Hyperfine Interactions 95 (1995) 199-225

199

Characterization oflaser-nitrided iron and sputtered iron nitride films Peter Schaaf, ChristofIllgner, M a t t h i a s N i e d e r d r e n k and Klaus Peter Lieb IL Physikalisches [nstitut, Universitdt G~ttingen, Bunsenstrasse 7/9, D-37073 Gdttingen, Germany Laser-nitriding may be a promising technique for substituting conventional nitriding processes. We have irradiated pure iron with pulses of an excimer laser and achieved high nitrogen contents in a thin surface layer. We found that the nitrogen is dissolved into T-Fe, leading to a large amount of retained austenite. This was also verified by X-ray diffraction (XRD) measurements. Three subspectra can be resolved in the M6ssbauer spectra (CEMS) for this nitrogen austenite. The nitrogen concentration can be calculated in terms of site occupation, indicating a content as high as 16(1) at%, which is consistent with the results of Rutherford backscattering spectroscopy (RBS), resonant nuclear reaction analysis (RNRA) and Auger electron spectroscopy (AES) measurements. This is more than the solubility limit for 7-Fe(N). By reactive magnetron-sputtering it is possible to produce thin iron nitride i'rims of various stoichiometries. We report on the production of e-FexN and FeNy films. These films were again characterized by CEMS, RBS, RNRA (15N(p, aT)) and XRD. For e-FexN, produced in the range 2 ~0.5 is produced. This phase is not reported in the F e N phase diagram. 1. I n t r o d u c t i o n T h e i r o n - n i t r o g e n system plays an i m p o r t a n t role in technical applications concerning iron based materials. Since the early iron age m a n has tried to p u t n i t r o g e n into his iron based materials and tools in order to improve their hardness, tribological properties, c o r r o s i o n resistance or lifetime [I]. A m o n g m a n y processes, which have been d e v e l o p e d to improve the wear resistance o f surfaces o f materials, nitriding o f iron and steel is one o f the m o s t frequently used. As examples salt b a t h nitriding, p l a s m a nitriding and nitrogen i m p l a n t a t i o n [2-6] should be m e n t i o n e d . W e have f o u n d that irradiating iron with pulses o f an excimer laser in air or in p u r e n i t r o g e n a t m o s p h e r e results in a r e m a r k a b l e nitriding effect in the i r r a d i a t e d surface [7]. In section 4 details o f this nitriding effect will be presented. I f the effect can be i m p r o v e d to a technical process, it would be superior to the c o n v e n t i o n a l nitriding processes, because it is very fast, it can be used as a finishing process a n d does only affect the surface o f the material w i t h o u t any t h e r m a l i m p a c t on the bulk 9 J.C. Baltzer AG, Science Publishers

200

P. Schaaf et al. / La~er nitrided iron and sputtered iron nitride films

[8,9]. Section 5 deals with the F e - N phase diagram as obtained after reactive magnetron sputtering. If we want to apply M6ssbauer spectroscopy to the investigation of these processes and materials, it is important to have a good "M6ssbauer knowledge" of the iron-nitrogen system. Therefore, this paper aims to enlarge the knowledge on this system of great technological importance. Alternative and complementary information has been obtained from depth profiling methods (RBS, RNRA, AES) and X-ray diffraction. 2. Analyzing methods Conversion electron M6ssbauer spectra (CEMS) were taken at room temperature using a 57Co/Rh source with an activity of about 500 MBq, a constant acceleration drive and a He/CH4 gas flow proportional counter [10,11]. Spectra were stored in a multichannel scaler using 1024 channels [12]. Calibration was performed using a 25 gm a-Fe foil, to which all isomer shifts (6) are related. Spectra were fitted according to a least squares routine by superimposing Lorentzian lines [13]. XRD spectra were taken with a copper anode at fixed incidence angle of 0 = 5~ (Seemann-Bohlin). Rutherford backscattering (RBS) was performed using the G6ttinger 530 kV implanter IONAS [14], with 900 keV a-particles and a backscattering angle of 165 ~ The spectra were analyzed using the R U M P code [15,16]. The most accurate determination of the nitrogen depth profile is enabled by resonant nuclear reaction analysis (RNRA) via the reaction ISN(p, a3')12C with a resonance energy of 429.6 keV and a resonance width of F = 124 eV [17,18]. 3. M 6 s s b a u e r overview o f the F e N system The iron-nitrogen system consists of several interstitial solutions (a, 7, ~), chemical compounds (7'-Fe4N,~-Fe2N) and metastable phases (a'-martensite, tz"-Fe16N2), according to the actually accepted phase diagram [19]. There will be a detailed discussion of the 3' and e regions in the following sections, therefore, here only the M6ssbauer parameters of u', a", 3" and ~ are given in tables 1-4, using published data [20-25] and own measurements. 4. Laser nitriding 4.1. EXPERIMENTAL Samples of Armco iron (purity 99.8+%, diameter 25 mm, thickness 1 mm, polished to 1 gin) were irradiated with pulses of a Siemens XP2020 XeC1 excimer

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitridefilms

201

Table 1 Hyperfme parameters ofa'- martensite a. Ref.

Subspectrum

6 (ram/S)

e (rnm / s)

H CI')

RA (%)

CN(at%) (remarks)

[2o]

Fe0 Fel Fe2 Fe3

0.00 -0.11 -0.12 -0.06

34.0 31.6 34.6 37.0

17.5 22.1 30.6 29.8

(T = 77K)

-0.32 0.12 0.00

10.4

"6, isomer shift relative to a-Fe at room temperature; e, quadrupole splitting, e = ~eQV:,~/1 + ~72 for non-magnetic, e = ~eQV:z(3 cos2 0 - 1 + 77sin2 0 cos 2~) for magnetic subspectra (fi~st order perturbation theory); H, magnetic hyperfine field acting at the nucleus; RA, relative area of the subspectrum. The measuring temperature T is always room temperature unless otherwise stated. Table 2 Hyperfme parameters ofct"-Fel6N2. Ref.

Subspectrum

6 (ram / s)

e (ram / s)

H (T)

RA (%)

[20]

A B C

-0.09 -0.06 -0.15

-0.09 -0.09 -0.17

39.90 31.40 28.80

25.0 50.0 25.0

[21]

A B C

0.25 0.21 0.05

-0.18 0.21 -0.49

40.60 31.60 30.70

A B C

0.14 0.17 0.09

-0.07 0.08 -0.16

39.81 31.55 29.46

thiswork

cN (at%) (remarks)

(T = 1 5 K )

17.3 36.2 15.5

Table 3 Hyperfme parameters of 3/-Fe4N. Ref.

Sub-

6

e

H

Fa

RA

cN

spectrum

(minis)

(ram/s)

CO

(ram/s)

(%)

(at%)

[22,23]

A B C

0.24 0.3 0.3

0.00 0.22 0.43

[24]

A B C

0.25 0.31 0.31

0.00 0.16 -0.24

34.1 21.7 21.7

0.30 0.30 0.30

thiswo~

A A2 B C

0.16 -0.02 0.25 0.25

-0.01 0.02 0.02 -0.03

34.04 32.54 21.24 22.65

0.25 0.25 0.39 0.32

* T', line width (FWHM).

34.06 21.55 21.92

25.0 50.0 25.0

18.5 11.4 50.1 20.0

202

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitride films

Table 4 HyperFme parameters of (-Fe2N. Ref.

Subspectrum

6 (ram / s)

r (ram / s)

[25]

Fe-III

0.41

0.28

H (T)

F (ram / s)

RA (%)

cN (at%)

0.26

laser (A = 308 nm, pulse width tp = 55 ns). The irradiation was carried out at ambient temperature either in a vacuum chamber, evacuated and subsequently filled with nitrogen of 0.1 MPa pressure, or, for comparison, in air. The energy density of the laser irradiation was set to 40 m J / m m 2, the spot size was set to approximately 5 x 5 mm 2, the whole sample surface was irradiated by setting the spots side by side with almost no overlap. To enhance the effect of the laser treatment, each surface area was irradiated with 32 subsequent pulses at a frequency of 2 Hz. It should be noted that the energy density can be controlled with an accuracy of about 5-10%. The samples were characterized without any further sample preparation using several methods. CEMS and XRD gave information on the phases present in the near surface layers of the sample, and to a limited extent, on its nitrogen content. More information about the concentrations and the depth profiles was obtained from RBS, AES and RNRA measurements. 4.2. RESULTS OBTAINED BY MOSSBAUER SPECTROSCOPY (CEMS)

The CEMS spectra recorded at room temperature of the iron irradiated in air and in pure nitrogen atmosphere at ambient pressure are displayed in fig. 1. It is easily recognized that dramatic changes occurred relative to the original pure a-Fe spectrum. The hyperfine parameters of the corresponding subspectra are summarized in tables 5 and 6. According to their hyperfine parameters the subspectra A0 and A1 have to be attributed to austenite, although the quadrupole splitting ~ of A1 is slightly too small compared to parameters given in the literature [26]. Since there is no carbon in the pure iron samples, the austenite has to be nitrogen austenite, thus the doublet A1 is due to iron atoms having nitrogen interstitials in their nearest neighbourhood. The parameters agree quite well with literature data of nitrogen austenite [20,27-30], which are summarized in table 7. The nitrogen content of this austenite should be above 8.6 at% because only then its martensitic transformation is prevented at room temperature, as the martensite start temperature Ms decreases with increasing nitrogen content. The subspectra M0, M1, and M2 correspond to magnetic phases. M0 is equivalent to the pure a-Fe of the non-irradiated sample. The additional magnetic subspectra M1 and M2 in fig. lb hint to the existence of a'-martensite and T'-Fe4N phases [20,26] in the irradiated surface layer. Altogether, about 55% of nitrogen

203

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitridefilms I

I

I

I

I

I

I

I

0

1.04

ID•DI

1.02

~ --U..

9 .~. r . ~ . f f

1.00

..LAO ukl , I 01

I-Z tu

i

I

l

iMO

i

W

b

.~ 1 . o 6

1,03

t .00

u AI I

I

I i -8

I -6

I

I I -4

I I -2

~A

I

I

I I 0

I

I I 2

I 4

IH2 I 6

I 8

VELOCITY [mm/s]

Fig. 1. CEMS spectra of laser nitrided iron: (a) sample irrradiated in air, 0a) sample irradiated in nitrogen atmosphere.

austenite for the sample irradiated in nitrogen in addition to some magnetic nitrides and 35% nitrogen austenite in the sample irradiated in air are found. This means that there is a remarkable nitriding effect due to the laser treatment. The additional subspectrum O1 for the sample irradiated in air has to be attributed to the iron oxide wiistite (FexO) and its parameters agree with those given in refs. [31-33]. No oxides were found in the sample irradiated in pure nitrogen. In order to get more information on the nitrogen content, the same samples Table 5 Hyperfine parameters of the sample irradiated in air (fig. la). Subspectrum

6 (ram / s)

~ (ram / s)

A0 A1 O1 M0

-0.05(3) 0.15(3) 1.06(3) 0.02(2)

0.36(5) 0.87(4) 0.00(1)

H (T)

F (ram / s)

RA (%)

33.28(25)

0.35(2) 0.35(2) 0.72(1) 0.35(1)

11.6(1.5) 23.6(2.8) 20.6(1.4) 44.2(2.6)

204

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitride films

Table 6 Hyperffme parameters of the sample irradiated in pure nitrogen atmosphere (fig. lb). Sub-

~

9

H

F

spectrum

(,.m/s)

Cram/s)

03

(ram/s)

(%)

A0 A1 M0 M1 M2

-0.05(3) 0.10(3) 0.02(2) 0.11(4) 0.23(6)

33.27(08) 29.30(17) 22.38(28)

0.36(2) 0.36(2) 0.53(2) 0.53(2) 0.53(2)

22.8(1.2) 32.6(1.7) 30.8(1.9) 8.4(2.2) 5.4(2.7)

0.40(5) 0.00(1) -0.07(2) -0.03(3)

RA

were measured with a freer velocity scale. Thus a better resolution in the center of the spectra was obtained. These spectra are shown in fig. 2. Now it was observed, that the nitrogen austenite had to be fitted with three subspectra denoted as A0, A1 and A2. The corresponding hyperfme parameters are summarized in tables 8 and 9 for the sample irradiated in air and in pure nitrogen. Some parameters of the magTable 7 Hyperfme parameters of nitrogen austenite, 7-Fe(N). Ref.

Subspectrum

~ (mr,/s)

9 (mm/s)

[27]

A0 A1 A2

-0.10 0.15 0.30

A0 A1 A2

[28]

[20]

[29]

[30]

this work

F (ram/s)

RA (%)

CN (at%)

0.25 0.40

23.0 75.0 2.0

8.9

0.01 0.08 0.20

0.39 0.72

47.3 49.6 3.1

9.0

A0 A1 A2

-0.18 -0.39

0.29

55 45

A0 A0m All Aim A2m

-0.13 0.02 0.08 0.02 0.19

0.68 0.37 0.95

33.3 28.3 14.0 19.2 5.2

A0 A0m A10 Alm A2m

-0.11 0.06 0.20 0.11 0.20

0.67 0.37 0.95

0.3 21.8 8.5 66.4 3.0

A0 A1 A2

-0.09(2) 0.15(3) 0.21(4)

0.29(2) 0.59(3)

0.30(2) 0.30(2) 0.30(2)

25.5(0.6) 32.0(0.9) 4.2(1.5)

8.7

7.1

I0.0

12.0

205

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitridefilma I

I

I

I

I

[

I

1.06

1.03 'a

1.00

-L

)-

t

t/) Z uJ IZ D-4

AO i

)

kl

iA2

I

I

b

~1.10 J W

t .05

.

1.00

.

.

I

I

-1.5

I

-i

I

I

.

t

I

I

I

i

iA2

I

H(

i I

-0.5 0 0.5 VELOCITY [mm/s]

I

I

I

1.5

Fig. 2. Same CEMS spectra of laser nitrided iron as in fig. 1 but taken with f'mer velocity scale: (a) sample irradiated in air, Co)sample irradiated in nitrogen atmosphere.

nefic phases had to be fixed according to the results of the former measurements, due to their limited "visibility" with the small velocity amplitude. What sites do these three subspectra correspond to? In iron-carbon austenlte we have ordering of the carbon interstitials (repulsive model) [26,34], thus allowing only one nearest carbon neighbour and up to four next nearest carbon neighTable 8 Hyperfme parameters of the sample irradiated in air, fmer velocity scale (fig. 2a) a Subspectrum

6 (ram/s)

e (ram/s)

A0 A1 A2 O1 M0

-0.09(1) 0.16(1) 0.23(2) 1.07(2) 0.02(4)

0.28(1) 0.59(3) 0.84(3) 0.00( )

a No error means that parameter was fixed.

H (T)

F (ram/s)

RA (%)

33.04(14)

0.29(2) 0.29(2) 0.29(2) 0.66(4) 0.27(3)

11.9(1.0) 23.2(1.2) 7.3(1.2) 21.1(1.9) 36.5(3.1)

206

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitridefilms

Table 9 Hyperfine parameters of the sample irradiated in pure nitrogen atmosphere, freer velocity scale (fig. 2b)" Sub-

6

E

H

r

RA

spectrum

(mm/s)

(-,-,/s)

OD

Cram/s)

(%)

A0 A1 A2 M0 M1 M2

-0.09(1) 0.15(2) 0.21(3) 0.02( )

0.29(2) 0.59(3) 0.00( )

33.25( )

0.30(1) 0.30(1) 0.30(1) 0.34(2)

25.5(0.5) 32.0(0.8) 4.2(1.2) 21.9(2.4)

0.11( )

0.00( )

29.28( )

0.34(2)

6.0(3.4)

0.23( )

0.00( )

22.37( )

0.34(2)

10.4(2.7)

a No error means that parameter was fixed. bouts. In iron-nitrogen austenite there is no ordering (hl atoms are smaller than C atoms), leading to a random distribution of nitrogen atoms on the interstitial sublattice (octahedral sites) [27,26]. Although some authors also claim a weak ordering in the nitrogen austenite [28], we first assume no such ordering. If we write the nitrogen austenite as FeNx, with x = cN/(100 - cN), where c~ is the nitrogen concentration in atomic percent (at%), we have with x the probability that a certain octahedral site is occupied by a nitrogen atom. Since an iron atom has six nearest neighbouring interstitial sites, the probabilityp, (x) for an iron atom to have n nearest nitrogen neighbours is given by p,(x)=

(:)(x)"(1-x)

6-"

for

n=0-6.

(1)

Taking the maximum solubility of CN = 10.5 at%, or x = 0.117, we should have up to three nearest nitrogen neighbours (P4 ~<0.1%). The influence of next-nearest nitrogen atoms to the hyperfine parameters should be negligible, because at a distance which is larger by a factor of v~, their influence is very small as compared to nearest nitrogen neighbours. Consequently, they cannot be resolved in the M6ssbauer spectra. All attempts to correlate the measured areas An with the probabilities p , failed. Therefore we considered the different configuration of the nitrogen neighbours, which are displayed in fig. 3, together with their reduced quadrupole interaction parameters v= and % resulting from point charge calculations (PCM). Obviously there is only one arrangement for the a0 and al cases. For two nitrogen neighbours, we have to consider two different arrangements: The two nitrogen neighbours forming an angle of 90 ~ (the a2-90 arrangement), results in the same Vzz as the al site, but with opposite sign. Since one cannot distinguish the sign of v= in a quadrupole split doublet without external field, this results in the same quadrupole splitting. If the two nitrogen atoms form an angle of 180 ~ twice the quadrupole splitting of the A1 or a2-90 should arise in the spectrum. Dealing with three nitrogen neighbours again two cases have to be distinguished. The a3-T arrangement causes Vzz = 3 and 7/= 1, leading to almost the same value ofr as for a2-180. Finally the a3-D arrangement should have no quadrupole splitting due to the van-

P. Schaaf et al. I Laser nitrided iron and sputtered iron nitride films

"

/

207

~---2

I et==o a2-180

/

9

eta=O

Fig. 3. Various configurations of nitrogen neighbours for an iron atom in nitrogen austenite with reduced values ofquadrupole interaction (see text).

islfing electric field gradient (efg). If the real quadrupole splittings are calculated by means of the point charge model (PCM) and the adapted value of the Sternheimer factor %0, the values given in table 10 are calculated, which fairly well agree with the measured values. Thus we should add up the al and a2-90 fractions to the A1 and the a2-180 and a3-T fractions to the A2 quadmpole doublet. Furthermore, the isomer shift should increase with the number of nitrogen neighbours, but this cannot always be resolved in the spectrum. The fraction of the single line a3-D is very small, so that it cannot be distinguished from the A2 fraction. Accounting for the probabilities of these arrangements (a2-90 : a2-180 = 4 : 1, a3-D : a3-T = 3 : 2), we now expect the following fractions for the three sites in the spectrum: Table 10 Measured and calculated quadrupole splittings for the various iron sites in nitrogen austenite (Z = 0.5,%o = -13.4) a. Site a0 al a2-90 a2-180 a3-T a3-D

Calculated

Measured

(,-m/s)

(mm/s)

0.000 0.294 0.294 0.587 0.508 0.000

0.00( ) 0.29(1) 0.59(3)

208

P. Schaaf et ai. / Laser nitrided iron and sputtered iron nitride films

A0 =p(a0) =p0, A1 = p(al) +p(a2-90) = Pl + 4p2, A2 =p(a2-180) +p(a3-T) (+p(a3-D)) : lp2 -{'- ~P3

("~5P3) 9

(2)

Plotting these equations with the measured data, as displayed in fig.4, a good agreement is found. It should be mentioned that the higher concentrations were achieved for irradiationin air.Nevertheless,also from the A 0 fractionthe nitrogen content can be determined almost independently of the model used. N o w the total amount of nitrogen in the laserirradiatedsurfacelayersas measured by C E M S can be calculated.The resultsare summarized in table 1I. 4.3. RESULTS FROM XRD, RBS, AES AND RNRA The M6ssbauer results will now be compared with the results of other methods. In fig. 5 the X R D spectra of the laser irradiated samples are shown. In accordance with the results of the M6ssbauer data, the most prominent reflexes arc due to anstenite and ferrite. Additional reflexes of FexO can be seen in the case of the sample irradiated in air (fig. 5a). Contrary to the CEMS measurements no hints to magnetic nitride phases were found in the X-ray spectra. This could be due to the peaks overlapping with stronger peaks of the other phases or being too weak for detection. 1001~

90-~

'

'

'

nit]'~

80

''

'

[2~

'

'

A

",4

o.oo

o.;5

I

[] AO t A2

1

o.lo

o. 5

o. o

o.2

x in F e N x Fig. 4. Distribution for the iron sites in nitrogen austenite (see text): open symbols- literature data (referencesare given);closedsymbols- this work.

209

P. Schaaf et aL I Laser nitrided iron and sputtered iron nitridefilms

Table 11 M6ssbauer phase analysis of the samples irradiated in air and in nitrogen. Phase fractions

Nitrogen (%)

Air (%)

a-Fe 7-Fe(N) Fel-xO nitrides (e, a')

29(3) 55(2) 0(2) 16(3)

44(2) 35(2) 21(2) 0(2)

Nitrogen concentration

Nitrogen (at%)

Air (at%)

CNin u cN total

12(1)

16(I)

12(2)

7(2)

8000

[

!

c< 11o

c<: ~-Fe

y: y-Fe(N) 6000

F '(X ',200

C 0

u 4000 ~. >-

2000

Y 2oo

Y

-o -

14000

111 I

FeOi

II

'

O

y y z2o,

220

311 222

',

!

I

Fe lasered in N2

y: y-Fe(N)

8000

cx ; Y

220 '

I

10000

E

O~ 211

:

FeO: I

II

111,

12000

0 U

I

Fe lasered in air 40 m J / m m z 32 pulses

40 m J / r a m 2 32 pulses (X 110

Y

'(X

111

1211 i

6000

y zoo

4000

',~

,200

X

220

:

~ y v.1

Ylcx,

2OOO 0 20

40

60

80

100

28 [o]

Fig. 5. XRD spectra of laser nitrided iron: (a) sample irradiated in air (top), (b) sample irradiated in nitrogen atmosphere (bottom).

210

P. $chaaf et al. / Laser nitrided iron and sputtered iron nitridefilms

According to Ruhl and Cohen [35], the nitrogen content in y-Fe(N) is related to the lattice constant a via a = 3.572A + 0.0078 A x cry(at%).

(3)

F r o m the measured lattice constants of a = 4.645(8) A and a = 3.638(8) A for the sample irradiated in nitrogen and air, nitrogen concentrations of cN = 9(1) at% and cN = 8 (1) at% were deduced. In the XRD-spectrum of the sample irradiated in air, additional information can be extracted from the (111) reflex of the FexO phase, which yields a lattice constant of a = 4.26(1) A. The relation between the stoichiometric parameter x of the FexO-phase and a as given by M c C a m m o n and Liu [36], a = 3.856A + 0.478A x x

(4)

yields x = 0.86(5). When comparing the results of M6ssbauer spectroscopy and X R D , differences in the nitrogen content of the samples irradiated in air and in nitrogen are evident. In order to understand these differences the concentration profiles of iron, nitrogen, and oxygen were determined by means ofRBS, AES, and R N R A . The RBS measurements sensitive to the iron content are shown in fig. 6. N o differentiation between nitrogen and oxygen in the sample is possible at the used a-particle energy of 900 keV. However, the spectra show that the laser-affected zone reaches depths of more than 500 nm from the surface. The results of AES sputtering measurements are displayed in fig. 7. The oxygen content in the sample irradiated in air (see fig. 7a) is limited to the first 100 nm of the surface, whereas nitrogen follows a fairly homogeneous depth prof'de exceeding the measured depth of 350 nm. The nitrogen concentration is cN = 14(5) at%. The sample irradiated in nitrogen, as shown in fig. 7b, does not exhibit oxygen (within the errors of AES) and its nitrogen content is 12(5) at% with a slight increase towards the surface. The most accurate determination of the nitrogen content in the samples is enabled by R N R A via the resonance reaction 15N(p, ay) 12C. The y-ray yield is displayed in fig. 8 versus the incident proton energy. The resulting nitrogen depth profiles are shown in fig. 9. Again we see, more pronounced than in the AES spectra, the increase of the nitrogen content towards the surface of the sample irradiated in nitrogen, reaching nitrogen contents of up to 20 at%. For the sample irradiated in air, it can be seen that the nitrogen content decreases towards the surface. This m a y be caused by the increase of the oxygen content. In depths greater than 100 nm the nitrogen contents in both cases become identical with a value of 8 at%. R N R A measurements thus help to explain the differences in the nitrogen content as measured by X R D and by M6ssbauer spectroscopy. It should be noted that the information depth for the used geometry is about 300 nm for X R D compared to about 100 nm for the CEMS measurements. The

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitride films

Energy (MeV) 0.5

0.4

400

I

211

0.6

I

I

o) ~",,..~'V~ . ~~,~",,~,,~

.........

Fe r e f e r e n c e Fe Iosered in oir

300 (D

>--(3

N ~ 200

5 E o z

100

0 30c

i '!

I

I

400

i

I

500 Channel

600

700

Energy (MeV) 0.4

4-00

0.5

~ , b) _ " ~ t ) ~" t ~ -

. ~

0.6

,

..........

0.7

Fe r e f e r e n c e Fe lasered in N 2

_

3O0

~N 200

E 0

z IO0

0 300

Fe-concent

.,

{i

?

t"

u 400

-

v 500

Channel

i 600

700

Fig. 6. RBS spectra of laser nitrided iron: (a) sample irradiated in air, (b) sample irradiated in nitrogen atmosphere, together with the pure iron reference sample and the iron concentration profile

(inset).

212

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitride film~ 3rofile

AES s

]

100

I

80 cO

]

Fe

L

j

I

w 0

i

60

Fe lasered in air {U tO U

[]

40

40/32

i

oo!o o~

20

0

1

N

A 0

100

300

200

400

sputtering depth [nm]

1oo

AES s

I

l

[

)rofile

,

80 - - - . ~ - ~ - ~ ~

1_o_ [J_D_ Fe

w tO

k.

E

60 -7 , 40 ~_~_ i ;

U cO U

, ;

Fe lasered in N z 40/32

I ; I

i

20 --

0

i

=

,

100

~

I

200

<> ~> / <> <> o 300

400

sputtering depth [nm]

Fig. 7. AES sputtering profiles of laser nitrided iron: (a) sample irradiated in air, (b) sample irradiated in nitrogen atmosphere.

difference in nitrogen content can be understood by noting the fact that the nitrogen is not homogeneously distributed in the sensitive region of these methods. The R N R A measurements show that the nitrogen content rises towards the surface of the samples. Clearly, CEMS and XRD measurements can only give an average value of the nitrogen content of our samples. Due to the different information depths, a high nitrogen content is measured on the surface (CEMS), whereas the XRD results represent the lower nitrogen contents at deeper layers.

213

P. Schaaf et aL / Laser nitrided iron and sputtered iron nitridefilms 10

I

Fe lasered in N2 4.0 m J / m m 2

T. il

,~ 8 o ::1.

.

.

.

!

32 pulses

.

6 ,

--~

-

.L

l

. L__

I__~-

i

I

r

I

2 r

0

430

i

I

i

i

L

~

I

it

44-0

~

450

i'

i-

,

460

-

i

470

480

Proton energy [keV]

Fig. 8. RNRA 7-yieldversusproton energyfor laser nitrided iron (in nitrogen). 4.4. CONCLUSION We have shown that there is a strong nitriding effect when irradiating iron with pulses of an excimer laser in nitrogen containing atmosphere, which could be interesting for technical applications. Furthermore, it was shown that the combined application of phase-sensitive CEMS and X R D together with depth-sensitive RBS and R N R A gives a consistent picture of the iron surface after laser irradiation in nitrogen and in air. Further experiments are to be carried out in order to achieve a better understanding of the basic mechanisms of the nitrogen and oxygen uptake during laser irradiation and Of the influence of the processing parameters.

I

I

!

'

I

'

I

I

[

28 - Fe lasered with 40 m J / m m 2 and 32 pulses .'~ 24 r~

E E

20 16

O

u

tQI

12

o Z

0

40

80

120 160 Depth Into]

200

240

280

Fig. 9. RNRA nitrogen depth profilesof laser nitrided iron.

-

214

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitride films

5. Magnetron sputtered iron nitride films 5.1. REACTIVE MAGNETRON SPUTTERING Iron nitride thin films were produced by reactive magnetron sputtering, using a permanent magnet planar type rfmagnetron from I O N T E C H Ltd. The system was equipped with a 500 W if-generator, whereas the iron nitride films were produced with an if-power of about 110 W. The base pressure of the vacuum chamber was better than 10 -4 Pa; due to the gas flow during reactive sputtering, the working pressure was about 0.5 Pa. A high purity Fe sputter target (99.999% Fe) with a diameter of 76 ram and a thickness of 5 m m was used. The target substrate distance was fixed to be 7 cm. The gas flow of the pure argon (5N8) and nitrogen (6N) gas was set by two independent mass flow controllers. The argon flow was fixed at 27 seem, whereas the nitrogen flow was varied between 0 and 27 seem; the N 2 / A r ratio was varied between 0 : 1 and 1 : 1. The accuracy of the mass flow controller was +0.3 sccm. A Si wafer of 0.5 m m thickness and SiO2 plates of 0.1 mm thickness were used as substrate materials. The substrate temperature was varied by means of a heated substrate holder between 300 and 850 K; the temperature was controlled and variations during deposition were smaller than 10 K. The growth rate was about 0.2 A/s. This way, iron nitride films were produced with thicknesses of about 300 nm; several samples were deposited in one run under identical conditions. The samples were analyzed using M6ssbauer spectroscopy, X R D , RBS and R N R A without any further sample preparation. By measuring some samples immediately after deposition and again one year later (kept dry and at r o o m temperature), no changes in structure or nitrogen content were found. 5.2. MEDIUM NITROGEN FLOWS: e-FexN The diffraction pattern of an iron-nitride film prepared at 660 K with a nitrogen flow of 2.4 sccm is displayed in fig. 10. All peaks can be attributed to the hexagonal e-FexN [37,38]. This implies that a single phase nitride layer has been produced. The analysis of the peak locations led to the lattice parameters a = 2.753(8) A and c = 4.408(8) A ( c / a -- 1.60(8)). Kunze [19] gives the following formula for calculating the atomic ratio YN: a---- 2 . 5 1 9 A + 0 . 5 0 A • YN, c / a = 1.633 A - 0.005 A x YN,

(5)

where one can easily convert the different nomenclatures for these e-FexN nitrides (FexN -- FeNy) with x being the stoichiometry and y or YN being the atomic ratio (the probability of an interstitial site being occupied by a nitrogen atom). Our own compilation of the literature summarized by Wriedt [38] yields the following equations:

P. Schaaf et al. / La~er nitrided iron and sputtered iron nitridefilms

215

3,0X104 2,5xlo 4

~ ' 2.0x104 ~P "1

~,-~ 1,5x104

.Z2 C

9~ 1,0x104 o

5.0x10~

o

L

0.0

.

40

60

I

8O

9

L..

100

2~ Fig. 10. X R D measurement of the sample prepared at 660 K with a nitrogen flow of 2.4 scorn.

a = 2.525(9) + 0.00747(29)/at% • CN, c = 4.238(7) + 0.00565(22)/at% • CN,

c/a =

1.680(3) --0.0025(1) x cN.

(6)

Using eq. (6) we can now calculate the stoichiometry of the nitride film to be E-FexN, with x = 2.26(17). Analyzing the RBS spectra of this sample, a nitrogen concentration of CN -----32.9(21) at% of x = 2.04(18) was found. This is also in accordance with the R N R A measurement displayed in fig. 11, which also exhibits a nitrogen concentration of 32.7(11) at%, or x = 2.06(10). In addition to that, the RBS and R N R A measurements show that we have homogeneous layers of the e nitride phase. The mean value resulting from these three types of measurements indicates that we have the nitride E-Fe2.12(8)N. Now we ask how this nitride appears in the M6ssbauer spectrum. Looking at the values from literature [24,39,40] given in table 12 we should expect a single quadrupole split doublet for this E-Fezl2N. The CEMS spectrum of this film shown in fig. 12 indicates that it cannot be fitted with a single quadrupole split doublet, but contains two doublets. In the literature such a spectrum is often interpreted as the asymmetric doublet of E or ~ nitride due to texture effects [24], but neither was a texture observed in X R D analysis nor could the spectrum be fitted satisfactorily with only one asymmetric doublet, nor were changes found upon changing the angle between incident y-rays and the film's surface normal. Consequently, the asymmetry should result from the Fe-II doublet, which has to be included into the fit. The hyperfine parameters of the spectral analysis are summarized in table 13.

216

P. Schaaf et aL / Laser nitrided iron and sputtered iron nitride films

40 .m t'30 .o_ t-

O E o o to') o I,,-

I I L I _ !

!__!~]!--I--I--!-~--I

--''!~I

32.7(1.1) at.%N Fe2.0600) N

20

10

IK

RNRA 15N(p,7) i

ci

i

I

=

I

=

I

~

I

~

I

425 ' -~ 430 435 440 445 450 455 460 I proton energy [keV] i Fig. 11. RNRA depth profde of the sample prepared at 660 K with a nitrogen flow of 2.4 seem.

The subspectrum denoted Fe-III, i.e. iron with three nitrogen neighbours, agrees well with the parameters given for the only present "ideal" Fe-III sites in FezN [39,25,24]. Table 12 Hyperfme parameters ofe-Fe2+,N. Ref.

Subspectrum

8 (nun/s)

Fe-I Fe-II Fe-III

0.24 0.33

29.8 23.8

Fe-I Fe-II Fe-III

0.26 0.34 0.40

27.30 20.50 9.95

0.68 0.76

~3 63 37

[39]

Fe-II Fe-III

0.35 0.41

18.6 8.4

0.74 0.74

52 48

2.47

[39]

Fe-II Fe-III

0.29

0.34

17 83

2.20

0.42

Fe-II Fe-III

0.28

0.74

0 100

2.08

0.42

Fe-III

0.43

0.28

0.30

[40]

[39]

[39]

[24]

e (mm/s)

H (T)

/" (ram/s)

RA

x in E-FexN

(%)

10.0 73.3

3.20

2.67

2.00

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitride films I

I

I

I

!

I

!

t

i 1.5

217

t .24

>I7

~1.J2 i-.J

1.00 I

-1.5

I

-1

I I -0.5 0 VELOCITY

t

J Fe-lj

i 0.5 [mm/s]

Fig. 12. CEMS measurement of the sample prepared at 660 K with a nitrogen flow of 2.4 sccm. Due to the deviation from the Fe2N stoichiometry, we have to deal with two different iron sites, including Fe-II sites, i.e. iron atoms with two nitrogen neighhours. Furthermore, comparing the isomer shift 8 of the paramagnetic Fe-II site with the values for magnetic spectra of the E nitride (table 12), it is found that they are quite close to each other. Thus also pure paramagnetic spectra of e-FexN have to be fitted with two subspectra for the Fe-III and Fe-II sites, provided that x > 2. According to Jack [41,39], an ordering of the nitrogen atoms should also be effective, leading to the following probabilities of the different iron sites: p(Fe-III) = - - 6 2,

(7)

X

p(Fe-II)

= 3 --

6

(8)

X

with x in e-FexN. Calculating then the stoichiometry from the spectral areas of the two sites, the nitride is calculated to have x = 2.116(10). That means M6ssbauer spectroscopy is the most accurate method for determining the stoichiometry of e-FexN, provided the spectral areas (no thickness effects, identical or known f-factors) are well known and the ordering proposed by Jack is fully effective. Another sample was prepared with the same nitrogen flow of 2.4 sccm but at a temperature of 740 K. The X-ray diffraction analysis, as displayed in fig. 13, Table 13 Hyperfme parameters of E-Fe2.12N. Sub6 spectrum (ram/s) Fe-III 0.41(2) Fe-II 0.30(3)

e (ram/s) 0.27(2) 0.46(3)

H (T)

F (ram/ s) 0.28(2) 0.28(2)

RA (%) 83.5(0.7) 16.5(1.3)

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitridefilms

218

6.0xl 0 a

I

I

1

I

5.0xl 0 3

4.0x10 3 C

3,0x10 3

C e

2.0x10 3

.=_

o ~

o

1.0x10 3

O

O

t'~

0.0 ,

I

i

I

40

i

I

60

,

80

I

1~

2~ Fig. 13. X R D measurement of the sample prepared at 740 K w i t h a nitrogen flow of 2.4 seem.

showed again that only e-nitride is present. The stoichiometry of e-Fe2.62(21)N is derived from the lattice parameters, whereas the RBS analysis reveals a nitrogen concentration of 30.5(20) at%, yielding a mean value of e-Fe2.4509)N. The corresponding CEMS spectrum of this nitride film is shown in fig. 14, with its hyperfine parameters summarized in table 14. Here again the two iron sites Fe-III and Fe-II can be well resolved and from their spectral areas and eq. (7) the stoichiometry e-Fe2.44(2)N is calculated in excellent agreement with the value determined by X R D and RBS.

>k-

I

I

I

I

I

I

I

I

I

t .04

Cn Z UJ

~i.02

.J

i.O0

I I -6

-4

I -2

I

i

I

0

VELOCITY

i

I

I

2

liFe4

~

I 6

[mm/s]

Fig. 14. C E M S measurement of the sample prepared at 740 K w i t h a nitrogen flow of 2.4 seem.

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitride films

219

Table 14 Hyped'me parameters of ~-Fe2.4sN. Subspectrum

6

(ram/s)

e (ram / s)

H (q')

F (ram / s)

RA (%)

Fe-III Fe-H

0.38(2) 0.31(2)

0.00(2) -0.01(2)

9.75(12) 20.62(09)

0.96(6) 0.82(6)

45.6(1.2) 54.4(1.2)

When the nitrogen flow is reduced to 1.5 seem and deposition is carried out at a substrate temperature of 300 K, the nitride film reveals the CEMS spectrum shown in fig. 15. It had to be fitted with three magnetic split subspectra, whose hyperf'me parameters are summarized in table 15. In addition to the Fe-III and FeII sites the Fe-I site appears, its parameters well coinciding with the data given in table 12. As RBS analysis reveals a stoichiometry of ~-Fez59(20)N, this is contradictory to the ordered model of Jack, which does not permit the existence of the Fe-I sites for x < 3. Nevertheless, the f'im was deposited at room temperature and additionally XRD reveals broad peaks of the hexagonal phase. Therefore, it can be assumed that this film is not in its equilibrium state. This would coincide with observations of DeChristofaro and Kaplow [42], who found an ordering of the nitrogen interstitials during annealing of quenched ~-FexN samples. Annealing experiments have to be carried out with the samples deposited at room temperature in order to get more information on the ordering. Chen et al. [39] have doubted that the ordering model is effective in E-FexN, but one should take into account that they have not well resolved the paramagnetic spectra, i.e. not fitted the Fe-II site. Nevertheless, when including their value, together with the values obtained so far into fig. 16, one can see that there is a good agreement with the ordered model for x < 2.6. It should be noted that our fractions shown for x = 2.58 resulted from a sample which was deposited at room temperaI

I

I

I

I

l

I

I

1.04

P" I . O E Z M hl >

J t~ t . 0 0 Ig

I

-8

I

-6

1

-4

I

I

Fe-m

ill

! i I

t

i

I

1 I

-E VELOCITY

I E

i i

l 4

Fe-~ i Fe-~ B

I

fl

[mm/s]

Fig. 15. CEMS measurement of the sample prepared at 300 K with a nitrogen flow of 1.5 sccm.

220

P. $chaaf et al. / Laser nitrided iron and sputtered iron nitridefilms

Table 15 Hyped'me parameters ofe-Fe2.ssN (not thermally equilibrated). Sub-

6

e

spectrum

(ram/ s)

(mm/ s)

Fe-ITI Fe-II Fe-I

0.39(2) 0.33(2) 0.26(2)

0.01(2) -0.01(2) 0.02(2)

H

F

03

(mm/s)

10.17(12) 20.87(09) 27.08(09)

1.07(9) 1.07(9) 1.07(9)

RA

(%) 31.6(1.7) 53.7(1.7) 14.7(1.7)

ture and is not in thermal equilibrium. Above this value there is at least a preferential site occupation. However, further annealing measurements should also bring more information on that region. We claim that the ordering proposed by Jack [37] is effective, at least for x-values below 2.6 and provided the sample is thermally equilibrated. 5.3. H I G H N I T R O G E N FLOWS: FeNy

Increasing the nitrogen flow to above 5 scem leads to nitride films with high nitrogen contents exceeding the concentrations normally plotted in the iron-nitrogen phase diagrams. The RBS spectrum for a nitride film prepared at 670 K and a nitrogen flow of 27 sccm as shown in fig. 17 reveals a nitrogen concentration of 50(9) at%, but RBS cannot distinguish between nitrogen in nitride phases and free nitrogen present in the sample. An XRD measurement as displayed in fig. 18 shows crystalline reflexes, which fit to an fcc structure with the lattice constant a = 4.50(2) A. 100 O

80"

Fell

~ o r d e d n g 7-

9"'%.~176176176'.

(~

60.

.=

~~--~.m..i~!.....F~!~..

t~ | ._>

40-

20"

0 2.0

2.2

2,4

2.6

2.8

3.0

x in e-FexN Fig. 16. Ordering of nitrogen intersitials in e-FexN. Jack's model - sofid lines, binomial distribution - dashed lines, data of ref. [39] - open symbols, this work - closed symbols (x = 2.58 not in thermal equilibrium).

P. Schaaf et aL / Laser nitrided iron and sputtered iron nitride films

221

Energy 2000.40

0.70 I

I

I

I

I

"6

0

I

I

I

400

800

Channel Fig. 17. RBS spectrum of the sample prepared at 670 K with a nitrogen flow of 27 sccm.

Nakagawa and coworkers [43] reported such a fcc nitride phase with the same lattice constant and denoted it as y'-FeNy with a value ofy ~ 0.65 and NaC1 structure together with another nitrogen rich phase y"-FeNy with y ~-, 0.91 and ZnS I

I

I

I

4,0x10 3

'•'c3,0X10

3

"~ 2,0x10 3

,,# O

.~_

O r

~

r r

1,0x10 3

0,0

r

i

40

,

i

i

60

O O

I

80

,

I

100

2~ Fig. 18. XRD measurement of the sample prepared of 670 K with a nitrogen flow of 27 sccm.

222

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitride films

~. 1 . 1 2 z ~ l.OB

.J w i1- t . 0 0

A I

I -1.5

I -!

I I I -0.5 0 VELOCITY

I I 0.5 (mm/s]

B

I f T t

I 1.5

Fig. 19. CEMS measurement of the sample prepared at 670 K with nitrogen flow of 27 sccm, fitted according to ref. [43].

structure (a = 4.33 A). They analyzed the corresponding M6ssbauer spectra for y"-FeNy with two single lines (the second being weak) with an isomer shift around 0.1 mm/s. The ~"-FeNy was fitted with two doublets of isomer shifts around 0.45-0.50 mm/s. The CEMS spectrum obtained for our sputtered nitride fdm is shown in fig. 19, together with a spectra analysis according to the reported model with quite a low value for the goodness of fit (X2). The hyperfine parameters are given in table 16. That would mean that we have a mixture of y" and y" in the film, whereas only reflexes for y" are found in the XRD analysis. Nevertheless, the spectrum can also be fitted with an even slightly better value for X2 according to a model similar to that ofwiistite [33], which is shown in fig. 20, with the hyperfine parameters given in table 17. Assuming NaC1 structure for y", the subspectra should be attributed to the nitrogen-deficient FeNy in the following way: A - Fe 3+ with all nitrogen neighbours, B - Fe 2+ with all nitrogen neighbours, C - Fe 2+, D - Fe 3+ with missing nitrogen neighbours. The quadrupole splittings can be approximated fairly well by PCM calculations (C: 0.43 mm/s, D: 0.64 mm/s). Based on the assumption of a binomial distribution for the nitrogen vacancies, the stoichiometry of FeN0.91(]0) can be calculated from the relative areas in agreement with the RBS measurement. Table 16 Hyperfine parameters of~'- and/or y"-FeNy fitted according to ref. [43]. Subspectrum

6 (ram/s)

~ (minis)

A B C

0.16(1) 0.27(2) 0.47(3)

0.79(5) 0.83(6)

H (3")

F (mm/s)

RA (%)

0.45(3) 0.31(3) 0.31(3)

42.9(1.4) 38.4(1.2) 18.6(1.6)

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitride films

223

t.t2 I-Z w

~t.06 uJ M

~ t.oo t I

I --

.5

I

-1

-0.5

I I

I

0

0.5

VELOCITY

Jam/s]

)

A ~B t[ tD

I

1

1,5

Fig. 20. CEMS measurement of the sample prepared at 670 K with a nitrogen flow of 27 scorn, fitted equivalent to wustite structure.

Nevertheless, this is only a hypothesis which has to be proven by further investigations on these nitrogen rich nitrides. Evidently the F e N phase diagram has to be extended, since there is at least one new crystalline (metastable) phase, prepared at 670 K with nitrogen contents around 50 at%. 5.4. CONCLUSION

Iron-nitride fdms of various stoichiometries can be produced by reactive magnetron sputtering. For e-FexNit has been shown that it has to be fitted with two different iron sites, even in the paramagnetic state below x = 2.3. The measurements show that the nitrogen ordering in that phase is effective at least for x < 2.6, and even above this value preferential site occupation seems to exist. A new crystalline phase (stable at least until 670 K) was found with nitrogen contents around 50 a t ~ Thus the F e N phase diagram has to be extended. Nevertheless, more work has to be done in order to have a complete "M6ssbauer picture" of the complete F e N system. Table 17 Hyperfine parameters ofy'-FeNy fitted similar to wiistite". Sub-

6

e

H

F

RA

spectrum

(mm / s)

(ram / s)

(T)

(ram / s)

(%)

A B C D

0.12(1) 0.92(3) 0.63(4) 0.27(3)

0.43() 0.29() 0.29() 0.29()

52.1(1.1) 6.2(1.2) 6.5(1.4) 35.2(1.4)

0.39(5) 0.80(7)

a No error means that parameter was fixed.

224

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitridefilms

Acknowledgement The excimer laser treatments were carried out in cooperation with Drs. A. Emmel and E. Schubert (ATZ-EVUS, Vilseck). We are indebted to them for their fruitful cooperation. The authors also would like to thank O. Schulte and F. Rose (I. Physikalisches Institut, Universit~t G6ttingen) for performing the X R D measurements, T. Kacsich and Dr. W. Bolse for their help in running the magnetron and analyzing the RBS and R N R A data and D. Purschke for operating the IONAS implanter. This work was partly supported by the Deutsche Forschungsgemeinschaft (DFG).

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [ 10] [1 l] [12] [13] [14] [15] [16] [17] [18] [I 9] [20] [21] [22] [23] [24] [25] [26] [27]

VDI, VDI-Lexikon Werkstofftechnik (VDI Verlag, Dfisseldorf, 1990). D. Liedtke, Nitrieren, Merkblatt 447 (VdEh, Dfisseldorf, 1974). H. Kunst and D. Liedtke, Tribologie, Reibung- Verschleiss- Schm/erung 7 (1983) 39. L. Abada, G. Rixecker, F. Aubertin, P. Schaaf and U. Gonser, Phys. Stat. Sol (a) 139 (1993) 181. D.L. Williamson, R. Wei and P.J. Wilbur, Nucl. Instr. Meth. B 56/57 (1991) 625. G. Marest, Defect and Diffusion Forum 57-58 (1988) 273. P. Schaaf, A. F.mmel, C. Illgner, K.P. Lieb, E. Schubert and H.W. Bergmann~ Mater. Sci. Eng. ALett., submitted (I 994). P. Schaaf, V. Biehl, M. Bamberger, P. Bauer and U. Gonser, J. Mater. Sci. 26 (199 l) 5019. E. Schubert and H.W. Bergmann; Lasers in Engineering 2 (1993) 111. P. Schaaf, A. Kr~mer, F. Aubertin and U. Gonser, Z. Metallk. 82 (1991) 815. U. Gonser, P. Schaaf and F. Aubertin, Hyp. Int. 66 (199 I) 95. P. Schaaf, T. Wenzel, K. Schemmerling and ICP. Lieb, Hyp. Int. 92 (1994) 1189. W. Kfindig, Nucl. Instr. Meth. 75 (1969) 336. M. Uhrmacher, K. Pampus, F. Bergmeister, D. Purschke and K.P. Lieb, Nucl. Instr. Meth. B 9 (1985) 234. L.R. Doolitfle, Nucl. Instr. Meth. B 9 (1985) 344. L.R. Doollttle, Nucl. Instr. Meth. B 15 (1986) 227. K.P. Lieb, W. Bolse, T. Colts, A. Kehrel, M. Uhrmacher and T. Weber, in: Plasma Surface Engineering, eds. E. Broszeit et al. (DGM Informationsgesellschaft, Oberursel, 1989) p. 1055. M. Borowski, A. Battistig, W. Bolse and K.P. Lieb, Z. Phys. A, submitted (1994). J. Kunze, Nitrogen and Carbon in Iron andSteel(Akademie Verlag, Berlin, 1990). N. DeChristofaro and R. Kaplow, Metall. Trans. A 8 (1977) 35. J.M.D. Coey, K. O'Donnell, Q. Qinian, E. Touchais and K.H. Kack, J. Phys.: Condens. Matter 6 (1994) L23. A.J. Nozik, J.C. Wood and G. Haacke, Solid State Commun. 7 (1969) 1677. M.I. Clauser, Solid State Commun. 8 (1970) 781. R.S. Figueiredo and V. Drago, Solid State Commun. 80 (1991) 757. J. Bainbridge, D.A. Channing, W.H. Whitlow and R.E. Pendlebury, J. Phys. Chem. Solids 34 0973) 1579. M. Ron, in: Applications of M~ssbauer Spectroscopy II, ed. R.L. Cohen (Academic Press, New York, 1980) pp. 329-392. J. Foct, P. Rocbegude and A. Hendry, Acta Metall. 36 (1988) 501.

P. Schaaf et al. / Laser nitrided iron and sputtered iron nitride films

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225

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