Mass-transport driven by surface instabilities under high-flux, low-energy nitrogen ion irradiation at elevated temperatures

Mass-transport driven by surface instabilities under high-flux, low-energy nitrogen ion irradiation at elevated temperatures

ARTICLE IN PRESS Vacuum 72 (2004) 161–168 Mass-transport driven by surface instabilities under high-flux, low-energy nitrogen ion irradiation at elev...

310KB Sizes 2 Downloads 14 Views

ARTICLE IN PRESS

Vacuum 72 (2004) 161–168

Mass-transport driven by surface instabilities under high-flux, low-energy nitrogen ion irradiation at elevated temperatures L. Praneviciusa,*, L.L. Praneviciusa, D. Milciusb, S. Muzardc, C. Templierc, J.-P. Rivierec a

Vytautas Magnus University, 8 Vileikos St., LT-3035 Kaunas, Lithuania Lithuanian Energy Institute, 3 Breslaujos St., LT-3035 Kaunas, Lithuania c Laboratoire de M!etallurgie Physique, Universit!e de Poitiers, Bd. Marie et Pierre CURIE, B.P. 30179, 86962 Futuroscope Chasseneuil Cedex, France b

Abstract Flux effects in ion nitrided AISI 304 stainless steel have been investigated in an attempt to understand the mechanism of nitrogen transport. It is concluded that an interaction between a highly activated surface layer, the internal interfaces and the bulk is critical. Under conditions of non-equilibrium present on the surface, the nitrogen atoms are driven into the grain boundaries and highly compressive stress is formed. The stress relaxation processes initiate plastic flow of atoms in the grains and a corresponding flow of nitrogen atoms. r 2003 Elsevier Ltd. All rights reserved. Keywords: Ion nitriding; Surface instabilities; Compressive stress; Stress relaxation; Nitrogen transport

1. Introduction During the last 10 years, significant progress has been made in the development of high-flux, lowenergy ion beam and plasma technologies for a variety of applications. For example, it is shown [1,2] that improvements in tribological behavior and corrosion resistance of stainless steel may be obtained by combining a high current density with elevated temperatures and nitrogen ion energy of 0.5–1.0 keV.

*Corresponding author. Tel.: +370-7-203858; fax: +370-7203775. E-mail address: liudvikas [email protected] (L. Pranevicius).

The use of a dense plasma is potentially economically more desirable, but results in significant effects that are not negligible under highflux, low-energy ion irradiation. Dense plasmas are usually produced by the application of a sufficiently high level of energy, e.g. in the form of arcs, sparks or glow discharges. Plasma includes individual types of particles, e.g. electrons, ions and neutral particles in the form of non-excited and excited atoms, molecules and radicals. Plasma treatment technologies have limited control over the energy of ions, ion current density and target temperature. The modification of solids affected by dense plasma treatment takes place through interaction with incident particles when the parameters of incident particles are not strictly determined. A number of physical processes, such

0042-207X/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0042-207X(03)00126-X

ARTICLE IN PRESS

as adsorption and desorption, preferential sputtering, implantation, mixing, or radiation-enhanced diffusion and segregation, contribute to the formation of an altered layer in the near-surface region. The kinetics of ion nitriding has been the focus of much research. Experimentally it is established that under high-flux, low-energy nitrogen ion irradiation, a highly defected metastable f.c.c. phase (gN ) containing a high concentration of nitrogen in solid solution is formed. However, of great interest is an CrN phase, which exists in thin (about 300 nm) near-surface region of the nitrided layer depending on the process parameters [3,4]. The kinetics of precipitation of this phase and the role of this layer in the nitriding mechanism are still not well known. The nitrogen saturated f.c.c. structure, sometimes described as expanded austenite, can be obtained by different processes [5]. In this paper, a nitrogen plasma torch at atmospheric pressure is used to allow high-flux, low-energy nitrogen ion irradiation at elevated temperatures and the formation of a highly nitrogen enriched surface layer with nitrogen concentration higher than in the expanded austenite phase.

Concentration of nitrogen, %at

L. Pranevicius et al. / Vacuum 72 (2004) 161–168

30

20

10 1 2 3

0

0

500

1500 1000 Depth, nm

o

1 - 500 C, 10 min o 2 - 500 C, 25 min

20

1

2

10

100

2. Experimental technique The samples were prepared from an austenitic Fe70Cr19Ni11 (in at%) stainless steel (AISI 304) with an average grain size 25 mm in the form of 2 mm-thick circular discs. Prior to plasma torch nitriding, the surfaces were polished to get a mean roughness less than 10 nm. The samples were cleaned in alcohol and acetone before loading into the sample holder. The installation included an in situ temperature-monitoring device. Nitrogen ion irradiation was made using a broad beam Kaufman ion source with acceleration voltage 1.2 kV and the ion current density 0.5– 1 mA cm2 [6]. The flux of incident nitrogen ions was equal to about 1016 cm2 s1. The fluxes of incident nitrogen atoms and molecules up to 1020 cm2 s1 have been obtained employing plasma torch technique operating at atmospheric pressure in the ambient of nitrogen [7].

2500

2000

Fig. 1. NRA depth profiles of nitrogen in AISI 304 after ion beam nitriding at 400 C for fluence 4  1019 cm2 and current densities 0.5, 0.75 and 1.0 mA cm2 (curves 1–3, respectively).

Concentration, %at

162

200 Depth, µm

300

Fig. 2. GDOES depth profiles of nitrogen in AISI 304 after plasma torch nitriding at 500 C for 10 min (curve 1) and 25 min (curve 2).

The nitrogen depth profiles were measured using nuclear reaction analysis (NRA) up to 2.5 mm and glow discharge optical emission spectroscopy (GDOES) for thicker nitrided layers. The formation of the nitrogen solid solution phase (expanded austenite gN ) was registered by quantitative X-ray diffraction (XRD) technique. Fig. 1 shows the experimental NRA depth profiles of nitrogen in AISI 304 stainless steel after nitriding using 1.2 keV Nþ 2 irradiation with current density 0.5, 0.75, and 1.0 mA cm2 (curves 1–3, respectively) for fixed irradiation fluence equal to 4  1019 cm2 and 400 C. Fig. 2 shows the GDOES depth profiles in the plasma torch nitrided samples during 10 and 25 min (curves 1

ARTICLE IN PRESS L. Pranevicius et al. / Vacuum 72 (2004) 161–168

163

19 2  Fig. 3. Cross-sectional SEM micrographs of nitrided layers in AISI 304 after (a) ion beam nitriding (1.2 keV Nþ 2 ; 400 C, 4  10 cm 2  and 1 mA cm ) and (b) plasma torch nitriding (500 C for 10 min).

Fig. 4. SEM surface images of stainless steel samples after ion nitriding with parameters indicated in Fig. 3, respectively.

and 2, respectively) for 500 C. Fig. 3 shows typical SEM cross-sectional views of ion beam (Fig. 3a) and plasma torch (Fig. 3b) nitrided samples. The corresponding SEM surface views are presented in Fig. 4. The XRD analysis showed the presence of an expanded austenite phase, consisting of an f.c.c. solid solution, gN ; in samples affected by ion beam nitriding. A supersaturated austenite phase, consisting of a f.c.c. solid solution, gSN ; which has a higher nitrogen concentration than the expanded austenite phase, has been observed in plasma torch nitrided samples. In addition, the expanded austenite peaks are large, this being due to the presence of dislocations stacking faults and other defects, high internal stresses and the concentration gradient between layers [8].

The experimental results show that nitriding efficiency of stainless steel is highly flux dependent. It sharply increases with the increase in the flux of incident ions equal to 1016 cm2 s1 for ion beam nitriding and 1020 cm2 s1 for plasma torch nitriding.

3. Formation of nitrogen saturated layer Compositional modification of solid surfaces through the interaction with low-energy ions has been widely studied [9,10]. The problem becomes more complicated under high-flux, high-dose plasma irradiation when parameters of incident particles are not strictly determined. To avoid these difficulties the ‘‘black box’’ approach is used.

ARTICLE IN PRESS 164

L. Pranevicius et al. / Vacuum 72 (2004) 161–168

The target is divided into three layers: (1) the top emission layer, (2) the beneath altered layer and (3) the homogeneous bulk material. The thickness of the emission layer is e and the thickness of the altered layer is d. Let us denote the distribution profile of i atoms as NiðAÞ ðxÞ which is usually not a known function when irradiation parameters are not well determined. Index i indicates the type of atoms and index A indicates that they belong to the altered layer. The number # ðAÞ of R x2i atoms in the altered layer is equal to Ni ¼ x1 Ni ðxÞ dx; where x2  x1 ¼ d is the thickness of the altered layer and x1 and x2 are the corresponding boundaries. The mean concentration of i atoms in the altered layer /NiðAÞ S ¼ N# ðAÞ i =d: The number of i atoms in the emission layer is equal to N# ðEÞ ¼ eNiðEÞ ; where NiðEÞ is the concentration of i i atoms in the emission layer. The thickness of the emission layer e is equal to the depth from which particles are emitted. Usually, the thickness of the emission layer is equal to 1–2 monolayers [11]. If the flux of incident ions is equal to I and N# ðEÞ is the number of atoms in the emission layer, the frequency probability of sputtering of i atoms is equal to wi;sp ¼ ðYi IÞ=N# ðEÞ ; where Yi is the sputtering yield of i atoms. The frequency of jumps of thermally activated atoms can be evaluated using the Arrhenius equation as to wi;th ¼ wi0 expðEi =kTÞ; where wi0 E1012  1013 s1 ; and Ei is the energy required to form surface vacancy for i atom. The total emission rate of i atoms is the sum of ballistically and thermally relocated atoms:   Yi I Ei wi ¼ þ wi0 exp  : ð1Þ kT N# ðEÞ The balance equation for i atoms of the altered layer can be written as dN# ðAÞ dx2 i ¼ wi N# ðEÞ þ bi Ii ; þ Ni i dt dt

ð2Þ

where the first term on the right-hand side of Eq. (2) defines the removal rate of i atoms from the altered layer by sputtering and thermal evaporation; the second term defines the increase rate of i atoms in the altered layer as the result of arrival of i atoms from the bulk as the back boundary of the altered layer x2 is continuously moving, where Ni

is the concentration of i atoms in the bulk; and the third term defines the accommodation rate of incident (nitrogen) ions in the altered layer, where bi is the accommodation probability and Ii is the flux of incident i atoms. The recession rate of surface is defined by the flux of atoms leaving the emission layer and is equal to dx1 1 X ¼ ðEÞ wj N# ðEÞ ð3Þ j ; N dt j where N ðEÞ is the concentration of atoms in the emission layer. Assuming that boundaries x1 and x2 are moving with the same velocity (dx1/dt=dx2/dt) and after introduction notations nðEÞ ¼ NjðEÞ =N ðEÞ ¼ cj and j ðAÞ ðAÞ # /ni S ¼ N =ðN  dÞ Eq. (2) may be rewritten as d ðAÞ ð/nðAÞ Þ ¼  w0i N ðEÞ nðEÞ i SN i dt X þ Nni w0j nðEÞ þ b0i Ii ; ð4Þ j j

  ¼ bi =d and ¼ wi e=d : After the where summation (index i) the equation is obtained which defines the relationship between the concentrations of atoms in the altered layer N ðAÞ ; the emission layer N ðEÞ and in the bulk N as X X 0 dN ðAÞ  ¼ N  N ðEÞ wi ci þ b0i Ii : ð5Þ dt i i b0i

w0i

The steady-state concentration of atoms in the emission layer is equal to ! X X ðEÞ 0 0 bi Ii = wi ci : Nst:st: ¼ N þ ð6Þ i

i

The surface composition in steady state is obtained using Eqs. (6) and (5). They give the system of equations " !# X X Nþ b0j Ij = w0j cj w0i ci j

 Nni

X

j

w0j cj

¼ b0i Ii :

ð7Þ

j

For one type (i=m) of incident species b0m ¼ b0 and b0i ¼ 0 if iam.P 0 Let us denote j wj cj ¼ A: The steady-state surface concentration of matrix atoms (iam) is

ARTICLE IN PRESS L. Pranevicius et al. / Vacuum 72 (2004) 161–168

expressed as ci;st:st: ¼

A2 ni  ; A þ a w0i

ð8Þ

where a ¼ b0 Im =N: The ratio of surface concentrations for matrix elements is equal to ci;st:st: ni ¼  cj;st:st: nj

w0i ; w0j

ð9Þ

where iam and jam. It follows that the accommodated incident atoms do not change the ratio of surface concentrations of the matrix atoms at steady state. The accommodation of incident atoms changes the absolute values concentrations. The surface concentration of i atoms (iam) is equal to ni =w0i ci ¼ ð1  cm Þ P 0: j nj =wj

ð10Þ

165

which can be rewritten as h i d/nðAÞ ðAÞ ðEÞ i S ¼ Nðni  /nðAÞ S þ N /n SÞ i i dt X 0 ðEÞ 0  w j cj  N w i ci j

1 þ di  d/nðAÞ ð13Þ i S ðAÞ ; N P where di ¼ bi Ii and d ¼ di : If the concentrations of atoms in the emission and altered layers are equal N ðEÞ ¼ N ðAÞ ; and P using notations Z ¼ N ðAÞ =N; oi ¼ bi Ii =N and o ¼ oi ; Eq. (13) and Eq. (5) give the system of equations for calculation of the kinetics of surface composition and kinetics of accommodation of incident particles: P dci ½ni þ ci ðZ  1Þ w0j cj  Zw0i ci þ oi  oci ¼ ; dt Z ð14Þ

The following equation is obtained for the parameter A:

X dZ ¼ ð1  ZÞ w0j cj þ o: dt

jA2 þ ða  w0m ÞA  aw0m ¼ 0; ð11Þ P where j ¼ w0m ðni =w0i Þ is the dimensionless parameter which is proportional to the removal rate of accommodated atoms. Eqs. (11) and (6) give that the relative increase in concentration of atoms (densification) for the emission layer is equal to

Fig. 5 illustrates the kinetics of accommodation of incident particles in the altered layer calculated according to Eq. (15) for different values of the parameter o=0.01; 0.02; 0.05 and 0.1 (curves 1–4, respectively). It is seen that the concentration of accommodated atoms in the altered layer approaches steady-state concentration and it depends on the flux of incident particles.

ðEÞ Nst:st: N 2jx ¼ ; ð12Þ N 1  x þ ½ð1  xÞ2 þ 4jx 1=2

where x ¼ a=w0m ¼ ðb0 Im Þ=ðw0m NÞ is the dimensionless parameter proportional to the flux of the incident particles. For the low-flux treatment yBjx: In the case of high fluxes Im-N the ratio y-N and does not depend on the parameter j dependent on the removal rate of incident particles. It follows that the steady-state concentration of atoms in the emission layer depends in a complex way on the flux of incident particles and their removal rate. Kinetics of accommodation of incident atoms in the altered layer can be calculated using Eq. (4)

0.4 4

Partial concentration



ð15Þ

0.3

0.2

3

0.1

2 1

0

0

5

10

15

Time in rel.u. Fig. 5. Kinetics of accommodation of nitrogen atoms in the near-surface layer calculated for different values of the parameter w.

ARTICLE IN PRESS 166

L. Pranevicius et al. / Vacuum 72 (2004) 161–168

Fig. 6. Schematic illustration of the flow of atoms into grain boundaries in response to irradiation induced increase in the surface chemical potential (a) and SEM cross-sectional view of the AISI 304 after plasma torch nitriding at 450 C for 10 min (b).

The theoretically predicted results are in qualitative agreement with the experimental ones: nitriding is a flux dependent process and high-flux nitrogen ion irradiation leads to the formation of supersaturated expanded austenite with the nitrogen content of up to 20 at% and the expansion of the interplanar spacing that can reach 12% [12].

4. Discussions In previous publications [6], the role of the processes on the surface and in the near-surface layer were distinguished in the mechanism of ion nitriding of stainless steel. The surface defect production and annihilation processes under irradiation lead to a quasidynamic equilibrium between production, conversion and disappearance mechanisms that leads in turn to the formation of highly activated altered layer on the surface. Chemisorption of nitrogen is modified by changes in surface morphology and the formation of chemical bonds is perturbed by surface defect structures. The dynamic state of the surface supported by high-flux nitrogen irradiation is thus perceived to be able to create, at least dynamically, an imperfect near-surface region highly enriched with nitrogen. The concentration of nitrogen in this layer is flux dependent and saturates. Hence densification of the layer takes place. The nitrogen

saturated surface layer is both chemically and physically distinct from the underlying bulk material. A difference in chemical potentials between activated surface layer, bulk and grain boundaries is established. The excess chemical potential of the surface relative to the grain boundaries produces a net flux of nitrogen into the grain boundaries that generates compressive stress in the grains [13]. If the stress exceeds the limit of plasticity, stress relaxation occurs through the production of dislocations. If dislocation movement is prevented, by pinning at grain boundaries, then the formation of subgrains within the original grain structure takes place. In this way the uptake of nitrogen atoms through the grain boundaries in open contact with the highly activated nitrogen saturated surface occurs. Thus, the high surface chemical potential is not only imposed on the external surface but also on the internal grain interfaces. Fig. 6a schematically illustrates the flow of nitrogen atoms into grain boundaries in response to flux induced changes in the surface chemical potential. An estimate of internal stresses in the nitrided layer shows that accommodated biaxial stresses are of the order of 3 GPa [14,15]. The yield stress for stainless steel under biaxial compression is on the order of 200 MPa. Thus, compressive stress provides strong driving force for the plasticity in the grains. The plastic deformation of grains

ARTICLE IN PRESS L. Pranevicius et al. / Vacuum 72 (2004) 161–168

results in their highly defected structure and fragmentation and as a consequence, the lateral inward diffusion of nitrogen through the walls of grain boundaries takes place. Lateral diffusion paths along the boundaries of subgrains and activated by plastic flow of material have been observed experimentally (Fig. 6b). The experimentally observed non-homogeneous nitrogen distribution profiles (Fig. 2) are the result of non-homogeneous stress and defect distribution profiles across the nitrided layer. The increase in the incident flux changes the microstructure of the nitrided layer (Fig. 3) and surface morphology (Fig. 4). With the increase in the flux of incident nitrogen ions the efficiency of nitriding sharply increases. The thickness of the nitrided layer reaches up to 250 mm by employing plasma torch nitriding. The surface microstructure that initially consists of grains with a mean size equal to 25 mm, becomes granular (Fig. 4b). The XRD patterns after plasma torch nitriding [16] show that the near-surface layer of stainless steel has a nanocrystalline structure ranging in the size from a few nanometers to 200 nm after plasma torch nitriding. The nanocrystalline surface with highly developed system of grain boundaries is transparent for nitrogen atoms driven by the excess of surface chemical potential. Thus, the role of surface is two-fold: (1) it is the source of nitrogen atoms which are trapped in the oversaturated thermodynamically nonequilibrium layer, and (2) when the chemical potential of the surface under irradiation exceeds the chemical potential of the grain boundaries, adatoms are driven into the grain boundaries generating compressive stresses in grains. The relaxation processes include plastic flow and fragmentation. The final size of grains depends on the irradiation parameters and the nitrogen supply rate.

5. Conclusion A tentative explanation of the plasma nitriding of stainless steel under high-flux, low-energy ion irradiation at elevated temperatures is presented. It becomes clear that the interaction between the

167

bulk and its free surfaces and internal interfaces is critical. The high-flux external irradiation increases the surface chemical potential, and dynamic behavior of adatoms forms a highly nitrogen saturated altered layer. Under highly non-equilibrium conditions, the difference in chemical potentials between an activated altered layer and the grain boundaries is established and this acts as the driving force for the movement of atoms, including nitrogen, from the surface into the grain boundaries with the creation of compressive stresses. The flow of nitrogen atoms will continue until the resultant compressive stress forces the chemical potential of the grain boundary to equal the chemical potential of the activated surface layer.

Acknowledgements The authors would like to thank the Forschungszentrum Rossendorf (Germany) for the NRA measurements and Dr. P. ValatkeviWius and Dr. V. ValinWius (Lithuanian Energy institute) for the plasma torch experiments.

References [1] Michel A, Czerwiec T, Gantois M, Ablitzer D, Ricard A. Surf Coat Technol 1995;75:103–14. [2] Williamson DL, Ozturk O, Wei R, Wilbur PJ. Surf Coat Technol 1994;65:15–23. [3] Bourjot A, Foos M, Frantz C. Plasma Surf Eng 1989;2:777–84. [4] Randal NX, Renevier N, Michel H, Colligon P. Vacuum 1997;48:849–55. [5] Renevier N, Michel H, Colligon P, Czerwiec T. Surf Coat Technol 1997;95:1–19. [6] Pranevicius L, Templier C, Riviere J-P, Meheust P, Pranevicius LL, Abrasonis G. Surf Coat Technol 2001;135:250–7. [7] Pranevicius L, Pranevicius LL, Valatkevicius P, Valincius V. Surf Coat Technol 2000;123:122–9. [8] Saker A, Leroy C, Michel H, Frantz C. Mater Sci Eng A 1991;140:702–11. [9] Sigmund P. Nucl Instrum Methods Phys Res B 1987;18:375–81. [10] Galdikas A, Pranevicius L. Interactions of ions with condensed matter. New York: Nova Science Publishers, Inc, 2000. p. 176.

ARTICLE IN PRESS 168

L. Pranevicius et al. / Vacuum 72 (2004) 161–168

[11] Pranevicius L, Templier C, Delafond J, Muzard S. Surf Coat Technol 1995;72:51–62. [12] M.andl S, Rauschenbach B. Deff Diffus Forum 2001;188– 190:125–31. [13] Floro JA, Chason E, Camarata RC, Srolowitz DJ. MRS Bulletin 2002;27(1):19–27.

[14] Grigull S, Parascandola S. J Appl Phys 2000;88: 6925–30. [15] Sienz S, M.andal S, Rauschenbach B. Surf Coat Technol 2002;156:185–9. [16] Pranevicius L, Milcius D, Templier C, Riviere J-P, Pranevicius LL. Surf Eng 2002;18(3):182–7.