Theoretical consideration on composite oxide scales and coatings

The present paper discussed some fundamental aspects on composite oxide scales and coatings for protection of alloys from high temperature oxidation, the related thermodynamic conditions, special mechanical characteristics and a sealing mechanism. It was proposed that the oxide scales and coatings with a composite structure should possess superior mechanical properties than that with a single phase oxide. It also showed that the Al2O3 scales or coatings doped with Y2O3 and ZrO2 (or YSZ)-Al2O3 composite coatings possessed superior properties at high temperatures. In such composite oxide scales and coatings, the fracture resistance of the scales was increased by the toughening effect, the thermal stress was decreased owing to the increase of thermal-expansion coefficients, and Al2O3 phase could seal the alloy substrate well. In addition, the kinetic equation of thermal growth oxide on alloy covered with composite oxide coatings was derived.     

Effects of Ce(III) on Rate of Formation and Phase Compositions of Ceramic Coatings Formed on Surface of ZAlSi12Cu2Mg1 by Microarc Oxidation

Components of electrolyte has an important effect on the process of microarc oxidation on the surface of aluminum alloys. In this work, ceramic coatings were prepared on ZAlSi12Cu2Mg1 alloy by microarc oxidation through varying content of rare earth Ce(III) from 0 g·L⁻¹ to 0.125 g·L⁻¹ in NaOH-Na₂SiO₃ electrolyte. The influences of the content of Ce (III) on the rate of formation and phase compositions of ceramic coatings were investigated. The electric vortex thickness indicater was employed to measure the thickness of the ceramic coatings. The phase compositions of ceramic coatings were analyzed by XRD. It was found that the rate of formation firstly increased to some extent and then decreased with the content of Ce(III) increasing. The rate of formation reached the maximum while the content of Ce(III) was 0.025 g·L⁻¹. The maximum rate is c. a. 8.3 μm · min⁻¹. In addition, the phase compositions of ceramic coatings vary with the rate of formation. Ceramic coating was mainly consisted of lots of amorphousness and Al while it formed at the maximum rate. Meanwhile, it was mainly composed of α-Al₂O₃, γ-Al₂O₃, Al₂SiO₅, Al and amorphousness while the coatings formed at the others.     

Microstructural characterization

Rapidly solidified microstructures of Fe-Cr-W-C quaternary alloy deposited on low-carbon steel by laser cladding were investigated. The clad-coating alloy, a powder mixture of Fe, Cr, W, and C with a weight ratio of 10:5:1:1, was processed with a high-power continuous wave CO₂ laser. The developed clad coatings possessed fine microstructures, uniform distributions of al- loying elements, and high microhardness. Analytical electron microscopy and energy dispersive X-ray spectroscopy were used to characterize the crystal structures and microchemistries of the various phases in the clad coatings. The laser processed microstructure comprised fine primary dendrites of a face-centered cubic (fcc) austenitic y phase and interdendritic eutectic consisting of a network of pseudohexagonal M₇C₃ carbides rich in Cr in an fcc austenitic γ phase. The interlamellar spacing in the eutectic matrix was about 20 nm. The relatively high microhardness, about 900 kg_f/mm², of such fine microstructures is attributed to the formation of complex ter- nary carbides uniformly distributed in the eutectic matrix. In situ transmission electron micros- copy (TEM) of thermally treated clad coatings revealed that transformation of the primary γ phase to body-centered cubic (bcc) ferrite (α phase) commenced after heating at 843 K for about 7 minutes. The transformation initiated at the interface of the primary dendrites and the eutectic and propagated gradually into the primary phase. Phase change of the interdendritic γ austenite to a bcc α ferrite occurred after about 30 minutes of hold period at 843 K. Transformation of the M₇C₃ carbides did not occur even after heating at 843 K for about 3.2 hours. The growth of a thin M₂O₃ (M = Fe, Cr) oxide scale was detected after heating at 843 K for approximately 24 minutes. After cooling gradually to room temperature, the softened (723 kgy/mm²) micro- structure consisted of primary dendrites with a bcc α ferrite crystal structure and interdendritic ternary eutectic of untransformed M₇C₃ carbides in α ferrite.     

Oxidation behavior of a fine-grained rapidly solidified 18-8 stainless steel

The cyclic oxidation behavior of a fine-grained, rapidly solidified 303 stainless steel was determined at 900 °C in pure oxygen. The rapidly solidified alloy exhibited superior resistance to oxidation compared with that of a wrought 304 stainless steel; its oxidation resistance was as good as that of a wrought 310 stainless steel, even though the latter alloy contained more Cr and Ni. The matrix of the rapidly solidified steel contained a uniform dispersion of fine MnS precipitates (0.2 to 0.5 μm), which were effective in inhibiting grain growth at elevated temperatures. The enhanced resistance to oxidation of the rapidly solidified alloy is attributed to two factors: (1) the formation and growth of protective Cr₂O₃ and SiO₂ scales were promoted by the fine alloy grain size (5 to 8 =gmm) and by the presence of the MnS dispersion, and (2) the adherence of the scale was increased by the formation of intrusions of SiO₂ from the external scale into the alloy, which formed around MnS precipitates and along closely-spaced alloy grain boundaries, and which acted to key the scale mechanically to the alloy.     

Effect of small concentrations of zirconium on high temperature oxidation behaviour of Fe-15 Cr-4 Al

Effect of 1% Zr on oxidation behaviour of Fe-15 Cr-4 Al alloy under isothermal conditions in air, O₂ and O₂-10% H₂O environments in the temperature range 1000–1150°C was investigated. The effect of zirconium concentration was studied at 1 200°C in air. Oxidation rate increases with increase in zirconium concentration. Parabolic rate of oxidation was observed. Limited study on cyclic oxidation was carried out at 1150°C in air. The cycle consisted of one hour holding at isothermal temperature followed by half an hour air cooling. The oxidised samples were examined by X-Ray diffractometry, SEM, EDAX. Extensive spalling was observed in the base alloy, Fe-15 Cr-4 Al in all environments. Zirconium additions eliminated the spalling of the scale. The EDAX analysis of a spalled region shows the presence of iron and chromium while the unspalled region is aluminium rich. A common structural feature, localised formation of granules/nodules was observed in the scale of zirconium containing alloys in all the environments. The number of granules increased with increase in zirconium concentration and was observed to be a maximum in 1% Zr and also increases with increasing temperature. The observations reveal that 1% Zr alloy shows lower oxidation rate in O₂-H₂O environment under isothermal conditions. X-Ray diffraction analysis shows the additional presence of Fe₂O₃ and Cr₂O₃ in α-Al₂O₃ scale which have not been detected in the α-Al₂O₃ scale formed in other environments, air and O₂. 0.2% Zr is most effective in increasing oxidation resistance of Fe-15 Cr-4 Al alloy both under isothermal and cyclic oxidation conditions.     

Structural and phase characteristics and physicomechanical properties of detonation coatings in the system aluminum oxide-titanium oxide

Conclusions Powdered A1₂O₃—TiO₂ of the highest quality, characterized by a high degree of purity and homogeneity, are obtained by the method of crystallizing melts in a cold crucible. Detonation coatings from such powders have a uniform structure, uniform phase composition, and they have a complex of good physicomechanical properties: density, great separating strength of the substrate, mechanical strength. An addition of TiO₂ to powdered aluminum oxide stabilizes the crystal lattice of the phase -Al₂O₃ which is decisive for the heat and wear resistance of coatings. Moreover, an addition of TiO₂ increases the interparticular bond in the coating and its adhesive strength with the substrate. It is indispensable to point out the selectivity of the properties of detonation coatings type AT-40 in regard to the surfaces of the protected metals. The greatest strength of adheion with the coatings is found in chromium and titanium substrates; in steel, brass, and aluminum ones the strength is somewhat lower. The high selectivity of the properties is most probably due to the difference in sublimation energy and melting points of the materials of the substrate, their hardness and brittleness. It was noted that the adhesive bond between aluminum titanate coatings and the substrates is anomalously strong; this is due to the uniqueness of its properties. Coatings and substrates have a mutual effect on the strength and plastic properties. As a rule, coatings lower the bending strength of the substrate. An exception are chromium alloys which only become embrittled without loss of strength, and substrates of ductile low alloy steels which are somewhat strengthened by coatings.Translated from Poroshkovaya Metallurgiya, No. 5(281), pp. 59–64, May, 1986.     

Up‐scaling of a Low Temperature PACVD Process for the Deposition of α/γ‐Nanocrystalline Alumina Coated Tools for Thixocasting of 1.2208 steel

An industrial‐scale pulsed plasma‐assisted chemical vapor deposition (PACVD) process for crystalline alumina growth was developed. To obtain a homogeneous coating thickness distribution over complex geometries and large dimensions, a suitable gas injection system was designed. A phase formation diagram for alumina coatings as a function of pulse length and cathode voltage has been compiled, allowing for the deposition of dense α/γ‐Al₂O₃ coatings at a substrate temperature of 590 °C. Moulds coated with an α/γ‐Al₂O₃ coating were utilized in steel thixocasting at temperatures of ～1400 °C. The coatings were intact after thixocasting and showed significantly improved chemical wear resistance compared to plain steel moulds.     

Oxidation of TiAl intermetallic

The oxidation kinetics of TiAl intermetallic at 500–900 °C in air is studied using a gravimetric method, and the phase composition of the scale is studied using an x-ray phase analysis. At t > 600 °C, the kinetics of oxidation is described by a parabolic equation. The oxides TiO₂ (rutile), γ-Al₂O₃, α-Al₂O₃, Ti₂O₃ are found in the scale. It is shown that at the first stage the γ-Al₂O₃ and low-titanium oxides form on the sample surface at t < 70 °C. At t ≥ 850 °C, the Ti₂O₃ forms on the external surface of the scale, TiAl₃ is found in the sublayer at the alloy/scale interface. It is shown that at t ≤ 800 °C the process is controlled by oxygen diffusion. At t > 800 °C, the oxidation mechanism changes: counterdiffusion of titanium ions through interstitial sites in TiO₂ lattice occurs.     

Oxidation and corrosion resistance of TiAl3 coatings

Titanium aluminides based on TiAl, TiAl₃ and Ti₃Al are potential materials for high temperature aerospace applications. Their low density, high temperature creep resistance, high temperature strength and high oxidation resistance make them excellent coating materials. However these coatings are likely to be subjected to high temperature and corrosive environments during service. Hence it is aimed to study the oxidation and corrosion resistance of TiAl₃ coatings on various types of substrates. In the present work, TiAl₃ is coated on high speed steel, stainless steel 304, stainless steel 316, copper and aluminum substrates by physical vapor deposition technique. X-ray diffraction analysis confirms the presence of TiAl₃ phase. The hardness studies reveal that better hardness can be achieved with thick coatings. The oxidation behavior of the coatings is studied by carrying out step stress experiments at elevated temperatures. Coated samples are heated up from 400°C in the steps of 100°C for 1h in each step to 1000°C. The mass gain caused by oxidation was determined. The oxidation curve drawn as a function of mass gain versus temperature reveals that TiAl₃ film started to oxidize above 800°C, where as oxidation of the uncoated substrates began at a much lower temperature of 550°C. The excellent oxidation resistance of the coatings can be attributed to the formation of an amorphous Al₂O₃ film. Scanning electron microscope (SEM) and EDAX analysis confirm the presence of an amorphous Al₂O₃ film. The corrosion behavior of TiAl₃ coatings are investigated by the polarization resistance experiments in NaCl aqueous solution at ambient temperature. According to the Tafel plot analysis, the coatings show lower corrosion rate than the untreated substrates. The major corrosion in the coatings arose from electrolyte penetration into the pores of the coatings. In fact, a dense coating showed a high corrosion resistance in an aqueous medium.     

Relative performance of alumina coatings prepared by micro arc oxidation and detonation gun spray on AA 6063 under plain fatigue and fretting fatigue loading

The present study compares the performance of alumina coatings prepared by two different methods (micro arc oxidation (MAO) and detonation gun (D-gun) spray) on AA 6063 (Al alloy) fatigue test samples under plain fatigue and fretting fatigue loading. While MAO coating had comparable proportions of γ-Al₂O₃ and α-Al₂O₃, D-gun sprayed coating contained γ-Al₂O₃ with minimal quantities of α-Al₂O₃. MAO coating was relatively harder than D-gun sprayed coating. As both types of coated samples were ground, they exhibited almost the same surface roughness. D-gun sprayed alumina coated samples exhibited slightly higher magnitude of surface residual compressive stress compared with MAO coated specimens. Both types of coated samples experienced almost the same friction force. D-gun spray coated samples exhibited superior plain fatigue and fretting fatigue lives compared with MAO coated specimens. This may be attributed to layered structure of the D-gun sprayed coating.     

Correlation of microstructure and wear resistance of ferrous coatings fabricated by atmospheric plasma spraying

The correlation of microstructure and wear resistance in ferrous coatings applicable to diesel engine cylinder bores was investigated in this study. Seven kinds of ferrous spray powders, two of which were stainless steel powders and the others blend powders of ferrous powders mixed with Al₂O₃-ZrO₂ powders, were sprayed on a low-carbon steel substrate by atmospheric plasma spraying. Microstructural analysis of the ferrous coatings showed that various Fe oxides such as FeO, Fe₂O₃, and γ-Fe₂O₃ were formed in the martensitic (or austenitic) matrix as a result of the reaction with oxygen in air. The blend coatings containing γ-Al₂O₃ and t-ZrO₂ oxides, which were formed as Al₂O₃-ZrO₂ powders, were rapidly solidified during plasma spraying. The wear test results revealed that the blend coatings showed better wear resistance than the ferrous coatings because they contained a number of hard Al₂O₃-ZrO₂ oxides. However, delamination occurred when cracks initiated at matrix/oxide interfaces and propagated parallel to the worn surface in the case of the large hardness difference between the matrix and oxide. The wear rate of the coating fabricated with STS316 powders was slightly higher than other coatings, but the wear rate of the counterpart material was very low because of the smaller matrix/oxide hardness difference due to the presence of many Fe oxides. In order to reduce the wear of both the coating and its counterpart material, the matrix/oxide hardness difference should be minimized, and the hardness of the coating should be increased over a certain level by forming an appropriate amount of oxides.     

Oxidation behaviour of Al enhanced stainless steel coatings produced by cryomilling and spark plasma sintering

Abstract

Nanostructured 316L stainless steel coatings with and without Al were deposited on stainless steel substrates to evaluate the potential gain in oxidation resistance. Compared to the base alloy, nanostructured samples had thicker thermally grown oxides, independent of tested temperature. The oxides formed, however, were enriched in Cr, which was found to increase oxidation resistance. The addition of Al (2 and 6 wt-%) yielded an Al₂O₃/Cr₂O₃ oxide scale, which significantly increased the oxidation resistance compared to other samples. The nanostructured alloys had roughly half the activation energy for oxidation, and lower oxidation rate constants, than the conventional coatings.

On a déposé des revêtements nanostructurés d’acier inoxydable 316L, avec ou sans Al, sur des substrats en acier inoxydable (SS) afin d’évaluer le gain potentiel de résistance à l’oxydation. Comparés à l’alliage de base, les échantillons nanostructurés avaient des oxydes à croissance thermique plus épais, indépendamment de la température évaluée. Cependant, les oxydes formés étaient enrichis de chrome, ce qui augmentait la résistance à l’oxydation. L’addition d’Al (2 ou 6% en poids) produisait une écaille oxyde d’Al₂O₃/Cr₂O₃, ce qui augmentait significativement la résistance à l’oxydation par rapport aux autres échantillons. Les alliages nanostructurés avaient approximativement la moitié de l’énergie d’activation pour l’oxydation et des constantes de vitesse d’oxydation inférieures à celles des revêtements conventionnels.     

Oxidation Behavior of NiAl-30.75Cr-3Mo-0.25Ho Alloy at High Temperatures

The oxidation behavior of NiAl-30.75Cr-3Mo-0.25Ho alloy from 1300 to 1500 K in air atmosphere was investigated. The results reveal that oxidation resistance of the alloy is improved by the addition of Ho. At 1500 K, the oxidation kinetic curve obeys the parabolic law (n≈0.5), whereas the oxidation kinetic curve of the tested alloy follows the cubic relations (n≈0.3～0.4) from 1300 to 1450 K. An activation energy of about 261 kJ·mol-1 was determined for the tested alloy. It is found that a continuous and compact Al2O3 layer has formed on the surface of NiAl-30.75Cr-3Mo-0.25Ho alloy after oxidation 100 h at various tested temperatures. A rich-Ho solution formed on the boundaries of Cr(Mo) phase. Doped little amount of Ho in NiAl-31Cr-3Mo alloy promotes the transformation from θ-Al2O3 phase to α-Al2O3 phase, and decreases the size of Al2O3 and the crack forming in the oxidation scale prolongs the spalling time of the film at high temperature. The volatilizing oxides of Cr, Mo and the reactive element effects (REEs) make the mass gain lower than that of pure NiAl.     

Nanoparticle-Induced Superior Hot Tearing Resistance of A206 Alloy

Al-Cu alloys (such as A206) offer high strength and high fracture toughness at both room and elevated temperatures. However, their widespread applications are limited because of their high susceptibility to hot tearing. This article presents a nanotechnology approach to enhance hot-tearing resistance for A206. Specifically, γ-Al₂O₃ nanoparticles were used, and their effects on the hot-tearing resistance of the as-cast Al-4.5Cu alloy (A206) were investigated. While it is well known that grain refinement can improve the hot-tearing resistance of cast Al alloys, the current study demonstrated that nanoparticles can be much more effective in the case of A206. The hot-tearing susceptibilities (HTSs) of A206 alloy and its Al₂O₃ nanocomposite were evaluated by constrained rod casting (CRC) with a steel mold. Monolithic A206 and M206 (the Ti-free version of A206) alloys with the B contents of 20, 40, and 300 ppm from an Al-5Ti-1B master alloy addition were also cast under the same conditions for comparison. The results showed that with an addition of 1 wt pct γ-Al₂O₃ nanoparticles, the extent of hot tearing in A206 alloys was markedly reduced to nearly that of A356, an Al-Si alloy highly resistant to hot tearing. As compared with grain-refined A206 or M206, the hot-tearing resistance of the nanocomposites was significantly better, even though the grain size was not reduced as much. Microstructural analysis suggested that γ-Al₂O₃ nanoparticles modified the solidification microstructure of the eutectic of θ-Al₂Cu and α-Al, as well as refined primary grains, resulting in the enhancement of the hot-tearing resistance of A206 to a level similar to that of A356 alloy.     

Effects of an α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> thin film on the oxidation behavior of a single-crystal Ni-based superalloy

A ∼150-nm-thick coating layer consisting of α-Al₂O₃ as the major phase with a minute amount of Φ-Al₂O₃ was deposited on the surface of a single-crystal Ni-based superalloy by chemical vapor deposition (CVD). Within 0.5 hours of oxidation at 1150°C, the resulting thermally grown oxide (TGO) formed on the coated alloy surface underwent significant lateral grain growth. Consequently, within this time scale, the columnar nature of the TGO became established. After 50 hours, a network of ridges was clearly observed on the TGO surface instead of equiaxed grains typically observed on the uncoated alloy surface. Comparison of the TGO morphologies observed with and without the CVD-Al₂O₃ layer suggested that the transient oxidation of the alloy surface was considerably reduced. Also, the CVD-Al₂O₃ layer significantly reduced the growth rate of the TGO and improved its spallation resistance, while slowing the internal oxidation of Ta-rich areas that were present in the superalloy as-casting defects. These results demonstrated that this thin α-Al₂O₃ coating could be used as a means of favorably altering the TGO morphology and growth kinetics for no bond coat thermal barrier coating (TBC) applications. Y.-F. SU, formerly Doctoral Candidate     

Effects of an α-Al2O3 thin film on the oxidation behavior of a single-crystal Ni-based superalloy

A ∼ 150-nm-thick coating layer consisting of α-Al₂O₃ as the major phase with a minute amount of θ-Al₂O₃ was deposited on the surface of a single-crystal Ni-based superalloy by chemical vapor deposition (CVD). Within 0.5 hours of oxidation at 1150 °C, the resulting thermally grown oxide (TGO) formed on the coated alloy surface underwent significant lateral grain growth. Consequently, within this time scale, the columnar nature of the TGO became established. After 50 hours, a network of ridges was clearly observed on the TGO surface instead of equiaxed grains typically observed on the uncoated alloy surface. Comparison of the TGO morphologies observed with and without the CVD-Al₂O₃ layer suggested that the transient oxidation of the alloy surface was considerably reduced. Also, the CVD-Al₂O₃ layer significantly reduced the growth rate of the TGO and improved its spallation resistance, while slowing the internal oxidation of Ta-rich areas that were present in the superalloy as-casting defects. These results demonstrated that this thin α-Al₂O₃ coating could be used as a means of favorably altering the TGO morphology and growth kinetics for no bond coat thermal barrier coating (TBC) applications.     

Structure and properties of thin ceramic coatings in the system AlN-TiCrB2

The structure, composition and properties of coatings on Si, Al₂O₃ and GaAs single crystals prepared by radio-frequency magnetron sputtering of a AlN-TiCrB₂ target prepared by powder metallurgy are studied. Coating phase composition is different from that of the target material due to oxidation of aluminum nitride and its dissociation under ion bombardment conditions. The coatings have a very fine structure and marked resistance to high-temperature oxidation due to formation of solid solutions in the systems Al₂O₃-TiO₂-Cr₂O₃ and Al₂O₃-B₂O₃. The increase in mass of the target material at 1300°C is 1.4 mg/cm². After high-temperature oxidation the reinforced fine structure of the coating forms as interwoven Al₂O₃ fibers. Coatings of (AlN-TiCrB₂)-Al₂O₃ and (AlN-TiCrB₂)-GaAs are thermally stable up to 900°C and have a high microhardness (H _μ) and crack resistance (K_Ic = 4.7–3.3 MN/m^3/2). With an increase in annealing temperature (T ≥ 1000°C) coating mechanical properties worsen, but their adhesive strength increases. The AlN-TiCrB₂ target material may be recommended for preparing wear-and corrosion-resistant coatings on tools, and also on critical components operating under extreme conditions. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(449), pp. 39–47, May–June, 2006.     

The effects of alloy microstructure refinement on the short-term thermal oxidation of NiCoCrAlY alloys

Three γ + β NiCoCrAlY alloys (a cast alloy, a laser-surface-melted (LSM) alloy, and a coating as deposited by electron beam-physical vapor deposition (EB-PVD)) with similar average composition (Ni-20Co-19Cr-24Al-0.2Y in at. pct), but with different microstructures prior to oxidation, were oxidized for 0.5 and 1 hours at 1373 K in an Ar 20 vol pct O₂ atmosphere (i.e., at a partial oxygen pressure of 20 kPa). It was found that on the alloy with β precipitates larger than 20 μm, the oxide layer was nonuniform in thickness, and had a laterally inhomogeneous composition and phase constitution. In this case, the oxide layer developed on top of the γ phase was thicker than that formed on top of the β phase and consisted of a NiCr₂O₄/Cr₂O₃ outer and an α-Al₂O₃ inner layer. For the thinner oxide formed on top of the β phase, the outer layer was constituted of a Cr and Co containing NiAl₂O₄ spinel and the inner layer also consisted of α-Al₂O₃. For the alloys with β precipitates smaller than 3 μm, a uniform and laterally homogeneous oxide formed, consisting of a Cr and Co containing NiAl₂O₄ outer layer on top of an α-Al₂O₃ inner layer. After oxidation, Y was distributed as numerous, small precipitates within the oxide layer for a homogeneous Y distribution prior to oxidation, or as a few, very large pegs along the γ/β phase boundaries of the alloy for an inhomogeneous Y distribution prior to oxidation. The performance of the alloys upon thermal cycling was improved for smaller β precipitates and for a more homogeneous Y distribution in the alloy prior to oxidation.     

The oxidation behavior of some Ni-Cr-Al alloys at high temperatures

Oxidation of ternary Ni-Cr-Al alloys containing different Cr/Al ratios has been studied in the temperature range 800° to 1300°C. Most of the studies were performed in 1 atm oxygen or air, but the oxygen pressure dependence for one of the alloys was also investigated. The experimental methods included thermogravimetric measurements of oxidation rates and studies on reacted specimens by means of X-ray diffraction, metallographic techniques, electron microprobe analysis, and electron microscopy. In general, the oxidation rates decrease faster with time than that for an ideal parabolic behavior. The major reaction products were NiO, Cr₂O₃,α-Al₂O₃, and Ni(Cr,Al)₂O₄. The relative amounts of these were a function of composition, temperature, oxygen pressure, and reaction time. The Ni-9Cr-6Al alloy has the best oxidation resistance due to the formation ofα-Al₂O₃ at all temperatures investigated. The oxidation mechanism of the alloy is discussed.     

Some water vapor effects during the oxidation of alloys that are <Emphasis Type="Italic">α</Emphasis>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> formers

Oxidation studies were performed at 1100 °C in dry air and air containing fixed partial pressures of water vapor on a number of alloys and coatings that form α-Al₂O₃ scales under oxidizing conditions. The alloys investigated included RENé N5, PWA 1484, diffusion aluminide coatings (with and without Pt modification) on RENé N5, and a Ni-8 wt pct Cr-6 wt pct Al model alloy. The water vapor affected the oxidation of the alloys in three important ways: (1) The scales spalled more profusely during cyclic oxidation in wet air than in dry air, particularly for those alloys with alumina scales, which are only moderately adherent under dry conditions. The results were consistent with the mechanism previously proposed (Reference 1), whereby the water molecules decrease the fracture toughness of the alumina/alloy interface. (2) Thicker oxides are formed during oxidation in wet air than dry air. This effect comes primarily from accelerated transient oxidation during exposure in wet air. (3) Spinel was found to form on top of the alumina scales during long-term exposure. This phenomenon occurred in all atmospheres but was much more pronounced for exposures in wet atmospheres. Mechanisms for the preceding observations are proposed.