Oxidation Behavior Of MCrAlY Coatings Research Articles

Synergistic effect of CeO2 and Al2O3 nanoparticle dispersion on the oxidation behavior of MCrAlY coatings deposited by HVOF

In this study, the as-received and nano-scaled oxide dispersion strengthened (ODS) MCrAlY coatings were deposited using high-velocity oxy-fuel (HVOF) spraying process. The high-energy planetary ball-milling process was utilized to prepare CeO2 and Al2O3 nanoparticles. ODS-NiCoCrAlY feedstock powders were also developed using the ball-milling process. The various formulations of Al2O3 and CeO2 nanoparticles (0.5 and 1.0 wt%) were chosen to apply different types of ODS-NiCoCrAlY coatings. The microstructure of the as-received and ODS coatings were evaluated by field emission scanning electron microscope (FESEM) as well as the commercial and ODS powders. Furthermore, the microhardness of different compositions of ODS coatings was accordingly investigated and the obtained results were compared with as-received coating. On account of the measurement of oxidation kinetics, the freestanding as-received and ODS coatings were exposed to air at 1000 °C up to 500 h and the thickness growth rate of the α-Al2O3 oxide layer was simultaneously examined. The results exemplified that NiCoCrAlY+1.0 wt% nano-CeO2+0.5 wt% nano-Al2O3 coating had a better oxidation resistance and lower oxide scale growth rate under the synergistic effects of both CeO2 and Al2O3 nanoparticles.

Effect of Surface Morphologies on the Isothermal Oxidation Behavior of MCrAlY Coatings Fabricated by High-velocity Oxyfuel Processes

Effect of Surface Morphologies on the Isothermal Oxidation Behavior of MCrAlY Coatings Fabricated by High-velocity Oxyfuel Processes

Effect of Sandblasting Process on the Oxidation Behavior of HVOF MCrAlY Coatings

Surface characteristics, such as topography and roughness (R a), are important in affecting the isothermal oxidation behavior of MCrAlY coatings. In this study, the effect of sandblasting on the oxidation behavior of HVOF thermal-sprayed MCrAlY coatings was investigated. Oxidation tests were conducted under isothermal conditions at 1050 °C for different periods. The R a of the bondcoat was approximately 12 μm. Before and after oxidation, the thermally-grown scale composition and morphology were analyzed using scanning electron microscope (SEM/EDX) and X-ray diffraction. The results showed that the oxide scale that formed on the as-sprayed coatings was a mixture of Al2O3 and spinel, while the oxide formed on the sandblasted coating was composed mainly of Al2O3. In addition, with increasing R a of the bondcoat, the oxidation rate progressively decreased and the amount of Al2O3 increased compared with the as-sprayed coating. Scanning electron microscope analysis showed that the thickness of the scale layer on the sandblasted CoNiCrAlY coating at 1050 °C was much thinner than that on the as-sprayed one. Isothermal oxidation kinetics of HVOF CoNiCrAlY coatings with different surface modifications at 1050 °C. This figure indicates the effect of the surface process on the oxidation kinetics of the CoNiCrAlY coating at the temperature of 1050 °C. As it can be observed, in both samples, oxidation rate is initially high and gradually lowers with the passage of time. The reason for this reduction in the longer times is the formation of a uniform and dense oxide layer at the beginning of the process within which the rate of oxygen penetration reduces and gets harder with lasting oxidation time. The type of oxidation kinetics of these coatings is parabolic. According to the results obtained, the oxidation rate constant (Kp) for the coating in the as-sprayed and sandblasted states equals 1.75 × 10−13 and 0.63 × 10−13 mg/cm2, respectively. The reduction in the oxidation rate signifies the effect of the surface morphology on the oxidation behavior of these coatings.

Effect of atmosphere composition on the oxidation behavior of MCrAlY coatings

AbstractIn the present work the effect of atmosphere composition on the growth rate and adherence of the alumina scales was studied using free‐standing MCrAlY‐coatings and TBC‐specimens with MCrAlY‐bondcoats. The exposures comprised isothermal and cyclic exposures in laboratory air and Ar‐H2‐H2O at 1100 °C.It is shown that minor Zr‐addition to the bondcoat results in enhanced scale growth and internal oxidation. This effect is independent of the atmosphere composition. As a consequence of the rapid oxide formation the times to TBC failure on the Zr‐containing bondcoat in both atmospheres were much shorter compared to those with Zr‐free bondcoat. In the latter case the formation of a thin compact alumina TGO was slower in H2/H2O than in air resulting in significantly longer TBC‐lifetime in the former atmosphere.

On the high-temperature oxidation of MCrAlY coatings

AbstractIn this review, recent progress in understanding the short- and long-term oxidation behaviour of MCrAlY coatings and the instruments that can be employed to influence the oxidation behaviour of the MCrAlY's are discussed. Particular attention is given on the transient oxidation behaviour, the initial distribution of the Y in the oxide scale and how these initial oxide products influence the long-term oxide growth rate and adhesion of the oxide scale. It is shown that the growth kinetics and adhesion of the oxide scale are to a large extent determined by the amount and distribution of Y-oxide inclusions in the scale. Oxide scales with a regularly distributed and larger amount of small Y-oxide inclusions are faster growing, but perform better in thermal cycle tests. Transient oxide products like metastable aluminas (γ- or θ-Al2O3) and spinels do not have a large influence on long-term oxidation behaviour except when the oxide scale is virtually free of Y-oxide inclusions due to Y depletion. Then, the nucleation of metastable aluminas at the onset of oxidation may be beneficial due to a slower-growing alumina scale with a larger grain size after transformation into α-Al2O3. Spinel oxides are considered detrimental for long-term oxidation performance, but their amount is usually limited on most types of MCrAlY coatings due to the rapid supply of Al along the numerous grain and phase boundaries in the fine-grained coatings. The key instruments that can be employed to manipulate the oxidation behaviour of MCrAlY coatings are: (i) the composition and size of the main phases in the coating (γ-Ni and β-NiAl), (ii) the initial distribution, chemical state (metallic or oxidized) and reservoir of the Y in the coating, (iii) the composition and roughness at the coating surface, and (iv) the oxidation conditions during post-deposition heat treatment (temperature, time, partial oxygen pressure).

Effect of surface condition on the oxidation behaviour of MCrAlY coatings

Coatings of the MCrAlY type (M = Ni, Co) are commonly used as overlay coatings and as bond coats (BC's) for ceramic thermal barrier coatings (TBC's) in industrial gas turbines. During high temperature service the MCrAlY coatings form aluminium based surface oxide scales. The technologically most relevant properties of the oxide scales, growth rate and adherence do not only depend on the exact MCrAlY composition but also on the surface condition after coating manufacturing. Depending on the coating manufacturing process, the MCrAlY surfaces may substantially differ in roughness. In addition the coatings may be subjected to surface treatments (e.g. shot peening, grinding or smoothening prior to deposition of TBC). In the present work the effect of the surface roughness commonly prevailing in the case of vacuum plasma sprayed MCrAlY coatings on the morphology and composition of the alumina scales formed during high-temperature oxidation was studied. For this purpose free standing coatings with rough and polished surfaces were oxidised in the temperature range from 800 to 1100 °C with exposure times up to 1000 h. The surface scale composition and morphology were analysed by optical metallography and SEM. Fluorescence spectroscopy was used for stress measurements in the oxide scales. It has been found that the oxide scales formed on rough surfaces of MCrAlY coatings have an intrinsically different morphology and growth rate compared to those formed on the flat ground surfaces.