R. CHEGROUNE et al.: CHARACTERIZATION AND KINETICS OF PLASMA-PASTE-BORIDED AISI 316 STEEL
263–268
CHARACTERIZATION AND KINETICS OF
PLASMA-PASTE-BORIDED AISI 316 STEEL
KARAKTERIZACIJA IN KINETIKA PLAZMA BORIRANJA S
PASTO JEKLA AISI 316
Redouane Chegroune1, Mourad Keddam1, Zahra Nait Abdellah2, Sukru Ulker3,
Sukru Taktak3, Ibrahim Gunes3
1Laboratoire de Technologie des Matériaux, Faculté G.M. et G.P., USTHB, B.P. 32, 16111 El-Alia, Bab-Ezzouar, Algiers, Algeria
2Département de Chimie, Faculté des sciences Université Mouloud Mammeri, Tizi-Ouzou, 15000, Algeria
3Department of Metallurgical and Materials Engineering, Faculty of Technology, Afyon Kocatepe University, 03200, Afyonkarahisar, Turkey
chegroune_redouane@yahoo.fr, rchegroune@usthb.dz
Prejem rokopisa – received: 2015-01-31; sprejem za objavo – accepted for publication: 2015-03-11
doi:10.17222/mit.2015.031
In this work, AISI 316 steel was plasma-paste borided in a gas mixture of 70 % H2 – 30 % Ar using a mixture of 30 % SiC +
70 % B2O3 as a boron source. The samples were treated at temperatures of (700, 750 and 800) °C for (3, 5 and 7) h. The
morphology of the formed boride layers was examined by light microscope and scanning electron microscope coupled to an
EDS analyser. The borides present in the boride layer were identified by means of XRD analysis. The boron-activation energy
for the AISI 316 steel was found to be equal to 250.8 kJ mol–1. This value for the energy was compared to the literature data. A
regression model based on a full factorial design was used to estimate the boride layers’ thicknesses as a function of the boriding
parameters: time and temperature. A comparison was made between the values of the boride layers’ thicknesses estimated from
the regression model with those given by an empirical relation. In addition, an iso-thickness diagram was plotted to predict the
boride-layer thickness as a function of the processing parameters. This iso-thickness diagram can serve as a simple tool to select
the optimum values for the boride layers’ thicknesses for a practical utilisation in industry for this kind of steel.
Keywords: plasma paste boriding, kinetics, borides, transition zone, activation energy, regression model
V tem delu je bilo jeklo AISI 316 borirano s plazmo v me{anici plinov 70 % H2 – 30 % Ar, z uporabo me{anice (30 % SiC +
70 % B2O3), kot vir bora. Vzorci so bili obdelani pri treh temperaturah (700, 750 in 800) °C v trajanju (3, 5 in 7) h. Morfologija
nastale boridne plasti je bila preiskovana s svetlobnim mikroskopom in z vrsti~nim elektronskim mikroskopom, opremljenim z
EDS analizatorjem. Boridi v borirani plasti so bili identificirani z rentgensko analizo. Aktivacijska energija bora v jeklu AISI
316 je bila 250,8 kJ mol–1. Ta vrednost je bila primerjana s podatki iz literature. Za dolo~anje debeline borirane plasti v
odvisnosti od parametrov boriranja (~as in temperatura), je bil uporabljen regresijski model, ki temelji na upo{tevanju faktorjev.
Izvr{ena je bila primerjava debeline borirane plasti, dolo~ene z regresijskim modelom in primerjava s tisto, ki je bila dolo~ena
empiri~no. Za napovedovanje debeline borirane plasti je bil postavljen diagram enake debeline v odvisnosti od procesnih
parametrov. Diagram enake debeline je uporaben kot enostavno orodje pri izbiri optimalne debeline borirane plasti, za prakti~no
uporabo v industriji za to vrsto jekla.
Klju~ne besede: plazma boriranje s pasto, kinetika, boridi, prehodno podro~je, aktivacijska energija, regresijski model
1 INTRODUCTION
The boriding process is widely used in industry
because of its broad application range.1 This thermo-
chemical treatment is generally performed in the range
700 °C to 1050 °C. The boriding process results in a
metallic boride layer of about 20 μm – 300 μm thickness.
The boriding treatment can be carried out in solid, liquid,
gaseous or plasma media.2-5 Due to their relatively small
size and very mobile nature, boron atoms can diffuse into
the surface material to form hard borides. In the case of
ferrous materials, the boriding treatment leads to the for-
mation of either a single layer (Fe2B) or a double-layer
(FeB+Fe2B) with a definite composition. The boride-
layer thickness is determined by the temperature and
treatment time. The characteristics of this boride layer
depend on the physical state of the boron source used,
the boriding temperature, the treatment time, and the
chemical composition of the material to be borided.6-9
In order to lower the boriding temperature and the
process time, ion-implantation boriding10, and plasma-
assisted boriding11 have been employed. Although the
plasma boriding process has an advantage over the
powder- or paste-boriding process, the use of expensive
gases such as B2H6 and BCl3 for the plasma-boriding
process poses a major problem related to their toxicity.
To overcome this problem, an alternative was recently
proposed by using the plasma paste boriding (PPB)
process. This recently developed method uses inert gases
such as hydrogen, argon and nitrogen, making it very
advantageous.5
The objective of this work was to investigate the bo-
riding kinetics of AISI 316 steel by plasma paste borid-
ing. The boron-activation energy for the AISI 316 steel
was also estimated basing on our experimental results.
Materiali in tehnologije / Materials and technology 50 (2016) 2, 263–268 263
UDK 669.14:621.793/.795:533.9 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(2)263(2016)
2 MATERIALS AND METHODS
In this study the AISI 316 austenitic steel used for the
plasma paste boriding has the following chemical com-
position given in Table 1.
Table 1: Chemical composition of AISI 316 stainless steel (in mass
fractions, w/%)
Tabela 1: Kemijska sestava nerjavnega jekla AISI 316 (v masnih
dele`ih, w/%)
C Cr Mo Mn Si Ni P S Fe
0.08 16 2 2 0.75 10 0.045 0.03 balance
The samples had a cylindrical shape and were 20 mm
in diameter and 5 mm in height. The surfaces of the AISI
316 steel were mechanically polished in sequence with
(600, 800, 1200 and 2400) grit wet SiC emery paper,
followed by fine polishing with an alumina slurry. The
samples were cleaned in alcohol before the plasma paste
boriding. In this study, a mixture of powder (30 % SiC +
70 % B2O3) constituting a boron source was applied,
where silicon carbide (SiC) was used as a catalyst for the
B2O3 paste. The plasma paste boriding was carried out in
a dc plasma system, which is described in detail in the
work by Gunes et al.5 Argon gas is important for the
plasma system. It is used to change the plasma para-
meters such as the electron temperature and the electron
density, which influence the production of active species
by inelastic collisions, plasma reactions and plasma
surface interactions. During the plasma paste boriding,
H2 gas plays an important role. Borax reacts with active
hydrogen (H+) in a glow discharge. Atomic boron was
produced through the decomposition of the boron
hydride (BxHy) from the paste, and this atomic boron
became the active boron, B+1, within the molten borax or
in the glow discharge. Finally, this active boron B+1
diffused and reacted with the Fe to form the boride layer.
The samples of AISI 316 austenitic steel were plasma
paste borided at (700, 750 and 800) °C for (3, 5 and 7) h
in a gas mixture of 70 % H2 – 30 % Ar under a constant
pressure of 10 mbar. The temperature of the samples was
measured by means of a chromel–alumel thermocouple,
placed at the bottom of the treated samples. After the
plasma paste boriding process, each borided sample was
cleaned in an ultrasonic bath with alcohol. The treated
samples were chemically etched using a chemical solu-
tion consisting of 20 mL glycerine, 10 mL HNO3 and 30
mL HCl. The morphology of boride layers was observed
using a light microscope (Olympus Vanox AHMTS) and
a LEO 1430 VP model Scanning Electron Microscope
(SEM). The values of boride layers’ thicknesses were
taken as averages of at least 10 measurements.
The presence of different borides formed in the bo-
ride layer was confirmed by means of an XRD analysis.
This analysis was carried out using a Philips X-ray
diffractometer with Cu-K radiation (Cu = 0.154 nm).
The distribution of alloying elements was analysed by
energy-dispersive X-ray spectroscopy (EDS) from the
surface towards the substrate.
EDS line analyses were performed to determine
which element accumulated across the boride layer and
the transition zone.
3 RESULTS AND DISCUSSION
3.1 Observation of the morphology of the boride layers
Figure 1 shows the etched cross-sections of boride
layers formed on the surfaces of AISI 316 steel at
different temperatures and for various treatment times. It
reveals the formation of a bilayer configuration com-
posed of FeB and Fe2B. A transition zone exists in all the
optical micrographs due to the accumulation of un-
dissolved elements beneath the boride layer. The
morphology of the boride-layer/transition-zone interface
exhibited a flat diffusion front due to the effects of the
main alloying elements such as Cr, Mo and Ni. This fact
can be explained by the reduction of the active boron
flux in the diffusion zone by the presence of these ele-
ments. It is clear that the thicknesses of the boride layer
and the transition zone are affected by the process para-
meters (time and temperature). For instance, the boride
layer’s thickness reached a value of 10.4 μm at 750 °C
for 7 h, while its corresponding value was 5.11 μm at a
temperature of 700 °C during 7 h of treatment. Further-
more, some precipitates (i.e., chromium carbides) were
also observed along the austenitic grain boundaries
revealed by the chemical etchant composed of 20 mL
glycerine, 10 mL HNO3 and 30 mL HCl.
Figure 2 gives the cross-sectional views of micro-
structures of borided AISI 316 steel. The FeB and Fe2B
layers are discernable from a difference in contrast. The
inner layer Fe2B is clearer than the outer layer of FeB.
The transition zone is also observed on the all SEM
images. It is clear that the obtained boride layers are
compact and continuous. The smooth morphology of the
R. CHEGROUNE et al.: CHARACTERIZATION AND KINETICS OF PLASMA-PASTE-BORIDED AISI 316 STEEL
264 Materiali in tehnologije / Materials and technology 50 (2016) 2, 263–268
Figure 1: Light micrographs showing the microstructures of boride
layers formed on the AISI 316 steel for different boriding parameters:
a) 700 °C for 7 h, b) 750 °C for 7 h, c) 800 °C for 3 h, d) 800 °C for 5 h
Slika 1: Svetlobni posnetki mikrostruktur boriranih plasti na AISI 316
jeklu, pri razli~nih parametrih boriranja: a) 700 °C, 7 h, b) 750 °C, 7 h,
c) 800 °C, 3 h, d) 800 °C, 5 h
boride-layer/transition-zone interface is observed in
Figure 2.
Figure 3 shows the SEM image (for the EDS analy-
sis) of cross-sections of the boride layer formed on AISI
316 steel at 800 °C for a treatment time of 3 h. The EDS
line scan taken perpendicularly from the surface of the
borided sample at 800 °C for 3 h showed a qualitative
distribution of different elements along the boride layer
and the transition zone. It indicates the precipitation of
metallic borides inside the boride layer and an accumu-
lation of certain elements (such as Cr, C and Mo) in the
transition zone (Figure 3b). In particular, molybdenum
has a lower tendency to dissolve in the boride layer and
tends to concentrate in the diffusion zone.12 In addition,
the boride layer thickness was found to increase with the
boriding temperature.
3.2 XRD analysis
Figure 4 gives the XRD pattern recorded at the sur-
face of the borided AISI 316 steel at 800 °C for 3 h of
treatment. To confirm the presence of iron and metallic
R. CHEGROUNE et al.: CHARACTERIZATION AND KINETICS OF PLASMA-PASTE-BORIDED AISI 316 STEEL
Materiali in tehnologije / Materials and technology 50 (2016) 2, 263–268 265
Figure 3: Cross-section of the borided AISI 316 steel at 800 °C for a
treatment time of 3 h: a) SEM image of the cross-section of a borided
sample for EDS analysis, b) EDS line scan across the borided zone, c)
EDS spectrum of the selected zone (Figure 3a)
Slika 3: Presek jekla AISI 316, boriranega pri 800 °C, v trajanju 3 h:
a) SEM-posnetek preseka borirane plasti za EDS-analizo, b)
EDS-linijska analiza skozi borirano podro~je, c) EDS-spekter v izbra-
nem podro~ju, ki ga ka`e Slika 3a
Figure 2: SEM micrographs of the cross-sections of boride layers
formed on the AISI 316 steel for different boriding parameters: a) 750
°C for 5 h, b) 750 °C for 7 h, c) 800 °C for 3 h
Slika 2: SEM-posnetki preseka borirane plasti, nastale na AISI 316
jeklu pri razli~nih parametrih boriranja: a) 750 °C, 5 h, b) 750 °C, 7 h,
c) 800 °C, 3 h
borides, the corresponding files taken from the JCPDS
database13 were used.
The iron borides (FeB and Fe2B) were identified as
well as the presence of metallic borides as precipitates
within the boride layers such as CrB, Cr2B and Ni3B. In
addition, Cr and Ni tend to dissolve in the boride layer
and form independent metallic borides (CrB, Cr2B and
Ni3B).
3.3 Estimation of the boron activation energy
The growth kinetics of boride layers is controlled by
the boron diffusion into the substrate. The boride-layer
thickness varies parabolically14–17 with the process time
given by Equation (1):
u k t= ' (1)
where u is the boride-layer thickness (in μm), k’ is a
parabolic growth constant (in μm s–1/2) at a given tem-
perature, D is the diffusion coefficient of boron in the
boride layer, and t is the boriding time. The time depen-
dence of the boride layer thickness for increasing
temperatures is given in Figure 5. It is clear that the
boride layer varies linearly with the square root of time,
which proves that the growth kinetics of the boride layer
is governed by the diffusion phenomenon of boron
atoms inside the substrate.
The relationship between the parabolic growth con-
stant k’ (in μm s–1/2) and the boriding temperature T in
Kelvin, can be expressed using an Arrhenius-type equa-
tion as follows:
k k D
Q
RT
' exp= −⎛⎝
⎜ ⎞
⎠
⎟
0 (2)
where D0 is the diffusion coefficient of boron extrapo-
lated at a value of 1/T = 0. The Q parameter is the
activation energy that indicates the amount of energy
(kJ mol–1 ) required for the reaction to occur, and R is
the ideal gas constant (R = 8.314 J mol–1 K–1). Taking
the natural logarithm of Equation (2), we obtain Equa-
tion (3):
ln( ' ) ln( ) ln( )k k D
Q
RT
2 2
0= + −
⎛
⎝
⎜ ⎞
⎠
⎟ (3)
The activation energy Q can be easily deduced from
the slope of the curve relating ln (k’2) to the inverse of
the temperature. Figure 6 provides the temperature de-
pendence of the natural logarithm of the square of the
parabolic growth constant k’2.
The reported values for boron-activation ener-
gies4,18–25 for borided steels are listed in Table 2 together
with the value of the boron-activation energy (250.8
kJ mol–1) estimated from this work. However, these
R. CHEGROUNE et al.: CHARACTERIZATION AND KINETICS OF PLASMA-PASTE-BORIDED AISI 316 STEEL
266 Materiali in tehnologije / Materials and technology 50 (2016) 2, 263–268
Figure 4: XRD pattern obtained at the surface of borided AISI 316
steel at 800 °C for 3 h of treatment
Slika 4: Rentgenogram, dobljen na povr{ini jekla AISI 316,
boriranega 3 h na 800 °C
Figure 6: Temperature dependence of the square of the parabolic
growth constant
Slika 6: Odvisnost temperature od kvadratnega korena paraboli~ne
konstante rasti
Figure 5: Evolution of the boride layer thickness versus the square
root of time for increasing temperatures
Slika 5: Rast debeline borirane plasti pri nara{~ajo~i temperaturi v
odvisnosti od kvadratnega korena iz ~asa
values for the boron-activation energies for different
steels depended on various factors such as: the boriding
method, the chemical composition of the base steel, the
nature of the boriding agent, and the mechanism of
boron diffusion. For the paste plasma boriding of AISI
316 steel, the high activation energy shown in Table 2 is
ascribed to the formation of FeB, Fe2B, CrB, Cr2B and
Ni3B phases in the boride layer. The obtained value of
the boron-activation energy (250.8 kJ mol–1) from this
work, can also be interpreted as the required barrier to
allow boron diffusion inside the steel substrate. Thus, the
diffusion phenomenon of boron atoms can occur along
the grains boundaries and also in the volume to form the
boride layer on the AISI 316 steel.
Table 2: Values of boron-activation energies obtained in the case of
borided steels using different methods
Tabela 2: Vrednosti aktivacijske energije bora, dobljene pri boriranju
jekel z razli~nimi metodami
Material Boridingmethod Q (kJ mol
–1) References
AISI 304 Salt Bath 253.35 18
AISI H13 Salt bath 244.37 18
AISI 4140 Paste 168.5 19
AISI H13 Powder 186.2 20
AISI 1040 Powder 168 21
AISI440C Powder 340.4 22
AISI 316 Powder 199 23
AISI 51100 Plasma 106 24
AISI 304 Plasma PasteBoriding 123 4
AISI8620 Plasma PasteBoriding 124.7–138.5 25
AISI 316 Plasma Pasteboriding 250.8 This work
3.4 Prediction of the boride-layer thickness with a re-
gression model
A full factorial design with two factors at three le-
vels26 was used to predict the boride-layer thickness as a
function of the boriding parameters (time and tempera-
ture).
Using this approach, Equation (4) was obtained as
follows:
u T t
tT T
= − − +
+ + +
761.50 2.0856 11.763
0.015825 0.001435 0.2 108333t 2
(4)
where t is the boriding time (h) and T is the temperature
in degree Celsius.
Table 3: Comparison between the experimental values of the boride-
layer thicknesses and those given by Equation (1) and the regression
model (Equation 4) in the temperature range 700–800 °C
Tabela 3: Primerjava eksperimentalnih vrednosti debeline borirane
plasti z vrednostmi iz ena~be (1) in iz regresijskega modela (ena~ba
4), v temperaturnem obmo~ju 700–800 °C
T
(°C)
Time
(h)
Predicted
(FeB+Fe2B)
Layer thickness
(μm)
Equation (1)
Predicted
(FeB+Fe2B)
Layer thickness
(μm)
Equation (4)
Experimental
(FeB+Fe2B) layer
thickness
(μm)
700 3 3.59 3.34 3.30
700 5 3.92 4.32 4.20
700 7 5.11 5.11 5.84
750 3 5.72 7.15 6.60
750 5 7.63 9.23 8.50
750 7 10.40 10.92 9.45
800 3 15.02 14.20 15.13
800 5 18.51 18.34 18.16
800 7 22.86 21.70 24.00
Table 3 compares between the experimental values
of the boride layers’ thicknesses and the predicted values
using Equations (1) and (4) in the temperature range 700
°C – 800 °C. A good agreement was observed between
the simulated values of the boride layers’ thicknesses and
those obtained experimentally. Equation (1) can be used
to plot the iso-thickness diagram shown in Figure 7. The
iso-thickness diagram can serve as a simple tool to select
the optimum value of the boride-layer thickness for a
practical use of the plasma paste borided AISI 316 steel
in industry. As a rule, thin layers (e.g., 15 μm – 20 μm)
are used to protect against adhesive wear (such as chip-
less shaping and metal stamping dies and tools), whereas
thick layers are recommended to combat abrasive wear
(extrusion tooling for plastics with abrasive fillers and
pressing tools for the ceramics industry). In the case of
low-carbon steels and low-alloy steels, the optimum
boride-layer thicknesses are between 50 μm and 250 μm.
4 CONCLUSIONS
In this work AISI 316 steel was plasma paste borided
in a gas mixture of 70 % H2 – 30 % Ar using a mixture
of 30 % SiC + 70 % B2O3. The boride-layer/transition-
R. CHEGROUNE et al.: CHARACTERIZATION AND KINETICS OF PLASMA-PASTE-BORIDED AISI 316 STEEL
Materiali in tehnologije / Materials and technology 50 (2016) 2, 263–268 267
Figure 7: Iso-thickness diagram describing the evolution of the
boride-layer thickness as a function of the boriding parameters (time
and temperature)
Slika 7: Diagram enakih debelin opisuje razvoj debeline borirane
plasti v odvisnosti od parametrov boriranja (~as in temperatura)
zone interface had a smooth morphology with a boride
layer thickness ranging from 3.3 μm to 24 μm.
The boride layer formed on the surface of the AISI
316 was composed of FeB, Fe2B, CrB, Cr2B and Ni3B
phases. This result was confirmed by XRD analysis and
a transition zone was also visible beneath the boride
layer. The boron-activation energy for the AISI 316 steel
was estimated to be 250.8 kJ mol–1. This value was inter-
preted as the required quantity of energy to stimulate the
boron diffusion in the preferential direction (0 0 1). This
value of the boron-activation energy was comparable to
values found in the literature.
In addition, an iso-thickness diagram describing the
evolution of the boride-layer thickness as a function of
the boriding parameters was proposed. It can serve as a
simple tool to select the optimum boride layer thickness
according to the practical use of borided AISI 316 steel.
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