Nitrogen ion implantation for improvement of the mechanical surface properties of aluminum

Nitrogen ion implantation for improvement of the mechanical surface properties of aluminum

ARTICLE IN PRESS Vacuum 81 (2007) 1154–1158 www.elsevier.com/locate/vacuum Nitrogen ion implantation for improvement of the mechanical surface prope...

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ARTICLE IN PRESS

Vacuum 81 (2007) 1154–1158 www.elsevier.com/locate/vacuum

Nitrogen ion implantation for improvement of the mechanical surface properties of aluminum P. Budzynskia,, A.A. Youssefb, Z. Surowiecc, R. Palucha a

Mechanical Faculty, Lublin University of Technology, Nadbystrzycka Street 36, 20-618 Lublin, Poland Nuclear Physics Department, Nuclear Research Center, Atomic Energy Authority, 13759 Cairo, Egypt c Institute of Physics, M. Curie-Sklodowska University, 20-031 Lublin, Poland

b

Abstract The results of the surface treatment of commercial aluminum by nitrogen ion implantation at 120 keV and implanted fluences ranging from 3  l017 to 1.1  1018 ions/cm2 are reported. The treatment was found to lead to the formation of the hexagonal phase AIN, a decrease in strain and an increase of crystallite sizes. The modification of the surface layer so produced was thought to be a cause of a measured increase in surface microhardness and corresponding decrease in friction coefficient and wear measured in pure methanol. Oxygen found in the surface layers was also thought to play a significant role in determining tribological performance. r 2007 Published by Elsevier Ltd. Keywords: Aluminum; Nitrogen implantation; Oxide; Surface modification; Friction and wear; Hardness; Microstrains; Crystallite size

1. Introduction Ion implantation has for many years been used to modify the tribological behavior of aluminum and its alloys. The majority of reported work focuses on nitrogen implantation in order to form a hard wear-resistance layer of AIN on the surface [1,2], although in some reports this compound could not in fact be detected [3–5]. Nitrogen bubbles and voids have also been observed in the nitrogenimplanted region, this being considered detrimental to surface hardening applications [6,7]. The results of X-ray diffraction (XRD) and transmission electron diffraction (TED) measurements [8] have shown that the structure of AIN is hexagonal. Aluminum nitride with the fcc structure has also been found; the fcc crystal is a transient structure which decreases with increasing fluence of the implanted nitrogen ions. The effect of a high fluence of nitrogen implantation into aluminum has been investigated by Jagielski et al. [9]. So far, less attention has been paid to the changes of strain and crystallite sizes produced by implantation and their effect on the tribological properties of aluminum. The present work concentrates on these Corresponding author. Tel.: +48 81 5384 256; fax: +48 81 5384 247.

E-mail address: [email protected] (P. Budzynski). 0042-207X/$ - see front matter r 2007 Published by Elsevier Ltd. doi:10.1016/j.vacuum.2007.01.072

effects and includes a study of the influence of different atmospheres (air, oxygen, argon, vacuum) on the change of friction coefficient of implanted aluminum. Standard measurements of friction coefficient were also performed in methanol lubrication.

2. Experimental Experiments detailed in this work are based on commercial aluminum alloy 99.01% Al (wt%). The main contaminants in this alloy are Fe: 0.303%; Mn: 0.209%; Ca: 0.202%; Zn: 0.101% and Cu: 0.08%. Nitrogen ion implantation was conducted at an energy of 120 keV + with no mass separation (80% N+ 2 ; 20% N ) with 17 18 fluences ranging from 3  l0 to 1.1  10 N/cm2. The residual vacuum, p, in the implantation chamber was o1.3  104 Pa. The ion current density was limited to 1 mA/cm2 at room temperature in order to restrict the substrate temperature to around 55–60 1C. The ion beam was scanned over the target to achieve a uniform dose. The depth profiles of the implanted nitrogen and any oxygen present in the surface were measured using Rutherford back-scattering (RBS) using a 2.04 MeV He beam scattered at angle 1701.

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Friction and wear tests were carried out on a pin-on-disk tester [10]. A steel ball bearing with a diameter of 2.5 mm was used as a counter-sample loaded with 0.25 N force. The measurements were conducted in the ambient environment (room temperature 22 1C and humidity 25%) with methanol lubrication. Investigations of changes in the wear of this soft material are difficult due to severe adhesive wear and cladding. Therefore, pure methanol was used as a lubricant in order to diminish the contribution of adhesive wear [11]. Friction tests were also made in, air, oxygen and argon at atmospheric pressure and in vacuum. The depth and width of the wear tracks were measured with a Taylor Hobson profilometer. Micro-hardness measurements were made for the surfaces of the implanted and the nonimplanted samples using a micro-hardness tester with loads ranging from 50 to 249 mN. 3. Results and discussion 3.1. Depth distribution The depth distribution profiles of the nitrogen and oxygen atoms evaluated from the RBS measurements at fiuences 5  l017 and 8  l017 N/cm2 are presented in Fig. 1. The figure also shows the predicted distribution of nitrogen atoms calculated by means of the program SATVAL [12]. With the increase of implanted ions fluence, more nitrogen atoms are found in deeper layers than predicted by the theoretical distribution. After implantation with the fluence 8  1017 N/cm2, their amount decreases significantly on the surface layer. The changes are attributed to two possible factors, namely, diffusion of nitrogen atoms (D ¼ 1010 cm2/s [13]) inside the sample (deep down and towards the sample surface) and sputtering of the sample surface layer during implantation together with nitrogen

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atoms adsorbed there. This means that the diffusion coefficient for free nitrogen atoms is large enough to allow migration from the implanted layer towards the nitrogen with lower atomic N/Al ratio. As follows from the calculations made using the program Satval [12], the thickness of sputtered layer is 0.02 and 0.012 mm for a fluence of implanted ions 8  1017 and 5  1017 N/cm2, respectively. A radiation-enhanced diffusion process has also been observed at or below room temperature [14–16]. Nitrogen diffusion results in smoothing of the distribution curve of nitrogen atoms and disappearance of a subsidiary peak which should be visible at a depth of about 0.35 mm (Fig. 1) according to the theoretical calculations for ions N+ which constitute 20% of the implanted fluence. The main peak at a depth of 0.22 mm comes from N+ 2 molecules. The surface layers contain also significant amounts of oxygen, confirming the role of residual gas composition during ion implantation [17]. The concentration of oxygen atoms decreases progressively from 20% at the surface to 0% at depth 0.2 mm as can be seen in Fig. 1. This oxygen was introduced from the surface layer during implantation (knock-on effects). The RBS measurements for the unimplanted samples showed that the aluminum oxidized layer was p0.05 mm thick before implantation. It was shown in Ref. [18] that oxygen atoms migrate toward deeper regions of Al as more nitrogen ions are implanted in Al. 3.2. X-ray diffraction The surface characterization of Al samples implanted with nitrogen was performed using XRD with Cu Ka radiation (l ¼ 1.5406 A˚) at y–2y geometry. The diffraction patterns were refined by the FULLPROF Rietveld refinement program [19]. The surface layer of the sample

60 Satval φ = 5.0E17 N/cm2 Satval φ = 8.0E17 N/cm2 RBS (N) φ = 5.0E17 N/cm2 RBS (N) φ = 8.0E17 N/cm2

Concentration (%)

50

RBS (O) φ = 5.0E17 N/cm2 RBS (O) φ = 8.0E17 N/cm2

40

30

20

10

0

0.0

0.1

0.2

0.3

0.4

0.5

Depth (µm)

Fig. 1. Comparison of the distribution of nitrogen (and oxygen) atoms in aluminum measured by RBS and calculated using the program Satval [12].

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contains cold rolled Al, A1N and Al2O3. After implantation additional diffraction peaks appear at 2y ¼ 35.81, 37.81 and 58.41. These values correspond to (0 0 2), (1 0 1) and (1 1 0) of aluminum nitride crystals with hcp structure when compared with the ASTM file. The content of this phase increases with the increasing fluence of implanted nitrogen ions. To determine crystallite, the size and the microstrains during implantation process the Hall and Williamson’s method has been applied [20]. This integral breadth method for deconvoluting size and strain contributions makes it possible to show the line broadening as a function of 2y. The total diffraction line broadening b is obtained by the sum of broadening related to the lattice imperfections and the crystallite size D:

2 2 2

Fig. 2. Williamson–Hall plot for implanted and non-implanted aluminum. 2 1=2

b ¼ Ahz i

1 sþ , D

(1)

where A is a coefficient which depends on the distribution of microstrains (it is almost equal to one for dislocations), /z2S1/2 the mean square deformation (microstrain), s ¼ ð2 sin y=lÞ and l denote the wavelength. The graphical representation of Eq. (1) b(s) is called the Williamson–Hall plot. The diffraction line broadening b is the difference between the measured width of diffraction line for the sample and the standard one. Simultaneously, b depends on the shape of diffraction line. Assuming both the size and the strain-broadened profiles are approximated by the pseudo-Voigt function. b is obtained by the formula qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi b ¼ b2exp:  b2pattern: . (2) The grain size D and the microstrain z for the implanted and unimplanted samples were calculated from the Wiliamson–Hall plot (Fig. 2) and tabulated in Table 1. As can be seen from Table 1, the grain size increases and the microstrain decreases with increasing implantation fluence. The decrease in microstrain after implantation is surprising and not easily explained. The XRD measurements show the total decrease of stresses in the layer whose thickness exceeds the range of implanted ions by an order of magnitude. We suppose that during implantation of ions with energy p120 keV at the sample temperature of 55–60 1C, stresses formed in the process of technical aluminum production are removed. Stress removal is of local character and covers the area around the track of the implanted nitrogen ion. If the energy of the implanted atoms is sufficiently large (X180 keV for aluminum [21]), then an increase in strain with increasing fluence is observed [21,22]. 3.3. Microhardness The relative microhardness is displayed in Fig. 3 as a function of the ion implantation damage estimated in displacement per atom (dpa) units at 50 mN. Fig. 3 shows that the relative microhardness increases with increasing

Table 1 Grain size D and microstrain z for the implanted and unimplanted Al samples Dose (N/cm2)

D7DD (nm)

z7Dz

Unimplanted D ¼ 3  1017 D ¼ 8  1017 D ¼ 1.1  1018

7377 93713 9478 146736

(1.0170.09)  105 (6.2770.7)  106 (3.5970.4)  106 (1.2970.5)  106

implantation fluence for an applied load of 50 mN. It is suggested that this may be attributed to increasing radiation damage and the formation of a crystalline hcpAIN layer at the Al surface. The surface aluminum layer modified during the implantation process is 0.6 mm thick. 3.4. Friction and wear The mean values of the friction coefficient for the implanted samples at fluence f ¼ 8  1018 N/cm2 as a function of the sliding cycles are presented in Fig. 4. With abrasion of the polished surface layer and wear of the layer containing oxygen (thickness p0.2 mm, see Fig. 1), a pronounced increase in friction coefficient occurs over the first 120–200 cycles in all atmospheres examined. However, for samples immersed in methanol the friction coefficient increases slowly from a relatively low value over the first 500 cycles and then at an even slower rate for subsequent cycles. Methanol has the effect of excluding oxygen but also acting as a lubricant by removing heat from contacting micro-asperities. It is difficult to predict its precise effect but it would appear that reduction of the adhesive component of wear is dominating in these experiments. Oxygen is found close to the surface as detected by RBS but further work is needed to assess its precise effect on surface mechanical parameters. Wieser et al. [11] found that oxygen implanted into aluminum reduced wear up to a factor of 10 times in an ethanol environment and increased microhardness (1.5–2.5 times). Other studies have found

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1.6 φ = 1.1E18

Relative microhardness

1.5 1.4 1.3 φ = 8.0E17

1.2 φ = 5.0E17 φ = 3.0E17

1.1 1 0

5E-17

1E-16

1.5E-16

2E-16

Dpa Fig. 3. Relative microhardness as a function of the implantation damage, (dpa).

φ=8x1017 N/cm2

1.2

Friction coefficient

VACUUM

ARGON

1.0 0.8

OXYGEN

0.6

AIR

0.4

METHANOL

0.2 0.0 0

500

1000

1500

2000

2500

Cycle Fig. 4. Friction coefficient for implantation aluminum as a function of sliding cycles.

that for non-implanted aluminum, even under vacuum conditions, an increase in near surface oxide can increase wear and friction of the surface layer [23]. It is clear that even small amounts of oxygen in the test environment can have an effect depending on load and velocity conditions which will determine the degree of mixing of surface constituents and oxidation processes [24]. With the increase of cycle number X500 (in various atmospheres) the friction coefficient is seen to decrease (Fig. 4), possible reasons being: broadening of the weartrack and micro-smoothing at contact points. Nitrogen implantation was found to significantly diminish wear of the samples tested in methanol. After implantation with a fluence of (f ¼ 5  1018 N/cm, wear diminishes by about 23 times compared to that of the

unimplanted sample. However, the differences in the wear of unimplanted samples in different atmospheres are small. The cause of these effects is not at present understood but will be the subject of future studies. 4. Conclusions Nitrogen implantation significantly improves the tribological properties of aluminum when tested with pure methanol lubrication. Decrease in friction and wear after implantation, when tested in atmospheres of air, oxygen, argon and vacuum, is small. For the conditions examined here there is evidence that implanted nitrogen ions diffuse deeper into the sample from their projected range and away from the surface. After implantation the surface layers of

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the sample consisted of cold rolled Al, Al2O3 and crystalline phase hcp-AlN. Oxygen was found in the surface layers and is believed to play a role in determining tribological properties. Acknowledgments This study has been supported by the grant from the Ministry of High Education and Scientific Research (Poland) No. 1454/H03/2006/31. The authors wish to thank Dr. A.P. Kobzev, JINR, Dubna, for his help in RBS analysis. References [1] Rodriguez RJ, Sanz A, Medrano A, Garcia-Lorente JA. Vacuum 1999;52:187. [2] Blawert C, Mordike BL. Nucl Instrum Methods B 1997;127–128:873. [3] Pavlov PV, Zorin EI, Tetelbaum DI, Lesnikov VP, Ryzhkov GM, Pavlov AV. Phys Stat Sol A 1973;19:373. [4] Ma E, Liu BX, Chen X, Li HD. Thin Solid Films 1987;147:49. [5] Gautier M, Duraud JP, Le Gressus C. J Appl Phys 1987;61:574. [6] Mecunc RC, Donlon WT, Plummer HK, Toth L, Kunz FW. Thin Solid Films 1989;168:263.

[7] Matthews AP, Iwaki M, Horion Y, Satou M, Yabe K. Nucl Instrum Methods B 1991;59–60:671. [8] Lu HL, Sommer WF, Borden MJ, Tesmer JR, Wu XD. Thin Solid Films 1996;289:17. [9] Jagielski J, Piatkowska A, Aubert P, Legrand-Buscema C, Le Paven C, Gawlik G, et al. Werner Z Vacuum 2003;70:147. [10] Tarkowski P, Budzynski P, Kasietczuk W. Vacuum 2005;78:683. [11] Wieser E, Reuther H, Richter E. Nucl Instrum Methods B 1996;111:271. [12] Sielanko J, Szyszko W. Nucl Instrum Methods B 1986;16:340. [13] Reier T, Schultze JW, Osterle W, Buchal Chr. Surf Coat Technol 1998;103–104:415. [14] Piller RC, Marwick AD. J Nucl Matter 1978;71:309. [15] Marwick AD, Piller RC, Sivell PM. J Nucl Matter 1979;83:35. [16] Battels A, Dworschak F, Meurer HP, Abromeit C, Wollenberg H. J Nucl Matter 1979;83:24. [17] Moller W, Parascandola S, Telbizova T, Prokert E, Gunzel R, Richter E. Surf Coat Technol 2001;136:73. [18] Ohira S, Iwaki M. Nucl Instrum and Methods 1987;B19/20:162. [19] Moller W, Parascandola S, Telbizova T, Prokert E, Gunzel R, Richter E. Surf Coat Technol 2001;136:73. [20] Ritveld HM. J Appl Crysteallogr 1969;2:65. [21] Williamson C, Hall WH. Acta Metallica 1953;1:22. [22] Youssef AA, Budzynski P, Filiks J, Surowiec Z. Vacuum 2005;78:599. [23] Trejo-Luna R, De La Vega LR, Rickards J, Falcony C, Jergel M. J Mater Sci 2001;36:503. [24] Kim HJ, Emge A, Karthikeyan S, Rigney DA. Wear 2005;259:501.