Boriding is a thermochemical treatment in which boron atoms are diffused into the surface of a workpiece and form borides with the base metal. Apart from constructional materials, which meet these high demands, processes have been developed which have a positive effect on the tribological applications including abrasive, adhesive, fatigue and corrosion wear of the component surface.

1. Boriding kinetics and mechanical behaviour of
AISI O1 steel
M. Elias-Espinosa1
, M. Ortiz-Domı´nguez2
, M. Keddam*3
, O. A. Go´mez-Vargas4
,
A. Arenas-Flores5
, F. R. Barrientos-Herna´ndez6
, A. R. West6
and D. C. Sinclair6
In this work, American Iron and Steel Institute (AISI) O1 steel was pack borided in the temperature
range of 1123–1273 K for treatment times between 2 and 8 hours. A kinetic model was proposed
for estimating the boron diffusion coefficients through the Fe2B layers. As a result, the boron
activation energy for the AISI O1 steel was estimated as 197?2 kJ mol21
. This value of energy was
compared to the literature data. In addtion, to extend the validity of the present model, two
additional boriding conditions were done. The Fe2B layers grown on AISI O1 steel were
characterised by use of the following experimental techniques: scanning electron microscopy,
X-ray diffraction analysis and Daimler-Benz Rockwell-C indentation technique. Finally, the scratch
and pin on disc tests for wear resistance were respectively performed using an LG Motion Ltd and
a CSM tribometer under dry sliding conditions.
Keywords: Incubation time, Diffusion model, Activation energy, Growth kinetics, Adherence, Tribology
This paper is part of a special issue on Diffusion
Introduction
Boriding is a thermochemical diffusion process in which
the boron atoms are extracted from boron containing
media and deposited onto the sample surface.1
It has
been applied to a wide range of materials, including
ferrous, non-ferrous and some super alloys. The borid-
ing process results in some important improvements in
surface properties, such as high hardness, resistance
against wear and corrosion.2–4
The diffusion of atomic
boron into the material substrate results in the
formation of a wear resistant iron boride layer on the
surface of steel parts. The boride layer is composed of
either a single phase layer of Fe2 B or a double phase
layer (Fe2B þ FeB) according to the Fe–B phase
diagram. The monolayer configuration (Fe2B) is
desirable to the double phase layer since the FeB phase is
more brittle and harder than Fe2B and has a coefficient
of thermal expansion superior than that of Fe2B. This
situation can cause cracks when a double phase layer
is formed. In practice, there are many boriding methods
to form boride layers on different substrates, such as
gas boriding,5,6
liquid boriding,7
powder or paste
boriding,8,9
and laser boriding.10
The most frequently
used method is pack boriding because of its technical
advantages and cost effectiveness.11,12
In a kinetic point
of view, several mathematical models concerning the
growth kinetics of Fe2B layers on different substrates9,13–26
were suggested in the literature. These approaches
are considered as a tool to select the adequate boride layers
thicknesses for practical applications. Until now, no
studies were reported in the literature about the boriding
kinetics of AISI O1 steel.
The AISI O1 steel is a high quality non-distorting cold
work tool steel. It is suitable for a wide variety of cold
work applications. It has a certain resı´stance against
abrasion with a good machinability and sufficient
toughness for normal tool and die applications.
The pack boriding treatment was used to improve the
tribological properties and extend the lifetime of the
workpieces made of AISI O1 steel.
The aim of this work was to investigate the growth
kinetics and mechanical behaviour of the pack borided
AISI O1 steel. A diffusion model was proposed to
determine the boron diffusion coefficients in the Fe2B
layer at the surface of AISI O1 steel considering a
constant boride incubation time. As a result, the boron
activation energy for the AISI O1 steel was estimated
using this diffusion model basing on our experimental
results. For metallurgical investigation, scanning
electron microscopy (SEM) and X-ray diffraction
(XRD) analysis were used to characterise the pack
borided AISI O1 steel. For mechanical characteris-
ations, the Daimler-Benz Rockwell-C indentation
technique was used to characterise the cohesion of
1
Instituto Tecnolo´gico y de Estudios Superiores de Monterrey-ITESM
Campus Santa Fe, Av. Carlos Lazo No. 100, Del. A´ lvaro Obrego´n, CP.
01389, D. F., Me´xico
2
Universidad Auto´noma del Estado de Hidalgo, Campus Sahagu´n,
Carretera Cd. Sahagu´n-Otumba s/n, Hidalgo, Me´xico
3
Laboratoire de Technologie des Mate´riaux, Faculte´ de Ge´nie Me´canique
et Ge´nie des Proce´de´s, USTHB, B.P. No. 32, 16111 El-Alia,
Bab-Ezzouar, Algiers, Algeria
4
Instituto Tecnolo´gico de Tlalnepantla-ITTLA. Av., Instituto Tecnolo´gico,
S/N. Col. La Comunidad, Tlalnepantla de Baz. CP. 54070. Estado de
Me´xico, Me´xico
5
Universidad Auto´noma del Estado de Hidalgo-AACTyM, Carretera
Pachuca Tulancingo Km. 4·5, Mineral de la Reforma. CP. 42184.
Hidalgo, Me´xico
6
University of Sheffield-Department of Engineering Materials, Sheffield S1
3JD, U.K.
*Corresponding author, email keddam@yahoo.fr
588
Ñ 2015 Institute of Materials, Minerals and Mining
Published by Maney on behalf of the Institute
Received 30 January 2015; accepted 12 April 2015
DOI 10.1179/1743294415Y.0000000065 Surface Engineering 2015 VOL 31 NO 8

2. boride layer on the surface of AISI O1 steel. In addition,
the sample borided at 1123 K with 4 hours of
exposure time was tested in an LG Motion Ltd (scratch)
and a CSM tribometer (pin on disc) in ambient air at
room temperature. Friction coefficient and wear
behaviour were finally compared with the unborided
AISI O1 steel.
Diffusion model
Mass balance equation
The model takes into account the growth of Fe2B
layer on a saturated substrate with boron atoms,
as shown in Fig. 1. The f(x,t) function illustrates the
boron distribution in the ferritic matrix before
the nucleation of Fe2B phase. tFe2B
0 corresponds to the
incubation time required to form the Fe2B phase
when the matrix reaches a saturation state with boron
atoms. CFe2B
up represents the upper limit of boron
content in Fe2B ( ¼ 60|103
mol m23
), CFe2B
low Tð Þ is the
lower limit of boron content in Fe2B with
CFe2B
low Tð Þ ¼ (20?0004Tþ60?373)|103
mol m23
and
the point x(t ¼ t) ¼ v refers to the Fe2B layer thick-
ness.9,24–26
The term CB
ads is the effective adsorbed
boron concentration during the boriding process.27
Likewise, the homogeneity of the distribution of boron
inside the Fe2B layer can be described by the parameter
a1 ¼ CFe2B
up 2 CFe2B
low ðTÞ. a2 ¼ CFe2B
low ðTÞ 2 C0 is the
miscibility gap, and C0 is the boron solubility in the
matrix. This diffusion zone in the substrate underneath
the compound layer can be ignored by setting
(C0 < 0mol m23
)28,29
due to the lower solubility of
boron in the matrix. The assumptions taken into
account during the formulation of the diffusion model
can be found elsewhere.9
The initial and boundary conditions for the diffusion
problem are represented as
t ¼ 0; x . 0; with : CFe2B½xðtÞ; t ¼ 0Š ¼ C0 < 0 ð1Þ
Boundary conditions
CFe2B x t ¼ tFe2B
0
À Á
¼ v0;t ¼ tFe2B
0
 Ã
¼ CFe2B
up ðthe upper
boron concentration is kept constantÞ
for CB
ads . 60 £ 103
mol m23
9
>>=
>>;
ð2Þ
CFe2B½xðt ¼ tÞ ¼ v; t ¼ tŠ ¼ CFe2B
low ðTÞ ðthe boron
concentration at the interface is kept constantÞ
CB
ads , 59·8 £ 103
mol m2 3
9
>>=
>>;
ð3Þ
v0 is a thin layer with a thickness of <5 nm that formed
during the nucleation stage.30
Thus, v0(<0) when
compared to the thickness of the Fe2B layer (v). The
mass balance equation at the (Fe2B/substrate) interface
can be formulated by equation (4) as follows
CFe2B
up þ CFe2B
low ðTÞ 2 2C0
2
" #
ðAÁdvÞ
¼ JFe2Bðx ¼ v; t ¼ tÞðAÁdtÞ
2 JFeðx ¼ v þ dv; t ¼ tÞðAÁdtÞ
ð4Þ
where A ( ¼ 1?1) is defined as the unit area, and C0
represents the boron concentration in the matrix. The
flux JFe2
B and JFe are obtained from Fick’s first law as
JFe2B xðt ¼ tÞ ¼ v; t ¼ t
 Ã
¼ 2{DFe2BLCFe2B xðt ¼ tÞ ¼ v; t ¼ t
 Ã
=Lx}x¼v ð5Þ
and
JFe xðt ¼ tÞ ¼ v þ dv; t ¼ t
 Ã
¼ 2{DFeLCFe xðt ¼ tÞ ¼ v þ dv; t ¼ t
 Ã
=Lx}x¼vþdv
ð6Þ
The term JFe is null since the boron solubility in the
matrix is very low (<0 mol m23
).28,30
Thus, equation (4) can be written as
CFe2B
up þ CFe2B
low ðTÞ 2 2C0
2
" #
dxðtÞ
dt
xðtÞ¼v
¼ 2DFe2B
LCFe2B½xðt ¼ tÞ; t ¼ tŠ
Lx
xðtÞ¼v
ð7Þ
If the boron concentration profile in Fe2B is constant for
the treatment time, Fick’s second law is reduced to an
ordinary second order differential equation as follows
LCFe2B½xðtÞ; tŠ
Lt
¼ DFe2B
L2
CFe2B½xðtÞ; tŠ
Lx2
: ð8Þ
By solving equation (8) and applying the boundary
conditions proposed in equations (2) and (3), the boron
concentration profile in Fe2B is expressed by equation
(9) if the boron diffusion coefficient in Fe2B is constant
for a particular temperature
CFe2B½xðtÞ; tŠ ¼ CFe2B
up
þ
CFe2B
low ðTÞ 2 CFe2B
up
erf v
2 DFe2Bt
À Á1=2
# erf
x
2 DFe2Bt
À Á1=2
#
ð9Þ1 Schematic boron concentration profile through Fe2B layer
M. Elias-Espinosa et al. Boriding kinetics and mechanical behaviour of AISI O1 steel
Surface Engineering 2015 VOL 31 NO 8 589

3. By substituting the derivative of equation (9) with
respect to the diffusion distance x(t) into equation (7),
equation (10) is obtained as
CFe2B
up þ CFe2B
low ðTÞ 2 2C0
2
#
dv
dt
¼
DFe2B
pt
1=2
CFe2B
up 2 CFe2B
low ðTÞ
erf v
2 DFe2Bt
À Á1=2
# exp 2
v2
4DFe2Bt
ð10Þ
for 0xv.
By substituting the derivative of the parabolic growth
law (v ¼ 21D
1=2
Fe2Bt1=2
) with respect to time t into
equation (10), equation (11) is deduced as
CFe2B
up þ CFe2B
low ðTÞ 2 2C0
2
#
1
¼
1
p
1=2
CFe2B
up 2 CFe2B
low ðTÞ
erf 1ð Þ
exp ð212
Þ ð11Þ
The normalised growth parameter (1) for the (Fe2B/
substrate) interface can be estimated numerically by
the Newton–Raphson method. It is assumed that the
expressions CFe2B
up , CFe2B
low ðTÞ and C0 do not depend
significantly on temperature (in the considered
temperature range).26
Experimental procedure
Boriding process
The material to be pack borided is the AISI O1 steel.
It has a nominal chemical composition of 0?85–1?00%C,
0?30–0?50%Si, 1?00–1?40%Mn, 0?40–0?60%Cr,
0?40–0?60%W, 0?10–0?30%V, 0?030%P and 0?030%S.
The samples are cubic shaped of dimension
10|10|10 mm. Prior to the boriding process, the
samples were polished, ultrasonically cleaned in an
alcohol solution and deionised water for 15 min at room
temperature and then dried and stored under clean room
conditions. The samples were embedded in a closed
cylindrical case (AISI 304L), containing a fresh Durborid
powder mixture. The boriding agent, with an average
particle size of 30 mm, was composed of an active source
of boron (B4C), an inert filler (SiC) and an activator
(KBF4). The boriding process was carried out in
the temperature range of 1123–1273 K for a variable
time (2, 4, 6 and 8 hours). Once the treatment was
finished, the container was removed from the furnace
and slowly cooled to room temperature.
Experimental techniques
The borided and etched samples were cross-sectioned,
for microstructural investigations, to be observed by
SEM (JEOL JSM 6300). For a kinetic study, the boride
layer thickness was automatically measured with the aid
of an MSQ PLUS software. To ensure the reproduci-
bility of the measured layers, 50 measurements were
taken from different sections of the borided samples to
estimate the Fe2B layer thickness; defined as an average
value of the long boride teeth.31–32
The presence of
different borides formed at the surface of AISI O1 steel
was determined by means of an XRD equipment
(Equinox 2000) using Co Ka radiation at l ¼ 0?179 nm.
The Daimler-Benz Rockwell-C technique was performed
to attain qualitative information on the cohesive
strength of the boride layers to the substrate. The well
known Rockwell-C indentation test is prescribed by the
VDI 3198 norm as a destructive quality test of coated
compounds.33–35
A load of 1471 N was applied to cause
coating damage adjacent to the boundary of the inden-
tation. Three indentations were made for each borided
sample to assess the cohesion test. The indentation
craters on the surfaces of samples were observed by
SEM (JEOL JSM 6300). The pin on disc wear tests were
achieved at ambient conditions without lubrication.
Before the test, the samples were cleaned with acetone in
order to remove contaminants from the surface. The
tested samples had a disc shape with a diameter of
25?4 mm and a thickness of 10 mm. Tribological tests
were performed with a diamond made indenter with a
10 mm diameter hemispheric and wear test machine
(CSM tribometer (pin on disc)) at room temperature and
a relative humidity of 40%. All tests were conducted for
a total sliding distance of 500 m with a sliding speed of
0?08 m s21
, and the covered radial distance was 14 mm
under a normal load of 5 N. The pin on disc test was
achieved on a Revetest device equipped with an acoustic
emission sensor that measures the loads in situ when
damage occurs and another that permits direct recording
of the tangential force that gives the instantaneous
friction coefficient. Before the scratch wear tests,
the samples with a rectangular shape of dimensions
12|7|7 mm were cleaned with acetone in order to
remove the contaminants from the surface. The test
consists of scratching the sample surface by using an LG
Motion Ltd (scratch) with a single pass under increasing
normal load at a rate of 10 N mm21
of covered distance.
The scratch wear tests were realised at ambient
conditions without lubrication.
Experimental results and discussions
Microscopical observations of boride layers
Figure 2 shows the cross-sections of boride layers
formed on the surfaces of AISI O1 steel at different
temperatures and for 6 hours of exposure time. The
resultant microstructure of Fe2B layers looks very dense,
compact and homogenous, with reduced sawtooth
morphology. This peculiar morphology is ascribed to
the presence of carbon and alloying elements in AISI O1
steel. In fact, the carbon content in the steel influences
the nature of (boride layer/substrate) interface.
Generally, with an increase in the steel’s carbon content,
there is a tendency to reduce the formation of a jagged
interface between the steel substrate and the boride
layer. In addition, the alloying elements present in the
steel tend to concentrate in the tips of boride layers,
reducing the boron flux in this zone.
X-ray diffraction analysis
Figure 3 shows the XRD patterns obtained at the
surfaces of borided AISI O1 steel at 1273 K for 6 and
8 hours of treatment. The formation of Fe2B layer at the
surface of AISI O1 steel is confirmed by XRD analysis.
It is noted that the diffractions peaks are different in
intensity and depend on the crystallographic orien-
tations of Fe2B crystals. In a kinetic point of view, the
Fe2B crystals begin to nucleate when the matrix reaches
M. Elias-Espinosa et al. Boriding kinetics and mechanical behaviour of AISI O1 steel
590 Surface Engineering 2015 VOL 31 NO 8

4. the saturation level by boron atoms. Furthermore, the
boron solubility in iron is very low and strongly
dependent on irregularities in the crystal lattice and
therefore also on the purity of the same metal.
Afterwards, the boride layer becomes compact and
continuous with a prolonged treatment time. The
crystallographic direction [001] is the easiest path for
the boron diffusion in the Fe2B phase because of the
tendency of boride crystals to grow along a direction of
minimum resistance. For this reason, the growth of Fe2B
layer is of a highly anisotropic nature.36
Rockwell-C cohesion test
An indenter hardness tester was used to assess the
Daimler-Benz Rockwell-C cohesion as a destructive
quality test for the examined layers; it was employed for
the determination of cohesion. A conical diamond
indenter penetrated into the surface of an investigated
layer, thus inducing massive plastic deformation to the
substrate and fracture of the boride layer. The damage
to the boride layer was compared with the cohesion
strength quality maps HF1–HF6. In general, the cohe-
sion strengths HF1 to HF4 are defined as sufficient
cohesion, whereas HF5 and HF6 represent insufficient
cohesion.33
Fig. 4 shows the SEM images of the inden-
tation craters after the VDI cohesion test on the surfaces
of AISI O1 steel borided at 1123 K during 2 and 5 hours.
The indentation craters obtained on the surface of the
pack borided AISI O1 steel revealed that they were
radial cracks at their perimeter. However, a small
quantity of spots with flaking resulting from dela-
mination was visible, and the cohesion strength quality
of this boride layer is related to the HF3 standard.
3 X-ray diffraction patterns obtained at surfaces of borided
AISI O1 steel at 1273 K for 6 and 8 hours of treatment
2 Images (SEM) of cross-sections of borided AISI O1 steel during 6 hours of treatment at different boriding temperatures:
a 1123 K; b 1173 K; c 1223 K; d 1273 K
M. Elias-Espinosa et al. Boriding kinetics and mechanical behaviour of AISI O1 steel
Surface Engineering 2015 VOL 31 NO 8 591

5. Tribological characterisation
The pin on disc tests were carried out in dry sliding
conditions using a CSM tribometer. This machine is
used to determine the magnitude of friction coefficient
and wear as two surfaces rub together. In one
measurement method, a pin or a sphere is loaded onto
the test sample with a precisely known force. The pin is
mounted on a stiff lever designed as a frictionless force
transducer. The friction coefficient is determined during
the test by measuring the deflection of the elastic arm.
A diamond made indenter with a 10 mm diameter
hemispheric, commonly employed, was used to slide
against the surface of the borided AISI O1 steel.
Figure 5 shows the variation in friction coefficient of the
borided and unborided surfaces under dry sliding
conditions against a diamond made indenter. Figure 5
points out that the borided sample exhibited a friction
coefficient lower than that of the unborided substrate.
The average friction coefficient, for the borided sample
at 1123 K for 4 hours, ranged from 0?398 to 0?375, while
the average friction coefficient was between 0?637 and
0?612 for the unborided substrate. These results agree
with those obtained in other works.37–39
Figs. 6 gives the
SEM images of the unborided and borided surfaces
obtained at 1123 K with exposure time of 4 hours
respectively. A defined wear scar is produced, which
has a width of approximately 0?902 and 0?610 mm
respectively. In Fig. 6a, the wear debris and scratching
lines are observed, whereas common wear mechanisms,
like plastic deformation, are viewed on the unborided
surface. Figure 6b shows the wear scar formed on the
borided surface, where some pits and scratching lines are
observed. The scratch tests were carried out in dry
sliding conditions using an LG Motion Ltd. In this
technique, the tip material (commonly diamond or hard
metal (WC)) is drawn across the borided surface under
constant, incremental or progressive load. Figures 7
displays the SEM images of the borided surfaces of AISI
O1 steel at 1123 and 1273 K with exposure time of
4 hours respectively. It is noticed that the cracks pro-
pagate in depth along the scratch trails. They have either
a curvilinear form (see Fig. 7b) or a mosaic (see Fig. 7d).
According to the literature, this type of cracks is
characteristic of a Hertzian fracture on brittle solids
when a blunted indenter is used. These cracks propagate
in depth in a semiconical shape and start at flaws near
the contact surface where high tension stresses
develop.40,41
There was not a case of cohesive scaling at
the (Fe2B /substrate) interface was observed. This was
expected since it is well established that the coatings
achieved at high temperature present a good cohesion
due to the interdiffusion phenomenon ensuring the
continuity of metallic interface.
Estimation of boron activation energy
The growth kinetics of Fe2B layers formed on the AISI
O1 steel was used to estimate the boron diffusion coef-
ficient in the Fe2B layers by applying the suggested dif-
fusion model. The determination of 1 parameter by
solving equation (11) is required to deduce the value of
boron diffusion coefficient in the Fe2B layer at each
boriding temperature. Figure 8 gives the time depen-
dence of the squared value of the Fe2B layer thickness
for increasing temperatures. The slopes of the straight
lines, displayed in Fig. 8, give the values of growth
constants ( ¼ 41 2
DFe2
B).
Table 1 provides the values of growth constants
( ¼ 41 2
DFe2
B) along with the squared normalised value
4 Images (SEM) showing indentations of VDI cohesion test
on surfaces of AISI O1 steel borided at 1123 K for variable
treatment times: a 2 hours; b 8 hours
5 Variation of friction coefficient of diamond indenter
during sliding against borided surface of AISI O1 steel at
1123 K during 4 hours and unborided substrate
M. Elias-Espinosa et al. Boriding kinetics and mechanical behaviour of AISI O1 steel
592 Surface Engineering 2015 VOL 31 NO 8

6. of 1 determined from equation (11). To estimate the
boron activation energy for the AISI O1 steel, it is
necessary to plot lnDFe2
B as a function of reciprocal
boriding temperature following the Arrhenius equation
(see Fig. 9). So, the time dependence of boron diffusion
coefficient in the Fe2B layer was obtained from a linear
fitting as follows
DFe2B ¼ 3·03 £ 1022
exp 2197·2 kJmol21
=RT
À Á
ð12Þ
where R ¼ 8?314 J mol21
K21
, and T the absolute
temperature in Kelvin.
The growth kinetics of Fe2B layers proposed by the
diffusion model was also verified by estimating the Fe2B
layers thicknesses for additional boriding conditions.
Figures 10 shows the SEM images of the cross-sections
of the samples borided at 1148 and 1248 K for 1 and
4 hours, respectively. Table 2 compares the value of
boron activation energy for AISI O1 steel with the
values found in the literature for some borided
steels.9,32,42–44
It is concluded that the reported values of
boron activation energy depended on various factors,
such as the nature of boriding agent, the chemical
composition of base steel and the kinetic approach used.
However, some reported values of boron activation
energies in the literature for borided steels are very
different. For indication, the value of boron activation
energy obtained by Ipek et al.45
on the pack borided
AISI 51100 that contains 0?9 % C was 106 kJ mol21
.
This value of energy is found to be lower when
compared to the values listed in Table 2.
This obtained value of boron activation energy for
the AISI 51100 steel45
(with the presence of alloying
elements) is incompatible with the data estimated for the
borided Armco iron.19,25,46
A high value of boron
activation energy is obtained for the alloy steel with
6 Images (SEM) of wear scar on surfaces of AISI O1 steel: a unborided surface and b borided surface at 1123 K for 4 hours
M. Elias-Espinosa et al. Boriding kinetics and mechanical behaviour of AISI O1 steel
Surface Engineering 2015 VOL 31 NO 8 593

7. increasing alloying elements.47
As a consequence,
the boride layer thickness is also reduced because of the
effect of alloying elements.
The value of boron activation energy found in
this work (i.e. 197?2 kJ mol21
) was interpreted for the
borided AISI O1 steel as the amount of energy for the
movement of boron atoms in the easiest path direction
[001] along the boride layer that minimises the growth
stresses. This value of energy can also be interpreted as
the required barrier to allow boron diffusion inside the
7 Images (SEM) of wear scar on surfaces of AISI O1 steel for two boriding conditions: a 1123 K for 4 hours; b 1123 K for
4 hours; c 1273 K for 4 hours; d 1273 K for 4 hours
8 Square of boride layer thickness versus boriding time at
different temperatures
Table 1 Squared value of normalised growth parameter
and growth constants as function of boriding
temperature
Temperature/K
Type of
layer
1 2
(Dimensionless) 41 2
DFe2
B/mm2
s21
1123 Fe2B 1.74714161023
1.3061021
1173 4.0061021
1223 8.0061021
1273 16.2561021
9 Temperature dependence of boron diffusion coefficient in
Fe2B layer
M. Elias-Espinosa et al. Boriding kinetics and mechanical behaviour of AISI O1 steel
594 Surface Engineering 2015 VOL 31 NO 8

8. Table 2 Comparison of boron activation energy of AISI O1 steel with other borided steels
Material Boriding method
Boron activation
energy/kJ mol21
References
AISI 316 Powder 198 [32]
AISI D2 Powder 201.5 [9]
AISI M2 Powder 207 [42]
AISI H13 Powder 186.2 [43]
AISI 420 Powder 206.1 [44]
AISI O1 Powder 197.2 Present study
10 Images (SEM) of boride layers formed at surface of AISI O1 steel for different boriding conditions: a 1148 K for 1 hour;
b 1148 K for 4 hours; c 1248 K for 1 hour; d 1248 K for 4 hours
Table 3 Predicted and experimental values of boride layers thicknesses obtained for different boriding conditions
Temperature/K
Type of
layer
Boride layer
thickness
(mm) estimated
by equation (13)
for exposure
time of 1 hour
Boride layer
thickness
(mm) estimated
by equation (13)
for exposure
time of 4 hours
Experimental boride
layer thickness
(mm) for exposure
time of 1 hour
Experimental
boride layer
thickness
(mm) for exposure
time of 4 hours
1148 Fe2B 28.06 56.12 30.05¡2.4 53.83¡4.9
1248 64.21 128.42 61.37¡5.3 125.97¡8.4
M. Elias-Espinosa et al. Boriding kinetics and mechanical behaviour of AISI O1 steel
Surface Engineering 2015 VOL 31 NO 8 595

9. metal substrate. Thus, the diffusion phenomenon of
boron atoms can occur along the grain boundaries and
also in volume to form the Fe2B layer on the steel’s
substrate.48
From equation (13), the boride layers
thicknesses are described as follows
v ¼
17DFe2Bt
2500
1=2
ð13Þ
The predicted values of boride layers thicknesses (using
equation (13)) are in good agreement with the exper-
imental results, as shown in Table 3. For industrial
applications for this kind of steel, knowledge of the
variables that control the boriding treatment is of great
importance for obtaining the optimum value of Fe2B
layer thickness.
Conclusion
1. In this work, the growth kinetics and some
mechanical properties of Fe2B layers formed at the
surface of AISI O1 steel were investigated in
the temperature range of 1123–1273 K for a vari-
able exposure time between 2 and 8 hours.
2. A simple kinetic model was proposed to evaluate
the boron diffusion coefficient in Fe2B. As a result,
the value of boron activation energy was estimated
as 197?2 kJ mol21
for the AISI O1 steel.
3. Validity of the present model was verified by
comparing the experimental Fe2B layer thicknesses
with the predicted values for the samples borided at
1148 and 1248 K for 1 and 4 hours respectively.
A good agreement was then observed between the
predicted values of Fe2B layers thickness and those
obtained experimentally.
4. The interfacial adherence of the boride layers on
the AISI O1 steel (obtained at 1123 K during 2
and 4 hours), by the Daimler-Benz Rockwell-C
indentation technique, was found to be related to
HF3 category according to VDI 3198 norm.
5. The average friction coefficient for the borided
sample was between 0?398 and 0?375 while the
corresponding value for the unborided substrate
ranged from 0?637 to 0?612.
6. The characteristic wear mechanism for the unborided
surface was plastic deformation; debris and scratch-
ing lines areobserved. For the borided surface of AISI
O1 steel, some pits and scratching lines are noticed.
Acknowledgements
The work described in this paper was supported by a
grant of CONACyT and PROMEP Me´xico. Also, the
authors want to thank Ing. Martı´n Ortiz Granillo, who
is in charge as Director of the Escuela Superior de Ciudad
Sahagu´n, which belongs to the Universidad Auto´noma
del Estado de Hidalgo, Me´xico, for all the facilities to
accomplish this research work.
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