H_2O和Y(O)对NiCoCrAl热障涂层高温氧化的影响

分别利用真空等离子沉积和超音速火焰喷涂技术制备含有Y和含Y氧化物的NiCoCrAl涂层,用差热分析和光学及电子显微镜研究两种涂层在Ar-16.7%O_2,Ar-3.3%H_2O和Ar-0.2%H_2-0.9%H_2O气氛中1100℃时的氧化动力学和断面微观结构,通过第一性原理计算对比在不同气氛中含Y氧化物对涂层氧化的影响机理。结果表明:对于NiCoCrAl+Y涂层,Y倾向于向界面扩散并在界面富集导致Al_2O_3膜生成更多有利于内氧化的孔洞,水蒸气更会对内氧化产生促进作用。而对于NiCoCrAl+Y(O)涂层,由于Y在涂层制备过程中被氧钉扎,导致NiCoCrAl+Y(O)涂层在上述气氛中生成了平直而均匀的Al_2O_3层,不同气氛对其氧化行为影响较小。上述研究进一步揭示NiCoCrAl涂层中活性元素Y的存在状态和氧化气氛中的水蒸气对氧化铝组织结构和生长速率有重要影响。     

Effect of oxygen partial pressure on the oxidation behaviour of an yttria dispersion strengthened NiCr-base alloy

An yttria dispersion strengthened NiCr-base alloy was studied with respect to isothermal oxidation behaviour at 1000 °C and 1050 °C in high- and low-pO₂ gases, i.e. Ar–O₂ and Ar(−H₂)–H₂O. The scale growth kinetics, morphology and composition were studied by thermogravimetry in combination with SEM/EDX and SNMS. Due to Y doping the surface scale is very protective and initially grows predominantly by inward oxygen diffusion. Local formation of mainly outwardly growing oxide nodules occurs after longer oxidation times and is related to metallic protrusions formed as a result of internal oxidation of the minor alloying addition aluminium. The differences in scale morphology in the various environments are related to the effect of the gas composition on scale grain size and on the relative amounts of inward scale growth. Possibly the pO₂ dependence of the Ti-solubility in the chromia scale and/or hydrogen doping of the oxide plays an additional role in the scale growth process.     

High-temperature behavior of oxide dispersion strengthening CoNiCrAlY

Abstract

To fabricate oxide dispersion strengthened bond coatings, commercial Co–30wt-%Ni–20Cr–8Al–0?4Y powder was milled with 2% additions of Al₂O₃, Y₂O₃ or Y₂O₃ + HfO₂. Low-pressure plasma sprayed, free-standing specimens were oxidised in air + 10%H2O at 1100 °C both isothermally (100 h) and in 500, 1?h cycles. Dry air cyclic testing conducted at both ORNL and FZJ showed remarkably similar results. In general, the water vapour addition caused more scale spallation. Two LPPS specimens without oxide additions were tested for comparison. The specimens with 2%Al₂O₃ addition exhibited the best behaviour as the powder already contained 0?4%Y. Additions of 2%Y₂O₃ and especially 1%Y₂O₃ + 1%HfO₂ resulted in over-doping as evidenced by high mass gains and the formation of Y- and Hf-rich pegs. Scanning transmission electron microscopy of the isothermal specimens showed no Hf and/or Y segregation to the alumina scale grain boundaries in the over-doped specimens.     

Experience with the use of the acoustic emission technique for monitoring scale failure in wet and dry environments

Abstract

A brief survey of the acoustic emission technique for monitoring scale cracking and failure on 2.25–24% Cr steels in wet and dry environments is given. A number of acoustic emission test rigs are described. Some of the more simple test rigs are used for testing small oxidation coupons during isothermal oxidation. More sophisticated rigs have been used for testing full size heat exchanger tubes during thermal cycling.

Most acoustic emission measurements in a wet environment come from testing at temperatures below 650°C. There are examples from Alloy 800 and thermal barrier coatings that were tested at higher temperatures, 900°C and 1100°C, respectively. Through the years acoustic emission tests have been performed in dry air, dry air+10%H₂O, dry air+0.5%SO₂, and Ar+5%H₂+50%H₂O. Consequently, a wide variety of exposure temperatures and atmospheres can be investigated using acoustic emission techniques.

Qualitative acoustic emission results can detect when scale cracking occurs at exposure temperature, where such cracks are produced by growth stress. Acoustic emission signals have been measured during sample cooling, where the signal arises from scale cracking that is caused by the thermal expansion mismatch stress. Measured results have clearly shown that scale cracking caused by both growth stress and thermal expansion mismatch stress are affected by water vapor in the exposure environment. Post-test metallographic investigations show that crack orientation and the oxide scale phases are also affected by the gas composition in the test rig. Additionally the sample mass gain and scale thickness is affected by water vapor content.

Finally, acoustic emission techniques are helpful for understanding the phenomena of breakaway oxidation and spallation/exfoliation.     

The influence of the moisture content of the atmosphere on alumina scale formation and growth during high temperature oxidation of PM2000

Abstract

Preliminary studies have been undertaken on cyclic and isothermal oxidation at 1,300°C of thin (125 μm) samples of commercial ODS alloy PM2000 for up to 350h in two different oxidising environments; dry and moist air. Scanning electron microscopy (SEM) and electron microprobe analysis (EPMA) have been used to study the influence of such environments on alumina scale formation and growth. Initial mass gain observations showed that the alumina scale, which formed on the samples oxidised in air+2.5vol% H₂O grew faster in the early stages of oxidation than in the case of dry air. However the SEM analysis revealed that the scale morphologies in both dry air and air+2.5vol% H₂O were similar. In both cases the scales consisted of equiaxed grains at the scale–gas interface with Ti-rich particles in the outermost part of the scale. The major factor for the total scale failure, the formation of non-protective iron oxide, is the depletion of Al levels to a critical value, below which no protective alumina scale can form; and this occurred slightly faster in moist air than in dry air     

Protective coatings on orthorhombic Ti2AlNb alloys

Abstract

The oxidation behaviour of an orthorhombic Ti–22Al–25Nb alloy, bare and with protective coatings, was investigated at 750°C in air under quasi-isothermal and thermal cycling conditions. As found by post-oxidation analysis, the uncoated substrate material was severely degraded by formation of spalling oxide scales and ingress of oxygen and nitrogen causing nitride precipitation, internal oxidation and interstitial embrittlement. Metallic Ti–51Al–12Cr coatings as well as nitride coatings based on Ti–Al–Cr–Y–N, either monolithically grown or with superlattice structure, provided an effective diffusion barrier against oxygen. The excellent oxidation resistance of the TiAlCr coatings was associated with the ternary Ti(Al,Cr)₂ Laves phase promoting the formation of a protective alumina scale. The different intermetallic phases formed in the interdiffusion zone caused neither cracking nor spallation of the protective coating. Both, monolithically grown TiAlCrYN and superlattice TiAlYN/CrN coatings, exhibited slow, but nearly linear oxidation kinetics at 750°C in air. In the subsurface region of the substrate a niobium rich phase and the α₂-phase formed. At the coating/substrate interface pores and a thin, fine-grained TiN layer were found.     

Vacuum plasma spray deposition of YSZ–NiO–Ni coatings at different Ar and H2 gas flow rates

The work presents results of the experimental investigation of vacuum sprayed yttria stabilized zirconia, nickel oxide, nickel (YSZ–NiO–Ni) ceramic composite coatings deposited on Al₂O₃ ceramic and stainless steel substrates produced at different Ar and H₂ gas flow rates. The Ar and H₂ gas flow was varied according to the factorial plan design. It is shown that for the used vacuum plasma spray YSZ and NiO powder mixture the produced coatings were composed of three phases mainly: cubic YSZ (c-YSZ), cubic NiO (c-NiO), and cubic Ni (c-Ni). The quantitative X-ray diffraction (XRD) analysis was used to evaluate each phase amount in the coatings. It was found that the vacuum spray technique enables formation of composite layers with a variable composition and that phase content in the coatings can be controlled choosing the Ar and H₂ gas flow rates. The electrical conductivity measurements revealed that a variation of the phase content in the YSZ–NiO–Ni composites is responsible for the existence of different electrical conduction mechanism and rapid change in the conductivity of coatings with the used powder content. The surface morphology and the cross-section analysis by scanning electron microscope (SEM) have shown porous structures of the deposited coatings.     

Influence of the oxidation environment on scale morphology and oxidation rate of Fe–22Cr

Abstract

The oxidation behaviour of a commercial Fe–Cr alloy with 22 wt% Cr was investigated at 1173K in Ar–9 H₂ with 1% H₂O (pO₂ = 9.8 × 10^?19), in air with 1% H₂O (pO₂ = 0.208), and in a combination of the two atmospheres. The oxide morphology was investigated with X-ray diffraction and scanning electron microscopy. The oxide layer consisted of MnCr₂O₄ on top of Cr₂O₃.

Small oxide whiskers were present at the surface after oxidation in Ar–9 H₂ with 1% H₂O but not after oxidation in air with 1% H₂O. For samples initially oxidised in air with 1% H₂O, the oxide/alloy interface was wrinkled and covered with a SiO₂ layer. SiO₂ particles had developed at a rather flat oxide/alloy interface for samples initially oxidised in Ar–9% H₂ with 1% H₂O. The results obtained can be explained assuming that oxide growth occurs by cation diffusion only in Ar–9 H₂ with 1% H₂O, whereas both cation and anion diffusion contribute to the growth in air/H₂O.     

Effect of water vapour on growth and adherence of chromia scales formed on Cr in high and low pO2-environments at 1000 and 1050°C

Abstract

The oxidation behaviour of pure Cr at 1000 and 1050°C was studied in Ar–O₂ and Ar–H₂–H₂O mixtures. It was found that in the low-pO₂ gases the oxide scales exhibited higher growth rates than in the high-pO₂ gases. The scales formed in the low-pO₂ gases showed substantially better adherence during cooling, than scales formed in the high-pO₂ gases. These differences in growth rate and adherence can be correlated with differences in size and location of the in-scale voids formed during the isothermal exposure. Exposures in Ar-O₂-H₂O mixtures revealed that the differences in scale growth rates as well as in scale void formation and growth are not primarily related to differences in the oxygen partial pressure of the atmosphere but to the presence of water vapour in the test gas. At sufficiently high H₂O/O₂-ratios, water vapour promotes oxide formation at the scale/metal interface thereby suppressing excessive growth of existing voids, and also as a consequence improved scale adherence. Whether the enhancement of inward scale growth is related to transport of H₂O- or H₂-molecules or due to OH^? ions, cannot be derived with certainty from the present results.     

The behaviour of commercial FeCrAlRE alloys in nitrogen-oxygen – water vapour bioxidant environments

Abstract

The corrosion behaviour of a range of commercial FeCrAlRE alloys (MA956, ODM751, PM2000, Kanthal AF, Kanthal APM and Aluchrom YHf) have been examined in nitrogen–oxygen-H₂O or N₂–H₂–H₂O bioxidant environments, at temperatures between 1100°C and 1350°C. The corrosion behaviour is governed by the competition between oxidation leading to protective alumina formation/ maintenance and nitrogen ingress leading to nitridation of the matrix alloy. Key issues addressed by four series of experiments, have included: the influence of a pre-formed protective alumina scale; the oxidant level required to form/reheal a protective oxide scale; the role of mechanical failure of the scale above the critical thickness for cracking/spallation in oxygen rich environments; chemical failure of the protective oxide scale leading to breakaway (non-protective) attack and in particular, the potential roles in such failure processes of nitridation concurrent with, and following defective oxide scale formation, and of oxidation following nitridation.

Detailed characterisation of the chemical composition and physical microstructure of the attack of the respective alloys was undertaken using a range of surface analytical techniques, including X-ray diffraction, optical and scanning electron microscopy and energy dispersive X-ray analysis.     

Microstructural changes upon annealing in ODS-strengthened ultrafine grained ferritic steel

In this study, the stability of grain size and oxide nanoparticles in the ODS steel upon annealing at high temperature (650–1350 °C) has been evaluated. The ODS Fe–Cr–W–Ti–Y₂O₃ steel has been manufactured by powder metallurgy, consolidated by hot isostatic pressing and processed by hydrostatic extrusion. Such a processing brings about ultrafine grain structure reinforced with oxide nanoparticles (few nm in diameter) and results in superior mechanical properties. The stability of nano-oxides has been analyzed by small angle X-ray scattering together with transmission electron microscopy. The results obtained revealed excellent thermal stability of ultrafine grained ODS ferritic steel, which was attributed to the resistance of oxides against coarsening.     

High-temperature degradation mechanism of aluminide coatings on titanium alloys under cyclic oxidation at 750°C

Aluminide coatings prepared on Ti-6Al-4V substrate were able to improve oxidation resistance of the alloy under cyclic oxidation at 750°C both in dry and moist air conditions due to aluminide’s ability to form a stable alumina oxide scale. However, degradation of the coating due to spallation, cracking, internal oxidation and formation of voids with increased cyclic oxidation reduced the lifespan of the coating and the underneath substrate. The main cause of coating degradation for hot-dip specimens is cracks that initiated and propagated perpendicular to the surface. For the plasma spray specimens, the cracks are parallel to the surface. Initiation of cracks in hot-dip coatings are more accredited to residual stresses due to cooling and presence of brittle intermetallic phases TiAl₂ and TiAl. For plasma spray coatings, initiation and propagation of cracks are attributed to presence of entrapped oxides, pores and grain boundaries of the deposited splats whose flattened edges are parallel to the surface of the coating. Presence of water vapor, too, acts as an oxygen carrier and thus promotes oxidation internally, inhibits growth of continuous protective alumina oxide scales and weakens the scale/alloy interfacial toughness. Water vapor therefore accelerates degradation by increasing spallation and cracking rate of the coating.     

Microstructure characterisation and tensile properties of 18Cr-4.5Al-0.3Zr- oxide dispersion strengthened steel

The 18Cr–4.5Al–0.3Zr–oxide dispersion strengthened (ODS) steel was fabricated by mechanical alloying (MA) and spark plasma sintering (SPS) technique. A microstructural characterisation was performed on an 18Cr–4.5Al–0.3Zr–ODS steel using high angle annual dark field (HAADF) and synchrotron small angle X-ray scattering (SAXS). HAADF and SAXS results showed that high-density nanoscale oxides are formed in 18Cr–4.5Al–0.3Zr–ODS steel. The oxides in the specimen can be roughly divided into two categories according to their compositions: (1) core/shell structure oxides with Al–O oxide cores and Y shells; (2) nm-scale trigonal-phase Y₄Zr₃O₁₂ oxides. In addition, tensile testing results revealed that the specimen exhibited better tensile strength and ductility as compared with another commercial ODS steels with similar composition.     

Characterization of alumina interfaces in TBC systems

Interfacial segregants in thermally grown α-Al₂O₃ scales formed during high temperature exposure of thermal barrier coating systems reflect the oxygen-active dopants present in the bond coating and substrate, such as Y and Hf. These dopants diffuse outward and segregate to the substrate-alumina interface and the alumina grain boundaries. Related studies suggest that these segregants affect the growth and mechanical properties of the alumina-scale; however, the characterization of segregation in alumina formed on coated superalloy systems has been limited. Segregation examples evaluated using analytical transmission electron microscopy are given from traditional Pt-modified aluminide coatings and newer Pt diffusion coatings. Model systems are used to illustrate that grain boundary segregants on the columnar alumina boundaries are not because of the reverse diffusion of cations from the Y₂O₃-stabilized ZrO₂ top coating, and that interstitial elements in the substrate likely affect the outward flux of cation dopants. The dynamic nature of this segregation and oxygen-potential gradient-driven diffusion is discussed in light of observations of substrate dopant and interstitial contents affecting coating performance.     

High temperature isothermal oxidation behaviour of an oxide dispersion strengthened derivative of IN625

Abstract

Gas atomised IN625 powder was mechanically alloyed with <1·0 Wt.% nano-yttria and consolidated by spark plasma sintering (SPS) to produce an oxide dispersion strengthened (ODS) alloy. The isothermal oxidation rate constant of the ODS alloy, and wrought IN625, was determined by thermogravimetric analysis. This was performed at 900 °C in static laboratory air for exposure times of up to 1000 h. It was found that the ODS alloy oxidised ~40x slower than wrought IN625, which is attributed to the reactive element effect. It is further proposed that the improvement in oxidation resistance of the ODS alloy, and the superior morphology of the oxide scale formed on the ODS alloy, may be related to the presence of Nb carbide, rather than δ-phase, in the ODS alloy.     

Transient oxidation of alumina forming Ti–Al–Ag-based alloys and coatings studied by SEM,AFM, XPS and LRS

Abstract

γ-TiAl based intermetallics possess poor oxidation properties at temperatures above approximately 700°C. Previous studies showed that protective alumina scale formation on γ-TiAl can be obtained by small additions (around 2 at.%) of Ag. Recently, this type of materials has therefore been proposed as oxidation resistant coatings for high strength TiAl alloys. In the present study, a number of cast Ti–Al–Ag alloys and magnetron sputtered Ti–Al–Ag coatings were investigated in relation to transient oxide formation in air at 800°C. After various oxidation times the oxide composition, microstructure and morphology were studied by combining a number of analysis techniques, such as SEM, ESCA, AFM and LIOS-RS. The γ-TiAl–Ag alloys and coatings appear to form an α-Al₂O₃ oxide scale from the beginning of the oxidation process, in spite of the relatively low oxidation temperature of 800°C. The formation of metastable alumina oxides seems to be related to the presence of Ag-rich precipitates in the alloy matrix.     

High temperature carbon corrosion in solid oxide fuel cells

Abstract

Operating conditions in a current design for a planar geometry oxide fuel cell plant are briefly reviewed and the danger of encountering “metal dusting” conditions identified. Laboratory tests were designed to produce accelerated metal dusting by exposing heat resisting alloys to a CO–26 H₂–6 H₂O (vol. pct) gas mixture at 680°C under thermal cycling conditions. The hot gas composition corresponded to a_c = 2.9 and an oxygen potential high enough to oxidise chromium and aluminium, but not iron or nickel. The alloys tested included ferritic and austenitic chromia formers and two ferritic alumina formers, all with electropolished surfaces. Thermal cycling of the chromia formers led to oxide scale damage followed by internal carburisation, metal dusting and coking. This failure occurred very rapidly on most austenitic materials (Alloy 800, Inconel 601, 690, 693, Alloy 602CA), but did not commence until after approximately 50 one-hour cycles for the ferritic steel Fe–27Cr–0.001Y (wt %). The alloy with the best performance was Inconel 625, which was still protected by its Cr₂O₃ scale after 500 cycles. The alumina forming alloys showed superior performance, with no damage apparent after 1200 cycles. Additional tests using ground metal surfaces showed that they were more resistant to dusting in the case of chromia formers, but more susceptible in the case of alumina formers, metal dusting.     

Vacuum plasma spray deposition of YSZ-NiO-Ni coatings at different Ar and H2 gas flow rates

The work presents results of the experimental investigation of vacuum sprayed yttria stabilized zirconia, nickel oxide, nickel (YSZ-NiO-Ni) ceramic composite coatings deposited on Al₂O₃ ceramic and stainless steel substrates produced at different Ar and H₂ gas flow rates. The Ar and H₂ gas flow was varied according to the factorial plan design. It is shown that for the used vacuum plasma spray YSZ and NiO powder mixture the produced coatings were composed of three phases mainly: cubic YSZ (c-YSZ), cubic NiO (c-NiO), and cubic Ni (c-Ni). The quantitative X-ray diffraction (XRD) analysis was used to evaluate each phase amount in the coatings. It was found that the vacuum spray technique enables formation of composite layers with a variable composition and that phase content in the coatings can be controlled choosing the Ar and H₂ gas flow rates. The electrical conductivity measurements revealed that a variation of the phase content in the YSZ-NiO-Ni composites is responsible for the existence of different electrical conduction mechanism and rapid change in the conductivity of coatings with the used powder content. The surface morphology and the cross-section analysis by scanning electron microscope (SEM) have shown porous structures of the deposited coatings.     

In situ formation of Al2O3–ZrO2–Y2O3 composite ceramic coating by plasma electrolytic oxidation on ZAlSi12Cu3Ni2 alloy

In situ formation of Al₂O₃–ZrO₂–Y₂O₃ composite ceramic coating on ZAlSi12Cu3Ni2 aluminum alloy was successfully prepared by plasma electrolytic oxidation (PEO) technology in a zirconate electrolytic solution. The morphologies, phase components, the thermal diffusion coefficient and thermal conductivity of the composite coatings were investigated by scanning electron microscope, energy dispersive spectroscopy, X-ray diffraction and laser pulse tester. The results indicate that the composite coatings are relatively dense and uniform in thickness, and predominantly composed of Al₂O₃, c-Y0.15Zr0·85O1·93Vo0·07(Vo-oxygen vacancies), monoclinic ZrO₂ (m-ZrO₂) and littleY₂O₃. The composite coatings exhibit a gradient distribution in phase component from the surface to the inner part. With the increase of the applied voltage, the micropores, the discharges products, thickness and the ZrO₂ content of the composite coatings increase. With the oxidation time increasing, the surface of coating generates oxide ceramic particles around the holes and accumulates repeatedly. The content of zirconium is the higher on the surface and interface. The content of Al is less and it shows that the ceramic coating contains mainly the zirconium oxide. This is attributed to the presence of micropores and microcracks, plus the extremely fine grain size and the presence of an amorphous phase. When considered in conjunction with the possible thickness range, it’s clear that this PEO coatings offer considerable promise as thermal barriers.     

Transparent amorphous In-Ga-Zn-O thin film as function of various gas flows for TFT applications

The electrical and optical properties of amorphous indium gallium zinc oxide (a-IGZO) films, which can be used as a channel layer, deposited by radio frequency (rf) magnetron sputtering system at room temperature (RT), were investigated as function of various gas flows. The optical transmittance of films deposited under Ar, O₂ / Ar + O₂ and O₂ / Ar-4% H₂ + O₂ atmospheres in the visible wavelength was consistently above 90% at a wavelength of 550 nm at all gas flows, although the film deposited under Ar-4% H₂ atmosphere exhibited a transmittance of below 50%. The carrier concentration and mobility of the a-IGZO films fabricated under Ar and Ar-4% H₂ were observed slight decrease as a function of the flow, respectively. The thin film transistors (TFTs) with an a-IGZO channel deposited under Ar and Ar-4% H₂ atmosphere exhibited the following good characteristics: V_th of 0.34 V, µ_FE of 3.6 cm² V^− 1 s^− 1, on/off ratio of 10⁶, and S value of 0.04 V decade^− 1.