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Microstructures and wear properties of in situ formed composite coatings produced by laser alloying technique
Materials Letters 61 (2007) 635 – 638 www.elsevier.com/locate/matlet
Microstructures and wear properties of in situ formed composite coatings produced by laser alloying technique Y.S. Tian ⁎, C.Z. Chen School of Materials Science and Engineering, Shandong University, Jinan Shandong 250061, PR China Received 18 December 2004; accepted 14 May 2006 Available online 8 June 2006
Abstract Laser-surface alloying of titanium alloy Ti–6Al–4V with C and Si mixed powders has been carried out. The composite coatings, thickness of about 0.7 mm, mainly consisting of titanium carbides and silicides, have a hardness of about 1500 HV0.1, and the wear resistance is 4 times more than that of the as-received. © 2006 Elsevier B.V. All rights reserved. Keywords: Laser alloying; Titanium carbides and silicides; Wear resistance
1. Introduction In view of the excellent combination of high temperature strength and lightweight properties, titanium-based alloys are more attractive for the manufacture of structural components in aerospace, chemical, petrochemical and marine industries. However, they are currently restricted to non-tribological applications due to their poor friction and wear resistance and a high tendency to galling. Surface modification is one of the most efficient means to improve the tribological properties of titanium alloys and of all the surface modification techniques, laser alloying is finding increasing use in surface modification of many kinds of metals [1–3]. Laser nitriding is an effective method to improve the wear and corrosion resistance of titanium and its alloys. But the cracks in the alloyed layers produced by laser nitriding in a pure nitrogen gas environment are a bane to the strength and toughness of the alloy [4,5]. Titanium carbides and silicides such as TiC and Ti5Si3 are confirmed to be good reinforcements for titanium alloys because of their excellent capabilities including high hardness and wear resistance in many tribological systems, thermal stability at high temperature, high elastic modulus and the similar density to
⁎ Corresponding author. Tel.: +86 531 8395991; fax: +86 531 8392313. E-mail address: [email protected] (Y.S. Tian). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.05.035
pure titanium [6–8] and surface modification of titanium alloys with titanium carbides or silicides is considered to be effective method to improve the wear resistance of titanium alloys [7–9]. So, in the present study, the laser alloying with graphite and silicon mixed powders was performed to modify the surface properties of titanium alloy Ti–6Al–4V. The microstructure, the phase composition and the wear resistance of the surface alloyed layers were investigated.
Fig. 1. The surface morphology of the laser alloyed layer of sample 1.
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2. Experimental procedures
Fig. 2. XRD spectrum of the sample 1.
The samples of titanium alloy Ti–6Al–4V, 10× 10× 30 in size, were abraded with SiC grit paper prior to the coating operation. Graphite and silicon mixed powders, an average particle size of 10μm, with different weight ratios (2:1, 1:1 and 1:2), blended with diluted polyvinyl alcohol solution were coated on the surface of the samples in a thickness of approximately 0.5 mm. A 1500 W continuous wave CO2 laser, output power of 1200 W, beam size of 2 mm, scanning speed of 3.5 mm/s was employed to melt the preplaced coatings together with the surface layer of the samples. To protect the melt pool from oxidation during processing, argon gas shield at a pressure of 0.35 MPa was fed through a nozzle
Fig. 3. EMPA micrographs of the samples.
Y.S. Tian, C.Z. Chen / Materials Letters 61 (2007) 635–638
637
Fig. 4. EDS of elements Ti, C and Si of sample 2.
diameter of 40 mm was selected as the wear couple. The weight loss was evaluated every 5 min using an electronic balance with an accuracy of 0.1 mg.
which was coaxial with the laser beam. In addition, there was a side argon gas flow through another nozzle at an angle of 30° to the melt pool. Metallographic samples were prepared using standard mechanical polishing procedures and then etched in a solution of HF, HNO3 and H2O in volume ratio of 2:1:47 to reveal the growth morphologies of the compounds. The phase composition of the coatings was identified using D/max-rC XRD with Cu Kα radiation operated at a voltage of 40 kV, a current of 40 mA, and a scanning rate of 5°/min. Microstructures were characterized using JXA-8800R EMPA and the chemical compositions of the compounds in the coating were analyzed by EDS of EMPA. The hardness of the samples was evaluated using Shimadzu Vickers hardness tester. Sliding wear tests were performed using MM200 wear test machine with a load of 4 kg. A sintered carbide abrasive wheel (rotation speed: 400 rpm) with a
Fig. 1 is the surface morphology of the laser alloyed layer of the sample 1 and the XRD spectrum of it indicates that the compounds in the coating mainly consist of TiC and Ti5Si3 (see Fig. 2). From Fig. 3 it is seen that the compounds in the top layers of the coatings are mainly dendritic and block-like pre-eutectic and chrysanthemum-like eutectic, however, in the inner layer are nearly all the chrysanthemum-like. On the other hand, with the ratios of graphite to silicon increasing, the morphology of the pre-eutectic compounds turns from dendritic to block-like, which shows that the variation of the concentration of the alloying elements has an effect on the morphology of the compounds. The EDS of EMPA shown in Fig. 4 indicates that the pre-eutectic compounds are rich in carbon and the
Fig. 5. Microhardness profiles of the coatings.
Fig. 6. Weight loss as a function of the distance in sliding wear test.
3. Results and discussions
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microhardness decreases from about 1500 HV0.1 in the top layer gradually to about 1200 HV0.1 and then drastically down to about 400 HV0.1 for the matrix. Results of the sliding wear test, as shown in Fig. 6, indicate that the wear resistance of the coatings is 4 times more than the as-received. Fig. 7 is an EMPA micrograph of the worn surface of the samples. Clearly, under the sliding wear conditions with the sintered carbide as the counterpart, the worn surface of the original Ti–6Al–4V is easily plastically deformed and grooved. Since the microhardness of the original Ti–6Al–4V (about 400 HV0.1) is much lower than that of the counterpart (about 1200 HV0.1), the hard asperities on the surface of the counterpart can easily penetrate into the surface of titanium alloy Ti–6Al–4V, resulting in effective microcutting. Thus, the wear behavior of the original Ti–6Al–4V is featured as abrasive and adhesive wear (see Fig. 7a). While the very high hardness of titanium carbides and silicides makes the composite coating very difficult to be grooved and plastically deformed, the worn surface is characterized by the presence of much shallower grooves and little adhesive features, as shown in Fig. 7b. That is, titanium carbides and silicides reinforced composite coatings exhibit excellent abrasive and adhesive wear resistance under sliding wear test conditions because the strength and hardness of laser-alloyed layer are significantly enhanced by the in situ formed compounds.
4. Conclusion
Fig. 7. EMPA micrographs of the worn surface of the samples: (a) as-received, (b) laser treated.
eutectic compounds rich in silicon. It means that the pre-eutectic compounds are mainly titanium carbides and the eutectic compounds mainly titanium silicides. The different compounds formed in different areas of the same melt pool may be attributed to the different diffusion ability of the alloy elements and the different solidification rate of the areas in the same melt pool. During laser alloying processing, the mixed powders dissolve into the melt pool leading to reacting with titanium:Ti +C=TiC; 5Ti +3Si =Ti5Si3. With the laser beam move away, the melt pool solidifies quickly. According to the Ti–C and Ti–Si binary diagrams, the melting points of TiC and Ti5Si3 are 3140 °C and 2120 °C, respectively. In addition, at 1339 °C the eutectic reaction takes place in the Ti–Si system to form eutectic phases Ti5Si3–β(Ti) [10]. So, it can be inferred that the combination of C with Ti at relatively higher temperature results in a shorter diffusion time of the C atoms in the melt pool and thus the titanium carbides formed in the top layer of the coatings. On the other hand, the titanium silicides formed at a relatively lower temperature results in the Si atoms having more time to diffuse deep into the melt pool. So, the compounds in the inner layer of the coatings are mainly titanium silicides. The microhardness profiles along the depth direction of the composite coatings are shown in Fig. 5. It can be found that the total depth of the coatings is about 0.7 mm and with the depth increasing the
A composite strengthening layer was fabricated by laser alloying of preplaced Si and graphite powders on the surface of titanium alloy Ti–6Al–4V. The phase composition, microstructure, hardness and wear resistance of the composite coatings can be summarized as follows: XRD result confirms that the composite coating mainly consists of TiC and Ti5Si3 compounds. EDS indicates that the dendritic and the block-like compounds are mainly titanium carbides and the chrysanthemum-like mainly titanium silicides. The high microhardness and excellent wear resistance of the composite coatings are attributed to the formation of hard ceramic compounds. Thus, it can be deduced that the coatings with such an excellent wear resistance can significantly improve the load-bearing capability of the substrate. References [1] H.Y. Chen, S.J. Huang, International Journal of Machine Tools and Manufacture 44 (2004) 1653. [2] G. Thawari, G. Sundarararjan, S.V. Joshi, Thin Solid Films 423 (2003) 41. [3] I. García, J.J.D. Damborenea, Corrosion Science 40 (1998) 1411. [4] C. Hu, T.N. Baker, Materials Science & Engineering. A, Structural Materials: Properties, Microstructure and Processing 265 (1999) 268. [5] Tian, Y. PhD Thesis, Shandong University, Jinan, Shandong, P.R. China, 2006, p 66. [6] Y. Chen, H.M. Wang, Journal of Alloys and Compounds 351 (2003) 304. [7] H.M. Wang, Y.F. Liu, Materials Science & Engineering. A, Structural Materials: Properties, Microstructure and Processing 338 (2002) 126. [8] P. Jiang, J.J. Zhang, G.L. Yu, Rare Metal Materials and Engineering 29 (2000) 269 (Chinese). [9] E. Richter, J. Piekoszewski, F. Prokert, J. Stanislawski, L. Walis, E. Wieser, Vacuum 63 (2001) 523. [10] Y.S. Tian, C.Z. Chen, L.X. Chen, Q.H. Huo, Materials Letters 60 (2006) 109.
Microstructures and wear properties of in situ formed composite coatings produced by laser alloying technique Y.S. Tian ⁎, C.Z. Chen School of Materials Science and Engineering, Shandong University, Jinan Shandong 250061, PR China Received 18 December 2004; accepted 14 May 2006 Available online 8 June 2006
Abstract Laser-surface alloying of titanium alloy Ti–6Al–4V with C and Si mixed powders has been carried out. The composite coatings, thickness of about 0.7 mm, mainly consisting of titanium carbides and silicides, have a hardness of about 1500 HV0.1, and the wear resistance is 4 times more than that of the as-received. © 2006 Elsevier B.V. All rights reserved. Keywords: Laser alloying; Titanium carbides and silicides; Wear resistance
1. Introduction In view of the excellent combination of high temperature strength and lightweight properties, titanium-based alloys are more attractive for the manufacture of structural components in aerospace, chemical, petrochemical and marine industries. However, they are currently restricted to non-tribological applications due to their poor friction and wear resistance and a high tendency to galling. Surface modification is one of the most efficient means to improve the tribological properties of titanium alloys and of all the surface modification techniques, laser alloying is finding increasing use in surface modification of many kinds of metals [1–3]. Laser nitriding is an effective method to improve the wear and corrosion resistance of titanium and its alloys. But the cracks in the alloyed layers produced by laser nitriding in a pure nitrogen gas environment are a bane to the strength and toughness of the alloy [4,5]. Titanium carbides and silicides such as TiC and Ti5Si3 are confirmed to be good reinforcements for titanium alloys because of their excellent capabilities including high hardness and wear resistance in many tribological systems, thermal stability at high temperature, high elastic modulus and the similar density to
⁎ Corresponding author. Tel.: +86 531 8395991; fax: +86 531 8392313. E-mail address: [email protected] (Y.S. Tian). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.05.035
pure titanium [6–8] and surface modification of titanium alloys with titanium carbides or silicides is considered to be effective method to improve the wear resistance of titanium alloys [7–9]. So, in the present study, the laser alloying with graphite and silicon mixed powders was performed to modify the surface properties of titanium alloy Ti–6Al–4V. The microstructure, the phase composition and the wear resistance of the surface alloyed layers were investigated.
Fig. 1. The surface morphology of the laser alloyed layer of sample 1.
636
Y.S. Tian, C.Z. Chen / Materials Letters 61 (2007) 635–638
2. Experimental procedures
Fig. 2. XRD spectrum of the sample 1.
The samples of titanium alloy Ti–6Al–4V, 10× 10× 30 in size, were abraded with SiC grit paper prior to the coating operation. Graphite and silicon mixed powders, an average particle size of 10μm, with different weight ratios (2:1, 1:1 and 1:2), blended with diluted polyvinyl alcohol solution were coated on the surface of the samples in a thickness of approximately 0.5 mm. A 1500 W continuous wave CO2 laser, output power of 1200 W, beam size of 2 mm, scanning speed of 3.5 mm/s was employed to melt the preplaced coatings together with the surface layer of the samples. To protect the melt pool from oxidation during processing, argon gas shield at a pressure of 0.35 MPa was fed through a nozzle
Fig. 3. EMPA micrographs of the samples.
Y.S. Tian, C.Z. Chen / Materials Letters 61 (2007) 635–638
637
Fig. 4. EDS of elements Ti, C and Si of sample 2.
diameter of 40 mm was selected as the wear couple. The weight loss was evaluated every 5 min using an electronic balance with an accuracy of 0.1 mg.
which was coaxial with the laser beam. In addition, there was a side argon gas flow through another nozzle at an angle of 30° to the melt pool. Metallographic samples were prepared using standard mechanical polishing procedures and then etched in a solution of HF, HNO3 and H2O in volume ratio of 2:1:47 to reveal the growth morphologies of the compounds. The phase composition of the coatings was identified using D/max-rC XRD with Cu Kα radiation operated at a voltage of 40 kV, a current of 40 mA, and a scanning rate of 5°/min. Microstructures were characterized using JXA-8800R EMPA and the chemical compositions of the compounds in the coating were analyzed by EDS of EMPA. The hardness of the samples was evaluated using Shimadzu Vickers hardness tester. Sliding wear tests were performed using MM200 wear test machine with a load of 4 kg. A sintered carbide abrasive wheel (rotation speed: 400 rpm) with a
Fig. 1 is the surface morphology of the laser alloyed layer of the sample 1 and the XRD spectrum of it indicates that the compounds in the coating mainly consist of TiC and Ti5Si3 (see Fig. 2). From Fig. 3 it is seen that the compounds in the top layers of the coatings are mainly dendritic and block-like pre-eutectic and chrysanthemum-like eutectic, however, in the inner layer are nearly all the chrysanthemum-like. On the other hand, with the ratios of graphite to silicon increasing, the morphology of the pre-eutectic compounds turns from dendritic to block-like, which shows that the variation of the concentration of the alloying elements has an effect on the morphology of the compounds. The EDS of EMPA shown in Fig. 4 indicates that the pre-eutectic compounds are rich in carbon and the
Fig. 5. Microhardness profiles of the coatings.
Fig. 6. Weight loss as a function of the distance in sliding wear test.
3. Results and discussions
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Y.S. Tian, C.Z. Chen / Materials Letters 61 (2007) 635–638
microhardness decreases from about 1500 HV0.1 in the top layer gradually to about 1200 HV0.1 and then drastically down to about 400 HV0.1 for the matrix. Results of the sliding wear test, as shown in Fig. 6, indicate that the wear resistance of the coatings is 4 times more than the as-received. Fig. 7 is an EMPA micrograph of the worn surface of the samples. Clearly, under the sliding wear conditions with the sintered carbide as the counterpart, the worn surface of the original Ti–6Al–4V is easily plastically deformed and grooved. Since the microhardness of the original Ti–6Al–4V (about 400 HV0.1) is much lower than that of the counterpart (about 1200 HV0.1), the hard asperities on the surface of the counterpart can easily penetrate into the surface of titanium alloy Ti–6Al–4V, resulting in effective microcutting. Thus, the wear behavior of the original Ti–6Al–4V is featured as abrasive and adhesive wear (see Fig. 7a). While the very high hardness of titanium carbides and silicides makes the composite coating very difficult to be grooved and plastically deformed, the worn surface is characterized by the presence of much shallower grooves and little adhesive features, as shown in Fig. 7b. That is, titanium carbides and silicides reinforced composite coatings exhibit excellent abrasive and adhesive wear resistance under sliding wear test conditions because the strength and hardness of laser-alloyed layer are significantly enhanced by the in situ formed compounds.
4. Conclusion
Fig. 7. EMPA micrographs of the worn surface of the samples: (a) as-received, (b) laser treated.
eutectic compounds rich in silicon. It means that the pre-eutectic compounds are mainly titanium carbides and the eutectic compounds mainly titanium silicides. The different compounds formed in different areas of the same melt pool may be attributed to the different diffusion ability of the alloy elements and the different solidification rate of the areas in the same melt pool. During laser alloying processing, the mixed powders dissolve into the melt pool leading to reacting with titanium:Ti +C=TiC; 5Ti +3Si =Ti5Si3. With the laser beam move away, the melt pool solidifies quickly. According to the Ti–C and Ti–Si binary diagrams, the melting points of TiC and Ti5Si3 are 3140 °C and 2120 °C, respectively. In addition, at 1339 °C the eutectic reaction takes place in the Ti–Si system to form eutectic phases Ti5Si3–β(Ti) [10]. So, it can be inferred that the combination of C with Ti at relatively higher temperature results in a shorter diffusion time of the C atoms in the melt pool and thus the titanium carbides formed in the top layer of the coatings. On the other hand, the titanium silicides formed at a relatively lower temperature results in the Si atoms having more time to diffuse deep into the melt pool. So, the compounds in the inner layer of the coatings are mainly titanium silicides. The microhardness profiles along the depth direction of the composite coatings are shown in Fig. 5. It can be found that the total depth of the coatings is about 0.7 mm and with the depth increasing the
A composite strengthening layer was fabricated by laser alloying of preplaced Si and graphite powders on the surface of titanium alloy Ti–6Al–4V. The phase composition, microstructure, hardness and wear resistance of the composite coatings can be summarized as follows: XRD result confirms that the composite coating mainly consists of TiC and Ti5Si3 compounds. EDS indicates that the dendritic and the block-like compounds are mainly titanium carbides and the chrysanthemum-like mainly titanium silicides. The high microhardness and excellent wear resistance of the composite coatings are attributed to the formation of hard ceramic compounds. Thus, it can be deduced that the coatings with such an excellent wear resistance can significantly improve the load-bearing capability of the substrate. References [1] H.Y. Chen, S.J. Huang, International Journal of Machine Tools and Manufacture 44 (2004) 1653. [2] G. Thawari, G. Sundarararjan, S.V. Joshi, Thin Solid Films 423 (2003) 41. [3] I. García, J.J.D. Damborenea, Corrosion Science 40 (1998) 1411. [4] C. Hu, T.N. Baker, Materials Science & Engineering. A, Structural Materials: Properties, Microstructure and Processing 265 (1999) 268. [5] Tian, Y. PhD Thesis, Shandong University, Jinan, Shandong, P.R. China, 2006, p 66. [6] Y. Chen, H.M. Wang, Journal of Alloys and Compounds 351 (2003) 304. [7] H.M. Wang, Y.F. Liu, Materials Science & Engineering. A, Structural Materials: Properties, Microstructure and Processing 338 (2002) 126. [8] P. Jiang, J.J. Zhang, G.L. Yu, Rare Metal Materials and Engineering 29 (2000) 269 (Chinese). [9] E. Richter, J. Piekoszewski, F. Prokert, J. Stanislawski, L. Walis, E. Wieser, Vacuum 63 (2001) 523. [10] Y.S. Tian, C.Z. Chen, L.X. Chen, Q.H. Huo, Materials Letters 60 (2006) 109.