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Electrical contact strengthening of induction-clad Ni–40% WC composite coatings on 40Cr substrates
Surface & Coatings Technology 279 (2015) 32–38
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Electrical contact strengthening of induction-clad Ni–40% WC composite coatings on 40Cr substrates Mengkuo Xu, Shigen Zhu ⁎, Hao Ding College of Mechanical Engineering, Engineering Research Center of Advanced Textile Machinery, Ministry of Education, Donghua University, Shanghai 201620, China
a r t i c l e
i n f o
Article history: Received 29 January 2015 Revised 5 July 2015 Accepted in revised form 14 August 2015 Available online 15 August 2015 Keywords: Electrical contact strengthening Induction cladding Ni–40% WC composite coating Coating microstructure Wear resistance
a b s t r a c t Ni–40% WC composite coatings were prepared on 40Cr surfaces by high frequency induction cladding and then were treated by electrical contact strengthening (ECS) to further improve the coating properties. The effects of ECS on the coating microstructure, microhardness, phase transformation and wear behaviour were investigated using optical microscopy, field emission scanning electron microscopy, Vickers hardness measurements, X-ray diffraction and rolling contact wear tests. As the contact current increased, the porosity and grain size of the resulting composite coating decreased, and its cohesion increased. The hard phase in the coatings also diffused further. After ECS treatment, the average hardness of the Ni–40% WC composite coatings ranged from 652 HV0.1 up to 906 HV0.1. As demonstrated by the results of the rolling contact wear tests, the composite coatings treated by ECS exhibited improved wear resistance. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The surface of work piece serves an important role in contacting other work piece, especially under harsh conditions. Most surface damage failure results in high costs [1]. Because tungsten carbide (WC) has high hardness, superior mechanical properties and excellent tribological properties, WC composite coatings have been used to improve the wear resistance of various surfaces. Traditional thermal spraying technologies have been employed to prepare WC composite coatings on metal surfaces [2–7]. However, thermally sprayed coatings do not adhere to the substrate as strongly as coatings prepared by induction cladding [8], and the low-density structures of these coatings can cause them to flake under harsh conditions, thus limiting their application. Compared to composite coatings prepared by thermal spraying, coatings subjected to electrical contact strengthening (ECS) exhibit improved properties [9]. ECS, which is a technology based on the electrical contact resistance heating between an electrode and a workpiece, can enhance the coating properties and bonding strength [10]. Wiemer [11] and Polyachenko [12] reported that the coated materials can be strips, wire rods or powder–paste mixtures. Stachowiak and Batchelor [13] demonstrated that a sacrificial roller made of a metal with a lower melting point could be used to deposit coatings on substrates with higher melting points by ECS. Yalin Wang et al. [14] applied ECS to a thermally sprayed WC–Co coating to improve its adhesion strength. These authors showed that electrical resistance heating is an inexpensive, ⁎ Corresponding author. E-mail address: [email protected] (S. Zhu).
http://dx.doi.org/10.1016/j.surfcoat.2015.08.029 0257-8972/© 2015 Elsevier B.V. All rights reserved.
efficient method for strengthening clad layers. However, the final coating performance after the ECS process depended on the pre-coating preparation method. Induction cladding can be used to prepare metallurgically bonded WC composite coatings on metal surfaces [15]. However, the coating porosity and defects in the coating during the cladding process affects its performance, limiting the application of this method. In this work, Ni–40% WC composite powder was used as a pre-coating prepared by induction cladding, then the Ni–40% WC coatings were treated by ECS. 2. Experimental procedures 2.1. Materials and induction cladding The substrate used in this investigation was alloy steel (40Cr) with a chemical composition of 0.37–0.44 wt.% C, 0.17–0.37 wt.% Si, 0.5– 0.8 wt.% Mn, 0.8–1.1 wt.% Cr, ≤ 0.03 wt.% Ni, ≤ 0.035 wt.% P, ≤ 0.03 wt.% S, and ≤ 0.03 wt.% Cu, Fe constituted the balance. Samples with a diameter of 60 mm and length of 100 mm were machined from a rolling workpiece. A Ni-based alloy powder with particle sizes ranging from 15 to 45 μm (see Table 1 for its chemical composition) was used as a binder during the coating. The WC powder, which had particle sizes ranging from 20 to 40 μm, was composed of 12.09 wt.% cobalt and balance tungsten carbide. The composite powder, which was composed of 60 wt.% Ni-based alloy and 40 wt.% WC, was prepared by a mechanical ball-milling method under an argon gas atmosphere for 3 h. The ball-to-powder weight ratio was 10:1, and the rotation speed of the mill was 350 rpm. Both the vial and milling balls (10 mm in diameter) were made of cemented carbide materials.
M. Xu et al. / Surface & Coatings Technology 279 (2015) 32–38 Table 1 Chemical composition (wt.%) of the Ni-based alloy powder. C
Cr
B
Si
Fe
Ni
0.6–1.0
14–17
2.5–4.5
3–4.5
≤15
Balance
33
and electrode; the small area of the contact severely constricted the electric current, which led to intense resistive heating close to the contact. During ECS, the electric current is an important parameter used to control the level of resistance heating. Here, electric currents of 15, 20 and 25 kA were employed. The contact pressure affects the coating porosity and the number of defects in the coating, and the coating speed also affects the surface heating region and the cooling rate of the surface [16]. In this study, ECS was performed using a contact pressure of 2.0 kN, rotation speed of 0.5 rpm, and electrode speed of 0.2 mm/min. 2.3. Metallographic characterisation of the ECS coating
Fig. 1. Schematic diagram of the induction cladding process.
Cellulose acetate was dissolved in acetone, and the resulting solution was then mechanically mixed with the Ni-based alloy and 40% WC powders. After the powder–paste mixtures were prepared, they were brushed on the substrate surface at a thickness of 0.3 mm as a prelayer. Fig. 1 presents a schematic diagram of the induction cladding process in which the pre-layer-coated workpiece was placed into the highfrequency induction coil. At the beginning of the induction cladding process, the induced current was focused on the workpiece surface, which resulted in the rapid heating of the pre-layer and workpiece to the melting temperature. During the formation of the induction cladding, the surface heating depth was 0.5–2.5 mm. The high frequency induction cladding was performed using a power of 25 kW, oscillation frequency of 100 kHz, output current of 1.2 kA, induction coil–workpiece distance of approximately 2 mm, and time of 24 s. 2.2. ECS of the induction coating After high frequency induction cladding, the surface of Ni–40% WC composite coatings were smoothed, then were reinforced by using a self-developed electrical contact surface strengthening device, which is illustrated in Fig. 2. The resistive heating was localised by passing an electric current through the contact point between the rotating workpiece
Cross sections of the coatings were prepared using standard polishing procedures and were then chemically etched in a 4% HNO3 and C2H5OH solution. The microstructure was examined by optical microscopy (Axiovert 200, Carl Zeiss, Germany) and field emission scanning electron microscopy (FESEM) with energy dispersive X-ray spectroscopy (EDS) (S-4800, Hitachi, Japan). The phases present in the coating were identified by X-ray diffraction (D/Max-2550PC, Rigaku, Japan). The coating porosity and grain size were determined by analysing the FESEM images with the Image-Pro Plus software. Specifically, the porosity was determined by calculating the percentage of pores in the coating, and the grain size was determined based on GB/T 6394-2002 national standards and grain area calculations. The microhardness along a cross section of the substrate-coating interface was measured using a microhardness tester (HXS-1000A, Shanghai Optics Apparatus Ltd., China) with a load of 100 g and loading time of 15 s. Thermal shock tests were performed to compare the cohesion of samples with and without the ECS Ni–40% WC coating during rapid temperature changes. Cross sections were removed from the workpieces with and without the ECS coating. In the thermal shock experiments, the coated samples were heated at 600 °C in a resistor furnace for 5 min. After quickly quenching the samples in cold water, they were dried under airflow and then reheated in the furnace. This process was repeated until cracks formed in the coating surface, and the number of thermal shocks performed up to that point was recorded. 2.4. Wear tests A rolling wear testing machine (MJP-20, JingCheng Ltd., China), which is shown schematically in Fig. 3, was used to test the wear resistance of the samples at room temperature (25 °C). The upper sample of the hardened rubbing pair was GCr15 steel with an average hardness of HRC60, and the lower sample was Ni–WC-coated 40Cr and GCr15. The ring shape of the rubbing pairs had an external diameter of 60 mm, inner diameter of 30 mm, length of 20 mm, and contact width of
Fig. 2. Schematic diagram of the ECS device.
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M. Xu et al. / Surface & Coatings Technology 279 (2015) 32–38
Fig. 3. Schematic of the rolling wear testing machine.
5 mm. The lower sample rotated at 400 rpm with an applied load of 1.0 kN. The test was conducted for 6 h; after each hour of testing, the samples were cleaned with acetone, dried, and weighed. The sample weight loss was measured on an electronic balance (TG328B, Shanghai LiNeng Electronic Instrument Co., China) with a sensitivity of ±0.1 mg. The worn surfaces were also analysed by FESEM. 3. Results and discussion 3.1. Phase analysis
of different phases, including Co6W6C, W2C, Cr23C6, Fe6W6C, Co6W6C, Ni3Fe, FeNi, and Fe5C2. These different chromium carbide and tungsten carbide materials were evenly distributed throughout the Ni matrix. Some researchers showed that WC particles tend to sink to the bottom of the coating due to a stirring motion in the melting pool during laser cladding [17,18], which is important because the WC particle distribution within the coating affects its quality and performance. However, the formation of Fe6W6C and Co6W6C in this study showed that the WC particles partially decomposed and rapidly combined with other elements during the induction cladding process. The formation of these new compounds might enhance the cohesive force of the coating. After ECS, the major phases of the coatings changed slightly. As the contact current increased, the Fe6W6C, Cr23C6, and W2C peak intensities in the XRD patterns gradually increased, whereas the Ni3Fe, FeNi, and Fe5C2 peak intensities decreased. These changes in the peak intensities might be associated with changes in the grain size. The ECS treatment might have caused elemental species to diffuse and recrystallize in the coating. Under high contact resistance heating and contact force, the Ni–40% WC pre-coatings might have melted instantly and sintered to become more compact. 3.2. Coating microstructure The cross-sectional microstructures of the induction-clad and ECSprocessed Ni–40% WC composite coatings are shown in Figs. 5 and 6, respectively. The coating–substrate interface appeared as a bright white band, which is referred to as the diffusion transfer belt (DTB) [8]. The presence of the DTB indicates that metallurgical bonding between the coating and substrate occurred. Fig. 5a shows that it was impossible to
The Ni–40% WC composite powder mainly consisted of Ni, W, C, Co, Cr and Fe and might form a multiple eutectic point mixture after induction cladding. As shown in Fig. 4, the XRD results indicated the presence
Fig. 4. XRD analysis of the Ni–40% WC coatings before and after ECS treatment.
Fig. 5. Cross-sectional (a) metallographic and (b) FESEM micrographs of the coating–substrate interface (before ECS).
M. Xu et al. / Surface & Coatings Technology 279 (2015) 32–38
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Fig. 7. Microhardness distribution along the cross sections of the coatings subjected to different contact currents.
Fig. 6. Cross-sectional (a) metallographic and (b) FESEM micrographs of the coating–substrate interface (after the 25 kA ECS treatment).
obtain a non-porous, defect-free coating due to the uneven distribution of the composite powder in the cellulose acetate binder and to binder evaporation during the rapid heating in the induction cladding process. Fig. 5b presents a detailed FESEM image of the coating–substrate interface. During induction cladding, part of the tungsten carbide combined with other elements to form mixed chromium carbide–tungsten carbide dendritic, twin, whisker and eutectic colony microstructures. The XRD microstructure analyses (Fig. 4) indicate that these various carbide structures were the primary strengthening components in the coating.
Fig. 6a and b shows that after the 25 kA ECS treatment, the various eutectic carbides in the coating were finer and diffused to become more dispersed. Thus, ECS led to a more dispersed, stronger coating with smaller, more densely packed grains. During ECS, the resistance heating caused the coating temperature to change rapidly, resulting in the rapid recrystallisation of the duplex grain structure in the coating, and the appearance of another solid phase in the coating caused the mixed grains to become scattered and to partially disappear. Furthermore, the application of higher pressures reduced the coating porosity and enhanced the coating cohesion. Therefore, as the microstructure changed, the coating performance improved. According to the image analysis, the average porosity of the induction-clad coating was 3.8%. After the 25 kA ECS treatment, the average porosity was reduced to 1.7%. The change in the coating grain size with increasing contact current is shown in Table 2. As the contact current increased, the average grain size decreased. During the ECS treatment, an instantaneously high temperature and high pressure can reduce the coating porosity and grain size, which might cause the composite coating to become more compact. In the thermal shock tests, the sample treated with a 25 kA current had a better thermal shock performance (205 thermal shocks were applied before cracking was observed) than the samples that were not subjected to ECS (136
Table 2 Change in the grain size with contact current. Unstrengthened
Grain size (μm)
Contact current
9.4
15 kA
20 kA
25 kA
6.7
5.6
4.7
Table 3 EDS analysis of the DTB microstructure. Microstructure characteristics
Composition (wt.%) Fe
Ni
C
W
Co
Cr
DTB
72.8 64.9
9.9 15.7
0.4 0.6
9.0 10.2
4.2 4.6
3.7 4.0
Without ECS With 25 kA ECS
Fig. 8. Weight loss during the rolling wear tests for different samples.
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M. Xu et al. / Surface & Coatings Technology 279 (2015) 32–38
Fig. 9. FESEM micrographs of the 40Cr surface after (a) 1 h, (b) 3 h and (c) 6 h; the untreated Ni–40% WC-coated surface after (d) 1 h, (e) 3 h and (f) 6 h; and the Ni–40% WC-coated surface treated at 25 kA after (g) 1 h, (h) 3 h and (i) 6 h of rolling wear testing.
M. Xu et al. / Surface & Coatings Technology 279 (2015) 32–38
thermal shocks led to cracking). These results show that ECS can improve the cohesion of the Ni–40% WC composite coating. EDS analysis reveals the element distributions in the DTB before and after 25 kA ECS (Table 3). Each element diffused during cladding. After 25 kA ECS, the amount of Ni and Co in the DTB increased, improving the bonding strength between the coating and substrate. The amount of W, Cr and C in the DTB also increased, which enhanced the coating strength. 3.3. Coating microhardness and wear resistance The microhardness distributions in the cross sections of the samples subjected to different contact currents are shown in Fig. 7. The results show that as the contact current increased, the coating hardness increased. The coating hardness changed slightly after the 20 kA ECS treatment and had an average hardness of 763 HV0.1, which was 8% higher than that of the unstrengthened induction-clad coating. However, the 25 kA ECS treatment led to a substantial increase in the coating hardness to an average of 906 HV0.1, which was 28% higher than that of the unstrengthened induction-clad coating. These results indicate that the Ni–40% WC coating became denser due to the rapid change in the resistance heat. Therefore, the increase in the coating hardness could be attributed to the increasing contact current, which decreased the coating grain size and porosity. Fig. 7 also shows that as the contact current increased, the microhardness throughout the cross section at the coating–substrate interface generally increased relative to that for the unstrengthened coating. The hardness of the DTB also increased with increasing contact current, which is consistent with the EDS analysis. Near the interface, the substrate was also heated to the transition temperature. The heated substrate area, which was denoted the heat affected zone (HAZ) [2], had a higher hardness. The microhardness gradient at the interface not only helped to improve the wear properties but also increased the bonding strength between the two neighbouring layers [19]. To investigate the effect of the contact current on the coating wear resistance, rolling wear tests were performed using the same rolling parameters for each sample, and the weight losses of the samples were measured. Fig. 8 shows the weight losses of the coating samples, 40Cr sample and GCr15 sample after 6 h under loads of 1.0 kN. In the first hour, the samples quickly lost weight, but the rate of weight loss decreased over time, indicating that the surfaces became stable under rolling wear at longer times. The weight losses of the samples with the Ni–40% WC coatings were much lower than those of 40Cr. The weight loss of the unstrengthened induction-clad sample was higher than that of the GCr15 sample and the weight loss of the coating formed during the 15 kA ECS treatment was similar to that of the GCr15 sample. As the contact current increased, the coating weight loss decreased below that of the GCr15 sample. The Ni–40% WC-coated sample that was treated at 25 kA had the lowest weight loss of all the coated samples. To understand the wear mechanisms of the coated samples, the worn surfaces of the samples were examined by FESEM, and the micrographs are shown in Fig. 9. As shown in Fig. 9a, sheared areas were observed on the worn surface of the uncoated 40Cr substrate during the grinding (Fig. 9a). As the rolling time increased, the sheared areas became larger, and fatigue spalling occurred (Fig. 9b and c). The 40Cr substrate microstructure consisted of ferrite, pearlite and cementite, and the rubbing pair had a higher hardness. Thus, when the rolling contact friction was applied to the surface, it was subjected to the action of the shearing force and compressive stress, which caused the surface to undergo plastic deformation. As the wear time increased, the plastic deformation might have caused the worn surface to flake. The flaking particles would then cause abrasive wear, resulting in large areas peeling from the sample surface [20]. When the 40Cr surface was cladded with the Ni–40% WC coating, the wear modes changed. Previous studies showed that four coating failure modes are possible: surface abrasion,
37
spalling, delamination within the coating and interfacial delamination [21]. After induction cladding, the high microhardness of the coating and the metallurgical bonding between the coating and substrate limited the delamination of the coating materials. As shown in Fig. 9d–f, as the wear time increased, the coatings failed via interfacial delamination. During the rolling wear tests, interfacial delamination, which resulted in the formation of wear debris, might have originated in the pores in the coating. Shear plastic deformation, crack initiation, and crack propagation near the surfaces would lead to lamination of the debris [22]. After the ECS treatment, the Ni–40% WC coating had a higher hardness than the rubbing pair and fewer pores, stronger cohesion and smaller grain size than the coating that was not treated by ECS. Thus, fewer particles flaked from the ECS-treated coating, limiting its interfacial delamination under high stress or heavy loads. As shown in Fig. 9g–i, the Ni–40% WC powders cladded by induction and reinforced by ECS had desirable qualities that provided superior wear resistance. 4. Conclusions In this study, alloy steel (40Cr) surfaces were induction-cladded with a Ni–40% WC composite powder as a pre-coating, and then ECS was employed to obtain high-quality, wear-resistant coatings. The coating microstructure, hardness and wear resistance were examined. Based on the results, the following conclusions were made: 1. ECS can be used to reinforce Ni–40% WC composite coatings to obtain high-performance surfaces. After ECS, the coating porosity, the number of other defects in the coating, and the coating grain size decreased. 2. Examination of the microstructure indicated that chromium carbide and tungsten carbide constituted the main hard phase in the Ni–40% WC induction coatings. ECS resulted in a more compact Ni–40% WC composite coating. As the contact current increased, the coating hardness increased. 3. Under rolling wear, peeling and delamination were observed at the sample surfaces. After ECS, the Ni–40% WC composite coatings had superior cohesion, higher hardness and lower porosity, and the metallurgical bonding between the coating and substrate was improved. These enhanced properties might limit the peeling of the coating material under high stress or heavy loads. References [1] M. Franco, W. Sha, S. Malino, H. Liu, Micro-scale wear characteristics of electroless Ni–P/SiC composite coating under two different sliding conditions, Wear 317 (2014) 254–264. [2] M. Afzal, M. Ajmal, et al., Surface modification of air plasma spraying WC–12%Co cermet coating by laser melting technique, Opt. Laser Technol. 56 (2014) 202–206. [3] V.K. Balla, S. Bose, A. Bandyopadhyay, Microstructure and wear properties of laser deposited WC–12%Co composites, Mater. Sci. Eng. A 527 (2010) 6677–6682. [4] HungHua Sheu, et al., Effects of plating parameters on the Ni–P–Al2O3 composite coatings prepared by pulse and direct current plating, Surf. Coat. Technol. 235 (2013) 529–535. [5] Hong Liu, RongXin Guo, Liu Zhu, Characteristics of microstructure and performance of laser-treated electroless Ni–P/Ni–W–P duplex coatings, Trans. Nonferrous Metals Soc. China 22 (2012) 3012–3020. [6] Y. Akiyama, et al., Local deposition of polypyrrole on aluminium by anodizing, laser irradiation, and electrolytic polymerization and its application to the fabrication of micro-actuators, Electrochim. Acta 51 (2006) 4834–4840. [7] Shengfeng Zhou, Xiaoqin Dai, Microstructure evolution of Fe-based WC composite coating prepared by laser induction hybrid rapid cladding, Appl. Surf. Sci. 256 (2010) 7395–7399. [8] Yuan Gao, Chenglei Wang, et al., Microstructure and wear resistance of Ni60 layer prepared by high-frequency induction cladding, Rare Metal Mater. Eng. 40 (2011) 309–312. [9] Y.L. Wang, et al., Electric contact strengthening to improve the bonding between thermally sprayed 316 stainless steel coating and 45# steel substrate, Exp. Tech. 35 (2011) 66–70. [10] Xiaoben Qi, Shigen Zhu, Study on electric contact heating for nodular cast iron 6003, Appl. Mech. Mater. 152–154 (2012) 316–321. [11] K. Wiemer, An investigation into cladding by electrical resistance heating, Proceeding of 10th International Conference on Surface Modification Technologies, Singapore. Surface Modification Technologies X [C] 1997, pp. 891–898. [12] A.V. Polyachenko, Electric resistance surfacing-optimum method of reconditioning and hardening precision machine components, Weld. Int. 8 (1994) 226–228.
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[13] G.W. Stachowiak, A.W. Batchelor, Surface hardening and deposition of coatings on metals by a mobile source of localized electrical resistive heating, J. Mater. Process. Technol. 57 (1996) 288–297. [14] Yalin Wang, et al., Electric contact strengthening to improve the bonding between WC–Co coating and 45# steel substrate, J. Therm. Spray Technol. 19 (2010) 1142–1146. [15] Ding-yong He, et al., Properties of WC reinforced Ni60 coating prepared by high frequency induction cladding, Trans. Mater. Heat Treat. 29 (2008) 138–141. [16] X. Qi, et al., Microstructure and wear behaviors of WC–12%Co coating deposited on ductile iron by electric contact surface strengthening, Appl. Surf. Sci. 282 (2013) 672–679.
[17] P. Wu, et al., Influence of WC particle behavior on the wear resistance properties of Ni–WC composite coatings, Wear 257 (2004) 142–147. [18] P. Wu, et al., Laser alloying of a gradient metal–ceramic layer to enhance wear properties, Surf. Coat. Technol. 73 (1995) 111–114. [19] S.Y. Chen, et al., Preparation of a novel Ni/Co-based alloy gradient coating on surface of the crystallizer copper alloy by laser, Appl. Surf. Sci. 258 (2011) 1443–1450. [20] De Mello, et al., Influence of surface texturing and hard chromium coating on the wear of steels used in cold rolling mill rolls, Wear 302 (2013) 1295–1309. [21] R. Ahmed, M. Hadfied, Failure modes of plasma sprayed WC–15%Co coated rolling elements, Wear 230 (1999) 39–55. [22] R.B. Waterhouse, D.E. Taylor, Fretting debris and the delamination theory of wear, Wear 29 (1974) 337–344.
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Electrical contact strengthening of induction-clad Ni–40% WC composite coatings on 40Cr substrates Mengkuo Xu, Shigen Zhu ⁎, Hao Ding College of Mechanical Engineering, Engineering Research Center of Advanced Textile Machinery, Ministry of Education, Donghua University, Shanghai 201620, China
a r t i c l e
i n f o
Article history: Received 29 January 2015 Revised 5 July 2015 Accepted in revised form 14 August 2015 Available online 15 August 2015 Keywords: Electrical contact strengthening Induction cladding Ni–40% WC composite coating Coating microstructure Wear resistance
a b s t r a c t Ni–40% WC composite coatings were prepared on 40Cr surfaces by high frequency induction cladding and then were treated by electrical contact strengthening (ECS) to further improve the coating properties. The effects of ECS on the coating microstructure, microhardness, phase transformation and wear behaviour were investigated using optical microscopy, field emission scanning electron microscopy, Vickers hardness measurements, X-ray diffraction and rolling contact wear tests. As the contact current increased, the porosity and grain size of the resulting composite coating decreased, and its cohesion increased. The hard phase in the coatings also diffused further. After ECS treatment, the average hardness of the Ni–40% WC composite coatings ranged from 652 HV0.1 up to 906 HV0.1. As demonstrated by the results of the rolling contact wear tests, the composite coatings treated by ECS exhibited improved wear resistance. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The surface of work piece serves an important role in contacting other work piece, especially under harsh conditions. Most surface damage failure results in high costs [1]. Because tungsten carbide (WC) has high hardness, superior mechanical properties and excellent tribological properties, WC composite coatings have been used to improve the wear resistance of various surfaces. Traditional thermal spraying technologies have been employed to prepare WC composite coatings on metal surfaces [2–7]. However, thermally sprayed coatings do not adhere to the substrate as strongly as coatings prepared by induction cladding [8], and the low-density structures of these coatings can cause them to flake under harsh conditions, thus limiting their application. Compared to composite coatings prepared by thermal spraying, coatings subjected to electrical contact strengthening (ECS) exhibit improved properties [9]. ECS, which is a technology based on the electrical contact resistance heating between an electrode and a workpiece, can enhance the coating properties and bonding strength [10]. Wiemer [11] and Polyachenko [12] reported that the coated materials can be strips, wire rods or powder–paste mixtures. Stachowiak and Batchelor [13] demonstrated that a sacrificial roller made of a metal with a lower melting point could be used to deposit coatings on substrates with higher melting points by ECS. Yalin Wang et al. [14] applied ECS to a thermally sprayed WC–Co coating to improve its adhesion strength. These authors showed that electrical resistance heating is an inexpensive, ⁎ Corresponding author. E-mail address: [email protected] (S. Zhu).
http://dx.doi.org/10.1016/j.surfcoat.2015.08.029 0257-8972/© 2015 Elsevier B.V. All rights reserved.
efficient method for strengthening clad layers. However, the final coating performance after the ECS process depended on the pre-coating preparation method. Induction cladding can be used to prepare metallurgically bonded WC composite coatings on metal surfaces [15]. However, the coating porosity and defects in the coating during the cladding process affects its performance, limiting the application of this method. In this work, Ni–40% WC composite powder was used as a pre-coating prepared by induction cladding, then the Ni–40% WC coatings were treated by ECS. 2. Experimental procedures 2.1. Materials and induction cladding The substrate used in this investigation was alloy steel (40Cr) with a chemical composition of 0.37–0.44 wt.% C, 0.17–0.37 wt.% Si, 0.5– 0.8 wt.% Mn, 0.8–1.1 wt.% Cr, ≤ 0.03 wt.% Ni, ≤ 0.035 wt.% P, ≤ 0.03 wt.% S, and ≤ 0.03 wt.% Cu, Fe constituted the balance. Samples with a diameter of 60 mm and length of 100 mm were machined from a rolling workpiece. A Ni-based alloy powder with particle sizes ranging from 15 to 45 μm (see Table 1 for its chemical composition) was used as a binder during the coating. The WC powder, which had particle sizes ranging from 20 to 40 μm, was composed of 12.09 wt.% cobalt and balance tungsten carbide. The composite powder, which was composed of 60 wt.% Ni-based alloy and 40 wt.% WC, was prepared by a mechanical ball-milling method under an argon gas atmosphere for 3 h. The ball-to-powder weight ratio was 10:1, and the rotation speed of the mill was 350 rpm. Both the vial and milling balls (10 mm in diameter) were made of cemented carbide materials.
M. Xu et al. / Surface & Coatings Technology 279 (2015) 32–38 Table 1 Chemical composition (wt.%) of the Ni-based alloy powder. C
Cr
B
Si
Fe
Ni
0.6–1.0
14–17
2.5–4.5
3–4.5
≤15
Balance
33
and electrode; the small area of the contact severely constricted the electric current, which led to intense resistive heating close to the contact. During ECS, the electric current is an important parameter used to control the level of resistance heating. Here, electric currents of 15, 20 and 25 kA were employed. The contact pressure affects the coating porosity and the number of defects in the coating, and the coating speed also affects the surface heating region and the cooling rate of the surface [16]. In this study, ECS was performed using a contact pressure of 2.0 kN, rotation speed of 0.5 rpm, and electrode speed of 0.2 mm/min. 2.3. Metallographic characterisation of the ECS coating
Fig. 1. Schematic diagram of the induction cladding process.
Cellulose acetate was dissolved in acetone, and the resulting solution was then mechanically mixed with the Ni-based alloy and 40% WC powders. After the powder–paste mixtures were prepared, they were brushed on the substrate surface at a thickness of 0.3 mm as a prelayer. Fig. 1 presents a schematic diagram of the induction cladding process in which the pre-layer-coated workpiece was placed into the highfrequency induction coil. At the beginning of the induction cladding process, the induced current was focused on the workpiece surface, which resulted in the rapid heating of the pre-layer and workpiece to the melting temperature. During the formation of the induction cladding, the surface heating depth was 0.5–2.5 mm. The high frequency induction cladding was performed using a power of 25 kW, oscillation frequency of 100 kHz, output current of 1.2 kA, induction coil–workpiece distance of approximately 2 mm, and time of 24 s. 2.2. ECS of the induction coating After high frequency induction cladding, the surface of Ni–40% WC composite coatings were smoothed, then were reinforced by using a self-developed electrical contact surface strengthening device, which is illustrated in Fig. 2. The resistive heating was localised by passing an electric current through the contact point between the rotating workpiece
Cross sections of the coatings were prepared using standard polishing procedures and were then chemically etched in a 4% HNO3 and C2H5OH solution. The microstructure was examined by optical microscopy (Axiovert 200, Carl Zeiss, Germany) and field emission scanning electron microscopy (FESEM) with energy dispersive X-ray spectroscopy (EDS) (S-4800, Hitachi, Japan). The phases present in the coating were identified by X-ray diffraction (D/Max-2550PC, Rigaku, Japan). The coating porosity and grain size were determined by analysing the FESEM images with the Image-Pro Plus software. Specifically, the porosity was determined by calculating the percentage of pores in the coating, and the grain size was determined based on GB/T 6394-2002 national standards and grain area calculations. The microhardness along a cross section of the substrate-coating interface was measured using a microhardness tester (HXS-1000A, Shanghai Optics Apparatus Ltd., China) with a load of 100 g and loading time of 15 s. Thermal shock tests were performed to compare the cohesion of samples with and without the ECS Ni–40% WC coating during rapid temperature changes. Cross sections were removed from the workpieces with and without the ECS coating. In the thermal shock experiments, the coated samples were heated at 600 °C in a resistor furnace for 5 min. After quickly quenching the samples in cold water, they were dried under airflow and then reheated in the furnace. This process was repeated until cracks formed in the coating surface, and the number of thermal shocks performed up to that point was recorded. 2.4. Wear tests A rolling wear testing machine (MJP-20, JingCheng Ltd., China), which is shown schematically in Fig. 3, was used to test the wear resistance of the samples at room temperature (25 °C). The upper sample of the hardened rubbing pair was GCr15 steel with an average hardness of HRC60, and the lower sample was Ni–WC-coated 40Cr and GCr15. The ring shape of the rubbing pairs had an external diameter of 60 mm, inner diameter of 30 mm, length of 20 mm, and contact width of
Fig. 2. Schematic diagram of the ECS device.
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Fig. 3. Schematic of the rolling wear testing machine.
5 mm. The lower sample rotated at 400 rpm with an applied load of 1.0 kN. The test was conducted for 6 h; after each hour of testing, the samples were cleaned with acetone, dried, and weighed. The sample weight loss was measured on an electronic balance (TG328B, Shanghai LiNeng Electronic Instrument Co., China) with a sensitivity of ±0.1 mg. The worn surfaces were also analysed by FESEM. 3. Results and discussion 3.1. Phase analysis
of different phases, including Co6W6C, W2C, Cr23C6, Fe6W6C, Co6W6C, Ni3Fe, FeNi, and Fe5C2. These different chromium carbide and tungsten carbide materials were evenly distributed throughout the Ni matrix. Some researchers showed that WC particles tend to sink to the bottom of the coating due to a stirring motion in the melting pool during laser cladding [17,18], which is important because the WC particle distribution within the coating affects its quality and performance. However, the formation of Fe6W6C and Co6W6C in this study showed that the WC particles partially decomposed and rapidly combined with other elements during the induction cladding process. The formation of these new compounds might enhance the cohesive force of the coating. After ECS, the major phases of the coatings changed slightly. As the contact current increased, the Fe6W6C, Cr23C6, and W2C peak intensities in the XRD patterns gradually increased, whereas the Ni3Fe, FeNi, and Fe5C2 peak intensities decreased. These changes in the peak intensities might be associated with changes in the grain size. The ECS treatment might have caused elemental species to diffuse and recrystallize in the coating. Under high contact resistance heating and contact force, the Ni–40% WC pre-coatings might have melted instantly and sintered to become more compact. 3.2. Coating microstructure The cross-sectional microstructures of the induction-clad and ECSprocessed Ni–40% WC composite coatings are shown in Figs. 5 and 6, respectively. The coating–substrate interface appeared as a bright white band, which is referred to as the diffusion transfer belt (DTB) [8]. The presence of the DTB indicates that metallurgical bonding between the coating and substrate occurred. Fig. 5a shows that it was impossible to
The Ni–40% WC composite powder mainly consisted of Ni, W, C, Co, Cr and Fe and might form a multiple eutectic point mixture after induction cladding. As shown in Fig. 4, the XRD results indicated the presence
Fig. 4. XRD analysis of the Ni–40% WC coatings before and after ECS treatment.
Fig. 5. Cross-sectional (a) metallographic and (b) FESEM micrographs of the coating–substrate interface (before ECS).
M. Xu et al. / Surface & Coatings Technology 279 (2015) 32–38
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Fig. 7. Microhardness distribution along the cross sections of the coatings subjected to different contact currents.
Fig. 6. Cross-sectional (a) metallographic and (b) FESEM micrographs of the coating–substrate interface (after the 25 kA ECS treatment).
obtain a non-porous, defect-free coating due to the uneven distribution of the composite powder in the cellulose acetate binder and to binder evaporation during the rapid heating in the induction cladding process. Fig. 5b presents a detailed FESEM image of the coating–substrate interface. During induction cladding, part of the tungsten carbide combined with other elements to form mixed chromium carbide–tungsten carbide dendritic, twin, whisker and eutectic colony microstructures. The XRD microstructure analyses (Fig. 4) indicate that these various carbide structures were the primary strengthening components in the coating.
Fig. 6a and b shows that after the 25 kA ECS treatment, the various eutectic carbides in the coating were finer and diffused to become more dispersed. Thus, ECS led to a more dispersed, stronger coating with smaller, more densely packed grains. During ECS, the resistance heating caused the coating temperature to change rapidly, resulting in the rapid recrystallisation of the duplex grain structure in the coating, and the appearance of another solid phase in the coating caused the mixed grains to become scattered and to partially disappear. Furthermore, the application of higher pressures reduced the coating porosity and enhanced the coating cohesion. Therefore, as the microstructure changed, the coating performance improved. According to the image analysis, the average porosity of the induction-clad coating was 3.8%. After the 25 kA ECS treatment, the average porosity was reduced to 1.7%. The change in the coating grain size with increasing contact current is shown in Table 2. As the contact current increased, the average grain size decreased. During the ECS treatment, an instantaneously high temperature and high pressure can reduce the coating porosity and grain size, which might cause the composite coating to become more compact. In the thermal shock tests, the sample treated with a 25 kA current had a better thermal shock performance (205 thermal shocks were applied before cracking was observed) than the samples that were not subjected to ECS (136
Table 2 Change in the grain size with contact current. Unstrengthened
Grain size (μm)
Contact current
9.4
15 kA
20 kA
25 kA
6.7
5.6
4.7
Table 3 EDS analysis of the DTB microstructure. Microstructure characteristics
Composition (wt.%) Fe
Ni
C
W
Co
Cr
DTB
72.8 64.9
9.9 15.7
0.4 0.6
9.0 10.2
4.2 4.6
3.7 4.0
Without ECS With 25 kA ECS
Fig. 8. Weight loss during the rolling wear tests for different samples.
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Fig. 9. FESEM micrographs of the 40Cr surface after (a) 1 h, (b) 3 h and (c) 6 h; the untreated Ni–40% WC-coated surface after (d) 1 h, (e) 3 h and (f) 6 h; and the Ni–40% WC-coated surface treated at 25 kA after (g) 1 h, (h) 3 h and (i) 6 h of rolling wear testing.
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thermal shocks led to cracking). These results show that ECS can improve the cohesion of the Ni–40% WC composite coating. EDS analysis reveals the element distributions in the DTB before and after 25 kA ECS (Table 3). Each element diffused during cladding. After 25 kA ECS, the amount of Ni and Co in the DTB increased, improving the bonding strength between the coating and substrate. The amount of W, Cr and C in the DTB also increased, which enhanced the coating strength. 3.3. Coating microhardness and wear resistance The microhardness distributions in the cross sections of the samples subjected to different contact currents are shown in Fig. 7. The results show that as the contact current increased, the coating hardness increased. The coating hardness changed slightly after the 20 kA ECS treatment and had an average hardness of 763 HV0.1, which was 8% higher than that of the unstrengthened induction-clad coating. However, the 25 kA ECS treatment led to a substantial increase in the coating hardness to an average of 906 HV0.1, which was 28% higher than that of the unstrengthened induction-clad coating. These results indicate that the Ni–40% WC coating became denser due to the rapid change in the resistance heat. Therefore, the increase in the coating hardness could be attributed to the increasing contact current, which decreased the coating grain size and porosity. Fig. 7 also shows that as the contact current increased, the microhardness throughout the cross section at the coating–substrate interface generally increased relative to that for the unstrengthened coating. The hardness of the DTB also increased with increasing contact current, which is consistent with the EDS analysis. Near the interface, the substrate was also heated to the transition temperature. The heated substrate area, which was denoted the heat affected zone (HAZ) [2], had a higher hardness. The microhardness gradient at the interface not only helped to improve the wear properties but also increased the bonding strength between the two neighbouring layers [19]. To investigate the effect of the contact current on the coating wear resistance, rolling wear tests were performed using the same rolling parameters for each sample, and the weight losses of the samples were measured. Fig. 8 shows the weight losses of the coating samples, 40Cr sample and GCr15 sample after 6 h under loads of 1.0 kN. In the first hour, the samples quickly lost weight, but the rate of weight loss decreased over time, indicating that the surfaces became stable under rolling wear at longer times. The weight losses of the samples with the Ni–40% WC coatings were much lower than those of 40Cr. The weight loss of the unstrengthened induction-clad sample was higher than that of the GCr15 sample and the weight loss of the coating formed during the 15 kA ECS treatment was similar to that of the GCr15 sample. As the contact current increased, the coating weight loss decreased below that of the GCr15 sample. The Ni–40% WC-coated sample that was treated at 25 kA had the lowest weight loss of all the coated samples. To understand the wear mechanisms of the coated samples, the worn surfaces of the samples were examined by FESEM, and the micrographs are shown in Fig. 9. As shown in Fig. 9a, sheared areas were observed on the worn surface of the uncoated 40Cr substrate during the grinding (Fig. 9a). As the rolling time increased, the sheared areas became larger, and fatigue spalling occurred (Fig. 9b and c). The 40Cr substrate microstructure consisted of ferrite, pearlite and cementite, and the rubbing pair had a higher hardness. Thus, when the rolling contact friction was applied to the surface, it was subjected to the action of the shearing force and compressive stress, which caused the surface to undergo plastic deformation. As the wear time increased, the plastic deformation might have caused the worn surface to flake. The flaking particles would then cause abrasive wear, resulting in large areas peeling from the sample surface [20]. When the 40Cr surface was cladded with the Ni–40% WC coating, the wear modes changed. Previous studies showed that four coating failure modes are possible: surface abrasion,
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spalling, delamination within the coating and interfacial delamination [21]. After induction cladding, the high microhardness of the coating and the metallurgical bonding between the coating and substrate limited the delamination of the coating materials. As shown in Fig. 9d–f, as the wear time increased, the coatings failed via interfacial delamination. During the rolling wear tests, interfacial delamination, which resulted in the formation of wear debris, might have originated in the pores in the coating. Shear plastic deformation, crack initiation, and crack propagation near the surfaces would lead to lamination of the debris [22]. After the ECS treatment, the Ni–40% WC coating had a higher hardness than the rubbing pair and fewer pores, stronger cohesion and smaller grain size than the coating that was not treated by ECS. Thus, fewer particles flaked from the ECS-treated coating, limiting its interfacial delamination under high stress or heavy loads. As shown in Fig. 9g–i, the Ni–40% WC powders cladded by induction and reinforced by ECS had desirable qualities that provided superior wear resistance. 4. Conclusions In this study, alloy steel (40Cr) surfaces were induction-cladded with a Ni–40% WC composite powder as a pre-coating, and then ECS was employed to obtain high-quality, wear-resistant coatings. The coating microstructure, hardness and wear resistance were examined. Based on the results, the following conclusions were made: 1. ECS can be used to reinforce Ni–40% WC composite coatings to obtain high-performance surfaces. After ECS, the coating porosity, the number of other defects in the coating, and the coating grain size decreased. 2. Examination of the microstructure indicated that chromium carbide and tungsten carbide constituted the main hard phase in the Ni–40% WC induction coatings. ECS resulted in a more compact Ni–40% WC composite coating. As the contact current increased, the coating hardness increased. 3. Under rolling wear, peeling and delamination were observed at the sample surfaces. After ECS, the Ni–40% WC composite coatings had superior cohesion, higher hardness and lower porosity, and the metallurgical bonding between the coating and substrate was improved. These enhanced properties might limit the peeling of the coating material under high stress or heavy loads. References [1] M. Franco, W. Sha, S. Malino, H. Liu, Micro-scale wear characteristics of electroless Ni–P/SiC composite coating under two different sliding conditions, Wear 317 (2014) 254–264. [2] M. Afzal, M. Ajmal, et al., Surface modification of air plasma spraying WC–12%Co cermet coating by laser melting technique, Opt. Laser Technol. 56 (2014) 202–206. [3] V.K. Balla, S. Bose, A. Bandyopadhyay, Microstructure and wear properties of laser deposited WC–12%Co composites, Mater. Sci. Eng. A 527 (2010) 6677–6682. [4] HungHua Sheu, et al., Effects of plating parameters on the Ni–P–Al2O3 composite coatings prepared by pulse and direct current plating, Surf. Coat. Technol. 235 (2013) 529–535. [5] Hong Liu, RongXin Guo, Liu Zhu, Characteristics of microstructure and performance of laser-treated electroless Ni–P/Ni–W–P duplex coatings, Trans. Nonferrous Metals Soc. China 22 (2012) 3012–3020. [6] Y. Akiyama, et al., Local deposition of polypyrrole on aluminium by anodizing, laser irradiation, and electrolytic polymerization and its application to the fabrication of micro-actuators, Electrochim. Acta 51 (2006) 4834–4840. [7] Shengfeng Zhou, Xiaoqin Dai, Microstructure evolution of Fe-based WC composite coating prepared by laser induction hybrid rapid cladding, Appl. Surf. Sci. 256 (2010) 7395–7399. [8] Yuan Gao, Chenglei Wang, et al., Microstructure and wear resistance of Ni60 layer prepared by high-frequency induction cladding, Rare Metal Mater. Eng. 40 (2011) 309–312. [9] Y.L. Wang, et al., Electric contact strengthening to improve the bonding between thermally sprayed 316 stainless steel coating and 45# steel substrate, Exp. Tech. 35 (2011) 66–70. [10] Xiaoben Qi, Shigen Zhu, Study on electric contact heating for nodular cast iron 6003, Appl. Mech. Mater. 152–154 (2012) 316–321. [11] K. Wiemer, An investigation into cladding by electrical resistance heating, Proceeding of 10th International Conference on Surface Modification Technologies, Singapore. Surface Modification Technologies X [C] 1997, pp. 891–898. [12] A.V. Polyachenko, Electric resistance surfacing-optimum method of reconditioning and hardening precision machine components, Weld. Int. 8 (1994) 226–228.
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