The Microstructure and Oxidation Resistance of The Microstructure and Oxidation Resistance of Slurry Method on Rene 80 Superalloy

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Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 719-723

Published Online July 2012 (http://www.SciRP.org/journal/jmmce)

The Microstructure and Oxidation Resistance of

Aluminide MeCrAlY-Modified Coatings Obtained by

Slurry Method on Rene 80 Superalloy

Marek Goral

Research and Development Laboratory for Aerospace Materials, Rzeszow University of Technology,

Rzeszow, Poland

Email: [email protected]

Received April 16, 2012; revised May 31, 2012; accepted June 20, 2012

ABSTRACT

The slurry method is one of the oldest techniques of deposition of aluminide coating on the nickel superalloy, titanium

alloys and steel. It is characterized by relatively low costs of its realisation and necessary equipment. This method en-

ables a simple modification of chemical composition of the coating through addition of different powders. The author

showed study on the possibility of modification of the Al-Si slurry chemical composition used for aluminide coating

deposition by addition of MeCrAlY powder. The slurry was deposited by immersion than the diffusion treatment at

950˚C for two hours was applied. The thickness of obtained coatings was in the range of 30 - 65 μm.

Keywords: Slurry Method; Aluminide Coatings; Nickel Superalloy

1. Introduction

The diffusion aluminide coatings are used for protection

of the turbine blades made from nickel superalloys and

used as elements of aircraft and industrial gas turbines,

against oxidizing influence of exhaust gases [1]. The

most common technique used for aluminizing is the pack

cementation method. It involves putting the elements in

metal container and covering them with a powder which

contains an active powder (source of aluminium), neutral

filler material as well as halide activator. The process is

performed in protective atmosphere of argon, in the

chamber furnace [2]. The gas methods (out of pack, gas

phase aluminizing) are currently used in the aerospace

industry. The blades are located in those methods over

the powder or granules containing aluminum. The alu-

minum fluoride is most commonly used as an activating

agent. The aluminizing is conducted in retort furnace in

the temperature of 900˚C - 1050˚C and the argon and

hydrogen protective atmosphere for 2 - 6 hours [3,4]. The

most modern method of obtaining aluminide coatings is

the chemical vapour deposition (CVD method). The

halogenide Al activator (aluminium chloride) is created

in CVD process—as opposed to other methods—during

chemical reaction between HCl and aluminum granules,

in the external heated generator. The generated gas is

introduced to the retort, where the coated elements are

placed. The further development of diffusion aluminizing

methods is focused on the modification of aluminide

coatings by other elements—zirconium, hafnium, palla-

dium, platinum and silicon [5,6]. The slurry method has

been used, among other described methods, for a couple

of decades. It involves the deposition of organic or non-

organic slurry, which contains the aluminum powder, on

the blades surface, followed by the drying process. The

aluminide coatings are formed as a result of a diffusion

heat treatment process, in the atmosphere of argon, hy-

drogen or in vacuum. The slurry method is used for

preparation of coatings on the turbine blades. It enables

to create the aluminide coating on the surface of regener-

ated blade. It is possible to modify the chemical compo-

sition of the slurry by adding the powders with different

chemical composition mainly by silicon. Praxair com-

pany is one of the companies which manufactures this

kind of slurry (J. Sermaloy) [7]. The requirements con-

cerning the usage of chromium compounds, due to their

carcinogenic properties, led to development of new kinds

of slurries, based on organic binders. Product manufac-

tured by Ceral company—LSR SIALOY is an example

of this kind of slurry [8]. The slurry method enabled the

production of high temperature resistant coatings on dif-

ferent materials. This technique is used to obtain, except

the nickel superalloys, the diffusion coatings on the cast

steel or intermetallic γ-TiAl alloys [9,10]. The conducted

till now research on modifying the diffusion aluminide

coating obtained with slurry method, by addition of Me-

M. GORAL

720

CrAlY to commercial slurry containing aluminum and

silicon powders [11]. The authors present the techniques

of obtaining the annealed coatings in a reductive hydro-

gen atmosphere (instead of argon used so far) involving

the modification of components ratio.

2. Experiments

The process of aluminide coating deposition was per-

formed on the Rene 80 nickel superalloy. Its chemical

composition is presented in Table 1. The cylinder-shaped

samples, with a diameter of 14 mm, were sandblasted

and washed. The MeCrAlY powder was obtained from

the typical AMDRY 995C powder used for plasma spray-

ing during the milling process. The chemical composi-

tion of powder is presented in Table 1. After the milling

process, the final grain size in the MeCrAlY powder was

approx. 20 μm. The samples were coated by immersion

in the slurry. Addition of the MeCrAlY powder into a

typical commercial slurry Ceral 10 were done in 1:2 ratio.

The drying process was conducted for 24 h in the tem-

perature of 80˚C. The diffusion heat treatment process

was conducted in the retort furnace for 2 hours into hy-

drogen atmosphere condition (flow 2 l/min) and in the

temperature of 950˚C. The last stage of coating deposi-

tion process was sandblasting and washing to remove the

slurry residue.

The microstructure analysis was conducted with a use

of the Hitachi S-3400 scanning electron microscope con-

nected with chemical composition microprobe analysis

(EDS) manufactured by Thermo company.

The cyclic oxidation test was performed in chamber

furnace in static laboratory air in the temperature 1100˚C.

The 23-hour heating cycles were used. The mass meas-

urements were conducted on the analytical balance, with

accuracy of the order 10−4 g. The moment of mass de-

crease below the initial mass is considered to be a resis-

tance criterion.

3. Results

The microstructure analysis of obtained coatings con-

firmed a presence of many cracks. The thickness of coat-

ing was influenced by addition of MeCrAlY powder. In

case of slurry obtained from Al-Si, the thickness was

approx. 65 μm. With an increase of MeCrAlY powder

content, there was a decrease of coating thickness. In the

case of adding the MeCrAlY powder into Al-Si slurry

with the 1:2 ratio, the coating thickness was approx. 34

μm.

3.1. The Microstructural Analysis

Three zones could be observed (Figure 1) in a structure

of the coating obtained from the commercial Al-Si slurry.

The thickness of the outer zone presented on Figure 1(a)

(Zone 1) was of approx. 5 μm. The chemical composition

analysis, confirmed presence of 44 at% of silicon and 34

at% of chromium in this point. Moreover, the titanium,

molybdenum and nickel in the amount of 4 - 5 at% were

also present in this area. In the lower Zone 2 with a

thickenss of approx. 55 μm many precipitates with the

high silicon content (Figure 1(a), Table 2 pt 2) were

observed. Those precipitates were located uniformly in

the whole area of Zone 2 (Figure 1(b), Table 2 pt 4). The

precipitate matrix (Figure 1(a), Table 2 pt 5) contained

Table 1. The nominal chemical composition of Rene 80 alloy [wt%].

Material Ni Co Cr W Mo Al Ti Zr C Y

Rene 80 Bal. 9.5 14 4 4 3 5 0.06 0.17 -

MeCrAlY AMDRY 995C powder Bal. 38.5 21 - - 8 - - - 0.5

Table 2. The results of chemical composition analysis in areas presented on Figure 1.

Element at%

Point Al Si Ti Cr Co Ni Mo W

1 6.04 44.13 4.79 34.13 - 1.43 5.32 4.16

2 40.89 13.47 1.26 3.86 5.08 34.60 0.50 0.33

3 42.30 10.32 1.10 3.29 5.39 36.33 0.55 0.73

4 28.96 11.13 1.17 28.53 3.06 23.24 1.89 2.01

5 47.22 0.25 0.72 2.76 4.84 43.48 0.43 0.30

6 20.82 - 11.10 27.76 6.64 26.72 3.27 3.70

7 11.31 - 10.83 6.69 6.27 62.30 1.24 1.36

M. GORAL 721

Zone 1

Zone 2

(a)

Zone 2

Zone 3

(b)

Figure 1. Microstructure of the coating obtained on the

Rene 80 superalloy from a slurry contained 100 wt% of

Al-Si powder. The coating was obtained during diffusion

heat treatment at 950˚C/2 h under hydrogen atmosphere

condition with marked points of chemical composition mi-

croanalysis.

approx. 47 at% of aluminum, 43 at% of nickel, approx. 5

at% Co as well as 3 at% of Cr. Presence of Ti, Si, Mo

(with a concentration <1 at%) was also confirmed. In the

inner zone (marked as Zone 3 on Figure 1(b) pt 6, 7) the

existence of silicon was not observed. In the 6th point

(Figure 1(b), Table 2) the aluminum concentration was

approx. 20 at%. For chromium and nickel it was approx.

27 at%.

In the interface zone, between inner Zone 2 and base

material (Figure 1(b), Table 2 pt 7), the concentration of

aluminum was approx. 11 at%, and for nickel –62 at%.

The titanium content in this area was approx. 10 at%. For

chromium and cobalt it was approx. 6 at%.

The coating obtained form the slurry which contains

the addition of MeCrAlY powder were characterized by

a lack of outer zone rich in silicon and chromium. Its

smaller thickness was also observed. Two basic zones

were observed in theirs structure (Figure 2). The layers

with 33% (mass) content of MeCrAlY powder had pre-

cipitates in its outer zone (marked as “1” on Figures 2

and 3) and in the inner zone with the thickness of approx.

5 μm (marked as “2” on Figure 2). The chemical com-

position analysis conducted for precipitates in the outer

zone (marked as “1”) of a coating formed form the slurry

with an addition of 33% (mass) of NiCrAlY powder re-

vealed a presence of the silicon (approx. 19 at%) and

chromium (approx. 45 at%, Figure 2(a) pt 1, Table 3).

In the analyzed area, the aluminum content was approx.

19 at% and approx. 12 at% for nickel. The analysis of the

basis in the outer zone “1” of the coating (Figure 2(a) pt

2, Table 3) revealed a high aluminum content—approx.

52 at% and 37 at% for nickel. The chemical composition

Zone 1

(a)

Zone 2

(b)

Figure 2. Microstructure of the coating obtained on Rene 80

superalloy from slurry containe d 66 wt% Al-Si and 33 wt%

of NiCrAlY powder. The coating was created during diffu-

sion heat treatment at 950˚C/2 h under hydrogen atmos-

phere condition with marked points of chemical composi-

tion microanalysis.

M. GORAL

722

analysis conducted in the area of the inner Zone 2 (Fig-

ure 2(b) pt 3, 4, Table 3) showed a aluminum concentra-

tion of 44 - 51 at%. The precipitate matrix in this area

was characterized by nickel content of approx. 40 at% (pt

4 on Figure 2(b), Table 3) as well as by the existence of

chromium and cobalt. In the inner zone (Figure 2(b) pt 5

and 6) the aluminum content was approx. 13.6 - 14.88

at%. Moreover, the chemical composition analysis by

EDS method, revealed a high chromium amount (34 - 45

at%) and a presence of titanium, molybdenum and tung-

sten.

3.2. Results of Oxidation Test

The results of mass change measurement during cyclic

oxidation is presented on Figure 3. Tests performed in

the temperature of 1100˚C revealed the increase of cor-

rosion resistance of the Rene 80 alloy with deposited

aluminide layers. The mass of the sample without the

protective coating dropped below the initial mass after

the 4th cycle. The samples with the protecting coating

increased above the initial mass after the 4th cycle. There

were no mass change observed for the samples covered

with commercial Ceral slurry as well as the for those

modified with the multicomponent MeCrAlY powder.

4. Discussion

In the article authors presented new method of chemical

composition modification of the aluminide coatings ob-

tained by slurry method. The conducted research showed

a possibility of introduction of the conventional Al-Si

Table 3. The results of chemical composition analysis in areas presented on Figure 2.

Element at%

Point Al Si Ti Cr Co Ni Nb Mo W

1 52.08 1.60 0.83 3.09 5.28 37.12 - - -

2 19.42 19.34 1.85 44.55 2.06 11.54 0.04 1.20 -

3 44.41 - 6.49 19.11 3.78 19.06 - 3.07 4.08

4 51.46 - 0.87 2.52 5.52 39.64 - - -

5 14.88 - 7.67 45.18 4.92 17.67 - 5.22 4.46

6 13.60 - 8.85 33.85 7.69 26.88 - 4.87 4.26

Figure 3. The results of mass change measurement during the cyclic oxidation process in the temperature of 1100˚C.

M. GORAL 723

slurry consisting of multicomponent MeCrAlY alloy.

However, one has met some difficulties resulting from a

high density of introduced MeCrAlY powder. One has

observed the sedimentation of heavier powder compo-

nents on the bottom of the vessel during the application

process. All obtained coatings were characterized by a

structure, which is typical for highly active process and

was created during the inward diffusion of aluminium.

After the diffusion heat treatment process one has also

observed a presence of many fractures in the coating re-

sulting from internal stresses. It could be caused by a

short time of diffusion annealing process (2 h), fast cool-

ing and using of hydrogen atmosphere, instead of argon

atmosphere used till now. It caused in the coatings with

the addition of NiCoCrAlY powder fractures and exfo-

liation of the outer layer with high content of silicon and

chromium. In the structure of all obtained aluminide

coatings, a few characteristic zones can be observed. In

the layer obtained form a commercial slurry containing

only aluminum and silicon powders, the outer zone was

rich in silicon and chromium, which could be a proof of

silicide formation. In all layers, one observed the zone

with fine porous precipitates with the diameter less than

1 μm. They contained silicon on the basis of the β-NiAl

intermetallic phase, with 50 at% of aluminum. In all lay-

ers, the inner zone with a thickness of approx. 5 μm was

observed. The result of cyclic oxidation tests showed an

increase of oxidation resistance of samples with alu-

minide layers. The research did not proved a significant

difference in oxidation resistance between layer made

from commercial Al-Si slurry and layer modified with

MeCrAlY powder. It was caused by the observed sedi-

mentation process of the powder. One observed no in-

fluence of the additional outer layer with high silicon and

chromium content, on the increase of oxidation resis-

tance of the layer created from the commercial Al-Si

slurry. The presented results indicate the significant dif-

ficulties in obtaining the aluminide coatings modified by

powders, which are made of metals being components of

MeCrAlY alloy and have higher density. It is necessary

to prepare a new binder, which ensures the possibility of

homogenization of the slurry and prevents the sedimen-

tation process. Searching for new methods of obtaining

thermal barrier coatings is a significant reason for con-

tinuing the research. The European Project “Particoat” is

a good example of it.

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