Characteristics and performance of hard Ni60 alloy coating produced with supersonic laser deposition technique

Materials & Design 83 (2015) 26–35

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Characteristics and performance of hard Ni60 alloy coating produced with supersonic laser deposition technique Jianhua Yao ⇑, Lijing Yang, Bo Li, Zhihong Li Research Center of Laser Processing Technology and Engineering, Zhejiang University of Technology, No. 18 Chaowang Road, Hangzhou 310014, China Zhejiang Provincial Collaborative Innovation Center of High-end Laser Manufacturing Equipment, No. 18 Chaowang Road, Hangzhou 310014, China

a r t i c l e

i n f o

Article history: Received 8 December 2014 Revised 30 March 2015 Accepted 31 May 2015

Keywords: Supersonic laser deposition Ni60 coating Hardness Wear Corrosion Dilution

a b s t r a c t A novel coating fabrication technique, known as supersonic laser deposition (SLD), which combines cold spray (CS) with laser technology, is applied to produce hard Ni60 (58–62 HRC) coating on medium carbon steel (AISI 1045 steel) substrate. Different process parameters are investigated to obtain the optimal. The Ni60 coating specimens prepared by SLD process are studied microstructurally using scanning electron microscope (SEM), energy dispersive spectrum (EDS) and X-ray diffraction (XRD). The microstructures of the coatings are compared with those of the coatings produced using laser cladding (LC). The hardness, tribological property and corrosion resistance of the Ni60 coatings produced by SLD and LC with the optimal process parameters are evaluated under Vickers hardness, pin-on-disk wear and electrochemical corrosion tests. It is demonstrated that the Ni60 coating with SLD exhibits some characteristics, such as fine microstructure as cast, stable phases and less dilution; it surpasses the coating produced with conventional LC process in sliding wear resistance; but in 1 mol/L H2SO4 solution, the SLD and LC coatings performed similarly in corrosion resistance. This research has proved that SLD technique enables depositing hard Ni60 on steel substrate, which is impossible for CS technique. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Ni60 alloy is one of the most widely used Ni–Cr–B–Si alloys as self-fluxing powders for conventional laser cladding and thermal spray. Adding Si, B, Cr elements in a Ni-based self-fluxing alloy can effectively reduce its melting point down to 1050–1080 °C and enhance the alloy hardness up to 58–62 HRC (647–734 HV) [1–3]. The obtained coating exhibits excellent resistance to wear, corrosion and high temperature oxidation over a wide range of temperature [4]. Conventional Ni60 coatings were often prepared by the following deposition technologies: flame spraying, high-velocity oxygen fuel spraying (HVOF), electric arc spraying, plasma spraying, plasma-transferred arc welding, and laser cladding (LC) [5,6]. In thermal spray process, plasma jets, electric arc or high-velocity oxygen fuel are used to melt or partially melt the particles and also to accelerate the powder to impact on the substrate. Ni60 coatings deposited by thermal spray have some problems such as more oxides, high porosity, non-uniform microstructure, weak coating adhesion strength, low deposition efficiency, owing to un-melting powder and high thermal stress [1,7,8]. Recently, laser cladding technique, owing to the ⇑ Corresponding author. E-mail address: [email protected] (J. Yao). http://dx.doi.org/10.1016/j.matdes.2015.05.087 0264-1275/Ó 2015 Elsevier Ltd. All rights reserved.

characteristics of metallurgical bonding, uniform microstructure and less oxides, has been utilized to fabricate better Ni60 coating. LC technology uses high energy density of laser beam to melt feedstock material and the surface layer of the substrate to form metallurgical bonding between the coating and substrate [9,10]. Ni60 coatings deposited by LC showed that compounds such as FeB, Cr3C7, Ni3Si easily concentrated at the grain boundary, the structure stress and heat stress of the coating were generated due to dendritic growth and high thermal gradients were induced during rapid cooling and solidification, which can all result in the initiation of cracks [11–14]. Although metallurgical combination between the coating and substrate in laser cladding can improve coating adhesion strength, element component dilution of the coating may affect the properties of the coating and form a low hardness zone [15]. In order to cope with these problems and improve the properties of the coatings, cold spray, which is the newest spray technique, has attracted much attention of researches. Cold spray (CS) is a process where metal powder particles in solid state, accelerated by a low temperature supersonic gas stream, are plastically deformed to form a coating by means of ballistic impingement upon a suitable substrate [16–18]. The metal powder particles range from 5 to 50 lm. In the CS process the particles are accelerated by a high-velocity jet stream that is

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J. Yao et al. / Materials & Design 83 (2015) 26–35

generated via the expansion of pressurized, preheated nitrogen or air in a vonverging–diverging nozzle to exceed critical plastic deformation velocity [17,19]. Currently, cold spray is mainly employed to deposit relatively soft, heat sensitive and oxidation sensitive materials such as Al, Cu, Ti, 316, Ni-based 25 alloy, soft matrix composite powder such as WC/Co, diamond/Ni, and amorphous material [20–27], but it is difficult to deposit high hardness particles such as Stellite 6, Ni60, diamond, WC, and metal–ceramic materials, or prohibitively expensive to process by cold spray [28]. In order to achieve good hard alloy coatings, the idea of combining cold spray with laser technology was proposed by O’Neill et al. [29], known as supersonic laser deposition (SLD). SLD technology combines the advantages of cold spray and laser technology becoming a potential coating approach for hard materials. In the SLD process, laser is used to heat the deposition zone in order to soften both substrate and powder particles thus allowing the formation of a coating at a much reduced impact velocity [29,30]. Luo et al. [31,32] and Lupoi et al. [33] studied the wear resistance of Stellite 6 coatings deposited by LC and SLD, the results showed that the properties of the SLD layers of Stellite 6 were superior to those of their conventionally manufactured counterparts. Although SLD technique has been proposed for depositing hard materials, the SLD process has not been well studied, in particular, the performance and characteristics of the coatings of hard and low melting point Ni-based alloys. Since SLD can reduce particle deposition temperature and critical deposition velocity of hard particles, Ni60 particles can be deposited at a lower jet velocity. Meanwhile, the low temperature and high velocity deposition features of SLD may effectively suppress coarse microstructure, burning loss and oxidation of Ni60 particles and improve the performance of Ni60 coating. In the present research, an attempt was made to deposit Ni60 alloy powder onto a medium carbon steel substrates using SLD and LC, respectively; the obtained coatings were studies in microstructure, hardness, tribological property and corrosion resistance.

a

b

18

c

16

2.1. Coating materials Commercially available Ni60 alloy powder was used to create the SLD and LC Ni60 coating specimens. The chemical compositions of Ni-based 60 alloy are given in Table 1. The particles’ shape and size distribution of the Ni60 alloy powder are shown in Fig. 1, the powder has spherical morphology and its mean size is 18.87 lm. The substrate is medium carbon steel (AISI 1045 steel). The surfaces of substrate specimens were prepared by blasting using 24 Mesh Al2O3, then ultrasonic cleaning in alcohol medium.

Volume fraction (%)

2. Materials and experimental procedures

Mean size: 18.87 μm

14 12 10 8 6 4 2 0 0

2.2. SLD coating fabrication

5

10

15

20

25

30

35

40

45

50

55

60

Particle size (μm) The schematic diagram of the SLD system used in this research is shown in Fig. 2. In the cold spray system, laser irradiation is added in the deposited zone to soften both substrate and powder particles. In the cold spray process, high pressure (10–35 bar) nitrogen gas supply is split and delivered to a converging nozzle directly and also via a spiral high pressure powder feeder where the metal powder particles are entrained. The feedstock stream and carrier gas mix and pass through the nozzle where they are

Table 1 Chemical compositions of Ni60 alloy. Elements

C

Cr

Si

B

Fe

Ni

wt.%

0.1

18.5

4.5

3.0

14.5

Bal.

Fig. 1. Ni60 powder: (a) SEM morphology, (b) cross-section microstructure, and (c) size distribution.

accelerated to supersonic speeds. The high-speed flying particles impact a region of the substrate which is synchronously irradiated by the continuous wave diode laser (LDF-4.000, Laserline) with a maximum power of 4 kW. The sprayed zone and laser spot should well match each other. The temperature of the SLD zone is monitored by a pyrometer (LASON_PC_Local). Because of the softened substrate and powder particles by means of laser heating, the Ni60 coating can be easier formed at a much reduced impact velocity. In this experiment, nitrogen was used to accelerate the Ni60 powder. The optimized process parameters for the single-track coating and multiple-track coating are reported in Table 2.

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J. Yao et al. / Materials & Design 83 (2015) 26–35 Table 3 Optimal LC process parameters for Ni60 coating. Feeding rate (g/min)

Scanning velocity (mm/s)

Laser power (kW)

Laser spot diameter (mm)

15

10

1.3

4

made of 98 wt.% H2SO4 with deionized water. The polarization curves were obtained at a scanning rate of 0.001 V/s, starting from 1.2 V to zero. 3. Results and discussions 3.1. Microstructure

Fig. 2. Schematic diagram of the SLD system.

2.3. LC coating fabrication The LC system basically has a powder/laser beam, with which the coating is formed by means of melting the alloy powder. The powder is introduced into the beam with a coaxial powder-feed system. The carrier gas of powder-feed and the shielding gas of molten pool are nitrogen and argon, respectively. The laser used in this experiment was the continuous wave diode laser (LDF-2.000, Laserline) with a maximum power of 2 kW. The optimal process parameters for fabricating the coating are given in Table 3. 2.4. Coating characterization and performance tests In order to investigate the microstructures of Ni60 coatings, the specimens were etched by the corrosive solution of 1 mL FeCl3: 12 mL HCl: 4 mL HNO3. The effects of different laser energy inputs on the microstructure, phase composition, dilution and properties of Ni60 coatings produced with the optimal process parameters were studied using scanning electron microscopy (SEM, RIGMA HV-01-043, Carl Zeiss), energy dispersive spectrum (EDS, Nano Xflash Detector 5010, Bruker) and X-ray diffraction (XRD, D8 Advance, Bruker). The hardness of the coatings was measured on micro level from the coating surface to the substrate at a constant interval of 0.06 mm using the Digital Micro-Hardmeter (HMV-2T, SHIMADZU). The effects of laser energy on processing the coating were investigated and discussed with respect to wear and corrosion resistance. Pin-on-disk wear test was conducted on the SLD and LC Ni60 coatings at room temperature under dry sliding condition. The pin was a Si3N4 ceramic ball with 4 mm diameter. The disk was the coating specimen which was polished, cleaned in an ultrasonic bath, and finally dried. The test was performed under a normal load of 500 g at a rotational speed of 500 rpm of the specimen. The abrasion loss was measured by a three-dimensional microscopic system super depth of field (VHX-5000). Electrochemical test was conducted on the coating specimens using an electrochemical system (CHI660E) in 1 mol/L H2SO4 solution, which was

The SEM microstructures of the Ni60 coating specimens produced with SLD and LC are presented in Fig. 3. Since the SLD process did not lead to the formation of molten pool, the coating microstructure exhibits the characteristic of solid-state deposition structure, see Fig. 3a. On the contrary, the LC process caused the Ni60 powder molten and this resulted in composition dilution, see Fig. 3b. It is also observed that the bonding interface and the heat affected zone (HAZ) of the SLD Ni60 coating are smoother and smaller than that of LC coating. The composition dilution will significantly affect the microstructure and properties of the coating. From Fig. 3c the microstructure of the SLD Ni60 coating shows accumulated plastic deformation of the Ni60 particles with the microstructure unchanged, while in Fig. 3d the microstructure of LC coating exhibits a typical cladding dendritic structure. In addition, some pores are observed in the SLD microstructure at high magnification, which indicates that the structure of the SLD specimen is not as dense as that of the LC specimen. The SEM/BSE images at high magnification of the microstructures near the top surfaces of the SLD and LC coatings in cross section are shown in Fig. 3e and f, respectively. It is obvious that the microstructure of the SLD coating is finer than that of the LC one. EDS analysis was performed on the two distinct phases in the microstructures of the two coatings, as labeled in Fig. 3e and f; the results are reported in Table 4. It is shown that areas 1 and 3 contain higher C, Cr, Si contents and lower Fe, Ni contents. Based on the observation of previous research [1,34,35], Cr-rich carbides/borides and Ni-rich silicide may be present in these areas. Therefore it can be inferred that areas 1 and 3 are the eutectic phase. Areas 2 and 4 contain high Ni so that they are c-Ni solid solution. These observations indicate that the SLD Ni60 coating has similar microstructure to the LC Ni60 coating in phase, but the SLD coating has finer as-cast structure ascribing to high cooling rate during gas atomization process of Ni60 powder. 3.2. Phases XRD analysis was conducted on Ni60 powder, the SLD coating specimen and the LC coating specimen, in order to further identify the phases in the microstructures of the Ni60 coatings; the XRD spectra are presented in Fig. 4. According to the microstructure, EDS analysis results, and the XRD pattern in Fig. 4, c-Ni, FeNin, CrmCn, Cr3B and Ni3Si2 are the main phases in the SLD coating, and the CrmCn and Cr3B are the main strengthening phases of

Table 2 Optimal SLD process parameters for Ni60 coating. N2 pressure (bar)

N2 temperature (°C)

Spraying distance (mm)

Traverse rate (mm/s)

Deposited temperature (°C)

Laser spot diameter (mm)

Laser power (kW)

30

550

30

30

980

5

1.2

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J. Yao et al. / Materials & Design 83 (2015) 26–35

a

b Coating

Coating HAZ

HAZ Substrate

Substrate

c

d

pores

typical cladding dendrite

original particle structure

f

e

3 2

1 4

3

Fig. 3. SEM images of cross-section of Ni60 coatings: (a) morphology of the SLD coating, (b) morphology of the LC coating, (c) microstructure of the SLD coating at low magnification, (d) microstructure of the LC coating at low magnification, (e) BSE image of microstructure of the SLD coating at high magnification, and (f) BSE image of microstructure of the LC coating at high magnification.

Table 4 Elemental concentrations (wt.%) of the selected areas in the SLD and LC Ni60 coatings from EDS analysis.

Area Area Area Area

1 2 3 4

C

Si

Cr

Fe

Ni

4.34 1.20 18.78 12.92

6.15 4.56 2.26 1.04

23.10 14.51 15.14 7.87

12.11 19.31 48.05 52.56

54.31 60.43 15.77 25.61

Ni-based self-fluxing alloy. However, the phases in the XRD pattern of the LC coating differ from those of the SLD coating in that the LC process generated a new phase Fe5C2, due to dilution where excessive carbon and ferrum contents diffused into the Ni60 coating from the substrate. Based on the EDS and XRD analysis results reported in Table 4 and Fig. 4, areas 1 and 3 contain higher C, Cr, Si contents and lower Fe, Ni contents comparing with areas 2 and 4, as marked in Fig. 3e and f, therefore, in the SLD coating area 1 mainly consists of CrmCn, Cr3B and Ni3Si2, area 2 contains c-Ni and FeNi solid solution; while in the LC coating areas 3 is composed of Cr5Si3, Fe5C2 and CrmCn, and area 4 contains c-Ni and FeNi solid solution. Fig. 4 also shows that the phases in the SLD coating are similar to those of the Ni60 powder. This confirms that

the SLD Ni60 coating has retained the original particles’ structure and solid-state deposition characteristics. 3.3. Dilution behavior According to the microstructural analyses, the microstructure and phases of the SLD-Ni60 coating are distinct from the LC one, which may be caused by the dilution of the base material. The line scanning of Ni, Cr, Si and Fe elements from the coating surface to the substrate was performed with EDS in the cross section of the SLD and LC coating specimens; the results of elemental concentration are illustrated in Fig. 5. It is shown that Ni, Cr, and Si contents in the SLD coating are obviously higher than those in the LC one, while the Fe content of the LC coating is obviously higher than that of the SLD one, indicating that Fe dilution occurred more in the former than the latter. The noticeable difference in dilution behavior between the two coatings can be found at the coating/substrate interfaces, comparing the element fraction distribution curves in Fig. 5. For the SLD coating there is no transient region of element distribution, that is, the contents of Ni, Cr, Si and Fe elements drop down and rise up straightly from the coating layer to the substrate, whereas there is an obvious transient region at the interface where the contents of Ni, Cr, Si and Fe elements gradually change from

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J. Yao et al. / Materials & Design 83 (2015) 26–35

30

40

50

60

70

80

90

100

-Ni FeNin Cr mC n

Ni 5 Si 3

LC

Fe 5 C 2

Intensity (a.u.)

-Ni FeNi n

CrmCn Cr3B

SLD

Ni3Si2

γ-Ni

FeNin

CrmCn Cr3B

Powder

30

40

Ni3Si2

50

60

70

80

90

100

2θ (Degree) Fig. 4. XRD patterns of Ni60 powder and Ni60 coatings.

the coating layer to the substrate of the LC coating, due to convection and stirring of metal elements in the molten pool, which agrees with the research results of Luo et al. [32]. The studies of Hemmati et al. [36] found that dilution from the steel substrate suppressed the precipitation of the Ni–Si–B eutectics and Cr borides, further explaining the role of dilution in evolution of the microstructure and properties of Ni–Cr–B–Si coatings. 3.4. Hardness of coatings The hardness variation curves for the Ni60 coatings obtained under an indentation load of 2.942 N (HV0.3) are presented in Fig. 6. It can be seen that the average hardness of the SLD coating is 867 ± 24 HV0.3 and that of the LC one is 625 ± 55 HV0.3. The hardness of the SLD coating exhibits less variation within the coating layer than the LC one, but it decreases remarkably across the coating/substrate interface in the heat-affected zone (HAZ) of the substrate. As for the LC coating hardness shows a moderate decreasing trend, which is defined as gradient character, owing to generating compositional variation by laser stirring. For both the coatings hardness values of the bonding interface decrease gradually from the higher values to the hardness value of the substrate material (steel), but the LC coating has larger HAZ than the SLD one. According to the studies of Hemmati et al., the higher hardness of the SLD coating is attributed to the presence of more hard CrB ceramic and CrmCn carbides, which distribute within the c-Ni solid solution [36]. 3.5. Wear resistance The wear test duration was 60 min and the diameter of wear track was 5 mm. To evaluate the abrasion loss, a

three-dimensional microscopic system super depth of field was used to measure the cross-section profile of the wear track and scan the cross profile of the wear track, as shown in Fig. 7. The volume loss was estimated by calculating the volume of the wear track (worn scratch), which is the area of the cross profile multiplied by the periphery of the wear track. Two locations along each wear were measured to obtain the cross-sectional area of the wear track and the average value was taken to calculate the volume of the wear track. The wear loss results of the two coatings are reported in Table 5. It is clear that the SLD coating has better wear resistance than the LC one. The evolutions of friction coefficient of the specimens were recorded during the wear tests, thus the variations of friction coefficient with time could be obtained. The results of friction coefficient for the coating specimens are plotted in Fig. 8. As illustrated, the friction coefficient of the SLD specimen is much smaller and more stable than that of the LC specimen. The average coefficient of the SLD specimen was about 0.68 after 60 min sliding, however, the average coefficient of the LC specimen was about 0.82 at the same time. To help understand the wear test results, the worn surfaces of the coating specimens were examined using SEM, and the images are shown in Fig. 9. Deep plough scars can be observed obviously in the wear track of the LC specimen, while the SLD specimen looks smoother. Also; the width of the wear track of the LC specimen is wider than that of the SLD one. In both the worn surfaces, there are two distinct areas in dark and light, respectively, but the dark area in the LC coating is more than in the SLD coating. EDS analysis was performed on the dark and light areas, the results are presented in Fig. 10. It is shown that the dark areas (2 and 4) contain Ni, Cr, Fe, Si, C and a high content of O, in contrast, no O is detected in the light areas (1 and 3). This implies that oxidation had occurred in the dark areas during the wear tests. More dark areas in the worn surface of the SLD coating indicate more friction heat generated in its surface during wear, which is consistent with the friction coefficient results in Fig. 8 that the SLD specimen has lower friction coefficient, that is, less friction thus less friction heat. 3.6. Polarization behavior The conventional three-electrode cell was utilized, with a C(K2SO4)-1 electrode, a platinum electrode and the coating sample as the reference, counter and working electrodes, respectively. During the electrochemical corrosion test, the platinum electrode produced plenty of bubbles, and the solution was gradually turning from colorless to yellow, which can be explained as follows. The anodic dissolution took place, expressed by Eqs. (1) and (2), which was balanced by hydrogen evolution of platinum electrode, represented by Eq. (3).

Fe-2e ! Fe2þ

ð1Þ

Cr-2e ! Cr2þ

ð2Þ

2H2 O þ 2e ! H2 þ 2OH

ð3Þ

Therefore the platinum electrode generated the bubbles of H2; the 1 mol/L H2SO4 solution can oxidize Fe2+ to Fe3+, resulting in the solution color turning from colorless to yellow. The obtained polarization curves of the SLD Ni60 and LC Ni60 coatings are presented in Fig. 11. There are some differences in corrosion kinetics between the two coatings. The anodic branch of the SLD specimen is characterized by a larger slope and smaller passivation zone. But the SLD passivation film is easier to be repaired seeing from Fig. 11 comparing to the LC passivation zone. Both the Tafel curves demonstrate that the SLD coating exhibits a lower

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J. Yao et al. / Materials & Design 83 (2015) 26–35

100

a

90

HAZ

Coating

80

Element fraction (wt.%)

Coating

Substrate

70 60 50 40

SLD-Ni60 Ni Cr Fe Si

30 20 10

Substrat

0

0

10

20

30

40

50

60

70

80

Distance (µm) 100

b

90

Element fraction (wt.%)

Coating

HAZ

Coating

Substrate

80 70 60 50 40

LC-Ni60 Ni Cr Fe Si

30 20 10 0

0

10

20

30

40

50

60

70

80

Distance (µm) Fig. 5. Dilution analyses of elements from the coating layer to the substrate using EDS: (a) SLD coating, and (b) LC coating.

900

Micorhardness (HV0.3)

density p of laser is determined by output power of laser P and the diameter of laser spot d,

BZ

1000 LC-Ni60 coating SLD-Ni60 coating

800

P¼

4P

ð4Þ

pd2

While the acting time t is determined by d and scanning speed

700

v,

600 500

t¼

Coating

Substrate

d

ð5Þ

v

400

If the energy that laser beam inputs into the coating for an area,

300

pd2 , when scanning across, can be defined as energy inputting rate, e, then

200

HAZ HAZ

100 -0.48

-0.36

-0.24

-0.12

0.00

0.12

0.24

0.36

Distance (mm) Fig. 6. Microhardness of Ni60 coating specimens.

corrosion current (2.049 ± 0.04E05) and higher corrosion potential (0.78 ± 0.016) than the LC one. The polarization resistance (Rp) of the SLD coating is slightly higher than that of the LC one, as reported in Table 6. 3.7. Discussion Laser irradiation in SLD process provides heat which can synchronously soften the high-speed particles and substrate, while the heat in LC process melts the particles. In LC process, the energy

e ¼ p:t ¼

4P

pdv

ð6Þ

This formula can also be applied to calculate energy inputting rate (e) of SLD and LC. Using the parameters in Tables 2 and 4, they are 10.2 and 41.4, respectively. The energy inputting rate (e) of SLD process is lower than that of LC process. Since SLD process has less laser energy input than LC one, the coating/substrate interface and HAZ of the SLD specimen are smoother and smaller; the coating microstructure exhibits similar fine as-cast structure to original Ni60 particles. Differently, the LC coating has coarser dendritic structure. The Ni60 powder used in this research was commercially produced with high pressure gas atomization. During the powder production process higher gas pressure can rapidly solidify the melting alloy to form finer particles. As a result of excessive cooling rate, spherical particles were obtained during solidification. The Ni60 particles had solidified before the primary crystals of the molten alloy started to grow into big dendrites. In LC process the laser

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J. Yao et al. / Materials & Design 83 (2015) 26–35

Fig. 7. Cross-sectional profile of wear track: (a) SLD coating, and (b) LC coating.

beam can generate molten pool and larger temperature gradient from the coating/substrate interface to the top surface, thus planar, cellular, dendritic and equiaxed grains may be formed [37].

From Fig. 4, the XRD spectra of the SLD Ni60 coating and Ni60 powder are similar, but additional phases were induced in the LC coating due to dilution of the base material in the LC process.

33

J. Yao et al. / Materials & Design 83 (2015) 26–35 Table 5 Wear test results of the SLD and LC Ni60 coatings. Volume loss (mm3)

Maximum depth of wear track (lm)

0.46 ± 0.06 0.98 ± 0.08

21.3 ± 3.1 30.6 ± 2.8

1.2

SLD-Ni60 coating LC-Ni60 coating

Friction coefficient

1.0

60

Area 1 Area 2

50

Content (wt.%)

SLD LC

a

40

30

20

10 0.8 0 C

0.6

Si

Cr

Fe

Ni

Fe

Ni

Elements

b

0.4

60

Area 3 Area 4

50 0

10

20

30

40

50

60

Duration times (min) Fig. 8. Variations of friction coefficient with sliding time.

a

Content (wt.%)

0.2

O

40

30

20

10

1 0 C

O

Si

Cr

Elements Fig. 10. Element concentrations of the dark and light areas in wear track from EDS analysis: (a) SLD coating, and (b) LC coating.

2 -0.5

LC coating SLD coating

-0.6

E (V vs SCE)

b

-0.7

-0.8

4 3

-0.9

-1.0 -8

-7

-6

-5

-4

-3

-2

log(I) (A/cm2 ) Fig. 11. Polarization curves of Ni60 coatings in 1 mol/L H2SO4 solution.

Fig. 9. SEM morphologies of worn surfaces: (a) wear track of SLD coating, and (b) wear track of LC coating.

From Fig. 5, Fe content varies significantly from the coating layer to the substrate. The excessive C and Fe diffused into the Ni60 coating from the steel substrate, leading to higher Fe content and lower Ni,

Cr, Si contents in the SLD coating, these results are in agreement with the EDS analysis, as shown in Table 4. The EDS analysis results also indicate that the C and Fe contents of the SLD coating are lower; but the Si, Cr, Ni contents of the LC coating are lower, which confirms the occurrence of dilution in the LC process. The higher C content also enhanced the formation of Fe carbides in the LC coating. However, no Cr borides detected in the LC coating can be

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J. Yao et al. / Materials & Design 83 (2015) 26–35

Table 6 Summary of polarization test results of the SLD and LC Ni60 coatings in 1 mol/L H2SO4 solution.

SLD LC

Ecorr (V vs SCE)

Icorr (lA/cm2)

Rp (O cm2)

0.78 ± 0.016 0.845 ± 0.006

2.049 ± 0.04E05 3.169 ± 0.05E05

1115 ± 22.1 1064 ± 16.5

ascribe to the lower phase concentration owing to additional iron (Fe) dilution from the steel substrate. Hemmati et al. [36] also reported that Fe content over 40 wt.% entirely suppressed the precipitation of primary Cr borides. According to the previous research results [1,34–38], areas 1 and 2 of the SLD coating, labeled in Fig. 3e, are CrmCn/Cr3B/Ni3Si2/FeNin eutectics and c-Ni/FeNin solid solution, respectively; while areas 3 and 4 of the LC coating, labeled in Fig. 3f, are CrmCn/Cr5Si3/Fe5C2/FeNin eutectics and c-Ni/FeNin solid solution, respectively. The micro-hardness test results show that the dilution occurring in the LC process seriously reduced the hardness of the coating comparing with the SLD coating, because the dilution of steel base material can lead to the alteration of alloy element contents in the coating and thus cut down the concentration of hard phases such as Cr borides and carbides in the coating. According to the research of Hemmati et al., high Fe content changed and suppressed the precipitation of Cr borides at the beginning and the formation of Ni–Si–B eutectics at the end of solidification [36]. Reduction of the strengthening components was accompanied by a decrease in hardness from 867 ± 24 HV to 625 ± 55 HV. All of these indicate that controlling laser energy input and dilution in the deposition process are crucial and significant for improving the quality of coatings. From the sliding wear test, the SLD-Ni60 coating has superior wear resistance to the LC one. The wear scars on the SLD coating surface are much shallower, compared with the deep plough on the LC coating surface. Also the worn surface of the LC coating exhibits more severe oxidation than the SLD one. These observations conform with the reports of Luo et al. [32]. Since the oxidation occurrence in sliding wear is mainly caused by friction heat, more oxidation of the LC coating surface means more severe friction of its surface against the counterpart (pin). From the microstructures of the two coatings in Fig. 3, the SLD one has finer structures of the phases, that is, the carbides and borides are more homogeneously distributed in the c-Ni matrix. As concerns the electrochemical test results (Table 6), the SLD coating has a higher corrosion potential and polarization resistance (Rp), but lower corrosion current density compared with the LC one. The LC coating has a larger passivation region (Fig. 11). The Fe dilution of the steel substrate in the LC process resulted in higher Fe and C concentrations and lower Ni and Cr concentrations in the coating, which can affect the corrosion resistance of the coating. In addition, the previous research discovered that higher B content can strengthen the grain boundary and restrain nonhomogeneous composition and porosity of coatings [39]. He et al. reported [40] that the anticorrosion performance of Ni-based coating containing 2–4% B was better than those without B, which indicates that coatings containing moderate B content have improved anticorrosion resistance. More Fe dilution from the steel substrate in the LC coating resulted in decrease of B concentration, further causing the worse corrosion resistance. These results demonstrate that the different corrosion resistance both two coatings attributes to the element concentration alteration in the coating. 4. Conclusions Hard Ni60 alloy coating on medium carbon steel substrate were achieved by SLD process or LC process with optimal process

parameters. Since the energy inputting rate of the SLD process was lower than that of the LC process, the Ni60 coating with SLD exhibited special characteristics, such as unchanged fine powder microstructure, unchanged phases and low dilution rate. With these features the SLD Ni60 coating showed higher micro-hardness to that of the LC one. In 1 mol/L H2SO4 solution, the SLD and LC coatings performed similarly in corrosion resistance. Supersonic laser deposition (SLD) technique allowed fabricating hard Ni60 powder coating on medium carbon steel substrate, which exhibited good sliding wear resistance. This novel deposition technique surpasses conventional laser cladding (LC) technique when used to deposit hard materials such as Ni60 alloy, in that it can suppress the dilution of the steel substrate.

Acknowledgements The authors would like to appreciate the financial supports from the National Natural Science Foundation of China (51475429), and the Commonweal Technology Research Industrial Project of Zhejiang Province (2013C31012). The authors also would like to specially appreciate Prof. Rong Liu from Carleton University for her kind help in article modification.

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