Ion beam analysis of steel surfaces modified by nitrogen ion implantation

Nuclear Instruments and Methods in Physics Research B66 (1992) 242-249 North-Holland

NuclearInstruments gMethods in Physics Research Section B

Ion beam analysis of steel surfaces modified by nitrogen ion implantation C . Neelmeijer, R . Grötzschel, E . Hentschel, R . Klabes, A. Kolitsch and E. Richter Zentralinstitut für Kernforschung Rossendorf, Postfach 19, 0-8051 Dresden, Germany

Conventional ERDA in combination with RBS and the 14N(d, a)12C nuclear reaction hasbeen applied to understand thewear behaviour of 210Cr46 tool steel after nitrogen implantation. However, the presence of the neighbouring elements 12C and 16 0 within the 14 N-implanted layers results in both strong limitations for the conventional ERDA and the necessity to registrate them analytically. The ERDA with Bragg ionization chamber proved useful allowing depth profile measurements with element separation in a single run. The solid-state lubrication caused by a nonmetallic carbon covering of the steel surface was deduced to increase the wear resistance drastically. 1. Introduction The modification of steel surfaces by ion beams has been established as a favourable method to improve the mechanical properties as well as the resistance against chemical reactions [1]. For example, the mechanical and chemical abrasion of cutters chopping up plastic materials can be reduced by nearly an order of magnitude due to the implantation of nitrogen ions; the operating time of nitrogen-implanted extruder shafts increases strongly . In connection with the implantation process analytical methods which ascertain both the number of embedded nitrogen atoms and their real depth profile are necessary. For this reason the t4 N(d, a) 1zC nuclear reaction at Ed = 1 .4 MeV deuteron energy [2] is suitable usingoblique alpha-particle detection . However, the wear-resistance behaviour of steel surfaces was found [3] to be influenced not only by nitride formation but also favoured by the presence of C-N phases, where the carbon originates from the rest gas inside the implanter recipient. To correlate the mechanical steel properties with the chemical composition an analysis of both nitrogen and carbon atoms is of interest. Generally, the elastic recoil detection (ERD) [4] offers such a possibility and provides informations on further elements, i .e. oxygen, hydrogen, possibly incorporated within the nitride or carbonitride surface layers. The present work reports on tool steel surfaces hardened by nitrogen-ion implantation at different implanter conditions . In dependence on the nitrogen fluency and the carbon partial pressure during implantation, pin-on-disk measurements informed on the characteristic layer resistance to wear. Corresponding compositional analysis by conventional ERD [5] turned

out to be rather restricted due to strong overlaps of N, C and O signals. Separated nitrogen depth profiles deduced from complementary 14 N(d, a 1 )12 C experiments enabled a rough understanding of the complex ERD spectra. Moreover, Rutherford backscattering (RBS) [6] measurements allowed to distinguish between C and O surface deposition or matrix incorporation . The difficulties were overcome by using ERD in combination with a Bragg chamber [7] which allows to observe spectra well resolved in atomic number and energy of the recoiled species. 2. Experimental 2.1. Ion implantation

In our institute routine modification of steel surfaces takes place using a noncommercial high current implanter without mass separator. Nitrogen ions with 50 kV acceleration voltage were choosen to prepare a test series of polished 210Cr46 tool steel . Two samples labelled with Il and 12 were implanted at high ion current density IIA = 30 WA/cm2 and a low carbon partial pressure pc =1.3 x 10 -1 Pa, adjusting the nitrogen fluency to rß 1 = 1 x 10 17 at/cmZ and Y'2 =8 X 10 17 at/cm2, respectively. A further sample labelled with IC was prepared by 02 = 8 x 10 17 at/cm2 operating with a moderate ion-beam intensity I/A = 4.6 WA/cmZ but increased carbon partial pressure pc= 3.2 X 10-1 Pa. A comparative sample labelled with M was implanted with 50 keV N* (d,2 = 8 x 10 17 N/cm Z) ions of I/A = 4.3 pA/cm2 current density at a commercial implanter including mass separation .

0168-583X/92/$05.00 0 1992 - Elsevier Science Publishers B.V . All rights reserved

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C Neelmeijer et al. / Ion beam analysis ofsteel surfaces

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Fig . 1 . 14N test structure (a) and implantation profiles (b-d) examined by the 14 N(d, at)12C nuclear reaction at Ed =1 .4 MeV, oblique deuteron incidence a = 52 .5° (target normal) and glancing -particle detection ,9 =168 .5°; angles are given with respect to the beam direction.

2.2. Wear-resistance measurements The wear behaviour of the nonimplanted 210Cr46 tool steel and the nitrogen doped samples 11, 12, IC

and M was examined by means of a pin-on-disk arrangement. The volume abrasion was determined as a function of the sliding distance of a fixed hardened (HR H = 60) bearing ball (3 mm diameter) on the rotat-

Fig . 2 . ERD (conventional) energy spectrum of a sandwich structure produced by deposition of SiN(H) and a-Si : H layers of 20 nm thickness; a = 20°, 0 = 30° . III. CONTRIBUTED PAPERS

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ing steel disk. The pin ball moved on the disk with 1 m/s velocity and a load of 0.2 N. All wear experiments were performed at 30°C without lubrication. 2.3. Compositional analysis 2.3.1. Ruthetford backscattering

To decide on the presence of surface coatings RBS spectra of both the nonimplanted steel and the implanted samples were recorded using a 1.7 MeV °He ion beam (1= 50 nA, Q = 50 WC, ,fl/4ar = 1.1 msr) and a detection angle of ,9 = 170' . 2.3.2. t4N(d, a,) t2 C analysis

The 14 N(d, IUli)12 C nuclear reaction was applied in thevicinity of the local crosssection plateau at Ed = 1.4 MeV deuteron energy (Q 1 = 9.14 MeV, do,/d,O = 2 mb/sr, -e9 = 168.5°) [8]. A target tilting angle of a = 52 .5° - target normal with respect of the incident beam - provided for a suitable depth resolution. This was tested by examining thin silicon nitride layers (d = 10 nm) deposited on a Si substrate or buried below a 100 nm Si film. The measured spectra of the a1 group are depicted in fig . la. Moreover, fig . 1 presents the a1 spectra gained from nitrogen depth profiles produced by 50 keV NZ (0 = 1.5 x 10 17 N2/cm2, fig . lb)or N+ (45 = 3 x 10 17 N/cm 2, fig. 10 implantation into Si. The profiles were found to agree well with the theoretical predictions [9]. Fig. Id shows the analytical result of a 50 keV N+ plus 50 keV N2 double implantation into Si . As can be seen in this figure, the resulting a1 spectrum correlates well with the energy distribution composed by a proportional superposition of the N+ and Nz profiles presented in figs . lb and lc.

energy (channels) Fig. 3. ERD analysis of the steel sample IC (0 = 8x 10 17 at/cm2 nitrogen implantation without mass separation, 50 kV terminal voltage, increasedcarbon partial pressure pc = 3.2 x 10-3 Pa). Bragg ionization chamber record for35 MeV Cl", a =15°, 0 = 30°. 2.3.3. ERDA - conventional

Our conventional ERD arrangement including a semiconductor detector with Mylar foil has been outlined recently [10]. A CI6+ ion beam of 30 MeV energy was obtained from the 5 MV Rossendorf Tandem accelerator striking the target surface at a = 20° incident angle. The recoil atoms were observed at 6 = 30° with respect to the projectile beam . A special target sandwich structure produced by plasma CVD, alternating SiNH and a-Si : H layers of 20 nm thickness (insert

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Fig. 4. Volume abrasion versus sliding distance for nonimplanted and 1°N-implanted 210Cr46 tool steel . Implantation without mass separation: Il - ~A =1 x 10 17 at/cm2, 12 - 0= 8x 1017 at/cm2, IC - 46 = 8x 10 17 at/cm2 with increased Cpartial pressure; with mass separation : M- 46 =8x 10 17 N/cm 2.

245 of fig. 2), has served for estimating the depth resolution as a function of depth. Fig. 2 presents two regions of the corresponding ERD energy spectrum : the high energy part resolving the 14 N recoil signals of the upper three SiNH films and the low energy part originating from a superposition of the 1H group and the 14N signal of the deepest SiNH layer. 2.3.4. ERDA - Bragg ionization chamber ERD analysis with a Bragg chamber [71 allows simultaneous depth profiling of atoms and separation with respect to the atomic number via the Bragg peak height. As an example, fig. 3 depicts the data set of the steel sample IC (8 x 10 17 at/cmZ nitrogen implantation at increased carbon partial pressure) for 35 MeV Cl ions at a =15° and 19 = 30°. Besides a large number of elastically scattered projectiles the well separated branches of C, O and N recoils are discernable. Regarding ref. [71 the energy resolution was improved by using a more homogeneous polypropylene window of about 40 wg/cmZ thickness . Now typicalvalues DE/E are 0.7 to 0.9%. This corresponds to depth resolution values within 5 and 10 nm .

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3. Results and discussion 3.1. Wear resistance Fig. 4 shows the volume abrasion in relation to the sliding distance as measured by the pin-on-disk experiment . As obvious the wear decreases systematically with increasing fluency of implanted nitrogen atoms. Previous investigations [11 provided 0 = 8 x 10 17 N/cmZ as an optimum fluency for the nitrogen-steel system. At the same number of embedded nitrogen atoms a farther drastic wear reduction at increased carbon partial pressure during implantation was found as outlined by the curve IC in fig . 4. Such a behaviour had already been observed for carbon implantation in steel [111 . In connection with the following compositional analysis it is worth while to mention that implantation of 0 = 8 x 10 17 at/cmZ with mass separated N+ ions (fig. 4, sample M) and implantation without mass separation does not induce marked differences in the wear characteristic. 3.2. Compositional analysis Starting from the wear behaviour of the steel samples doped nominally with equal nitrogen fluencies of d, = 8 x 10 17 at/cmZ but using different implantation conditions (see section 2) it had been important to get information on (i) the amount and the depth distribution of possibly incorporated carbon atoms, especially for the sample IC ; and (ii) the composition identity or

Fig. 5. RBS spectra observed for 1 .7 MeV 4He bombardment of nonimplanted and 14 N-implanted tool steel (see text), random incidence, 0=170°; sample characterization, see fig. 4. the kind of differences within the surface region of the samples 12 and M. For the samples of interest figs. 5, 6 and 7 compare corresponding spectra as observed by RBS, conventional ERD and 14 N(d, a t )1z C analysis. As outlined in fig. 5 (RBS) the Fe surface signal is shifted towards lowerenergies only in the spectrum IC, note that the spectrum of sample M, here not shown, is quasi identical to that one of sample 12.To explain the shift of the Fe signal carbon growth was thought to take place during the implantation process. This assumption was confirmed regarding the corresponding ERD spectra in fig. 6. A broad carbon surface peak of high intensitydominates for sample IC in contrast to 12 and M. Supposing a pure carbon coating a layer thickIII. CONTRIBUTED PAPERS

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Fig. 7. Nitrogen depth profiles (implantation of 8X1017 at/cmZ , (I= 50 kV) measured by means of the 14N(d, a1) 12 C nuclear reaction ; Ed =1 .4 MeV, a=52.5* (target normal) deuteron incidence, 0 =168 .5° a-particle detection .

ness of 1.6 x 10 18 C/cm 2 was estimated from the shift of the Fe edge in the RBS spectrum (fig. 5 sample IC. Strong overlaps of N, O and C signals, especially in the spectra 12 and IC, do not allow to deduce the nitrogen depth profiles by conventional ERDA (see fig. 6) wit'`tout additional information . For this reason the 14yv selective results from the 14N(d, a t ) 12C nuclear re6ction as depicted in fig . 7were helpful. Taking spectrum M as the reference sample (0 = 8 x 10 1 N/cm2, implanted with mass separation and a low current density //A = 4.3 pA/cm2, the C partial pressure pc =1.3 x 10-3 Pa) from fig . 7 it is obvious that only sample IC bears a rough resemblance to the amount and the depth profile of the N atoms. Remember that 12 was

implanted with the same N fluency but using a high current intensity. Surely target heating during implantation results in a N diffusion as indicated in fig. 7, sample 12, by the loss of counts within the peak structure accompanied by the presence of an intense low energy tail . This knowledge allows a better understanding of the complex ERD spectra of fig. 6. Spectrum M is composed by a superposition of signals caused from C inside the steel bulk material (compare fig. 6, nonimplanted), the implanted N depth profile and O (+H) surface peaks originating from a thin oxide (+water) film on the sample surface due to storing on air. To obtain concentration depth profiles a simulation procedure was developed which is discussed in more detail in ref. [10] . Starting from an initial depth profile described by an analytical function (e .g. Gaussian shape) for each element the program calculates the ERD energy spectrum . Adjustable parameters of these functions are changed and the process is iterated until satisfactory agreement between the experimental and calculated spectrum is achieved . Fig. 6, nonimplanted and fig . 6, sample M, include simulated energy spectra obtained under the assumption of a depth constant carbon content and an oxide surface layer. The N distribution in fig. 6, sample M, was assumed to be a Gaussian implantation profile. The quantitative analysis results in oxygen areal densitiesof 5.9 x 10 16 O/cm2 and 2.8 x 10 16 O/cmZ for the implanted sample (M) and nonimplanted steel, respectively. The range at the maximum value and the standard deviation of the N+ ions (fig. 6, sample M) was found to be 60 and 45 nm, respectively . Because of the more complex character of the other spectra in fig . 6 there was no chance to analyse those with the simulation program. The complexity of these spectra arises either from the surpassing carbon peak at the high energy part of the ERD spectrum IC or from the nitrogen diffusion into deeper regions (fig. 6, sample I2). Additionally, a substantial O content was present in the sample 12 originating from a higher oxygen partial pressure within the implanter ion source . The absence of mass separation allows the ionized oxygen to be also implanted into the sample . The utilization of a Bragg ionization chamber [7] permits to overcome these difficulties. Figs . 8 and 9 show energy spectra recorded by the Bragg chamber, and depth distributions of N and O evaluated quantitatively by forward fitting [12] of adopted model functions. Supposing Rutherford cross section the number of projectiles scattered from the steel matrix atoms served for normAization . The qualitative behaviour of the measured 1°N spectra is in correspondence with the results of 1°N(d, a t ) t2C analysis (fig . 7). The calculated N distribution for the sample 12 shows the maximum shifted to 111 . CONTRIBUTED

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energy (channels) energy (channels) Fig. 8. Energy spectra of N and O recoiled atoms measured by ERDA with Bragg ionization chamber; 35 MeV C1 7+ , a =15°, 0 =30°. Samples, 14 N (50 kV) implantation into 210Cr46 tool steel, .0 =8x 10 17 at/cm2, M/1-implantation with/without mass separation, respectively. the surface and a drastic falling off with increasing depth. A comparable shape of the profile is also observed for the implanted oxygen atoms. 4. Conclusions As known from the literature [13], wear-resistant surface regions are created in steel due to the implantation of N atoms which partly move towards the bulk

during surface abrasion. Regarding the samples 12 and M it could be shown that in spite of significant differences in the shape of the N profile an identical wear behaviour was obtained . However, differences in the wear characteristic due to the different initial concentration of N atoms have to be expected when increasing the sliding way. A completely changed wear behaviour occurs in the case of nitrogen-implanted steel covered with a carbon surface layer. The drastic growth of thewear resistance in the case of sample IC we attribute to a solid-state lubrication [14] effect. This assumption is supported by the RBS result (fig . 5, sample IC) which shows a nonmetallic C-coating free of coexistence with Fe atoms. Consequently, there is no formation of hard Fe carbonitrides but a soft carbon surface layer of lubricating properties . Acknowledgements

Fig. 9. N and O depth profiles for sample 12 deduced from theERDenergy spectra of fig. 8.

The authors express their thanks to the collaborators of the Rossendorf Accelerator Laboratory for high beam quality on both the 2 MV van de Graaff and the 5 MW Tandem accelerator. Moreover, we thank Mrs. L. Kumpf and Mr. K. Brankoff for fruitful cooperation in electronics and computer-assisted control of the experiments.

C. Neelmeijer et al. / Ion beam analysis of steel surfaces References [11 U. Scholz, E. Richter and H. Reuther, Schmierungstechnik 20 (1989) 202. [2] G. Debras and G. Deconnick, J. Radioanal. Chem. 38 (1977) 193 . [3] B . Rauschenbach and K. Hohmuth, Crystallogr . Res. and Technol. 10 (1984) 1425 . [4] J .P. Thomas, M. Fallavier and A . Ziani, Nucl. Instr. and Meth . B15 (1986) 443. [5] C. N61schner, K. Brenner, R. Knauf and W. Schmidt, Nucl . Instr. and Meth. 218 (1983) 116. [6] W . K. Chu, J.W. Mayer and M.A . Nicolet, Backscattering Spectrometry (Academic Press, New York, 1978). [7] E. Hentschel, R. Kotte, H.G . Ortlepp, F. Stary and D. Wohlfarth, Nucl . Instr. and Meth. B43 (1989) 82 . [8] R .A . Jadis, Nuclear Cross Section Data for Surface

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Analysis, Department of Physics, Sch,, --ier Laboratory, University of Manchester (1979) . [9] J .F . Ziegler, J.F. Biersack and U . Littmark, The Stopping and Range of Ions in Solids (Pergamon, New York, 1985). [10] C. Neelmeijer, R. Grdtzschel, R . Klabes, U . Kreissig and G. Sobe, Proc. 10th Int. Conf. on Ion Beam Analysis, Eindhoven, The Netherlands, 1-5 July 1991, Nucl. Instr. and Meth. B64 (1992). [11] C . Neelmeijer, E. Hensel, P. Knothe, M. Posselt and E. Richter, Nucl. Instr. and Meth . B42 (1989) 369. [12] E. Hentschel, EBIC - A simulation p.--f!ram for fitting ERD energy spectra measured with a Bragg ionization chamber, to be published in Nucl. Instr. and Meth. B. [13] G . Dienel, U . Kreissig and E . Richter, Vacuum 36 (1986) 813 . [14] J.M . Williams, Nucl . Instr. and Meth. B10/11(1985) 539.

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