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Surface & Coatings Technolog

Microstructural and topographical studies of DC-pulsed

plasma nitrided AISI 4140 low-alloy steel

P. Corengiaa,T, G. Ybarraa, C. Moinaa, A. Cabob, E. Broitmanc

aInstituto Nacional de Tecnologıa Industrial, C.C. 157, (B1650WAB) San Martın, ArgentinabIonar S.A., Buenos Aires, Argentina

cDepartment of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA

Received 11 May 2004; accepted in revised form 10 January 2005

Available online 20 April 2005

Abstract

The influence of DC-pulsed plasma nitriding time on the surface properties of AISI 4140 low-alloy steel has been investigated. The

samples were nitrided in an industrial equipment using a gas mixture consisting of 25% N2+75% H2 and the DC-pulsed glow discharge time

was varied between 1 and 28 h. Optical and scanning electron microscopy as well as X-ray diffraction, electron probe microanalysis and

microhardness measurements have been used to study the ion-nitrided surfaces.

It was found that the compound layer thickness does not follow a parabolic law with treatment time. An equation has been derived

according to the experimental data that can predict the thickness under the nitriding conditions studied in the present investigation.

Furthermore, when the nitriding time is increased, the compound layer passes from a dual phase (e-Fe2–3N+gV-Fe4N) to a monophase

gV-Fe4N.The topographical evolution and roughness, studied by atomic force microscopy (AFM), have shown that all the roughness parameters

increase with the ion nitriding time. The rate of increase is higher during the first hours of nitriding, which could be related to the relative

intensity of sputtering and redeposition.

The microformations developed during nitriding show a conical aspect with different morphologies. Their growth direction changes from

a perpendicular orientation in the center of the sample to an inclination close to the sample edge, as a consequence of the change of the

direction of the electric field lines close to the edges.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Pulsed plasma nitriding; Low alloy steel; Topography

1. Introduction

Plasma nitriding is a plasma-activated thermochemical

method widely used to increase the fatigue strength,

hardness and wear resistance of low-alloy steels, tool steels

and stainless steels [1–3].

During the ion nitriding process, the nitriding reaction

takes place at the surface and also in the subsurface region

of the metal. As a result, two different structures have been

identified, the so-called bwhiteQ or bcompoundQ layer and

0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2005.01.060

T Corresponding author. Tel.: +54 11 4724 6297; fax: +54 11 4724 6313.

E-mail address: corengia@inti.gov.ar (P. Corengia).

the bdiffusion zoneQ. The first one is the outermost layer,

and consists of one or two iron nitrides (Fe4N–Fe2–3N),

depending on the process parameters. The second layer

results from reactions produced by the N diffusion, such as

precipitation of nitrides, a-Fe saturation, changes in the

residual tensions and C redistributions [3].

There have been many investigations aiming the pre-

diction of the resulting structures and, in consequence, their

service behavior. However, the results are contradictory and

the phenomena involved in the structural evolution of the

compound layer, its thickness and their surface topography

are still not well understood. Celik and Karadeniz [4] did

not observe variations of the formed compounds with the

processing time during the ion nitriding of an AISI 4140

y 200 (2005) 2391–2397

Table 1

Elemental chemical analysis of AISI 4140 steel

Element C Mn Si Cr Ni Mo P S Fe

Composition (wt.%) 0.41 0.82 0.30 0.90 0.04 0.17 0.014 0.011 balance

P. Corengia et al. / Surface & Coatings Technology 200 (2005) 2391–23972392

steel at different temperatures, being in all cases observed

only one phase. However, Kurny et al. [5] reported for an

En40B steel the transformation of phases when the treat-

ment time was increased from 0.5 to 10 h. Other researchers

[6] have observed in the white layer surface of API X65L

the opposite transformation observed by Kurny.

In the present work, we report the microstructural and

topographical studies of DC-pulsed plasma nitrided AISI

4140 low-alloy steel treated for different periods of time.

The effect of nitriding time on structural and mechanical

properties of the nitrided layer was investigated by optical

microscopy, scanning electron microscopy (SEM), X-ray

diffraction (XRD), electron probe microanalysis (EPMA)

and microhardness. The topographical evolution and

roughness, studied by atomic force microscopy (AFM),

are correlated to the mechanisms observed during the

nitriding.

2. Experimental

The chemical composition of an AISI 4140 steel samples

employed in the experiments can be found in Table 1. A bar

made of this steel was oil quenched from 1113 K, then

tempered at 823 K for 2 h, and finally cooled in air, resulting

in a material with a final hardness of 320 HV0.25N.

Specimens, machined into discs of 45 mm in diameter and

8 mm in thickness, were mechanically ground to 600 grade

emery paper; some of them were finally polished with 1 Amalumina grinding paste for topographical studies. After

cleaning with acetone in an ultrasonic bath, the substrates

were placed into a DC-pulsed plasma nitriding industrial

equipment [7]. The process parameters used in the nitriding

chamber are shown in Table 2. In order to reduce dimen-

sional distortions of the substrates, the treatment temper-

ature, measured by a thermocouple attached to the back of

one sample, was carefully controlled by the control of

plasma parameters and heating by electrical resistors placed

inside the chamber. After nitriding, the samples were slowly

cooled down to room temperature in a nitrogen atmosphere.

Table 2

Main nitriding parameters

Parameter Value

Nitriding time 1, 2, 4, 15, 20, and 28 h

Voltage between electrodes 750 V

Pulse on/off time 70–200 AsCurrent density 1.03 mA cm�2

Pressure 60 Pa

Temperature 773 K

Plasma chamber atmosphere 75% H2–25%N2

The treated samples were nickel electroplated and

sectioned for examination and hardness profile determina-

tion. The microstructure was revealed using 3% nital reagent

and examined with a Zeiss Axiotech optical microscope and a

Philips SEM 505 scanning electron microscope. The micro-

hardness was measured with a Vickers tester (Akashi, MVK-

H2) using loads of 0.25 and 1 N. The X-ray diffraction

measurements were carried out in a Philips PW 1810 X-ray

diffractometer using Co Ka radiation.

AFM measurements were carried out in the center and

edge of the samples with an UltraObjetive scanning probe

microscope (Surface Imaging Systems GmbH) in tapping

mode.

The carbon content of the samples after nitriding was

determined by electron probe microanalysis (EPMA) in

cross sectioned nitrided specimens. The employed apparatus

was a Cameca SX100 instrument equipped with five

wavelength dispersive spectrometers.

3. Results and discussion

3.1. Microhardness

Fig. 1 shows the microhardness profile of an AISI 4140

DC-pulsed plasma nitrided samples treated at 773 K during

2, 15, and 28 h. As seen in the figure, when we increase the

processing time, the interface case/core is less pronounced

and the hardening reaches a higher depth. We also observe a

decrease in the core hardness for long process time.

Fig. 2 shows the surface hardness as a function of

treatment time. The surface hardness increases to reach a

Fig. 1. Microhardness profile of DC-pulsed plasma nitrided samples for (n)

2 h, (o) 15 h, and (�) 28 h.

Fig. 2. The surface microhardness as a function of DC-pulsed plasma

nitriding time for AISI 4140 low-alloy steel.

Fig. 4. Variation of the thickness of the compound layer with ion nitriding

time.

P. Corengia et al. / Surface & Coatings Technology 200 (2005) 2391–2397 2393

maximum value at 2 h, and then lower values are obtained

for longer treatment periods. This result can be explained by

the time dependence of the nitrides precipitates dispersion

within the matrix [8]. Precipitates with a certain size will be

the most effective in obstructing the movement of dis-

locations and in producing the maximum strengthening and

hardening. At longer times, the precipitates particles are

larger in size and more prone to coarsening, leading to a

lower precipitate density and hence lower hardness. There-

fore, an optimum nitriding time exists at which the highest

hardness may be achieved.

3.2. Microstructure

3.2.1. Compound layer

Fig. 3 shows cross-sectional SEM images of an AISI

4140 DC-pulsed plasma nitrided samples done at 1, 4, 15

Fig. 3. Cross-sectional SEM micrographs of AISI 4140 steel DC-pulsed

and 28 h. We observe the presence of a continuous and

uniform layer, the compound layer, which has not been

attacked by the nital reagent. Under this layer, we observe

the microstructure of the diffusion zone, where the nitrogen

is found in solution or forming nitrides of the alloy. In Fig.

3(a) and (b) we can also observe that the compound layer

has grown along grain boundary.

The development of the compound layer is determined

by treatment temperature and time, as well as nitriding

potential [8]. We clearly observe in Fig. 3 that the

thickness of the compound layer increases with the

nitriding time. Fig. 4 shows the thickness of the compound

layer as a function of nitriding time. The thickness

ion-nitrided at 773 K for (a) 1 h, (b) 4 h, (c) 15 h and (d) 28 h.

Table 3

Diffraction intensities for the lines corresponding to the e and gV phases fordifferent processing times

Ion nitriding time [h] cV(200) e(100) U = e/cV

0 – – –

1 74 26 0.35

2 76 24 0.32

4 84 16 0.19

15 93 7 0.08

20 95 5 0.05

P. Corengia et al. / Surface & Coatings Technology 200 (2005) 2391–23972394

increases very fast in the first hours of process, but then

the rate of growth diminishes. This behavior, which has a

considerable deviation from a parabolic growth observed

in gaseous nitriding [9], can be explained in terms of the

sputtering which produces the elimination of the formed

nitrides [10], the denitriding [5] and the role of the layer as

a barrier for the formation of free Fe atoms [11].

Marciniak [12] proposed a model to explain the thickness

evolution of the compound layer. In our case, the kinetics of

Fig. 5. X-ray diffractograms of AISI 4140 steel (a) untreated and DC-pulsed

ion-nitrided at 773 K for (b) 2 h and (c) 20 h. (o) a-Fe, (x) g-Fe4N and (�)

E-Fe2 – 3N.

28 96 4 0.04

growth for the compound layer can be explained by the

equation:

Th ¼ a t1=2 þ b t þ c ð1Þ

where Th is the thickness of the compound layer in [Am], a

is a diffusion growth coefficient in [Am h�1/2], b is the

sputtering rate in [Am h�1], and c is a compound zone

thickness formed during the ramp-up time in [Am] [13].

The sputtering rate (SR) depends on the gas pressure,

accelerating voltage, discharge current density and material

to be removed. If we consider that the SR remained constant

during the process, and that in our experiments the nitriding

started when the samples reached the process temperature

(773 K) because of the presence of the auxiliary heaters in

the chamber, then c =0 (there is no compound layer at t =0).

If we fit the experimental values of Fig. 4 with Eq. (1) we

obtain a =2.278 Am h�1/2 and b =�0.168 Am h�1.

Furthermore, the SR calculated from our experiments is in

good agreement with the value calculated for similar steels

[13].

Fig. 5 shows typical X-ray diffraction spectra from

unnitrided samples (Fig. 5a), and treated for 2 h (Fig. 5b)

and 20 h (Fig. 5c). The analysis shows that two nitrides, e-Fe2–3N and gV-Fe4N, are present at short treatment time,

while the e phase can not be detected for processing periods

longer than 15 h.

Similar to the analysis done by Kurny et al. [5], a

semiquantitative idea of the variation and distribution of

these phases can be obtained comparing the intensities of

the (100) and (200) diffraction lines for e and gVrespectively, which do not overlap and are intense enough.

Table 3 shows that the intensity of the e phase gradually

decreases with the treatment time, indicating a decrease in

the amount of this phase, while the intensity of the gVdiffraction line increases with the time. For treatment time

longer than 15 h, we can consider that the samples have a

compound layer of one phase: gV.It is well known that better characteristics in ductility

and fatigue are found when the compound layer consists of

only one nitride phase, i.e. either gV-Fe4N or e-Fe2–3N [14].

When the (gV+ e) dual phase is present, internal stresses

develop as a consequence of the difference in lattice

structures. On that basis, the samples treated for more than

15 h are expected to present a better performance as they

Fig. 6. The case depth versus the square root of time for AISI 4140 DC-

pulsed plasma nitrided.

P. Corengia et al. / Surface & Coatings Technology 200 (2005) 2391–2397 2395

show a high homogeneity and consist of only one phase

(gV-Fe4N).A similar phase variation was observed by Kurny et al.

[5] for En40B steels. However, Celik and Karadeniz [4]

did not observe compound variations during the ion

nitriding of an AISI 4140 steel for treatments between 1

and 10 h at different temperatures, gV-Fe4N being the

observed phase. Other researchers [6] have observed in the

white layer surface of API X65L, the opposite trans-

formation (Fe4NYFe2–3N) with the increased time of

Fig. 7. Surface morphology of untreated AISI 4

Fig. 8. Surface topography of ASI 4140 after nitriding at 7

nitriding (0.5 to 28 h). They found the formation of an

e-Fe2–3N sub-layer at the higher nitriding times, and

interpreted this transformation as a result of the

accumulation of N in the compound layer produced by

the lower N diffusion coefficient on the compound layer

than on a-Fe.

Two factors influence the kinetics of the formation of

the compound layer during ion nitriding: the SR

[12,13,15] and the amount of C in the steel [8]. In our

research, the higher amount of e in the beginning of the

process can be explained in terms of the amount of C in

the steel, because it stabilizes this phase. The evolution

(e+gV)YgV, that we observe in Table 3, could be a

consequence of the sputtering and/or generated by a phase

transformation eYgV. If we consider that for the first

hours of the process the compound layer is divided in two

sub-layers, an upper layer richer on e phase and a lower

layer richer on gV phase, the cathode sputtering will

remove the e phase during the first hours of the process.

This assumption is supported by Marciniak [12] and Sun

et al. [10], who showed the distribution of phases as a

function of the distance to the surface of the sample

taking into account the sputtering effect. Moreover,

simulations of the time evolution of (e+gV) bilayers

formed on steel by gaseous nitriding have shown that

the e/gV front moves much slower than the gV/a front [16];

thus, if the outer e phase is removed, the bilayer may

eventually become a gV layer.

140 (a) SEM image and (b) AFM image.

73 K for 1 h. (a) SEM image and (b) AFM image.

Fig. 9. SEM and AFM images of surface topography of AISI 4140 after nitriding at 773 K for 20 h.

P. Corengia et al. / Surface & Coatings Technology 200 (2005) 2391–23972396

Regarding the phase evolution (e+gV)YgV due to the

phase transformation eYgV, some researchers [17] suggest

that it could be the result from the diffusion of C out of the

steel during nitriding, leading to the decarburization of the

steel surface and the destabilization of the e phase. In our

experiments this mechanism can not be considered because

the C profiles obtained by EPMA did not show variations in

the amount of C in the surface of the samples at different

processing time.

3.2.2. Diffusion zone

As expected, higher depth values of the diffusion zone

are obtained as the treatment time increases. The diffusion

length d, defined as the depth where the hardness has

decreased to 400 HV, its near surface value, can be

estimated as [18]:

d ¼ De tð Þ1=2 þ c ð2Þ

where De is the effective diffusion coefficient which

depends on the in-diffusion and trapping (nitride formation)

of N. The constant c in Eq. (2) takes into account the onset

time of the nitriding process, and t is the elapsed time.

If the width of the nitrided case is associated to the

diffusion zone d, then it can be estimated from the

microhardness profiles at different processing time t (Fig.

Fig. 10. Roughness parameters versus DC-pulsed plasma nitriding time.

(E) Maximum Height Difference, (o) Mean and (�) Root Mean Square.

1). A linear relationship was found between d and t1/2 (with

a calculated effective diffusion coefficient of nitrogen of

3.18�10�12 m2 s�1 and an onset c of 0.11 h), as expected

for a diffusion controlled process (Fig. 6).

3.3. Topography

Fig. 7 shows the SEM and AFM images from the central

region of untreated steel. The surface has the typical

morphology of fine grinding with regularly spaced grinding

marks. The mean roughness value of the surface determined

by AFM is approximately 14 nm with a maximum peak to

valley height Hpv of 35 nm over the entire mapped area of

25 Am2.

Fig. 8 shows the SEM and AFM images of the central

region for a nitrided sample treated during 1 h. The original

grinding marks have vanished and the surface appears to be

uniformly covered with small microformations or droplets.

The mean roughness increased to 111 nm with Hpv=232 nm

after nitriding for 1 h.

The nucleation and growth of the microformations on the

nitrided surface are determined by relative intensity of

sputtering and redeposition [19,20].

Fig. 9 shows the SEM and AFM images after 20 h of ion

nitriding, with mean roughness value of 213 nm and

Hpv=451 nm. The increase of nitriding time produces an

increase of the microformations which start to overlap,

Fig. 11. SEM image of AISI 4140 DC-pulsed plasma nitrided during 20 h.

The image was obtained at 50 Am from the edge.

P. Corengia et al. / Surface & Coatings Technology 200 (2005) 2391–2397 2397

giving as a result bigger cones of different morphologies

and lower surface density.

Fig. 10 shows the evolution of the roughness parameters

(Maximum Height Difference, Mean and Root Mean

Square) measured by AFM in an area of 5�5 Am2 of the

central region of the samples. The surface roughness

apparently increases with the square root of the treatment

time (Fig. 10). Similar dependence was detected on ion

nitriding of an austenitic stainless steel using a broad beam

Kaufman ion source [19].

It is known that during plasma nitriding the SR is much

higher around the edges than at the central region of the

samples [20]. Fig. 11 shows an SEM image taken at the

edge of a sample nitrided for 20 h. We can observe

columnar microformations that, instead of being perpendic-

ular to the substrate as in the center of the sample (Fig. 9),

are leaning out of the center. This variation in the growth

direction can be explained as a result of the change in the

electric field direction close to the edge of the sample. The

same effect has been observed for samples treated under

different nitriding time.

4. Conclusions

– The compound layer thickness does not follow a para-

bolic law with treatment time. An equation has been

derived according to the experimental data that can

predict the thickness under the nitriding conditions

studied in the present investigation.

– When the nitriding time is increased, the compound layer

passes from a dual phase {e-Fe2–3N+gV-Fe4N} to a

monophase gV-Fe4N. It is expected that samples treated

for longer periods of times (N15 h) will present better

ductility and fatigue resistance, because they show a

highly homogeneous compound layer consisting of only

gV-Fe4N phase.

– A linear relationship was found between the case depth

and the square root of the nitriding time, as expected for

a diffusion controlled process. The effective diffusion

coefficient of nitrogen has been determined to be

3.18�10�12 m2 s�1, according to the experimental

data.

– The surface hardness depends on the nitriding time. It

increases in the first 2 h when it reaches a maximum

value, and then decreases because of the formation of

coarser nitride precipitates.

– All the roughness parameters increase with the ion

nitriding time. The rate of increase is higher during the

first hours of nitriding, which could be related to the

relative intensity of sputtering and redeposition.

– The increase of nitriding time produces an increase of the

microformationsproducingbiggerconesof lower surfacedensity.

– The growth direction of the microformations changes

close to the sample edge. This could be a consequence of

the change of the direction of the electric field lines close

to the edges.

Acknowledgements

This investigation was carried out at the National

Institute of Industrial Technology (INTI). The authors want

to thank the collaboration received from Dr. Adolfo

Rodrigo, Mr. Fernando Rodriguez and Lic. Fabian Alvarez

from CNEA for discussions and microhardness testing;

Prof. Horacio De Rosa and Eng. Hernan Svoboda from the

University of Buenos Aires for discussions and optical

micrographs; Mrs. S. Haug from Max Planck Insitute for

Metallforschung (Germany) for EPMA analysis, and Dr.

Marlete Zampronio from the State University of Maringa

(Brazil) for discussions.

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