To develop new thermal protective coatings (TPCs) for high-temperature applications, single- and dual-ceramiclayer systems based on the rare-earth zirconate Pr2(Zr0.7Ce0.3)2O7 (PZ7C3) were prepared using atmospheric plasma spraying (APS). The study investigated the influence of phase stability, thermal physical properties, mechanical performance, coating structure, and infrared radiation characteristics on the thermal cycling and calcium-magnesium-alumino-silicate (CMAS) corrosion resistance of the coatings, along with an analysis of failure mechanisms. The PZ7C3 coating exhibited excellent phase stability at 1600 degrees C and low thermal conductivity, measuring 0.68 W & sdot;m-1 & sdot;K-1 at 1200 degrees C. It also demonstrated an appropriate coefficient of thermal expansion (CTE) of 10.54x10-6 K-1 (600-1500 degrees C) and high emissivity (0.971 at 1000 degrees C). However, the thermal cycling life of the PZ7C3 single- and dual-ceramic-layer systems at 1100 degrees C was relatively short, primarily due to CTE mismatch and diminished mechanical properties of the PZ7C3 coating. Following CMAS exposure at 1300 degrees C for 20 h, a protective reaction layer formed on the coating, indicating favorable resistance to CMAS penetration.

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.

In this work, the Y3Al5O12 (YAG) coatings prepared by atmospheric plasma spraying were found to have a thermal expansion coefficient of 8.51 × 10-6K-1 from 473 to 1673K and a thermal conductivity of 2.41 W/(m∙K) at 1273K as well as a Young's modulus of 89.4 ± 9 GPa and an average hardness of 6.79 ± 0.24 GPa. Their excellent mechanical and thermal-physical properties have promoted them as thermal/environmental barrier coating (T/EBC) materials. Multi-layered Y3Al5O12/Yb2SiO5/Yb2Si2O7 (YAG/YbMS/YbDS) T/EBCs were deposited on SiC based substrate. Thermal cycling and water vapor corrosion behavior of the YAG/YbMS/YbDS T/EBCs were also investigated. The cyclic oxidation tests at 1573K showed that the coatings had excellent resistance to thermal cycling, with an average lifetime of 442 ± 10 cycles. The initial oxidation of the Si layer at the edge, resulting in the formation of a 4.8 μm thick thermally grown oxide (TGO), coupled with the volumetric changes and the associated phase transformation stress, led to the delamination of the coatings from the edges. Due to the very low oxygen diffusion rate of YAG layer, the thickness of TGO was only 1.2 μm after steam corrosion for 40 h. The amorphous crystallization as well as thermal mismatch stresses were the main reasons for the coating failure.

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.

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.

Defects such as interconnected pores and cracks can improve the abradability of ceramic-based abradable sealing coatings (ASCs) but may reduce the lifetime. Self-healing can potentially close cracks and transform interconnected pores into isolated ones through filling and sintering effects. Ti3AlC2 (TAC) was incorporated into LaMgAl11O19 (LMA) as both the self-healing agent and sintering aid, and plasma-sprayed LMA-based composite coatings were annealed at 1200 °C to assess their self-healing capabilities and then subjected to oxidation in air and corrosion in steam at 1300 °C to study their long-term stability. Results indicated that increasing TAC content significantly enhances self-healing effectiveness, evidenced by the closure of cracks and the isolation of pores. Oxidation and corrosion at 1300 °C led to significant grain growth and the formation of equiaxed grains with an aspect ratio of approximately 3, which may impair the toughening mechanism. Meanwhile, due to the preferential volatilization of Al in a steam environment, LTA decomposed into α-La2/3TiO3 and La4Ti3O12 phases, and the accelerated mass transfer also resulted in grain coarsening. Interestingly, the L20T composite coating with a porosity of 32.17 ± 0.94% and a hardness of 74.88 ± 1.55 HR15Y showed great potential for abradable applications due to its stable phase composition and uniform pore distribution.

In this work, the Y 3 Al 5 O 12 (YAG) coatings prepared by atmospheric plasma spraying were found to have a thermal expansion coefficient of 8.51 x 10 -6 K -1 from 473 to 1673K and a thermal conductivity of 2.41 W/(m center dot K) at 1273K as well as a Young ' s modulus of 89.4 +/- 9 GPa and an average hardness of 6.79 +/- 0.24 GPa. Their excellent mechanical and thermal -physical properties have promoted them as thermal/environmental barrier coating (T/EBC) materials. Multi -layered Y 3 Al 5 O 12 /Yb 2 SiO 5 /Yb 2 Si 2 O 7 (YAG/YbMS/YbDS) T/EBCs were deposited on SiC based substrate. Thermal cycling and water vapor corrosion behavior of the YAG/YbMS/YbDS T/ EBCs were also investigated. The cyclic oxidation tests at 1573K showed that the coatings had excellent resistance to thermal cycling, with an average lifetime of 442 +/- 10 cycles. The initial oxidation of the Si layer at the edge, resulting in the formation of a 4.8 mu m thick thermally grown oxide (TGO), coupled with the volumetric changes and the associated phase transformation stress, led to the delamination of the coatings from the edges. Due to the very low oxygen diffusion rate of YAG layer, the thickness of TGO was only 1.2 mu m after steam corrosion for 40 h. The amorphous crystallization as well as thermal mismatch stresses were the main reasons for the coating failure.

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.

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.

Multi-layered LaMgAl11O19/Yb2Si2O7/Yb2Si2O7-Si/Si (LMA/YbDS/YbDS-Si/Si) thermal/environmental barrier coatings (T/EBCs) on SiC fibre-reinforced SiC ceramic matrix composites (SiCf/SiC-CMCs) were subjected to post heat treatment at 1200 or 1350 & DEG;C for 5 h. The thermal cycling and water-vapour/oxygen corrosion of the multilayered T/EBCs after annealing at different temperatures were studied. The results showed the thermal cycling lifetime of the 1350 & DEG;C-annealed T/EBCs was shorter than those of the 1200 & DEG;C-annealed and as-sprayed T/EBCs, while the 1350 & DEG;C-annealed T/EBCs exhibited relatively outstanding resistance to steam corrosion. The degraded thermal cycling lifetime may be associated with horizontal cracks generated at the interfaces of the LMA and YbDS layers after annealing at 1350 & DEG;C, which weakened the bonds between the coating layers. The outstanding resistance to steam corrosion of the 1350 & DEG;C-annealed T/EBCs could be attributed to the formation of a Yb3Al5O12 reaction layer with low-oxygen permeability on the upward sides of the YbDS layers in the 1350 & DEG;Cannealed T/EBCs. This could reduce the oxidation of Si inclusions within the YbDS layers, leading to less silica volatilisation.

LaMgAl11O19/Yb2Si2O7 (LMA/YbDS) thermal/environmental barrier coatings (T/EBCs) on SiCf/SiC composites were annealed at 1200 degrees C for 5 h in air or Ar atmosphere. The effect of post-annealing in different atmospheres on the microstructure, thermal shock and steam corrosion of the LMA/YbDS T/EBCs was investigated. Results indicated heat treatment in air and argon eliminated the amorphous in the coatings, avoiding the aging stress associated with the crystallization. And the argon-annealed layers (LMA and YbDS) exhibited the less elastic moduli than the air-annealed ones, resulting in the lower thermal mismatch stress in the argon-annealed T/EBCs upon subsequent thermal cycling. Thus, the argon-annealed T/EBCs exhibited an improved thermal cycling lifetime than the as-sprayed and the air-annealed ones. In addition, the unbroadened vertical cracks in the argon-annealed LMA-TBC layer limited the reactant (water-vapor) access to the silica-TGO, leading to the greater resistance of the argon-annealed T/EBCs against steam corrosion than the other two systems.

The hot corrosion behavior of plasma-sprayed HfSiO4 coatings exposed to molten calcium magnesium aluminosilicate (CMAS) and steam was investigated. No reactive crystallization was observed in the CMAS-attacked HfSiO4 coating which consist of tetragonal HfSiO4 and monoclinic HfO2. The penetration did not bring obvious volume expansion thanks to the buffering effect of the pores/microcracks. The bilayer environmental barrier coating (EBC) systems using HfSiO4 as top coat and Si as bond coat have the great resistance against steam corrosion. The coating degradation at high temperatures was associated with TGO volatilization and phase transformation as well as thermomechanical incompatibility.

A novel multi-layer HfO2-Si/YbSi1.5/Yb2SiO5/LaMgAl11O19 environmental barrier coatings (EBCs) was prepared using atmospheric plasma spraying method to restrain spallation failure and improve lifetimes of coating. The thermal cycling behaviors of SiCf/SiC completely covered with multi-layer EBCs were comparatively investigated at 1300 degrees C and 1350 degrees C in air. Meanwhile the water vapor corrosion behavior of coated sample was also investigated at 1300 degrees C for 100 h in 90% H2O-10% O2 steam. The coating at 1300 degrees C showed better thermal shock resistance than the coating at 1350 degrees C, due to the increase of tensile stresses at 1350 degrees C. The coating showed different forms of crack penetration due to the influence of thermal cycling temperature and water vapor. The formation of HfSiO4 phase thanks to solid-state reaction between HfO2 and SiO2 alleviate the beta ->alpha-cristobalite phase change and avoid the crack initiation. However, the thermal mismatch stress was contributed to the spallation of coating along the surface of the SiCf/SiC substrate.

Sm2O3-HfO2 series ceramics were synthesized at high temperature using the solid-state reaction. The phase stability, thermo-physical and infrared emission properties of Sm2Hf2O7 (SHO) and Sm2Hf2O7-44.83 wt%HfO2 (25S/H) composite ceramics were comparatively investigated. Furthermore, their calcium magnesium alumi-nosilicate (CMAS) corrosion was conducted at 1250 degrees C for different times. The results reveal that both SHO and 25S/H ceramics have excellent phase stability at 1600 degrees C as well as excellent sintering resistance. SHO still ex-hibits slightly lower thermal conductivity and lower hardness and Young's modulus, higher thermal expansion coefficient (CTE) and fracture toughness as well as higher infrared emittance (0.899 at 800 degrees C) than 25S/H composite with the excessive HfO2 inside. Both SHO and 25S/H ceramics react with CMAS to form a relatively compact reaction layer, which can effectively prevent the penetration of CMAS. These results preliminarily indicate that SHO ceramic can be proposed as an alternative material of the traditional YSZ for high-temperature thermal protective applications thanks to its compatible performance of low thermal conductivity and high infrared radiation, etc.

Silicon-doped aluminide (Al-Si) coatings were prepared on IN738 superalloy by hot dip. The microstructure and oxidation performance of the Al-Si coatings was investigated at 1050 degrees C in air and air plus water vapour. Water vapour can significantly reduce the oxidation resistance of Al-Si coatings. The Kirkendall voids were formed under the oxide layer in water vapour environment. The formation of a continuous Si/Ti-rich (SiCr3/Si3Ti5 phases) phase below the oxide layer could possibly slow down the internal oxidation, which increased the coating's oxidation resistance in air. In addition, the Si/Ti-rich phase was unstable in air plus water vapour atmosphere.

A Pt modified aluminide coating is prepared on the nickel-based alloy DZ125 containing Hf. After adding Pt, the continuous distribution of HfO2 could be observed at the oxide-scale/coating interface. Continuous HfO2 at the oxide-scale/coating interface could reduce the growth rate of the oxide scale. The local undulation of the oxide scale is enhanced due to the formation of continuous HfO2. Compared with the existence of crack in the oxide scale for the Pt free aluminide coating, compact alumina scale is formed on the surface of the Pt-doped aluminide coating, indicating the enhanced adhesion of the Pt modified aluminide coating.

The interfacial reactions of the plastic Cu-metal barrier layer in the Al2O3-40 wt% TiO2 (AT40) composite coating prepared by plasma spraying and its effect on the crack propagation behaviour during thermal cycling were investigated. The phenomena of Cu dendrites appearing on the upper surface of the Cu layer and the Cu whiskers growing along the crack of the ceramic coating under the Cu layer after thermal cycling (900 degrees C, Ar), and the formation mechanism are explained using FE-SEM, XRD, EPMA, and EBSD. The Cu dendritic and Cu whiskers changed the stress state of the interface and enhanced the anti-crack-propagation ability of the coating. In addition, nanoparticles surrounding the Cu layer using EBSD were observed in the Cu particles added to the AT40 coatings (C-AT coating) by plasma spraying. The calculations of surface energy indicated that the nanoparticles improved the wettability of the interface and promoted the interfacial coupling behaviour. A model of the relationship between stress and crack propagation in the process of heating and cooling was used to analyse the effect of the plastic metallic-barrier layer (Cu) on crack propagation. This work demonstrates that the Cu layer improved the anti-crack-propagation ability of the coating. Such C-AT coatings may find potential applications in high-temperature materials and wear-resistant material surfaces.