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Microstructure and wear resistance of thermal sprayed steel coatings ion beam implanted with nitrogen
Wear 267 (2009) 1757–1761
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Short communication
Microstructure and wear resistance of thermal sprayed steel coatings ion beam implanted with nitrogen Alexey V. Byeli a,∗ , Marat A. Belotserkovskii b , Vladimir A. Kukareko b a b
Physical-Technical Institute, Department of Plasma and Beam Processing, 10, Kuprevich Street, 220141 Minsk, Belarus Joint Institute of Mechanical Engineering, Belarus
a r t i c l e
i n f o
Article history: Received 16 November 2008 Received in revised form 1 April 2009 Accepted 14 May 2009 Available online 28 May 2009 Keywords: Steel Thermal spray coatings Nitrogen Ion implantation Wear resistance
a b s t r a c t In the present study, the high-current-density nitrogen ion implantation technique is applied to enhance mechanical properties of thermal sprayed steel coatings. XRD measurements and optical microscopy of ion implanted coatings show clearly the presence of nitrogen solid solutions and precipitates of new phases in the surface layers of coatings. Phases formed are controlled by the temperature of ion beam processing, initial chemical composition and microstructure of coatings. Wear tests demonstrate that properly selected parameters of ion implantation dramatically improve wear and score-resistance of coatings. The influence of the microstructure and phase composition of nitrogen ion implanted layers on tribological properties is discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction A promising technique of thermal spraying has been successfully developed to improve the mechanical properties of surfaces [1–3]. The two main drawbacks of this technique for tribological applications are generally considered to be the small surface hardness and low wear resistance. These drawbacks significantly restrict applicability of thermal spraying. For these reasons there has been an increasing interest in developing hardening processes that improve the mechanical surface properties of thermal sprayed coatings. Nitriding is one very common process of surface thermalchemical treating, which improves the wear resistance of steels. Technology of low energy, high-current-density ion implantation developed recently is an efficient instrument for surface engineering [4–8]. Investigations have shown that ion implantation at low energies (1–5 keV), but with high-current densities (1–3 mA/cm2 ) can produce several microns thick modified layers with a high nitrogen content, when the processing is performed at moderate temperature. Low energy high-current-density ion implantation is especially effective in processing of alloys with high concentration of chemically active elements and structure defects. It makes this technique prospective for surface engineering of thermal sprayed high-alloyed steel coatings. The article attempts to briefly sum-
∗ Corresponding author. Tel.: +375 17 284 18 22; fax: +375 17 284 03 75. E-mail address: [email protected] (A.V. Byeli). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2009.05.003
marize the recent progress in our understanding of low energy high-current-density nitrogen ion beam processing on tribological properties of the thermal sprayed coatings. The discussion in this paper is concentrated on a number of Fe-based coatings, where fruitful experimental data have been accumulated. 2. Materials and experimental techniques The specimens for wear tests were 6 mm × 8 mm rectangular blocks with height of 5 mm. The blocks were cut from low-carbon steel coated with 40X13, X18H10T and Cв-08 steel coatings. Table 1 shows the chemical composition of 40X13, X18H10T and Cв-08 steels. Vickers hardness measurements were performed at a load of 300 N and surface microhardness measurements were done at a 0.1 N load with a 10 s dwell time. X-ray diffraction analysis (monochromatic CoK␣ irradiation, U = 30 kV, I = 10 mA) was carried out to study the initial microstructure of coatings and microstructural changes induced by the ion implantation. Measurements and phase identification were done using Bregg-Brentanno focusing, 0.1◦ step scanning with a 40 s counting time, and standard PDF. Optical microscopy was used for the metallographic study of blocks. Concentration of oxides in the coatings was determined using method of secants [9], with estimated accuracy of 5%. The following techniques have been used for coatings deposition: melting of steel wires in propane-oxygen torch with subsequent spraying by high speed air-blast (flame coatings); and
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Table 1 Chemical composition of steels. Material
40X13 X18H10T Cb-08 60
Content, wt.% C
Cr
Ni
Ti
Mn
Si
Fe
0.42 0.06 0.07 0.65
13.4 18.40 0.08 0.20
0.54 925 0.10 0.25
0.13 0.62 – –
0.44 0.20 0.40 0.80
0.27 0.20 0.26 0.25
Base Base Base Base
electric arc melting of the wires with spraying by the reaction jet formed by air–propane combustion gas (arc coatings). The speed of melted drops was 100–130 m/s for flame deposition and 400–500 m/s for deposition. Drop size ranged from 5 to 40 m and the thickness of coatings was around 1 mm. Nitrogen ion implantation was performed with a Hall type accelerator. This type of high-power large aperture accelerators has been developed recently [10–12]. Steel blocks were implanted with 2 keV nitrogen ions at ion current density of 0.9 mA/cm2 for 90 min. The temperature of the blocks was monitored during implantation using a thermocouple located 2 mm from the surface being treated. Blocks were held at 620, 670, 720, or 770 K during implantation. A reciprocating wear tester was used to measure the tribological properties of blocks under 1.5 MPa pressure for dry friction and under 25 MPa pressure in the case of boundary lubrication. The experiments were conducted at a mean sliding speed of 0.1 m/s. The coefficient of friction and weight wear of the steel block were monitored with accuracy estimated to be ±5%, and ±10%, respectively. Preliminary quenched 60 steel blocks (chemical composition is presented in Table 1) with HV = 7800–8000 MPa were used as a counter bodies. The counter bodies were 40 mm × 90 mm blocks of 2 mm height. 3. Microstructure Typical microstructures of coatings are presented in Fig. 1. It is easy to see that high-speed molten drops have flattened out and got mixed on the steel surface. The coating has attained a particular wavy microstructure, with high residual porosity. Arc coatings formed by supersonic drops reveal a relatively dense dispersive structure with a residual porosity in the range of 3–5 vol.%. Measured hardness of the coatings was 3500 MPa for 40X13 steel, 3250 MPa for X18H10T steel, and 2250 for Cb-08 steel. The structure of the coatings strongly depends on their chemical composition. It consists of ␣-phase, ␥-phase and Fe3 O4 inclusions for 40X13 and X18H10T steels. Cb-08 coatings phase composition includes ␣-phase, FeO, and Fe3 O4 . In the case of flame coatings the speed of drops and deposition temperature are comparatively low. Because of low temperature
coatings contain small concentration of oxides and a lot of alloying elements. Coatings are formed by big particles which experience small flattening. Coatings porosity is around 10 vol.%, and hardness equals 2000, 1500, and 1900 MPa for 40X13, X18H10T, and Cb-08 steels correspondingly. Nitrogen ion beam processing of 40X13 steel coatings yields 5–40 m modified surface layers with H = 9000–15,000 MPa. Processing at 620–670 K results in formation of -(Fe,Cr)3 N, ␣ (Fe,Cr)8 N and ␥ -Fe4 N phases. Processing at high temperatures (720–770 K) leads predominantly to the formation of ␥ -Fe4 N nitrides and high strength inclusions of CrN. Nitrogen ion beam processing of X18H10T steel coatings yields 3–25 m modified surface layers with H = 6500–12,500 MPa. Surface layers modified at 620–670 K contain predominantly ␥ N phase (solid solution of nitrogen in austenite with distorted tetragonal fcc lattice). Temperature increase up to 720 K results in additional formation of CrN and ␥ -Fe4 N phases. High temperature processing of the flame coatings at 770 K results predominantly in formation of CrN and ␣-Fe. For high temperature processing of arc coatings predominant formation of ␥ -Fe4 N nitrides with low concentration of nitrogen has been observed. Ion beam processing of low-alloyed steel Cb-08 brings to formation of comparatively mild surface layers with the thickness 7–45 m. Surface layers contain ␣-Fe, oxides, -Fe3 N and ␥ Fe4 N nitrides. Increase in the processing temperature results in high ␥ -Fe4 N nitrides concentration with simultaneous decrease of the -Fe3 N nitrides concentration. Low surface microhardness is explained by the small concentration of alloying elements in this steel. Data reported for nitrogen implanted thermal sprayed coatings significantly differ from the data for the same nitrogen implanted bulk steels presented elsewhere [13,14]. Essentially strong differences were observed in the chromium containing coatings. Ion beam modified coatings exhibit high concentration of nitrogen poor ␥ -Fe4 N phase, large thickness of modified layer, and small surface microhardness. Different phase composition is explained by the small concentration of unbounded Cr atoms in 40X13 coatings. High concentration of crystal lattice defects formed in the process of
Fig. 1. Microstructure of 40 × 13 flame coating ion implanted at 770 K.
A.V. Byeli et al. / Wear 267 (2009) 1757–1761
Fig. 2. Coefficient of friction versus number of friction cycles: (a) 40 × 13 coatings (1 unimplanted arc coating; 2 ion beam implanted at 770 K are coating); (b) X18H10T coatings (unimplanted flame coating; 2 ion beam implanted at 770 K flame coating; 3 –unimplanted arc coating).
high-speed crystallization favors fast diffusivity and large thickness of the modified layer. 4. Tribological properties of coatings Tribological properties of thermal sprayed coatings were evaluated using dry friction tests and tests with boundary lubrication. Results of the dry friction tests are presented in Fig. 2 and Table 2. Data presented show that unimplanted coatings demonstrate poor
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wear resistance. Essentially intensive wear took place for Cb-08 coatings. Ion beam processing at 620 K did not noticeably improve tribological properties of the surface. High wear intensity of the coatings implanted at 620 K can be attributed to the shallow nitrided depth. Consequently, intensive plastic deformation of the near surface matrix material and intensive damage of thin hardened surface layer take place during friction [15]. Microcracks initiation and propagation in materials with hard surface layers is schematically illustrated in Fig. 3. Incompatibility of friction induced elastic deformations in the hard surface layer and plastic flow in the bulk material results in high tensile stresses, structure defect formation and subsequent initiation of microcracks in the interface area. Crack initiation and subsequent propagation of cracks into the bulk eventually result in debris formation. Processing at high temperatures dramatically improved wear resistance of coatings. Hard phases and inclusions formed in the surface layer are considered the main reasons for high wear resistance [16]. Blocks made from chromium steels showed the best wear resistance after processing at 770 K, whereas the best wear resistance of Cb-08 coatings was observed after processing at 720 K. Smaller wear resistance of Cb-08 coatings processed at 770 K can be explained by the fact that modified surface layer of these blocks contains mainly inclusions of nitrogen poor ␥ -Fe4 N phase. Under dry friction adhesive interaction of surfaces results in severe plastic deformation of contact spots, high local temperatures, dissociation and solution of thermally unstable ␥ -Fe4 N nitrides [14]. Blocks processed at 670 and 720 K primarily contain thermally stable -(Fe, Cr)3 N nitrides and ensure better wear resistance. Results of boundary lubricated wear tests are presented in Table 3. Data presented show that during initial (running-in) period of tests coefficient of friction for unimplanted 40X13 arc coatings equals 0.11–0.12. Further interaction results in increase in the friction coefficient and intensive seizure of the contacting surfaces after 8200–8400 cycles (410–420 m of sliding). Similar behavior was observed in the case of flame coatings, but seizure took place after 6000 cycles (300 m of sliding). Ion beam processing (especially at 720 and 770 K) dramatically improved score-resistance of surfaces. Unimplanted flame X18H10T coatings demonstrate poor scoreresistance. Seizure of contacting surfaces took place after 20 friction cycles. Ion beam processing at 770 K resulted in sharp increase of score-resistance and seizure was observed only after 24,000 cycles (1200 m of sliding). Arc deposited X18H10T coatings demonstrated significantly better score-resistance compared with the flame coatings and unimplanted 40X13 coatings. Seizure of contact surfaces was not observed after 26,000 friction cycles. High score-resistance of coatings is explained by ␥ → ␣ phase transformation during friction and high concentration of oxide inclusions. It was mentioned above, that deposition of arc coatings is accompanied by inten-
Table 2 Tribological properties for nitrogen ion beam implanted coatings (dry friction). Deposition technique
Temperature of processing, K
Coating 40X13 Intensity of wear, mg/m
X18H10T Coefficient of friction
Intensity of wear, mg/m
Cb-08 Coefficient of friction
Intensity of wear, mg/m
Coefficient of friction
Flame coatings
Unimplanted 620 670 720 770
1.6 × 10−3 1.6 × 10−3 1.4 × 10−3 1.4 × 10−3 1.1 × 10−3
0.9–1.0 0.9–1.0 1.1–1.2 1.1–1.2 1.1–1.2
50.0 × 10−3 52.0 × 10−3 40.0 × 10−3 2.5 × 10−3 1.6 × 10−3
0.8–0.9 0.8–0.9 1.0–1.2 1.0–1.2 1.1–1.2
43.0 × 10−3 40.0 × 10−3 18.5 × 10−3 10.5 × 10−3 32 × 10−3
0.7–0.8 0.7–0.8 0.6–0.8 0.6–0.8 0.7–0.8
Arc coatings
Unimplanted 620 670 720 770
4.5 × 10−3 4.3 × 10−3 0.7 × 10−3 1.1 × 10−3 1.3 × 10−3
0.9–1.0 0.9–1.0 1.1–1.2 1.1–1.2 1.1–1.2
12.5 × 10−3 12.8 × 10−3 4.1 × 10−3 1.6 × 10−3 1.4 × 10−3
0.7–0.8 0.7–0.8 1.1–1.2 1.1–1.3 1.1–1.2
29.0 × 10−3 28.0 × 10−3 15.2 × 10−3 8.4 × 10−3 25.4 × 10−3
0.6–0.8 0.6–0.7 0.6–0.8 0.6–0.7 0.6–0.8
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Fig. 3. Schematic for friction induced surface cracking in materials with modified surface layers.
Table 3 Tribological properties for nitrogen ion beam implanted coatings (boundary lubrication). Deposition technique
Temperature of processing, K
Coating 40X13 Hardness, MPa
X18H10T Coefficient of friction
Distance before seizure, m
Hardness, MPa
Cb-08 Coefficient of friction
Distance before seizure, m
Hardness, MPa
Coefficient of friction
Distance before seizure, m
Flame coatings
Unimplanted 670 720 770
2000 15,000 15,000 11,000
0.11 0.12 0.11 0.12
300 600 800 1550
1500 – 12,500 12,000
0.12 – 0.12 0.12
1–2 – 1000 1200
1900 – 4400 4200
0.12 – 0.11 0.12
80 – 350 280
Arc coatings
Unimplanted 670 720 770
3500 14,000 13,700 10,000
0.12 0.12 0.12 0.11
410 600 1750 1850
3250 – 12,000 11,500
0.11 – 0.12 0.12
1550 – 1400 1250
2250 – 720 770
0.11 – 5000 4500
120 – 400 310
sive oxidation of small melted drops and burning up of alloying Cr and Ni atoms. Burning up of Cr and Ni atoms results in low stability of austenite phase and stimulates friction induced ␥ → ␣ phase transformation with ␣-phase concentration increasing from 15 to 30 vol.%. High concentration of ␣-phase and Fe3 O4 inclusions ensures surface layer hardening and score-resistance improvement [17]. In the case of flame deposition of coatings oxidation and burning up are not so intensive and austenite phase keeps high stability. Friction interaction of flame coatings does not result in intensive surface hardening and surface score-resistance is comparatively low. Ion beam processing of X18H10T coatings arc deposited at 720–770 K does not influence their score-resistance. 5. Conclusions 1. High-current-density nitrogen ion beam processing effectively influences the microstructure and mechanical properties of thermal sprayed coatings. 2. In the case of dry friction nitrogen ion beam processing increases wear resistance of arc coatings at a factor of 3–8. Wear resistance of flame coatings increases at a factor of 1.5–30.
3. In the case of boundary lubricated tests nitrogen ion beam processing of 40X13 coatings increases surface score-resistance by a factor of 4–5. Especially high growth of score-resistance was observed in the flame deposited X18H10T coatings. Durability of coatings increased by a factor of 500–1000 and reached levels typical for high-strength martensite 40X13 coatings.
References [1] M.A. Belotserkovskii, Journal of Friction and Wear 21 (2000) 534. [2] H.L. Liao, Y.L. Zhu, R. Bolot, C. Coddet, S.N. Ma, Surface and Coatings Technology 200 (2005) 2123. [3] A. Killinger, M. Kuhn, R. Gadow, Surface and Coatings Technology 201 (2006) 1922. [4] A.V. Byeli, V.A. Kukareko, A.A. Kolesnikova, S.K. Shykh, Proceedings of the XIV International Conference on Surface Modification Technologies, Paris, France, 2000, p. 262. [5] P.J. Wilbur, B.W. Buchholtz, Surface and Coatings Technology 79 (1996) 1. [6] R. Wei, Surface and Coatings Technology 83 (1996) 218. [7] J.P. Riviere, P. Meheust, J.A. Garsia, R. Martinez, R. Sancher, R. Rodriguez, Surface and Coatings Technology 158–159 (2002) 295. [8] J.P. Riviere, P. Meheust, J.P. Villain, C. Templier, M. Cahoreau, J. Abrasonis, L. Pranevicius, Surface and Coatings Technology 158–159 (2002) 99. [9] G. Shultce, Physics of Metals, Moscow Mir 327 (1971) (in Russian).
A.V. Byeli et al. / Wear 267 (2009) 1757–1761 [10] B.B. Straumal, N.F. Vershinin, M. Friesel, T.V. Ishenko, S.A. Polyakov, V. Gust, Defect and Diffusion Forum 194–199 (2001) 1457. [11] N. Vershinin, B. Straumal, K. Filonov, R. Dimitriou, W. Gust, M. Benmalek, Thin Solid Films 351 (1999) 190. [12] N. Vershinin, R. Dimitriou, M. Benmalek, B. Straumal, W. Gust, J. Vigas, J. Shulga, Surface and Coatings Technology 125 (2000) 35. [13] A.V. Byeli, V.A. Kukareko, Surface and Coatings Technology 127 (2–3) (2000) 174.
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[14] A.V. Byeli, V.A. Kukareko, E.G. Bilenko, Journal of Friction and Wear 24 (2003) 497. [15] A.V. Byeli, V.A. Kukareko, V.E. Rubtsov, A.V. Kolubaev, Physical Mesomechanics 5 (2002) 41. [16] A.V. Byeli, V.A. Kukareko, I.V. Boyarenko, Friction and Wear 20 (1999) 378. [17] S.V. Korotkevich, A.M. Dubravin, S.M. Martynenko, Friction and Wear 21 (2000) 519–526.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Short communication
Microstructure and wear resistance of thermal sprayed steel coatings ion beam implanted with nitrogen Alexey V. Byeli a,∗ , Marat A. Belotserkovskii b , Vladimir A. Kukareko b a b
Physical-Technical Institute, Department of Plasma and Beam Processing, 10, Kuprevich Street, 220141 Minsk, Belarus Joint Institute of Mechanical Engineering, Belarus
a r t i c l e
i n f o
Article history: Received 16 November 2008 Received in revised form 1 April 2009 Accepted 14 May 2009 Available online 28 May 2009 Keywords: Steel Thermal spray coatings Nitrogen Ion implantation Wear resistance
a b s t r a c t In the present study, the high-current-density nitrogen ion implantation technique is applied to enhance mechanical properties of thermal sprayed steel coatings. XRD measurements and optical microscopy of ion implanted coatings show clearly the presence of nitrogen solid solutions and precipitates of new phases in the surface layers of coatings. Phases formed are controlled by the temperature of ion beam processing, initial chemical composition and microstructure of coatings. Wear tests demonstrate that properly selected parameters of ion implantation dramatically improve wear and score-resistance of coatings. The influence of the microstructure and phase composition of nitrogen ion implanted layers on tribological properties is discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction A promising technique of thermal spraying has been successfully developed to improve the mechanical properties of surfaces [1–3]. The two main drawbacks of this technique for tribological applications are generally considered to be the small surface hardness and low wear resistance. These drawbacks significantly restrict applicability of thermal spraying. For these reasons there has been an increasing interest in developing hardening processes that improve the mechanical surface properties of thermal sprayed coatings. Nitriding is one very common process of surface thermalchemical treating, which improves the wear resistance of steels. Technology of low energy, high-current-density ion implantation developed recently is an efficient instrument for surface engineering [4–8]. Investigations have shown that ion implantation at low energies (1–5 keV), but with high-current densities (1–3 mA/cm2 ) can produce several microns thick modified layers with a high nitrogen content, when the processing is performed at moderate temperature. Low energy high-current-density ion implantation is especially effective in processing of alloys with high concentration of chemically active elements and structure defects. It makes this technique prospective for surface engineering of thermal sprayed high-alloyed steel coatings. The article attempts to briefly sum-
∗ Corresponding author. Tel.: +375 17 284 18 22; fax: +375 17 284 03 75. E-mail address: [email protected] (A.V. Byeli). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2009.05.003
marize the recent progress in our understanding of low energy high-current-density nitrogen ion beam processing on tribological properties of the thermal sprayed coatings. The discussion in this paper is concentrated on a number of Fe-based coatings, where fruitful experimental data have been accumulated. 2. Materials and experimental techniques The specimens for wear tests were 6 mm × 8 mm rectangular blocks with height of 5 mm. The blocks were cut from low-carbon steel coated with 40X13, X18H10T and Cв-08 steel coatings. Table 1 shows the chemical composition of 40X13, X18H10T and Cв-08 steels. Vickers hardness measurements were performed at a load of 300 N and surface microhardness measurements were done at a 0.1 N load with a 10 s dwell time. X-ray diffraction analysis (monochromatic CoK␣ irradiation, U = 30 kV, I = 10 mA) was carried out to study the initial microstructure of coatings and microstructural changes induced by the ion implantation. Measurements and phase identification were done using Bregg-Brentanno focusing, 0.1◦ step scanning with a 40 s counting time, and standard PDF. Optical microscopy was used for the metallographic study of blocks. Concentration of oxides in the coatings was determined using method of secants [9], with estimated accuracy of 5%. The following techniques have been used for coatings deposition: melting of steel wires in propane-oxygen torch with subsequent spraying by high speed air-blast (flame coatings); and
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Table 1 Chemical composition of steels. Material
40X13 X18H10T Cb-08 60
Content, wt.% C
Cr
Ni
Ti
Mn
Si
Fe
0.42 0.06 0.07 0.65
13.4 18.40 0.08 0.20
0.54 925 0.10 0.25
0.13 0.62 – –
0.44 0.20 0.40 0.80
0.27 0.20 0.26 0.25
Base Base Base Base
electric arc melting of the wires with spraying by the reaction jet formed by air–propane combustion gas (arc coatings). The speed of melted drops was 100–130 m/s for flame deposition and 400–500 m/s for deposition. Drop size ranged from 5 to 40 m and the thickness of coatings was around 1 mm. Nitrogen ion implantation was performed with a Hall type accelerator. This type of high-power large aperture accelerators has been developed recently [10–12]. Steel blocks were implanted with 2 keV nitrogen ions at ion current density of 0.9 mA/cm2 for 90 min. The temperature of the blocks was monitored during implantation using a thermocouple located 2 mm from the surface being treated. Blocks were held at 620, 670, 720, or 770 K during implantation. A reciprocating wear tester was used to measure the tribological properties of blocks under 1.5 MPa pressure for dry friction and under 25 MPa pressure in the case of boundary lubrication. The experiments were conducted at a mean sliding speed of 0.1 m/s. The coefficient of friction and weight wear of the steel block were monitored with accuracy estimated to be ±5%, and ±10%, respectively. Preliminary quenched 60 steel blocks (chemical composition is presented in Table 1) with HV = 7800–8000 MPa were used as a counter bodies. The counter bodies were 40 mm × 90 mm blocks of 2 mm height. 3. Microstructure Typical microstructures of coatings are presented in Fig. 1. It is easy to see that high-speed molten drops have flattened out and got mixed on the steel surface. The coating has attained a particular wavy microstructure, with high residual porosity. Arc coatings formed by supersonic drops reveal a relatively dense dispersive structure with a residual porosity in the range of 3–5 vol.%. Measured hardness of the coatings was 3500 MPa for 40X13 steel, 3250 MPa for X18H10T steel, and 2250 for Cb-08 steel. The structure of the coatings strongly depends on their chemical composition. It consists of ␣-phase, ␥-phase and Fe3 O4 inclusions for 40X13 and X18H10T steels. Cb-08 coatings phase composition includes ␣-phase, FeO, and Fe3 O4 . In the case of flame coatings the speed of drops and deposition temperature are comparatively low. Because of low temperature
coatings contain small concentration of oxides and a lot of alloying elements. Coatings are formed by big particles which experience small flattening. Coatings porosity is around 10 vol.%, and hardness equals 2000, 1500, and 1900 MPa for 40X13, X18H10T, and Cb-08 steels correspondingly. Nitrogen ion beam processing of 40X13 steel coatings yields 5–40 m modified surface layers with H = 9000–15,000 MPa. Processing at 620–670 K results in formation of -(Fe,Cr)3 N, ␣ (Fe,Cr)8 N and ␥ -Fe4 N phases. Processing at high temperatures (720–770 K) leads predominantly to the formation of ␥ -Fe4 N nitrides and high strength inclusions of CrN. Nitrogen ion beam processing of X18H10T steel coatings yields 3–25 m modified surface layers with H = 6500–12,500 MPa. Surface layers modified at 620–670 K contain predominantly ␥ N phase (solid solution of nitrogen in austenite with distorted tetragonal fcc lattice). Temperature increase up to 720 K results in additional formation of CrN and ␥ -Fe4 N phases. High temperature processing of the flame coatings at 770 K results predominantly in formation of CrN and ␣-Fe. For high temperature processing of arc coatings predominant formation of ␥ -Fe4 N nitrides with low concentration of nitrogen has been observed. Ion beam processing of low-alloyed steel Cb-08 brings to formation of comparatively mild surface layers with the thickness 7–45 m. Surface layers contain ␣-Fe, oxides, -Fe3 N and ␥ Fe4 N nitrides. Increase in the processing temperature results in high ␥ -Fe4 N nitrides concentration with simultaneous decrease of the -Fe3 N nitrides concentration. Low surface microhardness is explained by the small concentration of alloying elements in this steel. Data reported for nitrogen implanted thermal sprayed coatings significantly differ from the data for the same nitrogen implanted bulk steels presented elsewhere [13,14]. Essentially strong differences were observed in the chromium containing coatings. Ion beam modified coatings exhibit high concentration of nitrogen poor ␥ -Fe4 N phase, large thickness of modified layer, and small surface microhardness. Different phase composition is explained by the small concentration of unbounded Cr atoms in 40X13 coatings. High concentration of crystal lattice defects formed in the process of
Fig. 1. Microstructure of 40 × 13 flame coating ion implanted at 770 K.
A.V. Byeli et al. / Wear 267 (2009) 1757–1761
Fig. 2. Coefficient of friction versus number of friction cycles: (a) 40 × 13 coatings (1 unimplanted arc coating; 2 ion beam implanted at 770 K are coating); (b) X18H10T coatings (unimplanted flame coating; 2 ion beam implanted at 770 K flame coating; 3 –unimplanted arc coating).
high-speed crystallization favors fast diffusivity and large thickness of the modified layer. 4. Tribological properties of coatings Tribological properties of thermal sprayed coatings were evaluated using dry friction tests and tests with boundary lubrication. Results of the dry friction tests are presented in Fig. 2 and Table 2. Data presented show that unimplanted coatings demonstrate poor
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wear resistance. Essentially intensive wear took place for Cb-08 coatings. Ion beam processing at 620 K did not noticeably improve tribological properties of the surface. High wear intensity of the coatings implanted at 620 K can be attributed to the shallow nitrided depth. Consequently, intensive plastic deformation of the near surface matrix material and intensive damage of thin hardened surface layer take place during friction [15]. Microcracks initiation and propagation in materials with hard surface layers is schematically illustrated in Fig. 3. Incompatibility of friction induced elastic deformations in the hard surface layer and plastic flow in the bulk material results in high tensile stresses, structure defect formation and subsequent initiation of microcracks in the interface area. Crack initiation and subsequent propagation of cracks into the bulk eventually result in debris formation. Processing at high temperatures dramatically improved wear resistance of coatings. Hard phases and inclusions formed in the surface layer are considered the main reasons for high wear resistance [16]. Blocks made from chromium steels showed the best wear resistance after processing at 770 K, whereas the best wear resistance of Cb-08 coatings was observed after processing at 720 K. Smaller wear resistance of Cb-08 coatings processed at 770 K can be explained by the fact that modified surface layer of these blocks contains mainly inclusions of nitrogen poor ␥ -Fe4 N phase. Under dry friction adhesive interaction of surfaces results in severe plastic deformation of contact spots, high local temperatures, dissociation and solution of thermally unstable ␥ -Fe4 N nitrides [14]. Blocks processed at 670 and 720 K primarily contain thermally stable -(Fe, Cr)3 N nitrides and ensure better wear resistance. Results of boundary lubricated wear tests are presented in Table 3. Data presented show that during initial (running-in) period of tests coefficient of friction for unimplanted 40X13 arc coatings equals 0.11–0.12. Further interaction results in increase in the friction coefficient and intensive seizure of the contacting surfaces after 8200–8400 cycles (410–420 m of sliding). Similar behavior was observed in the case of flame coatings, but seizure took place after 6000 cycles (300 m of sliding). Ion beam processing (especially at 720 and 770 K) dramatically improved score-resistance of surfaces. Unimplanted flame X18H10T coatings demonstrate poor scoreresistance. Seizure of contacting surfaces took place after 20 friction cycles. Ion beam processing at 770 K resulted in sharp increase of score-resistance and seizure was observed only after 24,000 cycles (1200 m of sliding). Arc deposited X18H10T coatings demonstrated significantly better score-resistance compared with the flame coatings and unimplanted 40X13 coatings. Seizure of contact surfaces was not observed after 26,000 friction cycles. High score-resistance of coatings is explained by ␥ → ␣ phase transformation during friction and high concentration of oxide inclusions. It was mentioned above, that deposition of arc coatings is accompanied by inten-
Table 2 Tribological properties for nitrogen ion beam implanted coatings (dry friction). Deposition technique
Temperature of processing, K
Coating 40X13 Intensity of wear, mg/m
X18H10T Coefficient of friction
Intensity of wear, mg/m
Cb-08 Coefficient of friction
Intensity of wear, mg/m
Coefficient of friction
Flame coatings
Unimplanted 620 670 720 770
1.6 × 10−3 1.6 × 10−3 1.4 × 10−3 1.4 × 10−3 1.1 × 10−3
0.9–1.0 0.9–1.0 1.1–1.2 1.1–1.2 1.1–1.2
50.0 × 10−3 52.0 × 10−3 40.0 × 10−3 2.5 × 10−3 1.6 × 10−3
0.8–0.9 0.8–0.9 1.0–1.2 1.0–1.2 1.1–1.2
43.0 × 10−3 40.0 × 10−3 18.5 × 10−3 10.5 × 10−3 32 × 10−3
0.7–0.8 0.7–0.8 0.6–0.8 0.6–0.8 0.7–0.8
Arc coatings
Unimplanted 620 670 720 770
4.5 × 10−3 4.3 × 10−3 0.7 × 10−3 1.1 × 10−3 1.3 × 10−3
0.9–1.0 0.9–1.0 1.1–1.2 1.1–1.2 1.1–1.2
12.5 × 10−3 12.8 × 10−3 4.1 × 10−3 1.6 × 10−3 1.4 × 10−3
0.7–0.8 0.7–0.8 1.1–1.2 1.1–1.3 1.1–1.2
29.0 × 10−3 28.0 × 10−3 15.2 × 10−3 8.4 × 10−3 25.4 × 10−3
0.6–0.8 0.6–0.7 0.6–0.8 0.6–0.7 0.6–0.8
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Fig. 3. Schematic for friction induced surface cracking in materials with modified surface layers.
Table 3 Tribological properties for nitrogen ion beam implanted coatings (boundary lubrication). Deposition technique
Temperature of processing, K
Coating 40X13 Hardness, MPa
X18H10T Coefficient of friction
Distance before seizure, m
Hardness, MPa
Cb-08 Coefficient of friction
Distance before seizure, m
Hardness, MPa
Coefficient of friction
Distance before seizure, m
Flame coatings
Unimplanted 670 720 770
2000 15,000 15,000 11,000
0.11 0.12 0.11 0.12
300 600 800 1550
1500 – 12,500 12,000
0.12 – 0.12 0.12
1–2 – 1000 1200
1900 – 4400 4200
0.12 – 0.11 0.12
80 – 350 280
Arc coatings
Unimplanted 670 720 770
3500 14,000 13,700 10,000
0.12 0.12 0.12 0.11
410 600 1750 1850
3250 – 12,000 11,500
0.11 – 0.12 0.12
1550 – 1400 1250
2250 – 720 770
0.11 – 5000 4500
120 – 400 310
sive oxidation of small melted drops and burning up of alloying Cr and Ni atoms. Burning up of Cr and Ni atoms results in low stability of austenite phase and stimulates friction induced ␥ → ␣ phase transformation with ␣-phase concentration increasing from 15 to 30 vol.%. High concentration of ␣-phase and Fe3 O4 inclusions ensures surface layer hardening and score-resistance improvement [17]. In the case of flame deposition of coatings oxidation and burning up are not so intensive and austenite phase keeps high stability. Friction interaction of flame coatings does not result in intensive surface hardening and surface score-resistance is comparatively low. Ion beam processing of X18H10T coatings arc deposited at 720–770 K does not influence their score-resistance. 5. Conclusions 1. High-current-density nitrogen ion beam processing effectively influences the microstructure and mechanical properties of thermal sprayed coatings. 2. In the case of dry friction nitrogen ion beam processing increases wear resistance of arc coatings at a factor of 3–8. Wear resistance of flame coatings increases at a factor of 1.5–30.
3. In the case of boundary lubricated tests nitrogen ion beam processing of 40X13 coatings increases surface score-resistance by a factor of 4–5. Especially high growth of score-resistance was observed in the flame deposited X18H10T coatings. Durability of coatings increased by a factor of 500–1000 and reached levels typical for high-strength martensite 40X13 coatings.
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