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Surface modification of Ti6Al4V alloy by PIII at high temperatures: Effects of plasma potential
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 257 (2007) 722–726 www.elsevier.com/locate/nimb
Surface modification of Ti6Al4V alloy by PIII at high temperatures: Effects of plasma potential M.M. Silva a, M. Ueda a
b,*
, L. Pichon c, H. Reuther d, C.M. Lepienski
e
Divisa˜o de Engenharia Mecaˆnica-Aerona´utica, Instituto Tecnolo´gico de Aerona´utica, S.J. Campos, SP, Brazil b Laborato´rio Associado de Plasma, Instituto Nacional de Pesquisas, S.J. Campos, SP, Brazil c Laboratoire de Me´tallurgie Physique, UMR 6630-CNRS, Universite´ de Poitiers, France d Institute of Ion Beam Physics and Materials Research, Center Rossendorf, Dresden, Germany e Departamento de Fı´sica Universidade Federal do Parana´, Curitiba, Brazil Available online 25 January 2007
Abstract The present work is aimed to analyzing the influence of the plasma potential in the efficiency of plasma immersion ion implantation (PIII) process with nitrogen, at high temperatures (550 C and 800 C), applied to the Ti6Al4V alloy to increase its wear resistance. Treatments with plasma potentials (PP) at 420 V and 90 V were carried out. In the first case, in accordance with AES (Auger Electron Spectroscopy) analysis, nitrogen rich layers of 100 nm and 150 nm of thickness had been obtained, for total treatment times of 60 min and 120 min, respectively. For the treatments with lower PP of 90 V, the treated layers thicknesses have been measured by GDOS (Glow Discharge Optical Spectroscopy) and their values are 1 lm and 1.5 lm for treatments of 120 min and 240 min, respectively. The hardness values were determined for the samples treated with high PP by nanoindentation technique and a significant increase was observed for this treatment, reaching 11 GPa (60 min) and 19 GPa (120 min), which can be compared to 3.5–4.0 GPa obtained for the untreated samples. Pin-on-disk wear tests show that wear resistance increases after all these treatments. The friction coefficient as well as the wear rates are measured with a tribometer. 2007 Elsevier B.V. All rights reserved. Keywords: Ti6Al4V; Plasma immersion ion implantation; Surface modification; Nitrogen; High temperature treatment; Plasma potential
1. Introduction Because of its excellent combination of mechanical, toughness, corrosion resistance and chemical stability properties, Ti6Al4V alloy is one of the mostly used titanium alloys in aeronautical and biomedical applications [1,2]. Its excellent corrosion resistance is attributed to the formation of a passive titanium oxide film. This protection layer, formed by the contact of metal surface with air, avoids further oxidation of the bulk, thus providing alloy passivity and biocompatibility. However, Ti6Al4V alloy *
Corresponding author. Address: National Institute for Space Research, Av. dos Astronautas 1758, Jd. Granja, CEP 12227-010, Sa˜o Jose´ dos Campos, Sa˜o Paulo, Brazil. Tel.: +55 12 39456715; fax: +55 12 39456710. E-mail address: [email protected] (M. Ueda). 0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.135
presents inadequate tribological properties. Excluding the wear problem, this titanium alloy possesses excellent corrosion resistance in many environments including saline solutions, similar to corporeal fluids. However, problems arise when localized wear starts, causing intense corrosion. To improve corrosion, wear resistance and fatigue strength, surface treatments such as ionic implantation were previously reported [3,4]. The present work aims to investigate the improvement of Ti6Al4V alloy surface properties through the plasma assisted process of plasma immersion ion implantation (PIII) at high temperature and to verify the plasma potential influence in the surface treatment of this alloy. The thicker implanted layers can be achieved by controlling plasma potential and the treatment duration. The PIII treatment of Ti6Al4V has been shown to produce incorporation of nitrogen in its surface for all treatment temperatures or
M.M. Silva et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 722–726
without heating during the processes, but one can verify in various previous works that its thickness remains very small and a little hardness increase are achieved in the cases of a treatment without heating [4–8], or those carried out at temperatures below 500 C [9]. An increase in treatment temperature results in a thickness increase in the modified layer and in its surface hardness. The diffusion of nitrogen in titanium is known to be effective at temperatures higher than 500 C. At 800 C, the PIII process shows much effective results, regarding deeper nitrogen penetration, because in this temperature the diffusion rate becomes much higher [9,10]. Based on our previous work [4,8] and in the good results obtained in high temperatures [9–11], we are verifying the alloy behavior in 550 C and 800 C, and also the influence of plasma potential in the quality of layer obtained. So, in the present work, we studied the effects of low and high PP, with the objective to find the best high temperature PIII condition. The specimens are implanted in a reactor where only the sample holder is heated, according to the scheme developed in LAP/INPE that will be described in Section 2. 2. Experimental procedures The experiments described here were carried out at LAP (Laborato´rio Associado de Plasma) of INPE (Instituto Nacional de Pesquisas Espaciais). The samples have been prepared as disks of 15 mm in diameter by 1.5 mm in thickness, and then polished to mirror-like surface. They were mounted in a sample holder, inside of which was incorporated a heating system made of tungsten filaments passing through a perforated stainless steel bar of circular cross section, isolated by ceramic tubes. Then the sample holder is taken into the reactor chamber where it is heated using a Variac (220 V, 10 A) connected to the system, according to the scheme presented in Fig. 1. The plasma chamber is initially pumped down to 3 · 105 mbar, then the Variac is turned on until the desirable temperature is reached. Next, the nitrogen is introduced to produce the plasma by a glow
Fig. 1. Details of the sample holder with heated tungsten filament inside.
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discharge and the RUP-4 pulser is turned on and the treatments are carried out. The experimental conditions for the PIII treatments at high temperatures are presented in Table 1. The sample cooling down is performed inside the vacuum chamber, until reaching approximately the ambient temperature. In our PIII system we utilize a glow discharge plasma as the source of ions and normally the plasma potential is high. Injection of electrons from heated tungsten filament is carried out to control this plasma potential. When the treatment occurs with electron shower hot filament off, the plasma potential in the support is high, around 420 V. However, when the filament is turned on (Fig. 1), the potential decreases, and it was set down to about 90 V. The samples were then treated in the both conditions of high and low potentials. The treated surface hardness was measured by nanoindentation technique, conducted with a Triboscope nanomechanical indentation tester from Hysitron (UFPR), by the Bas Maur 2000 method, with indenter Berko 3. Each impression was made by eight successive increasing loadings of 0.315 g, 0.625 g, 1.25 g, 2.5 g, 5 g, 10 g, 20 g and 40 g. The final hardness value is the average of 9 indentation for each sample. The atomic composition profiles were obtained by Auger electron spectroscopy (AES) to find out the nitrogen concentration inside the samples treated at high PP (IIBPMR) and glow discharge optical spectroscopy (GDOS) to find out the nitrogen concentration inside the samples treated at low PP. And the friction coefficient as well as the wear are being measured with a computer controlled, CSMInstruments Pin-on-disk Tribometer, model SN 18-313. Roughness was measured by scanning probe microscope (SPM), SPM-9500, from Shimadzu. The temperature of the sample holder was monitored by an infrared pyrometer from MIKRON, model M90-Q. 3. Results and discussion Fig. 2 presents the AES results that show the atomic composition profiles for PIII treatments for 550 C and 800 C for the case high PP. The treated layer thickness increases with the treatment temperature, in general. For 550 C and high PP treatment, the nitrogen rich layer reaches approximately 40 nm, with 26% of maximum nitrogen atomic concentration. And as expected, the process becomes more efficient with the temperature increase to 800 C, when the nitrogen was implanted for up to approximately 60 nm from the surface and its maximum atomic concentration reached 25%, for treatment of 60 min. When the treatment time was increased to 120 min, the layer depth reached about 110 nm. Fig. 3 presents the GDOS result for 550 C (1 h) and 800 C (120 min and 240 min) and low PP (90 V) treatments. Here the nitrogen rich layer reaches less than 0.1 lm (100 nm) for 550 C, 1.0 lm for sample treated at 800 C for 120 min and 2.5 lm for 240 min, respectively.
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M.M. Silva et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 722–726
Table 1 Experimental conditions used Sample #
1
2
3
4
5
6
7
8
Temperature (C) Plasma potential (V) Duration (min) High voltage (kV) Pressure (mbar) Frequency (Hz) Pulse (ls)
550 420 60 5 6 · 103 400 40
550 90 60 5 6 · 103 400 40
800 420 60 5 6 · 103 400 40
800 90 120 5 6 · 103 400 40
800 90 150 5 6 · 103 400 40
800 90 240 5 6 · 103 400 40
800 420 120 5 6 · 103 400 40
800 90 60 5 6 · 103 400 40
30
# 1 - 550°C - 60 min - high PP # 3 - 800°C - 60 min - high PP # 7 - 800°C - 120 min - high PP
20
15
10
5
0 0
50
100
150
200
250
300
Depth (nm)
Fig. 2. AES analysis results: atomic concentration of nitrogen for samples treated at high PP at 550 C during 60 min, 800 C during 60 min, 800 C during 120 min.
45 40
Concentration At (%)
35 30 25
22
20
# 6 - 800°C - 240 min 20
15
# 4 - 800°C - 120 min
10
Reference #1 - 550°C - 60 min - high PP #3 - 800°C - 60 min - high PP #7 - 800°C - 120 min - high PP
18
# 2 - 500°C - 60 min
5
16
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Depth (μm)
Fig. 3. GDOS analysis results: atomic concentration of nitrogen for samples treated at low PP at 550 C during 60 min, 800 C during 120 min, 800 C during 240 min.
Hardness (GPa)
Concentration (At.-%)
25
greater depths in the treatment with low PP, because the plasma density is higher in this case. Ueda et al. [12] carried out a study of the effects of the nitrogen plasma etching during PIII process using Si wafers and they had verified that, when the plasma potential is low, the ion implantation becomes dominant whereas if the plasma potential is high, the etching is very large. The etching occurs even though the temperature is high, where the diffusion rate is appreciable, because the sputtering process removes an amount of ions of nitrogen that would have diffused into the material. Fig. 4 presents the hardness profiles obtained by nanoindentation for some of the samples subjected to the treatments above. The untreated sample (reference), presents values of hardness varying from 4.0 GPa to 5.5 GPa. For 550 C case with high PP, we can see that the hardness reaches 11.5 GPa, which amounts to enhancement of about 110%. Meanwhile, for PIII at 800 C with high PP and for 60 min treatment, we find that the hardness of the sample is 14 GPa at maximum, reaching an enhancement of 180% over the untreated one and, when the treatment time is increased to 120 min, the hardness values increased to 19 GPa, i.e. 280%. Measurements of hardness for PIII at low PP, in high temperature of 800 C, are under way. In this profile the hardness value remains high till deeper layers than the layer implanted with nitrogen (measured by
14 12 10 8 6 4
It should be reminded that the spatial resolution of the used GDOS system is of the order of 0.1 lm, therefore the result obtained for 550 C by GDOS is consistent with the results obtained by AES. The nitrogen atoms can reach
0
250
500
750
1000
1250
1500
1750
2000
Displacement into surface (nm)
Fig. 4. Surface hardness (nanoindentation) of the samples treated at high PP at 550 C during 60 min, 800 C during 60 min, 800 C during 120 min.
M.M. Silva et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 722–726
AES). This occurs due to the ‘‘long-range-effect’’ caused by a change in the defect structure and physico-mechanical properties, caused by ion treatment of metallic materials and semiconductors. This effect consists of the formation of a defect structure in the near-surface layer of the irradiated target whose depth is essentially higher than that of the surface layer doped by ion implantation. Sharkeev et al. [13] presented a review including results of many papers and made a deep study of this phenomenon. The samples #2, 3 and 4 was analyzed by SPM to determine their roughness. The data was obtained in an area of 1.0 · 1.0 lm. The results are shown in Table 2. Rms represents the distance of the deepest valley point and the average line. Ry is the vertical distance between the highest peak and the deepest valley points. Fig. 5 presents the results of pin-on-disk wear tests for untreated and treated specimens at 800 C, with low and high PP, #4 and #5 samples, respectively. The untreated sample (reference), presents values of friction coefficient (l) around 0.30–0.40. The friction coefficient of the sample #5 treated during 150 min, high PP, is around 0.25 until around 1500 cycles, decreasing quickly afterwards. The sample #4 treated with low PP at 800 C, during 120 min presents a higher coefficient than the other two samples, reaching value of 0.65 in the first 1000 cycles, afterwards decreasing gradually reaching 0.12 at 10,000 cycles. However, the fretting wear obtained in the sample #4 was as small as that in the sample #5. Probably this occurs, because the roughness is very large in the specimen #4,
Table 3 Pin-on-disk conditions and results Sample
R (mm)
r (mm)
d (mm)
Disk volume loss, V (mm3)
Wear rate, k (mm3/Nm)
Reference #4 #5
3.0 3.0 3.0
3.0 3.0 3.0
0.82 0.30 –
1310.49 478.43 –
6.50 2.37 –
and therefore the friction coefficient is also large. As the number of cycles increases, we can notice that the friction coefficient decreases rapidly. The same phenomenon can be seen in the sample #5 for which the friction coefficient values are greater in the beginning of the test, decreasing after some cycles, and reaching very small values (below error bars in the measurements). We can observe from these tests that, the bigger the roughness, i.e. Rms and Ry, the greater friction coefficient in the initial cycles which decreases quickly to values much smaller than for the reference samples. The volume loss is calculated by the Eq. (3.1), that is used when there is no significant pin wear [14]. The used pin is made of stainless steel 420. The disk volume loss is d d 1 2 1=2 2 2 V ¼ 2pR r sin ð4r d Þ ; ð3:1Þ 2r 4 where R, wear track radius; r, pin end radius and d, wear track width. The wear rate (k) is given by the Eq. (3.2): k¼
Table 2 SPM results to area of 1.0 · 1.0 lm Sample
Rms (nm)
Ry (nm)
#8 #3 #4
3.3 1.6 4.8
24.7 12.3 41.5
#4 - 800°C-120 min - low PP
Coefficient of friction, μ
0.5 0.4
V ; NL
ð3:2Þ
where N, sliding distance; V, disk volume loss and L, applied load. The conditions utilized in this test and the result of volume loss is presented in Table 3. For the reference sample, the volume loss is 1310.49 mm3 of material and its wear rate is around 6.5 mm3/Nm. After PIII treatment, at 800 C during 150 min with high PP (#4), the volume loss decreases to 478.43 mm3, with wear rate of 2.37 mm3/Nm. When the sample is treated at the same conditions of temperature, with low PP, during 120 min (#5), the wear is much smaller than that observed in the sample #4. Also, the wear track width is thin and not uniform, being difficult to carry out the measurement, therefore, it is difficult to calculate its volume loss and as a consequence the wear rate. For this reason the data are not presented.
0.7 0.6
725
reference
0.3 0.2
4. Conclusions
0.1
#5 - 800°C - 150 min high PP
0.0
0
2000
4000
6000
8000
10000
Cicles
Fig. 5. Pin-on-disk wear tests for samples treated at 800 C, with high and low PP, including reference sample.
The improvements of surface properties achieved by PIII nitrogen implantation in Ti6Al4V sample at high temperatures are confirmed by an increase of surface hardness, due to a significant enrichment of nitrogen in the structure, as verified by the elemental profiles measured by AES and GDOS techniques. The process becomes more efficient with the temperature increase, as the experimental results
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M.M. Silva et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 722–726
show, in which at 800 C the diffusion process is substantially greater than at 550 C. The hardness increases with the temperature and the treatment duration, reaching an enhancement of 110% in the sample #1, 180% in the sample #3 and 280% in the sample #7, when compared to the reference sample. When plasma potential decreases, i.e. at low PP, the sputtering of the layer with implanted ions is reduced and therefore fewer atoms are removed from the surface before they diffuse to deeper layers, when compared with the case with high PP. Thus the final layer obtained by the process with low PP is much thicker than in the treatment with high PP. We could confirm this result by comparing the layers thicknesses measured by GDOS and AES techniques for both conditions. The wear resistance is improved, because the fretting wear decreases for both conditions of high and low PP, even though the friction coefficient are greater in the first cycles of the pin-on-disk tests. The wear rate for the sample #4 treated at 800 C, high PP is much lower than for the reference sample and visually the sample #5, treated with low PP, were even less than the sample #4. However, it was not calculated because it was not possible to measure the wear track width satisfactorily. These tests confirm the improvement of the surface mechanical properties of the alloy treated by both processes (high and low PP), however with deeper layers and higher wear resistance, when treated with low PP.
Acknowledgments This project is partially sponsored by the FAPESP and MCT. References [1] M.A. Khan, R.L. Williams, D.F. Williams, Biomaterials 20 (1999) 631. [2] Z. Cai et al., Biomaterials 20 (1999) 183. [3] D. Muster et al., MRS Bull. (2000) 25. [4] M. Ueda, M.M. Silva, C. Otani, H. Reuther, M. Yatsuzuka, C.M. Lepienski, L.A. Berni, Surf. Coat. Technol. 169–170 (2003) 408. [5] K. Volz et al., Surf. Coat. Technol. 103–104 (1998) 257. [6] F. Sidel, H.R. Stock, P. Mayr, Surf. Coat. Technol. 98 (1998) 1174. [7] B.Y. Tang et al., Surf. Coat. Technol. 103–104 (1998) 248. [8] M.M. Silva, M. Ueda, C. Otani, H. Reuther, C.M. Lepienski, P.C. Soares Jr., J. Otubo, Mater. Res. 9 (1) (2006) 97. [9] V. Fouquet, L. Pichon, A. Straboni, M. Drouet, Surf. Coat. Technol. 186 (2004) 34. [10] M. Ueda, M.M. Silva, C.M. Lepienski, P.C. Soares Jr., J.A.N. Gonc¸alves, H. Reuther, Surf. Coat. Technol., in press. [11] L. Marot, M. Drouet, F. Berneau, A. Straboni, Surf. Coat. Technol. 156 (2002) 155. [12] M. Ueda, L.A. Berni, G.F. Gomes, et al., J. Appl. Phys. 86 (1999) 9. [13] Yu.P. Sharkeev, A.N. Didenko, E.V. Kozlov, Surf. Coat. Technol. 65 (1994) 112. [14] ASTM Standards, G99-95a Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus.
Nuclear Instruments and Methods in Physics Research B 257 (2007) 722–726 www.elsevier.com/locate/nimb
Surface modification of Ti6Al4V alloy by PIII at high temperatures: Effects of plasma potential M.M. Silva a, M. Ueda a
b,*
, L. Pichon c, H. Reuther d, C.M. Lepienski
e
Divisa˜o de Engenharia Mecaˆnica-Aerona´utica, Instituto Tecnolo´gico de Aerona´utica, S.J. Campos, SP, Brazil b Laborato´rio Associado de Plasma, Instituto Nacional de Pesquisas, S.J. Campos, SP, Brazil c Laboratoire de Me´tallurgie Physique, UMR 6630-CNRS, Universite´ de Poitiers, France d Institute of Ion Beam Physics and Materials Research, Center Rossendorf, Dresden, Germany e Departamento de Fı´sica Universidade Federal do Parana´, Curitiba, Brazil Available online 25 January 2007
Abstract The present work is aimed to analyzing the influence of the plasma potential in the efficiency of plasma immersion ion implantation (PIII) process with nitrogen, at high temperatures (550 C and 800 C), applied to the Ti6Al4V alloy to increase its wear resistance. Treatments with plasma potentials (PP) at 420 V and 90 V were carried out. In the first case, in accordance with AES (Auger Electron Spectroscopy) analysis, nitrogen rich layers of 100 nm and 150 nm of thickness had been obtained, for total treatment times of 60 min and 120 min, respectively. For the treatments with lower PP of 90 V, the treated layers thicknesses have been measured by GDOS (Glow Discharge Optical Spectroscopy) and their values are 1 lm and 1.5 lm for treatments of 120 min and 240 min, respectively. The hardness values were determined for the samples treated with high PP by nanoindentation technique and a significant increase was observed for this treatment, reaching 11 GPa (60 min) and 19 GPa (120 min), which can be compared to 3.5–4.0 GPa obtained for the untreated samples. Pin-on-disk wear tests show that wear resistance increases after all these treatments. The friction coefficient as well as the wear rates are measured with a tribometer. 2007 Elsevier B.V. All rights reserved. Keywords: Ti6Al4V; Plasma immersion ion implantation; Surface modification; Nitrogen; High temperature treatment; Plasma potential
1. Introduction Because of its excellent combination of mechanical, toughness, corrosion resistance and chemical stability properties, Ti6Al4V alloy is one of the mostly used titanium alloys in aeronautical and biomedical applications [1,2]. Its excellent corrosion resistance is attributed to the formation of a passive titanium oxide film. This protection layer, formed by the contact of metal surface with air, avoids further oxidation of the bulk, thus providing alloy passivity and biocompatibility. However, Ti6Al4V alloy *
Corresponding author. Address: National Institute for Space Research, Av. dos Astronautas 1758, Jd. Granja, CEP 12227-010, Sa˜o Jose´ dos Campos, Sa˜o Paulo, Brazil. Tel.: +55 12 39456715; fax: +55 12 39456710. E-mail address: [email protected] (M. Ueda). 0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.135
presents inadequate tribological properties. Excluding the wear problem, this titanium alloy possesses excellent corrosion resistance in many environments including saline solutions, similar to corporeal fluids. However, problems arise when localized wear starts, causing intense corrosion. To improve corrosion, wear resistance and fatigue strength, surface treatments such as ionic implantation were previously reported [3,4]. The present work aims to investigate the improvement of Ti6Al4V alloy surface properties through the plasma assisted process of plasma immersion ion implantation (PIII) at high temperature and to verify the plasma potential influence in the surface treatment of this alloy. The thicker implanted layers can be achieved by controlling plasma potential and the treatment duration. The PIII treatment of Ti6Al4V has been shown to produce incorporation of nitrogen in its surface for all treatment temperatures or
M.M. Silva et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 722–726
without heating during the processes, but one can verify in various previous works that its thickness remains very small and a little hardness increase are achieved in the cases of a treatment without heating [4–8], or those carried out at temperatures below 500 C [9]. An increase in treatment temperature results in a thickness increase in the modified layer and in its surface hardness. The diffusion of nitrogen in titanium is known to be effective at temperatures higher than 500 C. At 800 C, the PIII process shows much effective results, regarding deeper nitrogen penetration, because in this temperature the diffusion rate becomes much higher [9,10]. Based on our previous work [4,8] and in the good results obtained in high temperatures [9–11], we are verifying the alloy behavior in 550 C and 800 C, and also the influence of plasma potential in the quality of layer obtained. So, in the present work, we studied the effects of low and high PP, with the objective to find the best high temperature PIII condition. The specimens are implanted in a reactor where only the sample holder is heated, according to the scheme developed in LAP/INPE that will be described in Section 2. 2. Experimental procedures The experiments described here were carried out at LAP (Laborato´rio Associado de Plasma) of INPE (Instituto Nacional de Pesquisas Espaciais). The samples have been prepared as disks of 15 mm in diameter by 1.5 mm in thickness, and then polished to mirror-like surface. They were mounted in a sample holder, inside of which was incorporated a heating system made of tungsten filaments passing through a perforated stainless steel bar of circular cross section, isolated by ceramic tubes. Then the sample holder is taken into the reactor chamber where it is heated using a Variac (220 V, 10 A) connected to the system, according to the scheme presented in Fig. 1. The plasma chamber is initially pumped down to 3 · 105 mbar, then the Variac is turned on until the desirable temperature is reached. Next, the nitrogen is introduced to produce the plasma by a glow
Fig. 1. Details of the sample holder with heated tungsten filament inside.
723
discharge and the RUP-4 pulser is turned on and the treatments are carried out. The experimental conditions for the PIII treatments at high temperatures are presented in Table 1. The sample cooling down is performed inside the vacuum chamber, until reaching approximately the ambient temperature. In our PIII system we utilize a glow discharge plasma as the source of ions and normally the plasma potential is high. Injection of electrons from heated tungsten filament is carried out to control this plasma potential. When the treatment occurs with electron shower hot filament off, the plasma potential in the support is high, around 420 V. However, when the filament is turned on (Fig. 1), the potential decreases, and it was set down to about 90 V. The samples were then treated in the both conditions of high and low potentials. The treated surface hardness was measured by nanoindentation technique, conducted with a Triboscope nanomechanical indentation tester from Hysitron (UFPR), by the Bas Maur 2000 method, with indenter Berko 3. Each impression was made by eight successive increasing loadings of 0.315 g, 0.625 g, 1.25 g, 2.5 g, 5 g, 10 g, 20 g and 40 g. The final hardness value is the average of 9 indentation for each sample. The atomic composition profiles were obtained by Auger electron spectroscopy (AES) to find out the nitrogen concentration inside the samples treated at high PP (IIBPMR) and glow discharge optical spectroscopy (GDOS) to find out the nitrogen concentration inside the samples treated at low PP. And the friction coefficient as well as the wear are being measured with a computer controlled, CSMInstruments Pin-on-disk Tribometer, model SN 18-313. Roughness was measured by scanning probe microscope (SPM), SPM-9500, from Shimadzu. The temperature of the sample holder was monitored by an infrared pyrometer from MIKRON, model M90-Q. 3. Results and discussion Fig. 2 presents the AES results that show the atomic composition profiles for PIII treatments for 550 C and 800 C for the case high PP. The treated layer thickness increases with the treatment temperature, in general. For 550 C and high PP treatment, the nitrogen rich layer reaches approximately 40 nm, with 26% of maximum nitrogen atomic concentration. And as expected, the process becomes more efficient with the temperature increase to 800 C, when the nitrogen was implanted for up to approximately 60 nm from the surface and its maximum atomic concentration reached 25%, for treatment of 60 min. When the treatment time was increased to 120 min, the layer depth reached about 110 nm. Fig. 3 presents the GDOS result for 550 C (1 h) and 800 C (120 min and 240 min) and low PP (90 V) treatments. Here the nitrogen rich layer reaches less than 0.1 lm (100 nm) for 550 C, 1.0 lm for sample treated at 800 C for 120 min and 2.5 lm for 240 min, respectively.
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M.M. Silva et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 722–726
Table 1 Experimental conditions used Sample #
1
2
3
4
5
6
7
8
Temperature (C) Plasma potential (V) Duration (min) High voltage (kV) Pressure (mbar) Frequency (Hz) Pulse (ls)
550 420 60 5 6 · 103 400 40
550 90 60 5 6 · 103 400 40
800 420 60 5 6 · 103 400 40
800 90 120 5 6 · 103 400 40
800 90 150 5 6 · 103 400 40
800 90 240 5 6 · 103 400 40
800 420 120 5 6 · 103 400 40
800 90 60 5 6 · 103 400 40
30
# 1 - 550°C - 60 min - high PP # 3 - 800°C - 60 min - high PP # 7 - 800°C - 120 min - high PP
20
15
10
5
0 0
50
100
150
200
250
300
Depth (nm)
Fig. 2. AES analysis results: atomic concentration of nitrogen for samples treated at high PP at 550 C during 60 min, 800 C during 60 min, 800 C during 120 min.
45 40
Concentration At (%)
35 30 25
22
20
# 6 - 800°C - 240 min 20
15
# 4 - 800°C - 120 min
10
Reference #1 - 550°C - 60 min - high PP #3 - 800°C - 60 min - high PP #7 - 800°C - 120 min - high PP
18
# 2 - 500°C - 60 min
5
16
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Depth (μm)
Fig. 3. GDOS analysis results: atomic concentration of nitrogen for samples treated at low PP at 550 C during 60 min, 800 C during 120 min, 800 C during 240 min.
Hardness (GPa)
Concentration (At.-%)
25
greater depths in the treatment with low PP, because the plasma density is higher in this case. Ueda et al. [12] carried out a study of the effects of the nitrogen plasma etching during PIII process using Si wafers and they had verified that, when the plasma potential is low, the ion implantation becomes dominant whereas if the plasma potential is high, the etching is very large. The etching occurs even though the temperature is high, where the diffusion rate is appreciable, because the sputtering process removes an amount of ions of nitrogen that would have diffused into the material. Fig. 4 presents the hardness profiles obtained by nanoindentation for some of the samples subjected to the treatments above. The untreated sample (reference), presents values of hardness varying from 4.0 GPa to 5.5 GPa. For 550 C case with high PP, we can see that the hardness reaches 11.5 GPa, which amounts to enhancement of about 110%. Meanwhile, for PIII at 800 C with high PP and for 60 min treatment, we find that the hardness of the sample is 14 GPa at maximum, reaching an enhancement of 180% over the untreated one and, when the treatment time is increased to 120 min, the hardness values increased to 19 GPa, i.e. 280%. Measurements of hardness for PIII at low PP, in high temperature of 800 C, are under way. In this profile the hardness value remains high till deeper layers than the layer implanted with nitrogen (measured by
14 12 10 8 6 4
It should be reminded that the spatial resolution of the used GDOS system is of the order of 0.1 lm, therefore the result obtained for 550 C by GDOS is consistent with the results obtained by AES. The nitrogen atoms can reach
0
250
500
750
1000
1250
1500
1750
2000
Displacement into surface (nm)
Fig. 4. Surface hardness (nanoindentation) of the samples treated at high PP at 550 C during 60 min, 800 C during 60 min, 800 C during 120 min.
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AES). This occurs due to the ‘‘long-range-effect’’ caused by a change in the defect structure and physico-mechanical properties, caused by ion treatment of metallic materials and semiconductors. This effect consists of the formation of a defect structure in the near-surface layer of the irradiated target whose depth is essentially higher than that of the surface layer doped by ion implantation. Sharkeev et al. [13] presented a review including results of many papers and made a deep study of this phenomenon. The samples #2, 3 and 4 was analyzed by SPM to determine their roughness. The data was obtained in an area of 1.0 · 1.0 lm. The results are shown in Table 2. Rms represents the distance of the deepest valley point and the average line. Ry is the vertical distance between the highest peak and the deepest valley points. Fig. 5 presents the results of pin-on-disk wear tests for untreated and treated specimens at 800 C, with low and high PP, #4 and #5 samples, respectively. The untreated sample (reference), presents values of friction coefficient (l) around 0.30–0.40. The friction coefficient of the sample #5 treated during 150 min, high PP, is around 0.25 until around 1500 cycles, decreasing quickly afterwards. The sample #4 treated with low PP at 800 C, during 120 min presents a higher coefficient than the other two samples, reaching value of 0.65 in the first 1000 cycles, afterwards decreasing gradually reaching 0.12 at 10,000 cycles. However, the fretting wear obtained in the sample #4 was as small as that in the sample #5. Probably this occurs, because the roughness is very large in the specimen #4,
Table 3 Pin-on-disk conditions and results Sample
R (mm)
r (mm)
d (mm)
Disk volume loss, V (mm3)
Wear rate, k (mm3/Nm)
Reference #4 #5
3.0 3.0 3.0
3.0 3.0 3.0
0.82 0.30 –
1310.49 478.43 –
6.50 2.37 –
and therefore the friction coefficient is also large. As the number of cycles increases, we can notice that the friction coefficient decreases rapidly. The same phenomenon can be seen in the sample #5 for which the friction coefficient values are greater in the beginning of the test, decreasing after some cycles, and reaching very small values (below error bars in the measurements). We can observe from these tests that, the bigger the roughness, i.e. Rms and Ry, the greater friction coefficient in the initial cycles which decreases quickly to values much smaller than for the reference samples. The volume loss is calculated by the Eq. (3.1), that is used when there is no significant pin wear [14]. The used pin is made of stainless steel 420. The disk volume loss is d d 1 2 1=2 2 2 V ¼ 2pR r sin ð4r d Þ ; ð3:1Þ 2r 4 where R, wear track radius; r, pin end radius and d, wear track width. The wear rate (k) is given by the Eq. (3.2): k¼
Table 2 SPM results to area of 1.0 · 1.0 lm Sample
Rms (nm)
Ry (nm)
#8 #3 #4
3.3 1.6 4.8
24.7 12.3 41.5
#4 - 800°C-120 min - low PP
Coefficient of friction, μ
0.5 0.4
V ; NL
ð3:2Þ
where N, sliding distance; V, disk volume loss and L, applied load. The conditions utilized in this test and the result of volume loss is presented in Table 3. For the reference sample, the volume loss is 1310.49 mm3 of material and its wear rate is around 6.5 mm3/Nm. After PIII treatment, at 800 C during 150 min with high PP (#4), the volume loss decreases to 478.43 mm3, with wear rate of 2.37 mm3/Nm. When the sample is treated at the same conditions of temperature, with low PP, during 120 min (#5), the wear is much smaller than that observed in the sample #4. Also, the wear track width is thin and not uniform, being difficult to carry out the measurement, therefore, it is difficult to calculate its volume loss and as a consequence the wear rate. For this reason the data are not presented.
0.7 0.6
725
reference
0.3 0.2
4. Conclusions
0.1
#5 - 800°C - 150 min high PP
0.0
0
2000
4000
6000
8000
10000
Cicles
Fig. 5. Pin-on-disk wear tests for samples treated at 800 C, with high and low PP, including reference sample.
The improvements of surface properties achieved by PIII nitrogen implantation in Ti6Al4V sample at high temperatures are confirmed by an increase of surface hardness, due to a significant enrichment of nitrogen in the structure, as verified by the elemental profiles measured by AES and GDOS techniques. The process becomes more efficient with the temperature increase, as the experimental results
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show, in which at 800 C the diffusion process is substantially greater than at 550 C. The hardness increases with the temperature and the treatment duration, reaching an enhancement of 110% in the sample #1, 180% in the sample #3 and 280% in the sample #7, when compared to the reference sample. When plasma potential decreases, i.e. at low PP, the sputtering of the layer with implanted ions is reduced and therefore fewer atoms are removed from the surface before they diffuse to deeper layers, when compared with the case with high PP. Thus the final layer obtained by the process with low PP is much thicker than in the treatment with high PP. We could confirm this result by comparing the layers thicknesses measured by GDOS and AES techniques for both conditions. The wear resistance is improved, because the fretting wear decreases for both conditions of high and low PP, even though the friction coefficient are greater in the first cycles of the pin-on-disk tests. The wear rate for the sample #4 treated at 800 C, high PP is much lower than for the reference sample and visually the sample #5, treated with low PP, were even less than the sample #4. However, it was not calculated because it was not possible to measure the wear track width satisfactorily. These tests confirm the improvement of the surface mechanical properties of the alloy treated by both processes (high and low PP), however with deeper layers and higher wear resistance, when treated with low PP.
Acknowledgments This project is partially sponsored by the FAPESP and MCT. References [1] M.A. Khan, R.L. Williams, D.F. Williams, Biomaterials 20 (1999) 631. [2] Z. Cai et al., Biomaterials 20 (1999) 183. [3] D. Muster et al., MRS Bull. (2000) 25. [4] M. Ueda, M.M. Silva, C. Otani, H. Reuther, M. Yatsuzuka, C.M. Lepienski, L.A. Berni, Surf. Coat. Technol. 169–170 (2003) 408. [5] K. Volz et al., Surf. Coat. Technol. 103–104 (1998) 257. [6] F. Sidel, H.R. Stock, P. Mayr, Surf. Coat. Technol. 98 (1998) 1174. [7] B.Y. Tang et al., Surf. Coat. Technol. 103–104 (1998) 248. [8] M.M. Silva, M. Ueda, C. Otani, H. Reuther, C.M. Lepienski, P.C. Soares Jr., J. Otubo, Mater. Res. 9 (1) (2006) 97. [9] V. Fouquet, L. Pichon, A. Straboni, M. Drouet, Surf. Coat. Technol. 186 (2004) 34. [10] M. Ueda, M.M. Silva, C.M. Lepienski, P.C. Soares Jr., J.A.N. Gonc¸alves, H. Reuther, Surf. Coat. Technol., in press. [11] L. Marot, M. Drouet, F. Berneau, A. Straboni, Surf. Coat. Technol. 156 (2002) 155. [12] M. Ueda, L.A. Berni, G.F. Gomes, et al., J. Appl. Phys. 86 (1999) 9. [13] Yu.P. Sharkeev, A.N. Didenko, E.V. Kozlov, Surf. Coat. Technol. 65 (1994) 112. [14] ASTM Standards, G99-95a Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus.