Bond Coat Research Articles

Effect of laser cladding layers on microstructure and mechanical properties of NiCoCrAlY bond coat

The NiCoCrAlY alloy is usually used as a bonding layer material in thermal barrier coatings (TBCs) system. Owing its excellent high-temperature oxidation resistance and corrosion resistance, it plays a crucial role in maintaining the structural integrity and functionality of TBCs. Laser cladding technology is a flexible and efficient processing technique for the manufacturing of NiCoCrAlY bond coat, with the characteristics of precisely control the layers of the laser cladding and further modulate the quality of the bond coat. Therefore, the investigation on the influence of cladding layers on the microstructure and mechanical properties of the bond coat is of great significance for improving the service performance and lifetime of aircraft engines. To this end, the scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) were used in this study to explore the influence of cladding layers on the microstructure and mechanical properties of the NiCrAlCoY bond coat. The results indicate that the microstructure of the cladded alloy can be divided into substrate, heat-affected zone, and cladded zone. The hardness of both the heat-affected zone and the cladded zone increases as the number of cladding layers increases, while the hardness of the substrate maintains constant. In addition, as the number of laser cladding layers increases, so does the residual stress of the cladded alloy.

Synergistic effects of Pt and Y addition in (Ni, Pt)CrAlY bond coat on oxide spallation resistance and growth of interdiffusion zone between bond coat and Ni-based single crystal superalloy

This study shows the beneficial effect of Pt addition to the (β+γ)-NiCrAlY coating. Adding Y in NiCrAl increases oxide growth kinetics but improves spallation resistance. Pt with Y does not change the oxidation rate compared to only adding Y but significantly reduces spallation when cooled to room temperature. Continuous layers of Al2O3 and Cr2O3, along with other oxides such as YAlO3, NiCr2O4, and NiAl2O4, are observed. Moreover, Pt addition reduces the thickness of the interdiffusion zone where Al goes through an uphill diffusion profile, minimizing Al loss and significantly decreasing the growth of deleterious TCP phases.

Microstructural evolution and high temperature hot corrosion behaviour of AlCoCrFeNiTi high-entropy alloy coatings

The fuels used contain undesirable impurities such as Na, V, K, S, and Mg in gas turbine engines. These impurities react with hot components under service conditions, leading to the deterioration of the thermal barrier coatings (TBCs) structure and the formation of hot corrosion damage. In this study, metallic powders containing CoNiCrAlY were deposited as bond coat on Inconel 718 superalloy material using high velocity oxygen fuel (HVOF) coating technique. High entropy alloy (HEA) powder containing AlCoCrFeNiTi was synthesized as coating material using mechanical alloying (MA) method. AlCoCrFeNiTi powders were produced using atmospheric plasma spray (APS) coating technique. The produced coating system was subjected to hot corrosion tests at 700 °C and 800 °C temperatures for different time periods in corrosive environment conditions containing Na2SO4 (sodium sulfate) and V2O5 (vanadium pentoxide). Hot corrosion behavior of the coating system was determined, damage formations and degradation processes were investigated in detail. As a result of the determination and synthesis of HEA powder materials, production of coatings, experimental studies and hot corrosion test studies under service conditions, it was seen that the Ti-containing AlCoCrFeNi HEA coating system has usable qualities without any damage such as cracking, buckling or spalling.

Thermal life assessment of laser powder-directed energy deposited NiCrAlY/CuCrZr bimetallic clad for rocket nozzle applications

To enhance the thermal life of rocket exhaust nozzles, the hot side of copper liners is coated with thermal barrier coatings (TBCs) to provide thermal insulation and oxidation resistance. However, interface failures often occur between M-CrAlY bond coats and nozzle liners due to significant differences in their thermal expansion coefficients (CTE). This study explores the use of Laser Powder-Directed Energy Deposition (LP-DED) to clad NiCrAlY onto a CuCrZr substrate, as the process offers localized heating which can offer better bond strength. Optimization trials were conducted using single and multi-track studies to identify optimal parameters. Due to the low energy absorption of the CuCrZr substrate to 1070 nm laser sources, cladding was performed at a high energy density of 135 J/mm2 with a 1.2 g/min feed rate to achieve defect-free clads with sufficient diffusion. The bulk of the NiCrAlY clads showed γ′-Ni3Al, β-NiAl, and γ-Ni phases, while Y4Al2O9 and Y2O3 oxides formed on the top surface due to aluminum and yttrium depletion at high temperatures. The clads exhibited cellular dendritic microstructures at the bulk region, and planar microstructures were observed at the dilution zone. EBSD-KAM maps showed higher dislocation density near the interface due to CTE mismatch across substrate and clad. Scratch tests confirmed strong adhesion with no interface cracks, though crack propagation was observed from the edges after 50 isothermal cycles, driven by copper erosion. With Cu diffusion, interface region exhibited a graded microstructure which could enhance CTE, improving compatibility compared to standard NiCrAlY alloys.

Assessment of dwell-fatigue properties of nickel-based superalloy coated with a multi-layered thermal and environmental barrier coating

A three-layered experimental thermal and environmental barrier coating (TEBC) was deposited using air plasma spraying technology on cylindrical specimens of nickel-based superalloy MAR-M247. TEBC consists of mullite (Al6Si2O13) and hexacelsian (BaAl2Si2O8) upper layer, Y2O3 stabilised ZrO2 interlayer and CoNiCrAlY bond coat deposited on the grit-blasted surface of MAR-M247. The cyclic plastic response and damage mechanisms in uncoated and TEBC-coated MAR-M247 have been studied in isothermal dwell-fatigue tests conducted under strain control with constant strain amplitude at 900 °C. In each cycle, 5-minute dwells were introduced in both tensile and compression peaks of the hysteresis loop. Fatigue hardening/softening curves, stress relaxation curves, cyclic stress-strain curves and fatigue life curves are reported. Ceaseless mild softening has been found in both uncoated and TEBC-coated MAR-M247. TEBC-coated MAR-M247 showed an improved lifetime in the whole range of tested strain amplitudes. Data obtained from stress relaxation curves were used to assess the fraction of creep damage. The generalised damage accumulation rule was used to evaluate damage due to fatigue-creep-environment interaction. A study of the surface relief and internal microstructure using SEM and TEM helped to interpret the specifics of fatigue behaviour of uncoated and TEBC-coated material. The effectiveness of this newly developed TEBC, together with the dwell sensitivity of MAR-M247, were discussed from the perspective relevant to dwell-fatigue cyclic straining.

Data-driven AI for the automated classification of the isothermal heat-treated thermal barrier coatings using pulsed infrared thermography

Abstract Thermal barrier coating (TBC) is a multilayer coating applied to metallic structures exposed to high temperatures, such as aero-engine parts and gas turbine blades. These thermally insulating materials are used to extend the component life by minimizing the high-temperature exposure of structural components. As a result of the high-temperature oxidation tests, a thermally grown oxide (TGO) layer formed at the interface between the ceramic topcoat layer and the bond coat layer. The primary cause of TBC performance decline and structure failure is the growth of the oxide layer. Hence, it is crucial to regularly monitor the formation of the oxide layer and the degradation of coatings. This can be achieved by developing coating-life prediction models, which are vital for quality control, health monitoring of TBCs, maintenance decision-making, and the prevention of catastrophic failures. Existing methods for life estimation, which are dependent on empirical methods and microstructural analysis, often fail due to various material compositions and service conditions. This study proposes an automated classification of iso-thermal heat-treated TBC samples using temperature data, which helps in predicting the TBC life and monitoring the TBC degradation. TBC-coated samples are isothermal heat-treated at 1000 °C, and the initial growth of TGO is monitored using a non-destructive thermal imaging technique. Identifying distinctive features in each class corresponding to layer thickness measurement and service hours is challenging and time-consuming. So, we integrate data-driven AI models and feature extraction techniques to interpret complex thermal patterns. The performance of deep learning models and classical machine learning models demonstrates that our method can achieve maximum classification accuracy and F1-score of 89.2% and 89.0%, respectively. This automated classification framework promises a powerful tool for predictive maintenance and remaining useful lifetime estimation of hot section components reliant on TBCs.

The Effect of Shot Blasting Abrasive Particles on the Microstructure of Thermal Barrier Coatings Containing Ni-Based Superalloy

Grit particles remaining on the substrate surface after grit blasting are generally considered to impair the thermal performance of thermal barrier coatings (TBCs). However, the specific mechanisms by which these particles degrade the multilayer structure of TBCs during thermal cycling have not yet been fully elucidated. In this study, the superalloy substrate was grit-blasted using various processing parameters, followed by the deposition of thermal barrier coatings (TBCs) consisting of a metallic bond coat (BC) and a ceramic top coat (TC). After thermal shock tests, local thinning or discontinuities in the thermally grown oxide (TGO) layer were observed in TBCs where large grit particles were embedded at the BC/substrate interface. Moreover, cracks originated at the concave positions of the TGO layer and propagated vertically towards BC; these cracks may be associated with additional stress imposed by the foreign grit particles during thermal cycling. At the BC/substrate interface, crack origins were observed in the vicinity of large grit particles (~50 μm).

Thermal cycling and steam corrosion behavior of Yb3Al5O12‐based multilayered environmental barrier coatings for SiCf/SiC

AbstractTo alleviate thermal mismatch problem of environmental barrier coatings (EBCs) on SiC fiber‐reinforced SiC ceramic matrix composites (SiCf/SiC CMCs) surface and improve their high‐temperature durability for aircraft engines, by fully utilizing the appropriate thermal expansion coefficient, low oxygen transmittance, low thermal conductivity, and other advantages of ytterbium aluminum garnet (Yb3Al5O12), the double ceramic layered Yb3Al5O12/Yb2Si2O7 (DCL‐Yb3Al5O12) and the triple ceramic layered Yb3Al5O12/Yb2SiO5/Yb2Si2O7 (TCL‐Yb3Al5O12) EBC systems were prepared on SiCf/SiC CMCs by atmospheric plasma spraying (APS). Their phase composition and microstructures were investigated comparatively. The thermal cycling and water vapor/oxygen corrosion behavior of these coating systems were compared at 1300°C. The results showed that the thermal cycling life of the DCL‐Yb3Al5O12 and TCL‐Yb3Al5O12 EBC systems were 295 and 320 times, respectively. The failure of both the coating systems occurred between the Yb2Si2O7 layer and Si bond coat. The strength retention rate of DCL‐Yb3Al5O12 and TCL‐Yb3Al5O12 EBC systems after water vapor/oxygen corrosion for 70 h was 12.8% and 23.1%, respectively, and the fracture modes of both the systems exhibited “pseudoplastic” characteristic. TCL‐Yb3Al5O12 EBC systems with a gradient structure of more layers exhibit more excellent high‐temperature durability than DCL‐Yb3Al5O12 EBCs.

Revealing the oxidation growth mechanism and crack evolution law of Si-HfO2/Yb2Si2O7/Yb2SiO5/high-entropy hafnate thermal/environmental barrier coatings during thermal cycling

The Si-HfO2/Yb2Si2O7/Yb2SiO5/(Dy0.2Ho0.2Er0.2Tm0.2Lu0.2)2Hf2O7 thermal/environmental barrier coating (T/EBC) protects ceramic matrix composites (CMCs) from turbine multi-corrosive media erosion. Challenges persist in the thermal cycling performance of multi-layered T/EBC, notably in understanding the oxidation of the Si-HfO2 bond coat, compatibility of its mixed thermally grown oxide (m-TGO) with adjacent layers, and the evolution of cracks caused by thermal cycling. Utilizing plasma spraying physical vapor deposition (PS-PVD), this T/EBC on CMC substrates withstands up to 200 hours at 1450 ℃ to 1550 ℃. The m-TGO oxidation follows a parabolic growth curve, with oxygen diffusion activation energies of 160.31 kJ/mol from 1450 ℃ to 1500 ℃, and 125.16 kJ/mol from 1500 ℃ to 1550 ℃. Thermo-mechanical calculations indicate that elastic strain energy accumulation causes interlaminar cracks between m-TGO and adjacent layers. Controlling mud crack density is key to preventing the stress attraction at the tips of bifurcated cracks, thereby avoiding the formation of interlaminar cracks.

Investigating Hot corrosion, CMAS, and Thermal Shock Behaviour of Double-layer YSZ/La2Ce2O7 + YSZ Thermal Barrier Coatings

AbstractIn this work, new double-layer YSZ/La2Ce2O7 (LC) + YSZ coatings were developed using air plasma spraying (APS). The surface of the prepared coatings was relatively smooth and consisted of melted and partially melted areas. Their resistance to hot corrosion, CaO-MgO-Al2O3-SiO2 (CMAS), and thermal shock were examined. YSZ was added to the upper layer to enhance the lanthanum cerate (La2Ce2O7, LC) properties. During the hot corrosion tests, the corrosion salt reacted with the upper layer, and the CeO2 phase and new corrosion products were identified. The main phase was LaVO4, and the secondary phases were CeVO4 and YVO4. SEM confirmed the formation of new, cuboidal-shaped corrosion products. The infiltration of CMAS led to the formation of additional new products: Ca4MgxAl4Si(6-x-γ)O14 and Ca2.8(LaxCe1-x).6(SiO4)O6-4x. SEM revealed CMAS infiltration through the upper layer in the form of islands. Following the thermal shock resistance tests, the upper layer gradually peeled off, and the coating survived 67 cycles. Possible failure mechanisms were identified, and failure was attributed to the spallation of the upper layer from the surface layer by layer. After all tests, the top layer showed partial spalling and delamination. This was mainly caused by the reaction of corrosive salt or CMAS with the top layer, which changed its composition, leading to the formation and propagation of cracks and, ultimately, the separation of part of the upper layer. Peeling of the upper layer through mainly horizontal cracks was observed after hot corrosion, CMAS and thermal shocks. The NiCrAlY bond coat and YSZ interlayer remained undamaged.

Environmental barrier coatings on alumina-reinforced CMCs: Experimental validation and numerical modeling of thermal stability

Recent advancements in ceramic matrix composites (CMCs), including SiC/SiC, C/SiC, C/C, Al2O3 reinforced Al2O3, face various challenges at high temperature applications, including grain growth, sintering, and creep deformation which are leading to strength loss. The new generation of Environmental Barrier Coatings (EBCs) must allow the protection of these CMCs when operating under harsh conditions, which can reach up to 1100 °C in some applications, such as gas turbines and certain reforming processes. In this work, 8 wt% yttria stabilized zirconia (8YSZ) applied by Atmospheric Plasma Spraying (APS) on top of a porous alumina oxide matrix with reinforced alumina fibers Al2O3/Al2O3-CMC (OCMC), considering different interfaces, has been studied. Surface morphology and pre-treatment of CMCs is the most challenging part for thermal spray applications. Hence, two different surface structure were used in this work. The results show that the roughness of OCMC can be reduced from Rz = 41.8±6.8 μm to 20.3±1.1 μm with the new porous bond coat and plasma sprayed coating, resulting in a homogeneous surface morphology.. In terms of thermal stability, a numerical model that combines Object Oriented FEM (OOF) and Cohesive Zone Model CZM tools, has been developed to evaluate the effect of the porosity of the real microstructure and the characteristics of the interfacial roughness profile, respectively, in CMC/EBC structures. In addition, other materials, such as La2Zr2O7 and Y2O3, have been numerically studied in order to evaluate their suitability as EBC.

Physical and microstructural properties of YSZ + Cr2O3 thermal barrier coatings developed using HVOF technique

A composite coating material of Yttria-Stabilized Zirconia (YSZ) and Chromium Oxide (Cr2O3) was developed on the surface of Al-6061 substrates. The coating consisting of equal measures of the composite material of YSZ and Cr2O3 was successfully obtained using the High-Velocity Oxy Fuel (HVOF) technique. The surface roughness and coating thickness of the composite coatings are evaluated. The surface roughness (Ra) was evaluated showing an average of 6.0848 µm using the Surface Roughness Tester (SRT). The coating thickness was evaluated showing an average of 220.8 µm using the Coating Thickness Tester (CTT). The surface roughness and the coating thickness obtained are comparable to typical physical properties of Thermal Barrier Coatings (TBC’s). The microstructural evaluation of the coated specimens was performed through the Scanning Electron Microscope (SEM), showing the clear unification of materials The determination of the elemental composition of surface coatings was performed through the EDAX tests, which clearly established the elements in the coatings. This article is indicative of the achievement of TBC’s consisting of composite materials though HVOF technique as an alternative to TBC’s obtained through surface coats and bond coats.

Comprehensive study of the effect of Hf and Ta co-doped MCrAlY bond coat on the high-temperature properties of thermal barrier coating

The MCrAlY bond coat is modified by reactive element (RE) Hf and refractory element Ta, and the high-temperature oxidation resistance of the thermal barrier coating (TBC) before and after the modification are investigated by comparative experiments. The experiments use high-velocity oxygen-fuel (HVOF) to prepare NiCoCrAlY and NiCoCrAlYHfTa bond coats on the surface of GH4099 high-temperature alloy, and then yttrium stabilized zirconia (YSZ) ceramic layers are prepared on the surface of the bond coat by atmospheric plasma spraying (APS), and cyclic oxidation is carried out in air at 1050 °C until failure. It is shown that the oxidation lifetime of the modified TBC reached 1992 h, which is one time higher than that of the unmodified TBC. The doping of Hf and Ta elements reduces the growth rate of the oxide layer, and these diffuse externally and combine with the internally diffused O to form HfO2 and Ta2O5 in the thermally grown oxide (TGO) layer, thereby improving the adhesion and mechanical properties of the oxide layer. In addition, the bond strength and thermal shock resistance of the TBCs are significantly improved after doping with Hf and Ta elements, which prolongs the lifetimes of the TBCs. In this paper, the morphology, structure, and properties of the modified TBCs are characterized to investigate the mechanism of the oxidation lifetime extension.

Understanding the deposition of multilayered hydroxyapatite-bioactive glass/hydroxyapatite/titanium dioxide coatings on PEEK substrates by plasma spray

Poly-ether-ether-ketone (PEEK) is a polymeric material that is often used in biomedical applications due to its excellent mechanical properties and radiolucency; this material is, in general, biologically inert. In recent decades, thermally sprayed biocompatible coatings have been widely employed in metallic implants to improve the osseointegration process in the human body. Thus, not surprisingly, thermally sprayed biocompatible coatings can be envisaged as an excellent alternative to improve the material's surface properties. However, there is little information in the literature about the deposition of thermally sprayed bioactive coatings on polymeric substrates. In the present study, a titanium dioxide (TiO2) powder was plasma-sprayed on PEEK substrates acting as a bond coat to further deposition of hydroxyapatite (HA)/bioactive glass multilayered coatings. The design of experiments methodology was employed to produce coatings with tailored properties. A key aspect in this work is how the experiments were carried out to understand the effect of powder features, such as chemical composition, on the deposition of the ceramic coatings on the PEEK substrate. Heating/cooling physical laws were employed to discuss the results obtained. The methodology employed in this work allowed to deposit a multilayered coating with adhesion strength of about 50 MPa on PEEK substrates measured using scratch test. The coatings' structural, morphological, chemical, thermal, and mechanical evaluation was performed using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, differential scanning calorimetry, and micro indentation test, respectively. In-vitro tests were also performed on these coatings under static simulated conditions. The results show that the PEEK's surface can be successfully functionalized, turning it from showing an inert behavior to displaying bioactivity by applying a HA-based bilayer or multilayered coating.

Development and characterization of corrosion-resistant plasma-sprayed Ni-Al composite coatings via graphene oxide fillers

In this study, the authors propose a procedure to enhance pure Ni-Al powders through graphene oxide dispersion in water for plasma spraying. The enhanced powders were utilized to coat A36 steel via plasma spraying. Next, the powders and the coatings were characterized by energy-dispersive X-ray spectroscopy, X-ray diffraction analysis, micro-Raman spectroscopy, and field emission scanning electron microscopy. Given the capability of the generated X-ray to characterize near surfaces to a depth of a few micrometers, the performed X-ray microanalyses convey that the carbon species incorporated within the powders through the graphene oxide dispersion can act as nanofillers reducing the number of holes/pores in the plasma-sprayed coatings. Potentiodynamic polarization testing and electrochemical impedance spectroscopy were used to investigate the corrosion behavior of the samples at two intervals: after 1 h and after 30 days of exposure to a 3.5 % NaCl solution. According to the results of the corrosion testing, the enhanced coatings can display about 2.35 times lower corrosion current density compared to pure Ni-Al coatings, with the corrosion potentials of the enhanced coatings being higher than that of pure Ni-Al coatings (i.e., improved corrosion resistance). This improvement in corrosion resistance is also evident when the enhanced coatings are compared to the A36 substrate after 30 days of exposure to the solution. It appears that the incorporated carbon nanofillers, by reducing deep holes and pores that communicate with the surface of the substrate, ultimately determine the highest corrosion resistance. Thus, the outcome offers enhanced Ni-Al coatings possessing superior anticorrosion properties that typical plasma-sprayed Ni-Al bond coats may not exhibit.

Simulation and experimental study of femtosecond laser ablation mechanisms of NiCoCrAlY coatings

To enhance the adhesion of turbine blade thermal barrier coating systems using the LST method, a thorough understanding of the ablation mechanism of NiCoCrAlY bond coat material by femtosecond laser systems is essential for producing high-quality textured grooves. This study systematically investigates the ablation mechanisms of NiCoCrAlY material, exploring the effects of laser energy density, laser scanning speed, and the number of laser scans on the ablation of NiCoCrAlY. Numerical simulations based on the two-temperature model were conducted, providing a comprehensive analysis of thermal effects, heat accumulation, and material response during the laser ablation process. The experimental results indicate that 1) the ablation phenomenon caused by heat accumulation becomes evident as the laser energy density increases from 1.948 J/cm2 to 4.521 J/cm2, with the accumulated heat reaching 1525.2 K, leading to distinct melting residues and heat-affected zones on the groove walls. 2) The change in laser scanning speed also affects heat accumulation. Using a laser scanning speed of 800 mm/s results in a high material removal rate, smooth machined walls, and a uniform surface with no significant heat-affected zones. 3) Excessively high numbers of laser scans shift the laser focus to the bottom of the groove. The high concentration of laser energy causes intense localized ablation, forming sharp bases with numerous cracks and melting residues. To achieve efficient and high-quality laser ablation, it is necessary to ensure that the number of scans remains below 40.

Thermal response of TBC systems subjected to non-uniform thermal shocks and periodic thermal load: Effect of Biot number and interface imperfection

This study analytically examines the temperature variations within thermal barrier coating (TBC) systems under diverse kinds of thermal loadings, such as concentrated/distributed convective thermal shocks and oscillating thermal loads. In this study, the TBC system comprises ceramic top coat (TC) and NiCrAlY alloy bond coat (BC). The transient behavior of the TBC system facing various kinds of non-uniform convective thermal shocks is elaborated via the dual-phase-lag (DPL) heat conduction model. The effect of imperfect bonding of the TC/BC interface on transient responses of the regions in the neighborhood of the TC/BC interface is studied by considering impermeable, partially permeable and completely permeable heat flux flows. Laplace and Fourier integral transforms are employed to derive the exact solutions of time-dependent temperature fields governing equations. The prominent effects of sudden thermal impulse and steady alternating thermal loading on the variations of temperature within the TC, BC and substrate are inspected. The analysis reveals that the kind of thermal load, loading parameters, thermal properties and thickness of ceramic TC directly affect the transient/periodic temperature fields inside the TBC constituents. Meanwhile, the interface imperfection and the Biot number of convective thermal load play an impressive role in the severity of thermal response of the TBC constituents.

Polymer-derived coatings with La2Zr2O7 and glass fillers: Preparation and characterisation

This study investigated the impact of La2Zr2O7 (LZ) and different types of glass on the performance of polymer-derived ceramic (PDC) coatings on AISI 441 stainless steel substrates. Four double-layer PDC-based glass-ceramic coatings containing LZ and different glass fillers were prepared by dip coating. The LZ powder was synthesised by solid-state reaction (SSR): powder morphology, crystal structure, and thermal stability were analysed. X-ray diffraction (XRD) detected a LZ pyrochlore phase after annealing at 1300 and 1400 °C with a trace of t-ZrO2. Four different glass compositions, namely BaO-Al2O3-SiO2 (BAS), BaO-Al2O3-La2O3-B2O3-SiO2 (BALBS), CaO-B2O3-SiO2 (CBS), and BaO-ZnO-MgO-B2O3-SiO2 (BZMBS), were also synthesised as fillers for PDC coatings. The glass transition and crystallisation temperatures of the glasses were determined using differential scanning calorimetry (DSC). The coating systems, consisting of a Durazane 2250 bond coat and a top coat (Durazane 1800 + LZ filler + different glass sealants), were prepared. After pyrolysis of the coatings at 900 °C, some of the glasses partially crystallised. Scanning electron microscopy (SEM) revealed that the layers containing BAS, BALBS and CBS glass were dense, with good adhesion to the substrate, and with occasional presence of larger pores and cracks. Delamination of the upper layer was observed in the coating with the BZMBS glass filler.

Heterogeneous multilayered delamination of thermally grown oxide accelerating spallation of thermal barrier coatings

Long lifetime of thermal barrier coatings (TBCs) is limited by the localized thermally grown oxide (TGO) accumulation. Herein, the heterogeneous multilayered delamination mechanism of TGO is revealed. TGO cracking preferentially occurs at the peak of bond coat due to mismatch and extends toward the flank region under TGO growth. The thinner sublayer after first delamination is mainly attributed to loss of ceramic constraint. The delamination accumulation increases the YSZ crack driving force and provides a fast propagation channel for YSZ crack to pass through TGO. These results provide a guidance for the development of advanced TBC with higher oxidation resistance.

A non-destructive measurement method for thermal barrier coating thickness in a robot polishing process based on point cloud processing

The adhesion state between the bond coat and top coat of thermal barrier coatings (TBCs) is influenced by the surface roughness of the bond coat. Under certain operating conditions, polishing is employed to reduce the surface roughness of the bond coat, thereby enhancing the adhesion strength between the top coat and bond coat and improving the TBC’s resistance to high-temperature oxidation. In order to accurately control the thickness of the bond coat during the polishing process, this study proposes a non-destructive measurement method for coating thickness based on point clouds. By preprocessing, registering, and calculating the distance between the point cloud generated by laser scanning the turbine blade, the thickness of the bond coat was determined. The method exhibits greater accuracy in regions with small curvature variations. Compared to measurements obtained using a metallographic microscope, the maximum thickness deviation between the two methods is 4.004 μm, and the relative error is less than 5%. Experimental results demonstrate that this method can accurately measure coating thickness, offering advantages such as real-time processing, flexibility in application scenarios, high measurement efficiency, and relatively low requirements for coating materials.