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Laser clad TiC reinforced NiCrBSi composite coatings on Ti–6Al–4V alloy using a CW CO2 laser
Surface & Coatings Technology 203 (2009) 1395–1399
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Laser clad TiC reinforced NiCrBSi composite coatings on Ti–6Al–4V alloy using a CW CO2 laser R.L. Sun a,b,⁎, Y.W. Lei a, W. Niu a a b
School of Mechanical and Electronic Engineering, Tianjin Polytechnic University, Tianjin 300160, China Advanced Mechatronics Equipment Technology Tianjin Area Major Laboratory, Tianjin 300160, China
a r t i c l e
i n f o
Article history: Received 28 July 2008 Accepted in revised form 14 November 2008 Available online 27 November 2008 Keywords: Titanium alloys TiC–NiCrBSi composite coatings Laser cladding Microstructure Wear resistance
a b s t r a c t TiC–NiCrBSi composite coatings were fabricated on Ti–6Al–4V substrate by laser cladding. Microstructure and wear properties in vacuum (10− 5 Pa) and in atmosphere of the coatings were investigated. Three zones can be distinguished of the coatings: the clad zone, the bonding zone and the heat affected zone of the substrate. The clad zone is formed of undissolved TiC particles, precipitated TiC particles and dendrites, Cr23C6, CrB and TiB2 dispersed in a matrix consisting of primary γ-Nip and γ-Nie + Ni3B eutectics. The bonding zone and heat affected zone exhibited small dendrites and acicular martensite, respectively. The wear weight loss of the coatings was 11.4% as that of Ti–6Al–4V alloys in atmosphere, and 47.9% in vacuum. Analysis of the worn surfaces showed that the wear mechanisms of the coatings were abrasion wear in atmosphere, while adhesion wear and ploughed wear in vacuum. © 2008 Published by Elsevier B.V.
1. Introduction Ti-based alloys are very important structural material for aircraft industry due to their superior strength-to-weight ratio and excellent corrosion resistance. However, their poor resistance to sliding wear imposes a limitation on these alloys for a wider range of applications [1,2]. Surface modification is one of the most efficient means to improve wear resistance of titanium alloys. As a surface modification technique, laser cladding is finding increasing use in surface modification of titanium alloys [3–15]. Vreeling et al. [12] reinforced the surface of Ti–6Al–4V alloys by laser melt injection of WCp particles and the wear properties of the alloys are greatly enhanced. Tian and Chen [13] fabricated Ti–SiC composite coatings by laser cladding on Ti–6Al–4V substrate with C and Si mixed powders and found that the wear resistance of the coatings containing titanium carbides and silicides is significantly improved. Jiang et al. [14] prepared composite coatings reinforced by in-situ TiN dendrites on Ti–6Al–4V substrate by laser nitriding and the results show that the samples are much more resistant to wear than the untreated. Wang et al. [15] manufactured composite coatings of Ti–6Al–4V containing different volume fractions of TiC by laser cladding using Ti–6Al–4V wire and TiC powder. TiC particles are uniformly distributed in the coatings and the surface tribological properties are significantly improved when the volume fraction of TiC reinforcements is larger than 24%.
⁎ Corresponding author. School of Mechanical and Electronic Engineering, Tianjin Polytechnic University, Tianjin 300160, China. Tel.: +86 22 24528671; fax: +86 22 24528064. E-mail address: [email protected] (R.L. Sun). 0257-8972/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2008.11.012
In our previous study [16,17], the influence of laser cladding parameters on the quality of the coatings, the distribution and the morphology of TiC phase in the coatings were reported. The present study evaluates the microstructure of the coatings in detail. According to the work condition (rarefied gas) of the spacecraft, the wear resistance of laser clad TiC–NiCrBSi composite coatings in vacuum are also investigated and compared with that of the coatings in atmosphere. The wear mechanisms based on the clad worn surfaces examinations were discussed. 2. Experimental procedures Cylindrical samples of Ti–6Al–4V alloy with size of ϕ30 mm× 20 mm were used as the substrate. The clad surfaces were ground with emery paper to remove the oxide scale, and rinsed with alcohol before laser cladding. The powder mixture of TiC and NiCrBSi pre-alloyed powder in volume ratio of 1:3 was used as the clad material, NiCrBSi pre-alloyed powder with the composition (wt.%) of: 17.0Cr, 3.5B, 4.0Si, 1.0C, b12Fe, and balance Ni, with particle size of 50–100 μm. Clad powders were preplaced on the surface of substrates using an organic binder, to form a layer of 1.0 mm thickness. Laser cladding was performed on a CW ML-108 CO2 laser processing system. Process parameters of laser cladding were: laser power P = 4 kW, scanning velocity V = 8 mm/s and laser beam diameter D = 6 mm. An overlap of 50% between successive tracks was selected. The microstructures of the laser clad coatings were examined using a scanning electron microscope (SEM) and transmission electron microscopy (TEM) with an energy dispersive spectroscopy (EDS). The TEM samples were cut along the direction parallel to the clad coating, and the foils were prepared by ion-thinning. The wear
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Fig. 1. SEM micrographs showing the cross-section morphology and microstructure of the laser clad coatings on Ti–6Al–4V. (a) cross-section morphology of clad coatings, (b) crosssection microstructure of clad coatings, (c) microstructure of clad zone (CZ), (d) microstructure of bonding zone (BZ) and heat-affected zone (HAZ).
properties of coatings were investigated on УТИ ТВ-100 pin-on-disc wear tester. Laser clad Ti–6Al–4V alloy samples of ϕ9 × 20 mm were used as the pins, and YG8B cemented carbides with hardness of HRA 89.5 and size of ϕ40 × 10 mm as the disks. The wear test surfaces were ground to a surface finish of Ra = 0.2 μm. Specifications of the wear tests were as follows: environment pressure P = 105 Pa and10− 5 Pa, normal load F = 50 N, sliding speed V = 0.75 m/s and sliding distance L = 540 m. The wear weight loss was measured using an analytical balance with a sensitivity of 10− 5 g.
as determined by SEM EDS analysis is (wt.%): 48.06Ti, 3.51Al, 2.33V, 30.56Ni, 5.72Cr, 0.76Si, 9.06Fe. The bonding zone is a coagulate region of the substrate and clad material which leads to a good metallurgical bond at the coating/substrate interface. Fig. 2a is a TEM bright field image showing the morphology of the matrix in the clad zone, which is composed of bulk primary phase and lamellar eutectic. The composition of the primary phase as determined by TEM EDS analysis is (wt.%): 64.5Ni, 21.4Fe, 5.3Cr, 4.5Si, 3.7Ti, 0.5Al, which implies that the primary phase is γ-Ni solid solution (notated as γ-Nip). Fig. 2b, c shows the SADP of dark phase and
3. Results and discussion 3.1. Microstructure of laser clad coatings Fig. 1a is a macrograph of single laser track transverse crosssection. The clad coating is free from pores and cracks, with a thickness of 0.9 mm and width of 6.1 mm. Fig. 1b is a SEM morphology of cross-section of the clad coating. According to microstructure difference, the laser clad coatings can be divided into three regions: clad zone (CZ), bonding zone (BZ), and heat affected zone of the substrate (HAZ). Fig. 1c and d are the higher magnification micrographs of CZ, BZ and HAZ. The clad zone is formed of irregular TiC particles and acicular phase dispersed in Ni matrix. The composition (excluding carbon and boron, the same as follows) of the acicular phase as determined by SEM EDS analysis is (wt.%): 72Cr, 18.7Fe, 4.6Ni, 2.6Ti, 1.2Si, 0.9Al, which indicates that the acicular phase was enriched in Cr. The bonding zone possesses a microstructure of small dendrites with thickness of approximately 70–80 μm and the heat affected zone is acicular martensite. The composition of the bonding zone
Fig. 2. TEM bright field image of the matrix (a) and SADP of γ-Nie (b) and Ni3B (c).
R.L. Sun et al. / Surface & Coatings Technology 203 (2009) 1395–1399
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Fig. 3. TEM bright field images of undissoved TiC particles (a) and CrB phase (b).
Fig. 4. TEM bright field images of precipitated TiC phase (a) and the TiB2 phase (b).
bright phase in the eutectics, respectively. The dark phase can be characterized as γ-Ni solid solution with face centered cubic structure (notated as γ-Nie) and the bright phase is Ni3B with orthorhombic structure. Fig. 3a shows the morphologies of undissolved TiC particles in clad zone. The TiC particles appear spherical shape. The interface
between TiC particles and γ-Ni matrix is close and free from crack and porous. Fig. 3b is the high magnification TEM morphology and the SADP of arrow notated phase in Fig. 3a. Analysis results indicate that the arrow notated phase is CrB with orthorhombic structure. The paralleled fringe contrast reveals that there are many twins in CrB phase.
Fig. 5. TEM bright field image (a) and SADP (b) of Cr23C6 phase.
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determined by the TEM EDS, is (wt.%): 6.6Ni, 77.6Cr, 1.2Si, 9.6Fe, 4.4Ti, 0.7Al. Analysis results indicate that the acicular phase is Cr23C6 with facial-centered cubic structure. The clad zone was composed of undissolved TiC particles, precipitated TiC particles and dendrites, Cr23C6, CrB and TiB2 dispersed in a matrix consisting of primary γ-Nip and γ-Nie + Ni3B eutectics, according to the results mentioned above. 3.2. Wear resistance of the laser clad coatings
Fig. 6. Weight loss of the coatings and Ti–6Al–4V alloys in atmosphere and in vacuum.
TiC particles were partially dissolved in molten Ni matrix (fine TiC particles and the margin of coarse TiC particles) during laser irradiation heating and then dissolving Ti and C atoms leads to the precipitation of TiC phase during solidification. The TEM morphologies and SADP of the precipitated TiC phase are shown in Fig. 4a, which exhibits two different morphologies: faceted particles and small dendrites. In addition, TEM analysis found a small quantity of TiB2 phase in the clad zone which nucleates and grows on TiC dendrites, as shown in Fig. 4b. The formation of TiB2 phase due to the reaction between Ti and B elements during laser cladding. Clad material NiCrBSi alloy provides the source of B element, while the element of Ti came from the melting of Ti substrate and dissolution of TiC particles. Fig. 5a, b shows the TEM morphologies and SADP of the acicular phase of the clad zone. The composition of the acicular phase as
Fig. 6 shows the wear weight loss of laser clad coatings and Ti–6Al–4V alloys in atmosphere and in vacuum. The weight loss of laser clad coatings in atmosphere (2.54 mg) is less than that in vacuum (3.25 mg), while Ti–6Al–4V alloys are on the contrary (22.34 mg in atmosphere and 6.78 mg in vacuum). Comparing the weight loss of laser clad coatings with Ti–6Al–4V alloys, the former is less than the latter, no matter in vacuum or in atmosphere. The wear weight loss of the laser clad coatings was 11.4% as that of Ti–6Al–4V alloys in atmosphere, and 47.9% in vacuum. 3.3. Wear mechanism of laser clad coatings Fig. 7a, b shows the worn surface morphologies of YG8B cemented carbide disc after dry sliding wear test coupling with clad coatings in atmosphere and in vacuum, respectively. There is much wear debris on the worn surface of the disc in atmosphere (Fig. 7a), and material transition on worn surface of the disc in vacuum (Fig. 7b). The worn surface morphologies of the coatings in atmosphere and in vacuum are shown in Fig. 8a,b, respectively. The ploughed furrows on worn surface of laser clad coating in atmosphere are narrower and shallower (Fig. 8a), while the furrows in vacuum are wider and deeper with the ridges at the furrow borders (Fig. 8b). In addition, the worn
Fig. 7. Worn surface morphologies of the disc coupling with clad coatings: (a) in atmosphere, (b) in vacuum.
Fig. 8. Worn surface morphologies of the coatings: (a) in atmosphere, (b) in vacuum.
R.L. Sun et al. / Surface & Coatings Technology 203 (2009) 1395–1399
surface of laser clad coating exhibits local detaching features in vacuum (arrow notated region). The distinctness of worn surface morphologies of the clad coatings in atmosphere and in vacuum indicates that the wear mechanisms of the laser clad coatings are different as dry sliding under various environmental pressures. In atmosphere, the oxide film and gas adsorbed film on the wear couples surface decrease the adhesion between them. The wear proceeds mainly due to the plough of the convex on YG8B cemented carbide disc surface and TiC separated from the coating and the wear mechanism was mainly abrasion wear in atmosphere. On the contrary, the oxide film and gas adsorbed film on the wear couple surface can not be regenerated during the wear process and the heat can not irradiate freely in vacuum which accelerates the adhesion and leads to a transfer film on the YG8B cemented carbide disc. The transfer material was rolled repeatedly and strengthened during the wear process, which acts as the hard convex and plough the clad coatings during wear process. The wear mechanism was mainly adhesion wear and ploughed wear in vacuum [12]. In view of the experimental results, laser clad TiC reinforced NiCrBSi composite coatings exhibit excellent wear resistance in atmosphere and in vacuum conditions. The strengthening of the clad coatings is as follows: (1) According to the microstructure analysis results mentioned above, many hard particles, such as TiC, Cr23C6, CrB and TiB2 particles, were dispersed in the matrix, which significantly enhance the hardness of the clad coatings. (2) Due to the quench in laser cladding process, the microstructure of clad coatings is fine. The spacing between the lamella of the eutectic is 20–30 nm (Fig. 3), which results in fine-crystal strengthening and enhances the strength and toughness of the matrix. (3) According to the EDS analysis results, large quantity of alloy elements, such as Cr, Fe and Ti etc. were dissolved into the matrix, which results in solution strengthening and enhances the strength and hardness of the matrix. 4. Conclusions Laser clad TiC–NiCrBSi composite coatings on Ti–6Al–4V alloys are divided into three regions: clad zone, bonding zone and heat
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affected zone of the substrate. The clad zone is mainly composed of undissolved TiC particles, precipitated TiC particles and dendrites, Cr23C6, and a small quantity of CrB and TiB2 dispersed in a matrix consisting of primary γ-Nip and γ-Nie + Ni3B eutectics. The bonding zone exhibited small dendrites and the heat affected zone is acicular martensite. Laser clad TiC–NiCrBSi composite coatings exhibit excellent wear resistance in atmosphere and in vacuum conditions. The wear weight loss of the laser clad coatings was 11.4% as that of Ti–6Al–4V alloys in atmosphere, and 47.9% in vacuum. The wear mechanisms of the coatings were mainly abrasion wear in atmosphere, while adhesion wear and ploughed wear in vacuum. Acknowledgement The authors would like to thank the financial support provided by the Foundation of University Science and Technique Development Tianjin (20060912). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
A. Molinari, G. Straffelini, B. Tesi, Wear 208 (1997) 105. G. Straffelini, A. Molinari, Wear 236 (1999) 328. E.W. Douglas, S. Jogender, S. Jeff, Adv. Mater. Process. 8 (2000) 41. J.A. Folks, K. Shibata, J. Laser Appl. 6 (1994) 88. J.H. Abboud, D.R.F. West, R.H. Hibberd, Surf. Eng. 9 (1993) 221. P.A. Molian, L. Hualum, Wear 130 (1989) 337. J.H. Abboud, D.R.F. West, R.D. Rawlings, Mater. Sci. Technol. 10 (1994) 848. S. Mridha, T.N. Baker, Mater. Sci. Technol. 12 (1996) 595. A. Mehlmann, S.F. Dirnfeld, I. Minkoff, Surf. Coat. Technol. 42 (1990) 275. J.D. Ayers, Wear 97 (1984) 249. C.L. Sexton, G. Byrne, K.G. Watkins, Laser Appl. 13 (2001) 2. J.A. Vreeling, V. Ocelík, J.T.M. De Hosson, Acta Mater. 50 (2002) 4913. Y.S. Tian, C.Z. Chen, Mater. Lett. 61 (2007) 635. P. Jiang, X.L. He, X.X. Li, L.G. Yu, Surf. Coat. Technol. 130 (2000) 24. F. Wang, J. Mei, H. Jiang, X. Wu, Mater. Sci. Eng. A 445 (2007) 461. R.L. Sun, D.Z. Yang, L.X. Guo, S.L. Dong, Surf. Coat. Tech. 135 (2001) 307. R.L. Sun, J.F. Mao, D.Z. Yang, Surf. Coat. Tech. 155 (2002) 203.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Laser clad TiC reinforced NiCrBSi composite coatings on Ti–6Al–4V alloy using a CW CO2 laser R.L. Sun a,b,⁎, Y.W. Lei a, W. Niu a a b
School of Mechanical and Electronic Engineering, Tianjin Polytechnic University, Tianjin 300160, China Advanced Mechatronics Equipment Technology Tianjin Area Major Laboratory, Tianjin 300160, China
a r t i c l e
i n f o
Article history: Received 28 July 2008 Accepted in revised form 14 November 2008 Available online 27 November 2008 Keywords: Titanium alloys TiC–NiCrBSi composite coatings Laser cladding Microstructure Wear resistance
a b s t r a c t TiC–NiCrBSi composite coatings were fabricated on Ti–6Al–4V substrate by laser cladding. Microstructure and wear properties in vacuum (10− 5 Pa) and in atmosphere of the coatings were investigated. Three zones can be distinguished of the coatings: the clad zone, the bonding zone and the heat affected zone of the substrate. The clad zone is formed of undissolved TiC particles, precipitated TiC particles and dendrites, Cr23C6, CrB and TiB2 dispersed in a matrix consisting of primary γ-Nip and γ-Nie + Ni3B eutectics. The bonding zone and heat affected zone exhibited small dendrites and acicular martensite, respectively. The wear weight loss of the coatings was 11.4% as that of Ti–6Al–4V alloys in atmosphere, and 47.9% in vacuum. Analysis of the worn surfaces showed that the wear mechanisms of the coatings were abrasion wear in atmosphere, while adhesion wear and ploughed wear in vacuum. © 2008 Published by Elsevier B.V.
1. Introduction Ti-based alloys are very important structural material for aircraft industry due to their superior strength-to-weight ratio and excellent corrosion resistance. However, their poor resistance to sliding wear imposes a limitation on these alloys for a wider range of applications [1,2]. Surface modification is one of the most efficient means to improve wear resistance of titanium alloys. As a surface modification technique, laser cladding is finding increasing use in surface modification of titanium alloys [3–15]. Vreeling et al. [12] reinforced the surface of Ti–6Al–4V alloys by laser melt injection of WCp particles and the wear properties of the alloys are greatly enhanced. Tian and Chen [13] fabricated Ti–SiC composite coatings by laser cladding on Ti–6Al–4V substrate with C and Si mixed powders and found that the wear resistance of the coatings containing titanium carbides and silicides is significantly improved. Jiang et al. [14] prepared composite coatings reinforced by in-situ TiN dendrites on Ti–6Al–4V substrate by laser nitriding and the results show that the samples are much more resistant to wear than the untreated. Wang et al. [15] manufactured composite coatings of Ti–6Al–4V containing different volume fractions of TiC by laser cladding using Ti–6Al–4V wire and TiC powder. TiC particles are uniformly distributed in the coatings and the surface tribological properties are significantly improved when the volume fraction of TiC reinforcements is larger than 24%.
⁎ Corresponding author. School of Mechanical and Electronic Engineering, Tianjin Polytechnic University, Tianjin 300160, China. Tel.: +86 22 24528671; fax: +86 22 24528064. E-mail address: [email protected] (R.L. Sun). 0257-8972/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2008.11.012
In our previous study [16,17], the influence of laser cladding parameters on the quality of the coatings, the distribution and the morphology of TiC phase in the coatings were reported. The present study evaluates the microstructure of the coatings in detail. According to the work condition (rarefied gas) of the spacecraft, the wear resistance of laser clad TiC–NiCrBSi composite coatings in vacuum are also investigated and compared with that of the coatings in atmosphere. The wear mechanisms based on the clad worn surfaces examinations were discussed. 2. Experimental procedures Cylindrical samples of Ti–6Al–4V alloy with size of ϕ30 mm× 20 mm were used as the substrate. The clad surfaces were ground with emery paper to remove the oxide scale, and rinsed with alcohol before laser cladding. The powder mixture of TiC and NiCrBSi pre-alloyed powder in volume ratio of 1:3 was used as the clad material, NiCrBSi pre-alloyed powder with the composition (wt.%) of: 17.0Cr, 3.5B, 4.0Si, 1.0C, b12Fe, and balance Ni, with particle size of 50–100 μm. Clad powders were preplaced on the surface of substrates using an organic binder, to form a layer of 1.0 mm thickness. Laser cladding was performed on a CW ML-108 CO2 laser processing system. Process parameters of laser cladding were: laser power P = 4 kW, scanning velocity V = 8 mm/s and laser beam diameter D = 6 mm. An overlap of 50% between successive tracks was selected. The microstructures of the laser clad coatings were examined using a scanning electron microscope (SEM) and transmission electron microscopy (TEM) with an energy dispersive spectroscopy (EDS). The TEM samples were cut along the direction parallel to the clad coating, and the foils were prepared by ion-thinning. The wear
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Fig. 1. SEM micrographs showing the cross-section morphology and microstructure of the laser clad coatings on Ti–6Al–4V. (a) cross-section morphology of clad coatings, (b) crosssection microstructure of clad coatings, (c) microstructure of clad zone (CZ), (d) microstructure of bonding zone (BZ) and heat-affected zone (HAZ).
properties of coatings were investigated on УТИ ТВ-100 pin-on-disc wear tester. Laser clad Ti–6Al–4V alloy samples of ϕ9 × 20 mm were used as the pins, and YG8B cemented carbides with hardness of HRA 89.5 and size of ϕ40 × 10 mm as the disks. The wear test surfaces were ground to a surface finish of Ra = 0.2 μm. Specifications of the wear tests were as follows: environment pressure P = 105 Pa and10− 5 Pa, normal load F = 50 N, sliding speed V = 0.75 m/s and sliding distance L = 540 m. The wear weight loss was measured using an analytical balance with a sensitivity of 10− 5 g.
as determined by SEM EDS analysis is (wt.%): 48.06Ti, 3.51Al, 2.33V, 30.56Ni, 5.72Cr, 0.76Si, 9.06Fe. The bonding zone is a coagulate region of the substrate and clad material which leads to a good metallurgical bond at the coating/substrate interface. Fig. 2a is a TEM bright field image showing the morphology of the matrix in the clad zone, which is composed of bulk primary phase and lamellar eutectic. The composition of the primary phase as determined by TEM EDS analysis is (wt.%): 64.5Ni, 21.4Fe, 5.3Cr, 4.5Si, 3.7Ti, 0.5Al, which implies that the primary phase is γ-Ni solid solution (notated as γ-Nip). Fig. 2b, c shows the SADP of dark phase and
3. Results and discussion 3.1. Microstructure of laser clad coatings Fig. 1a is a macrograph of single laser track transverse crosssection. The clad coating is free from pores and cracks, with a thickness of 0.9 mm and width of 6.1 mm. Fig. 1b is a SEM morphology of cross-section of the clad coating. According to microstructure difference, the laser clad coatings can be divided into three regions: clad zone (CZ), bonding zone (BZ), and heat affected zone of the substrate (HAZ). Fig. 1c and d are the higher magnification micrographs of CZ, BZ and HAZ. The clad zone is formed of irregular TiC particles and acicular phase dispersed in Ni matrix. The composition (excluding carbon and boron, the same as follows) of the acicular phase as determined by SEM EDS analysis is (wt.%): 72Cr, 18.7Fe, 4.6Ni, 2.6Ti, 1.2Si, 0.9Al, which indicates that the acicular phase was enriched in Cr. The bonding zone possesses a microstructure of small dendrites with thickness of approximately 70–80 μm and the heat affected zone is acicular martensite. The composition of the bonding zone
Fig. 2. TEM bright field image of the matrix (a) and SADP of γ-Nie (b) and Ni3B (c).
R.L. Sun et al. / Surface & Coatings Technology 203 (2009) 1395–1399
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Fig. 3. TEM bright field images of undissoved TiC particles (a) and CrB phase (b).
Fig. 4. TEM bright field images of precipitated TiC phase (a) and the TiB2 phase (b).
bright phase in the eutectics, respectively. The dark phase can be characterized as γ-Ni solid solution with face centered cubic structure (notated as γ-Nie) and the bright phase is Ni3B with orthorhombic structure. Fig. 3a shows the morphologies of undissolved TiC particles in clad zone. The TiC particles appear spherical shape. The interface
between TiC particles and γ-Ni matrix is close and free from crack and porous. Fig. 3b is the high magnification TEM morphology and the SADP of arrow notated phase in Fig. 3a. Analysis results indicate that the arrow notated phase is CrB with orthorhombic structure. The paralleled fringe contrast reveals that there are many twins in CrB phase.
Fig. 5. TEM bright field image (a) and SADP (b) of Cr23C6 phase.
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determined by the TEM EDS, is (wt.%): 6.6Ni, 77.6Cr, 1.2Si, 9.6Fe, 4.4Ti, 0.7Al. Analysis results indicate that the acicular phase is Cr23C6 with facial-centered cubic structure. The clad zone was composed of undissolved TiC particles, precipitated TiC particles and dendrites, Cr23C6, CrB and TiB2 dispersed in a matrix consisting of primary γ-Nip and γ-Nie + Ni3B eutectics, according to the results mentioned above. 3.2. Wear resistance of the laser clad coatings
Fig. 6. Weight loss of the coatings and Ti–6Al–4V alloys in atmosphere and in vacuum.
TiC particles were partially dissolved in molten Ni matrix (fine TiC particles and the margin of coarse TiC particles) during laser irradiation heating and then dissolving Ti and C atoms leads to the precipitation of TiC phase during solidification. The TEM morphologies and SADP of the precipitated TiC phase are shown in Fig. 4a, which exhibits two different morphologies: faceted particles and small dendrites. In addition, TEM analysis found a small quantity of TiB2 phase in the clad zone which nucleates and grows on TiC dendrites, as shown in Fig. 4b. The formation of TiB2 phase due to the reaction between Ti and B elements during laser cladding. Clad material NiCrBSi alloy provides the source of B element, while the element of Ti came from the melting of Ti substrate and dissolution of TiC particles. Fig. 5a, b shows the TEM morphologies and SADP of the acicular phase of the clad zone. The composition of the acicular phase as
Fig. 6 shows the wear weight loss of laser clad coatings and Ti–6Al–4V alloys in atmosphere and in vacuum. The weight loss of laser clad coatings in atmosphere (2.54 mg) is less than that in vacuum (3.25 mg), while Ti–6Al–4V alloys are on the contrary (22.34 mg in atmosphere and 6.78 mg in vacuum). Comparing the weight loss of laser clad coatings with Ti–6Al–4V alloys, the former is less than the latter, no matter in vacuum or in atmosphere. The wear weight loss of the laser clad coatings was 11.4% as that of Ti–6Al–4V alloys in atmosphere, and 47.9% in vacuum. 3.3. Wear mechanism of laser clad coatings Fig. 7a, b shows the worn surface morphologies of YG8B cemented carbide disc after dry sliding wear test coupling with clad coatings in atmosphere and in vacuum, respectively. There is much wear debris on the worn surface of the disc in atmosphere (Fig. 7a), and material transition on worn surface of the disc in vacuum (Fig. 7b). The worn surface morphologies of the coatings in atmosphere and in vacuum are shown in Fig. 8a,b, respectively. The ploughed furrows on worn surface of laser clad coating in atmosphere are narrower and shallower (Fig. 8a), while the furrows in vacuum are wider and deeper with the ridges at the furrow borders (Fig. 8b). In addition, the worn
Fig. 7. Worn surface morphologies of the disc coupling with clad coatings: (a) in atmosphere, (b) in vacuum.
Fig. 8. Worn surface morphologies of the coatings: (a) in atmosphere, (b) in vacuum.
R.L. Sun et al. / Surface & Coatings Technology 203 (2009) 1395–1399
surface of laser clad coating exhibits local detaching features in vacuum (arrow notated region). The distinctness of worn surface morphologies of the clad coatings in atmosphere and in vacuum indicates that the wear mechanisms of the laser clad coatings are different as dry sliding under various environmental pressures. In atmosphere, the oxide film and gas adsorbed film on the wear couples surface decrease the adhesion between them. The wear proceeds mainly due to the plough of the convex on YG8B cemented carbide disc surface and TiC separated from the coating and the wear mechanism was mainly abrasion wear in atmosphere. On the contrary, the oxide film and gas adsorbed film on the wear couple surface can not be regenerated during the wear process and the heat can not irradiate freely in vacuum which accelerates the adhesion and leads to a transfer film on the YG8B cemented carbide disc. The transfer material was rolled repeatedly and strengthened during the wear process, which acts as the hard convex and plough the clad coatings during wear process. The wear mechanism was mainly adhesion wear and ploughed wear in vacuum [12]. In view of the experimental results, laser clad TiC reinforced NiCrBSi composite coatings exhibit excellent wear resistance in atmosphere and in vacuum conditions. The strengthening of the clad coatings is as follows: (1) According to the microstructure analysis results mentioned above, many hard particles, such as TiC, Cr23C6, CrB and TiB2 particles, were dispersed in the matrix, which significantly enhance the hardness of the clad coatings. (2) Due to the quench in laser cladding process, the microstructure of clad coatings is fine. The spacing between the lamella of the eutectic is 20–30 nm (Fig. 3), which results in fine-crystal strengthening and enhances the strength and toughness of the matrix. (3) According to the EDS analysis results, large quantity of alloy elements, such as Cr, Fe and Ti etc. were dissolved into the matrix, which results in solution strengthening and enhances the strength and hardness of the matrix. 4. Conclusions Laser clad TiC–NiCrBSi composite coatings on Ti–6Al–4V alloys are divided into three regions: clad zone, bonding zone and heat
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affected zone of the substrate. The clad zone is mainly composed of undissolved TiC particles, precipitated TiC particles and dendrites, Cr23C6, and a small quantity of CrB and TiB2 dispersed in a matrix consisting of primary γ-Nip and γ-Nie + Ni3B eutectics. The bonding zone exhibited small dendrites and the heat affected zone is acicular martensite. Laser clad TiC–NiCrBSi composite coatings exhibit excellent wear resistance in atmosphere and in vacuum conditions. The wear weight loss of the laser clad coatings was 11.4% as that of Ti–6Al–4V alloys in atmosphere, and 47.9% in vacuum. The wear mechanisms of the coatings were mainly abrasion wear in atmosphere, while adhesion wear and ploughed wear in vacuum. Acknowledgement The authors would like to thank the financial support provided by the Foundation of University Science and Technique Development Tianjin (20060912). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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