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Effects of graphite and graphene spatial structure on the TiC crystal structure and the properties of composite coatings
Surface & Coatings Technology 377 (2019) 124909
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effects of graphite and graphene spatial structure on the TiC crystal structure and the properties of composite coatings Tengfei Hana, Meng Xiaob, Ying Zhanga, Yifu Shena, a b
T
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College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics (NUAA), 211106 Nanjing, PR China Laser Equipment and Machining Technology R&D Center, Nanjing Institute of Advanced Laser Technology, 210038 Nanjing, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser cladding Graphene TiC crystal structure Microstructure Hardness Wear resistance
To investigate the uniqueness of graphene, compared with graphite, in the preparation of TiC reinforced composite coating, the composite coatings were obtained from Ni-Cr-Ti-graphene/graphite powder by laser cladding. The phase composition and microstructure of composite coatings are detected by X-ray diffraction (XRD) and scanning electron microscope (SEM) with energy dispersive spectrometry (EDS), respectively. The phase composition of the composite coatings both mainly consist of Ni-Cr-Fe, Ni3Ti, Cr2Ti and TiC. The research results indicate that the crystal structures of in-situ synthesized TiC in the composite coatings have great difference, in detail, TiC crystals in the graphene-coating and the graphite-coating are dominated by equiaxed and dendrite crystals, severally, which is due to the reaction of two-dimensional graphene with Ti has better diffusion kinetics than that of three-dimensional graphite under rapid solidification of molten pool. The maximum microhardness of graphene-coating is 1000.3 HV0.1, which is about 4 times as hard as the substrate. The composite coatings show significant improvement in hardness and wear resistance than the substrate. The graphene-coating possesses more uniform hardness distribution and better wear resistance than that of graphite-coating, which is benefited from the uniform distribution of TiC equiaxed crystals in the graphene-coating.
1. Introduction
Rahman Rashid et al. [8] reported laser cladding 316 L stainless steel on mild steel. Hardness values as high as 400–450 HV and 320–380 HV in clads of thicknesses 0.5 mm and 1 mm, respectively. The hardness of cladding coating has been greatly improved than the mild steel. The previous literature has been proved that laser cladding can effectively enhance the surface properties of materials (such as hardness and wear resistance). Metal-ceramic composite coating [9,10] has been extensively used to improve the properties of materials. The remarkable properties of the metal-ceramic composite coating rely on high hardness of ceramic reinforcements [11,12]. Among various ceramic particles, TiC [13–15] is a potential reinforcement for composite coating because of its exceptional hardness (3000 HV) and high temperature stability. Ceramic reinforcements are introduced into coating through direct addition or insitu synthesis. Compared to direct addition, the in-situ synthesized ceramic reinforcements possess finer size and better wettability with coating [16]. Graphene is a peculiar two dimensional structure material, which is often used in energy applications. Elisa Sani et al. [17] reported characters of functionalized graphene nanoplatelet-nanofluids in solar thermal collectors. The addition of graphene nanoparticles improves the absorption of pure base fluid to sunlight. Selvakumar et al.
Mild steel is a well-known metal material with excellent comprehensive properties. By virtue of its high cost performance, mild steel is widely used in machinery manufacturing, automobile and ship industry, and used as construction material [1,2]. Unfortunately, the hardness of mild steel is low and its wear resistance is poor, which limits the service range of mild steel and reduces its service life. Laser cladding is a novel and effective technical method for surface modification. This technology has the advantages of simple operation, low energy consumption and small pollution. Compared with other surface modification technologies, the laser cladding coating can acquire finer microstructure, smaller heat affected zone and better bonding performance [3–5], hence, laser cladding has become a popular technology for surface modification. El-Labban et al. [6] produced TiC reinforced composite coating by laser cladding on low carbon steel. The improvement in wear resistance was reached to about 25 times of the low carbon steel. Popoola et al. [7] researched that tribological evaluation of mild steel with ternary alloy Zn-Al-Sn. The laser treated samples obtained higher hardness than the mild steel. Laser alloying of mild steel with Zn-Al-Sn can promise for improving the wear resistance.
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Correspondence author. E-mail address: [email protected] (Y. Shen).
https://doi.org/10.1016/j.surfcoat.2019.124909 Received 26 April 2019; Received in revised form 6 August 2019; Accepted 14 August 2019 Available online 14 August 2019 0257-8972/ © 2019 Published by Elsevier B.V.
Surface & Coatings Technology 377 (2019) 124909
T. Han, et al.
powder are shown in Fig. 2. Subsequently, put the mixed powder into drying oven at 110 °C for 2 h. Q235 hot rolled mild steel with size of 40 mm × 20 mm × 10 mm, was utilized as substrate. The chemical composition of Q235 mild steel is shown in Table 1. Prior to laser cladding, the substrate was polished by sandpaper to clean the rust and greasy dirt. The applied laser power (P) is 1300 W, laser scan speed is 6 mm/s and laser beam diameter is 3 mm. To avoid oxidation of the molten pool, argon gas was used as protective gas with flow velocity of 15 L/min. After laser cladding, specimens were cut from the transversal crosssections of coating. Phase identification was operated via X-ray diffractometer (XRD) with Cu Kα radiation (λ = 0.1540598 nm) at 40 kV and 40 mA. Cross-section microstructure of coatings were observed by scanning electron microscope (SEM) with an energy dispersive spectrometry (EDS). Micro-hardness, distributed from nearby surface of coating to substrate, was detected by a HXS-1000A micro-hardness tester with a load of 100 g and duration time of 15 s, and the distance between adjacent points was 0.1 mm. Wear resistance of substrate and coatings were carried out on ML-10 friction and wear tester under dry sliding wear condition at room temperature, and 400# sandpaper was utilized as friction pair. The experimental parameters of wear testing are shown in Table 2.
[18] researched and fabricated graphene/Cu/graphene absorber to improve optical absorption with tunable spectral selectivity. This absorber achieves the conversion from high transmittance to high absorptivity on the quartz substrate. Vaqueiro-Contreras et al. [19] studied and discussed the stability of graphene oxide passivation and the possibility of a cheap alternative to current passivation materials applied to the solar cell manufacturing. Graphene is also used as reinforcements [20] in metal matrix by means of its special nanostructure and superior mechanical properties. However, the agglomeration of graphene occurs easily in metal matrix, which reduces the reinforcing effect of graphene on metal matrix [21]. Luo et al. [22] investigated that reaction synthesis of TiC/graphene composite thin film on titanium foil by plasma enhanced chemical vapor deposition. The hardness of composite layer increased twice as that of titanium foil. Sadeghi et al. [23] reported that mechanical milling approach and in-situ reactive synthesis to manufacture TiC/Graphene layer/Cu nanocomposites. The results show that the Vickers microhardness of the sintered nanocomposites was in the range of 85.6–149.7 HV0.1, and the nanocomposites obtained excellent tribological characteristics depended on cooperation effect of graphene layers and TiC nanoparticles. The existing researches have shown that the synthesis of Ti and graphene for TiC reinforced composites is feasible. However, to date, TiC, in situ synthesized from graphene with Ti, reinforced composite coating by laser cladding has been rarely reported. In this experiment, laser cladding was utilized to fabricate the composite coatings. In order to avert the agglomeration of graphene in metal matrix, and investigate the difference between graphene and the frequently used graphite in microstructure and properties of composite coatings, the graphene-coating and graphite-coating were obtained by laser cladding Ni-Cr-Ti-graphene/graphite powder. In this research, the microstructure, wear resistance of composite coatings, as well as formation mechanism of different morphology of TiC were detailedly explored.
3. Results and discussion The macroscopic morphologies of the coatings are demonstrated in Fig. 3. The coatings surfaces are smooth and show satisfactory formability without obvious cracks. The phase composition of composite coatings is exhibited in Fig. 4. It can be observed that the graphenecoating and graphite-coating both mainly consist of TiC, solid solution Ni-Cr-Fe and intermetallic compound (IMC) Ni3Ti, Cr2Ti, which declares that graphene and graphite have no influence on the phase composition of composite coatings. The phase composition of composite coatings is totally different from the original powder, which indicates that complex chemical reactions thoroughly occurred in the molten pool during laser cladding. Fig. 5 exhibits the representative microstructure of coatings. As seen in the Fig. 5(a) and (c), the dark grey coarse fishbone or petal shape phases distribute in the graphite-coating, yet the dark grey fine punctaform phases in the graphene-coating. To further investigate the microstructure of the composite coatings, high power microstructure of
2. Experimental procedures The pure powder particles, including 32, 18, 49 and 1 wt% of Ni, Cr, Ti and graphene (or graphite), were employed as cladding materials. The morphology and size of raw powder was shown in Fig. 1. The raw powder was adequately mixed in planetary ball mill for 3 h at 300 rpm/ min to obtain the mixed powder, the morphology and size of the mixed
Fig. 1. Morphology and size of raw powder: (a) Ni, (b) Ti, (c) Cr, (d) graphite, (e) graphene, (f) high magnification morphology of graphene. 2
Surface & Coatings Technology 377 (2019) 124909
T. Han, et al.
Fig. 2. Morphology and size of mixed powder: (a) mixed powder of graphite-coating, (b) mixed powder of graphene-coating. Table 1 The chemical composition of Q235 mild steel (wt%). Element
C
Si
Mn
Cr
Ni
Cu
Fe
Content
0.12–0.20
≤0.30
0.30–0.70
≤0.03
≤0.03
≤0.03
Bal.
Table 2 The experimental parameters of wear testing. Parameter
Load (g)
Sliding speed (r·min−1)
Duration time (min)
Temperature (°C)
Value
500
60
15
25 ± 5
them were presented in Fig. 5(b) and (d), and the chemical composition of phases are detected by EDS. As shown in Fig. 5(b) and (d), in graphite-coating, the dark grey coarse phase is dendrites, and a small amount of equiaxed crystals (marked of brown dotted lines), in sharp contrast, the dark grey fine phase in graphene-coating is uniformly distributed equiaxed crystals. According to the test result of EDS, phases 1, 2 and 4 are mainly consisted of Ti element and C element in a ratio of about 1:1, and the phases 3 and 5 mainly contain metallic elements. Combining XRD analysis results, the phases 1, 2 and 4 are TiC, and phase 3, 5 are eutectic alloy. It is noted that phase 2 (secondary dendrite, 3.45 at.% Cr, 6.41 at.% Fe, 2.68 at.% Ni) contains more metal elements than that of phase 1 (primary dendrite, 1 at.% Cr, 0.88 at.% Fe, 0.23 at.% Ni), which is owing to the concentration of TiC decreased and some metal atoms dissolved into secondary dendrites. The bottom of coatings and bonding interfaces between coatings and substrates are demonstrated in Fig. 5(e) and (f), respectively. From the figure, the coatings both achieved excellent metallurgical bonding with the substrates without cracks. When the molten pool solidified, the solidified structure of the coating presents a planar crystal structure at the interface, due to the high temperature gradient (G) and the low solidification rate (R). With the appearance of planar crystals, the G decreases
Fig. 4. XRD patterns of the coatings of the composite coatings.
and the R increases, the coating structure grown as columnar crystals, and more, the growth direction of the columnar crystals was almost perpendicular to that of the planar crystal (i.e. the bonding interface), because this direction of the vertical planar crystal is most conducive to the heat dissipation and solidification of the molten pool. At the bottom of the graphite-coating, the TiC mainly exists as cross-shaped dendrite and equiaxed crystals. Emphatically, compared with the inner graphitecoating, the amount of the TiC equiaxed crystals obviously increases at the bottom of graphite-coating. The molten pool was not sufficiently stirred with very short lifetime, thus, there were fluctuations of the elements content in molten pool and graphite floated easily by virtue of its small density, which leaded to the content of graphite at the bottom of the coating is relatively low compared with that inner graphitecoating. Therefore, after the nucleation of TiC, it is unable to grow into developed dendrites due to the lack of sufficient TiC. In addition, the
Fig. 3. Macro-morphologies of coatings surfaces: (a) graphite-coating, (b) graphene-coating. 3
Surface & Coatings Technology 377 (2019) 124909
T. Han, et al.
Fig. 5. SEM microstructure of the coatings: (a) graphite-coating, (b) high magnification microstructure of graphite-coating, (c) graphene-coating, (d) high magnification microstructure of graphene-coating, (e) interface between the graphite-coating and substrate, (f) interface between the graphene-coating and substrate.
to continue to react with graphite, which reduces the nucleation rate of TiC. Besides, a graphite can provide more carbon than the graphene, which is easy to encounter component over-cooling of TiC around graphite. These reasons cause the TiC tends to grow into dendrite in the graphite-coating. By virtue of the two dimensional structure, graphene has a large specific surface area, so liquid Ti can easy contact with graphene to obtain a large reaction interface, which is beneficial to increase the nucleation rate of TiC. Under the action of high-energy laser irradiation, graphene with high surface energy is in an unstable state and has good activity, which is conducive to reducing the Gibbs free energy of reaction between graphene and Ti, thus improving the ability of reaction binding between graphene and Ti, and increasing the nucleation rate of TiC. In this case, dendrites of TiC are inhibited, so that TiC grows as equiaxed crystals in graphene-coating. Fig. 7 illuminates the microhardness curves of graphene-coating and graphite-coating. It visually could be seen that the hardness of graphitecoating and graphene-coating both have been enhanced obviously than the substrate (> 4 times). Specifically, the maximum hardness of graphite-coating and graphene-coating is 989 HV0.1 and 1000.3 HV0.1, separately. The high hardness TiC reinforcements contribute greatly to improve the hardness of coatings. According to the Hall-Petch theory, fine microstructure can also bring high hardness to the materials. The laser cladding coatings obtained fine microstructure under extremely rapidly cooling rate [26], which facilitates the coatings to obtain high hardness. Furthermore, the hardness fluctuation of graphene-coating is less than that of graphite-coating, the main reason can be ascribed to
solidification rate at the bottom of the coating, near the substrate, is fast, which gives rise to it easier for the equiaxed crystals of TiC to appear at the bottom of the graphite-coating. Previous researches also have found similar patterns [24,25]. Fig. 6 shows the schematic diagram of the synthesis TiC from graphite and graphene with Ti. During laser cladding, the high temperature of the molten pool can satisfy the thermodynamic conditions of TiC in-situ synthesis reaction, nevertheless, kinetics becomes the restrictive link in the formation of TiC owing to the life of molten pool is incredibly short. Because of three dimensional multilayer structure of graphite, liquid Ti first reacts with graphite surface to form a product layer of TiC, whereafter, liquid Ti needs to go through the product layer
Fig. 6. Schematic diagram of TiC synthesis from graphite and graphene with Ti. 4
Surface & Coatings Technology 377 (2019) 124909
T. Han, et al.
Table 3 The EDS analysis results of composite coatings wear debris. Element
at.% P1
at.% P2
at.% P3
at.% P4
C Ti Ni Cr Fe Al O
– – – – – 62.3 38.2
1.83 15.28 33.31 39.43 10.15 – –
41.44 55.44 1.81 0.80 0.51 – –
2.77 14.06 31.54 41.90 9.73 – –
uniform distributed equiaxed TiC particles in the graphene-coating. In addition, along a straight line from the top of the coatings to the substrate, the hardness value of coatings decreased significantly. In molten pools, TiC particles with small density (4.93 g/cm3) float up easily under the effect of surface tension and buoyancy, so the concentration
Fig. 7. Micro-hardness distribution from the composite coatings to the substrate.
(a)
(b) Furrow
Plastic deformation
(d)
(c)
P2
Furrow
P3 P1
(e)
(f)
P4 Furrow
Fig. 8. Worn surfaces morphologies: (a) worn morphology of substrate, (b) high magnification worn morphology of substrate, (c) worn morphology of graphitecoating, (d) high magnification worn morphology of graphite-coating, (e) worn morphology of graphene-coating, (f) high magnification worn morphology of graphene-coating. 5
Surface & Coatings Technology 377 (2019) 124909
T. Han, et al.
Fig. 9. Two-dimensional contour and three-dimensional morphology of substrate and coatings worn surface: (a) contour and three-dimensional morphology of substrate, (b) contour and three-dimensional morphology of graphite-coating, (c) contour and three-dimensional morphology of graphene-coating.
steel). The wear mechanism of the substrate shows a typical plough cutting. According to the law of Archard [25], the improvement of hardness is beneficial to improve the wear resistance of the materials. Therefore, with the hardness increasing, the worn surfaces of composite coatings are smoother than that of the substrate, and sparse scratches instead of dense furrows on composite coating surfaces. It is worth emphasizing that there is no plastic deformation on the surface of composite coatings, which also benefits from the improvement of coatings hardness. The uniformly distributed TiC particles not only act as skeleton to disperse and resist friction but also can conquer the plastic deformation and plough cutting of the coating through pinning effect. The obtained fine microstructure of coatings matrix can also be used to reduce the plough cutting action of coatings by more grain boundaries. Compared with the worn surface of the graphene-coating, relatively dense scratches and more abrasive debris appear on the worn surface of graphite-coating. The binding force between TiC equiaxed crystals and coating is stronger than that of dendritic TiC crystals. During the frictional wear test, the dendritic TiC is subjected to the tangential force perpendicular to the dendrite direction, while the brittleness of TiC is high, thus, the dendritic TiC is broken and flaked to form wear debris. During the wear test, the abrasive debris, especially the high-hardness TiC debris, will also have grinding effect on the
Fig. 10. The schematic diagram of composite coatings wear process.
of TiC particles increases from the bottom to the top of the coatings. With the melting of the surface layer of the substrate, Fe elements enter into the molten pool from the substrate, and the Fe content at the bottom of the coating is significantly higher than that at the top of the coating, while Fe elements can obviously reduce the hardness of the coating. The worn surface of composite coatings and substrate are exhibited in Fig. 8. From the Fig. 8(a) and (b), deep and dense furrows and severe plastic deformation appear on the surface of substrate, which is caused by the low hardness and poor wear resistance of the substrate (i.e. mild 6
Surface & Coatings Technology 377 (2019) 124909
T. Han, et al.
References
graphite-coating by three-body wear mechanism, thus the wear of the graphite-coating is aggravated. Under the same wear test conditions, the point-like TiC equiaxed crystals have no obvious orientation, and the effect of force on that is isotropic, so the point-like TiC equiaxed crystals are different to break and flake. Hence, the graphene-coating gained better wear resistance than that of graphite-coating. As seen in Fig. 8(c), (d) and (f), P1 wear debris appears as spherical black granule, P2 and P4 wear debris show white sheet, and P3 wear debris shows irregular black granular. The EDS analysis results of composite coatings wear debris are exhibited in Table 3. As shown in Table 3, P1 wear debris only contains Al and O elements, thus, P1 wear debris is assumed to be Al2O3, which is an exfoliated particle from the friction pair (i.e. 400# metallographic sandpaper). P2 and P4 wear debris mainly contains Ti, Ni, and Cr elements, so it can be concluded that these wear debris come from the coating matrix (i.e. eutectic alloy). P3 wear debris mainly consists of Ti and C elements, combined with XRD analysis results, P3 is TiC particle. The different shapes of P3 and P2 are mainly due to the huge difference in hardness between TiC and eutectic alloy. Fig. 9 exhibits the two-dimensional contour and three-dimensional morphology of substrate and coatings worn surface. From this figure, the wear depth of the substrate, graphene-coating and graphene-coating (i.e. the distance from the highest point to the lowest point of the contour of the wear surface) are 3.444, 1.994 and 1.437 μm, respectively. Compared with the worn surface of substrate, the worn surface of graphite-coating and graphene-coating is relatively smooth, and the graphene-coating has the smoothest worn surface, which indicates that the wear resistance of the coatings is significantly improved compared with the substrate, and the wear resistance of the graphene-coating is better than that of graphene-coating. The schematic diagram of composite coatings wear process is presented in Fig. 10. During wear test, TiC reinforcements are stripped from the coatings to form granular wear debris by the Al2O3 pin of the friction pair under the action of friction. When the coating matrix is subjected to the wear of Al2O3 pin, the eutectic alloy is gradually peeled off from the coating matrix under the cutting action. However, due to the good plasticity and toughness of the coating matrix (i.e. eutectic alloy), the stripped alloy and the coating will not completely peel off. When the friction force is greater than the connection between the stripped alloy and the coating matrix, the stripped alloy will completely peel off from the coating matrix, and then produce sheet wear debris.
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4. Conclusions The graphene-coating and graphite-coating were obtained by laser cladding using Ni-Cr-Ti-graphene/graphite. The in-situ synthesized TiC, Ni-Cr-Fe, Ni3Ti and Cr2Ti make up the graphene-coating and graphitecoating. With the same process parameters, the TiC in graphene-coating and graphite-coating are fine equiaxed crystals and coarse dendrites, respectively. The maximum hardness of graphite-coating and graphenecoating is 989 and 1000.3 HV0.1 respectively. The hardness and wear resistance of graphene-coating and graphite-coating are markedly improved than the substrate. Besides, the graphene-coating acquired more uniform hardness distribution and better wear resistance than the graphite-coating by virtue of the equiaxed crystals of TiC. Acknowledgments This research was supported financially by the National Natural Science Foundation of China [grant numbers 51475232, 51605473].
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Effects of graphite and graphene spatial structure on the TiC crystal structure and the properties of composite coatings Tengfei Hana, Meng Xiaob, Ying Zhanga, Yifu Shena, a b
T
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College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics (NUAA), 211106 Nanjing, PR China Laser Equipment and Machining Technology R&D Center, Nanjing Institute of Advanced Laser Technology, 210038 Nanjing, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser cladding Graphene TiC crystal structure Microstructure Hardness Wear resistance
To investigate the uniqueness of graphene, compared with graphite, in the preparation of TiC reinforced composite coating, the composite coatings were obtained from Ni-Cr-Ti-graphene/graphite powder by laser cladding. The phase composition and microstructure of composite coatings are detected by X-ray diffraction (XRD) and scanning electron microscope (SEM) with energy dispersive spectrometry (EDS), respectively. The phase composition of the composite coatings both mainly consist of Ni-Cr-Fe, Ni3Ti, Cr2Ti and TiC. The research results indicate that the crystal structures of in-situ synthesized TiC in the composite coatings have great difference, in detail, TiC crystals in the graphene-coating and the graphite-coating are dominated by equiaxed and dendrite crystals, severally, which is due to the reaction of two-dimensional graphene with Ti has better diffusion kinetics than that of three-dimensional graphite under rapid solidification of molten pool. The maximum microhardness of graphene-coating is 1000.3 HV0.1, which is about 4 times as hard as the substrate. The composite coatings show significant improvement in hardness and wear resistance than the substrate. The graphene-coating possesses more uniform hardness distribution and better wear resistance than that of graphite-coating, which is benefited from the uniform distribution of TiC equiaxed crystals in the graphene-coating.
1. Introduction
Rahman Rashid et al. [8] reported laser cladding 316 L stainless steel on mild steel. Hardness values as high as 400–450 HV and 320–380 HV in clads of thicknesses 0.5 mm and 1 mm, respectively. The hardness of cladding coating has been greatly improved than the mild steel. The previous literature has been proved that laser cladding can effectively enhance the surface properties of materials (such as hardness and wear resistance). Metal-ceramic composite coating [9,10] has been extensively used to improve the properties of materials. The remarkable properties of the metal-ceramic composite coating rely on high hardness of ceramic reinforcements [11,12]. Among various ceramic particles, TiC [13–15] is a potential reinforcement for composite coating because of its exceptional hardness (3000 HV) and high temperature stability. Ceramic reinforcements are introduced into coating through direct addition or insitu synthesis. Compared to direct addition, the in-situ synthesized ceramic reinforcements possess finer size and better wettability with coating [16]. Graphene is a peculiar two dimensional structure material, which is often used in energy applications. Elisa Sani et al. [17] reported characters of functionalized graphene nanoplatelet-nanofluids in solar thermal collectors. The addition of graphene nanoparticles improves the absorption of pure base fluid to sunlight. Selvakumar et al.
Mild steel is a well-known metal material with excellent comprehensive properties. By virtue of its high cost performance, mild steel is widely used in machinery manufacturing, automobile and ship industry, and used as construction material [1,2]. Unfortunately, the hardness of mild steel is low and its wear resistance is poor, which limits the service range of mild steel and reduces its service life. Laser cladding is a novel and effective technical method for surface modification. This technology has the advantages of simple operation, low energy consumption and small pollution. Compared with other surface modification technologies, the laser cladding coating can acquire finer microstructure, smaller heat affected zone and better bonding performance [3–5], hence, laser cladding has become a popular technology for surface modification. El-Labban et al. [6] produced TiC reinforced composite coating by laser cladding on low carbon steel. The improvement in wear resistance was reached to about 25 times of the low carbon steel. Popoola et al. [7] researched that tribological evaluation of mild steel with ternary alloy Zn-Al-Sn. The laser treated samples obtained higher hardness than the mild steel. Laser alloying of mild steel with Zn-Al-Sn can promise for improving the wear resistance.
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Correspondence author. E-mail address: [email protected] (Y. Shen).
https://doi.org/10.1016/j.surfcoat.2019.124909 Received 26 April 2019; Received in revised form 6 August 2019; Accepted 14 August 2019 Available online 14 August 2019 0257-8972/ © 2019 Published by Elsevier B.V.
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powder are shown in Fig. 2. Subsequently, put the mixed powder into drying oven at 110 °C for 2 h. Q235 hot rolled mild steel with size of 40 mm × 20 mm × 10 mm, was utilized as substrate. The chemical composition of Q235 mild steel is shown in Table 1. Prior to laser cladding, the substrate was polished by sandpaper to clean the rust and greasy dirt. The applied laser power (P) is 1300 W, laser scan speed is 6 mm/s and laser beam diameter is 3 mm. To avoid oxidation of the molten pool, argon gas was used as protective gas with flow velocity of 15 L/min. After laser cladding, specimens were cut from the transversal crosssections of coating. Phase identification was operated via X-ray diffractometer (XRD) with Cu Kα radiation (λ = 0.1540598 nm) at 40 kV and 40 mA. Cross-section microstructure of coatings were observed by scanning electron microscope (SEM) with an energy dispersive spectrometry (EDS). Micro-hardness, distributed from nearby surface of coating to substrate, was detected by a HXS-1000A micro-hardness tester with a load of 100 g and duration time of 15 s, and the distance between adjacent points was 0.1 mm. Wear resistance of substrate and coatings were carried out on ML-10 friction and wear tester under dry sliding wear condition at room temperature, and 400# sandpaper was utilized as friction pair. The experimental parameters of wear testing are shown in Table 2.
[18] researched and fabricated graphene/Cu/graphene absorber to improve optical absorption with tunable spectral selectivity. This absorber achieves the conversion from high transmittance to high absorptivity on the quartz substrate. Vaqueiro-Contreras et al. [19] studied and discussed the stability of graphene oxide passivation and the possibility of a cheap alternative to current passivation materials applied to the solar cell manufacturing. Graphene is also used as reinforcements [20] in metal matrix by means of its special nanostructure and superior mechanical properties. However, the agglomeration of graphene occurs easily in metal matrix, which reduces the reinforcing effect of graphene on metal matrix [21]. Luo et al. [22] investigated that reaction synthesis of TiC/graphene composite thin film on titanium foil by plasma enhanced chemical vapor deposition. The hardness of composite layer increased twice as that of titanium foil. Sadeghi et al. [23] reported that mechanical milling approach and in-situ reactive synthesis to manufacture TiC/Graphene layer/Cu nanocomposites. The results show that the Vickers microhardness of the sintered nanocomposites was in the range of 85.6–149.7 HV0.1, and the nanocomposites obtained excellent tribological characteristics depended on cooperation effect of graphene layers and TiC nanoparticles. The existing researches have shown that the synthesis of Ti and graphene for TiC reinforced composites is feasible. However, to date, TiC, in situ synthesized from graphene with Ti, reinforced composite coating by laser cladding has been rarely reported. In this experiment, laser cladding was utilized to fabricate the composite coatings. In order to avert the agglomeration of graphene in metal matrix, and investigate the difference between graphene and the frequently used graphite in microstructure and properties of composite coatings, the graphene-coating and graphite-coating were obtained by laser cladding Ni-Cr-Ti-graphene/graphite powder. In this research, the microstructure, wear resistance of composite coatings, as well as formation mechanism of different morphology of TiC were detailedly explored.
3. Results and discussion The macroscopic morphologies of the coatings are demonstrated in Fig. 3. The coatings surfaces are smooth and show satisfactory formability without obvious cracks. The phase composition of composite coatings is exhibited in Fig. 4. It can be observed that the graphenecoating and graphite-coating both mainly consist of TiC, solid solution Ni-Cr-Fe and intermetallic compound (IMC) Ni3Ti, Cr2Ti, which declares that graphene and graphite have no influence on the phase composition of composite coatings. The phase composition of composite coatings is totally different from the original powder, which indicates that complex chemical reactions thoroughly occurred in the molten pool during laser cladding. Fig. 5 exhibits the representative microstructure of coatings. As seen in the Fig. 5(a) and (c), the dark grey coarse fishbone or petal shape phases distribute in the graphite-coating, yet the dark grey fine punctaform phases in the graphene-coating. To further investigate the microstructure of the composite coatings, high power microstructure of
2. Experimental procedures The pure powder particles, including 32, 18, 49 and 1 wt% of Ni, Cr, Ti and graphene (or graphite), were employed as cladding materials. The morphology and size of raw powder was shown in Fig. 1. The raw powder was adequately mixed in planetary ball mill for 3 h at 300 rpm/ min to obtain the mixed powder, the morphology and size of the mixed
Fig. 1. Morphology and size of raw powder: (a) Ni, (b) Ti, (c) Cr, (d) graphite, (e) graphene, (f) high magnification morphology of graphene. 2
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Fig. 2. Morphology and size of mixed powder: (a) mixed powder of graphite-coating, (b) mixed powder of graphene-coating. Table 1 The chemical composition of Q235 mild steel (wt%). Element
C
Si
Mn
Cr
Ni
Cu
Fe
Content
0.12–0.20
≤0.30
0.30–0.70
≤0.03
≤0.03
≤0.03
Bal.
Table 2 The experimental parameters of wear testing. Parameter
Load (g)
Sliding speed (r·min−1)
Duration time (min)
Temperature (°C)
Value
500
60
15
25 ± 5
them were presented in Fig. 5(b) and (d), and the chemical composition of phases are detected by EDS. As shown in Fig. 5(b) and (d), in graphite-coating, the dark grey coarse phase is dendrites, and a small amount of equiaxed crystals (marked of brown dotted lines), in sharp contrast, the dark grey fine phase in graphene-coating is uniformly distributed equiaxed crystals. According to the test result of EDS, phases 1, 2 and 4 are mainly consisted of Ti element and C element in a ratio of about 1:1, and the phases 3 and 5 mainly contain metallic elements. Combining XRD analysis results, the phases 1, 2 and 4 are TiC, and phase 3, 5 are eutectic alloy. It is noted that phase 2 (secondary dendrite, 3.45 at.% Cr, 6.41 at.% Fe, 2.68 at.% Ni) contains more metal elements than that of phase 1 (primary dendrite, 1 at.% Cr, 0.88 at.% Fe, 0.23 at.% Ni), which is owing to the concentration of TiC decreased and some metal atoms dissolved into secondary dendrites. The bottom of coatings and bonding interfaces between coatings and substrates are demonstrated in Fig. 5(e) and (f), respectively. From the figure, the coatings both achieved excellent metallurgical bonding with the substrates without cracks. When the molten pool solidified, the solidified structure of the coating presents a planar crystal structure at the interface, due to the high temperature gradient (G) and the low solidification rate (R). With the appearance of planar crystals, the G decreases
Fig. 4. XRD patterns of the coatings of the composite coatings.
and the R increases, the coating structure grown as columnar crystals, and more, the growth direction of the columnar crystals was almost perpendicular to that of the planar crystal (i.e. the bonding interface), because this direction of the vertical planar crystal is most conducive to the heat dissipation and solidification of the molten pool. At the bottom of the graphite-coating, the TiC mainly exists as cross-shaped dendrite and equiaxed crystals. Emphatically, compared with the inner graphitecoating, the amount of the TiC equiaxed crystals obviously increases at the bottom of graphite-coating. The molten pool was not sufficiently stirred with very short lifetime, thus, there were fluctuations of the elements content in molten pool and graphite floated easily by virtue of its small density, which leaded to the content of graphite at the bottom of the coating is relatively low compared with that inner graphitecoating. Therefore, after the nucleation of TiC, it is unable to grow into developed dendrites due to the lack of sufficient TiC. In addition, the
Fig. 3. Macro-morphologies of coatings surfaces: (a) graphite-coating, (b) graphene-coating. 3
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Fig. 5. SEM microstructure of the coatings: (a) graphite-coating, (b) high magnification microstructure of graphite-coating, (c) graphene-coating, (d) high magnification microstructure of graphene-coating, (e) interface between the graphite-coating and substrate, (f) interface between the graphene-coating and substrate.
to continue to react with graphite, which reduces the nucleation rate of TiC. Besides, a graphite can provide more carbon than the graphene, which is easy to encounter component over-cooling of TiC around graphite. These reasons cause the TiC tends to grow into dendrite in the graphite-coating. By virtue of the two dimensional structure, graphene has a large specific surface area, so liquid Ti can easy contact with graphene to obtain a large reaction interface, which is beneficial to increase the nucleation rate of TiC. Under the action of high-energy laser irradiation, graphene with high surface energy is in an unstable state and has good activity, which is conducive to reducing the Gibbs free energy of reaction between graphene and Ti, thus improving the ability of reaction binding between graphene and Ti, and increasing the nucleation rate of TiC. In this case, dendrites of TiC are inhibited, so that TiC grows as equiaxed crystals in graphene-coating. Fig. 7 illuminates the microhardness curves of graphene-coating and graphite-coating. It visually could be seen that the hardness of graphitecoating and graphene-coating both have been enhanced obviously than the substrate (> 4 times). Specifically, the maximum hardness of graphite-coating and graphene-coating is 989 HV0.1 and 1000.3 HV0.1, separately. The high hardness TiC reinforcements contribute greatly to improve the hardness of coatings. According to the Hall-Petch theory, fine microstructure can also bring high hardness to the materials. The laser cladding coatings obtained fine microstructure under extremely rapidly cooling rate [26], which facilitates the coatings to obtain high hardness. Furthermore, the hardness fluctuation of graphene-coating is less than that of graphite-coating, the main reason can be ascribed to
solidification rate at the bottom of the coating, near the substrate, is fast, which gives rise to it easier for the equiaxed crystals of TiC to appear at the bottom of the graphite-coating. Previous researches also have found similar patterns [24,25]. Fig. 6 shows the schematic diagram of the synthesis TiC from graphite and graphene with Ti. During laser cladding, the high temperature of the molten pool can satisfy the thermodynamic conditions of TiC in-situ synthesis reaction, nevertheless, kinetics becomes the restrictive link in the formation of TiC owing to the life of molten pool is incredibly short. Because of three dimensional multilayer structure of graphite, liquid Ti first reacts with graphite surface to form a product layer of TiC, whereafter, liquid Ti needs to go through the product layer
Fig. 6. Schematic diagram of TiC synthesis from graphite and graphene with Ti. 4
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Table 3 The EDS analysis results of composite coatings wear debris. Element
at.% P1
at.% P2
at.% P3
at.% P4
C Ti Ni Cr Fe Al O
– – – – – 62.3 38.2
1.83 15.28 33.31 39.43 10.15 – –
41.44 55.44 1.81 0.80 0.51 – –
2.77 14.06 31.54 41.90 9.73 – –
uniform distributed equiaxed TiC particles in the graphene-coating. In addition, along a straight line from the top of the coatings to the substrate, the hardness value of coatings decreased significantly. In molten pools, TiC particles with small density (4.93 g/cm3) float up easily under the effect of surface tension and buoyancy, so the concentration
Fig. 7. Micro-hardness distribution from the composite coatings to the substrate.
(a)
(b) Furrow
Plastic deformation
(d)
(c)
P2
Furrow
P3 P1
(e)
(f)
P4 Furrow
Fig. 8. Worn surfaces morphologies: (a) worn morphology of substrate, (b) high magnification worn morphology of substrate, (c) worn morphology of graphitecoating, (d) high magnification worn morphology of graphite-coating, (e) worn morphology of graphene-coating, (f) high magnification worn morphology of graphene-coating. 5
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Fig. 9. Two-dimensional contour and three-dimensional morphology of substrate and coatings worn surface: (a) contour and three-dimensional morphology of substrate, (b) contour and three-dimensional morphology of graphite-coating, (c) contour and three-dimensional morphology of graphene-coating.
steel). The wear mechanism of the substrate shows a typical plough cutting. According to the law of Archard [25], the improvement of hardness is beneficial to improve the wear resistance of the materials. Therefore, with the hardness increasing, the worn surfaces of composite coatings are smoother than that of the substrate, and sparse scratches instead of dense furrows on composite coating surfaces. It is worth emphasizing that there is no plastic deformation on the surface of composite coatings, which also benefits from the improvement of coatings hardness. The uniformly distributed TiC particles not only act as skeleton to disperse and resist friction but also can conquer the plastic deformation and plough cutting of the coating through pinning effect. The obtained fine microstructure of coatings matrix can also be used to reduce the plough cutting action of coatings by more grain boundaries. Compared with the worn surface of the graphene-coating, relatively dense scratches and more abrasive debris appear on the worn surface of graphite-coating. The binding force between TiC equiaxed crystals and coating is stronger than that of dendritic TiC crystals. During the frictional wear test, the dendritic TiC is subjected to the tangential force perpendicular to the dendrite direction, while the brittleness of TiC is high, thus, the dendritic TiC is broken and flaked to form wear debris. During the wear test, the abrasive debris, especially the high-hardness TiC debris, will also have grinding effect on the
Fig. 10. The schematic diagram of composite coatings wear process.
of TiC particles increases from the bottom to the top of the coatings. With the melting of the surface layer of the substrate, Fe elements enter into the molten pool from the substrate, and the Fe content at the bottom of the coating is significantly higher than that at the top of the coating, while Fe elements can obviously reduce the hardness of the coating. The worn surface of composite coatings and substrate are exhibited in Fig. 8. From the Fig. 8(a) and (b), deep and dense furrows and severe plastic deformation appear on the surface of substrate, which is caused by the low hardness and poor wear resistance of the substrate (i.e. mild 6
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References
graphite-coating by three-body wear mechanism, thus the wear of the graphite-coating is aggravated. Under the same wear test conditions, the point-like TiC equiaxed crystals have no obvious orientation, and the effect of force on that is isotropic, so the point-like TiC equiaxed crystals are different to break and flake. Hence, the graphene-coating gained better wear resistance than that of graphite-coating. As seen in Fig. 8(c), (d) and (f), P1 wear debris appears as spherical black granule, P2 and P4 wear debris show white sheet, and P3 wear debris shows irregular black granular. The EDS analysis results of composite coatings wear debris are exhibited in Table 3. As shown in Table 3, P1 wear debris only contains Al and O elements, thus, P1 wear debris is assumed to be Al2O3, which is an exfoliated particle from the friction pair (i.e. 400# metallographic sandpaper). P2 and P4 wear debris mainly contains Ti, Ni, and Cr elements, so it can be concluded that these wear debris come from the coating matrix (i.e. eutectic alloy). P3 wear debris mainly consists of Ti and C elements, combined with XRD analysis results, P3 is TiC particle. The different shapes of P3 and P2 are mainly due to the huge difference in hardness between TiC and eutectic alloy. Fig. 9 exhibits the two-dimensional contour and three-dimensional morphology of substrate and coatings worn surface. From this figure, the wear depth of the substrate, graphene-coating and graphene-coating (i.e. the distance from the highest point to the lowest point of the contour of the wear surface) are 3.444, 1.994 and 1.437 μm, respectively. Compared with the worn surface of substrate, the worn surface of graphite-coating and graphene-coating is relatively smooth, and the graphene-coating has the smoothest worn surface, which indicates that the wear resistance of the coatings is significantly improved compared with the substrate, and the wear resistance of the graphene-coating is better than that of graphene-coating. The schematic diagram of composite coatings wear process is presented in Fig. 10. During wear test, TiC reinforcements are stripped from the coatings to form granular wear debris by the Al2O3 pin of the friction pair under the action of friction. When the coating matrix is subjected to the wear of Al2O3 pin, the eutectic alloy is gradually peeled off from the coating matrix under the cutting action. However, due to the good plasticity and toughness of the coating matrix (i.e. eutectic alloy), the stripped alloy and the coating will not completely peel off. When the friction force is greater than the connection between the stripped alloy and the coating matrix, the stripped alloy will completely peel off from the coating matrix, and then produce sheet wear debris.
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4. Conclusions The graphene-coating and graphite-coating were obtained by laser cladding using Ni-Cr-Ti-graphene/graphite. The in-situ synthesized TiC, Ni-Cr-Fe, Ni3Ti and Cr2Ti make up the graphene-coating and graphitecoating. With the same process parameters, the TiC in graphene-coating and graphite-coating are fine equiaxed crystals and coarse dendrites, respectively. The maximum hardness of graphite-coating and graphenecoating is 989 and 1000.3 HV0.1 respectively. The hardness and wear resistance of graphene-coating and graphite-coating are markedly improved than the substrate. Besides, the graphene-coating acquired more uniform hardness distribution and better wear resistance than the graphite-coating by virtue of the equiaxed crystals of TiC. Acknowledgments This research was supported financially by the National Natural Science Foundation of China [grant numbers 51475232, 51605473].
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