Ytterbium aluminum garnet (Yb3Al5O12, YbAG) is commonly employed as a topcoat material in thermal/environmental barrier coatings (T/EBCs), for its low oxygen transmittance. To meet the demands of highertemperature and fatigue-resistant service environments, this study introduced neodymium aluminate (NdAlO3) as a ferroelastic second phase into YbAG to develop a YbAG-based composite material (NdAlO3-Yb3Al5O12, NY). With NY as the top coat, a multilayered (NdAlO3-Yb3Al5O12)/Yb2SiO5/Yb2Si2O7/Si coating system (NY/EBC) was deposited onto silicon carbide fiber reinforced silicon carbide ceramic matrix composites (SiCf/SiC CMCs) substrates via atmospheric plasma spraying. The thermophysical, thermal-optical and mechanical properties of the NY coatings and the thermal-cycling and water vapor/oxygen corrosion resistance of NY/EBC at 1350 degrees C were examined and the associated failure mechanisms were elucidated. The results indicate that the NY coating exhibited a higher infrared emissivity than the YbAG coating in both the near- and mid-infrared regions. The fracture toughness of the heat-treated NY coating (2.22 +/- 0.49 MPa m1/2) increased by 28.32% compared with heat-treated YbAG (1.78 +/- 0.18 MPa m1/2), because of the dissipation of the crack-propagation energy through domain reorientation and crack deflection. Additionally, the NY/EBC coatings endured 298 thermal cycles at 1350 degrees C. After 30 h water vapor/oxygen corrosion, failure occurred at the Yb2Si2O7/Si coating interface, caused by the rapid penetration of corrosion media through cracks generated by NY-layer crystallization, thermalexpansion mismatch stresses, growth of thermally grown oxides and phase-transition-related stresses.

In order to find a coating material that primarily combines high temperature resistance, low thermal conductivity, thermal shock and high-temperature corrosion resistance, YbTaO4 powders were prepared by high-temperature solid-state reaction synthesis and YbTaO4 mechanically mixed with Ta2O5 powders were then sintered into dense bulks using spark plasma sintering (SPS) technology. The phase composition, microstructure, thermophysical and mechanical properties of the composite bulks with different Ta2O5 contents were studied in details. The results showed that YbTaO4-Ta2O5 ceramics have a low coefficient of thermal expansion and thermal conductivity (1.93-2.38 Wm(-1)K-1, 1200 degrees C), simultaneously possessing low elastic modulus, high Vickers hardness and fracture toughness. Further research was conducted on the thermal cycling performance of the composite bulks insulated at 1350 degrees C and then quickly thrown into water for quenching. The main reasons for bulks failure (similar to 300 cycles) were found to be the crack formation caused by thermal stress during the alternating process of cold and hot cycles and by the phase transition stress generated by Ta2O5 phase transformation. In addition, the water oxygen corrosion resistance of the bulks at 1350 degrees C was also evaluated. After 250 h water oxygen corrosion, although the porosity of the bulks slightly increased, their macro-quality remained essentially unchanged with stable phase compositions, showing good resistance to water oxygen corrosion. YbTaO4-Ta2O5 composites could be proposed as integrated thermal environmental barrier coating (TEBC) materials, since they have excellent high-temperature durability.

YbTaO4-Ta2O5 thermal environmental barrier coatings (YbT-T TEBCs) were fabricated by atmospheric plasma spraying technology (APS) using composite feedstock with different constitution ratios, which protected SiCf/ SiC-CMCs from water vapor corrosion and high-temperature ablation. By comparing the thermophysical and mechanical properties of YbT-T TEBCs, the optimal ratio was determined as 48.4 wt.%YbT + 51.6 wt.%T. Moreover, the thermal cycling performance and water oxygen corrosion resistance at 1350 degrees C of the SiCf/SiCCMCs fully-cladded with optimized YbT-T TEBCs were investigated. The findings indicated that the thermal cycling life of YbT-T TEBCs was greater than 500 cycles, which was mainly attributed to the coefficient of thermal expansion (CTE) being relatively matched with that of the substrate. After 60 h water oxygen corrosion at 1350 degrees C, the strength retention rate of YbT-T TEBCs-cladded SiCf/SiC-CMCs (73.91%) was about 8.1 times higher than that of SiCf/SiC-CMCs (8.10%). YbT-T TEBCs as integrated protective coating systems offer outstanding resistance to both high-temperature shocking and water oxygen corrosion.

Ytterbium disilicide-silicon (YSi2-Si, YS) as a novel bond coat was investigated as a potential alternative to the conventional Si bond coat in thermal/environmental barrier coating (T/EBC) systems, which were applied to the surface of SiC fiber-reinforced SiC ceramic-matrix composites (SiCf/SiC CMCs) hot-end components of the advanced gas turbines operating at higher temperatures. A multilayered coating architecture composed of YSi2Si/Yb2Si2O7/Yb2SiO5/Yb3Al5O12 was fabricated via atmospheric plasma spraying. The microstructure and phase constitution of the as-sprayed YS bond coat were first characterized, and its thermal cycling behavior at 1350 degrees C as well as high-temperature corrosion performance in a steam environment were systematically evaluated. The results suggest that coefficient of thermal expansion (CTE) mismatch is one of the factors contributing to the thermal cycling failure of the T/EBC system. After exposure to high-temperature steam at 1350 degrees C for 40 h, the YS-based T/EBC system exhibited a 128.24% increase in strength retention compared with the bare corroded CMCs. However, the hydrolysis and oxidation of the YS bond coat ultimately led to coating degradation and failure.

Si/Yb2Si2O7-Yb2SiO5/Yb2SiO5/LaMgAl11O19 thermal and environmental barrier coatings (TEBCs) were fabricated on the SiCf/SiC using atmospheric plasma spraying. The thermal stability of coating was evaluated by thermogravimetric-differential scanning calorimetery. The phase and microstructure evolution of coating was investigated at 950 degrees C-1350 degrees C to study the crack propagation and healing mechanisms of TEBCs during heat-treatment. Porosities and crack width of coating were characterized. The crystallization behavior and phase transformation were also investigated. The results indicated that viscous flow, phase transformation and solidstate sintering could be contributed to the crack healing in the TEBCs but the crack healing is limited. The formation of vertical crack in TEBCs is caused by bridging of crack in each layer. The crack continued to propagate and widen due to the release of thermal mismatch stress. The optimized heat treatment is about 1200 degrees C, due to ratio of crack width to porosity of coating is lower than other temperature.

To improve the poor oxidation resistance of MoSi2 coating on TZM alloys at 1500 degrees C, MoSi2-HfB2 and MoSi2-ZrB2 composite coatings were fabricated using atmospheric plasma spraying. The oxidation behavior, phase evolution, and self-healing mechanisms were systematically investigated through isothermal oxidation tests, viscosity analysis, and thermodynamic calculations. Compared to conventional MoSi2 coatings, boride-modified MoSi2 coatings exhibit exceptional oxidation resistance and structural stability. The excellent oxidation resistance originates from the formation of low viscosity SiO2-ZrO2 (4.89 x 106 Pa & sdot;s) and SiO2-HfO2 (4.94 x 106 Pa & sdot;s) glass scales. These scales significantly improve crack-healing efficiency by reducing viscosity and accelerating oxygen diffusion (D approximate to 10- 28). The synergistic interaction between viscosity and oxygen permeability provides guidelines for novel design of ceramic. These findings demonstrate significant potential for application in thermal protection components of hypersonic vehicles.

Based on ZrSi2 (ZS) or TaSi2-Ta2O5 (TST) bond coats, different multilayered ZS/YbDS/YbMS/YbAG and TST/ YbDS/YbMS/YbAG thermal/environmental barrier coatings (T/EBCs) were prepared through plasma spraying to protect silicon carbide fiber-reinforced silicon carbide ceramic matrix composites (SiCf/SiC-CMCs). The phase compositions of the plasma-sprayed ZS and TST coatings were investigated. It was found that the powders were partially oxidized during the spraying process, but the main components of the coatings remained ZS and TST, respectively. Furthermore, the thermal cycling behavior of the corresponding T/EBCs in air at 1300 degrees C and their corrosion behavior in 90 % H2O-10 % O2 steam at 1350 degrees C were also investigated. It was observed that the thermal cycling failure was mainly related to the thermal stresses formed due to the mismatch in coefficients of thermal expansion (CTEs). When these T/EBCs were subjected to high-temperature water vapor, degradation of the bond coats ultimately led to delamination of the systems.

Dual-layer YbTaO4-Ta2O5/Si (YbT-T/Si) and triple-layer YbTaO4-Ta2O5/Yb2Si2O7/Si (YbT-T/YbDS/Si) environmental barrier coatings (EBCs) were fabricated on SiC fiber reinforced SiC composites (SiCf/SiC-CMCs) by atmospheric plasma spraying (APS). The resistance of the coating systems to isothermal oxidation (1350 degrees C, 300 h) was estimated by monitoring the strength retention rate of the SiCf/SiC composites covered by EBCs. Furthermore, calcium-magnesium-aluminosilicate (CMAS) corrosion behavior (1350 degrees C) of YbT-T coating was studied. Results show that the SiCf/SiC composites with YbT-T-based EBCs had significantly higher strength retention rate than the composites without coating protection, indicating the effectiveness of the YbT-T-based EBCs. After CMAS corrosion for 50 h, the interaction layer consisting of large pores, CaTa2O6 products, YbTaO4 and dispersed CMAS was formed on top of the YbT-T coating.

SiC f /SiC composites were annealed at 1300 degrees C for 5 h in argon and then fully cladded by LaMgAl 11 O 19 /Yb 2 Si 2 O 7 (LMA/YbDS) thermal/environmental barrier coatings (T/EBCs). The annealed and coated system, directly -coated one and non -coated composites were isothermally oxidized at 1350 degrees C for 300 h. The oxidation resistance was estimated by strength retention rates. Compared to serious strength degradation in the non -coated composites, high strength retention rates were obtained for the coated ones, indicating LMA/YbDS provided effective protection for SiC f /SiC. Additionally, the annealed and coated system had higher strength retention rates than directly -coated one, resulting from weakened interfacial bonding strength between SiC matrix and fibers.

This study proposes cordierite (Mg2Al4Si5O18, MAS) as a novel bond coat material for thermal/environmental barrier coatings (T/EBC), deposited using atmospheric plasma spray on SiC fiber reinforced SiC matrix composites (SiCf/SiC-CMCs) coupons, forming a Mg2Al4Si5O18/Yb2Si2O7/Yb2SiO5/LaMgAl11O19 coating system. The phase composition and thermal properties of the as-sprayed MAS coating were investigated, revealing cordierite as the primary phase in the as-sprayed coating with minor amounts of spinel and quartz phases which disappeared after heat treatment. Additionally, the as-sprayed MAS coating exhibited a low thermal expansion coefficient (3.19 x 10-6 K-1). The thermal cycling behavior and steam corrosion performance of the T/EBC coating system with cordierite as the bond coat were studied. Detailed analysis revealed that during thermal cycling tests, the coating failure was primarily related to liquid-phase sintering caused by eutectic reactions. In steam corrosion tests, the coating delamination was linked to the formation and growth of cracks and corrosion voids due to the residual stress release.

YbTaO4-Ta2O5 composites were designed and different multi-layered YbTaO4-Ta2O5/Yb2Si2O7/Si (YbT-T/YbDS/Si) as well as YbTaO4-Ta2O5/Si (YbT-T/Si) environmental barrier coatings (EBCs) were put forward to protect SiC fiber reinforced silicon carbide (SiCf/SiC) ceramic matrix composites (CMCs). The thermal cycling and the water-vapor/oxygen corrosion behavior of different multi-layered EBCs were compared. Results indicated that the thermal cycling lifetime of the bi-layered YbT-T/Si EBCs was similar with the tri-layered YbT-T/YbDS/Si ones. The ultimate failure of these two EBCs was induced by the cracking of the SiCf/SiC substrates as well as the vertical cracks associated with the thermal mismatch stress among different layers. However, the tri-layered EBCs exhibited greater resistance to water-vapor/oxygen corrosion than the bi-layered ones, primarily due to that the tri-layered YbT-T/YbDS/Si EBCs contained significantly less cracks than the bi-layered ones, limiting the transport of reactant (oxygen and/or water vapor) to reach Si bond coat.

SiCf/SiC composites were annealed at 1300 °C for 5 h in argon and then fully cladded by LaMgAl11O19/Yb2Si2O7 (LMA/YbDS) thermal/environmental barrier coatings (T/EBCs). The annealed and coated system, directly-coated one and non-coated composites were isothermally oxidized at 1350 °C for 300 h. The oxidation resistance was estimated by strength retention rates. Compared to serious strength degradation in the non-coated composites, high strength retention rates were obtained for the coated ones, indicating LMA/YbDS provided effective protection for SiCf/SiC. Additionally, the annealed and coated system had higher strength retention rates than directly-coated one, resulting from weakened interfacial bonding strength between SiC matrix and fibers.

A series of Ce(4+-)doped Pr2Zr2O7 ceramics were produced using a solid reaction method to utilize novel ceramic materials as potential candidates for thermally protective coatings. The effects of Ce4+ doping on the phase composition and mechanical, infrared radiative, and thermophysical properties were studied in detail. The results indicated the good phase stability and sintering resistance of the Pr-2(Zr-1- xCex)(2)O(7 )ceramics. Moreover, Ce4+-doped Pr2Zr2O7 bulk materials exhibited higher infrared emissivity (0.904) at wavelengths of 3-5 mu m and 1000 degrees C for Pr-2(Zr0.5Ce0.5)(2)O-7) owing to the introduction of impurity energy levels. Pr-2(Zr0.7Ce0.3)(2)O-7 exhibited an appropriate average coefficient of thermal expansion of 12.4 x 10-6 K- 1 and a low thermal conductivity of 1.14 W m- 1 K- 1 at 1000 degrees C, owing to the reduction of lattice energy and the presence of more oxygen vacancies and substitution atoms.

To alleviate thermal mismatch problem of environmental barrier coatings (EBCs) on SiC fiber-reinforced SiC ceramic matrix composites (SiCf/SiC CMCs) surface and improve their high-temperature durability for aircraft engines, by fully utilizing the appropriate thermal expansion coefficient, low oxygen transmittance, low thermal conductivity, and other advantages of ytterbium aluminum garnet (Yb3Al5O12), the double ceramic layered Yb3Al5O12/Yb2Si2O7 (DCL-Yb3Al5O12) and the triple ceramic layered Yb3Al5O12/Yb2SiO5/Yb2Si2O7 (TCL-Yb3Al5O12) EBC systems were prepared on SiCf/SiC CMCs by atmospheric plasma spraying (APS). Their phase composition and microstructures were investigated comparatively. The thermal cycling and water vapor/oxygen corrosion behavior of these coating systems were compared at 1300 degrees C. The results showed that the thermal cycling life of the DCL-Yb3Al5O12 and TCL-Yb3Al5O12 EBC systems were 295 and 320 times, respectively. The failure of both the coating systems occurred between the Yb2Si2O7 layer and Si bond coat. The strength retention rate of DCL-Yb3Al5O12 and TCL-Yb3Al5O12 EBC systems after water vapor/oxygen corrosion for 70 h was 12.8% and 23.1%, respectively, and the fracture modes of both the systems exhibited "pseudoplastic" characteristic. TCL-Yb3Al5O12 EBC systems with a gradient structure of more layers exhibit more excellent high-temperature durability than DCL-Yb3Al5O12 EBCs.

Thermal radiation coating with low thermal conductivity and high infrared emissivity are desirable. In this work, a novel rare-earth aluminate (PrAlO3(+)(delta) with rhombohedral structure) was synthesized via solid-state reaction method, and the corresponding coating was fabricated using atmospheric plasma spraying (APS). The study encompassed an investigation into the phase structure, high-temperature phase stability, thermo-mechanical characteristics, and infrared properties. The results illustrate that as-deposited coating contained amorphous phase, but recrystallized coating presented phase stability up to 1600 degrees C. Furthermore, the recrystallized coating exhibited low thermal conductivity of 1.35 W/m K at 900 degrees C as well as an average coefficient of thermal expansion of 10.36 x 10(-6)/K. Also, the coating exhibited high infrared emissivity related to high valance state (Pr4+) and oxygen vacancies. And the average infrared emissivity within 2-14 mu m was 0.822 at 1300 degrees C and presented slight decrease with the increase of temperature (the average infrared emissivity of 0.795 at 1800 degrees C).

The novel potential environmental barrier coating materials, two high-entropy rare earth disilicates (HEDs) (Gd0.2Ho0.2Er0.2Yb0.2Lu0.2)2Si2O7 and (Sc0.2Ho0.2Er0.2Yb0.2Lu0.2)2Si2O7 were prepared by the solid-phase method. Both the HEDs showed remarkable high-temperature phase stability and an appropriate thermal expansion coefficient (CTE) that matched that of SiCf/SiC. In addition, both the HEDs possessed above 20% lower thermal conductivity at 1000 °C (1.37 and 1.42 W·m-1·K-1, respectively) compared to traditional single-component rare earth disilicate material YbDS, and they also exhibited higher mechanical properties and good resistance to water vapour corrosion at 1300 °C. Moreover, the corresponding free-standing coatings obtained after deposition by atmospheric plasma spraying (APS) showed excellent resistance to CMAS corrosion, primarily because of the formation of an apatite layer.

In order to develop thermal-protection coatings, rare earth hafnates Sm2Hf2O7 (SHO) and Pr2Hf2O7 (PHO) coatings with pyrochlore structure were prepared on different substrates by atmospheric plasma spraying (APS). The infrared radiation, flame-ablation resistance, and thermal cycle performance were investigated in relationship with their morphologies, phase stabilities, thermophysical and mechanical properties. The results showed that both SHO and PHO coatings had compatible thermal-protection performance with lower thermal conductivity (0.711 and 0.779 W center dot m(-1)center dot K-1 at 1000 degrees C) and higher infrared emissivity (0.814 and 0.894 at 1000 degrees C) in the 3-5 mu m band. Moreover, the supersonic flame ablation resistance of PHO coatings at high temperatures was greater than that of SHO coatings. Both SHO and PHO coatings have long thermal cycling life at low and medium temperatures and short thermal cycling life at high temperatures. The failure mechanisms of both type coatings were mainly associated with thermal expansion mismatch stress and fatigue stress generated during multiple thermal cycling.

The mismatch of thermal expansion coefficient (CTE) between thermal barrier coatings (TBCs) and environmental barrier coatings (EBCs) is the key factor that affects the durability of T/EBCs in aero-engine environments. To alleviate the CTE mismatch, the novel TBC materials with relatively low CTE are needed. In this study, Lu4Hf3O12 with delta-phase rhombohedral structure was synthesized by solid-state reaction using Lu2O3 and HfO2 as the raw materials. Lu4Hf3O12 ceramics exhibited outstanding phase stability up to 1600 degrees C and comparable mechanical properties to 8 wt% Y2O3 stabilized ZrO2 (8YSZ) as well as great resistance against water-vapour/ oxygen corrosion. Additionally, it possessed the thermal conductivity of 1.20 W/m & sdot;K at 1000 degrees C, about 43% lower compared to traditional 8YSZ material (2.1-2.22 W/m & sdot;K), and coefficient of thermal expansion (CTE) of 8.46 x 10-6 K-1 (10.38 x 10-6 K-1 for 8YSZ). These results preliminarily reveal that Lu4Hf3O12 could be considered as a potential material for thermal insulation top-coat in the field of T/EBCs.

In this work, Yb3Al5O12 (YbAG) garnet, as a new material for environment barrier coating (EBC) application, was synthesized and prepared by atmospheric plasma spraying (APS). The phases and microstructures of the coatings were characterized by XRD, EDS and SEM, respectively. The thermal stability was measured by TG-DSC. The mechanical and thermal-physical properties, including Vickers hardness (H-v), fracture toughness (K-IC), Young's modulus (E), thermal conductivity (kappa) and coefficient of thermal expansion (CTE) were also measured. The results showed that the as-sprayed coating was mainly composed of crystalline Yb3Al5O12 and amorphous phase which crystallized at around 917 degrees C. Moreover, it has a hardness of 6.81 +/- 0.23 GPa, fracture toughness of 1.61 +/- 0.18 MPa m(1/2), as well as low thermal conductivity (0.82-1.37 W/m.K from RT-1000 degrees C) and an average coefficient of thermal expansion (CTE) (similar to 6.3 x 10(-6) K-1 from RT to 660. C). In addition, the thermal shock and water-vapor corrosion behaviors of the Yb3Al5O12-EBC systems on the SiCf/SiC substrates were investigated and their failure mechanisms were analyzed in details. The Yb3Al5O12 coating has an average thermal shock lifetime of 72 +/- 10 cycles as well as an excellent resistance to steam. These combined properties indicated that the Yb3Al5O12 coating might be a potential EBC material. Both the thermal shock failure and the steam recession of the Yb3Al5O12-EBC systems are primarily associated with the CTE mismatch stress.

A tri-layer Si/Yb2SiO5/LaMgAl11O19 thermal and environmental barrier coatings (TEBCs) was prepared for protecting SiCf/SiC substrate by using atmospheric plasma spraying technology (APS). While the introduction of the thermal barrier coatings (TBCs) improves the temperature capability of the environmental barrier coatings (EBCs), it generates vertical crack in the as-deposited TEBCs due to the mismatch of coefficient of thermal expansion (CTE) between Yb2SiO5 EBCs and LaMgAl11O19 TBCs. Isothermal oxidation tests of coated samples were performed at 1300 degrees C in air for 50, 100, 200, and 300 h. The oxidation behavior and interface evolution of TEBCs were investigated. After 50 h of isothermal oxidation, the crack healing was observed in the coating due to the crystalline of amorphous phase and grain growth. However, the width of vertical crack increased with the oxidation times due to the increasing of residual stresses, which provide rapid diffusion channel for oxygen into surface of Si, resulting in the formation of SiO2 layer. Eventually, the mechanical spallation of coating was observed in the surface of bond coat due to the thermal mismatch and the growth of SiO2 layer. After isothermal oxidation, the Yb3Al5O12 reaction layer was formed in the interface between Yb2SiO5 and LaMgAl11O19 layers due to the inter-diffusion of atoms. Furthermore, the growth of thermally grown oxide (TGO) and Yb2Si2O7 layer at the interface between Si and Yb2SiO5 layers followed approximately the parabolic law.