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28 pages, 10592 KiB

Open AccessArticle

Modelling Thermal Conductivity of Porous Thermal Barrier Coatings

by Ramandeep Singh Ghai, Kuiying Chen and Natalie Baddour

Coatings 2019, 9(2), 101; https://doi.org/10.3390/coatings9020101 - 07 Feb 2019

Cited by 15 | Viewed by 4702

Thermal conductivity of porous thermal barrier coatings was evaluated using a newly developed five-phase model. It was demonstrated that porosities distributed in coating strongly affect thermal conductivity. The decisive reason for this change in thermal conductivity can be traced back to defect morphology [...] Read more.

Thermal conductivity of porous thermal barrier coatings was evaluated using a newly developed five-phase model. It was demonstrated that porosities distributed in coating strongly affect thermal conductivity. The decisive reason for this change in thermal conductivity can be traced back to defect morphology and its orientation, depending on the coating deposition technique and process parameters used during deposition. In this paper, the Bruggeman’s two-phase model was used as a reference, and a five-phase model was developed to evaluate the thermal conductivity of porous coatings. This approach uses microstructural details of the shape, size, orientation and volumetric fraction of defects of coatings as input parameters. The proposed model can predict thermal conductivity values better than the previous two-phase model. Full article

(This article belongs to the Special Issue Thermal Barrier Coatings)

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14 pages, 7988 KiB

Open AccessArticle

Nondestructive Interface Morphology Characterization of Thermal Barrier Coatings Using Terahertz Time-Domain Spectroscopy

by Dongdong Ye, Weize Wang, Jibo Huang, Xiang Lu and Haiting Zhou

Coatings 2019, 9(2), 89; https://doi.org/10.3390/coatings9020089 - 02 Feb 2019

Cited by 30 | Viewed by 4045

Abstract

In this work, a terahertz time-domain spectroscopy (THz-TDS) system was used to measure the thickness of thermal barrier coatings (TBCs) and characterize the interface morphology of TBCs after erosion. Reflection mode, with an angle of incidence of 0, was used for inspection before [...] Read more.

In this work, a terahertz time-domain spectroscopy (THz-TDS) system was used to measure the thickness of thermal barrier coatings (TBCs) and characterize the interface morphology of TBCs after erosion. Reflection mode, with an angle of incidence of 0, was used for inspection before and after erosion. The refractive index, thickness, and internal structure evolution tendency of the yttria-stabilized zirconia (YSZ) top coat were estimated under consideration of the interaction between the pulsed THz waves and the TBCs. The surface roughness of the top coat surface was considered for the errors analysis in the refractive index and thickness measurement. To reduce the errors introduced by the refractive index change after erosion, two mathematical models were built to assess the thickness loss. Then, the thickness loss was compared with results estimated by the micrometer inspection method. Finally, the basic erosion sample profile with R_a roughness was obtained, and the broadening of THz pulses were suggested as a possible measure for the top coat porosity change, showing that THz waves can be a novel online non-destructive and non-contact evaluation method that can be widely utilized to evaluate the integrity of TBCs applied to gas turbine blades. Full article

(This article belongs to the Special Issue Thermal Barrier Coatings)

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19 pages, 11132 KiB

Open AccessArticle

A Trial to Design γ/γ′ Bond Coat in Ni–Al–Cr Mode TBCs Aided by Phase-Field Simulation

by Na Ta, Lijun Zhang and Yong Du

Coatings 2018, 8(12), 421; https://doi.org/10.3390/coatings8120421 - 23 Nov 2018

Cited by 11 | Viewed by 2932

Abstract

Phase-field modeling coupled with calculation of phase diagram (CALPHAD) database was utilized to perform a series of two-dimensional phase-field simulations of microstructure evolution in the γ + γ′/γ + γ′ Ni–Al–Cr mode bond coat/substrate systems. With the aid of phase-field simulated microstructure evolution, [...] Read more.

Phase-field modeling coupled with calculation of phase diagram (CALPHAD) database was utilized to perform a series of two-dimensional phase-field simulations of microstructure evolution in the γ + γ′/γ + γ′ Ni–Al–Cr mode bond coat/substrate systems. With the aid of phase-field simulated microstructure evolution, the relationship between the interdiffusion microstructure and the cohesiveness/aluminum protective property with different alloy compositions and bond coat thicknesses was fully discussed. A semi-quantitative tie-line selection criteria for alloy composition of the bond coat/substrate system with the identical elements, i.e., that the equilibrium Al concentrations of γ′ and γ phases in the bond coat should be similar to those in substrate, while the phase fraction of γ′ in the bond coat tends to be higher than that in the substrate, was then proposed to reduce the formation of polycrystalline structure and thermal shock from the temperature gradient. Full article

(This article belongs to the Special Issue Thermal Barrier Coatings)

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18 pages, 3758 KiB

Open AccessFeature PaperArticle

Quantification of Thermal Barrier Efficiency of Intumescent Coatings on Glass Fibre-Reinforced Epoxy Composites

by Piyanuch Luangtriratana, Baljinder K. Kandola, Sophie Duquesne and Serge Bourbigot

Coatings 2018, 8(10), 347; https://doi.org/10.3390/coatings8100347 - 29 Sep 2018

Cited by 13 | Viewed by 3691

Abstract

In this work, the thermal barrier efficiency of three commercial intumescent coatings of varying thicknesses on glass fibre-reinforced epoxy (GRE) composites has been studied using cone calorimetric parameters and temperature profiles through the thicknesses, obtained by inserting thermocouples in the sample during the [...] Read more.

In this work, the thermal barrier efficiency of three commercial intumescent coatings of varying thicknesses on glass fibre-reinforced epoxy (GRE) composites has been studied using cone calorimetric parameters and temperature profiles through the thicknesses, obtained by inserting thermocouples in the sample during the experiment. The methodologies developed to measure char expansion of the three coatings during the cone experiment as well under slow heating conditions using an advanced rheometric expansion system have been discussed. While the expansion ratios in the two experiments were different, the trends were similar. Thermal conductivities of the chars as a function of time were measured, which could be related to the intumescence steps of respective coatings. The accurate measurements of these parameters are important in predicting the surface requirements of an ideal coating that can enable a given composite structure to survive a defined thermal threat for a specified period of time. Full article

(This article belongs to the Special Issue Thermal Barrier Coatings)

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18 pages, 7042 KiB

Open AccessArticle

Oxidation Simulation of Thermal Barrier Coatings with Actual Microstructures Considering Strength Difference Property and Creep-Plastic Behavior

by Chen Lin, Yijun Chai and Yueming Li

Coatings 2018, 8(10), 338; https://doi.org/10.3390/coatings8100338 - 25 Sep 2018

Cited by 15 | Viewed by 3283

Abstract

A scanning electron microscope (SEM) image based direct finite element (FE) mesh reconstruction method is employed to reflect microstructure features of thermal barrier coatings (TBC). The creep-plastic assumption of thermally grown oxide (TGO) scale and metallic bond coat (BC) as well as the [...] Read more.

A scanning electron microscope (SEM) image based direct finite element (FE) mesh reconstruction method is employed to reflect microstructure features of thermal barrier coatings (TBC). The creep-plastic assumption of thermally grown oxide (TGO) scale and metallic bond coat (BC) as well as the strength difference (SD) property of ceramic top coat (TC) are considered to simulate the mechanical behavior. A diffusion oxidation model considering oxygen consumption is proposed to characterize TGO growth. The oxidation simulation of TBC is carried out under the consideration of actual microstructure features. The results revealed that the interface defects increase the surface-area-to-volume ratio of BC exposed to oxygen anion. This leads to the non-uniform TGO growth, which has also been observed in experimental studies. The microstructures and mechanical behavior strongly affect stress evolution in TBC. The consideration of actual microstructure features and reasonable mechanical behaviors, including the creep-plastic behavior and SD property, is helpful for the accurate evaluation of interface stress. Full article

(This article belongs to the Special Issue Thermal Barrier Coatings)

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12 pages, 1480 KiB

Open AccessArticle

Capturing the Competing Influence of Thermal and Mechanical Loads on the Strain of Turbine Blade Coatings via High Energy X-rays

by Albert Manero, Kevin Knipe, Janine Wischek, Carla Meid, John Okasinski, Jonathan Almer, Anette M. Karlsson, Marion Bartsch and Seetha Raghavan

Coatings 2018, 8(9), 320; https://doi.org/10.3390/coatings8090320 - 10 Sep 2018

Cited by 9 | Viewed by 4316

Abstract

This paper presents findings of synchrotron diffraction measurements on tubular specimens with a thermal barrier coating (TBC) system applied by electron beam physical vapor deposition (EB-PVD), having a thermally grown oxide (TGO) layer due to aging in hot air. The diffraction measurements were [...] Read more.

This paper presents findings of synchrotron diffraction measurements on tubular specimens with a thermal barrier coating (TBC) system applied by electron beam physical vapor deposition (EB-PVD), having a thermally grown oxide (TGO) layer due to aging in hot air. The diffraction measurements were in situ while applying a thermal cycle with high temperature holds at 1000 °C and varying internal air cooling mass flow and mechanical load. It was observed that, during high temperature holds at 1000 °C, the TGO strain approached zero if no mechanical load or internal cooling was applied. When applying a mechanical load, the TGO in-plane strain (e22) changed to tensile and the out of plane TGO strain (e11) became compressive. The addition of internal cooling induced a thermal gradient, yielding a competing effect, driving the e22 strain to compressive and e11 strain to tensile. Quantifying TGO strain variations in response to competing factors will provide a path to controlling the TGO strain, and further improving the lifetime assessment and durability design strategies for TBC systems. Full article

(This article belongs to the Special Issue Thermal Barrier Coatings)

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14 pages, 3441 KiB

Open AccessArticle

Structure and Properties of High-Hardness Silicide Coatings on Cemented Carbides for High Temperature Applications

by Samuel Humphry-Baker and Jessica Marshall

Coatings 2018, 8(7), 247; https://doi.org/10.3390/coatings8070247 - 12 Jul 2018

Cited by 6 | Viewed by 4977

Abstract

Cemented tungsten carbides (cWCs) are routinely used in mining and manufacturing but are also candidate materials for compact radiation shielding in fusion power generation. In both applications, there is a need for oxidation to be minimized at operating temperatures. In a recent study, [...] Read more.

Cemented tungsten carbides (cWCs) are routinely used in mining and manufacturing but are also candidate materials for compact radiation shielding in fusion power generation. In both applications, there is a need for oxidation to be minimized at operating temperatures. In a recent study, Si-based coatings deposited by pack cementation were demonstrated to improve the oxidation resistance of cWCs by up to a factor of 1000. In this work, these coatings are further characterized, with the focus on growth kinetics, phase composition, and hardness. By combining quantitative X-ray diffraction, electron microscopy, and instrumented micro-indentation, it is shown that the coating layer has a 20% higher hardness than the substrate, which is explained by the presence of a previously-unknown distribution of very hard SiC laths. To interpret the coating stability, a coating growth map is developed. The map shows that the structure is stable under a broad range of processing temperatures and cWC compositions, demonstrating the wide-ranging applicability of these coatings. Full article

(This article belongs to the Special Issue Thermal Barrier Coatings)

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11 pages, 5281 KiB

Open AccessArticle

Laser Cladding of Embedded Sensors for Thermal Barrier Coating Applications

by Yanli Zhang, Daniel Emil Mack, Georg Mauer and Robert Vaßen

Coatings 2018, 8(5), 176; https://doi.org/10.3390/coatings8050176 - 04 May 2018

Cited by 10 | Viewed by 5866

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Thermal Barrier Coatings)

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17 pages, 28603 KiB

Open AccessArticle

Investigation of Element Effect on High-Temperature Oxidation of HVOF NiCoCrAlX Coatings

by Pimin Zhang, Ru Lin Peng, Xin-Hai Li and Sten Johansson

Coatings 2018, 8(4), 129; https://doi.org/10.3390/coatings8040129 - 03 Apr 2018

Cited by 9 | Viewed by 5529

Abstract

MCrAlX (M: Ni or Co or both, X: minor elements) coatings have been used widely to protect hot components in gas turbines against oxidation and heat corrosion at high temperatures. Understanding the influence of the X-elements on oxidation behavior is important in the [...] Read more.

MCrAlX (M: Ni or Co or both, X: minor elements) coatings have been used widely to protect hot components in gas turbines against oxidation and heat corrosion at high temperatures. Understanding the influence of the X-elements on oxidation behavior is important in the design of durable MCrAlX coatings. In this study, NiCoCrAlX coatings doped with Y + Ru and Ce, respectively, were deposited on an Inconel-792 substrate using high velocity oxygen fuel (HVOF). The samples were subjected to isothermal oxidation tests in laboratory air at 900, 1000, and 1100 °C and a cyclic oxidation test between 100 and 1100 °C with a 1-h dwell time at 1100 °C. It was observed that the coating with Ce showed a much higher oxidation rate than the coating with Y + Ru under both isothermal and cyclic oxidation tests. In addition, the Y + Ru-doped coating showed significantly lower β phase depletion due to interdiffusion between the coating and the substrate, resulting from the addition of Ru. Simulation results using a moving phase boundary model and an established oxidation-diffusion model showed that Ru stabilized β grains, which reduced β-depletion of the coating due to substrate interdiffusion. This paper, combining experiment and simulation results, presents a comprehensive study of the influence of Ce and Ru on oxidation behavior, including an investigation of the microstructure evolution in the coating surface and the coating-substrate interface influenced by oxidation time. Full article

(This article belongs to the Special Issue Thermal Barrier Coatings)

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5433 KiB

Open AccessArticle

Development of YSZ Thermal Barrier Coatings Using Axial Suspension Plasma Spraying

by Dapeng Zhou, Olivier Guillon and Robert Vaßen

Coatings 2017, 7(8), 120; https://doi.org/10.3390/coatings7080120 - 10 Aug 2017

Cited by 75 | Viewed by 11330

Abstract

The axial injection of the suspension in the atmospheric plasma spraying process (here called axial suspension plasma spraying) is an attractive and advanced thermal spraying technology especially for the deposition of thermal barrier coatings (TBCs). It enables the growth of columnar-like structures and, [...] Read more.

The axial injection of the suspension in the atmospheric plasma spraying process (here called axial suspension plasma spraying) is an attractive and advanced thermal spraying technology especially for the deposition of thermal barrier coatings (TBCs). It enables the growth of columnar-like structures and, hence, combines advantages of electron beam-physical vapor deposition (EB-PVD) technology with the considerably cheaper atmospheric plasma spraying (APS). In the first part of this study, the effects of spraying conditions on the microstructure of yttria partially-stabilized zirconia (YSZ) top coats and the deposition efficiency were investigated. YSZ coatings deposited on as-sprayed bond coats with 5 wt % solid content suspension appeared to have nicely-developed columnar structures. Based on the preliminary results, the nicely developed columnar coatings with variations of the stand-off distances and yttria content were subjected to thermal cycling tests in a gas burner rig. In these tests, all columnar structured TBCs showed relatively short lifetimes compared with porous APS coatings. Indentation measurements for Young’s modulus and fracture toughness on the columns of the SPS coatings indicated a correlation between mechanical properties and lifetime for the SPS samples. A simplified model is presented which correlates mechanical properties and lifetime of SPS coatings. Full article

(This article belongs to the Special Issue Thermal Barrier Coatings)

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6 pages, 566 KiB

Open AccessCommentary

Revisiting the Birth of 7YSZ Thermal Barrier Coatings: Stephan Stecura ^†

by James L. Smialek and Robert A. Miller

Coatings 2018, 8(7), 255; https://doi.org/10.3390/coatings8070255 - 22 Jul 2018

Cited by 41 | Viewed by 5401

Abstract

Thermal barrier coatings are widely used in all turbine engines, typically using a 7 wt.% Y₂O₃–ZrO₂ formulation. Extensive research and development over many decades have refined the processing and structure of these coatings for increased durability and reliability. [...] Read more.

Thermal barrier coatings are widely used in all turbine engines, typically using a 7 wt.% Y₂O₃–ZrO₂ formulation. Extensive research and development over many decades have refined the processing and structure of these coatings for increased durability and reliability. New compositions demonstrate some unique advantages and are gaining in application. However, the “7YSZ” (7 wt.% yttria stabilized zirconia) formulation predominates and is still in widespread use. This special composition has been universally found to produce nanoscale precipitates of metastable t’ tetragonal phase, giving rise to a unique toughening mechanism via ferro-elastic switching under stress. This note recalls the original study that identified superior properties of 6–8 wt.% yttria stabilized zirconia (YSZ) plasma sprayed thermal barrier coatings, published in 1978. The impact of this discovery, arguably, continues in some form to this day. At one point, 7YSZ thermal barrier coatings were used in every new aircraft and ground power turbine engine produced worldwide. 7YSZ is a tribute to its inventor, Dr. Stephan Stecura, NASA retiree. Full article

(This article belongs to the Special Issue Thermal Barrier Coatings)

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