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Surface modification of stainless steel drills using plasma-immersion nitrogen ion implantation
ARTICLE IN PRESS
Vacuum 81 (2007) 1385–1388 www.elsevier.com/locate/vacuum
Surface modification of stainless steel drills using plasma-immersion nitrogen ion implantation R. Lo´pez-Callejasa,b,, A.E. Mun˜oz-Castroa, R. Valencia A.a, S.R. Barocioa, E.E. Granda-Gutie´rrezb, O.G. Godoy-Cabreraa,b, A. Mercado-Cabreraa, R. Pen˜a-Eguiluza, A. de la Piedad-Beneitezb a
Instituto Nacional de Investigaciones Nucleares, Plasma Physics Laboratory, AP 18-1027, 11801 Mexico, D.F., Mexico b Instituto Tecnolo´gico de Toluca, AP 890, Toluca, Mexico
Abstract Better protective and functional coatings on dental drills are increasingly demanded for their desired mechanical, tribological or chemical properties. We have studied the surface modification of steel dental drills, with an aim to increasing both their cutting life and corrosion resistance in dentistry using low-energy plasma-immersion nitrogen implantation over different temperature ranges. The temperature of the drills was controlled by varying the implantation voltage. Following implantation, tests showed that the microhardness of the nitrided drill surfaces was approximately one order higher than that of untreated drills and the corrosion tolerance was found to increase with growing temperature of the plasma-immersion process. r 2007 Elsevier Ltd. All rights reserved. Keywords: PIII; SEM; XRD; Knoop microhardness
1. Introduction The high processing efficiency of plasma-immersion ion implantation (PIII) has been shown to be particularly effective in optimizing the surface properties such as hardness, wear and corrosion resistance, of both basic materials and industrial components [1, 2]. One of the most attractive advantages of this technique is the capability of treating irregularly shaped objects without resorting to any target manipulation [3,4]. The plasma-based low-energy PIII process has more recently been applied under either low or elevated temperature conditions [5–7]. Parameters such as pulse duration, implantation voltage, plasma density, pulse frequency, etc. have been found to have considerable influence on the implanted dose and the ensuing target temperature. These parameters in turn, almost unequivocally determine the final properties of the treated surfaces. In the present study we assess the results of the Corresponding author. Fax: +52 5329 7301.
E-mail address: [email protected] (R. Lo´pez-Callejas). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.01.051
treatment of stainless steel drills, as regularly used in dentistry, using nitrogen based PIII with a view to increasing the tool resistance to wear and corrosive attack. 2. Experimental set-up A 35 l cylindrical vacuum chamber made of SS-304 steel, pumped down to 10 6 mbar base pressure, was used for the experiments on a general layout previously described [8]. A 1 10 4 mbar operational pressure was then attained by admitting nitrogen gas through a needle valve and voltage applied from a current controlled DC power supply allowing a plasma to be formed. Its typical main parameters, measured by double electric probes, were a density in the 1 1.5 1010 cm 3 range and an electron temperature within 1–2 eV. The temperature of the drill sample holder was measured with an electrically isolated thermocouple. Many of the dental drills available in Mexico fall into the 0.7 mm diameter three edge standard (see Fig. 1) with a 3.7 mm body length and a total size of 32 mm. Table 1 shows a typical chemical analysis of an AISI-316 stainless
ARTICLE IN PRESS R. Lo´pez-Callejas et al. / Vacuum 81 (2007) 1385–1388
1386
drills, as well as from drills implanted at three selected temperature values (350, 450 and 500 1C) are displayed in Fig. 2. At higher temperatures, the microhardness achieved is generally greater but with a tendency to decrease with the load increment. This fact suggests that nitrogen concentration is diminishing inwards the drill volume. The maximum Knoop microhardness of the gN phase is about nine times higher than that of the g phase of the austenitic stainless steel at 500 1C (see Table 1). The nitrided drills were sectioned in order to uncover their circular cross sections, to be analyzed by SEM following polishing. Table 1 shows the elements present on the sample surface, in at%, for the selected process temperatures. The nitrogen concentration is seen to increase with increasing temperature, although the implantation depth is greater in the medium (8 mm) and high temperature cases (12 mm) than the low temperature case
Fig. 1. Lateral drill view.
Table 1 Elemental quantitative chemical analysis of the SS-316 drills Element
Untreated
350 1C
450 1C
500 1C
at %
at%
at%
at%
1.78 1.52 18.54 1.77 12.09 64.3 100
16.04 1.41 1.39 16.37 1.50 10.68 52.6 100
17.11 1.67 1.04 16.06 1.49 10.21 53.42 100
21.75 1.76 1.45 14.66 1.35 9.59 49.44 100
20
steel dental drill using energy dispersive X-ray spectroscopy (EDS) on an EDAXTM system. This steel is an excellent material applicable in a wide range of atmospheric environments and many corrosive media. However, under conditions encountered in dentistry which can involve a hot chloride environment, it can be subject to pitting and crevice corrosion with stress corrosion cracking occurring at temperatures above some 600 1C [9]. The drills under treatment were biased by means of a modulator capable of a 5 kV maximum implantation voltage, with a variable 5–100 ms pulse intended to maintain the sample temperature at 350, 450 and 500 1C at a repetition rate of 1 kHz. With these parameters the implantation dose lies between 1016 to 1017 ions/cm2. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and Knoop microhardness measurements were used to characterize the modified surface after implantation, while the corrosion resistance was determined by the potentiodynamic polarization method.
Microhardness [GPa]
18 N Si Mo Cr Mn Ni Fe Total
16
Untreated
14
350°C
12
450°C
10
500°C
8 6 4 2 0
100
200
300
400
500
Load [g] Fig. 2. Load-dependent microhardness for all the drill samples.
3. Results and discussion The Knoop microhardness tests were performed on a Buehler digital microhardness tester. Multiple measurements were taken along the body length of each drill’s handle longitudinal section using different loads applied for 10 s. The results from a control batch of untreated
Fig. 3. SEM cross-sectional image of the 3 mm nitrogen penetration on the drill land.
ARTICLE IN PRESS R. Lo´pez-Callejas et al. / Vacuum 81 (2007) 1385–1388
γ 650
Untreated γ γ
γ
γ
γ
0 130
Intensity [U.A]
γN Drill treated at 450 °C
0 380
γN
Drill treated at 500 °C
-150 -200 Treated at 500 °C -250 Potential [mV]
(3 mm). This could be attributed to the enhanced diffusion induced by the combined effect of sample thermalization and ion penetration, as suggested by Fig. 3, where a 3 mm penetration depth is observed. The microhardness of the gN-phase layer found on the treated drills reached about 8 GPa (0.1 kg load) which amounts to about two times that of the low-temperature treated samples and three times that of the high temperature ones. The most important parts of a drill are its cutting edges, i.e. the sharp borders between its concave faces, or ‘flutes’, and its convex ones often known as ‘lands’, as seen in Fig. 1. Because of the difficulty in applying the Knoop indentor onto the edges themselves or the very narrow flutes, the microhardness was measured on the lands. This approach was followed on the assumption that the nitrided layer was practically uniform over the drill surface, as suggested by SEM crosssections such as shown in Fig. 3. The crystal structure of the nitrided layers was investigated using XRD, as shown in Fig. 4, for the untreated and implanted drills for only two different process temperatures, namely, 450 and 500 1C. The results were recorded at a 21 glancing angle. No appreciable variations are observed between the drills treated at 350 and 450 1C. The presence
1387
-300 Treated at 450 °C
-350 -400
Untreated
-450 -500 1E-8
1E-7
1E-6 Log current density
1E-5
1E-4
[A/cm2]
Fig. 5. Potentiodynamic polarization curves for the drills.
of the peaks at 39.61 and 44.181, indicates the formation of an expanded austenitic phase in all treated cases. On the other hand, for the higher temperatures, an asymmetry has been detected on the peak at 39.61 on the higher 2y side. This effect can be associated to a nitrogen concentration gradient in the treated layer. In all the implanted samples, the CrN and Cr2N phases have not been observed by XRD. Electrochemical potentiodynamic polarization tests were carried out within a cell containing 1 l of de-aerated 1.0N solution of H2SO4 as electrolyte. The measured results for untreated, as well as treated samples are shown in Fig. 5. The corrosion potential of the second ones appears clearly increased while the polarization curves are shifted to the left of that of the untreated sample. These facts point to an improvement in the corrosion resistance. The corrosion potential of the untreated sample reached about 378 mV, whereas that of all the treated samples was recorded as follows: 305 mV for 450 1C and 295 mV for 500 1C. This fact can be accounted for by the formation of the gN–phase in the low-energy PIII process [2]. These curves show that the corrosion resistance was improved, though slightly, with respect to that of the untreated drills. Bearing in mind the results obtained by SEM (see Table 1) the Cr/Fe and Ni/Fe ratios diminished as the temperature increased, while the N/Cr and N/Ni ratios grew with it. 4. Conclusions
0 30 40 50 60 70 80 90 100 110 120 2θ [degress] Fig. 4. X-ray diffraction patterns obtained from drills treated at different temperatures.
A low-energy PIII process has been carried out on dental drills made of AISI-316 stainless steel. It is concluded that low energy ion implantation results in an expansion of the iron lattice due to nitrogen supersaturation. Surface layers produced by 350–500 1C implantations, having a large amount of this supersaturated iron phase, exhibit significantly increased microhardness without compromising
ARTICLE IN PRESS 1388
R. Lo´pez-Callejas et al. / Vacuum 81 (2007) 1385–1388
their corrosion resistance. Indeed, the expanded austenitic phase produced at these temperatures appears to have a higher corrosion resistance than in the case of untreated drills. Acknowledgments This work received partial financial support from CONACYT and DGEST, Mexico. The following technical collaborators: Leticia Carabias, Domingo Fuentes, Maria Teresa Torres M., Isaı´ as Contreras V. and Pedro A´ngeles E, are especially thanked. The authors are particularly grateful to Professor R. Hurley for many valuable observations.
References [1] Zeng Zhoaming, Fu RickyKY, Tian Xiubo, Chu Paul K. Surf Coat Technol 2004;186:260M–4M. [2] Lei K, Zhu XM. Surf Coat Technol 2005;193:22–8. [3] Conrad JR, Radtke JL, Dodd RA, Worzala FJ, Tran NC. J Appl Phys 1987;62(11):4591–6. [4] Ueda M, Berni LA, Castro RM, Beloto AF, Abromof E, Rossi JO, et al. Surf Coat Technol 2002;156:71–6. [5] Ma¨ndl S, Sader R, Thorwarth G, Krause D, Zeilhofer HF, Horch HH, et al. Biomol Eng 2002;19:129–32. [6] Tian XB, Leng YX, Kwok TK, Wang LP, Tang BY, Chu PK. Surf Coat Technol 2001;135:178–83. [7] Ley MK, Zhang ZL, Ma TC. Surf Coat Technol 2000;131:317–25. [8] Lo´pez-Callejas R, Valencia-Alvarado R, Mun˜oz-Castro AE, GodoyCabrera OG, Tapia-Fabela JL. Rev Sci Instr 2002;73–12:4277–82. [9] Neville A, Hodgkiess T. Brit Corr J 2000;35–3:183–8.
Vacuum 81 (2007) 1385–1388 www.elsevier.com/locate/vacuum
Surface modification of stainless steel drills using plasma-immersion nitrogen ion implantation R. Lo´pez-Callejasa,b,, A.E. Mun˜oz-Castroa, R. Valencia A.a, S.R. Barocioa, E.E. Granda-Gutie´rrezb, O.G. Godoy-Cabreraa,b, A. Mercado-Cabreraa, R. Pen˜a-Eguiluza, A. de la Piedad-Beneitezb a
Instituto Nacional de Investigaciones Nucleares, Plasma Physics Laboratory, AP 18-1027, 11801 Mexico, D.F., Mexico b Instituto Tecnolo´gico de Toluca, AP 890, Toluca, Mexico
Abstract Better protective and functional coatings on dental drills are increasingly demanded for their desired mechanical, tribological or chemical properties. We have studied the surface modification of steel dental drills, with an aim to increasing both their cutting life and corrosion resistance in dentistry using low-energy plasma-immersion nitrogen implantation over different temperature ranges. The temperature of the drills was controlled by varying the implantation voltage. Following implantation, tests showed that the microhardness of the nitrided drill surfaces was approximately one order higher than that of untreated drills and the corrosion tolerance was found to increase with growing temperature of the plasma-immersion process. r 2007 Elsevier Ltd. All rights reserved. Keywords: PIII; SEM; XRD; Knoop microhardness
1. Introduction The high processing efficiency of plasma-immersion ion implantation (PIII) has been shown to be particularly effective in optimizing the surface properties such as hardness, wear and corrosion resistance, of both basic materials and industrial components [1, 2]. One of the most attractive advantages of this technique is the capability of treating irregularly shaped objects without resorting to any target manipulation [3,4]. The plasma-based low-energy PIII process has more recently been applied under either low or elevated temperature conditions [5–7]. Parameters such as pulse duration, implantation voltage, plasma density, pulse frequency, etc. have been found to have considerable influence on the implanted dose and the ensuing target temperature. These parameters in turn, almost unequivocally determine the final properties of the treated surfaces. In the present study we assess the results of the Corresponding author. Fax: +52 5329 7301.
E-mail address: [email protected] (R. Lo´pez-Callejas). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.01.051
treatment of stainless steel drills, as regularly used in dentistry, using nitrogen based PIII with a view to increasing the tool resistance to wear and corrosive attack. 2. Experimental set-up A 35 l cylindrical vacuum chamber made of SS-304 steel, pumped down to 10 6 mbar base pressure, was used for the experiments on a general layout previously described [8]. A 1 10 4 mbar operational pressure was then attained by admitting nitrogen gas through a needle valve and voltage applied from a current controlled DC power supply allowing a plasma to be formed. Its typical main parameters, measured by double electric probes, were a density in the 1 1.5 1010 cm 3 range and an electron temperature within 1–2 eV. The temperature of the drill sample holder was measured with an electrically isolated thermocouple. Many of the dental drills available in Mexico fall into the 0.7 mm diameter three edge standard (see Fig. 1) with a 3.7 mm body length and a total size of 32 mm. Table 1 shows a typical chemical analysis of an AISI-316 stainless
ARTICLE IN PRESS R. Lo´pez-Callejas et al. / Vacuum 81 (2007) 1385–1388
1386
drills, as well as from drills implanted at three selected temperature values (350, 450 and 500 1C) are displayed in Fig. 2. At higher temperatures, the microhardness achieved is generally greater but with a tendency to decrease with the load increment. This fact suggests that nitrogen concentration is diminishing inwards the drill volume. The maximum Knoop microhardness of the gN phase is about nine times higher than that of the g phase of the austenitic stainless steel at 500 1C (see Table 1). The nitrided drills were sectioned in order to uncover their circular cross sections, to be analyzed by SEM following polishing. Table 1 shows the elements present on the sample surface, in at%, for the selected process temperatures. The nitrogen concentration is seen to increase with increasing temperature, although the implantation depth is greater in the medium (8 mm) and high temperature cases (12 mm) than the low temperature case
Fig. 1. Lateral drill view.
Table 1 Elemental quantitative chemical analysis of the SS-316 drills Element
Untreated
350 1C
450 1C
500 1C
at %
at%
at%
at%
1.78 1.52 18.54 1.77 12.09 64.3 100
16.04 1.41 1.39 16.37 1.50 10.68 52.6 100
17.11 1.67 1.04 16.06 1.49 10.21 53.42 100
21.75 1.76 1.45 14.66 1.35 9.59 49.44 100
20
steel dental drill using energy dispersive X-ray spectroscopy (EDS) on an EDAXTM system. This steel is an excellent material applicable in a wide range of atmospheric environments and many corrosive media. However, under conditions encountered in dentistry which can involve a hot chloride environment, it can be subject to pitting and crevice corrosion with stress corrosion cracking occurring at temperatures above some 600 1C [9]. The drills under treatment were biased by means of a modulator capable of a 5 kV maximum implantation voltage, with a variable 5–100 ms pulse intended to maintain the sample temperature at 350, 450 and 500 1C at a repetition rate of 1 kHz. With these parameters the implantation dose lies between 1016 to 1017 ions/cm2. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and Knoop microhardness measurements were used to characterize the modified surface after implantation, while the corrosion resistance was determined by the potentiodynamic polarization method.
Microhardness [GPa]
18 N Si Mo Cr Mn Ni Fe Total
16
Untreated
14
350°C
12
450°C
10
500°C
8 6 4 2 0
100
200
300
400
500
Load [g] Fig. 2. Load-dependent microhardness for all the drill samples.
3. Results and discussion The Knoop microhardness tests were performed on a Buehler digital microhardness tester. Multiple measurements were taken along the body length of each drill’s handle longitudinal section using different loads applied for 10 s. The results from a control batch of untreated
Fig. 3. SEM cross-sectional image of the 3 mm nitrogen penetration on the drill land.
ARTICLE IN PRESS R. Lo´pez-Callejas et al. / Vacuum 81 (2007) 1385–1388
γ 650
Untreated γ γ
γ
γ
γ
0 130
Intensity [U.A]
γN Drill treated at 450 °C
0 380
γN
Drill treated at 500 °C
-150 -200 Treated at 500 °C -250 Potential [mV]
(3 mm). This could be attributed to the enhanced diffusion induced by the combined effect of sample thermalization and ion penetration, as suggested by Fig. 3, where a 3 mm penetration depth is observed. The microhardness of the gN-phase layer found on the treated drills reached about 8 GPa (0.1 kg load) which amounts to about two times that of the low-temperature treated samples and three times that of the high temperature ones. The most important parts of a drill are its cutting edges, i.e. the sharp borders between its concave faces, or ‘flutes’, and its convex ones often known as ‘lands’, as seen in Fig. 1. Because of the difficulty in applying the Knoop indentor onto the edges themselves or the very narrow flutes, the microhardness was measured on the lands. This approach was followed on the assumption that the nitrided layer was practically uniform over the drill surface, as suggested by SEM crosssections such as shown in Fig. 3. The crystal structure of the nitrided layers was investigated using XRD, as shown in Fig. 4, for the untreated and implanted drills for only two different process temperatures, namely, 450 and 500 1C. The results were recorded at a 21 glancing angle. No appreciable variations are observed between the drills treated at 350 and 450 1C. The presence
1387
-300 Treated at 450 °C
-350 -400
Untreated
-450 -500 1E-8
1E-7
1E-6 Log current density
1E-5
1E-4
[A/cm2]
Fig. 5. Potentiodynamic polarization curves for the drills.
of the peaks at 39.61 and 44.181, indicates the formation of an expanded austenitic phase in all treated cases. On the other hand, for the higher temperatures, an asymmetry has been detected on the peak at 39.61 on the higher 2y side. This effect can be associated to a nitrogen concentration gradient in the treated layer. In all the implanted samples, the CrN and Cr2N phases have not been observed by XRD. Electrochemical potentiodynamic polarization tests were carried out within a cell containing 1 l of de-aerated 1.0N solution of H2SO4 as electrolyte. The measured results for untreated, as well as treated samples are shown in Fig. 5. The corrosion potential of the second ones appears clearly increased while the polarization curves are shifted to the left of that of the untreated sample. These facts point to an improvement in the corrosion resistance. The corrosion potential of the untreated sample reached about 378 mV, whereas that of all the treated samples was recorded as follows: 305 mV for 450 1C and 295 mV for 500 1C. This fact can be accounted for by the formation of the gN–phase in the low-energy PIII process [2]. These curves show that the corrosion resistance was improved, though slightly, with respect to that of the untreated drills. Bearing in mind the results obtained by SEM (see Table 1) the Cr/Fe and Ni/Fe ratios diminished as the temperature increased, while the N/Cr and N/Ni ratios grew with it. 4. Conclusions
0 30 40 50 60 70 80 90 100 110 120 2θ [degress] Fig. 4. X-ray diffraction patterns obtained from drills treated at different temperatures.
A low-energy PIII process has been carried out on dental drills made of AISI-316 stainless steel. It is concluded that low energy ion implantation results in an expansion of the iron lattice due to nitrogen supersaturation. Surface layers produced by 350–500 1C implantations, having a large amount of this supersaturated iron phase, exhibit significantly increased microhardness without compromising
ARTICLE IN PRESS 1388
R. Lo´pez-Callejas et al. / Vacuum 81 (2007) 1385–1388
their corrosion resistance. Indeed, the expanded austenitic phase produced at these temperatures appears to have a higher corrosion resistance than in the case of untreated drills. Acknowledgments This work received partial financial support from CONACYT and DGEST, Mexico. The following technical collaborators: Leticia Carabias, Domingo Fuentes, Maria Teresa Torres M., Isaı´ as Contreras V. and Pedro A´ngeles E, are especially thanked. The authors are particularly grateful to Professor R. Hurley for many valuable observations.
References [1] Zeng Zhoaming, Fu RickyKY, Tian Xiubo, Chu Paul K. Surf Coat Technol 2004;186:260M–4M. [2] Lei K, Zhu XM. Surf Coat Technol 2005;193:22–8. [3] Conrad JR, Radtke JL, Dodd RA, Worzala FJ, Tran NC. J Appl Phys 1987;62(11):4591–6. [4] Ueda M, Berni LA, Castro RM, Beloto AF, Abromof E, Rossi JO, et al. Surf Coat Technol 2002;156:71–6. [5] Ma¨ndl S, Sader R, Thorwarth G, Krause D, Zeilhofer HF, Horch HH, et al. Biomol Eng 2002;19:129–32. [6] Tian XB, Leng YX, Kwok TK, Wang LP, Tang BY, Chu PK. Surf Coat Technol 2001;135:178–83. [7] Ley MK, Zhang ZL, Ma TC. Surf Coat Technol 2000;131:317–25. [8] Lo´pez-Callejas R, Valencia-Alvarado R, Mun˜oz-Castro AE, GodoyCabrera OG, Tapia-Fabela JL. Rev Sci Instr 2002;73–12:4277–82. [9] Neville A, Hodgkiess T. Brit Corr J 2000;35–3:183–8.