NiCoCrAlY Coatings Research Articles

Hot corrosion behavior of AIP NiCoCrAlY(SiB) coatings on nickel base superalloys

NiCoCrAlY(SiB) coatings were deposited on the nickel base superalloys DZ125 and DSM11 by arc ion plating (AIP). The corrosion behavior of the bare alloys and the protective coatings in molten 75 wt.% Na 2SO 4+25 wt.% K 2SO 4 at 900 °C in air was tested for 90 h. The results indicated that DZ125 superalloy suffered catastrophic corrosion after 5 h exposure and a thick porous oxide scale (500–600 μm) interspersed with Ni 3S 2 and alloy particles formed on the alloy. For the Cr-rich DSM11 alloy, a mild corrosion attack happened, but severe internal Al-rich oxidation occurred inside the alloy. With NiCoCrAlY and NiCoCrAlYSiB coatings, the hot corrosion resistance of the systems was improved due to the formation of an α-Al 2O 3 scale. The hot corrosion resistance of the coatings with additions of Si and B can be improved by promoting the growth of a dense and continuous α-Al 2O 3 layer in the initial stage and improving the adherence of the oxide scale to the coating in the subsequent hot corrosion process. The hot corrosion degradation mechanism of the NiCoCrAlYSiB coatings and the influence of the substrate materials on the behavior of the whole system were also investigated.

Hot corrosion behavior of AIP NiCoCrAlY(SiB) coatings on nickel base superalloys

NiCoCrAlY(SiB) coatings were deposited on the nickel base superalloys DZ125 and DSM11 by arc ion plating (AIP). The corrosion behavior of the bare alloys and the protective coatings in molten 75 wt.% Na2SO4+25 wt.% K2SO4 at 900 °C in air was tested for 90 h. The results indicated that DZ125 superalloy suffered catastrophic corrosion after 5 h exposure and a thick porous oxide scale (500–600 μm) interspersed with Ni3S2 and alloy particles formed on the alloy. For the Cr-rich DSM11 alloy, a mild corrosion attack happened, but severe internal Al-rich oxidation occurred inside the alloy. With NiCoCrAlY and NiCoCrAlYSiB coatings, the hot corrosion resistance of the systems was improved due to the formation of an α-Al2O3 scale. The hot corrosion resistance of the coatings with additions of Si and B can be improved by promoting the growth of a dense and continuous α-Al2O3 layer in the initial stage and improving the adherence of the oxide scale to the coating in the subsequent hot corrosion process. The hot corrosion degradation mechanism of the NiCoCrAlYSiB coatings and the influence of the substrate materials on the behavior of the whole system were also investigated.

Microstructural Observation of Scales formed on HVOF-sprayed NiCoCrAlY Coatings

High velocity oxy-fuel sprayed NiCoCrAlY coatings were oxidized between 1000 and in air, and the oxide scales were examined by XRD, SEM/EDS, and EPMA. The unoxidized coatings consisted mainly of `Al, with some -Ni. The major oxide formed on the coatings was . Additionally, (CoCr, ) spinels and coexisted. NiO was not found, despite of high amount of Ni in the coating. Below the oxide layer, internally formed existed.

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-Al 2O 3, NiAl 2O 4, NiO, as well as a small amount of NiMoO 4 and MoO 2. 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 MoO 3. After the NiCoCrAlY coating was deposited, the scales mainly contained α-Al 2O 3, when the specimens were oxidized at 900 °C, and α-Al 2O 3and Cr 2O 3 at 1050 °C. The formation of α-Al 2O 3 and Cr 2O 3 scales on NiCoCrAlY coating was directly responsible for improving oxidation resistance of the alloy IC-6.

The behavior of MCrAlY coatings on Ni 3Al-base superalloy

This work is concerned with NiCrAlY and NiCoCrAlY coatings deposited on the superalloy IC-6 (Ni 3Al-base superalloy) by arc ion plating (AIP). The results indicated that the presence of Al and Mo in alloy IC-6 impeded Cr atoms moving from coatings to substrates during the deposition process. As a consequence, the distribution of Cr is well proportioned in both NiCrAlY and NiCoCrAlY coatings. Vacuum heat treatment drastically increased diffusivities and the coating became more uniform. The interdiffusion also led to the phase transformation in the coatings. Although γ′-Ni 3Al and γ-Ni were still the major phases in the coating, their lattice constant a 0 decreased. The results of isothermal oxidation showed that the oxidation behavior of coatings obeyed the parabolic rate law. The oxidation resistance was not influenced very much by the presence of Co in the NiCrAlY coating at 900 °C static atmosphere. NiCoCrAlY coating had poorer oxidation resistance than NiCrAlY coating when they were exposed at 1000 °C.

High Temperature Damping Behavior of Plasma Sprayed NiCoCrAlY Coatings

There is a trend to design the turbine coating and the substrate as in integral, layered, engineering assembly. Under the harsh environment of the turbine engine, a failure in one component can quickly lead to failure in other components. Materials that are used in structural applications are prone to mechanical vibration, which, when not attenuated, will lead to fatigue of components and shortening of life cycle. Therefore, it is necessary to examine the thermal stability and dynamic mechanical properties of coatings under dynamic conditions. In addition to these noise reduction and vibration amplitude control motivated objectives, however, mechanical energy dissipation processes also find intrinsic applications in cases for which a thorough understanding of the mechanisms responsible for the damping response of the material is required. This article describes the damping behavior and mechanisms that exist in plasma sprayed NiCoCrAlY coatings.

Viscoelastic properties of plasma sprayed NiCoCrAlY coatings

A dynamic mechanical analyzer (DMA) was employed to study the damping behavior of plasma sprayed NiCoCrAlY coatings. Results revealed that at low frequencies (0.01–0.1 Hz), the coatings exhibited viscoelastic behavior (enhanced compliant elastic deformation above a certain transition temperature). At higher frequencies (1–100 Hz), anisotropic elastic behavior was observed. This observation confirmed the viscoelastic nature of the coating. A master curve for the storage modulus of NiCoCrAlY coating was successfully constructed using appropriate shift-factors. This master curve can then be used for finite element or design analyses to provide important data input. Results also indicated that the anisotropic elastic behavior of the NiCoCrAlY coating resulted from nickel diffusion (apparent activation energy of 270 kJ/mol), however, the mechanism for the low frequency viscoelastic behavior was not clearly established.

Why do EB-PVD NiCoCrAIY Coatings Oxidize Faster than their LPPS Counterparts?

Bead-peened unannealed EB-PVD and LPPS NiCoCrAlY coatings showed differing oxidation kinetics on isothermal heating at 1000°C. The unoxidized starting materials were investigated by DTA, XRD, TEM and EDS. The differences in oxidation kinetics are attributed to the formation of different TGOs. Whereas α-Al 2 O 3 formation is assumed to start on the LPPS coating at the beginning it is seemingly formed at a later stage on the EB-PVD coating. Early α-Al 2 O 3 formation on LPPS coatings is hypothetically attributed to the beneficial nucleation conditions of the dominating γ phase on the surface and to the advantageous fast diffusive transport conditions in the coating. The fast provision of elements for TGO formation originates from grain boundary diffusion provided by the small-grained LPPS coating, from accelerated diffusion through amorphous phases and by highly supersaturated phases and grains and along directed and narrowly spaced eutectic phase boundaries. The oxidation behavior of the EB-PVD coating is ruled by dominating β particles in γ and less diffusive supply of Al.

Plasma Spraying of Functionally Graded Yttria Stabilized Zirconia/NiCoCrAlY Coating System Using Composite Powders

Pre-alloyed and plasma spheroidized composite powders were used as the feedstock in the plasma spraying of functionally graded yttria stabilized zirconia (YSZ)/NiCoCrAlY coatings. The ball milling parameters of the composite powders and the plasma spraying parameters for preparing functionally graded materials (FMGs) coatings were optimized to obtain the best performance for the thermal barrier coatings (TBCs). Microstructure, physical, mechanical, and thermal properties of YSZ/NiCoCrAlY FGMs coatings were investigated and compared with those of traditional duplex coatings. Results showed that the advantages of using pre-alloyed composite powders in plasma spraying were to ensure chemical homogeneity and promote uniform density along the graded layers. Microstructure observation showed the gradient distribution of YSZ and NiCoCrAlY phases in the coating, and no clear interface was found between two adjacent different layers. Oxidation occurred during plasma spray and the resultant aluminum oxide combines with YSZ in a wide range of proportions. The bond strength of functionally graded coatings was about twice as high as that of the duplex coatings because of the significant reduction of the residual stresses in the coatings. The thermal cycling resistance of functionally graded coating was much better than that of duplex coating.

Reaction Diffusion Behaviors for Interface Between Ni-Based Super Alloys and Vacuum Plasma Sprayed MCrAlY Coatings

The object of this study is overlay coatings of MCrAlY alloy sprayed by a vacuum plasma spray (VPS) process for the protection against high-temperature corrosion and oxidation in the field of gas turbine components. Reaction diffusion behaviors at the interface between the MCrAlY coatings and the substrate, which have an important effect on coating degradation, have not always been clarified. Three kinds of substrate, equiaxis IN738LC, directional solidified CM247LC and single-crystal CMSX-2, and the four kinds of vacuum plasma sprayed MCrAlY coating have been selected for these experiments. The experimental results showed that the reaction diffusion layers consisted of aluminum compound layer and aluminum depleted layer, excepting that the aluminum depleted layer could not be observed in the case of CoNiCrAlY and NiCoCrAlY coatings. It also indicated that the diffusion thickness could be observed to follow a parabolic time dependence. The order of reaction diffusion rate was NiCrAlY > CoCrAlY > CoNiCrAlY > NiCoCrAlY independent of the substrates. A convenient computer-aided system was developed for analyzing the reaction diffusion behaviors at the interface between coating and substrate. It was also clear that the estimated results of long time diffusion behaviors by simulation analysis was in good agreement with experiments.

ガスタービン用コーティング設計システムの開発 （第３報） Ｎｉ基超合金とＭＣｒＡｌＹコーティング皮膜界面の反応拡散解析

A computer-aided interactive system for coating design has been developed, which enables to analyze conveniently the reaction diffusion of bonded materials. The object of this study is the overlay coatings of MCrAlY alloy sprayed by a low-pressure-plasma spray (LPPS) process for protection against high-temperature corrosion and oxidation in the field of gas turbine components. However, the reaction diffusion behavior at the interface between the MCrAlY coating and the substrate, which has an important effect on coating degradation, has not always been clarified. Three kinds of substrate, such as equiaxis IN738LC, directional solidified CM247LC and single-crystal CMSX-2, and four kinds of low-pressure-plasma sprayed MCrAlY coating were selected for the experiments.The experimental results showed that the reaction diffusion layers consisted of aluminum compound layer and aluminum decrease layer, basically. However, the aluminum decrease layer could not be observed in the cases of CoNiCrAlY and NiCoCrAlY coatings. It was also indicated that each diffusion thickness changed in parabolic time dependence. And the order of reaction diffusion rate was NiCrAlY>CoCrAlY>CoNiCrAlY>NiCoCrAlY independent of the kind of substrate. It was also clear that the estimation of long time diffusion behavior by simulation analysis was possible in comparison with the experiments.

X-ray photoelectron spectroscopy studies on the surface of NiCoCrAlY coatings before and after high-temperature oxidation

Abstract The identification of the chemical composition of the surface layer of NiCoCrAlY coatings prepared by electron-beam alloying, before and after oxidation at 1050 °C for 10 h, was carried out using X-ray photoelectron spectroscopy (XPS). The results show that, before oxidation, predominantly α-Al 2 O 3 is formed, but also other oxides, such as Cr 2 O 3 , NiFe 2 O 4 and CoAl 2 O 4 , when aluminium is lacking or depleted due to inappropriate selection of the parameters of electron-beam melting. After oxidation, α-Al 2 O 3 , Cr 2 O 3 , TiO 2 and SiO 2 are formed on the surface of the coatings; titanium and silicon diffuse outward to the surface of the coatings from the substrate during oxidation. Segregation of yttrium on the surface layer after 10 h of oxidation at 1050 °C is probably beneficial to the stabilization of Cr 2 O 3 at temperatures in excess of 1000 °C.

Laser Melting of Plasma Sprayed Nicocraly Coatings

MCrAlY coatings (M = Ni, Co, Fe alone or in combination) are currently used to protect gas turbine blades and vanes from oxidation and hot corrosion. The coatings are required to be fully dense and well bonded to the substrate. They are formed either by electron beam evaporation or by low pressure plasma spraying. Another possible route is to form them by normal atmospheric plasma spraying, followed by remelting with a laser to remove porosity and other defects. The feasibility of this approach has been investigated by melting plasma sprayed NiCoCr AIY coatings on a Nimonic 75 substrate. This work focuses mainly on the effect of various laser processing parameters on the structure of remelted layers. Preliminary data on the hot corrosion properties of the laser melted coatings are also presented.

Characterization of unmelted powder particles in vacuum plasma sprayed NiCoCrAlY coatings

Characterization of unmelted powder particles in vacuum plasma sprayed NiCoCrAlY coatings

Thermal fatigue resistance of heat-resistant alloys with protective coatings

1. Among coatings of NiCrAlY, CoCrAlY, and NiCoCrAlY the thermal fatigue resistance is highest for coatings of the NiCoCrAlY type under conditions of 100⇌950 °, th=1.5 min, Δɛ= 0.6-3.0%. 2. Thermal fatigue fracture of samples with protective coatings is multicentered in character. The type of fracture and crack development depend on the deformation in the cycle and the composition of the coating. 3. In coatings of CoCrAlY and Ni-15Al-35Cr-Y, with the lowest thermal fatigue resistance, all cracks propagate radially and pass into the base metal. The characteristic feature of the NiCoCrAlY coating with a β + γ structure, which has the highest thermal fatigue resistance, is branching of cracks and slow propagation. 4. The reduction in the ductility of the surface layer of the material with a coating leads to a 30–50% drop of the thermal fatigue resistance in air in the range of Δɛ> 0.8–1.0%. When deformation is reduced in the range of N>103(Δɛ < 0.6%) the thermal fatigue resistance is the same for coated and uncoated samples.

Fatigue crack initiation and propagation in electron beam vapor-deposited coatings for gas turbine superalloys

Thermal fatigue crack initiation and propagation in NiCoCrAlY-coated nickel base superalloys have been evaluated. At relatively low strain ranges, the cycles required to initiate coating cracking are a significant fraction of the total fatigue life of a component. Crack initiation occurs preferentially at pit and flake defects in the coating. The strain-temperature cycle shape has a significant effect on coating crack initiation; cycle I, which peaks tensile strains at the minimum temperature, is the most severe from a coatings standpoint. Cycle I thermal fatigue crack initiation and propagation in NiCoCrAlY coatings are dependent on the thermal expansion mismatch with the substrate. The rate of coating crack propagation is also dependent on coating thickness.