Surface layer characteristics of ion-implanted metals

Thhl Solid Fihns, 101 (1983) 223-23 I

223

METALLURGICAL AND PROTECTIVE COATINGS

SURFACE LAYER CHARACTERISTICS OF ION-IMPLANTED METALS MASAYA IWAK1

The Institute of Physieal and Chemical Research (Rikagaku Kenkyusho), Hirosawa 2-1, Wako-shi, Saitama 351 (Japan) (Received December 22, 1981 ; accepted November 2, 1982)

Recently ion implantation into metals has been carried out mainly for fundamental studies of non-electronic properties in surface layers such as friction, wear, corrosion etc. Many results have shown that the technique is useful for the improvement of surface layer properties such as wear and corrosion resistance. In this report we describe recent results obtained with the implantation process and the mechanical and chemical properties of implanted iron or iron-based alloys.

I. INTRODUCTION

Since the mid-1960s, ion implantation has been used successfully in the fabrication of semiconductor devices as a method of introducing controlled amounts of dopants into the surface layers of semiconductor substrates. The principal advantages of this doping technique include improved controllability and reproducibility for device fabrication in comparison with those obtained with thermal diffusion. The absolute concentration of an implanted element and its uniformity across the specimen surface may be controlled to better than 5% and 1% respectively. A number of books ~-a have been written about research on the application of ion implantation to semiconductors. In the early 1970s the initial application of ion implantation for modifying the non-electronic properties of materials, such as the chemical or mechanical properties of metals, led to unexpectedly large effects4. Until recently, these effects have been studied at only a limited number of laboratories, e.g. the Atomic Energy Research Establishment, Harwell, Gt. Britain, and the U.S. Naval Research Laboratory, but they are now receiving increasing interest from researchers and commercial users. A number of very important material properties are markedly affected by the composition and structure of the surface layers of a metal within about 1 lam from the surface: friction, wear, hardness, corrosion resistance, electrochemistry, catalysis, bonding, lubrication, adhesion etc. All these properties have been shown to be altered by ion implantation. In this report we describe the ion implantation process, and the tribologicai, mechanical and electrochemical properties of ion-implanted iron and iron-based alloys, on which most effort has been concentrated. 0040-6090/83/0000-0000/$03.00

© ElsevierSequoia/Printedin The Netherlands

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2. THE ION I M P L A N T A T I O N PROCESS

Implanted ions come to rest at depths of 0.01-1 ~tm in the host material because of the loss of their incident energy during collisions with substrate atoms. The resultant concentration profiles of implanted dopant atom~ can be calculated for most projectile-target combinations from well-established theoretical considerations 5. At low ion doses the concentration profiles are usually characterized by a gaussian distribution centred around an average projected range. At higher ion doses various effects such as sputtering and ion-beam-induced migration of atoms (radiation-enhanced diffusion) can significantly limit or alter the ultimate concentration profiles attainable. Figure 1 shows the concentration profile of as-implanted copper in low carbon steel 6. The broken curve indicates the depth distribution predicted by the Lindhard Scharff-SchiCtt (LSS) theory s, which makes no allowance for sputtering and enhanced diffusion. The depth of the peak i~ observed to be shallower than that predicted by the theory and, moreover, the profile shows the deeper penetration of implanted copper. These phenomena may be explained as due to sputtering and radiation-enhanced diffusion.

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Fig. 1. C o n c e n t r a t i o n profile of a s - i m p l a n t e d c o p p e r in mild steel m e a s u r e d by s e c o n d a r y ion mass analysis (dose, 1017 ions c m - z; ion energy, 150 keV). The d i s t r i b u t i o n s including ( ....... ) and e x c l u d i n g (- - -) the effects of s p u t t e r i n g r e m o v a l and e n h a n c e d diffusion are shown. (After ref. 6.)

If the sputtering removal thickness is assumed to be 27 nm, the calculated profile becomes shallower by 13.5 nm, and agrees with the real profile at depths shallower than the peak. The removal thickness of 27 nm with a dose of 1 × 10 t7

SURFACELAYERCHARACTERISTICSOF ION-IMPLANTEDMETALS

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ionscm -2 corresponds to the sputtering ratio of about 2 atoms i o n - l . The migration of implanted copper into deeper regions than the calculated range, as shown in the figure, is thought to be caused by radiation-enhanced diffusion. The diffusion coefficient, obtained from the best fit to the real profile of the distribution calculated from the radiation-enhanced diffusion equations, is nearly equal to that for thermal equilibrium at about 550 °C 7. Reynolds e t al. 8 have observed surface sputtering during binary alloy production by ion implantation, which they monitored by measuring the intensity of the optical photon emission from the excited neutrals leaving the surface of the sample. Emission from both the implanted species and the substrate species was monitored during ion implantation and was compared with the change in the surface atomic fraction. The results show that the chromium optical signals obtained during the implantation of chromium into iron at 90 keV began to increase at 1 x 1016 Cr atoms c m - 2 and saturated at 2 x 10' 7 Cr atoms cm 2. The behaviour of the chromium signals corresponded to the decrease and saturation in the intensity of the iron signals. If the sputtering ratio is 2 atoms ion- 1, the sputtering removal thickness is 54 nm at 2 x 1017 ions cm 2, and it corresponds to a shift in the peak of 27nm towards the surface. This value coincides with the projected range of chromium implanted into iron at 90 keV as predicted by the range theory. The absolute concentration of chromium therefore becomes the maximum at the surface. The typical concentration profiles are shown in Fig. 2. Ion implantation into iron was performed with 4 x 10' 7 Cr atoms cm-2 at 90 keV. The chromium profile was measured independently by three different techniques: Rutherford backscattering, nuclear reaction analysis and optical emission combined with sputtering. It should be noted that the three profiles agree with each other within the inherent uncertainties in each technique. The figure shows that the maximum concentration occurs at the surface due to sputtering and that implanted chromium migrates into the deeper regions by radiation-enhanced diffusion.

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Fig. 2. Steadystate concentrationprofileof chromium implanted in iron at 90 keV,as calculated ( ) and as measured by Rutherfordbackscattering(O), nuclear reaction analysis (A) and optical emission combined with sputtering([7). This profilecorrespondsto a fluenceof 4 x 1017atoms cm- 2.

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These results suggest that the effects of sputtering and radiation-enhanced diffusion on the concentration profile at high ion implantation doses should not be ignored. The profile can be predicted by assuming values for the sputtering ratio and the diffusion coefficient. 3.

I'RIBOLOGICAL AND MECHANICAL PROPERTIES

Tribology is the study of the friction, lubrication and wear of engineering surfaces with a view to understanding surface interactions in detail; it is therefore interdisciplinary, covering physics, chemistry, materials science and mechanics. The majority of the investigations of ion-implanted surfaces has concerned friction and wear properties, because this is the most obvious area in which relatively small concentrations of added elements could be expected to produce effects. The friction coefficient can be written as the ratio of two quantities representing the resistance to plastic flow of the weaker of the contacting materials in shear and in compression 9. However, friction behaves in a complicated way under the influence of such physical properties as adhesion and such engineering operations as ploughing. Hartley et al. 1° carried out the first published friction test on ion-implanted surfaces using a comparatively simple slow speed sliding apparatus based on the Bowden-Leben-type friction machine. Implantation with various ions (Sn ~, In Ag +, Pb +, Mo +) at doses in excess of 1016 ions cm 2 and energies of typically 120 keV resulted in macroscopic changes in the friction coefficient. The majority of the implanted ions produced a decrease in the friction coefficient, and Hartley e t al. assumed that the dominant behaviour of an ion-implanted species is to weaken the shear strength of the implanted layer by promoting the formation of oxide films. More recently, however, we have implanted nickel, copper and chromium into pure iron and steels, and we found that the implantation of chromium into steels caused a decrease in friction coefficient under dry sliding at low speed. In this case, the chromium had oxidized, forming C r 2 0 3 , as detected by electron spectroscopy for chemical analysis (ESCA). We have proposed that the reduction in friction due to chromium implantation is a result of surface hardening caused by the presence of chromium-based oxide particles. The implanted layers had indeed been hardened, as determined from Vickers microhardness tests. In all solid solid contacts, the frictional properties will be influenced by the characteristics of the materials from the surface to a certain depth below the surface. The friction coefficients of implanted materials are influenced by the acceleration energy and the total dose. Figure 3 shows the relative change in friction as a function of the total dose at various implantation energies for copper-implanted iron ~~. The friction coefficient tends to increase as the implantation energy is decreased and the total dose is increased; when the implantation energy is reduced, the relative change in friction becomes more pronounced. An investigation of which concentration contributes to the frictional properties has demonstrated that the important parameter is the amount of implanted ions in the surface layer and not the concentration at a specific depth such as the surface and maximum concentrations; the relative change in friction varies as a curve according to the amount of implanted ions down to 4 0 n m from the surface. This result suggests that the amount of

227

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implanted ions remaining in the surface layer (0-40 nm) is of prime importance in the frictional properties for copper-implanted steel plates t 1. Wear is a complex process involving parameters that can alter by several orders of magnitude depending on the dominant mechanisms and their interactions with the environment. Ion implantation has a particular role to play in the science and technology of wear because the unexpected benefits which result from the shallow depths of treatment can lead to a new understanding of wear processes and because ion implantation can improve wear resistance. Two types of wear test, which are best suited to the measurement of adhesive wear, are the pin-on-disc and the pin-on-ring tests. The object in these tests is to measure the total amount of wear material removed over a given period of time as a function of the load and the speed. The test geometries are well suited to ion-implanted samples since in each case the test arrangement is simple and the contact surface is easily bombarded by an ion beam. Hartley t2 and Hirvonen 13 have reported significant changes in the wear rate of steels implanted with relatively high doses of nitrogen. Figure 4 shows for a typical steel studied by Dearnaley and Hartley 14 the dose dependence of the factor by which the volumetric wear rate is reduced. As shown in the figure, the dose dependence of the relative improvement in wear is insensitive to doses in excess of about 3 × 1017 ions cm -2. The results show that it is presumably only necessary to achieve complete saturation of the surface microstructure in order to ensure that every contact eyent is altered by the presence of the implanted ions. In wear tests, measurements of the wear track on the ion-implanted disc showed, at an early stage in this research, that the depth of the track far exceeds the range of ion implantation. Recently, Hartley 15 has determined by nuclear reaction analysis the nitrogen content at the base of a wear track 1.2 pm deep and has shown the presence of 10~o-20~o N. This depth is about ten times the ion range. The

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migration of nitrogen may be caused by a heating effect such as the flash temperature induced during sliding. Reference has frequently been made to hardness changes induced by ion implantation, usually in the context of wear reduction. Hardness tests are easy to perform, as has been pointed out, and are probably the most obvious way of establishing whether a surface has been improved. Li et al. t6 have determined the relative change in hardness of implanted steel as a function of the dose of nitrogen implanted into steels. The behaviour of the hardness of nitrogen-implanted steel corresponds closely to the wear behaviour indicated in Fig. 4. 40

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4.

CORROSION PHENOMENA

It is convenient to divide corrosion phenomena into dry and wet corrosion. Dry corrosion generally involves metal-gas or metal vapour reactions at elevated temperatures; it includes such terms as hot corrosion and high temperature oxidation. Wet corrosion consists of an electrochemical reaction between a metal and moisture in its environment, usually at an ambient temperature; the term wet corrosion thus includes both corrosion in aqueous environments and atmospheric corrosion. An important class of corrosion problems arises when a metal is exposed to an oxidizing environment at an elevated temperature, e.g. in nuclear reactors and aircraft engines. The simplest method of protection is to use certain oxides that are very stable at high temperatures and exhibit a high degree of stoichiometry. The result is that vacancy concentrations are low and diffusion through the oxide is inhibited; oxide films such as AI203, C r z O 3 and SiO2 play an important role in protection. o. 4 54, °'AI -0.1J~,Y " Bernabaietal.tVchosetotakethesubcriticalalloyFe 23,1,Cr-1.

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and to investigate the effect of raising the surface concentration of aluminium by ion implantation. Oxidation was carried out in dry 0 2 at 1100 °C for periods of up to 24 h and the mass gain was recorded continuously by means of a microbalance. Figure 5 shows the mass gains for the unimplanted specimen and the implanted specimens. Aluminium-implanted specimens show a very high degree of oxidation resistance. Analysis of the oxide by ESCA confirmed that the oxide in the ionimplanted specimens contained higher concentrations of A120 3 and lower amounts of chromium, iron and manganese. 5

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Fig. 5. The mass gain of a sampleof the alloy Fe 24%Cr-I.45%AI-0.1%Yin oxygenat 1100°C showing the effectof various doses of ion-implanted aluminium.(After ref. 17.) The effect of ion implantation on the electrochemical properties of metals has mainly b e e n investigated with a view to improving their corrosion resistance. According to Ashworth et al.~ 8, ~9 the ion implantation of appropriate ions, such as chromium, tantalum and lead ions, can lead to a marked modification of the corrosion behaviour of iron in neutral aqueous electrolytic solutions. In aqueous sulphuric acid solutions, the corrosion behaviour of ion-implanted electrodes has been investigated by Ferber et al. 2°, who have also dealt with the effect of ion implantation on the exchange currents of hydrogen evolution reactions. Figure 6 shows polarization curves of chromium- and argon-implanted steel plates, where the cyclic voltamogram experiment was carried out in a 0.5 M acetate buffer solution (pH 5.0) at room temperature 2 ~. For chromium implantation at low doses the curves are similar to those of unimplanted and argon-implanted steel, whereas at a high dose the curve is similar to that for Fe-18~oCr alloy. This fact suggests that steel implanted with chromium at a high dose is electrochemically inert. These electrochemical properties were influenced by implanted ions, doses and energies 22.

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5. SUMMARIZING REMARKS

Ion implantation results in the improvement of many metal surface properties. Interest in ion implantation into metals has increased and is reflected in a number of conferences in this area as well as special symposia. These meetings include Application of" Ion Beams to Metals, Albuquerque, NM, 19734, Applications o[ hm Beams to Materials, Warwick, 1975 z 3, and Surface Mod(fication of Materials b v Ion Implantation, Cambridge, MA, 197924. In Japan, the Riken Symposia oil Ion Implantation Effects in Metals were held in 1977 and 1978. These proceedings have covered not only tribological effects but also mechanical properties, such as rolling behaviour, yield point, tensile strength and the bending angle, of implanted steel plates. I hope that the work presented in this review and these proceedings may stimulate more research into the surface layer characteristics of ion-implanted metals and encourage more researchers to take up such work. REFERENCES l

M a y e r , L. Ericksson and J. A. Davies, Ion hnphmtation in Semicomhu'tor< Silicon and 1970. G . D e a r n a l e y , J. H . F r e e m e n , R. S. Nelson and J. Stephen. Ion Implantation, North-Holland, Amsterdam, 1973.

J.W.

Germanium, Academic Press, New York,

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SURFACE LAYER CHARACTERISTICS OF ION-IMPLANTED METALS

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 ~18 19 20 21 22 23 24

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P. D. Townsend, J. C. Kelly and N. E. W. Hartley, Ion Implantation. Sputtering. and their Applications, Academic Press, New York, 1976. S, T. Picraux, E. P. EerNisse and F. L. Vook, Application ~['lon Beams to Metals, Plenum, New York, 1974. J. Lindhard, M. Scharffand H. F. SchiCtt, K. Dan. Vidensk. Selsk.. Mat.-Fys. Med., 33 (1963) 14. M. lwaki, S. Namba, K. Yoshida, N. Soda, T. SatoandK. Yukawa, J. Vac. Sci. Technol., 15(1978) 1089. M. lwaki, S. Namba, K. Yoshida, N. Soda, T. Sato and K. Yukawa, Proc. 7th Int. Vacuum Congr. and 3rd lnt. Con[] on Solid Sur[~wes. Vienna, 1977, Berger, Vienna, 1977, p. 1429. G . W . Reynolds, A. R. K n u d s o n and C. R. Gossett, Nucl. lnstrum. Methods, 182 183 (1981) 179. F.P. Bowden and D. Tabor, The Friction and Lubrication olSolid*, Part 1l, Oxford University Press, London, 1964. N. E. W. Hartley, G. Dearnaley and J. F. Turner, in B. L. Crowder (ed.), Ion hnplantation in Semiconductors and Other Materials, Plenum, New York, 1973, p. 423. M. Iwaki, H. Hayashi, A. Kohno and K. Yoshida, Jpn. J. Appl. Phys., 20 ( 1981 ) 3 I. N . E . W . Hartley, Wear, 34(1975)427. J . K . Hirvonen, J. Vac. Sci. Technol., 15 (1978) 1662. G. Dearnaley and N. E, W. Hartley, Thin Solid Fihns, 54 (1978) 215. N . E . W . Hartley, Thin Solid Films, 64(1979) 177. H.T. Li, P.S. Liu, S.C. Chang, H.C. Lfi, H . H . W a n g a n d K . Tao, Nucl. lnstrum. Methods',182 183 (1981)915. U. Bernabai, M. Cavallini, G. Bombara, G. Dearnaley and M. A. Wilkins, Corros. Sci., 20 (1980) 19. V. Ashworth, D. Baxter, W. A. Grant and R. P. M. Proctor, Corros. Sci., 16 (1976) 7 7 5 : 1 7 (1977) 947. V. Ashworth. W. A. Grant, R. P. M. Proctor and E. J. Wright, Corros. Sci., 20 (1978) 681. H. Ferber, H. Kasten, G. K. Wolf, W. J. Lorentz, H. Schweickert and H. Folger, Corros. Sei., 20(1980) 117. Y. Okabe, M. lwaki, K. Takahashi, H. Hayashi, S. N a m b a and K. Yoshida, Sur[~ Sci., 86 (1979) 257. K. Takahashi, Y. Okabe and M. lwaki, Nucl. lnstrum. Methods, 182-183 (1981) 1009. G. Carter, J. J. Colligon and W. A. Grant, Proc. Con[i on Applications orlon Beams to Materials, in Inst. Phys. Con£ Set'. 28 (1975}. C . M . Preece and J. K. Hirvonen, Ion hnplantation Metallurgy, Metallurgy Society, New York, 1980.