Air Plasma Sprayed Thermal Barrier Research Articles

A modified single edge V-notched beam method for evaluating surface fracture toughness of thermal barrier coatings

The surface fracture toughness is an important mechanical parameter for studying the failure behavior of air plasma sprayed (APS) thermal barrier coatings (TBCs). As APS TBCs are typical multilayer porous ceramic materials, the direct applications of the traditional single edge notched beam (SENB) method that ignores those typical structural characters may cause errors. To measure the surface fracture toughness more accurately, the effects of multilayer and porous characters on the fracture toughness of APS TBCs should be considered. In this paper, a modified single edge V-notched beam (MSEVNB) method with typical structural characters is developed. According to the finite element analysis (FEA), the geometry factor of the multilayer structure is recalculated. Owing to the narrower V-notches, a more accurate critical fracture stress is obtained. Based on the Griffith energy balance, the reduction of the crack surface caused by micro-defects is corrected. The MSEVNB method can measure the surface fracture toughness more accurately than the SENB method.

Recognizing Artifacts in Air Plasma Spray Thermal Barrier Coating Microstructures

Abstract Identifying metallographic TSC artifacts, specifically in air plasma spray zirconia-based thermal barrier coatings.

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.

CMAS attack behavior of air plasma sprayed thermal barrier coatings with big pores

Previous studies indicated that big globular pores in air plasma sprayed (APS) thermal barrier coatings (TBCs) are more likely to remain after calcium-magnesium-alumino-silicates (CMAS) attack than are small ones. In this research, therefore, the CMAS attack behavior of an APS TBC with more big globular pores was investigated. It was demonstrated that the more-big-globular-pores APS TBC exhibited better resistance to exfoliation and intergranular corrosion, and chemical degradation than normal APS TBCs, while its pores can be easily filled by CMAS. Analysis on pore structures and CMAS infiltration characteristics indicated that the morphologies of the transition regions between the globular and the crack network pores may play an import role in determining the CMAS filling behavior for globular pores in APS TBCs.

Degradation of Air Plasma-Sprayed Thermal Barrier Coatings Due to Ultra-High Temperature Melt Exposure at ~2500 °C

Degradation of Air Plasma-Sprayed Thermal Barrier Coatings Due to Ultra-High Temperature Melt Exposure at ~2500 °C

Durability of plasma sprayed Thermal Barrier Coatings with controlled properties part II: Effects of geometrical curvature

The present investigation elucidates the effects of substrate curvature on the durability of Air Plasma Sprayed (APS) Thermal Barrier Coatings (TBCs). Traditionally, planar disk specimens are utilized in Furnace Cycle Testing (FCT). In most cases, delamination is initiated at the disk's free edge and then propagates along the TBC-bond coat interface. However, in turbine components (e.g., blades/vanes), they lack significant free edges and the coatings are deposited on non-flat surfaces of varying radii of curvature. These geometrical discrepancies imply significant differences in the stress states as compared to planar disk specimens. Therefore, the part geometry inevitably affects the TBC failure mechanisms in the turbine system, and consequently influences TBC durability. Nevertheless, these effects have not been fully explored in the past, despite the anecdotal knowledge which suggests the leading edge of turbine blades to be one of the most common failure/spallation locations. In this study, representative TBC systems were deposited onto superalloy disks with flat substrate and rods with curved substrate. The experimental results from FCT suggest TBC durability on rods is significantly lower than when sprayed on disks. Furthermore, the durability of TBCs on rods appears to increase with higher porosity, which is contradictory to reported trends for TBCs on disks. In addition, the effects of various bond coats are not consistent with those observed in disks. To clarify the underlying effects of curved geometries, detailed stress analyses were performed. They revealed a state of thermal stresses which is unique to curved geometries. It was found that TBC on rods experience tensile radial stresses as well as tensile hoop stresses during the cooling. These stresses result in a complex interplay in failure processes in rods, which explains the different observed trends in the durability from those obtained with disks.

Effect of bond coat topography on the fracture mechanics and lifetime of air-plasma sprayed thermal barrier coatings

A modified four-point bending test was conducted on air-plasma sprayed (APS) thermal barrier coatings (TBC) to investigate the impact of bond coat (BC) topography tortuosity on the TBC fracture mechanics. The degree of BC tortuosity was quantified by the novel total thresholded summit area surface descriptor (Ssth) and was found to influence the crack path configuration, critical energy release rate (Gc) and crack propagation velocity (νprop). Specimens with higher BC interface tortuosity and more compliant TBC microstructures displayed a reduced strain energy release rates (Gc ≤ 79.90 J. m−2) and crack propagation velocities (vprop ≤ 0.90 mm. s−1). Delamination was primarily governed by the degree of inter-splat cohesion and intra-splat segmentation, although the topography-induced inter-splat tortuosity still had a relevant impact on fracture mechanics. The correlation observed between these results and the lifetime of analogous thermally cycled specimens tested herein suggests that the BC interface can affect the effectiveness of the microstructural stress relaxation mechanisms and by extension the TBC durability.

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

AbstractThe 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.

Influence of crack features on heat transfer characteristic and cracking behaviour of APS-YSZ coating: A numerical simulation study

Spallation of air plasma sprayed (APS) thermal barrier coatings (TBCs) is a decisive factor that dominates the lifetime of APS-TBCs. It is noticeable that coating spallation is correlated with the crack initiation and propagation. The cracks will influence the temperature field and other characteristics of the coating. It is of great importance to sufficiently comprehend and specify these characteristic indexes (such as temperature gradient, thermal conductivity and elastic modulus) of coating changes that the crack evolution brings. In this study, the numerical reconstruction models of TBCs with different major crack lengths and openings were obtained. By using the thermal lattice Boltzmann method (TLBM) and finite element model, the precise temperature field and heat flux distribution of coatings were obtained. The thermal and mechanical properties of coatings were also investigated. The results showed that, the effective thermal conductivity and Young’ modulus of the top coat decrease with the increase of major crack length and opening. At the same time, the energy release rate shows a faster increase for small cracks and its increase trend slows with crack growth. Crack opening further increases the growth of energy release rate. Although the horizontal crack can reduce the thermal conductivity and modulus of the coating, the crack can induce coatings failure. In addition, the extension and expansion of crack will lead to more non-uniform distributed heat flux and larger temperature gradient in the top coat. These results can provide an informed basis for reliable assessment of performance integrity of APS-TBCs. The structural reconstruction and properties analysis models can also be applicable to the field of porous media.

Multifarious Defects Co-Determined Thermal Conductivity in Air Plasma Sprayed Thermal Barrier Coatings

Yttria-stabilized zirconia (YSZ) is the most widely applied thermal barrier coatings (TBCs) materials in gas turbines and aviation engines to prevent components from overheating and to increase the combustion temperature and turbine efficiency. Plasma sprayed coatings usually contain about 15~25% pores, mainly including globular pores, cracks and amorphous phase that to be classified as multifarious and multi-scale defects. They all significantly lower the thermal conductivity, but in drastically different ways depending on their morphologies and orientations. Establishing an accurate correlation between the total porosity and the thermal conductivity requires not only a precise separation and estimation of different kinds of defects but also a reasonable mathematic model to describe their effect on the thermal conductivity. In the present research, cross-section ion polishing and image analysis were chosen as a reliable assembly for visualizing and characterizing multifarious defects of thermal sprayed coatings, through which tiny pores and cracks can be distinctly expressed without any mechanical damage of the microstructure. The effect of different micro-scale defects such as pores and cracks on the thermal conductivity was respectively and quantitatively calculated according to a Bruggeman-based iterative model. Additionally, the nanoscale defects, especially the amorphous phase in the matrix was also taken into consideration, which is proven to be of great importance. The predicted thermal conductivity based on this amorphous-phase-concerned iterative model has achieved a successful evaluation compared to the measured results of coating specimens originated from various spraying processes. This research confirms the applicability of image-analysis-based modeling as a simple, reliable and versatile method for thermal conductivity prediction of porous coating systems.

Interface detection from laser drilling of air plasma-sprayed thermal barrier coatings by laser-induced breakdown spectroscopy

To understand and control the laser machining process of cooling holes on thermal barrier coatings (TBCs) deposited on turbine blades, we introduced interfacial analysis based on laser-induced breakdown spectroscopy (LIBS). An integrated LIBS experimental setup was used to generate a series of craters under different laser shots to simulate the laser machining process. Based on the spectral data from LIBS, three characteristic atomic emission lines of Zr, Al, and Cr were selected for further analysis. We developed and revised a mathematical model based on laser ablation to understand the intensity variation of the three atomic emission lines. The experimental and model results confirmed a sharp intensity decrease of Zr at the ceramic layer/bond coat interface and observable intensity increases of Al and Cr at both the ceramic layer/bond coat and bond coat/substrate interfaces. We propose a first-order derivative method to further quantitatively obtain the interface location that can be used in practical applications.

Study on Sol–Gel Synthesized IN800 Thermal Barrier Coatings Subjected to Thermal Cyclic Loading: Effect of Metallic Substrates

In this work, the effect of sol–gel deposited top-coat on thermal fatigue resistance of thermal barrier coatings (TBCs) subjected to thermal fatigue loading is evaluated experimentally. To obtain non-conventional sol–gel thermal barrier coatings (SGTBC), coated samples underwent thermal fatigue loading at 1100 °C for 10 min heating and cooling. The tested sol–gel thermal barrier coatings were then compared to conventional air plasma sprayed (APS) thermal barrier coatings as well. The life of samples was investigated as a function of number of sustaining thermal cycles to times. Furthermore, the performed experiment was analyzed using scanning electron microscope, energy dispersive spectroscopy and X-ray diffractometer. The obtained results indicated that the non-conventional sol–gel thermal barrier coatings exhibited 1.46 times better thermal fatigue life in IN800SGTBC against 1.31 times thermal fatigue life of IN718SGTBC but overall thermal fatigue life was found to be better in IN718 SGTBC, signified effects of metallic substrates in thermal fatigue life determination. However, the nanostructured SGTBC had higher thermal cyclic resistance than conventional APS TBC, resulted in improved lifetime indicating the increased adherence at the substrate interface. Results also showed that the dominant failure mechanism of TBCs was destabilization of top-coat (YSZ), resulting composition of (Al, Cr)2O3 and spinel as reaction products for depleting Y2O3 producing from yttria stabilized zirconia (YSZ). Furthermore, the results showed that the amount of the porosity percent in the sol–gel TBCs was 2.3% higher than the conventional TBCs. Figure: Material Processing, and thereafter testing analysis for present work.

Investigation of the bond coat interface topography effect on lifetime, microstructure and mechanical properties of air-plasma sprayed thermal barrier coatings

The effect of bond coat/thermal barrier coating (BC/TBC) interface topography on the lifetime of air-plasma sprayed (APS) TBCs was comprehensively investigated in the present work. A quantitative description of the interface topography was achieved by utilising multiple surface texture parameters obtained from confocal microscopy, including a newly formulated parameter, denominated as total thresholded summit area, Ssth. Thermal cycling fatigue (TCF) testing showed a clear correlation between the TBC lifetime and the interface topography, especially for the novel Ssth parameter. The topographical and microstructural analysis revealed that deposition of a TBC over tortuous BC topographies leads to a highly curved TBC splat morphology, which in turn affected the crack path configuration. Mechanical testing of as-deposited and heat-treated specimens evidenced that the aforementioned microstructural changes promoted a reduced local and global elastic moduli, effectively linking the more compliant TBC microstructures with more beneficial fracture mechanics behaviours and higher TCF lifetimes.

Pore filling behavior of air plasma spray thermal barrier coatings under CMAS attack

The pore filling behavior in yttria-stabilized zirconia air plasma sprayed (APS) thermal barrier coatings (TBCs) during calcium-magnesium-alumino-silicate (CMAS) infiltration process was investigated. After fully infiltration, the total porosity of the coating decreased. However, the total porosity of the top region increased due to severe microstructural change. For the bottom region, whose microstructure did not change significantly, its total porosity dropped and almost all crack network disappeared, but 48 vol. % of globular pores still existed and larger globular pores were more likely to remain. This study indicates that introducing relatively large globular pores into APS TBCs may mitigate CMAS damage.

Numerical Investigation into the Effect of Splats and Pores on the Thermal Fracture of Air Plasma-Sprayed Thermal Barrier Coatings

The effect of splat interfaces on the fracture behavior of air plasma-sprayed thermal barrier coatings (APS-TBC) is analyzed using finite element modeling involving cohesive elements. A multiscale approach is adopted in which the explicitly resolved top coat microstructural features are embedded in a larger domain. Within the computational cell, splat interfaces are modeled as being located on a sinusoidal interface in combination with a random distribution of pores. Parametric studies are conducted for different splat interface waviness, spacing, pore volume fraction and fracture properties of the splat interface. The results are quantified in terms of crack nucleation temperature and total microcrack length. It is found that the amount of cracking in TBCs actually decreases with increased porosity up to a critical volume fraction. In contrast, the presence of splats is always detrimental to the TBC performance. This detrimental effect is reduced for the splat interfaces with high waviness and spacing compared to those with low waviness and spacing. The crack initiation temperature was found to be linearly dependent on the normal fracture properties of the splat interface. Insights derived from the numerical results aid in engineering the microstructure of practical TBC systems for improved resistance against thermal fracture.

Stress evolution in ceramic top coat of air plasma-sprayed thermal barrier coatings due to CMAS penetration under thermal cycle loading

Failure in the top coat (TC) induced by CaO-MgO-Al2O3-SiO2 (CMAS) penetration is one of the most serious threats to the performance of air plasma-sprayed thermal barrier coatings (APS TBCs) under thermal cycle loading. Thus, understanding the evolution of CMAS penetration induced stress in TCs under such loading is crucial to prolong the life of APS TBCs. In this study, a multi-layer model of an APS TBCs system incorporating the microstructures of the TC is established to numerically investigate the cyclic stress behavior in the TC during 10 thermal cycles under CMAS attack. The results show that thermal cycle loading aggravates the stress state around the microstructure in the TC during cooling and may further initiate mixed crack types here. Prolonging the holding time of thermal cycles also aggravates the stress state in the TC due to the accumulated unrecoverable creep deformation around the microstructure. The cyclic stress behavior in the TC under different depths of CMAS penetration demonstrates that the TC/bottom coat (BC) interface has a significant influence on the stress level of the CMAS-penetrated interface, especially as the latter approaches the TC/BC interface.

Tracking the calcium-magnesium-alumino-silicate (CMAS) infiltration into an air-plasma spray thermal barrier coating using X-ray imaging

The progressive infiltration of a free-standing air plasma spray (APS) thermal barrier coating (TBC) by calcium-magnesium-alumino-silicate (CMAS) at 1250 °C is followed non-destructively by time-lapse X-ray micro-computed tomography (µ-CT). The top ∼50 µm of the TBC expands rapidly (within the first 20 min) by as much as 70% due to the physical ingress of the CMAS along the splat boundaries and becomes exfoliated. The ingress and resulting exfoliation are shown to be driven by capillary forces with the exfoliated depth dependent on the CMAS loading. The remainder of the TBC swells at a slower rate (ultimately by 15%) but remains intact.

The evolution of fracture process zones in as-received and oxidized air plasma sprayed TBCs

The evolution of the fracture process zones as precursor to fracture was elucidated in as-received and oxidized air plasma sprayed (APS) Thermal Barrier Coating (TBC) specimens. Multiple techniques of Digital Image Correlation (DIC), Acoustic Emission (AE), Finite Element Analysis (FEA) and Scanning Electron Microscope (SEM) micrographs were combined together to obtain in-depth understanding of the failure mechanisms involved in a TBC component under uniform bending stress field. Both sides of specimens were monitored simultaneously during loading and it is shown that within a uniform applied strain, the localized zones of non-symmetric strains with the different lengths were initiated and growth within the ceramic top coat perpendicular to the principle stresses. The process zones were initiated in both as-received and oxidized specimens upon yielding the substrate. However, different strain localization areas were observed on two sides of each specimen. It was observed that separations in each localized zone were resisted by cohesive tractions controlled by the material microstructural features and not the applied external load. One of these localized strains was then rotated parallel to the interface propagating until the final delamination. In the oxidized specimen, the delamination was faster and spallation of top coat ceramic layer followed immediately after cracks reached the bond coat interface. The cohesive properties were determined using a hybrid experimental-numerical approach.

Modelling and analysis of the oxide growth coupling behaviour of thermal barrier coatings

A chemo-transport-mechanics model is developed to study the growth of thermally grown oxide (TGO) and its impact on deformation and stress in air plasma-sprayed thermal barrier coatings (TBCs). As the driving force for oxygen transport, the chemical potential consists of contributions from both species concentration and hydrostatic pressure. The model suggests that both the concentration boundary condition and the transport process of the oxygen are affected by hydrostatic stress. Since oxygen has smaller diffusion coefficient in TGO than in BC, the retarding effect of the formed TGO on oxygen transport is considered and clarified by the coupled model. The competition between geometrical imperfection (i.e. concave morphology) and the chemo-mechanics coupling to influence the transport of oxygen is also identified numerically. The geometrical imperfection can introduce additional oxygen transport at the margin of the concave imperfection due to the horizontal component of the gradient of the chemical potential of the oxygen, which plays a dominant role in the TGO growth kinetics for the studied TBCs. Consequently, there is a limited effect of the chemo-mechanics coupling on the growth kinetics of a concaved TGO. The amplitude change of the concave portion is found to be up to 0.36 µm after 600-h exposure at 1150 °C, which leads to large tensile stress above the concave portion potentially causing micro-cracks.

Determination of local residual stress in an air plasma spray thermal barrier coating (APS-TBC) by microscale ring coring using a picosecond laser

A picosecond laser for incremental annular trench cutting is combined with digital image correlation (DIC) to extend the incremental ring-core method to the profiling of residual stress in thick (>100 μm) coatings. In this case the local residual stress in a TBC is depth profiled after exposure to 1150 °C for 190 h. The topcoat was found to be in compression with an average compressive stress of −94 ± 8 MPa which is representative of the stresses that would be generated elastically on cooling from a stress-free temperature of ~970 °C. The stress profile measurements have been validated by high-energy synchrotron X-ray diffraction measurements.