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A study of Ni3Si-based composite coating fabricated by self-propagating high temperature synthesis casting route
Surface & Coatings Technology 205 (2011) 4249–4253
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
A study of Ni3Si-based composite coating fabricated by self-propagating high temperature synthesis casting route Muye Niu a,b, Qinling Bi a,⁎, Lingqian Kong a,b, Jun Yang a, Weimin Liu a a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate University of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e
i n f o
Article history: Received 20 January 2011 Accepted in revised form 9 March 2011 Available online 16 March 2011 Keywords: SHS Coating Nickel-silicide Wear resistance
a b s t r a c t A Ni3Si–Cr7C3 composite coating was fabricated on AISI 1020 steel substrate by self-propagating hightemperature synthesis (SHS) casting route. Phase composition, microhardness and dry sliding wear behavior of the coating were studied. The results indicated that the coating was mainly consisted of Ni3Si and Cr7C3. The microhardness of coating is about 900 HV. The friction and wear tests showed that although the friction coefficient had no obvious change, the wear resistance of the AISI 1020 steel matrix was improved greatly. Also, the corrosion test result showed that the coating had an excellent corrosion resistance. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Some ordered intermetallics show attractive properties in hostile environments [1–3]. Nickel silicide (Ni3Si) is one of these inspiring intermetallic alloys for its many excellent properties – an increasing yield stress with increasing temperature [4], good oxidation resistance and excellent corrosion resistance in acidic aqueous solutions, etc. The major drawback of nickel silicide is that it exhibits low roomtemperature ductility. In 1990, it was found that ductility was improved by the addition of Ti [5], and further improvement of the ductility was found by the addition of boron, niobium, chromium, etc. [6–17]. Moreover, some research results showed that alloying and microstructural control can substantially improve the ambient- and elevated-temperature ductility and strength [18–22]. Furthermore, some Ni3Si-based composite materials possessed excellent hightemperature wear resistance, structural stability and certain oxidation resistance [23–26]. Chromium carbide (Cr7C3) is well known for its high hardness, strong covalent atomic bonding and excellent high-temperature stability, so it has been widely used as an reinforcing phase in the composite coatings [27,28]. On the other hand, the excellent performance of the chromium carbide makes it an excellent candidate as a wear-resistant reinforcing phase for wear-resistant composite coatings [29–33]. Naturally, the Ni3Si-matrix composite coatings reinforced by Cr7C3 carbides are expected to possess good service properties under wear conditions.
⁎ Corresponding author. Tel.: +86 931 4968193; fax: +86 931 8277088. E-mail address: [email protected] (Q. Bi). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.031
In this study, a new type of Ni3Si – Cr7C3 composite coating was fabricated by self-propagating high temperature synthesis (SHS) casting route. Microstructures, mechanical, tribological as well as corrosion properties of the coating were studied.
2. Experimental 2.1. Fabrication of the SHS coating Starting materials used in this study were Cr2O3, CrO3, carbon, aluminum, nickel, and silicon powders. The powders were weighted in the stoichiometric proportion of chemical reaction Eqs. (1) and (2), respectively. The characteristics of the reactant powders are given in Table 1. Cr2 O3 þ 5CrO3 þ 3C þ 12A1 ¼ Cr7 C3 þ 6A12 O3 −1
¼ −6229kJmol 3Ni þ Si ¼ Ni3 Si
o
ΔHf ;298K ð1Þ
o
ΔHf ;298K ¼ −37:2kJmol
−1
ð2Þ
These raw material powders were mechanically milled for 8 h by a planetary ball mill for homogeneous mixture of the reactants. The AISI 1020 carbon steel substrate was 65 mm in diameter and 5 mm in thickness. Before coating, the surface of the substrates was polished and then cleaned with acetone and a small amount of nickel powders were spread on the surface of the substrate to improve the wettability between the coating and AISI 1020 carbon steel.
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Table 1 Properties of the starting powders. Powder
Size (mesh)
Purity (%)
Impurity
Ni Si Cr2O3 CrO3 Al C (graphite)
200 200 b200 b200 100–200 ≥500
99.5 99.0 99.0 99.0 99.0 99.85
Co, Si, C, S, Ca, Mn Fe, Cu, Zn, Sb Cl, SO4, CrO3, Fe Na, Al, K, Fe, Cl, SO4 Fe, Si, Cu Ignition residue
Ni and Si powders of 35 g were filled manually on the nickel powders, and 35 g of the Cr–C reactants were subsequently set on the Ni–Si layer. In order to generate a steady combustion wave in the SHS process and remove gases in the reactants [34], the reactant powders were cold pressed under a uniaxial pressure of 30 MPa. An igniting agent was then put on the surface of the Cr–C reactants, and subsequently, the steel crucible was put into the reactor. The reactor was cleaned by argon gas at room temperature and refilled with argon gas to 5 MPa when heated to 523 K. The igniter reaction started at this temperature. The compacted Cr–C powders were ignited by the heat released from the igniter, and the heat released by the Cr–C reactants then ignited the Ni–Si compacted powders. The combustion reactions were finished in a few seconds and thus the coating was formed [35–37]. The thickness of the coating on the substrate was about 2–3 mm. Several samples were produced using the same processing method. Specimens with dimensions of 18.5 × 18.5 × 8 mm were cut from the samples for testing the mechanical and tribological properties.
Fig. 2. SEM micrographs of transverse cross-sections of the coatings.
duration of 20 min. The counterpart was an AISI 52100 steel ball (hardness 62–63 HRC, Ra ≈ 0.01 μm) with a diameter of 9.60 mm. The wear rate is defined as W = V/PS, where V is the wear volume, P is the applied load and S is the total sliding distance. The wear volume V was measured using the Micro-XAM-3D non-contact surface profiler. In addition, the wear properties of the substrate and the bearing steel were tested under the same conditions, for analysis and comparison. The corrosion property of the coating was studied through salt spray test. The salt spray test was performed according to BSI 7479: the NaCl concentration of the sprayed solution was 50 g/L, and temperature 35 °C. The samples were salt sprayed for 72 h continuously.
2.2. Microstructure examination 3. Results and discussion The morphology of composite coatings was observed by means of JSM-5600LV scanning electron microscopy (SEM) at an acceleration voltage of 20 kV. The Shimadzu Dmax-RB X-ray diffraction (XRD) diffractometer (Japan) was adopted to analyze the phase compositions of the coating. 2.3. Mechanical, tribological and corrosion properties of the coating Vickers hardness of the coating was tested by using an AVKSHI MVK-1 hardness tester. The applied load was 500 g and the dwell time was 10 s. The sliding wear test was carried out on the Optimol (Germany) SRV oscillating friction and wear tester with an oscilliating amplitude of 1 mm, frequency of 15 Hz, loads 40 to 80 N, and a
Fig. 1. X-ray diffraction patterns of the SHS coatings.
3.1. Microstructure and phase composition of Ni3Si-based composite coating The phase compositions of the coating have been identified by Xray diffraction analysis. Fig. 1 shows the XRD pattern of the coating. The X-ray diffraction profile reveals that the coatings are mainly consisted of Ni3Si and Cr7C3 phases. Apart from these main phases, the presences of some other phases like Ni2Si, CrSi, and Ni phases have also been identified. The Al2O3 phase was not detected in the coating, and this proved that Al2O3 was completely separated and removed from the composite [16,38,39]. Fig. 2 presents the morphology of the cross-sections of the coating. It can be seen that the coating and the substrate bond well. In addition, there are no pores and cracks near the interface region, which proves
Fig. 3. SEM micrograph of the top surface of the coatings.
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Fig. 4. EDS elemental mappings for the top surface of the coatings.
that the composite coating layer is well adherent to the substrate. The growth of the columnar grains is observed almost perpendicular to the interface of the deposits. In order to evaluate the bonding strength of the coating, the thermal shock test was performed. The sample was heated to 1000 °C for 15 min, and then immediately dropped it into water of 20 °C, the coating did not peel off after 50 cycles thermal shock. The result indicates that the coating possesses excellent bonding strength [40].
Fig. 3 shows the microstructure of the top surface of the coating observed under SEM. It is found that there are mainly two phases – the dark phase (marked “A”) and gray phase (marked “B”) coexistence in the coating. EDS results (Fig. 4) reveal that the phase “A” is chromium-rich; the phase “B” contains nickel and silicon. The content of carbon is not clear from the results by EDS, but the high hardness suggests the formation of Cr7C3. Furthermore, the XRD results prove that the coating consists of Ni3Si and Cr7C3. Therefore,
0.7
1000
Coatings Bearing steel AISI 1020 steel
800
Coefficient of friction
Microhardness, HV
900
700 600 500 400
0.6
0.5
0.4
300 200 0.3 0
300
600
900
1200
Distance from the interface/µm Fig. 5. Microhardness profile of the Ni3Si–Cr7C3 coatings.
1500
40
50
60
70
80
Load (N) Fig. 6. Variation of COFs with normal load for same sliding speeds.
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Wear rate 10-6 mm3 /Nm
40 35 30 25 Coating Bearing steel AISI 1020 steel
20 15 10 5 0 40
50
60
70
80
Load (N) Fig. 7. Variation of wear rate with normal load.
the phase “A” is mainly Cr7C3 and the phase “B” is Ni3Si. Fig. 3 also shows that a lot of dendrites inside the coating. This should be a direct consequence of the higher cooling rates of the SHS process. 3.2. Vickers hardness, friction and wear properties of the SHS composite coating Fig. 5 shows the microhardness distribution from the interface to the coating. The formation of a hard Cr7C3 phase improves the mechanical properties of Ni3Si intermetallics, so the microhardness of
the coating is much higher than that of the substrate. With the increase of the distance from the interface, the volume fraction of the Cr7C3 increases and therefore the microhardness increases significantly. At the top surface of the composite coating, the vickers hardness (HV) is up to 9.70 ± 0.50 GPa. High hardness of the composite coating is due to the presence of hard phase Cr7C3. The coating consists of nickel and Ni3Si–Cr7C3 composite adjacent to the interface; and far from the interface it consists of Ni3Si–Cr7C3 composite. The hardness of nickel is lower than that of Ni3Si–Cr7C3 composite. Therefore the hardness of the coating adjacent to the interface is low. As the coating thickness increases, the amount of nickel in the coating decreases and therefore the hardness increases with coating thickness. As the thickness continues to increase, the content of nickel in the coating decreases further and the hardness of the coating increases to a maximum value, which is close to the hardness of Ni3Si–Cr7C3 composite. The friction and wear properties of the substrates are significantly improved by the composite coatings. Fig. 6 shows the variation of coefficients of friction (COF) with the normal loads of the coating, AISI 1020 steel substrate, as well as AISI 52100 bearing steel. It can be observed that the COFs vary from 0.48 to 0.53, and the COFs of the coatings are lower than that of the substrates and the bearing steels under high loads. Moreover, with the increase of the normal loads, the COFs of the coatings, the substrates and the bearing steels decrease in the same trend. The wear rates of the coatings, the substrates and the bearing steels tested at same sliding speeds have been plotted against applied normal loads in Fig. 7. The plots of wear rates against normal loads show that the wear rates of the three materials increased with increasing load. The wear rates of the substrates are about an order
Fig. 8. SEM morphologies of worn surfaces of the coatings tested with the same sliding speed 0.03 m/s at different applied loads: (a) 40 N, (b) 50 N, (c) 60 N, (d) 70 N, (e) 80 N.
M. Niu et al. / Surface & Coatings Technology 205 (2011) 4249–4253
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Fig. 9. The micrographs of transverse cross-sections of the coatings immersed in salt spray corrosion for 72 h: (a) optical image, (b) SEM image.
higher than that of the composite coatings, and even better than that of the bearing steels. The excellent wear resistance of the coatings could be explained as follows. First, Ni3Si possesses high hardness and good oxidation resistance; second, the very high hardness reinforced Cr7C3 phase is uniformly distributed in the matrix, and prevents the coatings from severe wear. Therefore, the composite coating exhibits excellent wear resistance ability. SEM morphologies of worn surfaces of the coatings tested at different applied loads are given in Fig. 8. It is found that applied loads have important influence on the morphologies of the worn surfaces. As shown in Fig. 8a, the worn surface of the coating is relatively smooth at a load of 40 N. Because the primary Cr7C3 phase has very high hardness and is uniformly distributed in the matrix, the wear resistance of the coating is excellent, and therefore there are nearly no scratches visible on the worn surface. As the load increasing, there are only shallow grooves visible on the worn surface, and a transfer layer consisting of iron oxide is formed on the worn surface (Fig. 8b and c), this indicates that the AISI 52100 steel ball has oxide and the formed iron oxide transferred to the coating surface. The transfer layer, in some extent, is helpful to reduce the COF and wear rate of the coating. With the load further increasing, delamination happens on the surface of the coating; and the wear mechanism expresses as delamination as shown in Fig. 8d and e. 3.3. Corrosion properties of the SHS composite coating The results of the salt spray test on the samples show that the substrates suffered severe corrosion after 72 h (Fig. 9). The red rust can be seen on the substrates. However, the coatings withstood 72 h exhibit weak corrosion. The excellent corrosion resistance of the coating is due to the good corrosion resistance of Ni3Si main phases. In addition, during the process of solidification, the rapid cooling rate in the melt pool makes the microstructure of the coatings fine and uniform, which also led to the excellent corrosion resistance of the coating. 4. Conclusions (1). A Ni3Si–Cr7C3 composite coating was fabricated by the SHS method. The coating is well adherent to the AISI 1020 steel substrate. The microhardness of the composite coating is reinforced by the formation of the hard phase Cr7C3. (2). The wear resistance of AISI 1020 steel was improved significantly. The wear rates of the composite coatings are lower than
that of the substrates, and even lower than that of the bearing steel in some extent. The main wear mechanism of the coating is delamination. (3). The coating has excellent corrosion resistance. References [1] E.P. George, M. Yamaguchi, K.S. Kumar, C.T. Liu, Annu. Rev. Mater. Sci. 24 (1994) 409. [2] C.T. Liu, J. Stringer, J.N. Mundy, L.L. Horton, P. Angelini, Intermetallics 5 (1997) 579. [3] N.S. Stoloff, C.T. Liu, S.C. Deevi, Intermetallics 8 (2000) 1313. [4] P. Thornton, R. Davies, Metall. Trans. 1 (1970) 549. [5] T. Takasugi, M. Nagashima, O. Izumi, Acta Metall. Mater. 38 (1990) 747. [6] T. Takasugi, M. Yoshida, J. Mater. Sci. 26 (1991) 3032. [7] T. Takasugi, O. Izumi, M. Yoshida, J. Mater. Sci. 26 (1991) 1173. [8] T. Takasugi, M. Yoshida, J. Mater. Sci. 36 (2001) 643. [9] T. Takasugi, C.L. Ma, S. Hanada, Mater. Sci. Eng. A 192–193 (1995) 407. [10] T. Takasugi, C.T. Liu, L. Heatherly, E.H. Lee, E.P. George, Intermetallics 6 (1998) 369. [11] T. Takasugi, H. Kawai, Y. Kaneno, Mater. Sci. Eng. A 329–331 (2002) 446. [12] T. Takasugi, H. Kawai, Y. Kaneno, Mater. Sci. Technol. 17 (2001) 671. [13] T. Takasugi, S. Hanada, Intermetallics 8 (2000) 47. [14] J.S.C. Jang, C.Y. Cheng, S.-K. Wang, Mater. Chem. Phys. 72 (2001) 66. [15] J.S.C. Jang, C.J. Ou, C.Y. Cheng, Mater. Sci. Eng. A 329–331 (2002) 455. [16] Q.L. Bi, P.Q. La, W.M. Liu, Q.J. Xue, Y.T. Ding, Metall. Mater. Trans. A 36 (2005) 1301. [17] Y.Q. Pu, Q.L. Bi, J. Yang, J.M. Chen, Q.J. Xue, Mater. Sci. Eng. A 496 (2008) 316. [18] W. Oliver, Mater. Res. Soc. Symp. Proc. 133 (1988) 397. [19] T.G. Nieh, W.C. Oliver, Scr. Metall. Mater. 23 (1989) 851. [20] L.M. Pike, C.T. Liu, Scr. Mater. 42 (2000) 265. [21] C.T. Liu, E.P. George, W.C. Oliver, Intermetallics 4 (1996) 77. [22] C.T. Liu, W.C. Oliver, Scr. Metall. Mater. 25 (1991) 1933. [23] X.D. Lu, H.M. Wang, Thin Solid Films 472 (2005) 297. [24] X.D. Lu, H.M. Wang, Acta Mater. 52 (2004) 5419. [25] X.D. Lu, H.M. Wang, Appl. Surf. Sci. 214 (2003) 190. [26] Q.L. Bi, W.M. Liu, J. Yang, J.Q. Ma, Q.J. Xue, Tribol. Int. 43 (2010) 136. [27] X.B. Liu, H.M. Wang, Appl. Surf. Sci. 252 (2006) 5735. [28] H.R. Karimi Zarchi, M. Jalaly, M. Soltanieh, H. Mehrjoo, Steel Res. Int. 80 (2009) 859. [29] X.B. Liu, H.M. Wang, Wear 262 (2007) 514. [30] F. Cheng, Y. Wang, T. Yang, Mater. Charact. 59 (2008) 488. [31] P.Q. La, Q.J. Xue, W.M. Liu, S.R. Yang, Wear 240 (2000) 1. [32] P.Q. La, M.W. Bai, Q.J. Xue, W.M. Liu, L.G. Yu, Mater. Sci. Technol. 16 (2000) 110. [33] X.B. Liu, Y.J. Gu, Mater. Lett. 60 (2006) 577. [34] A. Varma, A.S. Rogachev, A.S. Mukasyan, S. Hwang, in: W. James (Ed.), Advances in Chemical Engineering, Academic Press, 1998, p. 79. [35] V.I. Yukhvid, Pure Appl. Chem. 64 (1992) 977. [36] V. Yukhvid, in: A.A. Borisov, L.D. Luca, A. Merzhanov (Eds.), Self-Propagating HighTemperature Synthesis of Materials, Taylor and Francis Inc., New York, 2002, p. 238. [37] A.V. Yukhvid, A.M. Stolin, V.I. Yukhvid, L.S. Stelmakh, Int. J. Appl. Mech. Eng. 6 (2001) 107. [38] I. Gordopolova, T. Ivleva, K. Shkadinskii, V. Yukhvid, Int. J. SHS 10 (2001) 177. [39] I.S. Gordopolova, T.P. Ivleva, K.G. Shkadimskii, V.I. Yukhvid, Int. J. SHS 8 (1999) 137. [40] L. Lin, Q.J. Xue, Int. J. SHS 4 (1995) 171.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
A study of Ni3Si-based composite coating fabricated by self-propagating high temperature synthesis casting route Muye Niu a,b, Qinling Bi a,⁎, Lingqian Kong a,b, Jun Yang a, Weimin Liu a a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate University of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e
i n f o
Article history: Received 20 January 2011 Accepted in revised form 9 March 2011 Available online 16 March 2011 Keywords: SHS Coating Nickel-silicide Wear resistance
a b s t r a c t A Ni3Si–Cr7C3 composite coating was fabricated on AISI 1020 steel substrate by self-propagating hightemperature synthesis (SHS) casting route. Phase composition, microhardness and dry sliding wear behavior of the coating were studied. The results indicated that the coating was mainly consisted of Ni3Si and Cr7C3. The microhardness of coating is about 900 HV. The friction and wear tests showed that although the friction coefficient had no obvious change, the wear resistance of the AISI 1020 steel matrix was improved greatly. Also, the corrosion test result showed that the coating had an excellent corrosion resistance. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Some ordered intermetallics show attractive properties in hostile environments [1–3]. Nickel silicide (Ni3Si) is one of these inspiring intermetallic alloys for its many excellent properties – an increasing yield stress with increasing temperature [4], good oxidation resistance and excellent corrosion resistance in acidic aqueous solutions, etc. The major drawback of nickel silicide is that it exhibits low roomtemperature ductility. In 1990, it was found that ductility was improved by the addition of Ti [5], and further improvement of the ductility was found by the addition of boron, niobium, chromium, etc. [6–17]. Moreover, some research results showed that alloying and microstructural control can substantially improve the ambient- and elevated-temperature ductility and strength [18–22]. Furthermore, some Ni3Si-based composite materials possessed excellent hightemperature wear resistance, structural stability and certain oxidation resistance [23–26]. Chromium carbide (Cr7C3) is well known for its high hardness, strong covalent atomic bonding and excellent high-temperature stability, so it has been widely used as an reinforcing phase in the composite coatings [27,28]. On the other hand, the excellent performance of the chromium carbide makes it an excellent candidate as a wear-resistant reinforcing phase for wear-resistant composite coatings [29–33]. Naturally, the Ni3Si-matrix composite coatings reinforced by Cr7C3 carbides are expected to possess good service properties under wear conditions.
⁎ Corresponding author. Tel.: +86 931 4968193; fax: +86 931 8277088. E-mail address: [email protected] (Q. Bi). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.031
In this study, a new type of Ni3Si – Cr7C3 composite coating was fabricated by self-propagating high temperature synthesis (SHS) casting route. Microstructures, mechanical, tribological as well as corrosion properties of the coating were studied.
2. Experimental 2.1. Fabrication of the SHS coating Starting materials used in this study were Cr2O3, CrO3, carbon, aluminum, nickel, and silicon powders. The powders were weighted in the stoichiometric proportion of chemical reaction Eqs. (1) and (2), respectively. The characteristics of the reactant powders are given in Table 1. Cr2 O3 þ 5CrO3 þ 3C þ 12A1 ¼ Cr7 C3 þ 6A12 O3 −1
¼ −6229kJmol 3Ni þ Si ¼ Ni3 Si
o
ΔHf ;298K ð1Þ
o
ΔHf ;298K ¼ −37:2kJmol
−1
ð2Þ
These raw material powders were mechanically milled for 8 h by a planetary ball mill for homogeneous mixture of the reactants. The AISI 1020 carbon steel substrate was 65 mm in diameter and 5 mm in thickness. Before coating, the surface of the substrates was polished and then cleaned with acetone and a small amount of nickel powders were spread on the surface of the substrate to improve the wettability between the coating and AISI 1020 carbon steel.
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Table 1 Properties of the starting powders. Powder
Size (mesh)
Purity (%)
Impurity
Ni Si Cr2O3 CrO3 Al C (graphite)
200 200 b200 b200 100–200 ≥500
99.5 99.0 99.0 99.0 99.0 99.85
Co, Si, C, S, Ca, Mn Fe, Cu, Zn, Sb Cl, SO4, CrO3, Fe Na, Al, K, Fe, Cl, SO4 Fe, Si, Cu Ignition residue
Ni and Si powders of 35 g were filled manually on the nickel powders, and 35 g of the Cr–C reactants were subsequently set on the Ni–Si layer. In order to generate a steady combustion wave in the SHS process and remove gases in the reactants [34], the reactant powders were cold pressed under a uniaxial pressure of 30 MPa. An igniting agent was then put on the surface of the Cr–C reactants, and subsequently, the steel crucible was put into the reactor. The reactor was cleaned by argon gas at room temperature and refilled with argon gas to 5 MPa when heated to 523 K. The igniter reaction started at this temperature. The compacted Cr–C powders were ignited by the heat released from the igniter, and the heat released by the Cr–C reactants then ignited the Ni–Si compacted powders. The combustion reactions were finished in a few seconds and thus the coating was formed [35–37]. The thickness of the coating on the substrate was about 2–3 mm. Several samples were produced using the same processing method. Specimens with dimensions of 18.5 × 18.5 × 8 mm were cut from the samples for testing the mechanical and tribological properties.
Fig. 2. SEM micrographs of transverse cross-sections of the coatings.
duration of 20 min. The counterpart was an AISI 52100 steel ball (hardness 62–63 HRC, Ra ≈ 0.01 μm) with a diameter of 9.60 mm. The wear rate is defined as W = V/PS, where V is the wear volume, P is the applied load and S is the total sliding distance. The wear volume V was measured using the Micro-XAM-3D non-contact surface profiler. In addition, the wear properties of the substrate and the bearing steel were tested under the same conditions, for analysis and comparison. The corrosion property of the coating was studied through salt spray test. The salt spray test was performed according to BSI 7479: the NaCl concentration of the sprayed solution was 50 g/L, and temperature 35 °C. The samples were salt sprayed for 72 h continuously.
2.2. Microstructure examination 3. Results and discussion The morphology of composite coatings was observed by means of JSM-5600LV scanning electron microscopy (SEM) at an acceleration voltage of 20 kV. The Shimadzu Dmax-RB X-ray diffraction (XRD) diffractometer (Japan) was adopted to analyze the phase compositions of the coating. 2.3. Mechanical, tribological and corrosion properties of the coating Vickers hardness of the coating was tested by using an AVKSHI MVK-1 hardness tester. The applied load was 500 g and the dwell time was 10 s. The sliding wear test was carried out on the Optimol (Germany) SRV oscillating friction and wear tester with an oscilliating amplitude of 1 mm, frequency of 15 Hz, loads 40 to 80 N, and a
Fig. 1. X-ray diffraction patterns of the SHS coatings.
3.1. Microstructure and phase composition of Ni3Si-based composite coating The phase compositions of the coating have been identified by Xray diffraction analysis. Fig. 1 shows the XRD pattern of the coating. The X-ray diffraction profile reveals that the coatings are mainly consisted of Ni3Si and Cr7C3 phases. Apart from these main phases, the presences of some other phases like Ni2Si, CrSi, and Ni phases have also been identified. The Al2O3 phase was not detected in the coating, and this proved that Al2O3 was completely separated and removed from the composite [16,38,39]. Fig. 2 presents the morphology of the cross-sections of the coating. It can be seen that the coating and the substrate bond well. In addition, there are no pores and cracks near the interface region, which proves
Fig. 3. SEM micrograph of the top surface of the coatings.
M. Niu et al. / Surface & Coatings Technology 205 (2011) 4249–4253
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Fig. 4. EDS elemental mappings for the top surface of the coatings.
that the composite coating layer is well adherent to the substrate. The growth of the columnar grains is observed almost perpendicular to the interface of the deposits. In order to evaluate the bonding strength of the coating, the thermal shock test was performed. The sample was heated to 1000 °C for 15 min, and then immediately dropped it into water of 20 °C, the coating did not peel off after 50 cycles thermal shock. The result indicates that the coating possesses excellent bonding strength [40].
Fig. 3 shows the microstructure of the top surface of the coating observed under SEM. It is found that there are mainly two phases – the dark phase (marked “A”) and gray phase (marked “B”) coexistence in the coating. EDS results (Fig. 4) reveal that the phase “A” is chromium-rich; the phase “B” contains nickel and silicon. The content of carbon is not clear from the results by EDS, but the high hardness suggests the formation of Cr7C3. Furthermore, the XRD results prove that the coating consists of Ni3Si and Cr7C3. Therefore,
0.7
1000
Coatings Bearing steel AISI 1020 steel
800
Coefficient of friction
Microhardness, HV
900
700 600 500 400
0.6
0.5
0.4
300 200 0.3 0
300
600
900
1200
Distance from the interface/µm Fig. 5. Microhardness profile of the Ni3Si–Cr7C3 coatings.
1500
40
50
60
70
80
Load (N) Fig. 6. Variation of COFs with normal load for same sliding speeds.
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45
Wear rate 10-6 mm3 /Nm
40 35 30 25 Coating Bearing steel AISI 1020 steel
20 15 10 5 0 40
50
60
70
80
Load (N) Fig. 7. Variation of wear rate with normal load.
the phase “A” is mainly Cr7C3 and the phase “B” is Ni3Si. Fig. 3 also shows that a lot of dendrites inside the coating. This should be a direct consequence of the higher cooling rates of the SHS process. 3.2. Vickers hardness, friction and wear properties of the SHS composite coating Fig. 5 shows the microhardness distribution from the interface to the coating. The formation of a hard Cr7C3 phase improves the mechanical properties of Ni3Si intermetallics, so the microhardness of
the coating is much higher than that of the substrate. With the increase of the distance from the interface, the volume fraction of the Cr7C3 increases and therefore the microhardness increases significantly. At the top surface of the composite coating, the vickers hardness (HV) is up to 9.70 ± 0.50 GPa. High hardness of the composite coating is due to the presence of hard phase Cr7C3. The coating consists of nickel and Ni3Si–Cr7C3 composite adjacent to the interface; and far from the interface it consists of Ni3Si–Cr7C3 composite. The hardness of nickel is lower than that of Ni3Si–Cr7C3 composite. Therefore the hardness of the coating adjacent to the interface is low. As the coating thickness increases, the amount of nickel in the coating decreases and therefore the hardness increases with coating thickness. As the thickness continues to increase, the content of nickel in the coating decreases further and the hardness of the coating increases to a maximum value, which is close to the hardness of Ni3Si–Cr7C3 composite. The friction and wear properties of the substrates are significantly improved by the composite coatings. Fig. 6 shows the variation of coefficients of friction (COF) with the normal loads of the coating, AISI 1020 steel substrate, as well as AISI 52100 bearing steel. It can be observed that the COFs vary from 0.48 to 0.53, and the COFs of the coatings are lower than that of the substrates and the bearing steels under high loads. Moreover, with the increase of the normal loads, the COFs of the coatings, the substrates and the bearing steels decrease in the same trend. The wear rates of the coatings, the substrates and the bearing steels tested at same sliding speeds have been plotted against applied normal loads in Fig. 7. The plots of wear rates against normal loads show that the wear rates of the three materials increased with increasing load. The wear rates of the substrates are about an order
Fig. 8. SEM morphologies of worn surfaces of the coatings tested with the same sliding speed 0.03 m/s at different applied loads: (a) 40 N, (b) 50 N, (c) 60 N, (d) 70 N, (e) 80 N.
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Fig. 9. The micrographs of transverse cross-sections of the coatings immersed in salt spray corrosion for 72 h: (a) optical image, (b) SEM image.
higher than that of the composite coatings, and even better than that of the bearing steels. The excellent wear resistance of the coatings could be explained as follows. First, Ni3Si possesses high hardness and good oxidation resistance; second, the very high hardness reinforced Cr7C3 phase is uniformly distributed in the matrix, and prevents the coatings from severe wear. Therefore, the composite coating exhibits excellent wear resistance ability. SEM morphologies of worn surfaces of the coatings tested at different applied loads are given in Fig. 8. It is found that applied loads have important influence on the morphologies of the worn surfaces. As shown in Fig. 8a, the worn surface of the coating is relatively smooth at a load of 40 N. Because the primary Cr7C3 phase has very high hardness and is uniformly distributed in the matrix, the wear resistance of the coating is excellent, and therefore there are nearly no scratches visible on the worn surface. As the load increasing, there are only shallow grooves visible on the worn surface, and a transfer layer consisting of iron oxide is formed on the worn surface (Fig. 8b and c), this indicates that the AISI 52100 steel ball has oxide and the formed iron oxide transferred to the coating surface. The transfer layer, in some extent, is helpful to reduce the COF and wear rate of the coating. With the load further increasing, delamination happens on the surface of the coating; and the wear mechanism expresses as delamination as shown in Fig. 8d and e. 3.3. Corrosion properties of the SHS composite coating The results of the salt spray test on the samples show that the substrates suffered severe corrosion after 72 h (Fig. 9). The red rust can be seen on the substrates. However, the coatings withstood 72 h exhibit weak corrosion. The excellent corrosion resistance of the coating is due to the good corrosion resistance of Ni3Si main phases. In addition, during the process of solidification, the rapid cooling rate in the melt pool makes the microstructure of the coatings fine and uniform, which also led to the excellent corrosion resistance of the coating. 4. Conclusions (1). A Ni3Si–Cr7C3 composite coating was fabricated by the SHS method. The coating is well adherent to the AISI 1020 steel substrate. The microhardness of the composite coating is reinforced by the formation of the hard phase Cr7C3. (2). The wear resistance of AISI 1020 steel was improved significantly. The wear rates of the composite coatings are lower than
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