On the Microstructure of the Initial Oxide Grown by Controlled Annealing and Oxidation on a NiCoCrAlY Bond Coating

Different pre-annealing and pre-oxidation treatments were conducted on a dual phase γ+β Ni–21Co–18Cr–22Al–0.2Y (at.%) bond coating for 1 hr at 1373 K (i) with or without a native oxide upon heating, (ii) in two different atmospheres upon heating, and (iii) under various oxygen partial pressures (pO₂) in the range of 0.1–10⁵ Pa during oxidation. The chemical composition, structure, morphology and phase constitution of the resulting oxide layers were investigated using a range of analytical techniques. It is found that the exclusive formation of a continuous α-Al₂O₃ layer without the simultaneous formation of NiAl₂O₄ spinel was promoted for oxidation at low pO₂. The formation of metastable θ-Al₂O₃ was suppressed for a low fraction of the β phase, coupled with a high fraction of segregated Y at the initial bond coat surface. Initial Y segregation and incorporation of Y₂O₃ and Y₃Al₅O₁₂ within the developing oxide layer was promoted in the absence of a native oxide and for heating in an inert atmosphere. The development of protrusions (i.e. pegs) at the oxide/coating interface, as a result of the incorporation of internal Y₂O₃ precipitates by the inward growing oxide layer, was most pronounced upon heating in an inert atmosphere, followed by oxidation at an intermediate pO₂.     

Development of a Pre-Oxidation Treatment to Improve the Adhesion between Thermal Barrier Coatings and NiCoCrAlY Bond Coatings

High-temperature coating systems, consisting of a René N5 superalloy, a Ni–23Co–23Cr–19Al–0.2Y (at.%) bond coating (BC), and a yttria (7 wt%)-stabilized zirconia (YSZ) thermal barrier coating (TBC), were thermally cycled to failure for seven different controlled pre-oxidation treatments and one commonly employed industrial pre-oxidation treatment to establish the preferred microstructures of the thermally-grown oxide (TGO) on a NiCoCrAlY bond coating after pre-oxidation. It was found that the failure of the coating system occurred along the TGO/BC interface when the TGO attained a critical thickness, except if a NiAl₂O₄ spinel layer developed contiguous to the TBC/TGO interface. Then, the coating system failed at a smaller TGO thickness along the NiAl₂O₄/α-Al₂O₃ interface. The value for the TGO thickness at failure increased for a larger area fraction of Y-rich oxide pegs at the TGO/BC interface after pre-oxidation. A desired slow-growing oxide layer on the BC surface was promoted when the presence of the oxides NiAl₂O₄, θ-Al₂O₃, Y₃Al₅O₁₂ at the TGO surface after pre-oxidation was avoided. The α-Al₂O₃ layer, which developed adjacent to the BC upon thermal cycling, grew at a low rate if the initial oxide at the onset of oxidation consisted of θ-Al₂O₃ instead of α-Al₂O₃. Based on these results a pre-oxidation treatment is proposed for which the lifetime of the entire coating system during service is enhanced.     

Isothermal Oxidation Behavior of Ti-50Al Alloy with Y Additions at 800 and 900°C

The isothermal-oxidation behavior of Al-rich TiAl alloys containing Y up to 1.0 at.% was studied in synthetic air with a flow of 200–250 mL/min at 800 and 900°C. Oxidation kinetics and scale adherence were studied in terms of the morphological features and microstructural evolution of the oxide scale. In the specimens oxidized at 800°C, all alloys containing 0.3–1.0 at.%Y showed reduced mass gain compared to the Y-free alloy, especially for the 0.3 at.%Y alloy. Under isothermal exposure at 900°C, the addition of small amounts of Y (0.1 and 0.3 at.%) was effective in enhancing the oxidation resistance. The alloys with higher Y contents (0.6 and 1.0 at.%), on the contrary, had a reverse effect on the oxidation resistance by providing rapid diffusion paths in the form of coarse Y₂O₃ particles close to the substrate. The improvement of oxidation resistance of the alloy with Y additions was due partly to the improved adhesion of the scale and due partly to the formation of a continuous α-Al₂O₃ layer in the outer scale. Y segregation and/or Y₂O₃ precipitation at the oxide grain boundaries was effective in decreasing the oxidation rate and refining the oxide grains. The thinner scale was responsible for relaxing the thermal stress and, thus the cohesion between the scale and substrate was greatly improved in Y-containing alloys.     

In Situ Study of Oxidation-Induced Growth Strains In a Model NiCrAlY Bond-Coat Alloy

Synchrotron radiation has been used to study in situ the evolution of growth strains in an Al₂O₃ scale (the so-called TGO or thermally grown oxide) on a model bond-coat alloy (Ni-19.7 Cr-19.2 Al-0.1 Y at.%) as oxide growth proceeds in air at 950–1100°C, and the changes in these strains due to thermal-expansion mismatch as the samples are cooled. Tensile growth stresses develop in the oxide scales during the initial stages of oxidation, a result of initially formed transition aluminas converting to the stable α-Al₂O₃ form, but large residual compressive stresses are present at room temperature due to thermal-expansion mismatch between the scale and the bond-coat.     

Carburization Behavior of Electrodeposited Ni3 Al–CeO2-Base Coatings on Fe–Ni–Cr Alloys

The cyclic carburization of electrodeposited pure and CeO₂-dispersed Ni₃Al intermetallic coatings on Fe–Ni–Cr alloys has been investigated at 850 and 1050°C for periods up to 500 h in a reducing 2%CH₄–H₂ atmosphere. At 850°C, all Ni₃Al-base-coating samples showed excellent carburization resistance and slow mass increases due to the formation of a thin γ-Al₂O₃ scale and a low carbon activity (a _c = 0.73). At 1050°C and a high carbon activity (a _c = 3.21), all coatings are superior to the uncoated Fe–Ni–Cr alloy in terms of carburization resistance. A thin α-Al₂O₃ scale slowly formed on all Ni₃Al coatings effectively blocked the carbon attack. The addition of CeO₂ particles in the Ni₃Al coatings significantly mitigated the cracking of the α-Al₂O₃ scale and the resultant internal oxidation and carburization. For all coatings, Ni-rich particles were found to be formed on the α-Al₂O₃ scale during oxidation, which had led to the deposition of catalytic coke.     

Oxidation Behavior of a Sputtered Ni–5Cr–5Al Nanocrystalline Coating

A NiO-forming Ni–5Cr–5Al (at.%) alloy has been developed anddeposited as a sputtered nanocrystalline coating. The oxide formation andoxidation behavior of this coating have been studied at 1000°C inair. The oxidation rate markedly decreased with time and the oxidationkinetics obeyed the fourth power law. Complex oxide scales, consisting ofNiO, NiAl₂O₄ and -Al₂O₃,were formed during 200 hr oxidation. The outer oxide layer consisted of NiOand NiAl₂O₄ and an inner oxide layer of-Al₂O₃. The sputtered Ni–5Cr–5Alnanocrystalline coating showed good oxidation resistance due to theformation of an -Al₂O₃ inner layer andexcellent adhesion of the complex oxide scales.     

Oxidation Behavior of Cast Ni3Al Alloys and Microcrystalline Ni3Al+5% Cr Coatings with and without Y Doping

Ni₃Al+5% Cr and Ni₃Al+5% Cr+0.3% Y (wt.%) microcrystalline coatings were produced using a close-field, unbalanced magnetron-sputter deposition (CFUMSD) technique. Isothermal and cyclic-oxidation tests were carried out to assess the oxidation resistance of the coatings. The results showed that Al₂O₃ formed on the coatings as the main oxidation products, with the formation of - and -Al₂O₃ scales at 900 and 1200°C, respectively. The spallation resistance of the Al₂O₃ scales formed on the coatings was superior to the oxide scales formed on cast Ni₃Al. After oxidation, interfacial voids were observed on the oxide–metal interface of the cast alloy while no voids were found on the coating surfaces. On the basis of the enhancement of Al diffusion, because of the high density of grain boundaries in the coatings, oxidation mechanisms were proposed.     

Oxidation Behavior of In Situ-Synthesized (TiB+TiC)/Ti6242 Composites

The oxidation behaviors of TiB and TiC particle-reinforced, titanium-matrix composites (TMCs) were studied in air at 550–650°C, The oxidation kinetics follow approximately a parabolic rate law. The oxidation rates, which were lower than those of Ti6242, decrease gradually as oxidation proceeds. The oxide scales formed on TMCs were predominantly rutile and α-Al₂O₃. No B₂O₃ and other oxides were observed within the oxide scale. The in situ-synthesized TiB and TiC reinforcements can increase the oxidation resistance of TMCs. The oxide scales that formed exhibited excellent spallation resistance under all testing conditions. No scale cracking or spallation could be observed, implying that growth and thermal stresses generated during heating and cooling have been effectively released. The mechanisms of the decrease in oxidation rate and the improvement on spallation resistance are discussed based on microstructure studies.     

High-temperature oxidation of nickel-based alloys and estimation of the adhesion strength of resulting oxide layers

The kinetics of isothermal oxidation (1100°C) of commercial nickel-based alloys with different content of sulfur (0.22–3.2 wt ppm) is studied. The adhesion strength in a metal/oxide system is estimated as a function of sulfur content and duration of high-temperature exposure. The scratch-test technique is proposed to quantitatively estimate the work of adhesion of resulting oxide films. It is found that the film microstructure is composed of an inner α-Al₂O₃ layer and an outer NiAl₂O₄ spinel layer, which are separated by discrete inclusions of TiO₂. Residual stresses in the oxide film are experimentally determined by X-ray diffraction.     

Oxidation Resistance of a Cr0.50Al0.50N Coating Prepared by Magnetron Sputtering on Alloy K38G

A Cr_0.50Al_0.50N coating has been prepared by a reactive-magnetron-sputtering method on alloy K38G. The coating possesses mainly the B1 type with a small amount of B4-type crystal structure phase. Isothermal oxidation tests were performed at 900–1,100 °C for 20 h by thermogravimetric analysis (TGA) in air. The results reveal that the coated samples have much lower mass gain than that of the bare alloy. The parabolic rate constants of the coated samples decrease by 2 orders of magnitude compared with the bare alloy at 1,000 and 1,100 °C. During the oxidation of the coated samples below 1,000 °C, the main oxide is Cr₂O₃, but above 1,000 °C, the scale changes to α-Al₂O₃. The observed oxidation behaviors demonstrate that the Cr_0.50Al_0.50N coating can provide good protection against corrosion over a wide temperature range.     

Cyclic oxidation behavior of Ni3Al-7.8% Cr-1.3% Zr-0.8% Mo-0.025% B between 900 and 1100°C

A Ni₃Al-based alloy, the composition of which was Ni-16.0% Al-7.8% Cr-1.3% Zr-0.8% Mo-0.025%B, was cyclically oxidized in the temperature range of 900 to 1100°C in air for up to 500 hr. The alloy displayed good cyclic oxidation resistance up to 1000°C, with little scale spallation. It, however, lost cyclic oxidation resistance during oxidation at 1100°C after about 200 hr, displaying large weight losses due to serious scale spallation. NiO, α-Al₂O₃, NiAl₂O₄ and ZrO₂ were formed. The oxide scales consisted primarily of an outer Ni-rich layer which was prone to spallation, and (Al, Cr, Zr, Mo, Ni)-containing internal oxides which were adherent due mainly to the formation of (Al₂O₃, ZrO₂)-containing oxides that keyed the oxide scale to the matrix alloy.     

Oxidation behaviour of the alloy IC-6 and protective coatings

In this work, NiCoCrAlY coatings were deposited on a new Ni-base alloy, IC-6. The oxidation kinetic curves of alloy IC-6, K17 and NiCoCrAlY coatings on alloy IC-6 at 900-1100 °C were obtained. The results indicated that the oxide scales consisted of α-Al₂O₃, NiAl₂O₄, NiO, as well as a small amount of NiMoO₄ and MoO₂. These scales occurred after alloy IC-6 exposure at 900 °C for 100 h. The weight loss occurred when alloy IC-6 were exposed at 1050 and 1100 °C due to the formation of volatile MoO₃. After the NiCoCrAlY coating was deposited, the scales mainly contained α-Al₂O₃, when the specimens were oxidized at 900 °C, and α-Al₂O₃and Cr₂O₃ at 1050 °C. The formation of α-Al₂O₃ and Cr₂O₃ scales on NiCoCrAlY coating was directly responsible for improving oxidation resistance of the alloy IC-6.     

Cyclic Oxidation Behavior of Electrodeposited Ni3Al–CeO2Base Coatings at 850°C

The cyclic-oxidation behavior of electrodeposited pure, nano CeO₂ (9–15 nm)- and micron CeO₂ (5 μm)-modified Ni₃Al coatings on Fe–Ni–Cr substrates have been investigated at 850°C for periods up to 1000 hr. All the coatings showed good oxidation resistance in the early stage, but decreased oxidation protection during the intermediate stage of exposure. The formation of slow-growing γ-Al₂O₃ scales provided the coatings with good oxidation resistance in the early stage. However, a high volume fraction of pores in the coatings decreased oxidation resistance in the intermediate stage by forming oxidized channels between the pores. After long-term exposure, however, the pores in the coatings were healed by coating–substrate interdiffusion, and an improvement in cyclic-oxidation resistance was observed. The CeO₂ particles in the coating affected the stability of the protective oxide scale and the pore-healing behavior. The most significant effect was from nano size CeO₂ particles, which improved the stability of the protective oxide scale but retarded the rate of pore healing.     

Oxidation resistance of aluminized coatings on a directionally solidified Ni-Al-Cr3C2 eutectic alloy

Although a directionally solidified Ni-Al-Cr₃C₂ eutectic alloy has good high-temperature mechanical properties, it does not have adequate oxidation resistance for prolonged exposure to high surface temperatures. Thus the oxidation behavior of several aluminized coating systems on this alloy in flowing air at temperatures of 900 to 1100°C under isothermal and thermal cycling conditions has been investigated. Attempts to produce an oxidation-resistant system by direct aluminizing have not been successful since removal of carbide fibers results in a porous coating which gives little protection to the alloy. The deposition of a layer of nickel or a Ni-20%Co-10%Cr-4%Al alloy on the eutectic prior to aluminizing gives improved isothermal oxidation resistance for prolonged exposure to high surface temperatures. Thus the eutectic alloy substrate occur during thermal cycling. A more successful system has been produced by depositing a thin layer of platinum on the eutectic alloy prior to aluminizing. Protective -Al₂O₃ scales are formed and maintained during isothermal and thermal cycling oxidation at 900 and 1000°C. Similar scales are developed at 1100° C although these do break down during thermal cycling. However, surface -Al₂O₃ scales are able to re-form rapidly, thereby preventing excessive oxidation of the coating.     

The influence of yttrium additions on the oxidation resistance of a directionally solidified Ni-Al-Cr3C2 eutectic alloy

Isothermal oxidation of a directionally solidified Ni-Al-Cr₃C₂ eutectic alloy results in development of an external -Al₃O₃-rich scale. However, this scale breaks down after relatively short times at temperature and a less protective Cr₂O₃-rich scale is formed, together with substantial internal oxide in the alloy. In an attempt to maintain the external -Al₂O₃-rich scale and prevent damaging subscale oxidation, modified yttrium-containing directionally solidified alloys have been developed. The oxidation resistance of these alloys at 1000 and 1100°C in flowing air has been investigated and found to be considerably better than that of the corresponding yttrium-free alloy. At both temperatures an external -Al₂O₃-rich scale is produced and is retained for much longer periods than on the yttrium-free alloys during isothermal and thermal cycling oxidation. Some scale breakdown does occur during thermal cycling at 1100°C, but -Al₂O₃ is able to re-form as the surface oxide. However, although external -Al₂O₃-rich scales are retained for long periods on these alloys, some oxide penetration into the alloy beneath these scales does occur where coarse carbide fibers intersect the alloy surface. This is associated with relatively poor scale integrity at these intersections.     

高硼/钇合金化提高FeAl基金属间化合物多孔材料的抗氧化性能（英文）

利用反应合成法制备高铬(Cr)、硼(B)、钇(Y)复合合金化的B2结构FeAl基金属间化合物多孔材料,通过研究FeAl基多孔材料氧化后的孔结构演变、氧化动力学和氧化膜构型,探讨其高温氧化行为。结果显示,添加高含量合金元素Cr、B和Y后,FeAl多孔材料在600～800℃下氧化增质显著降低。富集在氧化膜表面的B和偏聚在氧化膜与基体界面处的Y共同促进多孔材料表面形成薄且具有优异防护性的结节状α-Al₂O₃氧化膜。研究表明,引入较高含量的活性元素如B和Y有利于FeAl金属间化合物在较低氧化温度和无预处理的情况下选择性生长单一的α-Al₂O₃氧化膜。     

High-temperature oxidation behavior of pure Ni3Al

The high-temperature oxidation behaviour of pure Ni₃Al alloys in air was studied above 1000°C. In isothermal oxidation tests between 1000 and 1200°C, Ni₃Al showed parabolic oxidation behavior and displayed excellent oxidation resistance. In cyclic oxidation tests between 1000 and 1300°C, Ni₃Al exhibited excellent oxidation resistance between 1000 and 1200°C, but drastic spalling of oxide scales was observed at 1300°C. When Ni₃Al was oxidized at 1000°C, Al₂O₃ was present as -Al₂O₃ in a whisker form. But, at 1100°C the gradual transformation of initially formed metastable -Al₂O₃ to stable -Al₂O₃ was observed after oxidation for about 20 hr. After oxidation at 1200°C for long times, the formation of a thick columnar-grain layer of -Al₂O₃ was observed beneath a thin and fine-grain outer layer of -Al₃O₃. The oxidation mechanism of pure Ni₃Al is described.     

The Effect of an Applied External Tensile Stress on the Oxidation Behavior of a Nickel-Base Alloy with a Re-base Diffusion-barrier-coating

A diffusion-barrier-coating system with a duplex layer structure comprised of an inner Re-base alloy layer and an outer β-NiAl layer was formed on the Ni–Mo alloy, Hastelloy-X. Alloy specimens with and without the coating were oxidized at 970 °C in air for up to 200 h with an imposed tensile stress of 22.5 MPa. The oxidation behavior under the stress-free condition was also investigated for comparison purposes. Strain rates of the specimens with a diffusion-barrier-coating system decreased rapidly for about 5 h, followed by a slow creep-deformation with a strain of 3.5% and strain rates of (0.7–0.2) × 10⁻⁷/s for 200 h. There was little change in both the coating structure and the composition (at%) of the inner Re-base alloy layer. Considering the creep behavior of the uncoated alloy, as well as the fact that there were few cracks and flaws in the Re-base alloy layer, it was concluded that this inner layer was subject to creep-deformation along with the alloy substrate. The external scale on the coated alloy consisted mainly of θ-Al₂O₃ at the early stage of the oxidation/deformation, and with further oxidation the surface scale formed a duplex layer structure consisting of outer plate-like θ-Al₂O₃ and inner equi-axed Al₂O₃. There was exfoliation of the outer θ-Al₂O₃ scale during the creep deformation. After the 200 h oxidation the outer β-NiAl contained (40–50)% Al, while the alloy substrate near the inner layer had less than 1 at% Al. It was found that the Re-base alloy layer acted an effective barrier against inward Al diffusion and outward diffusion of alloying elements.     

Characterization of oxide scales formed on high-velocity oxyfuel-sprayed Ni-Co-Cr-Al-Y + ReTa coatings

A high-velocity oxyfuel-sprayed 30 wt.% Ni-20 wt.% Co-30 wt.% Cr-10 wt.% Al-2 wt.% Y-4 wt.% Re-4 wt.% Ta coating was oxidized between 1000 and 1200 °C for up to 200 h in air, and the oxide scales were examined. The dense, sprayed coating consisted mainly of Cr₃Ni₂, Ni₃Al, Ni₃Ta, Ni, NiO, Al₅Y₃O₁₂, and Cr₂O₃. Intermetallics and some oxides formed during spraying. During oxidation, mainly αAl₂O₃, along with some Al₅Y₃O₁₂, CoAl₂O₄, CoCr₂O₄, Ta₂O₅, and Ta₂O_2.2 formed on the coating. The preferential oxidation of Al to form the Al-rich scales resulted in the formation of an Al-depleted region beneath the scales. Rhenium, being the most noble element, was distributed throughout the oxide scale and the coating, without forming any independent oxides.     

Analytical Electron-Microscopy Study of the Breakdown of α-Al2O3 Scales Formed on Oxide Dispersion-Strengthened Alloys

Alumina scales formed during cyclic oxidation at 1200°C on three Y₂O₃–Al₂O₃-dispersed alloys: Ni₃Al, -NiAl, and FeCrAl (Inco alloy MA956) were characterized. In each case, the Y₂O₃ dispersion improved the -Al₂O₃ scale adhesion, but in the case of Ni₃Al, an external Ni-rich oxide spalled and regrew, indicating a less-adherent scale. A scanning-transmission electron microscope (STEM) analysis of the scale near the metal–scale interface revealed that the scale formed an ODS FeCrAl showed no base metal-oxide formation. However, the scale formed on ODS Ni₃Al showed evidence of cracking and Ni-rich oxides were observed. The microstructures and mechanisms discussed may be relevant to a thermal-barrier coating with an Al-depleted aluminide bond coat nearing failure.