Modified Aluminide Coatings Research Articles

Hafnium Modified Aluminide Coatings Obtained by the CVD and PVD Methods

Zirconium, hafnium or platinum modification of NiAl phase increases the oxidation resistance of diffusion aluminide coatings. Small hafnium addition to aluminide coatings decreases the oxidation rate of nickel superalloys at 1100 °C.The paper presents comparison of structures of hafnium modified aluminide coatings deposited in two different ways on pure nickel. In the first way double layers of hafnium 3 μm thick and aluminum 3 μm thick were deposited by the EB-PVD on the nickel substrate. The double layers were subjected to diffusion treatment at 1050 °C for 6 h and 20 h. In the second method, a hafnium layer was deposited by the EB-PVD method, whereas aluminum was deposited by the CVD method. The obtained coatings were examined by the use of an optical microscope (microstructure and coating thickness) and a scanning electron microscope (chemical composition on the cross-section of the modified aluminide coating). Microstructures and phase compositions of coatings obtained by different methods differ significantly. Diffusion treatment for 6 h leads into formation of the Ni5Hf phase. The elongation of the diffusion time from 6 to 20 h decrease the volume fraction of the Ni5Hf phase. An aluminide coating deposited by the CVD method at 1050 °C at the nickel substrate with prior hafnium layer (3 μm thick) has a triple zone structure. An outer zone consists of the NiAl phase, a middle zone consists of the Ni3Al phase, and the Ni(Al) phase forms an inner zone, close to the substrate. An NiHf intermetallic phase is between the outer and the middle zone, whereas Ni3Hf is between the inner zone and the substrate.

Zirconium Modified Aluminide Coatings Obtained by the CVD Method

The paper presents the comparison of the structures of the zirconium modified aluminide coatings deposited on pure nickel by the CVD and method for different conditions, that is the gas flow and the time of deposition. The time of the aluminizing processes varied from 1.5 to 10 hours and the gas (HCl) flow varied from 0.4 to 1.4 l/min. Aluminum was deposited from the AlCl3 and zirconium from the ZrCl3 gas phases at 1040 oC. The obtained coatings were examined using an optical microscope (microstructure and coating thickness) a scanning electron microscope (chemical composition on the cross-section of the modified aluminide coating) and an XRD phase analyzer. Microstructures and phase compositions of coatings obtained at different process parameters do not differ significantly. In all cases, it is a triple zone structure. Chemical compositions of zones correspond to β-NiAl, γ’-Ni3Al and γ-Ni (Al) phases. The elongation of the time of zirconium-aluminizing process from 1.5 to 10 hours leads to the increase of the coating thickness from 30 to about 60 μm. The EDS analysis and concentration profiles of the cross-section of the coating showed the nickel outward diffusion from the substrate and the aluminum inward diffusion from the surface to the nickel substrate. In coatings deposited at a slow gas flow porosity was observed on the border between β-NiAl and γ’-Ni3Al layers. In coatings deposited at fast gas flow, zirconium does not form any inclusions but dissolves in the matrix. The Kirkendall porosity was not observed.

Microstructure and High Temperature Oxidation Behaviour of Zr-Doped Aluminide Coatings Fabricated on Nickel-based Super Alloy

In recent years, addition of rare earth elements such as Zr, Y and Hf to pack diffused aluminide coatings has been studied and practiced aiming to improve high temperature oxidation resistance of the coatings. In this work cementation packs containing ZrOCl2.8H2O as the activator and source of zirconium as well as mechanically alloyed 30% Al -Cr compound as the source of aluminum plus alumina as filler material were used to produce zirconium doped aluminide coatings. The cyclic oxidation of the coatings was measured at 1100°C and each cycle was consisted of one hour heating in furnace and 10minutes cooling in the air.The results showed that the Zr modified aluminide coatings provide a better resistance against high temperature oxidation and the highest resistance belonged to the coating that was achieved by 5hours diffusion process in a pack that contained 2% activator.

Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700°C

Two Co modified aluminide coatings with different Co contents were prepared by pack cementation process and above-the-pack process. The hot corrosion tests of the two coatings were performed in mixed salts of 75 wt.% Na2SO4 + 25 wt.% K2SO4 and 75 wt.% Na2SO4 + 25 wt.% NaCl at 700 °C, with a simple aluminide coating as the reference coating. X-Ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM) with energy dispersive X-ray spectrometer (EDS) were used to characterize the coatings and the corrosion scales. Results indicate that the addition of Co improves the hot corrosion resistance of the simple aluminide coating in the mixed sulfate salts, for the sulfide as well as its eutectic of cobalt are more stable, and possess higher melting points than those of nickel. While in the mixed salt containing chloride, the coating with medium Co content possesses the best corrosion resistance, primarily because the nitrides formed in the deposition process deteriorate the corrosion resistance of the coating with highest Co content.

The isothermal and cyclic oxidation behaviour of two Co modified aluminide coatings at high temperature

The isothermal and cyclic oxidation behaviour of two Co modified aluminide coatings together with the simple aluminide coating were performed at 1000°C and 1100°C. All the three coatings show a much lower oxidation rate compared with the bare alloy. Results also indicate the addition of Co to the aluminide coating decreases the oxidation resistance slightly. It can be ascribed to that Co is easier to be oxidized than Ni at high temperature, and the Cr(W) rich phases which could act as a diffusion barrier are less in the coating with higher Co content.

Preparation and hot corrosion behaviour of two Co modified NiAl coatings on a Ni-based superalloy

Abstract Two Co modified aluminide coatings with different Co contents were prepared by a combined method of pack cementation and chemical vapour deposition. The type-I hot corrosion tests of the two Co modified coatings together with the simple aluminide coating were carried out at 900 °C. Results indicate that the addition of cobalt could improve the corrosion resistance of the coatings in the sulphate salts, but exacerbate the degradation of the coatings when chloride salt gets involved. The formation mechanism of the Co modified aluminide coatings and the corrosion mechanism during the corrosion test have been discussed as well.

Zirconia Modified Aluminide Coatings Deposited by VPA and CVD Methods

The article presents the experimental trial to connect aluminizing methods: VPA and CVD. Inconel 713 alloy was used as the base material. The purpose of the process was to obtain a layer modified with more Zr than the coatings deposited by CVD. The conducted tests revealed intercrystalline corrosion in the area of β-NiAl phase granules, which indicates that precise selection of processing conditions is the key factor for Zr-modified aluminizing. The resultant microstructural faults of the layer, however, didn’t impair the properties PS-PVD-deposited ceramic layer. The zirconium content in the outer zone equalled 2-3 at.%. in the second variety of the process. In future the described coating may form an alternative for Pt-modified aluminide coatings.

Characterization of the Microstructural Degradation of Platinum Modified Aluminide Coating

This work presents the degradation mechanism of the platinum modified aluminide diffusion coating of the GTD 111 SC Ni-base superalloy turbine blades after 16000 h of exposition at different thermal cycles (critical heating temperatures reported ~1000°C and 1120°C). The initial coating condition and the evolution of degradation were characterized applying conventional microscopy and backscatter scanning electron microscopy. The initial microstructure condition consisted of a two phase coating (intermetallics PtAl2 dispersed in a matrix β-(Ni,Pt)Al). The major microstructure degradation was associated with: intermediate and interdiffusion zones growing, partial transformation of β-(Ni,Pt)Al to γ´-Ni3Al, and the dissolution of the intermetallic PtAl2 resulting in a more brittle single phase β-(Ni,Pt)Al coating. The degradation facilitates spallation and crack initiation, resulting in the loss of the coating and by consequence the blade failure.

The Influence of Turbine Blade Geometry and Process Parameters on the Structure of Zr Modified Aluminide Coatings Deposited by CVD Method on the ZS6K Nickel Superalloy

The Zr modified aluminide coatings is an alternative concept for replacing Pt-modified aluminide bondcoat for thermal barrier coatings. In the paper the influence of process parameters on the chemical composition and the thickness of aluminide coatings will be presented. The zirconia-doped aluminide coating was deposited on turbine blades made from ZS6K nickel superalloy during the low-activity CVD process. In recent work the influence of turbine blade geometry on thickness of coating was observed. The thickest coating was observed on the trailing and leading edge on the blade cross-section. In the conducted research, the light and scanning electron microscopy were used as well as the EDS chemical composition microanalysis.

Zirconium Modified Aluminide Coatings Obtained by the CVD and PVD Methods

The paper presents the comparison of the structures of the zirconium modified aluminide coatings deposited on pure nickel by the CVD and PVD methods. In the CVD process, zirconium was deposited from the ZrCl3 gas phase at the 1000°C. Zirconium thin layer (1 or 7 μm thick) and aluminum thin layer (1.0, 0.7 or 0.5 μm thick) were deposited by the EB-PVD method. Deposition velocity was about 1 ?m/min. The layers obtained by the Electron Beam Evaporation method were subjected to diffusion treatment for 2 h in the argon atmosphere. The obtained coatings were examined by the use of an optical microscope (microstructure and coating thickness) a scanning electron microscope (chemical composition on the cross-section of the modified aluminide coating) and XRD phase analysis. Microstructures and phase compositions of coatings obtained by different methods differ significantly. NiAl(Zr), Ni3Al and Ni(Al) phases were found in the CVD aluminide coatings, whereas Ni5Zr, Ni7Zr2 and γNi(Al,Zr) were observed in coatings obtained by the PVD method. The results indicate that the microstructure of the coating is strongly influenced by the method of manufacturing.

Microstructure degradation of simple, Pt- and Pt + Pd-modified aluminide coatings on CMSX-4 superalloy under cyclic oxidation conditions

The paper presents results of simple, Pt/Pd- and Pt-modified aluminide coatings' cyclic oxidation-induced degradation analysis. The coatings were deposited by Pt and Pt+Pd electroplating, followed by vapor phase aluminizing at 1050°C. Cyclic oxidation tests were performed at 1100°C in 23hcycles. Microstructural and phase analysis conducted using XRD, SEM, EDS and EBSD methods revealed that the oxide layers that formed on the coatings were composed of three distinctive types of oxides growing according to a specific pattern which is described. The oxide layer that formed on the simple aluminide coating exhibited low adhesion in comparison to Pt- and Pt,Pd-modified aluminide coatings which managed to maintain an adherent oxide layer that contained higher amount of desirable α-alumina. The Al-depleted βNiAl grains remained much larger in the modified aluminide coatings, even after failure. What is more, stripes characteristic for martensitic transformation were discovered in the β phase grains in all coatings.

Microstructure and Oxidation Behavior of Modified Aluminide Coating on Ni3Al-based Single Crystal Superalloy

Al, Al-Si and Cr-Al-Si coatings are prepared onto IC20 alloy by powder pack cementation to improve the oxidation resistance of the alloy. The isothermal oxidation behaviors of the coatings and the uncoated IC20 alloy at 1 100 °C are comparatively investigated. For the coatings, less weight gains are obtained compared to the uncoated alloy after 100 h thermal exposure. The Al-Si coating exhibits the best oxidation resistance, while the addition of Cr accelerates the scale spallation. Phase transformation from β-NiAl to γ and γ′ occurs in the coating after oxidation for extended periods. The oxidation protection and degradation mechanisms for these coatings are discussed.

Cyclic-Oxidation of RexOy-Modified Aluminide Coating

Reactive reactive element oxide RexOy (Re=Ce, Y)-modified aluminide coatings were developed by aluminizing the as-codeposited Ni-RexOy composite film using pack cementation method at 1100°C for 4 h. By comparison, aluminizing was also performed with the same condition on an as-deposited Ni film without RexOy particles. SEM/EDAX and TEM results indicated that the co-deposited CeO2 or Y2O3 particls were homogeneously dispersed in the finer-grain nanocrystalline Ni grains. The cyclic oxidation in air at 900°C indicated that the RexOy -modified aluminide coatings were profoundly spallation resistance as compared to the RexOy -free coatings due to the formation of a continuous adherent α-Al2O3 scale.

Improving high temperature oxidation and hot corrosion resistance of Ni-base super alloy INC738LC with Cr-Si-RE modified aluminide coatings produced by single step pack cementation process

The Cr-Si-Zr (or Y) modified aluminide coatings were produced on Ni-base INC738 by a novel single step pack cementation process and their behavior was evaluated in cyclic oxidation and hot corrosion conditions. A dual layer pack with different composition was applied to produce Cr-Si-Zr or Cr-Si-Y modified aluminide coatings. A mixture of salts 75%Na2SO4-20%NaCl-5%V2O5 at 950°C was used as a hot corrosion environment. The results show that oxidation and corrosion rate of Cr-Si-Zr or Cr-Si-Y modified aluminide coatings produced by dual layer pack process are lower than those of uncoated or simple aluminide coatings. Coatings with higher Al content and uniformly dispersed Cr-Si rich phases show better hot corrosion resistance.

Some Properties of Platinum and Palladium Modified Aluminide Coatings Deposited by CVD Method on Nickel-Base Superalloys

Some Properties of Platinum and Palladium Modified Aluminide Coatings Deposited by CVD Method on Nickel-Base Superalloys

The Influence of Silicon Amount on Structure of Si Modified Aluminide Coating Deposited on Ti46Al7Nb Alloy by Slurry Method

The alloys based on the intermetallic phases from Ti-Al system are materials which, on the grounds of their resistance characteristics, could be widely used in automotive and aerospace applications. The insufficient oxidation resistance of this alloys could be increased by the coating silicon modification. The technology used in this investigation was the immersion in water-based slurry containing metal powders. MHI high-Nb (14 wt %) alloy has been used as the research material for the coatings produced. The water-based slurries containing Al and Si powders have been prepared with 0-80 wt % Si content. The diffusion treatment has been done at 950oC in 4h in the Ar atmosphere. The investigation has showed that the thickness of the coatings ranged from 30 to 40 m. The structure of the Si-modified aluminide coatings is as follows: (a) outer zone consisting of TiAl3 phase and titanium silicides, (b) middle zone consisting of columnar titanium silicides in phase TiAl3 matrix (c) the inner zone consisting of TiAl2 phase. The XRD phase analysis has confirmed Ti5Si3 creation and, in case of the high silicon content (above 20 wt %), also other silicides: types Ti5Si4,TiSi2,TiSi

Development and Oxidation of RexOy-Modified Aluminide Coating

Reactive reactive element oxide RexOy (Re=Ce, Y)-modified aluminide coatings have been developed by the first step of co-electrodeposition of Ni with CeO2 or Y2O3 particls on pure Ni and the subsequent step of diffusional aluminizing on electrodeposited Ni-CeO2/or Y2O3 composite films using pack cementation method at 1100OC for 4 hr. By comparison, aluminizing was also performed with the same condition on an as-deposited Ni film without RexOy particles. The oxidation in air at 900OC indicated that the RexOy (Re=Ce, Y)-modified aluminide coatings were profoundly oxidation resistance as compared to the RexOy (Re=Ce, Y)-free coatings. The effect of CeO2 or Y2O3 on the oxidation behavior is discussed in detail.

On oxidation behaviour of platinum aluminide coated nickel based superalloy CMSX-4

The effects of platinum modification on the isothermal oxidation performance of a chemical vapour deposition aluminide coating are examined. Platinum was electrodeposited to varying thicknesses (to a maximum of 16 μm) onto the CMSX-4 single crystal superalloy, before applying a high temperature/low activity aluminisation process. The oxidation performance at 1100°C improved monotonically with increasing Pt thickness; this was confirmed by thermal gravimetric analysis. The degree of rumpling of the alumina scale was also decreased with increasing platinum. The superior oxidation resistance of platinum modified aluminide coatings is a consequence of the β-NiAl formed during aluminisation containing an enhanced Al/Ni ratio, reduced levels of Co and Ti and decreased concentrations of the refractory elements Re, W and Ta. This seems to be due at least in part to the microstructural effects caused by the prior diffusion of platinum into the surface, which causes the generation of a γ-(Ni,Pt)/γ′-(Ni,Pt)3Al microstructure; the upper portion of the Pt diffused coating comprises predominantly the γ′-(Ni,Pt)3Al phase which is rich in Pt and Al and depleted in Co and the refractory elements W and Re. The ramifications of our findings for the design of bond coats required for thermal barrier coating systems are discussed.

Factors affecting the microstructure of platinum-modified aluminide coatings during a vapor phase aluminizing process

Platinum (Pt)-modified aluminide coatings were developed by electroplating a thin layer of Pt followed by an industrial vapor phase aluminizing process. The goal of this work was to systematically investigate the effect of critical coating process parameters (such as the electroplated Pt thicknesses, Al contents in Cr–Al nuggets, diffusion heat treatments) and substrates on the final Pt-modified aluminide coatings. Surface morphology and cross-section microstructure of the developed coatings were inspected and compared by using Optical Microscope, Scanning Electron Microscope (SEM) equipped with energy dispersive spectroscopy (EDS). Experimental results showed that the Al and/or Pt increase shall favor the formation of ξ-PtAl 2 phase; transformation of ξ-PtAl 2 into β-(Ni,Pt)Al phase can be obtained via a heat treatment process; Cr, Co elements in the studied Ni-base superalloy substrates did not show significant influence on coating outer layer microstructure; while substrate elements affect the microstructure of the coating interdiffusion layer.

Rumpling of Platinum Modified Aluminide Coatings during Thermomechanical Testing

The evolution in surface morphology of platinum modified nickel aluminide (Ni,Pt)Al oxidation coatings during thermo-mechanical testing has been evaluated. One type of test consisted of cyclic oxidation between an upper temperature of 1150°C and a lower temperature varying from room temperature to 1050°C. The other type of test was cycling between 1000°C/1150°C under an applied compressive stress. Profilometry using optical interferometry was used to quantify the surface “rumpling”. First and second-order statistical parameters including RMS roughness and the auto-correlation function were calculated from the profilometry measurements. The results indicate that the grain structure of the aluminide coating plays a major role in the early stages of rumpling and set its wavelength. Also, the superimposed compressive stress during thermal cycling leads to an asymmetry in the rumpling pattern with respect with the loading axis as well as cracking along the applied stress direction.