Top Coat Thickness Research Articles

Thermal cycle behaviour of plasma sprayed thermal barrier coatings on cast iron substrate for the application of liner of internal combustion engine

In the present scenario, research on Thermal Barrier Coatings (TBCs) is attracting significant attention in high-temperature applications due to the rising demand in industrial applications, such as gas turbine blades and internal combustion engine components. To meet these demands, it is essential to enhance the mechanical and oxidation properties of thermal barrier coatings. The atmospheric plasma spray methodology for producing these coatings has become a central focus in recent times. In this work, TBCs were prepared by blending pure alumina and Yttria-Stabilized Zirconia (YSZ) in equal proportions. Extensive study of TBCs carried out under high-temperature conditions through thermal cycle tests at elevated temperatures, around 850 °C. A total of 350 thermal cycles were conducted, and microstructures of the coating surface were examined at 150, 250, and 350 cycles. High-temperature stress, generated due to the sintering of the coating, also influences the life of the coatings. Specimen with a topcoat thickness of 300 μm, exhibited good thermal shock resistance compared to the specimens with topcoats of thickness of 100 μm and 200 μm. The effects of process parameters, porosity, and thermal shock resistance are presented in detail. The topcoat with a thickness of 300 μm demonstrated excellent thermal shock resistance and reduced formation of thermally grown oxide (TGO). The porosity of the coating, found to be between 2% and 3%, contributed to its dense nature, effectively reducing oxidation rates and enhancing the coating's lifespan under cyclic high-temperature conditions.

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.

Interfacial failure induced by dynamic evolutional stress of NiCoCrAlY-4YSZ TBCs during gradient thermal cycling: Effect of 4YSZ top coat thickness

As the continuation of the previous work, the dynamic evolution of stress field of NiCoCrAlY-4YSZ TBCs with the top coat thickness of 100 and 200 μm at 1000 cycle is constructed based on the real interfacial morphologies of the TC/BC. Results indicated that interfacial stress usually exhibits higher value at the heating stage than cooling stage, which is completely different from the previous results during isothermal cyclic oxidation. Furthermore, the formed (Ni,Co)(Cr,Al)2O4 spinel oxide is unable to promote the high stress level at the TC/BC interface. The relationship between the stress state and formed (Ni,Co)(Cr,Al)2O4 spinel oxide as well as relative roughness was discussed.

Effect of Top-Coat Thickness and Interface Fluctuation on the Residual Stress in APS-TBCs

This study focused on the numerical simulation of the distribution of residual stress in yttria-stabilized zirconia (YSZ) coatings prepared with atmospheric plasma spraying (APS). We particularly investigated the stress distribution around the interface between the top coat and bond coat. During thermal spray deposition, droplets and particles deposit on the substrate in a complex manner, causing interface waviness and non-uniform stress distribution. Therefore, residual stress is an important consideration when preparing thermal barrier coatings (TBCs). Residual stresses directly affect the performance of bond coats (BCs) and ceramic top coats (TCs). To accurately evaluate residual stress, we considered interface waviness and the thickness of the ceramic top coat and conducted a detailed analysis of stress distribution. The results show that compressive stress exists at the TC/BC interface, which may be caused by the mismatch in the thermal expansion coefficient between the YSZ top coat and the substrate, potentially leading to coating delamination. Moreover, the residual stress at the TC/BC interface significantly increases with an increasing YSZ thickness. When the top-coat thickness exceeds 300 μm, stress concentration and failure of the coating are likely to occur. Meanwhile, the optimized thermal spray experiment results confirm that the residual stress at the BC/YSZ interface of the thermal barrier coating is tensile stress, with a maximum value of 155 MPa, which is consistent with the finite element calculation results. Furthermore, the failure modes of TBCs with rough interface conditions are discussed in detail. Our research provides important guidance for TBC design and optimizing their performance.

THzResNet: A Physics-Inspired Two-Stream Residual Network for Thermal Barrier Coating Thickness Measurement

The uneven and complicated porose microstructure of thermal barrier coatings (TBCs) brings a big challenge for accurate and reliable thickness measurement due to the resultant change of refractive index. In this work, a novel insight into the physics, the first two peaks are originally adopted as the preferred signal, and a physics-inspired two-stream deep learning framework, termed as the terahertz (THz) residual network (THzResNet) is proposed to accurately infer topcoat thickness by extracting refractive index online. Specifically, the division and reciprocal layers are figured out and incorporated into the first-stream, yielding the reciprocal of refractive index. Then, a product layer connects it with time-of-flight from the second-stream to generate topcoat thickness, which endows the THzResNet in accord with the THz physics. Finally, the experiments demonstrate the advantages of THzResNet in accuracy and efficiency. The presented approach allows for addressing the challenge for accurate thickness measurement of coatings with complicated microstructures.

Investigating thermal shock and corrosion resistance of Inconel 601 super alloy after thermal barrier coating with %8 YSZ powder

Superalloys, which are categorized in three groups as iron-based, nickel-based and cobalt-based, are used especially in high temperature applications. Inconel 601 alloy, a nickel-based superalloy, is widely used in applications such as chemical processing, aerospace, power generation, heat treatment, chemical refining and gas turbine engines. Although the mechanical properties of superalloy materials and their resistance to wear, corrosion and oxidation are better than other metallurgical materials, these properties are not satisfactory in some applications. In such cases, the desired properties can be obtained by applying heat treatment and coating processes to superalloys. In this study, approximately 100, 200 and 300 µm ceramic top coat thickness thermal barrier coating system with NiCrAlY and 8%YSZ powder coating materials, were created for the Inconel 601 substrate material and SEM, EDS and XRD datas are given. Then, electrochemical corrosion test with 3.5% NaCl solution and 8 cycles of thermal shock test at 1200 °C using FCT thermal shock method were applied to the samples. Considering the corrosion rate values and SEM images after the electrochemical corrosion test, it was determined that the sample with the highest corrosion resistance was the sample with a ceramic top coating thickness of 100 µm. The reason for this is that as the coating thickness increases in thermal barrier coatings, the tension increases and accordingly, a change in pore and crack density occurs. The fact that the samples with a ceramic top coating of 100 µm thickness have the highest corrosion resistance can be explained in this way. Considering that the pore and crack ratio decreases due to the sintering effect in the SEM images of the samples after the thermal shock test, but considering that the TGO layer thickness will increase at higher temperatures and longer service conditions, in regards to thermal barrier coating deformation, it was concluded that the samples with 100 and 200 µm ceramic top coating thicknesses were safer in terms of thermal shock.

Interaction of air plasma-sprayed spinel and yttria coatings with ultra-high temperature ceramic-metal hot-melt

Air Plasma Sprayed (APS) ceramic coatings are conventionally used for protecting metallic parts of hot section components in aero- and land-based gas turbines. Owing to their capability to withstand very high temperatures high specific heat capacity, and thermal shock resistance, such APS coatings are also investigated for the potential to protect the core catcher in nuclear reactors during the high-temperature core-meltdown accidents resulting in corium hot-melt. In this study, candidate plasma-sprayed spinel and yttria coatings on SS 316LN substrates were subjected to an ultra-high temperature (UHT) ceramic-metal hot-melt at ∼2500 °C. Uncoated steel substrates showed melting, whereas coated substrates were found un-attacked at the expense of degradation of the coatings such as surface melting, topcoat sintering, columnar grain growth and thermal shock cracks. The evolution of interfacial fusion was observed in the case of yttria coatings due to the formation of primary YAG and Al2O3-YAG eutectic at the interface. Spinel coatings with a comparatively lower thermal conductivity could generate a higher ΔT across the topcoat thickness with limited grain growth in the substrate. Evidence of local melting at the interface and evolution of temperature gradient between the hot-melt and substrate are comprehensively illustrated. Out of the two coatings tested, spinel was found to be more protective to the steel substrate.

Influence of top-coat thickness of thermal barrier coating on time–frequency domain and dispersion characteristics of laser-induced surface acoustic wave

Thermal barrier coating is commonly used to insulate the hot end components of aero engine and gas turbine in order to meet extreme high temperature service conditions. The performance of the thermal barrier coating will be significantly impacted by thinning or even spalling during the service process. Given the uniqueness of thermal barrier coating in structure and composition materials, and the fact that laser-induced surface acoustic waves are extremely sensitive to material surface characteristic parameters, laser-induced ultrasonic technology is used to detect the thermal barrier coating in this paper. To characterize the size and defect of thermal barrier coating, the excitation mechanism and propagation law of surface acoustic waves are very crucial. Firstly, the surface acoustic wave propagation law is investigated using spectral analysis and wavelet transform. Then, to study the influence of top-coat thickness on laser-induced surface acoustic wave, the surface acoustic wave velocity dispersion curves of thermal barrier coating with different top-coat thicknesses are extracted using wavelet analysis. The simulative and experimental results indicate that the time–frequency domain and dispersion characteristics of laser-induced surface acoustic wave in multilayered heterostructure are strongly affected by the top-coat thickness. By the establishment of the relationship between phase velocity of laser-induced surface acoustic wave and top-coat thickness, a theoretical basis is provided to characterize the top-coat thickness of multilayered heterostructure.

Millimeter Wave Thickness Evaluation of Thermal Barrier Coatings (TBCs) Using Open-Ended Waveguide Probes

ABSTRACT Nondestructive testing (NDT) of thermal barrier coatings (TBCs) is a critical and ongoing topic of research and development. In particular, inspection techniques that determine the thickness of ceramic topcoat and thermally grown oxide (TGO) are of interest. This work investigates the utility of open-ended rectangular waveguide probes in the millimeter wave frequency range of 26.5–110 GHz for evaluation of topcoat and TGO thicknesses through a compressive set of electromagnetic (EM) simulations. In addition, these EM simulations are used to illustrate the influence of probe size and TBC substrate curvature on the complex reflection coefficient properties and the subsequent thickness estimation. The impact of volumetric porosity level on the same is also investigated. A standing-wave probe at V-band (50–75 GHz) is constructed and used to measure the topcoat thickness on three button-type TBC samples. This probe eliminates the need for using expensive and bulky vector network analyzers (VNA), which is quite desirous from a practical point-of-view. The experimental results indicate the capability of estimating the topcoat thickness to within ±15 μm (0.6 mils).

Topcoat thickness measurement of thermal barrier coating using grating laser acoustic spectrum method

The thickness of top coating is a crucial factor to characterize the performance of thermal barrier coating. Aiming at the difficulty of thickness measurement with conventional ultrasonic method, a noncontact method based on grating laser ultrasonic acoustic spectrum is proposed. Firstly, the dispersion curve of surface acoustic waves in thermal barrier coating was obtained. The numerical simulation of the established three-layer media model based on finite element method was then performed. Experiments with thermal barrier coating specimens in different thicknesses of top coating were conducted. The theoretical results and simulative results were corrected by ultrasound attenuation theory to fit experimental results. It is found that experimental results show great agreement with the theoretical curve and numerical simulation results after correction, which illustrates the usefulness and potential of the proposed method when applied to the thickness measurement of top coating in service.

The photothermal wave field and high-resolution photothermal pulse compression thermography for ceramic/metal composite solids

Pulse compression thermography has been recently developed to increase the signal-to-noise ratio (SNR) and enhance depth resolvability. However, a distinctively high SNR and depth resolvability is usually required for the accurate characterization of the (thin) topcoat thickness and defects of multilayer ceramic/metal composite solids (CMCSs). To this end, based on the Green function approach, a photothermal wave model for three-layer CMCSs consisting of a semitransparent ceramic coating and two-layer opaque metal solids is first derived, where a spatially varied volumetric heat source and an interior interface heat source described as the Dirac delta function are considered. The dependence of the thermal wave behaviors on both the thickness and photothermal properties of the semitransparent coating is investigated using the model. Inspired by high-precision radar target detection, two nonlinear frequency modulation (NLFM) waveforms, whose instantaneous frequency curves are respectively convex and concave quadratic function, are explored. Results indicate the NLFM waveform with the instantaneous frequency curve of concave quadratic function outperforms conventional LFM and Barker waveforms in terms of the SNR and depth resolvability. Finally, we reveal that a proper window function can further significantly improve the pulse compression quality. Our investigation can provide direct guidance for the accurate and reliable non-destructive testing and evaluation of multilayer CMCSs.

Research on the Influence of Different Paint Film Thickness Combination on Corrosion Resistance of Sandblasting Steel Plate

To guide the selection of paint film thickness scheme with high cost performance when spraying sandblasting steel plate. In this paper, we sprayed different film thickness with the combination of primer and topcoat on the sandblasting steel plate. The corrosion resistance was evaluated by neutral salt spray test and electrochemical test of different film thickness combinations. In addition, the corrosion morphology was analyzed by automatic inverted metallographic microscope. Furthermore, potentiodynamic polarization and electrochemical impedance (EIS) curves were used to analyze the corrosion mechanism. The self-corrosive current density (Icorr) is 1.999 × 10-12 A·cm-2 with the combination of primer and topcoat thickness (20 μm+20 μm) and the self-corrosive current density (Icorr) of low carbon steel matrix is 1.855 × 10-5 A·cm-2 from the analysis of polarization curves from kinetic potential. The impedance value on 0.01 Hz (|Z|0.01Hz) (4.56 × 1010 Ω·cm2) of the combination of primer and topcoat thickness (20 μm+20 μm) after immersing in 3.5 wt. % NaCl for 2 hours was more than eight orders of magnitude higher than the value (|Z|0.01Hz 6.52 × 102 Ω·cm2) of the low carbon steel plate after immersing for 0.5 hours. Even dipping in 3.5 wt. % NaCl for 168 hours, the |Z|0.01Hz value of the combination of primer and topcoat thickness (20 μm+20 μm) was still three order of magnitude higher than the low carbon steel plate with 0.5 hours immersion. Increasing the film thickness of the topcoat can improve the anticorrosion performance under the condition of a certain thickness of the primer. However, too thick coating will generally bring greater internal stress, which leads to the cracking of the coating in the process of use because of the change of external force and temperature, thus affecting the anticorrosion performance. When the total film thickness is certain, with the increase of the proportion of primer increase of the proportion of primer film.

Influence of Bond Coat on Thermal Shock Resistance and Thermal Ablation Resistance for Polymer Matrix Composites

Polymer matrix composites (PMCs) have been widely used in aero industry because of its low density and high strength-to-weight ratio. However, the application of PMCs is still limited by poor abrasion resistance, weak oxidation resistance and low operation temperature. In this study, Cu (Al)/NiCrAlY/YSZ triple layer coating system was deposited on glass fiber reinforced polyimide matrix composites (FPM) by means of High Velocity Oxygen Fuel (HVOF) for metallic coatings and Atmospheric Plasma Spraying (APS) for YSZ coating. The influences of different bond coats and different thickness of the top coat on the thermal shock resistance and thermal ablation resistance of the coating system was investigated. Compared with Al particle, Cu particle has a high density and correspondingly a high kinetic energy during spraying by HVOF, resulting in a high bonding strength between the Cu coating and FPM substrate. In the thermal shock test, the coating Cu/NiCrAlY/YSZ has a much longer lifetime than the coating Al/NiCrAlY/YSZ. After high-speed impact on the substrate, there is a great compressive stress at the interface, which makes a plastic deformation to the substrate, and the particles are closely embedded into the substrate to form a strong mechanical interlock. The coating system consisting of 50 μm Cu, 50 μm NiCrAlY, and 200 μm 8YSZ exhibited the best thermal shock resistance, thermal ablation resistance and bonding strength. The increase of the top coat thickness will lead to the increase of residual stress and the decrease of bonding strength. The failure mechanism of the coating is mainly attributed to the residual stress in the deposition process and the thermal stress caused by thermal expansion mismatch.

The Influence of Difference of Top-Coat Composition and Thickness on the Residual Stress of Ebcs Fabricated by APS: A Finite Element Simulation Study

The Influence of Difference of Top-Coat Composition and Thickness on the Residual Stress of Ebcs Fabricated by APS: A Finite Element Simulation Study

Nondestructive evaluation of thermal barrier coating thickness degradation using pulsed IR thermography and THz-TDS measurements: A comparative study

Thermal barrier coatings are extensively used in aircraft engines. During service, the TBC coatings degrade because of erosion and sintering by hot gas flow and also by localized wear due to rubbing of flaps with spacers. It is necessary to assess the condition of the coatings as a function of service life through suitable non-destructive means. Pulse Thermography (PT) and Terahertz-Time Domain Spectroscopy (THz-TDS) techniques are used to evaluate the degree of degradation of the thin Air Plasma Sprayed (APS) TBCs top coat thickness. Infrared thermography has the advantage of fast inspection of a large area. In this work, we used a simplified analytical model aided calibration and development of a regression model to quantitatively analyze the thickness degradation in real-world TBC samples that have endured varying service life. These measurements were later verified using THz-TDS imaging, an emerging technique for accurate thickness measurements. Assuming the refractive index of the topcoat material, Yttria-Stabilized Zirconia (YSZ) as 4.8, the topcoat thickness of the entire specimen has been estimated using THz-TDS reflection mode setup. Results show that, the thickness values are varying between 94.94 μm - 114.96 μm for 500 h serviced samples and 32.5 μm - 91.96 μm in the case of 1000 h serviced samples demonstrating loss of TBC with increased service life. Comparison of the pulse thermography results with THz-TDS reveals a mean relative error of less than 10.3% in TBC thickness estimation. Further the results of both the techniques are cross-validated with Eddy current testing and optical microscopy. The proposed non-destructive techniques for TBC estimation will aid in the accurate creation of engine digital twin and will help in scheduling preventive maintenance measures thus increasing the life and safety of key aircraft engine components.

Study of wear behaviour of atmospheric plasma spraying composite coating on MS C40

The SiC + ZrO2 + La2O3 composite coating was deposited on the surface of Mild steel C-40 substrate by using air plasma spraying technology, which was done to relieve the wear of substrate during operation. Commercially available Nickel Chromium (Ni-Cr) is selected as bond coat, for top coat a composite of Silicon Carbide (SiC), Zirconium Oxide (ZrO2) and Lanthanum Oxide (La2O3) is selected. Bond coat is done for 75 µm. Top coat thickness is varied between 100 and 300 µm. APS ceramic coating is selected as it gives more importance to obtain the uniform thickness on the substrate. In this work the surface roughness is measured by using Surfcom FLEX 50-A measuring instrument, Morphology of the above TBC system are studied with the help of scanning electron microscope, Coating thickness is done by using image analysis technique, the porosity measured by using image analysis software and the wear test has been performed on Pin-on-disc at, room temperature. From the results and discussion, it can be stated that coating shows good wear and heat resistance.

Mechanisms and control of edge interfacial delamination in a multilayer system containing a functionally graded interlayer

The edge interfacial delamination mechanisms for a system comprising of a metallic substrate (316 L stainless steel), stepwise functionally graded (FG) interlayer (316 L stainless steel and alumina) and top coat (silicon carbide (SiC)) were studied for the first time utilizing an axisymmetric cylindrical model and interfacial failure criteria. Finite element analysis was used for this investigation with the assumptions of ideal bonding between layers and an initial stress-free temperature from which the model was cooled down (as might be achieved from plasma spray coating). The influence of graded interlayer composition factor on delamination behaviour of the two system interfaces was investigated using four separate interfacial failure criteria. The SiC top coat thickness was then considered and studied as another critical parameter in the system. Finally, a novel delamination failure map was obtained for the studied system with a FG layer based on interfacial fracture toughness ratios as a function of SiC thickness and composition factor. It was shown using this map that delamination in a multilayer system comprising a FG interlayer and top coat can be controlled by modifying the FG interlayer composition factor and thickness of the top coat.

Numerical Analysis of Composite Coating(Al2O3+ZrO2·5CaO) applied on Cast Iron

A new method, the rule of the mixture is developed for the determination of thermal conductivity (K), allied properties viz.density and the volume of the individual composite coating powders. Optimum coating thickness through steady-state thermal analysis of composite coating (Al2O3+ZrO2·5CaO) applied on cast iron surface is carried out. Also, the thermodynamic behaviour of TGO and bond coat were simulated. The thickness of the topcoat, TGO, and the bond coat was varied independently in the range of 0.1 to 0.04 mm. Results show that topcoat thickness doesn’t have much influence on the heat transfer. However, TGO and the bond coat plays a crucial role in heat transfer study. Maximum heat resistance, 171.47 ºC observed at 1 mm TGO thickness, compared to 88.94 ºC in case of bond coat for the same coating thickness. It is also understood that 1 mm of optimal TGO thickness may be beneficial in the design of thermal barrier composite coating systems.

Determination of Thermal Barrier Coatings Layers Optimum Thickness via PSO-SA Hybrid Optimization Method concerning Thermal Stress

Turbine entry temperature of turbo-engines has been increased to improve proficiency. Consequently, protecting the hot section elements experiencing aggressive service conditions necessitates the applying of thermal barrier coatings (TBC). Developing TBC systems and improving performance is an ongoing endeavour to prolong the lifetime. Thus, various studies have been conducted to find the optimum properties and dimensions. In this paper, the optimum thickness of intermediate bond coat (BC) and top coat (TC) have been determined via a novel hybrid particle swarm and simulated annealing stochastic optimization method. The optimum thicknesses have been achieved under the constraint of thermal stress induced by thermal fatigue, creep, and oxidation in the TC while minimizing the weight during twenty cycles. The solutions for BC and TC thicknesses are respectively 50 μm and 450 μm. Plane stress condition has been adopted for theoretical and finite element stress analysis, and the results are successfully compared.

Experimental investigation of the wear mechanisms of thin PDA/PTFE coatings

The wear mechanisms of thin polydopamine (PDA)/polytetrafluoroethylene (PTFE) coatings on stainless steel substrates were studied. Scratch, durability, and wear progression tests were performed on samples with three PTFE topcoat thicknesses. It was found that the wear process involved plastic deformation, plowing, adhesive spalling, large-area delamination, and abrasive wear. Thicker PTFE coatings had a lower initial coefficient of friction but a higher initial wear rate, which was attributed to the larger amount of loosely connected PTFE nanoparticles. The thickest coating had a slightly longer wear life due to the delayed delamination from the PDA layer.