Papers by Keyword: MCrAlY Coating

Abstract: Plasma spraying technique was used to prepare MCrAlY coating on TiAl alloy surface, the coating was treated through the laser remelting process, and then scanning electron microscope (SEM) and X-ray diffractometer (XRD) were used to analyze surface topography and phase composition before and after hot corrosion behavior of the coating. Results indicate that after laser remelting treatment, lamellar structure of the coating disappears with improved compactness and most defects like pores and inclusion eliminated. Plasma sprayed MCrAlY coating can significantly improve hot corrosion property of TiAl alloy, which can be further improved after laser remelting. MCrAlY coating mainly experiences surface oxidation reaction and internal sulfurization reaction during the hot corrosion in the high-temperature molten salt, which mainly generate corrosion products such as Al₂O₃, Cr₂O₃, NiO, NiCr₂O₄, Ni₃S₂ and CrS.

Abstract: To study effects of nano-Al2O3p (nanometric Al2O3 particles) on high temperature frictional wear behaviors of the NiCoCrAlY coating (a high temperature protective coating), three NiCoCrAlY coatings strengthened by different contents of nano-Al2O3p were prepared on a Ni-based super alloy by using laser cladding technology and their dry frictional wear behaviors at 500 °C in static air were investigated. The comparison was made with the coating without nano-particles. The results show that the frictional coefficient of the coatings increased with the hardness after adding nano-Al2O3p and the wear resistance of the coatings was enhanced. Among nano-Al2O3p-added coatings, the coating added with 1.0 wt% nano-Al2O3p performs best. The wear rate of the coating is only 58% of the coating without nano-particles. Moreover, the improvement of the oxidation behavior plays an important role in improving the high temperature wear resistance.

Abstract: Superalloy coating was deposited by cold-spraying using a commercial NiCoCrAlTaY powder. The coating microstructure was investigated by scanning electron microscopy and X-ray diffraction to reveal the change of the b-NiAl phase in the as-received powder particle during coating deposition. The oxidation behavior of the cold-sprayed MCrAlY coating and its microstructural evolution during the isothermal treatment were examined. The results show that significant microstructural change occurred to NiCoCrAlTaY superalloy during cold spraying and the thermal exposure. The intensive plastic deformation upon high velocity impact of spray particles results in transformation of b-NiAl to the matrix phase, forming metaltable b-NiAl depletion zones (b-PDZs) which are distributed around the boundaries of deposited particles in the coating. The central part of the deposited particles with limited deformation retained the original phase constitutions of the starting powder. The b-phase with fine grains is re-precipitated uniformly in the areas in b-PDZs in the as-sprayed coating during high temperature exposure. A stable Al2O3 scale was formed on cold-sprayed NiCoCrAlTaY during oxidation possibly due to active b-PDZs on the top surface of the coating.

Abstract: TiAl based alloys are promising candidates for structural applications at high temperature. However, the poor oxidation resistance above 800oC obviously restrains their applications. Although NiCrAlY overlay coatings can remarkably improve the high temperature oxidation resistance of TiAl, serious inward diffusion of Ni from the coating to the substrate occurs which could reduce the lifetime of the coating/substrate system. Apparently, the development of interdiffusion barrier could overcome the disadvantage of the NiCrAlY/TiAl system. In this work, Ta, TiN and Cr2O3 interlayers were deposited between NiCrAlY coating and γ-TiAl substrate as diffusion barrier (DB). The interdiffusion behavior of the TiAl/DB/NiCrAlY system was investigated at 1000°C. The results showed that the metallic and nitride interlayers cannot retard the interdiffusion of Ni effectively. As an active diffusion barrier, the oxide interlayer obviously suppressed the inward diffusion of Ni from the coating to the substrate by the formation of alumina-rich layers at both the TiAl/DB and DB/NiCrAlY interfaces.

Abstract: MCrAlY coatings were deposited on Ni-based IC6 superalloy by HVOF(high velocity oxy-fuel), and the ZrO2 coatings were prepared using plasma spray technique. The oxidation kinetic curves of the IC6 superalloy were obtained after the heat treatment. The results indicated that the oxidation resistance of IC6 superalloy was markedly improved by coatings with MCrAlY and MCrAlY+ZrO2. The oxidation product of the MCrAlY coating mainly consisted of α-Al2O3,NiO at 1100°C, oxidation product was not found on the MCrAlY+ZrO2 coating at 1100°C.

Abstract: A cast nickel based superalloy M963 was coated by high-velocity oxy-fuel (HVOF) spraying process. The effect of HVOF MCrAlY coating on thermo-mechanical fatigue (TMF) and isothermal fatigue (IF) in M963 was studied to understand fatigue life and failure mechanisms in coated and uncoated M963 alloy. Two types of TMF tests, i.e. in-phase (IP) and out-of-phase (OP), were performed in temperature range of 450~900°C, and IF tests were conducted at 900°C. It was found that the coating had a detrimental effect on fatigue life under OP TMF, while a beneficial effect of the coating existed under IP TMF and IF. Crack initiation time in the coated specimen was shorter than that in the uncoated specimen and the former’s crack density was higher than the latter’s one under OP TMF. The relationship of deformation and fracture response with fatigue life was discussed based on microscopic analysis.

Abstract: In this work we have evaluated the effect of platinum (Pt) pre-coating on microstructure evolution of MCrAlY coating during isothermal oxidation, to improve high temperature oxidation and corrosion resistance of the nickel-based superalloy MAR M247-DS. Pt was deposited on the superalloy by electroplating before the MCrAlY coating by a vacuum plasma spray, which was compared to the superalloy without Pt pre-coating. The samples were subjected to isothermal oxidation in a box furnace at 1100°C for 25–150 hrs. The weight gain and loss caused by the oxidation of samples was recorded, and the variation of surface layer and interface morphology were observed before and after the oxidation tests. The length decrement of cross section is remarkably low on according to the precipitation of (Ni,Pt)Al phase by Pt pre-coating. Also, the amount of voids at interface between the coating and the superalloy, which would be introduced as a processing defect, is reduced by Pt pre-coating.

Abstract: Two MCrAlY overlay coatings (NiCrAlYSi and NiCoCrAlYHf) were deposited on a DS superalloy of Ni-Al-Cr-Co-W-Mo-Ta-Hf system by vacuum arc deposition method. The two coatings show a good protection for the DS superalloy during the isothermal oxidation test at 1150 for 100 h. A continuous alumina-based scale was formed at the surfaces of the coated specimens after oxidation. Y2O3, NiO and SiO2 are also detectable in the oxide scale. For the Hf-bearing coating, some HfO2 particles exist at the interface between the coating and the substrate. But after the cyclic thermal oxidation at 1100°C, the Hf-free coating exhibited better oxidation resisitance than the Hf-bearing coating. The HfO2 blocks between the coat and the substrate may be the crack initiation which leads to the failure of the coating.

Abstract: Rotating components used in the hot sections of land-based gas turbine are exposed to severe environment of several ten thousands operating hours above 1100
. To protect such components against high temperature oxidation an intermediate bond coat is applied, typical of a MCrAlY-type metal alloy. Various processing methods have been studied for bond coat deposition. This study is concerned with the cyclic oxidation behavior of CoNiCrAlY coatings. Coatings were deposited by a vacuum plasma spray and high-velocity oxygen fuel method on a nickel-based superalloy (GTD-111). Cyclic thermal oxidation test condition is at 1100
in ambient air for various periods of time. Tests were used to evaluate the oxidation resistance of the spray-coated specimens. The microstructure and morphology of as-sprayed and of tested specimens were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The oxide phases formed on the coating surface are NiO, CoCr2O4, and Al2O3. The nickel oxide of them was to be dominant with increasing cycles. The differences in microstructure and phase composition in the interface with coating layer are reported. The influence of coating process methods on coating characteristics and degradation mechanisms is discussed. The HVOF coating with the splats was more resistant on the high temperature oxidation than the VPS coating.