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.     

Thermal Oxidation of Silicon Carbide Substrates

Thermal oxidation was used to remove the subsurface damage of silicon carbide (SiC) surfaces. The anisotrow of oxidation and the composition of oxide layers on Si and C faces were analyzed. Regular pits were observed on the surface after the removal of the oxide layers, which were detrimental to the growth of high quality epitaxial layers. The thickness and composition of the oxide layers were characterized by Rutherford backscat-tering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS), respectively. Epitaxial growth was performed in a metal organic chemical vapor deposition (MOCVD) system. The substrate surface morphol-ogy after removing the oxide layer and gallium nitride (GaN) epilayer surface were observed by atomic force microscopy (AFM). The results showed that the GaN epilayer grown on the oxidized substrates was superior to that on the unoxidized substrates.     

Microstructural evolution and degradation modes in cyclic and isothermal oxidation of an EB-PVD thermal barrier coating

Abstract

Thermal barrier coatings constituted of a partially stabilised zirconia layer (～100 μm thick) deposited by EB-PVD on top of a platinum-modified aluminide bondcoat to protect a single crystal superalloy substrate, have been studied in cyclic and isothermal oxidation at 1,100°C in air. Close examination by high resolution SEM (with field emission gun) of the oxide scale and of the metallic surfaces after various durations, up to 600 hours at 1,100°C, show several morphological features which may affect the integrity of the systems: partial decohesion regions with large grain areas at the inner surface of the scale, subsequently replaced by a cellular morphology, cavities within the oxide scale as well as at the zirconia–alumina interface. The nature and distribution of these features depend on the type of test (isothermal or cyclic oxidation) and on the systems studied (standard superalloy or modified to lower the sulphur content). Scenarios are proposed to explain the observations reported.     

Application of reaction synthesis principles to thermal spray coatings

Reaction synthesis principles have been extended to plasma spraying to obtain coatings consisting of mixed oxide phases and iron aluminides. Elemental powders of iron and aluminium were fed through a d.c. plasma torch to deposit intermetallic coatings on carbon steel substrates. Carbon steel substrates were also pre-heated with a plasma flame to create an iron oxide surface on the substrate such that an exothermic thermite reaction takes place when molten splats of aluminium impinge the pre-heated substrate at sub- or supersonic velocities. A thermite reaction between iron oxide and aluminium allowed the formation of alumina, FeAl2O4, iron, and iron aluminide phases. The presence of FeAl2O4 and Al2O3 increased the surface hardnesses of the coating, and the hardnesses of the coatings are significantly higher than the hardnesses of steel substrate, and aluminium particles. X-ray analysis of the coatings, microstructural observations, and microhardness measurements suggest that plasma spraying conditions can be tailored to obtain coatings with high hardness values with in situ synthesized reinforcements (spinel and alumina) or iron aluminide phases. Aluminium-rich phases were observed in the as-deposited coatings when a mixture of aluminium and iron or aluminium and nickel were fed through the plasma gun in ratios equivalent to Fe3Al, FeAl, Ni3Al, and NiAl. In some cases, annealing allowed the formation of iron-rich or nickel-rich aluminide phases. High solidification rates of molten splats allowed very limited diffusional reactions between the splats of aluminium and iron, or aluminium and nickel because the available diffusional time for exothermic interfacial reactions is limited to a fraction of a second at best. Oxidation of part of the aluminium led to the formation of alumina in the as-deposited coatings, and therefore, a vacuum plasma spraying technique is desirable to obtain intermetallic phases. The results suggest that reactive spraying will allow deposition of coatings by utilizing the heats of reaction between the constituents, and reactive spraying will broaden the engineering applications of reaction synthesis techniques.     

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.     

Investigation of the microstructure and oxidation behavior of Cr-modified aluminide coatings on γ-TiAL alloys

The microstructure and oxidation behavior of aluminide coatings are investigated. The layers are examined by the optical microscopy, scanning electron microscopy (SEM) equipped with EDS, and the X-ray diffraction method. The isothermal oxidation behaviors of samples are investigated at 950°C for 200 h. The results show that TiAl₃ is formed on the substrate. In addition, the aluminide coating improves the oxidation resistance of γ-TiAl alloys by forming a protective alumina scale. Moreover, during oxidation treatment, the interdiffusion of the TiAl₃ layer with γ-TiAl substrate results in the depletion of aluminum in the TiAl₃ layer and the growth of the TiAl₂ layer. After the oxidation treatment, the coating layer preserves a microstructure with phases including TiAl₃, TiAl₂, and Al₂O₃.     

Coatings of Aluminide Intermetallic Compounds on Steel Utilizing a Hybrid Technique of Spraying and IR-Laser Fusion

Titanium aluminide coatings were produced using a hybrid technique of arc-spraying followed by IR-laser fusion in an argon atmosphere. A titanium coating free of oxides was deposited onto a low-alloy steel by DC-arc spraying in argon. Optimal laser irradiation conditions and the amount of preplaced aluminum powder on the sprayed titanium were determined to obtain a composite coating of TiAl₃+Al of 150 urn thickness. Metallurgical and mechanical properties were examined using acoustic emission. The oxidation resistance of the coating was excellent up to 1173 K because of a protective alumina layer. Growth of the TiAl₃interlayer by diffusion of aluminum into titanium improved the corrosion resistance. The intermetallic coadng showed microcracking at ambient temperature, but possessed capability for filling and healing of cracks with alumina and titanium nitride during high-temperature exposures. However, at temperatures higher than 1200 K, the oxidation performance decreased by diffusion of iron into the coating     

Atomic layer deposition of aluminum oxide films for carbon nanotube network transistor passivation

Ultra-thin (2-5 nm thick) aluminum oxide layers were grown on non-functionalized individual single walled carbon nanotubes (SWCNT) and their bundles by atomic layer deposition (ALD) technique in order to investigate the mechanism of the coating process. Transmission electron microscopy (TEM) was used to examine the uniformity and conformality of the coatings grown at different temperatures (80 degrees C or 220 degrees C) and with different precursors for oxidation (water and ozone). We found that bundles of SWCNTs were coated continuously, but at the same time, bare individual nanotubes remained uncoated. The successful coating of bundles was explained by the formation of interstitial pores between the individual SWCNTs constituting the bundle, where the precursor molecules can adhere, initiating the layer growth. Thicker alumina layers (20-35 nm thick) were used for the coating of bottom-gated SWCNT-network based field effect transistors (FETs). ALD layers, grown at different conditions, were found to influence the performance of the SWCNT-network FETs: low temperature ALD layers caused the ambipolarity of the channel and pronounced n-type conduction, whereas high temperature ALD processes resulted in hysteresis suppression in the transfer characteristics of the SWCNT transistors and preserved p-type conduction. Fixed charges in the ALD layer have been considered as the main factor influencing the conduction change of the SWCNT network based transistors.     

Temperature dependence of the oxide growth on aluminized 9-12%Cr ferritic-martensitic steels exposed to water vapour oxidation

High temperature steam oxidation resistance of aluminide coatings obtained on 9-12% Cr ferritic-martensitic steels, which have been developed by chemical vapour deposition in fluidized bed reactor (CVD-FBR) using a bed modified with Zr particles, is presented here. The resulting coatings composed of (Fe,Cr)₂Al₅ intermetallic phase provide a high temperature steam oxidation resistance, which depends on the oxidation temperature. At 650 °C, after 1000 h of exposure, the alumina formed on the surface acts as a protective barrier. However, when the oxidation temperature increases up to 800 °C, the alumina scale fails before 1000 h of exposure giving rise to the formation of the Cr₂O₃ and (Fe,Mn)₃O₄, due to the high atomic diffusion rate at this temperature.     

Under-surface observation of thin-film alumina on NiAl(100) with scanning tunneling microscopy

Scanning tunneling microscopy (STM) was used to investigate the oxide structures underlying the surface of alumina thin-film grown on NiAl(100). At a bias voltage (on the sample) below 2.0 V, STM topography images of the alumina layer beneath the surface were obtained. A probe with depth of 2-8 Å was readily attained. The under-surface observation shows that the film consists of stacked layers of alumina whereas the layered alumina unnecessarily comprises entire θ-Al₂O₃ unit cells. The lattice orientation of the upper alumina layer differs from that of the lower one by 90° — the newly grown oxide structurally matching the horizontal oxide rather than the lower oxide. The results indicate a growth process competing with the typical mode of epitaxial growth for the growth of alumina film.     

Oxidation behaviour in air of thin alloy 601 fibres

Abstract

The oxidation behaviour in air of 12 μm diameter continuous alloy 601 fibres has been studied using thermo-gravimetric analysis (TGA) for kinetics identification and transmission electron microscopy (TEM) for determination of the nature of the oxide layers. The TGA allows two stages in the formation of the oxide layer to be distinguished: the first stage corresponds to the growth of a continuous layer of NiO above a discontinuous sub-layer of Cr₂O₃ whereas the second stage is attributed to the parabolic growth of the Cr₂O₃ sub-layer, from the time it becomes continuous. A third stage can be observed for high oxidation temperatures. The TEM observations of oxide layers formed after 30 min at 650, 750 and 900°C confirm these results. One common characteristic of these 3 oxidation conditions is the appearance of large cavities under the oxide layer. These cavities seem to be the consequence of the oxidation mechanism of Cr and of the particular morphology of the material (i.e. small diameter cylinders).     

Film Structure of Epitaxial Graphene Oxide on SiC: Insight on the Relationship Between Interlayer Spacing,Water Content,and Intralayer Structure

Chemical oxidation of multilayer graphene grown on silicon carbide yields films exhibiting reproducible characteristics, lateral uniformity, smoothness over large areas, and manageable chemical complexity, thereby opening opportunities to accelerate both fundamental understanding and technological applications of this form of graphene oxide films. Here, we investigate the vertical inter‐layer structure of these ultra‐thin oxide films. X‐ray diffraction, atomic force microscopy, and IR experiments show that the multilayer films exhibit excellent inter‐layer registry, little amount (<10%) of intercalated water, and unexpectedly large interlayer separations of about 9.35 Å. Density functional theory calculations show that the apparent contradiction of “little water but large interlayer spacing in the graphene oxide films” can be explained by considering a multilayer film formed by carbon layers presenting, at the nanoscale, a non‐homogenous oxidation, where non‐oxidized and highly oxidized nano‐domains coexist and where a few water molecules trapped between oxidized regions of the stacked layers are sufficient to account for the observed large inter‐layer separations. This work sheds light on both the vertical and intra‐layer structure of graphene oxide films grown on silicon carbide, and more in general, it provides novel insight on the relationship between inter‐layer spacing, water content, and structure of graphene/graphite oxide materials.     

Oxidation behaviour of an intermetallic Ti–Al–Cr–Zr bond coat on a γ–TiAl based TNB alloy with 7YSZ thermal barrier coating

Abstract

Intermetallic titanium aluminide alloys are attractive light-weight materials for high temperature applications in automotive and aero engines. The development of γ-TiAl alloys over the past decades has led to their successful commercial application as low pressure turbine blades. The operating temperatures of γ-TiAl based alloys are limited by deterioration in strength and creep resistance at elevated temperatures as well as poor oxidation behaviour above 800 °C. Since improvement in oxidation behaviour of γ-TiAl based alloys without impairing their mechanical properties represents a major challenge, intermetallic protective coatings have aroused increasing interest in the last years.

In this work, a 10 μm thick intermetallic Ti–46Al–36Cr–4Zr (in at.-%) coating was applied on a TNB alloy using magnetron sputtering. This layer provided excellent oxidation protection up to 1000 °C. Microstructural changes in this coating during the high temperature exposure were extensively investigated using scanning and transmission electron microscopy. The coating developed a three-phase microstructure consisting of the hexagonal Laves-phase Ti(Cr,Al)₂, the tetragonal Cr₂Al phase and the cubic τ-TiAl₃ phase. After long-term exposure the three-phase microstructure changed to a two-phase microstructure of the hexagonal α₂-Ti₃Al phase and an orthorhombic body-centred phase, whose crystal structure has not yet been definitely identified. On the coating, a thin protective alumina scale formed. Applying this intermetallic layer as bond coat, thermal barrier coatings (TBCs) of yttria partially stabilized zirconia were deposited on γ-TiAl based TNB samples using electron-beam physical vapour deposition. The results of cyclic oxidation testing (1 h at elevated temperature, 10 min. cooling at ambient temperature) revealed a TBC lifetime of more than 1000 h of cyclic exposure to air at 1000 °C. The ceramic topcoat exhibited an excellent adhesion to the thermally grown alumina scale which contained fine ZrO₂ precipitates.     

Chromium oxide nanolayers on gallium arsenide

Ultrathin chromium oxide layers (nanostructures) are grown on (100) and (110) GaAs substrates by molecular layering (atomic layer deposition). The effect of process conditions on the layer composition and growth mechanism is analyzed. The dielectric properties of the layers and the quality of the dielectric-semiconductor interface are evaluated.     

The influence of reactive element additions to β-NiAlCr alloys on the morphology of thermally grown oxides

Abstract

The effect of the reactive elements (REs), Y and Zr, on oxidation of β-NiCrAl alloy at 1373 K in a gas mixture of argon with 20 vol.% oxygen at atmospheric pressure was evaluated using X-ray diffractometry, scanning electron microscopy, electron probe X-ray microanalysis and secondary ion mass spectroscopy. The oxide surfaces and interface morphologies, compositions and growth kinetics were studied for alloys with 0.32 at.% Zr and 0.24 at.% Y additions and for an undoped alloy. The oxide layer produced on the three different alloys contains mainly α-alumina and some intermediate alumina modification, Cr₂O₃ and RE-oxides. A needle-like morphology was seen on top of the oxide layer for the undoped and Zr alloy. Needle formation on the Y alloy was suppressed by the formation of a thin Y₂O₃ layer during the initial stage of oxidation. Needles were maintained to long oxidation times for the undoped alloy, but disappeared on the doped alloys indicating that some cation diffusion is possible when REs are present. Fewer intermediate alumina modifications are seen for the oxide layers on the RE alloys showing that the REs promote the formation of the α-alumina phase. Oxide layer growth occurs in two stages for all alloys. Initially, oxide growth is rapid with outward diffusion of aluminium. The second stage of oxidation is slow and is initiated by the formation of a closed α-alumina layer limiting further oxidation to inward oxygen diffusion. This stage is characterised by parabolic growth kinetics associated with a constant aluminium interface concentration. The oxide layer is thinnest for the Y alloy due the fine Y₂O₃ layer acting as a diffusion barrier. The oxide/alloy interface for the undoped alloy is flat and shows many voids, whereas voids are not seen for the RE alloys. This is due to the promotion of a closed α-alumina layer giving predominantly inward growth early in the oxidation process. Oxide pegs of the RE are also seen growing into the alloys. The lack of voids and the oxide pegs are advantageous for oxide layer adhesion to the doped alloys.     

A surface modified ODS superalloy by thermal oxidation for potential implant applications

In the present work attention is paid on the composition, structure and protective properties of alumina layer produced by high temperature oxidation on MA 956 superalloy (Fe-20Cr-4.5Al-0.5Ti-0.5Y₂O₃ (wt %)). The combination of good mechanical properties of this material and the excellent biocompatibility, the good wear and corrosion behavior of an outer -alumina layer, limiting the release of ionic species and wear debris from the bulk material into the body-fluid environment, can make this material a candidate alloy for medical applications. Isothermal oxidation at 1100 °C in air of the alloy has led to the formation of a fine-grained, compact and adherent -alumina scale. Oxide nodules rich in Ti, Y, Cr, and Fe were found on the top of the surface. In vitro electrochemical corrosion experiments showed good protective properties of the oxide scale. Moreover, no spallation of the alumina layer was observed. This feature is significant considering that the alumina layer has to withstand very high compressive stresses resulting from both growth and thermal stresses incorporated during cooling. © 2001 Kluwer Academic Publishers     

Investigation of sub-nm ALD aluminum oxide films by plasma assisted etch-through

A new technique, called “plasma defect etching” (PDE), is proposed for studying the continuity of ultra-thin layers. The PDE technique utilizes the extremely high selectivity in the deep reactive ion etching (DRIE) process, thus achieving visualization of the defects in the layer, because etching of substrate happens only through voids and microholes of the layer. The etch profile generally reproduces the non-continuous structure of the layer. This PDE technique was applied for the investigation of thin, sub-nm aluminum oxide films grown on silicon wafers by atomic layer deposition (ALD) technique. Silicon substrate was etched by SF₆ at cryogenic temperatures in an inductively coupled plasma (ICP) reactor, exploiting the extremely high ratio of silicon/aluminum oxide etch rates in fluorine plasmas. The surface morphology was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The PDE method shows that in the case of water as an oxidation precursor, separate islands of aluminum oxide form during the five first ALD cycles. On the other hand, the use of ozone precursor helps to oxidize silicon surface and facilitates growth of a uniform layer.     

Breakdown of protective scales during the oxidation of thin foils of Fe–20Cr–5Al alloys at high temperatures

Abstract

The oxidation behaviour of alumina-forming Fe–20Cr–5Al and similar alloys containing small concentrations of lanthanum or lanthanum plus molybdenum in air at 1,150°C has been studied, with emphasis on thin (0.05 mm) specimens, where the aluminium reservoir in the substrate is soon depleted to a very low value. Oxidation of these alloys involves establishment and growth of protective alumina scales. However, once the residual aluminium concentration in the alloy drops below a critical level, a layer of chromia is able to develop and grow at the alumina–alloy substrate interface. Eventually, breakaway oxidation occurs and iron-rich oxides form and engulf the specimen.

This paper presents some kinetics of oxidation of these alloys and discusses the growth and breakdown of the protective scales, drawing on the results of detailed examinations of the oxidized specimens using analytical scanning and transmission electron microscopy in cross section. It has been shown that lanthanum increases the time to the onset of breakaway oxidation, probably due to beneficial effects on the mechanical integrity of the scale. Molybdenum additions have been found to decrease significantly the rate at which breakaway oxides are able to penetrate and engulf the alloy substrate. Such additions stabilize the ferrite phase in the substrate at the alloy–scale interface, thereby maintaining a high rate of diffusion of chromium to the interface and facilitating establishment of a healing and partially protective chromium-rich oxide layer at the base of the breakaway oxide scale. In the absence of such additions, depletions of chromium in the substrate adjacent to the alloy/scale interface, arising from oxidation of chromium, enable the austenite phase to be stabilized. The relatively low rate of diffusion of chromium in this phase allows chromium-rich oxide to form as internal precipitates in the alloy rather than as a continuous, healing layer; hence, the breakaway oxide scale is able to penetrate and consume the substrate more rapidly than in the presence of molybdenum additions.     

In situ surface oxidation of Ti-44Al-11Nb alloy at room temperature

The in situ surface oxidation of polycrystalline Ti-44Al-11Nb (compositions are in atomic %) alloy was studied at room temperature in an ultrahigh vacuum (66.6 to 80.0 nPa). The native oxide of Ti-44Al-11Nb was removed by sputtering the surface using 2.8 kV argon ions and then high-purity oxygen was carefully dosed onto the aluminide surface. After each oxygen dosing the surface species, metallic states and oxidized states, were detected by X-ray photoelectron spectroscopy (XPS). After analyzing the binding energy shifts and the concentration of oxidized species, aluminum oxidized first, then titanium followed by niobium. A reaction sequence model, incorporating the rate-determining step of a dissociative adsorption of oxygen, shows to be consistent with the initial oxidation rate of the Ti-44Al-11Nb intermetallic.     

Oxidation Resistance of the Aluminide Coating Formed on Carbon Steels

Low and medium carbon steels were aluminized by the pack aluminizing technique using halideactivated pure-Al and Fe-Al packs. The effect of mixture composition, aluminizing temperatureand time and C content of the steel substrate on the structure and thickness of the aluminidelayer, and on the oxidation resistance was investigated. The optimum oxidation resistance canbe achieved with a low carbon steel substrate when the intermetallic phases Fe3Al and FeAlform the surface of the aluminide layer. In this case, the Al concentration at the surface of thealuminide coating is at least ≥15 wt pct. Formation of high Al concentration phases (FeAl3 andFe2Al5) during aluminizing should be avoided as they tend to embrittle the aluminide layer andreduce its oxidation resistance.