**6.1. Ceramic top coat**

Vassen et al. [45] investigated three zirconate materials as potential TBC materials. They deposited 150 μm Ni-Co-Cr-Al-Y bond coat on IN738 substrate before deposition of zirconate (thickness-240 μm) as top coat. They indicated that SrZrO3 can not be used as a top coat in TBC systems as the coating showed a phase transition with a volume expansion at ~730°C that led to the failure of the samples. BaZrO3 showed relatively poor thermal and chemical stability resulting in early failure in thermal cycling tests. On the other hand, Young's modulus of the pyrochlore La2Zr2O7 was found to be lower than that of YSZ. Fracture toughness of this material was comparable to the toughness of plasma-sprayed YSZ coatings. Furthermore, La2Zr2O7 has favorable thermal conductivity at elevated temperatures, which is ~20% lower than that of YSZ. Failure of La2Zr2O7 coating was not observed after the first thermal cycling tests at temperatures >1,200°C and the coating showed thermal stability. Thus, La2Zr2O7 is a very promising material for advanced TBCs. Moskal et al. [46] studied a double-ceramic-layered (DCL) coating consisting of monolayer coatings Nd2Zr2O7 and 8YSZ. The coatings had ~300 μm thickness and porosities of ~5%. The chemical and phase composition analysis of the DCL layers revealed an external Nd2Zr2O7 ceramic layer (~80 μm thick), a transitional zone (~120 μm thick), and an internal 8YSZ layer (100 μm thick). The Nd2Zr2O7 pyrochlore phase was the only one-phase component. The surface topography of both TBC systems was typical for plasma sprayed coatings, and compressive stress state had a value in the range of ~5–10 MPa. Measurements of the thermal parameters, i.e., thermal diffusivity indicated better thermal insulation for both new types of layers as compared to the standard 8YSZ layers [46].

Yb2O3 (10 mol%) and Gd2O3 (20 mol%) doped SrZrO3 was investigated by Ma et al. [47] as a material for TBC applications. Measurement of thermal expansion coefficients (TECs) of sintered bulk Sr(Zr0.9Yb0.1)O2.95 and Sr(Zr0.8Gd0.2)O2.9 displayed a positive influence on phase transformations of SrZrO3 by doping Yb2O3 or Gd2O3. It was observed that both dopants can reduce the thermal conductivity of SrZrO3. Dense Sr(Zr0.9Yb0.1)O2.95 and Sr(Zr0.8Gd0.2)O2.9 had lower hardness, Young's modulus, and comparable fracture toughness as compared to YSZ. At operating temperatures <1,300°C, the cycling lifetimes of plasma sprayed Sr(Zr0.9Yb0.1)O2.95/ YSZ and Sr(Zr0.8Gd0.2)O2.9/YSZ double DLC were comparable to that of YSZ coating. However, at operating temperatures >1,300°C, the cycling lifetime of Sr(Zr0.9Yb0.1)O2.95/YSZ DLC was about 25% longer than YSZ coating, while that was shorter for Sr(Zr0.8Gd0.2)O2.9/YSZ DLC compared to YSZ coating [47]. The rare earth zirconates (M2Zr2O7, M = La → Gd) have a low intrinsic thermal conductivity and high temperature phase stability, which make them attractive candidates for TBC applications. Electron-beam evaporation, directed-vapor deposition (EB-DVD) technique was used by Zhao et al. [48] to investigate the synthesis of Sm2Zr2O7 (SZO) coatings and to explore the relationships between the deposition conditions and the coating composition, pore morphology, structure, texture, and thermal conductivity. The coatings exhibited significant fluctuations in composition because of the vapor pressure differences of the constituent oxides. It was noticed that the coatings had a metastable fluorite structure due to kinetic limitations that hindered the formation of the equilibrium pyrochlore structure. The morphology of growth of EB-DVD SZO was identical to those of EB-DVD 7YSZ and EB-PVD Gd2Zr2O7. The conductivity values of the as-deposited SZO coatings were nearly one-half of their DVD 7YSZ counterparts. This may be ascribed to their lower intrinsic conductivity [48].

Gd2Zr2O7 phase after vacuum sintering at 1,000°C for 2 h while the content of the monoclinic phase in G-YSZ composite coatings increased with the increase of Gd2O3 concentration. It was found that after isothermal annealing at 1,000°C in air for 100 h, G-YSZ composite coatings were composed of tetragonal ZrO2 phase, monoclinic ZrO2 phase, and cubic phase whereas

Vassen et al. [45] investigated three zirconate materials as potential TBC materials. They deposited 150 μm Ni-Co-Cr-Al-Y bond coat on IN738 substrate before deposition of zirconate (thickness-240 μm) as top coat. They indicated that SrZrO3 can not be used as a top coat in TBC systems as the coating showed a phase transition with a volume expansion at ~730°C that led to the failure of the samples. BaZrO3 showed relatively poor thermal and chemical stability resulting in early failure in thermal cycling tests. On the other hand, Young's modulus of the pyrochlore La2Zr2O7 was found to be lower than that of YSZ. Fracture toughness of this material was comparable to the toughness of plasma-sprayed YSZ coatings. Furthermore, La2Zr2O7 has favorable thermal conductivity at elevated temperatures, which is ~20% lower than that of YSZ. Failure of La2Zr2O7 coating was not observed after the first thermal cycling tests at temperatures >1,200°C and the coating showed thermal stability. Thus, La2Zr2O7 is a very promising material for advanced TBCs. Moskal et al. [46] studied a double-ceramic-layered (DCL) coating consisting of monolayer coatings Nd2Zr2O7 and 8YSZ. The coatings had ~300 μm thickness and porosities of ~5%. The chemical and phase composition analysis of the DCL layers revealed an external Nd2Zr2O7 ceramic layer (~80 μm thick), a transitional zone (~120 μm thick), and an internal 8YSZ layer (100 μm thick). The Nd2Zr2O7 pyrochlore phase was the only one-phase component. The surface topography of both TBC systems was typical for plasma sprayed coatings, and compressive stress state had a value in the range of ~5–10 MPa. Measurements of the thermal parameters, i.e., thermal diffusivity indicated better thermal

insulation for both new types of layers as compared to the standard 8YSZ layers [46].

Yb2O3 (10 mol%) and Gd2O3 (20 mol%) doped SrZrO3 was investigated by Ma et al. [47] as a material for TBC applications. Measurement of thermal expansion coefficients (TECs) of sintered bulk Sr(Zr0.9Yb0.1)O2.95 and Sr(Zr0.8Gd0.2)O2.9 displayed a positive influence on phase transformations of SrZrO3 by doping Yb2O3 or Gd2O3. It was observed that both dopants can reduce the thermal conductivity of SrZrO3. Dense Sr(Zr0.9Yb0.1)O2.95 and Sr(Zr0.8Gd0.2)O2.9 had lower hardness, Young's modulus, and comparable fracture toughness as compared to YSZ. At operating temperatures <1,300°C, the cycling lifetimes of plasma sprayed Sr(Zr0.9Yb0.1)O2.95/ YSZ and Sr(Zr0.8Gd0.2)O2.9/YSZ double DLC were comparable to that of YSZ coating. However, at operating temperatures >1,300°C, the cycling lifetime of Sr(Zr0.9Yb0.1)O2.95/YSZ DLC was about 25% longer than YSZ coating, while that was shorter for Sr(Zr0.8Gd0.2)O2.9/YSZ DLC compared to YSZ coating [47]. The rare earth zirconates (M2Zr2O7, M = La → Gd) have a low intrinsic thermal conductivity and high temperature phase stability, which make them

the Gd2Zr2O7 phase disappeared [44].

**6.1. Ceramic top coat**

122 Advanced Ceramic Processing

**6. Relatively new developments of TBC materials**

Alumina-based ceramic coating with a composition of La2O3, Al2O3 and MgO (MMeAl11O19, M-La, Nd; Me-alkaline earth elements, magnetoplumbite structure) has been developed as TBC by the researchers [49, 50]. Lanthanum hexaaluminate (LHA) coating has long-term structural and thermo-chemical stability of up to 1673 K and significantly lower sintering rate than zirconia-based TBCs. The low thermal conductivity of LHA is ascribed to the random arrangement of LHA platelets leading to micro-porous coating. The insulating properties of the material are related to its crystallographic feature. To meet the demand of advanced turbine engines, LaTi2Al9O19 (LTA) was proposed and investigated as a novel TBC material for application at 1,300°C by Xie et al. [51]. LTA showed excellent phase stability up to 1,600°C. The thermal conductivities for LTA coating were in a range of 1.0–1.3 W m−1 K−1 (300–1,500°C). The values of thermal expansion coefficients increased from 8.0 to 11.2 × 10−6 K−1 (200–1,400°C), which were comparable to those of YSZ. Both the LTA and YSZ coatings had a microhardness value of about 7 GPa, whereas the fracture toughness value was relatively lower than that of YSZ. However, the double-ceramic LTA/YSZ layer design balanced the lower fracture toughness. The LTA/YSZ TBC showed thermal cycling life of ~700 h at 1,300°C [51]. Lathanum phosphate (LaPO4) is considered as a potential TBC material on Ni-based superalloys because of its high temperature stability, high thermal expansion, and low thermal conductivity [52]. Further, lanthanum phosphate is expected to have good corrosion resistance in environments containing sulfur and vanadium salts. However, plasma spraying can not be easily used to make this type of coating. Detailed research is needed to establish the suitability of LaPO4 as TBC. Rare earth oxide coatings (La2O3, CeO2, Pr2O3, and Nb2O5 as main phases) can be used as TBCs as they have lower thermal diffusivity and higher thermal expansion coefficient than ZrO2 [53]. Most of the rare earth oxides are polymorphic at elevated temperatures [54] and their phase instability affects the thermal shock resistance of these coatings to a certain extent. When zircon is used as a TBC material, it dissociates during plasma spraying and consequently coatings are composed of a mixture of crystalline ZrO2 and amorphous SiO2. For diesel engines, the decomposed SiO2 in the coating may cause problems due to the evaporation of SiO and Si(OH)2 [55]. The thermal barrier effect is supposed to be due to the ZrO2 phase in the coating [56]. However, few other silicates such as garnet almandine [Fe3Al2(SiO4)3], garnet pyrope [Mg3Al2(SiO4)3], garnet andradite-grossular [Ca3Al2(SiO4)3], and basalt (glass) have potential as TBC materials [57]. The composite oxide coating consisting of 2CaO.SiO2-10 to 30 wt% CaO.ZrO2 shows excellent resistance to thermal shock and hot corrosion [58].

Researchers have conceived garnets [Y3AlxFe5–xO12 (x=0, 0.7, 1.4, and 5)] as TBC materials [59]. YAG (Y3Al5O12) has superior high-temperature mechanical properties, low thermal conduc‐ tivity, excellent phase/thermal stability up to the melting point and significantly lower oxygen diffusivity than those of zirconia. However, the major drawback of this material is its low melting point and relatively low thermal expansion coefficient [59]. Guo et al. [60] produced BaLa2Ti3O10 (BLT) by solid-state reaction of BaCO3, TiO2, and La2O3 for 48 h at 1,500°C. BLT showed phase stability between room temperature and 1,400°C. BLT showed a linearly increasing thermal expansion coefficient with increasing temperature up to 1,200°C and the coefficients of thermal expansion (CTEs) were in the range of 1 × 10− 5–12.5 × 10− 6 K− 1, compa‐ rable to those of 7YSZ. BLT coatings with stoichiometric composition were developed by APS technique. The coating contained segmentation cracks and had a porosity of ~13%. The microhardness for the BLT coating was in the range of 3.9–4.5 GPa. The thermal conductivity at 1,200°C was about 0.7 W/mK and thereby, revealing it as a promising material in improving the thermal insulation property of TBC. Thermal cycling results showed that the BLT TBC had a lifetime of more than 1,100 cycles of about 200 h at 1,100°C. The failure of the coating occurred by cracking at the TGO layer due to severe bond coat oxidation. Based on the experimental results BLT can be considered as a promising material for TBC applications [60]. Xu et al. [61] deposited DCL TBCs consisting of La2(Zr0.7Ce0.3)2O7 (LZ7C3) and YSZ by EB-PVD method. They showed that the DCL coating had a much longer lifetime than the single layer LZ7C3 coating and much longer than that of the single layer YSZ coating. Similar thermal expansion behaviors of YSZ interlayer with LZ7C3 coating and TGO layer, high sintering-resistance of LZ7C3 coating and unique columnar growth within DCL coating led to the extension of thermal cycling life of DCL coating. The failure of DCL coating occurred due to the reductionoxidation of cerium oxide, the crack initiation, propagation and extension, the abnormal oxidation of bond coat, the degradation of *t*′-phase in YSZ coating, and the outward diffusion of Cr alloying element into LZ7C3 coating [61]. Dy2O3–Y2O3 co-doped ZrO2 exhibits lower thermal conductivity and higher coefficient of thermal expansion. Thus, it is a promising ceramic thermal barrier coating material for aero-gas turbines and high temperature applica‐ tions in metallurgical and chemical industry. Qu et al. [62] prepared Dy2O3–Y2O3 co-doped ZrO2 ceramics using solid state reaction methods. Dy0.06Y0.072Zr0.868O1.934 exhibited a lower thermal conductivity and higher coefficient of thermal expansion as compared with standard 8 wt% Y2O3-stabilized ZrO2 used in conventional TBCs. The compatibility between the TGO (Al2O3) and the new compositions is complicated to ensure the durability of TBCs. Dy0.06Y0.072Zr0.868O1.934 was found to be compatible with Al2O3 whereas YAlO3 and Dy3Al2(AlO4)3 were formed when Dy0.25Y0.25Zr0.5O1.75 and Al2O3 were mixed and sintered [62].

New alternative TBC materials to YSZ for applications above 1,473 K are being explored by researchers. Zhou et al. [63] prepared Y4Al2O9 (YAM) ceramics by solid state reaction at 1,873 K for 12 h. They investigated the phase stability, thermophysical properties, and sinteringresistance behavior of the material. XRD results revealed single monoclinic phase YAM. Even no new phase appeared after long-term annealing. The thermal conductivities of YAM ceramic decreased gradually with the increase of temperature ranges from room temperature to 1,273 K. The minimum value obtained was ~1.81 W m−1 K−1, which is lower than that of YSZ. YAM showed moderate thermal expansion coefficient, i.e., 8.91 × 10−6 K−1 in the temperature range of 300–1,473 K. In comparison to YSZ, YAM has lower density and higher sintering-resistance ability, which is very favorable for TBC applications. The results indicated that YAM is a promising ceramic material candidate for application in the TBC system [63]. YSZ is usually used as ceramic top coat for gas turbine blades and vanes. The accelerated phase transforma‐ tion and the intensified sintering of the YSZ top coat at temperatures between 1,200°C and 1,300°C lead to microstructural changes resulting in higher thermal stress generation and lifetime reduction. Additionally, thermal conductivity (λ) of the top coat increases. Therefore, lanthanum zirconate (La2Zr2O7) and gadolinium zirconate (Gd2Zr2O7) is being suggested by researchers as a top coat because of their high phase stability up to their melting points and the lower thermal conductivity compared to YSZ. Bobzin et al. [64] deposited single-(SCL) and DCL top coats consisting of 7 wt% yttria-stabilized zirconia (7YSZ), La2Zr2O7, or Gd2Zr2O7 using the EB-PVD method. They wanted to investigate the temperature-dependent phase behavior and change of thermal conductivity of SCL and DCL top coats, as well as the influence of different top coat materials and architectures on the growth of the TGO. Morphology and coating thickness were determined using SEM. The SCL and DCL systems showed a columnar microstructure with a coating thickness of about 150 μm. The thermal conductivity of SCL and DCL systems was measured between 400°C and 1,300°C by laser flash technique. The XRD of SCL and DCL systems were carried out after isothermal oxidation at 1,300°C. Finally, the TGO phase was identified by XRD and EDS analysis. Correlation between morphology, architec‐ ture, coating material, and TGO behavior can give details of oxygen diffusion processes [64].

Researchers have conceived garnets [Y3AlxFe5–xO12 (x=0, 0.7, 1.4, and 5)] as TBC materials [59]. YAG (Y3Al5O12) has superior high-temperature mechanical properties, low thermal conduc‐ tivity, excellent phase/thermal stability up to the melting point and significantly lower oxygen diffusivity than those of zirconia. However, the major drawback of this material is its low melting point and relatively low thermal expansion coefficient [59]. Guo et al. [60] produced BaLa2Ti3O10 (BLT) by solid-state reaction of BaCO3, TiO2, and La2O3 for 48 h at 1,500°C. BLT showed phase stability between room temperature and 1,400°C. BLT showed a linearly increasing thermal expansion coefficient with increasing temperature up to 1,200°C and the coefficients of thermal expansion (CTEs) were in the range of 1 × 10− 5–12.5 × 10− 6 K− 1, compa‐ rable to those of 7YSZ. BLT coatings with stoichiometric composition were developed by APS technique. The coating contained segmentation cracks and had a porosity of ~13%. The microhardness for the BLT coating was in the range of 3.9–4.5 GPa. The thermal conductivity at 1,200°C was about 0.7 W/mK and thereby, revealing it as a promising material in improving the thermal insulation property of TBC. Thermal cycling results showed that the BLT TBC had a lifetime of more than 1,100 cycles of about 200 h at 1,100°C. The failure of the coating occurred by cracking at the TGO layer due to severe bond coat oxidation. Based on the experimental results BLT can be considered as a promising material for TBC applications [60]. Xu et al. [61] deposited DCL TBCs consisting of La2(Zr0.7Ce0.3)2O7 (LZ7C3) and YSZ by EB-PVD method. They showed that the DCL coating had a much longer lifetime than the single layer LZ7C3 coating and much longer than that of the single layer YSZ coating. Similar thermal expansion behaviors of YSZ interlayer with LZ7C3 coating and TGO layer, high sintering-resistance of LZ7C3 coating and unique columnar growth within DCL coating led to the extension of thermal cycling life of DCL coating. The failure of DCL coating occurred due to the reductionoxidation of cerium oxide, the crack initiation, propagation and extension, the abnormal oxidation of bond coat, the degradation of *t*′-phase in YSZ coating, and the outward diffusion of Cr alloying element into LZ7C3 coating [61]. Dy2O3–Y2O3 co-doped ZrO2 exhibits lower thermal conductivity and higher coefficient of thermal expansion. Thus, it is a promising ceramic thermal barrier coating material for aero-gas turbines and high temperature applica‐ tions in metallurgical and chemical industry. Qu et al. [62] prepared Dy2O3–Y2O3 co-doped ZrO2 ceramics using solid state reaction methods. Dy0.06Y0.072Zr0.868O1.934 exhibited a lower thermal conductivity and higher coefficient of thermal expansion as compared with standard 8 wt% Y2O3-stabilized ZrO2 used in conventional TBCs. The compatibility between the TGO (Al2O3) and the new compositions is complicated to ensure the durability of TBCs. Dy0.06Y0.072Zr0.868O1.934 was found to be compatible with Al2O3 whereas YAlO3 and Dy3Al2(AlO4)3 were formed when Dy0.25Y0.25Zr0.5O1.75 and Al2O3 were mixed and sintered [62].

124 Advanced Ceramic Processing

New alternative TBC materials to YSZ for applications above 1,473 K are being explored by researchers. Zhou et al. [63] prepared Y4Al2O9 (YAM) ceramics by solid state reaction at 1,873 K for 12 h. They investigated the phase stability, thermophysical properties, and sinteringresistance behavior of the material. XRD results revealed single monoclinic phase YAM. Even no new phase appeared after long-term annealing. The thermal conductivities of YAM ceramic decreased gradually with the increase of temperature ranges from room temperature to 1,273 K. The minimum value obtained was ~1.81 W m−1 K−1, which is lower than that of YSZ. YAM showed moderate thermal expansion coefficient, i.e., 8.91 × 10−6 K−1 in the temperature range

Investigation of the ZrO2–YO1.5–TaO2.5 system reveals several promising aspects for TBC applications. Unique presence of a stable, non-transformable, tetragonal region in this ternary oxide system allows for phase stability to elevated temperatures, e.g.,1,500°C. Yttria- and tantala-containing compositions exhibited significantly high resistance to vanadate corrosion compared to 7YSZ. Further, yttria- and tantala-stabilized zirconia compositions within the non-transformable tetragonal phase field exhibited toughness values comparable or higher than those of 7YSZ and thereby, increasing their stability as TBCs. Pitek and Levi discussed about these promising attributes based on recent experimental works [65]. Liu et al. [66] prepared pyrochlore-type (La0.8Eu0.2)2Zr2O7 feedstocks by spray drying and used that to produce ceramic thermal barrier coatings. DCL TBCs with a first layer of 8 wt% YSZ and a top layer of (La0.8Eu0.2)2Zr2O7 were deposited by plasma spraying. Plasma-sprayed (La0.8Eu0.2)2Zr2O7 coatings were composed of a defect fluorite-type phase and a *t*-ZrO2 phase. However, after thermal shock tests at 1,250°C for 32 cycles, (La0.8Eu0.2)2Zr2O7 coatings exhibited a pyrochlore-type structure. The thermal shock failure of DCL (La0.8Eu0.2)2Zr2O7/YSZ coatings mainly occurred at the interface between the YSZ and (La0.8Eu0.2)2Zr2O7 layers. However, the TGO layer from the bond coat had no effect on the thermal shock failure [66]. Two kinds of rare earth zirconate (Sm0.5La0.5)2Zr2O7 and (Sm0.5La0.5)2(Zr0.8Ce0.2)2O7 ceramics were prepared by Hong-song et al. [67] through solid state reaction at 1,600°C for 10 h. They investigated the phase compositions, microstructures, and thermophysical properties of these materials. XRD results confirmed the formation of single phase (Sm0.5La0.5)2Zr2O7 and (Sm0.5La0.5)2(Zr0.8Ce0.2)2O7 with pyrochlore structure. Dense microstructures of these materials and absence of other phases among the particles were revealed by SEM studies. The TEC of the ceramic increased with the increasing temperature, while the thermal conductivity decreased. TECs of (Sm0.5La0.5)2Zr2O7 and (Sm0.5La0.5)2(Zr0.8Ce0.2)2O7 were lower than that of Sm2Zr2O7. The CeO2 addition resulted in the higher TEC of (Sm0.5La0.5)2(Zr0.8Ce0.2)2O7 than those of 8YSZ and (Sm0.5La0.5)2Zr2O7. Although the TEC of (Sm0.5La0.5)2Zr2O7 was lower than that of 8YSZ, still it can serve as a TBC. Doping with La2O3 or CeO2 led to phonon scattering resulting in much lower thermal conductivities of (Sm0.5La0.5)2Zr2O7 and (Sm0.5La0.5)2(Zr0.8Ce0.2)2O7 than that of Sm2Zr2O7. In comparison to the thermal conductivity of (Sm0.5La0.5)2Zr2O7 the thermal conductivity of (Sm0.5La0.5)2(Zr0.8Ce0.2)2O7 was relatively lower. The experimental results showed that (Sm0.5La0.5)2Zr2O7 and (Sm0.5La0.5)2(Zr0.8Ce0.2)2O7 are novel candidate materials for TBCs in near future [67].
