Ion implantation in metals—structure investigations and applications

Vacuum/volume 38/number Printed in Great Britain

11 /pages

987 to 971 I1 988

0042-207X/88$3.00+.00 Pergamon Press plc

Ion implantation in metals-structure investigations and applications H Reuther, 6 Rauschenbach and E Richter, Centrallnstitute of Sciences of the GDR, PO Box 79, Dresden, 8057, GDR

for Nuclear Research Rossendorf, Academy

ton implantation is a method for modifying surface sensitive properties of different materials. it is well known for doping semiconductors, and in recent years also for metals. The surface effects are caused by structural changes in the uppermost surface layers (some l&l00 nm). in this paper, ion implantation in iron and steel is investigated. Experimental methods used here to get information on such structural changes are high voltage electron microscopy and conversion electron Mossbauer spectroscopy. Dependent on the implanted ion (boron, nitrogen, phosphorus) and the implantation conditions, compound formation andjor amorphization takes place. For industrial applications, an ion implanter was constructed. Results of some practical applications are also presented.

1. Introduction

Ion implantation has been established as a method to influence the surface properties of metals. In this way, it is possible to modify their mechanical, chemical, electrical and thermal properties. Ion implantation possesses several distinct advantages over other techniques for the surface treatment of materials. These are the ability to introduce any ion into the surface region of any substrate without the constraints of thermodynamic phase equilibrium, the ability to perform the implantation at a low process temperature, the ability to perform the treatment without further need of annealing or refinishing, and the absence of coating adhesion problems. The disadvantages are the relatively high process costs and the restriction to visible areas because it is a line-of-sight process. This precludes ion implantation treatment of parts with obscured or re-entrant featuresle4. Ion implantation means energy deposition in the target. The incoming ions lose their energy as a result of interactions with the target electrons and elastic collisions with the target atoms. For heavier ions the latter process dominates at energies up to 200 keV which are generally used for ion implantation. Through elastic collisions, target atoms are displaced from their lattice positions and a highly disordered region with a cascade of vacancies and interstitial atoms is formed along the way of the penetrating ion until the projectile comes to rest. The strong rearrangement of atoms in the cascade may result in an amorphous or a distorted crystalline zone. Relaxation takes place within a period of about lo-‘* s, and re-distribution is possible only in parts. It should be noted that the implanted region is not homogeneously doped in depth, but there is an approximately Gaussian concentration distribution with a maximum at 10-100 nm depending on the ion energy. So it is possible to get structures in the implanted region having different short and/or long range order which can overlap each other or be mixed. For low implantation doses, range distributions can be cal-

culated according to the theory of Lindhard et al’ which predicts a Gaussian implantation profile. Rp denotes the mean projected range and ARp the standard deviation. The implantation depth decreases with increasing ion mass and increasing the energy results in an increase of the penetration depth. Figure 1 shows a comparison between calculated and measured nitrogen profiles in iron. The doses are 1 x IO”, 5 x 10” and 10 x IO” ions cmm2, respectively, and the implantation energy is 50 keV6. The solid lines indicate the predicted distributions

0

0 0

20

Lo

25 25

60 60 DEPTH Cm1 Yl

50

loo

120

75 kS~~N’an-2$5

75

ll0 1z 5XlnN*an-2) MEAsuRbni TIME hhl

Figure 1. Comparison between calculated and measured profiles in iron after implantation at 50 keV.

. 175

nitrogen

depth

967

H Reuther et al: ion implantation in metals

according to LSS theory; points and dashed lines represent measurements by Auger electron spectroscopy. It is seen that the accuracy of LSS theory is sufficient up to ion doses of IO” cm-2. For higher implantation doses it is obvious from Figure 1 that the maximum concentration in the peak is lower than calculated. The profile is also much broader with a plateau region and the concentration close to the surface is higher. 2. Structural investigations of metalloid implanted iron There are several analytical techniques used to obtain information on submicrometre layers, e.g. Rutherford backscattering spectroscopy, Auger electron spectroscopy and nuclear reaction analysis, but they give no. information about microscopic structures. Electron diffraction carried out with the high voltage electron microscope and iron-57 conversion electron Mtissbauer spectroscopy (CEMS) are the most suitable techniques to study thin surface layers. We present some results obtained by these methods where iron was implanted with different metalloid ions.

I

,

, ,/
I .,.mIl lo’*

10” Blcni*

the sample is annealed to temperatures of 400°C and more one obtains a completely changed diagram which is shown in Figure 2b. Here, the sharp peaks clearly can be ascribed to different iron borides, that is, the amorphous fractions were converted into crystalline structures. By varying the dose and the temperature it is possible to obtain the state diagram shown in Figure 3’. Depending on temperature, there is a critical dose which is sufficient to amorphize the implanted region. Annealing at temperatures of 400°C and above, always gives rise to crystalline compounds. Therefore it may be concluded that the implantation has to be carried out at sufficiently low temperatures if amorphous fractions are desired for the example discussed here. 2.2. Conversion electron Miissbauer spectroscopy. CEMS is a special case of the Miissbauer spectroscopy because it is not a transmission but an emission method. The probed depth is some 200 nm, a range which is generally used for ion implantation%’ ‘.

lb)

(a)

968

I,

Figure 3. State diagram of iron implanted with boron at 50 keV.

2.1. Electron microscopic investigations. Polycrystalline iron films with a thickness of about 1 pm were implanted at room temperature with 50 keV boron ions. The ion dose was varied between lOI6 and 10” cmb2. Studies were made with the selectedarea diffraction technique using a 1 MeV JEM-1000 electron microscope’. To investigate heating effects, the samples were annealed in situ in the microscope up to 600°C. The identification of phases after implantation and annealing was made with the help of electron scattering diffraction diagrams. Figure 2a shows the diffraction pattern of a sample implanted with 1 x 10” ions cme2. Only diffuse rings are seen which are produced by amorphous fractions. There are no sharp peaks. If

Figure 2. Electron

I

O10’6

diffraction

pattern

of iron implanted

with IO ” cnl-’

boron

ions at 50 keV (a) before and (b) after annealing

for 10 min at 400°C.

H Reuther

et a/: ion implantation

in metals

Table 1. Parameters for nitrogen implantation in iron

Sample

Dose cm-2

Nl

5x lO”N+

N2 N3 N4

10” N+ 10” N+ lOI8 N+

2

Energy keV

Current density PA cm-’

100

5

50 50 50

5 5 32

Vacuum Pa

Remarks*

< 1o-4 N 1o-4 5 x lo-’ > 5 x 10-l

MS, no cooling MS, water cooled Interval implantation, no MS

* MS = mass separation.

In a first series of experiments CEMS was used to test different ion implantation machines ” . The implanted ion was nitrogen. Dose and implantation energy were the same for all samples but other parameters such as temperature during implantation, residual gases and vacuum were varied. The implantation conditions are given in Table 1. In one case (sample Nl) the energy was 100 keV but molecular nitrogen ions were implanted. When a diatomic Nz ion strikes the sample surface it dissociates into two SOkeV N+ ions and produces the same effect as implanting the two-fold dose of SOkeV single ions. Implantation was carried out with three different ion implanters. The machine without mass separation is used for industrial applications. Without mass separation, the ion beam contains both single and molecular nitrogen ions and residual gas ions such as oxygen or carbon. Figure 4 shows the Miissbauer spectra of the implanted sam-

ples together with the spectrum of normal alpha-iron. Alphairon produces a six-line pattern which is found in all spectra. However there are pronounced differences between the spectra, although the investigated layers contain the same amount of nitrogen. From the analyses of the spectra it is evident that the iron environment is different in all samples and th:ct different iron nitrides are formed. The central quadrupole doublet found in samples N2 and N3 is ascribed to Fe,N. A small fraction of this phase could also be detected in samples Nl and N4. But the major part is due to the high temperature phases a- and y’-iron nitride. Why is there such a difference? Samples N2 and N3 were little affected thermally during implantation while samples Nl and N4 were heated considerably as a consequence of the implantation conditions. From the fact that a- and y’-iron nitride were formed one can conclude that substrate temperatures of 300°C and more were reached during implantation. Furthermore, the spectra of samples N3 and N4 show additional lines. These could be ascribed to iron oxide. This oxide is formed as a consequence of the lack of mass separation. So oxygen from the residual gas could reach and oxidize the samples.

.

I -8

-4

0

I

8

velocity in mm/s

Figure 4. CEM spectra of nitrogen-implanted keV).

iron (IO’* ions cm-*, 50

VELOCITY [mm/s I Figure 5. CEM spectra of iron implanted with phosphorus (5 x 10” ions cm-‘, 100 keV) before and after annealing for 1 h at 400 and 500°C. 969

H Reuther

Table 2.

et al: Ion implantation

Implanter

for industrial

in metals

‘Meise’

applications

Energy Max 50 keV Current Max6mA Current density Max 50 PA cm-* Implanted area Max9x 15cm’ Pressure in the target chamber 5x IO-‘Pa No mass separation! Possible target motion, rotation and cooling Ion production by a slot source (Bemas) for gases (N, P, Ar, . . .) and a sputter source (Reuther) for solids (Ti, Cu, Al,. .) 1

3

5

7

9

1017N ‘cme2 implantion dose

It can be assumed

that carbon was also implanted because Auger electron spectroscopic measurements always show the presence of carbon under implantation conditions similar to

those used here. However, it is very difficult to distinguish iron nitrides and carbonnitrides by Mossbauer spectroscopy. In a second series, implantation of phosphorus into iron was studied. Electron microscopic investigations have shown that high implantation doses lead to the implanted layer becoming amorphous”. With the aid of Miissbauer spectroscopy it is possible to observe the short-range order of the amorphous structure. To be sure to obtain an amorphous region, a high dose of 5 x 10” phosphorus ions cm-’ at 100 keV was implanted. The Mossbauer spectra are shown in Figure 5. The spectrum of the as-implanted sample is seen at the bottom. The six-line pattern of alpha-iron is overlapped by a broad central line. The latter could be resolved into two quadrupole doublets with the hyperfine parameters of Fe,P but with very broad lines. An amorphous structure is formed in the implanted layer with a short-range order similar to Fe,P which is paramagnetic. Up to now only amorphous-iron phosphorus alloys, which are ferromagnetic with a short range order similar to Fe,P were known’%“. This it is possible by ion implantation to produce new amorphous components which are paramagnetic16. Annealing the sample at temperatures of up to 400°C results in a narrowing of the doublet lines, i.e. precipitation of Fe,P takes place. Between 400 and 500°C the paramagnetic Fe,P.is converted into ferromagnetic Fe3P, represented by four sextets in the Miissbauer spectrum. The transformation is abrupt without any coexistence between the two phosphides. 3. Applications There are a lot of successful practical applications of ion implanHowever, the obtained improvements tation in metals4.‘““. often cannot yet be explained successfully. Great Britain, USA and Japan offer industrial equipment for tool implantation whose success is undisputable. The technical investigations presented here were made using

Figure 6. Microhardness of steel 100 W 1 after implantation with different doses of nitrogen at 50 keV.

an implanter built especially for industrial applications. The operating parameters of this implanter are given in Table 2. Besides the test of finished tools, a systematic study of tool steels (for compositions see Table 3) was carried out. They were implanted with nitrogen and examined by the microhardness and by the pin-on-disk method. In the latter case, the sample is a rotating disk on which a pin is allowed to slide. The volume abrasion is determined in relation to the sliding distance. The load on the pin-ball can be varied and the sliding process can be carried out in air, in an inert gas atmosphere or with a lubricant. Figure 6 shows microhardness results obtained by nitrogen implantation of a carbon steel (100 W 1). Although the indenter pushes far through the implanted layer the effect of the ion implantation is remarkable. There is an optimum ion dose for which the microhardness has a maximum. At this dose the wear rate is found to have minimum”. The next figures show results obtained by the pin-on-disk method on tool steels implanted with nitrogen ions and compared with non-implanted materials. The diagrams show the sliding distance vs the volume abrasion”. Figure 7 shows the results of a bearing steel (100 Cr 6). For a dose of 4 x 10” ions cmm2 no change in the wear behaviour could be detected. Implantation with 8 x 10” ions cm-’ leads to a large decrease of the wear rate but implantation with 12 x 10” ions cm-* makes the sample worse than initially. The tool and spring steel (67 Si Cr 5) shows a similar behaviour (Figure 8). Again there is an optimum dose for which the implantation effect is especially high (8 x 10” ions cm-*). The merging of the lines at large sliding distances has a simple explanation : the implanted layer is completely removed and one observes the behaviour of the non-implanted bulk material. As a practical example, a forming tool for cigarettes is shown in Figure 9. The lifetime of the tool could be increased by a factor

Table 3. Chemical composition of the investigated steels (WT%) Steel

C

Si

Mn

P

S

Cr

Ni

61 Cr Si 5 1oow 1 100 Cr 6

0.67 0.10 0.10

1.30 0.20 0.17

0.50 0.20 0.20

0.025 0.015 0.25

0.025 0.020 0.020

0.50 0.15 1.50

0.20

970

H Reuther

et al: Ion implantation in metals

to*

2

slldlng

way Im)

5

103 r

without

lubrication

2

5

Figure 9. Cigarette forming tool implanted with nitrogen.

wL

Figure 7. Volume abrasion vs sliding distance for steel 100 Cr 6.

of 4 as the result of implantation with 8 x 10” cm-’ nitrogen ions at an energy of 50 keV with the implanter ‘Meise’. Until now, the best results were achieved with cutting tools for plastics. ‘The lifetime of these tools could be increased by a factor of 10. 4. Conclusions In conclusion, some general remarks are made on the optimum conditions of ion implantation for industrial applications: the optimum effects were obtained by nitrogen implantation with energies of 40-80 keV. Indeed, the implanted layer only reaches a thickness of about 100-200 nm but the effect often considerably exceeds this range. Optimum ion dose was found at 4-8 x 10” cme2. Lower doses produce no visible effect and higher doses sometimes result in a change of the initial state for the worse. The current density should not be higher than 5-8 PA cme2.

Higher currents heat the samples too much, lower currents lead to longer irradiation times. An essential point is the vacuum in the target chamber since residual gases may have a significant influence on the implanted layer. Finally, ion implantation can be used successfully for reducing the wear or improving the lifetime of steel and hard metal tools. But it is ineffective for all applications involving high process temperatures. This is because of the instability at higher temperatures of the nitrogen-induced defects and precipitates formed in the surface layer during implantation which are thought to be responsible for the beneficial influence. In the case of phosphorus or boron implantation, the amorphous layers seem to cause the positive changes and they are destroyed if the sample becomes too warm.

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Figure 8. Volume abrasion vs sliding distance for steel 67 Cr Si 5.

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