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Synthesis and characterization of Ni60-hBN high temperature self-lubricating anti-wear composite coatings on Ti6Al4V alloy by laser cladding
Optics & Laser Technology 78 (2016) 87–94
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Synthesis and characterization of Ni60-hBN high temperature self-lubricating anti-wear composite coatings on Ti6Al4V alloy by laser cladding Xiao-Long Lu a, Xiu-Bo Liu a,b,n, Peng-Cheng Yu a, Shi-Jie Qiao a, Yong-Jie Zhai a, Ming-Di Wang a, Yao Chen a, Dong Xu c,d a
School of Mechanical & Electric Engineering, Soochow University, 178 East Ganjiang Road, Suzhou 215006, PR China Jiangsu Key Laboratory of Materials Surface Science & Technology, Changzhou University, Changzhou 213164, PR China School of Materials Science & Engineering, Jiangsu University, Zhenjiang 212013, PR China d State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, PR China b c
art ic l e i nf o
a b s t r a c t
Article history: Received 3 August 2015 Received in revised form 20 September 2015 Accepted 5 October 2015
Ni60-hBN composite coatings with varying hBN content were prepared on Ti6Al4V substrates by laser cladding. The composite coatings with no cracks and few pores are bonded metallurgically with the substrates. The phase composition and microstructure of the composite coatings were investigated. The tribological properties of the composite coatings were evaluated under dry sliding wear test conditions at 20 °C, 300 °C and 600 °C, respectively. The microhardness gradually increased from the bottom to the top of the coating and increased with increasing of hBN content. The laser clad Ni60-10%hBN coating exhibits excellent tribological behavior at high temperatures (300 °C and 600 °C). & 2015 Elsevier Ltd. All rights reserved.
Keywords: Laser cladding Microstructure Self-lubrication Wear
1. Introduction Titanium and its alloys are extensively used in industrial fields owing to their high strength-to-weight ratio, excellent corrosion resistance, good mechanical properties and biocompatibility [1,2]. Nevertheless, due to their low surface hardness and poor tribological properties, the application of titanium and its alloys under severe wear and friction conditions is severely restricted [3]. In order to improve their surface properties, recent studies have been focused on the development of various surface modification techniques to prepare ceramic particle reinforced composite coatings [4]. In particular, laser cladding technique of the ceramic particles (such as Al2O3) reinforced composite coatings is a viable approach to improving the surface performance of titanium and its alloys, such as wear resistance, high temperature oxidation resistance, etc. [5,6]. Most composite coatings improve the wear resistance through increasing surface hardness [7,8]. However, due to their high friction coefficient under conditions of high contact stress and high temperature, which is harmful to the durability of the counter-body tribological components. Fabricating a selfn
Corresponding author. Fax: þ 86 512 6716 5607. E-mail addresses: [email protected], [email protected] (X.-B. Liu).
http://dx.doi.org/10.1016/j.optlastec.2015.10.005 0030-3992/& 2015 Elsevier Ltd. All rights reserved.
lubrication wear resistant coating on the surface of the component is an effective and economical approach to circumventing the problems [9,10]. In our previous work [11], self-lubricating antiwear γ-NiCrAlTi/TiC þ TiWC2/CrSþ Ti2CS composite coatings had been synthesized on Ti–6Al–4V substrate by laser cladding, the preliminary results showed that the composite coatings have excellent self-lubricating property due to the existence of the lubricious CrS and Ti2CS sulfides. In fact, due to their excellent properties, such as high hardness, wear resistance and corrosion resistance, Ni-based alloy coatings have been applied widely in mechanical parts through surface modified technology [12–15]. However, under more severe wear and friction conditions, the Ni-based alloy coatings will be fail to meet the requirements. On the other hand, hexagonal boron nitride (hBN) has a graphite-like lamellar structure, excellent lubrication and thermal stability up to 900 °C [16–18], so it can be a promising lubricating additive at relative higher temperatures [19]. Rajnesh et al. [20] found that the friction coefficient of powder metallurgy Ni-silver-hBN composite coating decreases with the increasing of both temperature and the amount of solid lubricant. Zhang et al. [21] had investigated the laser clad Ni/hBN composite coating on 1Cr18Ni9Ti stainless steel substrate, the results indicated that the Ni/hBN coating has excellent tribological properties at elevated temperatures up to below 800 °C. Hence, it will be a promising strategy to fabricate self-lubricating anti-wear
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composite coatings, which combine the excellent properties (high hardness, wear resistance and corrosion resistance) of Ni-based alloy with the self-lubricating property of hBN. Nevertheless, to the best knowledge of the authors, only few studies had reported about Ni-based alloy/hBN self-lubricating anti-wear composite coating on Ti6Al4V and 1Cr18Ni9Ti [22]. In the present study, the Ni-based alloy composite coatings modified by varying content of hBN were designed and prepared on the surface of Ti6Al4V alloy by laser cladding. Moreover, the microstructure, microhardness and the wear mechanisms of the substrate and the composite coatings were investigated systemically. It is expected to explore novel self-lubricating anti-wear composite coatings on titanium alloys and promote their industrial application.
Table 2 Chemical compositions of Ni60 powders (wt%). Ni
Cr
B
Si
C
Fe
Bal.
16
3.3
4.5
0.9
r 8.0
respectively. The counter-body was a Si3N4 ceramic ball with a diameter of 4 mm and a hardness of 1700 HV. The test parameters were listed in Table 3. The wear volume was measured by a surface mapping profiler. The wear rate was calculated by the equation below:
W=
V LS
where W is wear rate; V is wear volume (mm3); L is load (N); S is sliding distance (m).
2. Experimental procedures A number of cuboid Ti6Al4V alloys with a dimension of 50 mm 40 mm 8 mm were used as substrates (Table 1). Before laser cladding, the substrates were polished with sandpaper and ultrasonically with acetone. Ni60 alloy with the average powder grain size of 140 μm was used as matrix, whose chemical composition was listed in Table 2. The hBN particles with the average powder grain size of 1 μm were used as self-lubricating phase. Fig. 1 shows the morphology of (a) Ni60 alloy powder and (b) hBN particles. Ni-based alloy powder with 5% and 10% weight percent of hBN particles are homogenously mixed to prepare the composite powders, respectively. The composite powder was pre-placed onto the surface of Ti6Al4V substrates with a thickness of about 2 mm. The specimens for laser cladding were processed on a DILAS SD3000L–3 kW diode laser. The criteria for determining the optimum quality for the coating were based on a compromise of high hardness, low dilution, better homogeneity and lower occurrence of pores and cracks. On the basis of these criteria, the best processing parameters of laser processing were selected as: the output power P ¼1.5 kW, beam scanning speed V¼ 4 mm/s and laser beam size 4 mm 4 mm. Fig. 2 is the schematic drawing of laser cladding experiment and surface morphologies of laser clad Ni60 and Ni60-hBN composite coatings. After laser cladding treatment, metallographic samples were prepared by electrical discharge machining, mounted, polished and etched with etchant (with volume ration of HF: HNO3: H2O¼ 1: 3: 9). An automatic X-ray diffractometer (XRD, X’Pert-Pro MPD, Analytical Almelo, Netherlands) was used to analyze the phase compositions of the coatings using 40 kV, 40 mA and Cu Kα radiation in the scanning range of 2θ from 10° to 90°. The microstructure of the composite coating was examined using a KYKYEM3200S-4700 scanning electron microscope (SEM, Japan) equipped with an energy dispersive X-ray system (EDS). MH-5 type Vickers microhardness was measured with a 200 g load and dwell time of 15 s on the cross section from the coating surface to the substrate. Before wear testing, the surfaces of the specimens were smoothed by mechanical milling and polished to acquire the surface roughness of Ra ¼0.8 μm. The tribological properties of the composite coatings and substrate were performed with a ball-on-disk tribo-meter (HT-1000 tester, Lanzhou Zhongkekaihua science and technology Co., Ltd., China) under dry sliding conditions at room temperature (20 °C), 300 °C, 600 °C, Table 1 Chemical composition of Ti6Al4V substrate (wt%). Materials
Ti
Al
V
Fe
C
N
O
Ti6Al4V
balance
6.3
4.2
0.11
0.03
0.03
0.15
3. Results and discussion 3.1. Microstructure of laser clad coatings As a result of the direct laser beam irradiation, most of the preplaced mixed Ni60 and hBN powders were melted with certain dilution of the substrate, producing a hybrid complex Ni–Cr–B–Si– N–Ti–Al–C alloy molten pool on the surface of the substrate and subsequently, leading to the formation of the rapid solidification composite coating. As shown in Fig. 2(b), it can be clearly seen that the surface of Ni60-hBN coatings is rough and exhibit some spherical particles. Since the density of hBN (2.3 g/cm3) is much lower than Ni60 (4.6 g/cm3), part of hBN particles in the molten pool can rapid rise and splash during laser cladding, then they adhere to the surface of the composite coating to form spherical particles. Fig. 3 is XRD analysis results of the laser clad composite coatings. It should be noted that it is difficult to identify all phases in laser clad coatings, because of the laser cladding belongs to a rapid melting and rapid non-equilibrium solidification process, usually, multi-phases coexist and some diffraction peaks overlap and deviate from the equilibrium position. In addition, smaller size and less quantity of some phases (especially precipitations) resulted in difficultly identifying all phases [23]. The main phases of the Ni60 coating consist of TiO2, TiC carbides, TiB2 borides and γNiCrAlTi matrix. Interestingly, TiO2 has been found in laser clad Ni60 coating, which cannot be found in laser clad Ni60 þhBN coatings, the possible reason may be the violent disturbance of the molten pool and the decreasing dilution with the addition of hBN, which can prevent the oxidation of Ti6Al4V to some extent. Since titanium is a strong carbide- and boride-forming element, it could react easily with carbon and boron, forming TiC and TiB2 through the following reactions during laser cladding [24]: TiþC-TiC Tiþ2B-TiB2 Fig. 4(a) is macrograph of a single laser track transverse crosssection of laser clad Ni60 coating, which shows the convex crescent shape and free from holes and cracks. The composite coating is bonded metallurgically with the substrate, as demonstrated in Fig. 4(b). Because of the rapid and non-equilibrium solidification during laser cladding, cross-section microstructure of the layer can be divided into three regions corresponding to the heat-affected zone (HAZ), bonding zone (BZ), and cladding zone (CZ), respectively. The BZ possesses microstructure of dendrites and columnar crystal as displayed in Fig. 4(b) [25]. Some acicular martensite can
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Fig. 1. Morphology of (a) Ni60 alloy powder and (b) hBN particles.
be seen in the HAZ. Based on the relevant studies [26,27], while the temperature decreases from 882 °C to 850 °C during the β-α transformation in Ti6Al4V alloy and the cooling rate is more than 200 °C/s, the acicular martensite will be appeared in the HAZ near the cladding coating. The macrographs of laser clad Ni60þ hBN coatings can be seen in Fig. 4(c and d), it is obviously observed that the thicknesses of laser clad Ni60 þhBN coatings are greatly smaller than the laser clad Ni60 coating, because of the existence of splashing and the decreasing of dilution, the addition of hBN during laser cladding results in the thickness loss of the coatings. Fig. 5 is the microstructure of the laser clad Ni60-5%hBN composite coating, which is mainly composed of gray continuous matrix (A), black dendrite or short-stick morphology (B), polygon morphology (C) and sheet shape structure (D). Phases with different morphological features were analyzed by EDS. As shown in Fig. 6, the gray continuous matrix marked as area (A), which is riched in Ti, Ni, Cr and Al. The black dendrite or short-stick morphology marked as area (B) is enriched in Ti and C, and their content is relatively even. Area (C) is polygon morphology and its compositions are mainly Ti and B. The sheet shape structure (D) dispersed in the matrix, and marked as area (D), rich in Ti, C, B and N. Together with the previous results of XRD findings, it can be deduced that gray continuous matrix (A) is mainly γ-NiCrAlTi solid solution. Black dendrite or short-stick morphology (B) and polygon morphology (C) are TiC carbide and TiB2 boride, respectively, which can be expected to improve the microhardness and wear resistance of the coating. Finally, it can be inferred that the sheet shape structure (D) comprises the hybrid compounds of TiC, TiB2 and the un-melted hBN. Because of its hexagonal layered structure, the un-melted hBN could act as a potential solid lubricant. 3.2. Microhardness of laser clad coatings Fig. 7 shows the microhardness profile along the depth direction of laser clad coatings. There is an intergradation of microhardness in the interface between the coating and the substrate. It
Table 3 Experimental parameters of wear test. Load/N Temperature/°C Wear time/ min
Rotation radius/mm
Linear velocity/(m min 1)
5
2
16.89
20,300,600
30
Fig. 3. XRD analysis results of the laser clad composite coatings.
can be seen that the microhardness of Ti6Al4V substrate is improved by laser clad Ni-based coating due to the existence of large amount of reinforced carbides and borides and the dispersed fine grains. It is worthwhile to note that the average microhardness of composite coating with 10% hBN particles (approximately 1155.32HV0.2) is about three times as high as the substrate (approximately 370HV0.2), which is attributed to the formation of
Fig. 2. (a) Schematic drawing of laser cladding experiment; (b) surface morphologies of laser clad Ni60 and Ni60-hBN composite coatings.
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Fig. 4. (a) SEM macrograph of laser clad Ni60 coating; (b) magnification morphology of laser clad Ni60 coating; (c) SEM macrograph of laser clad Ni60þ 5%hBN coating; (d) SEM macrograph of laser clad Ni60 þ10%hBN coating.
the hybrid Ni–Cr–B–Si alloy molten-pool, reacted with Ti to form TiB2. Then the reinforced TiB2 boride phase randomly interweaved to act as the “skeleton” to improve the microhardness of the coating. 3.3. Friction and wear properties of laser clad coatings
Fig. 5. Microstructures of laser clad Ni60-5%hBN composite coating.
reinforced TiC carbides and TiB2 borides during laser cladding process. It is also evident that the microhardness of the laser clad coating is relatively uniform in the coating zone. However, the microhardness in the bonding zone has a sharp falling, as observed in Fig. 7, the microhardness falls greatly near the coating/substrate interface, because the bottom of coating was diluted by the substrate material. Besides, the microhardness of the HAZ is more than that of the initial Ti6Al4V alloys because of the formation of the acicular martensite in the HAZ. Moreover, the average microhardness increases with the increasing content of hBN particles. Instead of decreasing, the microhardness of laser clad coating increases after adding the soft solid lubrication. The following reason may be responsible for this phenomenon. During the laser cladding process, part of hBN decomposed into B and N, the decomposed B atoms dissolved into
Fig. 8 shows the variation of friction coefficients for Ti6Al4V alloy and composite coatings sliding against Si3N4 ball at different temperatures under load of 5 N. It could be seen that the friction coefficients of Ti6Al4V substrate considerably decreased with the increase of temperature. The reason may be the formation of many oxide debris, such as TiO2 (see Fig. 9) with the rutile structure [28], which have solid lubricating effect and can reduce the friction coefficient to certain extent. On the contrary, the friction coefficients of Ni-based composite coatings increase with the increasing temperature up to 600 °C. The possible reason is that γ-NiCrAlTi matrix is softened at high temperature, which could result the asperities of the hard counter-body Si3N4 ceramic ball penetrate into the surface of the coating. This abnormal friction-temperature relationship implies that the laser clad Ni60 coating is disadvantageous when used in high-temperature sliding wear service. It could be found from Fig. 8 that the laser clad Ni60-hBN coatings possess the lowest friction coefficient at all temperatures from room temperature to 600 °C. It indicates that the lubricant hBN particles are beneficial to reduce friction. Moreover, the more addition of hBN is, the lower friction coefficient is obtained in each temperature, which can be seen from Fig. 8. As indicated in Fig. 10, the composite coatings exhibited outstanding wear resistance in comparison to that of Ti6Al4V alloy under dry sliding wear conditions. The wear rate of Ti6Al4V alloy has a little increase with increasing temperature from ambient temperature to 300 °C and then slightly decreases when the temperature increases to 600 °C. However, with the increase of the temperature, the wear rate of the three composite coatings all decrease. It can also be seen from Fig. 10, with the increasing of
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Fig. 6. EDS analysis of different phases: (a) phase A; (b) phase B; (c) phase C; (d) phase D.
hBN, the wear rate of the three laser clad composite coatings firstly decrease and then gradually increase slightly at RT. The possible reason maybe that with the increasing content of reinforced TiB2 borides in the coating after adding 10%hBN, threebody abrasive wear would occur during dry sliding wear test, which may lead to the worse wear resistance of the coating at RT. In addition, with the addition of hBN, the laser clad coatings can present better wear resistance at elevated temperature up to
800 °C [21], while the effects of anti-wear at RT are not obviously. While at 300 °C, with the increase of hBN, the wear rate of the coatings slightly increases and then decreases. However, at 600 °C the wear rate of the coatings decreases monotonously with the increase of hBN. In one word, it could be inferred that laser clad Ni60-10%hBN coating exhibits the best tribological behavior at relatively high temperatures (300 °C and 600 °C).
Fig. 7. Microhardness profiles of the laser clad coatings: (a) Ni60; (b) Ni60-hBN.
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Fig. 8. Friction coefficients of the substrate and laser clad Ni60 and Ni60-hBN coatings vs temperature.
3.4. Analysis of wear mechanisms The worn surfaces of the laser clad coatings and Ti6Al4V substrate after wear test at 600 °C are shown in Fig. 11. It appears that the morphology of worn surface of Ti6Al4V substrate is quite rough with severe abrasive wear and plastic deformation (Fig. 11a). It is because that the asperities of the very hard counter-body Si3N4 ceramic ball can effectively penetrate into the soft surface of Ti6Al4V alloy to give deep plowing and cutting. These facts indicate severe abrasive wear, micro-plough and plastic deformation occurred during the wear tests for Ti6Al4V substrate and Si3N4 ceramic ball friction pairs [29]. The worn surfaces of the composite coatings are much smoother than that of Ti6Al4V substrate, as shown in Fig. 11(b–d). The difference of the tribological behavior results from the existence of reinforced TiC carbide and TiB2 boride and self-lubricating hBN in the coatings. It is well-known that microhardness is closely related to the wear resistance of materials. Usually, the higher hardness means the higher wear resistance [30]. Moreover, hBN with excellent self-lubrication properties can diffuse and
Fig. 10. Wear rates of the substrate and laser clad Ni60 and Ni60-hBN coatings vs temperature under the dry sliding wear test.
accumulate on rubbing surface to form a lubricating film and finally reduce the friction coefficient. Therefore, this material exhibits the properties of lower wear rate and friction coefficient, which results in longer service life. As shown in Fig. 11(b), the surface clad Ni60 coating shows a wear mechanism with adhesive wear and spalling fatigue. With the addition of hBN particles in the composite coating, some grooves, micro-cracks and brittle fracture can be obviously detected from Fig. 11(c). Because of some removed brittle wear debris are produced between wear couples during dry sliding wear test at high temperature, which could plough into the bulk coating and causing the micro-cutting and grooving [31,32]. The wear mechanisms of the laser clad Ni60-5%hBN coating are mainly slight abrasive wear and brittle fracture as well as the breakdown of the transferred layer. With the addition of 10% weight percent of hBN, delamination and slight grooves are present in the worn surface of the coatings. The resistance to shearing would be weakened because of the delamination of the transferred layer, which could lead to some mild grooves along the sliding direction present on the worn surface. At the same time, the ductile and tough γ-
Fig. 9. EDS pattern and quantitative results of area A in Fig. 11(a).
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Fig. 11. Typical SEM micrographs showing the worn surface morphologies of the substrate and laser clad coatings at 600 °C: (a) Ti6Al4V substrate, (b) Ni60 coating, (c) Ni605%hBN coating, (d) Ni60-10%hBN coating.
NiCrAlTi solid solution is also beneficial to reduce the wear volume loss of the coating. So the wear mechanisms of the laser clad Ni6010%hBN coating are slight abrasive wear and the breakdown of the transferred layer.
4. Conclusions In this article, laser clad Ni60 composite coating and Ni60-hBN composite coatings were successfully fabricated on Ti6Al4V alloy substrates by laser cladding and their tribological properties were evaluated under dry sliding wear test conditions at room temperature (RT), 300 °C and 600 °C, respectively. The following conclusions can be drawn: (1) The composite coatings have no cracks and few pores and bonded metallurgically with the substrates. (2) The laser clad Ni60 composite coating mainly consists of TiO2, γ-NiCrAlTi, TiC and TiB2 phases. The laser clad Ni60-hBN composite coatings are mainly composed of hBN, γ-NiCrAlTi, TiC and TiB2 phases. (3) The microhardness of Ti6Al4V substrate is improved by laser clad Ni-based coating, and the average microhardness increases with the increase of hBN particles. The average microhardness of composite coating with 10%hBN particles (approximately 1155.32HV0.2) is about three times as high as the substrate (approximately 370HV0.2), which is attributed to the existence of TiC and TiB2 hard phase. (4) The composite coatings exhibited outstanding wear resistance in comparison to Ti6Al4V alloy under dry sliding wear test conditions. The laser clad Ni60-10% hBN coating exhibits the best tribological behavior at relatively high temperatures (300 °C and 600 °C). (5) The wear mechanism of Ti6Al4V substrate is the combination of severe micro-ploughing and abrasive wear, the surface clad Ni60 coating shows a wear mechanism of adhesive wear and
spalling fatigue. The wear mechanism of the laser clad Ni605%hBN and 10%hBN coatings are mainly slight abrasive wear and brittle fracture as well as the breakdown of transfer layer, respectively.
Acknowledgments The authors acknowledge the financial supports from the Natural Science Foundation of Jiangsu Province (Grant nos. BK20141194 and BK20131155), the Opening Foundation of Key Laboratory of High-end Structural Materials of Jiangsu Province (Grant no. hsm1401) and Tribology Science Fund of State Key Laboratory of Tribology (SKLTKF13B08). One of the authors (M. D. Wang) is also grateful for the financial supports from the National Natural Science Foundation of China (Grant no. 51205266)
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Synthesis and characterization of Ni60-hBN high temperature self-lubricating anti-wear composite coatings on Ti6Al4V alloy by laser cladding Xiao-Long Lu a, Xiu-Bo Liu a,b,n, Peng-Cheng Yu a, Shi-Jie Qiao a, Yong-Jie Zhai a, Ming-Di Wang a, Yao Chen a, Dong Xu c,d a
School of Mechanical & Electric Engineering, Soochow University, 178 East Ganjiang Road, Suzhou 215006, PR China Jiangsu Key Laboratory of Materials Surface Science & Technology, Changzhou University, Changzhou 213164, PR China School of Materials Science & Engineering, Jiangsu University, Zhenjiang 212013, PR China d State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, PR China b c
art ic l e i nf o
a b s t r a c t
Article history: Received 3 August 2015 Received in revised form 20 September 2015 Accepted 5 October 2015
Ni60-hBN composite coatings with varying hBN content were prepared on Ti6Al4V substrates by laser cladding. The composite coatings with no cracks and few pores are bonded metallurgically with the substrates. The phase composition and microstructure of the composite coatings were investigated. The tribological properties of the composite coatings were evaluated under dry sliding wear test conditions at 20 °C, 300 °C and 600 °C, respectively. The microhardness gradually increased from the bottom to the top of the coating and increased with increasing of hBN content. The laser clad Ni60-10%hBN coating exhibits excellent tribological behavior at high temperatures (300 °C and 600 °C). & 2015 Elsevier Ltd. All rights reserved.
Keywords: Laser cladding Microstructure Self-lubrication Wear
1. Introduction Titanium and its alloys are extensively used in industrial fields owing to their high strength-to-weight ratio, excellent corrosion resistance, good mechanical properties and biocompatibility [1,2]. Nevertheless, due to their low surface hardness and poor tribological properties, the application of titanium and its alloys under severe wear and friction conditions is severely restricted [3]. In order to improve their surface properties, recent studies have been focused on the development of various surface modification techniques to prepare ceramic particle reinforced composite coatings [4]. In particular, laser cladding technique of the ceramic particles (such as Al2O3) reinforced composite coatings is a viable approach to improving the surface performance of titanium and its alloys, such as wear resistance, high temperature oxidation resistance, etc. [5,6]. Most composite coatings improve the wear resistance through increasing surface hardness [7,8]. However, due to their high friction coefficient under conditions of high contact stress and high temperature, which is harmful to the durability of the counter-body tribological components. Fabricating a selfn
Corresponding author. Fax: þ 86 512 6716 5607. E-mail addresses: [email protected], [email protected] (X.-B. Liu).
http://dx.doi.org/10.1016/j.optlastec.2015.10.005 0030-3992/& 2015 Elsevier Ltd. All rights reserved.
lubrication wear resistant coating on the surface of the component is an effective and economical approach to circumventing the problems [9,10]. In our previous work [11], self-lubricating antiwear γ-NiCrAlTi/TiC þ TiWC2/CrSþ Ti2CS composite coatings had been synthesized on Ti–6Al–4V substrate by laser cladding, the preliminary results showed that the composite coatings have excellent self-lubricating property due to the existence of the lubricious CrS and Ti2CS sulfides. In fact, due to their excellent properties, such as high hardness, wear resistance and corrosion resistance, Ni-based alloy coatings have been applied widely in mechanical parts through surface modified technology [12–15]. However, under more severe wear and friction conditions, the Ni-based alloy coatings will be fail to meet the requirements. On the other hand, hexagonal boron nitride (hBN) has a graphite-like lamellar structure, excellent lubrication and thermal stability up to 900 °C [16–18], so it can be a promising lubricating additive at relative higher temperatures [19]. Rajnesh et al. [20] found that the friction coefficient of powder metallurgy Ni-silver-hBN composite coating decreases with the increasing of both temperature and the amount of solid lubricant. Zhang et al. [21] had investigated the laser clad Ni/hBN composite coating on 1Cr18Ni9Ti stainless steel substrate, the results indicated that the Ni/hBN coating has excellent tribological properties at elevated temperatures up to below 800 °C. Hence, it will be a promising strategy to fabricate self-lubricating anti-wear
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composite coatings, which combine the excellent properties (high hardness, wear resistance and corrosion resistance) of Ni-based alloy with the self-lubricating property of hBN. Nevertheless, to the best knowledge of the authors, only few studies had reported about Ni-based alloy/hBN self-lubricating anti-wear composite coating on Ti6Al4V and 1Cr18Ni9Ti [22]. In the present study, the Ni-based alloy composite coatings modified by varying content of hBN were designed and prepared on the surface of Ti6Al4V alloy by laser cladding. Moreover, the microstructure, microhardness and the wear mechanisms of the substrate and the composite coatings were investigated systemically. It is expected to explore novel self-lubricating anti-wear composite coatings on titanium alloys and promote their industrial application.
Table 2 Chemical compositions of Ni60 powders (wt%). Ni
Cr
B
Si
C
Fe
Bal.
16
3.3
4.5
0.9
r 8.0
respectively. The counter-body was a Si3N4 ceramic ball with a diameter of 4 mm and a hardness of 1700 HV. The test parameters were listed in Table 3. The wear volume was measured by a surface mapping profiler. The wear rate was calculated by the equation below:
W=
V LS
where W is wear rate; V is wear volume (mm3); L is load (N); S is sliding distance (m).
2. Experimental procedures A number of cuboid Ti6Al4V alloys with a dimension of 50 mm 40 mm 8 mm were used as substrates (Table 1). Before laser cladding, the substrates were polished with sandpaper and ultrasonically with acetone. Ni60 alloy with the average powder grain size of 140 μm was used as matrix, whose chemical composition was listed in Table 2. The hBN particles with the average powder grain size of 1 μm were used as self-lubricating phase. Fig. 1 shows the morphology of (a) Ni60 alloy powder and (b) hBN particles. Ni-based alloy powder with 5% and 10% weight percent of hBN particles are homogenously mixed to prepare the composite powders, respectively. The composite powder was pre-placed onto the surface of Ti6Al4V substrates with a thickness of about 2 mm. The specimens for laser cladding were processed on a DILAS SD3000L–3 kW diode laser. The criteria for determining the optimum quality for the coating were based on a compromise of high hardness, low dilution, better homogeneity and lower occurrence of pores and cracks. On the basis of these criteria, the best processing parameters of laser processing were selected as: the output power P ¼1.5 kW, beam scanning speed V¼ 4 mm/s and laser beam size 4 mm 4 mm. Fig. 2 is the schematic drawing of laser cladding experiment and surface morphologies of laser clad Ni60 and Ni60-hBN composite coatings. After laser cladding treatment, metallographic samples were prepared by electrical discharge machining, mounted, polished and etched with etchant (with volume ration of HF: HNO3: H2O¼ 1: 3: 9). An automatic X-ray diffractometer (XRD, X’Pert-Pro MPD, Analytical Almelo, Netherlands) was used to analyze the phase compositions of the coatings using 40 kV, 40 mA and Cu Kα radiation in the scanning range of 2θ from 10° to 90°. The microstructure of the composite coating was examined using a KYKYEM3200S-4700 scanning electron microscope (SEM, Japan) equipped with an energy dispersive X-ray system (EDS). MH-5 type Vickers microhardness was measured with a 200 g load and dwell time of 15 s on the cross section from the coating surface to the substrate. Before wear testing, the surfaces of the specimens were smoothed by mechanical milling and polished to acquire the surface roughness of Ra ¼0.8 μm. The tribological properties of the composite coatings and substrate were performed with a ball-on-disk tribo-meter (HT-1000 tester, Lanzhou Zhongkekaihua science and technology Co., Ltd., China) under dry sliding conditions at room temperature (20 °C), 300 °C, 600 °C, Table 1 Chemical composition of Ti6Al4V substrate (wt%). Materials
Ti
Al
V
Fe
C
N
O
Ti6Al4V
balance
6.3
4.2
0.11
0.03
0.03
0.15
3. Results and discussion 3.1. Microstructure of laser clad coatings As a result of the direct laser beam irradiation, most of the preplaced mixed Ni60 and hBN powders were melted with certain dilution of the substrate, producing a hybrid complex Ni–Cr–B–Si– N–Ti–Al–C alloy molten pool on the surface of the substrate and subsequently, leading to the formation of the rapid solidification composite coating. As shown in Fig. 2(b), it can be clearly seen that the surface of Ni60-hBN coatings is rough and exhibit some spherical particles. Since the density of hBN (2.3 g/cm3) is much lower than Ni60 (4.6 g/cm3), part of hBN particles in the molten pool can rapid rise and splash during laser cladding, then they adhere to the surface of the composite coating to form spherical particles. Fig. 3 is XRD analysis results of the laser clad composite coatings. It should be noted that it is difficult to identify all phases in laser clad coatings, because of the laser cladding belongs to a rapid melting and rapid non-equilibrium solidification process, usually, multi-phases coexist and some diffraction peaks overlap and deviate from the equilibrium position. In addition, smaller size and less quantity of some phases (especially precipitations) resulted in difficultly identifying all phases [23]. The main phases of the Ni60 coating consist of TiO2, TiC carbides, TiB2 borides and γNiCrAlTi matrix. Interestingly, TiO2 has been found in laser clad Ni60 coating, which cannot be found in laser clad Ni60 þhBN coatings, the possible reason may be the violent disturbance of the molten pool and the decreasing dilution with the addition of hBN, which can prevent the oxidation of Ti6Al4V to some extent. Since titanium is a strong carbide- and boride-forming element, it could react easily with carbon and boron, forming TiC and TiB2 through the following reactions during laser cladding [24]: TiþC-TiC Tiþ2B-TiB2 Fig. 4(a) is macrograph of a single laser track transverse crosssection of laser clad Ni60 coating, which shows the convex crescent shape and free from holes and cracks. The composite coating is bonded metallurgically with the substrate, as demonstrated in Fig. 4(b). Because of the rapid and non-equilibrium solidification during laser cladding, cross-section microstructure of the layer can be divided into three regions corresponding to the heat-affected zone (HAZ), bonding zone (BZ), and cladding zone (CZ), respectively. The BZ possesses microstructure of dendrites and columnar crystal as displayed in Fig. 4(b) [25]. Some acicular martensite can
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Fig. 1. Morphology of (a) Ni60 alloy powder and (b) hBN particles.
be seen in the HAZ. Based on the relevant studies [26,27], while the temperature decreases from 882 °C to 850 °C during the β-α transformation in Ti6Al4V alloy and the cooling rate is more than 200 °C/s, the acicular martensite will be appeared in the HAZ near the cladding coating. The macrographs of laser clad Ni60þ hBN coatings can be seen in Fig. 4(c and d), it is obviously observed that the thicknesses of laser clad Ni60 þhBN coatings are greatly smaller than the laser clad Ni60 coating, because of the existence of splashing and the decreasing of dilution, the addition of hBN during laser cladding results in the thickness loss of the coatings. Fig. 5 is the microstructure of the laser clad Ni60-5%hBN composite coating, which is mainly composed of gray continuous matrix (A), black dendrite or short-stick morphology (B), polygon morphology (C) and sheet shape structure (D). Phases with different morphological features were analyzed by EDS. As shown in Fig. 6, the gray continuous matrix marked as area (A), which is riched in Ti, Ni, Cr and Al. The black dendrite or short-stick morphology marked as area (B) is enriched in Ti and C, and their content is relatively even. Area (C) is polygon morphology and its compositions are mainly Ti and B. The sheet shape structure (D) dispersed in the matrix, and marked as area (D), rich in Ti, C, B and N. Together with the previous results of XRD findings, it can be deduced that gray continuous matrix (A) is mainly γ-NiCrAlTi solid solution. Black dendrite or short-stick morphology (B) and polygon morphology (C) are TiC carbide and TiB2 boride, respectively, which can be expected to improve the microhardness and wear resistance of the coating. Finally, it can be inferred that the sheet shape structure (D) comprises the hybrid compounds of TiC, TiB2 and the un-melted hBN. Because of its hexagonal layered structure, the un-melted hBN could act as a potential solid lubricant. 3.2. Microhardness of laser clad coatings Fig. 7 shows the microhardness profile along the depth direction of laser clad coatings. There is an intergradation of microhardness in the interface between the coating and the substrate. It
Table 3 Experimental parameters of wear test. Load/N Temperature/°C Wear time/ min
Rotation radius/mm
Linear velocity/(m min 1)
5
2
16.89
20,300,600
30
Fig. 3. XRD analysis results of the laser clad composite coatings.
can be seen that the microhardness of Ti6Al4V substrate is improved by laser clad Ni-based coating due to the existence of large amount of reinforced carbides and borides and the dispersed fine grains. It is worthwhile to note that the average microhardness of composite coating with 10% hBN particles (approximately 1155.32HV0.2) is about three times as high as the substrate (approximately 370HV0.2), which is attributed to the formation of
Fig. 2. (a) Schematic drawing of laser cladding experiment; (b) surface morphologies of laser clad Ni60 and Ni60-hBN composite coatings.
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Fig. 4. (a) SEM macrograph of laser clad Ni60 coating; (b) magnification morphology of laser clad Ni60 coating; (c) SEM macrograph of laser clad Ni60þ 5%hBN coating; (d) SEM macrograph of laser clad Ni60 þ10%hBN coating.
the hybrid Ni–Cr–B–Si alloy molten-pool, reacted with Ti to form TiB2. Then the reinforced TiB2 boride phase randomly interweaved to act as the “skeleton” to improve the microhardness of the coating. 3.3. Friction and wear properties of laser clad coatings
Fig. 5. Microstructures of laser clad Ni60-5%hBN composite coating.
reinforced TiC carbides and TiB2 borides during laser cladding process. It is also evident that the microhardness of the laser clad coating is relatively uniform in the coating zone. However, the microhardness in the bonding zone has a sharp falling, as observed in Fig. 7, the microhardness falls greatly near the coating/substrate interface, because the bottom of coating was diluted by the substrate material. Besides, the microhardness of the HAZ is more than that of the initial Ti6Al4V alloys because of the formation of the acicular martensite in the HAZ. Moreover, the average microhardness increases with the increasing content of hBN particles. Instead of decreasing, the microhardness of laser clad coating increases after adding the soft solid lubrication. The following reason may be responsible for this phenomenon. During the laser cladding process, part of hBN decomposed into B and N, the decomposed B atoms dissolved into
Fig. 8 shows the variation of friction coefficients for Ti6Al4V alloy and composite coatings sliding against Si3N4 ball at different temperatures under load of 5 N. It could be seen that the friction coefficients of Ti6Al4V substrate considerably decreased with the increase of temperature. The reason may be the formation of many oxide debris, such as TiO2 (see Fig. 9) with the rutile structure [28], which have solid lubricating effect and can reduce the friction coefficient to certain extent. On the contrary, the friction coefficients of Ni-based composite coatings increase with the increasing temperature up to 600 °C. The possible reason is that γ-NiCrAlTi matrix is softened at high temperature, which could result the asperities of the hard counter-body Si3N4 ceramic ball penetrate into the surface of the coating. This abnormal friction-temperature relationship implies that the laser clad Ni60 coating is disadvantageous when used in high-temperature sliding wear service. It could be found from Fig. 8 that the laser clad Ni60-hBN coatings possess the lowest friction coefficient at all temperatures from room temperature to 600 °C. It indicates that the lubricant hBN particles are beneficial to reduce friction. Moreover, the more addition of hBN is, the lower friction coefficient is obtained in each temperature, which can be seen from Fig. 8. As indicated in Fig. 10, the composite coatings exhibited outstanding wear resistance in comparison to that of Ti6Al4V alloy under dry sliding wear conditions. The wear rate of Ti6Al4V alloy has a little increase with increasing temperature from ambient temperature to 300 °C and then slightly decreases when the temperature increases to 600 °C. However, with the increase of the temperature, the wear rate of the three composite coatings all decrease. It can also be seen from Fig. 10, with the increasing of
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Fig. 6. EDS analysis of different phases: (a) phase A; (b) phase B; (c) phase C; (d) phase D.
hBN, the wear rate of the three laser clad composite coatings firstly decrease and then gradually increase slightly at RT. The possible reason maybe that with the increasing content of reinforced TiB2 borides in the coating after adding 10%hBN, threebody abrasive wear would occur during dry sliding wear test, which may lead to the worse wear resistance of the coating at RT. In addition, with the addition of hBN, the laser clad coatings can present better wear resistance at elevated temperature up to
800 °C [21], while the effects of anti-wear at RT are not obviously. While at 300 °C, with the increase of hBN, the wear rate of the coatings slightly increases and then decreases. However, at 600 °C the wear rate of the coatings decreases monotonously with the increase of hBN. In one word, it could be inferred that laser clad Ni60-10%hBN coating exhibits the best tribological behavior at relatively high temperatures (300 °C and 600 °C).
Fig. 7. Microhardness profiles of the laser clad coatings: (a) Ni60; (b) Ni60-hBN.
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Fig. 8. Friction coefficients of the substrate and laser clad Ni60 and Ni60-hBN coatings vs temperature.
3.4. Analysis of wear mechanisms The worn surfaces of the laser clad coatings and Ti6Al4V substrate after wear test at 600 °C are shown in Fig. 11. It appears that the morphology of worn surface of Ti6Al4V substrate is quite rough with severe abrasive wear and plastic deformation (Fig. 11a). It is because that the asperities of the very hard counter-body Si3N4 ceramic ball can effectively penetrate into the soft surface of Ti6Al4V alloy to give deep plowing and cutting. These facts indicate severe abrasive wear, micro-plough and plastic deformation occurred during the wear tests for Ti6Al4V substrate and Si3N4 ceramic ball friction pairs [29]. The worn surfaces of the composite coatings are much smoother than that of Ti6Al4V substrate, as shown in Fig. 11(b–d). The difference of the tribological behavior results from the existence of reinforced TiC carbide and TiB2 boride and self-lubricating hBN in the coatings. It is well-known that microhardness is closely related to the wear resistance of materials. Usually, the higher hardness means the higher wear resistance [30]. Moreover, hBN with excellent self-lubrication properties can diffuse and
Fig. 10. Wear rates of the substrate and laser clad Ni60 and Ni60-hBN coatings vs temperature under the dry sliding wear test.
accumulate on rubbing surface to form a lubricating film and finally reduce the friction coefficient. Therefore, this material exhibits the properties of lower wear rate and friction coefficient, which results in longer service life. As shown in Fig. 11(b), the surface clad Ni60 coating shows a wear mechanism with adhesive wear and spalling fatigue. With the addition of hBN particles in the composite coating, some grooves, micro-cracks and brittle fracture can be obviously detected from Fig. 11(c). Because of some removed brittle wear debris are produced between wear couples during dry sliding wear test at high temperature, which could plough into the bulk coating and causing the micro-cutting and grooving [31,32]. The wear mechanisms of the laser clad Ni60-5%hBN coating are mainly slight abrasive wear and brittle fracture as well as the breakdown of the transferred layer. With the addition of 10% weight percent of hBN, delamination and slight grooves are present in the worn surface of the coatings. The resistance to shearing would be weakened because of the delamination of the transferred layer, which could lead to some mild grooves along the sliding direction present on the worn surface. At the same time, the ductile and tough γ-
Fig. 9. EDS pattern and quantitative results of area A in Fig. 11(a).
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Fig. 11. Typical SEM micrographs showing the worn surface morphologies of the substrate and laser clad coatings at 600 °C: (a) Ti6Al4V substrate, (b) Ni60 coating, (c) Ni605%hBN coating, (d) Ni60-10%hBN coating.
NiCrAlTi solid solution is also beneficial to reduce the wear volume loss of the coating. So the wear mechanisms of the laser clad Ni6010%hBN coating are slight abrasive wear and the breakdown of the transferred layer.
4. Conclusions In this article, laser clad Ni60 composite coating and Ni60-hBN composite coatings were successfully fabricated on Ti6Al4V alloy substrates by laser cladding and their tribological properties were evaluated under dry sliding wear test conditions at room temperature (RT), 300 °C and 600 °C, respectively. The following conclusions can be drawn: (1) The composite coatings have no cracks and few pores and bonded metallurgically with the substrates. (2) The laser clad Ni60 composite coating mainly consists of TiO2, γ-NiCrAlTi, TiC and TiB2 phases. The laser clad Ni60-hBN composite coatings are mainly composed of hBN, γ-NiCrAlTi, TiC and TiB2 phases. (3) The microhardness of Ti6Al4V substrate is improved by laser clad Ni-based coating, and the average microhardness increases with the increase of hBN particles. The average microhardness of composite coating with 10%hBN particles (approximately 1155.32HV0.2) is about three times as high as the substrate (approximately 370HV0.2), which is attributed to the existence of TiC and TiB2 hard phase. (4) The composite coatings exhibited outstanding wear resistance in comparison to Ti6Al4V alloy under dry sliding wear test conditions. The laser clad Ni60-10% hBN coating exhibits the best tribological behavior at relatively high temperatures (300 °C and 600 °C). (5) The wear mechanism of Ti6Al4V substrate is the combination of severe micro-ploughing and abrasive wear, the surface clad Ni60 coating shows a wear mechanism of adhesive wear and
spalling fatigue. The wear mechanism of the laser clad Ni605%hBN and 10%hBN coatings are mainly slight abrasive wear and brittle fracture as well as the breakdown of transfer layer, respectively.
Acknowledgments The authors acknowledge the financial supports from the Natural Science Foundation of Jiangsu Province (Grant nos. BK20141194 and BK20131155), the Opening Foundation of Key Laboratory of High-end Structural Materials of Jiangsu Province (Grant no. hsm1401) and Tribology Science Fund of State Key Laboratory of Tribology (SKLTKF13B08). One of the authors (M. D. Wang) is also grateful for the financial supports from the National Natural Science Foundation of China (Grant no. 51205266)
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