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Radio-frequency plasma nitriding and nitrogen plasma immersion ion implantation of Ti-6A1-4V alloy
filliIt E
COATING8
ELSEVIER
)TEH#OLOWt Surface and CoatingsTechnology93 (1997) 309-313
Radio-frequency plasma nitriding and nitrogen plasma immersion ion implantation of Ti-6A1-4V alloy S.Y. W a n g a,b, P . K . C h u a,., B . Y . T a n g a, X . C . Z e n g a, Y . B . C h e n b, X . F . W a n g a,b a Department of Physics and Materials Science, City UniversiO, ofHong Kong, Kowloon, Hong Kong b Harbin Institute of Technology, Harbin, People's Republic of China
Abstract
Nitrogen ion implantation improves the wear resistance of Ti-6AI-4V alloys by forming a hard TiN Superficial passivation layer. However, the thickness of the layer formed by traditional ion implantation is typically I00-200 nm and may not be adequate for many industrial applications. We propose to use radio-frequency (RF) plasma nitriding and nitrogen plasma immersion ion implantation (PIII) to increase the layer thickness. By using a newly designed inductively coupled RF plasma source and applying a series of negative high voltage pulses to the Ti-6AI-4V samples, RF plasma nitriding and nitrogen PIII can be achieved. Our process yields a substantially thicker modified layer exhibiting more superior wear resistance characteristics, as demonstrated by data from micro-hardness testing, pin-on-disc wear testing, scanning electron microscopy (SEM), as well as Auger electron spectroscopy (AES). The performance of our newly developed inductively coupled RF plasma source which is responsible for the success of the experiments is also described. © 1997 Elsevier Science S.A. Keywords: Plasma immersion ion implantation; Plasma nitriding
1. Introduction
Ti-6A1-4V alloys have many desirable features, such as biocompatibility, high corrosion resistance, excellent strength-to-weight ratio, and so on. They have thus been widely used in aerospace and orthopedic applications. Their high corrosion resistance can be attributed to a passivation layer on the surface of these alloys. However, it can be easily worn out and wear-corrosion failure can occur by electric cell reactions between the passivated and non-passivated layer. It has been shown that nitrogen ion implantation is an effective means to improve the wear and corrosion resistance of Ti-6A1-4V alloys by forming a TiN superhard layer on the surface [1]. As the line-of-sight restriction is circumvented in plasma immersion ion implantation (PIII), nitrogen PIII is thus an excellent way to treat complex-shape Ti-6AI-4V components [2-10]. In general, the thickness of the surface layer formed by conventional vacuum discharge plasma processes is only 100-200 nm and may not be enough to withstand the harsh working conditions encountered in many industrial applications. In order to increase the thickness of the layer to further improve * Correspondingauthor. Tel.: (852)-2788-7724; fax: (852)-2788-7830; e-mail: appkchu@cit!al.edn.hk 0257-8972/97/$17.00 © I997 ElsevierScienceS.A. All rights reserved. PII S0257-8972 (97) 00066-2
the wear and corrosion resistance, we propose to use radio-frequency ( R F ) plasma nitriding and nitrogen PIII to treat Ti-6A1-4V alloys. The properties and characteristics of the processed samples are assessed using micro-hardness testing, pin-on-disk wear measurements, scanning electron microscopy (SEM), and Auger electron spectroscopy (AES).
2. Experimental The experiments were conducted in a custom designed plasma immersion ion implanter with an inductively coupled RF plasma source [11]. The diameter and height of the main vacuum chamber are 100 and 120 cm, respectively. A 13.56 MHz, 2 kW RF plasma source is positioned on top of the main vacuum chamber to produce RF plasmas which diffuse from the pyrex glass discharge chamber into the main chamber. The full cusp magnetic field in the main chamber confines the plasma electrons to further boost the plasma density and enhance theuniformity. To accomplish nitrogen nitriding and PIII, a series of negative high voltage pulses We~:e applied to the Ti-6A1-4V alloy samples after a
X Y. l~ang et aL / Surface and Coatings Technology 93 (1997) 309-313
310 535
530 > -r'
J
J
525
520
515
510
300
350
400
450
RF input power, W Fig. t. Microhardness of the implanted sample after processing at various RF input power.
stable nitrogen plasma had been sustained. The implantation parameters were: • pulse width = 15 gs • pulse repetition rate = 100 Hz • working gas pressure-- 1 Pa • implantation voltage = 30 kV • implantation dose from 5x1016 to 5x 10 iv atoms cm -z The sample temperature during implantation could not be measured accurately by our pyrometer as it was <100°C. For the RF source, the optimal working parameters were: 400 W input power and 1 Pa pressure. Under these conditions, the plasma density was
2.87 x 10 l° cm -3, and the radial distribution uniformity of plasma density was better than 97% throughout a circular zone 60 cm in diameter. The composition of the Ti-6At-4V alloy samples by weight is: AI, 6.32%; V, 4.41%; Fe, 0.14%; O, 0.16%; N, 0.01%; C, 0.04%; H, 0.04%.
3. Results and discussion
The properties investigated are micro-hardness, mass loss due to wear, and coefficient of friction. The microhardness of the untreated sample is 304 HV and the
600
500
> 400 2: 8 {
300
25KeV
--i_30KeV .~5--35KeV
o 200
100
0 I i : ~ I ! [ 0.00E+ 5.00E+ 1.00E+ 1.50E+ 2.00E+ 2.50E+ 3.00E+ 3.50E+ 4.00E+ 4.50E+ 5.00E+ 00 16 17 17 17 17 17 17 17 17 17
Implanted dose,ion~cm2 Fig. 2. Microhardness of the implanted sample versus implantation dose for three implantation energies: 25 keV (v), 30 keV (n) and 35 keV (A). Applied load: 20 gf,
S.Y. Wang et aI. / Surface and Coatings Technology 93 [1997) 309-313
311
3.5 3 2.5 -
2
..
o
35KEY
- - I - - 30KEY 1.5
~
25KEY
1 0.5 0
i 0
5E+16
1E+17 1.5E+17 2E+17 2.5E+17 3E+17 3.5E+17 4E+17 4.5E+17 5E+17
Implanted dose, ionNcm2
Fig. 3. Mass loss due to wear for the implanted sample versus implantation dose for three implantation energies: 35 keV (v), 30 keV (11) and 25 keV (/~ ). mass loss is 5.7 mg after a 125.66m wear route. The measured micro-hardness of the treated samples versus the applied R F input power ranging from 300 to 450 W is plotted in Fig. 1 (load--20 g). The enhanced microhardness at a higher R F power is probably due to the higher plasma density. The resulting higher ion flux is believed to elevate the sample temperature thereby causing further penetration of nitrogen, even though the absolute temperature was still too low to be accurately measured using our equipment. The micro-hardness of the implanted samples as a function of the implantation dose and energy is displayed in Fig. 2. It can be observed that after implantation of a nitrogen dose of
4 x 1017 ions cm -2, the micro-hardness of the implanted sample is enhanced by 70%. The relationship between the associated mass loss and the implantation energy and dose is indicated in Fig. 3 which shows a monotonic decrease in the mass loss with increasing implantation dose up to the dose of 3 x 101~ ions cm -2 followed by a slight increase if the dose exceeds 3 x 1027 ions cm -2. This behaviour valid for 35 keV is less pronounced for the implantation energy of 25 keV. The enhanced wear at high doses can be explained by the assumption that the longer implantation time needed to achieve the higher dose leads to a higher temperature of the implanted samples, resulting in surface stress release and
0.7
0.6
0.5
O
0.4
Q
0 0 tO
0.3
o
y
..J
Implanted Unimplanted
X
0.2
0.1
200
400
600
800
1000
1200
1400
1600
Cycle numbers, r
Fig. 4. Friction coefficientof the unimplanted (11) and impianted samples (o) versus cycle numbers.
312
S. !/.. Wang et al. / Surface and Coatings Technology 93 (1997) 309-313
i-, Jg.'-"t
5e~8~3
tS,eu.
~eKV
[b]
(a)
Fig. 5. SEM micrographsof (a) untreated sample and (b) implantedsample.
grain growth. Consequently, the wear resistance of the samples is decreased. Fig. 4 depicts the dependence of the friction coefficient on the rotation cycle number. It can be seen that the friction coefficient rises after 240 cycles to 0.7 for the untreated sample but stays fairly constant at a value of 0.16 for the treated one. If the modified layer is worn away (after 1500 cycles), the coefficient of friction of the implanted sample increases to that of the untreated sample (Fig. 4). The TiO2 and TiN=O~, compounds formed on the surface of the samples by PIII contribute to the good wear resistance and low
friction coefficient, and N + implantation helps to stabilize the superficial layer. The surface morphology of samples was investigated by SEM. The SEM micrograph of the untreated sample shown in Fig. 5a reveals the typical microstructure consisting of c~ phase and fl phase. The c~ phase exists as small and round grains whereas the/? phase is composed of banding grains among the phases. The/~ grains have a larger size but a lower density. The dimension of the grains varies from 10 gm to several tens of gm. The SEM photo of the implanted sample (35keV,
70 6O
50
4o
---II.-- N .-Ii-- 0
3o
._~5.._ -I1 m m
20 A
10
J 500
1000 1500 Etching depth, A
2000
Fig. 6. Auger depth profiles of Ti, N and O in the implanted sample.
2500
S. Y. Wang et al. / Surface and Coatings Technology93 (1997) 309-313
3 x 1017 cm -2) is shown in Fig. 5b. It can be seen that the grains of the implanted sample are small and more uniform, and the/3 phase has disappeared. A new and smaller phase has formed among the c~ phase grains. The dimension of the grains is only several tens of rim. The larger grains shown in Fig. 5b were characterized by AES and identified to be the c~phase of Ti-6At-4V deduced from the composition. The phases of the implanted layer produced by the PIII process were investigated using X-ray photoelectron spectroscopy (XPS) and the presence of the TiN phase is confirmed. The Ti, O and N depth profiles obtained by AES are displayed in Fig. 6. From the nitrogen in-depth distribution, it can be inferred that the thickness of the modified layer is >210 nm.
4. Conclusion Using our new inductively coupled plasma source, RF plasma nitriding and nitrogen PIII have been successfully employed to enhance the wear resistance of Ti-6AI-4V alloy samples. Even at a relatively low implantation voltage of 30 kV, the treated samples show an increase of surface micro-hardness and improved tribological properties. The thickness of the hardened surface layer is measured by AES to be over 210 nm and larger than that attained by conventional vacuum discharge plasma processing.
313
Acknowledgement This research Work was supported by the City University of Hong Kong (Contract 7000621) and the Hong Kong Research Grants Council (Contracts 8730005 and 9040220).
References [I] B.L. Garside, Mater. Sci. Engng, A13(11) (1991) 179. [2] A. Chen, J.T. Scheure, C. Ritter, R.B. Alexander and J.R. Conrad, J. Appl. Phys., 70 (i991) 6757. [3] B.Y. Tang, Physics, 23 (1994) 42. [4] J.R. Conrad, Mater. Sci. Engng, Al16 (I989) 201. [5] J.R. Conrad, S. Baumann, R. Fleming and G.P. Meeker, 3". Appl. Phys., 65 (1989) 1701. [6] J.R. Conrad, R.A. Dodd, S. Han, M. Madapura, J.T. Scheure, K. Saidaram and F.J. Worzala, J. Vac. Sci. Technol., A8 (1992) 3146. [7] J.R. Conrad, J.L. Radtke, R.A. Dodd, F.J. Worzala and N.E. Tran, Y. Appl. Phys., 62 (1987) 4591. [8] J.R. Conrad, R.A. Dodd, F.J. Worzala and X. Qiu, S w f Coat. Technol., 36 (1986) 927. [9] L. Xie, F.J. Worzala, J:R. Conrad, R.A, Dodd and K. Sridharan, Mater. Sci. Engng, A139 (I99I) 179. [I0] J, Chen, J. Blanchard, J.R. Conrad and R.A. Dodd, Surf Coat. Technol., 52 (I992) 267. [ 11] P.K. Chu, B.Y. Tang, Y.C, Cheng and P.K, Ko, Proceedings Third bzternatiotml HZorkshop on Plasma-ba4ed lon h~plantation, Dresden, Germany, I-3 (1996).
COATING8
ELSEVIER
)TEH#OLOWt Surface and CoatingsTechnology93 (1997) 309-313
Radio-frequency plasma nitriding and nitrogen plasma immersion ion implantation of Ti-6A1-4V alloy S.Y. W a n g a,b, P . K . C h u a,., B . Y . T a n g a, X . C . Z e n g a, Y . B . C h e n b, X . F . W a n g a,b a Department of Physics and Materials Science, City UniversiO, ofHong Kong, Kowloon, Hong Kong b Harbin Institute of Technology, Harbin, People's Republic of China
Abstract
Nitrogen ion implantation improves the wear resistance of Ti-6AI-4V alloys by forming a hard TiN Superficial passivation layer. However, the thickness of the layer formed by traditional ion implantation is typically I00-200 nm and may not be adequate for many industrial applications. We propose to use radio-frequency (RF) plasma nitriding and nitrogen plasma immersion ion implantation (PIII) to increase the layer thickness. By using a newly designed inductively coupled RF plasma source and applying a series of negative high voltage pulses to the Ti-6AI-4V samples, RF plasma nitriding and nitrogen PIII can be achieved. Our process yields a substantially thicker modified layer exhibiting more superior wear resistance characteristics, as demonstrated by data from micro-hardness testing, pin-on-disc wear testing, scanning electron microscopy (SEM), as well as Auger electron spectroscopy (AES). The performance of our newly developed inductively coupled RF plasma source which is responsible for the success of the experiments is also described. © 1997 Elsevier Science S.A. Keywords: Plasma immersion ion implantation; Plasma nitriding
1. Introduction
Ti-6A1-4V alloys have many desirable features, such as biocompatibility, high corrosion resistance, excellent strength-to-weight ratio, and so on. They have thus been widely used in aerospace and orthopedic applications. Their high corrosion resistance can be attributed to a passivation layer on the surface of these alloys. However, it can be easily worn out and wear-corrosion failure can occur by electric cell reactions between the passivated and non-passivated layer. It has been shown that nitrogen ion implantation is an effective means to improve the wear and corrosion resistance of Ti-6A1-4V alloys by forming a TiN superhard layer on the surface [1]. As the line-of-sight restriction is circumvented in plasma immersion ion implantation (PIII), nitrogen PIII is thus an excellent way to treat complex-shape Ti-6AI-4V components [2-10]. In general, the thickness of the surface layer formed by conventional vacuum discharge plasma processes is only 100-200 nm and may not be enough to withstand the harsh working conditions encountered in many industrial applications. In order to increase the thickness of the layer to further improve * Correspondingauthor. Tel.: (852)-2788-7724; fax: (852)-2788-7830; e-mail: appkchu@cit!al.edn.hk 0257-8972/97/$17.00 © I997 ElsevierScienceS.A. All rights reserved. PII S0257-8972 (97) 00066-2
the wear and corrosion resistance, we propose to use radio-frequency ( R F ) plasma nitriding and nitrogen PIII to treat Ti-6A1-4V alloys. The properties and characteristics of the processed samples are assessed using micro-hardness testing, pin-on-disk wear measurements, scanning electron microscopy (SEM), and Auger electron spectroscopy (AES).
2. Experimental The experiments were conducted in a custom designed plasma immersion ion implanter with an inductively coupled RF plasma source [11]. The diameter and height of the main vacuum chamber are 100 and 120 cm, respectively. A 13.56 MHz, 2 kW RF plasma source is positioned on top of the main vacuum chamber to produce RF plasmas which diffuse from the pyrex glass discharge chamber into the main chamber. The full cusp magnetic field in the main chamber confines the plasma electrons to further boost the plasma density and enhance theuniformity. To accomplish nitrogen nitriding and PIII, a series of negative high voltage pulses We~:e applied to the Ti-6A1-4V alloy samples after a
X Y. l~ang et aL / Surface and Coatings Technology 93 (1997) 309-313
310 535
530 > -r'
J
J
525
520
515
510
300
350
400
450
RF input power, W Fig. t. Microhardness of the implanted sample after processing at various RF input power.
stable nitrogen plasma had been sustained. The implantation parameters were: • pulse width = 15 gs • pulse repetition rate = 100 Hz • working gas pressure-- 1 Pa • implantation voltage = 30 kV • implantation dose from 5x1016 to 5x 10 iv atoms cm -z The sample temperature during implantation could not be measured accurately by our pyrometer as it was <100°C. For the RF source, the optimal working parameters were: 400 W input power and 1 Pa pressure. Under these conditions, the plasma density was
2.87 x 10 l° cm -3, and the radial distribution uniformity of plasma density was better than 97% throughout a circular zone 60 cm in diameter. The composition of the Ti-6At-4V alloy samples by weight is: AI, 6.32%; V, 4.41%; Fe, 0.14%; O, 0.16%; N, 0.01%; C, 0.04%; H, 0.04%.
3. Results and discussion
The properties investigated are micro-hardness, mass loss due to wear, and coefficient of friction. The microhardness of the untreated sample is 304 HV and the
600
500
> 400 2: 8 {
300
25KeV
--i_30KeV .~5--35KeV
o 200
100
0 I i : ~ I ! [ 0.00E+ 5.00E+ 1.00E+ 1.50E+ 2.00E+ 2.50E+ 3.00E+ 3.50E+ 4.00E+ 4.50E+ 5.00E+ 00 16 17 17 17 17 17 17 17 17 17
Implanted dose,ion~cm2 Fig. 2. Microhardness of the implanted sample versus implantation dose for three implantation energies: 25 keV (v), 30 keV (n) and 35 keV (A). Applied load: 20 gf,
S.Y. Wang et aI. / Surface and Coatings Technology 93 [1997) 309-313
311
3.5 3 2.5 -
2
..
o
35KEY
- - I - - 30KEY 1.5
~
25KEY
1 0.5 0
i 0
5E+16
1E+17 1.5E+17 2E+17 2.5E+17 3E+17 3.5E+17 4E+17 4.5E+17 5E+17
Implanted dose, ionNcm2
Fig. 3. Mass loss due to wear for the implanted sample versus implantation dose for three implantation energies: 35 keV (v), 30 keV (11) and 25 keV (/~ ). mass loss is 5.7 mg after a 125.66m wear route. The measured micro-hardness of the treated samples versus the applied R F input power ranging from 300 to 450 W is plotted in Fig. 1 (load--20 g). The enhanced microhardness at a higher R F power is probably due to the higher plasma density. The resulting higher ion flux is believed to elevate the sample temperature thereby causing further penetration of nitrogen, even though the absolute temperature was still too low to be accurately measured using our equipment. The micro-hardness of the implanted samples as a function of the implantation dose and energy is displayed in Fig. 2. It can be observed that after implantation of a nitrogen dose of
4 x 1017 ions cm -2, the micro-hardness of the implanted sample is enhanced by 70%. The relationship between the associated mass loss and the implantation energy and dose is indicated in Fig. 3 which shows a monotonic decrease in the mass loss with increasing implantation dose up to the dose of 3 x 101~ ions cm -2 followed by a slight increase if the dose exceeds 3 x 1027 ions cm -2. This behaviour valid for 35 keV is less pronounced for the implantation energy of 25 keV. The enhanced wear at high doses can be explained by the assumption that the longer implantation time needed to achieve the higher dose leads to a higher temperature of the implanted samples, resulting in surface stress release and
0.7
0.6
0.5
O
0.4
Q
0 0 tO
0.3
o
y
..J
Implanted Unimplanted
X
0.2
0.1
200
400
600
800
1000
1200
1400
1600
Cycle numbers, r
Fig. 4. Friction coefficientof the unimplanted (11) and impianted samples (o) versus cycle numbers.
312
S. !/.. Wang et al. / Surface and Coatings Technology 93 (1997) 309-313
i-, Jg.'-"t
5e~8~3
tS,eu.
~eKV
[b]
(a)
Fig. 5. SEM micrographsof (a) untreated sample and (b) implantedsample.
grain growth. Consequently, the wear resistance of the samples is decreased. Fig. 4 depicts the dependence of the friction coefficient on the rotation cycle number. It can be seen that the friction coefficient rises after 240 cycles to 0.7 for the untreated sample but stays fairly constant at a value of 0.16 for the treated one. If the modified layer is worn away (after 1500 cycles), the coefficient of friction of the implanted sample increases to that of the untreated sample (Fig. 4). The TiO2 and TiN=O~, compounds formed on the surface of the samples by PIII contribute to the good wear resistance and low
friction coefficient, and N + implantation helps to stabilize the superficial layer. The surface morphology of samples was investigated by SEM. The SEM micrograph of the untreated sample shown in Fig. 5a reveals the typical microstructure consisting of c~ phase and fl phase. The c~ phase exists as small and round grains whereas the/? phase is composed of banding grains among the phases. The/~ grains have a larger size but a lower density. The dimension of the grains varies from 10 gm to several tens of gm. The SEM photo of the implanted sample (35keV,
70 6O
50
4o
---II.-- N .-Ii-- 0
3o
._~5.._ -I1 m m
20 A
10
J 500
1000 1500 Etching depth, A
2000
Fig. 6. Auger depth profiles of Ti, N and O in the implanted sample.
2500
S. Y. Wang et al. / Surface and Coatings Technology93 (1997) 309-313
3 x 1017 cm -2) is shown in Fig. 5b. It can be seen that the grains of the implanted sample are small and more uniform, and the/3 phase has disappeared. A new and smaller phase has formed among the c~ phase grains. The dimension of the grains is only several tens of rim. The larger grains shown in Fig. 5b were characterized by AES and identified to be the c~phase of Ti-6At-4V deduced from the composition. The phases of the implanted layer produced by the PIII process were investigated using X-ray photoelectron spectroscopy (XPS) and the presence of the TiN phase is confirmed. The Ti, O and N depth profiles obtained by AES are displayed in Fig. 6. From the nitrogen in-depth distribution, it can be inferred that the thickness of the modified layer is >210 nm.
4. Conclusion Using our new inductively coupled plasma source, RF plasma nitriding and nitrogen PIII have been successfully employed to enhance the wear resistance of Ti-6AI-4V alloy samples. Even at a relatively low implantation voltage of 30 kV, the treated samples show an increase of surface micro-hardness and improved tribological properties. The thickness of the hardened surface layer is measured by AES to be over 210 nm and larger than that attained by conventional vacuum discharge plasma processing.
313
Acknowledgement This research Work was supported by the City University of Hong Kong (Contract 7000621) and the Hong Kong Research Grants Council (Contracts 8730005 and 9040220).
References [I] B.L. Garside, Mater. Sci. Engng, A13(11) (1991) 179. [2] A. Chen, J.T. Scheure, C. Ritter, R.B. Alexander and J.R. Conrad, J. Appl. Phys., 70 (i991) 6757. [3] B.Y. Tang, Physics, 23 (1994) 42. [4] J.R. Conrad, Mater. Sci. Engng, Al16 (I989) 201. [5] J.R. Conrad, S. Baumann, R. Fleming and G.P. Meeker, 3". Appl. Phys., 65 (1989) 1701. [6] J.R. Conrad, R.A. Dodd, S. Han, M. Madapura, J.T. Scheure, K. Saidaram and F.J. Worzala, J. Vac. Sci. Technol., A8 (1992) 3146. [7] J.R. Conrad, J.L. Radtke, R.A. Dodd, F.J. Worzala and N.E. Tran, Y. Appl. Phys., 62 (1987) 4591. [8] J.R. Conrad, R.A. Dodd, F.J. Worzala and X. Qiu, S w f Coat. Technol., 36 (1986) 927. [9] L. Xie, F.J. Worzala, J:R. Conrad, R.A, Dodd and K. Sridharan, Mater. Sci. Engng, A139 (I99I) 179. [I0] J, Chen, J. Blanchard, J.R. Conrad and R.A. Dodd, Surf Coat. Technol., 52 (I992) 267. [ 11] P.K. Chu, B.Y. Tang, Y.C, Cheng and P.K, Ko, Proceedings Third bzternatiotml HZorkshop on Plasma-ba4ed lon h~plantation, Dresden, Germany, I-3 (1996).