Crystallization behavior of CMAS and NaVO3+CMAS mixture and its potential effect to thermal barrier coatings corrosion

Calcium-magnesium-alumina-silicate (CMAS) and molten salt corrosion pose great threats to thermal barrier coatings (TBCs), and recently, a coupling effect of CMAS and molten salt has been found to cause even severer corrosion to TBCs. In this study, the crystallization behavior of CMAS and CMAS+NaVO₃ is investigated for potentially clarifying their corrosion mechanisms to TBCs. Results indicated that at 1000 °C and 1100 °C, CMAS was crystallized to form CaMgSi₂O₆, while at 1200 °C, the crystallization products were CaMgSi₂O₆, CaSiO₃ and CaAl₂Si₂O₈. The introduction of NaVO₃ in CMAS reduced the crystallization ability, and as the NaVO₃ content increased, glass crystallization occurred at a lower temperature, with crystallization products mainly consisting of CaAl₂Si₂O₈ and CaMgSi₂O₆. At 1200 °C, CMAS+10 wt% NaVO₃ was in a molten state without any crystallization, which suggested that NaVO₃ addition in CMAS could reduce its melting point, indicating enhanced penetration ability in TBCs and thus increased corrosiveness.     

Microstructure analyses and thermophysical properties of nanostructured thermal barrier coatings

Nanostructured 8 wt% yttria partially stabilized zirconia coatings were deposited by air plasma spraying. Transmission electron microscopy, scanning electron microscopy, and X-ray diffraction were carried out to analyze the as-sprayed coatings and powders. Mercury intrusion porosimetry was applied to analyze the pore size distribution. Laser flash technique and differential scanning calorimetry were used to examine the thermophysical properties of the nanostructured coatings. The results demonstrate that the as-sprayed nanostructured zirconia coatings consist of the nonequilibrium tetragonal phase. The microstructure of the nanostructured coatings includes the initial nanostructure of powder and columnar grains. Moreover, micron-sized equiaxed grains were also exhibited in the nanostructured coatings. Their evolution mechanisms are discussed. The as-sprayed nanostructured zirconia coating shows a bimodal pore size distribution, and has a lower value of thermal conductivity than the conventional coating.     

Progress update on extending the durability of air plasma sprayed thermal barrier coatings

Air plasma sprayed thermal barrier coatings (TBCs) are widely used in gas turbines to provide thermal insulation for the metallic engine components. During service, the multi-layered and multi-material systems undergo thermal and mechanical degradation. The degradation mechanisms include sintering, phase transformation, residual stress, oxidation, erosion and CMAS attack. The degradation leads to the initiation and propagation of cracks at or near the interface between the topcoat and bond coat, eventually merging into large-scale delamination and resulting in failure of the TBCs. Recent progress in the development of methods for mitigating the detrimental impact of these failure mechanisms via composition and processing modifications has been reviewed. Meanwhile, the applications of newly-emerging materials with superior properties have also been discussed. The review emphasises the relationships between composition, microstructure and properties of TBCs, which is beneficial for the exploration of the advanced TBCs with higher durability.     

Stress-dependent sintering behavior of porous thermal barrier coatings

Thermal barrier coatings (TBCs) are subjected to high temperature and complex stress fields during service in gas turbines. In this process, densification and hardening take place as the result of sintering, which is sensitive to boundary condition/external load. The stress-dependent sintering behaviors of porous TBCs were investigated in this work using a customized four-point bending method. Furthermore, stress-dependent sintering model was developed and implemented in finite element analysis to elucidate sintering mechanisms. It was found that stress gradient induced nonlinear differential sintering behavior, due to the accelerating and retarding effects of compressive and tensile stresses, respectively. In addition, microstructure-mechanical property relation was determined following the exponential law and high-throughput method was proposed for the characterization of stress dependence. The in-depth understanding of stress-dependent sintering behavior could provide guidance to the design and failure analysis of TBCs applied on complex shaped components in the hot section of gas turbines.     

Effect of scandia content on the thermal shock behavior of SYSZ thermal sprayed barrier coatings

Nanostructured 4SYSZ (scandia (3.5 mol%) yttria (0.5 mol%) stabilized zirconia) and 5.5 SYSZ (5 mol% scandia and 0.5 mol% yttria) thermal barrier coatings (TBCs) were deposited on nickel-based superalloy using NiCrAlY as the bond coat by plasma spraying process. The thermal shock response of both as-sprayed TBCs was investigated at 1000 °C. Experimental results indicated that the nanostructured 5.5SYSZ TBCs have better thermal shock performance in contrast to 4SYSZ TBCs due to their higher tetragonal phase content and higher fracture toughness of this coating     

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.     

Substrate-constrained effect on the stiffening behavior of lamellar thermal barrier coatings

Thermal exposure would compromises the compliance and thermal insulating performance of thermal barrier coatings (TBCs). However, most publications were based on free-standing coatings in which the stress resulting from substrate is essentially different from TBCs on superalloy substrate. In this paper, the constrained effect of substrate on the ceramic top-coat of plasma sprayed lamellar TBCs was investigated. Results showed that the structural changes evolve from micro-scale to macro-scale during thermal exposure. In a relatively shorter thermal exposure stage, the inter-splat pores became narrowed, whereas the intra-splat cracks became widened. Consequently, the healing kinetics of inter-splat pores was much faster than that of the intra-splat cracks. In a relatively longer thermal exposure stage, some macroscale cracks appeared in coating surface owing to the gradually stiffening coatings. As a result, the microscale intra-splat cracks near the macroscale cracks were healed rapidly. In brief, the substrate constraint induced structural changes were stage sensitive.     

Microstructural characterization of GZ/CYSZ thermal barrier coatings after thermal shock and CMAS+hot corrosion test

In this study, first, Gd₂Zr₂O₇/ceria–yttria stabilized zirconia (GZ/CYSZ) TBCs having multilayered and functionally graded designs were subjected to thermal shock (TS) test. The GZ/CYSZ functionally graded coatings displayed better thermal shock resistance than multilayered and single layered Gd₂Zr₂O₇ coatings. Second, single layered YSZ and functionally graded eight layered GZ/CYSZ coating (FG8) having superior TS life time were selected for CMAS + hot corrosion test. CMAS + hot corrosion tests were carried out in the same experiment at once. Furthermore, to generate a thermal gradient, specimens were cooled from the back surface of the substrate while heating from the top surface of the TBC by a CO₂ laser beam. Microstructural characterizations showed that the reaction products were penetrated locally inside of the YSZ. On the other hand, a reaction layer having ∼6 μm thickness between CMAS and Gd₂Zr₂O₇ was seen. This reaction layer inhibited to further penetration of the reaction products inside of the FG8.     

Hot corrosion studies of nanostructured gadolinium zirconate thermal barrier coatings

Improvement of hot corrosion resistance is one of the important parameters governing the lifetime and efficiency of the thermal barrier coatings (TBCs). In this study, the Gadolinium Zirconate (GZ) was synthesized by ball milling method and deposited by Electron Beam-Physical Vapour Deposition (EB-PVD) on Ni-based superalloy substrate with NiCrAlY as an intermediate bond coat. The effect of nanostructured GZ TBCs on hot corrosion resistance were studied under three different salt mixture environments viz; SM1, SM2 and SM3 in isothermal condition at 900 °C for 12 h. The results indicated that EB-PVD coated nanostructured GZ TBCs have improved the hot corrosion resistance and performed well under SM1 and SM3 conditions with minimal weight gain and without any spallation, whereas, the TBC suffered severe spallation under of SM2 salt condition with higher weight gain among the other two conditions. The formation of microcracks along the columnar gaps of the topcoat were found in the SM2 condition, have allowed the molten salts infiltration up to the coating interface. The formation of dense corrosive products GdVO₄ and m-ZrO₂ phases were identified after hot corrosion in SM1 and SM3 condition, which were absent in SM2 condition.     

A comprehensive mechanism for the sintering of plasma-sprayed nanostructured thermal barrier coatings

Nanostructured thermal barrier coatings (TBCs) are being widely researched for their superior thermal barrier effect and strain compliance. However, the sintering occurs inevitably in nanostructured TBCs that comprise both nanozones and lamellar zones, although the mechanism of sintering in such bimodal coatings is not yet clear. This study investigates the changes in microstructure and properties of nanostructured TBCs during thermal exposure with the aim to reveal the sintering mechanism operative in these coatings. Results show that the sintering process occurs in two stages. It was found that in the initial shorter stage (~0–10 h), the properties increased rapidly; moreover, this change was anisotropic. The main structural change was the significant healing of the intersplat pores through multiconnection. During the subsequent longer stage, the change in the properties was much smaller, where it was observed that the pores continued to heal, albeit at a much lower rate. Furthermore, the faster densification of the nanozones induced during sintering became significant, resulting in an opening at the interface between the nanozones and the lamellar zones. In brief, the pore healing at the lamellar zones affects the properties, especially in the initial stage. The presence of nanozones has a positive effect in that the performance degradation during the overall thermal exposure is slowed down. An understanding of this competing sintering mechanism would enable the structural tailoring of nanostructured TBCs in order to increase their thermal insulation and thermal cycling lifetime.     

Thermal cycling and hot corrosion behavior of a novel LaPO4/YSZ double-ceramic-layer thermal barrier coating

LaPO₄ powders were produced by a chemical co-precipitation and calcination method. The ceramic exhibited a monazite structure, kept phase stability at 1400?°C for 100?h, and had low thermal conductivity (~ 1.41?W/m?K, 1000?°C). LaPO₄/Y₂O₃ partially stabilized ZrO₂ (LaPO₄/YSZ) double-ceramic-layer (DCL) thermal barrier coatings (TBCs) were fabricated by air plasma spray. The LaPO₄ coating contained many nanozones. Thermal cycling tests indicated that the spallation of LaPO₄/YSZ DCL TBCs initially occurred in the LaPO₄ coating. The failure mode was similar to those of many newly developed TBCs, probably due to the low toughness of the ceramics. LaPO₄/YSZ DCL TBCs were highly resistant to V₂O₅ corrosion. Exposed to V₂O₅ at 700–900?°C for 4?h, La(P,V)O₄ formed as the corrosion product, which had little detrimental effect on the coating microstructure. At 1000?°C for 4?h, a minor amount of LaVO₄ was generated.     

Graceful behavior during CMAS corrosion of a high-entropy rare-earth zirconate for thermal barrier coating material

The corrosion resistance to calcium-magnesium-alumino-silicates (CMAS) is critically important for the thermal barrier coatings (TBCs). High-entropy zirconate (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ (HEZ) ceramics with low thermal conductivity, high coefficient of thermal expansion and good durability to thermal shock is expected to be a good candidate for the next-generation TBCs. In this work, the CMAS corrosion of HEZ at 1300°C was firstly investigated and compared with the well-studied La₂Zr₂O₇ (LZ). It is found that the HEZ ceramics showed a graceful behavior to CMAS corrosion, obviously much better than the LZ ceramics. The HEZ suffered from CMAS corrosion only through dissolution and re-precipitation, while additional grain boundary corrosion existed in the LZ system. The precipitated high-entropy apatite showed fine-grained structure, resulting in a reaction layer without cracks. This study reveals that HEZ is a promising candidate for TBCs with extreme resistance to CMAS corrosion.     

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.     

Review of suspension and solution precursor plasma sprayed thermal barrier coatings

As one of the promising methods that can be employed to fabricate high-performance thermal barrier coatings (TBCs), suspension plasma spraying (SPS) or solution precursor plasma spraying (SPPS) has received significant attention in academic research. Enhanced performances have been shown in the SPS-/SPPS-coatings due to their special microstructures, such as uniformly distributed micro-pores, vertical cracks or columnar structures. Since there are more complexities than conventional plasma spraying methods, many works have been devoted to study the mechanism and properties of SPS-/SPPS-coatings during the past decades. In this work, the latest development of SPS or SPPS is reviewed in order to discuss some key issues in terms of preparation of suspension or solution precursor, injection mode of liquid phase, interaction between liquid and plasma jet, microstructure of as-sprayed coatings and corresponding deposition mechanism. Meanwhile, the potential application of SPS or SPPS in some new-type TBCs is introduced at the end of this paper.     

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.     

Effect of suspension characteristics on the performance of thermal barrier coatings deposited by suspension plasma spray

This paper investigates the influence of suspension characteristics on microstructure and performance of suspensions plasma sprayed (SPS) thermal barrier coatings (TBCs). Five suspensions were produced using various suspension characteristics, namely, type of solvent and solid load content, and the resultant suspensions were utilized to deposit five different TBCs under identical processing conditions. The produced TBCs were evaluated for their performance i.e. thermal conductivity, thermal cyclic fatigue (TCF) and thermal shock (TS) lifetime. This experimental study revealed that the differences in the microstructure of SPS TBCs produced using varied suspensions resulted in a wide-ranging overall TBC performance. All TBCs exhibited thermal conductivity lower than 1 W/(m. K) except water-ethanol mixed suspension produced TBC. The TS lifetime was also affected to a large extent where 10 wt % solid loaded ethanol and 25 wt % solid loaded water suspensions produced TBCs exhibited the highest and the lowest lifetime, respectively. On the contrary, TCF lifetime was not as significantly affected as thermal conductivity and TS lifetime, and all ethanol suspensions showed marginally better TCF lifetime than water and ethanol-water mixed suspensions deposited TBCs.     

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).     

Evolution of thermal insulation of plasma-sprayed thermal barrier coating systems with exposure to high temperature

The thermal insulation potential of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings is generally assessed via the evaluation of the ceramic layer. However, ageing of the complete system leads to microstructural transformations that may also play a role in the heat transport properties. This study thus investigated the microstructure-heat insulation relationships of different TBC systems in their as-deposited state and when aged under various conditions, through the systematic analysis of both microstructure and thermal diffusivity. The latter was measured from room temperature up to 1100 °C using the laser-flash technique, while the porous microstructure was assessed using image analysis. The different coatings exhibited relatively similar thermal diffusivity values that were shown to be mostly influenced by the thin porosities in contrast to larger defects. The thermal insulation of the TBC systems after exposure to high temperature was shown to be stable despite the microstructural variations introduced by cracks, oxidation and chemical degradations.     

Preparation of plasma sprayed nanostructured GdPO4 thermal barrier coating and its hot corrosion behavior in molten salts

Nanostructured GdPO₄ coatings, designed as the outer layer of double-ceramic-layer thermal barrier coatings (DCL-TBCs), were produced by air plasma spraying (APS). The coatings have close chemical composition to that of the agglomerated particles used for thermal spray. Nanozones with porous structure are embedded in the coating microstructure, having a percentage of ~30%. Hot corrosion tests of the coatings were carried out in V₂O₅ and Na₂SO₄+V₂O₅ salts at 900 °C for 4 h. Results indicate that dense reaction layers, consisting of GdVO₄ and Gd₄(P₂O₇)₃, form on the coating surfaces, which could suppress further penetration of the molten salts. In the V₂O₅ molten salt, the reaction layer is thicker and less molten salt trace could be found beneath the layer.     

Microstructure evolution of CMAS glass below melting temperature and its potential influence on thermal barrier coatings

CaO–MgO–Al₂O₃–SiO₂ (CMAS) deposition significantly degrades the performance of thermal barrier coatings (TBCs). In this study, the microstructure evolution of CMAS glass at temperatures below its melting point was investigated in order to study the potential influence of temperature on the applicability of CMAS glass in TBCs. The CMAS glass fabricated in this study had a melting point of 1240 °C, became opaque, and underwent self-crystallization when the temperature reached 1000 °C. After heat treatment at 1050 °C, diopside and anorthite phases precipitated from the glass; at a higher temperature (1150 °C), diopside, anorthite, and wollastonite were formed as the self-crystallization products. An increase in the dwelling time resulted in the transformation of diopside to wollastonite and anorthite. At 1250 °C, all products formed a eutectic microstructure and melted. The results indicate that even at low temperatures, CMAS glass underwent microstructure evolution, which could influence the coating surface and stress distribution when deposited on TBCs.