Thick Thermal Barrier Coatings Research Articles

Boundary Layer Analysis of a Transonic High-Pressure Turbine Vane Using Ultra-Fast-Response Temperature-Sensitive Paint

Abstract The focus of this article is the impact of surface roughness on the boundary layer caused by a 7YSZ thermal barrier coating (TBC). Experimental investigations are conducted on a NGV installed inside the wind tunnel for Straight Cascades Göttingen (EGG). The shape of the vane has been altered in a way that eliminates the influence of TBC's thickness. Therefore, it is expected that only the surface roughness is influencing the location of the separation and boundary layer transition. The transition next to the roughness can also be affected by positive and negative pressure gradients, separation, and interacting shocks. The impact of TBC on the turbulent wedges' appearance, separation bubble's position and length, and transition location is examined in this study. This research, combined with prior investigations, provides a comprehensive understanding of a turbine vane's aerothermodynamics. To investigate unsteady flow phenomena on a TBC-coated NGV, ultra-fast-response temperature-sensitive paint (iTSP) is utilized. This dataset will serve as a reference point for developing new turbine vane designs that include TBC and extensive cooling. Furthermore, the findings will be employed as a benchmark for improving numerical models.

Accurate Thickness Evaluation of Thermal Barrier Coatings using Microwave Resonator Sensor

Thermal barrier coatings (TBC) are frequently employed in aircraft engines and turbine blades for protection against high temperatures. Constant exposure to in-service stresses, however, leads to a variety of anomalies in TBC that could prove to be potentially catastrophic for the assemblies they protect. This raises the need for inspection using non-destructive techniques (NDT) to ensure the reliability and structural integrity of the coated assemblies. Thickness evaluation of the topcoat of the TBC is paramount to determining structural integrity of the TBC. Microwaves are particularly suited for such applications since they readily penetrate low loss TBC. However, the performance of previous microwave-based TBC thickness evaluation techniques was limited in accuracy to 15 μm. In this paper, accurate thickness evaluation of thermal barrier coating (TBC) is performed using a microwave resonator sensor operating at a relatively low frequency (<1 GHz). TBC thicknesses as low as 100 μm are evaluated based on shift in resonant frequency of the probe. Overall, practically relevant thicknesses ranging from 100 μm - 600 μm are evaluated herein and multiple trials are performed to ascertain repeatability and assess uncertainty in measurements. For a 250 μm thick TBC sheet, a mean error of 0.9 μm was accomplished which translates to 0.36 % error with a standard deviation of 1.5 μm. Compared to other microwave sensors reported in the literature, a better estimation accuracy is demonstrated in this work, in particular for thicknesses less than <250 μm.

Thermal insulation capability and adhesion strength of unpyrolyzed thick thermal barrier coatings

Thermal insulation capability and adhesion strength of thick thermal barrier coatings (TBC) produced by unpyrolyzed powder (UPTTBC) were compared with those of the conventional TBC, thick TBC (TTBC), and dense vertically cracked (DVC) TTBC. Thermal insulation capability was evaluated using temperature drop. The adhesion strengths of the coatings were also measured by pull-off test method according to ASTM C633. The results indicated that the adhesion strength of UPTTBC and DVC TTBC was 35% and 25% higher than that of the TTBC, respectively. This could imply that the non-melted areas with sub-micron dimensions in UPTTBC structure, act as a trap and prevent the cracks propagation. In addition, the results of thermal insulation tests showed that DVC TTBC coatings had the lowest thermal insulation capability with temperature drop rate of 0.28 °C/µm and UPTTBC coatings had the highest insulation capability with temperature drop rate of 0.40 °C/µm. Due to the presence of sub-micron zones in the microstructure of UPTTBCs, the coatings exhibited superior thermal insulation capability. In contrary, the DVC TTBC showed the lowest thermal insulation capability because of its dense structure.

The Influence of Microstructure Characteristics on Thickness Measurement of TBCs Using Terahertz Time-Domain Spectroscopy

Thermal barrier coatings (TBCs) exhibit excellent thermal insulation capabilities, proving crucial in enhancing the performance of turbine blades. Accurate measurement of TBC thickness is pivotal for the quality control and health monitoring of turbine blades. However, the absence of suitable non-destructive testing (NDT) methods poses a challenge in ensuring precise quality control and health assessment of TBCs. This study investigates the efficacy of terahertz time-domain spectroscopy (THz-TDS) in measuring TBCs thickness, specifically focusing on the microstructure characteristics of the top coat (TC), including grain morphology, internal porosity, surface roughness, and agglomerates. The findings emphasize the significance of grain morphology in determining thickness measurement due to the varied terahertz wave propagation modes. Moreover, the study involved polishing EB-PVD and APS samples to mitigate surface roughness. This process revealed a discernible linear correlation between reduced surface roughness and decreased measurement errors. The slopes of the error reduction curves ranged from 0.59 to 1.7 for EB-PVD and 2.17 to 5.79 for APS samples. Furthermore, the research observed THz light scattering within internal pores, resulting in diminished outgoing energies and subsequent increments in measurement errors.

Solid dust induced roughening and overheating of TBC-coated superalloy

Thermal barrier coatings (TBC) on superalloy blade/vane enable an increase in turbine inlet temperature (TIT) and aero-engine efficiency. However, the deposition of dust on TBC significantly ablates superalloys and becomes a bottleneck for advanced engine development. This paper presents a comprehensive modeling approach, based on an experiment involving dust deposition on a TBC-coated substrate, aimed at understanding the fundamental cause of substrate ablation. The results show that a significant increase in surface roughness is obtained experimentally with dust accumulation. The temperature field within the TBC-substrate system is disturbed by surface roughening. Consequently, the film cooling is seriously destroyed. While the dust accumulated on the TBC surface also contributes to heat insulation by increasing the effective TBC thickness, localized overheating of the substrate occurs and then leads to ablation and premature failure of the substrate. This elucidation of the overheating mechanism could inform the development of strategies to resist dust accumulation in advanced aero engines.

High precision thickness evaluation of thermal barrier coating with high frequency eddy current testing method

Consisting of a top ceramic coating (TC) and a bond coating (BC) layer, thermal barrier coating (TBC) is a key component of the heavy duty gas turbine blade to protect the substrate material from high working temperature. Thinning of the top coating and the thermally grown oxide (TGO) layer formed between TC and BC cause TBC thickness reduction, and may significantly affect the in-service performance of TBC. High frequency eddy current testing (HF ECT) is considered as an efficient technique for nondestructive evaluation (NDE) of the TBC thickness, but its measurement accuracy is not satisfied up to now. Aiming to realize a high precision TBC thickness evaluation with HF ECT signals, a numerical method for simulating HF ECT of TBC was developed and implemented with consideration of the dielectric properties of TC and TGO material. Through numerical simulations with the developed code, an ECT probe of optimized configuration and robust against lift-off variation was designed and fabricated for the TBC thickness evaluation. A HF ECT measurement system is then setup to investigate the validity of the prototype ECT probe and sizing method experimentally. It was found that the thickness of the TC layer can be evaluated within 10 μm accuracy for the plate TBC specimens, which reveals the HF ECT method and new ECT probe are efficient for NDE of the TBC thickness.

Through thickness changes to stiffness and thermal conductivity in thermal barrier coatings subjected to gradient exposure

Sintering-induced increases in the elastic modulus and thermal conductivity of thermally sprayed Thermal Barrier Coatings (TBCs) under constraint by a bond-coated substrate due to thermal gradient exposure have been measured. The results suggest the degree of sintering due to this constrained thermal exposure to be significantly lower when compared to free standing exposed specimens. Overall, a ∼30% increase in elastic modulus and ∼12% increase in thermal conductivity have been measured in a ∼ 450 µm thick TBC after 12 h of gradient exposure. Here, the top surface of the TBC was kept at 1250 °C, and the gradient was determined based on a constant backside cooling air flow rate. The elastic modulus, measured across the thickness of coating, reveals that in the region near the top surface, the elastic modulus increased by ∼ 50%. Contrastingly, no significant increase in elastic modulus was observed near the metal/coating interface. The dynamic evolution of thermal conductivity and sintering under gradient exposure and constraint was calculated using an analytical model which was then used to rationalize the experimental observations.

Effect of thermal barrier coatings and integrated cooling on the conjugate heat transfer and thermal stress distribution of nickel-based superalloy turbine vanes

Based on the comprehensive design of the thermal barrier coatings (TBCs) and advanced integrated cooling systems for the Ni-based superalloy turbine vanes. This study employs a multidisciplinary method to research the aerothermal performance and elastic thermal stress of the first stage Ni-based superalloy (DS GTD 111) turbine guide vane cooled by an integrated cooling system and TBC thickness of 0.5 mm. The results show that the highest temperature and average temperature on the substrate under TBC decreased by 126 K and 110 K, respectively. For the vanes without and with TBC, the thermal stresses are concentrated on the midchord region of the suction side, the upper edge of film holes outflow, and junction regions between the vane root, tip, and endwalls, which bear high-temperature gradient and high temperature. However, TBC reduces stress value by 50%∼95% in thermal stress concentration regions of the substrate, which significantly reduces the slip systems activated regions on the substrate. In addition, the activation of the dominant slip system (DSS) is a further optimization objective for cooling turbine vanes. This study provides macroscopic and microscopic perspectives for improving integrated cooling structures of actual vanes in the design process by comprehensively considering aerothermal and thermomechanical performance.

Nondestructive Thickness Measurement of Thermal Barrier Coatings for Turbine Blades by Terahertz Time Domain Spectroscopy

Owing to its high penetrability with dielectric materials, terahertz time domain spectroscopy (THz-TDS) is a promising nondestructive measurement technology. The coating thickness deviation and defect of thermal barrier coatings (TBC) will affect its thermal insulation performance and lifetime. In this work, THz-TDS was applied to measure the coating thickness distribution of TBC. The refractive index was obtained by THz-TDS transmission mode. To avoid the normal incidence THz signal loss, the THz signal was reflected from the TBC with a 10° incident angle, which also made the measurement result insensitive to the unevenness and tilt of the TBC sample. In the experiment, the yttria-stabilized zirconia (YSZ) TBC was measured by THz-TDS to estimate the thickness distribution. To validate the thickness measurements, metallography was introduced to correlate the TBC thickness result. The measurement deviation was within 12.1 µm, i.e., 3.45% for the THz-TDS and metallography result. A piece of turbine blade was measured by THz-TDS and a eddy current test. The maximum deviation was 8.48 µm, i.e., 2.36% of these two methods. Unlike the eddy current test, the THz-TDS thickness result was not affected by the cooling holes. The effectiveness of the nondestructive thickness measurement of TBC for turbine blades by THz-TDS was verified.

Thick thermal barrier coatings with different segmentation crack densities: Microstructure analysis and thermal oxidation behavior

Thick thermal barrier coatings (TTBCs) have been developed to increase the lifetime of hot section parts in gas turbines by increasing the thermal insulating function. The premeditated forming of segmentation cracks was found to be a valuable way for such an aim without adding a new layer. The TTBC introduced in the current study are coatings with nominal thickness ranging from 1 to 1.1 consisting of MCrAlY bond coat and 8YSZ top coat deposited by air plasma spray technique (APS). TTBCs with segmented crack densities of 0.65 mm−1 (type-A) and 1 mm−1 (type-B) were deposited on a superalloy substrate by adjusting the coating conditions. It was found that the substrate temperature has an influential role in creating the segmentation crack density. The crack density was found to increase with substrate temperature and liquid splat temperature. The two types of coatings (type-A and B) with different densities of segmentation crack were heat-treated at 1000 °C (up to 100 h) and 1100 °C (up to 500 h). The variation of hardness measured by indentation testing indicates a similar trend in both types of coatings after heat treatments at 1000 °C and 1100 °C. Weibull analysis of results demonstrates that higher preheating coating during the deposition results in a denser YSZ coating. The growth rate of TGO for TTBCs was evaluated for cyclic and isothermal oxidation routes at 1000 °C and 1100 °C. The TGO shows the parabolic trend for both two types of coatings. The Kps value for two oxidation types is between 5.84 × 10−17 m2/s and 6.81 × 10−17 m2/s. Besides, the type B coating endures a lifetime of more than 40 cycles at thermal cycling at 1000 °C.

Effect of Cracks on Thermal Shock Behavior of Plasma-Sprayed Thick Thermal Barrier Coatings

Effect of Cracks on Thermal Shock Behavior of Plasma-Sprayed Thick Thermal Barrier Coatings

A numerical evaluation and guideline for thermal barrier coatings on gasoline compression ignition engines

The application of thermal barrier coatings (TBCs) for internal combustion engines has drawn more research attention in recent years due to the improved coating technology and advanced material properties. This paper presents the combination of thermal barrier coatings with an advanced combustion concept: gasoline compression ignition (GCI). This study aims to evaluate the coating performance with GCI combustion on Aramco’s light-duty GCI engine. The thermodynamic engine model has been established and collaborated with CFD results to investigate thermal barrier coatings for GCI with a PPCI-diffusion combustion strategy primarily at a load of 23.5 bar IMEPg. Two real-world coating materials were pre-selected as Gen.1 and Gen.2 candidates. The investigation consists of three main topics: the effect of coating thickness on GCI, the effect of intake boost on the performance of traditional coatings and temperature swing coatings, and the effect of combustion chamber coverage on engine performance. The result shows that the TBC thickness became the key parameter to optimize for TBC with GCI combustion mode, and in the end, 250–500 microns is recommended based on the temperature limit of ceramic material. It was also found that the high level of intake boosting could significantly alleviate the charge heating penalty. The coverage of piston, engine head, and both intake and exhaust valves should be applied to achieve the highest efficiency gains. It was found that the gain ratio from each chamber component can be drastically different at different loads.

Investigation of cooling hole blockage in the plasma spraying of thermal barrier coatings on super-alloy

ABSTRACT Ceramic materials will get deposited inside pre-drilled cooling holes and partially or completely block during thermal barrier coating (TBC). In this study, the hole blockage produced by atmosphere plasma spraying (APS) was investigated using industrial computed tomography. The results show that the geometric features of the blockage owing to APS coating are significantly affected by the inclination angle of the pre-drilled holes. The coating deposition in the vertical holes is rotationally symmetric and mainly concentrates at the entrance. For the inclined holes, the deposition on the trailing edge is much more than that on the leading edge. The deposition height and length on the trailing edge increase with the inclination angle; however, they are little disturbed by the hole size. Therefore, the blockage ratio decreases with the increase of the hole size. In addition, greater TBC thickness causes larger deposition height and length and subsequently results in a larger blockage ratio.

Factors affecting cyclic lifetime of EB-PVD thermal barrier coatings with various bond coats

Abstract Standard EB-PVD (electron beam physical vapor deposition) thermal barrier coatings (TBCs) of partially yttria-stabilized zirconia were applied on Ni-base substrate alloys and thermally cycled with T max = 1100 °C. Two different bond coats were investigated: NiCoCrAlY and NiPtAl. The longest lifetimes have been achieved on the Hf-containing Rene142 substrate. A rough interface between TGO (thermally grown oxide) and bond coat with hafnia pegs is characteristic for these alloys. A bond coat heat treatment in Ar–H significantly improved the lifetime of the TBC system with IN100 + PtAl bond coats, whereas reduced lifetimes were found on IN100 + NiCoCrAlY. Spallation of the TBCs was correlated to TGO thickness and microstructure. The TGO growth is faster for NiCoCrAlY than for NiPtAl bond coats, while TBC lifetime is longer for the NiCoCrAlY systems. Tests with reduced cycle length yielded an increased number of cycles to failure for IN100+NiCoCrAlY, but left the time to failure at temperature unchanged. TBC thickness had only a limited influence on cyclic lifetime.

Simulation of residual stress in thick thermal barrier coating (TTBC) during thermal shock: A response surface-finite element modeling

The current study demonstrates a well-designed response surface methodology (RSM), based on the generated dataset of finite element method (FEM) to establish an integrated model for simulation of residual stress distribution in a thick thermal barrier coating (TTBC). In this study, typical TTBCs were applied on Hastelloy X Nickel-based superalloy using air plasma spray technique followed by thermal cycling. The recorded stress data of Raman spectroscopy was employed to verify the proposed FEM model. A relatively good agreement was obtained between predicted residual stresses and measured ones. Verified FEM model was used to carry out the parametric studies to evaluate the effects of such various parameters as interface amplitude, wavelength, thermally grown oxide thickness and preheating temperature on the stress distribution in the TTBC during the thermal cycling. The computed data were subsequently used for the development of RSM model. In conclusion, experimentally verified numerical data was used to construct a statistical model based on RSM and successfully used to predict the residual stress distribution field in TTBC during thermal cycling. The obtained results of hybrid FEM- RSM model were in acceptable conformity with Raman spectroscopy measurements.

A parametric modeling study of thermal barrier coatings in low-temperature combustion engines

• Low-temperature combustions are more favorable with the thick coating configurations. • In-cylinder thick thermal barrier coating improves the indicated thermal efficiency by 4.6%. • Thick thermal barrier coating increases the volumetric efficiency due to the lower IVC temperature. • Thick thermal barrier coating increases the exhaust enthalpy and reduces intake temperature requirement. In-cylinder thermal barrier coatings (TBCs) reduce heat transfer losses and increase thermal efficiency. It has been shown that thick TBCs negatively impact the performance of conventional combustion modes by degrading volumetric efficiency and increasing the propensity for end-gas knock. Low-temperature combustion (LTC) is an advanced combustion strategy that offers high efficiencies and low emissions. Due to the nature of kinetics-driven autoignition, LTC is fundamentally different from the conventional combustion modes, where the benefits and tradeoffs of thick TBCs need to be re-evaluated. Previous experimental studies showed the feasibility and the efficiency gains associated with a 2 mm thick TBC applied to the piston surface, as well as the reduction in the required intake temperature with no observable deterioration on the high load limit. However, the effects of TBCs and their independent thermophysical properties on LTC have not been systematically explored. It is necessary to perform a comprehensive study on the effects of TBC on LTCs from a fundamental thermodynamic perspective, which serves as the motivation for the current study. This study couples a 0D engine thermodynamic model to a 1D transient heat transfer model of the coating and piston. The model was first validated against the metal piston baseline, followed by validation against experimental data of the TBC cases at different engine loads. With confidence established in the model’s fidelity, three parameters are investigated independently: thermal conductivity ( k ), coating thickness, and volumetric heat capacity ( s ). The results revealed that the volumetric efficiency actually increases by 7.4 percentage points with a thicker coating due to a reduction in heat transfer during the compression stroke, which allows for a lower intake temperature requirement to reach autoignition. However, there is a thickness limit before the intake temperature becomes impractically low. The results show that elevating surface temperature is directly proportional to higher efficiency. Therefore, the optimal coating configuration for kinetically-controlled LTC is a combination of the lowest k , thickest coating before reaching the limit, and the lowest s , where the low k and high thickness contribute the most thermal efficiency gains (4.1 percentage points) and increased exhaust enthalpy (5.7%).

Comparative investigation of the effect of composition and porosity gradient on thermo-mechanical properties of functionally graded thick thermal barrier coatings deposited by atmospheric plasma spraying

The present contribution aims to propose a modified thick thermal barrier coating (TTBC) to maximize the durability of convectional TTBC while maintaining the thermal insulation capacity by applying graded chemical composition layers and a porosity gradient to the ceramic top coat. To this end, a TTBC, a functionally graded TTBC (FGTTBC) with a structure consisting of five layers, and two modified FGTTBCs with a porous and porosity gradient top coat were designed. Resultant coatings were assessed for their microstructure, mechanical properties (hardness, elastic modulus, fracture toughness, and bonding strength), thermal shock lifetime, and thermal insulation capacity. The microhardness of the as-sprayed coatings was evaluated via the indentation method and the bonding strength testing was carried out according to ASTM C633-01 standard. Their thermal shock lifetime and thermal insulation capacity were also tested in a furnace at 1000 °C. The results indicated that the use of gradual NiCrAlY layers improved the fracture toughness of the coating to 8.07 MPa m0.5. The highest and lowest elastic modulus was determined to be in the 100 wt% NiCrAlY and porous 100 wt% YSZ layers, respectively. The FGTTBC with porosity gradient has better interface stability than the porous one due to its higher bonding strength (22.3 MPa compared to 19 MPa). The porous FGTTBC failed after 140 cycles, which is superior to TTBC after 50 cycles and conventional FGTTBC after 104 cycles. Ultimately, porous FGTTBC provides greater thermal insulation capacity than TTBC and reduces surface temperature by 91 °C.

Impact of Cooling with Thermal Barrier Coatings on Flow Passage in a Gas Turbine

Inlet temperature is vital to the thermal efficiency of gas turbines, which is becoming increasingly important in the context of structural changes in power supplies with more intermittent renewable power sources. Blade cooling is a key method for gas turbines to maintain high inlet temperatures whilst also meeting material temperature limits. However, the implementation of blade cooling within a gas turbine—for instance, thermal barrier coatings (TBCs)—might also change its heat transfer characteristics and lead to challenges in calculating its internal temperature and thermal efficiency. Existing studies have mainly focused on the materials and mechanisms of TBCs and the impact of TBCs on turbine blades. However, these analyses are insufficient for measuring the overall impact of TBCs on turbines. In this study, the impact of TBC thickness on the performance of gas turbines is analyzed. An improved mathematical model for turbine flow passage is proposed, considering the impact of cooling with TBCs. This model has the function of analyzing the impact of TBCs on turbine geometry. By changing the TBCs’ thickness from 0.0005 m to 0.0013 m, its effects on turbine flow passage are quantitatively analyzed using the proposed model. The variation rules of the cooling air ratio, turbine inlet mass flow rate, and turbine flow passage structure within the range of 0.0005 m to 0.0013 m of TBC thicknesses are given.

Thermal barrier coating for carbon fiber-reinforced composite materials

Carbon fiber-reinforced plastic (CFRP) composites are widely employed in lightweight and high performance applications including supercars, aero-vehicles, and space components. However, although carbon fibers are thermally stable, the low thermal endurance of the matrix materials remains a critical problem in terms of the performance of the material. In this study, we proposed a new, Al 2 O 3 -based thermal barrier coating (TBC) for the CFRP composites. The TBC comprised α-phase Al 2 O 3 particles with a mean diameter of 9.27 μm. The strong adhesion between the TBC and the CFRP substrate was evaluated using a three-point bending test. When the CFRP substrate was subjected to a 500–700 °C flame, the 1.45-mm thick TBC protected the CFRP substrate remarkably by reducing the surface temperature to 188–228 °C. The thermo-mechanical responses of this TBC/CFRP composite were analyzed after thermal shock tests. Surprisingly, 50% of the pristine flexural strength of the TBC/CFRP composite was preserved, whereas that of neat CFRP was reduced significantly by 95%.

Atmospheric plasma sprayed thick thermal barrier coatings: Microstructure, thermal shock behaviors and failure mechanism

Advanced thick thermal barrier coatings (TTBCs) have attracted extensive attentions due to the embedded segmentation-crack structure and excellent thermal insulation performance. The thermal shock life and failure behaviors of advanced TTBCs still remain a challenge. The objective of this work is to well understand the effect of microstructure on the coating cracking behavior in atmospheric plasma sprayed TTBCs system, in which the extended Raman spectroscopy (RS) techniques are used to obtain an in-depth understanding on how the oxidation affects the stress distribution in coatings and in turn, the causes of the observed cracking. Results show that the TTBCs deposited by the highly purified feedstock exhibit excellent thermal shock life at 1100 ℃ due to the well-proportioned and compact structure and built-in vertical cracks. After thermal shock test, the broken regions usually occur at the horizontal cracks and vertical cracks area within top-coat, and top-coat/bondcoat interface, which may suggest that each structural mutation parts are prone to causing the TTBCs instabilities. Cracks are propagated and extended under thermal tensile stress. The coating failure is most possibly dominated by the accumulated residual stress generated due to thermal expansion mismatch between the substrate and top-coat. In addition, the good thermal shock resistance have a close relationship with the newly formed vertical cracks at the bottom of ceramic coating near the top-coat/bondcoat interface.