CoNiCrAlY Bond Research Articles

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.

Structural evolution and enhanced thermal cycling life of HVAF-sprayed NiCrAlY bond coat modified by YSZ

Thermal barrier coatings with varied bond coat structures were deposited using high-velocity air fuel spraying. Highly hard YSZ particles are introduced into the NiCrAlY bond layer by accelerating to supersonic speeds at low temperatures to change the porous structure of the bond coat. We systematically increased the YSZ content in NiCrAlY bond coats and extensively investigated microstructure, oxidation behavior, thermal cycling life, and residual stress variation of the bond coats suing a combination of scanning electron microscopy, optical microscope, confocal Raman microprobe. Introduction of YSZ particles reduced the porosity of NiCrAlY bond coat from 2.69% to 0.45%, and altered the oxidation behavior of the bond layer. The NiCrAlY mixed YSZ composite structure induced the formation of a complex and thin thermal growth oxide layer on the bond coat during oxidation process, greatly alleviating and reducing the thermal stress generated of the coating and during the thermal cycling test. And the thermal cycling life of the NiCrAlY mixed YSZ composite structure coating is 2.8 times longer than that of the ordinary NiCrAlY coating. This finding highlights the significance of the bond coat structure in enhancing the service life of thermal barrier coatings.

Investigation on the Oxidation Behavior of the NiCrAlY Bond-Coat with Low Al Content Sprayed by High Velocity Oxygen Fuel Method

Investigation on the Oxidation Behavior of the NiCrAlY Bond-Coat with Low Al Content Sprayed by High Velocity Oxygen Fuel Method

Wear behavior at high temperature of ZrO2–Y2O3 (YSZ) plasma-sprayed coatings

The wear behavior of two plasma-sprayed zirconia–yttria coatings was studied at high temperatures. Agglomerated and sintered, as well as fused and crushed zirconia–yttria feedstock powders were used to manufacture bimodal and monomodal coatings by atmospheric plasma spraying onto an INCONEL 718 substrate previously coated with a NiCrAlY bond coat. The structure of the coatings was analyzed by SEM on their cross section and surface. The samples were subjected to wear conditions by sliding contact through a ball-on-disk test up to 1000 °C, using an alumina ball 6 mm in diameter as the counterbody, on which a load of 5 N was applied. The samples were rotated during 20000 cycles, reaching a speed of 0.10 m·s−1 at the contact area with the counterbody. The porosity, phase, and mechanical properties were determined before and after wear tests. The results indicate that at 25 °C, both coatings have enough mechanical resistance to withstand the tribological conditions they were exposed to. Therefore, low wear rates were produced by ductile deformation. The tribological conditions became more aggressive as the thermal stresses increased with the test temperature, producing cracking, and detaching particles in the coatings tested at 500 and 750 °C. Consequently, high wear rates related to brittle deformation were obtained. However, the transformation of the amorphous phase to the t′-zirconia phase, produced at 1000 °C, increased the hardness of both coatings and, consequently, their wear resistance; thus, the predominant mechanism of damage was ductile deformation, with wear rates similar to those obtained when the coatings were tested at 25 °C.

Role of bond coat deposition method in the hot corrosion behavior of CSZ thermal barrier coatings

In this research, a NiCrAlY bond coat was applied using two methods – namely, conventional air plasma spraying (APS) and new spark plasma sintering (SPS) technique – on an Inconel-738 substrate. The ceria-stabilized zirconia (CSZ) topcoat was similar in both samples and deposited using the APS method. The hot corrosion performance of the fabricated thermal barrier coatings (TBCs) was studied at 950°C in a sodium sulfate (Na2SO4)–55 wt.% vanadium (V) oxide (V2O5) corrosive salt environment. The results showed that the hot corrosion resistance of the coating system with an SPS-processed NiCrAlY bond coat and a CSZ topcoat was much better than that of the conventional TBC sample because the adhesion of the bond coat to the substrate was very good in this sample, and the interface was free of any defects such as cracks and porosities. According to these results, no spallation was observed in the SPS-processed coating up to 38 h, while for the CSZ topcoat with an APS-processed bond coat, spallation occurred after 24 h.

Improving the oxidation resistance of NiCrAlY-coated superalloy by Zr/ZrN diffusion barrier

The elemental interdiffusion between the bond-coat in thermal barrier coatings and the superalloy substrate has emerged as a critical factor in the failure of turbine blades. In order to mitigate this elemental interdiffusion, a Zr/ZrN diffusion barrier was strategically designed and deposited between the NiCrAlY bond-coat and the superalloy substrate using multi-arc ion plating. The performance of the as-deposited coatings was evaluated through annealing, quenching, thermal cycle oxidation, and isothermal oxidation tests. The microstructural evolutions of the Zr/ZrN diffusion barrier and the NiCrAlY bond-coat were investigated. The Zr/ZrN diffusion barrier exhibited remarkable bonding strength and diffusion barrier effect. Importantly, the structural integrity of the diffusion barrier remained unaffected by thermal stresses during quenching and thermal oxidation tests. Additionally, twins were observed within the ZrN layer after thermal cycle oxidation tests. The application of the Zr/ZrN diffusion barrier layer substantially enhanced the oxidation resistance of NiCrAlY by preventing the depletion of Al. These results suggest that ZrN holds promise as a favorable candidate material for diffusion barriers.

Hot corrosion behaviour of mullite thermal barrier coatings for marine diesel engines

Mullite's inherent qualities have made it a potential material for the application of thermal barrier coatings (TBCs) for diesel engine components. Hot corrosion at 600–800 °C can cause TBC degradation, thus significantly affecting the performance of engine components and reducing their service life. This work examines the hot corrosion behaviour of atmospheric plasma-sprayed (APS) mullite coating over NiCrAlY bond coat on mild steel substrates. The coated specimen surface was covered with a mixture of Na2SO4 (sodium sulphate) and V2O5 (vanadium pentoxide) in the form of paste and heated in a muffle furnace at 700 °C for up to 300 h. SEM, EDS, and XRD characterisations were used to investigate the mechanism of hot corrosion. Coatings remained intact after corrosion tests; however, it had reacted with corrosive salts, particularly sodium sulphate. This was evidenced by the removal of amorphous silica, followed by the formation of nosean as a major phase. During the reaction between sodium sulphate and mullite coating, vanadium pentoxide was found to be acting as a flux and mineraliser.

A Study on Long-Term Oxidation and Thermal Shock Performance of Nanostructured YSZ/NiCrAlY TBC with a Less Dense Bond Coat.

This paper focused on studying the performance of a nanostructured thermal barrier coating (TBC) system deposited by APS, which had a bond coat with inter-lamellar porosities that resulted during the manufacturing process. The higher porosity level of the bond coat was studied as a possible way to keep the thickness of the TGO under control, as it is distributed on a higher surface, thereby reducing the chance of top-coat (TC) spallation during long-term oxidation and high-temperature thermal shock. The TBC system consisted of nanostructured yttria partially stabilized zirconia (YSZ) as a top coat and a conventional NiCrAlY bond coat. Inter-lamellar porosities ensured the development of a TGO distributed on a higher surface without affecting the overall coating performance. Based on long-term isothermal oxidation tests performed at 1150 °C, the inter-lamellar pores do not affect the high resistance of nanostructured TBCs in case of long-term iso-thermal oxidation at 1150 °C. The ceramic layer withstands the high-temperature exposure for 800 h of maintaining without showing major exfoliation. Fine cracks were discovered in the ceramic coating after 400 h of isothermal oxidation, and larger cracks were found after 800 h of exposure. An increase in both ceramic and bond-coat compaction was observed after prolonged high-temperature exposure, and this was sustained by the higher adhesion strength. Moreover, in extreme conditions, under high-temperature thermal shock cycles, the TBC withstands for 1242 cycles at 1200 °C and 555 cycles at 1250 °C.

Investigation of Microstructural Characterization and Mechanical Properties of 8YSZ Thermal Barrier Coatings: Atmospheric Plasma Spray versus Electron Beam-Physical Vapour Deposition

In this study, the formation and growth of coating microstructure was studied and reported based on APS and EB-PVD deposition mechanism with the change in spray angle. A NiCrAlY bond coat with 8YSZ ceramic coat was applied by APS and EB-PVD deposition onto the Inconel 718 with the variation in spray angle. The investigation was divided into microstructural analysis and mechanical properties to determine the effect of different deposition mechanism and its variation in spray angle. The results obtained show that the APS with 60° spray angle has more presence of defects, followed by APS 90° and EB-PVD samples. The APS avg. grain size varies between ~ 400 to 800nm, whereas EB-PVD in between ~ 150 to 300nm. The EB-PVD with 90° spray angle results in low porosity i.e. 18.12 % followed by APS 90°, EB-PVD 60°, and APS 60° spray angle, respectively. The low porosity results in high compressive residual stress than medium and high porosity. The avg. Raman shift for EB-PVD 90° spray angle i.e. low porosity is 1.86 cm-1, for medium i.e. APS 90° and EB-PVD 60° is 1.33cm-1 and 1.53cm-1 respectively, and for high porosity is 0.93cm-1. The 90° spray angle showed better microstructural and mechanical properties compared to 60°.

Oxidation Behaviors of the NiCrAlY Bond Coats in the Thermal Barrier Coatings under External Loads

To understand the oxidation behaviors of the NiCrAlY bond coats in the thermal barrier coatings (TBCs) under external loads, uniaxial tensile and compressive experiments of the TBCs in air at 900 °C for 100 h were investigated. Then, the experimental results were explained by first-principles simulation. The results showed that the oxidation rate of the NiCrAlY bond coat was accelerated by external stresses. A 0.9 μm thickness, a thermally grown oxide (TGO) layer was found in the NiCrAlY bond coat without stress after oxidation, while a 1.2 μm thickness TGO layer was obtained in the NiCrAlY bond coat under tensile stress after oxidation. The main composition of the TGO layer was Al2O3 because Al was more easily oxidized without stress and under tensile stress. The thickest TGO layer with a thickness of 1.5 μm was formed in the NiCrAlY bond coat under compressive stress after oxidation, consisting of the oxide of Al, Ni, and Cr. The first-principles results showed that the oxidation of Ni and Cr under compressive stress was easier than that under tensile stress due to the role of the 3d orbital.

Design and realization of interface strengthening to GNPs/YSZ nanocomposite coating

Interfacial strengthening of ceramic coatings on metal surfaces is a goal that has been pursued by researchers in this field, but remains unsolved. In this work, the interface strengthening between ceramic coating and metal substrate was designed and successfully realized. GNPs were modified with silica-sol at the molecular scale to disperse in YSZ uniformly and the GNPs/YSZ nanocomposite coatings with high bonding strength were produced due to in-situ generation of interfacial carbides. Results show that the bonding strength of 9 wt% GNPs/YSZ nanocomposite coating increased by up to ∼54% compared to nanostructured YSZ coating without NiCrAlY bond-coat, and even 17% higher than that of nanostructured conventional type YSZ coating with NiCrAlY bond-coat. This technique could be applied to reinforce the interface between various materials, showing great application potential.

Influence of spray angle onto the microstructural and mechanical properties of air plasma sprayed 8YSZ TBCs

In this paper, the air plasma sprayed 8YSZ TBC was deposited onto the Inconel 800 with NiCrAlY bond coat using spray angle variation. The comparative study was conducted to evaluate the change in microstructural and mechanical properties with the effect of spray angle. The 8YSZ TBCs were deposited using 90° and 60° spray angles. The microstructural examination with elemental analysis was done by SEM and EDS. The mechanical properties such as porosity, hardness, and surface roughness were determined using SEM image analysis with greyscale threshold, indentation test, and surface profilometer technique. The experimental result shows the spray angle effect over the structure in grain growth behaviour, defects like pores, cracks, and surface irregularities. The 90° showed a uniform grain growth structure with few defects compared to the 60° spray angle. The spray angle 60° shows large amount of defects resulting in decreased hardness, surface roughness, and increased porosity. Also the shadow region obtained is high leading to large porous structure developing rough surface.

Thermal fatigue characteristics of 8Y2O3-ZrO2, La2Zr2O7, La2(Zr0.7Ce0.3)2O7 and La2Ce2O7 thermal barrier coatings in duplex, multilayer functionally graded and multilayer configurations

La2Zr2O7, La2(Zr0.7Ce0.3)2O7 and La2Ce2O7 pyrochlore plasma sprayable powders were synthesized and plasma spray coated on steel plates with NiCrAlY bond coat. Three different configurations were used: duplex, multilayer functionally graded and multilayer, with different combinations of commercial 8% yttria stabilized zirconia (8YSZ) and NiCrAlY (bond coat) layers. The prepared coatings were compared with the standard duplex 8YSZ thermal barrier coatings (TBCs) with a goal to study their suitability to serve as TBCs. TBCs? layer thicknesses and interfaces were studied via SEM on polished cross section metallographic samples removed from the spray coated TBCs. Thermal fatigue resistance was evaluated by directing a gas flame on the ceramic surface at 1200 and 1400 ?C, followed by its rapid withdrawal and forced cooling by pedestal fan. The maximum number of thermal shock cycles the coatings could withstand before failure was determined. The multilayered TBCs with lanthanum cerate composition stacked with 8YSZ exhibited the superior thermal fatigue resistance characteristics compared to all other studied TBCs. The findings were correlated with the crystalline phases of the ceramic coatings, obtained via XRD, and discussed in the light of existing literature.

Thermal expansion and microstructure evolution of atmospheric plasma sprayed NiCrAlY bond coat using in-situ high temperature X-ray diffraction

The paper focuses on in-situ high-temperature X-ray diffraction (HT-XRD) study on atmospheric plasma sprayed NiCrAlY coating. The sample was in-situ heated from 25 °C to 1150 °C in a controlled atmosphere (3 × 10−4 bar), and the corresponding X-ray diffraction patterns for different temperatures were recorded. The effect of temperature on crystallite size, lattice strain, and coefficient of linear thermal expansion was studied. Major phases identified are γ-Ni, γ’-Ni3Al, β-NiAl, and α-Cr. The formation of stable α-Al2O3 and spinel was found above 1000 °C. The transformation of β to γ’ and γ phase was observed as a function of temperature. The equilibrium phases and the thermal expansion of disordered Face Centered Cubic (FCC) and Body Centered Cubic (BCC) phases were predicted and supported by Thermo-Calc prediction for the stable temperature range. Results showed that the non-equilibrium microstructure produced by thermal spray process did not alter the thermal expansion behaviour. In-situ treatment resulted in microstructure and elemental homogenization. The thermal expansion and mechanism of phase evolution were discussed.

Experimental Investigation of Biohythane Performance on Thermal Barrier-Coated Compression Ignition Engine

Abstract The performance, combustion, and emission parameters of a thermal barrier-coated (TBC) compression ignition (CI) engine were experimentally examined using biohythane (biohydrogen + biomethane) as leading fuel and diesel as secondary fuel. The mixture of supplied fuel was blended with 85–95% biomethane and 5–15% biohydrogen, mixed with the inlet air, and supplied through the intake manifold for both coated and uncoated conditions. Yttria-stabilized zirconia with a thickness of 0.4 mm was used as the top coat on the valves and piston crown over 0.1 mm thickness NiCrAlY bond coat using the plasma spray method. Baseline data were obtained by running the engine with conventional diesel fuel, and the same was compared with dual-fuel operation at constant engine speed and variable loading conditions. Results indicated that 15% biohydrogen enrichment with TBC operation improved the engine brake thermal efficiency (BTE) by 6% compared to diesel-only mode. Further, peak cylinder pressure and heat release rate were improved up to 16.5–20% with TBC and biohythane under dual-fuel mode compared to diesel-only mode. Moreover, the emission characteristics such as hydrocarbon (HC), carbon monoxide (CO), and smoke were reduced up to 16.2, 29.1, and 62.6%, respectively, with TBC and biohythane compared to the base diesel operation owing to low carbon concentration and improved combustion characteristics.

Enhancing the Oxidation Resistance of NiCrAlY Bond Coat by High-Current Pulsed Electron Beam Irradiation

The bond coat of a NiCrAlY thermal barrier coating plays an important role in solving the thermal expansion mismatch between a metal matrix and a ceramic layer and in improving the oxidation resistance of the whole thermal barrier coating. However, the NiCrAlY bond coat prepared by low-pressure plasma spraying is not conducive to its oxidation resistance because its lamellar structure is loose, porous and the surface is rough. To improve the oxidation resistance of the bond coat, the NiCrAlY bond coat prepared by plasma spraying was modified by high-current pulsed electron beam with different energy densities. Under the electron beam irradiation, the surface of the coating became smooth, and there was a 3–5 μm thick remelting layer on the surface. Under the irradiation, the thickness of the thermal growth oxide layer decreased, and the oxidation resistance was significantly improved, the oxidation product being mainly Al2O3.

The effect of spray angle on the microstructural and mechanical properties of plasma sprayed8YSZ thermal barrier coatings

Thermal barrier coatings (TBCs) are favorable for better protection of gas turbines and aero engines at high temperatures. The TBCs were fabricated using NiCrAlY bond coat and 8% wt. yttria-stabilized zirconia (YSZ) topcoat onto the nickel-based superalloy Inconel 800 by atmospheric plasma spray. In this article, the investigation of microstructural and mechanical properties of 8YSZ TBCs with the effect of spray angle has been discussed. The microstructural and elemental analyses were conducted by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS). The porosity analysis was conducted based on SEM image analysis using a gray scale threshold. Mechanical properties such as coating hardness, surface roughness, and thickness are measured by indentation, surface profilometer, and optical microscopy. The result shows the effect of the spray angle over the coating surface in terms of pores and microcracks. The influence of the spray angle leads to different grain growth resulting in the shadow region. A large number of defects and a decrease in coating hardness were observed for 60 degrees pray angle compared to the 90 degrees pray angle. A large number of defects led to developing rough surfaces, resulting in low hardness and increased porosity. The experimental results showed that the plasma sprayed 8YSZ TBC with a 90 degree spray angle can improve the durability and performance of the TBCs, as it has better microstructural and mechanical properties.

Thermal shock resistance of a NiCrAlY-coated Alloy 625 system produced by laser powder bed fusion

Additive Manufacturing offers an innovative route for producing high-quality parts in various fields. A bi-material system, consisting in a NiCrAlY bond coat deposited onto a Ni-based Alloy 625 substrate, was manufactured by laser powder bed fusion (LPBF). Test samples were prepared and included sol-gel ceramic ZrO2(Y2O3) top coats as well as a specific grain boundary serration (GBS) heat treatment for promoting high temperature resistance. These specimens were subjected to very severe thermal shock cycles between 950 °C and 300 °C, characterized by steep heating and air quenching rates in a state-of-the-art burner rig designed to render gas turbine conditions, and their integrity was compared. While LPBFed NiCrAlY coatings were relatively spared from degradations due to thermal shocks, ceramic top coats exhibited clear spallation. Poor bonding was particularly experienced by specimens subjected to the GBS heat treatment due to the unavoidable formation therein of surface oxides. Numerous cracks were detected within NiCrAlY bond coats, both in tested specimens as well as in pre-cycled as-built ones, which suggested a dominant role of the LPBF process known for generating residual stress. Heat treated specimens exhibited nearly no cracking. Hardness was found to significantly increase within as-built NiCrAlY bond coats as a result of heat exposure during thermal shock cycling and was attributed to precipitations. The fully recrystallized microstructure of heat treated specimens, on the other hand, was found more stable. The present study completes a series of investigations demonstrating the great potential for manufacturing excellent high temperature structural components by means of LPBF as opposed to more constraining conventional routes.

Effect of spray angle onto the microstructure of EB-PVD enabled 8YSZ thermal barrier coatings

Abstract The microstructure plays an important role in the durability and performance of the Thermal Barrier Coatings (TBCs). The TBCs were deposited on Ti-6Al-4 V substrate with NiCrAlY bond coat using APS and 8YSZ using in-house EB-PVD. The paper presents the effect of spray angle onto the microstructural and mechanical properties of 8YSZ in detail. The surface microstructural and elemental composition are examined using SEM with EDS. The coating porosity was examined by SEM and ImageJ using the grayscale threshold technique. Also, the mechanical properties like hardness, surface roughness, and thickness were measured by indentation, surface profilometer, and optical microscopy in combination with ImageJ. The results show 90° TBC with the uniform structure in the presence of a few pores and cracks compared to 60° TBC. In 60° TBC, the dissimilar grain growth and elongated grains with average size 300 nm compared to 90° TBC grain size 150 nm was observed. In 60° TBC, porosity with 20.65% more was present which contributes to large shadow region leading to high porosity. Also, the decrease in coating hardness with 5% was observed with an inclined spray angle. In 60° TBC, the rough and non-uniform surface was obtained, but the overall coating thickness was almost similar for both the spray angle condition.

The influence of pyrolysing Al2O3 precursor on the high temperature properties of the YSZ-Al2O3 composite coating

ABSTRACT In this research, YSZ-Alumina top coat was deposited on the Hastelloy substrate with a NiCrAlY bond coat by Atmospheric plasma spraying process. YSZ-10 wt-% Al2O3 composite coatings with pyrolysed and un-pyrolysed Al2O3 powder were deposited. High temperature oxidation and thermal shock resistance of both coatings were investigated. Thermal shock test results showed that the lifetime of the YSZ-10 wt-% Al2O3 coating was increased by the pyrolysation of the un-pyrolysed Al2O3. The network of micro-segmented cracks produced by the pyrolysation of the un-pyrolysed Al2O3 improved the strain accommodation, introducing it as the main enhancement mechanism for TBC life extension. Using of pyrolysed alumina, resulted increased in the flattening degree of the splats, and so reduction of porosity and intra-splat cracks. This, in turn, reduced the permeability of the top coat and decreased the rate of oxide growth on the bond coat.