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Study on double-glow plasma niobium surface alloying of pure titanium
ARTICLE IN PRESS
Vacuum 81 (2007) 937–942 www.elsevier.com/locate/vacuum
Discussion
Study on double-glow plasma niobium surface alloying of pure titanium Wenbo Wang, Zhong Xu, Zhiyong He, Zhenxia Wang, Pingze Zhang Research Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, Shan xi, PR China Received 9 February 2006; received in revised form 19 September 2006; accepted 19 September 2006
Abstract A niobium modified layer on a pure titanium surface was obtained by means of a double-glow plasma surface alloying technique. Microstructure and phases resident in the alloy layer were analyzed. The processing parameters and effects of cathode sputtering before the diffusion process were also studied. The results show that the surface niobium content in the modified layer is similar to that in the Ti–45Nb alloy, and decreases gradually from the surface into the underlying substrate. The oxidation behavior of the modified pure titanium at 900 1C was noticeably improved after the niobium alloying process. Characterization was performed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The oxidation mechanism is also discussed. r 2006 Elsevier Ltd. All rights reserved. Keywords: Niobium; Niobium modified layer; Plasma surface alloying; High-temperature oxidation
1. Introduction Titanium and titanium alloys are excellent candidates for aerospace applications owing to their higher mechanical strength and excellent corrosion resistance [1]. However, one of the main shortcomings of titanium and its alloys is the severe oxidation at high temperature, which could lead to combustion or even explosion under certain circumstances [2,3]. Ti–45Nb is a typical titanium niobium alloy that possesses burn-resistant properties and superior corrosion resistance in addition to robust mechanical properties [4]. However, the high content of expensive niobium makes the production of this alloy very difficult and costly, and also reduces its specific strength. In this study, we prepared a niobium modified layer on the surface of pure titanium using a double-glow plasma surface alloying technique [5,6]. The modified surface possesses similar composition and properties of the Ti–45Nb alloy, while the bulk of the material maintains the high specific strength of pure titanium. The processing is conducted within a vacuum environment under ion bombardment of a glow discharge. Compared to conventional metallic Corresponding author. Tel.: +86 351 6010540; fax: +86 351 6018760.
E-mail addresses: [email protected] (W. Wang), [email protected] (Z. He). 0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.09.008
cementation processes, the treatment is highly efficient and controllable [6]. 2. Experimental 2.1. Apparatus and processing conditions The surface alloying experiments were performed in a double-glow plasma surface-alloying device, in which a low-temperature plasma was produced by a glow discharge process in a vacuum sputtering chamber. The sketch is shown in Fig. 1. A pure niobium plate is used as the target and pure argon is used as the discharge media. Glow discharge is ignited on the target and substrate separately with two sets of DC power supplies that heat them to high temperature. Niobium particles are sputtered away from the target and deposited on the substrate surface. The modified layer is formed thereafter through a thermal diffusion process. The thickness and surface composition of the niobium modified layer could be varied by controlling the processing parameters. Substrates are pure titanium disc specimens approximately 12 mm in diameter and 3 mm in thickness. The oxygen content of the material is 0.12%. The specimens were machined, ground, polished and ultrasonically cleaned in acetone and dried in air. A WDL-31
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90 80
Ti
70
Nb
Content(wt%)
60 50 40 30 20 10 0 0
5
10
15
20
25
30
Distance from surface(μm) Fig. 2. Composition of the niobium modified layer. Fig. 1. Sketch of double-glow plasma surface alloying technology.
photoelectric pyrometer was used to measure and control chamber temperature. In this experiment, processing parameters are: source voltage (650–1000 V); substrate voltage (360–500 V); processing temperature (900–1100 1C); holding time (2–4 h); working pressure (20–80 Pa); and distance between the target and substrate (15–17 mm). Oxidation tests were performed in ambient air at 900 1C for 100 h. Specimens, which were contained in crucibles, were placed in the furnace and weight gain was measured every 5 h initially and subsequently every 10 h, with an electronic balance. 2.2. Characterization The compositions of the modified layers were analyzed using a glow discharge spectrum analyzer (GDA). The microstructure of the alloying layer was observed with an image analyzer (Zeiss Axiovert 25CA). The scanning electron microscope (SEM) LEO438VP equipped with an EDX facility was used to observe the microstructure of the oxidized specimens. Phase characterization was conducted using a D/max2500 X-ray diffractometer (XRD). 3. Result and discussion 3.1. Composition and microstructure of the niobium modified layer Niobium and titanium are co-miscible at all compositions and exist in the same phase at temperatures above 885 1C. This is beneficial to the efficiency of the alloying process. The composition and microstructure of the niobium modified layer formed on commercially pure titanium are shown in Figs. 2 and 3. The niobium modified
Fig. 3. Microstructure of the niobium modified layer.
layer is uniform, continuous and dense. At the surface, the niobium content is nearly 40% and decreases gradually from surface to substrate bulk. In Fig. 4, an XRD spectrum of the niobium modified layer shows that the b-Ti substituted solid solution structure exists in the modified layer. 3.2. The influence of process parameters on the formation of the niobium modified layer Processing parameters need to be selected appropriately to form the niobium modified layer with desired surface composition and depth. The processing temperature is one of the key parameters. It is required that processing
ARTICLE IN PRESS W. Wang et al. / Vacuum 81 (2007) 937–942
temperature is higher than 885 1C, which is the critical temperature for niobium to dissolve in titanium. The temperature range we selected is 900–1100 1C. Fig. 5(a) indicated that the niobium content increased as processing temperature is increased. Second, the gas pressure is important. Fig. 5(b) demonstrates how the niobium content changes with gas pressure. It is found that 50–65 Pa is the optimized gas pressure range, where the highest niobium content is achieved. Substrate surface condition is another important factor in the formation of niobium modified layers. Titanium has an affinity for oxygen at room temperature, and there exists a continuous thin oxide film that hinders niobium from diffusing into the substrate. An independent argon sputtering process on the substrate before the diffusion process is suggested as a necessity to clean up the surface contamination and remove oxide film [7]. Fig. 6 shows the variation of the niobium content as the cathode sputtering time changes. Another effect of
939
sputtering on the substrate is that the collision of high-energy ions and atoms with the substrate surface enhances the density of lattice defects, which is very favorable for the adsorption and diffusion of niobium atoms [8]. It is reported that vacancy concentration under ion bombardment is 103–105 times that of the thermodynamic equilibrium condition [9]. 3.3. Oxidation properties Since oxidation of the titanium alloy accompanied combustion of titanium [10], it is necessary to study the oxidation behavior of the niobium modified layer to characterize the high-temperature stability of the asproduced alloy. Weight gain versus time curves during oxidation tests are presented in Fig. 7 for pure titanium and the niobium modified layer. It is inferred from the initial results obtained at 900 1C that niobium plays a key role in improving the oxidation resistance of pure titanium. 40
2000 1800
♦
♦ Ti
Δ
Δ Nb
40min
32
1600
60min
28 Content(wt%)
1400 1200 1000 800
24 20 16 12
600 ♦ Δ
400
♦ Δ
♦ Δ
200
8 4 0 0
0 10
20
30
40
50
60
70
80
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30 Distance from surface(μm)
2θ/(°) Fig. 6. Niobium content in the niobium modified layer change with cathode sputtering.
Fig. 4. XRD spectrum of the niobium modified.
60 70 60 50 40 30 20
25Pa 50Pa 65Pa 80Pa
50 Content(wt %)
900 °c 950 °c 1000 °c 1050 °c 1100 °c
80 Content(wt%)
Instensity(cps)
10min
36
40 30 20 10
10 0
0 0
10 20 30 40 50 60 70 Distance from surface(μm)
0
5 10 15 20 25 30 35 40 Distance from surface(μm)
Fig. 5. Niobium content in the niobium modified layer change with (a) temperature and (b) gas pressure.
ARTICLE IN PRESS W. Wang et al. / Vacuum 81 (2007) 937–942
940
The oxidation growth kinetics curve of pure titanium over a total period of 100 h can be partitioned into two stages. The first stage is associated with a parabolic growth law 100
pure titanium Ti-Nb alloyed layer
Mass gain (mg/cm2)
80
60
40
20
0 0
20
40
60
80
Oxidation Time (h) Fig. 7. Weight gain as oxidation time at 900 1C.
Fig. 8. Macroscopic views of oxidized specimens.
100
during the first 70 h and evolved into a linear law after 70 h. This change of the oxidation growth kinetics from parabolic to linear could be attributed to the breakdown of the protective layer. Fig. 8 shows macroscopic views of pure titanium (NO. 1) and the niobium modified layer (NO. 2) after oxidation at 900 1C for 100 h. On visual observation, the oxide scale on the untreated titanium was thicker than that on the niobium modified material. The color of the oxidation surface scale on pure titanium is reddish brown, while that of the niobium modified layer is dark gray. Fig. 9 shows optical micrograph cross sections of the oxidized surfaces. The scale on untreated titanium was much thicker (about 150 mm) than that of the niobium modified surface (about 46 mm). Fig. 10 illustrates the XRD spectrum of the subject specimens. Fig. 11(a) indicated that peaks originating from TiO2 were dominant; simultaneously, Ti2N and TiN existed together within the oxide layer. The surface of the niobium modified layer was mainly composed of TiO2; in addition, Nb2O5, Ti2N and TiN, also were present. In Fig. 11(a), the small-sized and compact oxide grains, which consisted mostly of TiO2, formed on the surface of the niobium modified layer. In Fig. 11(b), it should be noted that the scale is dense and continuous and adheres well to the substrate. Fig. 12 shows results of line scans of element distribution of the oxidized specimens. It is noticed that the oxygen content of the niobium modified specimen is lower than that of untreated titanium. This indicates that the niobium concentration is not uniformly distributed through out the scale. There was recognizable niobium enrichment around the metal/oxide interface area. The role of niobium on the oxidation behavior of titanium alloys has been explored by many researchers. It is generally believed that, the presence of substituted niobium in the lattice decreases oxygen vacancies and therefore retards the diffusion of oxygen thereby promoting increased oxidation resistance [11]. Niobium also lowers the solubility of oxygen in the titanium alloy, thus retarding the internal oxidation of the alloy [12]. Niobium also promotes the formation of TiN at the metal/oxide interface, which works as a diffusion barrier to titanium and oxygen
Fig. 9. Cross section microstructure of (a) the pure titanium and (b) the niobium modified layer oxidized at 900 1C for 100 h.
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TiO2
500
941
TiO2
500
Ti2N
Nb2O5
TiN
Ti2N
400 CPS
CPS
400 300 200
TiN
300 200
)
100
100 •
0
0 20
40
60
80
20
2θ/(°)
40
60
80
2θ/(°)
Fig. 10. XRD patterns of the pure titanium (a) and the niobium modified layer (b) oxidized at 900 1C for 100 h.
Fig. 11. SEM images of surface (a) and cross section (b) of the niobium modified layer oxidized at 900 1C for 100 h.
Fig. 12. Line scans of the pure titanium (a) and the niobium modified layer (b) oxidized at 900 1C for 100 h.
ions [11,13]. Actually, these processes may occur simultaneously [14] in a complex and coupled reaction mechanism. According to Stringer [15], the oxidation behavior of titanium is controlled by rutile oxide growth at temperatures below 600–650 1C, while at higher temperatures, the oxidation rate is controlled by the dissolution of oxygen in the alloy. The higher the temperature, the higher will be the contribution of oxygen dissolution to total mass gain. Based on results of this study, it is suggested that niobium possibly suppressed the dissolution of oxygen into the substrate through doping of the titania lattice by
Nb5+ions, which decreases the number of oxygen vacancies and/or interstitials. 4. Conclusions A plasma surface alloying approach was used successfully to form a niobium modified layer on a clean pure titanium substrate. The surface niobium content of the modified layer is similar to Ti–45Nb. Processing temperature and gas pressure effect the formation of the niobium modified layer and the niobium content. The influence of
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cathode sputtering time on the processing and properties of the alloy layer has been described. Experiments indicate that double-glow plasma surface modification of titanium through incorporation of niobium is a practical method to improve the oxidation resistance of titanium. Acknowledgments This project is financially supported through the China National Nature Science Fund (50271045), Shan xi province Nature Science Fund (20031051) and Study Abroad Foundation of Shan xi province (2004–20). References [1] Boyer RR. An overview on the use of titanium in the aerospace industry. J Mater Sci Eng 1996;A213:103–14. [2] Zhao Y-Q, Zhao X-M, Zhu K-Y, et al. Burn-resistant titanium alloy. J Rare Met Mater Eng 1996;25(5):1–6. [3] Zhao Y-Q, Zhou L, Deng J. Burn resistant behavior and mechanism of a Ti40 alloy. J Rare Met Mater Eng 1999;28(2):78–80.
[4] Hemson R. New titanium–niobium alloy solves autoclave problem. J Adv Mater Process 1995;14(5):27–8. [5] Z. Xu. US Patent, No. 4520268, 1985. [6] Z. Xu. US Patent, No. 4731539, 1988. [7] Kringhøja P, Budtz-Jørgensena CV, Find Nielsena J, et al. On glowdischarge sputtering of iron and steels in a commercial deposition plant. J Surf Coat Technol 2001;137:277–83. [8] Li CM, Xu Z. Diffusion mechanism of ion bombardment. J Surf Eng 1987;4(3):310–2. [9] Gao Y. The vacancy density distributed surface and affecting to spreading speed under double glow discharge surface alloying process. J Vac 1993;6:53–6. [10] Zhao Y-Q, Zhou L, Deng J. Burn resistant behavior and burn resistant mechanism of Ti40 alloy. J Ordnance Mater Sci Eng 1999; 22:19–24. [11] Roy TK, Balasubramaniam R, Ghosh A. Metall Mater Trans 1996; 27A:3993. [12] Quadakkers WJ, Zheng N, Wallura E. Werkst Korros 1997;48(1):28. [13] Perez P, Haanappel VAC, Stroosnijder MF. Mater Sci Eng A 2000;284:126. [14] Pe´z* P. Influence of nitriding on the oxidation behaviour of titanium alloys at 700 1C. J Surf Coat Technol 2005;191:293–302. [15] Stringer J. Acta Metall 1960;8:758.
Vacuum 81 (2007) 937–942 www.elsevier.com/locate/vacuum
Discussion
Study on double-glow plasma niobium surface alloying of pure titanium Wenbo Wang, Zhong Xu, Zhiyong He, Zhenxia Wang, Pingze Zhang Research Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, Shan xi, PR China Received 9 February 2006; received in revised form 19 September 2006; accepted 19 September 2006
Abstract A niobium modified layer on a pure titanium surface was obtained by means of a double-glow plasma surface alloying technique. Microstructure and phases resident in the alloy layer were analyzed. The processing parameters and effects of cathode sputtering before the diffusion process were also studied. The results show that the surface niobium content in the modified layer is similar to that in the Ti–45Nb alloy, and decreases gradually from the surface into the underlying substrate. The oxidation behavior of the modified pure titanium at 900 1C was noticeably improved after the niobium alloying process. Characterization was performed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The oxidation mechanism is also discussed. r 2006 Elsevier Ltd. All rights reserved. Keywords: Niobium; Niobium modified layer; Plasma surface alloying; High-temperature oxidation
1. Introduction Titanium and titanium alloys are excellent candidates for aerospace applications owing to their higher mechanical strength and excellent corrosion resistance [1]. However, one of the main shortcomings of titanium and its alloys is the severe oxidation at high temperature, which could lead to combustion or even explosion under certain circumstances [2,3]. Ti–45Nb is a typical titanium niobium alloy that possesses burn-resistant properties and superior corrosion resistance in addition to robust mechanical properties [4]. However, the high content of expensive niobium makes the production of this alloy very difficult and costly, and also reduces its specific strength. In this study, we prepared a niobium modified layer on the surface of pure titanium using a double-glow plasma surface alloying technique [5,6]. The modified surface possesses similar composition and properties of the Ti–45Nb alloy, while the bulk of the material maintains the high specific strength of pure titanium. The processing is conducted within a vacuum environment under ion bombardment of a glow discharge. Compared to conventional metallic Corresponding author. Tel.: +86 351 6010540; fax: +86 351 6018760.
E-mail addresses: [email protected] (W. Wang), [email protected] (Z. He). 0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.09.008
cementation processes, the treatment is highly efficient and controllable [6]. 2. Experimental 2.1. Apparatus and processing conditions The surface alloying experiments were performed in a double-glow plasma surface-alloying device, in which a low-temperature plasma was produced by a glow discharge process in a vacuum sputtering chamber. The sketch is shown in Fig. 1. A pure niobium plate is used as the target and pure argon is used as the discharge media. Glow discharge is ignited on the target and substrate separately with two sets of DC power supplies that heat them to high temperature. Niobium particles are sputtered away from the target and deposited on the substrate surface. The modified layer is formed thereafter through a thermal diffusion process. The thickness and surface composition of the niobium modified layer could be varied by controlling the processing parameters. Substrates are pure titanium disc specimens approximately 12 mm in diameter and 3 mm in thickness. The oxygen content of the material is 0.12%. The specimens were machined, ground, polished and ultrasonically cleaned in acetone and dried in air. A WDL-31
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W. Wang et al. / Vacuum 81 (2007) 937–942
90 80
Ti
70
Nb
Content(wt%)
60 50 40 30 20 10 0 0
5
10
15
20
25
30
Distance from surface(μm) Fig. 2. Composition of the niobium modified layer. Fig. 1. Sketch of double-glow plasma surface alloying technology.
photoelectric pyrometer was used to measure and control chamber temperature. In this experiment, processing parameters are: source voltage (650–1000 V); substrate voltage (360–500 V); processing temperature (900–1100 1C); holding time (2–4 h); working pressure (20–80 Pa); and distance between the target and substrate (15–17 mm). Oxidation tests were performed in ambient air at 900 1C for 100 h. Specimens, which were contained in crucibles, were placed in the furnace and weight gain was measured every 5 h initially and subsequently every 10 h, with an electronic balance. 2.2. Characterization The compositions of the modified layers were analyzed using a glow discharge spectrum analyzer (GDA). The microstructure of the alloying layer was observed with an image analyzer (Zeiss Axiovert 25CA). The scanning electron microscope (SEM) LEO438VP equipped with an EDX facility was used to observe the microstructure of the oxidized specimens. Phase characterization was conducted using a D/max2500 X-ray diffractometer (XRD). 3. Result and discussion 3.1. Composition and microstructure of the niobium modified layer Niobium and titanium are co-miscible at all compositions and exist in the same phase at temperatures above 885 1C. This is beneficial to the efficiency of the alloying process. The composition and microstructure of the niobium modified layer formed on commercially pure titanium are shown in Figs. 2 and 3. The niobium modified
Fig. 3. Microstructure of the niobium modified layer.
layer is uniform, continuous and dense. At the surface, the niobium content is nearly 40% and decreases gradually from surface to substrate bulk. In Fig. 4, an XRD spectrum of the niobium modified layer shows that the b-Ti substituted solid solution structure exists in the modified layer. 3.2. The influence of process parameters on the formation of the niobium modified layer Processing parameters need to be selected appropriately to form the niobium modified layer with desired surface composition and depth. The processing temperature is one of the key parameters. It is required that processing
ARTICLE IN PRESS W. Wang et al. / Vacuum 81 (2007) 937–942
temperature is higher than 885 1C, which is the critical temperature for niobium to dissolve in titanium. The temperature range we selected is 900–1100 1C. Fig. 5(a) indicated that the niobium content increased as processing temperature is increased. Second, the gas pressure is important. Fig. 5(b) demonstrates how the niobium content changes with gas pressure. It is found that 50–65 Pa is the optimized gas pressure range, where the highest niobium content is achieved. Substrate surface condition is another important factor in the formation of niobium modified layers. Titanium has an affinity for oxygen at room temperature, and there exists a continuous thin oxide film that hinders niobium from diffusing into the substrate. An independent argon sputtering process on the substrate before the diffusion process is suggested as a necessity to clean up the surface contamination and remove oxide film [7]. Fig. 6 shows the variation of the niobium content as the cathode sputtering time changes. Another effect of
939
sputtering on the substrate is that the collision of high-energy ions and atoms with the substrate surface enhances the density of lattice defects, which is very favorable for the adsorption and diffusion of niobium atoms [8]. It is reported that vacancy concentration under ion bombardment is 103–105 times that of the thermodynamic equilibrium condition [9]. 3.3. Oxidation properties Since oxidation of the titanium alloy accompanied combustion of titanium [10], it is necessary to study the oxidation behavior of the niobium modified layer to characterize the high-temperature stability of the asproduced alloy. Weight gain versus time curves during oxidation tests are presented in Fig. 7 for pure titanium and the niobium modified layer. It is inferred from the initial results obtained at 900 1C that niobium plays a key role in improving the oxidation resistance of pure titanium. 40
2000 1800
♦
♦ Ti
Δ
Δ Nb
40min
32
1600
60min
28 Content(wt%)
1400 1200 1000 800
24 20 16 12
600 ♦ Δ
400
♦ Δ
♦ Δ
200
8 4 0 0
0 10
20
30
40
50
60
70
80
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30 Distance from surface(μm)
2θ/(°) Fig. 6. Niobium content in the niobium modified layer change with cathode sputtering.
Fig. 4. XRD spectrum of the niobium modified.
60 70 60 50 40 30 20
25Pa 50Pa 65Pa 80Pa
50 Content(wt %)
900 °c 950 °c 1000 °c 1050 °c 1100 °c
80 Content(wt%)
Instensity(cps)
10min
36
40 30 20 10
10 0
0 0
10 20 30 40 50 60 70 Distance from surface(μm)
0
5 10 15 20 25 30 35 40 Distance from surface(μm)
Fig. 5. Niobium content in the niobium modified layer change with (a) temperature and (b) gas pressure.
ARTICLE IN PRESS W. Wang et al. / Vacuum 81 (2007) 937–942
940
The oxidation growth kinetics curve of pure titanium over a total period of 100 h can be partitioned into two stages. The first stage is associated with a parabolic growth law 100
pure titanium Ti-Nb alloyed layer
Mass gain (mg/cm2)
80
60
40
20
0 0
20
40
60
80
Oxidation Time (h) Fig. 7. Weight gain as oxidation time at 900 1C.
Fig. 8. Macroscopic views of oxidized specimens.
100
during the first 70 h and evolved into a linear law after 70 h. This change of the oxidation growth kinetics from parabolic to linear could be attributed to the breakdown of the protective layer. Fig. 8 shows macroscopic views of pure titanium (NO. 1) and the niobium modified layer (NO. 2) after oxidation at 900 1C for 100 h. On visual observation, the oxide scale on the untreated titanium was thicker than that on the niobium modified material. The color of the oxidation surface scale on pure titanium is reddish brown, while that of the niobium modified layer is dark gray. Fig. 9 shows optical micrograph cross sections of the oxidized surfaces. The scale on untreated titanium was much thicker (about 150 mm) than that of the niobium modified surface (about 46 mm). Fig. 10 illustrates the XRD spectrum of the subject specimens. Fig. 11(a) indicated that peaks originating from TiO2 were dominant; simultaneously, Ti2N and TiN existed together within the oxide layer. The surface of the niobium modified layer was mainly composed of TiO2; in addition, Nb2O5, Ti2N and TiN, also were present. In Fig. 11(a), the small-sized and compact oxide grains, which consisted mostly of TiO2, formed on the surface of the niobium modified layer. In Fig. 11(b), it should be noted that the scale is dense and continuous and adheres well to the substrate. Fig. 12 shows results of line scans of element distribution of the oxidized specimens. It is noticed that the oxygen content of the niobium modified specimen is lower than that of untreated titanium. This indicates that the niobium concentration is not uniformly distributed through out the scale. There was recognizable niobium enrichment around the metal/oxide interface area. The role of niobium on the oxidation behavior of titanium alloys has been explored by many researchers. It is generally believed that, the presence of substituted niobium in the lattice decreases oxygen vacancies and therefore retards the diffusion of oxygen thereby promoting increased oxidation resistance [11]. Niobium also lowers the solubility of oxygen in the titanium alloy, thus retarding the internal oxidation of the alloy [12]. Niobium also promotes the formation of TiN at the metal/oxide interface, which works as a diffusion barrier to titanium and oxygen
Fig. 9. Cross section microstructure of (a) the pure titanium and (b) the niobium modified layer oxidized at 900 1C for 100 h.
ARTICLE IN PRESS W. Wang et al. / Vacuum 81 (2007) 937–942
TiO2
500
941
TiO2
500
Ti2N
Nb2O5
TiN
Ti2N
400 CPS
CPS
400 300 200
TiN
300 200
)
100
100 •
0
0 20
40
60
80
20
2θ/(°)
40
60
80
2θ/(°)
Fig. 10. XRD patterns of the pure titanium (a) and the niobium modified layer (b) oxidized at 900 1C for 100 h.
Fig. 11. SEM images of surface (a) and cross section (b) of the niobium modified layer oxidized at 900 1C for 100 h.
Fig. 12. Line scans of the pure titanium (a) and the niobium modified layer (b) oxidized at 900 1C for 100 h.
ions [11,13]. Actually, these processes may occur simultaneously [14] in a complex and coupled reaction mechanism. According to Stringer [15], the oxidation behavior of titanium is controlled by rutile oxide growth at temperatures below 600–650 1C, while at higher temperatures, the oxidation rate is controlled by the dissolution of oxygen in the alloy. The higher the temperature, the higher will be the contribution of oxygen dissolution to total mass gain. Based on results of this study, it is suggested that niobium possibly suppressed the dissolution of oxygen into the substrate through doping of the titania lattice by
Nb5+ions, which decreases the number of oxygen vacancies and/or interstitials. 4. Conclusions A plasma surface alloying approach was used successfully to form a niobium modified layer on a clean pure titanium substrate. The surface niobium content of the modified layer is similar to Ti–45Nb. Processing temperature and gas pressure effect the formation of the niobium modified layer and the niobium content. The influence of
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cathode sputtering time on the processing and properties of the alloy layer has been described. Experiments indicate that double-glow plasma surface modification of titanium through incorporation of niobium is a practical method to improve the oxidation resistance of titanium. Acknowledgments This project is financially supported through the China National Nature Science Fund (50271045), Shan xi province Nature Science Fund (20031051) and Study Abroad Foundation of Shan xi province (2004–20). References [1] Boyer RR. An overview on the use of titanium in the aerospace industry. J Mater Sci Eng 1996;A213:103–14. [2] Zhao Y-Q, Zhao X-M, Zhu K-Y, et al. Burn-resistant titanium alloy. J Rare Met Mater Eng 1996;25(5):1–6. [3] Zhao Y-Q, Zhou L, Deng J. Burn resistant behavior and mechanism of a Ti40 alloy. J Rare Met Mater Eng 1999;28(2):78–80.
[4] Hemson R. New titanium–niobium alloy solves autoclave problem. J Adv Mater Process 1995;14(5):27–8. [5] Z. Xu. US Patent, No. 4520268, 1985. [6] Z. Xu. US Patent, No. 4731539, 1988. [7] Kringhøja P, Budtz-Jørgensena CV, Find Nielsena J, et al. On glowdischarge sputtering of iron and steels in a commercial deposition plant. J Surf Coat Technol 2001;137:277–83. [8] Li CM, Xu Z. Diffusion mechanism of ion bombardment. J Surf Eng 1987;4(3):310–2. [9] Gao Y. The vacancy density distributed surface and affecting to spreading speed under double glow discharge surface alloying process. J Vac 1993;6:53–6. [10] Zhao Y-Q, Zhou L, Deng J. Burn resistant behavior and burn resistant mechanism of Ti40 alloy. J Ordnance Mater Sci Eng 1999; 22:19–24. [11] Roy TK, Balasubramaniam R, Ghosh A. Metall Mater Trans 1996; 27A:3993. [12] Quadakkers WJ, Zheng N, Wallura E. Werkst Korros 1997;48(1):28. [13] Perez P, Haanappel VAC, Stroosnijder MF. Mater Sci Eng A 2000;284:126. [14] Pe´z* P. Influence of nitriding on the oxidation behaviour of titanium alloys at 700 1C. J Surf Coat Technol 2005;191:293–302. [15] Stringer J. Acta Metall 1960;8:758.