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Hard coatings based on thermal spray and laser cladding
Int. Journal of Refractory Metals & Hard Materials 27 (2009) 485–491
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Hard coatings based on thermal spray and laser cladding A.S. Khanna a,*, S. Kumari a, S. Kanungo a, A. Gasser b a b
Corrosion Science and Engineering, Indian Institute of Technology, Bombay, India Fraunhofer Institute of Laser Technique, Aachen, Germany
a r t i c l e
i n f o
Article history: Received 19 September 2008 Accepted 20 September 2008
Keywords: Hard coatings Thermal spray Laser glazing Laser cladding HVOF APS Oxidation Wear
a b s t r a c t Thermal spray and laser cladding have been used to develop hard coatings. HVOF and laser glazing techniques were used to form hard and corrosion resistant coatings, using WC/Co, impregnated in Ni–Cr powder, to protect the heat exchanger tubes from fireside erosion and corrosion, while PM 20 alloy (chromium carbide in Ni–Cr powder), WC/Co in Ni–Cr powder were used to develop a very hard and friction resistant coatings for engine groves, using plasma spray and laser cladding techniques. Results indicate that an optimized composition (15–30% of WC/Co in NiCr matrix) was best to control the erosion and corrosion of heat exchanger tubes and PM 20 alloy, applied using laser cladding, gave excellent hardness and adequate wear and friction resistance. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Many cutting tools use a very hard coating on its edge to enhance its life. There are umpteen examples in industry where tool edges, coming in contact with hard particles are protected by various hard coatings, which enhance the life of the tool or component [1,2]. The important feature of a protective hard coating is its high hardness and chemical inertness. Conventional methods such as carburizing, nitriding, electroplating are being used over a century to protect tools, however, the development of the hard protective coatings in the narrower sense, started in the sixties with the discovery of chemical vapor deposition (CVD) and physical vapor deposition (PVD) techniques. In recent times, modern deposition techniques such as thermal spray and laser cladding have become more popular and can give large throughput in shortest possible time. The present paper focuses two important problems where hard coating and strong corrosion resistance was required. The first problem is fireside erosion corrosion of heat exchanger tubes. For this, a hard ceramic dispersion of WC/Co in Ni–Cr matrix was chosen as the coating system. While ceramic dispersion of WC/Co, takes care of abrasion, the Ni–25Cr matrix protects the heat ex-
* Corresponding author. E-mail address: [email protected] (A.S. Khanna). 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.09.017
changer tubes from oxidation and hot corrosion. The objective of the work was to find an optimum concentration of WC/Co in Ni– Cr matrix which possesses the best erosion and abrasion resistance, along with strong corrosion resistance. For this, various coating compositions were prepared using high velocity oxy fuel (HVOF) method and laser glazing. In the second problem, an alternative to hard chrome coatings, used in many engine grooves was chosen. For this, two ceramic powders, WC/Co and chromium carbide powders were deposited by atmospheric plasma spray (APS) and laser cladding. This paper, discusses the development of WC/Co in Ni–25Cr matrix and its performance for wear and corrosion and Cr2C6 in Ni–Cr matrix and WC/Co coating and their performance in terms of hardness, wear and friction. 2. Experimental 2.1. Materials Chemical compositions of the powders, used for erosion corrosion coatings for fireside corrosion in power plants, are given in Tables 1 and 2 and the powders used for engine grooves wear studies, are given in Tables 2 and 3. HVOF coating, and HVOF coating, followed by laser glazing were used for erosion–corrosion studies, while APS plasma and laser cladding were used to make coatings for wear resistant engine grooves. Low carbon steel was used as the substrate material.
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Table 1 Chemical composition of NiCrBSiFe – powder (particle size 20–50 lm).
Table 5 Spray parameters used in HVOF technique.
C%
Si%
B%
Fe%
Cr%
Ni%
0.75
4.3
3.1
3.7
14.8
Bal.
Table 2 Composition of WC/Co powder (particle size 20–50 lm). C%
Co%
W%
5.5
12.0
Bal.
Fuel gas Fuel flow rate Oxygen flow rate Carrier gas Carrier gas flow rate Powder feed rate Nozzle diameter Spray distance Substrate temperature
Propane 50–60 l/min 80–90 l/min Argon 15–20l/min 55 g/min 8 mm 50 mm 140 °C
Table 6 Laser parameters.
Table 3 Composition of Cr3C2–Ni–Cr powder (PM 20 alloy) (particle size 40–50 lm). Cr3C2
Ni–Cr
50%
Mo
18%
32%
Table 4 Various compositions of WC/Co in Ni–Cr matrix used for making HVOF coatings. Sl. no.
Sample name
% of NiCrBSiFe
% of WC/Co
1 2 3 4 5
0WC 15WC 35WC 60WC 100WC
100 85 65 40 0
0 15 35 60 100
Laser power = 2000 W Laser beam = 10 1 mm2 Shielding gas = Ar at 30 lt/min Nozzle substrate distance = 260 mm Sample no. Sweep speed (mm/min)
Sample designation
1 2 3
400 500 600
0WC
4 5 6
1000 1200 800
100WC
7 8 9
800 1000 600
35WC
Table 7 APS plasma parameters used for plasma deposition.
2.2. Composition of the erosion corrosion coatings Both the powders (given in Tables 1 and 2) were mixed together in different proportions and blended at 500 rpm for 10 min at room temperature. Various composition formed are given in Table 4. 2.3. Deposition by high velocity oxy-fuel (HVOF) technique The schematic of the equipment used for the deposition was the HIPOJET-2100 Gun is shown in Fig. 1. The performance of thermally sprayed coating depends on various factors such as, temperature, pressure, gas composition, gas velocity, etc. Various parameters used in the present study are tabulated in Table 5. 2.4. Laser glazing methodology A few samples were also prepared by laser glazing of HVOF coatings, described above.
Plasma coating parameters
PM 20 alloy coating
WC/Co coating
Ar gas pressure (bar)/flow rate (m3/h) H2 gas pressure (bar)/flow rate (m3/h) Current (A) Voltages (V) Carrier gas flow (m3/h) Spray distance (mm) Spray rate (g/min) Coating thickness obtained (lm)
6.8/3.6 3.4/0.32 500 60–70 1.3 60–100 42 200
6.8/3.6 3.4/0.32 400 55–60 1.3 60–100 60 248
A continuous 2 kW Nd:YAG laser at Fraunhofer Institute for Lasertechnik, Germany was used for the laser glazing of the coating. The parameters used are given in Table 6. 2.5. Preparation of wear resistant coatings using APS plasma and laser cladding The parameters used for making plasma coatings are as given in Table 7. 2.6. Parameters for laser cladding
Fig. 1. Schematic of HVOF HIPOJET-2100 gun.
Three kilowatt Nd:Yag Laser with a fixed power of 1000 W and a beam diameter of 1.2 mm was used. Argon gas was used as the shielding gas to avoid any oxidation during coating. The powder was introduced using a feed gas at rate 20 m3/s. The other parameters are listed in Table 8. The coatings, so formed were characterized using several techniques including, optical, microscopy, SEM/EDAX, and X-ray diffraction. The performance of the coatings was estimated by determining the surface hardness, surface profile, and wear rate, friction and corrosion resistance.
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A.S. Khanna et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 485–491 Table 8 List of laser parameters used for laser cladding of PM 20 alloy and WC/Co coatings. Powder
Powder feed rate (%)
Flow rate of shielding gas (m3/s)
Laser sweep speed (mm/min)
Thickness of the coating achieved (lm)
PM 20 alloy
15
20
WC/Co
10
15
600 1000 1500 600 1000 1500
600 480 180 600 480 180
3. Results and discussion 3.1. Erosion corrosion resistant hard coatings using HVOF and laser glazing Fig. 2 shows detailed surface morphologies of different coatings (on the left), with varying composition of WC/Co in Ni–25Cr matrix, (from 100% WC/Co to pure Ni–25Cr), deposited by HVOF technique, and the corresponding coating cross-sections are given on the right. The morphology of the coating is relatively rough with more pores for pure Ni–25Cr coating, to relatively smooth coating for 100% WC/Co composition. The cross-section of the coating has a smooth interface with a little porosity. The coatings appear to be lamellar in nature. Table 9 shows a summary of the various properties, determined for these coatings, such as number of phases in the coating, determined by XRD; surface porosity, determined by image analysis; surface profile; and Vickers hardness. As expected, the hardness of the coating increased with increase in the WC/Co in the coating, the maximum when WC/Co was 100%. Out of various phases present in the coatings, the hardness can be attributed to the WC and some intermetallic phases of Co and W. The performance of the coatings were determined by oxidation and sulphidation studies which are the two main degrading corrosion mechanisms of heat exchanger tubes on fire side of a coal based power plants. Results of the oxidation tests in air are shown in Fig. 3 at three different temperatures, 600, 700, and 800 °C. From the results, it is clear that the coatings with composition of WC/Co from 15% to 35% show the lowest oxidation rate compared to those having no WC/Co or having more WC/Co. Similar results reflect from the sulphidation tests, carried out at two temperatures 700 and 800 °C (Fig. 4). The performance of the coatings in wear, also showed that the HVOF coatings with composition of 15–35% WC/Co exhibit the minimum wear rate compared to those with higher or no WC/Co. (Fig. 5). This confirms that the optimum coating composition for HVOF coatings, which give the maximum corrosion (oxidation and sulphidation resistance) as well as lowest wear rate is 15% WC/Co. The reason for this observation can be found in the composition of the coating and its morphology and porosity level. Although, 15% WC/Co does not give the hardest coating, it has suitable porosity that takes care of the wear properties. Higher hardness at 100%
Fig. 2. Surface morphologies (left) and coating cross-section (right), using SEM for various coatings of WC/Co in Ni–25Cr matrix: (a) 0%, (b) 15%, (c) 35%, (d) 60% and (e) 100% WC/Co.
WC/Co may not be very good for wear because of much higher porosity level. For high temperature oxidation resistance, again 15–35% WC/ Co, gives the lowest oxidation rate. From the analysis of various oxides formed by XRD, it was found that various oxides such as WOx, MWOx and MOx are formed along the cross-section of oxide layer, WOx being at the metal oxide interface. Lower levels of WOx is favourable. If the value of M in MWOx and MOX is more Co and Cr, the coatings are relatively more protective. This is obtained when WC/Co is between 15% and 35% [3,4]. Laser glazing of HVOF coating resulted in a smoother, pore free coating with a cross-section, which did not show any porosity. Wear tests carried out on various samples of HVOF coatings after laser glazing; show almost no difference in the wear behavior, irrespective of the concentration of WC/Co in that. And the wear rate of these coatings is much lower than that of the coatings having no WC/Co. A typical result is shown in Fig. 6 for a laser glazed sample, showing almost same wear rate for 15, 35, 60 and 100% WC/ Co–Ni–Cr coatings.
Table 9 A summary of various data characterized for HVOF coatings. Coatings
Phases (determined by XRD)
% Porosity
Surface roughness Ra (lm)
Surface hardness HV0.3
100% NiCrBSiFe (coating) 15%WC/Co 85%NiCrBSiFe (coating) 35%WC/Co 65%NiCrBSiFe (coating) 60%WC/Co 40%NiCrBSiFe (coating) 100%WC/Co (coating)
Ni3B, Fe5Si3, CrSi2, Ni3(BO3)2 WC, Ni3Si, Fe2SiO4 WC, Fe3W3C, Co6W6C, Co2O4, Ni3Si, Fe2SiO4 WC, Fe3W3C, Co6W6C, Co2O4, Ni3Si, Fe2SiO4 WC, Co2O4, Co6W6C
2.46 2.07 3.94 2.49 3.79
9.011 9.387 8.467 5.58 7.644
358.7 696.2 747.5 792 1003.5
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Fig. 5. Wear resistance of HVOF coated specimens determined using pin on disc machine with load: 2 kg, speed: 250 rpm, track radius: 90 mm, for 480 s.
Fig. 3. Oxidation behavior of various HVOF coatings at 600, 700, and 800 °C in air.
Fig. 4. High temperature sulphidation behavior of HVOF sprayed coatings at (a) 700 °C and (b) 8000 °C.
3.2. Wear resistant hard coatings for engine grooves Atmospheric plasma spray was used to deposit PM 20 alloy and WC/Co coatings on mild steel substrates to study the improvement
Fig. 6. Wear resistances of laser glazed specimens along with that of an HVOF sample, showing tremendous improvement after laser glazing of HVOF coatings.
in surface hardness, wear and co-efficient of friction of these coatings. A typical plasma sprayed coating profile of PM 20 alloy is shown in Fig. 7, along with the cross-section of the coating. The coating has some porosity and voids, not only on the surface, but also in the cross-section. The micro-hardness results of this coating, along with that of WC/Co coating, applied in the same manner, and those applied by laser cladding technique are given in Table 10. Micro-hardness was measured both on the coating surface as well as on the crosssection for the each individual coating. One striking difference is that the value of micro-hardness, measured on laser claddings are much higher than that coated by APS method. Further, it can be seen that, there is hardly any difference in the hardness, measured at the coating surface and that at cross-section for the APS coated samples, while there is a significant difference in the hardness values at the surface than at the cross-section for laser cladded samples. This can be attributed to the inter-diffusion of alloying elements as well as additional intermetallic phases formed during laser cladding [5]. It can be seen that compared to hard-chrome coating, deposited by electroplating, APS deposited coating has marginally lower hardness value, while laser cladding gives almost twice the hardness. Hence, laser cladding appears to be viable alternative coating method for forming hard coatings. These hard coatings were subjected to wear and friction testing. Table 11 lists the results of the wear and friction tests, carried out on plasma coated samples at various wear testing parameters such as load, track radius and time. From the results, it can be summarized that wear rate of PM 20 alloy coating is lower than that of WC/Co coatings, though the difference is marginal. There is not much difference in their co-efficient of friction values. However, when compared to hard-chrome coating, there is tremendous improvement (see Fig. 8).
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Fig. 7. (a) Surface morphology and (b) cross-section of the APS plasma coating of PM 20 alloys for a laser scan speed of 1500 mm/min.
Table 10 Micro-hardness results of the coating surface (S) and cross-section (CS) APS, laser cladded (LC) and hard chrome samples. Sl no.
Powder
Type
Surface (S)/X-sec (CS)
Thickness of coating in lm
Load (g)
10 average of 4 readings in kg/mm2
1 2 3 4 5 6 7 8 17 18 19 20 21 22 23 24 25
PM 20
APS
S CS S CS S CS S CS S CS S CS S CS S CS S CS
290
300 300 300 300 300 300 100 100 300 300 300 300 300 300 100 100 100 100
422 442 966 1005 971 945 1019 795 549 553 1241 1502 1181 1292 1092 1055 580 600
LC-600 LC-1000 LC-1500 WC/Co
APS LC-600 LC-1000 LC-1500
Hard chrome
600 400 180 490 660 480 180 310
Table 11 Summary of the wear and friction test results obtained for APS coatings at various wear measuring parameters. Coating type
Load (kg)
Track radius (mm)
Time (min)
Weight loss (g)
Loss in thickness (mm)
Wear rate g/m
Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr WC/Co WC/Co WC/Co Hard chrome
7 7 7 7 7 7 7
80 40 60 74 64 54 70
12 24 16 13 15 17 15
0.0011 0.0031 0.0010 0.0002 0.0006 0.0006 0.045
0.03 0.25 0.06 0.04 0.05 0.05 0.1
3.7 10 1.0 10 3.3 10 5.7 10 1.3 10 2.0 10 1.5 10
7 7 7 7 7 7 5
Co-efficient of friction (l) 0.48 0.65 0.52 0.48 0.56 0.59 0.12
Fig. 8. (a) Surface morphology and (b) cross-section of the laser cladding of PM 20 alloys for a laser scan speed of 1500 mm/min.
The corresponding results on the wear and friction of laser cladded samples are summarized in Table 12. There are no signif-
icant differences in the wear and friction characteristics of different cladded samples and at various wear testing parameters. Also, the
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Table 12 Wear results of laser coated samples. Coating type
Sweep speed (Mm/min)
Load (kg)
Track radius (mm)
Time (min)
Weight loss (g)
Loss in thickness (mm)
Wear rate (g/mt)
Co-efficient of friction (l)
Cr3C2
1500 1000 600 1500 1000 W 600
7.5 7.5 15 7.5 7.5 7.5
46 46 80 64 64 60
60 60 60 60 60 60
0.00426 0.00451 0.00567 0.0021 0.0022 0.0015
0.01 0.01 0.01 0.004 0.005 0.006
3.96 10 7 4.0 10 7 4.2 10 7 1.9 10 7 1.9 10 7 2.0 10 7
0.36 0.36 0.31 0.38 0.36 0.37
WC/Co
Fig. 9. Comparison of the wear behaviour of APS coatings and laser cladded coating with Hard Chrome and Ni–Cr matrix coatings.
Fig. 10. Comparison of the friction behaviour of APS coatings and laser cladded coating with hard chrome and Ni–Cr matrix coatings.
A.S. Khanna et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 485–491
overall wear rate obtained for laser cladded samples is not very different for coated samples formed by APS technique. Fig. 9 compares the above results given in Tables 7 and 8 in a histogram. The data in Fig. 9 also compares the wear rate of PM 20 alloy coatings with a NiCr matrix coating by APS and laser cladding. The results very clearly show that the developed coatings of PM 20 alloy as well of WC/Co by APS and laser cladding show a remarkable improvement over conventional hard chrome coating as well as from a pure Ni–25Cr coating, formed by the same techniques. In Fig. 10, the results of the friction tests on the two coatings are compared with hard chrome as well as that of Ni–Cr matrix coating made by APS and laser cladding. From the results, it is clear again that the friction co-efficient has not improved much compared to the conventional hard chrome coating but it is quite less than the Ni–Cr matrix coating. 4. Conclusions Hard coatings have been developed using thermal spray techniques and laser cladding processing. With the aim to improve upon the conventional hard chrome coatings for engine grooves, the results very clearly show that laser cladding process with PM 20 alloy or WC/Co is perhaps a better choice. APS plasma could
491
not enhance the hardness to the extent laser cladding did. For erosion corrosion resistant coatings, the optimum composition of the coating was found to be 15–35% of WC/Co in Ni–25Cr matrix. However, if HVOF coatings are laser glazed, even 100% pure WC/Co give excellent wear resistance. However such coatings have poor oxidation and corrosion resistance. Acknowledgements The authors gratefully acknowledge the help rendered by Ms. Modi Metalliizing Syndicate, Jodhpur for extending their facilities for carrying out the HVOF coatings. They also convey sincere thanks to Institute for Lasetechnik, Aachen, Germany for helping with laser glazing and laser cladding work. References [1] Djouadi MA et al. Surf Coat Technol 1999;116–119:508. [2] Lee Sega land Rosen Torbin, Sputtek – Thin film hard coatings, paper No. -013230. [3] Khanna AS, Kumari Smita. In: Proceedings of ITSC conference, May 15–18, Seattle, USA, organized by ASM International; 2006. [4] Kumari Smita, Khanna AS. In: Proceedings international surface engineering congress. Minnesota: ASM International; 2005. [5] Kanungo Sanjeet, Dixit SG, Khanna AS. Society for Tribology. London, UK: Institute of Mechanical Engineers; 2006. July 12–14.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Hard coatings based on thermal spray and laser cladding A.S. Khanna a,*, S. Kumari a, S. Kanungo a, A. Gasser b a b
Corrosion Science and Engineering, Indian Institute of Technology, Bombay, India Fraunhofer Institute of Laser Technique, Aachen, Germany
a r t i c l e
i n f o
Article history: Received 19 September 2008 Accepted 20 September 2008
Keywords: Hard coatings Thermal spray Laser glazing Laser cladding HVOF APS Oxidation Wear
a b s t r a c t Thermal spray and laser cladding have been used to develop hard coatings. HVOF and laser glazing techniques were used to form hard and corrosion resistant coatings, using WC/Co, impregnated in Ni–Cr powder, to protect the heat exchanger tubes from fireside erosion and corrosion, while PM 20 alloy (chromium carbide in Ni–Cr powder), WC/Co in Ni–Cr powder were used to develop a very hard and friction resistant coatings for engine groves, using plasma spray and laser cladding techniques. Results indicate that an optimized composition (15–30% of WC/Co in NiCr matrix) was best to control the erosion and corrosion of heat exchanger tubes and PM 20 alloy, applied using laser cladding, gave excellent hardness and adequate wear and friction resistance. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Many cutting tools use a very hard coating on its edge to enhance its life. There are umpteen examples in industry where tool edges, coming in contact with hard particles are protected by various hard coatings, which enhance the life of the tool or component [1,2]. The important feature of a protective hard coating is its high hardness and chemical inertness. Conventional methods such as carburizing, nitriding, electroplating are being used over a century to protect tools, however, the development of the hard protective coatings in the narrower sense, started in the sixties with the discovery of chemical vapor deposition (CVD) and physical vapor deposition (PVD) techniques. In recent times, modern deposition techniques such as thermal spray and laser cladding have become more popular and can give large throughput in shortest possible time. The present paper focuses two important problems where hard coating and strong corrosion resistance was required. The first problem is fireside erosion corrosion of heat exchanger tubes. For this, a hard ceramic dispersion of WC/Co in Ni–Cr matrix was chosen as the coating system. While ceramic dispersion of WC/Co, takes care of abrasion, the Ni–25Cr matrix protects the heat ex-
* Corresponding author. E-mail address: [email protected] (A.S. Khanna). 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.09.017
changer tubes from oxidation and hot corrosion. The objective of the work was to find an optimum concentration of WC/Co in Ni– Cr matrix which possesses the best erosion and abrasion resistance, along with strong corrosion resistance. For this, various coating compositions were prepared using high velocity oxy fuel (HVOF) method and laser glazing. In the second problem, an alternative to hard chrome coatings, used in many engine grooves was chosen. For this, two ceramic powders, WC/Co and chromium carbide powders were deposited by atmospheric plasma spray (APS) and laser cladding. This paper, discusses the development of WC/Co in Ni–25Cr matrix and its performance for wear and corrosion and Cr2C6 in Ni–Cr matrix and WC/Co coating and their performance in terms of hardness, wear and friction. 2. Experimental 2.1. Materials Chemical compositions of the powders, used for erosion corrosion coatings for fireside corrosion in power plants, are given in Tables 1 and 2 and the powders used for engine grooves wear studies, are given in Tables 2 and 3. HVOF coating, and HVOF coating, followed by laser glazing were used for erosion–corrosion studies, while APS plasma and laser cladding were used to make coatings for wear resistant engine grooves. Low carbon steel was used as the substrate material.
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Table 1 Chemical composition of NiCrBSiFe – powder (particle size 20–50 lm).
Table 5 Spray parameters used in HVOF technique.
C%
Si%
B%
Fe%
Cr%
Ni%
0.75
4.3
3.1
3.7
14.8
Bal.
Table 2 Composition of WC/Co powder (particle size 20–50 lm). C%
Co%
W%
5.5
12.0
Bal.
Fuel gas Fuel flow rate Oxygen flow rate Carrier gas Carrier gas flow rate Powder feed rate Nozzle diameter Spray distance Substrate temperature
Propane 50–60 l/min 80–90 l/min Argon 15–20l/min 55 g/min 8 mm 50 mm 140 °C
Table 6 Laser parameters.
Table 3 Composition of Cr3C2–Ni–Cr powder (PM 20 alloy) (particle size 40–50 lm). Cr3C2
Ni–Cr
50%
Mo
18%
32%
Table 4 Various compositions of WC/Co in Ni–Cr matrix used for making HVOF coatings. Sl. no.
Sample name
% of NiCrBSiFe
% of WC/Co
1 2 3 4 5
0WC 15WC 35WC 60WC 100WC
100 85 65 40 0
0 15 35 60 100
Laser power = 2000 W Laser beam = 10 1 mm2 Shielding gas = Ar at 30 lt/min Nozzle substrate distance = 260 mm Sample no. Sweep speed (mm/min)
Sample designation
1 2 3
400 500 600
0WC
4 5 6
1000 1200 800
100WC
7 8 9
800 1000 600
35WC
Table 7 APS plasma parameters used for plasma deposition.
2.2. Composition of the erosion corrosion coatings Both the powders (given in Tables 1 and 2) were mixed together in different proportions and blended at 500 rpm for 10 min at room temperature. Various composition formed are given in Table 4. 2.3. Deposition by high velocity oxy-fuel (HVOF) technique The schematic of the equipment used for the deposition was the HIPOJET-2100 Gun is shown in Fig. 1. The performance of thermally sprayed coating depends on various factors such as, temperature, pressure, gas composition, gas velocity, etc. Various parameters used in the present study are tabulated in Table 5. 2.4. Laser glazing methodology A few samples were also prepared by laser glazing of HVOF coatings, described above.
Plasma coating parameters
PM 20 alloy coating
WC/Co coating
Ar gas pressure (bar)/flow rate (m3/h) H2 gas pressure (bar)/flow rate (m3/h) Current (A) Voltages (V) Carrier gas flow (m3/h) Spray distance (mm) Spray rate (g/min) Coating thickness obtained (lm)
6.8/3.6 3.4/0.32 500 60–70 1.3 60–100 42 200
6.8/3.6 3.4/0.32 400 55–60 1.3 60–100 60 248
A continuous 2 kW Nd:YAG laser at Fraunhofer Institute for Lasertechnik, Germany was used for the laser glazing of the coating. The parameters used are given in Table 6. 2.5. Preparation of wear resistant coatings using APS plasma and laser cladding The parameters used for making plasma coatings are as given in Table 7. 2.6. Parameters for laser cladding
Fig. 1. Schematic of HVOF HIPOJET-2100 gun.
Three kilowatt Nd:Yag Laser with a fixed power of 1000 W and a beam diameter of 1.2 mm was used. Argon gas was used as the shielding gas to avoid any oxidation during coating. The powder was introduced using a feed gas at rate 20 m3/s. The other parameters are listed in Table 8. The coatings, so formed were characterized using several techniques including, optical, microscopy, SEM/EDAX, and X-ray diffraction. The performance of the coatings was estimated by determining the surface hardness, surface profile, and wear rate, friction and corrosion resistance.
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A.S. Khanna et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 485–491 Table 8 List of laser parameters used for laser cladding of PM 20 alloy and WC/Co coatings. Powder
Powder feed rate (%)
Flow rate of shielding gas (m3/s)
Laser sweep speed (mm/min)
Thickness of the coating achieved (lm)
PM 20 alloy
15
20
WC/Co
10
15
600 1000 1500 600 1000 1500
600 480 180 600 480 180
3. Results and discussion 3.1. Erosion corrosion resistant hard coatings using HVOF and laser glazing Fig. 2 shows detailed surface morphologies of different coatings (on the left), with varying composition of WC/Co in Ni–25Cr matrix, (from 100% WC/Co to pure Ni–25Cr), deposited by HVOF technique, and the corresponding coating cross-sections are given on the right. The morphology of the coating is relatively rough with more pores for pure Ni–25Cr coating, to relatively smooth coating for 100% WC/Co composition. The cross-section of the coating has a smooth interface with a little porosity. The coatings appear to be lamellar in nature. Table 9 shows a summary of the various properties, determined for these coatings, such as number of phases in the coating, determined by XRD; surface porosity, determined by image analysis; surface profile; and Vickers hardness. As expected, the hardness of the coating increased with increase in the WC/Co in the coating, the maximum when WC/Co was 100%. Out of various phases present in the coatings, the hardness can be attributed to the WC and some intermetallic phases of Co and W. The performance of the coatings were determined by oxidation and sulphidation studies which are the two main degrading corrosion mechanisms of heat exchanger tubes on fire side of a coal based power plants. Results of the oxidation tests in air are shown in Fig. 3 at three different temperatures, 600, 700, and 800 °C. From the results, it is clear that the coatings with composition of WC/Co from 15% to 35% show the lowest oxidation rate compared to those having no WC/Co or having more WC/Co. Similar results reflect from the sulphidation tests, carried out at two temperatures 700 and 800 °C (Fig. 4). The performance of the coatings in wear, also showed that the HVOF coatings with composition of 15–35% WC/Co exhibit the minimum wear rate compared to those with higher or no WC/Co. (Fig. 5). This confirms that the optimum coating composition for HVOF coatings, which give the maximum corrosion (oxidation and sulphidation resistance) as well as lowest wear rate is 15% WC/Co. The reason for this observation can be found in the composition of the coating and its morphology and porosity level. Although, 15% WC/Co does not give the hardest coating, it has suitable porosity that takes care of the wear properties. Higher hardness at 100%
Fig. 2. Surface morphologies (left) and coating cross-section (right), using SEM for various coatings of WC/Co in Ni–25Cr matrix: (a) 0%, (b) 15%, (c) 35%, (d) 60% and (e) 100% WC/Co.
WC/Co may not be very good for wear because of much higher porosity level. For high temperature oxidation resistance, again 15–35% WC/ Co, gives the lowest oxidation rate. From the analysis of various oxides formed by XRD, it was found that various oxides such as WOx, MWOx and MOx are formed along the cross-section of oxide layer, WOx being at the metal oxide interface. Lower levels of WOx is favourable. If the value of M in MWOx and MOX is more Co and Cr, the coatings are relatively more protective. This is obtained when WC/Co is between 15% and 35% [3,4]. Laser glazing of HVOF coating resulted in a smoother, pore free coating with a cross-section, which did not show any porosity. Wear tests carried out on various samples of HVOF coatings after laser glazing; show almost no difference in the wear behavior, irrespective of the concentration of WC/Co in that. And the wear rate of these coatings is much lower than that of the coatings having no WC/Co. A typical result is shown in Fig. 6 for a laser glazed sample, showing almost same wear rate for 15, 35, 60 and 100% WC/ Co–Ni–Cr coatings.
Table 9 A summary of various data characterized for HVOF coatings. Coatings
Phases (determined by XRD)
% Porosity
Surface roughness Ra (lm)
Surface hardness HV0.3
100% NiCrBSiFe (coating) 15%WC/Co 85%NiCrBSiFe (coating) 35%WC/Co 65%NiCrBSiFe (coating) 60%WC/Co 40%NiCrBSiFe (coating) 100%WC/Co (coating)
Ni3B, Fe5Si3, CrSi2, Ni3(BO3)2 WC, Ni3Si, Fe2SiO4 WC, Fe3W3C, Co6W6C, Co2O4, Ni3Si, Fe2SiO4 WC, Fe3W3C, Co6W6C, Co2O4, Ni3Si, Fe2SiO4 WC, Co2O4, Co6W6C
2.46 2.07 3.94 2.49 3.79
9.011 9.387 8.467 5.58 7.644
358.7 696.2 747.5 792 1003.5
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Fig. 5. Wear resistance of HVOF coated specimens determined using pin on disc machine with load: 2 kg, speed: 250 rpm, track radius: 90 mm, for 480 s.
Fig. 3. Oxidation behavior of various HVOF coatings at 600, 700, and 800 °C in air.
Fig. 4. High temperature sulphidation behavior of HVOF sprayed coatings at (a) 700 °C and (b) 8000 °C.
3.2. Wear resistant hard coatings for engine grooves Atmospheric plasma spray was used to deposit PM 20 alloy and WC/Co coatings on mild steel substrates to study the improvement
Fig. 6. Wear resistances of laser glazed specimens along with that of an HVOF sample, showing tremendous improvement after laser glazing of HVOF coatings.
in surface hardness, wear and co-efficient of friction of these coatings. A typical plasma sprayed coating profile of PM 20 alloy is shown in Fig. 7, along with the cross-section of the coating. The coating has some porosity and voids, not only on the surface, but also in the cross-section. The micro-hardness results of this coating, along with that of WC/Co coating, applied in the same manner, and those applied by laser cladding technique are given in Table 10. Micro-hardness was measured both on the coating surface as well as on the crosssection for the each individual coating. One striking difference is that the value of micro-hardness, measured on laser claddings are much higher than that coated by APS method. Further, it can be seen that, there is hardly any difference in the hardness, measured at the coating surface and that at cross-section for the APS coated samples, while there is a significant difference in the hardness values at the surface than at the cross-section for laser cladded samples. This can be attributed to the inter-diffusion of alloying elements as well as additional intermetallic phases formed during laser cladding [5]. It can be seen that compared to hard-chrome coating, deposited by electroplating, APS deposited coating has marginally lower hardness value, while laser cladding gives almost twice the hardness. Hence, laser cladding appears to be viable alternative coating method for forming hard coatings. These hard coatings were subjected to wear and friction testing. Table 11 lists the results of the wear and friction tests, carried out on plasma coated samples at various wear testing parameters such as load, track radius and time. From the results, it can be summarized that wear rate of PM 20 alloy coating is lower than that of WC/Co coatings, though the difference is marginal. There is not much difference in their co-efficient of friction values. However, when compared to hard-chrome coating, there is tremendous improvement (see Fig. 8).
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Fig. 7. (a) Surface morphology and (b) cross-section of the APS plasma coating of PM 20 alloys for a laser scan speed of 1500 mm/min.
Table 10 Micro-hardness results of the coating surface (S) and cross-section (CS) APS, laser cladded (LC) and hard chrome samples. Sl no.
Powder
Type
Surface (S)/X-sec (CS)
Thickness of coating in lm
Load (g)
10 average of 4 readings in kg/mm2
1 2 3 4 5 6 7 8 17 18 19 20 21 22 23 24 25
PM 20
APS
S CS S CS S CS S CS S CS S CS S CS S CS S CS
290
300 300 300 300 300 300 100 100 300 300 300 300 300 300 100 100 100 100
422 442 966 1005 971 945 1019 795 549 553 1241 1502 1181 1292 1092 1055 580 600
LC-600 LC-1000 LC-1500 WC/Co
APS LC-600 LC-1000 LC-1500
Hard chrome
600 400 180 490 660 480 180 310
Table 11 Summary of the wear and friction test results obtained for APS coatings at various wear measuring parameters. Coating type
Load (kg)
Track radius (mm)
Time (min)
Weight loss (g)
Loss in thickness (mm)
Wear rate g/m
Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr WC/Co WC/Co WC/Co Hard chrome
7 7 7 7 7 7 7
80 40 60 74 64 54 70
12 24 16 13 15 17 15
0.0011 0.0031 0.0010 0.0002 0.0006 0.0006 0.045
0.03 0.25 0.06 0.04 0.05 0.05 0.1
3.7 10 1.0 10 3.3 10 5.7 10 1.3 10 2.0 10 1.5 10
7 7 7 7 7 7 5
Co-efficient of friction (l) 0.48 0.65 0.52 0.48 0.56 0.59 0.12
Fig. 8. (a) Surface morphology and (b) cross-section of the laser cladding of PM 20 alloys for a laser scan speed of 1500 mm/min.
The corresponding results on the wear and friction of laser cladded samples are summarized in Table 12. There are no signif-
icant differences in the wear and friction characteristics of different cladded samples and at various wear testing parameters. Also, the
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Table 12 Wear results of laser coated samples. Coating type
Sweep speed (Mm/min)
Load (kg)
Track radius (mm)
Time (min)
Weight loss (g)
Loss in thickness (mm)
Wear rate (g/mt)
Co-efficient of friction (l)
Cr3C2
1500 1000 600 1500 1000 W 600
7.5 7.5 15 7.5 7.5 7.5
46 46 80 64 64 60
60 60 60 60 60 60
0.00426 0.00451 0.00567 0.0021 0.0022 0.0015
0.01 0.01 0.01 0.004 0.005 0.006
3.96 10 7 4.0 10 7 4.2 10 7 1.9 10 7 1.9 10 7 2.0 10 7
0.36 0.36 0.31 0.38 0.36 0.37
WC/Co
Fig. 9. Comparison of the wear behaviour of APS coatings and laser cladded coating with Hard Chrome and Ni–Cr matrix coatings.
Fig. 10. Comparison of the friction behaviour of APS coatings and laser cladded coating with hard chrome and Ni–Cr matrix coatings.
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overall wear rate obtained for laser cladded samples is not very different for coated samples formed by APS technique. Fig. 9 compares the above results given in Tables 7 and 8 in a histogram. The data in Fig. 9 also compares the wear rate of PM 20 alloy coatings with a NiCr matrix coating by APS and laser cladding. The results very clearly show that the developed coatings of PM 20 alloy as well of WC/Co by APS and laser cladding show a remarkable improvement over conventional hard chrome coating as well as from a pure Ni–25Cr coating, formed by the same techniques. In Fig. 10, the results of the friction tests on the two coatings are compared with hard chrome as well as that of Ni–Cr matrix coating made by APS and laser cladding. From the results, it is clear again that the friction co-efficient has not improved much compared to the conventional hard chrome coating but it is quite less than the Ni–Cr matrix coating. 4. Conclusions Hard coatings have been developed using thermal spray techniques and laser cladding processing. With the aim to improve upon the conventional hard chrome coatings for engine grooves, the results very clearly show that laser cladding process with PM 20 alloy or WC/Co is perhaps a better choice. APS plasma could
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not enhance the hardness to the extent laser cladding did. For erosion corrosion resistant coatings, the optimum composition of the coating was found to be 15–35% of WC/Co in Ni–25Cr matrix. However, if HVOF coatings are laser glazed, even 100% pure WC/Co give excellent wear resistance. However such coatings have poor oxidation and corrosion resistance. Acknowledgements The authors gratefully acknowledge the help rendered by Ms. Modi Metalliizing Syndicate, Jodhpur for extending their facilities for carrying out the HVOF coatings. They also convey sincere thanks to Institute for Lasetechnik, Aachen, Germany for helping with laser glazing and laser cladding work. References [1] Djouadi MA et al. Surf Coat Technol 1999;116–119:508. [2] Lee Sega land Rosen Torbin, Sputtek – Thin film hard coatings, paper No. -013230. [3] Khanna AS, Kumari Smita. In: Proceedings of ITSC conference, May 15–18, Seattle, USA, organized by ASM International; 2006. [4] Kumari Smita, Khanna AS. In: Proceedings international surface engineering congress. Minnesota: ASM International; 2005. [5] Kanungo Sanjeet, Dixit SG, Khanna AS. Society for Tribology. London, UK: Institute of Mechanical Engineers; 2006. July 12–14.