6
38 Nuclear Instruments and Methods in Physics Research B7/8 (1985) 38-43 North-Holland, Amsterdam CHARACTERIZATION OF AMORPHOUS SURFACE LAYERS IN Fe IMPLANTED WITH Ti AND C J.A. KNAPP, D.M. FOLLSTAEDT and B.L. DOYLE Sandia National Laboratories, Albuquerque, NM 87185, USA The amorphous layers produced when Ti alone or Ti and C are implanted into high purity Fe have been characterized by ion beam analyses and TEM. Ion channeling measurements on an Fe single crystal were used to monitor the amorphous layer thickness, while TEM was used to characterize the implanted alloy’s microstructure. The C and Ti profiles were directly measured by 6 MeV He backscattering. The C profile analysis took advantage of a highly non-Rutherford (a, a) scattering cross-section at high energy. For implanted concentrations 5 20 at.% Ti, both Ti and C are required to produce the amorphous phase. Lower limits on the Ti and C concentrations needed for amorphization have been determined; e.g. with 20 at.% Ti, 4 f 2 at.!% C is required. Ion-implanted C was found to be as effective as C which is incorporated into the sample during Ti implantation in forming the amorphous phase. This result shows how C implantation can be used to form a thicker amorphous layer on Ti-implanted steels in order to extend reductions in friction and wear to more severe wear regimes. 1. Introduction When Ti is implanted into Fe, an amorphous surface alloy forms [1,2]. The implanted layer is actually a ternary alloy including C which has been incorporated into the near-surface of the sample during Ti implanta- tion. The C is acquired from residual gases in the implant chamber [3], presumably when implanted Ti becomes exposed at the surface [4], and migrates into the sample. This process is referred to as “carburiza- tion”, and is believed to be enhanced by the high chemical affinity of Ti for C [1,2,4]. The presence of the C is essential informing the amorphous phase, at least for the Ti concentrations common in implanted alloys, usually = 20 at.% [l]. The carburized layer typically extends to - l/2 the depth of the Ti implanted to high fluences [3,4]. Thus the amorphous layer can be expected to increase in thick- ness if additional C is implanted ‘beyond the depth to which C is incorporated during Ti implantation. Maxi- mizing the amorphous layer thickness is important be- cause of the reduced friction and wear which result when a similar layer is formed on various steels [S-7]. In the experiments to be described here, we have characterized the amorphous alloy in Fe-Ti-C by ex- amining the results of Ti and C implants in pure Fe. Concentration profiles of Fe, Ti and C were measured by ion beam analysis, while the thickness of the amorphous layer was determined by ion beam channel- ing and its structure confirmed by TEM. Detailed ex- amination has allowed us to place minimum limits on the Ti and C concentrations needed to form the amorphous phase in implanted Fe-Ti-C. 0168-583X/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) 2. Sample preparation Ion beam analysis was done on an Fe (lOO)-oriented single crystal of 99.99% purity which was mechanically polished to remove its previous implanted layer, an- nealed - 20 h at 860°C and then electropolished. Iron foil of the same purity was identically annealed to produce - 100 pm grains and electropolished to make samples for TEM. Implants of Ti and C were performed in a vacuum of - 3 x lo-’ Torr at room temperature. Four separate implantation conditions will be discussed here: (I) a 2 x 10” C/cm* implant at 30 keV, (II) a 2 x 10” Ti/cm* implant at 180 keV, (III) 2 X 10” Ti/cm* at 180 keV followed by 2 x 10” C/cm* at 30 keV, and (IV) the previous implant followed by an additional 2 x 10” C/cm* at 50 keV. The total amount of 0 and C after these implants were measured by nuclear reaction analysis [160(d, p)l’O and ‘*C(d, p)13C], and are given in table 1. The high C content after implanting Ti alone reflects the signifi- cance of C incorporation during the Ti implant. The 0 was also detected by ion backscattering and was found to be confined to within a few nm of the surface. Thus the 0 is not expected to significantly influence the analysis at greater depths. 3. Channeling analysis and TEM Rutherford backscattering spectrometry (RBS) was used with channeling on the (100) samples to determine the depth distribution of the disorder resulting from the implants. A 1.5 MeV He+ beam was used with a

Characterization of amorphous surface layers in Fe implanted with Ti and C

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Page 1: Characterization of amorphous surface layers in Fe implanted with Ti and C

38 Nuclear Instruments and Methods in Physics Research B7/8 (1985) 38-43 North-Holland, Amsterdam

CHARACTERIZATION OF AMORPHOUS SURFACE LAYERS IN Fe

IMPLANTED WITH Ti AND C

J.A. KNAPP, D.M. FOLLSTAEDT and B.L. DOYLE

Sandia National Laboratories, Albuquerque, NM 87185, USA

The amorphous layers produced when Ti alone or Ti and C are implanted into high purity Fe have been characterized by ion beam analyses and TEM. Ion channeling measurements on an Fe single crystal were used to monitor the amorphous layer thickness, while TEM was used to characterize the implanted alloy’s microstructure. The C and Ti profiles were directly measured by 6 MeV He backscattering. The C profile analysis took advantage of a highly non-Rutherford (a, a) scattering cross-section at high energy. For implanted concentrations 5 20 at.% Ti, both Ti and C are required to produce the amorphous phase. Lower limits on the Ti and C concentrations needed for amorphization have been determined; e.g. with 20 at.% Ti, 4 f 2 at.!% C is required.

Ion-implanted C was found to be as effective as C which is incorporated into the sample during Ti implantation in forming the amorphous phase. This result shows how C implantation can be used to form a thicker amorphous layer on Ti-implanted steels in

order to extend reductions in friction and wear to more severe wear regimes.

1. Introduction

When Ti is implanted into Fe, an amorphous surface alloy forms [1,2]. The implanted layer is actually a ternary alloy including C which has been incorporated into the near-surface of the sample during Ti implanta- tion. The C is acquired from residual gases in the implant chamber [3], presumably when implanted Ti becomes exposed at the surface [4], and migrates into the sample. This process is referred to as “carburiza- tion”, and is believed to be enhanced by the high chemical affinity of Ti for C [1,2,4].

The presence of the C is essential informing the amorphous phase, at least for the Ti concentrations common in implanted alloys, usually = 20 at.% [l]. The carburized layer typically extends to - l/2 the depth of the Ti implanted to high fluences [3,4]. Thus the amorphous layer can be expected to increase in thick- ness if additional C is implanted ‘beyond the depth to which C is incorporated during Ti implantation. Maxi- mizing the amorphous layer thickness is important be- cause of the reduced friction and wear which result when a similar layer is formed on various steels [S-7].

In the experiments to be described here, we have characterized the amorphous alloy in Fe-Ti-C by ex- amining the results of Ti and C implants in pure Fe. Concentration profiles of Fe, Ti and C were measured by ion beam analysis, while the thickness of the amorphous layer was determined by ion beam channel- ing and its structure confirmed by TEM. Detailed ex- amination has allowed us to place minimum limits on the Ti and C concentrations needed to form the amorphous phase in implanted Fe-Ti-C.

0168-583X/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

2. Sample preparation

Ion beam analysis was done on an Fe (lOO)-oriented single crystal of 99.99% purity which was mechanically polished to remove its previous implanted layer, an- nealed - 20 h at 860°C and then electropolished. Iron foil of the same purity was identically annealed to

produce - 100 pm grains and electropolished to make samples for TEM. Implants of Ti and C were performed

in a vacuum of - 3 x lo-’ Torr at room temperature. Four separate implantation conditions will be discussed

here: (I) a 2 x 10” C/cm* implant at 30 keV, (II) a 2 x 10” Ti/cm* implant at 180 keV, (III) 2 X 10” Ti/cm* at 180 keV followed by 2 x 10” C/cm* at 30 keV, and (IV) the previous implant followed by an additional 2 x 10” C/cm* at 50 keV. The total amount of 0 and C after these implants were measured by nuclear reaction analysis [160(d, p)l’O and ‘*C(d, p)13C], and are given in table 1. The high C content after implanting Ti alone reflects the signifi- cance of C incorporation during the Ti implant. The 0 was also detected by ion backscattering and was found to be confined to within a few nm of the surface. Thus the 0 is not expected to significantly influence the analysis at greater depths.

3. Channeling analysis and TEM

Rutherford backscattering spectrometry (RBS) was used with channeling on the (100) samples to determine the depth distribution of the disorder resulting from the implants. A 1.5 MeV He+ beam was used with a

Page 2: Characterization of amorphous surface layers in Fe implanted with Ti and C

J.A. Knapp et al. / Characterization of amorphous surface layers in Fe 39

DEPTH hm)

200 150 100 50 0

<loo>A.xIs /“I _

,----L---i._+’ Ti,..,,lik.”

\ 2x10” C/cm2(30keV1

\ /?%_ 2x10’7 Clcm2(50k.V

-103nm -

2.5 2.0

DEPT”;:O1’

1.0 0.5 0

.tom./sm2)

Fig. 1. Backscattering spectra obtained from single-crystal Fe

samples along random and (loO)-axis directions using 1.5 MeV He. The shaded areas demarcate the surface amorphous layers.

backscattering angle of 120’ for improved depth resolu- tion. The results of the analysis for each of the four implants are shown in fig. 1. Two depth scales are shown by tic marks for each panel. The lower depth scale is in atoms per cm’, which is directly measured by RBS when the composition is known. Since the com-

Fig. 2. TEM micrographs and diffraction patterns obtained

from Fe samples with implants(I-III): (a) 2 x 10” C/cm’ (30

keV), (b) 2X10” Ti/cm2 (180 keV), and (c) implant (b)

followed by (a).

position necessarily varies in depth for these samples, the scale shown was calculated for each sample using an average composition over the region of interest, as dis- cussed below. The depth scale at the top of each panel is the corresponding depth in nm, assuming the mass density of Fe, 7.86 g/cm3. Although neither of these scales is precisely correct, since the composition varies

I. METALS Kinematics, Metastable phases

Page 3: Characterization of amorphous surface layers in Fe implanted with Ti and C

40

Table 1

J. A. Knapp et al. / Characterization o/amorphous surface layers in Fe

Nuclear reaction analysis of 0 and C content in Fe samples

I II III

Sample

Unimplanted 2x 10” C/cm* (30 keV) 2 x 10”Ti/cm2 (180 keV) 2 X 10” Ti/cm* (180 keV)

+ 2 x 10” C/cm* (30 keV)

Oxygen Carbon (10” atoms/cm’) (10” atoms/cm2)

0.11-0.34 0.21-0.4 0.49-0.63 2.29-2.11 0.33 2.41 0.78 4.12

IV 2 X 10” Ti/cm’ (180 keV) + 2 x 10” C/cm* (30 keV) + 2 x 10” C/cm* (50 keV)

1.13 6.29

in depth and the actual mass dens& are unknown, they are believed to be accurate to within - 108, and serve our purpose here.

Transmission electron microscopy (TEM) was used on samples of large-grain Fe foil which had been im- planted to the same fluences as the single crystal sam- ples. The phases present were thus identified, with a qualitative assignment of their depth distribution. Fig. 2 shows TEM micrographs and diffraction patterns for implants I-III.

Fig. la shows the results from the implant of 2 x 10” C/cm* alone (I). The near-surface region affected by the implant is highly disordered, but is not amorphous, since there is still a channeling effect. TEM examination of a foil treated with this same implant confirms this, as shown in fig. 2a. The dark-field micrograph exhibits precipitates identified by diffraction as Fe& in a bee matrix [8]; no amorphous material was detected. The depth of the disordered region is - 80 nm, assuming the mass density of pure Fe and an average composition of 30% C (as determined by RBS).

The result of implanting 2 X 10” Ti/cm’ alone (II) into the Fe is shown in fig. lb. A surface region with no channeling effect is seen to extend to a depth of - 60 nm. TEM confirms that this region is amorphous, with no other phases but the underlying bee present, as shown in fig. 2b. As is typical for these samples [1,2], the amorphous layer resists the electrochemical thinning process used to prepare electron-transparent thin areas and is left as a free-standing film at the edge of a hole. Thus it is easily confirmed that the near-surface region is a single-phase amorphous alloy.

As will be shown, the depth of the amorphous phase for Ti-implanted Fe is less than the range of the im- planted Ti. The amorphous layer thickness is de- termined by the depth to which sufficient C has migrated to form the amorphous phase with Ti and Fe. Fig. lc shows the results of implanting additional C which extends to a depth beyond the original C, but still overlaps the Ti profile. For this sample, 2 X 10” C/cm*

was implanted at 30 keV (III). The channeling results show that the amorphous region now extends to - 80 nm, a 33% increase in thickness. The corresponding TEM micrograph and diffraction pattern shown in fig. 2c again confirm the amorphous phase with the same diffuse ring pattern, with the important additional ob- servation that no carbide is formed. The layer which is formed with the additional implanted C is essentially identical to that formed with only “carburized” C, but is thicker. The increased thickness with the C implanta- tion could be directly confirmed in the TEM by noting that Fe and Ti X-ray count rates from the amorphous layer were - 2 X higher with the same electron beam current than those from the layer implanted with Ti alone.

A further increase in the layer thickness is possible by implanting even more C to a greater depth, overlap- ping the end of the Ti profile. Fig. Id shows the results of implanting another 2 X 10” C/cm*, but at 50 keV (IV). The amorphous layer now extends to = 100 nm, which is significantly thicker than the amorphous layer produced by Ti implantation alone (= 60 nm).

4. Depth profiles

Rutherford backscattering at 1.5 MeV, although use- ful for obtaining the depth of the amorphous layer, cannot be used to profile the implanted Ti or C. The Ti profile overlays the portion of the Fe spectrum which is reduced by the Ti and C, while the C cross-section is too low to be useful. However, by using a 6 MeV He+ beam, both Ti and C can be profiled. The higher energy separates the Ti signal sufficiently from the reduced Fe signal from the near surface region so that deconvolu- tion is no longer needed, and the cross-section for scattering from C is highly non-Rutherford. Fig. 3 shows the relevant portions of a spectrum obtained from a sample with implant III. The energy corresponding to the surface position is marked for each element. The Fe

Page 4: Characterization of amorphous surface layers in Fe implanted with Ti and C

J.A. Knapp et al. / Characterization of amorphous surjace layers in Fe 41

8 MeV It.+ Backmcatterlns

TI

iL~p; IMPLANTS:

~~:~~;,, 412 413 414 1 I I 1

. . . . . .

ENEFiGY (MeV)

Fig. 3. Portions of a backscattering spectrum obtained with 6 MeV He from an implanted Fe sample, showing the separate sections used to derive the Ti, C and Fe concentration profiles.

signal shows the reduction due to the presence of Ti and C from the leading edge down to - 4.4 MeV, while the Ti signal near 4.3 MeV is well separated, overlaying a flat background. The C signal near 1.5 meV is increased by 50 x compared to the Rutherford cross-section and also overlays a smooth background. A spectrum ob- tained from a pure C sample confirmed that the en-

s (.4) IMPLANTS:

2x10” Tl/cm2 (180 kav)

4

- TI

_---.-- c

, .____ , m I

2x10” Tl/cm2 (180 keV) r--x__. 2x10” C/cm2 (30 kd’)

0

ii,,^ ,

0 0.5 1.0 1.5 2.0

OEPT” (10’8 atom./sm2)

Fig. 4. Concentration profiles of Ti and C derived from 6 MeV backscattering. Panels (a), (b), and (c) correspond to implants II, III, and IV which produced an amorphous layer, with the depth of that layer measured by channeling (fig. 1) indicated by arrows.

hanced C cross-section is constant over an energy range much greater than the 0.2 MeV needed here for profil- ing C.

The concentration profiles were obtained from spec- tra such as fig. 3 by first subtracting a smooth back- ground from the C and Ti signals. An iterative calcula- tion of the concentrations versus depth was then per- formed, first using pure Fe for the stopping powers, then the resulting Fe-Ti-C ratios versus depth from the previous step. This procedure usually converged by the third step.

Fig. 4 shows the measured Ti (solid line) and C (dashed line) profiles for the three implants (II-IV) which produced an amorphous layer. Each case is plotted as concentration in at.% versus depth in atoms/cm2. The thickness of the amorphous layer in atoms/cm2, as measured by channeling, is indicated for each implant. After the initial Ti implant, the peak Ti concentration is - 20 at.%, with a C concentration of - 18 at.% near the surface. As more C is implanted, first at 30 and then at 50 keV, the relative concentration of Ti is reduced to 12-14 at.% while the peak C concentration reaches - 50 at.%.

It is clear from fig. 4 that the formation of the amorphous phase requires both Ti and C, at least for these Ti concentrations (5 20 at.%). For example, the Ti-only implant (II) is amorphous only to a depth of - 0.55 X 10’s atoms/cm2 even though the Ti profile extends well past that depth. Clearly, the amorphous layer thickness was limited by the depth to which C was incorporated. Another experiment which confirmed the need for C was monitoring the ion-induced Ti X-rays under channeling conditions. Although ion-induced X- rays do not provide depth resolution, the technique can give useful information vjhen the excited species has a known distribution, as in the present situation. By ob- serving the total Ti X-ray yield for both random and (100) directions, one obtains an approximate indication of the amount of Ti in the amorphous phase. The observed Ti X-ray channeling yield reduction for the Ti-only implant (I) [(l - xti) = 2283 was - 50% of that for Fe, and decreased to - 25% of that for Fe in the sample with both additional C implants (IV). This indicates that only a portion of the total Ti was con- tained in the amorphous phase for the Ti-only implant and that the C implantations increased that amount, just as the measured Ti and C profiles show. The Ti K, X-ray channeling scans had the same angular width as the Fe K,, suggesting that the Ti not in the amorphous phase was on substitutional sites of the bee Fe lattice.

5. Discussion

The results that we have obtained on implanted Fe-Ti-C alloys are summarized with the ternary con-

I. METALS Kinematics, Metastable phases

Page 5: Characterization of amorphous surface layers in Fe implanted with Ti and C

42 J.A. Knapp et al. / Characterization of amorphous surface layers in Fe

Amorphou8 Phase Boundary in

Fe-Ti-C Alloys

Fig. 5. A diagram showing the observed concentrations of the amorphous phase resulting from Ti and C implants into pure Fe. Filled circles are the peak concentrations in the amorphous alloy for implants II and III. Half&Bled circles with error bars are the concentrations at the lower (deep) edge of the amorphous layers, The square symbols are amorphous concentrations from previous work [1,2]. Open circles are concentrations of alloys observed to be crystalline (bee Fe+Fe&). The approximate boundary for minimum Ti and C concentrations needed to form the amorphous phase is indicated by cross hatching.

centration diagram in fig. 5. Points which correspond to our observations of the amorphous phase, and an ap- proximate boundary between that phase and crystalline phases are shown. The region that we have explored is bounded roughly by 22 at.% Ti and 50 at.% C.

Along the C axis (0% Ti) we have not observed an amorphous phase for C concentrations up to nearly 50 at.%, although an amorphous alloy which probably con- tained 2 50 at.% C implanted at room temperature has been reported [9]. Data are more difficult to obtain along the Ti axis (0% C) because of the carburization process. For our concentrations Ti alone does not pro- duce the amorphous phase; at 20 at.% Ti 4 f 2 at.% C is necessary for amorphization.

Interior to the diagram in fig. 5, the filled circles correspond to measured peak Ti and C concentrations known to be in the amorphous phase for implants II and III, and the filled squares are previously determined amorphous phase concentrations [1,2]. The half-filled circles with error bars are the concentrations at the back edges of the amorphous layers; the error bars were assigned from a f 8 nm uncertainty in the thickness of the amorphous phase. The hatched region is thus an approximate boundary of minimum Ti and C con- centrations needed to form the amorphous phase. For concentrations to the Fe-rich side of this boundary, we generally observed the bee Fe-based phase, with Fe,C precipitates at the higher C concentrations (open circles). In some samples with concentrations below those needed

for the amorphous phase, the material exhibits two phases, crystalline bee and amorphous [1.2]. The square dot at 3 at.% Ti/25 at.% C is the estimated concentra- tion of the amorphous phase in such a two-phase alloy. The minimum concentrations of Ti and C necessary to form an amorphous implanted alloy are seen to exhibit a well-behaved relationship to each other; an increase in the concentration of either one reduces the minimum concentration of the other needed for amorphization.

It is clear that either carburization or C implantation may be used to form the amorphous phase. It has been suggested that ion implanted C does not yield the same mechanical properties in 52 100 steel as does carburiza- tion [3]. However, since the implantation of either Ti or C to a fluence of 10” ions/cm2 induces many displace- ments per atom in the near surface (> 50 dpa for 80-180 keV Ti to a depth of R, + AR p in Fe; > 10 dpa for 30-50 keV C [lO,ll]), the source of the C should not be expected to make a difference. The increase in amorphous layer thickness by implanting additional C beyond the carburized layer is consistent with our re- cently reported result that reductions in friction and wear in 15-5 PH stainless steel with similar amorphous surface layers are extended to more severe wear regimes by C implantation [12]. Since such ttibological improve- ments are associated with the amorphous phase in all steels implanted with Ti and C examined to date [6], we expect that an appropriate C implantation will prove beneficial for every Ti-implanted steel.

This work was performed at Sandia National Laboratories and was supported by the U.S. Depart- ment of Energy under contract number DE-ACO4- 76DPOO789. We thank D.K. Brice for valuable calcula- tions. Technical assistance by R.E. Asbill, W.R. Ander- son, and N.D. Wing is gratefully acknowledged.

References

[l] D.M. Follstaedt, J.A. Knapp and ST. Picraux, Appl. Phys. Lett. 37 (1980) 330.

[2] J.A. Knapp, D.M. Follstaedt and ST. Picraux, in: Ion Implantation Metallurgy, eds., C.M. Preece and J.K. Hirvonen (AIME, Warrendale, PA, 1980) p.152.

[3] I.L. Singer and T.M. Barlak, Appl. Phys. Lett. 43 (1983) 457.

[4] I.L. Singer, J. Vat. Sci. Technol. Al (1983) 419. [5] L.E. Pope, F.G. Yost, D.M. Follstaedt. J.A. Knapp and

S.T. Picraux, in: Wear of Materials, ed., K.C. Ludema (ASME, New York, 1983) p. 280. The 15-5 PH and Nitronic 60 implanted sample data were interchanged in figs. 3 and 4 of this paper.

[6] D.M. Follstaedt, F.G. Yost and L.E. Pope, in: Ion Im- plantation and Ion Beam Processing of Materials. eds., G.K. Hubler, C.R. Clayton, O.W. Holland and C.W. White (North-Holland, New York, 1984) p. 655.

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/.A. Knapp et al. / Characterization of amorphow surface layers in Fe 43

[7] D.M. FoIlstaedt, F.G. Yost, L.E. Pope, ST. Picraux and J.A. Knapp, Appl. Phys. L&t. 43 (1983) 358.

[8] D.M. Foilstaedt, these Proceedings (IBMM ‘84) NucI. Instr. and Meth. B7/8 (1985) Il.

[9] B. Rauschenbach and K. Hobmuth, Phys. Stat. Sol. (a) 72 (1982) 667.

[lo] P. Sigmund, Appl. Phys. I&t. 14 (1%9) 114. [ll] D.K. Brice, J. Appl. Phys. 46 (1975) 3385. [12] D.M. Folhtaedt, J.A. Knapp, L.E. Pope, F.G. Yost and

ST. Picraux, Appl. Phys. Len. 45 (1984) 529.

1. METALS Kinematics, Me&stable phases