Materials Science and Engineering A362 (2003) 200–203

Characterization of surface mechanical properties and microstructureof H13 steel implanted by pulsed tungsten�

Jianhua Yanga,b,∗, Tonghe Zhangb

a Department of Physics, Nantong Institute of Technology, Jiangsu 226007, PR Chinab Radiation Beam and Materials Engineering Laboratory, Beijing Normal University, Beijing 100875, PR China

Received 31 October 2002; received in revised form 27 June 2003

Abstract

H13 steel samples were implanted with tungsten and the improvement in surface mechanical properties was investigated. The tungsten ionimplantation was done using a metal vapor vacuum arc (MEVVA) ion source, with an ion current density of 15�A cm−2. The implantationof tungsten showed a superior wear resistance surface. The relationship between surface mechanical property and microstructure for thetungsten-implanted samples was characterized by Rutherford backscattering spectroscopy (RBS), grazing-angle X-ray diffraction (GAXRD)and X-ray photoelectron spectroscopy (XPS). The tungsten carbides and tungsten oxides were observed in the doped region. The experimentalresults have shown that the formation of WC and WO3 improved the wear resistance of H13 steel.© 2003 Elsevier B.V. All rights reserved.

Keywords:Ion implantation; Wear; Microstructure

1. Introduction

Metal ion implantation has several specific advantages incomparing with other techniques for the surface modificationof materials. The influence of pulsed beam for ion implanta-tion is one of the interesting topics in both ion beam materialsmodification research and radiation effect[1,2]. For modify-ing the mechanical and chemical properties of metals, suchas wear and oxidation resistance, significant effects usuallyinvolve high dose ion implantation (>1×1017 cm−2). A ma-jor limitation for ion implantation with metallic ions was thelong processing time due to the small beam currents of con-ventional implanters. A metal vapor vacuum arc (MEVVA)ion source can produce metallic ion beams at a voltage of100 kV and with an ion current of 50 mA. Therefore, theMEVVA source will be a promising high-current metallicion source in industrial application[3,4]. In this paper, highdose ion implantation in H13 steel was performed. The wearresistance and wear mechanisms of the implanted layerswere studied. The composition and microstructure of the im-

� Supported by the Natural Science Foundation of China (59671051)and the Education Department of Jiangsu Province (02KJD490001).

∗ Corresponding author.E-mail address:yang.yangyangj@163.com (J. Yang).

planted samples were measured by Rutherford backscatter-ing spectroscopy (RBS), X-ray photoelectron spectroscopy(XPS) and grazing-angle X-ray diffraction (GAXRD). Therelationship between surface mechanical property and mi-crostructure was discussed.

2. Experiments and methods

Square H13 steel samples (0.36 wt.% C, 5.7 wt.% Cr,1.6 wt.% Mo, 1.1 wt.% V, 0.8 wt.% Si, 0.2 wt.% Mn) withthe size of 15 mm× 15 mm were used. The typical thick-ness of each sample was 0.4 mm. The samples were pol-ished to a metallgraphic finish before ion implantation. AMEVVA source can generate multi-charge state ions. Themeasured percentages of the tungsten ions with 1–5 elemen-tary charges were 6, 40, 36, 13 and 5%, respectively. Thetungsten ions were implanted into H13 steel to a dose of3 × 1017 cm−2 at an acceleration voltage of 48 kV, corre-sponding to an average ion energy of 130 keV since the av-erage charge state of tungsten ions measured was about 2.7.The beam current density of tungsten ions was maintainedat 15�A cm−2. The measured substrate temperature duringion implantation was 300◦C. The wear measurements ofthe samples were performed with a machine based on the

0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0921-5093(03)00586-0

J. Yang, T. Zhang / Materials Science and Engineering A362 (2003) 200–203 201

pin-on-disc principle[7] using a load of 0.7 N and a slid-ing velocity of 0.3 cm s−1. The wear tests were continuedto 800 circles. The maximum depth (D), the width (W) andS (D × W/2) were measured using an optical interferencemicroscope. Since all the experiment conditions were thesame for the unimplanted steel and implanted steel, theSofwear tracks were directly proportional to the wear rate. Thedry friction testing was carried out by a tribometer with abearing ball. The surface compositions of implanted layerswere measured by Rutherford backscattering spectroscopy.For the RBS measurement, 2.023 MeV alpha particles wereincident on the samples normally. The backscattering ionswere detected at the angle of 165◦. X-ray diffraction patternswere carried out on a Rigaku D/max-RB diffractometer, withthe grazing incidence geometry, using Cu K� radiation. Theincident beam was 2◦ with respect to the sample surface.The crystalline phases were identified using PDF cards. TheXPS spectra of the samples were taken with XSAM800 us-ing Al K� radiation. The experimental resolution of the XPSspectra was 1.0 eV. Electron spectra were calibrated againstcarbon in hydrocarbon layers. XPS of the sample surfaceswere measured after they were etched by argon ions for 30 sin order to remove the contaminative layer of hydrocarbonpartly. XPS signals from a deeper layer of the samples wereobtained after a 4 min argon ion sputtering.

3. Experimental results

3.1. Wear resistance and friction coefficient measurement

For wear testing, interferograms of wear tracks of im-planted and unimplanted H13 steel were obtained by anoptical interference microscope (Fig. 1). The values ofSwere 1.9 × 10−4 mm2 for the implanted sample and 4.2 ×10−4 mm2 for the unimplanted H13 steel, respectively. Theresults indicated that tungsten ion implantation could signif-icantly improve the wear resistance of the implanted sample.The measured friction coefficients were 0.7 for unimplantedH13 steel and 0.4 for the implanted sample, respectively.Besides the value ofS, ion implantation dramatically alteredthe texture of wear tracks. The track implanted, withoutwear patches, showed very little damage. However, severewear has been initiated and some debris like plates couldbe observed in the unimplanted track. It can be concludedthat the wear mode of the tungsten-implanted sample was

Fig. 1. Interferograms of wear tracks of implanted (right) and unimplanted(left) H13 steel samples.

a “plough” wear mode in 800 circles because the track wasonly made up of shallow and narrow grooves. The wear ofunimplanted sample has already been in a “shearing” wearmode when the wear test were continually measured up to800 turns because some debris like “plate” have appeared inthe unimplanted track. Obviously, tungsten ion implantationcan delay the onset of severe wear.

3.2. GAXRD analysis

The GAXRD spectrum from the unimplanted H13 steelsample was shown inFig. 2. Strong peaks belonging toFe(1 1 0) and Fe(2 0 0) were observed. The spectrum (Fig. 2)was the same as that of the pure Fe according to PDF cards.The XRD analysis of the implanted H13 steel showed thepresence of additional characteristic peaks corresponding toWC structure, though the intensity of the additional peakswas much lower than that of the Fe(1 1 0) peak. The Millerindexes of these peaks from the unimplanted and implantedsteel were marked inFigs. 2 and 3, respectively.

3.3. XPS measurement

The C 1s electron spectra from unimplanted H13 steelconsisted of one peak originating from the saturated hydro-carbon before and after a 4 min argon ion sputtering (Fig. 4cand d). The C 1s from the implanted sample showed twopeaks, one corresponding to WC and one originating from

20 30 40 50 60 70 80

0

1000

2000

3000

α-F

e(20

0)

--α

-Fe(

110)

coun

ts

2 θ

Fig. 2. The GAXRD of H13 steel.

20 30 40 50 60 70 80

0

1000

2000

coun

ts

2 θ

-- α

-Fe(

200)

α-F

e(11

0)

--W

C(1

01)

--W

C(1

00)

--W

C(0

01)

Fig. 3. The GAXRD of tungsten-implanted H13 steel.

202 J. Yang, T. Zhang / Materials Science and Engineering A362 (2003) 200–203

286 284 282

Inte

nsity

d

c

b

a

B.E/eV

Fig. 4. C 1s electron spectra from H13 steel: (a) implanted and sput-tered; (b) implanted and unsputtered; (c) unimplanted and unsputtersd;(d) unimplanted and sputtered.

the saturated hydrocarbon before a 4 min argon ion sputter-ing. The intensity of the WC peak from the carbide of tung-sten (WC) hardly lowered after a 4 min argon ion sputter-ing. The W 4f electron spectrum from the implanted sampleshowed three peaks before sputtering (Fig. 5a). The threepeaks could be resolved into two spins doublets so thateach peak had the same Gaussian/Lorentzian line shape andwidth [5], one corresponding to WC and one to WO3 ac-cording to their electron binding energies and GAXRD re-sults. The intensity of the peak originating from WO3 be-came much lower after a 4 min argon ion sputtering. TheXPS was mainly made up of one spins doublet correspond-ing to WC (Fig. 5b). The XPS results suggested the layerbearing WC was much thicker than that bearing WO3. TheFe 2p spectrum from unimplanted H13 steel was quite sim-ilar to that of Fe2O3. This suggested that the sample surfacewas oxidized into Fe2O3 (Fig. 6a and b). Fe 2p electronpeaks from iron metal in the H13 steel appeared along withthe same peak from Fe2O3 due to tungsten ion implantation(Fig. 6c). The Fe 2p electron peaks corresponding to ironmetal were only observed after a 4 min argon ion sputtering(Fig. 6d).

40 35 30

Inte

nsity

-b

-a

B.E/eV

Fig. 5. W 4f electron spectra from H13 steel: (a) unsputtering; (b) sput-tering.

740 730 720 710 700

Inte

nsity

abc

d

B.E/eV

Fig. 6. Fe 2p electron spectra from H13 steel: (a) unimplanted and unsput-tering; (b) unimplanted and sputtering; (c) implanted and unsputtering;(d) implanted and sputtering.

3.4. RBS analysis results

The depth distributions of tungsten in H13 steel was de-rived from RBS (Fig. 7). The charge state for tungsten iongenerated by the MEVVA source may be from 1 to 5, cor-responding to ion energies of 48, 96, 144, 192, 240 keV, re-spectively. This broad energy distribution led to a broaderimplantation depth distribution than would be the case forsingle ion energy. It could be shown that the maximum intungsten concentration was about 27% for the tungsten im-plantation. The position of the maximum in tungsten con-centration was near the surface of the implanted samplesdue to ion sputtering induced by the tungsten implantation.The tungsten dose retained in the implanted sample was1.1×1017 cm−2, which was much bigger than the value cal-culated by the normal collision theory[6] due to the beamheating and radiation enhanced diffusion. The thickness ofthe implanted layer, in which relative atomic concentrationwas bigger than 2%, was 90 nm. The thickness of the mod-ification layer was about 1�m because the thickness of themodification layer is usually 10 times greater than that ofthe implanted layer[7].

0 50 100 15

0

10

20

30

0

C/%

depth/nm

Fig. 7. Tungsten concentration depth profile in H13 steel (48 kV,15�A cm−2).

J. Yang, T. Zhang / Materials Science and Engineering A362 (2003) 200–203 203

4. Discussions

Wear testing results could be interpreted by the resultsfrom RBS, GAXRD and XPS shown above. Ion implanta-tion is a radiation damage process. Tungsten ion implan-tation can form a modification layer with WC precipitate(Fig. 3) in the surface at the suitable ion implantation en-ergies and doses because the tungsten element is a kind ofstronger carbide-forming element than iron. WC is a kindof hard compound and can make H13 steel to be strength-ened. These WC precipitations can be regarded as the stressconcentrators, which can enable a local stress relaxationprocess to occur and make a modified sub-layer developthrough the emission of dislocation. The thickness of thesub-layer may exceed several micrometers for high dosetungsten-implantation due to these relaxation processes.Meantime, the implantation of the heavy tungsten ion witha mean energy of 130 keV can make the crystal grain ofH13 steel become smaller, even form an amorphous surfacelayer in the implanted surface[8]. When the surface withthe sub-layer and small crystal grain was formed the fric-tion coefficient of the steel was dramatically reduced, andthe shearing wear was also reduced. Moreover, oxide oftungsten, WO3 formed by the tungsten implantation, mayweaken the oxidation of iron in the surface layer and im-proved the microstructure of the oxidation layer. There wasno diffraction peak from WO3 in the GAXRD spectrum ofthe tungsten-implanted H13 steel because XPS analysis ismore sensitive than GAXRD. Thus, the surface with lowfriction coefficient, WC and little oxidation of iron couldimprove the wear resistance of H13 steel implanted.

5. Conclusions

1. The tungsten ion implantation of 48 kV, corresponding tothe average ion energy of 130 keV, can improve the wearresistance of H13 steel significantly. The dose retainedin the implanted layer was much bigger than the valuecalculated by the normal collision theory.

2. The improvement mechanisms of wear resistance werephase precipitate strengthening of WC, little oxidationof iron, low friction coefficient surface and forming of ahigh density radiation damage.

Acknowledgements

The authors wish to thank Zhang Huixing for the ionimplantation (Beijing normal university) and Wang Xuemeifor the RBS (Beijing University).

References

[1] G. K Wolf, Surf. Coat. Technol. 83 (1996) 1.[2] J.R. Treglio, Nucl. Instrum. Methods B40–B41 (1989) 567.[3] I.G. Brown, et al., J. Appl. Phys. 63 (1988) 4899.[4] T. Zhang, et al., Surf. Coat. Technol. 128 (2000) 1.[5] L. Ranqvist, et al., J. Phys. Chem. Solids 30 (1969) 1835.[6] F. Schulz, Radiat. Eff. 29 (1976) 31.[7] I.L. Singer, R.A. Jeffries, Mater. Res. Soc. Symp. Proc. 27 (1984)

667.[8] G.C. Nelson, L.E. Pope, et al., J. Vac. Sci. Technol. A 1 (2) (1983)

496.