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Nitrogen atoms in ArN2 flowing microwave discharges for steel surface nitriding
Materials Science and Engineering, A 139 ( 1991 ) 9 14
9
Nitrogen atoms in A r - N 2 flowing microwave discharges for steel surface nitriding A. Ricard, J. Deschamps and J. L. Godard Plasrna Physique Lahoratoire. B(itiment 212. ('NRS, Paris-Sud UniverMt~;. F-91405 Orsay (l+ance)
L. Falk and H. Michel Science Genie Su(lhce Laboratoire. ('NRS, Ecole des Mines, F-54042Namy (France)
Abstract High nitrogen atom densities (10~5-10 " cm-3) have been produced in high pressure (20-760 Torr) Ar-N~ flowing post-discharges. Gas ionization is performed by 2.45 GHz surfatron and surfaguide exciters with 100-200 W transmitted power. Nitrogen atoms have been measured by NO titration and by the absolute intensities of the first positive system of N, in the afterglow. From kinetics analysis the nitrogen atom recombination rate coefficient has been found to decrease with gas temperature from (4 + 1) x 10--~3 cm" s ~ at 71,= 296 K to 1.3 x 10- ~3 cm" s ~ at f, = 600 K. The nitrogen atom density remains constant within a factor of 2 at T= 700 K. An increase of v'-Fe4N coating thickness with nitrogen atom density in the post-discharge reactor is reported.
1. Introduction Interest is growing in nitrogen flowing postdischarges for surface treatment processes such as Si3N 4 thin film deposition [1-3] and steel surface nitriding where thick coatings of Fe4N (5-8 /,m in thickness) have been obtained from d.c. [4] and microwave [5] post-discharges. T h e nitrogen atoms in the gas flow after N, and A t - N 2 microwave discharges produce the Lewis-Rayleigh afterglow following the reactions k.;(B. ['1
N+N+M, N2(B, V')
, N2(B, V')+ M~ *N2(A, V")+ h v ( f i r s t positive)
(a)
where kN(B , V') is the recombination rate for Ne(B, V') state excitation and M~ denotes the A r - N 2 gas mixture. Such an afterglow is well suited to study the nitrogen atom flows in post-discharge reactors. It is the purpose of the ,present work to report the results obtained in 2.45 G H z microwave ( 1 0 0 200 W) post-discharges in A r - x N 2 gas mixtures, where x is the percentage of N 2, at high gas pressures ( 3 - 7 6 0 Torr). T h e radiative states produced by reaction (a) have been detected by 0921-5(193/91/$3.50
emission spectroscopy. Two methods of diagnostics of nitrogen atom density have been used, the first by N O titration and the second from the absolute intensities of the first positive spectral bands produced by reaction (a). T h e special effect of gas temperature on nitrogen atom density in the flowing afterglow has been studied.
2. Experimental set-up T h e microwave post-discharge reactor for steel surface nitriding is reproduced in Fig. 1. T h e plasma is initiated in a quartz tube of internal diameter 4 or 5 mm by means of gap-type exciters: surfatron [6] and suffaguide [7]. T h e postdischarge runs into a reactor of diameter 3 cm and length 120 cm at a distance of 70 cm from the exciter gap. Fe-0.1%C samples of diameter 1 cm and length 1 cm can be heated to 900 K with a conventional heating device. T h e heating device, of length 60 cm, is located between Z = 8 0 cm and Z = 140 cm (see Fig. 1). With the surfaguide exciter, discharges have been performed in N, at 3 Tort (flow rate Q = 0 . 3 N l min i) and in A r - ( 1 . 5 % - 3 % ) N 2 at 2 0 - 7 5 Torr ( Q = 5 N I min ~) with a transmitted power of © Elsevier Sequoia/Prinled in The Netherlands
10 PRESSURE
--
r
SAMPLE {!:~mmmm
X
30 mm
x
X xx/
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--
Jr x x
X
F" L '~
<---- N2, At, H2
-"
MASSFLOWMETERS
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I~ 2• ~I 1
GA,~INLETPIPE
'1~ x
"x/N/N/N/ ~.'~: ROTARYVACUUM PUMP
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Ar-NO [ ~ ",
X
/
REACTO~ ~ \ ~ _t _ L_ . ~1200 mm
\AAA/
' I " ~
PICOAMMETER
E Fig. 1. Set-upof the 2.45 GHz microwavepost-discharge reactor. Z is the post-discharge distance (Z = 0 at the plasma end).
70 W. With the surfatron exciter, discharges in A r - 2 % N 2 at atmospheric gas pressure have been produced with 200 W. In this last experimental condition the gas temperature has been measured inside the gas flow just at the end of the 4 mm quartz tube. The afterglow has been analysed by emission spectroscopy using a Jobin-Yvon spectrometer of 60 cm focal length with a 1200 m m - 1 grating. For qualitative emission spectra a picoammeter and chart recorder are connected to a Hamatsu R636 photomultiplier. For quantitative measurements (absolute intensities) the photomultiplier is connected to a counting device, the noise background being calculated and subtracted in the computer. 3. D i a g n o s t i c s of nitrogen a t o m density in the flowing postdischarge
The nitrogen atom densities have been measured by introducing NO into the postdischarge [5, 8] and by measuring the absolute intensities of the first positive bands produced by reaction (a)[9].
1.3x10 ]5 cm -3 in A r - 3 % N 2 at 20 Torr have been determined at the end of the surfaguide 2.45 GHz post-discharge (5 mm quartz tube) with 64 W transmitted power in the discharge. In A r - 3 % N 2 at atmospheric pressure the nitrogen atom yield per watt was previously found to reach a maximum value of 8.5 x 10 ]3 cm -3 W -1 for a microwave power of about 100 W [8]. The NO titration method as investigated with the experimental set-up of Fig. 1 only gives a mean value in the post-discharge reactor with a 40% relative uncertainty. For local determination a quantitative determination of the first positive band intensities has been undertaken [9].
3.2. Nitrogen atom densities from absolute intensities of first positive bands in the afterglow The Nz(N, V') states which are produced by reaction (a) in the afterglow are mainly destroyed when [N2] > 1016 cm-3 by the quenching reaction ko(B. V')
N2(B , V') + M 2 ~
products
(b)
where ko(B, V') is the quenching rate constant and M 2 is the A r - N 2 gas mixture. Then the pseudosteady state density of N2(B , V') can be written as
3.1. Nitrogen atom densities by NO titration By introducing NO in the post-discharge (see Fig. 1), NO fl emission (violet colour) at low NO flow rates and NO 2 continuum emission (green colour) at high NO flow rates are observed. The colour change occurs when the NO and N 2 flow rates are equal. With this method, nitrogen atom densities of 2.5 x 10 xs cm -3 in N 2 at 3 Torr and
ky(B, V') [B, V'] = [N]2 kQ(B, l/)
(1)
V~' V " The first positive intensity l"BA is given by
i ~, v,, = C ~ ) [B, I/]A( V', V")
(2)
II
where 2 is the optical wavelength, C(2) is a factor related to the spectral response of the detection system and A( V', V") is the emission probability {10]. By combining eqns. (1) and (2), the first positive intensity can be expressed as ky(B, V)
N]z densities by absolute intensity measurements of first positive system in the A r - 2 % N 2 afterglow (200 W, atmospheric gas pressure)[9] Z (cm)
10
[N]~(10'~ cm ~)
2(1
5.5
3t)
3.5
2
(3) TABLE 2
INI2 ko(B" I/)
T ( Z ) variations in the A r - 2 % N 2 afterglow (4 mm discharge tube, 200 W, atmospheric pressure) [9]
where KB A
TABLE 1
--
Z (cm) T(K)
C(Z) A(V, V')
10 580
20 555
30 530
2
Moreover, from the gas flow of velocity V it is possible to derive a kinetic equation for the nitrogen atoms as a function of time t, with t = Z/V, from the reaction
K
(B)
crn6
s -~
10 3~
k~
N+N+M~
---," N~+M~
(c)
It follows that 1
1
[N]z {Nk,
2 kN[M2](t - t0)
(4)
By taking k N : 2 Y v, kN(B, V') as previously estimated [11, 12], eqns. (3) and (4) can be coupled to determine Zv, kN(B, V') from Zv, I~A v" absolute intensity measurements and the nitrogen atom density from eqn. (3)[9]• • • • V' V" The absolute mtensmes IBA (Z) are obtained by counting the number of photons per second for each (V', V") transition, taking into account the spectral response of the detection system• The results obtained with a 200 W excitation of an Ar-2%N2 gas mixture are reproduced in Table 1 for Z = 10-30 cm. The Z dependence of I~A v'' is obtained by moving a quartz fibre along the discharge tube• The measurements reported by the absolute intensity method bear a 25% relative uncertainty. It can be deduced from Table 1 that the nitrogen atom densities are decreasing with Z. Such a result could not be obtained by using the NO titration method• 4. Effect of gas temperature on nitrogen atom density under postdischarge conditions
With the heating device of Fig. 1, the gas temperature can be varied from 300 to 900 K in the post-discharge reactor at medium N 2 and
10~33
I
I
I
I
Fig. 2. k N/(B), the recombination rate of nitrogen atoms in the N2(B) state, vs. (T/To) 2, where T is the gas temperature at the end of the 4 mm discharge tube and To = 296 K.
A r - 2 % N 2 gas pressure (3-100 Torr). Also, the gas temperature of atmospheric pressure Ar-2%N: discharges has been measured inside the gas flow at the end of the discharge tube. 4.1. Variation of recombination rate of nitrogen atoms with gas temperature Variations of kN(B)(with kN(B)= Zv, kN(B, V') vs. gas temperature (T) have been determined from absolute intensity measurements in the Ar-2%N: post-discharge inside the 4 mm quartz discharge tube (Fig. 1) at atmospheric pressure• The gas temperature was measured in the gas flow at the end of the discharge tube, the Z dependence of T being obtained by moving the surfatron exciter along the discharge tube. The T ( Z ) variations are given in Table 2. The corresponding kN(B) variation is reproduced in Fig. 2 vs. (T/To) 2, with T0 = 296 K for the room temperature. The rate of recombination at room temperature is then obtained either by extrapolation of the kNT(B) values of Fig. 2 to T= 71~or by using
12 kT(B)
1
To
k N( B)o,
N2
o.a
o~
a) & / / ~~* ' ~ , ~ , .
o6
b)
-
3Torr
Y
~
• Ar-3.2% N2 N2
%
20Tor, 75Tort
05
0.,~ 0J
01
T(K) 0
I
J
I
I
400
500
600
700
Fig. 3. Normalized recombination rate of nitrogen atoms vs'. T (K): (a) from ref. 1 1; (b) from ref. 12; (c) present results.
the following empirical formula for the temperature dependence of the rate constant in the range 300-600 K [9]:
{ ()21
k s V ( B ) = 6 x l 0 - ~ exp - 0 . 4
cm~s -1
(5)
This results in a value of (4 _+1)x 10 -33 c m 6 S-1 at To 296 K, which is in good agreement with previously reported values [ 11, 12]. Variations of the normalized rate coefficient kNT(B)/kNTr(B) vs. gas temperature are reproduced in Fig. 3. Figure 3(a) is from the following empirical formula given in ref. 11:
0.2
i 300
[ 400
i
I
i
[
500
600
i
f
T(K) ,
700
Fig. 4. Normalized intensity of N 2 (B, V'= 8-A, V" =4) first positive band vs. T (K) for 2.45 GHz microwave postdischarge at 64 W.
The intensity ratio r 8-4 first increases in the 300 to 500 K temperature range before decreasing for Ar-3.2%N 2 and Ar-1.5%N 2 when T > 5 0 0 K . The r 8-4 intensity ratio can be expressed from eqn. (3) as
=
kNT(B)= 1.6 X 10 -33 exp
2~
(6)
Equation (6) was derived in ref. 11 from experimental results in Ar-xN2, with x--20%-100%, at T = 196-327K. Figure 3(b) is from calculations of ref. 12 and Fig 3(c) is from eqn. (5). A more marked decrease of kNT(B)/kNT°(B) is found by using eqn. (5).
By assuming a constant value of the B state quenching cross-section oo(B ) between 300 and 700 K, the quenching rate ko(B ) = oo(B ) V, where I? is the neutral relative velocity (17"= (8k T/erm)]/2), is proportional to T 1/2. The resulting quenching rate ratio in eqn. (7) is given by
ko'(B, 8)
(8)
Then the [N]T/[N]r0 density ratio can be expressed as
l'l 4
4.2. Variation of nitrogen atom density in the heating device
IN]r0
The first positive intensity has been recorded at the end of the heating device (Z = 140 cm, Fig. 1) as the temperature was increased from 300 to 700 K. The N~(B, 8-A, 4) band intensity ratios r~ -4= IT8-a/ITo are reproduced in Fig. 4 in the range T = 3 0 0 - 7 0 0 K for 64 W, 2.45 GHz post-discharges in N 2 (3Torr) and A r (1.5%-3%)N2 (20-75 Torr).
By taking the kNTiB)/kNT~(B) values reproduced in Fig. 3, the [N]r/[N]16 variations reproduced in Fig. 5 have been deduced. In pure N 2 and A r - x N 2 gas mixture (with x = 1.5% and 3.2%) the nitrogen atom densities increase with gas temperature up to 500 K by factors of 1.8 and 1.5 respectively.
kN'(B, 8))
~]
(9)
13
5. Correlation between nitrogen atom dissociation rate and thickness of }"-Fe4N nitrided layers
The ratio [N]~/[N]yI, = f ( T ) increases according to curve (c) in Fig. 3 but decreases according to curves (a) and (b) in the range 5 0 0 - 7 0 0 K for the two A r - x N 2 gas mixtures. In any case, [Nit= 700K is equal to or greater than [N]r=300K. Thus it can be concluded that the temperature increase in the heating device of the post-discharge reactor up to 700 K is not detrimental to the nitrogen atom density. Rather, a slow increase by a factor of 1.5-2 is observed.
()'N'T
(from fig.3ab)
By performing nitriding treatments of 1 h on Fe-0.1%C steel specimens located at Z = 100 cm (see Fig. 1) and heated up to 840 K, 7'-FerN layers have been obtained as shown on the micrograph of Fig. 6(a) for an N~ flowing post-discharge at 2.5 Torr, 70 W and a flow rate Q = 0.3 1 min- J. The y' layer thickness can be determined by direct measurement on the micrograph of Fig. 6(a) or from the (100),, and (200)V intensity ratio of diffraction patterns of Fig. 6(b) [5]. A y' layer thickness of 8 - 1 0 / ~ m has been determined with a thin e-Fe 2 3N layer on top of the y' layer and a diffusion zone of nitrogen in solution in a-Fe. After a metallographic etching, the a layer is revealed on cross-sectional micrographs by the presence of numerous 7' precipitates formed during the slow cooling of the Fe-0.1%C sample after treatment. The y' layer thickness is reproduced in Fig. 7 vs. the nitrogen atom density as measured by N O titration at the end of the discharge tube (Z = 70 cm, Fig. 1 ). The experiments have been performed in A r - l . 4 % N 2 gas mixtures with a microwave
(from fig.3c)
/
/
1.5
1
~A
Ar-3.2%N2
~,
Ar-1,5%N
(~
20Torr 2 75 Torr
N 2 3 Tort
T(K) I 300
400
i
I
500
600
Fig. 5. Normalized nitrogen atom density deduced from Fig. 3.
I
]l=
700
vs.
T
(K) as
(b)
Tvw o o ÷
"2
A ,t-
la
sb ~ 0
28 ,
(BPagg's
2~
24
2~
angle)
Fig. 6. (a) Cross-sectional micrograph of compound and diffusion layers in Fe 0.1%C. The steel specimen was nitrided in flowing N 2 post-discharge plasma (0.3 1 min J) for 1 h at 840 K, 2.5 Tort, 70 W and Z = 30 cm. (b) Diffracted X-ray intensity vs. Bragg angle 0 at the outer surface of the nitrided specimen (Co Ka).
14
d~,, ( t i m )
/ []
J 10 '4
F~
I
,
10 ~s
I
J 1 0 is
,
(
N] z= 70cm , I
c m -3 }
Fig. 7. y'-Fe4Nlayer thickness (dr,) vs. nitrogen sities in the post-discharge at distance Ar-l.4%N2 gas mixture, 2.45 GHz discharge at pressure from 40 Torr (A) to 500 Torr (D). The for e-Fe2N layers (thickness about 1/~m).
atom denZ = 7 0 cm. 70 W. Gas circles are
power of 70 W and a total gas pressure between 40 and 500 Torr. A good correlation is found between the ~,' layer thickness and the nitrogen atom density in the post-discharge reactor. The e layer appears for [N]z = 7ocm> (2-3) x 101 s cm- 3. H a gas was introduced in the first step of treatment (2-3 min) to prevent surface oxidation and was then cut off to obtain high nitrogen atom densities [5].
6. Concluding remarks High nitrogen atom densities have been produced in N2 at low gas pressure (3 Torr) and in Ar-N2 gas mixtures at high pressures (20-760 Torr) when the gas flows are ionized by means of 2.45 GHz surfaguide and surfatron exciters working with 1 0 0 - 2 0 0 W transmitted power. Two diagnostics of nitrogen atom density have
been performed in the post-discharges: the NO titration method and absolute intensity measurements of the first positive bands. The NO titration gives quick and accurate results (+ 20%) at medium gas pressures (3-200 Torr) but without spatial resolution. The measurement of absolute intensities is a more sophisticated method whose strength is to give local nitrogen atom densities at pressures as high as atmospheric pressure. Effects of gas temperature on nitrogen atoms have been analysed. From detailed kinetics studies the variation of the nitrogen atom recombination rate (kNr(B) to produce N2(B) radiative states) has been determined in the range T= 300-600 K. Nitrogen atom densities have been found to increase with T up to 500 K, with values multiplied by a factor of between 1 and 2 up to T= 700 K. A good correlation has been observed between the 7' layer thickness and the nitrogen atom density in the post-discharge reactor.
References 1 I. Kato, K. Noguchi and K. Numada, J. Appl. Phys., 62 (1987) 492. 2 G. Lucovsky and D, V. Tsu, J. Vac. Sci. Technol. A, 5 (1987) 2231. 3 L. Bardos, Vacuum, 38 (1988) 637. 4 A. Ricard, Rev. Phys. Appl., 24 (1989) 251. 5 A. Ricard, J. Oseguera, H. Michel and M. Gantois, IEEE Trans. Plasma Sci., 18 (1990) 940. 6 M, Moisan, P. Leprince, C. Beaudry and E. Bloyet, IEEE Trans. Plasma Sci., PS-3 (1975) 55; U.S. Patent 4,049,940, 1977. 7 V. M. M, Glaude, M. Moisan, R. Pantel, P. Leprince and J. Marec, J. Appl. Phys., 51 (1980) 5693. 8 A. Ricard, A. Besner, J. Hubert and M. Moisan, J. Phys. B: At. Mol. Phys., 21 (1988) L579. 9 G. Call6de, J. Godart and A. Ricard, submitted to J. Phys. D: Appl. Phys., in the press. 10 L. G. Piper, K. W. Holtzclaw and B. D. Green, J. Chem. Phys., 90(1989) 5337. 11 I. M. Campbell and B. A. Trush, Proc. R. Soc. A, 296 (1967) 201. 12 H. Partridge, S. R. Langhoff and C. W. Bauschlicher, J. Chem. Phys., 88 (1988) 3174.
9
Nitrogen atoms in A r - N 2 flowing microwave discharges for steel surface nitriding A. Ricard, J. Deschamps and J. L. Godard Plasrna Physique Lahoratoire. B(itiment 212. ('NRS, Paris-Sud UniverMt~;. F-91405 Orsay (l+ance)
L. Falk and H. Michel Science Genie Su(lhce Laboratoire. ('NRS, Ecole des Mines, F-54042Namy (France)
Abstract High nitrogen atom densities (10~5-10 " cm-3) have been produced in high pressure (20-760 Torr) Ar-N~ flowing post-discharges. Gas ionization is performed by 2.45 GHz surfatron and surfaguide exciters with 100-200 W transmitted power. Nitrogen atoms have been measured by NO titration and by the absolute intensities of the first positive system of N, in the afterglow. From kinetics analysis the nitrogen atom recombination rate coefficient has been found to decrease with gas temperature from (4 + 1) x 10--~3 cm" s ~ at 71,= 296 K to 1.3 x 10- ~3 cm" s ~ at f, = 600 K. The nitrogen atom density remains constant within a factor of 2 at T= 700 K. An increase of v'-Fe4N coating thickness with nitrogen atom density in the post-discharge reactor is reported.
1. Introduction Interest is growing in nitrogen flowing postdischarges for surface treatment processes such as Si3N 4 thin film deposition [1-3] and steel surface nitriding where thick coatings of Fe4N (5-8 /,m in thickness) have been obtained from d.c. [4] and microwave [5] post-discharges. T h e nitrogen atoms in the gas flow after N, and A t - N 2 microwave discharges produce the Lewis-Rayleigh afterglow following the reactions k.;(B. ['1
N+N+M, N2(B, V')
, N2(B, V')+ M~ *N2(A, V")+ h v ( f i r s t positive)
(a)
where kN(B , V') is the recombination rate for Ne(B, V') state excitation and M~ denotes the A r - N 2 gas mixture. Such an afterglow is well suited to study the nitrogen atom flows in post-discharge reactors. It is the purpose of the ,present work to report the results obtained in 2.45 G H z microwave ( 1 0 0 200 W) post-discharges in A r - x N 2 gas mixtures, where x is the percentage of N 2, at high gas pressures ( 3 - 7 6 0 Torr). T h e radiative states produced by reaction (a) have been detected by 0921-5(193/91/$3.50
emission spectroscopy. Two methods of diagnostics of nitrogen atom density have been used, the first by N O titration and the second from the absolute intensities of the first positive spectral bands produced by reaction (a). T h e special effect of gas temperature on nitrogen atom density in the flowing afterglow has been studied.
2. Experimental set-up T h e microwave post-discharge reactor for steel surface nitriding is reproduced in Fig. 1. T h e plasma is initiated in a quartz tube of internal diameter 4 or 5 mm by means of gap-type exciters: surfatron [6] and suffaguide [7]. T h e postdischarge runs into a reactor of diameter 3 cm and length 120 cm at a distance of 70 cm from the exciter gap. Fe-0.1%C samples of diameter 1 cm and length 1 cm can be heated to 900 K with a conventional heating device. T h e heating device, of length 60 cm, is located between Z = 8 0 cm and Z = 140 cm (see Fig. 1). With the surfaguide exciter, discharges have been performed in N, at 3 Tort (flow rate Q = 0 . 3 N l min i) and in A r - ( 1 . 5 % - 3 % ) N 2 at 2 0 - 7 5 Torr ( Q = 5 N I min ~) with a transmitted power of © Elsevier Sequoia/Prinled in The Netherlands
10 PRESSURE
--
r
SAMPLE {!:~mmmm
X
30 mm
x
X xx/
]
--
Jr x x
X
F" L '~
<---- N2, At, H2
-"
MASSFLOWMETERS
\
I
:41
0
I~ 2• ~I 1
GA,~INLETPIPE
'1~ x
"x/N/N/N/ ~.'~: ROTARYVACUUM PUMP
SURFAGUlDE 2450 MHz
Ar-NO [ ~ ",
X
/
REACTO~ ~ \ ~ _t _ L_ . ~1200 mm
\AAA/
' I " ~
PICOAMMETER
E Fig. 1. Set-upof the 2.45 GHz microwavepost-discharge reactor. Z is the post-discharge distance (Z = 0 at the plasma end).
70 W. With the surfatron exciter, discharges in A r - 2 % N 2 at atmospheric gas pressure have been produced with 200 W. In this last experimental condition the gas temperature has been measured inside the gas flow just at the end of the 4 mm quartz tube. The afterglow has been analysed by emission spectroscopy using a Jobin-Yvon spectrometer of 60 cm focal length with a 1200 m m - 1 grating. For qualitative emission spectra a picoammeter and chart recorder are connected to a Hamatsu R636 photomultiplier. For quantitative measurements (absolute intensities) the photomultiplier is connected to a counting device, the noise background being calculated and subtracted in the computer. 3. D i a g n o s t i c s of nitrogen a t o m density in the flowing postdischarge
The nitrogen atom densities have been measured by introducing NO into the postdischarge [5, 8] and by measuring the absolute intensities of the first positive bands produced by reaction (a)[9].
1.3x10 ]5 cm -3 in A r - 3 % N 2 at 20 Torr have been determined at the end of the surfaguide 2.45 GHz post-discharge (5 mm quartz tube) with 64 W transmitted power in the discharge. In A r - 3 % N 2 at atmospheric pressure the nitrogen atom yield per watt was previously found to reach a maximum value of 8.5 x 10 ]3 cm -3 W -1 for a microwave power of about 100 W [8]. The NO titration method as investigated with the experimental set-up of Fig. 1 only gives a mean value in the post-discharge reactor with a 40% relative uncertainty. For local determination a quantitative determination of the first positive band intensities has been undertaken [9].
3.2. Nitrogen atom densities from absolute intensities of first positive bands in the afterglow The Nz(N, V') states which are produced by reaction (a) in the afterglow are mainly destroyed when [N2] > 1016 cm-3 by the quenching reaction ko(B. V')
N2(B , V') + M 2 ~
products
(b)
where ko(B, V') is the quenching rate constant and M 2 is the A r - N 2 gas mixture. Then the pseudosteady state density of N2(B , V') can be written as
3.1. Nitrogen atom densities by NO titration By introducing NO in the post-discharge (see Fig. 1), NO fl emission (violet colour) at low NO flow rates and NO 2 continuum emission (green colour) at high NO flow rates are observed. The colour change occurs when the NO and N 2 flow rates are equal. With this method, nitrogen atom densities of 2.5 x 10 xs cm -3 in N 2 at 3 Torr and
ky(B, V') [B, V'] = [N]2 kQ(B, l/)
(1)
V~' V " The first positive intensity l"BA is given by
i ~, v,, = C ~ ) [B, I/]A( V', V")
(2)
II
where 2 is the optical wavelength, C(2) is a factor related to the spectral response of the detection system and A( V', V") is the emission probability {10]. By combining eqns. (1) and (2), the first positive intensity can be expressed as ky(B, V)
N]z densities by absolute intensity measurements of first positive system in the A r - 2 % N 2 afterglow (200 W, atmospheric gas pressure)[9] Z (cm)
10
[N]~(10'~ cm ~)
2(1
5.5
3t)
3.5
2
(3) TABLE 2
INI2 ko(B" I/)
T ( Z ) variations in the A r - 2 % N 2 afterglow (4 mm discharge tube, 200 W, atmospheric pressure) [9]
where KB A
TABLE 1
--
Z (cm) T(K)
C(Z) A(V, V')
10 580
20 555
30 530
2
Moreover, from the gas flow of velocity V it is possible to derive a kinetic equation for the nitrogen atoms as a function of time t, with t = Z/V, from the reaction
K
(B)
crn6
s -~
10 3~
k~
N+N+M~
---," N~+M~
(c)
It follows that 1
1
[N]z {Nk,
2 kN[M2](t - t0)
(4)
By taking k N : 2 Y v, kN(B, V') as previously estimated [11, 12], eqns. (3) and (4) can be coupled to determine Zv, kN(B, V') from Zv, I~A v" absolute intensity measurements and the nitrogen atom density from eqn. (3)[9]• • • • V' V" The absolute mtensmes IBA (Z) are obtained by counting the number of photons per second for each (V', V") transition, taking into account the spectral response of the detection system• The results obtained with a 200 W excitation of an Ar-2%N2 gas mixture are reproduced in Table 1 for Z = 10-30 cm. The Z dependence of I~A v'' is obtained by moving a quartz fibre along the discharge tube• The measurements reported by the absolute intensity method bear a 25% relative uncertainty. It can be deduced from Table 1 that the nitrogen atom densities are decreasing with Z. Such a result could not be obtained by using the NO titration method• 4. Effect of gas temperature on nitrogen atom density under postdischarge conditions
With the heating device of Fig. 1, the gas temperature can be varied from 300 to 900 K in the post-discharge reactor at medium N 2 and
10~33
I
I
I
I
Fig. 2. k N/(B), the recombination rate of nitrogen atoms in the N2(B) state, vs. (T/To) 2, where T is the gas temperature at the end of the 4 mm discharge tube and To = 296 K.
A r - 2 % N 2 gas pressure (3-100 Torr). Also, the gas temperature of atmospheric pressure Ar-2%N: discharges has been measured inside the gas flow at the end of the discharge tube. 4.1. Variation of recombination rate of nitrogen atoms with gas temperature Variations of kN(B)(with kN(B)= Zv, kN(B, V') vs. gas temperature (T) have been determined from absolute intensity measurements in the Ar-2%N: post-discharge inside the 4 mm quartz discharge tube (Fig. 1) at atmospheric pressure• The gas temperature was measured in the gas flow at the end of the discharge tube, the Z dependence of T being obtained by moving the surfatron exciter along the discharge tube. The T ( Z ) variations are given in Table 2. The corresponding kN(B) variation is reproduced in Fig. 2 vs. (T/To) 2, with T0 = 296 K for the room temperature. The rate of recombination at room temperature is then obtained either by extrapolation of the kNT(B) values of Fig. 2 to T= 71~or by using
12 kT(B)
1
To
k N( B)o,
N2
o.a
o~
a) & / / ~~* ' ~ , ~ , .
o6
b)
-
3Torr
Y
~
• Ar-3.2% N2 N2
%
20Tor, 75Tort
05
0.,~ 0J
01
T(K) 0
I
J
I
I
400
500
600
700
Fig. 3. Normalized recombination rate of nitrogen atoms vs'. T (K): (a) from ref. 1 1; (b) from ref. 12; (c) present results.
the following empirical formula for the temperature dependence of the rate constant in the range 300-600 K [9]:
{ ()21
k s V ( B ) = 6 x l 0 - ~ exp - 0 . 4
cm~s -1
(5)
This results in a value of (4 _+1)x 10 -33 c m 6 S-1 at To 296 K, which is in good agreement with previously reported values [ 11, 12]. Variations of the normalized rate coefficient kNT(B)/kNTr(B) vs. gas temperature are reproduced in Fig. 3. Figure 3(a) is from the following empirical formula given in ref. 11:
0.2
i 300
[ 400
i
I
i
[
500
600
i
f
T(K) ,
700
Fig. 4. Normalized intensity of N 2 (B, V'= 8-A, V" =4) first positive band vs. T (K) for 2.45 GHz microwave postdischarge at 64 W.
The intensity ratio r 8-4 first increases in the 300 to 500 K temperature range before decreasing for Ar-3.2%N 2 and Ar-1.5%N 2 when T > 5 0 0 K . The r 8-4 intensity ratio can be expressed from eqn. (3) as
=
kNT(B)= 1.6 X 10 -33 exp
2~
(6)
Equation (6) was derived in ref. 11 from experimental results in Ar-xN2, with x--20%-100%, at T = 196-327K. Figure 3(b) is from calculations of ref. 12 and Fig 3(c) is from eqn. (5). A more marked decrease of kNT(B)/kNT°(B) is found by using eqn. (5).
By assuming a constant value of the B state quenching cross-section oo(B ) between 300 and 700 K, the quenching rate ko(B ) = oo(B ) V, where I? is the neutral relative velocity (17"= (8k T/erm)]/2), is proportional to T 1/2. The resulting quenching rate ratio in eqn. (7) is given by
ko'(B, 8)
(8)
Then the [N]T/[N]r0 density ratio can be expressed as
l'l 4
4.2. Variation of nitrogen atom density in the heating device
IN]r0
The first positive intensity has been recorded at the end of the heating device (Z = 140 cm, Fig. 1) as the temperature was increased from 300 to 700 K. The N~(B, 8-A, 4) band intensity ratios r~ -4= IT8-a/ITo are reproduced in Fig. 4 in the range T = 3 0 0 - 7 0 0 K for 64 W, 2.45 GHz post-discharges in N 2 (3Torr) and A r (1.5%-3%)N2 (20-75 Torr).
By taking the kNTiB)/kNT~(B) values reproduced in Fig. 3, the [N]r/[N]16 variations reproduced in Fig. 5 have been deduced. In pure N 2 and A r - x N 2 gas mixture (with x = 1.5% and 3.2%) the nitrogen atom densities increase with gas temperature up to 500 K by factors of 1.8 and 1.5 respectively.
kN'(B, 8))
~]
(9)
13
5. Correlation between nitrogen atom dissociation rate and thickness of }"-Fe4N nitrided layers
The ratio [N]~/[N]yI, = f ( T ) increases according to curve (c) in Fig. 3 but decreases according to curves (a) and (b) in the range 5 0 0 - 7 0 0 K for the two A r - x N 2 gas mixtures. In any case, [Nit= 700K is equal to or greater than [N]r=300K. Thus it can be concluded that the temperature increase in the heating device of the post-discharge reactor up to 700 K is not detrimental to the nitrogen atom density. Rather, a slow increase by a factor of 1.5-2 is observed.
()'N'T
(from fig.3ab)
By performing nitriding treatments of 1 h on Fe-0.1%C steel specimens located at Z = 100 cm (see Fig. 1) and heated up to 840 K, 7'-FerN layers have been obtained as shown on the micrograph of Fig. 6(a) for an N~ flowing post-discharge at 2.5 Torr, 70 W and a flow rate Q = 0.3 1 min- J. The y' layer thickness can be determined by direct measurement on the micrograph of Fig. 6(a) or from the (100),, and (200)V intensity ratio of diffraction patterns of Fig. 6(b) [5]. A y' layer thickness of 8 - 1 0 / ~ m has been determined with a thin e-Fe 2 3N layer on top of the y' layer and a diffusion zone of nitrogen in solution in a-Fe. After a metallographic etching, the a layer is revealed on cross-sectional micrographs by the presence of numerous 7' precipitates formed during the slow cooling of the Fe-0.1%C sample after treatment. The y' layer thickness is reproduced in Fig. 7 vs. the nitrogen atom density as measured by N O titration at the end of the discharge tube (Z = 70 cm, Fig. 1 ). The experiments have been performed in A r - l . 4 % N 2 gas mixtures with a microwave
(from fig.3c)
/
/
1.5
1
~A
Ar-3.2%N2
~,
Ar-1,5%N
(~
20Torr 2 75 Torr
N 2 3 Tort
T(K) I 300
400
i
I
500
600
Fig. 5. Normalized nitrogen atom density deduced from Fig. 3.
I
]l=
700
vs.
T
(K) as
(b)
Tvw o o ÷
"2
A ,t-
la
sb ~ 0
28 ,
(BPagg's
2~
24
2~
angle)
Fig. 6. (a) Cross-sectional micrograph of compound and diffusion layers in Fe 0.1%C. The steel specimen was nitrided in flowing N 2 post-discharge plasma (0.3 1 min J) for 1 h at 840 K, 2.5 Tort, 70 W and Z = 30 cm. (b) Diffracted X-ray intensity vs. Bragg angle 0 at the outer surface of the nitrided specimen (Co Ka).
14
d~,, ( t i m )
/ []
J 10 '4
F~
I
,
10 ~s
I
J 1 0 is
,
(
N] z= 70cm , I
c m -3 }
Fig. 7. y'-Fe4Nlayer thickness (dr,) vs. nitrogen sities in the post-discharge at distance Ar-l.4%N2 gas mixture, 2.45 GHz discharge at pressure from 40 Torr (A) to 500 Torr (D). The for e-Fe2N layers (thickness about 1/~m).
atom denZ = 7 0 cm. 70 W. Gas circles are
power of 70 W and a total gas pressure between 40 and 500 Torr. A good correlation is found between the ~,' layer thickness and the nitrogen atom density in the post-discharge reactor. The e layer appears for [N]z = 7ocm> (2-3) x 101 s cm- 3. H a gas was introduced in the first step of treatment (2-3 min) to prevent surface oxidation and was then cut off to obtain high nitrogen atom densities [5].
6. Concluding remarks High nitrogen atom densities have been produced in N2 at low gas pressure (3 Torr) and in Ar-N2 gas mixtures at high pressures (20-760 Torr) when the gas flows are ionized by means of 2.45 GHz surfaguide and surfatron exciters working with 1 0 0 - 2 0 0 W transmitted power. Two diagnostics of nitrogen atom density have
been performed in the post-discharges: the NO titration method and absolute intensity measurements of the first positive bands. The NO titration gives quick and accurate results (+ 20%) at medium gas pressures (3-200 Torr) but without spatial resolution. The measurement of absolute intensities is a more sophisticated method whose strength is to give local nitrogen atom densities at pressures as high as atmospheric pressure. Effects of gas temperature on nitrogen atoms have been analysed. From detailed kinetics studies the variation of the nitrogen atom recombination rate (kNr(B) to produce N2(B) radiative states) has been determined in the range T= 300-600 K. Nitrogen atom densities have been found to increase with T up to 500 K, with values multiplied by a factor of between 1 and 2 up to T= 700 K. A good correlation has been observed between the 7' layer thickness and the nitrogen atom density in the post-discharge reactor.
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