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Use of cathodic cage in plasma nitriding
Surface & Coatings Technology 201 (2006) 2450 – 2454 www.elsevier.com/locate/surfcoat
Use of cathodic cage in plasma nitriding C. Alves Jr. a,⁎, F.O. de Araújo a , K.J.B. Ribeiro a , J.A.P. da Costa a , R.R.M. Sousa b , R.S. de Sousa c a b
Labplasma, Departamento de Física - UFRN, Campus Universitário, 59072-970 Natal, RN, Brazil Centro Federal de Educação Tecnológica do Piauí, Departamento de Mecânica, Teresina PI, Brazil c Centro Federal de Educação Tecnológica da Paraíba, Cajazeira PB, Brazil Received 15 February 2006; accepted in revised form 18 April 2006 Available online 12 June 2006
Abstract Cylindrical samples of 1020 steel and 316 stainless steel were nitrided under the conditions by conventional dc plasma nitriding (DCPN) and by a new technique denominate cathodic cage plasma nitriding (CCPN). The 1020 and 316 stainless steel samples were treated during 3 h and 5 h, respectively, in 773 K and 360 Pa. The samples were characterized by optical microscopy, X-ray diffraction and microhardness testing. All the samples nitrided by DCPN process presented erosion rings on the surface exposed to the plasma. In comparison, in samples nitrided by CCPN, the erosion rings were completely eliminated, without loss of the mechanical properties in the different phases of existence in the nitrided layer. © 2006 Elsevier B.V. All rights reserved. Keywords: Cathodic cage; Plasma nitriding; Edge effect
1. Introduction The DCPN has been industrially accepted, being used to improve several physical properties of metallic surfaces as hardness, wear and corrosion resistance that contribute to increase the use of the nitrided samples. This process presents advantages in comparison with the conventional nitriding processes, for example, the non-emission of pollutants, energy economy and lesser treatment time [1], although there are some inconveniences to treat components with complex geometry. The components treated are submitted to a high cathodic potential to produce the plasma directly in its surface. Due to distortions of the electric field around the corners and edges, the shape of plasma sheath, which is connected to the shape of samples, determines the ion flux distribution, which, in turn, affects the uniformity, hardness and surface phases of coating, erosion rings occur, characterized by the reduction of hardness [2,3]. This effect happens mainly in treatments made with high working pressure (> 100 Pa) and ⁎ Corresponding author. Tel.: +55 84 215 3800; fax: +55 84 215 3791. E-mail address: [email protected] (C. Alves). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.04.014
in materials with high content of alloy elements that produce nitrides, as chromium [4]. In this work, the CCPN technique was used, derived from the active screen plasma nitriding (ASPN), recently developed [5–7], to eliminate totally such edge effect. In this process, the sample was involved in a cage, in which the cathodic potential was applied. The samples were placed on an isolant substrate, remaining in a floating potential, and then it was treated in a post-discharge. In this process, the edge effect was completely eliminated, since the plasma was formed on the cage and not directly onto the samples. Radiation from the heated cage supplies the heat necessary to the temperature for treatment. 2. Experimental setup The materials used in this study were 1020 steel with ferritic structure and low content of alloy elements, and 316 stainless steel with austenitic structure, containing alloy elements able to produce nitrides. The samples (diameter: 8 mm and height: 10 mm) were machined and then annealed. The metallographic analysis was accomplished, where the top surfaces of the samples were
C. Alves Jr. et al. / Surface & Coatings Technology 201 (2006) 2450–2454
ground, using sandpapers from 220 to 1200 mesh, and polished with alumina (0.3 and 1 μm) and diamond slurry (1 and 3 μm). They were further cleaned ultrasonically in acetone bath and dried before placement into to vacuum chamber. The system used for plasma nitriding consists of a high voltage DC source (maximum output 1500 V, 2 A), a vertically mounted cylindrical vacuum chamber (40 cm in diameter and 40 cm in height, made of stainless steel), but with cathodic cage, gas input and evacuation components and process parameter sensors and controllers, as shown in Fig. 1. The plasma was generated in a negatively polarized cage (cathode) and the anode (rest of the chamber) held at ground potential. Samples were positioned onto isolating substrate as indicated in Fig. 1b to guarantee the same distance from the top and from lateral of the cage. The cage was made of austenitic 316 stainless steel (diameter: 76 mm and height: 25 mm), containing a removable cover. The cage walls thickness is 0.8 mm, with holes diameter of 7.6 mm and distance between the centers of adjacent holes of 9.2 mm. The working temperature was 773 K for the 1020 steel and to 316 stainless steel. The nitriding time was 3 h to 1020 steel and
2451
Fig. 2. Visual aspect of steel samples treated by (A) DCPN and (B) CCPN.
5 h to 316 stainless steel. Before nitriding, the system was pumped down by a two-stage rotary pump until a residual pressure of about 1 Pa was reached. Then, gas mixture with the composition of 20% N2–H2 for DCPN process and 80% N2–H2 for CCPN was introduced and its flow adjusted to 10 sccm using a mass flow controller. The treatment pressure of 360 Pa, measured by a barocel capacitance manometer, was adjusted manually. The phase composition and texture was analysed using X-ray diffraction (XRD). The analyses described here were performed using Cu Kα lines (wavelength: 0.154 nm), operated at 40 kV in a XRD instrument (Shimadzu, XRD-6000). Optical microscope was used to observe morphology and thickness of nitrided layer. Finally, microhardness profile was carried through to evaluate uniformity and the appearance of edge effect. 3. Results and discussion
Fig. 1. (A) The reactor scheme of plasma ionic nitride and (B) configuration of cathode used onto CCPN process.
The macroscopic analysis of the samples shows that the DCPN process produces a non-uniform surface. The samples presented different colors in the central and peripheric regions. This phenomenon is known as edge effect, a common problem associated to the DCPN nitriding. In comparison, the plasma nitriding by CCPN produces a uniform dark gray color in the whole surface of the samples (Fig. 2). In this process, as the plasma is not formed directly on the samples surface, these do not suffer with the active sputtering and the defects caused for the edge effects are eliminated. Furthermore, the heat used to warm up the samples to reach the treatment temperature is supplied by the radiation coming from the cage, what promotes a higher homogeneity of the temperature in the treated samples [5]. The thickness and morphology of the layers in nitrided samples had not present variation with the nitriding method
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C. Alves Jr. et al. / Surface & Coatings Technology 201 (2006) 2450–2454
Fig. 3. The samples micrograph of stainless steel AISI 316 and AISI 1020 nitrided samples, in (A) conventional method and (B) CCPN method.
used, in the same temperature of treatment (Fig. 3), allowing to obtain similar thickness and compositions using the cage technique, without the inconveniences of the DCPN method. Comparative micrographs for sample AISI 316 show austenite grains for all samples. The thickness of the composite layer was 18 μm. For the steel 1020 samples, the average thickness for both processes measure was 5.2 μm. Microhardness analysis of the top surface was done in the area of the erosion rings of the DCPN samples. For comparison effect, the indentation points in the CCPN samples were made in the same distance of the measures in the DCPN samples. Fig. 4 presents the comparative results of microhardness, with three indentations points for each sample, in both processes. In the first technique (DCPN), a hardness reduction in the erosion ring region was observed, compared with the central area. Furthermore, the microhardness magnitude is much bigger in edges area, due to thermal gradient [2]. This can cause the coating to break along the edge line. In CCPN samples, on the contrary, the hardness values stayed uniform in the whole sample and present the same order of magnitude that the central region of samples treated in DCPN. The X-ray diffraction patterns shown in Fig. 5 indicate that, for both DCPN and CCPN, similar microstructures were produced. However, in DCPN process occurs a larger formation of γ′-Fe4N phase, due to the direct interaction of the plasma species with the sample surface, that promotes a larger rate sputtering and decarburizing [8–10]. In the CCPN process, the sputtering does not occur on the samples
surface, but in the cage, eliminating the surface decarburizing, that contributes to the largest formation of ε-Fe2–3N phase. The concentration of nitrogen in the ε-Fe2–3N (7.7– 11%) phase is larger than the γ′-Fe4N (5.9%) phase that promotes a higher concentration of surface nitrogen in CCPN process [7,11]. 4. Conclusions The plasma nitriding using CCPN produces similar properties, such as microhardness, composition and thickness that central area of samples treated in DCPN process. However, the CCPN nitriding did not present the common problems associated to the DCPN process. According to the hardness profile was possible to observe a great homogeneity in the obtained layers, proving the elimination of the edge effects in this process. For the 316 stainless steel, a thickness of the composite layers with 18 μm (5 h) was obtained in both DCPN and CCPN processes, while for 1020 steel, the thickness of the composite layers for same time (3 h) was 5.2 μm. The CCPN process favors the formation of ε nitride, due to the non-occurrence of sputtering on the samples surface. Acknowledgements This work was partially supported by the Brazilian agencies, CAPES and CNPq.
C. Alves Jr. et al. / Surface & Coatings Technology 201 (2006) 2450–2454
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Profile along the surface (µm) Fig. 4. The upper part: top view of the samples at the same scale as the x-axis on the graph. The lower part: radial microhardness in the top surface of the samples (A) DCPN and (B) CCPN. The dashed vertical lines show the borders of the distinctive DCPN regions and connect the two parts of the figure.
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C. Alves Jr. et al. / Surface & Coatings Technology 201 (2006) 2450–2454
References [1] C. Alves Jr., Plasma Nitriding: Foundations and Applications, EDFRN, Natal, 2001. [2] R.S. Sousa, DE, Influence of the geometry of the pieces and process parameters about the characteristics of the nitrided layer in plasma, Doctor thesis. UFRN, Natal/RN, 2005 (In Portuguese). [3] F.O. Araújo, Influence of plasma parameters in ionic nitride. Master thesis. UFRN, Natal/RN, 2001 (In Portuguese). [4] C. Alves Jr., E.F. Silva, A.E. Martinelli, Surf. Technol. 139 (1) (2001) 1. [5] C.X. Li, J. Georges, X.Y. Li, Surf. Eng. 18 (6) (2002) 453. [6] J. George, Heat Treat. Met. 28 (2) (2001) 33. [7] C.X. Li, T. Bell, H. Dong, Surf. Eng. 18 (3) (2002) 174. [8] C. Ruset, Surf. Coat. Technol. 174–175 (2003) 1201. [9] C.A. Santos, et al., Mater. Sci. Eng., A 256 (1998) 60. [10] T. Lampe, S. Eisenberg, G. Landien, Surf. Eng. 9 (1) (1993) 69. [11] D. Cleugh, Surf. Eng. 18 (2) (2002) 133.
Fig. 5. X-ray spectroscopy of the samples DCPN and CCPN for (A) AISI 1020 and (B) AISI 316.
Use of cathodic cage in plasma nitriding C. Alves Jr. a,⁎, F.O. de Araújo a , K.J.B. Ribeiro a , J.A.P. da Costa a , R.R.M. Sousa b , R.S. de Sousa c a b
Labplasma, Departamento de Física - UFRN, Campus Universitário, 59072-970 Natal, RN, Brazil Centro Federal de Educação Tecnológica do Piauí, Departamento de Mecânica, Teresina PI, Brazil c Centro Federal de Educação Tecnológica da Paraíba, Cajazeira PB, Brazil Received 15 February 2006; accepted in revised form 18 April 2006 Available online 12 June 2006
Abstract Cylindrical samples of 1020 steel and 316 stainless steel were nitrided under the conditions by conventional dc plasma nitriding (DCPN) and by a new technique denominate cathodic cage plasma nitriding (CCPN). The 1020 and 316 stainless steel samples were treated during 3 h and 5 h, respectively, in 773 K and 360 Pa. The samples were characterized by optical microscopy, X-ray diffraction and microhardness testing. All the samples nitrided by DCPN process presented erosion rings on the surface exposed to the plasma. In comparison, in samples nitrided by CCPN, the erosion rings were completely eliminated, without loss of the mechanical properties in the different phases of existence in the nitrided layer. © 2006 Elsevier B.V. All rights reserved. Keywords: Cathodic cage; Plasma nitriding; Edge effect
1. Introduction The DCPN has been industrially accepted, being used to improve several physical properties of metallic surfaces as hardness, wear and corrosion resistance that contribute to increase the use of the nitrided samples. This process presents advantages in comparison with the conventional nitriding processes, for example, the non-emission of pollutants, energy economy and lesser treatment time [1], although there are some inconveniences to treat components with complex geometry. The components treated are submitted to a high cathodic potential to produce the plasma directly in its surface. Due to distortions of the electric field around the corners and edges, the shape of plasma sheath, which is connected to the shape of samples, determines the ion flux distribution, which, in turn, affects the uniformity, hardness and surface phases of coating, erosion rings occur, characterized by the reduction of hardness [2,3]. This effect happens mainly in treatments made with high working pressure (> 100 Pa) and ⁎ Corresponding author. Tel.: +55 84 215 3800; fax: +55 84 215 3791. E-mail address: [email protected] (C. Alves). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.04.014
in materials with high content of alloy elements that produce nitrides, as chromium [4]. In this work, the CCPN technique was used, derived from the active screen plasma nitriding (ASPN), recently developed [5–7], to eliminate totally such edge effect. In this process, the sample was involved in a cage, in which the cathodic potential was applied. The samples were placed on an isolant substrate, remaining in a floating potential, and then it was treated in a post-discharge. In this process, the edge effect was completely eliminated, since the plasma was formed on the cage and not directly onto the samples. Radiation from the heated cage supplies the heat necessary to the temperature for treatment. 2. Experimental setup The materials used in this study were 1020 steel with ferritic structure and low content of alloy elements, and 316 stainless steel with austenitic structure, containing alloy elements able to produce nitrides. The samples (diameter: 8 mm and height: 10 mm) were machined and then annealed. The metallographic analysis was accomplished, where the top surfaces of the samples were
C. Alves Jr. et al. / Surface & Coatings Technology 201 (2006) 2450–2454
ground, using sandpapers from 220 to 1200 mesh, and polished with alumina (0.3 and 1 μm) and diamond slurry (1 and 3 μm). They were further cleaned ultrasonically in acetone bath and dried before placement into to vacuum chamber. The system used for plasma nitriding consists of a high voltage DC source (maximum output 1500 V, 2 A), a vertically mounted cylindrical vacuum chamber (40 cm in diameter and 40 cm in height, made of stainless steel), but with cathodic cage, gas input and evacuation components and process parameter sensors and controllers, as shown in Fig. 1. The plasma was generated in a negatively polarized cage (cathode) and the anode (rest of the chamber) held at ground potential. Samples were positioned onto isolating substrate as indicated in Fig. 1b to guarantee the same distance from the top and from lateral of the cage. The cage was made of austenitic 316 stainless steel (diameter: 76 mm and height: 25 mm), containing a removable cover. The cage walls thickness is 0.8 mm, with holes diameter of 7.6 mm and distance between the centers of adjacent holes of 9.2 mm. The working temperature was 773 K for the 1020 steel and to 316 stainless steel. The nitriding time was 3 h to 1020 steel and
2451
Fig. 2. Visual aspect of steel samples treated by (A) DCPN and (B) CCPN.
5 h to 316 stainless steel. Before nitriding, the system was pumped down by a two-stage rotary pump until a residual pressure of about 1 Pa was reached. Then, gas mixture with the composition of 20% N2–H2 for DCPN process and 80% N2–H2 for CCPN was introduced and its flow adjusted to 10 sccm using a mass flow controller. The treatment pressure of 360 Pa, measured by a barocel capacitance manometer, was adjusted manually. The phase composition and texture was analysed using X-ray diffraction (XRD). The analyses described here were performed using Cu Kα lines (wavelength: 0.154 nm), operated at 40 kV in a XRD instrument (Shimadzu, XRD-6000). Optical microscope was used to observe morphology and thickness of nitrided layer. Finally, microhardness profile was carried through to evaluate uniformity and the appearance of edge effect. 3. Results and discussion
Fig. 1. (A) The reactor scheme of plasma ionic nitride and (B) configuration of cathode used onto CCPN process.
The macroscopic analysis of the samples shows that the DCPN process produces a non-uniform surface. The samples presented different colors in the central and peripheric regions. This phenomenon is known as edge effect, a common problem associated to the DCPN nitriding. In comparison, the plasma nitriding by CCPN produces a uniform dark gray color in the whole surface of the samples (Fig. 2). In this process, as the plasma is not formed directly on the samples surface, these do not suffer with the active sputtering and the defects caused for the edge effects are eliminated. Furthermore, the heat used to warm up the samples to reach the treatment temperature is supplied by the radiation coming from the cage, what promotes a higher homogeneity of the temperature in the treated samples [5]. The thickness and morphology of the layers in nitrided samples had not present variation with the nitriding method
2452
C. Alves Jr. et al. / Surface & Coatings Technology 201 (2006) 2450–2454
Fig. 3. The samples micrograph of stainless steel AISI 316 and AISI 1020 nitrided samples, in (A) conventional method and (B) CCPN method.
used, in the same temperature of treatment (Fig. 3), allowing to obtain similar thickness and compositions using the cage technique, without the inconveniences of the DCPN method. Comparative micrographs for sample AISI 316 show austenite grains for all samples. The thickness of the composite layer was 18 μm. For the steel 1020 samples, the average thickness for both processes measure was 5.2 μm. Microhardness analysis of the top surface was done in the area of the erosion rings of the DCPN samples. For comparison effect, the indentation points in the CCPN samples were made in the same distance of the measures in the DCPN samples. Fig. 4 presents the comparative results of microhardness, with three indentations points for each sample, in both processes. In the first technique (DCPN), a hardness reduction in the erosion ring region was observed, compared with the central area. Furthermore, the microhardness magnitude is much bigger in edges area, due to thermal gradient [2]. This can cause the coating to break along the edge line. In CCPN samples, on the contrary, the hardness values stayed uniform in the whole sample and present the same order of magnitude that the central region of samples treated in DCPN. The X-ray diffraction patterns shown in Fig. 5 indicate that, for both DCPN and CCPN, similar microstructures were produced. However, in DCPN process occurs a larger formation of γ′-Fe4N phase, due to the direct interaction of the plasma species with the sample surface, that promotes a larger rate sputtering and decarburizing [8–10]. In the CCPN process, the sputtering does not occur on the samples
surface, but in the cage, eliminating the surface decarburizing, that contributes to the largest formation of ε-Fe2–3N phase. The concentration of nitrogen in the ε-Fe2–3N (7.7– 11%) phase is larger than the γ′-Fe4N (5.9%) phase that promotes a higher concentration of surface nitrogen in CCPN process [7,11]. 4. Conclusions The plasma nitriding using CCPN produces similar properties, such as microhardness, composition and thickness that central area of samples treated in DCPN process. However, the CCPN nitriding did not present the common problems associated to the DCPN process. According to the hardness profile was possible to observe a great homogeneity in the obtained layers, proving the elimination of the edge effects in this process. For the 316 stainless steel, a thickness of the composite layers with 18 μm (5 h) was obtained in both DCPN and CCPN processes, while for 1020 steel, the thickness of the composite layers for same time (3 h) was 5.2 μm. The CCPN process favors the formation of ε nitride, due to the non-occurrence of sputtering on the samples surface. Acknowledgements This work was partially supported by the Brazilian agencies, CAPES and CNPq.
C. Alves Jr. et al. / Surface & Coatings Technology 201 (2006) 2450–2454
2453
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Microhardness (HV 0.1 )
1200
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Profile along the surface (µm)
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Microhardness (HV0.1 )
550 500 450 400 350 300 0
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1000
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Profile along the surface (µm) Fig. 4. The upper part: top view of the samples at the same scale as the x-axis on the graph. The lower part: radial microhardness in the top surface of the samples (A) DCPN and (B) CCPN. The dashed vertical lines show the borders of the distinctive DCPN regions and connect the two parts of the figure.
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C. Alves Jr. et al. / Surface & Coatings Technology 201 (2006) 2450–2454
References [1] C. Alves Jr., Plasma Nitriding: Foundations and Applications, EDFRN, Natal, 2001. [2] R.S. Sousa, DE, Influence of the geometry of the pieces and process parameters about the characteristics of the nitrided layer in plasma, Doctor thesis. UFRN, Natal/RN, 2005 (In Portuguese). [3] F.O. Araújo, Influence of plasma parameters in ionic nitride. Master thesis. UFRN, Natal/RN, 2001 (In Portuguese). [4] C. Alves Jr., E.F. Silva, A.E. Martinelli, Surf. Technol. 139 (1) (2001) 1. [5] C.X. Li, J. Georges, X.Y. Li, Surf. Eng. 18 (6) (2002) 453. [6] J. George, Heat Treat. Met. 28 (2) (2001) 33. [7] C.X. Li, T. Bell, H. Dong, Surf. Eng. 18 (3) (2002) 174. [8] C. Ruset, Surf. Coat. Technol. 174–175 (2003) 1201. [9] C.A. Santos, et al., Mater. Sci. Eng., A 256 (1998) 60. [10] T. Lampe, S. Eisenberg, G. Landien, Surf. Eng. 9 (1) (1993) 69. [11] D. Cleugh, Surf. Eng. 18 (2) (2002) 133.
Fig. 5. X-ray spectroscopy of the samples DCPN and CCPN for (A) AISI 1020 and (B) AISI 316.