Effects of bond coat preoxidation on the properties of ZrO2-8wt.% Y2O3/Ni-22Cr-10Al-1Y thermal-barrier coatings

Plasma-sprayed thermal barrier coatings (TBCs) consist of an intermediate MCrAlY bond coat to protect the substrate superalloy from oxidation/hot corrosion and a thermal insulating zirconia-based ceramic top coat. This system is developed for advanced turbine-engine, hot-section components. In this study the as-sprayed Ni-22Cr-10Al-1Y bond coat was subjected to preoxidation treatment at 1000° C for 1, 50, 100, and 200 hr, also at 1100°C, 1200°C and 1300°C for 1 hr to form an oxide scale before subsequent deposition of a ZrO₂-8wt.% Y₂O₃ top coat. The oxidation kinetics were measured, and the phase constitution and morphology of pregrown oxides on the Ni-22Cr-1Y bond coat were analyzed by x-ray diffractometer and SEM/EDS to elucidate the improvement and degradation mechanisms of the new system. The results of the experiments in this study showed that the tentative ZrO₂-8wt.% Y₂O₃ TBC specimens with preoxidized Ni-22Cr-10Al-1Y bond coat, when properly processed, exhibited lower oxidation rates and generally longer lifetime compared with traditional TBC specimens.     

Effect of bond-coat pre-aluminization and pre-oxidation duplex pretreatment on the performance of ZrO2-8 wt.% Y2O3/Co-29Cr-6Al-1Y thermal-barrier coatings

A modification to the conventional thermal-barrier coating (TBC) system was made. In this study, the low-pressure, plasma-sprayed Co-29Cr-6Al-1Y bond coat received a duplex pretreatment of bond-coat pre-aluminizing and pre-oxidation prior to overlaying the air-plasma sprayed ZrO₂-8 wt.% Y₂O₃ top coat. The effects of this treatment on the properties of the TBC system were evaluated by thermal-cyclic test at 1050° C in air. The results of cyclic-oxidation tests showed that the proposed processes could remarkably improve the performance of ZrO₂-8 wt.% Y₂O₃/Co-29Cr-6Al-1Y TBC. The improvement was attributed to a modification of bond-coat oxides and the associated reduction of the oxidation rate of the MCrAlY bond coat.     

Effect of pre-aluminization on the properties of ZrO2-8wt.% Y2O3/Co-29Cr-6Al-1Y thermal-barrier coatings

Specimens of investment-cast Mar-M247 superalloy were vacuum-plasma sprayed with Co-29Cr-6Al-1Y bond coat, and part of the specimens were further pre-aluminized at 980°C for 2, 4, 6, 8, and 10 hours. All the specimens were then deposited with ZrO₂-8 wt.% Y₂O₃ thermal-barrier coatings (TBCs) and thermally cycled at 1050°C to evaluate the effect of time of the prealuminizing treatment on the performance and failure mechanism of the modified system. Results showed that TBC specimens with pre-aluminized bond coatings exhibited lower oxidation rates and significantly higher cyclic life when compared with unaluminized specimens. The failure of bond-coat pre-aluminized TBC specimens was observed to propagate mainly along the lamellar splats of the top coat, whereas the failure of conventional TBC specimens occurred mainly along the top-coat/spinel oxides interface.     

The Oxidation Behavior of TBC with Cold Spray CoNiCrAlY Bond Coat

Cold gas dynamic spray (CGDS) has been considered a potential technique to produce the metallic bond coat for TBC applications, because of its fast deposition rate and low deposition temperature. This article presents the influence of spray processes for bond coat, including air plasma spray, high velocity oxy-fuel, and in particular CGDS, on the oxidation performance of TBCs with a Co-32Ni-21Cr-8Al-0.5Y (wt.%) bond coat and an air plasma sprayed topcoat. Oxidation behavior of the TBCs was evaluated by examining the coating microstructural evolution, TGO growth behavior, and crack propagation during thermal exposure at 1050 °C. The relationship between the TGO growth and crack propagation will be discussed.     

Nickel-chromium plasma spray coatings: A way to enhance degradation resistance of boiler tube steels in boiler environment

Boiler tube steels, namely low carbon steel ASTM-SA-210-Grades A1 (GrA1), 1Cr-0.5Mo steel ASTM-SA213-T-11 (T11), and 2.25Cr-1 Mo steel ASTM-SA213-T-22(T22), were used as substrate steels. Ni-22Cr-10Al-1Y powder was sprayed as a bond coat 150 μm thick before a 200 μm final coating of Ni-20Cr was applied Coatings were characterized prior to testing in the environment of a coal fire boiler. The uncoated and coated steels were inserted in the platen superheater zone of a coal fired boiler at around 755°C for 10 cycles, each 100 h. Coated steels showed lower degradation (erosion-corrosion) rate than uncoated steels showed. The lowest rate was observed in the case of Ni-20Cr coated T11 steel. Among the uncoated steels, the observed rate of degradation was the lowest for the T22 steel.     

The spalling modes and degradation mechanism of ZrO2-8 wt.% Y2O3/CVD-Al2O3/Ni-22Cr-10Al-1Y thermal-barrier coatings

State-of-the-art conventional thermal-barrier coatings consist of a thermalinsulating, partially-stabilized ZrO₂ top coat and a bond coat. In this study, a continuous alumina-diffusion-barrier layer was deposited and interposed between the top coat and bond coat by chemical-vapor deposition (CVD). Both the conventional and the experimental TBC systems were cyclically tested at 1000°C, 1050°C, 1100°C, and 1150°C to evaluate and compare oxidation, performance, and fracture behavior. Introduction of the intermediate CVD-Al₂O₃ layer effectively suppressed the oxidation rate of the bond coat and sufficiently altered its oxidation behavior. The thermal-cyclic life of TBCs was improved by the new system. The failure of the ZrO₂-8 wt.% Y₂O₃/CVD-Al₂O₃/Ni-22Cr-10Al-1Y TBC specimens was observed to propagate mainly along the lamellar splats of the top coat, and secondarily along the top coat/CVD-Al₂O₃ interface.     

Analytical Studies on the Behavior of Nickel- and Cobalt-Base Shrouded Plasma Spray Coatings at Elevated Temperature in Air

Nickel- and Cobalt-base coatings were developed on boiler-tube steels by the argon-shrouded, plasma spray process. The cyclic behavior of bare and coated boiler-tube steels has been studied in air at 900 °C. Four types of coatings were used: Ni-22Cr-10Al-1Y (NiCrAlY), Ni-20Cr, Ni₃Al and Stellite-6 (St-6). The NiCrAlY has also been used as a bond coat of approximately 150 μm thick before the final coating in all the cases. Oxidation products analyzed in the scale are mostly oxides of the elements present in the coatings and substrate steels. NiCr₂O₄ and CoCr₂O₄ spinels are the most-common observed phases in the scale for nickel- and cobalt-base coatings. The internal oxidation of coatings and diffusion of iron from the base steel to the upper scale occurred during the study. Cracks formed in the scale and coating and may be due to differences in composition of coatings, bond coat, substrate and oxides formed.     

Diffusion and phase transformation on interface between substrate and NiCrAlY in Y-PSZ thermal barrier coatings

NiCrAlY/Y₂O₃-Y-PSZ (yttria-partially stabilized zirconia) thermal barrier coatings were developed on a superalloy (Ni-10Co-9Cr-7W-5Al, wt.%) surface. The superalloys were first coated with a bond coat of Ni-19Cr-8Al-0.5Y (wt.%) alloy that was deposited by low-pressure plasma spraying and then covered with a top coat of ZrO₂-8wt.%Y₂O₃ by air plasma spraying. The microstructure near the interface was analyzed using an optical microscope, a scanning electron microscope, microhardness measurements, and x-ray diffraction, and the phases of composition were measured using an electron probe microanalyzer after exposure at 1100°C for different times in air or a vacuum. The reaction processes also were simulated using diffusion-controlled transformation (DICTRA) software in which diffusion was considered as being only the γ phase, and the γ′ phase was treated as spheroidal particles in γ. From the authors’ results, it can be concluded that a γ′-phase layer is observed at the interface between substrate and bond coat, and its thickness increases with increasing exposure times in air at 1100 °C. This layer showed good cohesion with the substrate and bond coat. It can also be concluded that the formation of the γ′-phase layer can be predicted from DICTRA simulation. The simulation also shows the same trend of the composition profiles as experimental data.     

Influence of Heat Treatment on the Bond Coat Cyclic Oxidation Behaviour in an Air-plasma-sprayed Thermal Barrier Coating System

THE METALLIC BOND COAT is an importantconstituent in a TBC system.It enhances the adhesionof the ceramic thermal barrier layer(the topcoat)to thesubstrate and also provides oxidation protection to thesubstrate metal.The composition of the bond coat,generalized as M-Cr-Al-Y,where M represents Ni,Coand/or Fe,generally allows a layer of alumina(A12O3)to form during high temperature exposure.If acontinuous scale of A12O3forms along the interfacebetween the bond coat and the ceramic to…     

TGO Growth and Crack Propagation in a Thermal Barrier Coating

In thermal barrier coating (TBC) systems, a continuous alumina layer developed at the ceramic topcoat/bond coat interface helps to protect the metallic bond coat from further oxidation and improve the durability of the TBC system under service conditions. However, other oxides such as spinel and nickel oxide, formed in the oxidizing environment, are believed to be detrimental to TBC durability during service at high temperatures. It was shown that in an air-plasma-sprayed (APS) TBC system, postspraying heat treatments in low-pressure oxygen environments could suppress the formation of the detrimental oxides by promoting the formation of an alumina layer at the ceramic topcoat/bond coat interface, leading to an improved TBC durability. This work presents the influence of postspraying heat treatments in low-pressure oxygen environments on the oxidation behavior and durability of a thermally sprayed TBC system with high-velocity oxy-fuel (HVOF)-produced Co-32Ni-21Cr-8Al-0.5Y (wt.%) bond coat. Oxidation behavior of the TBCs is evaluated by examining their microstructural evolution, growth kinetics of the thermally grown oxide (TGO) layers, and crack propagation during low-frequency thermal cycling at 1050 °C. The relationship between the TGO growth and crack propagation will also be discussed.     

Influence of Thermal Cycle Frequency on the TGO Growth and Cracking Behaviors of an APS-TBC

The durability of thermal barrier coatings (TBCs) is controlled by fracture near the interface between the ceramic topcoat and the metallic bond coat, where a layer of thermally grown oxide (TGO) forms during service exposure. In the present work, the influence of thermal cycle frequency on the oxidation performance, in terms of TGO growth and cracking behavior, of an air-plasma-sprayed (APS) Co-32Ni-21Cr-8Al-0.5Y (wt.%) bond coat was studied. The results show that while TGO growth exhibited an initial parabolic growth behavior followed by an accelerated growth stage, higher cycle frequency resulted in a faster TGO growth and a higher crack propagation rate. It is found that a power-law relationship exists between the maximum crack length and the TGO thickness, which is independent of the cycle frequency. This relationship may warrant a TBC life prediction methodology based on the maximum crack length criterion.     

Degradation behavior of Ni<Subscript>3</Subscript>Al plasma-sprayed boiler tube steels in an energy generation system

Boiler steels, namely, low-C steel, ASTM-SA210-Grade A1 (GrA1), 1Cr-0.5Mo steel, ASTM-SA213-T-11 (T11) and 2.25Cr-1Mo steel, ASTM-SA213-T-22 (T22) were plasma sprayed with Ni₃Al. The alloy powder was prepared by mixing Ni and Al in the stoichiometric ratio of 3 to 1. The Ni-22Cr-10Al-1Y alloy powder was used as a bond coat, with a 150 μm thick layer sprayed onto the surface before applying the 200 μm coating of Ni₃Al. Exposure studies have been performed in the platen superheater zone of a coal-fired boiler at around 755 °C for 10 cycles, each of 100 h duration. The protection to the base steel was minimal for the three steels. Scale spallation and the formation of a porous and nonadherent NiO scale were probably the main reasons for the lack of protection. In the case of T22-coated steel, cracks in the coatings have been observed after the first 100 h exposure cycle.     

Thermal shock characteristics of plasma sprayed mullite coatings

Commercially available mullite (3Al₂O₃·2SiO₂) powders containing oxides of calcium and iron as impurities, have been made suitable for plasma spraying by using an organic binder. Stainless steel substrates covered with Ni-22Cr-10Al-1.0Y bond coat were spray coated with mullite. The 425 μm thick coatings were subjected to thermal shock cycling under burner rig conditions between 1000 and 1200 °C and less than 200 °C with holding times of 1, 5, and 30 min. While the coatings withstood as high as 1000 shock cycles without failure between 1000 and 200 °C, spallation occurred early at 120 cycles when shocked from 1200 °C. The coatings appeared to go through a process of self erosion at high temperatures resulting in loss of material. Also observed were changes attributable to melting of the silicate grains, which smooth down the surface. Oxidation of the bond coat did not appear to influence the failure. These observations were supported by detailed scanning electron microscopy and quantitative chemical composition analysis, differential thermal analysis, and surface roughness measurements.     

EB-PVD热障涂层热循环过程中粘结层的氧化和相结构

采用磁控溅射方法在镍基单晶高温合金基体上沉积Ni-30Cr-12Al-0.3Y（质量分数，％）粘结层，采用电子束物理气相沉积方法（EB－VPD）沉积7％Y2O3（质量分数）－ZrO2陶瓷顶层，结果表明，在热循环过程中，非平衡相t′-ZrO2中的Y2O3含量逐渐减少，t′－ZrO2相逐渐分解成平衡相t-ZrO2（冷却时变转变成斜相）和立方组ZrO2,1050℃循环200次，粘结层氧化物（Al2O3)厚度约为3μm，表明Ni-Cr-Al-Y达宜作粘结层，继续热循环，陶瓷层中出现单斜阳，粘结层中Al贫化，氧化层中出现NiO及尖晶石等，引起应力集中，导致涂层失效。     

Heat treatment effects on the corrosion resistance of some HVOF-sprayed metal alloy coatings

The present study evaluates the effects of a 600 °C, 1 h heat treatment on the corrosion resistance of three High Velocity Oxygen Fuel (HVOF) flame-sprayed alloy coatings: a Co-28Mo-17Cr-3Si (similar to Tribaloy-800) coating, a Ni-20Cr-10W-9Mo-4Cu-1C-1B-1Fe (Diamalloy-4006) coating and a Ni-32Mo-16Cr-3Si-2Co (similar to Tribaloy-700) coating. Electrochemical polarization tests and free corrosion tests were performed in 0.1 M HCl aqueous solution. The corrodkote test (ASTM B380-97R02) was also performed, to evaluate the coatings qualitatively. The heat treatment improves the corrosion resistance of the Co-28Mo-17Cr-3Si coating and of the Ni-20Cr-10W-9Mo-4Cu-1C-1B-1Fe coating by enhancing their passivation ability. The precipitation of sub-micron sized secondary phases after the treatment may produce galvanic microcells at intralamellar scale, but the beneficial contribution provided by the healing of the very small but dangerous interlamellar defects (normally present in thermal spray coatings but not detectable using ordinary scanning electron microscopy) prevails. The effect on Ni-32Mo-16Cr-3Si-2Co coatings is more ambiguous: its sensitivity to crevice corrosion is worsened by the heat treatment.     

The hot corrosion of Co-25Cr-10Ni-5Ta-3Al-0.5Y alloy (S-57)

A cobalt-base alloy, Co-25 Cr-10 Ni-5 Ta-3 Al-0.5 Y (S-57), was subjected to hot corrosion in Mach 0.3 burner rig combustion gases at maximum alloy temperatures of 900 and 1000°C. Various salt concentrations were injected into the burner; 0.5, 2, 5 and 10 parts per million synthetic sea salt and 4 parts per million sodium sulphate (Na₂SO₄). The extent of corrosion was determined by measuring the maximum depth of corrosion in the alloy and the corrosion process was studied by metallography, X-ray diffraction, scanning electron microscopy, and electron microprobe analysis. While S-57 was found to possess only moderate oxidation resistance at these temperatures, this alloy resisted significant hot corrosion attack under all but the most severe test conditions. The process of the hot corrosion attack under the most severe conditions of this study was primarily sulphidation.     

Microstructural aspects of low-pressure plasma-sprayed CoNiCrAlY coating on Hastelloy X

A Co-32Ni-21Cr-8Al-0.5Y alloy coating was plasma sprayed on Hastelloy X. The microstructure of the coating layer consists of γ phase solid solution, γ′ phase, and Y-rich intermetallic phase. This coating exhibits excellent oxidation and sulfidation resistance after exposure in air and in sodium sulfate at 1,000°C for 60 h, due to the formation of α-Al₂O₃ oxide scale. However, the presence of chloride in the sodium sulfate leads to rupture of the aluminium oxide scale, and this results in the precipitation of chlorides and sulfides within the coating layer.     

The Sulfidation/Oxidation Resistance of Two Ni-Cr-Al-Y Alloys at 700℃

The high temperature corrosion resistance of Ni-25.9Cr-13.5Al-1.2Y-0.6Si and Ni-10.2Co-12.4Cr-16.0Al-0.5Y-0.2Hf alloys was assessed in sulfidation/oxidation environments.In the environment with a sulfur partial pressure of 1Pa. and an oxygen partial pressure of 10^-19Pα,both these alloys exhibited three distinct stages in the weight gain-time curve when tested at 700℃.In the initial stage, selective sulfidation of Cr suppressed the formation of the other metal sulfides,resulting in lower weight gains.In the transient stage, breakdown and cracking of Cr sulfides and insufficient concentration of Cr at the outer zone led to the rapid formation of Ni sulfides and a rapid increase in weight.In the steady-state stage, corrosion was controlled by the diffusion of anions and/or cations, which led to a parabolic rate law.     

Some interactions in the erosion-oxidation of alloys

The modes of interaction of erosion and high-temperature oxidation, occurring simultaneously on an alloy surface, have been studied using Ni-30Cr, MA754, Ni-20Al, and Co-22Cr-11Al-0.2Y alloys to examine the influence of chromia and alumina scale formation on the erosion of nickel and cobalt base alloys. The results have shown that, in the presence of a rapidly flowing oxidizing gas stream, the evaporation of volatile metal oxides becomes important at lower temperatures where normally it can be ignored. Otherwise, the erosion and oxidation of alloys parallels the behavior of pure metals but also introduces additional factors derived from lengthening of the period of transient oxidation and modification of concentration profiles in the alloy adjacent to the alloy/scale interface. Higher erosion intensities extend the transient oxidation behavior which adversely affects the formation of protective scales. As with pure metals, the presence of erosion and oxidation together always produced increased rates of degradation.     

High Temperature Alloy Corrosion by Halogens

Difficulties of alloy selection for high-temperature halogen service are described with reference to product morphologies, vaporization and the manner by which alloy degradation may occur. Selected results from work with alloy 600 (Ni-16 Cr-8Fe) and Havnes® Alloy 230 (Ni-22 Cr-14W-2Mo-0.3Al-0.02 La) are used to contrast various modes of attack. Oxidizing chlorine-containing gases often give rise to heavy volatilization and intergranular attack at 900°C (1650°F) whilst HF/steam mixtures result in more uniform internal oxidation at 750°C (1380°F).