Dry air cyclic oxidation of mixed Y/Yb disilicate environmental barrier coatings and bare silica formers

Mixed Y and Yb disilicate coatings (Y/Yb)DS have been proposed as dual function thermal and environmental barrier coatings (EBCs) for protecting SiC-based ceramic matrix composites in gas-turbine environments. As an initial step, the 1350 °C dry air cyclic oxidation of atmospheric plasma sprayed (Y_1.2/Yb_0.8)DS and ytterbium disilicate/ytterbium monosilicate (YbDS/YbMS) EBCs deposited onto Si bond coatings was compared. As a baseline for evaluating EBC oxidant permeability, the dry air cyclic oxidation scale growth rates for bare silica formers (SiC, Si) were also measured and were consistently higher than rates previously measured after isothermal oxidation. Regarding Si bond coat oxidation rates underlying (Y/Yb)DS and YbDS/YbMS EBCs, the thinner silica scale formed under the thinner and denser (Y/Yb)DS coatings suggested a lower oxidant permeability than YbDS/YbMS. After 500 1-h cycles, the (Y/Yb)DS coating was comprised of only the β-polymorph disilicate and minor amounts of the X-2 phase monosilicate phase. Negligible differences in oxidation kinetics for (Y/Yb)DS coatings over the 90 – 240 µm thickness range were observed.     

High temperature performances of tri-layer environmental barrier coating with Si bond layer modified by Yb2O3

Environmental barrier coatings (EBCs) have been expected to be applied on the surface of ceramic matrix composites (CMCs). However, the oxidation and propagation cracking of the silicon bond layer are the most direct causes to induce the failure of EBCs under high temperature service environment. The modification of silicon bond layer has become an important method to prolong the service life of EBCs. In this work, the Yb₂O₃ have been introduced to the silicon bond layer, and three kinds of tri-layer Yb₂SiO₅/Yb₂Si₂O₇/(Si-xYb₂O₃) EBCs with modified Si bond layer by different contents of Yb₂O₃ (x = 0, 10 vol%, 15 vol%) were prepared by vacuum plasma spray technique. The thermal shock performance and long-term oxidation resistance of the EBCs at 1350 °C were investigated. The results showed that the addition of appropriate amount of Yb₂O₃ (10 vol%) can improve the structural stability and reduce the cracks of the mixed thermal growth oxide (mTGO) layer by forming the oxidation product of Yb₂Si₂O₇ during long-term oxidation. The excessive addition of Yb₂O₃ increased the stress during thermal shock as well as accelerated the oxygen diffusion during long-term oxidation, leading to the failure of EBCs. Moreover, the distribution uniformity of Yb₂O₃ deserves further consideration and improvement.     

Effect of environmental pressure on the microstructure of YSZ thermal barrier coating via suspension plasma spraying

Suspension plasma spraying (SPS) as a potential technique to prepare thermal barrier coatings (TBCs) has been attracting more and more attention. However, most reports on SPS were carried out in the atmosphere. Given the unique features of in-flight particles and plasma jets under low pressure, the resulting coatings are expected to be different from those under atmospheric pressure. In this article, yttria-stabilized zirconia (YSZ) thermal barrier coatings were prepared using suspension plasma spraying under different environmental pressures. The results show that as the environmental pressure decreased, the column-like structural coating turned into a vertical crack segmented structure, as well as a dramatic decrease in surface roughness. More nanoparticle agglomerates were formed in the coating under lower environmental pressures. The real porosity of the coating increased with a decrease in environmental pressure.     

High-temperature interface stability of tri-layer thermal environmental barrier coatings

Thermal environmental barrier coatings (TEBCs) are capable of protecting ceramic matrix composites (CMCs) from hot gas and steam. In this paper, a tri-layer TEBC consisting of 16 mol% YO_1.5 stabilized HfO₂ (YSH16) as thermal barrier coating, ytterbium monosilicate (YbMS) as environmental barrier coating, and silicon as the bond coating was designed. Microstructure evolution, interface stability, and oxidation behavior of the tri-layer TEBC at 1300 °C were studied. The as-sprayed YSH16 coating was mainly comprised of cubic phase and ～3.4 vol% of monoclinic (M) phase. After 100 h of heat exposure, the volume fraction of the M phase increased to ～27%. The YSH16/YbMS interface was proved to be very stable because only slight diffusion of Yb to YSH16 was observed even after thermal exposure at 1300 °C for 100 h. At the YbMS/Si interface, a reaction zone including a Yb₂Si₂O₇ layer and a SiO₂ layer was generated. The SiO₂ grew at a rate of ～0.039 μm²/h in the first 10 h and a reduced rate of 0.014 μm²/h in the subsequent exposure.     

Challenging zircon coatings by suspension plasma spraying

Zircon is a ceramic material that decomposes at high temperature, limiting its use by conventional thermal spraying. In this work, it is intended to use thermal spraying from concentrated aqueous suspensions to evaluate the possibility of obtaining coatings in which a significant proportion of zircon could be preserved. For this purpose, stable concentrated suspensions of zircon have been prepared, which have been subsequently sprayed at two different spraying distances. The coatings were characterised in terms of microstructural features and the amount of zircon present in the coatings was quantified. All the coatings obtained display the typical microstructure derived from the deposition of liquid feedstocks by plasma spraying. In all cases, the XRD analysis demonstrates the partial decomposition of zircon into zirconia and residual silica, but also that a significant percentage (about 20%) is preserved without decomposing, which marks a strong difference with respect to reported data for atmospheric plasma spraying.     

Thermal shock resistance of thermal barrier coatings for nickel-based superalloy by supersonic plasma spraying

Double-layer thermal barrier coatings (TBCs), including a top ZrO₂ layer and an inner CoNiCrAlY layer, were deposited on nickel-based superalloy using supersonic atmospheric plasma spraying (SAPS). Thermal shock resistance of the TBCs between 1200 °C and room temperature was investigated. After thermal shock test, the adhesive strength of the coatings was evaluated through scratch test. The SAPS–TBCs present good thermal shock resistance, exhibiting only 0.26% mass gain up to 150-time thermal cycling. Before thermal cyclic treatment, SAPS–TBCs exhibited a strong adhesion with the absence of the thermally grown oxide (TGO) between out and inner layer. With the increasing of thermal cycles, the TGO layer was formed and its thickness firstly increased and then dropped down. The critical load fell down by about 32% for topcoat–bondcoat adhesion (up to 50 cycles) and 35% or so for TBCs–substrate adhesion (up to 150 cycles) compared to the counterpart of as-sprayed specimens. The strain introduced by the existence of TGO and mixed oxides resulted in a varied adhesion for TBCs on nickel-based alloy during thermal cycling.     

Thermal shock performance and failure behavior of Zr6Ta2O17-8YSZ double-ceramic-layer thermal barrier coatings prepared by atmospheric plasma spraying

Zr₆Ta₂O₁₇ has higher fracture toughness, better phase stability, thermal insulation performance and calcium-magnesium-alumino-silicates (CMAS) attack resistance than yttria-stabilized zirconia (8 YSZ, 7–8 wt%) at temperatures above 1200 °C. However, the thermal expansion coefficients between Zr₆Ta₂O₁₇ coating and bond coating do not match well. A double-ceramic-layer design is applied to alleviate the thermal stress mismatch. The Zr₆Ta₂O₁₇/8 YSZ double-ceramic-layer thermal barrier coatings (TBCs) are prepared by atmospheric plasma spraying (APS). During the thermal shock test, Zr₆Ta₂O₁₇/8 YSZ double-ceramic-layer TBCs exhibit a better thermal shock resistance than 8 YSZ and Zr₆Ta₂O₁₇ single-layer TBCs. The thermal shock performance and failure mechanism of TBCs in the thermal shock test are investigated and discussed in detail.     

High-temperature corrosion of spark plasma sintered Gd2SiO5 with volcanic ash for environmental barrier coatings

Hot corrosion behavior was evaluated by gadolinium monosilicate (Gd₂SiO₅) with volcanic ash for environmental barrier coatings (EBCs). Sintered Gd₂SiO₅ was prepared by the spark plasma sintering (SPS) method at 1400 °C for 20 min, and high-temperature corrosion resistance against of volcanic ash was evaluated at 1400 °C for 2 h, 12 h, and 48 h. The surface region of sintered Gd₂SiO₅ was partially dissolved in molten volcanic ash, creating a chemically reacted area. The formation of the elongated morphology of Ca₂Gd₈(SiO₄)₆O₂ grains observed in the reaction area is thicker with increasing heat-treatment time as the volcanic ash dissolves. In addition, high-temperature X-ray diffraction was carried out to identify the dynamics of phase evaluation in the volcanic ash and Gd₂SiO₅. According to the results, corrosion occurs due to reaction of the Gd₂SiO₅ phase and the Ca component of volcanic ash at 1300 °C, and the Ca₂Gd₈(SiO₄)₆O₂ phase is generated.     

High-entropy thermal barrier coating of rare-earth zirconate: A case study on (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 prepared by atmospheric plasma spraying

We report a double-ceramic-layer (DCL) thermal barrier coating (TBC) with high-entropy rare-earth zirconate (HE-REZ) as the top layer and yttria stabilized zirconia (YSZ) as the inner layer sprayed on Ni-based superalloy by atmospheric plasma spraying. La₂Zr₂O₇ (LZ) was selected as a reference for the HE-REZ. Thermal cycling test results demonstrate that the HE-REZ/YSZ DCL coating exhibited obviously improved thermal stability when compared to the LZ/YSZ DCL coating. The reasons for the improvement of the thermal shock resistance are considered to be the anti-sinterability of the HE-REZ ceramics during the thermal cycling test attributed to the sluggish diffusion effect and as well as the better match in the coefficient of thermal expansion of HE-REZ coating with the YSZ inner layer. In addition, the HE-REZ coating maintains fluorite structure after thermal cycling test. This study makes one step forward in the development and application of high-entropy rare-earth zirconate ceramic thermal barrier coatings.     

CMAS corrosion resistance,thermal shock resistance and numerical simulation of novel surface micromesh thermal barrier coatings

Thermal barrier coatings (TBCs) are widely used as insulating layers to protect the underlying metallic structure of gas turbine blades. However, the thermal cycling performance of TBCs is affected by their complex working environments, which may shorten their service life. Previous studies have shown that preparing a mesh structure in the bonding layer can relieve thermal stress and improve the bonding strength, thereby prolonging the service life of TBCs. In this paper, a micromesh structure was prepared on the surface of the bonding layer via wet etching. The microstructure and failure mechanism of the micromesh TBCs after CMAS (CaO-MgO-Al₂O₃-SiO₂) thermal erosion were investigated. Numerical simulation was combined with thermal shock experiments to study the stress distribution of the micromesh-structured TBCs. The results showed that the circular convex structure can effectively improve the CMAS corrosion resistance and thermal shock resistance of TBCs.     

Plasma spray deposition of tri-layer environmental barrier coatings

An air plasma spray process has been used to deposit tri-layer environmental barrier coatings consisting of a silicon bond coat, a mullite inter-diffusion barrier, and a Yb₂SiO₅ top coat on SiC substrates. Solidified droplets in as-deposited Yb₂SiO₅ and mullite layers were discovered to be depleted in silicon. This led to the formation of an Yb₂SiO₅ + Yb₂O₃ two-phase top coat and 2:1 mullite (2Al₂O₃*SiO₂) coat deposited from 3:2 mullite powder. The compositions were consistent with preferential silicon evaporation during transient plasma heating; a consequence of the high vapor pressure of silicon species at plasma temperatures. Annealing at 1300 °C resulted in internal bond coat oxidation of pore and splat surfaces, precipitation of Yb₂O₃ in the top coat, and transformation of 2:1 mullite to 3:2 mullite + Al₂O₃. Mud-cracks were found in the Yb₂SiO₅ layer and in precipitated Al₂O₃ due to the thermal expansion mismatch between these coating phases and the substrate.     

Influence of SiC fiber on thermal cycling lifetime of SiC fibers /YSZ thermal barrier coatings by atmospheric plasma spraying

The initiation and propagation of cracks under thermal stresses easily is one of the problems limiting the thermal cycling lifetime of thermal barrier coatings (TBCs). In order to improve the thermal cycling lifetime, SiC fibers were introduced to yttria stabilized zirconia (YSZ) coating deposited on In738LC substrate by atmospheric plasma spray (APS). Phase composition, thermal cycling behaviors and fiber toughening mechanisms of coatings were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal cycling test. Results showed that the thermal cycling lifetime and fracture toughness of SiC fibers/YSZ coatings could reach 442?±?13 and 1.54?±?0.19?MPa $\sqrt{m}$ respectively, which were 1.6 times and 1.3 times higher than that of conventional TBCs. There are two stages of fiber reinforced during thermal cycling, and the first is crack deflection and termination, the second is fiber debonding, pull-out, breakage and bridging. Meanwhile, SiC fibers could prevent the stress-activated ZrO₂ martensitic transformation by reducing the stress in the lattice.     

Fabrication and characterization of 8YSZ ceramic based abradable seal coatings by atmospheric plasma spraying

8 wt% yttria-stabilized zirconia (8YSZ) powders are fabricated as high-temperature based materials via a solid-state reaction method and ground into spheres in this paper. Following that, 4 wt% Nickle (Ni), 4 wt% Hexagonal Boron Nitride (hBN) and 4 wt% PHB (Polyphenyl ester) are added to 8YSZ for getting 8YSZ ceramic-based abradable seal powders (8YSZ CAS_p). Then, the 8YSZ CAS_p are sprayed on the stainless steel substrate with a NiCoCrAlY transition layer by an atmospheric plasma spraying (APS) technology. The phase structure, surface morphology and the cross-section topography of the fabricated are analyzed, the indentation hardness and nano-indentation test are conducted. The experiments of 8YSZ ceramic-based abradable seal coatings (8YSZ CAS_c) show that the deposition efficiency and porosity are respectively 78.5% and 21.8%, the bond strength is 4.6 MPa, the cycle number of thermal shock resistance is 37 times, those parameters prove that the fabricated 8YSZ CAS_c are promising abradable seal coatings.     

Hot corrosion behavior of multialkaline-earth aluminosilicate for environmental barrier coatings

Multialkaline-earth aluminosilicate Ba_1/3Sr_1/3Ca_1/3Al₂Si₂O₈ (BSCAS) were synthesized to serve as new environment barrier coatings. Their hot corrosion behavior in an Na₂SO₄ environment was studied in the temperature range of 900–1100 °C over a period of 100 h. The phase and cross-sectional morphology evolutions of the corroded samples were characterized via X-ray diffraction and scanning electron microscopy. Combined with the thermodynamic analysis of the possible reactions occurring during hot corrosion, the competitive out-diffusion of the alkaline-earth elements to react with Na₂SO₄ is believed to have a considerable influence on the hot corrosion behavior of BSCAS. The sluggish diffusion and the dense Ca₂Al₂SiO₇ layer, which originate from the competitive reactions of the multialkaline earth elements, lead to an improvement in the hot corrosion resistance of BSCAS. A model is proposed to describe the hot corrosion process.     

Crack-healing behaviour of MoSi2 dispersed Yb2Si2O7 environmental barrier coatings

MoSi₂ doped Yb₂Si₂O₇ composites were designed to extend the lifetime of Yb₂Si₂O₇ environmental barrier coatings (EBCs) via self-healing cracks during high-temperature applications. Yb₂Si₂O₇–Yb₂SiO₅–MoSi₂ composites with different mass fractions were prepared by applying spark plasma sintering. X-ray diffraction results confirmed that the composites consisted of Yb₂Si₂O₇, Yb₂SiO₅, and MoSi₂. The thermal expansion coefficients (CTEs) of the composites increased with an increase in the MoSi₂ content. The average CTE of the 15 wt% MoSi₂ doped Yb₂Si₂O₇ composite was 5.24 × 10^?6 K^?1, indicating that it still meets the CTE requirement of EBC materials. After being pre-cracked by using the Vickers indentation technique, the samples were annealed for 0.5 h at 1100 or 1300 °C to evaluate the crack-healing ability. Microstructural studies showed that cracks in 15 wt% MoSi₂ doped Yb₂Si₂O₇ composites were fully healed during annealing at 1300 °C. Two mechanisms may be responsible for crack healing. First, the cracks were filled with SiO₂ glass formed by MoSi₂ oxidation. Second, the formed SiO₂ continued to react with Yb₂SiO₅ to form Yb₂Si₂O₇, which can cause cracks to heal owing to volumetric expansion. The Yb₂Si₂O₇ formation with smaller volume expansion is more beneficial.     

Thermal cycling behavior and bonding strength of single-ceramic-layer Sm2Zr2O7 and double-ceramic-layer Sm2Zr2O7/8YSZ thermal barrier coatings deposited by atmospheric plasma spraying

The single-ceramic-layer (SCL) Sm₂Zr₂O₇ (SZO) and double-ceramic-layer (DCL) Sm₂Zr₂O₇ (SZO)/8YSZ thermal barrier coatings (TBCs) were deposited by atmospheric plasma spraying on nickel-based superalloy substrates with NiCoCrAlY as the bond coat. The mechanical properties of the coatings were evaluated using bonding strength and thermal cycling lifetime tests. The microstructures and phase compositions of the coatings were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results show that both coatings demonstrate a well compact state. The DCL SZO/8YSZ TBCs exhibits an average bonding strength approximately 1.5 times higher when compared to the SCL SZO TBCs. The thermal cycling lifetime of DCL SZO/8YSZ TBCs is 660 cycles, which is much longer than that of SCL 8YSZ TBCs (150 cycles). After 660 thermal cycling, only a little spot spallation appears on the surface of the DCL SZO/8YSZ coating. The excellent mechanical properties of the DCL LZ/8YSZ TBCs can be attributed to the underlying 8YSZ coating with the combinational structures, which contributes to improve the toughness and relieve the thermal mismatch between the ceramic layer and the metallic bond coat at high temperature.     

Experimental visualization of microstructure evolution during suspension plasma spraying of thermal barrier coatings

This paper investigates the evolution of microstructure of thermal barrier coatings (TBCs) produced by suspension plasma spraying (SPS) through a careful experimental study. Understanding the influence of different suspension characteristics such as type of solvent, solid load content and median particle size on the ensuing TBC microstructure, as well as visualizing the early stages of coating build-up leading to formation of a columnar microstructure or otherwise, was of specific interest. Several SPS TBCs with different suspensions were deposited under identical conditions (same substrate, bond coat and plasma spray parameters). The experimental study clearly revealed the important role of suspension characteristics, namely surface tension, density and viscosity, on the final microstructure, with study of its progressive evolution providing invaluable insights. Variations in suspension properties manifest in the form of differences in droplet momentum and trajectory, which are found to be key determinants governing the resulting microstructure (e.g., lamellar/vertically cracked or columnar).     

Study on thermal shock resistance of Sc2O3 and Y2O3 co-stabilized ZrO2 thermal barrier coatings

In order to reveal the effect of Sc₂O₃ and Y₂O₃ co-doping system on the thermal shock resistance of ZrO₂ thermal barrier coatings, Y₂O₃ stabilized ZrO₂ thermal barrier coatings (YSZ TBCs) and Sc₂O₃–Y₂O₃ co-stabilized ZrO₂ thermal barrier coatings (ScYSZ TBCs) were prepared by atmospheric plasma spraying technology. The surface and cross-section micromorphologies of YSZ ceramic coating and ScYSZ ceramic coatings were compared, and their phase composition before and after heat treatment at 1200 °C was analyzed. Whereupon, the thermal shock experiment of the two TBCs at 1100 °C was carried out. The results show that the micromorphologies of YSZ ceramic coating and ScYSZ ceramic coating were not much different, but the porosity of the latter was slightly higher. Before heat treatment, the phase composition of both YSZ ceramic coating and ScYSZ ceramic coating was a single T′ phase. After heat treatment, the phase composition of YSZ ceramic coating was a mixture of M phase, T phase, and C phase, while that of ScYSZ ceramic coating was still a single T′ phase, indicating ScYSZ ceramic coating had better T′ phase stability, which could be attributed to the co-doping system of Sc₂O₃ and Y₂O₃ facilitated the formation of defect clusters. In the thermal shock experiment, the thermal shock life of YSZ TBCs was 310 times, while that of ScYSZ TBCs was 370 times, indicating the latter had better thermal shock resistance. The difference in thermal shock resistance could be attributed to the different sintering resistance of ceramic coatings and the different growth rates of thermally grown oxide in the two TBCs. Furthermore, the thermal shock failure modes of YSZ TBCs and ScYSZ TBCs were different, the former was delamination, while the latter was delamination and shallow spallation.     

Preparation of crystalline ytterbium disilicate environmental barrier coatings using suspension plasma spray

In high-speed modern industries, high-temperature stability of materials is essential. A promising high-temperature material currently attracting attention is silicon carbide (SiC)-based ceramic matrix composites (CMC). However, a disadvantage of these materials is their reduced lifetime in an oxidizing atmosphere. To overcome this, environmental barrier coating can be employed. In this study, we aimed to fabricate an environmental barrier coating using suspension plasma spray with Yb₂Si₂O₇, which exhibits excellent oxidation resistance and a similar thermal expansion coefficient to SiC. To prepare the crystalline Yb₂Si₂O₇ coating layer, the gas concentration of the plasma spray was adjusted, and then the suspension manufacturing solvent was adjusted and sprayed. The prepared coating samples were analyzed by X-ray diffraction, scanning electron microscope, transmission electron microscopes, and energy dispersive X-ray spectroscopy to determine phase and microstructure changes. Highly crystalline ytterbium disilicate was observed at low plasma enthalpy with no hydrogen and 20% addition of water.