- Email: [email protected]
Formation of S-phase layer on plasma sprayed AISI 316L stainless steel coating by plasma nitriding at low temperature
Thin Solid Films 523 (2012) 11–14
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Formation of S-phase layer on plasma sprayed AISI 316L stainless steel coating by plasma nitriding at low temperature Shinichiro Adachi ⁎, Nobuhiro Ueda Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi-shi, Osaka 594‐1157, Japan
a r t i c l e
i n f o
Available online 30 May 2012 Keywords: Plasma nitriding Plasma spraying Austenitic stainless steel Expanded austenite Diffusion Surface treatment
a b s t r a c t Low-temperature nitriding of austenitic stainless steels can produce expanded austenite, known as S-phase, leading to improved surface hardness while maintaining corrosion resistance. Sprayed AISI 316L coatings include oxide layers and defects (pores and cracks), and their structure is considerably different from AISI 316L steel plate structures. In this paper, plasma sprayed AISI 316L coating was treated to produce S-phase by lowtemperature plasma nitriding. The effects of nitriding temperatures and spraying conditions (input electrical power and particle size of the spray powder) on the S-phase layer were investigated, and also the nitrogen diffusion process was discussed. Under optimized spraying conditions, the sprayed coatings formed thicker nitride layers than those formed on AISI 316L steel plates; this was the result of a small amount of trapping of nitrogen by dissolved Cr. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Thermal sprayed austenitic stainless steel coatings are applied to various mechanical machines and equipment components as lowcost corrosion protection [1]. However, the sprayed austenitic stainless steel coating cannot function as a wear-resistant coating because of its poor wear properties. If the surface hardness of the sprayed austenitic stainless steel coating could be improved, while maintaining the corrosion resistance, the sprayed coating would then be useful in a greater variety of applications, for example, as an alternative to chrome plating and ceramic coating. Low-temperature nitriding could be considered an appropriate method for hardening a sprayed austenitic stainless steel coating because it has been reported that low-temperature nitriding at temperatures below 773 K can improve the hardness of austenitic stainless steels by producing expanded austenite at the surface [2,3]. The expanded austenite, called S-phase, has been identified as a supersaturated nitrogen solid-solution fcc material by Christiansen et al. [4]. Furthermore, the corrosion resistances of low-temperature nitrided stainless steels are almost the same as those of un-nitrided stainless steels [3]. However, a thermal sprayed coating is stacked with melted particles, and contains cracks, pores and oxide layers, therefore the lowtemperature nitriding of the sprayed coating will be different from that of the bulk stainless steel. It is a concern that the thickness of the
⁎ Corresponding author. Tel.: + 81 725 51 2648; fax: + 81 725 51 2749. E-mail address: [email protected] (S. Adachi). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.05.062
S-phase on the sprayed coating is not sufficient to improve the wearresistance. It has been reported that the thickness of the S-phase formed by nitriding on bulk stainless steel plates varies from a few micrometers to 20 μm [5]. For practical use, the S-phase on the sprayed coating should be same thickness as, or thicker than, that on bulk stainless steel plates. In this paper, a plasma sprayed AISI 316L coating was treated by low-temperature plasma nitriding. The effects of the nitriding temperature and the plasma spraying conditions on the S-phase were examined, to produce a thick S-phase. Also, the differences between the nitrogen diffusion processes in the sprayed AISI 316L coating and in the AISI 316L steel plate are discussed.
2. Experimental procedure AISI 316L powder was used as the spray material; the average particle diameters were 90 μm and 140 μm. The substrate was an AISI 316L plate; the specimen size was 25 mm × 55 mm × 5 mm. The substrate was roughened by blasting with alumina grit before spraying. The plasma spraying was performed using an Aeroplasma Corporation APS-7050 system in an air atmosphere using an input electrical power to the plasma torch of 21 kW, 31.5 kW or 42 kW. The plasma gas mass flows were Ar (8.3 × 10 − 5 m 3/s) and CO2 (16.7 × 10 − 5 m 3/s) at 21 kW and 31.5 kW, or Ar (8.3 × 10 − 5 m 3/s) and CO2 (20 × 10 − 5 m 3/s) at 42 kW. The spray distance was 0.15 m. A typical coating thickness was about 300 μm. After spraying, the coated surfaces were polished using metallographic 3 μm diamond paste to remove the surface oxide layer and smooth the surface.
12
S. Adachi, N. Ueda / Thin Solid Films 523 (2012) 11–14
The plasma nitriding equipment used was a laboratory type with direct current (DC) electrical power. The specimen was placed on a holder in the chamber, and the holder was connected to the cathode. Nitriding was performed in a gas mixture of N2 and H2 (80:20); the pressure in the chamber was 633 Pa. Specimen temperatures were maintained at 623 K, 673 K, 723 K or 773 K by controlling the plasma current; the specimen temperature was measured by a thermocouple inserted in a 1 mm diameter hole drilled in the specimen side. The treatment time was 4 h. X-ray diffraction measurements in the conventional θ-2θ scan were performed to evaluate the crystal structure of the nitrided layers by Rigaku RINT2000 using Cu-Kα radiation with 40 kV and 150 mA. Compositions of the sprayed coatings were investigated by SEM with energy-dispersive X-ray (EDX) analysis, SEM of Elionix ERAX3000 and EDX of AMETEK GENESIS-4000, and the operating voltage of the SEM was 20 kV. EDX analysis was performed on two different spots (of area 2.38 mm × 1.78 mm) on the surface of the sprayed coatings, and the measurement errors were less than 1%. In addition, the nitrogen depth distribution profiles were obtained using glowdischarge optical emission spectroscopy (GDOES) of Rigaku System 3860. 3. Results and discussion 3.1. Sprayed AISI 316L coatings with various spraying conditions The effects of the spraying conditions on the sprayed AISI 316L coatings were investigated. X-ray diffraction patterns showed that the sprayed coatings before the plasma nitriding contained not only austenite (γ) but also oxides (Fe3O4 and CrO). The oxides were synthesized during particle flight and after the particles impacted on the substrate by reaction with air [6,7]. Then, the compositions of the sprayed coatings were examined using EDX analysis. As shown in Table 1, the oxygen contents of the sprayed coatings ranged from 3.1 mass% to 6.3 mass%. The oxygen content with a 140 μm powder and 42 kW power was higher than those with 21 kW and 31.5 kW. Meanwhile, the contents of Ni and Cr elements in the coatings sprayed using 42 kW power were a little smaller than those in the coatings sprayed using 21 kW and 31.5 kW power. It is considered that the plasma flame at the high-input electrical power heated the sprayed particles to a higher temperature, causing more oxidation of the particles and evaporation of the Ni and Cr during the particle flight. It is a concern that the oxides in the sprayed coatings prevent the formation of the S-phase. However, the results show that only a small amount of the spraying powder was oxidized, and the mass loss of Ni and Cr from the powder was quite small. That is, the sprayed coatings can form the S-phase, the same as bulk stainless steel. 3.2. Nitriding temperature for S-phase formation Fig. 1 shows the X-ray diffraction patterns of the nitrided coatings with 90 μm powder and 21 kW power. At a nitriding temperature of 623 K, the peaks of γ(111) and γ(200) were slightly shifted towards
Fig. 1. X-ray diffraction patterns of the nitrided coatings obtained with 90 μm powder and 21 kW input electrical power at various nitriding temperatures.
lower angles, while at nitriding temperatures of 673 K and 723 K, these peaks were clearly shifted to lower angles; the d spacing of the (111) and (200) planes were 2.23 Å and 1.95 Å, respectively. This shift of the γ-phase peaks is caused by the dissolution of nitrogen in the fcc lattice [8], and suggests that the S-phase exists in the nitride layer [9]. At a nitriding temperature of 773 K, Fe4N and CrN peaks were identified as well as the S-phase. These results show that the S-phase in the sprayed coating was formed at nitriding temperatures of 673 K and 723 K. These temperatures are the same as those used for nitriding AISI 316L steel plates. And also, the spraying conditions of the particle size and the input electrical power did not affect the X-ray diffraction patterns as much as the nitriding temperature did. That is, the formation of the S-phase on the sprayed coating was dominated mainly by the nitriding temperature, and oxides and dissolved oxygen in the sprayed coating did not affect the nitriding temperature required to form the S-phase, since the compositions of the sprayed coatings with the spraying conditions were a little different from those of the sprayed AISI 316L powder. 3.3. Micrographs and thickness of nitride layer Fig. 2 shows the cross-sectional micrographs of the sprayed coatings after nitriding at 723 K; Marble's reagent was used to reveal the microstructure. At the surface, a bright contrast layer was observed, which corresponded to the nitride layer. In contrast, the areas of dark contrast corresponded to the oxide. The thicknesses of the nitride layers in the sprayed coatings were measured by optical microscopic observations at five different points of the micrographs, and are shown in Table 2. The nitriding temperature had the largest effect on thickness, but it was recognized that the input electrical power to the plasma torch and the particle size of the spray powder also affected the thickness slightly. At a nitriding temperature of 673 K, the coatings sprayed using 31.5 kW and 42 kW power tended to be thicker nitride layers than the coatings obtained using 21 kW power. The coatings sprayed using
Table 1 Chemical compositions of the sprayed coatings (obtained using EDX). Particle size (μm)
Input electrical power to the plasma torch (kW)
Chemical composition (mass%) Si
Mn
Ni
Cr
Mo
Fe
O
90 μm
21 31.5 42 21 31.5 42
0.4 0.4 0.3 0.7 0.5 0.5
0.7 0.6 0.6 0.9 1.0 0.7
12.1 12.6 11.8 12.0 12.4 11.4
16.6 16.5 15.7 16.7 17.0 15.9
2.2 2.5 2.4 2.3 2.2 2.3
61.7 62.9 63.4 64.4 63.8 63.0
6.3 4.6 5.7 3.1 3.2 6.3
140 μm
S. Adachi, N. Ueda / Thin Solid Films 523 (2012) 11–14
(a)
13
(b) Nitride layer
Nitride layer Open pore 50 µm
(c)
50 µm
(d)
Nitride layer
Nitride layer
Oxide-layer 50 µ m
50 µ m
Oxide-layer Un-melted particle
Non-oxide layer
Fig. 2. Cross-sectional micrographs of the sprayed coatings at nitriding temperature of 723 K; (a) 90 μm and 21 kW, (b) 90 μm and 42 kW, (c) 140 μm and 21 kW, and (d) 140 μm and 42 kW.
21 kW power clearly showed un-melted particles (Fig. 2(a) and (c)); there were fewer un-melted particles in the coating sprayed using 42 kW (Fig. 2(b) and (d)). The plasma flame at 21 kW power did not fuse the sprayed particles completely, because the plasma flame at 21 kW power was lower temperature than those at 31.5 kW and 42 kW. The interfaces of the un-melted particles were covered by oxide films, therefore the sprayed coatings using 21 kW power contained more oxide films. In Fig. 2, the oxides at the surfaces of the sprayed coatings were recognized, this indicated that the oxide had not been nitrided. And also, EDX line scans of nitrogen in the crosssection showed that the oxide layers in the sprayed coatings interfered with the nitrogen diffusion. Therefore, the nitride layers formed using 21 kW power at 673 K would be thinner. With respect to the effect of particle size, the nitride layers in the coatings sprayed with 140 μm particles tended to be thicker than those sprayed with 90 μm particles. The non-oxide layers between the oxide layers were wider in the coatings sprayed with 140 μm particles. The sprayed coating with stacked large particles included wide non-oxide layers, since the sprayed particles would be oxidized at the surface during flight. The wider non-oxide layers in the sprayed coating with 140 μm particles allowed nitrogen to diffuse more effectively into the inside. The sprayed coatings contain pores and cracks; these defects have a possibility to promote nitrogen diffusion by providing diffusion paths. However, the distances between these defects are large compared with the thickness of the nitride layer. Also, the sizes of open pores at the surfaces of the sprayed coatings were only some micrometers; glow discharge of the plasma nitriding was not excited in the open pores because they were too narrow. These defects therefore could not promote nitrogen diffusion, and would not affect the thickness of the nitride layer.
Table 2 Thicknesses of the nitride layer in the sprayed coatings. Input electrical power to 21 kW the plasma torch
Particle size
31.5 kW 42 kW
Nitriding temperature
673 K
90 μm 140 μm
3.5 μm 9 μm 5.6 μm 3.7 μm 13 μm 6.5 μm
723 K
673 K
673 K
723 K
5.6 μm 10 μm 6.5 μm 13 μm
3.4. Nitrogen depth distribution profiles The differences between the nitrogen diffusion processes in spray coating and in AISI 316L steel plate by low-temperature plasma nitriding were examined. The nitrogen depth distribution profiles of sprayed coatings with 90 μm (21 kW and 42 kW) and 140 μm (21 kW and 42 kW) particles, and steel plates at nitriding temperatures of 673 K and 723 K, obtained using GDOES, are shown in Fig. 3. For the AISI 316L steel plates, the nitrogen profiles in Fig. 3(a) clearly show a plateau and step-like distribution, and did not obey Fick's law of diffusion. The plateau and step-like distribution has previously been attributed to the affinity of Cr for nitrogen, causing dissolved Cr atoms to bind to nitrogen [10,11]. In contrast, the nitrogen profiles of the sprayed coatings using 21 kW power and nitrided at 673 K (Fig. 3(b) and (c)) decreased towards the inside of the specimen, as expected for an errorfunctional. These nitrogen profiles seem to be fitted to Fick's law of diffusion, therefore, the simulated nitrogen profile was calculated by using Fick's law of diffusion, Eq. (1), and the associated boundary conditions as below. ∂C ∂2c x ¼ 0; t > 0; ¼D 2 ∂t ∂x x > 0; t ¼ 0;
C ¼ Ci þ C0 C ¼ Ci
ð1Þ
Eq. (1) can be transformed to x C ðx; t Þ ¼ Ci þ C0 1 erf pffiffiffiffiffiffi 2 Dt Ci C0 D x t
ð2Þ
is the initial nitrogen content in the sprayed coating. is the steady-state content. is the diffusion coefficient of nitrogen in pure fcc-Fe [12]. is depth. is time.
Fig. 3(d) shows the simulated and experimentally measured nitrogen profiles using 21 kW power and nitriding at 673 K. The experimental profiles were in relatively good agreement with those simulated using Fick's law of diffusion, and this indicated that the nitrogen trapping would not occur. Meanwhile, the coatings sprayed
14
S. Adachi, N. Ueda / Thin Solid Films 523 (2012) 11–14
(b)
4 3 2 723 K
1
673 K
0
100
200
4
Nitrogen intensity (arb.unit)
Nitrogen intensity (arb.unit)
(a)
300
400
21 kW 42 kW
3 723 K
2 1 673 K
500
0
GDS Sputtering time, t / sec
200
300
400
500
4 21 kW 42 kW
3 723 K
2 1 673 K
0
100
200
300
400
Nitrogen intensity (arb.unit)
(d)
(c) Nitrogen intensity (arb.unit)
100
GDS Sputtering time, t / sec
3 Measured 90 µ m particle 140 µ m particle
2.5 2
Simulated
1.5 1 0.5
500
0
GDS sputtering time, t / sec
100
200
300
GDS Sputtering time, t / sec
Fig. 3. Nitrogen depth distribution profiles; (a) AISI 316L plate, (b) sprayed coatings with 90 μm particles, (c) sprayed coatings with 140 μm particles, (d) simulation of Fick's law of diffusion of sprayed coatings with 21 kW power — nitrided at 673 K and values measured by GDOES.
using 42 kW power or nitrided at 723 K, the nitrogen profiles (Fig. 3(b) and (c)) showed a bulge in the middle, and these profiles did not fit Fick's law of diffusion due to nitrogen trapping by Cr probably occurred. However, the degree of the trapping was small compared with that in the AISI 316L steel plate. The reason of the small trapping in the sprayed coating was explainable in terms of thermodynamics. The standard free energies of formation of Cr2N and Cr2O3 show that Cr2O3 is more stable thermodynamically than Cr2N [13]. The sprayed coating contained oxides and dissolved oxygen, the oxygen would be bound to some of the Cr sites, decreasing their ability to trap nitrogen. The thicknesses of the nitride layers in the AISI 316L steel plates were 4.5 μm at a nitriding temperature of 673 K and 11 μm at 723 K respectively. Some sprayed coatings (Table 2) had thicker nitride layers than those of the steel plates. These thicker nitride layers on the sprayed coatings could be explained by the difference of the nitrogen trapping. For the steel plates, nitrogen was trapped tightly by Cr sites, and the nitrogen diffusion was interrupted. As a result, the thickness of the nitride layer would be restricted. As for the sprayed coatings, some Cr trapping sites would be occupied by oxygen, therefore nitrogen could diffuse easily, enabling a thick nitride layer to be formed. 4. Conclusion
profiles show that nitrogen was trapped tightly by Cr sites to be interfered with the diffusion. As for the sprayed coatings, some Cr trapping sites were occupied by oxygen to weaken the nitrogen trapping, and nitrogen could diffuse easily. As a result, some sprayed coatings had enabled the nitride layer slightly thicker than those formed on the steel plates. That is, oxygen in the sprayed coatings with optimized spraying conditions, higher inputs of electrical power to the plasma torch and using a spray powder of 140 μm particle size, could not interfere but promote the nitrogen diffusion. It is concluded that the lowtemperature plasma nitriding was effective in forming thick nitride layers on the sprayed AISI 316L coatings. Acknowledgment This work was supported financially by KAKENHI (22560737). References [1] [2] [3] [4] [5] [6]
Plasma sprayed AISI 316L stainless steel coatings were treated by low-temperature plasma nitriding to form the S-phase on their surface. The S-phase was formed at nitriding temperatures of 673 K and 723 K, which were the same temperatures as those used for AISI 316L steel plates, although the sprayed coatings contained oxide layers. And more, the thicknesses of the nitride layers on some sprayed coatings were slightly thicker than those on the AISI 316L steel plates. These thick nitride layers in the sprayed coatings were explainable in terms of the nitrogen trapping. For the steel plate, the nitrogen depth distribution
[7] [8] [9] [10] [11] [12] [13]
Z. Zeng, N. Sakoda, T. Tajiri, J. Therm. Spray Technol. 15 (2006) 431. K. Ichii, K. Fujimura, T. Takase, Tech. Rep. Kansai Univ. 27 (1986) 135. Z.L. Zhang, T. Bell, Surf. Eng. 1 (1985) 131. T. Christiansen, M.A.J. Somers, Scr. Mater. 50 (2004) 35. Motoo Egawa, Nobuhiro Ueda, Kazuhiro Nakata, Masato Tsujikawa, Manabu Tanaka, Surf. Coat. Technol. 205 (2010) S246. A.A. Syed, A. Denoirjean, P. Denoirjean, J.C. Labbe, P. Fauchais, J. Therm. Spray Technol. 14 (2005) 117. M.P. Planche, H. Liao, C. Coddet, Surf. Coat. Technol. 202 (2007) 69. T. Christiansen, M.A.J. Somers, Surf. Eng. 21 (2005) 445. E. Menthe, K.-T. Rie, Surf. Coat. Technol. 116–119 (1999) 199. W. Moller, S. Parascandola, T. Telbizova, R. Gunzel, E. Richter, Surf. Coat. Technol. 136 (2001) 73. J.P. Riviere, P. Meheust, J.P. Villain, C. Templier, M. Cahoreau, G. Abrasonis, L. Pranevicious, Surf. Coat. Technol. 158–159 (2002) 99. P. Grieveson, E.T. Turkdogan, Trans. Metall. Soc. AIME 230 (1964) 407. Chemical Handbook Basic Edition, The Chemical Society of Japan, MARUZEN, 1966, p. 822.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Formation of S-phase layer on plasma sprayed AISI 316L stainless steel coating by plasma nitriding at low temperature Shinichiro Adachi ⁎, Nobuhiro Ueda Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi-shi, Osaka 594‐1157, Japan
a r t i c l e
i n f o
Available online 30 May 2012 Keywords: Plasma nitriding Plasma spraying Austenitic stainless steel Expanded austenite Diffusion Surface treatment
a b s t r a c t Low-temperature nitriding of austenitic stainless steels can produce expanded austenite, known as S-phase, leading to improved surface hardness while maintaining corrosion resistance. Sprayed AISI 316L coatings include oxide layers and defects (pores and cracks), and their structure is considerably different from AISI 316L steel plate structures. In this paper, plasma sprayed AISI 316L coating was treated to produce S-phase by lowtemperature plasma nitriding. The effects of nitriding temperatures and spraying conditions (input electrical power and particle size of the spray powder) on the S-phase layer were investigated, and also the nitrogen diffusion process was discussed. Under optimized spraying conditions, the sprayed coatings formed thicker nitride layers than those formed on AISI 316L steel plates; this was the result of a small amount of trapping of nitrogen by dissolved Cr. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Thermal sprayed austenitic stainless steel coatings are applied to various mechanical machines and equipment components as lowcost corrosion protection [1]. However, the sprayed austenitic stainless steel coating cannot function as a wear-resistant coating because of its poor wear properties. If the surface hardness of the sprayed austenitic stainless steel coating could be improved, while maintaining the corrosion resistance, the sprayed coating would then be useful in a greater variety of applications, for example, as an alternative to chrome plating and ceramic coating. Low-temperature nitriding could be considered an appropriate method for hardening a sprayed austenitic stainless steel coating because it has been reported that low-temperature nitriding at temperatures below 773 K can improve the hardness of austenitic stainless steels by producing expanded austenite at the surface [2,3]. The expanded austenite, called S-phase, has been identified as a supersaturated nitrogen solid-solution fcc material by Christiansen et al. [4]. Furthermore, the corrosion resistances of low-temperature nitrided stainless steels are almost the same as those of un-nitrided stainless steels [3]. However, a thermal sprayed coating is stacked with melted particles, and contains cracks, pores and oxide layers, therefore the lowtemperature nitriding of the sprayed coating will be different from that of the bulk stainless steel. It is a concern that the thickness of the
⁎ Corresponding author. Tel.: + 81 725 51 2648; fax: + 81 725 51 2749. E-mail address: [email protected] (S. Adachi). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.05.062
S-phase on the sprayed coating is not sufficient to improve the wearresistance. It has been reported that the thickness of the S-phase formed by nitriding on bulk stainless steel plates varies from a few micrometers to 20 μm [5]. For practical use, the S-phase on the sprayed coating should be same thickness as, or thicker than, that on bulk stainless steel plates. In this paper, a plasma sprayed AISI 316L coating was treated by low-temperature plasma nitriding. The effects of the nitriding temperature and the plasma spraying conditions on the S-phase were examined, to produce a thick S-phase. Also, the differences between the nitrogen diffusion processes in the sprayed AISI 316L coating and in the AISI 316L steel plate are discussed.
2. Experimental procedure AISI 316L powder was used as the spray material; the average particle diameters were 90 μm and 140 μm. The substrate was an AISI 316L plate; the specimen size was 25 mm × 55 mm × 5 mm. The substrate was roughened by blasting with alumina grit before spraying. The plasma spraying was performed using an Aeroplasma Corporation APS-7050 system in an air atmosphere using an input electrical power to the plasma torch of 21 kW, 31.5 kW or 42 kW. The plasma gas mass flows were Ar (8.3 × 10 − 5 m 3/s) and CO2 (16.7 × 10 − 5 m 3/s) at 21 kW and 31.5 kW, or Ar (8.3 × 10 − 5 m 3/s) and CO2 (20 × 10 − 5 m 3/s) at 42 kW. The spray distance was 0.15 m. A typical coating thickness was about 300 μm. After spraying, the coated surfaces were polished using metallographic 3 μm diamond paste to remove the surface oxide layer and smooth the surface.
12
S. Adachi, N. Ueda / Thin Solid Films 523 (2012) 11–14
The plasma nitriding equipment used was a laboratory type with direct current (DC) electrical power. The specimen was placed on a holder in the chamber, and the holder was connected to the cathode. Nitriding was performed in a gas mixture of N2 and H2 (80:20); the pressure in the chamber was 633 Pa. Specimen temperatures were maintained at 623 K, 673 K, 723 K or 773 K by controlling the plasma current; the specimen temperature was measured by a thermocouple inserted in a 1 mm diameter hole drilled in the specimen side. The treatment time was 4 h. X-ray diffraction measurements in the conventional θ-2θ scan were performed to evaluate the crystal structure of the nitrided layers by Rigaku RINT2000 using Cu-Kα radiation with 40 kV and 150 mA. Compositions of the sprayed coatings were investigated by SEM with energy-dispersive X-ray (EDX) analysis, SEM of Elionix ERAX3000 and EDX of AMETEK GENESIS-4000, and the operating voltage of the SEM was 20 kV. EDX analysis was performed on two different spots (of area 2.38 mm × 1.78 mm) on the surface of the sprayed coatings, and the measurement errors were less than 1%. In addition, the nitrogen depth distribution profiles were obtained using glowdischarge optical emission spectroscopy (GDOES) of Rigaku System 3860. 3. Results and discussion 3.1. Sprayed AISI 316L coatings with various spraying conditions The effects of the spraying conditions on the sprayed AISI 316L coatings were investigated. X-ray diffraction patterns showed that the sprayed coatings before the plasma nitriding contained not only austenite (γ) but also oxides (Fe3O4 and CrO). The oxides were synthesized during particle flight and after the particles impacted on the substrate by reaction with air [6,7]. Then, the compositions of the sprayed coatings were examined using EDX analysis. As shown in Table 1, the oxygen contents of the sprayed coatings ranged from 3.1 mass% to 6.3 mass%. The oxygen content with a 140 μm powder and 42 kW power was higher than those with 21 kW and 31.5 kW. Meanwhile, the contents of Ni and Cr elements in the coatings sprayed using 42 kW power were a little smaller than those in the coatings sprayed using 21 kW and 31.5 kW power. It is considered that the plasma flame at the high-input electrical power heated the sprayed particles to a higher temperature, causing more oxidation of the particles and evaporation of the Ni and Cr during the particle flight. It is a concern that the oxides in the sprayed coatings prevent the formation of the S-phase. However, the results show that only a small amount of the spraying powder was oxidized, and the mass loss of Ni and Cr from the powder was quite small. That is, the sprayed coatings can form the S-phase, the same as bulk stainless steel. 3.2. Nitriding temperature for S-phase formation Fig. 1 shows the X-ray diffraction patterns of the nitrided coatings with 90 μm powder and 21 kW power. At a nitriding temperature of 623 K, the peaks of γ(111) and γ(200) were slightly shifted towards
Fig. 1. X-ray diffraction patterns of the nitrided coatings obtained with 90 μm powder and 21 kW input electrical power at various nitriding temperatures.
lower angles, while at nitriding temperatures of 673 K and 723 K, these peaks were clearly shifted to lower angles; the d spacing of the (111) and (200) planes were 2.23 Å and 1.95 Å, respectively. This shift of the γ-phase peaks is caused by the dissolution of nitrogen in the fcc lattice [8], and suggests that the S-phase exists in the nitride layer [9]. At a nitriding temperature of 773 K, Fe4N and CrN peaks were identified as well as the S-phase. These results show that the S-phase in the sprayed coating was formed at nitriding temperatures of 673 K and 723 K. These temperatures are the same as those used for nitriding AISI 316L steel plates. And also, the spraying conditions of the particle size and the input electrical power did not affect the X-ray diffraction patterns as much as the nitriding temperature did. That is, the formation of the S-phase on the sprayed coating was dominated mainly by the nitriding temperature, and oxides and dissolved oxygen in the sprayed coating did not affect the nitriding temperature required to form the S-phase, since the compositions of the sprayed coatings with the spraying conditions were a little different from those of the sprayed AISI 316L powder. 3.3. Micrographs and thickness of nitride layer Fig. 2 shows the cross-sectional micrographs of the sprayed coatings after nitriding at 723 K; Marble's reagent was used to reveal the microstructure. At the surface, a bright contrast layer was observed, which corresponded to the nitride layer. In contrast, the areas of dark contrast corresponded to the oxide. The thicknesses of the nitride layers in the sprayed coatings were measured by optical microscopic observations at five different points of the micrographs, and are shown in Table 2. The nitriding temperature had the largest effect on thickness, but it was recognized that the input electrical power to the plasma torch and the particle size of the spray powder also affected the thickness slightly. At a nitriding temperature of 673 K, the coatings sprayed using 31.5 kW and 42 kW power tended to be thicker nitride layers than the coatings obtained using 21 kW power. The coatings sprayed using
Table 1 Chemical compositions of the sprayed coatings (obtained using EDX). Particle size (μm)
Input electrical power to the plasma torch (kW)
Chemical composition (mass%) Si
Mn
Ni
Cr
Mo
Fe
O
90 μm
21 31.5 42 21 31.5 42
0.4 0.4 0.3 0.7 0.5 0.5
0.7 0.6 0.6 0.9 1.0 0.7
12.1 12.6 11.8 12.0 12.4 11.4
16.6 16.5 15.7 16.7 17.0 15.9
2.2 2.5 2.4 2.3 2.2 2.3
61.7 62.9 63.4 64.4 63.8 63.0
6.3 4.6 5.7 3.1 3.2 6.3
140 μm
S. Adachi, N. Ueda / Thin Solid Films 523 (2012) 11–14
(a)
13
(b) Nitride layer
Nitride layer Open pore 50 µm
(c)
50 µm
(d)
Nitride layer
Nitride layer
Oxide-layer 50 µ m
50 µ m
Oxide-layer Un-melted particle
Non-oxide layer
Fig. 2. Cross-sectional micrographs of the sprayed coatings at nitriding temperature of 723 K; (a) 90 μm and 21 kW, (b) 90 μm and 42 kW, (c) 140 μm and 21 kW, and (d) 140 μm and 42 kW.
21 kW power clearly showed un-melted particles (Fig. 2(a) and (c)); there were fewer un-melted particles in the coating sprayed using 42 kW (Fig. 2(b) and (d)). The plasma flame at 21 kW power did not fuse the sprayed particles completely, because the plasma flame at 21 kW power was lower temperature than those at 31.5 kW and 42 kW. The interfaces of the un-melted particles were covered by oxide films, therefore the sprayed coatings using 21 kW power contained more oxide films. In Fig. 2, the oxides at the surfaces of the sprayed coatings were recognized, this indicated that the oxide had not been nitrided. And also, EDX line scans of nitrogen in the crosssection showed that the oxide layers in the sprayed coatings interfered with the nitrogen diffusion. Therefore, the nitride layers formed using 21 kW power at 673 K would be thinner. With respect to the effect of particle size, the nitride layers in the coatings sprayed with 140 μm particles tended to be thicker than those sprayed with 90 μm particles. The non-oxide layers between the oxide layers were wider in the coatings sprayed with 140 μm particles. The sprayed coating with stacked large particles included wide non-oxide layers, since the sprayed particles would be oxidized at the surface during flight. The wider non-oxide layers in the sprayed coating with 140 μm particles allowed nitrogen to diffuse more effectively into the inside. The sprayed coatings contain pores and cracks; these defects have a possibility to promote nitrogen diffusion by providing diffusion paths. However, the distances between these defects are large compared with the thickness of the nitride layer. Also, the sizes of open pores at the surfaces of the sprayed coatings were only some micrometers; glow discharge of the plasma nitriding was not excited in the open pores because they were too narrow. These defects therefore could not promote nitrogen diffusion, and would not affect the thickness of the nitride layer.
Table 2 Thicknesses of the nitride layer in the sprayed coatings. Input electrical power to 21 kW the plasma torch
Particle size
31.5 kW 42 kW
Nitriding temperature
673 K
90 μm 140 μm
3.5 μm 9 μm 5.6 μm 3.7 μm 13 μm 6.5 μm
723 K
673 K
673 K
723 K
5.6 μm 10 μm 6.5 μm 13 μm
3.4. Nitrogen depth distribution profiles The differences between the nitrogen diffusion processes in spray coating and in AISI 316L steel plate by low-temperature plasma nitriding were examined. The nitrogen depth distribution profiles of sprayed coatings with 90 μm (21 kW and 42 kW) and 140 μm (21 kW and 42 kW) particles, and steel plates at nitriding temperatures of 673 K and 723 K, obtained using GDOES, are shown in Fig. 3. For the AISI 316L steel plates, the nitrogen profiles in Fig. 3(a) clearly show a plateau and step-like distribution, and did not obey Fick's law of diffusion. The plateau and step-like distribution has previously been attributed to the affinity of Cr for nitrogen, causing dissolved Cr atoms to bind to nitrogen [10,11]. In contrast, the nitrogen profiles of the sprayed coatings using 21 kW power and nitrided at 673 K (Fig. 3(b) and (c)) decreased towards the inside of the specimen, as expected for an errorfunctional. These nitrogen profiles seem to be fitted to Fick's law of diffusion, therefore, the simulated nitrogen profile was calculated by using Fick's law of diffusion, Eq. (1), and the associated boundary conditions as below. ∂C ∂2c x ¼ 0; t > 0; ¼D 2 ∂t ∂x x > 0; t ¼ 0;
C ¼ Ci þ C0 C ¼ Ci
ð1Þ
Eq. (1) can be transformed to x C ðx; t Þ ¼ Ci þ C0 1 erf pffiffiffiffiffiffi 2 Dt Ci C0 D x t
ð2Þ
is the initial nitrogen content in the sprayed coating. is the steady-state content. is the diffusion coefficient of nitrogen in pure fcc-Fe [12]. is depth. is time.
Fig. 3(d) shows the simulated and experimentally measured nitrogen profiles using 21 kW power and nitriding at 673 K. The experimental profiles were in relatively good agreement with those simulated using Fick's law of diffusion, and this indicated that the nitrogen trapping would not occur. Meanwhile, the coatings sprayed
14
S. Adachi, N. Ueda / Thin Solid Films 523 (2012) 11–14
(b)
4 3 2 723 K
1
673 K
0
100
200
4
Nitrogen intensity (arb.unit)
Nitrogen intensity (arb.unit)
(a)
300
400
21 kW 42 kW
3 723 K
2 1 673 K
500
0
GDS Sputtering time, t / sec
200
300
400
500
4 21 kW 42 kW
3 723 K
2 1 673 K
0
100
200
300
400
Nitrogen intensity (arb.unit)
(d)
(c) Nitrogen intensity (arb.unit)
100
GDS Sputtering time, t / sec
3 Measured 90 µ m particle 140 µ m particle
2.5 2
Simulated
1.5 1 0.5
500
0
GDS sputtering time, t / sec
100
200
300
GDS Sputtering time, t / sec
Fig. 3. Nitrogen depth distribution profiles; (a) AISI 316L plate, (b) sprayed coatings with 90 μm particles, (c) sprayed coatings with 140 μm particles, (d) simulation of Fick's law of diffusion of sprayed coatings with 21 kW power — nitrided at 673 K and values measured by GDOES.
using 42 kW power or nitrided at 723 K, the nitrogen profiles (Fig. 3(b) and (c)) showed a bulge in the middle, and these profiles did not fit Fick's law of diffusion due to nitrogen trapping by Cr probably occurred. However, the degree of the trapping was small compared with that in the AISI 316L steel plate. The reason of the small trapping in the sprayed coating was explainable in terms of thermodynamics. The standard free energies of formation of Cr2N and Cr2O3 show that Cr2O3 is more stable thermodynamically than Cr2N [13]. The sprayed coating contained oxides and dissolved oxygen, the oxygen would be bound to some of the Cr sites, decreasing their ability to trap nitrogen. The thicknesses of the nitride layers in the AISI 316L steel plates were 4.5 μm at a nitriding temperature of 673 K and 11 μm at 723 K respectively. Some sprayed coatings (Table 2) had thicker nitride layers than those of the steel plates. These thicker nitride layers on the sprayed coatings could be explained by the difference of the nitrogen trapping. For the steel plates, nitrogen was trapped tightly by Cr sites, and the nitrogen diffusion was interrupted. As a result, the thickness of the nitride layer would be restricted. As for the sprayed coatings, some Cr trapping sites would be occupied by oxygen, therefore nitrogen could diffuse easily, enabling a thick nitride layer to be formed. 4. Conclusion
profiles show that nitrogen was trapped tightly by Cr sites to be interfered with the diffusion. As for the sprayed coatings, some Cr trapping sites were occupied by oxygen to weaken the nitrogen trapping, and nitrogen could diffuse easily. As a result, some sprayed coatings had enabled the nitride layer slightly thicker than those formed on the steel plates. That is, oxygen in the sprayed coatings with optimized spraying conditions, higher inputs of electrical power to the plasma torch and using a spray powder of 140 μm particle size, could not interfere but promote the nitrogen diffusion. It is concluded that the lowtemperature plasma nitriding was effective in forming thick nitride layers on the sprayed AISI 316L coatings. Acknowledgment This work was supported financially by KAKENHI (22560737). References [1] [2] [3] [4] [5] [6]
Plasma sprayed AISI 316L stainless steel coatings were treated by low-temperature plasma nitriding to form the S-phase on their surface. The S-phase was formed at nitriding temperatures of 673 K and 723 K, which were the same temperatures as those used for AISI 316L steel plates, although the sprayed coatings contained oxide layers. And more, the thicknesses of the nitride layers on some sprayed coatings were slightly thicker than those on the AISI 316L steel plates. These thick nitride layers in the sprayed coatings were explainable in terms of the nitrogen trapping. For the steel plate, the nitrogen depth distribution
[7] [8] [9] [10] [11] [12] [13]
Z. Zeng, N. Sakoda, T. Tajiri, J. Therm. Spray Technol. 15 (2006) 431. K. Ichii, K. Fujimura, T. Takase, Tech. Rep. Kansai Univ. 27 (1986) 135. Z.L. Zhang, T. Bell, Surf. Eng. 1 (1985) 131. T. Christiansen, M.A.J. Somers, Scr. Mater. 50 (2004) 35. Motoo Egawa, Nobuhiro Ueda, Kazuhiro Nakata, Masato Tsujikawa, Manabu Tanaka, Surf. Coat. Technol. 205 (2010) S246. A.A. Syed, A. Denoirjean, P. Denoirjean, J.C. Labbe, P. Fauchais, J. Therm. Spray Technol. 14 (2005) 117. M.P. Planche, H. Liao, C. Coddet, Surf. Coat. Technol. 202 (2007) 69. T. Christiansen, M.A.J. Somers, Surf. Eng. 21 (2005) 445. E. Menthe, K.-T. Rie, Surf. Coat. Technol. 116–119 (1999) 199. W. Moller, S. Parascandola, T. Telbizova, R. Gunzel, E. Richter, Surf. Coat. Technol. 136 (2001) 73. J.P. Riviere, P. Meheust, J.P. Villain, C. Templier, M. Cahoreau, G. Abrasonis, L. Pranevicious, Surf. Coat. Technol. 158–159 (2002) 99. P. Grieveson, E.T. Turkdogan, Trans. Metall. Soc. AIME 230 (1964) 407. Chemical Handbook Basic Edition, The Chemical Society of Japan, MARUZEN, 1966, p. 822.