
For the development of high-performance laser protective coatings, this work innovatively proposes the idea of recombining PCS ablative coatings on the surface of traditional yttrium stabilized zirconia (YSZ) thermal insulation coatings. Based on characteristics of the consumption of large amount laser energy through decomposition of polycarbosilane (PCS) and production of high-temperature ceramic protective phases, NiCrAlY/YSZ/PCS-TiO2 (YPT) and NiCrAlY/YSZ/PCS-Y2O3 (YPY) coatings were prepared on the surface of Ni-based alloy by slurry method combined with atmospheric plasma spraying (APS). Based on the effect of TiO2 and Y2O3 addition on the pyrolysis behavior of PCS, the laser ablation resistance of YPT and YPY composite coatings on 10.6 μm CO2 laser were systematically studied and compared with that of single layer YSZ coating. The results show that YPY and YPT composite coatings have better laser protection effect than traditional YSZ coating, because in the early stage of laser ablation, the PCS decomposition on the coating surface can consume laser energy, and the residual Y2SiO5, SiC and SiO2 phases after ablation can be deposited on the YSZ coating, forming a dense protective layer, which can continue to play a role in laser protection of the YSZ coating. YPY coating has better laser protection performance than YPT coating, because Y2O3 has higher thermal conductivity and lower coefficient of thermal expansion. Smaller temperature gradient can be produced in YPY coating, which can alleviate thermal stress. Moreover, Y2O3 participates in the pyrolysis of PCS to generate Y2SiO5 phase, which inhibits more volume expansion in PCS pyrolysis than that in TiO2. In addition, the core ablation temperature of YPY coating is higher, and the formation of PCS pyrolysis products SiC and SiO2 phase is faster, which can timely protect the lower coating, showing better laser ablation resistance. This study is expected to provide a new idea for the design of novel laser resistant composite coatings.
TiAl alloy has excellent properties of low density and high specific strength, which is a potential structural material for aero-engine. The service temperature of TiAl alloy is limited in the range of 700-900 ℃, which is further improved by preparation of high temperature thermal protection coating. In the present work, a new type of TiAlCrY/YSZ coating was prepared on the TiAl alloy by plasma spray technologies, of which long-term service performance at high temperature was compared with the traditional NiCrAlY/YSZ coating. It was found that the TiAlCrY/YSZ coating remained intact after served 300 h at 1100 ℃ in the air, showing excellent high temperature performance, while the service lifetime of the NiCrAlY/YSZ coating at 1100 ℃ was less than 100 h. The microstructure observation found that a continuous and dense TGO composed of Al2O3 was continuously formed on the TiAlCrY bonding coating, suggesting that TiAlCrY coating has good interfacial compatibility with the YSZ top coating. Moreover, the thickness of TGO was still less than 8 μm after served 300 h at 1100 ℃ in the air. As compared with traditional NiCrAlY/YSZ coating, the TiAlCrY/YSZ coating is a more suitable high temperature thermal coating ever for TiAl alloys.
Environmental barrier coatings (EBCs) are expected to be applied to the hot-section components of a new generation of high thrust-to-weight ratio aero-engines. Rare-earth silicates have been acknowledged as promising alternatives to EBC materials due to their superior high-temperature phase stability, suitable coefficient of thermal expansion, and long durability in water vapor. However, the calcium-magnesium-alumino-silicates (CMAS) molten salt corrosion under service conditions has become a bottleneck that limits the application of rare-earth silicates in EBCs. Factors such as the composition of CMAS and the crystal structures of rare-earth silicates have a significant impact on their corrosion behavior. In this paper, X1-Gd2SiO5 and X2-RE2SiO5 (RE=Y, Er) coatings with different crystal structures, were prepared by atmospheric plasma spraying (APS) technique. Their corrosion behaviors and mechanisms of the three kinds of coatings under CMAS melt environment at 1400 ℃ were explored. Results showed that the corrosion resistance of X2-RE2SiO5 coatings were better than that of X1-Gd2SiO5 coating due to their phase compositions and stability of crystal structure. After corrosion by CMAS, X1-Gd2SiO5 coating dissolved in CMAS melt and formed apatite phase, while the X2-RE2SiO5 coatings not only formed apatite phase, but also formed garnet phase from reaction of the RE2O3 in the coatings with Al2O3 in CMAS. Formation of generate garnet phase could increase relative content of CaO and SiO2 in CMAS, and promote formation of dense apatite layer, thereby improving corrosion resistance. This study provides a strategy for designing EBC systems with excellent CMAS corrosion resistance.
Environmental barrier coating (EBC) is essential for protection of ceramic matrix composite hot-sections in future gas turbine engines with high thrust-to-weight ratio. Rare-earth silicates, such as Yb2SiO5 and Yb2Si2O7, have been developed for potential application as EBC. However, the corrosion behaviors and mechanisms of EBC in molten salt environment such as Na2SO4 at high temperature are not clear. In this work, the Yb2SiO5/Yb2Si2O7/Si coating was prepared by vacuum plasma spraying (VPS). The molten salt (Na2SO4+25% NaCl, in mass) corrosion behaviors and mechanisms of the coating at 900 ℃ for 60-240 h were investigated. Results showed that the Yb2SiO5/Yb2Si2O7/Si coating exhibited dense structure with good bonding between the triple ceramic layers. The molten salt of Na2SO4+25% NaCl penetrated the Yb2SiO5 top layer and enriched in the Yb2Si2O7 interlayer, while the interfacial bonding between the coating and substrate still remained good after corrosion for 240 h. The Yb2SiO5 phase in the top layer exhibited good stability, while the second phase of the Yb2O3 reacted with molten salt. The content of the Yb2O3 decreased with the increase of corrosion time. The Yb2Si2O7 phase in the interlayer reacted with molten salt to form apatite phase of NaYb9Si6O26 and sodium silicate as well as volatile species such as Cl2 and SO2, which might shorten the service life of the coating. Moreover, there was almost no molten salt in the silicon bond layer, which remained intact. The Yb2SiO5/Yb2Si2O7/Si coating exhibited good resistance to molten salt corrosion.
LaMeAl11O19 ceramics is a kind of thermal barrier coating (TBC) material with promising application prospect due to its unique crystal structure, excellent thermodynamic properties, low thermal conductivity, and high temperature phase stability. Here, LaMeAl11O19/YSZ (Me=Mg, Cu, Zn) thermal barrier coatings were prepared by atmospheric plasma spraying (APS). Failure analysis of the coating was carried out by burner rig test and other analysis techniques. The results show that LaMgAl11O19 (LMA), LaZnAl11O19 (LZA) and LaCuAl11O19 (LCA) powders are decomposed during the plasma spraying, resulting in different contents of magnetoplumbite phase in the coatings, which may be an important factor responsible for their distinction of thermal cycling lifetimes. The LaMeAl11O19 layer is delaminated upon YSZ layer due to mismatch of thermal expansion coefficient between LaMeAl11O19 layer and YSZ layer and volume shrinkage caused by recrystallization of amorphous phase. Then the YSZ layer is exposed high temperature, accelerating sintering and TGO growth, and promoting the delamination of the YSZ layer from the bond coat. At low temperature, with the increase of the atomic number of the divalent Me2+, the thermal conductivity of the LaMeAl11O19 decreases. At high temperature, LCA coating has better infrared emissivity (0.88, 600 ℃) than both LMA and LZA, which weakens the contribution of photon conduction to thermal conductivity and leads to the reduction of thermal conductivity. Therefore, LCA coating has potential application in high temperature infrared radiation coating.
LaMgAl11O19 (LMA) coating prepared by atmospheric plasma spraying has large amounts of amorphous phase, which seriously affects the service life of coating. Effects of microstructure, such as grain size, porosity and amorphous phase content, on mechanical, thermophysical and thermal shock resistance properties of LMA coatings after heat-treatement at 900-1600 ℃ for 12 h were investigated. The results show that the as-sprayed LMA coating possesses two crystallization temperature points, 900 and 1163 ℃. After heat-treatement at 900 ℃, the lowest thermal diffusivity of 0.53 mm2/s was obtained for LMA coating at 1000 ℃ due to the large amount of amorphous phase and the highest porosity of (18.88±2.15)%. LMA coatings, heat-treated at 1100-1400 ℃, exhibited higher hardness owing to reduced amorphous content and porosity through recrystallization and sintering with the maximum hardness of (12.08±0.58) GPa at 1100 ℃. After heat-treatement at 1300 ℃, the coating displayed the highest average thermal cycling life (588 times), which attributed to abundant micron flake crystals with high strain tolerance. When the heat-treatment temperature reached 1500 ℃, the grain thickness increased rapidly due to parallel stacking of lamellar crystals, porosity increased and mechanical properties significantly decreased. During the thermal shock, grain breaking and crack propagation occurred in the coating due to the repeated thermal stress, resulting in final failure of the coating.