Architecture designs for extending thermal cycling lifetime of suspension plasma sprayed thermal barrier coatings

Suspension plasma spraying (SPS) as a relatively new spraying technology has great potential on depositing high performance thermal barrier coatings (TBCs). In some cases, however, columnar SPS TBCs show premature failure in thermal cycling test. To explain the reasons of such failure, a failure mechanism for columnar SPS TBCs was proposed in this work. The premature failure of TBCs might be related to the radial stresses in the vicinity of top coat/bond coat interface. These radial stresses were introduced by the thermal misfit and the roughness of bond coat. According to this mechanism, two architecture designs of SPS TBCs were applied to improve the thermal cycling lifetime. One was a double layered top coat design with a lamellar atmospheric plasma sprayed (APS) sub-layer and a columnar SPS top-layer. The other one was a low roughness bond coat design with a columnar SPS top coat deposited on a low roughness bond coat which was grinded before the spraying. With both designs, lifetimes of SPS TBCs were significantly extended. Especially, a lifetime even better than conventional APS TBCs was achieved with the double layered design.     

Thermal shock resistance of air plasma sprayed thermal barrier coatings

The spallation resistance of an air plasma sprayed (APS) thermal barrier coating (TBC) to cool-down/reheat is evaluated for a pre-existing delamination crack. The delamination emanates from a vertical crack through the coating and resides at the interface between coating and underlying thermally grown oxide layer (TGO). The coating progressively sinters during engine operation, and this leads to a depth-dependent increase in modulus. Following high temperature exposure, the coating is subjected to a cooling/reheating cycle representative of engine shut-down and start-up. The interfacial stress intensity factors are calculated for the delamination crack over this thermal cycle and are compared with the mode-dependent fracture toughness of the interface between sintered APS and TGO. The study reveals the role played by microstructural evolution during sintering in dictating the spallation life of the thermal barrier coating, and also describes a test method for the measurement of delamination toughness of a thin coating.     

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.     

Laser surface modification of plasma sprayed CYSZ thermal barrier coatings

In this study, Inconel 738 LC superalloy coupons were first sprayed with a NiCoCrAlY bond coat and then with a ceria and yttria stabilized zirconia (CYSZ) top coat by air plasma spraying (APS). After that, the plasma sprayed CYSZ thermal barrier coatings (TBCs) were treated using a Nd:YAG pulsed laser. The effect of laser glazing on the microstructure of the coatings was investigated. The microstructures and surface topographies of both as-sprayed and laser glazed samples were investigated using field emission scanning electron microscope (FESEM) and atomic force microscope (AFM). The phases of the coatings were analyzed with X-ray diffractometry (XRD). The microstructural analysis results revealed that laser surface glazing of ceramic top coat reduced the surface roughness considerably, eliminated the surface porosities and produced a network of continuous cracks perpendicular to the surface. XRD patterns also showed that both as-sprayed and laser glazed top coats consisted of nonequibrium tetragonal (T′) phase.     

Thermal conductivity of air plasma sprayed yttrium heavily-doped lanthanum zirconate thermal barrier coatings

The yttrium heavily doped La₂Zr₂O₇ solid solutions coatings, with a Y to La molar ratio of 1:1, have been successfully prepared by air plasma spraying technique. The evolution of phase composition, phase structure and thermal conductivity of such coatings with annealing at 1300?°C has been investigated. The results show that, a single pyrochlore structure can be retained for coating after annealing up to 48?h, beyond which the fluorite phase begins to precipitate out. By comparing thermal conductivities to those undoped counterparts at a similar porosity level, we find a considerably flat thermal conductivity versus temperature (k-T) curve, suggesting the existence of a strong phonon scattering source, which is inferred as rattlers. In addition, after the segmentation of the fluorite phase, the thermal conductivity of corresponding coatings rises considerably, indicating that the fluorite phase has a higher thermal conductivity than that of pyrochlore phase. Moreover, while the as-sprayed coatings show a clear indication of radiative thermal conduction beyond 1000?°C, the thermal conductivity of annealed coatings do not show such an uprising trend after 1000?°C, suggesting that the radiative thermal conduction has been greatly suppressed. The reason is proposed as the formation of local dipoles due to local enrichment of certain elements influences the propagation of electromagnetic waves and thus suppresses the radiative thermal conduction.     

Design of high lifetime suspension plasma sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) fabricated by suspension plasma spraying (SPS) have shown improved performance due to their low thermal conductivity and high durability along with relatively low production cost. Improvements in SPS TBCs that could further enhance their lifetime would lead to their widespread industrialisation. The objective of this study was to design a SPS TBC system with optimised topcoat microstructure and topcoat–bondcoat interface, combined with appropriate bondcoat microstructure and chemistry, which could exhibit high cyclic lifetime. Bondcoat deposition processes investigated in this study were high velocity air fuel (HVAF) spraying, high velocity oxy fuel spraying, vacuum plasma spraying, and diffusion process. Topcoat microstructure with high column density along with smooth topcoat–bondcoat interface and oxidation resistant bondcoat was shown as a favourable design for significant improvements in the lifetime of SPS TBCs. HVAF sprayed bondcoat treated by shot peening and grit blasting was shown to create this favourable design.     

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.     

Laser thermal gradient testing of air plasma sprayed multilayered,multi-material thermal barrier coatings

Air plasma sprayed (APS) thermal barrier coatings (TBCs) are a widely used technology in the gas turbine industry to thermally insulate and protect underlying metallic superalloy components. These TBCs are designed to have intrinsically low thermal conductivity while also being structurally compliant to withstand cyclic thermal excursions in a turbine environment. This study examines yttria-stabilized zirconia (YSZ) TBCs of varying architecture: porous and dense vertically cracked (DVC), which were deposited onto bond-coated superalloys and tested in a novel CO₂ laser rig. Additionally, multilayered TBCs: a two-layered YSZ (dense + porous) and a multi-material YSZ/GZO TBC were evaluated using the same laser rig. Cyclic exposure under simulative thermal gradients was carried out using the laser rig to evaluate the microstructural change of these different TBCs over time. During the test, real-time calculations of the normalized thermal conductivity of the TBCs were also evaluated to elucidate information about the nature of the microstructural change in relation to the starting microstructure and composition. It was determined that porous TBCs undergo steady increases in conductivity, whereas DVC and YSZ/GZO systems experience an initial increase followed by a monotonic decrease in conductivity. Microstructural studies confirmed the difference in coating evolution due to the cycling.     

Preparation of long-lifetime thermal barrier coatings and toughening mechanism by using hierarchy structured zirconia-based microspheres

The thermal cycling lifetime of thermal barrier coatings was doubled when deposited by electro-sprayed (ESP) microspheres instead of by commercial hollow spherical powders. It was believed that partial-molten nodules with featured microstructures inherited from the feedstock microspheres were the main contributor for prolonged thermal cycling durability due to improved fracture toughness and strain tolerance. The maximum lifetime was observed on samples with 20?30 vol.% of partial-molten microspheres. The hierarchy pores may both slow down the crack propagation by triggering multi-deflecting and promote cracking by reducing the tendency of interfacial deflection, the net effect depends on situation. The ESP coatings exhibited bimodal Weibull moduli upon indentation, which was regarded as originated from the hierarchy porous structure. Finally, the criterion was verified by micro-indentation and residual stain-stress evaluation by Raman spectroscopy.     

Formation of vertical cracks in air plasma sprayed YSZ coatings using unpyrolyzed powder

Thick thermal barrier coatings (TTBCs) with vertical cracks deposited by air plasma spray (APS) and solution precursor plasma spray (SPPS) techniques have been widely investigated to achieve good thermal insulation along with reasonable service life. In this study, synthesized unpyrolyzed YSZ powder was air plasma sprayed in order to produce segmentation crack TTBCs. The microstructure and hardness of the deposits were then compared with those of the conventional TTBCs and dense vertically cracked (DVC)TTBCs. In this regard, spraying parameters were optimized to achieve deposits with the appropriate amount of unpyrolyzed particles in them to assist inducing vertical cracks in the deposited layers. The effect of the unpyrolyzed particles on microstructure, porosity, and microhardness of plasma sprayed coatings were also evaluated and compared. The new fabricated coating showed a bimodal structure combining non-molten sub-micron size particles and conventional splats along with segmentation cracks with higher amount of porosity and lower hardness compared to those of the DVC coatings. The results implied that, depositing unpyrolyzed powder by APS, as a new approach for achieving segmentation crack TTBCs, is very promising.     

Hot corrosion resistance of air plasma sprayed ceramic Sm2SrAl2O7 (SSA) thermal barrier coatings in simulated gas turbine environments

Samarium strontium aluminate (Sm₂SrAl₂O₇-SSA) and Yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were developed on NiCrAlY bond coated Inconel 718 superalloy substrate using air plasma spray process. The hot corrosion study was conducted in simulated gas turbine environments (molten mixtures of 50?wt% Na₂SO₄ + 50?wt% V₂O₅ and 90?wt% Na₂SO₄ + 5?wt% V₂O₅ + 5?wt% NaCl) for two different temperatures of 700 and 900?°C. A developed SSA TBCs showed about 8% and 22% lower lifetime at 700 and 900?°C, respectively than YSZ TBCs in 50?wt% Na₂SO₄ +?50?wt% V₂O₅ (vanadate). The hot corrosion life of SSA TBCs being found about 13% and 39% lower than YSZ TBCs in 90?wt% Na₂SO₄ +?5?wt% V₂O₅ +?5?wt% NaCl (chloride) at 700 and 900?°C, respectively. X-ray diffraction results showed the formation of SmVO₄, SrV₂O₆, and SrSO₄ as a major hot corrosion product in 50?wt% Na₂SO₄ +?50?wt% V₂O₅ and 90?wt% Na₂SO₄ +?5?wt% V₂O₅ +?5?wt% NaCl environments respectively for SSA TBCs. Similarly, YSZ TBCs also showed YVO₄ as hot corrosion product in vanadate and chloride environments. Both the TBCs suffer a more severe hot corrosion attack in chloride environment at 900?°C. The leaching of Sr²⁺ and Y³⁺ ions from SSA and YSZ respectively play a vital role in the destabilization of coating in vanadate and chloride environments at 700 and 900?°C. In both SSA and YSZ TBCs, the leaching of ion has significantly low influence as compared to attack by chloride ions at the bond coat-top coat interface in the presence of chloride environment. The hot corrosion resistance of SSA TBCs was improved three times higher in the presence of MgO and NiO inhibitor in vanadate environment at 900?°C mainly due to the formation of a stable Ni₃V₂O₈ phase at the surface.     

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     

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).     

Influence of phase composition on microstructure and thermal properties of ytterbium silicate coatings deposited by atmospheric plasma spray

In this study, ytterbium silicate coatings with different compositions were designed by controlling the Yb₂O₃/ SiO₂ ratio and fabricated by atmospheric plasma spray. The microstructure and thermal properties of these coatings were characterized. Results showed that the Yb₂O₃-rich coatings contained Yb₂O₃ and Yb₂SiO₅ phases, which were characterized by Yb₂O₃ columnar grains, obvious interfaces between splats and many microcracks. The SiO₂-rich coatings were composed of Yb₂SiO₅ and Yb₂Si₂O₇ phases, which were composed of well bonded splats with many spherical pores. The Yb₂O₃-rich coatings had higher coefficient of thermal expansion values and lower thermal conductivities than the SiO₂-rich coatings. The SiO₂-rich coatings presented much better thermal cycling resistance than the Yb₂O₃-rich coatings. The relationship among phase composition, microstructure and thermal properties of ytterbium silicate coatings was analyzed. The results of this study may provide some clues for designs and applications of rare-earth silicates as environmental barrier coatings.     

Preparation of Pd-doped Y3Al5O12 thermal barrier coatings using cathode plasma electrolytic deposition

Here, noble metal Pd-doped Y₃Al₅O₁₂ thermal barrier coatings (TBCs) were efficaciously prepared by means of cathode plasma electrolytic deposition (CPED). The formation mechanism of the Y₃Al₅O₁₂ coatings and the difference in coating performance before and after doping with Pd were analyzed. The results indicated that the preparation of the Y₃Al₅O₁₂ TBCs by using the CPED method could be divided into three stages, and the phase compositions of the coatings obtained with different deposition times were different. A single-phase Y₃Al₅O₁₂ TBCs with a 115-μm thickness was obtained after a deposition time of 20 min. After Pd doping, the average surface roughness of the TBCs decreased from 27.72 to 13.84 μm, and the high-temperature oxidation resistance and thermal shock resistance at 1050 °C improved significantly.     

Outstanding durability of sol-gel thermal barrier coatings reinforced by YSZ-fibers

Thermal barrier coatings (TBC) were fabricated with commercial powders of yttria stabilized zirconia with spherical and fiber-like morphologies. The influence of fiber percentage and sintering temperature on the thermomechanical behavior was studied. TBCs with 60%–80% fibers content had the best lifetime in cyclic oxidation with less than 10% of coating spallation after 1000 cycles, with very good reproducibility. They reached lifetimes higher than industrial TBCs made by EB-PVD. The enhancement of durability is believed to be due to an increase in the thermomechanical constraints accommodation thanks to higher porosity and higher tenacity due to the presence of well anchored fibers, indeed deviation of the cracks were observed. Moreover, the morphology of the thermally grown oxide (TGO) layer is also favorable as it includes anchorage points of the TGO with fibers. This increased the adherence at the substrate interface and improved lifetime.     

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.     

Effect of splat-interface discontinuity on effective thermal conductivity of plasma sprayed thermal barrier coating

The thermal barrier coating obtained by atmospheric plasma spraying (APS TBCs) has a distinct lamellar microstructure, in which the splats discontinuous interfaces running parallel to the metal/ceramic interface contribute largely to the reduction in the effective thermal conductivity of APS TBCs. The dependency of such contribution on the topological structure of the interface discontinuity is investigated in the present work. Firstly, the concept of discontinuity of splats interfaces was defined to quantify the splats discontinuous interfaces revealed by microscopic observations. Then, the microstructure model with a random distribution of discontinuous interfaces was established by utilizing the finite element simulation method to investigate the effect of interlayer discontinuity on thermal conductivity of the APS TBCs. Finally, an optimal topological structure of the interface discontinuity was found to be responsible for the lowest effective thermal conductivity of the APS TBCs and typical parametrical tendencies demonstrated.     

Present status and future prospects of plasma sprayed multilayered thermal barrier coating systems

Thermal barrier coatings (TBCs) play a pivotal role in protecting the hot structures of modern turbine engines in aerospace as well as utility applications. To meet the increasing efficiency of gas turbine technology, worldwide research is focused on designing new architecture of TBCs. These TBCs are mainly fabricated by atmospheric plasma spraying (APS) as it is more economical over the electron beam physical vapor deposition (EB-PVD) technology. Notably, bi-layered, multi-layered and functionally graded TBC structures are recognized as favorable designs to obtain adequate coating performance and durability. In this regard, an attempt has been made in this article to highlight the structure, characteristics, limitations and future prospects of bi-layered, multi-layered and functionally graded TBC systems fabricated using plasma spraying and its allied techniques like suspension plasma spray (SPS), solution precursor plasma spray (SPPS) and plasma spray –physical vapor deposition (PS-PVD).