Optimizing the structure and properties of Y2O3 stabilized zirconia: An atmospheric plasma spray (APS) and solution precursor plasma spray (SPPS) based comparative study

The yttria stabilized zirconia (8%YSZ) is widely used to insulate the metallic components of the engine from high temperature and improve the operating temperature of gas turbine engines. With different processing parameters, 8YSZ coatings are prepared by atmospheric plasma spray (APS) and solution precursor plasma spray (SPPS) techniques and the microstructural features and thermodynamics properties are compared. The electron back scattered diffraction (EBSD) analysis indicate that the substitutional point defects (Zr_0.86Y_0.14O_1.93) in the 8YSZ APS coatings are considerably higher than the corresponding SPPS coatings. The replacement of Zr⁴⁺ by Y³⁺ disturbs the charge neutrality of the system which might be compensated by the creation of oxygen vacancy. Both the substitutional point defects and the oxygen vacancies are the sources of phonon scattering, modifying the thermal conductivity of the coating. Pores and cracks are qualitatively and quantitatively analyzed in the microstructure of 8YSZ coatings. Strain tolerant and high thermal cycling life coatings are prepared by SPPS due to the existence of vertical cracks in the microstructure. Comparing the thermal insulation properties of the coatings, the APS coating provided lower thermal conductivities relative to the SPPS coatings which might be due to the high concentration of point defects and low concentration of the mixed oxide phase.     

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.     

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.     

Dense Alumina–Zirconia Coatings Using the Solution Precursor Plasma Spray Process

For the first time, dense coatings have been made by the solution precursor plasma spray (SPPS) process. The conditions are described for the deposition of dense Al₂O₃–40 wt% 7YSZ (yttria-stabilized zirconia) coatings; the coatings are characterized and their thermal stability is evaluated. X-ray diffraction analysis shows that the as-sprayed coating is composed of α-Al₂O₃ and tetragonal ZrO₂ phases with grain sizes of 72 and 56 nm, respectively. The as-sprayed coating has a 95.6% density and consists of ultrafine splats (1–5 μm) and unmelted spherical particles (<0.5 μm). The lamellar structure, typical of conventional plasma-sprayed coatings, is absent at the same scale in the SPPS coating. The formation of a dense Al₂O₃–40 wt% 7YSZ coating is favored by the lower melting point of the eutectic composition, and resultant superheating of the molten particles. Phase and microstructural thermal stabilities were investigated by heat treatment of the as-sprayed coating at temperatures of 1000°–1500°C. No phase transformation occurs, and the grain size is still in the nanometer range after the 1500°C exposure for 2 h. The coating hardness increases from 11.8 GPa in the as-coated condition to 15.8 GPa following 1500°C exposure due to a decrease in coating porosity.     

Influence of solution-precursor plasma spray (SPPS) processing parameters on the mechanical and thermodynamic properties of 8 YSZ

8?mol% yttria stabilized zirconia (8YSZ) coatings are prepared by solution precursor plasma spray (SPPS) technique under different spray conditions. Phase analysis is performed using X-ray diffraction (XRD) and Electron back scattered diffraction (EBSD) techniques. Irrespective of the processing conditions and the heat treatment temperatures, all samples displayed cubic and tetragonal zirconia phases. Vertical and horizontal cracks appeared in the microstructural analysis of the coatings. Coating prepared under spray conditions having 40?mm distance between the spray gun and the sample exhibit high hardness, both at 0?h and 10?h heat treatment holding time. To explore the suitability of the coatings for the heat insulating applications, the thermal diffusivity and thermal conductivity are calculated. The coating with 42?mm distance between the spray gun and the sample displayed lowest thermal conductivity from 400 to 1200?℃.     

Thermal Stability of Air Plasma Spray and Solution Precursor Plasma Spray Thermal Barrier Coatings

Yttria-stabilized zirconia (7YSZ) thermal barrier coatings (TBCs) were produced by conventional air plasma spray (APS) and solution precursor plasma spray (SPPS) processes. Both TBCs were isothermally heat treated from 1200° to 1500°C for 100 h. Changes in the phase content, microstructure, and hardness were investigated. The nontransformable tetragonal ( t ') phase is the predominant phase in both the as-sprayed APS and SPPS TBCs. APS and SPPS coatings exhibit similar thermal stability behavior such as densification rate, hardness increase, and grain coarsening rate. Both the as-received and heat-treated APS and SPPS TBCs show a bimodal pore size distribution with nano- and micro-size pores. After 1400°C/100 h heat treatment, equiaxed grains replace the columnar structure in APS TBCs and the splat structure disappears. Vertical cracks remain after the 1500°C/100 h exposure in SPPS TBCs. The monoclinic phase appears in APS TBCs after a 1400°C/100 h exposure and in SPPS coatings after a 1500°C/100 h exposure.     

Characterization of Thermal Barrier Coatings Produced by Various Thermal Spray Techniques Using Solid Powder,Suspension, and Solution Precursor Feedstock Material

Use of a liquid feedstock in thermal spraying (an alternative to the conventional solid powder feedstock) is receiving an increasing level of interest due to its capability to produce the advanced submicrometer/nanostructured coatings. Suspension plasma spraying (SPS) and solution precursor plasma spraying (SPPS) are those advanced thermal spraying techniques which help to feed this liquid feedstock. These techniques have shown to produce better performance thermal barrier coatings (TBCs) than conventional thermal spraying. In this work, a comparative study was performed between SPS‐ and SPPS‐sprayed TBCs which then were also compared with the conventional atmospheric plasma‐sprayed (APS) TBCs. Experimental characterization included SEM, porosity analysis using weight difference by water infiltration, thermal conductivity measurements using laser flash analysis, and lifetime assessment using thermo‐cyclic fatigue test. It was concluded that SPS coatings can produce a microstructure with columnar type features (intermediary between the columnar and vertically cracked microstructure), whereas SPPS can produce vertically cracked microstructure. It was also shown that SPS coatings with particle size in suspension (D₅₀) <3 μm were highly porous with lower thermal conductivity than SPPS and APS coatings. Furthermore, SPS coatings have also shown a relatively better thermal cyclic fatigue lifetime than SPPS.     

High velocity sand impact damage on CVD diamond

This paper describes a recent study of the damage mechanisms generated by high velocity-sand impact on diamond coatings deposited on tungsten substrates by chemical vapour deposition (CVD). The coatings were erosion tested using 90–355-μm diameter sand at a velocity of 268 m s⁻¹ and the eroded coatings examined by scanning electron and acoustic microscopy. The images indicate that the circumferential cracks and pinholes are the main erosion features and are only located on debonded areas of the coating. This suggests that they could be formed by stress waves reflected from the coating–substrate interface, which interact with surface waves to generate circumferential cracks, the precursor to pinholes. The high spatial resolution of scanning acoustic microscopy enables the resolution of individual pinholes, thus, providing important evidence for identifying the mechanism responsible for the formation of circumferential cracks, the precursor to the pinholes. However, the acoustic images must be interpreted with care; in particular, it is important to compare microstructural features observed by acoustic microscopy with other techniques.     

Molten silicate interactions with plasma sprayed thermal barrier coatings: Role of materials and microstructure

Ingestion of siliceous particulate debris into both propulsion and energy turbines has introduced significant challenges in harnessing the benefits of enhanced operation efficiencies through the use of higher temperatures and thermal barrier coatings (TBCs). The so-called CMAS (for calcium-magnesium alumino-silicate) particles can melt in the gas path at temperatures greater than 1200C, where they will subsequently impact the coating surface and infiltrate through the carefully engineered porosity or cracks in a TBC. Ultimately, this CMAS attack causes premature spallation through its solidification and stiffening the ceramic during cooling. It has been noted in recent years, that TBCs based on yttria stabilized zirconia (YSZ) are completely non-resistant to CMAS attack due to their lack of reactivity with infiltrant liquid. New TBC ceramics such as Gadolinium Zirconate (GZO) show promise of CMAS resistance through rapid reaction-induced crystallization and solidification of the infiltrant, leading to its arrested infiltration. In both situations, the microstructure (porosity, micro and macro cracks) can be important differentiators in terms of the infiltration and subsequent failure mechanisms. This paper seeks to examine the interplay among microstructure, material, and CMAS attack in different scenarios. To do so, different types of YSZ & GZO single and multilayer coatings were fabricated using Air Plasma Spray (APS) and exposed to CMAS through isothermal and gradient mechanisms. In each of the cases, beyond their unique interactions with CMAS, it was observed the inherent microstructure and character of the porosity of the coating will have an additional role on the movement of the melt. For instance, vertical cracks can provide pathways for accelerated capillaric flow of the melt into both YSZ and GZO coatings. Based on these observations multilayer coatings have been proposed and realized toward potentially reducing complete coating failure and supporting multiple CMAS attack scenarios.     

Thermal cycling behavior of plasma-sprayed yttria-stabilized zirconia thermal barrier coating with La0.8Ba0.2TiO3−δ top layer

In this study, a La_0.8Ba_0.2TiO_3?δ (LBT) upper layer was deposited on an yttria-stabilized zirconia (YSZ) thermal barrier coating (TBC) through atmospheric plasma spraying. The thermal cycling behaviors of the YSZ single-ceramic-layer and LBT–YSZ double-ceramic-layer coatings at 1000 °C were investigated through a water quenching method. Moreover, phases, microstructural evolution, and elemental distributions were studied through by X-ray diffraction and scanning electron microscopy–energy-dispersive X-ray spectroscopy. The results showed that the thermal cycling lifetime of the LBT–YSZ coating was 27% higher than that of conventional YSZ coating. The conventional YSZ coating failed after 251 cycles because of the joining of the continuous horizontal and vertical cracks caused by the formation of thermal growth oxides and the bending effect of the single-ceramic-layer structure. The thermal cycling behavior of the LBT–YSZ coating was different from that of the YSZ coating at the edge and center. Although the former was similar to the failure behavior of the YSZ coating, the cracks in the vertical direction were deflected as a result of the bending effect of the double-ceramic-layer structure during quenching. This deflection led to the formation of slope cracks with longer propagation paths and slope spallation zones. The latter showed small-debris spallation on top of the LBT upper layer due to the lower fracture toughness of the LBT, which protected the central coating from the structural damage of the ceramic coating. These two behaviors would either release the thermal stress or increase the crack-propagation energy requirement in the ceramic coating, consequently improving the thermal cycling lifetime of the LBT–YSZ coating. In summary, depositing an LBT upper layer could potentially improve the thermal cycling lifetimes of TBCs.     

Microstructure and Polarization Studies on Interlayer Free La0.65Sr0.3Co0.2Fe0.8O3–δ Cathodes Fabricated on Yttria Stabilized Zirconia by Solution Precursor Plasma Spraying

Nano‐structured cathodes of La_0.65Sr_0.3Co_0.2Fe_0.8O_3–δ (LSCF) are fabricated by solution precursor plasma spraying (SPPS) on yttria stabilized zirconia (YSZ) electrolytes (LSCF‐SPPS‐YSZ). Phase pure LSCF is obtained at all plasma power. Performances of LSCF‐SPPS‐YSZ cathodes are compared with conventionally prepared LSCF cathodes on YSZ (LSCF‐C‐YSZ) and gadolinium doped ceria (GDC) (LSCF‐C‐GDC) electrolytes. High R_p is observed in the LSCF‐C‐YSZ (∼42 Ohm cm² at 700 °C) followed by LSCF‐C‐GDC (R_p ∼ 1.5 Ohm cm² at 700 °C) cathodes. Performance of the LSCF‐SPPS‐YSZ cathodes (R_p ∼ 0.1 Ohm cm² at 700 °C) is found to be even superior to the performance of LSCF‐C‐GDC cathodes. High performance in LSCF‐SPPS‐YSZ cathodes is attributed to its nano‐structure and absence of any interfacial insulating phase which may be attributed to the low temperature at the interaction point of LSCF and YSZ and low interaction time between LSCF and YSZ during SPPS process. In the time scale of 100 h, no change in the polarization resistances is observed at 750 °C. Based on the literature and from the present studies it can be stated that SOFC with YSZ electrolyte and LSCF‐SPPS‐YSZ cathode can be operated at 750 °C for a longer duration of time and good performance can probably be achieved.     

Atmospheric plasma sprayed 7%-YSZ thick thermal barrier coatings with controlled segmentation crack densities and its thermal cycling behavior

Yttria-stabilized zirconia (YSZ)-coatings are deposited on Ni-based superalloy IN738 by atmospheric plasma spraying (APS). For the first time, controlled segmentation crack densities are manually developed in the coatings, even after the APS deposition. This method allows to user to control segmentation densities as well as cracks depth, which could be designed as per coating thickness and required application. Thermal cycling test shows promising strain tolerance behavior for the segmented coatings, whereas coating without segmentation could not sustain even for its first thermal cycle period. Further, microstructural studies reveal that a very thin layer of TGO was formed and obvious no coating failure or spallation was observed after thermal cycling test at 1150 °C for 500 cycles.     

Preparation and characterization of YSZ abradable sealing coating through mixed solution precursor plasma spraying

Yttria-stabilized zirconia (YSZ) ceramic matrix abradable sealing coatings were prepared by plasma spraying of a blend of YSZ solution precursor with YSZ nano-particles. The microstructure and phase compositions of the prepared abradable sealing coatings were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). In addition, the mechanical, high-temperature oxidation, and tribological properties of the coatings were systematically investigated. The results show that addition of YSZ nano-particles increased porosity and bond strength and decreased the hardness of the coating. The optimum performance value was achieved by addition of 5?g nano-particles into the coating. The coatings maintained excellent thermal stability through a ten-cycle thermal shock test at 1150?°C. The 8YSZ-5 coating had an improved oxidation constant of 5.540?×?10^?4 and exhibited remarkable oxidation kinetics at 1150?°C. The friction coefficient of the mixed solution precursor coating was remarkably decreased compared with a traditional ceramic matrix abradable sealing coating. The results indicate that mixed solution precursor plasma spraying increased abradable sealing coating application performance.     

Evaluation of microstructure evolution of thermal barrier YSZ coating after thermal exposure

Understanding the microstructural transformation of plasma sprayed (APS) yttria-stabilized zirconia (YSZ) after experiencing the thermal shocking cycles is practically important for the coating optimization in terms of structure and performance. In this study, thermal shocking tests were conducted on the YSZ coated piston crown. The microscopic morphology and structure alteration across the YSZ coating interface over the piston crown was characterized by the ex-situ techniques. The results revealed that the YSZ coating primarily consisted of a stable tetragonal phase, without the monoclinic phase even after 800 cycles of thermal shocking. As the thermal shocking test continued, the pore number within the YSZ coating gradually decreased due to their spontaneous closure and the grain size correspondingly increased. Some visible cracks parallel to the interface consisting of YSZ and bonding layer happened at the localized regions of the YSZ coating. The stress state of YSZ coating was from originally tensile to compressive after thermal exposure, which helped prolonging the service lifetime of YSZ coating. In particular, the thermal shock resistance of plasma sprayed YSZ coated piston crown in association with the varying microstructure was also discussed.     

Effect of Processing Parameters on Cerium Oxide Coating Deposition in Solution Precursor Plasma Spray

Solution precursor plasma spray was used to deposit cerium oxide coating. This study is to understand the coating deposition mechanism in solution precursor plasma spray for cerium oxide using a cerium nitrate liquid precursor. This study includes the effect of various processing parameters on coating micro structure, such as plasma power, standoff distance, and solution concentration. Single scan experiments were performed to better understand the single splat formation and unpyrolyzed precursor deposition. X‐ray diffraction analysis was conducted to determine average crystallite size of the coating from different concentration and formation of single phase cerium oxide formation. Detailed microstructural characterizations of the coatings were carried out by scanning electron microscopy.     

The sintering behavior of plasma-sprayed YSZ coating over the delamination crack in low temperature environment

The sintering behavior of plasma-sprayed yttria-stabilized zirconia (YSZ) coating over the delamination crack and its influence on YSZ cracking were investigated via gradient thermal cycling test and finite element model (FEM). The gradient thermal cycling test was performed at a peak surface temperature of 1150 °C with a duration of 240 s for each cycle. A three-dimensional model including delamination cracks with different lengths was employed to elaborate the temperature evolution characteristics in YSZ coating over the delamination cracks. The temperature over the delamination crack increases linearly with the crack propagation, which continuously promotes the sintering of YSZ coating in the region. As a result, the YSZ coating over the delamination crack sinters dramatically despite of the low temperature exposure. Meanwhile, the temperature distribution difference in YSZ coating induces an nonuniform sintering along both free surface and thickness of YSZ coating. Correspondingly, the maximum vertical crack driving force locates at the YSZ free surface over the delamination crack center, which makes the vertical cracks generate in this region and propagate to the interface of YSZ /bond coat with YSZ further sintering. The vertical crack promotes the delamination crack propagation via accelerating the oxidation velocity of the bond coat. The influence of temperature rise on delamination crack propagation can be divided into two stages: the little contribution stage and the promotion stage. For the actual engine exposure to low temperature, the study of phase transformation of YSZ over the delamination crack is indeed needed because of an extended remarkable temperature rise period.     

Hot corrosion behaviour of atmospheric and solution precursor plasma sprayed (La0.9Gd0.1)2Ce2O7 coatings in sulfate and vanadate environments

Conventional and solution precursor plasma spraying (SPPS) techniques were employed for developing gadolinium oxide doped lanthanum cerate ((La_0.9Gd_0.1)₂Ce₂O₇, Gd-LC) based double-layered thermal barrier coatings (TBCs). Hot corrosion studies of the above coatings were carried out in molten Na₂SO₄ + V₂O₅ (1:1) environment at 900 °C. The state-of-the-art yttria-stabilized-zirconia (YSZ) coating was found to be completely delaminated after 120 h by forming yttrium vanadate (YVO₄) and m-ZrO₂. The accelerated delamination of YSZ can be attributed to the undesired phase transformation during exposure to corrosive species. Double-layered APS coatings were found to last for more than 300 h without densification of underlying YSZ layer and also, show better adherence with bond coat. SPPS Gd-LC coatings were found to be completely delaminated on densifying the underlying YSZ within 300 h. Lanthanum vanadate (LaVO₄) was found to be the main corrosive product along with minor amounts of gadolinium vanadate (GdVO₄) in Gd-LC double-layer coatings.     

Influence of in-flight particle state diagnostics on properties of plasma sprayed YSZ-CeO2 nanocomposite coatings

This article describes the influence of controlling in-flight hot particle characteristics on properties of plasma sprayed nanostructured yttria stabilized zirconia (YSZ) coatings. This article depicts dependence of adhesion strength of as-sprayed nanostructured YSZ coatings on particle temperature, velocity and size of the splat prior to impact on the metallic substrate. Particle temperature measurement is based on two-color pyrometry and particle velocities are measured from the length of the particle traces during known exposure times. The microstructure and adhesion strength of as-sprayed nano-YSZ coatings were studied. Field emission scanning electron microscopy results revealed that morphology of coating exhibits bimodal microstructure consisting of nano-zones reinforced in the matrix of fully melted particles. The coating adhesion strength is noticed to be greatly affected by the melting state of agglomerates. Maximum adhesion strength of 42.39 MPa has been experimentally found out by selecting optimum levels of particle temperature and velocity. The enhanced bond strength of nano-YSZ coating may be attributed to higher interfacial toughness due to cracks being interrupted by adherent nano-zones.     

Microstructure–Thermal Conductivity Relationships for Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

The microstructures of plasma-sprayed yttria-stabilized zirconia (YSZ) coatings are complex, contributing to challenges in establishing microstructure–thermal conductivity relationships. Furthermore, the dynamic evolution of microstructure and properties during service offers a significant challenge in defining design strategies and extended coating performance. In this paper, the relationship between microstructure and thermal conductivity is investigated for three sets of plasma-sprayed YSZ coating systems prepared using different morphology powders, different particle size distributions, and controlled modification of particle states through plasma torch parameters. Both ambient and temperature-dependent thermal conductivity were conducted in the as-sprayed and thermally aged states. The results suggest that a range of thermal conductivities can be achieved from the coatings, offering potential for microstructural tailoring for desired performance. The results also demonstrate that different as-deposited microstructures display varying propensity for sintering and these attributes need to be considered in the design and manufacturing cycle. This expansive study of a range of coatings has also allowed synthesis of the results through thermal conductivity–porosity maps and has allowed elucidation of the contributing microstructural components for both the ambient and high-temperature thermal conductivity. Considering that the operating thermal transport mechanisms are different at these two temperature extremes, such mapping strategies are of value to both science and technology.     

Fracture Toughness of Plasma‐Sprayed Thermal Barrier Ceramics: Influence of Processing,Microstructure, and Thermal Aging

Fracture toughness of thermal barrier coatings (TBCs) has gained significant interest in recent years as one of the dominant design parameters dictating selection of materials and assessing durability. Much progress has been made in characterizing and understanding fracture toughness of relevant TBC compositions in their bulk form, but it is also apparent that the toughness is significantly affected by process‐induced microstructural defects. In this investigation, a systematic study of the influence of coating microstructure on the fracture toughness of atmospheric plasma‐sprayed TBCs has been carried out. Yttria partially stabilized zirconia (YSZ) coatings were fabricated under different process conditions inducing different levels of porosity and defect densities. Fracture toughness was measured on free‐standing coatings in as‐processed and thermally aged conditions using the double torsion technique. Results indicate significant variance in fracture toughness among coatings with different microstructures including changes induced by thermal aging. Comparative studies were also conducted on an alternative composition, Gd₂Zr₂O₇ which, as anticipated, shows significantly lower fracture toughness compared to YSZ. The results not only point toward a need for process and microstructure optimization for enhancing TBC performance, but also a framework for establishing performance metrics for promising new TBC compositions.