Air Plasma Sprayed Thermal Barrier Coatings Research Articles

Advanced MCrAlY alloys with doubled TBC lifetime

Thermal barrier coatings (TBC) have been used in both aero engines and industrial gas turbines for a few decades. However, the most commonly used MCrAlY bond coats which largely determine air plasma sprayed (APS) TBC lifetime are still deposited by the powders developed in 1980s. APS TBC has been found predominate failure within top coat near interface/or at interface with thermally grown oxides (TGO). TBC lifetime can be increased by either increasing top coat fracture strength (strain tolerance) or reducing the tensile stress in the top coat or both. By focusing on the reduction of tensile stress in the top coats, three new bond coat alloys have been designed and developed. More than doubled TBC lifetime has been achieved by using the new alloys as bond coat. Extended thermal cycle lifetime is attributed to the unique properties of the new alloys, such as remarkably lower coefficient of thermal expansion (CTE), lower weight fraction of β phase, lower aluminum migration rate and the absence of mixed/spinel TGO oxides.

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

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.

Prediction of growth behavior of thermally grown oxide considering the microstructure characteristics of the top coating

Under steady-state conditions, the high-temperature oxidation of a thermal barrier coating (TBC) leads to the formation of a thermally grown oxide (TGO). This growth of TGO is a major cause of damage to the TBC, as it results in delamination of the top coating, which decreases the adhesion strength of the bond coating. In this study, an isothermal degradation test was performed to obtain the growth curve of the TGO of a TBC. The growth behavior of the TGO differs with respect to the microstructure of the top coating. Accordingly, if the growth of the TGO can be predicted universally for air plasma-sprayed (APS) TBCs, the time and economic costs can be saved as additional tests would not be required.In this study, a model that can predict capable of predicting the growth of the TGO on an APS TBC with various microstructures. An isothermal degradation test was performed on three specimens featuring substrates and top coatings with different microstructures. The results confirmed that the growth of the TGO was affected by the microstructure of the top coating, regardless of the type of substrate. A structure constant term accounting for the thickness and porosity of the top coating was defined. Furthermore, the reaction rate constant was derived based on the measured TGO thickness. Using these constants, a novel equation for predicting the growth behavior of the TGO with respect to temperature and the microstructure of the top coating was proposed. The validity of this prediction model was verified through comparisons with the previously reported results.

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.

Dynamic-stiffening-induced aggravated cracking behavior driven by metal-substrate-constraint in a coating/substrate system

Air plasma sprayed thermal barrier coatings (APS-TBCs) saw their wide application in high-temperature-related cutting-edge fields. The lamellar structure of APS-TBCs provides a significant advantage on thermal insulation. However, short life span is a major headache for APS-TBCs. This is highly related to the property changes and passive behaviors of the coatings during thermal service. Herein, a finite element model was developed to investigate the dynamic stiffening and substrate constraint on total spallation process. Results show that the stiffening accelerates the crack propagation of APS-TBCs. The driving force for crack propagation, which is characterized by strain energy release rate (SERR), is significantly enlarged. Consequently, the crack starts to propagate when the SERR exceeds the fracture toughness. In addition, the changing trends of SERR and crack propagation features are highly associated with temperatures. A higher temperature corresponds to more significant effect of stiffening on substrate constraint. In brief, temperature-dependent stiffening significantly aggravates the substrate constraint effect on APS-TBCs, which is one of the major causes for the spallation. Given that, lowering stiffening degree is essential to maintain high strain tolerance, and to further extend the life span of APS-TBCs. This understanding contributes to the development of advanced TBCs in future applications.

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.

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.

The Use of APS Thermal Barrier Coatings in Corrosive Environments

Thermal barrier coatings (TBC) can be used to reduce the metal temperature of gas turbine blades enabling higher Cr alloys (lower strength) to be used when gas turbines are to be used in corrosive environments (where hot corrosion resistance is required). However, the TBC must also be resistant to the corrosive environment and remain attached to the blade. A 1000 h test to evaluate air plasma-sprayed (APS) TBC adhesion to a low-pressure plasma-sprayed CoNiCrAlY bond coat (with and without through thickness cracking) under hot corrosion conditions at 850 °C has been carried out. The APS TBC significantly reduced the hot corrosion rate of the CoNiCrAlY; however, delamination cracking occurred with a thinner thermally grown oxide than would be expected from isothermal and cyclic oxidation testing.

Improved the Durability of Thermal Barrier Coatings with Interface Modified by Three‐Dimensional Mesh Patterns

An approach to improve the lifetime of air plasma sprayed (APS) thermal barrier coatings (TBCs) by modifying the interfacial microstructure has been reported. The laser powder deposition technique was employed to fabricate the mesh structure (with the same composition as the bond coat) at the ceramic–substrate interface. After thermal cycling test, the APS TBCs with the mesh exhibited a much less spallation degree (5%–10%) compared with the reference samples without mesh (>50%), implying that the mesh is effective in impeding the crack propagation along the interface. In addition, the effect of the mesh geometry parameters, e.g., height and spacing of mesh, on the spallation degree of TBCs was also investigated. Based on the results of experiment and calculation, the optimal mesh parameters were proposed.

Delamination Property of Modeled Air Plasma Sprayed-Thermal Barrier Coatings under Shear Loading: Effect of Difference in Chemical Composition of Bond Coat

In order to understand the effect of the difference in chemical composition and microstructure of bond coat (BC) layer on delamination properties, pushout tests were performed on modeled air plasma-sprayed thermal barrier coatings (APS-TBCs). Nickel-platinum-aluminides (Ni-Pt-Al), hafnium modified nickel-platinum-aluminides (Ni-Pt-Al-Hf) and NiCoCrAlY alloys were used as BC alloy. Hafnium and aluminum oxide were formed in needle like shape when the Ni-Pt-Al-Hf alloy was used. Vickers hardness of BC alloy decreased up to 50hours heat exposure and then increased with increasing heat exposure time. Interfacial delamination toughness increased and then decreased with the increase in heat exposure time.

Effects of Heat Exposure Time and Temperature on the Delamination Behavior of Air Plasma-Sprayed Thermal Barrier Coatings under Shear Loading

Effects of Heat Exposure Time and Temperature on the Delamination Behavior of Air Plasma-Sprayed Thermal Barrier Coatings under Shear Loading

Buckling failure in air-plasma sprayed thermal barrier coatings induced by molten silicate attack

The buckling phenomenon in air-plasma sprayed yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) induced by calcium–magnesium-alumino-silicate (CMAS) attack was investigated. The liquid CMAS infiltration due to capillary force weakened the interface and introduced a significant volume expansion (32%) in the top coat, leading to a large scale buckling of the TBC at high temperature. The dissolution and tetragonal-mononiclic phase transformation of YSZ induced by CMAS only has a negligible contribution.

Nondestructive testing of the fatigue properties of air plasma sprayed thermal barrier coatings by pulsed thermography1

The fatigue properties of air plasma sprayed (APS) thermal barrier coatings (TBC) were investigated by quantitative Pulsed Thermography (PT). This analysis principally concerned with microstructural evolution during the thermal cyclic. The microstructural features of the specimens were examined by scanning electron microscopy (SEM). The TBC fatigue life was correlated with the development of the thermally grown oxide (TGO) and defects cracks. For better understanding of the thermal response from the surface of TBC, APS TBC samples were inspected as a function of ageing time by PT. The changes in the thermal response amplitude (TRA) were found to correlate with the TGO layer growth and crack nucleation, which was verified by SEM results. The experimental results demonstrated that the thermal response decreased correspondingly along with the increase of the TGO and cracks thickness. The relationships of TRA obtained from the surface of TBC samples with the thickness of TGO and cracks were established. A simple and effective method for quantitative measuring TGO layer and cracks thicknesses using the nondestructive testing of PT was proposed. The experimental results provide a basis for the application of PT in the nondestructive testing of the fatigue properties of TBC thermal fatigue.

2ZrO2·Y2O3 Thermal Barrier Coatings Resistant to Degradation by Molten CMAS: Part II, Interactions with Sand and Fly Ash

Based on the application of OB considerations (Part I) to various major thermal barrier coating (TBC) compositions and two types of important calcium–magnesium–alumino–silicates (CMAS)—desert sand and fly ash—the 2ZrO2·Y2O3 solid solution (ss) TBC composition, with high CMAS‐resistance potential, is chosen for studying molten‐CMAS/TBC interactions. It is demonstrated that 2ZrO2·Y2O3(ss) air plasma sprayed (APS) TBCs are highly resistant to high‐temperature attack by both sand‐CMAS and fly‐ash‐CMAS. Despite the differences in the compositions of the two CMASs, the overall CMAS‐attack mitigation mechanisms in both cases appear to be similar, viz reaction between 2ZrO2·Y2O3(ss) APS TBC and the CMAS, and the formation of main reaction products of Y‐depleted c‐ZrO2 and nonstoichiometric Ca–Y apatite. Large differences in the OBs (ΔΛ) between the 2ZrO2·Y2O3(ss) and the CMASs are good predictors of ready reaction between this TBC and these CMASs. While the details of the CMAS‐mitigation mechanisms can depend critically on various other aspects, the OB difference (ΔΛ) calculations could be used for the initial screening of CMAS‐resistant TBC compositions.