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Effect of laser melting on plasma sprayed WC-12 wt.%Co coatings
Surface & Coatings Technology 266 (2015) 22–30
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effect of laser melting on plasma sprayed WC-12 wt.%Co coatings M. Afzal a,d,⁎, A. Nusair Khan b, T. Ben Mahmud d, T.I. Khan c,d, M. Ajmal a a
University of Engineering and Technology, G. T Road, Lahore, Pakistan Institute of Industrial Control System, Rawalpindi, Pakistan c Department of Mechanical & Industrial Engineering, Qatar University, Doha, Qatar d Department of Mechanical and Manufacturing Engineering, University of Calgary, 2500 University Dr NW, Calgary, Alberta T2N 1N4, Canada b
a r t i c l e
i n f o
Article history: Received 1 July 2014 Accepted in revised form 2 February 2015 Available online 9 February 2015 Keywords: Air plasma spraying WC-12%Co coating Laser treating Micro-hardness Microstructure Sliding wear
a b s t r a c t Tungsten carbide powder with 12% Co was deposited on AISI 321 stainless steel substrate by air plasma spraying. The coating was produced at 130 mm standoff distance. The coated samples were melted using a CO2 laser with an inert gas shroud. Four different laser speeds from 100 mm/min to 250 mm/min were used to melt the coatings. After laser melting the treated surfaces were characterized and compared with as sprayed surfaces. It was observed that the laser melting process produced defect free surfaces. The results showed that the thicknesses of the melted surfaces and wear resistance varied as a function of laser speed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction During the last several decades, tungsten carbide in a cobalt binder has been widely used in a number of engineering applications such as cutting tools and dies, drill bits for mining, and wear resistant nozzles. Due to the distinctive combination of high hardness, high resistance to abrasive/erosive wear and toughness of the WC-Co systems, this cermet is employed both as a bulk and coating for a variety of applications [1,2]. WC-Co cermets can be applied to surfaces by thermal spraying processes such as air plasma spraying (APS), high velocity oxygen fuel (HVOF) spraying and detonation gun methods [3,4]. These coating techniques use high temperatures during the spraying process and the microstructure of air plasma sprayed coatings in particular, can result in many defects such as high percentage of porosity, weak interconnection between the solidified splats and an inhomogeneous coating structure. The presence of porosity not only reduces the mechanical properties of the coating, but can also reduce the corrosion resistance of the coatings. This is attributed to the presence of micro-channels which allow the movement of corrosive media to the substrate/coating interface [5]. Laser melting is a well-established technique often used to reduce inherent defects, such as porosity and inter-splat boundaries of sprayed coatings. Furthermore, laser surface alloying (LSA) is a relatively new
⁎ Corresponding author at: University of Engineering and Technology (UET) G.T. Road, Lahore-54890, Pakistan. E-mail addresses: [email protected] (M. Afzal), [email protected] (A.N. Khan), [email protected] (T.B. Mahmud), [email protected] (T.I. Khan), [email protected] (M. Ajmal).
http://dx.doi.org/10.1016/j.surfcoat.2015.02.004 0257-8972/© 2015 Elsevier B.V. All rights reserved.
and important area of research, which is also used to enhance wear resistant properties of surfaces. The objective of these techniques is to fuse completely pores within the coatings and the process of melting and solidification results in a homogeneous coating. A number of researchers have worked in this field [6–14], but there are very few studies reported on the application of laser melting of WC based coatings [5]. The laser cladding and melting of WC-Co coatings could lead to WC dissolution and precipitation of brittle phases, depending on the laser parameters used [1,15–17]. Therefore, process parameters have to be carefully controlled to minimize the extent of carbide dissolution [16–18]. In the present study, the effect of laser melting of WC-12%Co coatings, sprayed by air plasma spraying technique is performed and the effect of laser speed on changes in microstructure, mechanical and wear properties are investigated. 2. Experimental 2.1. Air plasma spraying A WC-12%Co coating was deposited by Air Plasma Spraying onto AISI 321 stainless steel coupons (dimensions 5 mm thickness and 25.4 mm diameter). The feedstock powder used had an angular morphology and contained 76 wt.%WC, with a powder size range of 5 to 45 μm. X-ray diffraction analysis of the feedstock powder is provided elsewhere [19]. The composition of the substrate surface was determined using optical emission spectrometry (OES), see Table 1. The steel coupons were placed in an aluminum fixture and rotated during
M. Afzal et al. / Surface & Coatings Technology 266 (2015) 22–30
23
Table 1 Chemical composition of AISI-321 used as a substrate. Elements
Fe
Cr
Cu
Mn
Mo
Ni
Si
P
C
wt.%
Base
18.44
0,26
1.16
0.28
9.07
0.56
0.03
0.05
Table 2 Principal parameters used during plasma spraying. Parameter
Values
Current, A Voltage, V Primary gas (argon), liter/min Secondary gas (hydrogen), liter/min Anode diameter, mm Position of powder injector, degree Internal diameter of injector, mm No. of passes Carrier gas flow rate, liter/min Powder feed rate, g/s Spraying distance, mm
500 50 75.5 4.8 12.5 90 2 59 9.4 1.26 130
Fig. 1. Laser beads obtained from different laser speeds from 100 mm/min to 250 mm/min.
microscope. Vickers hardness was measured along the depth and width of the melted zone using a 9.8 N load. 2.4. Wear analysis
the spraying operation so that a uniform coating could be deposited over the coupon surfaces. The substrate surfaces were preheated to 150 °C using a plasma gun prior to plasma spraying. After preheating, the plasma gun was used to coat the substrates. These substrates were plasma sprayed at different spraying distances i.e. 80 to 130 mm. The microstructure of the sprayed coatings and other mechanical properties are discussed in earlier work [19]. The results showed that a 130 mm standoff distance produced a coating with more porosity and poor interfacial adhesion. Therefore, the coatings deposited at 130 mm standoff distance were selected for laser melting. Similarly, the high carrier gas flow rate was chosen so that poor microstructure can be produced. The spraying parameters used to deposit the coatings are shown in Table 2.
A pin-on-plate type sliding wear tests were performed (ASTM G133-05) on the steel substrate before coating, after spraying and after laser melting. A multi- channel 16-bit 100 kHz data acquisition device USB-1608FS model (microDAQ, OH) was used to acquire data and record a real time analysis of the wear process. All coupons were weighed before and after wear testing using an electronic analytical balance model-SCIENTECH ZSA210. All measurements were made after removing the debris by air blowing. The applied load was 20 N for each test and each test was carried out for 30 min. The load used for wear tests was selected on the basis of published work [20,21]. A total sliding distance of 53 m and a speed of 28.8 mm/s were recorded for each test. A Scanning Electron Microscope (SEM) model JXA-8200, JEOL was used for characterization of the worn surfaces. 3. Results and discussion
2.2. Laser treatment 3.1. Metallography of conventional coating The surface of plasma sprayed WC-12%Co coatings was melted using a 700 W transverse flow CO2 laser. A laser beam spot size of 2.5 mm diameter was focused on the work piece using a lens of 120 mm focal length. Sample movement under the laser beam was controlled using a CNC table. Nitrogen was used as a shielding gas to prevent the formation of oxides on the melted surface. The laser melting process was carried out at different speeds using a fixed laser power intensity and spot size as shown in Table 3. All the laser tracks were produced in a single pass. The formations of laser melted tracks on a coated surface are shown in Fig. 1. The nomenclature used is also provided in Table 4. 2.3. Characterization technique The sample was cut transversely through the coating, mounted, ground and polished for metallographic study. The microstructures of the laser treated zone were examined under optical and electron
The examination of the as sprayed WC-12%Co coating using SEM showed many features including the presence of porosity throughout the coating, un-melted WC particles, entrapped gas voids, interfacial defects, a rough surface and the presences of different phases within the coating. The percentage porosity within the coating was analyzed by image analyzer attached to the optical microscope. It was observed that the percentage porosity varied from place to place and the average value was about 12%. Similarly, other features were also characterized using image analyzer and it was found that the size of un-melted particles was about 2 μm to 6 μm. The presence of un-melted particles demonstrated that either the temperature was not high enough or the particles were too large to melt completely in the plasma flame. A few “rough voids” were also observed at some locations. These voids are normally associated with gas entrapment during the spraying process. These voids were not larger than 1 to 2 μm in size. The entrapment of gases within the coating can be attributed to the dissociation of
Table 3 Principal parameters used during laser treatment. Parameter (unit)
Values
Laser power (watts) Spot size (mm) Focal length of lens (mm) Working speed (mm/min) Shielding gas, nitrogen
700 2.5 120 100, 150, 200, 250 Flow rate, liter/minute Pressure, kPa
Table 4 Nomenclature showing laser speed in the manuscript.
10 50
Nomenclature
Laser speed, mm/min
CL1 CL2 CL3 CL4
100 150 200 250
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M. Afzal et al. / Surface & Coatings Technology 266 (2015) 22–30
a
b
Fig. 2. (a, b). Conventional APS coating deposited at 130 mm standoff distance.
WC particles and interaction of WC particles with air at high temperature during the spraying processes [22–24]. These interactions can consist of several steps as shown: 2WC→W2 C þ C
ð1Þ
W2 C→2W þ C:
ð2Þ
The loss of carbon in the coating [23] may be associated with oxidation in the flame according to the following reaction [22]:
Another typical feature of sprayed coating is the formation of a lamellar structure. The thickness of these lamellae ranged from 2.5 to 7.5 μm. This showed that the splat temperature during the spraying was relatively low. Furthermore, a poor interface between the sprayed coating and the substrate was observed and consisted of many interfacial defects as shown in Fig. 2. The surface roughness of the coating varied from Ra 5 to 6 μm and the hardness values ranged from 650 to 1050 VHN. A large variation in hardness was attributed to the presence of different phases in the coating. X-ray diffraction analysis of as sprayed coating revealed that 65% W2C phase was present with 25% Co3W9C4 and 10% WC phases. The details of the XRD scans have been discussed in references [19,25].
3.2. Metallography of laser treated coating 2C þ O2 →2COðgasÞ:
ð3Þ
Furthermore, during plasma spraying when hydrogen gas is used, the decomposition by reduction can also take place as shown [24]: 2WC þ 2H2 ¼ W2 C þ CH4 ðgasÞ
ð4Þ
2H2 þ O2 ¼ 2H2 O:
ð5Þ
It was observed that the laser speed used during melting influenced bead depth, as shown in Fig. 3. When the laser speed was 100 mm/min, the bead depth recorded was 600 μm, and as the speed was increased to 250 mm/min a depth of 313 μm was recorded. Laser speed is directly related to the laser input energy i.e. when the speed of the laser increases the laser input energy decreases, as shown in Fig. 4. This showed that bead depth decreased gradually when the laser melting speed was increased. The graphical representation for bead depth as a function of laser speed is shown in Fig. 3, and optical microstructures of each bead against their respective laser speed are shown in Fig. 5.
The evolution of CO or CH4 gas produced during spraying also entrapped within the coating resulting in gaseous voids.
Fig. 3. Comparison of bead depth verses laser speed.
Fig. 4. Laser energy input with respect to laser speed.
M. Afzal et al. / Surface & Coatings Technology 266 (2015) 22–30
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Fig. 5. Optical microscope showing the laser beads height, where (CL1) = laser speed 100 mm/min, (CL2) = laser speed 150 mm/min, (CL3) = laser speed 200 mm/min, (CL4) = laser speed 250 mm/min.
3.2.1. Composition variation Laser surface melting of various samples were carried out under the conditions given in Table 3. When the laser beam focuses on the coating surface, the temperature of the surface will increase. If the laser power density and working speed are selected in such a way that the temperature of the surface reaches to the melting point of the material then WC-Co layer starts to melt. The melted WC-Co layer intermixes with
the base metal substrate and results in good interfacial adhesion. It was observed that the melting depth and width decreased with an increase in the working speed of the laser. Fig. 6 shows the variations in composition in sample CL1 along the depth in the laser treated zone. The variation in composition along the depth of laser treated zone was almost homogeneous, and this suggests that uniform mixing of the WC-Co with base metal occurred. It was observed that the composition
Fig. 6. Variation of composition along the depth of the laser melted zone at laser speed 100 mm/min.
Fig. 7. Variation of composition along the depth of the laser melted zone at laser speed 250 mm/min.
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M. Afzal et al. / Surface & Coatings Technology 266 (2015) 22–30
Fig. 8. SEM microstructure at different laser speeds (CL1) = laser speed 100 mm/min, (CL2) = laser speed 150 mm/min, (CL3) = laser speed 200 mm/min, (CL4) = laser speed 250 mm/min.
Fig. 9. SEM microstructure at different laser speeds (CL1) = laser speed 100 mm/min, (CL2) = laser speed 150 mm/min, (CL3) = laser speed 200 mm/min, (CL4) = laser speed 250 mm/min.
M. Afzal et al. / Surface & Coatings Technology 266 (2015) 22–30
Fig. 10. The microhardness against melted depth for the different samples.
of cobalt in the laser treated zone was not detected except at one point. The concentration of cobalt in this region was very low because the depth and width both increased with a decrease in laser speed. The variation in composition along with the depth of the laser melted zone for sample CL4 is shown in Fig. 7. A uniform concentration for the major elements was observed through the depth of the laser treated zone, but cobalt was not detected at some points within the zone. 3.2.2. SEM observation of modified surfaces Metallographic examination revealed the presence of different zones in the alloyed surface. The first one is a zone rich in WC-Co particles embedded in the surface, i.e. laser treated area, as shown in
27
Fig. 8. The next zone can be considered as a transient zone between the alloyed zone and the base metal. This zone shows slightly changed structure compared to the substrate material and can be considered as a very narrow heat affected zone (HAZ). In the “as polished” condition, the cross-section of the samples revealed modified layers, Fig. 8. It was observed that the modified laser treated layer has a good appearance without any discontinuities as well as free of micro-cracks in all the samples. However, the bead demonstrated in CL1, showed a comparatively uniform distribution of cellular dendritic structure compared to other beads. The decrease in bead thickness by increasing the laser speed was probably due to insufficient time for surface melting. However, a homogeneous microstructure was observed in all laser melted tracks while the APS coating had some porosity along the coat/substrate interface. It can be concluded that laser treatment improved the properties of these carbide coatings deposited by APS techniques. Furthermore, it was also observed that after laser melting, a continuous and sound interface between the substrate and laser modified layer was achieved as shown in Fig. 8. No micro-cracks or delamination was observed in the laser modified zones compared to the as sprayed APS coatings. However, some micro-voids were seen and could be due to entrapped gases or the formation of gas pockets. These types of defects were also observed by other researchers [3]. At higher laser melting speeds these voids were more prominent and could be due to the insufficient time for gases to escape from the molten pool, as shown in Fig. 9 (CL4). A fine dendritic region, next to the HAZ, was observed in all samples (CL1 to CL4). It was observed that a finer dendrite structure was visible at higher laser speeds. This is because the higher speed created a smaller molten zone compared to surface melting at slower
Fig. 11. The microhardness against melted width for the different samples.
Fig. 12. Mass loss during the sliding wears where (a): substrate, (b): as sprayed APS coating, (c): laser modified at speed of 100 mm/min, (d): laser modified at speed of 150 mm/min, (e): laser modified at speed of 200 mm/min, and (f): laser modified at speed of 250 mm/min.
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M. Afzal et al. / Surface & Coatings Technology 266 (2015) 22–30
Fig. 13. Wear rate in terms of sliding distance where (a): substrate, (b): as sprayed APS coating, (c): laser modified at speed of 100 mm/min, (d): laser modified at speed of 150 mm/min, (e): laser modified at speed of 200 mm/min, and (f): laser modified at speed of 250 mm/min.
Fig. 14. Sliding wear resistant values where (a): substrate, (b): as sprayed APS coating, (c): laser modified at speed of 100 mm/min, (d): laser modified at speed of 150 mm/min, (e): laser modified at speed of 200 mm/min, and (f): laser modified at speed of 250 mm/min.
speeds. In the smaller molten zone less volume of metal results in faster solidification and this produces a finer dendritic structure. The brighter phase shown in Figs. 8 and 9 were analyzed and found to be rich in tungsten. 3.3. Micro-hardness testing Observations using the SEM showed that WC-12%Co layer completely melted and inter-mixed with the base metal. This was
indicated by the Energy Dispersive Spectroscopy (EDS) composition profiles, measured throughout the depth of the laser melted zone, see Figs. 6 and 7. The micro-hardness profiles measured along the depth and width of the laser melted zone at different speeds are shown in the Figs. 10 and 11. The depth of the laser melted zone decreased with increasing the laser speed. The amount of WC phase increased with increasing laser speed because less base metal dilution occurred with the coating. This increase in WC phase resulted in an increase in surface hardness.
Fig. 15. Wear depths as function of sliding distance during sliding wear, where (a): substrate, (b): as sprayed APS coating, (c): laser modified at speed of 100 mm/min, (d): laser modified at speed of 150 mm/min, (e): laser modified at speed of 200 mm/min, and (f): laser modified at speed of 250 mm/min.
M. Afzal et al. / Surface & Coatings Technology 266 (2015) 22–30
3.4. Wear characteristics Pin-on-plate sliding wear tests were conducted using a sliding distance of 53 m, and change in mass loss was measured by weighing the coupon before and after the wear test. The wear rate was determined by using the following equation [26]:Wear rateðkg=mÞ ¼ Mass loss or wear amount=Slidingdistance:
ð6Þ
While wear resistance was calculated as: −1 : Wear resistance ¼ 1=ðwear amount or mass lossÞ kg
ð7Þ
The wear test results showed that the substrate material (AISI 321) had higher mass loss value compared to the treated surfaces. Fig. 12 shows that the lowest mass loss was for laser surfaces, treated with the
29
lowest speed of 100 mm/min. As the laser speed was increased to 250 mm/min, a gradual increase in wear loss was recorded. The graph in Figs. 13 and 14 showed that an increase in wear resistance for laser melted surfaces at a speed of 100 mm/min, and a loss in wear resistance with increasing laser speed. Furthermore, the results showed that by using a slower speed results in less wear depth as shown in Fig. 15(c) and (d). The SEM micrographs in Fig. 16(a) to (f) are taken from the worn surfaces and show the mechanism of wear. The micrograph for the untreated steel substrate shows considerable fracture and loss of material from the surface. However, the extent of fracture decreased with the deposition of the harder WC-Co coating. The laser melting process increased the wear resistance of the coatings and much less surface fracture was observed as shown in Fig. 16(c) and (d). The extent of surface fracture increased with increase in laser speed. These results correspond with the wear test results shown in Figs. 12 to 14.
Fig. 16. Worn out surfaces and wear scars after sliding wear tests whereas (a): substrate, (b): as sprayed APS coating, (c): laser modified at speed of 100 mm/min, (d): laser modified at speed of 150 mm/min, (e): laser modified at speed of 200 mm/min, and (f): laser modified at speed of 250 mm/min.
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M. Afzal et al. / Surface & Coatings Technology 266 (2015) 22–30
4. Conclusions This study shows that surface coatings deposited by air plasma spraying can be improved by laser melting. It is observed that the surface hardness could be controlled by optimizing the speed of the laser gun. The melting and solidification processes reduce the inherent defects within the APS deposited coatings. The wear resistance of the coating is significantly enhanced when laser melting is performed at a speed of 100 mm/min, but this wear resistance decreased as the laser speed increased to 250 mm/min. Acknowledgment The authors gratefully acknowledge the financial support of UET, Lahore and technical support of IICS (09-Ph.D-Met-04) and Pakistan Institute of Laser and Optics (PILO). The authors are also thankful to the Surfaces and Joining Research Group within the Department of Mechanical & Manufacturing Engineering at the University of Calgary, Alberta, Canada. References [1] Y. Xiong, J.E. Smugeresky, J.M. Schoenung, J. Mater. Process. Technol. 209 (10) (2009) 4935–4941. [2] V.K. Balla, S. Bose, A. Bandyopadhyay, Mater. Sci. Eng. A 527 (24–25) (2010) 6677–6682.
[3] G. Xie, J. Zhang, Y. Lu, Z. He, B. Hu, D. Zhang, K. Wang, P. Lin, Appl. Surf. Sci. 253 (23) (2007) 9198–9202. [4] T.N. Rhys-Jones, 43–44(1) (1990) 402–415. [5] J. Mateos, J.M. Cuetos, E. Fernandez, R. Vijande, Wear 239 (2) (2000) 274–281. [6] M. Barletta, G. Rubino, A. Gisario, Wear 271 (9–10) (2011) 2016–2024. [7] T. Li, Q. Lou, J. Dong, Y. Wei, J. Liu, Appl. Surf. Sci. 172 (3–4) (2001) 331–344. [8] K. Wang, J. Liu, D. Han, Surf. Eng. 12 (3) (1996) 235–238. [9] S.C. Tjong, J.S. Ku, N.J. Ho, Mater. Sci. Technol. 13 (1) (1997) 56–60. [10] Y. Liu, H.M. Wang, Surf. Coat. Technol. 205 (2) (2010) 377–382. [11] Y.M. Zhang, M. Hida, A. Sakakibara, Y. Takemoto, Surf. Coat. Technol. 169–170 (2003) 384–388. [12] F.T. Cheng, C.T. Kwok, H.C. Man, Mater. Lett. 57 (4) (2002) 969–974. [13] S. Zhou, X. Dai, H. Zheng, Opt. Laser Technol. 44 (1) (2012) 190–197. [14] Z.Y. Taha-al, M.S. Hashmi, B.S. Yilbas, J. Mater. Process. Technol. 209 (7) (2009) 3172–3181. [15] P. Wu, C.Z. Zhou, X.N. Tang, Surf. Coat. Technol. 166 (1) (2003) 84–88. [16] Y. Yang, H.C. Man, Surf. Coat. Technol. 132 (2–3) (2000) 130–136. [17] J. Przybylowicz, J. Kusinski, J. Mater. Process. Technol. 109 (1–2) (2001) 154–160. [18] M. Riabkina-Fishman, E. Rabkin, P. Levin, N. Frage, M.P. Dariel, A. Weisheit, R. Galun, B.L. Mordike, Mater. Sci. Eng. A 302 (1) (2001) 106–114. [19] M. Afzal, M. Ajmal, A. Nusair Khan, Tribol. Trans. 57 (1) (2014) 94–103. [20] A. Mateen, G.C. Saha, T.I. Khan, F.A. Khalid, Surf. Coat. Technol. 206 (2011) 1077–1084. [21] P.G. Mukunda, A.S. Rao, S.S. Rao, Front. Mater. Sci. Chin. 3 (2009) 339. [22] J. Nerz, B. Kushner, A. Rotolico, J. Therm. Spray Technol. 1 (2) (1992) 147–152. [23] C. Verdon, A. Karimi, J.L. Martin, Mater. Sci. Eng. A 246 (1–2) (1998) 11–24. [24] J. He, J.M. Schoenung, Mater. Sci. Eng. A 336 (1–2) (2002) 274–319. [25] M. Afzal, M. Ajmal, A. Nusair Khan, A. Hussain, R. Akhtar, Opt. Laser Technol. 569 (2014) 202–206. [26] H. Czichos, T. Saito, L. Smith (Eds.), Springer, Wurzberg, Germany, 2006, p. 701.
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effect of laser melting on plasma sprayed WC-12 wt.%Co coatings M. Afzal a,d,⁎, A. Nusair Khan b, T. Ben Mahmud d, T.I. Khan c,d, M. Ajmal a a
University of Engineering and Technology, G. T Road, Lahore, Pakistan Institute of Industrial Control System, Rawalpindi, Pakistan c Department of Mechanical & Industrial Engineering, Qatar University, Doha, Qatar d Department of Mechanical and Manufacturing Engineering, University of Calgary, 2500 University Dr NW, Calgary, Alberta T2N 1N4, Canada b
a r t i c l e
i n f o
Article history: Received 1 July 2014 Accepted in revised form 2 February 2015 Available online 9 February 2015 Keywords: Air plasma spraying WC-12%Co coating Laser treating Micro-hardness Microstructure Sliding wear
a b s t r a c t Tungsten carbide powder with 12% Co was deposited on AISI 321 stainless steel substrate by air plasma spraying. The coating was produced at 130 mm standoff distance. The coated samples were melted using a CO2 laser with an inert gas shroud. Four different laser speeds from 100 mm/min to 250 mm/min were used to melt the coatings. After laser melting the treated surfaces were characterized and compared with as sprayed surfaces. It was observed that the laser melting process produced defect free surfaces. The results showed that the thicknesses of the melted surfaces and wear resistance varied as a function of laser speed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction During the last several decades, tungsten carbide in a cobalt binder has been widely used in a number of engineering applications such as cutting tools and dies, drill bits for mining, and wear resistant nozzles. Due to the distinctive combination of high hardness, high resistance to abrasive/erosive wear and toughness of the WC-Co systems, this cermet is employed both as a bulk and coating for a variety of applications [1,2]. WC-Co cermets can be applied to surfaces by thermal spraying processes such as air plasma spraying (APS), high velocity oxygen fuel (HVOF) spraying and detonation gun methods [3,4]. These coating techniques use high temperatures during the spraying process and the microstructure of air plasma sprayed coatings in particular, can result in many defects such as high percentage of porosity, weak interconnection between the solidified splats and an inhomogeneous coating structure. The presence of porosity not only reduces the mechanical properties of the coating, but can also reduce the corrosion resistance of the coatings. This is attributed to the presence of micro-channels which allow the movement of corrosive media to the substrate/coating interface [5]. Laser melting is a well-established technique often used to reduce inherent defects, such as porosity and inter-splat boundaries of sprayed coatings. Furthermore, laser surface alloying (LSA) is a relatively new
⁎ Corresponding author at: University of Engineering and Technology (UET) G.T. Road, Lahore-54890, Pakistan. E-mail addresses: [email protected] (M. Afzal), [email protected] (A.N. Khan), [email protected] (T.B. Mahmud), [email protected] (T.I. Khan), [email protected] (M. Ajmal).
http://dx.doi.org/10.1016/j.surfcoat.2015.02.004 0257-8972/© 2015 Elsevier B.V. All rights reserved.
and important area of research, which is also used to enhance wear resistant properties of surfaces. The objective of these techniques is to fuse completely pores within the coatings and the process of melting and solidification results in a homogeneous coating. A number of researchers have worked in this field [6–14], but there are very few studies reported on the application of laser melting of WC based coatings [5]. The laser cladding and melting of WC-Co coatings could lead to WC dissolution and precipitation of brittle phases, depending on the laser parameters used [1,15–17]. Therefore, process parameters have to be carefully controlled to minimize the extent of carbide dissolution [16–18]. In the present study, the effect of laser melting of WC-12%Co coatings, sprayed by air plasma spraying technique is performed and the effect of laser speed on changes in microstructure, mechanical and wear properties are investigated. 2. Experimental 2.1. Air plasma spraying A WC-12%Co coating was deposited by Air Plasma Spraying onto AISI 321 stainless steel coupons (dimensions 5 mm thickness and 25.4 mm diameter). The feedstock powder used had an angular morphology and contained 76 wt.%WC, with a powder size range of 5 to 45 μm. X-ray diffraction analysis of the feedstock powder is provided elsewhere [19]. The composition of the substrate surface was determined using optical emission spectrometry (OES), see Table 1. The steel coupons were placed in an aluminum fixture and rotated during
M. Afzal et al. / Surface & Coatings Technology 266 (2015) 22–30
23
Table 1 Chemical composition of AISI-321 used as a substrate. Elements
Fe
Cr
Cu
Mn
Mo
Ni
Si
P
C
wt.%
Base
18.44
0,26
1.16
0.28
9.07
0.56
0.03
0.05
Table 2 Principal parameters used during plasma spraying. Parameter
Values
Current, A Voltage, V Primary gas (argon), liter/min Secondary gas (hydrogen), liter/min Anode diameter, mm Position of powder injector, degree Internal diameter of injector, mm No. of passes Carrier gas flow rate, liter/min Powder feed rate, g/s Spraying distance, mm
500 50 75.5 4.8 12.5 90 2 59 9.4 1.26 130
Fig. 1. Laser beads obtained from different laser speeds from 100 mm/min to 250 mm/min.
microscope. Vickers hardness was measured along the depth and width of the melted zone using a 9.8 N load. 2.4. Wear analysis
the spraying operation so that a uniform coating could be deposited over the coupon surfaces. The substrate surfaces were preheated to 150 °C using a plasma gun prior to plasma spraying. After preheating, the plasma gun was used to coat the substrates. These substrates were plasma sprayed at different spraying distances i.e. 80 to 130 mm. The microstructure of the sprayed coatings and other mechanical properties are discussed in earlier work [19]. The results showed that a 130 mm standoff distance produced a coating with more porosity and poor interfacial adhesion. Therefore, the coatings deposited at 130 mm standoff distance were selected for laser melting. Similarly, the high carrier gas flow rate was chosen so that poor microstructure can be produced. The spraying parameters used to deposit the coatings are shown in Table 2.
A pin-on-plate type sliding wear tests were performed (ASTM G133-05) on the steel substrate before coating, after spraying and after laser melting. A multi- channel 16-bit 100 kHz data acquisition device USB-1608FS model (microDAQ, OH) was used to acquire data and record a real time analysis of the wear process. All coupons were weighed before and after wear testing using an electronic analytical balance model-SCIENTECH ZSA210. All measurements were made after removing the debris by air blowing. The applied load was 20 N for each test and each test was carried out for 30 min. The load used for wear tests was selected on the basis of published work [20,21]. A total sliding distance of 53 m and a speed of 28.8 mm/s were recorded for each test. A Scanning Electron Microscope (SEM) model JXA-8200, JEOL was used for characterization of the worn surfaces. 3. Results and discussion
2.2. Laser treatment 3.1. Metallography of conventional coating The surface of plasma sprayed WC-12%Co coatings was melted using a 700 W transverse flow CO2 laser. A laser beam spot size of 2.5 mm diameter was focused on the work piece using a lens of 120 mm focal length. Sample movement under the laser beam was controlled using a CNC table. Nitrogen was used as a shielding gas to prevent the formation of oxides on the melted surface. The laser melting process was carried out at different speeds using a fixed laser power intensity and spot size as shown in Table 3. All the laser tracks were produced in a single pass. The formations of laser melted tracks on a coated surface are shown in Fig. 1. The nomenclature used is also provided in Table 4. 2.3. Characterization technique The sample was cut transversely through the coating, mounted, ground and polished for metallographic study. The microstructures of the laser treated zone were examined under optical and electron
The examination of the as sprayed WC-12%Co coating using SEM showed many features including the presence of porosity throughout the coating, un-melted WC particles, entrapped gas voids, interfacial defects, a rough surface and the presences of different phases within the coating. The percentage porosity within the coating was analyzed by image analyzer attached to the optical microscope. It was observed that the percentage porosity varied from place to place and the average value was about 12%. Similarly, other features were also characterized using image analyzer and it was found that the size of un-melted particles was about 2 μm to 6 μm. The presence of un-melted particles demonstrated that either the temperature was not high enough or the particles were too large to melt completely in the plasma flame. A few “rough voids” were also observed at some locations. These voids are normally associated with gas entrapment during the spraying process. These voids were not larger than 1 to 2 μm in size. The entrapment of gases within the coating can be attributed to the dissociation of
Table 3 Principal parameters used during laser treatment. Parameter (unit)
Values
Laser power (watts) Spot size (mm) Focal length of lens (mm) Working speed (mm/min) Shielding gas, nitrogen
700 2.5 120 100, 150, 200, 250 Flow rate, liter/minute Pressure, kPa
Table 4 Nomenclature showing laser speed in the manuscript.
10 50
Nomenclature
Laser speed, mm/min
CL1 CL2 CL3 CL4
100 150 200 250
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a
b
Fig. 2. (a, b). Conventional APS coating deposited at 130 mm standoff distance.
WC particles and interaction of WC particles with air at high temperature during the spraying processes [22–24]. These interactions can consist of several steps as shown: 2WC→W2 C þ C
ð1Þ
W2 C→2W þ C:
ð2Þ
The loss of carbon in the coating [23] may be associated with oxidation in the flame according to the following reaction [22]:
Another typical feature of sprayed coating is the formation of a lamellar structure. The thickness of these lamellae ranged from 2.5 to 7.5 μm. This showed that the splat temperature during the spraying was relatively low. Furthermore, a poor interface between the sprayed coating and the substrate was observed and consisted of many interfacial defects as shown in Fig. 2. The surface roughness of the coating varied from Ra 5 to 6 μm and the hardness values ranged from 650 to 1050 VHN. A large variation in hardness was attributed to the presence of different phases in the coating. X-ray diffraction analysis of as sprayed coating revealed that 65% W2C phase was present with 25% Co3W9C4 and 10% WC phases. The details of the XRD scans have been discussed in references [19,25].
3.2. Metallography of laser treated coating 2C þ O2 →2COðgasÞ:
ð3Þ
Furthermore, during plasma spraying when hydrogen gas is used, the decomposition by reduction can also take place as shown [24]: 2WC þ 2H2 ¼ W2 C þ CH4 ðgasÞ
ð4Þ
2H2 þ O2 ¼ 2H2 O:
ð5Þ
It was observed that the laser speed used during melting influenced bead depth, as shown in Fig. 3. When the laser speed was 100 mm/min, the bead depth recorded was 600 μm, and as the speed was increased to 250 mm/min a depth of 313 μm was recorded. Laser speed is directly related to the laser input energy i.e. when the speed of the laser increases the laser input energy decreases, as shown in Fig. 4. This showed that bead depth decreased gradually when the laser melting speed was increased. The graphical representation for bead depth as a function of laser speed is shown in Fig. 3, and optical microstructures of each bead against their respective laser speed are shown in Fig. 5.
The evolution of CO or CH4 gas produced during spraying also entrapped within the coating resulting in gaseous voids.
Fig. 3. Comparison of bead depth verses laser speed.
Fig. 4. Laser energy input with respect to laser speed.
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Fig. 5. Optical microscope showing the laser beads height, where (CL1) = laser speed 100 mm/min, (CL2) = laser speed 150 mm/min, (CL3) = laser speed 200 mm/min, (CL4) = laser speed 250 mm/min.
3.2.1. Composition variation Laser surface melting of various samples were carried out under the conditions given in Table 3. When the laser beam focuses on the coating surface, the temperature of the surface will increase. If the laser power density and working speed are selected in such a way that the temperature of the surface reaches to the melting point of the material then WC-Co layer starts to melt. The melted WC-Co layer intermixes with
the base metal substrate and results in good interfacial adhesion. It was observed that the melting depth and width decreased with an increase in the working speed of the laser. Fig. 6 shows the variations in composition in sample CL1 along the depth in the laser treated zone. The variation in composition along the depth of laser treated zone was almost homogeneous, and this suggests that uniform mixing of the WC-Co with base metal occurred. It was observed that the composition
Fig. 6. Variation of composition along the depth of the laser melted zone at laser speed 100 mm/min.
Fig. 7. Variation of composition along the depth of the laser melted zone at laser speed 250 mm/min.
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Fig. 8. SEM microstructure at different laser speeds (CL1) = laser speed 100 mm/min, (CL2) = laser speed 150 mm/min, (CL3) = laser speed 200 mm/min, (CL4) = laser speed 250 mm/min.
Fig. 9. SEM microstructure at different laser speeds (CL1) = laser speed 100 mm/min, (CL2) = laser speed 150 mm/min, (CL3) = laser speed 200 mm/min, (CL4) = laser speed 250 mm/min.
M. Afzal et al. / Surface & Coatings Technology 266 (2015) 22–30
Fig. 10. The microhardness against melted depth for the different samples.
of cobalt in the laser treated zone was not detected except at one point. The concentration of cobalt in this region was very low because the depth and width both increased with a decrease in laser speed. The variation in composition along with the depth of the laser melted zone for sample CL4 is shown in Fig. 7. A uniform concentration for the major elements was observed through the depth of the laser treated zone, but cobalt was not detected at some points within the zone. 3.2.2. SEM observation of modified surfaces Metallographic examination revealed the presence of different zones in the alloyed surface. The first one is a zone rich in WC-Co particles embedded in the surface, i.e. laser treated area, as shown in
27
Fig. 8. The next zone can be considered as a transient zone between the alloyed zone and the base metal. This zone shows slightly changed structure compared to the substrate material and can be considered as a very narrow heat affected zone (HAZ). In the “as polished” condition, the cross-section of the samples revealed modified layers, Fig. 8. It was observed that the modified laser treated layer has a good appearance without any discontinuities as well as free of micro-cracks in all the samples. However, the bead demonstrated in CL1, showed a comparatively uniform distribution of cellular dendritic structure compared to other beads. The decrease in bead thickness by increasing the laser speed was probably due to insufficient time for surface melting. However, a homogeneous microstructure was observed in all laser melted tracks while the APS coating had some porosity along the coat/substrate interface. It can be concluded that laser treatment improved the properties of these carbide coatings deposited by APS techniques. Furthermore, it was also observed that after laser melting, a continuous and sound interface between the substrate and laser modified layer was achieved as shown in Fig. 8. No micro-cracks or delamination was observed in the laser modified zones compared to the as sprayed APS coatings. However, some micro-voids were seen and could be due to entrapped gases or the formation of gas pockets. These types of defects were also observed by other researchers [3]. At higher laser melting speeds these voids were more prominent and could be due to the insufficient time for gases to escape from the molten pool, as shown in Fig. 9 (CL4). A fine dendritic region, next to the HAZ, was observed in all samples (CL1 to CL4). It was observed that a finer dendrite structure was visible at higher laser speeds. This is because the higher speed created a smaller molten zone compared to surface melting at slower
Fig. 11. The microhardness against melted width for the different samples.
Fig. 12. Mass loss during the sliding wears where (a): substrate, (b): as sprayed APS coating, (c): laser modified at speed of 100 mm/min, (d): laser modified at speed of 150 mm/min, (e): laser modified at speed of 200 mm/min, and (f): laser modified at speed of 250 mm/min.
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Fig. 13. Wear rate in terms of sliding distance where (a): substrate, (b): as sprayed APS coating, (c): laser modified at speed of 100 mm/min, (d): laser modified at speed of 150 mm/min, (e): laser modified at speed of 200 mm/min, and (f): laser modified at speed of 250 mm/min.
Fig. 14. Sliding wear resistant values where (a): substrate, (b): as sprayed APS coating, (c): laser modified at speed of 100 mm/min, (d): laser modified at speed of 150 mm/min, (e): laser modified at speed of 200 mm/min, and (f): laser modified at speed of 250 mm/min.
speeds. In the smaller molten zone less volume of metal results in faster solidification and this produces a finer dendritic structure. The brighter phase shown in Figs. 8 and 9 were analyzed and found to be rich in tungsten. 3.3. Micro-hardness testing Observations using the SEM showed that WC-12%Co layer completely melted and inter-mixed with the base metal. This was
indicated by the Energy Dispersive Spectroscopy (EDS) composition profiles, measured throughout the depth of the laser melted zone, see Figs. 6 and 7. The micro-hardness profiles measured along the depth and width of the laser melted zone at different speeds are shown in the Figs. 10 and 11. The depth of the laser melted zone decreased with increasing the laser speed. The amount of WC phase increased with increasing laser speed because less base metal dilution occurred with the coating. This increase in WC phase resulted in an increase in surface hardness.
Fig. 15. Wear depths as function of sliding distance during sliding wear, where (a): substrate, (b): as sprayed APS coating, (c): laser modified at speed of 100 mm/min, (d): laser modified at speed of 150 mm/min, (e): laser modified at speed of 200 mm/min, and (f): laser modified at speed of 250 mm/min.
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3.4. Wear characteristics Pin-on-plate sliding wear tests were conducted using a sliding distance of 53 m, and change in mass loss was measured by weighing the coupon before and after the wear test. The wear rate was determined by using the following equation [26]:Wear rateðkg=mÞ ¼ Mass loss or wear amount=Slidingdistance:
ð6Þ
While wear resistance was calculated as: −1 : Wear resistance ¼ 1=ðwear amount or mass lossÞ kg
ð7Þ
The wear test results showed that the substrate material (AISI 321) had higher mass loss value compared to the treated surfaces. Fig. 12 shows that the lowest mass loss was for laser surfaces, treated with the
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lowest speed of 100 mm/min. As the laser speed was increased to 250 mm/min, a gradual increase in wear loss was recorded. The graph in Figs. 13 and 14 showed that an increase in wear resistance for laser melted surfaces at a speed of 100 mm/min, and a loss in wear resistance with increasing laser speed. Furthermore, the results showed that by using a slower speed results in less wear depth as shown in Fig. 15(c) and (d). The SEM micrographs in Fig. 16(a) to (f) are taken from the worn surfaces and show the mechanism of wear. The micrograph for the untreated steel substrate shows considerable fracture and loss of material from the surface. However, the extent of fracture decreased with the deposition of the harder WC-Co coating. The laser melting process increased the wear resistance of the coatings and much less surface fracture was observed as shown in Fig. 16(c) and (d). The extent of surface fracture increased with increase in laser speed. These results correspond with the wear test results shown in Figs. 12 to 14.
Fig. 16. Worn out surfaces and wear scars after sliding wear tests whereas (a): substrate, (b): as sprayed APS coating, (c): laser modified at speed of 100 mm/min, (d): laser modified at speed of 150 mm/min, (e): laser modified at speed of 200 mm/min, and (f): laser modified at speed of 250 mm/min.
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4. Conclusions This study shows that surface coatings deposited by air plasma spraying can be improved by laser melting. It is observed that the surface hardness could be controlled by optimizing the speed of the laser gun. The melting and solidification processes reduce the inherent defects within the APS deposited coatings. The wear resistance of the coating is significantly enhanced when laser melting is performed at a speed of 100 mm/min, but this wear resistance decreased as the laser speed increased to 250 mm/min. Acknowledgment The authors gratefully acknowledge the financial support of UET, Lahore and technical support of IICS (09-Ph.D-Met-04) and Pakistan Institute of Laser and Optics (PILO). The authors are also thankful to the Surfaces and Joining Research Group within the Department of Mechanical & Manufacturing Engineering at the University of Calgary, Alberta, Canada. References [1] Y. Xiong, J.E. Smugeresky, J.M. Schoenung, J. Mater. Process. Technol. 209 (10) (2009) 4935–4941. [2] V.K. Balla, S. Bose, A. Bandyopadhyay, Mater. Sci. Eng. A 527 (24–25) (2010) 6677–6682.
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