A TEM and EXAFS study of TiB2 coatings produced by dynamic ion mixing

Nuclear Instruments and Methods in Physics Research B 95 (1995) 327-333

&*H

NOMB

Beam Interactions

with Materials & Atoms

ELSEVIER

A TEM and EXAFS study of TiB, coatings produced by dynamic ion mixing P. Chartier

*,J.P. Riviere,

J. Mimault, T. Girardeau

Laboratoire de M&allurgie Physique. URA 131 CNRS, 40, avenue du Recteur Pineau, 86022 Poitiers Cedex, France

Received 7 October 1994; revised form received 18 November 1994 Abstract TiBz ceramic coatings have been deposited by the Dynamic Ion Mixing (DIM) technique, at room temperature, on different substrates. The depositing atom flux was obtained by co-evaporation of B and Ti using two electron beam evaporators. The characterisation of the films was performed by Transmission Electron Microscopy (TEM) and Conversion Electron Extended X-ray Absorption Fine Structure (CEEXAFS). Complementary information has been deduced from both TEM and CEEXAFS data. The influence of the mixing conditions and the deposition rate on the structure of the film have been observed. The TiB, coatings prepared by DIM are crystallised with an hexagonal structure while those prepared without mixing are principally amorphous. The columnar growth tendency is reduced when the mixing efficiency is increased or the deposition rate decreased. It is shown that CEEXAFS gives reliable and detailed information on the crystallisation state around titanium atoms. 1. Introduction Because the surface of an engineering component is a critical region, it is often necessary to improve its resistance against wear and corrosion. Titanium diboride (TiB,) coatings have recently received much attention [l-3] as a possible ceramic coating with excellent mechanical and chemical properties at high temperature. In addition, as a consequence of its metallic-like bonding, it exhibits good thermal and electrical properties. Different methods have been reported for the production of TiB, coatings [l-5]. However, several investigations [1,2,6] have demonstrated that the new method of Dynamic Ion Mixing (DIM) consisting in simultaneous deposition and bombardment by a high energy ion beam is of great interest for producing adherent TiB, coatings at low temperature. One of the unique features of DIM is to build up surface coatings which are thicker (l-2 pm> than those produced by direct ion implantation or conventional ion mixing of predeposited multilayers. Nevertheless, in spite of the limited depth of these latter techniques (0.1-0.2 pm) beneficial surface modifications have been achieved. The majority of the experimental work in the field of Ion Beam Assisted Deposition (IBAD) has been performed using low energy ion beams in the range 10 eV-10 keV [7-lo]. In recent

* Corresponding 49453759.

author.

Tel.

+33

49453631,

fax

+33

years, it appeared that there would be several advantages in using heavy ion beams of higher energy (100-300 keV) [6,11-131 for bombardment during deposition. Energetic ions produce dense displacement cascades and can induce an effective collisional mixing firstly at the coating-substrate interface and secondly in the growing film. The consequences are an increased adhesion to the substrate and the possibility of controlling the micro structural state of the coating. Concerning ceramic TiB, coatings, recent experiments provide evidence of the role of the high energy ion bombardment during deposition since Transmission Electron Microscopy (TEM) experiments show that TiB, is obtained in the amorphous state by codeposition alone while a nanocrystalline structure is formed by dynamic ion mixing [l]. The displacement per atom (dpa) is an essential parameter governing the crystallisation induced by DIM. It has also been demonstrated for this material that DIM induces interface mixing and improves the adhesion performance of the coating [6]. The high degree of control over the flux and energy of the incident ion beam allows the microstructural state of the film to be modified in a reproducible way. In this article, we report both TEM and CEEXAFS (Conversion Electron Extended X-ray Absorption Fine Structure) microstructural characterisation of TiB, coating prepared by DIM using electron beam codeposition. One of the main objectives of this study is to take advantage of the unique feature of the CEEXAFS technique to obtain information about the local structure around the titanium

0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(94)00601-6

328

P. Chartier et al. /Nucl. Instr. and Meth. in Phys. Res. B 95 (1995) 327-333 3.5

atoms. We focus the discussion on the importance of the bombardment conditions in relation with the microstructural final state of the coating.

-A3 -Al -AZ

3

Sample Sample Sample

I

I

2. Experimental details TiB, films were deposited on two types of substrates at room temperature: rock salt for TEM experiments and quartz for CEEXAFS study. The deposition apparatus, described in detail elsewhere [13] consists of an UHV chamber equipped with two automatically controlled electron guns in line with a 200 kV ion implanter. The base pressure is about lo-’ to lo-* Pa and is kept always lower than lop6 Pa during film deposition. The coating thickness, measured by a calibrated quartz crystal oscillator, is limited to 100 nm. The ion mixing was performed with Art ions at different energies as listed in Table 1, where Q is the ion per atom arrival rate ratio. The sample A0 was deposited without mixing. The evaporation rate u = 0.46 rims-’ and the flux 1.56 X 1Ol3 Ar+ ions/scm* were chosen as basic deposition conditions (Al sample). The rate was reduced to u = 0.3 nm s-l in order to enhance the mixing efficiency (A2 sample). Finally, in the last series of samples (sample A3), we have tried to obtain a relatively uniform damage profile throughout the thickness of the coating chiefly up to the near surface region. In order to construct such a profile we have decreased the ion energies from 160 keV at the beginning of the deposition process, to 80 keV and finally 50 keV at the end of the mixing process. We have calculated with the TRIM Monte Carlo program [14], the damage depth profile for the different mixing conditions selected in this study. The universal dpa damage unit is used for these calculations. We present in Fig. 1 the calculated dpa damage profiles for 100 nm thick coatings. They will be useful for comparing and discussing the influence of the mixing conditions on the microstructural state of the coating. The TEM diffraction patterns were performed using a Jeol 200 CX electron microscope working at 200 kV. CEEXAFS experiments were performed at LURE (Laboratoire pour I’Utilisation du Rayonnement Synchrotron, Orsay, France) using the synchrotron radiation from the DC1 storage ring operating with an energy of 1.85 GeV and a current of 230 rn4. The electron yield spectra

0

20

40 60 Thickness (urn)

I

100

Fig. 1. Damage depth profile in TiB, coatings prepared by different DIM conditions. were recorded by using the electron detector device described and improved by Tourillon et al. [1.5]. Absorption spectra were collected over 700 eV above titanium K-edge (4966 eV). The EXAFS oscillations were analysed according to a standard procedure described in detail in Ref. [16]. Fourier transformation (F.T.) was performed over the range 2-11 nm-’ using nodal points for the Hamming window function, k3 weighted. The EXAFS contributions of the first coordination shells (Ti-B and Ti-Ti bonds) were separated, singled out and backtransformed in k space; then their real part was simulated using the widely reported formula [17] given by: kXi(k)=

C[

fi(k2~>4j/r$]

x exp( - 2criTk2)exp(

Xsin[2krij

- 2kri,/T)

+ @ij(k)],

where Nijlfj(k,r)l is the backscattering amplitude of Nij atoms of type j surrounding the absorbing atom of species i, a distance rij apart; aiT is the mean square relative displacement (MSRD) assuming a Gaussian distribution; the term exp( - 2miTk2) is also called the DebyeWaller factor; exp( - 2kr,j/r) is a mean free path term taking into account inelastic effects and cbij(k) is the sum of the central and backscattering phase shift. The amplitude If,(k,n-)I and the total phase shifts &j(k) used here are theoretical values calculated by MacKale et al. [18]. It is important to mention that the CEEXAFS technique is a local probe which samples only the immediate surface

Table 1 Sample deposition conditions s-l

80

Sample

Deposition rate [nm

A0 Al A2

0.46 0.46 0.30

no mixing 3.4 x 10” Ar’/cm* (80 keV): Q = 32 X 10m4 5.6 x 10’*Ar+/cm2 (80 keV): Q = 48 X 10e4

A3

0.46

8.2 x lOI Ar+/cm*

(160 keV): Q = 12 X 10e4

5.6 x lOI Ar+/cm*

(80 keV): Q = 33 X 1O-4

9.4 x 1014 Ar+/cm’

(50 keV): Q = 33 X 10W4

DIM procedure

P. Chartier et al. / Nucl. Instr. and Meth. in Phys. Res. B 95 (1995) 327-333

0.8

-

a. 0.6

-

329

5 zo.4 z

-

1::ti’/ 0

50

p!ggqq 100 150 200 Thickness z (nm)

Fig. 2. Experimental Ti K-edge deposited layer thickness.

step height

250

300

versus

TiBz

alloy Fig. 4. Diffraction pattern and dark field of a TiBa coating deposited by MID with 3.4 X 10” Ar+/cm’ at 80 keV: Q = 32 X 10-j (Al sample).

of the film, while TEM gives rather uniform information on the whole film thickness (100 nm). We have recently established that the CEEXAFS signal is associated with a typical escape depth D [19]. In fact, for a depth limited film, the EXAFS K-edge jump is approximativaly matched by the following exponential law:

more efficient. We first present TEM results, which give the long range order of the structure as well as the grain size of the coatings. After that the CEEXAFS signals will be interpreted to provide a more local picture of the structure.

S(z)=S,(l-exp(-z/D)), where z is the probed film thickness, D is an effective escape depth and S, represents the signal related to bulk thickness (z 1, D). We carefully analysed, for Ti K-edge atomic threshold, the behaviour of the probed depth by examining the signal magnitude (pre-edge and edge data) versus thickness in the particular case of TiB, coatings (Fig. 2). We found that for these samples and at the Ti K-edge energy, the sampling depth (D) representative of the signal depth measured in conversion electron yield experiments is about D = 70 nm.

3. Results In this section we will describe and analyse the evolution of structural parameters as mixing becomes more and

Fig. 3. Diffraction pattern deposited (M sample).

and bright field of a TiB,

coating

as

3.1. TEM results One can compare in Figs. 3, 4 and 5 the evolution of TEM pictures as the dpa amount increases, in relation with the Q factor. A typical transmission electron micrograph of TiB, film, 100 nm thick produced without ion mixing by electron beam coevaporation (A0 sample) is presented in Fig. 3. The corresponding electron diffraction pattern exhibits only very diffuse rings, this indicates that the film is principally amorphous. As has been previously observed on unimplanted TiB, film [20] this amorphous state is stable up to about 1200 K. The contrast in the bright field electron micrograph in Fig. 3 is not uniform; rather it exhibits bright boundary regions which could be taken for cracks. However, these latter probably correspond to less

Fig. 5. Diffraction pattern and dark field of a TiB, coating deposited by MID with 5.6 X 10” Ar+/cm’ at 80 keV: Q = 48 X 10mJ (A2 sample).

330

P. Chartier et al. /Nucl. Instr. and Meth. in Phys. Res. B 95 (1995) 327-333

dense or unoccupied volumes between the columns of a columnar structure. Indeed we have already observed by cross sectional electron microscopy [20] such a columnar growth of TiB, coatings on Si or steel substrates. The micro structural state of the Al film (Q = 32 X 10w4) is shown in Fig. 4. The corresponding selected area diffraction pattern is also given. One can notice concentric rings characteristic of a crystalline hexagonal structure. On the dark field image taken with a part of the second ring, the crystallised grains appear in white contrast. The columnar growth is still observed; however, the boundary regions between columns are reduced as a consequence of the mixing effect. For the A2 film (Q = 49 X 10d4) the evaporation rate was decreased to u = 0.30 rims-‘. The diffraction pattern presented in Fig. 5 reveals the presence of the main rings corresponding to hexagonal TiB,, thinner than those found for the Al film indicating a more crystallised state. Nevertheless, it must be emphasised that the reflections on planes (00 1) are not observed which indicates clearly that the films have a preferential orientation along the [00 l] direction. Such a result has been observed many times for TiB, coatings deposited by various methods [3,20,21]. The polycrystalline grains of about 2-3 nm are imaged in bright contrast. An important point to notice is the reduction of the columnar growth type contrast as a consequence of the reduced deposition rate. Fig. 6 shows the results obtained for the more complicated mixing conditions (MID with three different energies). The TiB, hexagonal structure can also be identified. The diffraction rings are sharper, indicating the more crystallised state of all the deposited sample of this study. Nevertheless, the bright field image reveals the presence of columnar structures, which is more accentuated than for the last A2 sample. When comparing these two last samples it can be assumed that a reduced deposition rate (0.3 nm s- ‘) together with the mixing conditions at three energies would lead to a well crystallised state with a limited columnar growth. 3.2. CEEXAFS

results and analysis

We first present the experimental EXAFS spectrum (Fig. 7a) and its Fourier Transform (Fig. 7b) measured on a bulk TiB, used as a reference as well as a corresponding simulation. In this case, the signal is noise free up to a

Fig. 6. Diffraction pattern and bright field of a TiB, coating deposited by MID with three different energies (A3 sample): 8.2~10’~ Arf/cm2 at 160 keV: Q= 12~10~~; 5.6~10’~ Ar*/cm’ at 80 keV: Q = 33 x 10-4; keV: Q = 33 x 10m4.

9.4~ lOI

Ari/cm2

high energy value of about 700 eV (14 A-‘) above the absorption edge. Except in the very low energy range where there is a small shift of the signal phase, we get a very good fit with values close to the structural parameters found in the literature for the hexagonal structure (12B at 0.238 nm + 6Ti at 0.303 nm + 2Ti at 0.323 nm) as gathered in Table 2. In this table, the term NTie (NTiTi) represents the number of boron (titanium) atoms surrounding titanium at the distance RTi, (RTiTi). The terms aTi, and cTiTi are the corresponding mean square relative displacements. This fit also allows us to check out the correctness of the theoretical backscattering phase and amplitude calculated by McKale et al. [18] used in this work. In a second step, we separate the contribution of each peak in order to fit the backtransformed EXAFS function corresponding to Ti-B bonds on one hand and Ti-Ti bonds on the other hand. A very good fit is thus obtained with the same structural parameters used above (Table 2) but the spectrum amplitude cannot be well fitted before 4 A-’ indicating that in the very low energy range, the Ti-B signal overlaps the Ti-Ti one. We may also mention that when fitting the two peaks separately the interatomic distances become closer to the theoretical values. Then, we decided to separate the contributions of the two shells and to fit each spectrum in the restricted high energy range. The EXAFS signals measured on the four TiB, films

Table 2 Local environment of Ti atoms in TiB, coatings and bulk deduced from EXAFS spectra simulations Sample

Neil

RTiB

Al A2 Bulk TiB,

6 7 12

0.237 0.238 0.239

hnl

at 50

flTiBhd

NTiTl

R,iTi[nml

UTiTibml

0.0110 0.0085 0.0080

2 2.5 6 2

0.303 0.303 0.301 0.327

0.0115 0.0095 0.0075 0.0075

P. Chartier et al. / Nucl. Instr. and Meth. in Phys. Res. B 95 (1995) 327-333

studied here become noisy (the EXAFS oscillations amplitudes are of the same order as the noise) above a photoelectron energy value of about 460 eV (11 A-‘). Therefore, the Fourier Transforms corresponding to AO, Al, A2 films and to the bulk reference (Fig. 8), have*been performed over the restricted energy range (2-11 A-t). In a first step, one may give only the following qualitative observations: - The Fourier Transform signal of the bulk sample presents very well defined peaks up to a high distance range, implying a well crystallised state, and as mentioned above, the structure is found to be hexagonal. - The A3 and A2 samples provide almost exactly the same EXAFS signal (phase and amplitude). So, we will consider that fitting A3 and A2 will imply the same local environment. Their Fourier Transforms present a well defined structure: two well separated peaks which rise at the same interatomic distance as the reference, and large distance range peaks (up to 0.6 nm), largely decreased in comparison to the bulk. - The A0 sample Fourier Transform presents a main peak and a shoulder centred at lower distance than those

r

-A2

0

1

2

3

4

5

sample

6

7

8

Distance (A) Fig. 8. Comparison of Fourier Transforms measured at Ti K-edge on evaporated films and bulk sample (energy window: 2-11 A-‘,.

-0.4 2

4

6

(b,

10

12

14

k

lb) 14 ” =12

0

1

2

3

4

5

6

7

8

Distance (A) Fig. 7. (a) Experimental and simulated backtransformed EXAFS function of bulk reference TiB: (Energy window: 2-14 A-’ ). (b) Experimental and simulated Fourier Transform of bulk reference TiB, (energy window: 2-14 A-‘).

typical of the mixing samples. There is no structure up to 0.4 nm. Such characteristics are representative of an amorphous state (as observed with MET), and a separation of the two peaks is inadequate in regard to the reference. For this main reason its EXAFS spectrum simulation was not performed with the previous chosen procedure. - The Al sample appears to be in an intermediate state between (A2, A3) and AO. Two main peaks come out, but one must note the absence of peaks up to 0.3 nm. So, only the quantitative simulations of Al and A2 signals are presented here and discussed in detail. We present (Figs. 9a and 9b) the EXAFS contribution of each peak (Ti-B and Ti-Ti) for these two samples and for the reference which also exhibits well separated contributions: - From the comparison of the Ti-B EXAFS functions, one can deduce that the signals have almost the same phase (except for sample Al at high energy) but different amplitude. They are representative of a similar environment (only Ti-B bonds) with lower coordination numbers and/or greater disorder for the coatings. - The Ti-Ti EXAFS signal obtained from the bulk reference is representative of two Ti-Ti shells separated by about AR = 0.323 - 0.303 nm = 0.02 nm, which induces a beat in the EXAFS signal at about 5 A-‘. This beat is not very pronounced since the two distances are weighted very differently (6 neighbours at 0.303 nm and 2 at 0.323 nm) but it is obvious that this beat is not perceptible in the signals from the films. So, only one Ti-Ti shell for coatings has been considered instead of two distinct

P. Chartier et al. /Nucl. Instr. and Meth. in Phys. Res. B 95 (1995) 327-333

332

4. Discussion

Ia) 0.3

I

!

I

2

4

6

1 2

/

I

4

6

I

I

I

4

I

1

lo

l2

(b) 0.3

-0.3

k &.Q8

Fig. 9. (a) Comparison of the Ti K-edge backtransformed EXAFS function of Ti-B peak. (b) Comparison of the Ti K-edge back-

transformed EXAFS function of Ti-Ti peak.

ones as for the bulk. This assumption provides a significant simplification for the simulations. Moreover the fit quality is much improved with one Ti-Ti shell. The results are given in Table 2. We must recall here that when the local disorder becomes important the EXAFS formula [17] used in this work is no longer valid. One might take into account high disorder using other distribution functions (see for example Ref. [22]). The use of a Gaussian distribution in highly disordered compounds introduces progressive errors especially in the high energy range and generally yields smaller interatomic distances. The A2 signal simulation results show low coordination numbers (7Ti-B instead of 12 and 2.5Ti-Ti instead of 6) but the average interatomic distances remain close to those of the hexagonal TiB, structure. The MSRD values become larger than the bulk value but are still typical of a crystalline phase. Results obtained on the Al sample also show low coordination numbers and very large MSRD values which explain the phase shift at high energy (u = 0.011 nm is a typical value found for amorphous alloys [16]). Nevertheless, the interatomic distances remain close to the reference.

Structural conclusions based on analysis of EXAFS spectra bring an average but detailed characterisation of the crystallisation state around titanium atoms through the experimental coatings. EXAFS results yield a quantitative evaluation of the average structural state, via parameters which describe the local environment of an atom species. The TEM patterns provide an unambiguous but qualitative long range order information about the structural state, a good idea of the average mean grain size and the relative importance of the unoccupied volume. About the correlation between both experiments, some restrictions can however be pointed out; the CEEXAFS signal favors the structure of the surface layers while TEM patterns are typical of a more homogeneous average over the whole depth of the specimen. Thus, the EXAFS coordination numbers and values derived from the Debye-Waller term are certainly more typical of the region where the dpa profile is varying (Fig. 1). Nevertheless, a crude calculation, based on results previously shown in Fig. 2, indicates that for the 100 nm depth here tested, the 30 nm front layer contributes about 45% of the EXAFS signal instead of 30% for microscopy patterns. In spite of this difference one can consider the evolution of both EXAFS and TEM signals as complementary. From a detailed comparison between Al and A2 EXAFS results, one should notice the slight increase of the coordination number with the dpa parameter while an important increase of the Debye-Waller term occurs. This quantitative result indicates that the more defined concentric rings observed in the diffraction pattern of sample A2 must be interpreted rather as a better organisation of the occupied site in the crystalline zone than as an average grain growth. The Al sample yields EXAFS spectra with a Debye-Waller term still typical of amorphous material, while the A2 one is more representative of a disordered but crystalline state. Low coordination numbers (in comparison with the theoretical ones) could not be explained exclusively by the mean grain size, which remains close to 3 nm according to the TEM results. Such size can induce, at best, only a 20% decrease on the first and second neighbour shells. It is generally observed that for nanocrystalline materials, the crystallite sizes are in the range of a few nanometers: typically 3-20 nm and about 30% of the material consists of grain (or interphase) boundaries [23,24]. So, the dramatic reduction in Ti-Ti bonds must have partially another origin. One can think that an important structurally unorganised amount of titanium atoms is found in the grain boundaries. This assumption might be at the same time significant for the lack of boron atoms relative to the stoichiometric TiB, composition. This deficiency of boron atoms has been observed previously by Electron Probe X-ray Micro Analysis (EPMA) [25]. This assumption could also explain the apparently limited size of the grains,

P. Chartier et al. / Nucl. Instr. and Meth. in Phys. Res. B 95 (1995) 327-333

which are, however, typical of a TiB, hexagonal phase, according to the interatomic distance parameter, in particular the Ti-B one which is found constant for all samples, including AO. More surprising is the quasi identity of the EXAFS spectra obtained from samples A2 and A3, which were obtained with very different dpa amounts. Then we cannot conclude that the damage profiles alone represent a relevant physical parameter explaining the whole modification during the dynamical mixing process. This fact suggest ’ that the energy of impinging Arf ions is an important parameter in the crystallisation process. One must also underline that the structural state seems to reach the same limit of local order for both mixing conditions. This argument supports the previous conclusion that the effect of mixing is limited by the composition of the evaporated coating, which is not stoichiometric.

Acknowledgements The authors wish to thank C. Boisseaux and C. Templier for their technical assistance during dynamic ion mixing experiments as well as M.F. Denanot for her assistance with the operation of the electron microscope.

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