Characterisation of the microstructure and thermal properties of Nd2Zr2O7 and Nd2Zr2O7/YSZ thermal barrier coatings

The microstructure of following thermal barrier coatings (TBC) was characterised in this paper: monolayer coatings Nd₂Zr₂O₇ and 8YSZ; a double ceramic layered (DCL) coating. Coatings were characterised by thicknesses that did not exceed 300 μm and porosities of approx. 5%. The chemical and phase composition analysis of the DCL layers revealed an external Nd₂Zr₂O₇ ceramic layer approx. 80 μm thick, a transitional zone approx. 120 μm thick and an internal 8YSZ layer 100 μm thick. For the case of the monolayer coating, 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 of approx. 5–10 MPa. Measurements of the thermal parameters, i.e., thermal diffusivity, point to considerably better insulative properties for both new types of layers when compared to the standard 8YSZ layers.     

Thermal shock behavior of 8YSZ and double-ceramic-layer La2Zr2O7/8YSZ thermal barrier coatings fabricated by atmospheric plasma spraying

The single-ceramic-layer (SCL) 8YSZ (conventional and nanostructured 8YSZ) and double-ceramic-layer (DCL) La₂Zr₂O₇ (LZ)/8YSZ thermal barrier coatings (TBCs) were fabricated by plasma spraying on nickel-based superalloy substrates with NiCrAlY as the bond coat. The thermal shock behavior of the three as-sprayed TBCs at 1000 °C and 1200 °C was investigated. The results indicate that the thermal cycling lifetime of LZ/8YSZ TBCs is longer than that of SCL 8YSZ TBCs due to the fact that the DCL LZ/8YSZ TBCs further enhance the thermal insulation effect, improve the sintering resistance ability and relieve the thermal mismatch between the ceramic layer and the metallic layer at high temperature. The nanostructured 8YSZ has higher thermal shock resistance ability than that of the conventional 8YSZ TBC which is attributed to the lower tensile stress in plane and higher fracture toughness of the nanostructured 8YSZ layer. The pre-existed cracks in the surface propagate toward the interface vertically under the thermal activation. The nucleation and growth of the horizontal crack along the interface eventually lead to the failure of the coating. The crack propagation modes have been established, and the failure patterns of the three as-sprayed coatings during thermal shock have been discussed in detail.     

Protective properties of Al2O3 + TiO2 coating produced by the electrostatic spray deposition method

Mechanical resistance of Al₂O₃ + TiO₂ nanocomposite ceramic coating deposited by electrostatic spray deposition method onto X10CrAlSi18 steel to thermal and slurry tests was investigated. The coating was produced from colloidal suspension of TiO₂ nanoparticles dispersed in 3 wt% solution of Al₂(NO₃)₃, as Al₂O₃ precursor, in ethanol. TiO₂ nanoparticles of two sizes, 15 nm and 32 nm, were used in the experiments. After deposition, coatings were annealed at various temperatures, 300, 1000 and 1200 °C, and next exposed to cyclic thermal and slurry tests. Regardless of annealing temperature and the size of TiO₂ nanoparticles, the outer layer of all coatings was porous. The first five thermal cycles caused a rapid increase of aluminum content of the surface layer to 30–37 wt%, but further increase in the number of thermal cycles did not affect the aluminum content. The oxidation rate of coating-substrate system was lower during the thermal tests than during annealing. The oxidation rate was also lower for smaller TiO₂ particles (15 nm) forming the coating than for the larger ones (32 nm). The protective properties of Al₂O₃ + TiO₂ coating against intense oxidation of substrate were lost at 1200 °C. Slurry tests showed that coatings annealed at 1000 °C had the best slurry resistance, but thermal tests had weakened this slurry resistance, mainly due to decreasing adhesion of the coating.     

Microstructure and oxidation resistance of C-AlPO4–mullite coating prepared by hydrothermal electrophoretic deposition for SiC-C/C composites

Oxidation resistant C-AlPO₄–mullite coating for SiC pre-coated carbon/carbon composites (SiC-C/C) was prepared by a novel hydrothermal electrophoretic deposition process. The phase composition, surface and cross-section microstructure of the as-prepared multi-layer coatings were characterized by X-ray Diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The influence of deposition voltage on phase composition, microstructure and oxidation resistance of the as-prepared coatings was particularly investigated. Results show that the outer layer coating mainly composed of C-AlPO₄ and mullite phase can be achieved after the hydrothermal electrophoretic deposition. The thickness, density and anti-oxidation property of the C-AlPO₄–mullite coating was improved with the increase of deposition voltage from 160 V to 200 V. The multi-layer coating prepared at a voltage of 200 V exhibit excellent anti-oxidation property, which can effectively protect C/C composites from oxidation in air at 1773 K for 324 h with a weight loss of 1.01%. The failure of the multi-layer coatings is due to the generation of cross-holes in the coating, which cannot be self-cured by the metaphosphate and silicate glass layer after long time oxidation at 1773 K.     

A simple non-destructive method to indicate the spallation and damage degree of the double-ceramic-layer thermal barrier coating of La2(Zr0.7Ce0.3)2O7 and 8YSZ:Eu

Double-ceramic-layer (DCL) thermal barrier coatings (TBCs) of La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) and Eu³⁺-doped zirconia, which was partially stabilised by 8 wt% yttria (8YSZ:Eu), were prepared by atmospheric plasma spraying. A thermal cycling test was carried out. The 8YSZ:Eu sublayer exposed during thermal cycling could produce visible luminescence under ultraviolet (UV) illumination, providing an indication of the spallation and damage degree of the coating. The result shows that the application of a Eu³⁺-doped luminescence sublayer can be a very simple and useful non-destructive technique to indicate the spallation and damage degree of DCL coatings.     

Electron beam physical vapour deposition and mechanical properties of c-ZrO2-ZTA-coatings on alloy 617 substrates

Multilayered zirconia toughened alumina (ZTA) and c-zirconia coatings were prepared using electron beam physical vapour deposition (EB-PVD). Characterizations of the morphology and chemical composition of the deposited coatings were performed using scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). Scratch resistance, nano-indentation and bending strength were used for the evaluation of the mechanical properties. X-ray diffraction of the top ceramic TBC surface showed that it consists entirely of cubic ZrO₂ phase. The energy-dispersive X-ray spectroscopy analysis (EDS) showed that α-Al₂O₃ is the only oxide phase present at the interface, while SEM indicated the presence of columnar c-ZrO₂ as the only phase of the top coat. Delamination over a large region was observed in the case of double layer (ZTA) coating. In contrast, the multilayered (ZTA1 + ZTA2 + c-Z) coating showed neither delamination nor cracking. The hardness and scratch measurements showed that the top coat c-ZrO₂ layer is harder than the ZTA layers. The thermal conductivity of the multilayer coatings was estimated using the theoretical density and thermal conductivity values of zirconia toughened alumina (ZTA) and cubic-zirconia (c-ZrO₂) together with their experimentally measured data.     

Influence of the intermixed interfacial layers on the thermal cycling behaviour of atmospheric plasma sprayed lanthanum zirconate based coatings

In application as a thermal barrier coating (TBC), yttria stabilised zirconia (YSZ) approaches some limits of performance. To further enhance the efficiency of gas turbines, higher temperature capability and a longer lifetime of the coating are needed for the next generation of TBCs. Pyrochlore oxides of general composition, A₂B₂O₇, where A is a 3⁺ cation (La to Lu) and B is a 4⁺ cation (Zr, Hf, Ti, etc.) have high melting point, fair coefficient of thermal expansion, and low thermal conductivity which make them suitable for applications as high temperature thermal barrier coatings. Among those oxide materials lanthanum zirconate (LZ/La₂Zr₂O₇) offers very attractive properties. This work describes the fabrication, microstructure and high temperature (1280 °C) thermal cycling behaviour of lanthanum zirconate coatings with five different coating architectures, deposited using atmospheric plasma spray process. The coating architecture which had five layers with two intermixed interlayers had much longer life time than other considered architectures. The coatings were characterised using X-ray diffraction, energy dispersive spectrometry, optical and scanning electron microscopy, before and after thermal cycling tests, to study the coating failure mechanisms.     

A mullite/SiC oxidation protective coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites against oxidation, a mullite coating was prepared on SiC precoated C/C composites by a hydrothermal electrophoretic deposition process. The phase composition, microstructure and oxidation resistance of the prepared mullite/SiC coatings were investigated. Results show that hydrothermal electrophoretic deposition is an effective route to achieve crack-free mullite coatings. The mullite/SiC coating displays excellent oxidation resistance and can protect C/C composites from oxidation at 1773 K for 322 h with a weight loss rate of only 4.89 × 10^?4 g/cm² h. The failure of the multi-layer coatings is considered to be caused by the volatilization of silicate glass layer, the formation of microholes and microcracks on the coating surface and the formation of penetrative holes between the SiC bonding layer and the C/C matrix at 1773 K. The corresponding high temperature oxidation activation energy of the coated C/C composites at 1573–1773 K is calculated to be 111.11 kJ/mol.     

TaB2–SiC–Si multiphase oxidation protective coating for SiC-coated carbon/carbon composites

To improve the oxidation resistance of the carbon/carbon (C/C) composites, a TaB₂–SiC–Si multiphase oxidation protective ceramic coating was prepared on the surface of SiC coated C/C composites by pack cementation. Results showed that the outer multiphase coating was mainly composed of TaB₂, SiC and Si. The multilayer coating is about 200 μm in thickness, which has no penetration crack or big hole. The coating could protect C/C from oxidation for 300 h with only 0.26 × 10^?2 g²/cm² mass loss at 1773 K in air. The formed silicate glass layer containing SiO₂ and tantalum oxides can not only seal the defects in the coating, but also reduce oxygen diffusion rates, thus improving the oxidation resistance.     

Solution combustion of spinel CuMn2O4 ceramic pigments for thickness sensitive spectrally selective (TSSS) paint coatings

A series of spinel-type CuMn₂O₄ ceramic pigments were prepared by a facile and low-cost sol-gel solution combustion method and used as cost-effective materials to fabricate thickness sensitive spectrally selective (TSSS) paint coatings by a convenient spray-coating technique. The chemical component, crystalline morphology, and optical property of the copper manganese oxide ceramic pigment could be accurately controlled by altering the annealing temperature. X-ray diffraction (XRD) analysis confirmed that the ceramic pigments annealed at 500 °C for 1 h coincided well with the XRD patterns of crystalline CuMn₂O₄ in the JCPDS database, and there were segregated phases of CuO and Mn₂O₃. Furthermore, the pure spinel CuMn₂O₄ phase could be achieved at 900 °C for 1 h. The copper manganese oxide ceramic pigments could serve as an effective pigment for fabricating the TSSS paint coating, and the TSSS paint coatings based on ceramic pigments calcined at 900 °C showed solar absorptance of 0.895–0.905 and thermal emittance of 0.186–0.310. In addition, the accelerated thermal stability test revealed that the TSSS paint coating exhibited good thermal stability when it was exposed to air at a temperature of 300 °C for 300 h. Hence, the fabricated TSSS paint coating could be used as a solar absorber coating in the low-to-mid temperature domain.     

A novel Co-ions complexation method to synthesize pyrochlore La2Zr2O7

Pure pyrochlore Lanthanum zirconate (LZ) was synthesized by co-ions complexation method (CCM) at 1300 °C, which is 300 °C lower than that by solid-state method (SSM). At 1450 °C, the LZ prepared by CCM possessed lower thermal conductivity (1.15 W/m K) than that obtained by SSM (1.99 W/m K). This significant decrease may be caused by the different grain size, which is 300 nm and 2.5 μm synthesized by CCM and SSM, respectively. LZ precursor was belt-shaped and the belt shorten and the grain grown with the temperature increasing. Fourier transform infrared spectroscopy suggested the solidification in CCM forms from the complexation between La³⁺, Zr⁴⁺ and CH₃COO⁻, which is the key for solidification. Compared to SSM, CCM is a lower temperature and simpler technology to synthesize nano-size LZ and other rare-earth oxides.     

Thermophysical properties of Yb2O3 doped Gd2Zr2O7 and thermal cycling durability of (Gd0.9Yb0.1)2Zr2O7/YSZ thermal barrier coatings

(Gd_1−xYb_x)₂Zr₂O₇ compounds were synthesized by solid reaction. Yb₂O₃ doped Gd₂Zr₂O₇ exhibited lower thermal conductivities and higher thermal expansion coefficients (TECs) than Gd₂Zr₂O₇. The TECs of (Gd_1−xYb_x)₂Zr₂O₇ ceramics increased with increasing Yb₂O₃ contents. (Gd_0.9Yb_0.1)₂Zr₂O₇ (GYbZ) ceramic exhibited the lowest thermal conductivity among all the ceramics studied, within the range of 0.8–1.1 W/mK (20–1600 °C). The Young's modulus of GYbZ bulk is 265.6 ± 11 GPa. GYbZ/YSZ double-ceramic-layer thermal barrier coatings (TBCs) were prepared by electron beam physical vapor deposition (EB-PVD). The coatings had an average life of more than 3700 cycles during flame shock test with a coating surface temperature of ∼1350 °C. Spallation failure of the TBC occurred by delamination cracking within GYbZ layer, which was a result of high temperature gradient in the GYbZ layer and low fracture toughness of GYbZ material.     

Preparation of nanostructured Gd2Zr2O7-LaPO4 thermal barrier coatings and their calcium-magnesium-alumina-silicate (CMAS) resistance

Nanostructured 30 mol% LaPO₄ doped Gd₂Zr₂O₇ (Gd₂Zr₂O₇-LaPO₄) thermal barrier coatings (TBCs) were produced by air plasma spraying (APS). The coatings consist of Gd₂Zr₂O₇ and LaPO₄ phases, with desirable chemical composition and obvious nanozones embedded in the coating microstructure. Calcium-magnesium-alumina- silicate (CMAS) corrosion tests were carried out at 1250 °C for 1–8 h to study the corrosion resistance of the coatings. Results indicated that the nanostructured Gd₂Zr₂O₇-LaPO₄ TBCs reveals high resistance to penetration by the CMAS melt. During corrosion tests, an impervious crystalline reaction layer consisting of Gd-La-P apatite, anorthite, spinel and tetragonal ZrO₂ phases forms on the coating surfaces. The layer is stable at high temperatures and has significant effect on preventing further infiltration of the molten CMAS into the coatings. Furthermore, the porous nanozones could gather the penetrated molten CMAS like as an absorbent, which benefits the CMAS resistance of the coatings.     

Synthesis,spray granulation and plasma spray coating of lanthanum phosphate nanorods for thermal insulation coatings

Nanorods of lanthanum phosphate obtained by a wet chemical precipitation route were granulated to obtain sizes in the range of 10–15 µm by spray drying from aqueous slurry of 35 wt% solid loading and 2 wt% of PVA binder. The powders thus obtained displayed enhanced flowability and were plasma sprayed on to stainless steel substrates resulting in the formation of adherent coatings of 150–180 µm thickness. These coatings were characterized using electron microscopy, X-ray diffraction analysis and Raman spectroscopy. X-ray analysis indicated phase instability of LaPO₄ during plasma spraying resulting in the formation of oxy and polyphosphates of lanthanum (La₂P₄O₁₃ and La₃PO₇). However, post deposition heat treatment of coated samples at 1100 °C for 2 h resulted in the reversible formation of stoichiometric lanthanum orthophosphate (LaPO₄). Raman spectral analysis was used to confirm the phase structure of the coatings deposited at various plasma input powers. The coatings obtained were found to effectively lower the thermal conductivity of the substrates from ~24 W/mK to less than 19 W/mK (~10%) even at 200 °C.     

Microstructural and electrical characterization of Mn-Co spinel protective coatings for solid oxide cell interconnects

Electrophoretic deposition, thermal co-evaporation and RF magnetron sputtering methods are used for the preparation of Mn-Co based ceramic coatings for solid oxide fuel cell steel interconnects. Both thin and relatively thick coatings (1–15 μm) are prepared and characterised for their potential protective behaviour. Mn-Co coated Crofer22APU samples are electrically tested for 5000 h at 800 °C under a 500 mA cm⁻² current load to determine their Area Specific Resistance increase due to a growing chromia scale. After tests, samples are analysed by scanning and transmission electron microscopy. Analysis is focused on the potential chromium diffusion to or through the coating, the oxide scale thickness and possible reactions at the interfaces. The relationships between the coating type, thickness and effectiveness are reviewed and discussed. Out of the three Mn-Co coatings compared in this study, the one deposited by electrophoretic deposition presents the best protection against Cr diffusion and offers long term stability.     

La2O3-coated Li1.2Mn0.54Ni0.13Co0.13O2 as cathode materials with enhanced specific capacity and cycling stability for lithium-ion batteries

Li-rich layered oxides are the most promising cathode candidate for new generation rechargeable lithium-ion batteries. In this work, La₂O₃-coated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode materials were fabricated via a combined method of sol-gel and wet chemical processes. The structural and morphological characterizations of the materials demonstrate that a thin layer of La₂O₃ is uniformly covered on the surface of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ particles, and the coating of La₂O₃ has no obvious effect on the crystal structure of Li-rich oxide. The electrochemical performance of La₂O₃-coated Li-rich cathodes including specific capacity, cycling stability and rate capability has been significantly improved with the coating of La₂O₃. The Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ coated with 2.5 wt% La₂O₃ exhibits the highest discharge capacity, improved cycling stability and reduced charge transfer resistance, delivering a large discharge capacity of 276.9 mAh g⁻¹ in the 1st cycle and a high capacity retention of 71% (201.4 mAh g⁻¹) after 100 cycles. The optimal rate capability of the materials is observed at the coating level of 1.5 wt% La₂O₃ such that the material exhibits the highest discharge capacity of 90.2 mAh g⁻¹ at 5 C. The surface coating of La₂O₃ can effectively facilitate Li⁺ interfacial diffusion, reduce the structural change and secondary reactions between cathode materials and electrolyte during the charge-discharge process, and thus induce the great enhancement in the electrochemical properties of the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ materials.     

Constrained sintering of YSZ/Al2O3 composite coatings on metal substrates produced from eletrophoretic deposition

Yttria stabilized zirconia/alumina (YSZ/Al₂O₃) composite coatings were prepared from electrophoretic deposition (EPD), followed by sintering. The constrained sintering of the coatings on metal substrates was characterized with microstructure examination using electron microscopy, mechanical properties examination using nanoindentation, and residual stress measurement using Cr³⁺ fluorescence spectroscopy. The microstructure close to the coating/substrate interface is more porous than that near the surface of the EPD coatings due to the deposition process and the constrained sintering of the coatings. The sintering of the YSZ/Al₂O₃ composite coating took up to 200 h at 1250 °C to achieve the highest density due to the constraint of the substrate. When the coating was sintered at 1000 °C after sintering at 1250 °C for less than 100 h, the compressive stress was generated due to thermal mismatch between the coating and metal substrate, leading to further densification at 1000 °C because of the ‘hot pressing’ effect. The relative densities estimated based on the residual stress measurements are close to the densities measured by the Archimedes method, which excludes an open porosity effect. The densities estimated from the hardness and the modulus measurements are lower than those from the residual stress measurement and the Archimedes method, because it takes account of the open porosity.     

High emissivity MoSi2–ZrO2–borosilicate glass multiphase coating with SiB6 addition for fibrous ZrO2 ceramic

To develop a high emissivity coating on the low thermal conductivity ZrO₂ ceramic insulation for reusable thermal protective system, the MoSi₂–ZrO₂–borosilicate glass multiphase coatings with SiB₆ addition were designed and prepared with slurry dipping and subsequent sintering method. The influence of SiB₆ content on the microstructure, radiative property and thermal shock behavior of the coatings has been investigated. The coating prepared with SiB₆ included the top dense glass layer, the surface porous coating layer and the interfacial transition layer, forming a gradient structure and exhibiting superior compatibility and adherence with the substrate. The emissivity of the coating with 3 wt% SiB₆ addition was up to 0.8 in the range of 0.3–2.5 μm and 0.85 in the range of 0.8–2.5 μm at room temperature, and the “V-shaped grooves” surface roughness morphology had a positive effect on the emissivity. The MZB-3S coating showed excellent thermal shock resistance with only 1.81% weight loss after 10 thermal cycles between 1773 K and room temperature, which was attributed to the synergistic effect of porous gradient structure, self-sealing property of oxidized SiB₆ and the match of thermal expansion coefficient between the coating and substrate. Thus, the high emissivity MoSi₂–ZrO₂–borosilicate glass coating with high temperature resistance presented a promising potential for application in thermal insulation materials.     

Comparing the deposition mechanisms in suspension plasma spray (SPS) and solution precursor plasma spray (SPPS) deposition of yttria-stabilised zirconia (YSZ)

Thermal spraying using liquid feedstock has emerged as a promising technology for the deposition of finely structured ceramic coatings. In order to provide a comparative assessment of the deposition mechanisms occurring when spraying suspension or solution feedstock, suspensions of 300 nm-sized ZrO₂–4.5 mol.% Y₂O₃ particles dispersed in water and in ethanol and solutions of zirconium and yttrium salts, corresponding to ZrO₂–4.5 mol.% Y₂O₃ and ZrO₂–8 mol.% Y₂O₃ stoichiometries, were processed by plasma spraying using different parameter settings. In-flight diagnostics of sprayed droplets, together with the morphological, microstructural and phase analysis of individual lamellae collected onto polished substrates, performed by SEM, FIB, AFM and micro-Raman spectroscopy, led to the identification of deposition mechanisms, which were subsequently verified through the characterisation of complete coating layers.     

Effects of ZrO2-La2O3 co-addition on the microstructural and optical properties of transparent Y2O3 ceramics

Commercial Y₂O₃ powder was used to fabricate Y₂O₃ ceramics sintered at 1600 °C and 1800 °C with concurrent addition of ZrO₂ and La₂O₃ as sintering aids. One group with different contents of La₂O₃ (0–10 mol%) with a fixed amount of 1 mol% ZrO₂ and another group with various contents of ZrO₂ (0–7 mol%) with a fixed amount of 10 mol% La₂O₃ were compared to investigate the effects of co-doping on the microstructural and optical properties of Y₂O₃ ceramics. At low sintering temperature of 1600 °C, the sample single doped with 10 mol% La₂O₃ exhibits much denser microstructure with a few small intragranular pores while the samples with ZrO₂ and La₂O₃ co-doping features a lot of large intergranular pores leading to lower density. When the sintering temperature increases to 1800 °C, samples using composite sintering aids exhibit finer microstructures and better optical properties than those of both ZrO₂ and La₂O₃ single-doped samples. It was proved that the grain growth suppression caused by ZrO₂ overwhelms the acceleration by La₂O₃. Meanwhile, 1 mol% ZrO₂ acts as a very important inflection point with regard to the influence of additive concentration on the transmittance, pore structure and grain size. The highest in-line transmittance of Y₂O₃ ceramic (1.2 mm in thickness) with 3 mol% of ZrO₂ and 10 mol% of La₂O₃ sintered at 1800 °C for 16 h is 81.9% at a wavelength of 1100 nm, with an average grain size of 11.2 µm.