Kinetics Growth and Oxidation Resistance of Aluminide Coatings Deposited by the CVD Method on Re 80 Superalloy

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Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 853-857

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

Kinetics Growth and Oxidation Resistance of Aluminide

Coatings Deposited by the CVD Method on

Re 80 Superalloy

Marek Goral, Maciej Pytel, Kamil Dychton, Andrzej Nowotnik

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

Email: [email protected]

Received July 2, 2012; revised August 4, 2012; accepted August 19, 2012

ABSTRACT

The preliminary results of research on forming the aluminide coatings using CVD method were presented in the article.

The coatings were obtained in low activity process on the surface of Rene 80 superalloy. The microstructure analysis

and chemical composition analysis were performed applying different values of aluminizing process parameters. The

authors present in the article the results of oxidation resistance analysis of aluminide coatings which were obtained on

the surface of Rene 80 superalloy using various techniques. It was shown that the coating created during the CVD

process was characterized by a good oxidation resistance at the temperature of 1100˚C.

Keywords: CVD Aluminizing; Aluminide Coatings; Nickel Superalloy; Re 80

1. Introduction

Diffusion aluminizing is one of the basic techniques of

protecting the turbine blades surfaces against the oxidiz-

ing influence of fumes environment. The pack cementa-

tion-, out-of-pack-, slurry- and CVD methods of alumin-

izing is one of most commonly used [1,2]. Regardless of

the aluminizing technique the mechanism of the coating

formation consist in the chemical reaction between the

halide containing aluminium and the base material. The

β-NiAl phase is created as a result of aluminium diffu-

sion. Goward and Boone [3] used pack cementation me-

thod and showed two possible courses of growth mecha-

nism for aluminide coatings. Using the pack containing

15% of Al, 3% of amonium chloride and 82% of granular

Al2O3 it has been observed that the Ni2Al3 phase is cre-

ated in the first instance on the surface on the nickel su-

peralloy. During this high activity process, the inward

aluminium diffusion is the dominant growth mechanism.

By using the powder with 15% of Ni2Al3, 3% of NH4Cl

and 82% of Al2O3 it has been shown, that β-NiAl is the

basic phase constituent. During this low activity process,

the outward aluminium diffusion is the dominant growth

mechanism. During the high activity process it is neces-

sary to perform the additional thermal treatment in order

to obtain the transformation of the Ni2Al3 phase into the

β-NiAl phase. The out of pack and vapour phase alumin-

izing techniques are considered as the extension of pack

cementation method. The Al powder and Cr-Al granules

are the source of aluminium placed at at least 100 mm

away from the coated surface. The process is conducted

in the retort furnaces in the atmosphere of argon or hy-

drogen. The aluminium fluoride or ammonium fluoride is

used as the activating agent [3,4]. It has been proven that

there is a possibility of conducting the aluminizing proc-

ess of blades cooling channels with a use of out-of-pack

method.

There is a limitation of parameter control during the

aluminizing process using out-of-pack method. It is a

result of application of the activating agent introduced in

the solid state before the process. The chemical vapour

deposition method provides the largest ability of control-

ling the parameters values. Sun et al. [5] conducted the

aluminizing process with a CVD method applying the

flow of AlCl3 and hydrogen which are created in the ex-

ternal generator over the powder made of aluminium and

aluminum oxide. Howmet company develops the alu-

minizing process by chemical vapour deposition [6]. The

technological process is conducted as a high or low ac-

tivity process, like in the case of pack cementation. Dur-

ing the low activity process, the aluminium chloride is

created in the external generator by transferring hydrogen

chloride through pure aluminium granules. The obtained

aluminium chloride was introduced to the retort inside

the pit furnace. The Cr-Al were inside the baskets placed

around the central pipe which distributed the reactive gas

from the external aluminium generator. They were the

M. GORAL ET AL.

854

additional source of aluminium during the high activity

process [7]. It resulted from the influence of hydrogen

which caused the sulfur removal from the base material.

Kohlscheen and Storck [8] proposed different idea of

the aluminizing process using CVD method. The authors

used the internal generator, in which the Al-Cl granules

were the source of aluminium. AlCl3 was obtained by

transferring HCl through those granules. The model-

based calculations proved, that the increase of hydrogen

chloride amount didn’t ensure the thickness growth of

the aluminium coating. It was explained by the fact that

surface of the granules which reacted with hydrogen

chloride was to small to create AlCl3. The experimental

verification confirmed that the increase of hydrogen par-

ticipation ensured the thickness of the aluminide coating.

The literature analysis showed, that during the low ac-

tivity aluminizing process, it is necessary to decrease the

amount of used hydrogen chloride. The recent work fo-

cuses on the assessment of influence of the basic alu-

minizing parameters using CVD method on the micro-

structure of aluminide coating deposited on the high

temperature creep resisting nickel alloys. Application of

new BPXPro 325S device, which is available in the Re-

search and Development Laboratory for Aerospace Ma-

terials at the Rzeszów University of Technology required

the correlation of available literature data with the device

capabilities.

2. Experimental

The authors analysed the high temperature creep resisting

Rene 80 nickel superalloy. It’s chemical composition

was presented in the Table 1. Samples were made form

bar; they had thickness of 4 mm and diameter of 14 mm.

The samples were polished with water-resistant abrasive

paper with gradation of 500, degreased in isopropyl al-

cohol with a use of ultrasonic cleaner and dried before

the process.

The aluminizing process using CVD method was

conducted in the Research and Development Laboratory

for Aerospace Materials at Rzeszow University of Tech-

nology. The Bernex BPX Pro 325S device was used. It is

a conventional hot-wall CVD device. The aluminizing

process was performed during the HTLA process (high

temperature low activity). The aluminizing processes were

conducted for different parameters values to analyse the

growth kinetics of aluminide coating (Table 2). Parame-

ters used till now were used as a base values: HCl flow

of 1.4 NLPM, hydrogen flow of 10.5 NLPM, tempera-

ture of 1000˚C and pressure of 150 mbar. For the pur-

poses of further tests the pressure value from 100 mbar to

359 mbar and the time from 2 h to 8 h were chosen. The

HCl flow of 1.2 - 2.0 NLPM and the hydrogen flow of

10.5 NLPM was applied. The parameters concerning

particular processes are presented in Table 2. After fin-

ishing the analysis of the sample, the microstructure was

investigated with a use of S-3400 Scanning Electron Mi-

croscope (Hitachi) equipped with electron probe micro-

analysis (Thermo).

The oxidation test was conducted in static laboratory

air at the temperature of 1100˚C. 23 hours cycles of ex-

posure to high temperature were conducted. The mass

measurements of samples were taken after each cycle.

Table 1. The nominal chemical composition of Rene 80 alloy (wt%).

Material Ni Co Cr W Mo Al Ti Zr C

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

Table 2. The list of parameters used for aluminizing process implemented by CVD method.

Run No. Temp (˚C) P (mbar) HCl flow (l/min) H2 flow Time (h)

1 1000 150 1.2 10.5 4

2 1000 150 1.4 10.5 4

3 1000 150 2.0 10.5 4

4 1000 100 1.4 10.5 4

5 1000 350 1.4 10.5 4

6 1000 150 1.4 10.5 2

7 1000 150 1.4 10.5 8

M. GORAL ET AL. 855

3. Results

3.1. Microstructure of Coating

The conducted aluminizing processes implemented with

CVD method resulted in total coverage of the surface of

all samples made of Re 80 alloy (Figure 1 by the diffu-

sion coating. For the base parameters of aluminizing

process (1.4 NLPM HCl, 10.5 NLPM H2, 150 mbar, 4 h,

1000˚C) the aluminium content on the alloy surface was

of 44.5 at %.

Two characteristic zones could be observed in the

coating microstructure for the base parameters. Their

structure was typical for coatings obtained in low activity

process. The average aluminium content in the outer

zone (area 1 on Figure 2, Table 3) was approximation

39 at %. In the area near surface, the aluminium content

was higher and was at the level of 45 at % (point 3 on

Figure 2, Table 3). In the lower part of the outer zone

(point 4 on Figure 2, Table 3) the aluminium content

was of 35 at %. Chromium and cobalt were present, ex-

cept the aluminium and nickel, in the outer area of the

coating. In the outer diffusion zone the average alumin-

ium content was of 23 at %. The separations in compo-

nents of Re 80 alloy—Cr, Co, W, Ti were present (area 2

on Figure 2, Table 3). The white-coloured separations

contained 10 - 11 at % of Cr, Co and W for aluminium

content of 23 at %. The aluminium content was higher in

the base of diffusion zone and was at the level of 27 at %

(point 6 on Figure 2, Table 3).

3.2. Kinetic Growth of Coating

The performed microstructure analysis of aluminide

coatings created during CVD process conducted with

different parameters values didn’t prove that there is a

significant difference in their chemical composition. It

proved however that there is a difference as far as coating

thickness is concerned. The influence of pressure value

Figure 1. The surface morphology of aluminide coatings

deposited on Rene 80 superalloy by the CVD method.

Figure 2. The microstruture of aluminide coatings deposited

on Rene 80 superalloy by low-activity aluminizing. Process

parameters: 1.4 NLPM HCl, 10.5 NLPM H2, 150 mbar, 4 h,

1000˚C.

Table 3. The results of EDS microanalysis of areas pre-

sented on Figure 2.

Chemical composition (at %)

Area/

point Al Ti Cr Fe Co Ni W

1 39.290.63 2.84 1.09 6.67 49.48-

2 23.091.8410.64 - 10.18 43.4210.83

3 45.14- 0.81 1.15 4.21 48.510.18

4 35.181.42 4.16 0.87 7.02 50.590.76

5 23.101.4511.76 - 11.93 40.9510.82

6 27.711.92 8.51 0.86 9.52 46.275.21

in the retort, HCl flow and time on the thickness of the

coatings obtained during different processes is presented

on Figures 3-5. The thickness of coatings was in the range

of 16 - 18 μm. It has been showed, that decrease of pres-

sure in the retort during aluminizing process causes a small

increase of thickness of the aluminide coating by ap-

proximation 1 μm (Figure 3). It applied to the whole

coating as well as to particular zones (external and the

diffusion one). The authors noticed also a slight influence

of hydrogen chloride flow on the coating thickness. A

small decrease of the coating thickness (by approximation

1 μm) was observed in case of HCl flow of 1.4 NLPM

(Figure 4). The duration of aluminizing process had the

largest influence on the thickness of obtained coatings

(Figure 5). During the two-hour process, the thickness of

obtained coating was of 9 μm. Increase of aluminizing

time to 8 hours caused growth of thickness to 19 μm.

M. GORAL ET AL.

856

3.3. Oxidation Resistance

The cyclic oxidation test was conducted at the tempera-

ture of 1100˚C. The test results are presented on Figure 6.

Resistance to oxidation of the alloy without protective

coating and the previously investigated aluminide coat-

ing obtained using slurry method were investigated dur-

ing the research. The decrease of sample mass below the

initial value was a criterion of a process termination. The

obtained results showed a high oxidation resistance of

obtained coating during the CVD method with standard

parameters values. The trial was terminated after 49 cy-

cles (1127 hours) of exposure at high temperature. The

coatings fromed using slurry method, in spite of high

aluminium content, exceed the value of initial mass after

230 hours of oxidation and for Rene 80 alloy—after 4

cycles (92 hours).

Figure 3. The influence of pressure in retort on thickness of

aluminide coating obtained on Re 80 superalloy by CVD

method.

Figure 4. The influence of HCl flow on thickness of alu-

minide coating obtained on Re 80 superalloy by CVD

method.

Figure 5. The influence of aluminizing time on thickness of

aluminide coating obtained on Re 80 superalloy by CVD

method.

Oxidationresistanceat1100

‐4,60E‐05

‐3,60E‐05

‐2,60E‐05

‐1,60E‐05

‐6,00E‐06

4,00E‐06

1,40E‐05

2,40E‐05

0510 15 20 25 30 35 40 45 50

numberof23hcycles

weightchangemg/cm

‐5,60E‐05

Re80(Slurryaluminized)

Rene80(CVDaluminized)

Rene80barealloy

Figure 6. The results of cyclic oxidation test at 1100˚C of aluminide coatings deposited on Rene 80 nickel superalloy.

M. GORAL ET AL. 857

4. Summary

The CVD method is one of most modern techniques of

aluminizing of turbine blades in aircraft engines. The

conducted technological trials made with BPX Pro 325S

device showed a possibility of creating the aluminide

coatings with a structure typical for low activity process.

The coating consisted of the outer layer (β-NiAl phase)

containing approximation 45 at % of aluminium and the

transition diffusion zone. The coating structure indicates

the growth resulting from outward nickel diffusion. The

analysis of coating growth kinetics didn’t show the sig-

nificant influence of pressure in the retort and the HCl

flow on the thickness of formed coating. It is necessary

to change a value of hydrogen chloride flow in order to

increase the coating thickness.

The conducted research proved that the aluminizing

using CVD method is an efficient way of surface protec-

tion against oxidation for Rene 80 alloy. It was con-

firmed by the result of cyclic oxidation test. High heat

resistance of the coating obtained with CVD method can

be a result of application of the hydrogen atmosphere. The

method provides high quality of obtained coating [7].

5. Acknowledgements

The research was supported by National Centre for Re-

search and Development under grant No. PR NR15-

0121-10/2011.

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