- Email: [email protected]
AlN and intermetallic compound layers formed between aluminum and austenitic stainless steel using barrel nitriding
Progress in Organic Coatings 76 (2013) 1841–1845
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
AlN and intermetallic compound layers formed between aluminum and austenitic stainless steel using barrel nitriding J.H. Kong a , M. Okumiya a,∗ , Y. Tsunekawa a , S.G. Kim b , M. Yoshida c a b c
Toyota Technological Institute, 2-12-1, Hisakata, Tempaku, Nagoya 468-8511, Japan Korea Institute of Industrial Technology, 7-47, Songdo, Yeonsu, Incheon 406-840, South Korea Shizuoka Institute of Science and Technology, 2200-2, Toyosawa, Hukuroi 437-8555, Japan
a r t i c l e
i n f o
Available online 19 June 2013
a b s t r a c t Recently, a new nitriding process was proposed to produce the aluminum nitride on an aluminum surface using a barrel. After barrel nitriding, AlN nitride layer is formed on the aluminum surface and the surface hardness can be improved remarkably. In this study, barrel nitriding was performed to investigate the interface between aluminum substrate, with SUS304 austenitic stainless steel used for a physical catalyst. The barrel nitriding was carried out at 893 K for 18 ks, 25.2 ks and 36 ks, respectively with aluminum and aluminum–magnesium alloy powder. After barrel nitriding, aluminum nitride layer and Fe–Al intermetallic compound layers were formed at the interface between pure aluminum and austenitic stainless steel at the same time. The thickness of the aluminum nitride layer and intermetallic layer was increased by increasing the treatment time. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Pure aluminum and aluminum alloys have numerous industrial applications due to their outstanding properties such as light weight and high specific strength. Aluminum alloys are one of the most promising materials for sliding parts in automotive power trains. However, various surface treatments are indispensable to improve tribologic properties because the hardness and wear resistance of the aluminum alloys are inferior to those of steels. Aluminum nitride is one of the effective protection films with high hardness and high thermal conductivity [1,2]. Aluminum nitride film has been produced using various processing techniques such as plasma nitriding, CVD, PVD, spray coating and gas nitriding such studies have been reported by many researchers [3,4]. In the conventional process, the nitriding of the aluminum surface is very difficult because of the thin aluminum oxide layer existing on the surface. This layer prevents the diffusion of nitrogen into the aluminum substrate. Therefore, the growth rate of the aluminum nitride layer is very sluggish. To solve this problem, numerous studies have been conducted on the aluminum nitriding process using plasma nitriding. This method requires a sputtering of the aluminum surface to remove the oxide layer prior to the nitriding, and takes considerable time. The aluminum nitride film thickness obtained by this method is between 5 and 15 m, and typically
∗ Corresponding author. E-mail address: [email protected] (M. Okumiya). 0300-9440/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.porgcoat.2013.05.026
much thinner. Recently, a new nitriding process for Al, “barrel nitriding”, has been proposed [3]. The interfacial reaction between steel and aluminum such as brazing, welding, diffusion bonding and hot dip coating are known [5]. Studies have already examined the chemical compositions and growth kinetics of the intermetallic layers [5–7] and the effect of impurities on their growth [8,9] during hot dip coating of steels in molten aluminum. Diffusion bonding and hot dip coating are usually performed in the temperature range of solid Fe/solid Al diffusion from 873 to 973 K. Fe–Al intermetallic compound layers formed under such conditions are usually FeAl3 and Fe2 Al5 [1–10]. These intermetallic compound layers have high hardness and brittle matrix. In this study, we proposed the new process that a combined method of AlN and Fe–Al intermetallic compound layer at the interface between pure Al and SUS304 austenitic stainless steel using barrel nitriding.
2. Experimental procedures The substrates of JIS-A1050 commercial grade pure Al and SUS304 austenitic stainless steel measured 20 width × 50 length × 3 thickness (mm). Al2 O3 particles (average diameter 0.1 mm) and Al 50 wt.% Mg powder (average diameter 0.2 mm) were used for the barrel filler. The furnace temperature was adjusted with a temperature control unit. Nitrogen gas (N2 ) was introduced into the barrel; the N2
1842
J.H. Kong et al. / Progress in Organic Coatings 76 (2013) 1841–1845
Fig. 2. The Fe–Al phase diagram [11].
measurements (Akashi, HM-125) were performed under a load of 100 g on the cross-sectional region. 3. Results and discussion
Fig. 1. Schemetaic diagram of the barrel nitriding and treatment cycle.
flow rate was adjusted with a program control unit and a mass flow controller. The furnace was evacuated about 7.5 × 10–1 torr and the atmosphere was substituted by N2 . Before increasing temperature, a vacuum and nitrogen gas introduction process was repeated for sufficient N2 gas atmospheres without oxygen. After the preparation process at room temperature, barrel nitriding was carried out at 893 K for 18 ks, 25.2 ks and 36 ks, respectively. Then, the specimen was cooled in the barrel. The schematic diagram and treatment cycle is indicated as shown in Fig. 1. The inside temperature of the chamber with powder was measured by thermocouple; the temperature, and N2 gas flowrate were monitored with a data processor. The barrel nitriding process parameters are shown in Table 1. Optical microscopy was used for thickness measurements and structure observations of the AlN and Fe–Al intermetallic compound layers, and scanning electron microscopy (SEM-EDX) (Hitachi, SU6600) was used to observe the phases and modified layers. The micro Vickers hardness
Table 1 Treatment conditions. Nitriding temperature T (K) Heating time (ks) Nitriding time (ks) Nitriding gas Nitrogen gas flow (cc/min) Filler Al–Mg/Al2 O3 ratio (wt.%)
893 5.4 18, 25.2 and 36 Nitrogen (N2 ) 1500 Al2 O3 (average dia. 0.1 mm) Al–50wt.%Mg (average dia. 0.2 mm) 1.8
Fig. 2 is the Fe–Al phase diagram [11] which contains five types of Fe–Al intermetallic compound. FeAl2 , Fe2 Al5 and FeAl3 compounds are known to have a high aluminum composition with high hardness and brittleness. Fe3 Al and FeAl compounds also have good mechanical properties in terms of wear resistance, oxidation resistance, corrosion resistance and specific strength properties [12]. In this study, it can be expected that the intermetallic Fe–Al compound layer will be formed at the interface between pure aluminum and austenitic stainless steel by diffusion of Al with the nitride layer due to the permeation of nitrogen atom during barrel nitriding under melting temperature. Fig. 3 shows the variation in the nitride layer on the surface of the pure aluminum substrate after barrel nitriding at 893 K at different treatment times. After barrel nitriding, some parts of the layer begin to form aluminum nitride (AlN) at 893 K for 18 ks. The selected area EDX spectrum for the small part of the layer on the outmost surface, presented in Fig. 3(a), reveal K␣ peaks for N and Al at 0.38 keV and 1.48 keV, respectively, in addition to the peak for Mg as shown in Fig. 3(d). The weight percent of N and Al shows 14.2 wt.% and 84.8 wt.%, respectively, with a small amount of Mg and O under 1 wt.%. The thickness of the aluminum nitride layer is increased with increasing the treatment time. Finally, the layer covers the entire substrate surface up to the thickness of 200 m at 36 ks. Generally, the aluminum is acknowledged to have oxidation film on outmost surface. Therefore, the formation of nitride layer is very difficult due to the oxidation film. This thin film can be removed by the activation of Al2 O3 powder and Al–Mg alloy powder in this study. However, the formation of aluminum oxide occurs more rapidly than AlN, because Al has a strong affinity with oxygen compared to nitrogen. Thus the growth rate of nitride layer can be fast depending on the high vacuum rate. The SUS304 austenitic stainless steel was attached on pure aluminum to form the nitride layer and Fe–Al intermetallic compound layer. As shown in Fig. 4 optical micrographs, double layers are formed at the interface between pure aluminum and austenitic stainless steel after barrel nitriding at 893 K for 18 ks, 25.2 ks and 36 ks respectively. An aluminum nitride layer is considered due to the affinity between Al and N. The other one is a Fe–Al intermetallic compound layer by Al diffusion from the aluminum substrate and the AlN layer. The thickness of the aluminum nitride layer and Fe–Al intermetallic compound layer shows 90 m and
J.H. Kong et al. / Progress in Organic Coatings 76 (2013) 1841–1845
1843
Fig. 3. Cross-sectional optical micrographs on the surface of pure aluminum after barrel nitriding at 893 K for (a) 18 ks, (b) 25.2 ks, (c) 36 ks and (d) EDX spectra result of (a) nitride layer.
70 m, respectively, at the treatment time of 18 ks. The thickness of the double layer is gradually increased with increasing the treatment time. Therefore, total thickness of the double layer is also increased with increasing the treatment time from 160 m to 310 m. This double layer is formed very uniformly compared with the aluminum nitride single layer on the pure aluminum surface as shown in Fig. 3. Furthermore, the formation and growth rate of nitride layer are faster than those of a single layer without the problem of poor surface roughness. It is expected that the reactivity of nitrogen is more active at the interface between pure aluminum substrate and SUS304 austenitic stainless steel in comparison with the outside surface. Hence, the nitrogen atoms can be easily decomposed from N2 to N atoms due to the vibration in the narrow interface by crashing both sides of the substrates. At this time, the oxidation film is also removed more easily by the impact energy of nitrogen atoms. Thus, it is considered that the decomposed nitrogen atoms can react with Al easily after removing the oxidation film [13,14]. During the growth of the nitride layer,
the aluminum is diffused to the austenitic stainless steel surface from the nitride layer when the latter contacts the steel surface. The growth direction of the aluminum nitride layer is proceeding toward the aluminum substrate simultaneously. Fig. 5 shows a schematic illustration of the growth process for the nitride layer and Fe–Al intermetallic compound layer at the interface between the pure aluminum and austenitic stainless steel during barrel nitriding. Fig. 6 shows the SEM micrographs and EDX analysis results of the double compound layer between the pure aluminum and austenitic stainless steel. As shown in Fig. 4(a), the double layer is composed of the aluminum nitride layer and Fe–Al intermetallic compound layer by the diffusion of aluminum. Fig. 6(b) shows the result of line mapping. The profile of Al amount is seen to decrease toward the austenitic stainless steel from the pure aluminum substrate. The element of Al, which has a strong affinity with Fe, is thereby diffused to the austenitic stainless steel surface to form the Fe–Al intermetallic layer while the aluminum nitride layer is
Fig. 4. Optical micrographs of double compound layers between pure aluminum and austenitic stainless steel after barrel nitriding at 893 K for (a) 18 ks, (b) 25.2 ks and (c) 36 ks.
1844
J.H. Kong et al. / Progress in Organic Coatings 76 (2013) 1841–1845
Fig. 5. The schematic illustration about the growth process of aluminum nitride and Fe–Al intermetallic compound layer during barrel nitriding.
growing. Therefore, the Al-riched FeAl2 , Fe2 Al5 or FeAl3 compound is expected to exist near the aluminum nitride layer, while the Feriched Fe3 Al or FeAl compounds exist near the austenitic stainless steel. Fig. 7 shows the Vickers hardness profile on a cross-section of the modified double layers. The hardness of Pure Al and austenitic stainless steel shows about 30 HV (0.2) and 200 HV (0.2),
respectively. However, the area of the aluminum nitride layer shows over 200 HV (0.1) and Fe–Al compound layer shows about 800 HV (0.1). The maximum hardness of the AlN layer and Fe–Al intermetallic compound layer shows about 380 HV and 910 HV, respectively, after barrel nitriding at 893 K for 18 ks. The maximum hardness of the aluminum nitride layer and Fe–Al intermetallic compound layer is decreased with increasing the treatment time.
Fig. 6. SEM micrographs and EDX analysis of double layer after barrel nitriding at 893 K for 36 ks.
J.H. Kong et al. / Progress in Organic Coatings 76 (2013) 1841–1845
1845
The area of the aluminum nitride layer (II) and Fe–Al compound layer (III) is increased with increasing the treatment time. 4. Conclusion The aluminum nitride layer (AlN) and Fe–Al intermetallic compound layers formed between pure aluminum and austenitic stainless steel after barrel nitriding at 893 K for 18 ks, 25.2 ks and 36 ks, respectively. The obtained results are as follows. • The total thickness of double layer shows 160 m at 893 K for 18 ks treated specimen. The thickness of double layers is increased with increasing the treatment time. • The double layer is formed very uniformly and thickly at the interface of the pure aluminum and austenitic stainless steel compared with the aluminum nitride single layer on the outside surface of the pure aluminum. • The element of Al which has an affinity with Fe is diffused to the austenitic stainless steel surface to form the Fe–Al intermetallic layer while the aluminum nitride layer is growing. • The maximum hardness of the aluminum nitride layer and Fe–Al compound layer shows 380 HV and 910 HV, respectively, after barrel nitriding at 893 K for 18 ks. References [1] I. Yonegawa, A. Nikolaev, Y. Melnik, V. Dmitriev, J. Appl. Phys. Jpn. 40 (2001) 426. [2] I. Yonegawa, T. Shima, M.H.F. Sluiter, J. Appl. Phys. Jpn. 41 (2002) 4620. [3] M. Okumiya, Y. Tsunekawa, H. Sugiyama, Y. Tanaka, N. Takano, M. Tomimoto, Surf. Coat. Technol. 200 (2005) 35. [4] J.H. Kong, D.J. Lee, H.Y. On, S.J. Park, S.K. Kim, C.Y. Kang, J.H. Sung, H.W. Lee, Met. Mater. Int. 16 (2010) 857. [5] V.N. Yeremenko, Y.V. Natanzon, V.I. Dybkov, J. Mater. Sci. 16 (1981) 1748. [6] C.C. Lee, E.S. Machlin, H. Rathore, J. Appl. Phys. 71 (1992) 5877. [7] V.G. Gurtler, K. Sagel, Z. Metallkde. 46 (1955) 738. [8] T. Morinaga, Y. Kato, J. Jpn. Inst. Metals 19 (1955) 578. [9] S. Koda, S. Morozumi, A. Kanai, J. Jpn. Inst. Metals 26 (1962) 764. [10] M.V. Akdeniz, A.O. Mekhrabov, Acta Mater. 46 (1998) 1185. [11] U.R. Kattner, T.B. Massalski, in: H. Baker (Ed.), Binary Alloy Phase Diagrams, ASM International, Material Park, OH, 1990, p. 147. [12] S. Kobayashi, T. Yakou, Mater. Sci. Eng. A 338 (2002) 44. [13] M.A. Vasylyev, S.P. Chenakin, L.F. Yatsenko, Acta Mater. 60 (2012) 6223. [14] B.N. Mordyuk, G.I. Prokopenko, J. Sound Vib. 308 (2007) 855.
Fig. 7. Vickers hardness profile on a cross-section of the modified layers after barrel nitriding at 893 K for (a) 18 ks, (b) 25.2 ks and (c) 36 ks, respectively.
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
AlN and intermetallic compound layers formed between aluminum and austenitic stainless steel using barrel nitriding J.H. Kong a , M. Okumiya a,∗ , Y. Tsunekawa a , S.G. Kim b , M. Yoshida c a b c
Toyota Technological Institute, 2-12-1, Hisakata, Tempaku, Nagoya 468-8511, Japan Korea Institute of Industrial Technology, 7-47, Songdo, Yeonsu, Incheon 406-840, South Korea Shizuoka Institute of Science and Technology, 2200-2, Toyosawa, Hukuroi 437-8555, Japan
a r t i c l e
i n f o
Available online 19 June 2013
a b s t r a c t Recently, a new nitriding process was proposed to produce the aluminum nitride on an aluminum surface using a barrel. After barrel nitriding, AlN nitride layer is formed on the aluminum surface and the surface hardness can be improved remarkably. In this study, barrel nitriding was performed to investigate the interface between aluminum substrate, with SUS304 austenitic stainless steel used for a physical catalyst. The barrel nitriding was carried out at 893 K for 18 ks, 25.2 ks and 36 ks, respectively with aluminum and aluminum–magnesium alloy powder. After barrel nitriding, aluminum nitride layer and Fe–Al intermetallic compound layers were formed at the interface between pure aluminum and austenitic stainless steel at the same time. The thickness of the aluminum nitride layer and intermetallic layer was increased by increasing the treatment time. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Pure aluminum and aluminum alloys have numerous industrial applications due to their outstanding properties such as light weight and high specific strength. Aluminum alloys are one of the most promising materials for sliding parts in automotive power trains. However, various surface treatments are indispensable to improve tribologic properties because the hardness and wear resistance of the aluminum alloys are inferior to those of steels. Aluminum nitride is one of the effective protection films with high hardness and high thermal conductivity [1,2]. Aluminum nitride film has been produced using various processing techniques such as plasma nitriding, CVD, PVD, spray coating and gas nitriding such studies have been reported by many researchers [3,4]. In the conventional process, the nitriding of the aluminum surface is very difficult because of the thin aluminum oxide layer existing on the surface. This layer prevents the diffusion of nitrogen into the aluminum substrate. Therefore, the growth rate of the aluminum nitride layer is very sluggish. To solve this problem, numerous studies have been conducted on the aluminum nitriding process using plasma nitriding. This method requires a sputtering of the aluminum surface to remove the oxide layer prior to the nitriding, and takes considerable time. The aluminum nitride film thickness obtained by this method is between 5 and 15 m, and typically
∗ Corresponding author. E-mail address: [email protected] (M. Okumiya). 0300-9440/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.porgcoat.2013.05.026
much thinner. Recently, a new nitriding process for Al, “barrel nitriding”, has been proposed [3]. The interfacial reaction between steel and aluminum such as brazing, welding, diffusion bonding and hot dip coating are known [5]. Studies have already examined the chemical compositions and growth kinetics of the intermetallic layers [5–7] and the effect of impurities on their growth [8,9] during hot dip coating of steels in molten aluminum. Diffusion bonding and hot dip coating are usually performed in the temperature range of solid Fe/solid Al diffusion from 873 to 973 K. Fe–Al intermetallic compound layers formed under such conditions are usually FeAl3 and Fe2 Al5 [1–10]. These intermetallic compound layers have high hardness and brittle matrix. In this study, we proposed the new process that a combined method of AlN and Fe–Al intermetallic compound layer at the interface between pure Al and SUS304 austenitic stainless steel using barrel nitriding.
2. Experimental procedures The substrates of JIS-A1050 commercial grade pure Al and SUS304 austenitic stainless steel measured 20 width × 50 length × 3 thickness (mm). Al2 O3 particles (average diameter 0.1 mm) and Al 50 wt.% Mg powder (average diameter 0.2 mm) were used for the barrel filler. The furnace temperature was adjusted with a temperature control unit. Nitrogen gas (N2 ) was introduced into the barrel; the N2
1842
J.H. Kong et al. / Progress in Organic Coatings 76 (2013) 1841–1845
Fig. 2. The Fe–Al phase diagram [11].
measurements (Akashi, HM-125) were performed under a load of 100 g on the cross-sectional region. 3. Results and discussion
Fig. 1. Schemetaic diagram of the barrel nitriding and treatment cycle.
flow rate was adjusted with a program control unit and a mass flow controller. The furnace was evacuated about 7.5 × 10–1 torr and the atmosphere was substituted by N2 . Before increasing temperature, a vacuum and nitrogen gas introduction process was repeated for sufficient N2 gas atmospheres without oxygen. After the preparation process at room temperature, barrel nitriding was carried out at 893 K for 18 ks, 25.2 ks and 36 ks, respectively. Then, the specimen was cooled in the barrel. The schematic diagram and treatment cycle is indicated as shown in Fig. 1. The inside temperature of the chamber with powder was measured by thermocouple; the temperature, and N2 gas flowrate were monitored with a data processor. The barrel nitriding process parameters are shown in Table 1. Optical microscopy was used for thickness measurements and structure observations of the AlN and Fe–Al intermetallic compound layers, and scanning electron microscopy (SEM-EDX) (Hitachi, SU6600) was used to observe the phases and modified layers. The micro Vickers hardness
Table 1 Treatment conditions. Nitriding temperature T (K) Heating time (ks) Nitriding time (ks) Nitriding gas Nitrogen gas flow (cc/min) Filler Al–Mg/Al2 O3 ratio (wt.%)
893 5.4 18, 25.2 and 36 Nitrogen (N2 ) 1500 Al2 O3 (average dia. 0.1 mm) Al–50wt.%Mg (average dia. 0.2 mm) 1.8
Fig. 2 is the Fe–Al phase diagram [11] which contains five types of Fe–Al intermetallic compound. FeAl2 , Fe2 Al5 and FeAl3 compounds are known to have a high aluminum composition with high hardness and brittleness. Fe3 Al and FeAl compounds also have good mechanical properties in terms of wear resistance, oxidation resistance, corrosion resistance and specific strength properties [12]. In this study, it can be expected that the intermetallic Fe–Al compound layer will be formed at the interface between pure aluminum and austenitic stainless steel by diffusion of Al with the nitride layer due to the permeation of nitrogen atom during barrel nitriding under melting temperature. Fig. 3 shows the variation in the nitride layer on the surface of the pure aluminum substrate after barrel nitriding at 893 K at different treatment times. After barrel nitriding, some parts of the layer begin to form aluminum nitride (AlN) at 893 K for 18 ks. The selected area EDX spectrum for the small part of the layer on the outmost surface, presented in Fig. 3(a), reveal K␣ peaks for N and Al at 0.38 keV and 1.48 keV, respectively, in addition to the peak for Mg as shown in Fig. 3(d). The weight percent of N and Al shows 14.2 wt.% and 84.8 wt.%, respectively, with a small amount of Mg and O under 1 wt.%. The thickness of the aluminum nitride layer is increased with increasing the treatment time. Finally, the layer covers the entire substrate surface up to the thickness of 200 m at 36 ks. Generally, the aluminum is acknowledged to have oxidation film on outmost surface. Therefore, the formation of nitride layer is very difficult due to the oxidation film. This thin film can be removed by the activation of Al2 O3 powder and Al–Mg alloy powder in this study. However, the formation of aluminum oxide occurs more rapidly than AlN, because Al has a strong affinity with oxygen compared to nitrogen. Thus the growth rate of nitride layer can be fast depending on the high vacuum rate. The SUS304 austenitic stainless steel was attached on pure aluminum to form the nitride layer and Fe–Al intermetallic compound layer. As shown in Fig. 4 optical micrographs, double layers are formed at the interface between pure aluminum and austenitic stainless steel after barrel nitriding at 893 K for 18 ks, 25.2 ks and 36 ks respectively. An aluminum nitride layer is considered due to the affinity between Al and N. The other one is a Fe–Al intermetallic compound layer by Al diffusion from the aluminum substrate and the AlN layer. The thickness of the aluminum nitride layer and Fe–Al intermetallic compound layer shows 90 m and
J.H. Kong et al. / Progress in Organic Coatings 76 (2013) 1841–1845
1843
Fig. 3. Cross-sectional optical micrographs on the surface of pure aluminum after barrel nitriding at 893 K for (a) 18 ks, (b) 25.2 ks, (c) 36 ks and (d) EDX spectra result of (a) nitride layer.
70 m, respectively, at the treatment time of 18 ks. The thickness of the double layer is gradually increased with increasing the treatment time. Therefore, total thickness of the double layer is also increased with increasing the treatment time from 160 m to 310 m. This double layer is formed very uniformly compared with the aluminum nitride single layer on the pure aluminum surface as shown in Fig. 3. Furthermore, the formation and growth rate of nitride layer are faster than those of a single layer without the problem of poor surface roughness. It is expected that the reactivity of nitrogen is more active at the interface between pure aluminum substrate and SUS304 austenitic stainless steel in comparison with the outside surface. Hence, the nitrogen atoms can be easily decomposed from N2 to N atoms due to the vibration in the narrow interface by crashing both sides of the substrates. At this time, the oxidation film is also removed more easily by the impact energy of nitrogen atoms. Thus, it is considered that the decomposed nitrogen atoms can react with Al easily after removing the oxidation film [13,14]. During the growth of the nitride layer,
the aluminum is diffused to the austenitic stainless steel surface from the nitride layer when the latter contacts the steel surface. The growth direction of the aluminum nitride layer is proceeding toward the aluminum substrate simultaneously. Fig. 5 shows a schematic illustration of the growth process for the nitride layer and Fe–Al intermetallic compound layer at the interface between the pure aluminum and austenitic stainless steel during barrel nitriding. Fig. 6 shows the SEM micrographs and EDX analysis results of the double compound layer between the pure aluminum and austenitic stainless steel. As shown in Fig. 4(a), the double layer is composed of the aluminum nitride layer and Fe–Al intermetallic compound layer by the diffusion of aluminum. Fig. 6(b) shows the result of line mapping. The profile of Al amount is seen to decrease toward the austenitic stainless steel from the pure aluminum substrate. The element of Al, which has a strong affinity with Fe, is thereby diffused to the austenitic stainless steel surface to form the Fe–Al intermetallic layer while the aluminum nitride layer is
Fig. 4. Optical micrographs of double compound layers between pure aluminum and austenitic stainless steel after barrel nitriding at 893 K for (a) 18 ks, (b) 25.2 ks and (c) 36 ks.
1844
J.H. Kong et al. / Progress in Organic Coatings 76 (2013) 1841–1845
Fig. 5. The schematic illustration about the growth process of aluminum nitride and Fe–Al intermetallic compound layer during barrel nitriding.
growing. Therefore, the Al-riched FeAl2 , Fe2 Al5 or FeAl3 compound is expected to exist near the aluminum nitride layer, while the Feriched Fe3 Al or FeAl compounds exist near the austenitic stainless steel. Fig. 7 shows the Vickers hardness profile on a cross-section of the modified double layers. The hardness of Pure Al and austenitic stainless steel shows about 30 HV (0.2) and 200 HV (0.2),
respectively. However, the area of the aluminum nitride layer shows over 200 HV (0.1) and Fe–Al compound layer shows about 800 HV (0.1). The maximum hardness of the AlN layer and Fe–Al intermetallic compound layer shows about 380 HV and 910 HV, respectively, after barrel nitriding at 893 K for 18 ks. The maximum hardness of the aluminum nitride layer and Fe–Al intermetallic compound layer is decreased with increasing the treatment time.
Fig. 6. SEM micrographs and EDX analysis of double layer after barrel nitriding at 893 K for 36 ks.
J.H. Kong et al. / Progress in Organic Coatings 76 (2013) 1841–1845
1845
The area of the aluminum nitride layer (II) and Fe–Al compound layer (III) is increased with increasing the treatment time. 4. Conclusion The aluminum nitride layer (AlN) and Fe–Al intermetallic compound layers formed between pure aluminum and austenitic stainless steel after barrel nitriding at 893 K for 18 ks, 25.2 ks and 36 ks, respectively. The obtained results are as follows. • The total thickness of double layer shows 160 m at 893 K for 18 ks treated specimen. The thickness of double layers is increased with increasing the treatment time. • The double layer is formed very uniformly and thickly at the interface of the pure aluminum and austenitic stainless steel compared with the aluminum nitride single layer on the outside surface of the pure aluminum. • The element of Al which has an affinity with Fe is diffused to the austenitic stainless steel surface to form the Fe–Al intermetallic layer while the aluminum nitride layer is growing. • The maximum hardness of the aluminum nitride layer and Fe–Al compound layer shows 380 HV and 910 HV, respectively, after barrel nitriding at 893 K for 18 ks. References [1] I. Yonegawa, A. Nikolaev, Y. Melnik, V. Dmitriev, J. Appl. Phys. Jpn. 40 (2001) 426. [2] I. Yonegawa, T. Shima, M.H.F. Sluiter, J. Appl. Phys. Jpn. 41 (2002) 4620. [3] M. Okumiya, Y. Tsunekawa, H. Sugiyama, Y. Tanaka, N. Takano, M. Tomimoto, Surf. Coat. Technol. 200 (2005) 35. [4] J.H. Kong, D.J. Lee, H.Y. On, S.J. Park, S.K. Kim, C.Y. Kang, J.H. Sung, H.W. Lee, Met. Mater. Int. 16 (2010) 857. [5] V.N. Yeremenko, Y.V. Natanzon, V.I. Dybkov, J. Mater. Sci. 16 (1981) 1748. [6] C.C. Lee, E.S. Machlin, H. Rathore, J. Appl. Phys. 71 (1992) 5877. [7] V.G. Gurtler, K. Sagel, Z. Metallkde. 46 (1955) 738. [8] T. Morinaga, Y. Kato, J. Jpn. Inst. Metals 19 (1955) 578. [9] S. Koda, S. Morozumi, A. Kanai, J. Jpn. Inst. Metals 26 (1962) 764. [10] M.V. Akdeniz, A.O. Mekhrabov, Acta Mater. 46 (1998) 1185. [11] U.R. Kattner, T.B. Massalski, in: H. Baker (Ed.), Binary Alloy Phase Diagrams, ASM International, Material Park, OH, 1990, p. 147. [12] S. Kobayashi, T. Yakou, Mater. Sci. Eng. A 338 (2002) 44. [13] M.A. Vasylyev, S.P. Chenakin, L.F. Yatsenko, Acta Mater. 60 (2012) 6223. [14] B.N. Mordyuk, G.I. Prokopenko, J. Sound Vib. 308 (2007) 855.
Fig. 7. Vickers hardness profile on a cross-section of the modified layers after barrel nitriding at 893 K for (a) 18 ks, (b) 25.2 ks and (c) 36 ks, respectively.