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AbstractThe enhancement of turbine blade performance within turbo jet engines is crucial for prolonged engine life. Amidst various cooling strategies for turbine blades, thermal barrier coatings (TBCs) emerge as a highly effective method. Just as the choice of material for the turbine blade holds importance, so too do the materials comprising TBCs. The current research work aims to pursue finite element-based analyses of turbine blades with TBCs, an almost real complex-shaped geometric model comprising the blade body with a rabbet is essential, the stress generated during TGO growth which is crucial for analysis, to select the substrate from both a thermo-structural and cost perspective. In this research endeavor, TBCs were structured with a substrate, a bond coat (NiCoCrAlY), and a top coat of lanthanum cerate (La2Ce2O7) ceramic layers. A thermally grown oxide (TGO), namely α-Al2O3, forms between the bond coat and the top coat due to the elevated temperatures experienced. Two distinct substrates, Inconel 625 and Titanium-T6 alloy, were meticulously chosen as turbine blade materials. Commencing with NACA 4412 airfoil data, the turbine blade was meticulously designed using CATIA, followed by simulations conducted on ANSYS software through a static thermal structural model. The simulation aimed to determine the optimal top coat/thermally grown oxide (TC/TGO) and TGO/bond coat (TGO/BC) layer thicknesses to achieve minimal equivalent stress and total deformation in the turbine blade. The simulated results revealed that a combination of 400 µm TC/10 µm TGO and 10 µm TGO/100 µm BC provided the lowest equivalent stress and total deformation, thereby offering valuable insights for the current research. [ABSTRACT FROM AUTHOR]

The direct laser melting of plasma sprayed bond coat has the possibility of improving the surface topography and can be done with Yb:YAG laser. In the current paper, laser melting of plasma sprayed coatings of 50 wt. % standard Amdry 963 (Ni24.5Cr6Al0.4Y) and 50 wt. % standard Amdry 9621 (Ni22Cr10AlY) was carried out by using 600 W continuous wave Yb:YAG laser. Notably, the obtained melted tracks were investigated from the upper surface plan view and transverse section. The topography, microstructure, phases, hardness, and compositions of the plasma sprayed coatings and laserremelted tracks were characterized by scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), roughness, and microhardness. By controlling the dependent laser variables (power density and interaction time), a different effect can be obtained for the plasma sprayed bond coating (NiCrAlY). Typically, the results obtained show that at low specific energies, four different distinct zones were observed. These specific energies were not sufficient to produce a uniform melting. The MCrAlY coatings remelted with high specific energies were characterized by completely uniform melting and cellular/dendritic microstructures. The melted coatings have high roughness reduction (1.2 μm) and low hardness (275 HV0.5) in comparison with plasma-sprayed coatings (10.5 μm) and (315 HV0.5) respectively. [ABSTRACT FROM AUTHOR]

Thermal barrier coatings (TBCs) are commonly employed as thermal protection systems in harsh environments and are comprised of a ceramic top coat and a metallic bond coat. The traditional materials used for bond coats are MCrAlX alloys, where M represents metals or alloys (such as Ni, Co, or NiCo), and X represents reactive elements like Y, Hf, Ta, and Si. In high-temperature environments, high-entropy alloys (HEAs) have exhibited considerable potential as viable alternatives to bond coat materials, owing to their remarkable mechanical strength, thermal stability, and oxidation resistance. The distinctive compositional characteristics of HEAs can augment bond coat hot corrosion resistance by facilitating the formation of a thermally grown oxide (TGO) characterized by enhanced continuity, density, and uniformity, attributed to the synergistic effects arising from their unique alloying elements. Moreover, HEAs can modulate diffusion phenomena at the interface between the bond coat and substrate, particularly under elevated temperature conditions. This manuscript explores the thermodynamic, mechanical, and microstructural behavior of HEAs, as well as, the selection and application of HEAs as bond coats in comparison to conventional bond coats. [ABSTRACT FROM AUTHOR]

To reduce the formation of detrimental complex oxides, bond coatings in the thermal barrier coatings for gas turbines are typically fabricated using vacuum plasma spraying (VPS) or the high-velocity oxygen fuel (HVOF) process. Herein, VPS and HVOF processes were applied using NiCoCrAlY + HfSi-based powder to assess the oxidation behavior of the bond coatings for both coating processes. Each coated sample was subjected to 50 cyclic heat treatments at 950 °C for 23 h and cooling for 1 h at 20 °C with nitrogen gas, and the weight change during the heat treatment was measured to evaluate the oxidation behavior. After the oxidation test, the coating layer was analyzed with X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The VPS coating exhibited faster weight gain than the HVOF coating because the alumina particles generated during the initial formation of the HVOF coating inhibited oxidation and diffusion. The VPS coating formed a dense and thick thermal growth oxide (TGO) layer until the middle of the oxidation test and remained stable until the end of the evaluation. However, the HVOF coating demonstrated rapid weight loss during the final 20 cycles. Alumina within the bond coat suppressed the diffusion of internal elements and prevented the Al from being supplied to the surface. The isolation of the Al accelerated the growth of spinel TGO due to the oxidation of Ni, Co, and Cr near the surface. The as-coated VPS coating showed higher hardness and lower interfacial bonding strength than the HVOF did. Diffusion induced by heat treatment after the furnace cyclic test (FCT) led to a similar internal hardness and bonding strengths in both coating layers. To improve the quality of the HVOF process, the densification of the coating layer, suppression of internal oxide formation, and formation of a dense and uniform alumina layer on the surface must be additionally implemented. [ABSTRACT FROM AUTHOR]

Improving bonding and mechanical strengths is important for the application of bond coats used in the construction of steel deck bridges. Graphene nanoplatelets (GNPs) are attractive nanofillers for polymer modification because of their low cost, ultra-high aspect ratio, and extraordinary thermal and mechanical performance. In this paper, GNPs were used to reinforce the epoxy asphalt bond coat (EABC). The morphology, viscosity–time behavior, contact angle, dynamic mechanical properties, and mechanical and bonding strengths of GNP-reinforced EABCs were investigated using laser confocal microscopy, a Brookfield rotational viscometer, a contact angle meter, dynamic mechanical analysis, a universal test machine, and single-lap shear and pull-off adhesion tests. GNP dispersed non-uniformly in the asphalt phase of EABC. The viscosity of the neat EABC was lowered with the inclusion of GNPs and thus the allowable construction time was extended. The existence of GNPs enhances the hydrophobicity of the neat EABC. When adding more than 0.2% GNP, the storage modulus, crosslinking density and glass transition temperatures of both asphalt and epoxy of the neat EABC increased. The mechanical and bonding properties of the neat EABC were greatly enhanced with the incorporation of GNPs. Furthermore, the mechanical and bonding strengths of the modified EABCs increased with the GNP content. GNP-reinforced EABCs can be utilized in the pavement of long-span steel bridges with long durability. [ABSTRACT FROM AUTHOR]

Wear-resistant amorphous coatings Fe49.7Cr18Mn1.9 Mo7.4W1.6B15.2C3.8Si2.4 of about 300 μm thickness were deposited on mild steel substrate using a high-velocity oxygen fuel (HVOF) thermal spray method. The median Vickers hardness of the as-sprayed coating is 751.9 HV300 g, which further increases by 127% and by 112% when the coating is vacuum heat treated at 650 °C and 800 °C, respectively. The longer the amorphous phase is, the higher the Vickers hardness is. The relatively low porosity indicates excellent wear resistance of these amorphous coating, which were measured using a pin-on-disc method. The wear mechanism of the current Fe-based amorphous coating varies from oxidative wear to delamination wear when the sliding speed changes from low (0.4 cm/s) to high (4.0 cm/s). It is believed that the upsurge in hardness and the wear resistance of the applied coating is due to the optimized selection of the heat treatment process (i.e., at 650 °C), which has a significant impact on the microstructure, as well as the low porosity, with better coating adherence and fewer cracks. Under dry sliding wear circumstances, the coating demonstrated good wear resistance, with wear rates of (2.81–16.30) x 10−6 mm3/N m. Due to the properties, low porosity, and high hardness especially in case vacuum heat treatment 650 °C, these coatings produced by (HVOF) exhibit a high ability to resist better friction and wear performance that indicates to ability application it in facilities steel.

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 Al 2 O 3 and Cr 2 O 3 , along with other oxides such as YAlO 3 , NiCr 2 O 4 , and NiAl 2 O 4, 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. [Display omitted] • Pt addition to (β+γ)-NiCrAl improves spallation resistance without affecting oxide growth kinetics. • Y addition increases oxide growth kinetics but improves spallation resistance. • Combined Pt and Y additions markedly enhance oxide layer spallation resistance. • TEM reveals α-Al 2 O 3 and Cr 2 O 3 oxide layers, spinels and Y-Al-rich oxide pegs at the bond-coat/Al 2 O 3 interface. [ABSTRACT FROM AUTHOR]

Environmental Barrier Coatings (EBCs) can prevent SiC-based ceramic matrix composites from corrosion at high temperatures. However, the low melting point of conventional Si bond layers limits the operating temperature of EBCs to 1350 °C. Herein, a MoSi 2 -HfO 2 duplex bond coat was designed and showed robust adhesion on SiC substrate after corrosion at 1450 °C for 100 h in steam/oxygen atmosphere. A mixed oxide layer was observed to in-situ form at the interface between the MoSi 2 and HfO 2 layers, containing SiO 2 , Hf-Si-O glass and HfSiO 4. This oxide layer was proven to effectively inhibit further oxidation/corrosion of the coating and the substrate. The phase composition evolution and anti-oxidation mechanism were systematically analyzed. • MoSi 2 -HfO 2 duplex bond coat showed robust adhesion on SiC at 1450 °C/100 h in steam. • In-situ interface reaction at MoSi 2 /HfO 2 forms Hf-Si-O glass and HfSiO 4 phase. • SiO 2 , Hf-Si-O glass and HfSiO 4 composite TGO effectively inhibit steam corrosion. • MoSi 2 -HfO 2 duplex bond coat is expected to improve the working temperature of EBCs. [ABSTRACT FROM AUTHOR]

[Display omitted] • CoNiCrAlY coating has been deposited successfully on Inconel718 by Laser Cladding. • The cladding is dense, defect-free with minimum dilution and HAZ. • This cladding has better microhardness than that of HVOF spray-coating. • The cladding also has superior oxidation resistance than the HVOF spray-coating. This study delves into laser surface cladding of CoNiCrAlY onto Inconel718 employing a 6.6 kW continuous wave diode laser with varying process parameters. The resulting microstructure, microhardness, and isothermal oxidation resistance are meticulously assessed. Laser surface cladding yields a defect free microstructure with the presence of gamma (γ), gamma prime (γ') and beta (β) phases. There is negligible dilution at the interface with a minimum HAZ. The average microhardness of the clad zone ranges from 425 to 465 HV, increasing with increase in scan speed and decrease in power. The high temperature oxidation resistance property of the clad surface is superior to the same developed by high velocity oxy fuel spraying. [ABSTRACT FROM AUTHOR]

Optimal deposition parameters for the aerosol deposition of a ¢-SiAlON coating and the microstructure change of an EBC after heat exposure in air are investigated. Dense and crystalline SiAlON coating having developed texture, where the (0001) plane is declined approximately 10° from the coating plane is formed. The deposition rate increases with the gas flow rate when the rate is ranging from 12 to 16 L/min. Further increase of the gas flow rate decreases the deposition rate. Regarding the 15µm thick mullite coating deposited on SiAlON substrate heat exposed at 1573K over 30 h, delamination of the coating occurs due to the oxidation of SiAlON. 30µm thick mullite coating prevents the oxidation. As for the EBC deposited on SiSiC substrate, delamination occurs at SiSiC/SiAlON interface by the oxidation of SiC during heat exposure at 1573 K. At the bonded region during heat exposure, SiAlON prevents the mullite coating to become (SiO2+mullite) two-phase state by supping Al to the mullite. Residual Si at the substrate moves to SiAlON and mullite coating under heat exposure at 1673 K. Structure of EBC is maintained by using SiC substrate in which the Si will not move to the coating during exposure. [ABSTRACT FROM AUTHOR]

The bonding strength of the bond coat plays an important role in the composite action between the wearing surface and the deck plate of the orthotropic steel deck system. Poor bonding results in the delamination of the wearing surface from the deck plate. Graphene oxide (GO) possesses outstanding mechanical and thermal properties, as well as impressive multifunctional groups, which makes it an ideal reinforcement candidate for polymer matrices. In this study, graphene oxide was used to improve the bonding strength and toughness of the epoxy asphalt bond coat (EABC). The dispersion, hydrophobicity, viscosity–time behavior, phase-separated morphology, dynamic mechanical properties, pull-off strength, shear strength and mechanical performance of GO-modified EABCs were investigated using various techniques. The inclusion of GO improved the hydrophobicity of the unmodified EABC. The viscosity of the unmodified EABC was lowered with the addition of GO during curing. Moreover, the allowable construction time for the modified EABCs was extended with the GO loading. The incorporation of GO enhanced the stiffness of the unmodified EABC in the glassy and rubbery states. However, graphene oxide lowered the glass transition temperature of the asphalt of the unmodified EABC. Confocal microscopy observations revealed that GO was invisible in both the asphalt and epoxy phases of the EABC. The inclusion of GO improved the bonding strength, particularly at 60 °C, and mechanical properties of the unmodified EABC. [ABSTRACT FROM AUTHOR]

An HfO2‐doped Si bond layer is expected to work at higher temperature with better oxidation resistance of environmental barrier coatings for SiC‐based ceramic matrix composites. Establishing the relationship between process parameters and powder feeders with the property of coatings could help to further optimizing their performance. Three sets of plasma spray processes and two kinds of HfO2 powders were used to fabricate Si–HfO2 bond coats with a different Si/HfO2 ratio. The particle size of HfO2 greatly affects the composition of coat, where smaller one induced an uncontrollable Si/HfO2 ratio inside coatings. Larger HfO2 powder greatly improved the composition uniformity with an Si/HfO2 ratio similar to the starting feeder. However, phase segregation occurred for both cases, indicating the limit of mechanical mixing method to make composite feedstock. More strategies should be explored for precise control of the phase and composition of composite coatings from spray technique. [ABSTRACT FROM AUTHOR]

As the operating temperature of advanced gas turbines typically exceeds 1400 °C, it has been required to replace conventional Si bond coat in environmental barrier coatings (EBCs) with materials possessing higher thermal stability. Since HfSiO 4 has excellent thermal properties such as a high melting point, phase stability over 1400 °C, and CTE matches with that of the SiC-based ceramic matrix composites, it has attracted much attention as a next-generation bond coat material. In this study, HfSiO 4 bond coat was successfully formed by atmospheric plasma spray with pre-mixed HfO 2 -SiO 2 powders (molar ratios: 7:3 and 5:5) followed by heat treatment. Effect of molar ratios of the HfO 2 -SiO 2 and post-heat treatment temperature (1375 and 1475 °C) on the formation of HfSiO 4 were studied. An oxidation test of the HfSiO 4 coating was carried out at 1475 °C with the conventional Si bond coat to verify whether the new bond coat was suitable for use in a thermal environment of 1400 °C or higher. From the results, the HfO 2 /SiO 2 ratio of 5:5 was suitable for the formation of HfSiO 4 than that of 7:3. After heat treatment at 1475 °C, the ratio of HfSiO 4 phase was 84.35%. The higher content of HfSiO 4 formed under 1475 °C, meaning the higher heat treatment temperature accelerated the HfSiO 4 formation. In the oxidation test at 1475 °C, the new HfSiO 4 bond coat showed no cracks and maintained its integrity, but the Si bond coat was oxidized and cracked severely. Therefore, it can be concluded that the new HfSiO 4 bond coat formed from 5HfO 2 –5SiO 2 coating is a potential candidate as a next-generation bond coat material in EBCs. [ABSTRACT FROM AUTHOR]

Plasma sprayed alumina coatings find numerous applications owing to their high electrical resistivity and wear resistance. However, the high-temperature thermal cycling performance of such coatings is a concern in many applications. Inconel 600 collars used in the high-temperature fission chamber neutron detectors need to have a ceramic coating with high electrical resistivity and thermal cycling performance. Alumina is coated on Inconel 600 collars by plasma spray process with an intermediate Ni-50Cr bond coat deposited by HVOF technique. The coated collars exhibited superior thermal cycling performance and insulation resistance of 2 × 108 Ω at 600 V DC after 100 thermal cycles at 923 K. The satisfactory performance of coated Inconel 600 collars suggests its potential application as fast reactor neutron detector. Further, the durability of alumina coated samples with Ni-50Cr and NiCrAlY bond coats deposited by plasma spray coating were also studied. Inconel 600 samples coated with Ni-50Cr bond coat with and without alumina top coat subjected to isothermal aging at 923 K for 1000 h showed a higher weight gain than those coated with NiCrAlY bond coat. The isothermally aged Ni-50Cr bond coat showed undesirable epitaxial growth with protruding Cr2O3 crystals (13.5 µm) while the NiCrAlY bond coat showed uniform thermally grown oxide (TGO) of 1 µm with fine disc-shaped pegs. The results indicated higher long-term durability of plasma sprayed alumina coated samples with NiCrAlY bond coat and the mechanisms of formation of TGO with Ni-50Cr and NiCrAlY bond coats at the service temperature are elucidated. [ABSTRACT FROM AUTHOR]

In a high-temperature, high-flame-velocity, and high-pressure gas corrosion environment, the intercolumnar pores and gaps of electron beam–physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs) may serve as infiltration channels for molten calcium–magnesium–alumino–silicate (CMAS), leading to the severe degradation of TBCs. In order to clarify the relationship between the roughness of the bond coat and the CMAS corrosion resistance of the EB-PVD TBCs, 7 wt.% yttria-stabilized zirconia (7YSZ) TBCs were prepared on the surfaces of four different roughness-treated bond coats. The effect of the bond coat roughness on the columnar microstructure of the EB-PVD YSZ was investigated. The effect of the change of the bond coat's microstructure on the CMAS corrosion resistance of the EB-PVD YSZ was studied in detail. The results showed that the reduction in the roughness of the bond coat contributes to the improved formation of the EB-PVD YSZ columns. The small and dense columns are similar to a lotus leaf-like structure, which could reduce the wettability of CMAS and minimize the spread area between the coating and the CMAS melt. Thus, the CMAS corrosion resistance of the coating can be greatly improved. This preparation process also provides a reference for the preparation of other TBC materials, improving the resistance to CMAS hot corrosion. [ABSTRACT FROM AUTHOR]

In this study, the feasibility of using ZrO2 coatings, fabricated through the atmospheric pressure plasma jet technique, as top coats in thermal barrier systems was explored. Inconel 617 was used as the substrate material, and sputtered laminated RuAl/RuAlZr/Ru was used as the bond coat. >20-μm-thick ZrO2 coatings were deposited through several iterations of deposition at 500 °C and annealing at 600 °C; here, ZrO2 existed in the tetragonal phase. The tetragonal ZrO2 was stable after annealing at up to 700 °C but transformed into a mixture of tetragonal and monoclinic ZrO2 phases when annealed at >800 °C. The thermal stability of the ZrO2/RuAl/RuAlZr/Ru/Inconel 617 assembly was evaluated at 800–900 °C in air. The variations in volumetric fractions of the tetragonal and monoclinic ZrO2 phases were recorded; 44% of the tetragonal phase remained after 100 h of annealing at 800 °C, whereas 80% of the tetragonal phase had transformed into the monoclinic phase after 4 h of annealing at 900 °C. Observation of BC after annealing at 800 °C indicated that a dense thermally grown oxide scale, comprising O and Al, formed between the ZrO2 and RuAl layers, which impeded the inward diffusion of O. The inhibition of O diffusion declined with the crystallization of the thermally grown oxide scale into the δ-Al2O3 phase when the annealing time was extended.

Epoxy asphalt bond coat (EABC) is one of the thermosetting polymer-modified asphalts, which has been widely employed as a strong waterproof bonding layer between the asphalt concrete and the orthotropic steel bridge deck. In the present work, the influences of asphalt content on the phase separation, viscosity, thermal stability, viscoelastic behavior, mechanical performance, pull-off strength, and adhesive performance of EABCs were characterized by various instruments. Laser scanning confocal microscopy observations showed that phase separation occurring in EABCs depended on the curing time. The asphalt merged to form large spherical particles in the continuous epoxy phase during the cure reaction. The size of asphalt particles increased in the asphalt content. Viscosities of EABCs were higher than that of the pure epoxy and increased with the asphalt content in the initial curing stage. However, the opposite trend was observed in the latter curing stage. The presence of asphalt slightly improved the glass transition temperature and the damping properties of the pristine epoxy. For EABCs, the damping behaviors slightly increased with the asphalt content. The thermal stability of the pristine epoxy was enhanced by the incorporation of asphalt. The addition of asphalt decreased the mechanical, adhesive properties, and pull-off strength of the pristine epoxy. What's more, these properties of EABCs decreased in the asphalt content. [ABSTRACT FROM AUTHOR]

The potentials and limitations of a halide-activated pack cementation process on SiC/SiC Ceramic Matrix Composites for the development of bond coats as part of environmental barrier coating (EBCs) systems were investigated. Different pack compositions using chromium, aluminum and alloys of these elements were tested and the kinetics of coating formation were examined in addition to their microstructure. The results and their analogy to diffusion couples were discussed and it was shown that coating elements which form silicides and carbides are promising candidates for coatings deposited on SiC/SiC via pack cementation. Based on such considerations a two-step pack cementation was proposed, which used chromium, one of the suitable elements, in a first step, to finally achieve an alumina-forming coating. The oxidation resistance of the developed coating was tested via thermogravimetric analysis and compared to the uncoated material. The coating protected the fiber-matrix interface of the SiC/SiC Ceramic Matrix Composites from oxidation. [ABSTRACT FROM AUTHOR]

Improving bonding and mechanical strengths is important for the application of bond coats used in the construction of steel deck bridges. Graphene nanoplatelets (GNPs) are attractive nanofillers for polymer modification because of their low cost, ultra-high aspect ratio, and extraordinary thermal and mechanical performance. In this paper, GNPs were used to reinforce the epoxy asphalt bond coat (EABC). The morphology, viscosity–time behavior, contact angle, dynamic mechanical properties, and mechanical and bonding strengths of GNP-reinforced EABCs were investigated using laser confocal microscopy, a Brookfield rotational viscometer, a contact angle meter, dynamic mechanical analysis, a universal test machine, and single-lap shear and pull-off adhesion tests. GNP dispersed non-uniformly in the asphalt phase of EABC. The viscosity of the neat EABC was lowered with the inclusion of GNPs and thus the allowable construction time was extended. The existence of GNPs enhances the hydrophobicity of the neat EABC. When adding more than 0.2% GNP, the storage modulus, crosslinking density and glass transition temperatures of both asphalt and epoxy of the neat EABC increased. The mechanical and bonding properties of the neat EABC were greatly enhanced with the incorporation of GNPs. Furthermore, the mechanical and bonding strengths of the modified EABCs increased with the GNP content. GNP-reinforced EABCs can be utilized in the pavement of long-span steel bridges with long durability.

The bonding strength of the bond coat plays an important role in the composite action between the wearing surface and the deck plate of the orthotropic steel deck system. Poor bonding results in the delamination of the wearing surface from the deck plate. Graphene oxide (GO) possesses outstanding mechanical and thermal properties, as well as impressive multifunctional groups, which makes it an ideal reinforcement candidate for polymer matrices. In this study, graphene oxide was used to improve the bonding strength and toughness of the epoxy asphalt bond coat (EABC). The dispersion, hydrophobicity, viscosity–time behavior, phase-separated morphology, dynamic mechanical properties, pull-off strength, shear strength and mechanical performance of GO-modified EABCs were investigated using various techniques. The inclusion of GO improved the hydrophobicity of the unmodified EABC. The viscosity of the unmodified EABC was lowered with the addition of GO during curing. Moreover, the allowable construction time for the modified EABCs was extended with the GO loading. The incorporation of GO enhanced the stiffness of the unmodified EABC in the glassy and rubbery states. However, graphene oxide lowered the glass transition temperature of the asphalt of the unmodified EABC. Confocal microscopy observations revealed that GO was invisible in both the asphalt and epoxy phases of the EABC. The inclusion of GO improved the bonding strength, particularly at 60 °C, and mechanical properties of the unmodified EABC.

This paper investigated the solid particle erosion behavior of partially oxidized NiCrBSiFe and NiCr coatings by varying temperature and impact angle. The challenge in the current situation is to process a new system of powders containing metallic and oxide phases. Partially oxidized powders containing metallic and oxide phases were processed by flame spraying the alloy powders into distilled water and allowing the oxide layer to form while keeping the core in the middle of the particle. Partially oxidized coatings were developed on MDN321 steel using the plasma spray technique with feedstock of partially oxidized powders. An air jet erosion test was carried out using Al2O3 erodent of grit size 50 µm at room temperature, 200, 400, 600, and 800°C by varying 30, 45, 60, 75, and 90° impact angles. Coatings were characterized concerning bond strength, porosity, micro-hardness, and density. The effect of temperature and impact angle on volumetric erosion loss was studied using SEM, EDS, and XRD analysis. Partially oxidized NiCrBSiFe coating exhibited better erosion resistance compare with partially oxidized NiCr coating. NiCr coating demonstrates maximum volumetric erosion loss at 45° impact angle, whereas NiCrBSiFe at 60° impact angle under all tested temperatures. [ABSTRACT FROM AUTHOR]

Herein, a biomimetic coating of hydroxyapatite (HA)–Al2O3 and HA–ZrO2 was deposited on Ti–6Al–4V-alloy using vacuum plasma spray (VPS) technique. The bond-coat of ZrO2 has been introduced between the substrate and reinforced HA coatings to study the effect of bond-coat on structural, mechanical properties and electrochemical corrosion performance of the developed coatings. In addition, the impact of thermal treatment of coating was investigated on these properties too. Coating characteristics, such as morphology, porosity, surface roughness, and crystallinity were investigated. The corrosion performance of coatings was tested in Hank’s-based salt solution (HBSS). Significant enhancement in crystallinity and surface-hardness has been witnessed after heat treatment; nevertheless, porosity reduced. The electrochemical corrosion study revealed that the corrosion resistance of heat-treated samples was better than the as-sprayed coatings samples. The intensity of XRD peaks of all coatings increased after 24h immersion in HBSS for the electrochemical test in comparison to the intensity of peaks before the corrosion test. [ABSTRACT FROM AUTHOR]

This study sought to examine the relationship between the degradation mechanism, thermal stress, and crack propagation behavior in environmental barrier coating (EBC) systems. An EBC system composed of a mullite topcoat (TC), Si‐bond coat (BC), and SiC substrate was prepared by atmospheric plasma spraying. Heat exposure tests were conducted to evaluate the microstructure of the EBC system at 1300°C for 1, 10, 50, and 100 h. The fracture resistance of the Si BC for the in‐plane (direction parallel to each layer, 0.4–0.6 MPam) and through‐thickness directions (direction from the TC to substrate, 1.7–2.1 MPam) differed because a thermal compressive stress was induced for the in‐plane direction owing to the mismatch of the thermal expansion coefficients for each layer, which acted as a barrier for crack propagation. However, cracks tended to propagate in the in‐plane direction because they were not affected by the in‐plane compressive stress. These results clearly showed that Si BC exhibited in‐plane anisotropy and crack propagation after heat exposure, which were the major sources of delamination of the EBC system. [ABSTRACT FROM AUTHOR]

The world's experience in using hafnium in two important parts of high-temperature thermal barrier coatings, such as the top thermal barrier layer and bond coat layer, was analyzed. In the top thermal barrier layer, hafnium is present as HfO2 completely or partially stabilized by yttria (or other rare-earth oxides). Another approach is to use hafnium dioxide as an addition to conventional coatings based on ZrO2 stabilized completely or partially. Electron-beam physical vapor deposition (EB-PVD) and air plasma spray process (APS) are most common techniques for applying thermal barrier coatings containing hafnium dioxide. Magnetron sputtering turned out to be successful as well. Compared to the 8YSZ coating, the 7.5YSH coating showed reduced Young's modulus, 30% lower thermal conductivity (decreased to 0.5–1.1 W/(m · K)) at high temperatures for HfO2 stabilized with 27 wt.% Y2O3, and higher sintering resistance and heat resistance. Doping of ZrO2 and HfO2 by several stabilizers proved to be promising: specifically, doping by a mixture of one trivalent ion larger than Y3+ and another trivalent ion smaller than Y3+, preserving the metastable structure of the t′ phase. The importance of phase diagrams for a correct choice of the top coat composition and doping elements for the bond coat is shown. Doping the bond coat with a small amount (up to 1 wt.%) of hafnium improved its cyclic oxidation resistance and increased the adhesion of the thermally grown oxide layer to the bond coat and strength of the latter. [ABSTRACT FROM AUTHOR]

Plasma-sprayed chromia coatings are known to have excellent corrosion and wear properties in highly acidic conditions at ambient and elevated temperatures, but are not watertight due to their intrinsic porosity. Therefore, in applications involving aggressive environments, the whole component is usually made of a corrosion-resistant alloy, to which the Cr2O3 coating imparts the necessary wear resistance. However, in such aggressive environments, the survival of thermal spray metallic bond layers becomes an issue. The present study deals with the performance in sulfuric acid solutions of coated systems consisting of a Hastelloy C-276 substrate and a plasma-sprayed Cr2O3 top coating with four different intermediate bond coatings. The bond coatings were HVOF-sprayed Ni-20Cr, Hastelloy C-276 and Ultimet alloys and plasma-sprayed tantalum. Open-circuit measurement, electrochemical polarization and electrochemical impedance spectroscopy tests were carried out at room temperature (RT) in solutions with various concentrations. Also, static immersion tests were performed at RT and 60 °C. The results revealed that the HVOF-sprayed Ni-20Cr and Ultimet alloy coatings were significantly attacked by the sulfuric acid electrolyte, especially at 60 °C, whereas the HVOF-sprayed Hastelloy C-276 and plasma-sprayed Ta coatings performed significantly better. [ABSTRACT FROM AUTHOR]

This study proposes cordierite (Mg 2 Al 4 Si 5 O 18 , MAS) as a novel bond coat material for thermal/environmental barrier coatings (T/EBC), deposited using atmospheric plasma spray on SiC fiber reinforced SiC matrix composites (SiC f /SiC-CMCs) coupons, forming a Mg 2 Al 4 Si 5 O 18 /Yb 2 Si 2 O 7 /Yb 2 SiO 5 /LaMgAl 11 O 19 coating system. The phase composition and thermal properties of the as-sprayed MAS coating were investigated, revealing cordierite as the primary phase in the as-sprayed coating with minor amounts of spinel and quartz phases which disappeared after heat treatment. Additionally, the as-sprayed MAS coating exhibited a low thermal expansion coefficient (3.19 × 10−6 K−1). The thermal cycling behavior and steam corrosion performance of the T/EBC coating system with cordierite as the bond coat were studied. Detailed analysis revealed that during thermal cycling tests, the coating failure was primarily related to liquid-phase sintering caused by eutectic reactions. In steam corrosion tests, the coating delamination was linked to the formation and growth of cracks and corrosion voids due to the residual stress release. • Cordierite is innovatively introduced as a novel bond coat material for T/EBC. • The liquid-phase sintering of the coating occurred during the thermal cycle tests. • Cordierite material has good resistance to water vapor corrosion. [ABSTRACT FROM AUTHOR]

A quasi-continuous α-W-based diffusion barrier (DB) forms in-situ between the (Ni,Pt)Al and the inter-diffusion zone by adding a Ni-W plating layer before the coating preparation. The oxidation behavior of the DB-contained coatings is compared with the normal (Ni,Pt)Al coating at 1100 ℃. The results indicate that the (Ni,Pt)Al with DB exhibits lower oxidation rate, slighter inter-diffusion and flatter surface. Additionally, martensite was observed in (Ni,Pt)Al-DB coatings, which contributed to lower surface rumpling for its higher stiffness and better resistance to creep. The mechanism of the DB and its effects on the oxidation performance of the (Ni,Pt)Al coating were discussed. • A quasi-continuous α-W-based diffusion barrier (DB) forms in-situ between the (Ni,Pt)Al coating and substrate. • (Ni,Pt)Al-DB coating exhibits improved oxidation performance. • The inter-diffusion between the (Ni,Pt)Al coating and substrate is hindered by the α-W DB. • Martensite transformation is observed in (Ni,Pt)Al-DB coating, which contributes to a slight surface rumpling in the coating. [ABSTRACT FROM AUTHOR]

(Ni,Pt)Al coating with varying thicknesses of α-W-based diffusion barrier (DB) were prepared by adding various thicknesses of Ni W plating layer. The oxidation performance of the DB-contained coatings was compared to that of the normal (Ni,Pt)Al coating at 1100 °C. With the thicker of DB, the inter-diffusion of Ni, Al as well as the refractory elements was more effectively inhibited. Moreover, better oxidation resistance was observed on (Ni,Pt)Al coating with a thicker DB, exhibiting a lower oxidation rate, flatter surface rumpling extent and slower phase degradation rate. The effects of DB on the inter-diffusion behaviour and oxidation performance of the (Ni,Pt)Al coating are discussed, and the degradation mechanism of the DB is proposed. • (Ni,Pt)Al coatings with varying thicknesses of in situ formed α-W-based diffusion barrier (DB) are prepared. • (Ni,Pt)Al coatings with thicker DB exhibit better oxidation resistance. • Less inter-diffusion occurs for thicker DB-contained (Ni,Pt)Al coatings. • The DB gets degenerating when the intermittent channels between α-W become wide enough. [ABSTRACT FROM AUTHOR]

Ceramic coatings of Al2O3, Al2O3-TiO2, and Al2O3-ZrO2 commercial powders were thermally sprayed on the external surface of ASTM A106 Grade-B carbon steel pipelines (with and without a bond coat of a commercial nickel alloy) to reduce the loss of heat contained in hot fluids that are transported through this kind of ducts. The bond coat, as well as the ceramic coatings, were deposited by means of oxy-acetylene thermal spray; afterwards, the heat transfer in transitory state was evaluated by placing heated silicone oil at 160°C inside the pipelines. The temperature variation over time of the silicone oil and the external wall of each pipeline was measured. Besides the low thermal conductivity of the ceramic materials, a thicker and more porous coating provides a more effective means for thermal isolation. Additionally, the bond coat acts as a thermal resistance, which consumes heat and contributes to the reduction of the superficial temperature of the pipeline.

Metallic alloy bond coats are used as an interlayer to promote adhesion of ceramic Thermal Barrier Coatings (TBCs) to the superalloy substrates. These bond coats can be applied through a variety of processing means such as Air Plasma Spray (APS), High-Velocity Oxy Fuel spray (HVOF), and Vacuum Plasma Spray (VPS). While widely adopted in the industry, some uncertainties remain as to the specific microstructural attributes which are influenced by the processing routes and their implications on durability. This paper seeks to elucidate these subtleties between bond coat deposition processing routes through an integrated study of processing to connect to furnace cycle durability on a single bond coat chemistry of NiCoCrAlYHfSi. A standard porous APS TBC top coating based on yttria stabilized zirconia (7YSZ) was consistently applied on these distinct bond coats and tested for durability in cyclic furnace testing at 1100 °C. The results point to substantial differences in durability which are not readily explained only by the variances in coating roughness. Detailed assessment of deposited bond coat properties (modulus, thermal expansion, chemistry) provided some explanation relative to durability. The results suggest a complex interplay among roughness, compliance, aluminum diffusion pathway, and thermal expansion mismatch. Photo-stimulated luminescence piezospectroscopy (PLPS) provided additional insights enabling a comprehensive assessment of processing-performance linkages. • Direct study of one bond coat powder chemistry, sprayed by three main processes (APS/HVOF/VPS). • Phase analysis of the bond coats from the three processes – APS has a phase not present in the others. • Dilation experiments of bond coats – APS shows low temperature behavior deviating from others. • No clear trend between FCT Life of TBC and R a of the bond coat. • Stress maps of the bond coat oxide scale show there is a process dependence on scale growth behavior. [ABSTRACT FROM AUTHOR]