56 56

Volume 45

Issue 1

September 2010

Pages 56-60

International Scientific Journal

published monthly by the

World Academy of Materials

and Manufacturing Engineering

Archives of Materials Science and Engineering

© Copyright by International OCSCO World Press. All rights reserved. 2010

1.Introduction

Nickel-based superalloys are used as materials for the hot components of aeroengines, land based and marine gas turbines

[1-4]. These superalloys have a good mechanical properties but low oxidation resistance at high temperature and environments of aggressive gases.

Effect of diffusion on platinum coatings deposited on the surface of nickel based superalloy by the electroplating process

M. Yavorska*, J. SieniawskiR&D Laboratory for Aerospace Materials, Rzeszow University of Technology, ul. W. Pola 2, 35-959 Rzeszów, Poland * Corresponding author: E-mail address: mjavorska@rambler.ru

Received 15.06.2010; published in revised form 01.09.2010

ABSTRACT

Purpose: In this paper the effect of diffusion on platinum coatings deposited on the surface of nickel based superalloy was evaluated.Design/methodology/approach: The platinum coatings with thickness of 3 mm and 7 mm were deposited by electroplating process on Inconel 713 LC Ni-base superalloy. The heat treatment of electroplating coatings at the temperature 1050 °C during 2h under argon atmosphere was performed. The microstructure investigations of the heat treated coatings were conducted by the use of optical microscope (Nikon Epiphot 300) and a scanning electron microscope (Hitachi S-3400N) equipped with an X-radiation detector EDS (VOYAGER of NORAN INSTRUMENTS). The phase composition was identified by X-ray (ARL X’TRAX) diffractometer. The surface roughness parameter - Ra was evaluated by Perthometer S2 MAHR equipment.Findings: The microstructure of platinum electroplating coating with thickness of 3 µm after diffusion treatment consists of two phases: γ-Ni and (Al0.25Pt0.75)Ni3. The increase of platinum thickness from 3 µm to 7 µm does not influence the phase composition of heat treated coatings. Heat treatment of platinum electroplating coatings causes the increase of surface roughness parameter as a result of unequal mass flow of platinum and nickel.Research limitations/implications: The results will be used in the future investigations to explain the mechanism of reaction of platinum as a modifier in aluminide coatings.Practical implications: The platinum electroplating coatings after diffusion treatment and aluminizing process are widely used as coatings for turbine blades of aircraft engines.Originality/value: The paper includes the results of microstructure and surface roughness investigations of platinum electroplating coatings with 3 mm and 7 µm thickness after diffusion treatment.Keywords: Inconel 713 LC; Platinum electroplating; Diffusion treatment; Surface roughnessReference to this paper should be given in the following way: M. Yavorska, J. Sieniawski, Effect of diffusion on platinum coatings deposited on the surface of nickel based superalloy by the electroplating process, Archives of Materials Science and Engineering 45/1 (2010) 56-60.

SHORT PAPER

1. Introduction

Therefore to improve the lifetime it is necessary to protect superalloys against heat, oxidation and corrosion degradation. An aluminde coating deposited on nickel-based superalloy provides excellent oxidation and corrosion resistance of elements. Several studies showed that platinum additions improve the properties of both alloy and aluminide coating [5-15]. Pt-modified aluminide coatings affirm the protection of nickel-base superalloys from oxidative gases even at gas temperature above 1400ºC [11]. Platinum improves the oxide scale adherence [12]. Authors of [14] showed that platinum reduces void formation at the interface of metal/oxide. Other studies showed that platinum accelerates aluminium diffusion, then reduces the vacancies flux from the metal to the surface and inhibits vacancy coalescence and internal void formation. Authors [10] reported that platinum decreases the diffusive flux of alloying elements to the coating, promotes a selective alumina scale formation and accelerates healing after spallation. Pt slightly accelerates an alumina scale growth and delays transformation of metastable alumina oxides to α-Al2O3. The addition of platinum to an aluminide coatings improves the stability of ß-NiAl phase and delays the transformation of ß-NiAl to the ’-Ni3Al phase.

The important factor of platinum coatings after electroplating is the diffusion treatment. The application of diffusion treatment of platinum coatings provides their good adherence.

In this paper the effect of diffusion on platinum coatings deposited on the nickel based superalloys was investigated. The influence of platinum coating thickness on the depth and microstructure of heat treated coating was analyzed.

2. Experimental procedure

The alloy used in this study was Inconel 713LC. Chemical composition of the used melt is given in Table 1. Table 1. Chemical composition of the Inconel 713LC Ni-base superalloy

Elements content, % mass Ni Cr C Mo Nb Al Ti Co Fe S

74.7 12 0.05 4.6 1.96 5.7 0.7 0.08 0.19 0.02

Process of platinum electroplating was made in WSK “PZL-Rzeszów”. The material surface preparation for platinum electroplating process includes four basic operation. These are, in the usual order of execution: surface degreasing, surface etching, rinsing in cold water, surface activation.

The electrochemical degreasing was carried out in gluconate electrolyte at the temperature of 50ºC. The electrolyte contained 180-240 g/dm3 sodium gluconate - C6H11NaO7 and 18-240 g/dm3 sodium hydroxide - NaOH. The rinsing in a heat water was carried out in order to clean samples surface from C6H11NaO7 and NaOH. The samples surface activation was carried out by etching in the mixture of: 90 g/dm3 HF + 530 g/dm3 HNO3

Platinum electroplating process included the activation and appropriate electroplating. The activation electroplating was performed during 1h by means of tetraamineplatinum (II) composite bath - Pt(NH3)2(NO2)2 15 g/dm3, which enables the rapid growth of an adherent and relatively homogeneous coating. The current density during the activation electroplating process

was about of 10 A/dm2. Titanium was used as an anode to electroplating. The appropriate electroplating was done with the current density of 0.1 A/dm2. The thickness value of 3 µm and 7 µm were used to investigate the influence of Pt coating thickness on depth and microstructure of the heat treated coatings. The samples after electroplating were heat treated at the temperature of 1050ºC for 2 h under argon atmosphere (Fig. 1).

Fig. 1. Samples after platinum electroplating The platinum electroplating samples after diffusion treatment were cut study the cross-section. Polished sections were etched by the use of reagent with chemical composition as follows: 100 ml HNO3, 7 ml HF, 11 ml H2O. Microstructure investigations of samples after diffusion treatment were performed by the use of light microscope Nikon 300 and scanning electron microscope (SEM) HITACHI S-3400N equipped with EDS spectrometer. Evaluation of phase composition of the investigated coatings was made using ARL X’TRAX-ray diffractometer, equipped with filtered copper lamp with the voltage of 45 kV and heater current of 40 mA. Measurements were made in the range of 15 to 140º.

The surface roughness parameter - Ra was evaluated by Perthometer S2 MAHR. The average value of surface roughness parameter and standard deviation were calculated.

3. Results and discussion

Coating surfaces after platinum electroplating with thickness of 3 and 7 µm had a closely spaced “cauliflowers” appearance. The cauliflower morphology of the coatings depends on process, such as current density and potential [8].

The diffusion annealing of platinum electroplating coating with thickness of 3 m at 1050ºC for 2 h causes the formation of diffusion zone at the cross-section of the coating (Fig. 2). The thickness and morphology of the microstructure components were investigated. The depth of diffusion zone is approximately 16 µm.

Analysis of the chemical composition on the cross-section proved the interdiffusion of platinum and substrate elements. Platinum diffused into the substrate and substrate elements, e.g. Ni diffused into the platinum layer as a result of diffusion zone formation (Fig. 2). The original surface of the substrate can be identified by adjusting the contrast of spots that are surrounding the pores. The microstructure of the cross-section of heat treated coatings consisted of two phases. The white phase (%at: 14Al-8Cr-60Ni-17Pt) has a higher Pt and Al content and lower Cr concentration in comparison to the surrounding substrate (%at: 5Al-14Cr-80Ni) (Table 2, point 1,3) .

57READING DIRECT: www.archivesmse.org

1.Introduction

Nickel-based superalloys are used as materials for the hot components of aeroengines, land based and marine gas turbines

[1-4]. These superalloys have a good mechanical properties but low oxidation resistance at high temperature and environments of aggressive gases.

Therefore to improve the lifetime it is necessary to protect superalloys against heat, oxidation and corrosion degradation. An aluminde coating deposited on nickel-based superalloy provides excellent oxidation and corrosion resistance of elements. Several studies showed that platinum additions improve the properties of both alloy and aluminide coating [5-15]. Pt-modified aluminide coatings affirm the protection of nickel-base superalloys from oxidative gases even at gas temperature above 1400ºC [11]. Platinum improves the oxide scale adherence [12]. Authors of [14] showed that platinum reduces void formation at the interface of metal/oxide. Other studies showed that platinum accelerates aluminium diffusion, then reduces the vacancies flux from the metal to the surface and inhibits vacancy coalescence and internal void formation. Authors [10] reported that platinum decreases the diffusive flux of alloying elements to the coating, promotes a selective alumina scale formation and accelerates healing after spallation. Pt slightly accelerates an alumina scale growth and delays transformation of metastable alumina oxides to α-Al2O3. The addition of platinum to an aluminide coatings improves the stability of ß-NiAl phase and delays the transformation of ß-NiAl to the ’-Ni3Al phase.

The important factor of platinum coatings after electroplating is the diffusion treatment. The application of diffusion treatment of platinum coatings provides their good adherence.

In this paper the effect of diffusion on platinum coatings deposited on the nickel based superalloys was investigated. The influence of platinum coating thickness on the depth and microstructure of heat treated coating was analyzed.

2. Experimental procedure

The alloy used in this study was Inconel 713LC. Chemical composition of the used melt is given in Table 1. Table 1. Chemical composition of the Inconel 713LC Ni-base superalloy

Elements content, % mass Ni Cr C Mo Nb Al Ti Co Fe S

74.7 12 0.05 4.6 1.96 5.7 0.7 0.08 0.19 0.02

Process of platinum electroplating was made in WSK “PZL-Rzeszów”. The material surface preparation for platinum electroplating process includes four basic operation. These are, in the usual order of execution: surface degreasing, surface etching, rinsing in cold water, surface activation.

The electrochemical degreasing was carried out in gluconate electrolyte at the temperature of 50ºC. The electrolyte contained 180-240 g/dm3 sodium gluconate - C6H11NaO7 and 18-240 g/dm3 sodium hydroxide - NaOH. The rinsing in a heat water was carried out in order to clean samples surface from C6H11NaO7 and NaOH. The samples surface activation was carried out by etching in the mixture of: 90 g/dm3 HF + 530 g/dm3 HNO3

Platinum electroplating process included the activation and appropriate electroplating. The activation electroplating was performed during 1h by means of tetraamineplatinum (II) composite bath - Pt(NH3)2(NO2)2 15 g/dm3, which enables the rapid growth of an adherent and relatively homogeneous coating. The current density during the activation electroplating process

was about of 10 A/dm2. Titanium was used as an anode to electroplating. The appropriate electroplating was done with the current density of 0.1 A/dm2. The thickness value of 3 µm and 7 µm were used to investigate the influence of Pt coating thickness on depth and microstructure of the heat treated coatings. The samples after electroplating were heat treated at the temperature of 1050ºC for 2 h under argon atmosphere (Fig. 1).

Fig. 1. Samples after platinum electroplating The platinum electroplating samples after diffusion treatment were cut study the cross-section. Polished sections were etched by the use of reagent with chemical composition as follows: 100 ml HNO3, 7 ml HF, 11 ml H2O. Microstructure investigations of samples after diffusion treatment were performed by the use of light microscope Nikon 300 and scanning electron microscope (SEM) HITACHI S-3400N equipped with EDS spectrometer. Evaluation of phase composition of the investigated coatings was made using ARL X’TRAX-ray diffractometer, equipped with filtered copper lamp with the voltage of 45 kV and heater current of 40 mA. Measurements were made in the range of 15 to 140º.

The surface roughness parameter - Ra was evaluated by Perthometer S2 MAHR. The average value of surface roughness parameter and standard deviation were calculated.

3. Results and discussion

Coating surfaces after platinum electroplating with thickness of 3 and 7 µm had a closely spaced “cauliflowers” appearance. The cauliflower morphology of the coatings depends on process, such as current density and potential [8].

The diffusion annealing of platinum electroplating coating with thickness of 3 m at 1050ºC for 2 h causes the formation of diffusion zone at the cross-section of the coating (Fig. 2). The thickness and morphology of the microstructure components were investigated. The depth of diffusion zone is approximately 16 µm.

Analysis of the chemical composition on the cross-section proved the interdiffusion of platinum and substrate elements. Platinum diffused into the substrate and substrate elements, e.g. Ni diffused into the platinum layer as a result of diffusion zone formation (Fig. 2). The original surface of the substrate can be identified by adjusting the contrast of spots that are surrounding the pores. The microstructure of the cross-section of heat treated coatings consisted of two phases. The white phase (%at: 14Al-8Cr-60Ni-17Pt) has a higher Pt and Al content and lower Cr concentration in comparison to the surrounding substrate (%at: 5Al-14Cr-80Ni) (Table 2, point 1,3) .

2. Experimental procedure

3. Results and discussion

58 58

M. Yavorska, J. Sieniawski

Archives of Materials Science and Engineering

1

2

3

Interface after heat treatment

Pt diffusion zone

Fig. 2. Cross-section image of platinum electroplating coating with thickness of 3 µm after heat treatment process; a two-phase microstructure

Table 2. The results of EDS analysis from on area presented in Fig. 2

Al, % Cr, % Ni, % Pt, % Point at wt at wt at wt at wt 1 14.09 4.98 8.53 5.80 60.7 46.64 16.68 42.582 6.06 2.63 12.34 10.33 77.07 72.83 4.53 14.213 5.84 3.34 13.67 15.10 80.49 81.56 - -

Surface morphology characterized a fine-grain structure with

a typical grain size about 3-5 µm. The high content of aluminium nickel and platinum was observed on the surface of platinum coating after diffusion treatment (Figs. 3a,b).

a)

b)

Energy,

Inte

nsity

, im

p/s

keV

Fig. 3. Microstructure (a) and EDS analysis results (b) of the surface of platinum electroplating coating with thickness of 3 µm after heat treatment process: showing a fine grain structure

The chemical composition on the cross-section of platinum coating after heat treatment showed outward diffusion of nickel, chromium and aluminium elements and inward diffusion of platinum (Fig. 4).

0102030405060708090

3 5 7 9 11 13 15 17

PtAlNiCr

Distance from the surface, m

Elem

ents

con

tent

, % a

t

Fig. 4. Chemical composition on the cross-section of platinum electroplating coating with thickness of 3 µm after heat treatment process

X-ray diffraction of the phase analysis samples with a 3 µm-thick platinum coating after heat treatment revealed a two-phase γ-Ni and (Al0.25Pt0.75)Ni3 structure (Fig. 5). The authors [11] proved, that phase composition of heat treated coating do not seem to change with the annealing time.

Fig. 5. X-ray diffraction results of platinum electroplating coating with thickness of 3 µm after heat treatment process

The heat treatment of platinum electroplating coating with thickness of 7 µm causes the formation of diffusion zone. The depth of diffusion zone is about 18 µm (Fig. 6). The coating consists of two phases: white phase contains 25.83% at and 15.56% at platinum and a grey one which contains nickel approximately 81% at and 0% at platinum (Table 3).

Fig. 6. Cross-section image of platinum electroplating coating with thickness of 7 µm after heat treatment process: showing a two-phase microstructure Table 3. Elemental composition from EDS analysis of the area showed in Fig. 6

Al, % Cr, % Ni, % Pt, % Point at wt at wt at wt at wt

1 8.44 2.51 9.72 5.58 56.01 36.30 25.83 55.612 6.73 2.36 12.50 8.45 65.21 49.75 15.56 39.444 5.73 3.29 13.10 14.49 81.17 82.22 - -

A fine grained structure with the grain size of 5-7 µm is formed on the surface of the treated sample. The surface is enriched in platinum, nickel, chromium and aluminium (Pt 10-20%at, Ni 66-68%at, Cr 6-12%at, Al 4-15%at).

The chemical composition on the cross-section of 7 µm-thick platinum coating after diffusion treatment showed outward diffusion of nickel, chromium and aluminium elements and inward diffusion of platinum. This phenomena is similar to the platinum coating with thickness of 3 µm.

X1

X2

Fig. 7. Morphology of the surface of platinum electroplating coating with thickness of 7 µm after heat treatment process

The phases of γ and γ’(Al0.25Pt0.75)Ni3 were determined in the

platinum electroplating coating with thickness of 7 µm after heat treatment process (Fig. 8).

Table 4. Elemental composition from EDS analysis of the area showed in Fig. 7

Al, % Cr, % Ni, % Pt, % Point

at wt at wt at wt at wt X1 14.86 7.82 6.69 6.78 68.44 47.32 10.01 38.08X2 4.88 1.65 12.40 8.09 61.95 39.39 20.77 50.87

Fig. 8. Diffraction results of platinum electroplating coating with thickness of 7 µm after heat treatment process

The surface roughness parameter of specimens decreases with the increase of platinum deposition thickness (Table 5). Heat treatment of platinum electroplating causes the increase of surface roughness with the increase of platinum thickness. This phenomena is a result of unequal mass flow and internal stresses release caused by interdiffusion of platinum and nickel [15]. Table 5. Values of surface roughness parameter after platinum electroplating and diffusion treatment

Nickel alloy Ra Platinum

electroplating Diffusion treatment Inconel

713 LC 0 m 3 m 7 m 3 m 7 m

Average value 1.080 0.664 0.605 2.623 2.971

Standard deviation 0.0144 0.047 0.003 0.167 0.051

4. Conclusions

Platinum coatings with thickness of 3 and 7 m were produced by electroplating method. Electroplating samples were heat treated at the temperature 1050 ºC during 2 h under argon atmosphere. Heat treated coatings with thickness of 3 µm had 16 µm depth. The increase of coating thickness from 3 to 7 µm leds to the increase of coating depth from 16 to 18 µm. Diffusion

59

Effect of diffusion on platinum coatings deposited on the surface of nickel based superalloy by the electroplating process

Volume 45 Issue 1 September 2010

1

2

3

Interface after heat treatment

Pt diffusion zone

Fig. 2. Cross-section image of platinum electroplating coating with thickness of 3 µm after heat treatment process; a two-phase microstructure

Table 2. The results of EDS analysis from on area presented in Fig. 2

Al, % Cr, % Ni, % Pt, % Point at wt at wt at wt at wt 1 14.09 4.98 8.53 5.80 60.7 46.64 16.68 42.582 6.06 2.63 12.34 10.33 77.07 72.83 4.53 14.213 5.84 3.34 13.67 15.10 80.49 81.56 - -

Surface morphology characterized a fine-grain structure with

a typical grain size about 3-5 µm. The high content of aluminium nickel and platinum was observed on the surface of platinum coating after diffusion treatment (Figs. 3a,b).

a)

b)

Energy,

Inte

nsity

, im

p/s

keV

Fig. 3. Microstructure (a) and EDS analysis results (b) of the surface of platinum electroplating coating with thickness of 3 µm after heat treatment process: showing a fine grain structure

The chemical composition on the cross-section of platinum coating after heat treatment showed outward diffusion of nickel, chromium and aluminium elements and inward diffusion of platinum (Fig. 4).

0102030405060708090

3 5 7 9 11 13 15 17

PtAlNiCr

Distance from the surface, m

Elem

ents

con

tent

, % a

t

Fig. 4. Chemical composition on the cross-section of platinum electroplating coating with thickness of 3 µm after heat treatment process

X-ray diffraction of the phase analysis samples with a 3 µm-thick platinum coating after heat treatment revealed a two-phase γ-Ni and (Al0.25Pt0.75)Ni3 structure (Fig. 5). The authors [11] proved, that phase composition of heat treated coating do not seem to change with the annealing time.

Fig. 5. X-ray diffraction results of platinum electroplating coating with thickness of 3 µm after heat treatment process

The heat treatment of platinum electroplating coating with thickness of 7 µm causes the formation of diffusion zone. The depth of diffusion zone is about 18 µm (Fig. 6). The coating consists of two phases: white phase contains 25.83% at and 15.56% at platinum and a grey one which contains nickel approximately 81% at and 0% at platinum (Table 3).

Fig. 6. Cross-section image of platinum electroplating coating with thickness of 7 µm after heat treatment process: showing a two-phase microstructure Table 3. Elemental composition from EDS analysis of the area showed in Fig. 6

Al, % Cr, % Ni, % Pt, % Point at wt at wt at wt at wt

1 8.44 2.51 9.72 5.58 56.01 36.30 25.83 55.612 6.73 2.36 12.50 8.45 65.21 49.75 15.56 39.444 5.73 3.29 13.10 14.49 81.17 82.22 - -

A fine grained structure with the grain size of 5-7 µm is formed on the surface of the treated sample. The surface is enriched in platinum, nickel, chromium and aluminium (Pt 10-20%at, Ni 66-68%at, Cr 6-12%at, Al 4-15%at).

The chemical composition on the cross-section of 7 µm-thick platinum coating after diffusion treatment showed outward diffusion of nickel, chromium and aluminium elements and inward diffusion of platinum. This phenomena is similar to the platinum coating with thickness of 3 µm.

X1

X2

Fig. 7. Morphology of the surface of platinum electroplating coating with thickness of 7 µm after heat treatment process

The phases of γ and γ’(Al0.25Pt0.75)Ni3 were determined in the

platinum electroplating coating with thickness of 7 µm after heat treatment process (Fig. 8).

Table 4. Elemental composition from EDS analysis of the area showed in Fig. 7

Al, % Cr, % Ni, % Pt, % Point

at wt at wt at wt at wt X1 14.86 7.82 6.69 6.78 68.44 47.32 10.01 38.08X2 4.88 1.65 12.40 8.09 61.95 39.39 20.77 50.87

Fig. 8. Diffraction results of platinum electroplating coating with thickness of 7 µm after heat treatment process

The surface roughness parameter of specimens decreases with the increase of platinum deposition thickness (Table 5). Heat treatment of platinum electroplating causes the increase of surface roughness with the increase of platinum thickness. This phenomena is a result of unequal mass flow and internal stresses release caused by interdiffusion of platinum and nickel [15]. Table 5. Values of surface roughness parameter after platinum electroplating and diffusion treatment

Nickel alloy Ra Platinum

electroplating Diffusion treatment Inconel

713 LC 0 m 3 m 7 m 3 m 7 m

Average value 1.080 0.664 0.605 2.623 2.971

Standard deviation 0.0144 0.047 0.003 0.167 0.051

4. Conclusions

Platinum coatings with thickness of 3 and 7 m were produced by electroplating method. Electroplating samples were heat treated at the temperature 1050 ºC during 2 h under argon atmosphere. Heat treated coatings with thickness of 3 µm had 16 µm depth. The increase of coating thickness from 3 to 7 µm leds to the increase of coating depth from 16 to 18 µm. Diffusion

4. Conclusions

60 60 READING DIRECT: www.archivesmse.org

zone was obtained after heat treatment of platinum coatings with 3 µm and 7 µm thickness. It was found that platinum coatings both with 3 µm and 7 µm thickness consist of the two phases - γ-Ni and (Al0.25Pt0.75)Ni3. The chemical composition on the cross-section of platinum coating after diffusion treatment showed outward diffusion of nickel, chromium and aluminium elements and inward diffusion of platinum. Heat treatment of platinum electroplating causes the increase of surface roughness parameter with the increase of platinum thickness as a result of unequal mass flow of platinum and nickel.

Acknowledgements

The present work was supported by the Polish Ministry of Education and Science under research project U-8072/G.

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