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Microstructure and wear resistance of molybdenum based amorphous nanocrystalline alloy coating fabricated by atmospheric plasma spraying
Surface & Coatings Technology 228 (2013) S266–S270
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Microstructure and wear resistance of molybdenum based amorphous nanocrystalline alloy coating fabricated by atmospheric plasma spraying X.B. Zhao a, b,⁎, Z.H. Ye a a b
School of Materials Science and Engineering, Changzhou University, Changzhou 213164, PR China Key Laboratory of Advanced Metallic Materials of Changzhou City, Changzhou University, Changzhou, 213164, PR China
a r t i c l e
i n f o
Available online 7 June 2012 Keywords: Plasma spraying Molybdenum based coating Amorphous Nanocrystalline Wear resistance
a b s t r a c t Molybdenum (Mo) based amorphous nanocrystalline coating was deposited on the surface of plain carbon steel substrates by atmospheric plasma spraying (APS). X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the microstructures of Mo-based powder and coating. The surface hardness of substrate and coating were determined using a Vickers hardness tester, and microhardness profile of cross-section was realized by a Vickers microhardness tester. Wear tests were carried out using a pin on plate type tester by changing the load and linear speed. The results indicated that the as-prepared Mobased coating was mainly composed of Fe0.54Mo0.73, Fe3Mo, Cr9Mo21Ni20, Fe3B and Cr0.46Mo.4Si0.14 phases. The average grain size of Mo-based coating was 22–32 nm. The coating presented a dense layered structure and its thickness was about 250 μm. The surface hardness of Mo-based coating was as high as 9.2 GPa, which was 8 times of substrate. The microhardness along the cross-section gradually decreased from coating to substrate. The wear resistance of Mo-based coating was far better than that of substrate. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Metallic glass alloys have been of great interest not only for fundamental studies, but also for potential applications [1,2]. However, the applications of these amorphous alloys have been restricted due to the difficulties encountered in the production, such as hard to control in preparation process and inefficient shaping caused by slow cooling rate [3]. Owing to the sufficiently rapid rate of cooling, thermal spraying is one of the technologies to deposit amorphous coatings. Thermal spraying is an economical and versatile fabrication process to produce large surface coatings with unlimited types of materials [4]. In recent years, many thermal spraying methods have been applied to prepare amorphous metallic coatings, such as highvelocity oxygen fuel (HVOF) spraying [5–8] and plasma spraying [9–12]. Among them, atmospheric plasma spraying (APS) has attracted increasing attention due to the advantages of low cost, simplicity, and flexibility. Though the high cooling rate of ~ 10 6 K/s exists in the APS process, it is very hard to obtain fully amorphous coatings because of crystallization or oxidation [13]. Therefore, it will be better to prepare the mixed coatings of amorphous and nanocrystalline structures by APS, which exploit amorphous to enhance corrosion resistance and nanocrystalline to enhance wear resistance [14]. In this work, the Mo-based amorphous nanocrystalline coating was prepared by APS using Mo-based powder as feedstock. The ⁎ Corresponding author at: School of Materials Science and Engineering, Changzhou University, Changzhou 213164, PR China. E-mail address: [email protected] (XB. Zhao). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.05.127
microstructure, hardness and wear resistance of Mo-based coating were also investigated. 2. Material and methods 2.1. Material Mo-based metallic alloy powder (Beijing SunSpraying Technology Co., Ltd., China) was used as the feedstock material. The Mo-based powder was prepared by gas atomization in the argon atmosphere after the master alloy melted by a medium frequency vacuum induction furnace and passed through 160 mesh screen. The compositions of Mo-based powder are displayed in Table 1. 2.2. Spraying process The coatings were deposited onto plain carbon steel substrates by an atmospheric plasma spraying system, including a Sulzer Metco 9MB plasma gun mounted on an ABB robot. Prior to plasma spraying, all the substrates were sandblasted with brown corundum. The spraying parameters are listed in Table 2. Table 1 Chemical compositions of the Mo-based alloy powder (wt%). Cr
Si
B
Fe
Ni
Mo
6–7.5%
1–2%
1–2%
2–3.5%
26–28%
Balance
X.B. Zhao, ZH. Ye / Surface & Coatings Technology 228 (2013) S266–S270
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3. Results and discussion
Table 2 Processing parameters of atmospheric plasma spraying. Nozzle type Nozzle I.D. (mm) Powder Injector (°)
GE 5.5 90
3.1. Characterization of Mo-based amorphous nanocrystalline powder and coating
Power (kW) Argon gas (NLPM)a Hydrogen gas (NLPM) Powder feed rate (g/min) Spray distance (mm) Cooling Air (MPa) Gun vertical speed (mm/s)
35 60 6 30 100 0.5 10
Fig. 1 presents XRD patterns of Mo-based amorphous powder and the as-prepared coating. The powder is mainly composed of four phases: Fe3Mo, Fe0.54Mo0.73, Cr9Mo21Ni20 and Fe3B. Weak broad diffraction peaks are observed in the range of 40–45°, which indicate that there are partly amorphous or nanocrystalline particles in Mobased powder. The phase composition of the as-prepared Mo-based coating is mainly composed of Fe0.54Mo0.73, Fe3Mo, Cr9Mo21Ni20, Fe3B and Cr0.46Mo.4Si0.14; the differences are that a new phase of Cr0.46Mo.4Si0.14 is formed at 2θ of 40° and main crystal peak changes from Fe3Mo to Fe0.54Mo0.73. In addition, a clear broad halo peak is also displayed in the as-prepared coating, which is the same range as the powder. The amorphous phase was caused by molten particles cooling rapidly (cooling rate reach ~10 6 K/s). Furthermore, changes of phase compositions in the coating might be the result of recrystallization from amorphous state to crystal state or the molten particles deforming, becoming lamellae and solidifying into finegrained crystals in the plasma spraying process [19]. The average crystalline sizes of the coating were calculated by Scherrer formula (Scherrer constant was 0.9 and wavelength of Xray was 1.54056 Å in this experiment). The average crystalline sizes of the coating calculated from the XRD peaks are listed in Table 4. It can be seen that the average grain size of the Mo-based coating is 22–32 nm, which theoretically proved that the crystals in the coating are nanostructure. The particle size distribution curves of Mo-based powder are shown in Fig. 2. It can be seen that the median diameter (D50) of Mo-based powders is 29.51 μm; the particle size distribution is wide and the highest frequency distribution is 4.48% in the range of 40.15 μm–44.69 μm. The percentage content of particle size less than 10 μm is about 18.1%, while large than 100 μm is about 5.23%. These small particles and big particles are more propitious to deposit high dense coatings. The SEM image of the powder visually presents the particle size distribution (as shown in Fig. 3). Spraying experiment indicated that the Mo-based powder had good fluidity, which was suitable for plasma spraying. The surface and cross sectional morphologies of the as-sprayed Mo-based coating are displayed in Fig. 4. The as-sprayed Mo-based coating presents a typical surface morphology of plasma spraying, which has some holes, lamellae, partially melted and unmelted particles, as shown in Fig. 4(a). During plasma spraying, molten particles impacted on the surface of substrate, then deformed, solidified and transformed into lamellae. Then lamellae piled up one upon another to build up the coating. Expanding gases entrapped in the closed pores between lamellae might evacuate out to leave holes on the coating surface [19]. In addition, the melting degree of particles was uneven during the spraying process, which also affected the coating structure. Particles might include one or all of the following states on impact: fully molten, superheated, semi-molten and molten then resolidified. Fully melted particles at or just above the material melting point arrived at the substrate, impacted, flowed, and flattened; superheated particles may splatter on impact, sending out radial splashes of fine droplets that ended up in the coating as “satellites” or debris; particles that resolidify in flight after melted appeared as unmelted particles [20]. From the cross sectional morphology of the coating (Fig. 4(b)), it can be seen that the thickness of the coating is about 250 μm. The coating has a dense structure, although some pores and microcracks exist as very dark contrast regions. Generally, the big pores arising between lamellae were mainly caused by gas porosity phenomenon, while the small pores within the flattened particles were resulted from shrinkage porosity [21]. When the spraying gun moved over
a
NLPM stands for normal liters per minute.
2.3. Characterization of powder and coating The phase characterization of the powder and coating were conducted by X-ray diffraction (XRD, D/max 2500PC, Rigaku, Japan) with Cu Kα radiation (λ = 1.5418 Å) in the range of 20–90° (2θ). The average grain sizes of nanocrystalline particles were evaluated by the Scherrer formula [15]. The morphologies of the powder and as-prepared coatings were examined by scanning electron microscopy (SEM, JSM-6360LA, JEOL, Japan). The particle size distribution of the powder was measured by laser particle size analyzer (BT-9300s, Bettersize, China). 2.4. Hardness and wear tests Before hardness and wear tests, the specimens were ground and polished to reach the surface roughness of ~ 0.1 μm [16] (for hardness) and ~0.2 μm [17] (for wear). The surface hardness values of substrate and coating were determined using a Vickers hardness tester (HVS-5Z/LCD, Shanghai Taiming Co., China) with a load of 2.942 N for 10 s. Microhardness profile was realized on cross-section of coating and substrate by a Vickers microhardness tester (HXD-1000TMC, Shanghai Taiming Co., China) incorporated with microhardness measurement system and evaluated by a load of 0.9807 N for 15 s. The indentations were performed in a row at separations of 60 μm (for coating) and 250 μm (for substrate), which ensured that the separations of any two indentations were at least three times longer than the diagonal of the biggest indentation [16]. The indentation series were produced separately at distances of approximately 40 μm, 80 μm, 120 μm and 160 μm from the coating-substrate interface to both sides [18]. Wear test was carried out using a pin on plate type tester (MMW1A, Jinan Yihua Co., China). The rotating pin material of the friction pair was 45# steel with Φ 4.8 mm × 12.7 mm and its hardness was 44–46 HRC. The test was performed by changing the load and linear speed to obtain the friction coefficient and mass loss. The parameters of wear test are listed in Table 3. All wear tests were conducted under dry sliding condition and exposed to the air without cooling device. Before and post wear tests, specimens were ultrasonically cleansed with alcohol and dried in an oven at 80 °C. The mass of the substrates and coatings were weighed by an electronic precision balance (a minimum reading of 0.1 mg) to calculate the mass loss.
Table 3 Wear test parameters: A denotes changing the load and B for speed. Details
A
B
Load (N) Linear speed (m/s) Rotation speed (rpm) Wear time (min) Wear track diameter (mm, CONST) Wear distance (m, CONST)
50, 60, 70 0.5 400 36 23.86 1080
50 0.5, 1, 1.5 400, 800, 1200 36, 18, 12
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Fe0.54Mo0.73
Coating
Fe3Mo Cr9Mo21Ni20 Fe3B Cr0.46Mo0.4Si0.14
Intensity (a.u.)
SiO2
Powder
20
30
40
50
60
70
80
90
2θ (degree) Fig. 1. The XRD patterns of the Mo-based amorphous powder and coating.
Table 4 The average grain sizes of the Mo-based coating. 2θ (°)
FWHM
Grain size (nm)
40.079 40.800 43.180 44.419
0.283 0.332 0.404 0.336
31.2 26.7 22.1 26.8
the substrate, the surfaces of the lamellae were subjected to the action of the environment, i.e. cooling and/or oxidation. The cooling led to generation of residual stresses, which might result in the microcracks of coatings [19]. Despite the present of these defects, the as-prepared Mo-based coating express a low porosity (approximately 3.8%) measured by the image analysis program of Image-Pro Plus 6.0. Moreover, it can also be seen from the cross sectional
morphology that the bonding region of the coating and the substrate is flawless (Fig. 4(b)). Fig. 4(c) is the high magnification backscattered electron (BSE) image of the coating; it can be observed that some heterogeneous phases appear in the coating. Table 5 shows the EDS analysis results: section A and B for the powder marked in Fig. 3, while C, D, E and F for the coating marked in Fig. 4 (a) and (c) respectively. It is seen from the results of EDS analysis that the mass fraction of oxygen of inside has a downward trend, after the powder deposited to the coating. During the spraying process, the elements of B and Si had the function of deoxygenation, which easily formed oxide then gradually floated to the surface because of low density [22]. So the mass fraction of oxygen of section C is higher than that of section D and E. Moreover, the XRD analysis indicates that the surface of the coating contains SiO2, as shown in Fig. 1, while the mass fraction of oxygen between section D and E is also different, which may caused by partially melted particles because of the composition like in Section B. However compared those of with section D and E, the mass fraction of oxygen of section F is higher. When the spraying gun moved over the substrate,
Fig. 2. Particle size distribution of the Mo-based powder.
X.B. Zhao, ZH. Ye / Surface & Coatings Technology 228 (2013) S266–S270
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Fig. 3. SEM images of the Mo-based powder: surface and cross-section.
the surfaces of the lamellae were subjected to oxidation in the environment of high temperature and air. Therefore, the coating had the phenomenon of deoxidization, though the borders of the lamellae had been oxidized.
3.2. Hardness and wear resistance of Mo-based amorphous nanocrystalline coating Fig. 5 shows the microhardness profile of Mo-based coating along the cross-section. As a whole, the hardness decrease continuously from 8.8 GPa to 1.34 GPa. The highest one is still inferior to the coating surface hardness of 9.2 GPa; the lowest one is superior to the surface hardness (1.26 GPa) of substrate. Diamond indenter pressed into cracks and forced the lamellae to separate, which resulted in the lower hardness. The same reason was less impact on the surface of the coating because of the flattened layers. For the substrate, both grit blasting and molten particles impacting on the substrate enhance the hardness of substrate, especially on the interface like the point at 40 μm (1.46 GPa), which is higher than that of other points in the substrate. Fig. 6 presents the friction coefficient of Mo-based coating and substrate in the case of changing the load and linear speed. It is found obviously that the friction coefficients of coating and substrate decrease with the increase of the load or the linear speed, and the friction coefficients of Mo-based coating are higher than that of substrate. When increased the load, the interface of the friction pair generated elastic–plastic deformation, which resulted in the practical contact area decreased, so the friction coefficient went down. Increase of the linear speed would cause the temperature of interface to rise sharply, which changed the surface properties of the friction pair and further influenced the friction coefficient [23]. Fig. 7 shows the mass loss of Mo-based coating and substrate after wear test in the same condition. It can be seen that the mass loss of substrate increases more sharply than that of the coating. The mass loss of the coating is around 10 mg, which is far lower than that of the substrate (about 90 mg). However, the mass loss of the substrate at the linear speed of 0.5 m/s is lower than that of 1.0 m/s, reason of which might be the instability of the wear tester in the final 300 m. According to the results of the friction coefficient and mass loss, the wear resistance of the as-prepared Mo-based coating is far superior to the substrate.
Fig. 4. SEM images of as-sprayed Mo-based coating: (a) surface, (b) cross-section (SE) and (c) high magnification cross-section (BSE).
Table 5 EDS results (mass%) of the Mo-based powder and coating.
Powder Coating
Section
O
Si
Cr
Fe
Ni
Mo
A B C D E F
3.75 2.68 3.75 2.59 0.66 3.34
0.87 0.90 0.96 0.86 0.83 0.70
4.19 4.89 6.74 4.26 4.22 4.92
6.08 5.99 12.9 6.49 6.25 6.39
23.61 23.78 25.81 24.11 25.33 21.61
61.50 61.76 50.14 61.70 62.71 63.04
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10
90 Coating Substrate
80 70
Mass Loss (mg)
Microhardness / GPa
8
6 Coating
Substrate
4
(a) V = 0.5m/s
(b) F = 50N
60 50 40 30 20
2
10 0
0 -200
50 -150
-100
-50
0
50
100
150
75
Load (N)
200
100
0.5
1.0
1.5
Liner speed (m/s)
Distance from interface / μm Fig. 7. Mass loss vs. load and linear speed of the coating and substrate. Fig. 5. Microhardness profile of Mo-based coating along the cross-section.
References
4. Conclusions Molybdenum based amorphous nanocrystalline alloy coating was fabricated by atmospheric plasma spraying using the Mo-based powder as feedstock. The detail results were summarized as follows:
[1] [2] [3] [4] [5]
(1) The coating was mainly composed of Fe0.54Mo0.73, Fe3Mo, Cr9Mo21Ni20, Fe3B and Cr0.46Mo.4Si0.14 phases. The average grain size of coating was 22–32 nm. (2) The coating with thickness of about 250 μm had a dense structure. The bonding region of the coating and substrate was flawless and interface line was clearly visible. The coating had the phenomenon of deoxidization, though the borders of the lamellae had been oxidized. And the oxygen content of the coating decreased with the increase of melting degree of particles. (3) The surface hardness of the coating was 9.2 GPa, which was 8 times higher than that of the substrate. The microhardness along the cross-section gradually decreased from coating to substrate. (4) The friction coefficients straightly decreased with the increase of load or the linear speed. Both friction coefficient and mass loss of the coating were far superior to the substrate. The Mobased coating exhibited excellent wear resistance under dry sliding condition than that of the plain carbon steel.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
0.75 Coating Substrate
Friction Coefficient
0.70
0.65
0.60
0.55
0.50
(a) V = 0.5m/s 0.45
50
75
Load (N)
(b) F = 50N 100
0.5
1.0
1.5
liner speed (m/s)
Fig. 6. Friction coefficient vs. load and linear speed of the coating and substrate.
[23]
C.A. Schuh, T.C. Hufnagel, U. Ramamurty, Acta Mater. 55 (2007) 4067. A. Inoue, B.L. Shen, C.T. Chang, Acta Mater. 52 (2004) 4093. F.E. Luborsky, Amorphous Metallic Alloys, Butterworths, London, 1983. X.B. Zhao, Z.G. Chen, X.Y. Liu, in: X.Y. Liu, P.K. Chu (Eds.), Surface Modification of Materials by Coatings and Films, Research Signpost, Trivandrum, 2009, p. 244. A.H. Dent, A.J. Horlock, D.G. McCartney, S.J. Harris, J. Therm. Spray Technol. 8 (1999) 399. B. Movahedi, M.H. Enayati, C.C. Wong, J. Therm. Spray Technol. 19 (2010) 1093. Z. Zhou, L. Wang, D.Y. He, F.C. Wang, Y.B. Liu, J. Therm. Spray Technol. 19 (2010) 1287. A.P. Wang, X.C. Chang, W.L. Hou, J.Q. Wang, Mater. Sci. Eng. A 449–451 (2007) 277. K. Kishitake, H. Era, F. Otsubo, J. Therm. Spray Technol. 5 (1996) 145. K. Kishitake, H. Era, F. Otsubo, J. Therm. Spray Technol. 5 (1996) 283. Z. Zhou, L. Wang, D.Y. He, F.C. Wang, Y.B. Liu, J. Therm. Spray Technol. 20 (2011) 344. F. Otsubo, K. Kishitake, Mater. Trans. 46 (2005) 80. A.P. Wang, T. Zhang, J.Q. Wang, Philos. Mag. Lett. 86 (2006) 5. Z.S. Fan, D.B. Sun, H.Y. Yu, H.Q. Li, H.M. Meng, X.D. Wang, Trans. Mater. Heat Treat. 26 (2005) 90. H.P. Klug, L.E. Alexander, X-Ray Diffraction Procedures for Polycrystalline and Amorphous Materials, John Wiley & Sons, Inc, London, 1954, p. 491. GB/T 4340.1-1999, Metallic materials-Vickers hardness test Part 1:Test method, , 1999. ASTM G99-04, Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus, , 2004. M. Factor, I. Roman, Surf. Coat.Technol. 132 (2000) 184. L. Pawlowski, The Science and Engineering of Thermal Spray Coatings, Second ed., John Wiley & Sons, Inc, London, 2008, pp. 134, 221–222. J.R. Davis, Handbook of Thermal Spray Technology, ASM International, 2004, pp. 122–123. V.V. Sobolev, J.M. Guilemany, Mater. Lett. 18 (1994) 304. H.J. Wang, Thermal spraying materials and application, National Defense Industry Press, Beijing, 2008, pp. 35–36. S.Z. Wen, P. Huang, Principles of Tribology, Second Ed., Tsinghua University Press, Beijing, 2002, pp. 284–286.
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Microstructure and wear resistance of molybdenum based amorphous nanocrystalline alloy coating fabricated by atmospheric plasma spraying X.B. Zhao a, b,⁎, Z.H. Ye a a b
School of Materials Science and Engineering, Changzhou University, Changzhou 213164, PR China Key Laboratory of Advanced Metallic Materials of Changzhou City, Changzhou University, Changzhou, 213164, PR China
a r t i c l e
i n f o
Available online 7 June 2012 Keywords: Plasma spraying Molybdenum based coating Amorphous Nanocrystalline Wear resistance
a b s t r a c t Molybdenum (Mo) based amorphous nanocrystalline coating was deposited on the surface of plain carbon steel substrates by atmospheric plasma spraying (APS). X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the microstructures of Mo-based powder and coating. The surface hardness of substrate and coating were determined using a Vickers hardness tester, and microhardness profile of cross-section was realized by a Vickers microhardness tester. Wear tests were carried out using a pin on plate type tester by changing the load and linear speed. The results indicated that the as-prepared Mobased coating was mainly composed of Fe0.54Mo0.73, Fe3Mo, Cr9Mo21Ni20, Fe3B and Cr0.46Mo.4Si0.14 phases. The average grain size of Mo-based coating was 22–32 nm. The coating presented a dense layered structure and its thickness was about 250 μm. The surface hardness of Mo-based coating was as high as 9.2 GPa, which was 8 times of substrate. The microhardness along the cross-section gradually decreased from coating to substrate. The wear resistance of Mo-based coating was far better than that of substrate. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Metallic glass alloys have been of great interest not only for fundamental studies, but also for potential applications [1,2]. However, the applications of these amorphous alloys have been restricted due to the difficulties encountered in the production, such as hard to control in preparation process and inefficient shaping caused by slow cooling rate [3]. Owing to the sufficiently rapid rate of cooling, thermal spraying is one of the technologies to deposit amorphous coatings. Thermal spraying is an economical and versatile fabrication process to produce large surface coatings with unlimited types of materials [4]. In recent years, many thermal spraying methods have been applied to prepare amorphous metallic coatings, such as highvelocity oxygen fuel (HVOF) spraying [5–8] and plasma spraying [9–12]. Among them, atmospheric plasma spraying (APS) has attracted increasing attention due to the advantages of low cost, simplicity, and flexibility. Though the high cooling rate of ~ 10 6 K/s exists in the APS process, it is very hard to obtain fully amorphous coatings because of crystallization or oxidation [13]. Therefore, it will be better to prepare the mixed coatings of amorphous and nanocrystalline structures by APS, which exploit amorphous to enhance corrosion resistance and nanocrystalline to enhance wear resistance [14]. In this work, the Mo-based amorphous nanocrystalline coating was prepared by APS using Mo-based powder as feedstock. The ⁎ Corresponding author at: School of Materials Science and Engineering, Changzhou University, Changzhou 213164, PR China. E-mail address: [email protected] (XB. Zhao). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.05.127
microstructure, hardness and wear resistance of Mo-based coating were also investigated. 2. Material and methods 2.1. Material Mo-based metallic alloy powder (Beijing SunSpraying Technology Co., Ltd., China) was used as the feedstock material. The Mo-based powder was prepared by gas atomization in the argon atmosphere after the master alloy melted by a medium frequency vacuum induction furnace and passed through 160 mesh screen. The compositions of Mo-based powder are displayed in Table 1. 2.2. Spraying process The coatings were deposited onto plain carbon steel substrates by an atmospheric plasma spraying system, including a Sulzer Metco 9MB plasma gun mounted on an ABB robot. Prior to plasma spraying, all the substrates were sandblasted with brown corundum. The spraying parameters are listed in Table 2. Table 1 Chemical compositions of the Mo-based alloy powder (wt%). Cr
Si
B
Fe
Ni
Mo
6–7.5%
1–2%
1–2%
2–3.5%
26–28%
Balance
X.B. Zhao, ZH. Ye / Surface & Coatings Technology 228 (2013) S266–S270
S267
3. Results and discussion
Table 2 Processing parameters of atmospheric plasma spraying. Nozzle type Nozzle I.D. (mm) Powder Injector (°)
GE 5.5 90
3.1. Characterization of Mo-based amorphous nanocrystalline powder and coating
Power (kW) Argon gas (NLPM)a Hydrogen gas (NLPM) Powder feed rate (g/min) Spray distance (mm) Cooling Air (MPa) Gun vertical speed (mm/s)
35 60 6 30 100 0.5 10
Fig. 1 presents XRD patterns of Mo-based amorphous powder and the as-prepared coating. The powder is mainly composed of four phases: Fe3Mo, Fe0.54Mo0.73, Cr9Mo21Ni20 and Fe3B. Weak broad diffraction peaks are observed in the range of 40–45°, which indicate that there are partly amorphous or nanocrystalline particles in Mobased powder. The phase composition of the as-prepared Mo-based coating is mainly composed of Fe0.54Mo0.73, Fe3Mo, Cr9Mo21Ni20, Fe3B and Cr0.46Mo.4Si0.14; the differences are that a new phase of Cr0.46Mo.4Si0.14 is formed at 2θ of 40° and main crystal peak changes from Fe3Mo to Fe0.54Mo0.73. In addition, a clear broad halo peak is also displayed in the as-prepared coating, which is the same range as the powder. The amorphous phase was caused by molten particles cooling rapidly (cooling rate reach ~10 6 K/s). Furthermore, changes of phase compositions in the coating might be the result of recrystallization from amorphous state to crystal state or the molten particles deforming, becoming lamellae and solidifying into finegrained crystals in the plasma spraying process [19]. The average crystalline sizes of the coating were calculated by Scherrer formula (Scherrer constant was 0.9 and wavelength of Xray was 1.54056 Å in this experiment). The average crystalline sizes of the coating calculated from the XRD peaks are listed in Table 4. It can be seen that the average grain size of the Mo-based coating is 22–32 nm, which theoretically proved that the crystals in the coating are nanostructure. The particle size distribution curves of Mo-based powder are shown in Fig. 2. It can be seen that the median diameter (D50) of Mo-based powders is 29.51 μm; the particle size distribution is wide and the highest frequency distribution is 4.48% in the range of 40.15 μm–44.69 μm. The percentage content of particle size less than 10 μm is about 18.1%, while large than 100 μm is about 5.23%. These small particles and big particles are more propitious to deposit high dense coatings. The SEM image of the powder visually presents the particle size distribution (as shown in Fig. 3). Spraying experiment indicated that the Mo-based powder had good fluidity, which was suitable for plasma spraying. The surface and cross sectional morphologies of the as-sprayed Mo-based coating are displayed in Fig. 4. The as-sprayed Mo-based coating presents a typical surface morphology of plasma spraying, which has some holes, lamellae, partially melted and unmelted particles, as shown in Fig. 4(a). During plasma spraying, molten particles impacted on the surface of substrate, then deformed, solidified and transformed into lamellae. Then lamellae piled up one upon another to build up the coating. Expanding gases entrapped in the closed pores between lamellae might evacuate out to leave holes on the coating surface [19]. In addition, the melting degree of particles was uneven during the spraying process, which also affected the coating structure. Particles might include one or all of the following states on impact: fully molten, superheated, semi-molten and molten then resolidified. Fully melted particles at or just above the material melting point arrived at the substrate, impacted, flowed, and flattened; superheated particles may splatter on impact, sending out radial splashes of fine droplets that ended up in the coating as “satellites” or debris; particles that resolidify in flight after melted appeared as unmelted particles [20]. From the cross sectional morphology of the coating (Fig. 4(b)), it can be seen that the thickness of the coating is about 250 μm. The coating has a dense structure, although some pores and microcracks exist as very dark contrast regions. Generally, the big pores arising between lamellae were mainly caused by gas porosity phenomenon, while the small pores within the flattened particles were resulted from shrinkage porosity [21]. When the spraying gun moved over
a
NLPM stands for normal liters per minute.
2.3. Characterization of powder and coating The phase characterization of the powder and coating were conducted by X-ray diffraction (XRD, D/max 2500PC, Rigaku, Japan) with Cu Kα radiation (λ = 1.5418 Å) in the range of 20–90° (2θ). The average grain sizes of nanocrystalline particles were evaluated by the Scherrer formula [15]. The morphologies of the powder and as-prepared coatings were examined by scanning electron microscopy (SEM, JSM-6360LA, JEOL, Japan). The particle size distribution of the powder was measured by laser particle size analyzer (BT-9300s, Bettersize, China). 2.4. Hardness and wear tests Before hardness and wear tests, the specimens were ground and polished to reach the surface roughness of ~ 0.1 μm [16] (for hardness) and ~0.2 μm [17] (for wear). The surface hardness values of substrate and coating were determined using a Vickers hardness tester (HVS-5Z/LCD, Shanghai Taiming Co., China) with a load of 2.942 N for 10 s. Microhardness profile was realized on cross-section of coating and substrate by a Vickers microhardness tester (HXD-1000TMC, Shanghai Taiming Co., China) incorporated with microhardness measurement system and evaluated by a load of 0.9807 N for 15 s. The indentations were performed in a row at separations of 60 μm (for coating) and 250 μm (for substrate), which ensured that the separations of any two indentations were at least three times longer than the diagonal of the biggest indentation [16]. The indentation series were produced separately at distances of approximately 40 μm, 80 μm, 120 μm and 160 μm from the coating-substrate interface to both sides [18]. Wear test was carried out using a pin on plate type tester (MMW1A, Jinan Yihua Co., China). The rotating pin material of the friction pair was 45# steel with Φ 4.8 mm × 12.7 mm and its hardness was 44–46 HRC. The test was performed by changing the load and linear speed to obtain the friction coefficient and mass loss. The parameters of wear test are listed in Table 3. All wear tests were conducted under dry sliding condition and exposed to the air without cooling device. Before and post wear tests, specimens were ultrasonically cleansed with alcohol and dried in an oven at 80 °C. The mass of the substrates and coatings were weighed by an electronic precision balance (a minimum reading of 0.1 mg) to calculate the mass loss.
Table 3 Wear test parameters: A denotes changing the load and B for speed. Details
A
B
Load (N) Linear speed (m/s) Rotation speed (rpm) Wear time (min) Wear track diameter (mm, CONST) Wear distance (m, CONST)
50, 60, 70 0.5 400 36 23.86 1080
50 0.5, 1, 1.5 400, 800, 1200 36, 18, 12
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Fe0.54Mo0.73
Coating
Fe3Mo Cr9Mo21Ni20 Fe3B Cr0.46Mo0.4Si0.14
Intensity (a.u.)
SiO2
Powder
20
30
40
50
60
70
80
90
2θ (degree) Fig. 1. The XRD patterns of the Mo-based amorphous powder and coating.
Table 4 The average grain sizes of the Mo-based coating. 2θ (°)
FWHM
Grain size (nm)
40.079 40.800 43.180 44.419
0.283 0.332 0.404 0.336
31.2 26.7 22.1 26.8
the substrate, the surfaces of the lamellae were subjected to the action of the environment, i.e. cooling and/or oxidation. The cooling led to generation of residual stresses, which might result in the microcracks of coatings [19]. Despite the present of these defects, the as-prepared Mo-based coating express a low porosity (approximately 3.8%) measured by the image analysis program of Image-Pro Plus 6.0. Moreover, it can also be seen from the cross sectional
morphology that the bonding region of the coating and the substrate is flawless (Fig. 4(b)). Fig. 4(c) is the high magnification backscattered electron (BSE) image of the coating; it can be observed that some heterogeneous phases appear in the coating. Table 5 shows the EDS analysis results: section A and B for the powder marked in Fig. 3, while C, D, E and F for the coating marked in Fig. 4 (a) and (c) respectively. It is seen from the results of EDS analysis that the mass fraction of oxygen of inside has a downward trend, after the powder deposited to the coating. During the spraying process, the elements of B and Si had the function of deoxygenation, which easily formed oxide then gradually floated to the surface because of low density [22]. So the mass fraction of oxygen of section C is higher than that of section D and E. Moreover, the XRD analysis indicates that the surface of the coating contains SiO2, as shown in Fig. 1, while the mass fraction of oxygen between section D and E is also different, which may caused by partially melted particles because of the composition like in Section B. However compared those of with section D and E, the mass fraction of oxygen of section F is higher. When the spraying gun moved over the substrate,
Fig. 2. Particle size distribution of the Mo-based powder.
X.B. Zhao, ZH. Ye / Surface & Coatings Technology 228 (2013) S266–S270
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Fig. 3. SEM images of the Mo-based powder: surface and cross-section.
the surfaces of the lamellae were subjected to oxidation in the environment of high temperature and air. Therefore, the coating had the phenomenon of deoxidization, though the borders of the lamellae had been oxidized.
3.2. Hardness and wear resistance of Mo-based amorphous nanocrystalline coating Fig. 5 shows the microhardness profile of Mo-based coating along the cross-section. As a whole, the hardness decrease continuously from 8.8 GPa to 1.34 GPa. The highest one is still inferior to the coating surface hardness of 9.2 GPa; the lowest one is superior to the surface hardness (1.26 GPa) of substrate. Diamond indenter pressed into cracks and forced the lamellae to separate, which resulted in the lower hardness. The same reason was less impact on the surface of the coating because of the flattened layers. For the substrate, both grit blasting and molten particles impacting on the substrate enhance the hardness of substrate, especially on the interface like the point at 40 μm (1.46 GPa), which is higher than that of other points in the substrate. Fig. 6 presents the friction coefficient of Mo-based coating and substrate in the case of changing the load and linear speed. It is found obviously that the friction coefficients of coating and substrate decrease with the increase of the load or the linear speed, and the friction coefficients of Mo-based coating are higher than that of substrate. When increased the load, the interface of the friction pair generated elastic–plastic deformation, which resulted in the practical contact area decreased, so the friction coefficient went down. Increase of the linear speed would cause the temperature of interface to rise sharply, which changed the surface properties of the friction pair and further influenced the friction coefficient [23]. Fig. 7 shows the mass loss of Mo-based coating and substrate after wear test in the same condition. It can be seen that the mass loss of substrate increases more sharply than that of the coating. The mass loss of the coating is around 10 mg, which is far lower than that of the substrate (about 90 mg). However, the mass loss of the substrate at the linear speed of 0.5 m/s is lower than that of 1.0 m/s, reason of which might be the instability of the wear tester in the final 300 m. According to the results of the friction coefficient and mass loss, the wear resistance of the as-prepared Mo-based coating is far superior to the substrate.
Fig. 4. SEM images of as-sprayed Mo-based coating: (a) surface, (b) cross-section (SE) and (c) high magnification cross-section (BSE).
Table 5 EDS results (mass%) of the Mo-based powder and coating.
Powder Coating
Section
O
Si
Cr
Fe
Ni
Mo
A B C D E F
3.75 2.68 3.75 2.59 0.66 3.34
0.87 0.90 0.96 0.86 0.83 0.70
4.19 4.89 6.74 4.26 4.22 4.92
6.08 5.99 12.9 6.49 6.25 6.39
23.61 23.78 25.81 24.11 25.33 21.61
61.50 61.76 50.14 61.70 62.71 63.04
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10
90 Coating Substrate
80 70
Mass Loss (mg)
Microhardness / GPa
8
6 Coating
Substrate
4
(a) V = 0.5m/s
(b) F = 50N
60 50 40 30 20
2
10 0
0 -200
50 -150
-100
-50
0
50
100
150
75
Load (N)
200
100
0.5
1.0
1.5
Liner speed (m/s)
Distance from interface / μm Fig. 7. Mass loss vs. load and linear speed of the coating and substrate. Fig. 5. Microhardness profile of Mo-based coating along the cross-section.
References
4. Conclusions Molybdenum based amorphous nanocrystalline alloy coating was fabricated by atmospheric plasma spraying using the Mo-based powder as feedstock. The detail results were summarized as follows:
[1] [2] [3] [4] [5]
(1) The coating was mainly composed of Fe0.54Mo0.73, Fe3Mo, Cr9Mo21Ni20, Fe3B and Cr0.46Mo.4Si0.14 phases. The average grain size of coating was 22–32 nm. (2) The coating with thickness of about 250 μm had a dense structure. The bonding region of the coating and substrate was flawless and interface line was clearly visible. The coating had the phenomenon of deoxidization, though the borders of the lamellae had been oxidized. And the oxygen content of the coating decreased with the increase of melting degree of particles. (3) The surface hardness of the coating was 9.2 GPa, which was 8 times higher than that of the substrate. The microhardness along the cross-section gradually decreased from coating to substrate. (4) The friction coefficients straightly decreased with the increase of load or the linear speed. Both friction coefficient and mass loss of the coating were far superior to the substrate. The Mobased coating exhibited excellent wear resistance under dry sliding condition than that of the plain carbon steel.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
0.75 Coating Substrate
Friction Coefficient
0.70
0.65
0.60
0.55
0.50
(a) V = 0.5m/s 0.45
50
75
Load (N)
(b) F = 50N 100
0.5
1.0
1.5
liner speed (m/s)
Fig. 6. Friction coefficient vs. load and linear speed of the coating and substrate.
[23]
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