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Cr5Si3 metal silicide composite coatings
Applied Surface Science 253 (2006) 1584–1589 www.elsevier.com/locate/apsusc
Microstructure and wear properties of laser clad Cuss/Cr5Si3 metal silicide composite coatings Y.X. Yin, H.M. Wang * Laboratory of Laser Materials Processing and Manufacturing, School of Materials Science and Engineering, Beihang University (Formerly Beijing University of Aeronautics and Astronautics), 37 Xueyuan Road, Beijing 100083, China Received 20 January 2006; received in revised form 20 February 2006; accepted 20 February 2006 Available online 4 April 2006
Abstract Wear resistant Cu-based solid solution (Cuss) toughened Cr5Si3 metal silicide composite coatings were fabricated on austenitic stainless steel AISI321 by laser cladding process. Due to the rapidly solidified microstructural characteristics and the excellent toughening effect of Cuss on Cr5Si3, the Cuss/Cr5Si3 coatings have outstanding wear resistance and low coefficient of friction under room temperature dry sliding wear test conditions coupling with hardened 0.45% C steel. # 2006 Elsevier B.V. All rights reserved. PACS: 81.15.Fg; 81.40.Pq Keywords: Coating; Laser cladding; Metal silicide; Wear; Microstructure
1. Introduction Wear is of considerable importance to most moving mechanical components. High performance wear resistant materials are expected to have high hardness to resist plastic flow under high compressive load, good strength to resist elastic and plastic deformation under normal or shear load, and excellent anti-welding, anti-scoring and anti-seizing capabilities and good tribological compatibility. In recent years, much research is focused on the refractory transition metal silicides alloys as potential high- and/or ultrahigh-temperature candidate structural materials because of their outstanding balance of high melting point, high strength, low density, high elastic modulus and excellent creep and oxidation resistance [1–4]. Many ordered transition metal silicides alloys, such as Cr3Si, Cr5Si3, W5Si3, etc., have also demonstrated outstanding abrasive wear resistance and low coefficient of friction. They are expected to be a new class of wear resistant candidate materials for those moving mechanical components working under aggressive tribological conditions [5,6]. Unfortunately,
* Corresponding author. Tel.: +86 10 8231 7102; fax: +86 10 8233 8131. E-mail address: [email protected] (H.M. Wang). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.054
these metal silicides alloys are very brittle at ambient temperature, which restricted their practical applications as bulk tribological components. Addition of a ductile second phase is one of the most effective means to improve the toughness of intermetallics. Moreover, in this way, it also allows us to better control on the tribological properties of the metal silicides alloys by adjusting the trade-off between hardness and toughness, which the wear behavior of the metal silicide alloy is direct proportion to [7–11]. The copper-based solid solution (hereafter referred as Cuss) is well known for its high thermal conductivity, low coefficient of friction and excellent tribological compatibility and ductility, and is expected to be an ideal phase to toughen the wear resistant metal silicide alloys [12,13]. In our previous study, a wearresistant Cuss-toughened Cr5Si3/CrSi metal silicide alloy was designed and fabricated by the laser melting (Lasmelt) process. The Cuss-toughened alloys exhibited excellent wear resistance under room dry sliding wear test conditions [14]. Since wear is a surface-related gradual degradation phenomenon, fabricating a high performance wear-resistant coating using advanced surface engineering methods is one of the most economic and effective means to solve the wear problems for many tribological components. Among the different surface treatments, laser cladding is an efficient way to synthesize novel
Y.X. Yin, H.M. Wang / Applied Surface Science 253 (2006) 1584–1589
wear resistant coating for mechanical moving components made of cheap materials such as carbon steels [15]. Austenitic stainless steel AISI 321, which is cheap and has excellent combination of corrosion resistance and good high temperature mechanical properties up to 600 8C, is one of the most widely used class corrosion resistant structural materials in chemical industries. In this paper, a wear resistant Cuss/Cr5Si3 metal silicide composite coating was fabricated on austenitic stainless steel AISI321 by laser cladding. The laser clad coatings can be applied under the high speed and heavy load aggressive tribological service environments, such as sliding bearing, mechanical seals in valves, pumps in chemical, marine or petrochemical industries. Microstructure of the coatings was characterized by optical microscopy (OM), scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). Vickers microhardness indentations and the resulting cracks were used to estimate the toughness of the coating [16]. Wear resistance of the Cuss/ Cr5Si3 metal silicide composite coating was evaluated under room temperature dry sliding wear test conditions and the responding wear mechanism was discussed. 2. Experimental procedures A solution-treated commercial austenitic stainless steel AISI321 was selected as the substrate material because it is widely used as tribological components in valves and pumps in chemical and petrochemical industries. Alloy powders (particle size 45–125 mm) in nominal chemical composition (at.%) of 11% Cu–48 % Cr–41% Si were utilized as the laser cladding raw materials. The powders were preplaced on surface of the austenitic stainless steel specimens, 60 mm 20 mm 10 mm in size, prior to laser cladding with a powder-bed thickness of approximately 1.8 mm. Laser cladding was conducted on an 8 kW transverse-flow continuous-wave CO2 laser materials processing system equipped with a 4-axis computer numerical controlled (CNC) laser materials processing machine under ambient atmospheric environment. The optimized laser cladding parameters are as follows: laser power 3.5 kW, laser beam diameter 4 mm and beam traverse speed 500 mm/min. Six overlap tracks were clad side by side with an overlap ratio of approximately 25% in order to clad the whole surface of 60 mm 20 mm on the substrate specimen. Metallographic sections were prepared using standard mechanical polishing procedures and were etched in HNO3– 20 vol% CH3COOH–20 vol% H2O. The microstructure was characterized by Olypus BX51M optical microscope (OM) equipped with a Sisc IAS6.0 image analyzing software and Cambridge S250MK2 KYKY-2800B scanning electron microscopes. Phase constituents were identified by X-ray diffraction using a Rigaku D/max 2200 pc automatic X-ray diffractometer with Cu target Ka radiation. Chemical compositions of the phases were analyzed by energy dispersive spectrometer using Noran Ventage DSI spectrometer. Hardness profiles along the depth direction and Vickers indentations of the laser clad coatings were performed on polished coating and measured using a MH-6 semi-automatic Vickers microhardness tester
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with test loads of 200 and 500 g and a dwell time of 15 s. Volume fraction of the Cu-based solid solution (Cuss) in the coating was measured by quantitative metallographic analysis method on high-contrast SEM photographs (1000 magnifications) using a commercial contrast-based image analyzing software. Wear properties of the coatings were evaluated on a MM200 block-on-wheel dry sliding wear tester at room temperature, where a square block-like specimen (10 mm 10 mm 10 mm in size) is pressed under the applied test load against the outer periphery surface of a hardened 0.45% C steel wheel (HRc53) rotating at a speed of 400 rpm. The outer diameter of the steel wheel is 45 mm and the relative sliding speed between the specimen and the contact-coupling wheel is 0.942 m/s. The wear test cycle lasted for 60 min and the total wear sliding distance is approximately 3391 m. Details of the dry sliding wear tester and the test procedures were reported elsewhere [17]. The solid-solution treated austenitic stainless steel AISI321 was selected as the reference comparison test material for all the wear tests in order to rank the wear resistance of the laser clad Cuss/Cr5Si3 coating. Different levels of applied test loads, 98, 147 and 196 N, were selected. Each test was repeated two times. Wear mass loss was measured by an electronic balance (Sartorius BS110) with an accuracy of 0.1 mg and was utilized to rate the relative wear resistance of the coatings in comparison to the reference test material. Friction coefficient was calculated from the friction torque recorded during the dry sliding wear process. To assist the analysis of wear mechanisms, worn surfaces and the wear debris particles were collected during wear testing process and examined under SEM. 3. Results The laser clad Cuss/Cr5Si3 metal silicide composite coating, with a thickness of approximately 1.3 mm, low dilution to the substrate, has a uniform, fine dendritic structure consisting of primary dendrites and a small amount of interdendritic phase and is metallurgically bonded to the substrate, as shown in Figs. 1 and 2. Small amount of fine micro-porosities was visible and uniformly distributed in the interdendritic zone due to solidification shrinkage of the interdendritic residual liquid, as shown in Figs. 1b and 2a. After six overlap tracks were clad side by side, the microstructure of whole coating, including the overlap area and the tracks, is uniform and homogeneous. Results of XRD (as indicated in Fig. 3) and EDS analysis indicate that the main constitutional phases of the coating are Cr5Si3 having the tI38 W5Si3-type topologically closed packed (TCP) crystal lattice and Cu-based solid solution which is highly supersaturated with Cr, Si and Fe. Volume fraction of the Cu-based solid solution in the laser clad Cuss/Cr5Si3 metal silicide composite coating is approximately 17%. Hardness profile along the depth direction of the coating is shown in Fig. 4. Result of the Vickers indentation test shows that the coating has an excellent combination of the hardness and toughness and no cracks were observed at the corners of indentation prints under applied test loads of 200 and 500 g, as indicated in Fig. 5. Because of the high volume
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Fig. 2. SEM micrographs showing typical microstructure of the laser clad Cuss/ Cr5Si3 metal silicide composite coating, (a) low and (b) high magnifications. Fig. 1. OM photographs showing the overview transverse cross-section (a) and coating/substrate transition zone (b) of the laser clad Cuss/Cr5Si3 metal silicide composite coating.
fraction of the hard metal silicides Cr5Si3, the laser clad coating has an average Vickers hardness number of over HV1000 and uniform hardness distribution. Compared to the austenitic stainless AISI321 reference material, the laser clad Cuss/Cr5Si3 metal silicide composite coating exhibited excellent wear resistance and low friction coefficient under room temperature dry sliding wear test conditions coupling with the hardened 0.45% C steel wheel as the mating counterpart, as shown in Figs. 6 and 7. While compared to the laser melted Cuss-toughened Cr5Si3-CrSi metal silicide alloy with the same powder chemical composition, the laser clad coating has a little lower wear resistance. The wear mass losses of laser clad coating and laser melted alloy are very insensitive to the test load, which increases extremely slow as the applied load increases, whereas that of the reference material AISI321 increases drastically with the increasing load. The mean friction coefficient for the sliding friction couple
between the laser clad Cuss/Cr5Si3 metal silicide composite coating and the hardened 0.45% C steel is relatively low (approximately 0.49) and fluctuates between approximately 0.47 and 0.52, while that between austenitic stainless steel AISI321 and the hardened 0.45% C steel is high (approximately 0.71) and fluctuates between approximately 0.65 and 0.77.
Fig. 3. X-ray diffraction patterns of the laser clad Cuss/Cr5Si3 metal silicide composite coating.
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Fig. 4. Hardness profile along depth direction of the laser clad Cuss/Cr5Si3 coating.
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Fig. 7. Profiles of friction coefficient vs. wear test time for the laser clad Cuss/ Cr5Si3 coating and the reference material AISI321 during dry sliding wear process at an applied test load of 196 N.
Fig. 5. OM micrographs showing the indentation prints of Vickers hardness in the laser clad Cuss/Cr5Si3 coating.
Fig. 6. Wear mass loss of the laser clad Cus/Cr5Si3 coating, laser melted Cuss/ (Cr5Si3-CrSi) alloy and the reference material AISI321 as a function of test load under dry sliding wear conditions.
Fig. 8. SEM micrographs showing the worn surface morphologies of the Cuss/ Cr5Si3 coating after dry sliding wear test at an applied test load of 147 N, (a) low and (b) high magnifications.
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Fig. 11. SEM micrograph showing morphologies of the wear debris particles of the laser clad Cuss/Cr5Si3 coating after dry sliding wear test.
Fig. 9. SEM micrographs showing the worn surface morphologies of the Cuss/ Cr5Si3 coating after dry sliding wear test, at 98 N (a) and 196 N (b).
Fig. 10. SEM micrograph showing the worn surface morphologies of the austenitic stainless steel AISI321 after dry sliding wear test at an applied test load of 174 N.
As shown in Figs. 8 and 9, the worn surfaces of the coating are smooth, similar to a polished and deep-etched metallographic section on which the primary metal silicide phases are visible. On the contrary, noticeable grooves and adhesion and plastic deformation features are observable on the worn surface of the reference material AISI321, as illustrated in Fig. 10. As shown in Fig. 11, SEM observations indicate that wear debris is in fine powders and platelet-like consolidated powderagglomerates. EDS analysis (with average chemical composition (at.%) of approximately 44.45% Fe–26.54% O–11.61% Cu–11.61% Cr–5.80% Si) indicates that the fine wear debris particles are highly enriched in Fe and with high content of O and with minor amount of Cu, Cr and Si. Worn subsurface microstructure on longitudinal section indicates no evidence of local subsurface plastic deformation, brittle fragmentation to the Cuss/Cr5Si3 metal silicide composite coating, as shown in Fig. 12.
Fig. 12. SEM micrograph showing subsurface microstructure of the laser clad Cuss/Cr5Si3 coating on section perpendicular to worn surface after dry sliding wear test at an applied test load of 174 N.
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4. Discussions Because the volume fraction of the primary silicides Cr5Si3 is high, the coating has very high hardness (approximately HV1000). When sliding with the hardened 0.45% C steel, the contacting asperities of the coating are difficult to be plastically deformed due to its high hardness or adhered to the counterpart contact surface due to the strong covalent dominant atomic bonds of the metal silicides while the asperities of the slidemating counterpart are inevitable to plastic deformation and thus partially removed and formed the wear debris because of its relative low hardness. During the sliding process, the toughening phase Cuss, which is much softer than asperities of the steel-mating counterpart, is preferentially worn, making the Cr5Si3 dendrites slightly protrude above the matrix. Because the hardness of chromium silicides Cr5Si3 is higher than that of the steel-mating counterpart, the slight protrusion of Cr5Si3 dendrites can withstand the applied load, so as to lessen the true contract area between the coating and the counterpart and decrease the friction coefficient. Furthermore, it can also prevent the Cuss from wear by the sliding contacting asperities of the counterpart. Consequently, the coating was worn slowly through the mechanism of ‘‘soft abrasion’’ by the counterpart. The wear debris is mainly from the hardened 0.45% C steelmate counterpart, and minor is from the coating. This is proved by the EDS result of the wear debris, which is highly enriched in Fe and with high content of O with minor amount of Cu, Cr and Si. Meanwhile, the presence of Cuss in the interdendritic zone, well known for its excellent thermal conductivity, low coefficient of friction and very high ductility, has a positive effect on improving the toughness of the Cr5Si3. No microcracking was seen at the indentation corners during the Vickers microhardness indentation tests, as shown in Fig. 5. So when the load applied on the Cr5Si3 dendrites, no microcracking and microspalling were observed during the dry sliding wear process, as shown in Fig. 12. In addition, the rapidly solidified homogeneous microstructure of the coating, which has excellent combination strength and toughness, also improve the wear resistance of the coating. At the same time, during the wear process, some of wear debris were detached from the sliding surface while some still retained on the sliding contact surface. Because the hardness of the wear debris is higher than that of the Cuss, while lower than that of the Cr5Si3, the retained wear debris can participate in the wear process and play a role just like ‘‘three body abrasion’’ to polish the coating surface. As a result, the worn surface is very smooth without any noticeable cutting or plowing grooves and metallic adhesion features, as shown in Figs. 8 and 9. Both the wear mass loss and the friction coefficient of the coating are low compared to those of the reference metallic material austenitic stainless steel AISI321, as clearly shown in Figs. 6 and 7. Alpas et al. [18–20] observed that the composite with the coarser particulate size had better wear resistance during the wear process. Compared to laser melted Cuss/(Cr5Si3-CrSi) metal silicide alloy with the same
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powder chemical composition, the laser clad Cuss/Cr5Si3 coating has the smaller size structure. When sliding with the hardened 0.45% C steel, the metal silicides with the coarser size can carry the applied load and protect the Cuss effectively. So the laser melted Cuss/(Cr5Si3-CrSi) alloy has the slightly better wear resistance than the laser clad Cuss/Cr5Si3 coating. 5. Conclusions Wear resistant Cuss/Cr5Si3 metal silicide composite coating was fabricated on an austenitic stainless steel AISI321 by laser cladding. Due to the rapid solidification process and the positive toughening effect of the Cuss on the Cr5Si3, the Cuss/ Cr5Si3 coatings have good combination of strength and toughness, and exhibit excellent wear resistance and low coefficient of friction under dry sliding wear test conditions. The wear mechanism is mostly dominated by the ‘‘soft abrasion’’. Acknowledgements The work was supported by National Natural Science Foundation of China (Grant Nos. 50331010, 50471006), TransCentury Outstanding Talents Program of Ministry of Education of China, Basic Scientific Research Project of the Commission of Science and Technology and Industries for National Defense of China. The authors are indebted to Mr. Lingyun Zhang and Ms. Rongli Yu for their invaluable assistance during the laser melting and the metallographic experiments. References [1] E. Strom, S. Eriksson, H. Rundlof, J. Zhang, Acta Mater. 53 (2005) 357. [2] H. Bei, E.P. George, G.M. Pharr, Intermetallics 11 (2003) 283. [3] J.H. Schneibel, C.J. Rawn, E.A. Payzant, C.L. Fu, Intermetallics 12 (2004) 845. [4] J.R. Jokisaari, S. Bhaduri, S.B. Bhaduri, Mater. Sci. Eng. A 323 (2002) 478. [5] H.M. Wang, F. Cao, L.X. Cai, H.B. Tang, R.L. Yu, L.Y. Zhang, Acta Mater. 51 (2003) 6319. [6] J.A. Hawk, D.E. Alman, Mater. Sci. Eng. A 239–240 (1997) 899. [7] C.T. Liu, Mater. Chem. Phys. 42 (1995) 77. [8] H. Bei, E.P. George, E.A. Kenik, G.M. Pharr, Acta Mater. 51 (2003) 6241. [9] W.Y. Kim, H. Tanaka, A. Kasama, S. Hanada, Intermetallics 9 (2001) 827. [10] M.H. Yoo, K. Yoshimi, Intermetallics 8 (2000) 1215. [11] J.J. Petrovic, Intermetallics 8 (2000) 1175. [12] J.P. Tu, L. Meng, M.S. Liu, Wear 220 (1998) 72. [13] M.R. Bateni, F. Ashrafizadeh, J.A. Szpunar, R.A.L. Drew, Wear 253 (2002) 626. [14] Y.X. Yin, H.M. Wang, J. Mater. Res. 20 (2005) 1122. [15] X.D. Lu, H.M. Wang, Acta Mater. 52 (2004) 5419. [16] M.M. Lima, C. Godoy, J.C. Avelar-Bastista, P.J. Modenesi, Mater. Sci. Eng. A 357 (2003) 337. [17] H.M. Wang, G. Duan, Intermetallics 11 (2003) 755. [18] A.T. Alpas, J. Zhang, Metall. Mater. Trans. A 25 (1994) 969. [19] T.E. Raghy, P. Blau, M.W. Barsoum, Wear 238 (2000) 125. [20] J.H. Gong, H.Z. Miao, Z. Zhao, Mater. Lett. 53 (2002) 258.
Microstructure and wear properties of laser clad Cuss/Cr5Si3 metal silicide composite coatings Y.X. Yin, H.M. Wang * Laboratory of Laser Materials Processing and Manufacturing, School of Materials Science and Engineering, Beihang University (Formerly Beijing University of Aeronautics and Astronautics), 37 Xueyuan Road, Beijing 100083, China Received 20 January 2006; received in revised form 20 February 2006; accepted 20 February 2006 Available online 4 April 2006
Abstract Wear resistant Cu-based solid solution (Cuss) toughened Cr5Si3 metal silicide composite coatings were fabricated on austenitic stainless steel AISI321 by laser cladding process. Due to the rapidly solidified microstructural characteristics and the excellent toughening effect of Cuss on Cr5Si3, the Cuss/Cr5Si3 coatings have outstanding wear resistance and low coefficient of friction under room temperature dry sliding wear test conditions coupling with hardened 0.45% C steel. # 2006 Elsevier B.V. All rights reserved. PACS: 81.15.Fg; 81.40.Pq Keywords: Coating; Laser cladding; Metal silicide; Wear; Microstructure
1. Introduction Wear is of considerable importance to most moving mechanical components. High performance wear resistant materials are expected to have high hardness to resist plastic flow under high compressive load, good strength to resist elastic and plastic deformation under normal or shear load, and excellent anti-welding, anti-scoring and anti-seizing capabilities and good tribological compatibility. In recent years, much research is focused on the refractory transition metal silicides alloys as potential high- and/or ultrahigh-temperature candidate structural materials because of their outstanding balance of high melting point, high strength, low density, high elastic modulus and excellent creep and oxidation resistance [1–4]. Many ordered transition metal silicides alloys, such as Cr3Si, Cr5Si3, W5Si3, etc., have also demonstrated outstanding abrasive wear resistance and low coefficient of friction. They are expected to be a new class of wear resistant candidate materials for those moving mechanical components working under aggressive tribological conditions [5,6]. Unfortunately,
* Corresponding author. Tel.: +86 10 8231 7102; fax: +86 10 8233 8131. E-mail address: [email protected] (H.M. Wang). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.054
these metal silicides alloys are very brittle at ambient temperature, which restricted their practical applications as bulk tribological components. Addition of a ductile second phase is one of the most effective means to improve the toughness of intermetallics. Moreover, in this way, it also allows us to better control on the tribological properties of the metal silicides alloys by adjusting the trade-off between hardness and toughness, which the wear behavior of the metal silicide alloy is direct proportion to [7–11]. The copper-based solid solution (hereafter referred as Cuss) is well known for its high thermal conductivity, low coefficient of friction and excellent tribological compatibility and ductility, and is expected to be an ideal phase to toughen the wear resistant metal silicide alloys [12,13]. In our previous study, a wearresistant Cuss-toughened Cr5Si3/CrSi metal silicide alloy was designed and fabricated by the laser melting (Lasmelt) process. The Cuss-toughened alloys exhibited excellent wear resistance under room dry sliding wear test conditions [14]. Since wear is a surface-related gradual degradation phenomenon, fabricating a high performance wear-resistant coating using advanced surface engineering methods is one of the most economic and effective means to solve the wear problems for many tribological components. Among the different surface treatments, laser cladding is an efficient way to synthesize novel
Y.X. Yin, H.M. Wang / Applied Surface Science 253 (2006) 1584–1589
wear resistant coating for mechanical moving components made of cheap materials such as carbon steels [15]. Austenitic stainless steel AISI 321, which is cheap and has excellent combination of corrosion resistance and good high temperature mechanical properties up to 600 8C, is one of the most widely used class corrosion resistant structural materials in chemical industries. In this paper, a wear resistant Cuss/Cr5Si3 metal silicide composite coating was fabricated on austenitic stainless steel AISI321 by laser cladding. The laser clad coatings can be applied under the high speed and heavy load aggressive tribological service environments, such as sliding bearing, mechanical seals in valves, pumps in chemical, marine or petrochemical industries. Microstructure of the coatings was characterized by optical microscopy (OM), scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). Vickers microhardness indentations and the resulting cracks were used to estimate the toughness of the coating [16]. Wear resistance of the Cuss/ Cr5Si3 metal silicide composite coating was evaluated under room temperature dry sliding wear test conditions and the responding wear mechanism was discussed. 2. Experimental procedures A solution-treated commercial austenitic stainless steel AISI321 was selected as the substrate material because it is widely used as tribological components in valves and pumps in chemical and petrochemical industries. Alloy powders (particle size 45–125 mm) in nominal chemical composition (at.%) of 11% Cu–48 % Cr–41% Si were utilized as the laser cladding raw materials. The powders were preplaced on surface of the austenitic stainless steel specimens, 60 mm 20 mm 10 mm in size, prior to laser cladding with a powder-bed thickness of approximately 1.8 mm. Laser cladding was conducted on an 8 kW transverse-flow continuous-wave CO2 laser materials processing system equipped with a 4-axis computer numerical controlled (CNC) laser materials processing machine under ambient atmospheric environment. The optimized laser cladding parameters are as follows: laser power 3.5 kW, laser beam diameter 4 mm and beam traverse speed 500 mm/min. Six overlap tracks were clad side by side with an overlap ratio of approximately 25% in order to clad the whole surface of 60 mm 20 mm on the substrate specimen. Metallographic sections were prepared using standard mechanical polishing procedures and were etched in HNO3– 20 vol% CH3COOH–20 vol% H2O. The microstructure was characterized by Olypus BX51M optical microscope (OM) equipped with a Sisc IAS6.0 image analyzing software and Cambridge S250MK2 KYKY-2800B scanning electron microscopes. Phase constituents were identified by X-ray diffraction using a Rigaku D/max 2200 pc automatic X-ray diffractometer with Cu target Ka radiation. Chemical compositions of the phases were analyzed by energy dispersive spectrometer using Noran Ventage DSI spectrometer. Hardness profiles along the depth direction and Vickers indentations of the laser clad coatings were performed on polished coating and measured using a MH-6 semi-automatic Vickers microhardness tester
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with test loads of 200 and 500 g and a dwell time of 15 s. Volume fraction of the Cu-based solid solution (Cuss) in the coating was measured by quantitative metallographic analysis method on high-contrast SEM photographs (1000 magnifications) using a commercial contrast-based image analyzing software. Wear properties of the coatings were evaluated on a MM200 block-on-wheel dry sliding wear tester at room temperature, where a square block-like specimen (10 mm 10 mm 10 mm in size) is pressed under the applied test load against the outer periphery surface of a hardened 0.45% C steel wheel (HRc53) rotating at a speed of 400 rpm. The outer diameter of the steel wheel is 45 mm and the relative sliding speed between the specimen and the contact-coupling wheel is 0.942 m/s. The wear test cycle lasted for 60 min and the total wear sliding distance is approximately 3391 m. Details of the dry sliding wear tester and the test procedures were reported elsewhere [17]. The solid-solution treated austenitic stainless steel AISI321 was selected as the reference comparison test material for all the wear tests in order to rank the wear resistance of the laser clad Cuss/Cr5Si3 coating. Different levels of applied test loads, 98, 147 and 196 N, were selected. Each test was repeated two times. Wear mass loss was measured by an electronic balance (Sartorius BS110) with an accuracy of 0.1 mg and was utilized to rate the relative wear resistance of the coatings in comparison to the reference test material. Friction coefficient was calculated from the friction torque recorded during the dry sliding wear process. To assist the analysis of wear mechanisms, worn surfaces and the wear debris particles were collected during wear testing process and examined under SEM. 3. Results The laser clad Cuss/Cr5Si3 metal silicide composite coating, with a thickness of approximately 1.3 mm, low dilution to the substrate, has a uniform, fine dendritic structure consisting of primary dendrites and a small amount of interdendritic phase and is metallurgically bonded to the substrate, as shown in Figs. 1 and 2. Small amount of fine micro-porosities was visible and uniformly distributed in the interdendritic zone due to solidification shrinkage of the interdendritic residual liquid, as shown in Figs. 1b and 2a. After six overlap tracks were clad side by side, the microstructure of whole coating, including the overlap area and the tracks, is uniform and homogeneous. Results of XRD (as indicated in Fig. 3) and EDS analysis indicate that the main constitutional phases of the coating are Cr5Si3 having the tI38 W5Si3-type topologically closed packed (TCP) crystal lattice and Cu-based solid solution which is highly supersaturated with Cr, Si and Fe. Volume fraction of the Cu-based solid solution in the laser clad Cuss/Cr5Si3 metal silicide composite coating is approximately 17%. Hardness profile along the depth direction of the coating is shown in Fig. 4. Result of the Vickers indentation test shows that the coating has an excellent combination of the hardness and toughness and no cracks were observed at the corners of indentation prints under applied test loads of 200 and 500 g, as indicated in Fig. 5. Because of the high volume
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Fig. 2. SEM micrographs showing typical microstructure of the laser clad Cuss/ Cr5Si3 metal silicide composite coating, (a) low and (b) high magnifications. Fig. 1. OM photographs showing the overview transverse cross-section (a) and coating/substrate transition zone (b) of the laser clad Cuss/Cr5Si3 metal silicide composite coating.
fraction of the hard metal silicides Cr5Si3, the laser clad coating has an average Vickers hardness number of over HV1000 and uniform hardness distribution. Compared to the austenitic stainless AISI321 reference material, the laser clad Cuss/Cr5Si3 metal silicide composite coating exhibited excellent wear resistance and low friction coefficient under room temperature dry sliding wear test conditions coupling with the hardened 0.45% C steel wheel as the mating counterpart, as shown in Figs. 6 and 7. While compared to the laser melted Cuss-toughened Cr5Si3-CrSi metal silicide alloy with the same powder chemical composition, the laser clad coating has a little lower wear resistance. The wear mass losses of laser clad coating and laser melted alloy are very insensitive to the test load, which increases extremely slow as the applied load increases, whereas that of the reference material AISI321 increases drastically with the increasing load. The mean friction coefficient for the sliding friction couple
between the laser clad Cuss/Cr5Si3 metal silicide composite coating and the hardened 0.45% C steel is relatively low (approximately 0.49) and fluctuates between approximately 0.47 and 0.52, while that between austenitic stainless steel AISI321 and the hardened 0.45% C steel is high (approximately 0.71) and fluctuates between approximately 0.65 and 0.77.
Fig. 3. X-ray diffraction patterns of the laser clad Cuss/Cr5Si3 metal silicide composite coating.
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Fig. 4. Hardness profile along depth direction of the laser clad Cuss/Cr5Si3 coating.
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Fig. 7. Profiles of friction coefficient vs. wear test time for the laser clad Cuss/ Cr5Si3 coating and the reference material AISI321 during dry sliding wear process at an applied test load of 196 N.
Fig. 5. OM micrographs showing the indentation prints of Vickers hardness in the laser clad Cuss/Cr5Si3 coating.
Fig. 6. Wear mass loss of the laser clad Cus/Cr5Si3 coating, laser melted Cuss/ (Cr5Si3-CrSi) alloy and the reference material AISI321 as a function of test load under dry sliding wear conditions.
Fig. 8. SEM micrographs showing the worn surface morphologies of the Cuss/ Cr5Si3 coating after dry sliding wear test at an applied test load of 147 N, (a) low and (b) high magnifications.
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Fig. 11. SEM micrograph showing morphologies of the wear debris particles of the laser clad Cuss/Cr5Si3 coating after dry sliding wear test.
Fig. 9. SEM micrographs showing the worn surface morphologies of the Cuss/ Cr5Si3 coating after dry sliding wear test, at 98 N (a) and 196 N (b).
Fig. 10. SEM micrograph showing the worn surface morphologies of the austenitic stainless steel AISI321 after dry sliding wear test at an applied test load of 174 N.
As shown in Figs. 8 and 9, the worn surfaces of the coating are smooth, similar to a polished and deep-etched metallographic section on which the primary metal silicide phases are visible. On the contrary, noticeable grooves and adhesion and plastic deformation features are observable on the worn surface of the reference material AISI321, as illustrated in Fig. 10. As shown in Fig. 11, SEM observations indicate that wear debris is in fine powders and platelet-like consolidated powderagglomerates. EDS analysis (with average chemical composition (at.%) of approximately 44.45% Fe–26.54% O–11.61% Cu–11.61% Cr–5.80% Si) indicates that the fine wear debris particles are highly enriched in Fe and with high content of O and with minor amount of Cu, Cr and Si. Worn subsurface microstructure on longitudinal section indicates no evidence of local subsurface plastic deformation, brittle fragmentation to the Cuss/Cr5Si3 metal silicide composite coating, as shown in Fig. 12.
Fig. 12. SEM micrograph showing subsurface microstructure of the laser clad Cuss/Cr5Si3 coating on section perpendicular to worn surface after dry sliding wear test at an applied test load of 174 N.
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4. Discussions Because the volume fraction of the primary silicides Cr5Si3 is high, the coating has very high hardness (approximately HV1000). When sliding with the hardened 0.45% C steel, the contacting asperities of the coating are difficult to be plastically deformed due to its high hardness or adhered to the counterpart contact surface due to the strong covalent dominant atomic bonds of the metal silicides while the asperities of the slidemating counterpart are inevitable to plastic deformation and thus partially removed and formed the wear debris because of its relative low hardness. During the sliding process, the toughening phase Cuss, which is much softer than asperities of the steel-mating counterpart, is preferentially worn, making the Cr5Si3 dendrites slightly protrude above the matrix. Because the hardness of chromium silicides Cr5Si3 is higher than that of the steel-mating counterpart, the slight protrusion of Cr5Si3 dendrites can withstand the applied load, so as to lessen the true contract area between the coating and the counterpart and decrease the friction coefficient. Furthermore, it can also prevent the Cuss from wear by the sliding contacting asperities of the counterpart. Consequently, the coating was worn slowly through the mechanism of ‘‘soft abrasion’’ by the counterpart. The wear debris is mainly from the hardened 0.45% C steelmate counterpart, and minor is from the coating. This is proved by the EDS result of the wear debris, which is highly enriched in Fe and with high content of O with minor amount of Cu, Cr and Si. Meanwhile, the presence of Cuss in the interdendritic zone, well known for its excellent thermal conductivity, low coefficient of friction and very high ductility, has a positive effect on improving the toughness of the Cr5Si3. No microcracking was seen at the indentation corners during the Vickers microhardness indentation tests, as shown in Fig. 5. So when the load applied on the Cr5Si3 dendrites, no microcracking and microspalling were observed during the dry sliding wear process, as shown in Fig. 12. In addition, the rapidly solidified homogeneous microstructure of the coating, which has excellent combination strength and toughness, also improve the wear resistance of the coating. At the same time, during the wear process, some of wear debris were detached from the sliding surface while some still retained on the sliding contact surface. Because the hardness of the wear debris is higher than that of the Cuss, while lower than that of the Cr5Si3, the retained wear debris can participate in the wear process and play a role just like ‘‘three body abrasion’’ to polish the coating surface. As a result, the worn surface is very smooth without any noticeable cutting or plowing grooves and metallic adhesion features, as shown in Figs. 8 and 9. Both the wear mass loss and the friction coefficient of the coating are low compared to those of the reference metallic material austenitic stainless steel AISI321, as clearly shown in Figs. 6 and 7. Alpas et al. [18–20] observed that the composite with the coarser particulate size had better wear resistance during the wear process. Compared to laser melted Cuss/(Cr5Si3-CrSi) metal silicide alloy with the same
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powder chemical composition, the laser clad Cuss/Cr5Si3 coating has the smaller size structure. When sliding with the hardened 0.45% C steel, the metal silicides with the coarser size can carry the applied load and protect the Cuss effectively. So the laser melted Cuss/(Cr5Si3-CrSi) alloy has the slightly better wear resistance than the laser clad Cuss/Cr5Si3 coating. 5. Conclusions Wear resistant Cuss/Cr5Si3 metal silicide composite coating was fabricated on an austenitic stainless steel AISI321 by laser cladding. Due to the rapid solidification process and the positive toughening effect of the Cuss on the Cr5Si3, the Cuss/ Cr5Si3 coatings have good combination of strength and toughness, and exhibit excellent wear resistance and low coefficient of friction under dry sliding wear test conditions. The wear mechanism is mostly dominated by the ‘‘soft abrasion’’. Acknowledgements The work was supported by National Natural Science Foundation of China (Grant Nos. 50331010, 50471006), TransCentury Outstanding Talents Program of Ministry of Education of China, Basic Scientific Research Project of the Commission of Science and Technology and Industries for National Defense of China. The authors are indebted to Mr. Lingyun Zhang and Ms. Rongli Yu for their invaluable assistance during the laser melting and the metallographic experiments. References [1] E. Strom, S. Eriksson, H. Rundlof, J. Zhang, Acta Mater. 53 (2005) 357. [2] H. Bei, E.P. George, G.M. Pharr, Intermetallics 11 (2003) 283. [3] J.H. Schneibel, C.J. Rawn, E.A. Payzant, C.L. Fu, Intermetallics 12 (2004) 845. [4] J.R. Jokisaari, S. Bhaduri, S.B. Bhaduri, Mater. Sci. Eng. A 323 (2002) 478. [5] H.M. Wang, F. Cao, L.X. Cai, H.B. Tang, R.L. Yu, L.Y. Zhang, Acta Mater. 51 (2003) 6319. [6] J.A. Hawk, D.E. Alman, Mater. Sci. Eng. A 239–240 (1997) 899. [7] C.T. Liu, Mater. Chem. Phys. 42 (1995) 77. [8] H. Bei, E.P. George, E.A. Kenik, G.M. Pharr, Acta Mater. 51 (2003) 6241. [9] W.Y. Kim, H. Tanaka, A. Kasama, S. Hanada, Intermetallics 9 (2001) 827. [10] M.H. Yoo, K. Yoshimi, Intermetallics 8 (2000) 1215. [11] J.J. Petrovic, Intermetallics 8 (2000) 1175. [12] J.P. Tu, L. Meng, M.S. Liu, Wear 220 (1998) 72. [13] M.R. Bateni, F. Ashrafizadeh, J.A. Szpunar, R.A.L. Drew, Wear 253 (2002) 626. [14] Y.X. Yin, H.M. Wang, J. Mater. Res. 20 (2005) 1122. [15] X.D. Lu, H.M. Wang, Acta Mater. 52 (2004) 5419. [16] M.M. Lima, C. Godoy, J.C. Avelar-Bastista, P.J. Modenesi, Mater. Sci. Eng. A 357 (2003) 337. [17] H.M. Wang, G. Duan, Intermetallics 11 (2003) 755. [18] A.T. Alpas, J. Zhang, Metall. Mater. Trans. A 25 (1994) 969. [19] T.E. Raghy, P. Blau, M.W. Barsoum, Wear 238 (2000) 125. [20] J.H. Gong, H.Z. Miao, Z. Zhao, Mater. Lett. 53 (2002) 258.