Long-term High-temperature Oxidation Research Articles

Residual stress in air-plasma-sprayed thermal barrier coatings under long-term high-temperature oxidation

The accumulated residual stress in yttria-stabilized zirconia top coat (YSZ TC) and thermally grown oxide (TGO) is regarded as the key driving force for the delamination of air-plasma-sprayed thermal barrier coating system (APS-TBCs). In this study, the synchronous evolution of residual stress of both YSZ TC and TGO, as well as the TGO thickness, are characterized via the Raman spectroscopy (RS) and photoluminescence piezo-spectroscopy (PLPS) methods. For the APS-TBCs where one surface is coated with YSZ TC, while the other surface remains directly exposed to high temperature, the service life can be categorized into four distinct stages: the early stage with tensile stress state (0–40 h), the medium stage with reversal compressive stress state (40–100 h), the late stage with re-inverted tensile stress (100–140 h), and the final stage with compressive stress state (140–300 h). Such stress reversal phenomenon may be attributed to the competition between the growth of TGO and substrate oxide (SO). It is advisable to apply an oxidation-resistant coating on the inner wall of high-temperature blade cooling passages, to prevent the formation of SO and consequently avoid complex stress transitions during high-temperature service of the APS-TBC, ultimately preventing local stress concentration-induced cracking and delamination.

Improving the high-temperature oxidation resistance of CrN coating by gradient deposition considering internal stress effects

In this study, two different CrN coatings were deposited on the surface of Zr-4 via a multi-arc ion-plating method using gradient and non-gradient deposition processes. The effects of two deposition processes on the high-temperature oxidation resistance and internal stress of the coatings were investigated. The microstructure and phase structure of the coatings were characterized by scanning electron microscopy, X-ray diffraction (XRD), and energy dispersive spectrometry. The high-temperature oxidation resistance of the specimens was investigated by performing thermal shock experiments and simulated loss-of-coolant accident high-temperature steam oxidation experiments. The residual stresses of both coatings during deposition were determined by XRD analysis. In addition, the thermal stresses of both coatings during the high-temperature oxidation tests were modeled and calculated. Under long-term high-temperature oxidation conditions, the gradient-deposited CrN coating exhibited improved high-temperature oxidation resistance performance than that exhibited by non-gradient-deposited CrN coating. In addition, the residual stresses of gradient-deposited CrN coating during deposition and thermal stresses during high-temperature oxidation were significantly lower than those of the non-gradient-deposited CrN coating. The results obtained herein confirm the promising potential of gradient-deposited CrN coating as an accident-tolerant fuel cladding material.

Distinctive performance evolution of surface layer in TC4 alloy oxidized at high temperature: Softening or hardening?

The characteristics of metal surface layer at high-temperature significantly affect the service performance of TC4 titanium alloy components, so that the evolution mechanism of mechanical properties of metal surface layer under high-temperature oxidation conditions is an important scientific issue. In the present work, the mechanical behavior and the relevant mechanism of metal surface oxidized at the temperatures of 500 ℃, 700 ℃, and 900 ℃ for different times have been systematically investigated. The experimental results show that with the increase of temperature, the oxidation kinetics curve of the alloy changes from parabolic law to linear law, and the oxidation reaction changes from diffusion control to interfacial reaction control. Furthermore, as the temperature increases from 500 °C to 900 °C, the feature of oxide scale changes from the single-layer structure to the multi-layer alternating structure, and the thickness of oxide scales and depth of alpha case increased with increasing temperature. The surface oxide scale is mainly composed of low valent oxides such as Ti3O and Ti6O during low-temperature oxidation as well as short-term high temperature oxidation. After long-term high temperature oxidation, the oxide scale is mainly composed of high valent oxides such as TiO2, Al2O3, and TiVO4. Simultaneously, significant changes occur in the composition, microstructure and properties of metal surface layer. In general, the microstructure and mechanical properties of the surface-layer metal are strongly dependent upon the competitive influence of the outward diffusion of Al and the inward diffusion of O. In the temperature range of 500–900ºC, when the temperature is relatively low, the softening effect caused by the outward diffusion of Al is stronger than the strengthening effect caused by the inward diffusion of O, and a surface softening effect thus occurs. On the contrary, when the temperature is high, the strengthening effect caused by the inward diffusion of O is stronger than the softening effect caused by the outward diffusion of Al, and a surface strengthening effect accordingly happens.

Experimental Study of the Evolution of Creep-Resistant Steel’s High-Temperature Oxidation Behavior

This study shows that in an atmosphere containing water vapor, the oxide layer on the surface of the 9CrNB steel MarBN (Martensitic 9Cr steel strengthened by Boron and MX Nitrides) was formed by an outer layer of hematite Fe2O3 and Cr2O3 and an inner two-phase layer of Fe3O4 and Fe3O4 + (Fe, Cr)2O4, which was confirmed by XRD analysis. Part of the layer consisted of nodules and pores that were formed during the increase in oxides when the present H2O(g) acted on the steel surface. The diffusion mechanism at temperatures of 600 and 650 °C and at longer oxidation times supported the “healing process” with a growing layer of Fe oxides and the presence of Cr and minor alloying elements. The effects of alloying elements were quantified using a concentration profile of the oxide layer based on quantitative SEM analysis, as well as an explanation of the mechanism influencing the structure and chemical composition of the oxide layer and the steel-matrix–oxide interface. In addition to Cr, for which the content reached the requirement of exceeding 7.0 wt. % in the inner oxide layer, W, Co, Mn, and Si were also found in increased concentrations, whether in the form of the present Fe-Cr spinel oxide or as part of a continuously distributed layer of Mn2O3 and SiO2 oxides at the steel-matrix–oxide interface. After long-term high-temperature oxidation, coarser carbides of the M23C6 type (M = Fe,W) significantly depleted in Cr were formed at the oxide-layer/matrix interface. In the zone under the oxide layer, very fine particles of MC (M = V, Nb, and to a lesser extent also Cr in the particle lattice of the given phase) were observed, with a higher number of particles per unit area compared to the state before oxidation. This fact was a consequence of Cr diffusion to the steel surface through the subsurface zone.

Phase decompositions of Gd2Zr2O7 + 8YSZ TBC systems under the condition of long-term high-temperature oxidation

The article presents the results of research related to the assessment of the phase composition of two-phase TBC systems based on gadolinium zirconate GZO and a conventional zirconium oxide modified by the yttrium oxide 8YSZ. The coatings were made on a nickel superalloy of IN 625 type with a NiCrAlY base bond coat. The technique of atmospheric plasma spray was used for coatings depositions. Three types of composite coatings, made of a mixture of base powders with the proportion of components 25–75, 50–50 and 75–25, were evaluated. The thickness of the ceramic layers was approximately 300 μm. The TBC systems were then subjected to high temperature oxidation at 1100 °C for up to 2000 h in air. Changes in phase composition were evaluated by the XRD method after 48, 175, 500, 1000 and 2000 h of exposure. Detailed microstructural studies with the use of scanning microscopy methods were performed after 2000 h. The GZO + 8YSZ composite coatings were found to be characterized by high susceptibility to changes in phase composition, which is related to the formation of new zirconate phases with a lower content of gadolinium oxide (in the case of zirconates) and phases based on zirconium oxide with a higher content of gadolinium oxide. The observed changes are diffusive and take place predominantly within the GZO phase. Additional structural effects observed during SEM studies are the phenomena of the formation of a network of numerous small voids in the GZO phase, which suggests the course of nonequilibrium diffusion processes. Furthermore, a high concentration of nickel oxide and a slightly lower concentration of chromium oxide were found in the ceramic layer.

Effect of TiC on the high-temperature oxidation behavior of WMoTaNbV refractory high entropy alloy fabricated by selective laser melting

The refractory high-entropy alloys possess both high melting points and excellent high-temperature properties. As a result, they are expected to become a new generation of high-temperature materials. Along these lines, in this work, selective laser melting (SLM) was used to prepare WMoTaNbV and TiC/WMoTaNbV-based alloys, while the influence of the SLM process parameters on the surface morphology, internal defects distribution and microstructure of the alloy was thoroughly analyzed. On top of that, the long-term high-temperature oxidation behavior of the two alloys at 600°C was studied. The extracted results showed that the process parameters had a serious impact on the surface quality and internal defects of the refractory high-entropy alloys. By optimizing the process parameters, the final surface roughness value of the WMoTaNbV-based alloy was 9.148 μm (Sa). On the contrary, the surface roughness value of the TiC/WMoTaNbV compound was 14.218 μm (Sa). It is also interesting to notice that the incorporation of TiC could significantly reduce the number of cracks within the TiC/WMoTaNbV alloy. Moreover, the WMoTaNbV-based alloy was mainly composed of dendritic structures, while the TiC/WMoTaNbV-based alloy consisted of cellular structures. The main oxidation products of the WMoTaNbV-based alloy were the Ta16W18O94 and Nb14W3O47 ternary oxidation products. After the TiC particles were added, the Ti-containing oxidation products such as TiO2, Ti2Nb10O29. etc. within the TiC/WMoTaNbV-based alloy increased significantly, preventing thus effectively the alloy from being seriously oxidized. The cellular structures, carbides and TiO2 phase are helpful in improving the oxidation resistance of the TiC/WMoTaNbV-based alloy.

Study on oxidation-creep behavior of a Ni-based single crystal superalloy based on crystal plasticity theory

Long-term high temperature oxidation seriously affects the creep properties of the Ni-based single crystal superalloy. By using experimental research together with finite element simulation, the effects of high temperature oxidation on creep properties of a Ni-based single crystal superalloy (Ni-9.0Co-4.0Cr-5.7Al-7.0Ta-8.0W-2.2Re, wt.%) were studied. The specimens were pre-oxidized at 1100 °C for different periods of time, and then the pre-oxidized specimens were conducted by creep tests at 980 °C/270 MPa. By data fitting and analysis, the thickness evolution law and dynamic model of oxide layer/heat affected layer were established and the quantitative effect laws of pre-oxidation on creep life and deformation were established. The evolution and influence mechanism of oxide layer, heat-affected layer and substrate on creep failure were systematically studied by scanning electron microscope combined with energy dispersive spectrum analysis (SEM-EDS). Based on the thickening dynamic model of heat-affected layer and crystal plasticity theory, two oxidation-creep models were established. The initial damages caused by oxidation and microstructure evolution are both considered in the initial pre-oxidation damage model, which is fast and efficient. The distribution and evolution of stress, strain, resolved shearing stress and strain of the substrate and the heat-affected layer are obtained by another model, the two-phase model. The two models are compared and analyzed, which is also the uniqueness and innovation of this study. The simulations results of the two models are in good agreement with the creep tests.

Insights into the role of the layer architecture of Cr–Ti–N based coatings in long-term high temperature oxidation experiments in steam atmosphere

Knowledge on hard coatings has been applied in the energy field extending their use as protecting coatings of steam power generation plants components. The role of the layer architecture of Cr–Ti–N based coatings deposited by reactive cathodic arc evaporation on P92 steel substrates was studied with the focus on their oxidation resistance at 650 °C in 100% steam atmosphere up to 2000 h. Characterization of the coatings was performed by gravimetry, scanning electron microscopy, electron probe microanalysis, glow discharge optical emission spectroscopy, X-ray diffraction, thermodynamic simulations using the CALPHAD method, Rockwell C indentation and nanoindentation. The layered arrangement improves the oxidation resistance of TiN under the working conditions of steam power plants, as well as the mechanical properties of CrN. The produced architectures performance under the described working conditions boosts the understanding of the processes taking place at high temperature, making possible the design of optimal coatings combining the best behavior of both nitrides for each specific application, reaching a corrosion protection at high temperature in water vapor comparable to that of CrN and a hardness and Young's modulus as high as those of TiN.

Структурные особенности высокотемпературного окисления литого жаропрочного сплава HP40NbTi. Часть II. Эволюция микроструктуры и фазового состава

Optical and electron microscopy and x-ray spectrum microanalysis are used to study the transformation of the phases present in the structure of cast refractory alloy HP40NbTi under long-term high-temperature oxidation. It is shown that during the oxidation the chemical composition of the alloy varies continuously and by complex laws depending on the environment, on the temperature, and on the duration of the exposure; the chemical elements transfer intensely between different phases. After a long hold, this process reaches the limiting state.

Structure and properties of composite Ni–Co–Mn coatings on metal interconnects by electrodeposition

In order to obtain the high conductivity values and wide spinel stability region for solid oxide fuel cell interconnect, several multilayer Ni, Co and Mn coatings are electroplated and then oxidized in air to form spinel oxide layers. Potentiodynamic polarization curves in different simple solutions are tested to analyze the deposition behavior of Co and Mn. Microstructures and compositions of Ni–Co–Mn multi-layers by adjusting the thickness and deposition parameters are analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that area specific resistance value of sample B–Ni/Co after oxidation at 750 °C for 500h is the lowest among the coatings, and the resistance values at 700 °C and 800 °C are 35.3 and 31.7 mΩ‧cm2, respectively. When the Ni transition layer in the vicinity of coating/substrate interface is thick, it will lead to the outward diffusion and aggregation of element Fe to form Fe-rich oxide intermediate layer, which will affect the high-temperature performance of the coating. Pure Co and CoMn alloy coatings with a certain thickness can effectively prevent the inward diffusion of oxygen and the outward diffusion of Fe and Cr at high temperature. The thin Ni transition layer combined with the thick Co layer or CoMn layer has the best element diffusion inhibition and high temperature electrical properties during the long-term high-temperature oxidation process.

Effect of Long-Term High Temperature Oxidation on the Coking Behavior of Ni-Cr Superalloys.

The service time of an industrial cracker is strongly dependent on the long-term coking behavior and microstructure stability of the reactor coil alloy. Super alloys are known to withstand temperatures up to even 1400 K. In this work, several commercially available alloys have been first exposed to a long term oxidation at 1423 K for 500 h, so-called metallurgic aging. Subsequently, their coking behavior was evaluated in situ in a thermogravimetric setup under ethane steam cracking conditions (Tgasphase = 1173 K, Ptot = 0.1 MPa, XC2H6 = 70%, continuous addition of 41 ppmw S/HC of DMDS, dilution δ = 0.33 kgH2O/kgHC) and compared with their unaged coking behavior. The tested samples were also examined using scanning electron microscopy and energy diffractive X-ray for surface and cross-section analysis. The alloys characterized by increased Cr-Ni content or the addition of Al showed improved stability against bulk oxidation and anti-coking behavior after application of metallurgic aging due to the formation of more stable oxides on the top surface.

Fabrication of 4H-SiC PiN diodes without bipolar degradation by improved device processes

We developed a simple technology for fabricating bipolar degradation-free 6.5 kV SiC PiN diodes on the basal plane dislocation (BPD)-free areas of commercially available 4H-SiC wafers. In order to suppress process-induced basal plane dislocation, we first investigated the causes of BPD generation during fabrication and then improved the processes. We found that no BPD was induced on a flat Si-face, but a large number of BPDs were concentrated in the mesa edge after high-dose Al ions were implanted [p++ ion implantation (I. I.)] at room temperature (RT) followed by activation annealing. Therefore, we examined new technologies in device processes including (I) long-term high-temperature oxidation after the mesa process to remove etching damage in the mesa edge and (II) reducing the Al dose (p+ I. I.) in the mesa edge to suppress BPD generation. We investigated the effect of the Al dose in the mesa edge on BPD generation and bipolar degradation. The results indicated that no BPD appeared when the dose was lower than 1 × 1015 atoms/cm2 and when long-term high-temperature oxidation was applied after the mesa process. As a result, we successfully fabricated 6.5 kV PiN diodes without bipolar degradation on BPD-free areas. Moreover, the diodes are very stable when applying 270 A/cm2 for over 100 h. Photoluminescence (PL) observation indicated that no BPD was generated during the improved fabrication processes. Besides, the Ir-Vr measurements showed that the breakdown voltage was over 8 kV at RT. The leakage currents are as low as 7.6 × 10−5 mA/cm2 (25 °C) and 6.3 × 10−4 mA/cm2 (150 °C) at 6.5 kV. Moreover, this result is applicable not only for PiN diodes but also for MOSFETs (body diode), IGBTs, thyristors, etc.

MECHANICAL PROPERTIES AND HIGH TEMPERATURE OXIDATION BEHAVIOR OF Ti–Al COATING REINFORCED BY NITRIDES ON Ti–6Al–4V ALLOY

Ti–Al alloyed coating reinforced by nitrides was fabricated by laser surface alloying technique to improve mechanical properties and high temperature oxidation resistance of Ti–6Al–4V titanium alloy. Microstructures, mechanical properties and high temperature oxidation behavior of the alloyed coating were analyzed. The results show that the alloyed coating consisted of Ti3Al, TiAl2, TiN and Ti2AlN phases. Nitrides with different morphologies were dispersed in the alloyed coating. The maximum microhardness of the alloyed coating was 906[Formula: see text]HV. The friction coefficients of the alloyed coating at room temperature and high temperature were both one-fourth of the substrate. Mass gain of the alloyed coating oxidized at 800[Formula: see text]C for 1000[Formula: see text]h in static air was [Formula: see text][Formula: see text]mg/mm2, which was 1/35th of the substrate. No obvious spallation was observed for the alloyed coating after oxidation. The alloyed coating exhibited excellent mechanical properties and long-term high temperature oxidation resistance, which improved surface properties of Ti–6Al–4V titanium alloy significantly.

EFFECT OF SURFACE ALUMINIZING ON LONG-TERM HIGH-TEMPERATURE THERMAL STABILITY OF TC4 TITANIUM ALLOY

Aluminum (Al)–rich titanium (Ti)–Al alloyed coating was fabricated on the surface of TC4 Ti alloy to improve the high-temperature stability of TC4 Ti alloy by means of arc-added glow discharge plasma technology. Microstructure, room-temperature properties and long-term high-temperature oxidation behavior of the alloyed sample were investigated. The results show that a uniform and compact Ti–Al alloyed coating with about 30[Formula: see text][Formula: see text]m thickness formed on the surface of TC4 Ti alloy. Microhardness and room-temperature wear resistance of the alloyed sample improved significantly. Long-term oxidation behaviors of the samples in air at 800[Formula: see text]C for 1000[Formula: see text]h show that the mass gain of the alloyed sample was 0.3686[Formula: see text]mg/cm2, while that of the substrate was 18.2095[Formula: see text]mg/cm2. Arc-added glow discharge plasma aluminizing improved high-temperature oxidation resistance of TC4 Ti alloy significantly.

Detailed electrical and mechanical retention characteristics of MoSi2–RSiC composites exhibiting three-dimensional (3D) interpenetrated network structure during long-term high-temperature oxidation process

MoSi2–RSiC composites were prepared via a combination of precursor impregnation pyrolysis and high-temperature melt infiltration, in which re-crystallized silicon carbide (RSiC) was used as matrix. The composition, microstructure, oxidation resistance, electrical and mechanical retention characteristics of the composites during long-term high-temperature oxidation process were studied. SEM images revealed that dense MoSi2–RSiC composites exhibiting three-dimensional (3D) interpenetrated network structure were obtained. XRD patterns confirmed that the primary compositions of the composites were 6H-SiC and hexagonal MoSi2, as well as a small amount of Mo4.8Si3C0.6. The weight gain rate of the MoSi2–RSiC composites was about 50% lower than that of RSiC, indicating that the MoSi2–RSiC composites possess improved oxidation resistance, which was mainly attributed to the acute decrease in porosity of the composites and the oxidation only occurred on the surface of it. The electrical properties of the MoSi2–RSiC composites decreased slightly and then reached a flat with the increase in oxidation time, suggesting that the MoSi2–RSiC composites possessed an excellent electrical retention characteristic. The calculated infactor I of the modified mixture value indicated that the interface combination played a more important role than that of interpenetrated network structure on the electrical retention characteristic of the composites. The composites oxidized for 50h achieved the maximal flexural strength and elastic modulus, and values were 132.38MPa (flexural strength) and 335.45GPa (elastic modulus) for MoSi2-RSiC-2, respectively, exhibiting 31.30% and 27.7% improvement compared with their initial values. The mechanical properties of MoSi2–RSiC composites were higher than their original values even after 100h of oxidation. This phenomenon can be due to the dense 3D interpenetrated network structure of the composites, in which oxidation reaction only occurred on the external surface.

Long-term high-temperature oxidation of iridium coated rhenium by electrical resistance heating method

A continuous and compact iridium (Ir) coating with a thickness of ~100μm was electrodeposited on a rhenium (Re) rod in molten salt at 580°C for 4h. The oxidation resistance and failure mechanism of the Ir coated Re (Ir/Re) material were investigated by resistance heating method at 2000°C in air till the Ir coating failed. The results showed that the lifetime of the Ir/Re rod oxidized at 2000°C in air was 183min. After high-temperature oxidation, except for the failure position, the Ir coating in most of the heated regions kept dense and exhibited excellent adhesion on the substrate, with smooth surface and large grain size. The preferred orientation of the Ir coating changed from <220> to <111> after oxidation test. From the end to the center of the as-oxidized Ir/Re sample, the Ir coating became thinner, and the diffusion layer between Ir and Re got thicker. Meanwhile, the preferential oxidation of grain boundaries of Ir coating was more and more severe. It was found that the lifetime of the Ir/Re material in high-temperature oxidizing environment is closely related to the consumption rates of Ir coating by both the direct oxidation of Ir and the diffusion of Re into Ir coating. Based on the diffusion and oxidation kinetics of Re and Ir, the lifetime of the Ir/Re sample in the present study was calculated to be 242min. The difference between the calculated and real lifetimes can be attributed to the ignored fact that Re diffuses rapidly along the grain boundaries of Ir coating in the calculation.

Long-term high temperature oxidation of CrAl(Y)N coatings in steam atmosphere

The oxidation resistance of CrAl(Y)N coatings deposited by reactive magnetron sputtering on P92 steel substrates was tested at 650°C in 100% steam atmosphere up to 2000h of oxidation. Mass gain measurements and characterisation of coatings and scales after oxidation show the enhanced oxidation resistance provided by the coatings with respect to that of the substrate. The dominant influence of the film microstructure developed due to the presence of an adhesion interlayer of CrN at the coating/substrate interface over Y additions is evidenced. The best performance is achieved by a CrAlN dense coating of around 6μm without adhesion interlayer.

Influence of “Island-Like” Oxides in the Bond-Coat on the Stress and Failure Patterns of the Thermal-Barrier Coatings Fabricated by Atmospheric Plasma Spraying During Long-Term High Temperature Oxidation

Thermal-barrier coatings (TBCs) are very important ceramic-coating materials due to their excellent performance at high temperature. The inner zone of the bond-coat is often easily endured oxidized (internal oxidation) in the process of thermal spraying and the long-time exposure to the high temperature, and the “island-like” oxides can be formed. Especially, when the bond-coat was fabricated by atmospheric plasma spraying (APS), this trend is more evident. In this paper, the stress distribution around the thermally grown oxide (TGO) has been calculated by the finite element method when the “island-like” oxides have been considered. The simulation results indicate that the maximum tensile stress and compressive stress existed in the TGO, and the existence of the “island-like” oxides will further decrease the maximum tensile stress level in the TGO. While the “island-like” oxides in the bond-coat will decrease the effective thickness of the TGO at the metallic layer/ceramic layer interface due to the oxidation of the metallic elements in the bond-coat. The crack propagation equation has been established and the failure mechanism of the TBC due to the formation and growth of the TGO has also been discussed in detail. The lifetime of the TBCs which have experienced high temperature oxidation has been predicted and the theoretical results agreed well with the experimental data.

Improvement of high-temperature oxidation resistance and strength in alumina-forming austenitic stainless steels

A new alumina-forming austenitic stainless steel with greatly improved high-temperature oxidation resistance and strength was developed via alloying 3.0 wt.% Al in the Fe-25Ni-18Cr based alloy. Continuous, stable and exclusive alumina scale was formed in either dry air or air with 10% water vapor mixed environment at 800 °C. The long-term high-temperature oxidation performance is appreciably enhanced which is associated with the high density of the B2-NiAl precipitation phase maintaining the Al 2O 3 surface layer. Moreover, when tested at 750 °C in dry air environment, the new steel showed high yield and fracture tensile strength of 310–335 and 480–500 MPa, respectively.

Yttrium and oxygen distributions in MCrAlY coatings after deposition, annealing and oxidation

A novel technique was used to determine quantitatively the yttrium and oxygen distributions in MCrAlY coatings using electron microprobe X-ray analysis (EPMA). This method was used to assess the influence of annealing, pre-oxidation conditions, and long-term oxidation at high temperature on the Y and O distributions in a vacuum plasma sprayed Ni–23Co–17Cr–24Al –0.2Y (at.%) coating and in the oxide scales that developed on top of this coating. It was found that after deposition most of the Y in the coating was bonded to oxygen and preferentially enriched at the splat boundaries inside the coating. This effect became more pronounced after annealing and oxidation. Besides the internal oxidation of Y, also segregation of Y was observed near the coating surface. The amount of segregated Y near the coating surface increased with increasing oxidation temperature and time, and decreasing oxygen partial pressure. The MCrAlY coatings with a large amount of Y segregated near the surface during annealing and pre-oxidation formed oxide scales with a higher growth rate during long-term high temperature oxidation due to the presence of a larger fraction of Y oxide inclusions in the alumina scale.