Thermal Barrier Coatings Surface Research Articles

Influence of YSZ layer thickness on the durability of gadolinium zirconate/YSZ double-layered thermal barrier coatings produced by suspension plasma spray

In this work, three double layered thermal barrier coating (TBC) variations with different gadolinium zirconate (GZ) and YSZ thickness (400GZ/100YSZ, 250GZ/250YSZ and 100GZ/400YSZ respectively, where the prefixed numbers before GZ and YSZ represent the layer thickness in μm), were produced by suspension plasma spray (SPS) process. The objective was to investigate the influence of YSZ thickness on the thermal conductivity and thermal shock lifetime of the GZ/YSZ double layered TBCs. The as sprayed TBCs were characterized using SEM, XRD and porosity measurements. Thermal diffusivity measurements were made using laser flash analysis and the thermal conductivity of the TBCs was calculated. The double layered TBC with the lowest YSZ (400GZ/100YSZ) thickness showed lower thermal diffusivity and thermal conductivity. The double layered TBCs were subjected to thermal shock test at a TBC surface temperature of 1350 °C. Results indicate that the TBC with a higher YSZ thickness (100GZ/400YSZ) showed inferior thermal shock lifetime whereas the TBCs with low YSZ thickness showed comparatively higher thermal shock lifetimes. Failure of the TBCs after thermal shock test was analyzed using SEM and XRD to gain further insights.

Residual stress measurement of Sic fiber/YSZ composite thermal barrier coating by micro-Raman spectroscopy

Residual stress distribution in Sic fiber/YSZ composite thermal barrier coating (TBC) on a stainless substrate has been measured by micro-Raman spectroscopy. Stress-spectroscopic coefficient was calibrated on a independent TBC layer. The coefficient for all TBC system is Σ=33.92cm-1Gpa-1. The stress measurement along the TBC thickness shows compressive stress distribution is decrease progressively from substrate/top coat interface to TBC surface. And the fiber can effectively relaxing the residual stress of composite coating.

Laser Cladding of Embedded Sensors for Thermal Barrier Coating Applications

The accurate real-time monitoring of surface or internal temperatures of thermal barrier coatings (TBCs) in hostile environments presents significant benefits to the efficient and safe operation of gas turbines. A new method for fabricating high-temperature K-type thermocouple sensors on gas turbine engines using coaxial laser cladding technology has been developed. The deposition of the thermocouple sensors was optimized to provide minimal intrusive features to the TBC, which is beneficial for the operational reliability of the protective coatings. Notably, this avoids a melt pool on the TBC surface. Sensors were deposited onto standard yttria-stabilized zirconia (7–8 wt % YSZ) coated substrates; subsequently, they were embedded with second YSZ layers by the Atmospheric Plasma Spray (APS) process. Morphology of cladded thermocouples before and after embedding was optimized in terms of topography and internal homogeneity, respectively. The dimensions of the cladded thermocouple were in the order of 200 microns in thickness and width. The thermal and electrical response of the cladded thermocouple was tested before and after embedding in temperatures ranging from ambient to approximately 450 °C in a furnace. Seebeck coefficients of bared and embedded thermocouples were also calculated correspondingly, and the results were compared to that of a commercial standard K-type thermocouple, which demonstrates that laser cladding is a prospective technology for manufacturing microsensors on the surface of or even embedded into functional coatings.

Laser Multi-mode Scanning Thermography Method for Fast Inspection of Micro-cracks in TBCs Surface

Conventional non-destructive testing methods are difficult to be applied in defect detection of thermal barrier coating (TBCs) because of some of its characteristics, such as porosity and thin thickness, etc. For detecting surface cracks in TBCs, a laser multi-modes scanning thermography (SMLT) method has been developed in this paper, combining fast scan mode using linear laser with fine scan mode using point laser on the tested specimen surface. Linear scanning has a large detection range and detection speed, and point scanning has a higher sensitivity. Through the theoretical analysis, numerical simulation and experimental verification, five unique thermal response features of the cracks stimulated by two scanning modes were discovered and summarized. These features in the thermal images include temperature sharply rising in local region, distinct increase of the area of high temperature zone, obvious ‘tailing’, ‘dislocation’ and thermal obstruction phenomenon, respectively. Therefore, with the corresponding post-processing algorithm developed here, the location and shape of surface cracks in TBCs can be efficiently detected by analyzing the information of these thermal response features. Validation tests showed that the surface cracks with the width of more than $$20\,\upmu \hbox {m}$$ can be quickly detected in line-scan stage, while in point-scan stage, the $$9.5\,\upmu \hbox {m}$$ wide surface cracks can be accurately detected.

Cover Image, Volume 101, Issue 5

3D mesh patterns were deposited on the bond coat surface of thermal barrier coatings using laser powder deposition technique, which could dramatically improve the life of the coatings. The parameters of the mesh, e.g., height, spacing length, are crucial to the durability and failure behavior. (a) Buckling of air‐plasma sprayed thermal barrier coatings with mesh‐patterned substrate after thermal cycling; (b) surface topography measured from the buckled region using optical profilometry; (c) cross‐sectional image showing the coating delamination from the bond coat surface; (d) the height profile across the buckle center (dash line) and the corresponding residual stress along the line. L Luo, X Shan, Y Guo, et al. Thermal Barrier Coatings with Interface Modified by 3D Mesh Patterns: Failure Analysis and Design Optimization. DOI: 10.1111/jace.15386. image

Mechanisms governing the thermal shock and tensile fracture of PS-PVD 7YSZ TBC

Thermal barrier coating (TBC) system including NiCoCrAlYTa metallic bond coating and 7YSZ (7wt%Y2O3-ZrO2) ceramic top coating was deposited on nickel-based superalloy by plasma spray-physical vapor deposition (PS-PVD). Thermal shock property of 7YSZ TBC was characterized by water-quenching test at 1100℃ and its failure behaviors were investigated in detail. Besides, tensile test was performed for TBC sample and its cross-sectional fracture microstructure was studied as well. The results showed that after water-quenching test lots of pitting spallation took place in TBC surface, but no obvious microcracks were observed. Additionally, the tensile test indicated that fracture occurred in 7YSZ coating near the interface of ceramic-bond coating. After conduction of water-quenching and tensile testing, a lot of spherical particles and nano-sized agglomerated clusters were observed in the quasi-columnar structured 7YSZ coating. These lead to the formation of weak inter-column bonding and the failure of PS-PVD 7YSZ TBC. Moreover, in order to better understand the failure process, a deposition mechanism of coating was proposed.

Inverse Analysis of In-Cylinder Gas-Wall Boundary Conditions: Investigation of a Yttria-Stabilized Zirconia Thermal Barrier Coating for Homogeneous Charge Compression Ignition

Thermal barrier coatings (TBCs) applied to in-cylinder surfaces of a low temperature combustion (LTC) engine provide an opportunity for enhanced efficiency via two mechanisms: (i) positive impact on thermodynamic cycle efficiency due to combustion/expansion heat loss reduction, and (ii) enhanced combustion efficiency. Heat released during combustion increases the temperature gradient within the TBC layer, elevating surface temperature over combustion-relevant crank angles. Thorough characterization of this dynamic temperature “swing” at the TBC–gas interface is required to ensure accurate determination of heat transfer and the associated impact(s) on engine performance, emissions, and efficiencies. This paper employs an inverse heat conduction solver based on the sequential function specification method (SFSM) to estimate TBC surface temperature and heat flux profiles using sub-TBC temperature measurements. The authors first assess the robustness of the solution methodology ex situ, utilizing an inert, quiescent environment and a known heat flux boundary condition. The inverse solver is extended in situ to evaluate surface thermal phenomena within a TBC-treated single-cylinder, gasoline-fueled, homogeneous charge compression ignition (HCCI) engine. The resultant analysis provides crank angle resolved TBC surface temperature and heat flux profiles over a host of operational conditions. Insight derived from this work may be correlated with TBC thermophysical properties to determine the impact(s) of material selection on engine performance, emissions, heat transfer, and efficiencies. These efforts will guide next-generation TBC design.

Microstructure and Properties of Peg-nail Structured YSZ TBCs by Selective Laser Modification after Plasma Spraying

Yttria partially stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were deposited by an air plasma spraying (APS) method on nickel-based superalloy with initially sprayed NiCoCrAlYTa bond coat. After that, the surface of the plasma sprayed TBCs was subjected to selective laser modification using a Nd:YAG pulsed laser. The microstructure and performance of the plasma sprayed and peg-nail structured YSZ coatings by selective laser modification were investigated. The results reveal that selective laser modification helps to reduce the surface roughness considerably, eliminate surface porosities and produce a network of continuous cracks perpendicular to the surface. The microstructure of peg-nail structured units consists of columnar grains in the cross-section and equiaxed grains on the surface. XRD patterns show that both as-sprayed and the peg-nail structured coatings by selective laser modification consist of nonequilibrium tetragonal (T') phase; however, monoclinic (M) phase disappeared and intensity of T' phase slightly increases in the range of 72°∼76°(2 θ ) after selective laser modification. It has been found that the average bonding strength YSZ coatings is enhanced from 7.3 MPa to 13.3 MPa after selective laser modification. Thermal insulation capability of the peg-nail structured YSZ coating by selective laser modification, as compared to the as-sprayed YSZ coating, is decreased due to microstructure change in the ceramic top coat.

Estimation of Thermal Barrier Coating Surface Temperature and Heat Flux Profiles in a Low Temperature Combustion Engine Using a Modified Sequential Function Specification Approach

A modified form of the sequential function specification method (SFSM) is developed with specific consideration given to multiple time scales in an effort to avoid overregularization of the solution estimates. The authors extend their approach to solve the inverse heat conduction problem (IHCP) associated with the application of thermal barrier coatings (TBC) to in-cylinder surfaces of an internal combustion engine. Subsurface temperature measurements are used to calculate surface heat flux profiles. The modified inverse solver is validated ex situ using a custom fabricated radiation chamber. The solution methodology is extended in situ to evaluate temperature data collected from a single-cylinder research engine operating in homogeneous charge compression ignition (HCCI) mode. Crank angle resolved, thermal barrier coating surface temperature and heat flux profiles are produced—enabling correlation of thermal conditions at the gas-wall boundary with engine performance, emission, and efficiency metrics.

Predicting the gas-wall boundary conditions in a thermal barrier coated low temperature combustion engine using sub-coating temperature measurements

Low temperature combustion (LTC) engines exhibit potential to significantly reduce fuel consumption and nitric oxide emissions over traditional spark ignited (SI) engines. A prior study has shown that thermal barrier coatings (TBCs) can increase the temperature swing during combustion, thus bolstering both low load LTC operation and its combustion efficiency. However, no attempt has been made so far to maximise the benefits through optimisation of TBC thermal and morphological properties. In this work, a finite element model was developed to expeditiously determine crank-angle resolved TBC surface temperatures across a spectrum of potential TBC thermal and morphological properties and facilitate exploration over a large design space. The simulation was validated using crank-angle resolved temperature and heat flux data from TBC coated, fast-response thermocouples. Experiments were carried out using a metal piston, and a TBC coated piston. Subsequent numerical study characterises the sensitivity of the temperature swing on the surface to TBC conductivity.

Development of the Advanced TBC for High Efficiency Gas Turbine

Since 2004, Mitsubishi has been pursuing a 1,700°C gas turbine as part of the Japanese National Project [1][2]. One of the most important key technologies for the target is thermal barrier coatings (TBCs) which are capable of improving cooling efficiency of hot parts. With increasing the turbine inlet temperature, TBCs surface temperature is also rising up. In addition, the temperature gradient through TBCs thickness must steepen as a result of keeping metal temperature. Both have a significant effect on durability such as spallation and erosion of TBCs. To evaluate these issues, thermal cycle test and hot erosion test were introduced. After the screening of those component tests, the advanced TBCs coated in the first unit of M501J were verified at our pilot plant called T-point. Sound condition for row 1 blades and vanes had been confirmed after over 3 year operation.

On the resistance of rare earth oxide-doped YSZ to high temperature volcanic ash attack

Abstract Thermal barrier coatings (TBCs), such as 7–8 wt.% yttria stabilized zirconia (YSZ), are widely used to reduce the temperature of coated materials. However, when a turbine operates in a harsh environment, for example a volcanic ash attack, sand and ash particles ingested by the engine could be deposited on the TBC surfaces as molten calcium-magnesium-alumino-silicate (CMAS). CMAS melts and penetrates into TBCs at high temperatures, which causes a loss of strain tolerance and results in the premature failure of the top coat. It is recognized that the formation of CMAS is inevitable due to exposing the turbine to sand and ash; therefore, CMAS mitigation solutions are among the top challenges for materials scientists. To some extent, CMAS is the biggest weakness of the traditional 7-8YSZ TBC material. The thermochemical interactions between YSZ and real volcanic ash are investigated in this paper as a means to alleviate the volcanic ash attack by understanding the underlying mechanisms. 7YSZ, 0.5Gd-8YSZ and 3Er-7YSZ powders and free-standing plates were prepared, respectively. The effect of the volcanic ash on the three different samples was investigated using an X-ray diffraction, scanning electron microscopy, a scanning transmission electron microscopy, and a differential scanning calorimetry. The phase transformations and reaction of TBCs materials subjected to a volcanic ash attack were examined and the volcanic ash degradation and mitigation mechanisms were discussed. It was found that rare earth-doped YSZ samples suffered less damage from the volcanic ash attack than the standard 7YSZ. The results demonstrate that rare earth-doped YSZ may be effectively utilized to mitigate a volcanic ash attack.

Study on TBCs insulation characteristics of a turbine blade under serving conditions

It is a key problem to study thermal barrier coatings (TBCs) insulation and followed stresses for the coated blade. This article focused on the insulation characteristics of TBCs by coupling heat transfer and flow with a multilayer blade. We found that the coated blade can benefit more in the decline of average temperature than the decline of maximum temperature, compared to the uncoated case. Temperature fluctuation on TBCs surface is evident. The inlet temperature of main flow (Tin) more than the heat transfer coefficient of cooling passages (hcool) impacted the fluctuation. And there is a non-homogeneous distribution of the temperature decline (ΔT) across the coatings around the blade. At the suction side and the head, ΔT was generally higher than that of the pressure side and the tail. The TBCs thickness and Tin can affect ΔT more than hcool. We suggest that in the sequential TBCs stresses simulation the actual temperature distribution should be prescribed.

Enhanced properties of Al-modified EB-PVD 7YSZ thermal barrier coatings

7wt% yttria-stabilized zirconia (7YSZ) thermal barrier coating (TBC) prepared by electron beam-physical vapor deposition (EB-PVD) has been used in gas turbine engines for many years, where the TBC must successfully withstands the damage caused by a variety of environmental and mechanical aspects. The primary failure modes for TBC are oxidation of bond coating, particle erosion and CMAS (calcium-magnesium-alumina-silicates) corrosion. The lifetime of TBC associated with above three failure factors will be reduced significantly. In order to prolong the operation time, an alternative approach depositing Al film on 7YSZ TBC surface by magnetron sputtering is proposed. An α-Al2O3 overlay was in-situ synthesized on each 7YSZ column through reaction of Al and ZrO2 during vacuum heat treatment. And the results indicate that the Al-modified EB-PVD 7YSZ TBC shows better oxidation resistance, as well as lower particulate erosion and CMAS corrosion.

Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion

Feather-like columnar structured 7YSZ thermal barrier coating (TBC) was prepared by plasma spray-physical vapor deposition (PS-PVD). An aluminum (Al) film was deposited on the TBC surface by magnetron sputtering. The Al-deposited TBC was vacuum heat-treated at 665°C, 808°C and 900°C for 1h separately. Then an α-Al2O3 layer was observed on the top of 7YSZ coating, which resulted from in situ reaction of Al and ZrO2 at high temperature under vacuum condition. The microstructure evolution and phase composition of the TBC were characterized by FE-SEM and XRD respectively. The CMAS (CaO–MgO–Al2O3–SiO2) corrosion property of as-sprayed and treated Al-deposited TBC was compared, which was conducted at 1200°C for 24h. The results show that the Al-deposited TBC after vacuum heat treatment have better CMAS corrosion resistance than the as-sprayed TBC.

Surface Investigation of Laser Glazed Mullite Thin Films on Yttria-Stabilized Zirconia Coatings

Thermal barrier coatings (TBCs) are used in the cooled parts of gas turbines to reduce the temperature of blades and hot engine components, allowing higher operating temperatures and yielding increased efficiency. The corrosive gases which come from combustion of low grade fuels can penetrate into the TBCs and reach the metallic components and bond coat and cause hot corrosion and erosion damage. Mullite thin films have been applied onto the top layer of yttria-stabilized zirconia (YSZ) coatings by sol-gel dipping process in order to decrease the porosity as well as improve thermal stability of the coatings. Glazing the top layer by laser beam is an advanced approach to seal TBCs surface. The laser beam has the advantage of forming a dense thin layer composed of micrograins. CO2 laser beam assisted in the densification of the surface by remelting a thin layer of the exposed surface. The laser glazing converted the rough surface of TBCs into smooth micron-sized grains with size of 1-4 microns. Mullite compounds have remained on the surface after laser treatment since alumina and silica are still present, as indicated in EDX spectra. The results revealed that the roughness increases as the grain size decreases.

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.

CMAS-Resistant Plasma Sprayed Thermal Barrier Coatings Based on Y2O3-Stabilized ZrO2 with Al3+ and Ti4+ Solute Additions

The higher operating temperatures in gas-turbine engines made possible by thermal barrier coatings (TBCs) are engendering a new problem: environmentally ingested airborne silicate particles (sand, ash) melt on the hot TBC surfaces and form calcium-magnesium-alumino-silicate (CMAS) glass deposits. The molten CMAS glass degrades the TBCs, leading to their premature failure. Here, we demonstrate the use of a commercially manufactured feedstock powder, in conjunction with air plasma spray process, to deposit CMAS-resistant yttria-stabilized zirconia-based TBCs containing Al3+ and Ti4+ in solid solution. Results from the characterization of these new TBCs and CMAS/TBCs interaction experiments are presented. The CMAS mitigation mechanisms in these new TBCs involve the crystallization of the anorthite phase. Raman microscopy is used to generate large area maps of the anorthite phase in the CMAS-interacted TBCs demonstrating the potential usefulness of this method for studying CMAS/TBCs interactions. The ubiquity of airborne sand/ash particles and the ever-increasing demand for higher operating temperatures in future high efficiency gas-turbine engines will necessitate CMAS resistance in all hot-section components of those engines. In this context, the versatility, ease of processing, and low cost offered by the process demonstrated here could benefit the development of these new CMAS-resistant TBCs.

Improving the hot corrosion resistance of plasma sprayed ceria–yttria stabilized zirconia thermal barrier coatings by laser surface treatment

Ceria–yttria stabilized zirconia (CYSZ) thermal barrier coatings (TBCs) were deposited by air plasma spraying on NiCoCrAlY-coated Inconel 738LC substrates. After that, the surface of plasma sprayed CYSZ TBCs were glazed using a pulsed Nd:YAG laser. The effects of laser glazing on hot corrosion resistance of the coatings were evaluated in presence of 45wt%Na2SO4+55wt%V2O5 corrosive molten salt at 1000°C. The results revealed that the hot corrosion resistance of plasma sprayed CYSZ TBCs were enhanced more than twofold by laser surface glazing due to reducing specific reactive area of the dense glazed surface layer and consequently, decreasing the reaction between molten salt and zirconia stabilizers.

플라즈마 용사법으로 제작된 4mol% Y2O3-ZrO2열차폐코팅의 화산재에 의한 고온열화거동

The hot corrosion behavior of plasma sprayed 4 mol% Y₂O₃-ZrO₂ (YSZ) thermal barrier coatings (TBCs) with volcanic ash is investigated. Volcanic ash that deposited on the TBCs in gas-turbine engines can attack the surface of TBCs itself as a form of corrosive melt. YSZ coating specimens with a thickness of 430-440 μm are prepared using a plasma spray method. These specimens are subjected to hot corrosion environment at 1200℃ with five different duration time, from 10 mins to 100 h in the presence of corrosive melt from volcanic ash. The microstructure, composition, and phase analysis are performed using Field emission scanning electron microscopy, including Energy dispersive spectroscopy and X-ray diffraction. After the heat treatment, hematite (Fe₂O₃-TiO₂) and monoclinic YSZ phases are found in TBCs. Furthermore the interface area between the molten volcanic ash layers and YSZ coatings becomes porous with increases in the heat treatment time as the YSZ coatings dissolved into molten volcanic ash. The maximum thickness of this a porous reaction zone is 25 μm after 100 h of heat treatment.