Coating Surface Temperature Research Articles

High near-infrared reflectance Zn1-xMnxO yellow pigment prepared and properties by co-precipitation method

A successful synthesis of yellow pigments, namely Zn1-xMnxO (x = 0.001–0.009), was achieved using the co-precipitation method. To characterize the prepared pigments, various analytical techniques were employed, including powder X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric-differential scanning calorimetry (TG-DSC). The results showed that the Zn1-xMnxO series pigments prepared by co-precipitation method retained the ZnO fibrous zincite structure and all had a good yellow color (b*=30.14–40.56) and high NIR solar reflectance (80.66 %-87.80 %). The results of the simulated heat shielding experiments confirmed that the Zn0.997Mn0.003O (abbreviated as ZM3) pigment synthesized in the study was 5°C cooler than the commercial iron yellow pigment, and the thermal imaging experiments showed that the surface temperature of the sample coatings was reduced by 8°C, which further confirmed that the ZM3 pigment had good heat shielding properties. Furthermore, the pigment demonstrates excellent chemical and thermal stability. and can be used as a cold pigment in architectural coatings to obtain energy-saving effects, which has great potential for development.

Failure mechanisms for Gd2O3–Yb2O3 co-doped YSZ thermal barrier coatings under high-temperature gradient

In this study, Gd2O3–Yb2O3 co-doped YSZ thermal barrier coatings with a top coat thickness of 400 μm were prepared on a nickel-based superalloy by atmospheric plasma spraying. The samples underwent multiple short-term and single long-term thermal exposures through burner rig tests under high-temperature gradients. The short-term thermal exposure duration of the coating was 25 s and the long-term thermal exposure duration was 1200 s. The microstructures and phase compositions of the samples were characterized, and their failure mechanism was discussed. The results demonstrate that during the burner rig tests, the coating surface temperature reached 2600K, resulting in coating sintering and sintered zone formation comprising coarse equiaxed grains. The short-term thermal exposure life of the coating is 3 cycles, and the long-term life is more than 1200s. After the single 1200s thermal exposure, the coating exhibited a stable cubic phase. However, under multiple 25s thermal exposures, the depth of the sintered zone increased with increasing number of cycles, leading to the formation of various vertical and transverse cracks within the coating. Coating failure was due to a combination of sintering and thermal-mismatch-induced stress.The early-stage coating delamination was attributed to sintering-induced transverse cracks, while the overall spallation was primarily ascribed to thermal-mismatch-induced transverse cracks.

Multi-functional and durable anti-corrosion coatings with hydrophobic, freeze time retardation and photothermal properties by means of a simple spraying method

In this study, a composite material consisting of perfluorinated quaternary silane-modified graphene oxide and modified CeO2 nanoparticles was prepared, and a hydrophobic, corrosion-resistant, self-cleaning and anti-icing composite coating was prepared by combining the composite material with polyphenylene sulfide (PPS) through high-temperature curing in a simple spraying method for the protection of iron sheets. The results show that the composite coating has excellent corrosion resistance, strong mechanical properties and certain photothermal properties. When the content of perfluorinated quaternary silane modified CeO2 nanoparticles with graphene oxide (P-GKC) was 10 wt%, the composite coating had a water contact angle of 145.6 ± 3° and a sliding angle of 5 ± 1°, with good hydrophobicity and low surface energy. Meanwhile, the hydrophobic modification of perfluoroquaternary silanes reduced the surface energy of the particles in the composite coatings and greatly improved the hydrophobicity. Surface temperature and laser irradiation tests of the composite coatings showed that the photothermal properties of the composite coatings were better than those of the single PPS and graphene oxide (GO) coatings, with good photothermal cycling stability.The Tafel curves and the EIS results showed that the PPS/10 wt% P-GKC composite coatings had the best corrosion resistance, with a corrosion current density of 5.01 × 10−9A cm−2. After 200 h of salt spray test in 3.5 % NaCl environment, the PPS/10 wt% P-GKC composite coating still has good substrate protection effect, which provides a practical basis for the research of composite coating anticorrosion.

Solar reflective superhydrophobic coatings with phase change function

In this work, white Portland cement (WPC) based composite coatings consisting of WPC, building glue, phase change microcapsules, TiO2 particles, polymethylhydrosiloxane, and SiO2 particles were developed via a brushing method. After adjusting the variety and content of additives, the optimized WPC coating (PCM30S/WPCM) exhibits a phase transition temperature of 35 °C, solar reflectivity of 0.764, contact angle of 157.6°, and rolling angle of 7.1°. The coating shows stable superhydrophobicity in the corrosion test, mechanical abrasion test, ultraviolet (UV) aging experiment, and temperature fluctuation experiment. In addition, the superhydrophobic WPC composite coating exhibits a significant cooling effect of 8.5 °C under simulated solar radiation. Furthermore, the phase change coating can also prevent large fluctuation of coating surface temperature through phase change function, so as to improve building comfort. The simulation analysis indicates that the energy saving rate of the superhydrophobic phase change coating is the highest in Guangzhou, China, which is 23.2%.

Temperature Effects on Two-Cell Field Impedance Testing of Epoxy and Polysulfide-Modified Epoxy Coatings

Two-cell field impedance spectroscopy provides coating condition assessment in situ, i.e., on coated structures. This study evaluated the dependence of impedance data on environmental conditions encountered during testing, especially temperature. Field impedance spectroscopy measurements were applied to two coated steel substrates over the course of a diurnal heating-cooling cycle. One coated substrate provided a high-barrier epoxy and the second a low-barrier polysulfide-modified epoxy (polysulfide). The temperature range examined was approximately 80 degrees Fahrenheit (ºF) to 110ºF for air, surface, and solution temperature, with baseline measurements made at 70ºF. The results showed coating impedance decreases as the coating surface temperature increases. The coatings showed time-dependent recovery of their initial properties during the cooling cycle, with the greater hysteresis observed by the high-barrier epoxy. Linear regression of temperature and impedance provided a strong correlation, with R2 exceeding 0.80. Therefore, field impedance data with varied temperatures can be corrected via a linear regression formula derived by measuring one sample at two or more temperatures within the encountered range. Coating surface temperature is preferred based on a stronger correlation with impedance than atmospheric or test cell solution temperatures. An Arrhenius relationship is also acceptable for temperature correction and may be preferred where the highest accuracy is needed. This correlation had an R2 of 0.96 or higher, demonstrating a logarithmic dependence of impedance on the inverse of the absolute temperature, i.e., the temperature in Kelvin. The effect of humidity on the impedance was low and could not be resolved from the strongly correlated effect of temperature. The repeated impedance testing, about 40 repetitions at a single location, applied in this experiment showed no discernable effect on the coating impedance, suggesting the voltage amplitudes utilized of 100 mV for low-barrier polysulfide and 500 mV for high-barrier epoxy were nondestructive.

Super-amphiphobic coatings with sub-ambient daytime radiative cooling—Part 2: Cooling effect under real conditions

The cooling effect of a super-amphiphobic self-cleaning radiative cooling coating (RC coating) on various substrates was systematically studied under real working conditions and compared with that of conventional cool white coatings. Under direct sunlight, the surface temperature of the RC coatings was always below the ambient air temperature, even up to 12.9 °C on concrete, whereas that of the conventional cool white coatings on concrete reached 4.1 °C above the ambient air temperature. When the RC coatings were applied to industrial facilities such as carbon-steel-based substation cabinets, the temperatures of the coated ceiling surface and interior remained below the ambient temperature, yielding an interior temperature reduction of 13.7 °C relative to the temperature of an unpainted substation cabinet. When applied to outdoor living facilities, including safety helmets, sunshades, and Roman tents with open and closed curtains, the RC coatings could greatly improve the thermal comfort either by reducing the air temperature beneath the open facilities or by significantly lowering the interior temperature of the closed space of the living supplies. Upon application to concrete buildings, the RC coatings reduced the temperatures of the concrete surface, roof surface, and room interior by 25.7, 12.9, and 11.5 °C below the ambient air temperature, respectively, the decrease in the latter two being 9.8 and 4.8 °C, respectively, relative to the buildings coated with 0.88-solar-reflectance conventional cool white coatings. The measured sub-ambient cooling effect agreed with the theoretical prediction. In summary, RC coatings have wide application prospects in cooling energy conservation and emission reduction. Superamphiphobic self-cleaning radiative cooling coatings were applied to industrial and basic facilities, outdoor living supplies and real buildings. The coating surface temperatures over different basements were below ambient air temperature day and night. The air temperatures beneath the coated roofs/ceilings of facilities in open and closed environments were at least 2.5 °C and 9.0 °C lower than those beneath the conventional white facilities in open and closed environments, respectively. • The subambient daytime radiative cooling coatings were applied in various fields. • The coating greatly reduced outdoor products' surface and interior temperature. • The coating enabled the metals' surface temperature to be below that of the ambient. • The coating greatly reduced concrete buildings' surface and interior temperature.

Application of Nanoporous Super Thermal Insulation Material in the Prevention and Control of Thermal Hazards in Deep Mining of Metal Mines

As an important energy material, mineral resources have an important role in the rapid growth of the national economy. In recent years, with the enhancement of mining strength and the improvement of mechanized production level, the depth of mining has also increased year by year. Therefore, the number of heat hazards in deep mine mining has increased year by year, and the problem of heat loss in deep mines has become increasingly prominent, which has seriously affected the safe and efficient production of mines. In this paper, the application of nanoporous super insulation materials in the prevention of thermal damage in deep mining of metal mines is studied, and the related theories of nanoporous super insulation materials and the prevention of thermal damage in deep mining of metal mines are understood on the basis of literature data. Nanoporous thermal insulation materials refer to thermal insulation materials with pore diameters in the nanometer range. This material can effectively prevent the collision of gas molecules and prevent the heat conduction of the gas. Then, the application experiment of nanoporous super thermal insulation materials in the prevention and control of heat damage in deep mining of metal mines is carried out. Through experiments, it is concluded that the room temperature thermal conductivity of the experimental material decreases with the increase of the addition of SiO2 aerogel, and the downward trend is linear, and when the heat source temperature is 200°C, the coating surface temperature of the thermal insulation material (excluding aerogel) is 100°C, and the thermal insulation temperature difference is 100°C, while when the coating surface temperature of the aerogel thermal insulation coating is only 60°C, the thermal insulation temperature difference is 140°C. From these data, it can be seen that the thermal insulation performance of nanoporous super thermal insulation materials is better, which verifies the feasibility of its application in the prevention of heat damage in metal mine mining.

Microstructure and Ablation Behavior of Low-Pressure Plasma Sprayed ZrB2 Coatings Down to 100 Pa.

The effect of chamber pressure on the microstructure and ablation behavior of ZrB2 coatings deposited by low-pressure plasma spraying was investigated. The results showed that as the spray chamber pressure further was reduced to less than 50 kPa, the porosity of the coating deposited at the same distance decreased with the chamber pressure, and the coating prepared under 100 Pa presented the lowest porosity of about 0.89%. The ablation performance test subjected to high-temperature plasma jet revealed that the linear ablation rate of ZrB2 coating increased with the porosity of the coating. As a result, among the ZrB2 coatings deposited at chamber pressures of 100 Pa, 5 kPa, 10 kPa and 50 kPa, the dense coating deposited at 100 Pa showed the lowest ablation rate of 0.33 µm/s. The dense ZrB2 coating with a thickness of about 100 μm was able to withstand 300 s ablation by a plasma flame with a net power of 25 kW resulting in an ablating coating surface temperature of about 2000 °C. The ablation mechanism of the coating was also examined.

The Effect of Thermal Barrier Coating Surface Temperature on the Adhesion Behavior of CMAS Deposits

Abstract The interaction of thermal barrier coating’s surface temperature with calcium magnesium aluminosilicate (CMAS) like deposits in gas turbine hot flowpath hardware is investigated. Small Hastelloy X coupons were coated in thermal barrier coatings (TBC) using the air plasma spray (APS) method and then subjected to a thermal gradient via backside impingement cooling and frontside impingement heating using the High-Temperature Deposition Facility (HTDF) at The Ohio State University (OSU). A one-dimensional (1D) heat transfer model was used to estimate TBC surface temperatures and correlate them to intensity values taken from infrared (IR) images of the TBC surface. TBC frontside surface temperatures were varied by changing backside mass flow (kept at a constant temperature) while maintaining a constant hot-side gas temperature and jet velocity representative of modern commercial turbofan high-pressure turbine (HPT) inlet conditions (approximately 1600 K and 200 m/s, or Mach 0.25). In this study, Arizona Road Dust (ARD) was utilized to mimic the behavior of CMAS attack on TBC. To identify the minimum temperature at which particles adhere, the backside cooling mass flow was set to the maximum amount allowed by the test setup, and trace amounts of 0–10 µm ARD particles were injected into the hot-side flow to impinge on the TBC surface. The TBC surface temperature was increased through coolant reduction until noticeable deposits formed, as evaluated through an IR camera. Accelerated deposition tests were then performed where approximately 1 g of ARD was injected into the hot side flow while the TBC surface temperature was held at various points above the minimum observed deposition temperature. Surface deposition on the TBC coupons was evaluated using an infrared camera and a backside thermocouple. Coupon cross-sections were also evaluated under a scanning electron microscope for any potential CMAS ingress into the TBC. Experimental results of the impact of surface temperature on CMAS deposition, and deposit evolution and morphology are presented. In addition, an Eulerian–Lagrangian solver was used to model the hot-side impinging jet with particles at four TBC surface temperatures and deposition was predicted using the OSU deposition model. Comparisons to experimental results highlight the need for more sophisticated modeling of deposit development through a conjugate heat transfer and mesh morphing of the target surface. These results can be used to improve physics-based deposition models by providing valuable data relative to CMAS deposition characteristics on TBC surfaces, which modern commercial turbofan high-pressure turbines use almost exclusively.

Energy saving performance of thermochromic coatings with different colors for buildings

Thermochromic coatings are promising in lowering air conditioning energy consumption of buildings. In this paper, thermochromic coatings with five different colors are prepared and their optical properties are measured. A two-story office building model is established and simulation is conducted to evaluate the energy performance of thermochromic coatings by using a three-step control method based on their dynamic color-changing properties. A comparative analysis is carried out for thermochromic coatings, cool coating and common coating. It is observed that when the building is located in Shanghai, average daily surface temperature of thermochromic coatings is lower than the common coating during cooling period and higher than the cool coating during heating period. Annual energy consumption results indicate that thermochromic coatings can avoid the penalty of increasing heating load for cool coatings. In particular, the annual energy consumption of the office building using thermochromic coatings can be reduced by 4.28–5.02 kWh/m2 compared to the common coating, and 0.73–1.47 kWh/ m2 compared to the cool coating. Simulation of other cities in different climate zones in China reveals that thermochromic coatings have better energy saving effects especially for the areas with hot summers and cold winters, and relatively uniform distribution of cooling and heating demands.

Molecular Simulations of Sputtering Preparation and Transformation of Surface Properties of Au/Cu Alloy Coatings Under Different Incident Energies

The surface properties of coatings during deposition are strongly influenced by temperature, particle fluxes, and compositions. In addition, the precursor incident energy also affects the surface properties of coatings during sputtering. The atomistic processes associated with the microstructure of coatings and the surface morphological evolution during sputtering are difficult to observe. Thus, in the present study, molecular dynamics simulation was employed to investigate the surface properties of Au/Cu alloy coatings (Cu substrate sputtering by Au atoms) with different incident energies (0.15 eV, 0.3 eV, 0.6 eV). Subsequently, the sputtering depth of the Au atoms, the particle distribution of the Au/Cu alloy coating system, the radial distribution function of particles in the coatings, the mean square displacement of the Cu atoms in the substrate, and the roughness of the coatings were analyzed. Results showed that the crystal structure and the sputtering depth of Au atoms were hardly influenced by the incident energy, and the incident energy had little impact on the motion of deep-lying atoms in the substrate. However, higher incident energy resulted in higher surface temperature of coatings, and more Au atoms existed in the coherent interface. Moreover, it strengthened the motion of Cu atoms and reduced the surface roughness. Therefore, the crystal structure of coatings and the motions of deep-lying atoms in the substrate are not influenced by the incident energy. However, the increase in incident energy will enhance the combination of coatings and the base while optimizing the surface structure.

The effect of γ-ray irradiation on deuterium permeation through reduced activation ferritic steel and erbium oxide coating

Deuterium permeation through a fusion-relevant ferritic steel F82H with and without erbium oxide coatings under γ-ray irradiation has been investigated in a temperature range from 300 to 700 °C. The deuterium permeation flux through F82H sample increased by γ-ray irradiation at lower temperatures below 450 °C. The irradiation effect increased with dose rate, and the percentage of the permeation flux gain might be several percent under the dose rate of a few Gy s‒1. Temperature of the F82H sample surface rose by about 0.5 °C depending on the dose rate, and so the γ-ray irradiation effect is mainly attributed to γ-heating. On the other hand, at higher temperature above 500 °C, no appreciable change of the deuterium permeation was observed. Similarly, the deuterium permeation flux through erbium oxide coated samples increased under γ-ray irradiation at lower temperatures (350‒450 °C), but no appreciable change of permeation flux through coatings was observed at higher temperatures (600‒700 °C). The coating surface temperature increased at lower sample temperatures by γ-heating.

Preparation, characterization and thermal performance evaluation of coating colored with NIR reflective pigments: BiVO4 coated mica-titanium oxide

A series of environmentally nontoxic near-infrared reflective pigments of BiVO4 coated mica-titanium oxide were synthesized. The pigments were characterized by XRD, FE-SEM, UV–vis-NIR spectrophotometer and CIE L∗ a∗ b∗ color scales. The results illustrated that the BiVO4 particles with a size of ∼500 nm were firmly adhered to mica-titanium oxide substrate. Additionally, the composite pigments change the color from white to bright yellow with the increase of the BiVO4 loadings. All the pigments had higher NIR reflectance than conventional yellow pigment (≥70.0%). In the simulated experiment, the temperature difference of the inner surface temperature of coatings and the interior air temperature of boxes reached 4.5 °C and 2.3 °C, respectively, indicating that the good thermal insulation performance of the sample coating (30% BiVO4) could be used as building roof material to reduce the heat buildup and decrease the indoor temperature. Furthermore, we also evaluated aging resistance of the pigments. In conclusion, BiVO4 coated mica-titanium oxide composite pigments have great potential in serving as cool materials for building coatings and they also meet energy-saving and environmentally friendly requirements.

Disintegration of Composite Electroexplosive Coatings in Spark Erosion

A mathematical model is proposed for the destruction of electroexplosive W–Cu, Mo–Cu, W‒C–Cu, Mo–C–Cu, Ti–B–Cu, and TiB2–Cu composite coatings under spark erosion associated with the opening of electrical contacts. The model describes the evaporation of the electrodes under the action of the heat flux due to sparking when the electrical contacts open. In this model, the resistance of the electrical contacts during the tests is assumed to be 40–50 μΩ. The model corresponds to the heating of a half-space by a normal pulsed surface heat source uniformly distributed over an area of specified radius, with a specified heating time. The energy distribution of the pulse over time is approximated by a rectangular pulse. The finite pulse length is taken into account in solving the heat-conduction equation in cylindrical coordinates for a plane instantaneous heat source. The vapor pressure of the metal is determined from the surface temperature. The calculations assume a contact voltage of 380 V; a current of 3 A; a spark discharge of length 150 μs; and a discharge–surface contact spot of radius 152 μm. The following parameters are determined: the surface temperature of pure electrodes; the surface temperature of the composite coatings; the depth of the evaporation layer for pure electrodes; the mass loss of the composite coating after a single discharge pulse; the relative change in bulk resistance of the pure electrodes and composite coatings to electroerosion; and the relative change in mass resistance of the pure electrodes and composite coatings to electroerosion. The content of the elements in the composite coating is determined. The model results are in good agreement with experimental data, especially for the ternary systems W–C–Cu, Mo–C–Cu, and Ti–B–Cu. The results are also in adequate agreement with literature data. For the binary systems W–Cu and Mo–Cu, the discrepancies may be attributed to the approximations made in the model.

Multi-layered thermal barrier coatings on BMI polyimide matrix composite

Al/YSZ and (Cu-6Sn)/YSZ as the double layer thermal barrier coatings (TBCs) and Cu-6Sn/NiCrAlY/YSZ as the triple layer TBC with 100μm thick for bonding layers and ceramic top coat were deposited on bismaleimide (BMI) matrix composite substrates. The results showed the coherent and clean substrate/coating interface in the triple layer coating system. On the other hand, significant pores, cavities and cracks were appeared throughout substrate/bond coat interfaces when double layer coating systems were created on the polymeric substrate. Fourier Transform Infra-Red (FTIR) spectroscopy showed that decomposition of polymeric substrate took place for double layer coating system in spite of that for the triple layer coating system. The results of laser flash analysis showed that lower thermal diffusivity value of Cu-6Sn/NiCrAlY bonding layer in the triple layer coating system than the two other bonding layers in double layer coating systems may result in lower heat flux transmitted to the substrate during ceramic top coat deposition. NiCrAlY intermediate layer reduced the differences of microhardness and coefficient of thermal expansion (CTE) values between YSZ top coat and metallic inner layer resulting in reduced mismatch in the thermal and mechanical properties at the two interfaces of the triple layer coating system as compared to double layer coating systems. It can be inferred that the triple layer coating system with 300μm thick YSZ might increase the working temperature of polymeric substrate due to significant reduction in its thermal conductivity value. Thermal gradient cycling test at the coating surface temperature between 300°C to 450°C showed that the increase of surface temperature is followed by the significant decrease of TBC lifetime. The failure mode was the crack formation and delamination at the bond coat/substrate interface.

High-temperature thermal barrier-coated Sylramic-iBN/pyrolytic carbon/chemical vapor infiltration silicon carbide ceramic matrix composite behavior in a combustion environment

An environmental barrier-coated Sylramic-iBN/pyrolytic carbon/chemical vapor infiltration silicon carbide ceramic matrix composite was characterized in a harsh combustion environment under two conditions; (i) with a coating surface temperature of 1480℃, and (ii) with a combustion flame distance of 120 mm (constant flux). Three coating systems were evaluated; Coating “A” (ytterbium silicate using hafnium–silicate bond), Coating “B” (ytterbium silicate using alumina bond), and Coating “C” (ytterbium–silica compounds using alumina bond). All three coatings survived with essentially no mass loss up to 25 h when tested at 1480℃. At a flame distance of 120 mm coatings “A” and “B” experienced extensive degradation and strength loss while coating “C” had practically no degradation in retained strength. Coating “C” outperformed the other two coatings exposed to the harsh combustion environment. It performed best in terms of material loss, thermal barrier capability, and residual tensile strength.

Numerical investigation of transient coating build-up and heat transfer in cold spray

Since cold spray is widely considered as an additive manufacturing and damage repair technology, it is crucial to understand the coating build-up process and the temperature evolution. In this work, a 3D numerical model was developed to simulate the transient coating build-up process as well as the heat transfer in cold spray. By coupling the heat transfer with the ALE (Arbitrary Lagrangian–Eulerian) moving mesh and coating thickness model, this 3D model is able to investigate the temperature evolution of a coating which simultaneously grows according to the nozzle trajectory. The nozzle trajectory that represents the heat source and mass flux of particle impact is generated and simulated in the offline programming software RobotStudio™. By assigning the results of coating thickness distribution, the simultaneous build-up of coating computational domain is achieved by ALE moving mesh method. The validation of the FEA (finite element analysis) model was carried out by measuring the coating surface temperature via an infrared imaging camera. The effects of nozzle traverse speed on the coating surface temperature as well as coating thickness in cold spray were also investigated. With the proposed model, it is able to study the actual coating build-up process as well as the heat transfer phenomena, which may provide more insights for the application in additive manufacturing and damage repair.

Uncertainty determination in high-temperature spectral emissivity measurement method of coatings

The paper deals with uncertainty analyses of the new laboratory method for the measurement of spectral emissivity of high-temperature coatings. These coatings are intended to increase heat transfer in various industrial applications. The experimental set-up of the method is shortly introduced. The method is innovative in the application of scanning laser heating and in the coating surface temperature measurement using an infrared camera with a reference coating. Methods for total and partial uncertainty evaluation are described. As the uncertainty is always related to individual sample being measured, the DupliColor 800°C paint (MOTIP Dupli Ltd.) is used as an example to introduce the results. Except the absorption bands the uncertainty is below 4% with coverage factor k=2. Uncertainty spectral and temperature dependences are analyzed. Contribution of individual uncertainty sources as measured sample signal, measured laboratory blackbody signal, sample surface temperature, laboratory blackbody temperature, surroundings temperature, blackbody effective emissivity and surroundings emissivity and their sub-components are discussed.

Development of Advanced Thermal Barrier Coatings With Improved Temperature Capability

Continuously increasing hot gas temperatures in heavy duty gas turbines lead to increased thermal loadings of the hot gas path materials. Thermal barrier coatings (TBCs) are used to reduce the superalloys temperature and cooling air needs. Until now 6–8 wt. % yttria stabilized zirconia (YSZ) is the first choice material for such coatings, but it is slowly reaching its maximum temperature capability due to the phase transformation at high temperature and sintering. New thermal barrier coating material with increased temperature capability enables the next generation of gas turbine with >60% combined cycle efficiency. Such material solutions have been developed through a multistage selection process. In a first step, critical material performance requirements for thermal barrier coating performance have been defined based on the understanding of standard TBC degradation mechanisms. Based on these requirements, more than 30 materials were a preselected and evaluated as potential coating materials. After carefully reviewing their properties both from literature data and laboratory test results on raw materials, five materials were selected for coating manufacturing and laboratory testing. Based on the coating manufacturing trials and laboratory test results, two materials have been selected for engine testing, in a first step in GT26 Birr Test Power Plant and afterward in customer engines. For such tests, the original coating thickness has been increased such to achieve coating surface temperature ∼100 K higher than with a standard thermal barrier coating. Both coatings performed as predicted in both GT26 Birr Test Power Plant and customer engines.

Sintering‐induced delamination of thermal barrier coatings by gradient thermal cyclic test

AbstractLifetime is crucial to the application of advanced thermal barrier coatings (TBCs), and proper lifetime evaluation methods should be developed to predict the service lifetime of TBCs precisely and efficiently. In this study, plasma‐sprayed YSZ TBCs were subjected to gradient thermal cyclic tests under different surface temperatures, with the aim of elucidating the correlation between the coating surface temperature and the thermal cyclic lifetime. Results showed that the thermal cyclic lifetime of TBCs decreased with the increasing of surface temperatures. However, the failure modes of these TBCs subjected to thermal cyclic tests were irrespective of different surface/BC temperatures, that is, sintering‐induced delamination of the top coat. The thickness of thermally grown oxide (TGO) was significantly less than the critical TGO thickness to result in the failure of TBCs through the delamination of top coat. There was no phase transformation of the top coat after failure. In contrast, in the case concerning the top coat surface of the failure specimens, the elastic modulus and microhardness increased to a comparable level due to sintering despite of the various thermal cyclic conditions. Consequently, it is conclusive that the failure of TBCs subjected to gradient thermal cyclic test was primarily induced by sintering during high‐temperature exposure. A delamination model with multilayer splats was developed to assist in understanding the failure mechanism of TBCs through sintering‐induced delamination of the top coat. Based on the above‐described results, this study should aid in facilitating the lifetime evaluation of the TBCs, which are on active service at relatively lower temperatures, by an accelerated thermal cyclic test at higher temperatures in laboratory conditions.