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Influence of binder materials on the properties of low power plasma sprayed cermet coatings
Surface & Coatings Technology 199 (2005) 66 – 71 www.elsevier.com/locate/surfcoat
Influence of binder materials on the properties of low power plasma sprayed cermet coatings M.F. Morksa,b,*, Yang Gaob, N.F. Fahimc, F.U. Yingqingb, M.A. Shoeiba a
Central Metallurgical Research and Development Institute, P.O. Box 87 Helwan El-Tibben, Cairo, Egypt b Materials and Technology Institute, Dalian Maritime University, 116026 Dalian City, PR China c National Research Center, 12622 El-Dokki, Giza, Egypt Received 15 April 2004; accepted in revised form 11 February 2005 Available online 22 April 2005
Abstract Thermal spraying of cermet coatings is widely used for protection of machined parts against wear and corrosion. These coatings consist of WC particles in metal binders such as Co, Cr, and Ni. In this study, three kinds of WC powders with different metal binders (Co, NiCr and CoCr) were sprayed by low power plasma spray system on Al – Si – Cu alloy substrate. Fundamental aspects of sprayed cermet coatings, including (i) the effects of binder type on the coating structure, (ii) the microhardness properties and (iii) the microstructure were investigated. All the cermet coatings had the same phase structure such as WC and W2C. However, the phase intensities of these phases are different in each coating, mainly due to the different in solidification rate in each case. Moreover, the microhardness measurements were found to be different in each coating. The results show that the binder type has a significance effect of the physical and mechanical properties of the sprayed coatings. D 2005 Published by Elsevier B.V. Keywords: Tungsten carbide; Binders; Aluminum alloy substrate; Microhardness; Microstructure
1. Introduction Current trends in marine industry are focused on decreasing the weight of marine vehicles such as ships and submarines by using light materials such as aluminum and magnesium alloys. However, these materials exhibit poor wear resistance because of their softness. Also, they are often exposed to environments that are both corrosive and erosive mainly due to the salt water of the sea. This causes a reduction to the lifetime of these components and high maintenance costs. For that, surface modification is required to improve their wear and corrosion resistance. Al – Si– Cu alloys are one of the promising light materials currently used to fabricate many machined parts such as the cylinder engine block for their low specific gravity and high thermal conductivity. In previous studies [1– 4] authors used * Corresponding author. Central Metallurgical Research and Development Institute, P.O. Box 87 Helwan El-Tibben, Cairo, Egypt. Tel.: +20 2 7620596; fax: +20 2 5010639. E-mail address: [email protected] (M.F. Morks). 0257-8972/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2005.02.159
cast iron plasma spray coatings to improve the wear resistance of such alloys against the wear arising from the movement of piston rings in cylinder blocks. Although plasma cast iron coating is a good candidate for Al-alloys in lubricated engine environment, it cannot be used in case of marine components, because they may expose to corrosive environment such as seawater. Protection of light and soft metallic components by ceramic – metallic coatings (cermet) or hard oxide coatings is an effective method to reduce the wear and corrosion. These coatings consist of WC particles in a metal binder such as Co, Cr, and Ni. WC –Co cermet coatings have wide applications in machinery industries as a wear resistant-material due to the advantage of combination of microhardness and toughness [5– 8]. These coatings have good tribological properties due to their excellent adherence to the substrate, good cohesion, low porosity, and low tendency to form brittle phases (W2C, Co3W3C) during spraying. These excellent properties make these coatings a good choice for use in engineering applications [9 – 11]. Mechanistic studies of metal matrix composites with
M.F. Morks et al. / Surface & Coatings Technology 199 (2005) 66 – 71
composition similar to some WC – Co cermet coatings have been conducted by Human et al. [12] and they have demonstrated that C and W dissolved in the Co binder are effective in reducing the corrosion resistance of the binder. Corrosion behavior of thermal spray coatings (WC – CrNi and WC/CrC – CoCr) has been addressed by Souza and Neville [13] where it was shown that WC –CrNi coatings exhibit passivity and very low galvanic currents were measured which indicate that these coatings act as net anode. Many authors studied the performance of conventional WC – Co coatings, but a little works have been done on WC –Ni [14 –16] and WC –CrNi [17]. Moreover, the effect of binder materials (Co, Ni, and Cr) on the microstructure and microhardness properties of the cermet coatings has not been investigated. In this paper, three WC- (Co, NiCr, CoCr) powders were sprayed by low power plasma spray system. The influence of binder’s types on the coating microstructure, phase structure and microhardness properties has been investigated. Phase structure and microhardness distribution as a function of coating thickness was examined to try to understand the relation between microstructure and microhardness properties.
2. Experimental procedure 2.1. Materials and plasma spray system Three different cermet powders of WC particles in different metallic binder materials (Co, NiCr and Co –Cr) were used as feedstock. The powders composition and characteristics are presented in Table 1. The size distribution of these powders, determined with a laser diffusion sizer, was found to be in the range from 20 to 75 Am, with a mean value of 43 Am. The powders were sieved into size range of 38– 53 Am, using ultrasonic sieving machine, and plasma sprayed onto Al –Si – Cu alloys substrate. The sieved powders had a WC grain size of 3 – 5 Am. The substrates of diminution 30 30 2 mm3 were grit-blasted on one side to clean and roughen the surface (Ra 1 –2 Am) and followed by ultrasonic cleaning using acetone to remove any dusts. Spraying was achieved in air atmosphere using low power plasma spray system (Plasma
Table 1 Powders characteristics Compositions
Manufacturing method
Manufacturing company
WC – 12Co
Sintered crushed
WC – NiCr
Sintered crushed
WC – 4Co10Cr
Sintered crushed
Zigong Cemented Carbide Co., Ltd., China Zigong Cemented Carbide Co., Ltd., China Sumitomo Metal Mining Co., Ltd. Japan (SWCC-8614F)
67
Table 2 Spray conditions for the cermet powders Powder
WC – Co, WC – NiCr, WC – Co – Cr
Particle size (Am) Powder flow rate (g/min) Primary plasma gas (l/min) Secondary plasma gas (l/min) Arc current (A) Arc voltage (V) Powder carrier gas: (l/min) Spray distance (mm) Substrate temperature (K)
(38 – 53) 30 Ar: 21.6 H2: 3.3 130 50 N2: 6.6 70 ¨293
LE15), developed by Dalian Maritime University [18,19]. The spray conditions are presented in Table 2. The starting preheat substrate temperature (T S) was 293 K for all sprayed coatings. 2.2. Powder and coating characterization The different phase composition of the cermet powders and coatings were examined by X-ray diffraction (XRD) on a Rigaku D/max-IIIA X-ray diffractometer. The radiation ˚ , step size of 0.02- and dwell time source is CuKa (1.5406 A of 2 s). Powder morphology and coatings microstructure were showed by PHILIPS XL30 scanning electron microscope (utilizing secondary and backscattered electron and energy-dispersive X-ray (EDX) detector. Small species from as sprayed coatings were cut for phase identification by XRD. The cross-section samples suitable for scanning electron microscope (SEM) and microhardness measurements were prepared by mounting the cross-section pieces in conducting resin and then grinding and polishing the surface. The microhardness tests were performed using MH-6 microhardness tester at 200 g load and a dwell time of 5 s.
3. Results and discussion 3.1. Powder morphology and phase composition Typical scanning electron microscope morphologies and X-ray diffraction patterns of different cermet powders are shown in Fig. 1. The powder morphologies present the microstructural characteristics of sintered and crushed powders. From X-ray diffraction spectra, WC is the main phases of all the cermet powders. No peak from W2C phase was detected. 3.2. Coatings structure Typical X-ray diffraction patterns of different cermet coatings are shown in Fig. 2. WC is the major phase in all coatings. a-W2C phase was also detected in all coatings. However, the calculated intensity ratio of this phase is
WC (110) WC
WC (001)
Relative Intensity
400
WC-Co Powder
WC (111) WC (200) WC (102)
(a)
200
WC (101)
M.F. Morks et al. / Surface & Coatings Technology 199 (2005) 66 – 71 WC (100)
68
0 20
40
WC (111) WC (200) WC (102)
WC (110) WC
WC-Co-Cr Powder
WC (110) WC
Co6W6C (511)
WC (001)
Relative Intensity
400
WC-NiCr Powder
60
80
Diffraction angle, 2 θ /
WC (111) WC (200) WC (102)
(c)
200
WC (101)
WC (100)
0
Co
200
WC (101)
Co6W6C (511)
WC (001)
Relative Intensity
400
WC (100)
0
(b)
100
o
Fig. 1. SEM micrographs and XRD patterns of WC cermet powders of different binders: (a) WC – Co, (b) WC – NiCr and (c) WC – CoCr.
varied in each coating. The intensity ratio of W2C to WC was calculated by applying the following equation: Intensity ratio of W2 C ¼ IW2 Cð101Þ = IW2 Cð101Þ þ IWCð101Þ :
ð1Þ
The calculated intensity ratio of W2C for the different cermet coatings is shown in Fig. 3. WC – Co cermet coatings showed low intensity ratio of W2C phase. The amounts of W2C in the coatings depend on the amount of WC dissolution into the molten binder during spraying as well as the solubility of WC into the various binders. The solubility factor is greatly affected by the metallic binder due to the difference in physical properties such as viscosity, thermal conductivity and density. One can conclude from results that the metal binder type in the molten state affects the solubility of dissolved WC particles during spraying. The results reveal that WC – Co coating with Co binder exhibits lower solubility rate because the lowest intensity ratio of rabid solidified W2C phase was recognized in WC – Co coatings. On the other hand, the cermet coatings
of NiCr and CoCr binders nearly have the same intensity ratio of W2C. Appearance of W2C phase is mainly referring to the decarburization of WC phase in the hot zone of the flame as shown by the following equations: 2WC Y W2 C þ C
ð2Þ
The carbon is oxidized in the flame by reaction with oxygen as described by Eq. (3) [20]. 2C þ O2 Y 2COðgasÞ þ O2 Y 2CO2 ðgasÞ:
ð3Þ
3.3. Microhardness properties The microhardness was examined on cross-section samples of as-sprayed different cermet coatings. The average value of 20 measurements taken at different positions for each coating was measured. The results are shown in Fig. 4. WC –CoCr and WC –Co coatings had the highest hardness value of 1400 and 1300HV, respectively. However, WC – NiCr coating showed lower microhardness value of 1170HV.
M.F. Morks et al. / Surface & Coatings Technology 199 (2005) 66 – 71 2000
WC-Co Coating
Hardness of WC-different binder Coatings Arc Current = 130 A and Arc Voltage = 50 V
WC (101)
Hardness, HV
1800
WC (110)
WC (111) WC (200) WC (102)
WC (100) α -W2C (101)
125
WC (001)
Relative Intensity
250
69
1600 1400 1200 1000
0 WC-Co
WC-NiCr Coating WC (101)
WC (111) WC (200) WC (102)
α-W2 C (103)
WC (110)
0
WC-CoCr Coating
0 20
40
WC (110)
WC (111) WC (200) WC (102)
WC (101)
α-W 2C (101)
WC (100) WC (001)
Relative Intensity
250
125
WC-CoCr
Fig. 4. Influence of binder metal type on the microhardness value of cermet coatings.
α-W2 C (110)
WC (100)
α-W2 C (002) α-W2C (101)
125
WC (001)
Relative Intensity
250
WC-NiCr
Cermet Coatings
60
Diffraction angle, 2θ /
80
100
o
It is obvious from microhardness measurements that the binder type affects greatly the microhardness properties of cermet coatings. This probably due to the difference in physical properties of each metal binder as well as the way of distribution of WC particles in binder matrix. The microhardness values of WC – Co coating was measured at different distances from substrate to evaluate the coating microhardness distribution near and far from the substrate. The average value of 10 measurements, taken parallel to the substrate, at different distances from the substrate was evaluated as shown in Fig. 5. The microhardness value increases as the coating thickness increases. This may be related to the decrease in the amount of the brittle W2C in the coating as the coating thickness increases. The first coating lamella impact with the substrate surface achieves the highest solidification rate specially with spraying at room temperature. The solidification rate of molten metallic binder decreases as the coating thickness increases, mainly due to the decrease in heat transfer from the molten binder containing WC to the substrate. The dissolution of dissolved WC particles in the molten binder is
Fig. 2. Influence of binder type on phase structure of different cermet coatings. 1800
0.4
IW 2C (101) [ IW 2C (101) + IWC (101) ]
0.3
0.2
1400
1200
1000
0.1
0.0
1600
Hardness, HV
Intensity ratio of W2C
0.5
WC-Co
WC-NiCr
WC-CoCr
Cermet Coatings Fig. 3. Change in intensity ratio of W2C to the sum of W2C and WC on the X-ray diffraction pattern of cermet coatings with different binders.
800 100
200
300
400
500
600
Coating Thickness, µm Fig. 5. Change in coating microhardness as a function of WC – CoCr coating thickness.
70
M.F. Morks et al. / Surface & Coatings Technology 199 (2005) 66 – 71
affected by the molten binder temperature, which is affected by heat transfer rate. The decrease of intensity ratio of W2C phase as a function of coating thickness was proven by examination the phase structure at different coating thickness. For that, WC – Co coating was polished to 400 Am, 200 Am and 100 Am. The phase structure was examined at different coating thickness by X-ray diffraction. The intensity ratio of W2C was calculated from XRD spectra by applying the same equation (1) shown above. The change in intensity ratio of W2C phase at different coating thickness is shown in Fig. 6. The results show a decrease in intensity ratio of W2C phase as the coating thickness. 3.4. Coating morphology Cross-sections micrographs of cermet coatings with different binders analyzed via scanning electron microscope are shown in Fig. 7. The morphologies were taken at nearly the same coatings thickness of 300 Am. All coatings microstructure look dense without pore. However, there are some differences in microstructures for each coating. Although WC – Co and WC – CoCr coatings seem to have the same microstructure, the size of carbides in WC– CoCr coating is smaller (1 –2 Am) than that in WC –Co coating (> 2 Am). The fine carbides particles in these two coatings are arranged closely from each other and sometimes touch and combine with each other. This enforces the mechanical bound among the WC particles and binder metal and explains the high microhardness values of these coatings. The morphology of carbides in the case of WC –NiCr did not clearly appear. It seems that the carbides are covered with thick layers of Ni binder. This may explain the lower microhardness value of WC – NiCr coatings because the thick layer of soft Ni binder between WC particles isolate the WC particles from each other and prevent them from closing or touching, which results in a decrease in the microhardness values of the coatings. The morphologies of WC –Co coating near and far from the substrate are shown in Fig. 8. The carbides near the
Intensity Ratio of W2C
0.40
Fig. 7. Scanning electron micrographs of cross-sections of cermet coatings with different binders.
0.35 0.30 0.25 0.20 0.15
100
200
300
400
500
600
Coating Thickness, µm Fig. 6. Change in intensity ratio of W2C phase with WC – CoCr coating thickness.
Fig. 8. SEM morphologies of WC – Co coatings showing the microstructure near and far from the substrate.
M.F. Morks et al. / Surface & Coatings Technology 199 (2005) 66 – 71
substrate are coarser, larger in size and far from each other compared with the carbides far from the substrate. Near the substrate the carbide particles seem to be loosely and not engrossed in the binder matrix. The morphologies agree well with our explanation about the microhardness values taken at different coating thickness. From the above results it is clear that microstructure is closely linked to the binder materials, which affects greatly the coating microhardness.
4. Conclusions Three cermet coatings with different binders (– Co, – NiCr and – CoCr) were plasma sprayed via low power plasma spray system. The effect of binder materials on coatings microstructure and microhardness properties was examined. The conclusions can be summarized as follow: – The intensity ratio of W2C is lower in WC – Co coating than WC – NiCr and WC –CoCr coatings, which indicate lower solidification rate of WC – Co coatings. – The microhardness value is higher in WC– Co and WC – CoCr compared with WC – NiCr coating. – The intensity ratio of W2C decreases as the coating thickness increase mainly due to the decrease in solidification rate. – The coating microhardness increases as the coating thickness increases due to the decrease in intensity ratio of W2C. – The coating microstructure varied with coating thickness and extensive amount of W2C phase is formed near the substrate because of the rabid solidification of flight particles.
Acknowledgments This work has been supported by China PostDoc Foundation and the National Natural Science Foundation of China (project no. 50075011).
71
References [1] M.F. Morks, Y. Tsunekawa, M. Okumiya, M.A. Shoeib, Mater. Trans. 4 (2003) 743. [2] M.F. Morks, Y. Tsunekawa, M. Okumiya, M.A. Shoeib, J. Therm. Spray Technol. 12 (2) (2003) 282. [3] M.F. Morks, Y. Tsunekawa, M. Okumiya, M.A. Shoeib, J. Therm. Spray Technol. 11 (2) (2002 (June)) 226. [4] M.F. Morks, Y. Tsunekawa, M. Okumiya, M.A. Shoeib, Proc. The International Thermal Spray Conference Orlando, Florida 5 – 8 May, USA, 2003. [5] H.L. de Villiers Lovelock, P.W. Richter, J.M. Benson, P.M. Young, J. Therm. Spray Technol. 7 (1998) 97. [6] A.K. Akasawat, Proceedings of the 15th International Thermal Spray Conference, 25 – 29 May, Nice, France, 1998, p. 281. [7] H.M. Hawthorne, B. Arsenault, J.P. Immarigeon, J.G. Legoux, V.R. Parameswaran, Wear 225 – 229 (1999) 825. [8] A. Karimi, Ch. Verdon, J.L. Martin, R.K. Schmid, Wear 186 – 187 (1995) 480. [9] C. Edwards, P. Trester, B. Ryder, D. Wang, D. Marosok, et.al. In the Proceeding, ITSC 04 Thermal Spray Conference, May 10 – 12 Osaka, Japan. [10] J.M. Guilemany, J.M. dePaco, J. Nutting, J.R. Miguel, Metall. Mater. Trans., A 30A (1999) 1913. [11] A. Karimi, C. Verdon, G. Barbezat, Surface Modification Technologies VIII, Nice, France, 1995, p. 747. [12] A.M. Human, H.E. Exner, Int. J. Refract. Met. Hard Mater. 15 (1997) 65. [13] V.A.D. Souza, A. Neville, Mater. Sci. Eng., A 352 (2003) 202. [14] V.V. Sobolev, J.I. Guilemany, J.R. Miguel, J.A. Calero, Surf. Coat. Technol. 82 (1 – 2) (1996 (July)) 121. [15] J.M Guilemany, J.R. Miguel, J.A. Calero, V.V. Sobolev, Surf. Coat. Technol. 82 (1 – 2) (1996 (July)) 114. [16] J.M Guilemany, J. Nutting, J.R. Miguel, Z. Dong, Scr. Metall. Mater. 33 (1) (1995 (July 1)) 55. [17] V.A.D. Souza, A. Neville, Mater. Sci. Eng., A 352 (1 – 2) (2003 (July 15)) 202. [18] Y. Gao, China Surf. Eng. 2 (1999) 24. [19] Y. Gao, Low Power Plasma Apparatus, Chinese Patent, 2000, ZL99243441.6. [20] J.B. Nerz, A. Kushner, A.J. Rotolico, J. Therm. Spray Technol. 1 (2) (1992) 147.
Influence of binder materials on the properties of low power plasma sprayed cermet coatings M.F. Morksa,b,*, Yang Gaob, N.F. Fahimc, F.U. Yingqingb, M.A. Shoeiba a
Central Metallurgical Research and Development Institute, P.O. Box 87 Helwan El-Tibben, Cairo, Egypt b Materials and Technology Institute, Dalian Maritime University, 116026 Dalian City, PR China c National Research Center, 12622 El-Dokki, Giza, Egypt Received 15 April 2004; accepted in revised form 11 February 2005 Available online 22 April 2005
Abstract Thermal spraying of cermet coatings is widely used for protection of machined parts against wear and corrosion. These coatings consist of WC particles in metal binders such as Co, Cr, and Ni. In this study, three kinds of WC powders with different metal binders (Co, NiCr and CoCr) were sprayed by low power plasma spray system on Al – Si – Cu alloy substrate. Fundamental aspects of sprayed cermet coatings, including (i) the effects of binder type on the coating structure, (ii) the microhardness properties and (iii) the microstructure were investigated. All the cermet coatings had the same phase structure such as WC and W2C. However, the phase intensities of these phases are different in each coating, mainly due to the different in solidification rate in each case. Moreover, the microhardness measurements were found to be different in each coating. The results show that the binder type has a significance effect of the physical and mechanical properties of the sprayed coatings. D 2005 Published by Elsevier B.V. Keywords: Tungsten carbide; Binders; Aluminum alloy substrate; Microhardness; Microstructure
1. Introduction Current trends in marine industry are focused on decreasing the weight of marine vehicles such as ships and submarines by using light materials such as aluminum and magnesium alloys. However, these materials exhibit poor wear resistance because of their softness. Also, they are often exposed to environments that are both corrosive and erosive mainly due to the salt water of the sea. This causes a reduction to the lifetime of these components and high maintenance costs. For that, surface modification is required to improve their wear and corrosion resistance. Al – Si– Cu alloys are one of the promising light materials currently used to fabricate many machined parts such as the cylinder engine block for their low specific gravity and high thermal conductivity. In previous studies [1– 4] authors used * Corresponding author. Central Metallurgical Research and Development Institute, P.O. Box 87 Helwan El-Tibben, Cairo, Egypt. Tel.: +20 2 7620596; fax: +20 2 5010639. E-mail address: [email protected] (M.F. Morks). 0257-8972/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2005.02.159
cast iron plasma spray coatings to improve the wear resistance of such alloys against the wear arising from the movement of piston rings in cylinder blocks. Although plasma cast iron coating is a good candidate for Al-alloys in lubricated engine environment, it cannot be used in case of marine components, because they may expose to corrosive environment such as seawater. Protection of light and soft metallic components by ceramic – metallic coatings (cermet) or hard oxide coatings is an effective method to reduce the wear and corrosion. These coatings consist of WC particles in a metal binder such as Co, Cr, and Ni. WC –Co cermet coatings have wide applications in machinery industries as a wear resistant-material due to the advantage of combination of microhardness and toughness [5– 8]. These coatings have good tribological properties due to their excellent adherence to the substrate, good cohesion, low porosity, and low tendency to form brittle phases (W2C, Co3W3C) during spraying. These excellent properties make these coatings a good choice for use in engineering applications [9 – 11]. Mechanistic studies of metal matrix composites with
M.F. Morks et al. / Surface & Coatings Technology 199 (2005) 66 – 71
composition similar to some WC – Co cermet coatings have been conducted by Human et al. [12] and they have demonstrated that C and W dissolved in the Co binder are effective in reducing the corrosion resistance of the binder. Corrosion behavior of thermal spray coatings (WC – CrNi and WC/CrC – CoCr) has been addressed by Souza and Neville [13] where it was shown that WC –CrNi coatings exhibit passivity and very low galvanic currents were measured which indicate that these coatings act as net anode. Many authors studied the performance of conventional WC – Co coatings, but a little works have been done on WC –Ni [14 –16] and WC –CrNi [17]. Moreover, the effect of binder materials (Co, Ni, and Cr) on the microstructure and microhardness properties of the cermet coatings has not been investigated. In this paper, three WC- (Co, NiCr, CoCr) powders were sprayed by low power plasma spray system. The influence of binder’s types on the coating microstructure, phase structure and microhardness properties has been investigated. Phase structure and microhardness distribution as a function of coating thickness was examined to try to understand the relation between microstructure and microhardness properties.
2. Experimental procedure 2.1. Materials and plasma spray system Three different cermet powders of WC particles in different metallic binder materials (Co, NiCr and Co –Cr) were used as feedstock. The powders composition and characteristics are presented in Table 1. The size distribution of these powders, determined with a laser diffusion sizer, was found to be in the range from 20 to 75 Am, with a mean value of 43 Am. The powders were sieved into size range of 38– 53 Am, using ultrasonic sieving machine, and plasma sprayed onto Al –Si – Cu alloys substrate. The sieved powders had a WC grain size of 3 – 5 Am. The substrates of diminution 30 30 2 mm3 were grit-blasted on one side to clean and roughen the surface (Ra 1 –2 Am) and followed by ultrasonic cleaning using acetone to remove any dusts. Spraying was achieved in air atmosphere using low power plasma spray system (Plasma
Table 1 Powders characteristics Compositions
Manufacturing method
Manufacturing company
WC – 12Co
Sintered crushed
WC – NiCr
Sintered crushed
WC – 4Co10Cr
Sintered crushed
Zigong Cemented Carbide Co., Ltd., China Zigong Cemented Carbide Co., Ltd., China Sumitomo Metal Mining Co., Ltd. Japan (SWCC-8614F)
67
Table 2 Spray conditions for the cermet powders Powder
WC – Co, WC – NiCr, WC – Co – Cr
Particle size (Am) Powder flow rate (g/min) Primary plasma gas (l/min) Secondary plasma gas (l/min) Arc current (A) Arc voltage (V) Powder carrier gas: (l/min) Spray distance (mm) Substrate temperature (K)
(38 – 53) 30 Ar: 21.6 H2: 3.3 130 50 N2: 6.6 70 ¨293
LE15), developed by Dalian Maritime University [18,19]. The spray conditions are presented in Table 2. The starting preheat substrate temperature (T S) was 293 K for all sprayed coatings. 2.2. Powder and coating characterization The different phase composition of the cermet powders and coatings were examined by X-ray diffraction (XRD) on a Rigaku D/max-IIIA X-ray diffractometer. The radiation ˚ , step size of 0.02- and dwell time source is CuKa (1.5406 A of 2 s). Powder morphology and coatings microstructure were showed by PHILIPS XL30 scanning electron microscope (utilizing secondary and backscattered electron and energy-dispersive X-ray (EDX) detector. Small species from as sprayed coatings were cut for phase identification by XRD. The cross-section samples suitable for scanning electron microscope (SEM) and microhardness measurements were prepared by mounting the cross-section pieces in conducting resin and then grinding and polishing the surface. The microhardness tests were performed using MH-6 microhardness tester at 200 g load and a dwell time of 5 s.
3. Results and discussion 3.1. Powder morphology and phase composition Typical scanning electron microscope morphologies and X-ray diffraction patterns of different cermet powders are shown in Fig. 1. The powder morphologies present the microstructural characteristics of sintered and crushed powders. From X-ray diffraction spectra, WC is the main phases of all the cermet powders. No peak from W2C phase was detected. 3.2. Coatings structure Typical X-ray diffraction patterns of different cermet coatings are shown in Fig. 2. WC is the major phase in all coatings. a-W2C phase was also detected in all coatings. However, the calculated intensity ratio of this phase is
WC (110) WC
WC (001)
Relative Intensity
400
WC-Co Powder
WC (111) WC (200) WC (102)
(a)
200
WC (101)
M.F. Morks et al. / Surface & Coatings Technology 199 (2005) 66 – 71 WC (100)
68
0 20
40
WC (111) WC (200) WC (102)
WC (110) WC
WC-Co-Cr Powder
WC (110) WC
Co6W6C (511)
WC (001)
Relative Intensity
400
WC-NiCr Powder
60
80
Diffraction angle, 2 θ /
WC (111) WC (200) WC (102)
(c)
200
WC (101)
WC (100)
0
Co
200
WC (101)
Co6W6C (511)
WC (001)
Relative Intensity
400
WC (100)
0
(b)
100
o
Fig. 1. SEM micrographs and XRD patterns of WC cermet powders of different binders: (a) WC – Co, (b) WC – NiCr and (c) WC – CoCr.
varied in each coating. The intensity ratio of W2C to WC was calculated by applying the following equation: Intensity ratio of W2 C ¼ IW2 Cð101Þ = IW2 Cð101Þ þ IWCð101Þ :
ð1Þ
The calculated intensity ratio of W2C for the different cermet coatings is shown in Fig. 3. WC – Co cermet coatings showed low intensity ratio of W2C phase. The amounts of W2C in the coatings depend on the amount of WC dissolution into the molten binder during spraying as well as the solubility of WC into the various binders. The solubility factor is greatly affected by the metallic binder due to the difference in physical properties such as viscosity, thermal conductivity and density. One can conclude from results that the metal binder type in the molten state affects the solubility of dissolved WC particles during spraying. The results reveal that WC – Co coating with Co binder exhibits lower solubility rate because the lowest intensity ratio of rabid solidified W2C phase was recognized in WC – Co coatings. On the other hand, the cermet coatings
of NiCr and CoCr binders nearly have the same intensity ratio of W2C. Appearance of W2C phase is mainly referring to the decarburization of WC phase in the hot zone of the flame as shown by the following equations: 2WC Y W2 C þ C
ð2Þ
The carbon is oxidized in the flame by reaction with oxygen as described by Eq. (3) [20]. 2C þ O2 Y 2COðgasÞ þ O2 Y 2CO2 ðgasÞ:
ð3Þ
3.3. Microhardness properties The microhardness was examined on cross-section samples of as-sprayed different cermet coatings. The average value of 20 measurements taken at different positions for each coating was measured. The results are shown in Fig. 4. WC –CoCr and WC –Co coatings had the highest hardness value of 1400 and 1300HV, respectively. However, WC – NiCr coating showed lower microhardness value of 1170HV.
M.F. Morks et al. / Surface & Coatings Technology 199 (2005) 66 – 71 2000
WC-Co Coating
Hardness of WC-different binder Coatings Arc Current = 130 A and Arc Voltage = 50 V
WC (101)
Hardness, HV
1800
WC (110)
WC (111) WC (200) WC (102)
WC (100) α -W2C (101)
125
WC (001)
Relative Intensity
250
69
1600 1400 1200 1000
0 WC-Co
WC-NiCr Coating WC (101)
WC (111) WC (200) WC (102)
α-W2 C (103)
WC (110)
0
WC-CoCr Coating
0 20
40
WC (110)
WC (111) WC (200) WC (102)
WC (101)
α-W 2C (101)
WC (100) WC (001)
Relative Intensity
250
125
WC-CoCr
Fig. 4. Influence of binder metal type on the microhardness value of cermet coatings.
α-W2 C (110)
WC (100)
α-W2 C (002) α-W2C (101)
125
WC (001)
Relative Intensity
250
WC-NiCr
Cermet Coatings
60
Diffraction angle, 2θ /
80
100
o
It is obvious from microhardness measurements that the binder type affects greatly the microhardness properties of cermet coatings. This probably due to the difference in physical properties of each metal binder as well as the way of distribution of WC particles in binder matrix. The microhardness values of WC – Co coating was measured at different distances from substrate to evaluate the coating microhardness distribution near and far from the substrate. The average value of 10 measurements, taken parallel to the substrate, at different distances from the substrate was evaluated as shown in Fig. 5. The microhardness value increases as the coating thickness increases. This may be related to the decrease in the amount of the brittle W2C in the coating as the coating thickness increases. The first coating lamella impact with the substrate surface achieves the highest solidification rate specially with spraying at room temperature. The solidification rate of molten metallic binder decreases as the coating thickness increases, mainly due to the decrease in heat transfer from the molten binder containing WC to the substrate. The dissolution of dissolved WC particles in the molten binder is
Fig. 2. Influence of binder type on phase structure of different cermet coatings. 1800
0.4
IW 2C (101) [ IW 2C (101) + IWC (101) ]
0.3
0.2
1400
1200
1000
0.1
0.0
1600
Hardness, HV
Intensity ratio of W2C
0.5
WC-Co
WC-NiCr
WC-CoCr
Cermet Coatings Fig. 3. Change in intensity ratio of W2C to the sum of W2C and WC on the X-ray diffraction pattern of cermet coatings with different binders.
800 100
200
300
400
500
600
Coating Thickness, µm Fig. 5. Change in coating microhardness as a function of WC – CoCr coating thickness.
70
M.F. Morks et al. / Surface & Coatings Technology 199 (2005) 66 – 71
affected by the molten binder temperature, which is affected by heat transfer rate. The decrease of intensity ratio of W2C phase as a function of coating thickness was proven by examination the phase structure at different coating thickness. For that, WC – Co coating was polished to 400 Am, 200 Am and 100 Am. The phase structure was examined at different coating thickness by X-ray diffraction. The intensity ratio of W2C was calculated from XRD spectra by applying the same equation (1) shown above. The change in intensity ratio of W2C phase at different coating thickness is shown in Fig. 6. The results show a decrease in intensity ratio of W2C phase as the coating thickness. 3.4. Coating morphology Cross-sections micrographs of cermet coatings with different binders analyzed via scanning electron microscope are shown in Fig. 7. The morphologies were taken at nearly the same coatings thickness of 300 Am. All coatings microstructure look dense without pore. However, there are some differences in microstructures for each coating. Although WC – Co and WC – CoCr coatings seem to have the same microstructure, the size of carbides in WC– CoCr coating is smaller (1 –2 Am) than that in WC –Co coating (> 2 Am). The fine carbides particles in these two coatings are arranged closely from each other and sometimes touch and combine with each other. This enforces the mechanical bound among the WC particles and binder metal and explains the high microhardness values of these coatings. The morphology of carbides in the case of WC –NiCr did not clearly appear. It seems that the carbides are covered with thick layers of Ni binder. This may explain the lower microhardness value of WC – NiCr coatings because the thick layer of soft Ni binder between WC particles isolate the WC particles from each other and prevent them from closing or touching, which results in a decrease in the microhardness values of the coatings. The morphologies of WC –Co coating near and far from the substrate are shown in Fig. 8. The carbides near the
Intensity Ratio of W2C
0.40
Fig. 7. Scanning electron micrographs of cross-sections of cermet coatings with different binders.
0.35 0.30 0.25 0.20 0.15
100
200
300
400
500
600
Coating Thickness, µm Fig. 6. Change in intensity ratio of W2C phase with WC – CoCr coating thickness.
Fig. 8. SEM morphologies of WC – Co coatings showing the microstructure near and far from the substrate.
M.F. Morks et al. / Surface & Coatings Technology 199 (2005) 66 – 71
substrate are coarser, larger in size and far from each other compared with the carbides far from the substrate. Near the substrate the carbide particles seem to be loosely and not engrossed in the binder matrix. The morphologies agree well with our explanation about the microhardness values taken at different coating thickness. From the above results it is clear that microstructure is closely linked to the binder materials, which affects greatly the coating microhardness.
4. Conclusions Three cermet coatings with different binders (– Co, – NiCr and – CoCr) were plasma sprayed via low power plasma spray system. The effect of binder materials on coatings microstructure and microhardness properties was examined. The conclusions can be summarized as follow: – The intensity ratio of W2C is lower in WC – Co coating than WC – NiCr and WC –CoCr coatings, which indicate lower solidification rate of WC – Co coatings. – The microhardness value is higher in WC– Co and WC – CoCr compared with WC – NiCr coating. – The intensity ratio of W2C decreases as the coating thickness increase mainly due to the decrease in solidification rate. – The coating microhardness increases as the coating thickness increases due to the decrease in intensity ratio of W2C. – The coating microstructure varied with coating thickness and extensive amount of W2C phase is formed near the substrate because of the rabid solidification of flight particles.
Acknowledgments This work has been supported by China PostDoc Foundation and the National Natural Science Foundation of China (project no. 50075011).
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