Comparative study on thermal shock behavior of thick thermal barrier coatings fabricated with nano-based YSZ suspension and agglomerated particles

Two kinds of segmentation-crack structured YSZ thick thermal barrier coatings (TTBCs) were deposited by suspension plasma spraying (SPS) and atmospheric plasma spraying (APS) with nano-based suspension and agglomerated particles, respectively. The phase composition, microstructure evolution and failure behavior of both TBCs before and after thermal shock tests were systematically investigated. Microstructure of the APS coating exhibits typical segmentation-crack structure in the through-thickness direction, similar with the SPS coating. The densities of segmentation-crack in APS and SPS coatings were about 3 cracks mm⁻¹ and 4 cracks mm⁻¹_, respectively. The microstructure observation also showed that the columnar and equiaxed grains existed in the SPS coating. As for the thermal shock test, the spallation life of the APS TTBCs was 146 cycles, close to that of the SPS TTBCs (166 cycles). Failure of the APS coating is due to the spallation of fringe segments and splats.     

Thick thermal barrier coatings with different segmentation crack densities: Microstructure analysis and thermal oxidation behavior

Thick thermal barrier coatings (TTBCs) have been developed to increase the lifetime of hot section parts in gas turbines by increasing the thermal insulating function. The premeditated forming of segmentation cracks was found to be a valuable way for such an aim without adding a new layer. The TTBC introduced in the current study are coatings with nominal thickness ranging from 1 to 1.1 consisting of MCrAlY bond coat and 8YSZ top coat deposited by air plasma spray technique (APS). TTBCs with segmented crack densities of 0.65 mm^?1 (type-A) and 1 mm^?1 (type-B) were deposited on a superalloy substrate by adjusting the coating conditions. It was found that the substrate temperature has an influential role in creating the segmentation crack density. The crack density was found to increase with substrate temperature and liquid splat temperature. The two types of coatings (type-A and B) with different densities of segmentation crack were heat-treated at 1000 °C (up to 100 h) and 1100 °C (up to 500 h). The variation of hardness measured by indentation testing indicates a similar trend in both types of coatings after heat treatments at 1000 °C and 1100 °C. Weibull analysis of results demonstrates that higher preheating coating during the deposition results in a denser YSZ coating. The growth rate of TGO for TTBCs was evaluated for cyclic and isothermal oxidation routes at 1000 °C and 1100 °C. The TGO shows the parabolic trend for both two types of coatings. The Kps value for two oxidation types is between 5.84 × 10^?17 m²/s and 6.81 × 10^?17 m²/s. Besides, the type B coating endures a lifetime of more than 40 cycles at thermal cycling at 1000 °C.     

Comparison of temperature dependent deformation mechanisms of 8YSZ thermal barrier coatings prepared by air-plasma-spray and D-gun thermal spray: An in situ study

8 weight percent yttria stabilized zirconia (8YSZ) has gained widespread use in thermal barrier coatings for the hot sections of aero and power generation turbines due to its superb thermal and mechanical properties. In this study, in situ microcompression tests were conducted to evaluate the mechanical performance of 8YSZ coatings with dense vertically cracked (DVC) microstructures produced by detonation gun thermal spray to those deposited by air plasma spray (APS). At room temperature, the APS coatings showed high variability in fracture strength resulting from cracks and pores in the coating. DVC coatings, conversely, exhibited fracture strengths ranging from 3.9 to 6.6 GPa and less variability in fracture strength attributed to the relatively dense and less defective microstructure. At 500 °C, both coatings showed better consistency of fracture strength and enhanced deformability owing to deformable pores, ferroelastic domain switching, and dislocation activities.     

Microstructures and Properties of Plasma-Sprayed Segmented Thermal Barrier Coatings

Thermal barrier coatings (TBCs) with different levels of segmentation crack density were produced by spraying two types of ZrO₂–8Y₂O₃ powders. The fused and crushed powder has a greater capability of forming segmented coatings than the hollow sphere (HOSP) one. The highly segmented coatings reveal much lower porosity compared with traditionally sprayed coatings, thereby compromising the property of thermal insulation of TBCs. Microstructure and thermal conductivity of the HOSP coatings are more sensitive to the changes in spray conditions. Segmentation cracks had a strong influence in decreasing Young's modulus of coatings. Fifty hours heat treatment at 1250°C had little effect on the mechanical property of the highly segmented coatings.     

Effect of Hollow Spherical Powder Size Distribution on Porosity and Segmentation Cracks in Thermal Barrier Coatings

The effect of characteristics of hollow spherical (HOSP) powders on porosity and development of segmentation cracks in plasma-sprayed thick thermal barrier coatings (TBCs) was investigated. Three powders with particle size ranges of 20–45, 53–75, and 90–120 μm were selected from a commercial HOSP powder feedstock for spraying the TBCs. The 20–45 μm powder has a higher deposition efficiency and a greater capability of producing segmented coatings than the other larger powders. Diagnostics of in-flight particles show that the average surface temperature and velocity of the particles sprayed from the fine powder is higher by 250°C and 50 m/s compared with those sprayed from the 90 to 120 μm powder, respectively, due to its greater ratio of surface area to mass. The lower porosity of the coating sprayed from the fine powder is mainly attributed to the decreased volume of intersplat gaps and voids.     

Thermal and mechanical properties of crack-designed thick lanthanum zirconate coatings

Vertical cracks are beneficial in thermal barrier coatings due to enhanced thermo-mechanical compliance. Accordingly, an aqueous nitrate based precursor solution was atomized on stainless steel substrates by spray pyrolysis to deposit thick crack-designed lanthanum zirconate coatings. Coatings with designed crack patterns were deposited and characterized by electron microscopy, tribology, Vickers indentation, and thermal diffusivity. The crystallization of the coatings was investigated by in situ high temperature X-ray diffraction. The green coatings crystallized from 600 °C and the pyrochlore structure was formed after heat treatment at 1000 °C. Crystalline lanthanum zirconate multilayered coatings with small crack spacing and crack opening exhibited a higher density, a higher hardness, lower thermal diffusivities, and higher thermal conductivities compared to crystalline monolayered coatings of similar thickness with large crack spacing and crack opening. The thermal diffusivity of the coatings, ∼28 mm²/s at room temperature, was similar to the values reported for yttria-stabilized zirconia plasma sprayed coatings.     

Constrained sintering and cracking of air plasma sprayed thermal barrier coatings: Experimental observation and modeling

The development of vertical cracks in air plasma sprayed (APS) thermal barrier coatings (TBCs) during thermal cycling and constrained sintering under a temperature gradient is investigated. Microstructural analysis shows that the development of the vertical cracks is associated with multiple processes, including sintering during the hold period and cleavage during cooldown. Inspired by the experimental observations, an image-based sintering model is used to simulate the development of vertical cracks as the coating sinters while constrained by a substrate. The computational results show that microstructural imperfections can develop into vertical cracks, which then propagate toward the interface. A simple analytical model is presented for the threshold level of in-plane stress for the onset of propagation of a vertical crack during constrained sintering. By combining the results of these different modeling approaches, the cross-coupling of the material and geometric parameters, and how this determines the sintering response (microstructure evolution) and vertical crack formation is evaluated. In addition, the growth of vertical cracks by a cleavage mechanism during cooldown is examined and the coupling between sintering, cleavage crack growth, and TBC lifetime is explored.     

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.     

Microstructural,mechanical and tribological properties of nanostructured YSZ coatings produced with different APS process parameters

Plasma sprayed ceramic coatings can be used in turbine engines as thermal barrier or abradable coatings, in order to improve the durability of the components as well as the efficiency. The presence of nanostructures, deriving from partial melting of agglomerated nanostructured particles, represents an interesting technological solution in order to improve their functional characteristics. In this work nanostructured yttria stabilized zirconia (YSZ) coatings were deposited by air plasma spraying (APS). The influence of the main process parameters on their microstructural, mechanical and tribological properties was investigated by scanning electron microscopy (SEM), indentation techniques at micro- and nano-scale and wear tests, respectively. Their porous microstructure was composed of well melted overlapped splats and partially melted nanostructured areas. This bimodal microstructure led to a bimodal distribution of the mechanical properties. An increase of plasma power and spraying distance was able to produce denser coatings, with lower content of embedded nanostructures, which exhibited higher elastic modulus and hardness as well as lower wear rate.     

Sintering resistance of suspension plasma sprayed 7YSZ TBC under isothermal and cyclic oxidation

This study examines sintering resistance of a thermal barrier coating (TBC), composed of a 7YSZ suspension plasma sprayed (SPS) top coat (TC), an air plasma sprayed (APS) NiCoCrAl bond coat (BC), and an INCONEL 625 substrate, under isothermal and cyclic conditions with a peak temperature of 1080 °C for 400, 800, and 1300 h/cycles. Microstructure, phase composition and microstrain were examined using SEM and XRD. Mechanical properties of fracture toughness, hardness and elastic modulus were obtained using nano-indentation. Samples under cyclic conditions presented faster sintering rate than under isothermal condition due to larger compressive strain and frequent heating and cooling cycles. Faster degradation of mechanical properties due to sintering leads to shorter lifetime of SPS coating under cyclic conditions. Moreover, vertical cracks within SPS coatings reduces compressive stress leading to a greater lifetime as compared to APS coatings exposed to similar conditions.     

A highly strain and damage‐tolerant thermal barrier coating fabricated by electro‐sprayed zirconia hollow spheres

An approach to make air plasma sprayed (APS) thermal barrier coatings (TBCs) with the enhanced strain and damage tolerance was reported, using a novel hollow spheres produced by electro‐spraying (ESP) technique. Compared with agglomerated & sintered (A&S) and hollow spherical (HOSP) yttria‐stabilized zirconia (YSZ) powders, the ESP powder showed a unique network microstructure and the TBCs exhibited a 2‐3 times longer thermal cycling lifetime. The splat morphology and the top coats microstructure were investigated. Some semi‐melted ESP particles were observed in the as‐sprayed top coat. The indentation coupled with the Raman mapping technique was employed to evaluate the strain and damage tolerance of the TBCs. The coatings deposited by the ESP powder show a lower in‐plane stiffness determined by three‐point bending tests. It is proposed that the superior performance is attributed to the lower amount of the short microcracks (0.5‐4 μm) with low angle (<45°) and the semi‐melted ESP particles remained in the YSZ top coat.     

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.     

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.     

Structure and electrical properties of yttrium oxide sprayed by plasma torches from powders and suspensions

Yttrium oxide was sprayed by a plasma torch using a coarse thermal spray powder, which must be in size range of tens of micrometers to ensure good penetration into the plasma stream. Thick coatings on steel substrates were produced with two sprays systems facilitating gas stabilized plasma (GSP) and hybrid water-argon stabilized plasma (WSP–H) techniques. Additionally, an ultra-fine yttrium oxide powder was sprayed from a suspension. Hybrid water-argon stabilized plasma system was used for this purpose. Markedly thinner compact coatings were produced this way. All three sorts of plasma sprayed deposits were studied by the same methods. Dielectric properties were studied in a broad range of frequencies and temperatures. The microstructure aspects as well as crystallite size were analyzed and discussed in relation to electrical properties. All coatings exhibited stable dielectric parameters versus changing frequency and temperature, comparable with literature values for various samples. Concerning sintered bulks, and especially their thermal stability of capacitance, the plasma sprayed coatings were slightly worse. However due to shape and size variability of the plasma spraying are yttria coatings prospective for technical applications.     

Plasma-sprayed nanostructured YSZ thermal barrier coatings: Thermal insulation capability and adhesion strength

Adhesion strength and thermal insulation of nanostructured Yttria Stabilized Zirconia (YSZ) thermal barrier coatings (TBC) were investigated and compared with those of conventional YSZ TBCs. A Nickel based superalloy (IN-738LC) was used as the substrate with NiCrAlY bond coat, and nanostructured and conventional YSZ top coats were applied by using air plasma spray (APS). The adhesion strength of coatings was evaluated according to ASTM C633-01, and their thermal insulation capability was evaluated using a specially designed test setup at an electrical furnace. The results revealed the nanostructured YSZ coating to have a bimodal microstructure consisting of nanosized particles and microcolumnar grains. The bimodal microstructure of nanostructured coatings prevented crack propagation by splat boundaries and unmelted particles, thereby improving the bonding strength. Also, due to the presence of nano-zones in the microstructure of nano TBCs, coatings exhibited superior thermal insulation capability.     

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.     

Tribological Characterization of WC–Co Plasma Sprayed Coatings

Atmospheric plasma spraying of WC coatings is typically characterized by increased decarburization, with a consequent reduction of their wear resistance. Indeed, high temperature and oxidizing atmosphere promote the appearance of brittle crystalline and amorphous phases. However, by using a high helium flow rate in a process gas mixture, plasma spraying may easily be optimized by increasing the velocity of sprayed particles and by reducing the degree of WC dissolution. To this purpose, a comparative study was performed at different spray conditions. Both WC–Co powder and coating phases were characterized by X-ray difraction. Their microstructure was investigated by scanning electron microscopy. Mechanical, dry sliding friction, and wear tests were also performed. The wear resistance was highly related to both microstructural and mechanical properties. The experimental data confirmed that high-quality cermet coatings could be manufactured by using optimized Ar–He mixtures. Their enhanced hardness, toughness, and wear resistance resulted in coatings comparable to those sprayed by high velocity oxygen-fuel.     

Structure and mechanical properties of plasma sprayed coatings of titania and alumina

The paper concerns the use of traditional and depth-sensing indentation (DSI) for investigation of deposits produced from powders based on conventional and nano-sized particles by plasma spray technology.

Plasma sprayed coatings of titania and alumina were studied. Polished cross-section of each coating was prepared and matrices of nano-indents with Berkovich tip were applied onto both materials to explore local elastic behavior. Applied load was in the range of mN to create indents with the same size scale as the thickness of splats—the main building units of the coating. The hardness value as well as the load/unload curve for each indent was stored. Titania coating was sprayed from a novel type of nanoscale-size powder agglomerated to particles useful for plasma spraying, whereas fused and crushed conventional powder was utilized for alumina spraying and for titania coating used as reference. The effect of annealing on elastic properties of titania was studied as well. The values of elastic parameters as well as the character of the coating inhomogeneity seem to reflect: (i) the composition of material and the fabrication technique and (ii) microstructural differences between coatings that are partly inherited from the feedstock powders. The results of DSI tests are discussed also in comparison with common technique used for the investigation of plasma coatings hardness—Vickers microhardness measurement.     

Microstructure and Heat Transfer Phenomena in Ceramic Thermal Barrier Coatings

Comparably thick Y₂O₃-partially-stabilized ZrO₂ thermal barrier coatings were deposited by two different techniques, air plasma spray (APS) and electron beam physical vapor deposition (EB-PVD), on the same type of substrate. Microstructure and grain texture, as studied by SEM and XRD, were markedly different. The complex microstructure of the APS coatings, made of curled lamellar grains, was replaced in EB-PVD coatings by long columnar grains, aligned along the growth axis, with strong grain texture. Average porosity and other average or intrinsic properties, such as density and specific heat, were nearly the same for all studied coatings; phase composition ranged between 0 and 6 wt% of the m phase in a prevalent t '-phase matrix. The main difference was in the shape and orientation of porosity with respect to the thermal flux direction, which was responsible for the different thermal diffusivity that was three times higher in EB-PVD than in APS coatings. An appropriate modeling of the heat diffusion process, including open and closed porosity with orientation and shape factors, could explain the observed diffusivity values.     

Effects of different nano-agglomerated powders on the microstructures of PS-PVD YSZ coatings

Plasma spray physical vapor deposition (PS-PVD) is a technology that combines the advantages of traditional atmospheric plasma spraying (APS) and electron beam physical vapor deposition (EB-PVD). As the feedstock of the PS-PVD, nano-agglomerated powder is critical on determining the microstructure of the obtained coating. In this study, a method to characterize the cohesion of nano-agglomerated powders was investigated. The nano-agglomerated powders fractured into smaller particles under ultrasonic waves. Their particle size distributions were measured to quantitatively compare their cohesiveness. The change rate in the percentage of powders with particle size less than 5 μm was selected as the value for the cohesion comparison. A high change rate corresponded to a faster fracture and lower powder cohesion. Furthermore, the fracture behavior and heat and mass transfer process of nano-agglomerated powders in the plasma torch were studied by combining 3-D simulation and observation of the microstructures of PS-PVD coatings sprayed with different powders. To obtain a quasi-columnar coating, the nano-agglomerated powder required high cohesion. Finally, a suitable powder was selected and quasi-columnar structure coatings were obtained by optimizing the PS-PVD parameters.