As shown by AES and RBS methods, the saturation of a processed material by an alloying element (nitrogen) occurs in ε-area, the analysis of the final surface evidencing that the impurity concentration at the crystal-melt interface is determined mainly by the solubility of both nitrogen and carbon in liquid iron and remains constant for various combinations of processing modes and target materials

Astashynski, Valiantsin

Cherenda, Nikolai

English

Электронная библиотека БГУ

 
 
 
 
V International Conference 
 
 
 
 PLASMA PHYSICS 
AND 
PLASMA 
TECHNOLOGY 
 
PPPT-5 
 
Minsk, Belarus, September 18-22, 2006 
 
 
CONTRIBUTED 
PAPERS 
 
Volume II (Sections 4-6) 
 
 
Institute of Molecular and Atomic Physics 
National Academy of Sciences of Belarus 
 
THE IMPURITY DISTRIBUTION IN IRON AND STEEL 
UNDER THE ACTION OF COMPRESSION PLASMA FLOW 
 
V.V. Astashynski, N.N. Cherenda 
 
Belarusian State University, 
4 Nezalezhnastsi Ave., 220080 Minsk, Belarus, vincuk@mail.by 
 
Introduction 
 
The treatment of different materials with high-energy nitrogen-based com-
pression plasma flows opens up new opportunities for modification of their sur-
face characteristics aimed at obtaining layers unique in the structure, phase 
composition, and mechanical properties. Such a treatment is performed using 
compression plasma flows generated by quasi-stationary plasma accelerators 
such as a magnetoplasma compressor (MPC). 
Previous investigation has shown that exposure of iron samples to com-
pression plasma flows resulted in the formation of deep (up to 60 micrometers) 
modified layers /1-2/. However, precise identification of impurity concentration 
has not been performed. 
 
Experimental 
 
Iron (Armco-Fe) and carbon steel (0.8 wt. % C) samples were processed by 
varied-power compression plasma flows in the Institute of molecular and atomic 
physics, NAS of Belarus, using gas-discharge MPC of compact geometry /3/. 
The MPC employing hydrogen and nitrogen as plasma-forming gases, operates 
in a mode such that a pre-evacuated vacuum chamber is filled with working gas 
up to a predetermined pressure falling within the range of 400-7000 Pa. Accord-
ing to calorimetric measurements, the value of power density absorbed by a 
sample surface, depending on the distance from the tip of MPC discharge de-
vice, is 5-25 J/cm2 per pulse. 
The study of elemental composition by Auger spectroscopy (AES) has 
been carried out using Perkin Elmer PHI-660 Auger spectrometer. The setup 
was equipped with an ion gun both for surface cleaning and for analysing of im-
purity concentration profiles against depth. Energy of argon ions used for clean-
ing was 3.5 keV, ion current density — 50 µA/cm2, incidence angle of ion 
beam — 60 to 70º. Secondary-Ion Mass Spectrometry investigation has been 
carried out with Atomica Instruments Ion Microprobe 4100 using a beam of 
caesium ions. During the implantation, the beam current was kept below 1.5 nA. 
The nitrogen profiles have been obtained by Rutherford Backscattering Spec-
trometry (RBS) with a 6 MeV He++ beam at a backscattering angle of 170º. The 
data analysis was performed with the RUMP code /4/. 
 
607
Results and discussion 
 
When exposing the iron sample to compression plasma flows, a material 
being processed is saturated by an alloying element (nitrogen) in ε-area that is 
determined by values of plasma temperature close to the sample surface 
(5·104 K) and compression flow pressure (106 Pa) exerted on a molten layer. 
According to AES and RBS data, the iron sample near-surface layers proc-
essed in a standard mode (discharge duration — 120 µc; plasma velocity in a 
compression flow — 6·106 cm/s; concentration of the charged particles in a zone 
of the peak compression — 7·1017 cm-3; plasma temperature — 1-3 eV; energy 
density absorbed by the sample ~ 15 J/cm2), contain up to 20 at.% of nitrogen 
that corresponds to eutectoid composition. With a change in energy of action the 
nitrogen surface density varies within the limits of ±5 at.% (Fig. 1). More sig-
nificant nitrogen implantation is observed, when nitrogen initial pressure in 
MPC standard operating mode is increased up to 7000 Pa. In this case, accord-
ing to RBS data, nitrogen surface density attains 30 at.%. 
Apart from the changes in elemental composition caused by plasma-
forming substance implantation into a material, near-surface layers undergo a 
redistribution of impurities present in the sample before plasma processing. As 
was shown by RBS-studies of elemental composition of the samples processed 
by a plasma flow with hydrogen as a plasma-forming substance, redistributed in 
superficial area 10 µm thick is carbon contained in the initial sample, the carbon 
concentration decreasing by 3 and 1.5 times in layers up to 0.1 µm and 5 µm 
thick, respectively. The concentration decrease observed can be explained by 
dislodging of carbon by high-energy ions of hydrogen present in the compres-
sion flow. Such an assumption is confirmed by results of studies of a sample 
surface using Secondary-Ion Mass Spectrometry, which show a presence of hy-
drogen in a near-surface area. 
Comparison of AES- and RBS-data makes possible a suggestion, that the 
total penetration depth of nitrogen is more than 7 µm (Fig. 1). 
Upon subjecting a surface of carbon steel (0.8 wt. % C) samples to a com-
pression flow whose plasma-forming substance is nitrogen, their near-surface 
layers contain up to 20 at. % of nitrogen, like those in iron samples. Increasing 
the energy density in a standard mode obtained by the sample per pulse up to 
25 J/cm2 results in a decrease in surface density of nitrogen by 5 at. %, however 
the nitrogen total content in a near-surface zone increases essentially (Fig. 2). In 
particular, at a depth of 5 µm, nitrogen density was ~ 2 at. % at energy density of 
15 J/cm2 and ~ 4 at.% at 25 J/cm2. 
Such regularity can be explained by the replacement of carbon with nitro-
gen in solid solutions of the first zone of the sample modified layer. As the size 
of nitrogen atom is less than that of carbon, and concentration of nitrogen at a 
surface (as well as the initial content of carbon in steel) corresponds to eutectoid 
608
value in the course of crystallization from the melt, the diffusion of nitrogen will 
proceed more intensely as compared to carbon. 
 
  
Fig. 1. Concentration profile of nitrogen in the 
sample of iron processed by compres-
sion plasma flow. 
Fig. 2. Concentration profile of nitrogen in 
the sample of steel (0.8 wt.% C) 
processed by compression plasma 
flow. 
 
The process of nitrogen propagation deep into the sample will be analyzed 
as follows. Let us write down the equation of impurity mass transfer in the melt 
under condition that the diffusion coefficient D does not depend on the impurity 
concentration С (Fick’s second law): 
  
!C
!t
= D"
2
C .            (1) 
The solution of this equation for an impurity constant source is the expres-
sion: 
  
C(x, t) = C0[1!
2
"
exp(!z 2 )dz
0
x / 2 Dt
# ]= C0 efrc
x
2 Dt
,      (2) 
which is valid when the boundary conditions: 
C(0, t) = 0; C(x, 0) = 0 at x > 0; C (x, t) = C with at x > 0 and t > 0   (3) 
are satisfied. 
The diffusion coefficient of nitrogen in iron in Eq. (2) depending on the 
temperature T is defined by the expression /5/: 
  
D
N
= 6.6 !10
"7
!exp("18600 /RT )  m2/s,        (4) 
where R is the molar gas constant. 
609
The analysis of process of the compression flow action on a target shows 
that the boundary conditions (3) are satisfied throughout the life time of surface 
plasma which is an impurity source, i.e. at 0 < t < 100 µs. Thus, the nitrogen dif-
fusion depth profile in the sample over the processing duration calculated by 
means of Eq. (2) (see Fig. 2, “Calculated” curve), is obtained in the following 
approximation: the temperature of a surface layer 20 µm thick is constant and 
amounts to 103 K /6/, and the corresponding diffusion coefficient D of nitrogen 
equals 7·10-8 m2/s.  
Reasonable consistency of experimental results and calculation data makes 
it possible to conclude that nitrogen distribution in a surface layer of the proc-
essed samples is governed mainly by the diffusion process. 
Thus, as shown by AES and RBS methods, the saturation of a processed 
material by an alloying element (nitrogen) occurs in ε-area, the analysis of the 
final surface evidencing that the impurity concentration at the crystal-melt inter-
face is determined mainly by the solubility of both nitrogen and carbon in liquid 
iron and remains constant for various combinations of processing modes and 
target materials. The subsequent distribution of nitrogen in a surface layer of 
processed samples is governed mainly by the diffusion process. 
 
Acknowledgements 
 
We are indebted to our colleagues from the Laboratory of plasma accelera-
tors physics, the Institute of molecular and atomic physics of NAS, Belarus, for 
processing the samples by compression plasma flows. 
This work was supported by BRFFR (Projects T06M-193 and F06-112). 
 
References 
 
1. Uglov V.V., Anishchik V.M., Astashynski V.V., et al. Surf. Coat. Tech-
nol., 180-181(2004) 633-636. 
2. Anishchik V.M., Astashynski V.V., Uglov V.V., et al. Vacuum 78(2005) 
589-592. 
3. Astashinskii V.M., Efremov V.V., Kostyukevich E.A., et al. J. Plasma 
Phys. (Engl. Transl.) 17(1991) 545-549. 
4. Doolittle L. R. Nucl. Instrum. and Methods B 15(1986) 227-231. 
5. Fast J.D., Verrijp M.B. J. Iron and Steel Inst. 176(1954) 24-27. 
6. Astashynski V.V. Proceedings of the 6-th Int. Conf. “Interaction of Radia-
tion with Solids” (2005) 167-169. 
610


The impurity distribution in iron and steel under the action of compression plasma flow

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The impurity distribution in iron and steel under the action of compression plasma flow

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