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NiTi2 intermetallic composite coating on titanium alloy
Materials Science and Engineering A338 (2002) 126 /132 www.elsevier.com/locate/msea
Microstructure and wear resistance of laser clad Ti5Si3/NiTi2 intermetallic composite coating on titanium alloy H.M. Wang *, Y.F. Liu Laboratory of Laser Materials Processing and Surface Engineering, School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, 37 Xue Yuan Road, Beijing 100083, PR China Received 21 May 2001; received in revised form 14 January 2002
Abstract A wear resistant Ti5Si3/NiTi2 intermetallic composite coating was fabricated on substrate of a titanium alloy BT9 by laser cladding with (wt.%) Ni /30Ti /10Si elemental powder blends. The laser clad intermetallic composite coating has a rapidly solidified fine microstructure consisting of Ti5Si3 primary particles uniformly distributed in the NiTi2 matrix and is metallurgically bonded to the titanium substrate. The laser clad Ti5Si3/NiTi2 intermetallic composite coating has high hardness and excellent wear resistance under dry sliding wear test conditions. The excellent wear resistance of the laser clad Ti5Si3/NiTi2 intermetallic composite coating is attributed to the coating’s high hardness, strong intermetallic atomic bonding and refined microstructure. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser cladding; Intermetallic matrix composite coating; Ti5Si3; NiTi2; Wear resistance; Titanium alloy
1. Introduction Titanium alloys are widely used as structural components in aerospace, chemical, petrochemical and marine industries owing to their low density, high specific strength, exceptional corrosion resistance and hightemperature mechanical properties. However, titanium alloys suffer from the serious disadvantages of poor tribological properties such as high friction coefficient, difficult to lubricate, low adhesive and fretting wearresistance, which prevent them from applications as engineering tribological components [1,2]. Surface modification is one of the most efficient means to improve the tribological properties of titanium alloys. Plasma sprayed hydroxyapatite bioceramic coating was adopted in attempt to improve the fretting wear resistance of titanium alloys [1]. However, fretting wear resistance of the plasma sprayed bioceramic coatings was even inferior to the original Ti/6Al /4V substrate due to the porous and loose structure of hydroxyapatite coat-
* Corresponding author. Tel.: /86-10-82317102; fax: /86-1082328041. E-mail address: [email protected] (H.M. Wang).
ing. Hard chromium nitride films were deposited on Ti/ 6Al /4V alloy by both ion beam enhanced deposition and conventional PVD process [3,4] for improvement of fretting wear resistance. The ion beam enhanced deposited CrN films was found to be more wear resistant because of the finer microstructure and better interfacial bonding. The tribological properties of the titanium alloy Ti/6Al /4V alloy was found to be improved considerably after plasma nitriding treatment [5]. Dong et al. [6] utilized thermal oxidation heat treatment to enhance the sliding wear resistance of titanium alloys and the friction coefficient of the thermal oxidized Ti/ 6Al /4V alloy under ball-on-disc friction tests was significantly reduced. It is also reported that the wear resistance of titanium alloys can be improved by PVD and CVD thin wear resistant films such as TiN, TiC, etc. [7 /11]. Many laser surface modification technologies were also utilized to enhance the wear resistance of titanium alloys. Wear resistant composite coatings reinforced by rapidly solidified TiN dendrites were produced by laser surface alloying with gaseous nitrogen on substrate of Ti /6Al/4V alloy and wear resistance of the laser surface alloyed coating under two-body abrasive and block-on-ring full-sliding wear conditions were significantly enhanced (up to a factor of 90 under
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sliding wear test conditions) [12]. By means of laser surface alloying with Si, rapidly solidified composite coatings reinforced by primary and eutectic Ti5Si3 intermetallic phases were prepared on Ti/6Al /4V. The wear resistance both under dry sliding and twobody abrasive test conditions were significantly improved [13]. By laser surface alloying the Ti /6Al/4V alloy with SiC powder, rapidly solidified composite coatings co-reinforced by TiC and Ti5Si3 phases were produced on substrate of the Ti /6Al /4V titanium alloy and the wear resistance are enhanced by a factor of 85 compared to the original titanium alloy under dry sliding condition [14]. Majumdar et al. [15] reported that the tribological properties of commercial pure titanium were improved by laser surface alloying with Si, Al and Si/Al. Results demonstrated that laser surface alloying with Si is more effective in terms of wear resistance improvement than with Si/Al or Al. The improvement of wear resistance was attributed to the presence of Ti5Si3/Ti eutectics in the laser surface alloyed zone. The effects of laser beam assisted liquid nitridation treatment on wear resistance of a titanium alloy was also investigated by Weerasinghe et al., Akugun and Inal and Kloosterman and Dehosson [16 /18]. The technique of laser beam assisted solid-state diffusion also was adopted by Duverneix et al. [19] to improve the wear resistance of titanium alloy. The titanium silicide Ti5Si3 is regarded as one of the alternative high-temperature structural materials because of its excellent combination of high-temperature strength, high melting point, low density, good hightemperature stability and excellent oxidation resistance [20 /22]. The serious room temperature brittleness prevents Ti5Si3 from industrial applications as structural materials. Nevertheless, the very high hardness, strong covalent atomic bonding and excellent high-temperature stability do make Ti5Si3 a excellent candidate as a wear resistant reinforcing phase in a wear resistant composite material. The NiTi2 intermetallic compound has reasonably good combination of ductility due to its facecenter-cubic crystal structure [23] and wear resistance owing to its high hardness (HV700) [24]. Naturally, a NiTi2 intermetallic matrix composite coating reinforced by Ti5Si3 wear-resisting particles is naturally expected to have good wear resistance both under adhesive and abrasive wear conditions. In this paper, a Ti5Si3/NiTi2 wear resistant intermetallic matrix composite coating was fabricated on substrate of a titanium alloy BT9 by laser cladding with Ni /Ti /Si alloy powders. The microstructure of the laser clad intermetallic matrix composite coating was characterized by OM, scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and XRD. Wear resistance of the intermetallic composite coating was investigated under dry sliding wear conditions.
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Fig. 1. Schematic illustration of the block-on-wheel dry sliding wear tester.
2. Experimental procedures A commercial near alpha titanium alloy BT9 was selected as the starting material. The nominal chemical composition (wt.%) of the alloy was 5.8% Al, 2.8% Mo, 0.8% Zr, 0.2% Si and balance Ti. The samples, 50 / 20 /20 mm3 in size, were machined by electric discharge machining. Commercial pure elemental powder blends of Ni, Ti and Si in chemical composition (wt.%) of Ni30% Ti-10% Si (hereafter referred to as Ti30Si10Ni60) and with a particle size of approximately 140 mm were selected as the laser cladding powder materials. The elemental powder blends are pre-placed on the surface of the titanium alloy samples with a powder bed thickness of approximately 1.5 mm and are melted using a 5 kW continuous wave transverse flow CO2 laser material processing systems with a 4-axis CNC table. Argon gas was blown into the melt pool to provide shielding during the laser cladding process. The singletrack laser cladding parameters are: laser out power 4 kW, laser beam size 15 /1 mm and beam traverse speed 2 mm s 1. Microstructure of the laser clad composite coatings was analyzed using the Nephot II optical microscope (OM) and S-530 SEM. The phases presented in the laser clad intermetallic composite coating were identified by Dmax-rB X-ray diffractometer (using CuKa radiation and a scanning rate of 58 min 1) and the chemical composition of the phases were analyzed by LinkISIS EDS. The hardness profile along the depth direction of the laser clad intermetallic coating were measured by a MH-6 semi-automatic Vickers hardness tester with a testing load of 1.96 N and a loading time of 15 s. Wear resistance of the coating was evaluated on a block-on-wheel dry sliding wear tester (as shown in Fig. 1), where the laser clad titanium specimen, 10 /10 /10 mm3 in size, seats on a rotating wheel of hardened 0.45% C steel (HRC53). The testing parameters are: load 98 N, sliding speed 0.92 m s 1 and total wear sliding distance 3.02 /103 m. Relative wear resistance, i.e the ratio of wear weight loss of the original titanium alloy BT9 specimen to that of the laser clad specimen was utilized to evaluate the wear resisting property of the laser clad
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Fig. 2. XRD pattern of the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating on substrate of the titanium alloy BT9.
Ti5Si3/NiTi2 intermetallic composite coating. The larger the ratio, the higher the wear resistance of the laser clad intermetallic composite coating.
3. Results 3.1. Microstructure Fig. 2 is the XRD pattern of the laser clad Ti5Si3/ NiTi2 wear resistant intermetallic composite coating. After laser cladding with Ti30Si10Ni60 elemental powder blends, a new type of laser clad composite coating mainly composed of Ti5Si3 and NiTi2 phases was produced on substrate of the BT9 titanium alloy. The microstructure of the intermetallic composite coating is fine, uniform and free-from porosities and microcracks, as shown in Fig. 3. Fig. 4 shows the microstructural characteristics of the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating. As shown clearly in Fig. 4(a) and (b), the coarse irregular blocky or granular particles are uniformly distributed in the white matrix. Results of
Fig. 3. OM micrograph showing the overview cross-section of the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating on substrate of the titanium alloy BT9.
XRD and EDS analysis indicated that the dark gray coarse irregular blocks and the fine granular particles are the Ti5Si3 primary phase and the remaining white matrix phase is the intermetallics NiTi2. Fig. 5 is a high magnification SEM photograph showing the typical microstructures in the centre of the laser clad Ti5Si3/NiTi2 intermetallic coating. It can be seen that in some local areas minor b-Ti phase was observed outlining the fine granular Ti5Si3 particles. Since the intermetallic compound Ti5Si3 has the highest melting point (2393 K) and the most negative free
Fig. 4. (a) OM and (b) SEM micrographs showing the typical microstructures of laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating.
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Fig. 5. SEM micrograph showing the existence of minor amount of ‘bull-eye-like’ b-Ti phase around some primary Ti5Si3 granular particles in the central of the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating.
energy of formation in the Ni /Ti /Si system, the blocky or granular particles Ti5Si3 precipitate from the melt as the primary phase during rapid cooling of the melt pool. In some very local areas where the remaining liquid around some Ti5Si3 fine primary granular particles is rich in titanium, fine granular Ti5Si3 phase acts as a homogeneous crystal nucleus forb-Ti formation, and consequently, minor ‘bull-eye-like’ b-Ti phase was formed around the fine Ti5Si3 primary particles, as shown in Fig. 5. Accompanying the complete solidification of all the primary Ti5Si3 phase, the residual liquid is highly enriched in nickel content and the intermetallic
Fig. 6. SEM micrograph showing the existence of a few coarse TiN dendrites in the near free surface region of the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating.
compound NiTi2 was formed as the matrix of the laser clad intermetallic composite coating. Although the shielding gas of argon was utilized to protect melt pool during laser cladding process, a small amount of gaseous nitrogen coming from the air still goes into the melt pool, a few coarse TiN columnar dendrites was occasionally observed in the near-surface region of the laser clad intermetallic coating, as shown in Fig. 6.
Fig. 7. (a) OM and (b) SEM images showing the high quality metallurgical bonding of the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating to the titanium substrate.
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intermetallic composite coating to the substrate is of high-quality metallurgical bonding and with pronounced gradient microstructure in the transition zone. 3.2. Microhardness
Fig. 8. Microhardness profile across the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating.
Fig. 8 shows the microhardness profile along the depth direction of the laser clad Ti5Si3/NiTi2 intermetallic composite coating. The laser clad intermetallic coating has a high hardness distribution within the main coating except in the bottom of the coating where the volume fraction of Ti5Si3 primary phase is relatively low and the hardness decreases gradually to the substrate. 3.3. Wear resistance
Fig. 7 shows the microstructural characteristics of the bonding zone between the laser clad Ti5Si3/NiTi2 intermetallic composite coating and the substrate. The b-Ti dendrites grow directionally and epitaxially from the titanium substrate and the bonding of the laser clad
The laser clad Ti5Si3/NiTi2 intermetallic composite coating has excellent wear resisting property under dry sliding wear test conditions. Dry sliding wear test results indicated that the wear resistance of the laser clad
Fig. 9. SEM micrographs showing the worn surface morphologies of the original titanium alloy BT9 (a), (b) and the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating (c), (d).
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Fig. 10. Micrographs showing the worn subsurface microstructure of (a) the original titanium alloy BT9 OM and (b) the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating SEM.
intermetallic coating is up to 125 times higher than that of the original titanium alloy.
4. Discussions In the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating, the hard and wear resistant Ti5Si3 primary coarse blocks and particles are uniformly distributed in the hard, ductile, wear resistant NiTi2 intermetallic matrix. The unique microstructure of the laser clad intermetallic reinforced intermetallic matrix composite coating provided the coating excellent wear resistance especially under sliding wear conditions. During block-on-wheel dry sliding wear, the original titanium alloy BT9 suffered severe adhesive wear. Worn surface of the original titanium alloy BT9 specimen was very rough, with typical adhesive wear features evidenced by numerous adhesive craters, deep ploughing grooves and detached Ti debris (confirmed by EDS analysis), as shown in Fig. 9(a) and (b). Fig. 10(a) is the cross-section perpendicular to the worn surface of the original titanium BT9 alloy specimen. Severe plastic deformation occurred in the subsurface zone during sliding wear process. The original misorientated a/b acicular structure has transformed under the sliding wear action to a well-defined, severely deformed lamellar textured structure or severely bent to the sliding direction, as shown in Fig. 10(a). On the contrary, the worn surface of the laser clad Ti5Si3/NiTi2 intermetallic composite coating was very smooth, as shown in Fig.
9(c) and (d). SEM observation of the worn subsurface microstructure indicated that no noticeable plastic deformation or microcracks were detected in the subsurface zone, as shown in Fig. 10(b). Because the primary Ti5Si3 phase has very high hardness and is uniformly distributed in the both hard and ductile NiTi2 intermetallic matrix, the laser clad Ti5Si3/NiTi2 composite coating has very excellent abrasive wear resistance and consequently there are only slight scratches visible on the worn surface, as shown in Fig. 9(d). Since all the constitution phases in the laser clad Ti5Si3/NiTi2 wear resistant composite coating are intermetallic compounds, i.e. the Ti5Si3 and the NiTi2, the strong atomic bonding inherent to intermetallic compounds provides the composite coating excellent resistance to metallic adhesion during dry sliding wear. As a result, the intermetallic composite coating has excellent adhesive wear resistance when mating with a sliding metallic counterpart, which is evidenced by the fact that there is no significant adhesive worn morphologies visible on the worn surface of the laser clad composite coating, as shown in Fig. 9(c) and (d). Moreover, the rapidly solidified homogeneous fine microstructure imparts the coating good combination of high strength and toughness, which in turn provides the coating excellent resistance to spalling and delamination. Careful examination of worn subsurface microstructure indicates clearly that no selective wear and plastic deformation occured during the dry sliding wear process for the laser clad intermetallic coating, as shown in Fig. 10(b). This indicated that the excellent wear-resisting ability of the
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constitution phases of the laser clad Ti5Si3/NiTi2 composite coating, i.e. the reinforcing Ti5Si3 and the NiTi2 matrix, is fully exploited and consequently, the laser clad coating has excellent adhesive and abrasive wear resistance.
5. Conclusions A wear resistant Ti5Si3/NiTi2 intermetallic composite coating was produced on substrate of the titanium alloy BT9 by laser cladding with Ti30Si10Ni60 elemental powders blends. The laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating has a rapidly solidified fine microstructure with an uniform distribution of the primary Ti5Si3 intermetallic phase in the NiTi2 intermetallic matrix and is metallurgically bonded to the titanium substrate. The Ti5Si3/NiTi2 intermetallic coating has a high and uniform hardness distribution within the whole coating and exhibits excellent wear resistance under dry sliding wear test condition due to its combination of high hardness, strong intermetallic atomic bonding and fine microstructure. Laser cladding of Ti5Si3/NiTi2 intermetallic matrix composite coating is anticipated to be a promising wear-resisting surface modification technique for titanium alloys.
Acknowledgements The research was partly supported by the National Natural Science Foundation of China (Grant No. 50071004) and the Science Funds Office of Avic (Grant No. 00A51013). The authors acknowledge Prof. Xitong Sun for assistance during the dry sliding wear tests.
References [1] Y.Q. Fu, A.W. Batchelor, Y. Wang, K.A. Khor, Wear 217 (1998) 132 /139. [2] T. Bell, Proceedings of the First Asian International Conference on Tribology, 2 (1998) 421. [3] Y.Q. Fu, N.L. Loh, A.W. Batchelor, X.D. Zhu, K.W. Xu, J.W. He, Mater. Sci. Eng. A265 (1999) 224 /232. [4] Y.Q. Fu, N.L. Loh, A.W. Batchelor, D.X. Liu, X.D. Zhu, K.W. Xu, J.W. He, Surf. Coat. Technol. 106 (1998) 193 /197. [5] B.S. Yilbas, Surf. Coat. Technol. 80 (1996) 287 /292. [6] H. Dong, A. Bloyce, P.H. Morton, T. Bell, Surf. Eng. 13 (1997) 402 /406. [7] M.V. Stopper, M. Kerhofs, Surf. Coat. Technol. 62 (1993) 665 / 671. [8] A.D. Wilson, A. Leyland, A. Matthews, Surf. Coat. Technol. 114 (1999) 70 /80. [9] G. Heinrich, T. Groegler, S.M. Rosiwal, R.F. Singer, Proceedings of the 24th International Conference on Metallurgical Coatings and Thin Films, 94 /95 (1997) 514 /520. [10] T. Grogler, O. Plewa, S.M. Rosiwal, R.F. Singer, Int. J. Refractory Metals Hard Mater. 16 (1998) 217 /222. [11] T. Grogler, A. Franz, D. Klaffke, S.M. Rosiwal, R.F. Singer, Diamond Related Mater. 7 (1998) 1342 /1347. [12] P. Jiang, X.L. He, L.G. Yu, H.M. Wang, Surf. Coat. Technol. 130 (2000) 24 /28. [13] P. Jiang, J.J. Zhang, L.G. Yu, H.M. Wang, Rare Metal Mater. Eng. 29 (2000) 269 /272. [14] P. Jiang, J.J. Zhang, L.G. Yu, H.M. Wang, Proc. SPIE 3888 (2000) 334 /338. [15] J. Dutta Majumdar, B.L. Mordike, I. Manna, Wear 242 (2000) 18 /27. [16] V.M. Weerasinghe, D.R.F. West, J. Damborenea, J. Mater. Pro. Technol. 58 (1996) 79. [17] O.V. Akugun, O.T. Inal, J. Mater. Sci. 29 (1994) 159. [18] A.B. Kloosterman, M. Dehosson, Script. Metall. Mater. 33 (1995) 567. [19] T. Duverneix, T. Puig, F. Bataille, Proc. SPIE 1601 (1991) 525. [20] A.K. Vasudevan, J.J. Petrovic, Mater. Sci. Eng. 155 (1992) 1 /17. [21] J.J. Petrovic, MRS Bull. 7 (1993) 35. [22] G. Sauthoff, Intermetall. Alloys Bd. 81 (H.12) (1990) 855 /861. [23] J.Q. Yu, Hand Book of Binary Alloy Phase Diagrams, Shanghai Science and Technology Publishing Company, 1987, p. 496. [24] H. Hiraga, T. Inoue, H. Shimura, A. Matsunawa, Wear 231 (1999) 272 /278.
Microstructure and wear resistance of laser clad Ti5Si3/NiTi2 intermetallic composite coating on titanium alloy H.M. Wang *, Y.F. Liu Laboratory of Laser Materials Processing and Surface Engineering, School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, 37 Xue Yuan Road, Beijing 100083, PR China Received 21 May 2001; received in revised form 14 January 2002
Abstract A wear resistant Ti5Si3/NiTi2 intermetallic composite coating was fabricated on substrate of a titanium alloy BT9 by laser cladding with (wt.%) Ni /30Ti /10Si elemental powder blends. The laser clad intermetallic composite coating has a rapidly solidified fine microstructure consisting of Ti5Si3 primary particles uniformly distributed in the NiTi2 matrix and is metallurgically bonded to the titanium substrate. The laser clad Ti5Si3/NiTi2 intermetallic composite coating has high hardness and excellent wear resistance under dry sliding wear test conditions. The excellent wear resistance of the laser clad Ti5Si3/NiTi2 intermetallic composite coating is attributed to the coating’s high hardness, strong intermetallic atomic bonding and refined microstructure. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser cladding; Intermetallic matrix composite coating; Ti5Si3; NiTi2; Wear resistance; Titanium alloy
1. Introduction Titanium alloys are widely used as structural components in aerospace, chemical, petrochemical and marine industries owing to their low density, high specific strength, exceptional corrosion resistance and hightemperature mechanical properties. However, titanium alloys suffer from the serious disadvantages of poor tribological properties such as high friction coefficient, difficult to lubricate, low adhesive and fretting wearresistance, which prevent them from applications as engineering tribological components [1,2]. Surface modification is one of the most efficient means to improve the tribological properties of titanium alloys. Plasma sprayed hydroxyapatite bioceramic coating was adopted in attempt to improve the fretting wear resistance of titanium alloys [1]. However, fretting wear resistance of the plasma sprayed bioceramic coatings was even inferior to the original Ti/6Al /4V substrate due to the porous and loose structure of hydroxyapatite coat-
* Corresponding author. Tel.: /86-10-82317102; fax: /86-1082328041. E-mail address: [email protected] (H.M. Wang).
ing. Hard chromium nitride films were deposited on Ti/ 6Al /4V alloy by both ion beam enhanced deposition and conventional PVD process [3,4] for improvement of fretting wear resistance. The ion beam enhanced deposited CrN films was found to be more wear resistant because of the finer microstructure and better interfacial bonding. The tribological properties of the titanium alloy Ti/6Al /4V alloy was found to be improved considerably after plasma nitriding treatment [5]. Dong et al. [6] utilized thermal oxidation heat treatment to enhance the sliding wear resistance of titanium alloys and the friction coefficient of the thermal oxidized Ti/ 6Al /4V alloy under ball-on-disc friction tests was significantly reduced. It is also reported that the wear resistance of titanium alloys can be improved by PVD and CVD thin wear resistant films such as TiN, TiC, etc. [7 /11]. Many laser surface modification technologies were also utilized to enhance the wear resistance of titanium alloys. Wear resistant composite coatings reinforced by rapidly solidified TiN dendrites were produced by laser surface alloying with gaseous nitrogen on substrate of Ti /6Al/4V alloy and wear resistance of the laser surface alloyed coating under two-body abrasive and block-on-ring full-sliding wear conditions were significantly enhanced (up to a factor of 90 under
0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 2 ) 0 0 0 7 6 - X
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sliding wear test conditions) [12]. By means of laser surface alloying with Si, rapidly solidified composite coatings reinforced by primary and eutectic Ti5Si3 intermetallic phases were prepared on Ti/6Al /4V. The wear resistance both under dry sliding and twobody abrasive test conditions were significantly improved [13]. By laser surface alloying the Ti /6Al/4V alloy with SiC powder, rapidly solidified composite coatings co-reinforced by TiC and Ti5Si3 phases were produced on substrate of the Ti /6Al /4V titanium alloy and the wear resistance are enhanced by a factor of 85 compared to the original titanium alloy under dry sliding condition [14]. Majumdar et al. [15] reported that the tribological properties of commercial pure titanium were improved by laser surface alloying with Si, Al and Si/Al. Results demonstrated that laser surface alloying with Si is more effective in terms of wear resistance improvement than with Si/Al or Al. The improvement of wear resistance was attributed to the presence of Ti5Si3/Ti eutectics in the laser surface alloyed zone. The effects of laser beam assisted liquid nitridation treatment on wear resistance of a titanium alloy was also investigated by Weerasinghe et al., Akugun and Inal and Kloosterman and Dehosson [16 /18]. The technique of laser beam assisted solid-state diffusion also was adopted by Duverneix et al. [19] to improve the wear resistance of titanium alloy. The titanium silicide Ti5Si3 is regarded as one of the alternative high-temperature structural materials because of its excellent combination of high-temperature strength, high melting point, low density, good hightemperature stability and excellent oxidation resistance [20 /22]. The serious room temperature brittleness prevents Ti5Si3 from industrial applications as structural materials. Nevertheless, the very high hardness, strong covalent atomic bonding and excellent high-temperature stability do make Ti5Si3 a excellent candidate as a wear resistant reinforcing phase in a wear resistant composite material. The NiTi2 intermetallic compound has reasonably good combination of ductility due to its facecenter-cubic crystal structure [23] and wear resistance owing to its high hardness (HV700) [24]. Naturally, a NiTi2 intermetallic matrix composite coating reinforced by Ti5Si3 wear-resisting particles is naturally expected to have good wear resistance both under adhesive and abrasive wear conditions. In this paper, a Ti5Si3/NiTi2 wear resistant intermetallic matrix composite coating was fabricated on substrate of a titanium alloy BT9 by laser cladding with Ni /Ti /Si alloy powders. The microstructure of the laser clad intermetallic matrix composite coating was characterized by OM, scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and XRD. Wear resistance of the intermetallic composite coating was investigated under dry sliding wear conditions.
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Fig. 1. Schematic illustration of the block-on-wheel dry sliding wear tester.
2. Experimental procedures A commercial near alpha titanium alloy BT9 was selected as the starting material. The nominal chemical composition (wt.%) of the alloy was 5.8% Al, 2.8% Mo, 0.8% Zr, 0.2% Si and balance Ti. The samples, 50 / 20 /20 mm3 in size, were machined by electric discharge machining. Commercial pure elemental powder blends of Ni, Ti and Si in chemical composition (wt.%) of Ni30% Ti-10% Si (hereafter referred to as Ti30Si10Ni60) and with a particle size of approximately 140 mm were selected as the laser cladding powder materials. The elemental powder blends are pre-placed on the surface of the titanium alloy samples with a powder bed thickness of approximately 1.5 mm and are melted using a 5 kW continuous wave transverse flow CO2 laser material processing systems with a 4-axis CNC table. Argon gas was blown into the melt pool to provide shielding during the laser cladding process. The singletrack laser cladding parameters are: laser out power 4 kW, laser beam size 15 /1 mm and beam traverse speed 2 mm s 1. Microstructure of the laser clad composite coatings was analyzed using the Nephot II optical microscope (OM) and S-530 SEM. The phases presented in the laser clad intermetallic composite coating were identified by Dmax-rB X-ray diffractometer (using CuKa radiation and a scanning rate of 58 min 1) and the chemical composition of the phases were analyzed by LinkISIS EDS. The hardness profile along the depth direction of the laser clad intermetallic coating were measured by a MH-6 semi-automatic Vickers hardness tester with a testing load of 1.96 N and a loading time of 15 s. Wear resistance of the coating was evaluated on a block-on-wheel dry sliding wear tester (as shown in Fig. 1), where the laser clad titanium specimen, 10 /10 /10 mm3 in size, seats on a rotating wheel of hardened 0.45% C steel (HRC53). The testing parameters are: load 98 N, sliding speed 0.92 m s 1 and total wear sliding distance 3.02 /103 m. Relative wear resistance, i.e the ratio of wear weight loss of the original titanium alloy BT9 specimen to that of the laser clad specimen was utilized to evaluate the wear resisting property of the laser clad
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Fig. 2. XRD pattern of the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating on substrate of the titanium alloy BT9.
Ti5Si3/NiTi2 intermetallic composite coating. The larger the ratio, the higher the wear resistance of the laser clad intermetallic composite coating.
3. Results 3.1. Microstructure Fig. 2 is the XRD pattern of the laser clad Ti5Si3/ NiTi2 wear resistant intermetallic composite coating. After laser cladding with Ti30Si10Ni60 elemental powder blends, a new type of laser clad composite coating mainly composed of Ti5Si3 and NiTi2 phases was produced on substrate of the BT9 titanium alloy. The microstructure of the intermetallic composite coating is fine, uniform and free-from porosities and microcracks, as shown in Fig. 3. Fig. 4 shows the microstructural characteristics of the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating. As shown clearly in Fig. 4(a) and (b), the coarse irregular blocky or granular particles are uniformly distributed in the white matrix. Results of
Fig. 3. OM micrograph showing the overview cross-section of the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating on substrate of the titanium alloy BT9.
XRD and EDS analysis indicated that the dark gray coarse irregular blocks and the fine granular particles are the Ti5Si3 primary phase and the remaining white matrix phase is the intermetallics NiTi2. Fig. 5 is a high magnification SEM photograph showing the typical microstructures in the centre of the laser clad Ti5Si3/NiTi2 intermetallic coating. It can be seen that in some local areas minor b-Ti phase was observed outlining the fine granular Ti5Si3 particles. Since the intermetallic compound Ti5Si3 has the highest melting point (2393 K) and the most negative free
Fig. 4. (a) OM and (b) SEM micrographs showing the typical microstructures of laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating.
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Fig. 5. SEM micrograph showing the existence of minor amount of ‘bull-eye-like’ b-Ti phase around some primary Ti5Si3 granular particles in the central of the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating.
energy of formation in the Ni /Ti /Si system, the blocky or granular particles Ti5Si3 precipitate from the melt as the primary phase during rapid cooling of the melt pool. In some very local areas where the remaining liquid around some Ti5Si3 fine primary granular particles is rich in titanium, fine granular Ti5Si3 phase acts as a homogeneous crystal nucleus forb-Ti formation, and consequently, minor ‘bull-eye-like’ b-Ti phase was formed around the fine Ti5Si3 primary particles, as shown in Fig. 5. Accompanying the complete solidification of all the primary Ti5Si3 phase, the residual liquid is highly enriched in nickel content and the intermetallic
Fig. 6. SEM micrograph showing the existence of a few coarse TiN dendrites in the near free surface region of the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating.
compound NiTi2 was formed as the matrix of the laser clad intermetallic composite coating. Although the shielding gas of argon was utilized to protect melt pool during laser cladding process, a small amount of gaseous nitrogen coming from the air still goes into the melt pool, a few coarse TiN columnar dendrites was occasionally observed in the near-surface region of the laser clad intermetallic coating, as shown in Fig. 6.
Fig. 7. (a) OM and (b) SEM images showing the high quality metallurgical bonding of the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating to the titanium substrate.
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intermetallic composite coating to the substrate is of high-quality metallurgical bonding and with pronounced gradient microstructure in the transition zone. 3.2. Microhardness
Fig. 8. Microhardness profile across the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating.
Fig. 8 shows the microhardness profile along the depth direction of the laser clad Ti5Si3/NiTi2 intermetallic composite coating. The laser clad intermetallic coating has a high hardness distribution within the main coating except in the bottom of the coating where the volume fraction of Ti5Si3 primary phase is relatively low and the hardness decreases gradually to the substrate. 3.3. Wear resistance
Fig. 7 shows the microstructural characteristics of the bonding zone between the laser clad Ti5Si3/NiTi2 intermetallic composite coating and the substrate. The b-Ti dendrites grow directionally and epitaxially from the titanium substrate and the bonding of the laser clad
The laser clad Ti5Si3/NiTi2 intermetallic composite coating has excellent wear resisting property under dry sliding wear test conditions. Dry sliding wear test results indicated that the wear resistance of the laser clad
Fig. 9. SEM micrographs showing the worn surface morphologies of the original titanium alloy BT9 (a), (b) and the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating (c), (d).
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Fig. 10. Micrographs showing the worn subsurface microstructure of (a) the original titanium alloy BT9 OM and (b) the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating SEM.
intermetallic coating is up to 125 times higher than that of the original titanium alloy.
4. Discussions In the laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating, the hard and wear resistant Ti5Si3 primary coarse blocks and particles are uniformly distributed in the hard, ductile, wear resistant NiTi2 intermetallic matrix. The unique microstructure of the laser clad intermetallic reinforced intermetallic matrix composite coating provided the coating excellent wear resistance especially under sliding wear conditions. During block-on-wheel dry sliding wear, the original titanium alloy BT9 suffered severe adhesive wear. Worn surface of the original titanium alloy BT9 specimen was very rough, with typical adhesive wear features evidenced by numerous adhesive craters, deep ploughing grooves and detached Ti debris (confirmed by EDS analysis), as shown in Fig. 9(a) and (b). Fig. 10(a) is the cross-section perpendicular to the worn surface of the original titanium BT9 alloy specimen. Severe plastic deformation occurred in the subsurface zone during sliding wear process. The original misorientated a/b acicular structure has transformed under the sliding wear action to a well-defined, severely deformed lamellar textured structure or severely bent to the sliding direction, as shown in Fig. 10(a). On the contrary, the worn surface of the laser clad Ti5Si3/NiTi2 intermetallic composite coating was very smooth, as shown in Fig.
9(c) and (d). SEM observation of the worn subsurface microstructure indicated that no noticeable plastic deformation or microcracks were detected in the subsurface zone, as shown in Fig. 10(b). Because the primary Ti5Si3 phase has very high hardness and is uniformly distributed in the both hard and ductile NiTi2 intermetallic matrix, the laser clad Ti5Si3/NiTi2 composite coating has very excellent abrasive wear resistance and consequently there are only slight scratches visible on the worn surface, as shown in Fig. 9(d). Since all the constitution phases in the laser clad Ti5Si3/NiTi2 wear resistant composite coating are intermetallic compounds, i.e. the Ti5Si3 and the NiTi2, the strong atomic bonding inherent to intermetallic compounds provides the composite coating excellent resistance to metallic adhesion during dry sliding wear. As a result, the intermetallic composite coating has excellent adhesive wear resistance when mating with a sliding metallic counterpart, which is evidenced by the fact that there is no significant adhesive worn morphologies visible on the worn surface of the laser clad composite coating, as shown in Fig. 9(c) and (d). Moreover, the rapidly solidified homogeneous fine microstructure imparts the coating good combination of high strength and toughness, which in turn provides the coating excellent resistance to spalling and delamination. Careful examination of worn subsurface microstructure indicates clearly that no selective wear and plastic deformation occured during the dry sliding wear process for the laser clad intermetallic coating, as shown in Fig. 10(b). This indicated that the excellent wear-resisting ability of the
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constitution phases of the laser clad Ti5Si3/NiTi2 composite coating, i.e. the reinforcing Ti5Si3 and the NiTi2 matrix, is fully exploited and consequently, the laser clad coating has excellent adhesive and abrasive wear resistance.
5. Conclusions A wear resistant Ti5Si3/NiTi2 intermetallic composite coating was produced on substrate of the titanium alloy BT9 by laser cladding with Ti30Si10Ni60 elemental powders blends. The laser clad Ti5Si3/NiTi2 wear resistant intermetallic composite coating has a rapidly solidified fine microstructure with an uniform distribution of the primary Ti5Si3 intermetallic phase in the NiTi2 intermetallic matrix and is metallurgically bonded to the titanium substrate. The Ti5Si3/NiTi2 intermetallic coating has a high and uniform hardness distribution within the whole coating and exhibits excellent wear resistance under dry sliding wear test condition due to its combination of high hardness, strong intermetallic atomic bonding and fine microstructure. Laser cladding of Ti5Si3/NiTi2 intermetallic matrix composite coating is anticipated to be a promising wear-resisting surface modification technique for titanium alloys.
Acknowledgements The research was partly supported by the National Natural Science Foundation of China (Grant No. 50071004) and the Science Funds Office of Avic (Grant No. 00A51013). The authors acknowledge Prof. Xitong Sun for assistance during the dry sliding wear tests.
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