CoNiCrAlY Bond Coat Research Articles

Experimental investigation of Al2O3/8YSZ and CeO2/8YSZ plasma sprayed thermal barrier coating on diesel engine

In this research work, aluminium oxide/yttria stabilized zirconia (20%Al2O3/80%8YSZ) and ceria/yttria stabilized zirconia (20%CeO2/80%8YSZ) were coated through atmospheric plasma spray technique (APS) as thermal barrier coating (TBC) over CoNiCrAlY bond coat on aluminium alloy (Al-13%Si) substrate piston crown material and their thermal cycling behavior were studied experimentally. Thermal cycle test of both samples were conducted at 800 °C. Microstructural, phase and elemental analysis of the TBC coatings were experimentally investigated. The performance, combustion and emission characteristics of Al2O3/8YSZ, CeO2/8YSZ TBC coated and uncoated standard diesel engine were experimentally investigated. The test results revealed that CeO2/8YSZ based TBC has an excellent thermal cycling behavior in comparison to the Al2O3/8YSZ based TBC. The spallation of the Al2O3/8YSZ TBC occurred mainly due to the formation of thermally grown oxide (TGO), and growth of residual stresses at top coating and bond coating interface. The experimental results also revealed that the increase of brake thermal efficiency and reduction of specific fuel consumption for both TBC coated engine. Further reduction of HC, CO and smoke and increase of NOx emission were recorded for both TBC coated engine compared to the standard diesel engine.

Isothermal Oxidation Behavior of Gadolinium Zirconate (Gd2Zr2O7) Thermal Barrier Coatings (TBCs) produced by Electron Beam Physical Vapor Deposition (EB-PVD) technique

Abstract Thermal Barrier Coatings (TBCs) provide thermal insulation for gas turbine components operating at high temperatures. Generally, TBCs were produced on a MCrAlY bond coat with 7-8% Yttria Stabilized Zirconia (YSZ) using Atmospheric Plasma Spray (APS) technique. In this study, Inconel 718 substrate material was coated with CoNiCrAlY bond coat using high velocity oxygen fuel (HVOF) technique. Afterward, Gd2Zr2O7 was deposited on samples using Electron Beam Physical Vapor Deposition (EB-PVD) technique. Produced TBCs were exposed to isothermal oxidation tests at 1000°C for 8 h, 24 h, 50 h and 100 h in muffle furnace. Scanning electron microscopy-energy distribution X-ray (SEM-EDX) spectroscopy was used to investigate thermally grown oxide (TGO) layer and TGO growth behavior of TBCs. In addition, X-ray Diffractometer (XRD) analysis was performed to TBCs to understand whether phase transformation occurs or not before and after oxidation.

Microstructural Characterization and Room-Temperature Erosion Behavior of As-Deposited SPS, EB-PVD and APS YSZ-Based TBCs

The erosion behavior at room temperature of as-deposited suspension plasma spray (SPS), electron beam physical vapor deposition (EB-PVD) and air plasma spray (APS) ZrO2-Y2O3 (YSZ)-based TBCs was investigated. All coatings were deposited on Inconel 625 alloy coupons. The same APS CoNiCrAlY bond coat was employed for all SPS and APS TBCs. The erodent material was 50-µm alumina, and the impact angles were 15° and 90°. A total of 4 different types of SPS YSZ-based TBCs were tested, which consisted of two distinct columnar-segmented and two distinct columnar-grown microstructures. The EB-PVD and APS YSZ TBCs were employed as benchmarks. The erosion performance of the different TBCs in this study was ranked based on the coating volume loss after wear testing. The TBC microstructures and phase compositions were evaluated via scanning electron microscopy and X-ray diffraction. The erosion mechanisms of the different TBCs were compared by analyzing the cross-sectional and top surface microstructures of the as-deposited and eroded TBCs. These are released results from the Surftec Industrial R&D Group of the National Research Council of Canada.

Comparison of microstructural evolution and oxidation behaviour of NiCoCrAlY and CoNiCrAlY as bond coats used for thermal barrier coatings

NiCoCrAlY and CoNiCrAlY bond coats used for thermal barrier coatings, as well as 8 wt% Y2O3-stabilized ZrO2 (8YSZ) ceramic top coat, were air plasma sprayed on Inconel 718 superalloy and then subjected to thermal exposure at 950, 1050 °C in air. The microstructural evolution and oxidation behaviour of bond coats were comparatively studied using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the diffusion of Al is the key factor causing the microstructural changes, resulting in three zones in the bond coats: outer β-depleted zone (OBDZ), inner β-depleted zone (IBDZ) and β-left zone (BLZ). The thickness of BLZ decreases with the increasing of the heating time or temperature. NiCoCrAlY bond coat with higher Al content presents lower β-depletion rate and smaller Al interdiffusion coefficient, and the oxide scale is predominantly composed of Al2O3 whereas in the CoNiCrAlY coating, large amount of non alpha-alumina oxides have been found, resulting in the larger oxidation constant at the stable oxidation period and higher micro-crack density at the 8YSZ/TGO interface and 8YSZ coating. The results reveal that NiCoCrAlY bond coat presents better characteristics of oxidation resistance and crack resistance when used for thermal barrier coatings.

Oxidation behavior of thermal barrier coating systems with Al interlayer under isothermal loading

In the present study, the phenomena related to the Thermally Grown Oxides (TGO) in atmospheric plasma sprayed Thermal Barrier Coatings (TBCs) are discussed. CoNiCrAlY bond coatings were sprayed on Inconel 600 substrates. Subsequently, thin Al layers were deposited by DC-Magnetron sputtering. Finally, yttria-stabilized zirconia (YSZ) top coatings were deposited to form a three-layered TBC system. The thus produced aluminum interlayer containing thermal barrier coatings (Al-TBC) were subjected to isothermal exposure with different holding times at 1150 °C and compared with reference TBCs of the same kind, but without Al interlayers (R-TBC). The oxide film formation in the interface between bond coating (BC) and top coating (TC) was investigated by scanning electron microscope (SEM) after 100 and 300 h of high temperature isothermal exposure. The growth of this oxide film as a function of the isothermal exposure time was studied. As a result, the designed Al-TBC system exhibited better oxidation resistance in the BC/TC interface than the two-layered R-TBC system. This was lead back to the Al enrichment, which slows down the formation rate of transition metal oxides during thermal loading.

Low cycle fatigue performance of Ni-based superalloy coated with complex thermal barrier coating

Thermal barrier coatings (TBCs) are widely applied to protect high-temperature components against high temperatures in harsh environments. Nineteen cylindrical specimens of Inconel 713LC were manufactured using the investment castings technique, and 10 specimens were subsequently coated with a novel complex thermal barrier coating (TBC) system. The TBC system comprises a metallic CoNiCrAlY bond coat (BC) and a complex ceramic top coat (TC). The TC is a mixture of a near eutectic nanocrystalline ceramic made of zirconia (ZrO2), alumina (Al2O3), silica (SiO2) and conventional yttria stabilized zirconia (YSZ) ceramic in the ratio of 50/50 in wt%. Low cycle fatigue (LCF) tests were carried out in a symmetrical push-pull cycle under strain control at 900 °C. Cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves of the TBC-coated and uncoated material were assessed. Fatigue life curves in total strain representation showed transient behaviour. Fracture surfaces and polished sections parallel to the loading axis of the TBC-coated and uncoated specimens prior and after cyclic loading were observed by means of scanning electron microscopy (SEM) to study the degradation mechanisms during high-temperature LCF. TBC delamination was observed at the TC/BC interface, and rafting of precipitates occurred after high-temperature exposure. The microstructural investigations further the discussion of the differences in the stress-strain response and the fatigue life of the TBC-coated and uncoated superalloy.

High‐Temperature Low‐Cycle Fatigue Behaviour of MAR‐M247 Coated with Newly Developed Thermal and Environmental Barrier Coating

This study investigates the strain‐controlled low‐cycle fatigue (LCF) behaviour of an untreated and surface‐treated MAR‐M247 superalloy in a symmetrical push‐pull cycle with a constant strain rate at 900°C in laboratory air. A newly developed experimental thermal and environmental barrier coating (TEBC) system, consisting of a 170 μm thick CoNiCrAlY bond coat (BC) and a bilayer ceramic top coat (TC), with an interlayer and an upper layer, was deposited using air plasma spray techniques. The ceramic interlayer with an average thickness of 77 μm was formed from agglomerated and sintered yttria‐stabilized zirconia. An experimental mixture of mullite (Al6Si2O13) and hexacelsian (BaAl2Si2O8) at a ratio of 70/30 vol.% was sprayed as the upper layer. The average thickness of the TC was 244 μm. The specimen sections were investigated using a TESCAN Lyra3 XMU scanning electron microscope (SEM) to characterise the microstructure of both the TEBC and the substrate material. The fatigue damage mechanisms in the TEBC‐coated superalloy were studied. The fatigue life curves in the representation of the total strain amplitude versus the number of cycles to failure of the TEBC‐coated and uncoated superalloy were assessed. TEBC was found to have a slight, positive effect on the fatigue life of MAR‐M247.

Effect of alumina-silica-zirconia eutectic ceramic thermal barrier coating on the low cycle fatigue behaviour of cast polycrystalline nickel-based superalloy at 900 °C

Cylindrical specimens of nickel-based superalloy Inconel 713LC in an as-cast condition and coated with a thermal barrier coating (TBC) system consisting of a CoNiCrAlY bond coat and an eutectic ceramic alumina-silica-zirconia top coat were cyclically loaded under strain control at 900°C in air. TBC was deposited using atmospheric plasma spray and water-stabilised plasma spray techniques for the bond coat and the top coat, respectively. The microstructure of the substrate material and the TBC was characterised and the hardness of TBC was measured. The cyclic stress-strain response and fatigue life of both materials were assessed. The stress response of the TBC-coated superalloy is lower than the uncoated superalloy. The detrimental effect of TBC on the Basquin curve is documented. The specimen section and fracture surface observations revealed fatigue damage mechanisms in both the TBC-coated and uncoated specimens, and that finding helps explain the differences in the fatigue behaviour of both materials.

Effect of Double Electron Beam Remelting on Microstructure of HVOF and CGDS Bond Coat

This work focuses to investigate the influence of the parameters used in electron beam (EB) remelting including the effect of double remelting of CoNiCrAlY coatings fabricated on Nickel based super alloy substrates by using the high velocity oxygen-fuel (HVOF) and cold gas dynamic spraying (CGDS) methods. The microstructures of as sprayed and remelted coatings were investigated by scanning electron microscopy and the phase analysis by X-ray diffraction (XRD). The results obtained show that there are advantages at using the pulsed EB surface modification technique. Double EB treatment provides a smooth surface and low porosity level and at last but not least this study demonstrate that low-temperature processing of CoNiCrAlY bond coat represents an interesting and promising alternative for their manufacturing.

Protective pre‐deposited slurry dip‐coating alumina layer on TBC system

In the present study, a thin intermediate protective α‐alumina layer was pre‐deposited by slurry dip‐coating technique on APS–CoNiCrAlY bond coat prior depositing APS–YSZ as top coat. The alumina layer acts as barrier for oxygen diffusion to protect the metallic bond coat from excessive oxidation. The new TBC was compared with the standard TBC system which comprises of Ni‐superalloy substrate, CoNiCrAlY bond coat and YSZ top coat concerning the behavior under thermal cycling at 1150 °C. The samples were investigated before and after thermal cycling by optical microscopy, scanning electron microscopy (SEM) equipped with energy dispersive X‐ray spectroscopy (EDS), and X‐ray diffraction analysis (XRD). Image analysis was used to compare between the microstructure by measuring the crack lengths and the thermally grown oxide layer (TGO) average thickness after thermal cycling. It was observed that a more uniform and compact α‐Al2O3 TGO was formed in the new TBC system after thermal cycling comparing to the commercial one. It was, also, found that the alumina intermediate layer has the potential to reduce the length of the cracks and the average thickness of TGO by suppressing the detrimental mixed oxides, like Cr2O3, CoO, NiO, and (Ni, Co)(Cr, Al)2O4, thus, improving the TBC durability.

The Use of APS Thermal Barrier Coatings in Corrosive Environments

Thermal barrier coatings (TBC) can be used to reduce the metal temperature of gas turbine blades enabling higher Cr alloys (lower strength) to be used when gas turbines are to be used in corrosive environments (where hot corrosion resistance is required). However, the TBC must also be resistant to the corrosive environment and remain attached to the blade. A 1000 h test to evaluate air plasma-sprayed (APS) TBC adhesion to a low-pressure plasma-sprayed CoNiCrAlY bond coat (with and without through thickness cracking) under hot corrosion conditions at 850 °C has been carried out. The APS TBC significantly reduced the hot corrosion rate of the CoNiCrAlY; however, delamination cracking occurred with a thinner thermally grown oxide than would be expected from isothermal and cyclic oxidation testing.

An analytical approach to the β-phase coarsening behaviour in a thermally sprayed CoNiCrAlY bond coat alloy

This paper investigates the β-phase coarsening behaviour during isothermal heat treatment of free-standing CoNiCrAlY (Co-31.7%Ni-20.8%Cr-8.1%Al-0.5%Y, all in wt%) coatings prepared by high velocity oxy-fuel (HVOF) thermal spraying. The microstructure of the coatings was characterised using scanning electron microscopy with energy dispersive X-ray (EDX) analysis and electron backscatter diffraction (EBSD). It comprises a two phase structure of fcc γ-Ni matrix and bcc β-NiAl precipitates. The volume fraction of the γ-Ni and the β-NiAl phases were measured to be around 70% and 30% respectively, with grain sizes varying largely from 0.5 to 2 μm for both phases. Isothermal heat treatments of the free-standing coatings were carried out at 1100 C for times up to 250 h. The β-phase coarsening behaviour during isothermal heat treatments was analysed by quantitative metallography. It is shown that the coarsening behaviour of β phase in the CoNiCrAlY alloy followed the classical Lifshitz-Slyozov-Wagner (LSW) theory of Ostwald ripening. By incorporating a dimensionless factor which correlates with volume fraction of the β phase, a modified LSW model coupled with formulaic interfacial energy and effective diffusion coefficient of the CoNiCrAlY alloy was utilised to interpret the coarsening behaviour of the β phase. The coarsening rate coefficient obtained from the modified LSW model shows good agreement with the corresponding experimental result.

Comparison of oxidation behavior of YSZ and Gd2Zr2O7 thermal barrier coatings (TBCs)

Yttria Stabilized Zirconia (YSZ) is widely used as a traditional ceramic top coat material in gas turbine engine components. Nowadays, rare earth zirconates, as alternative materials to YSZ, are enhanced for use as top coat layer of TBC owing to their better thermal isolation properties. In the present study, metallic CoNiCrAlY bond coat powder was sprayed on Inconel 718 superalloy substrate by cold gas dynamic spray (CGDS) technique. After the deposition of bond coats, YSZ and Gd2Zr2O7 top coats were produced using EB-PVD process. TBCs were exposed to isothermal oxidation tests at 1100°C for 8, 24, 50 and 100h. Oxidation and growth behaviors of thermally grown oxide (TGO) layer were observed. TBC samples were investigated using scanning electron microscope (SEM), EDS elemental mapping and X-ray diffractometer (XRD) analysis before and after the oxidation tests. Oxidation performances of two different TBC systems were compared to each other according to the analysis results.

Formation of Thermally Sprayed Coatings on Grid-Like Structure Substrate

The main goal of this contribution is to investigate the influence of the substrate morphology on the resulting thermally sprayed coatings microstructure. Therefore, three different representative coating systems and/or thermal spray techniques were utilized to produce the coatings on grid-like structure substrates: (i) CoNiCrAlY bond coat (BC) sprayed by high velocity oxygen fuel (HVOF) technique and yttria stabilized zirconia (YSZ) top coat (TC) sprayed by means of atmospheric plasma spray (APS) technique, (ii) YSZ coating sprayed by means of APS and (iii) YSZ coating sprayed by means of nanoparticle colloid suspension plasma spraying (SPS). The shadowing effect of thermal spray coatings in relation with the grid-like substrate structure was investigated in detail. Resulting microstructure of sprayed samples was studied utilizing light microscopy, digital image analysis, scanning electron microscopy, energy-dispersive spectrometer and X-ray diffraction techniques.

Durability of Amorphous and Crystalline BMAS Thermal Barrier Coatings Produced by Plasma Spraying

Barium-Magnesium-Aluminium-Silicate (BMAS) powder was produced from a mixture of initial compounds BaO–MgO–Al2O3–SiO2 by means of solid state synthesis at the temperature of 1200 °C for 3 hours in a laboratory furnace. Synthetized powder was crushed into the fraction of 15-45 μm in a planetary ball mill. Thermal barrier coating system consisting of CoNiCrAlY (bond coat) and BMAS (top coat) was sprayed by atmospheric plasma spray technique onto the polycrystalline nickel-based superalloy substrate. During plasma spraying process, the BMAS underwent phase transformation and the amorphous phase within the top coat was produced. Therefore, after the spraying, several samples were crystallized via annealing in a furnace (4 hours at 1200 °C or 24 hours at 1000 °C) or by subjecting them to several passes of plasma jet. Both samples with an amorphous phase and fully-crystallized samples were subjected to the fire in a burner-rig test (propane-oxygen flame, single 3 + 3 minute cycle), where the top coat reached the temperature of 1150 °C. Top coat failure occurred during the cooling period due to the transformation of the amorphous phase into the crystalline one and/or due to the difference in thermal conductivity and expansion between the top coat and the bond coat.

Degradation of YSZ/EUCOR TBC Coating System during High Temperature Low Cycle Fatigue Tests

Thermal barrier coatings are widely used to protect the substrate from high temperature and extremely aggressive environments in gas engines. In the present article, authors have been studied degradation of complex thermal barrier coating system deposited on polycrystalline nickel superalloy IN 713LC. The substrate material was grit blasted with alumina (Al2O3) particles prior to air plasma deposition of CoNiCrAlY bond coat. Top coat consists of conventional zirconia (ZrO2) stabilized by yttria (Y2O3) -YSZ ceramic in combination with a eutectic nanocrystalline ceramic Eucor made of zirconia (ZrO2), alumina (Al2O3) and silicia (SiO2) –in the ratio of 50/50 in wt. %. The top coat was deposited using water stabilized plasma. Test specimens with the TBC coating system were fatigued under strain control condition in fully reversed symmetrical push-pull cycles at 900°C in air. The microstructure of TBC was characterized with scanning electron microscopy and energy dispersion X-ray analysis. The coating hardness and thickness were measured. Fracture surface and polished sections parallel to the specimen axis were examined to study damage mechanisms in coatings under cyclic loading at high temperature. TBC delamination was observed at the top coat/bond coat interface after cyclic loading at high temperature. Fatigue crack initiation sites are documented. Majority of fatigue cracks start from the surface and top coat/bond coat interface.

Isothermal oxidation behavior and kinetics of thermal barrier coatings produced by cold gas dynamic spray technique

The cold gas dynamic spray (CGDS) method was employed to deposit the CoNiCrAlY bond coats of thermal barrier coating (TBC) system. The oxidation behavior of the coatings were investigated under isothermal oxidation at 1000°C, 1100°C and 1200°C for 8, 24, 50 and 100h. Recent studies on TBCs have concentrated on the CGDS process and its properties under working conditions. The motivation of this study is to investigate the oxidation behavior of TBCs produced using CGDS technique under service conditions and to determine the oxidation growth kinetics of thermally grown oxide (TGO). The results show that the isothermal degradation of coatings was considerably influenced by microstructure of coating, interfacial oxide growth rate, oxidation temperature and time.

Microstructure studies of air-plasma-spray-deposited CoNiCrAlY coatings before and after thermal cyclic loading for high-temperature application

In the present study, bond-coats for thermal barrier coatings were deposited via air plasma spraying (APS) techniques onto Inconel 800 and Hastelloy C-276 alloy substrates. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and atomic force microscopy (AFM) were used to investigate the phases and microstructure of the as-sprayed, APS-deposited CoNiCrAlY bond-coatings. The aim of this work was to study the suitability of the bond-coat materials for high temperature applications. Confirmation of nanoscale grains of the γ/γ′-phase was obtained by TEM, high-resolution TEM, and AFM. We concluded that these changes result from the plastic deformation of the bond-coat during the deposition, resulting in CoNiCrAlY bond-coatings with excellent thermal cyclic resistance suitable for use in high-temperature applications. Cyclic oxidative stability was observed to also depend on the underlying metallic alloy substrate.

Microstructural evolution and growth kinetics of thermally grown oxides in plasma sprayed thermal barrier coatings

The formation of thermally grown oxide (TGO) during high temperature is a key factor to the degradation of thermal barrier coatings (TBCs) applied on hot section components. In the present study both the CoNiCrAlY bond coat and ZrO2-8wt.% Y2O3 (8YSZ) ceramic coat of TBCs were prepared by air plasma spraying (APS). The composition and microstructure of TGO in TBCs were investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis. The growth rate of TGO for TBC and pure BC were gained after isothermal oxidation at 1100°C for various times. The results showed that as-sprayed bond coat consisted of β and γ/γ′phases, β phase reducesd as the oxidation time increased. The TGO comprised α-Al2O3 formed in the first 2h. CoO, NiO, Cr2O3 and spinel oxides appeared after 20h of oxidation. Contents of CoO and NiO reduced while that of Cr2O3 and spinel oxides increased in the later oxidation stage. The TGO eventually consisted of a sub-Al2O3 layer with columnar microstructure and the upper porous CS clusters. The TGO growth kinetics for two kinds of samples followed parabolic laws, with oxidation rate constant of 0.344μm/h0.5 for TBCs and 0.354μm/h0.5 for pure BCs.

Microstructure Modification of CGDS and HVOF Sprayed CoNiCrAlY Bond Coat Remelted by Electron Beam

In the present work two techniques are combined to optimize bond coat properties before thermal barrier coating (TBC) application, the cold gas dynamic spraying (CGDS) and electron beam remelting (EB). Results of the work focused on comparison of high velocity oxygen fuel (HVOF) and CGDS CoNiCrAlY bond coats are firstly presented. Than the effect of the electron beam remelting of the CoNiCrAlY coating manufactured by HVOF and CGDS deposition techniques is deeply investigated. The CoNiCrAlY bond coat to Inconel substrate interface displayed locations with very poor bonding, in larger extent for the states prepared by HVOF comparing to CGDS. The bond coats prepared by both ways being EB remelted are typically removal of the defects on the substrate to bond coat interface. The microstructure of the bond coat after this treatment is formed by Inconel fine grain layer being followed by the surface layer consisting of elongated dendritic microstructure. An increased porosity has been observed in interdendritical space in larger extent for CGDS samples.