growth kinetics and mechanical properties of fe b layers...

Indian Journal of Engineering & Materials Sciences Vol. 22, April 2015, pp. 231-243

Growth kinetics and mechanical properties of Fe2B layers formed on AISI D2 steel

M Ortiz-Domíngueza, M Elias-Espinosab, M Keddamc*, O A Gómez-Vargasd, R Lewise, E E Vera-Cárdenasf & J Zuno-Silvaa aUniversidad Autónoma del Estado de Hidalgo, Campus Sahagún, Carretera Cd. Sahagún-Otumba s/n, Hidalgo, México

bInstituto Tecnológico y de Estudios Superiores de Monterrey-ITESM Campus Santa Fe, Av. Carlos Lazo No. 100, Del. Álvaro Obregón, CP. 01389, D. F., México

cDépartement de Sciences des Matériaux, Faculté de Génie Mécanique et Génie des Procédés, USTHB, B.P. No. 32, 16111 El-Alia, Bab-Ezzouar, Algiers, Algeria

dInstituto Tecnológico de Tlalnepantla-ITTLA. Av., Instituto Tecnológico, S/N. Col. La Comunidad, Tlalnepantla de Baz. CP. 54070. Estado de México, México

eDepartment of Mechanical Engineering, University of Sheffield, Mappin Street, UK. Sheffield S13JD fUniversidad Politécnica de Pachuca-UPP, Carretera Pachuca-Cd. Sahagún km. 20, Ex Hacienda de Santa Bárbara, CP 43830, Hidalgo, México

Received 15 April 2014; accepted 11 September 2014

In the present study, the growth kinetics and mechanical properties of Fe2B layers formed on the AISI D2 steel are investigated. A mathematical model for studying the growth kinetics of Fe2B

layers on the AISI D2 steel is proposed for the powder-pack boriding. This process is carried out in the temperature range of 1123-1273 K for treatment time ranging from 2 to 8 h. This kinetic model is based on solving the mass balance equation at the (Fe2B/ substrate) interface to evaluate the boron diffusion coefficients through the Fe2B

layers in the temperature range of 1123-1273 K. The formed boride layers are characterized by different experimental techniques such as light optical microscopy, scanning electron microscopy, XRD analysis and the Daimler-Benz Rockwell-C indentation technique. A tribological characterization is also performed using a Plint TE77 tribometer under dry sliding conditions. The boron activation energy for the AISI D2 steel is estimated as 201.5 kJ mol-1 based on our experimental results. This kinetic model is also validated by comparing the experimental Fe2B layer thickness with the predicted value at 1243 K for 5 h of treatment. A contour diagram relating the layer boride thickness to the boriding parameters is suggested to be used in practical applications.

Keywords: Incubation time, Diffusion model, Activation energy, Growth kinetics, Adherence, Tribology

Boriding is a well-known thermochemical surface hardening extensively used for many decades. It refers to a process in which the boron atoms are diffused into a metal substrate to form a hard metallic boride layer on the metal surface1. Boriding provides high wear resistance, corrosion resistance, high temperature oxidation resistance and 3-10 times increasing service life2. Boriding can be achieved with boron in different states such as solid powder, paste, liquid and gas. Different methods of boriding exist such as plasma boriding3, paste plasma boriding4, electrochemical boriding (called PHEB)5 , boriding in a fluidized bed6, gas-boriding7,8, solid boriding (powder or paste)9,10. Among the boriding methods, the pack-boriding has many advantages that make it potentially the most industrially efficient11,12. The boriding of ferrous materials results in the formation of either a single layer (Fe2B) or double-layer (FeB + Fe2B) with definite composition. In a kinetic pint of view, several approaches concerning the kinetics of

formation of Fe2B layers grown on different substrates13-23 were developed in the literature. These models offered the possibility of selecting the optimum boride layers thicknesses for practical applications in the industry.

The powder-pack boriding of AISI D2 steel has been investigated in this study. The growth kinetics of Fe2B layers formed on the AISI D2 steel has been studied. The boron diffusion coefficients through the Fe2B layers are estimated, in the temperature range of 1123-1273 K, using an original diffusion model. In addition, the obtained boron activation energy for the AISI D2 steel is compared with the literature data.

The microstructure of boride layers on AISI D2 steel was investigated by different experimental techniques such as: optical microscopy (OM), scanning electron microscopy (SEM) coupled to EDS analysis and XRD analysis. For the mechanical characterizations, the interfacial adhesion of boride layer on the AISI D2 steel’s substrate and its tribological behavior under dry sliding conditions were also investigated.

————— *Corresponding author (E-mail: [email protected])

INDIAN J. ENG. MATER. SCI., APRIL 2015

232

Diffusion Model Mass balance equation

The diffusion model considers the growth of Fe2B layer on a saturated substrate with boron atoms as displayed in Fig. 1. As boron is added to the surface, it is used completely to convert the Fe phase to 2Fe B .

The ( ( ))f x t function describes the evolution of boron concentration inside the matrix before the nucleation of Fe2B phase. 2Fe B

0t represents the incubation time necessary to form the Fe2B phase at a maximum saturation level of the matrix by the boron atoms. 2Fe B

upC

is the upper limit of boron content in the 2Fe B phase

( 3 360 10 mol m−= × ) and 2Fe BlowC represents the lower

limit of boron content in the 2Fe B phase

( 3 359 8 10 mol m.−−−−= ×= ×= ×= × ). ( ) vx t t= = is the position of

the (Fe2B/substrate) interface. A schematic representation of the 2Fe B

upC and 2Fe BlowC values obtained

from the Fe-B phase diagram for a range of temperatures is given in Fig. 2.

The term BadsC is the effective adsorbed boron

concentration during the boriding process24. In Fig. 1, 2 2Fe B Fe B

1 up lowa C C= − defines the homogeneity range of

the 2Fe B layer, 2Fe B2 low 0a C C= − is the miscibility gap

and C0 is the boron solubility in the matrix considered as null ( 0 0C ≈ 3mol m−−−− )25-27.

Certain assumptions are considered during the formulation of the diffusion model: − The diffusion model is not based on the boron

concentration profile 2Fe B[ ( )]C x t through 2Fe B

layer (Fig. 1). − The growth kinetics is controlled by the boron

diffusion in the 2Fe B layer.

− The 2Fe B iron boride nucleates after a certain period of incubation.

− The boride layer grows because of the boron diffusion perpendicular to the specimen surface.

− Boron concentrations remain constant in the boride layer during the treatment.

− The boride layer is thin compared to the sample thickness.

− A uniform temperature is assumed throughout the sample.

− Planar morphology is assumed for the phase interface.

With these assumptions, the initial and boundary conditions can be written as (Fig. 1): Initial condition:

2Fe B 00, 0, with: [ ( )] 0.t x C x t C= > = ≈

… (1)

Boundary conditions:

Fig. 1—Schematic boron concentration profile through the 2Fe B layer

Fig. 2—Schematic representation of the 2Fe BupC

and 2Fe B

lowC values

obtained from the Fe-B phase diagram for a range of temperatures

ORTIZ-DOMINGUEZ et al.: Fe2B LAYERS FORMED ON AISI D2 STEEL

233

( )2 2

2

Fe B Fe BFe B 0 0 up

B 3 3ads

v

(the upper boron concentration is kept constant),

for 60 10 mol m ,

C x t t C

C −

= = =

> ×

… (2)

2

2

Fe BFe B low

B 3 3ads

[ ( ) v]

(the boron concentration at the interface is kept constant),

for 59.8 10 mol m ,

C x t t C

C −

= = =

< ×

… (3)

where ( )2Fe Bv 0t t t= − is the effective growth time of the

2Fe B layer, t is the treatment time, 2Fe B0t is the boride

incubation time. v0 is a thin layer with a thickness of 5≈ nm that formed during the nucleation stage28.

However, the value of the 0v ( 0)≈ is small in comparison with actual measured thickness of Fe2B layer ( v ). Given the aforementioned conditions, and taking into account that the boride layer thickness v is governed by the parabolic

growth law ( )2

2 2

1/2Fe B1/2 1/2 1/2Fe B Fe B v 0v 2 2D t D t tε ε= = + ,

where ε is the normalized growth parameter of 2Fe B that will occur by simultaneous consumption of substrate at the (Fe2B/substrate) interface29.

2Fe BD denotes the diffusion coefficient of boron in the

Fe2B phase. By applying the principle of mass conservation at the (Fe2B/substrate) interface30,31, Eq. (4) is obtained as:

2 2

2

Fe B Fe Bup low 0

Fe B Fe

2

2

( v) ( v) ( ) ( v v) ( ),

C C C

A d J x A dt J x d A dt

+ −

⋅ = = ⋅ − = + ⋅

… (4)

where ( 1 1)A = ⋅ is defined as the unit area. The input

and output fluxes of boron atoms in the ( 2Fe B /substrate) interface during a lapse of time dt are defined as:

( )2 2 2Fe B Fe B Fe B vv { [ ( )] / }xJ x D dC x t dx == = −

, and ( )Fe Fe Fe v vv v { [ ( )] / }x dJ x d D dC x t dx +== + = −

respectively. Thus, Eq. (4) can be rewritten as:

2 2

2

2

Fe B Fe Bup low 0

Fe BFe B

( ) v ( ) v

2

2

[ ( )]( )

x t x t

C C C

dC x tdx tD

dt dx= =

+ −

= −

… (5)

Using the chain rule on the right term of Eq. (5) results in:

( )

2 2

2 2

Fe B Fe Bup low 0

2

Fe B Fe B ( ) v( ) v

2

2

( ) / [ ( )] .x tx t

C C C

dx t dt dt D dC x t==

+ −

= −

… (6)

Equation (7) can be obtained by replacing the derivative of parabolic growth law (

2

1/2 1/2Fe Bv ( ) 2x t D tε= = ) with respect to the time t into

Eq. (6).

2 2

2

Fe B Fe Bup low 0 2

Fe B ( ) v

2[ ( )] .

2 x t

C C C dtdC x t

tε

=

+ −= −

… (7)

A schematic representation of the square of the layer thickness as a function of the boriding time is depicted in Fig. 3.

Now, integrating both sides of Eq. (8) between limits of 2Fe B

0t to t and 2Fe BupC to 2Fe B

lowC , respectively on gets:

2 2

Fe B2low

2Fe B Fe B2 2

up0

Fe B Fe Bup low 0

2Fe B ( ) v

2

2

[ ( )] .Ct

x t

Ct

C C C

dtdC x t

tε

=

+ −

= −∫ ∫

… (8)

Fig. 3—Schematic representation of the square of the layer thickness as a function of the boriding time


234

The following solution was derived:

( ) ( )2 2 2 2 2Fe B Fe B Fe B Fe B Fe B2up low up low 0 02 / 2 / ,/C C C C C ln t tε = − + −

… (9) where ε is known as the normalized growth parameter

for the ( 2Fe B /substrate) interface, it is a dimensionless

parameter. It is assumed that expressions 2Fe BupC , 2Fe B

lowC ,

and C0, do not depend significantly on temperature (in the temperature range applied)28. Experimental Procedure

Boriding process

The material to be pack-borided was the AISI D2 steel. It had a nominal chemical composition of 1.40-1.60% C, 0.30-0.60% Si, 0.30-0.60% Mn, 11.00-13.00% Cr, 0.70-1.20% Mo, 0.80-1.10% V, 0.030% P and 0.030% S. The samples were sectioned into cubes with dimensions of 10 mm × 10 mm × 10 mm Before boriding, the samples were polished, ultrasonically cleaned in an alcohol solution and deionized water for 15 min at room temperature. Finally, the samples were dried and stored under clean-room conditions. The mean hardness was 287 HV. The samples were

embedded in a closed cylindrical case made of AISI 304L steel containing a fresh Durborid powder mixture. The powder boriding medium had an average particle size of 30 µm. This boriding agent was composed of an active source of boron (boron carbide-B4C), an inert filler (silicon carbide-SiC), and an activator (potassium fluoroborate-KBF4). The active boron is then supplied by the powder quantity placed over and around the material surface. The powder-pack boriding process was carried out in a conventional furnace under a pure argon atmosphere. The boriding process was carried out in the temperature range of 1123-1273 K for a variable time (2, 4, 6 and 8 h). The boriding temperatures were selected in accordance with the position of the solidus line in the Fe-B phase diagram26. Once the treatment was finished, the container was removed from the furnace and slowly cooled to room temperature. Microscopical observations of boride layers

The borided samples were sectioned for metallographic observations using a LECO VC-50 cutting precision machine. The cross-sectional morphology of the boride layers was viewed with the Olympus GX51 optical microscope in a clear field. Figure 4 gives the cross-sections of boride layers

Fig. 4—Optical micrographs of the cross-sections of borided AISI D2 steel at 1173 K for different treatment times: (a) 2 h, (b) 4 h, (c) 6 h and (d) 8 h


235

formed on the AISI D2 steel at 1173 K for different treatment times. The obtained microstructure is composed of a single phase layer (Fe2B) at the surface of AISI D2 steel. The formed layers appear to be compact and homogeneous, exhibiting a flat morphology. This peculiar morphology is attributed to the effect of alloying elements present in the AISI D2 steel. These elements tend to concentrate in the tips of boride layers, reducing the boron flux in this zone. It is noticed that the boride layer thickness increased with a change in the boriding time (Fig. 3). The boride layer thickness was automatically measured with the aid of MSQ PLUS software. To ensure the reproducibility of the measured layers, fifty measurements were collected in different sections of the borided samples to estimate the 2Fe B layer thickness; defined as an average value of the long boride teeth32,33.

All thickness measurements were taken from a fixed reference on the surface of the borided AISI D2 steel, as illustrated in Fig. 5. The phases present in the boride layers were identified by an X-ray diffraction (XRD) equipment (Equinox 2000) using αCoK radiation of 0.179 nm wavelength. The distribution of elements within the cross-section of boride layer was determined by electron dispersive spectroscopy (EDS) equipment (JEOL JSM 6300 LV) from the surface. The Daimler-Benz Rocwell-C was made to get a qualitative information on the adhesive 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 compounds34-36. The principle of this method was reported in the reference work36. A load of 1471 N was applied to cause coating damage adjacent to the boundary of the indentation. Three indentations were conducted for each borided sample and scanning electron microscopy (SEM) was used to assess the adhesion test.

The microhardness of the borided surface was measured at 5 different locations by means of a Vickers indenter with a load of 50 g and the average value was taken as representative of the hardness. The roughness values of borided and unborided samples were measured using a Mitutoyo Surftest Profilometer.

The tribological tests were carried out in dry sliding conditions using a PLINT TE77 high frequency friction machine (see Fig. 6). This machine is used to assess the dynamic wear and friction performance of lubricants, materials and surface coatings37. The contact consists of a fixed disc and reciprocating ball. The ball is mounted on the carrier head that is mechanically oscillated against the lower fixed specimen (disc). The normal load is applied via a spring balance through lever and stirrup mechanism. The force is transmitted directly onto the carrier head by means of a needle roller cam follower on the carrier head and a running plate on the loading stirrup. The oscillations are produced by a motor with an eccentric cam, scotch yoke and guide block arrangement. The fixed specimen is clamped to a block. The assembly is mounted on flexible supports that allow free movement in horizontal directions, but no movement vertically. This is connected to a stiff force transducer that measures tangential force in both directions.

AISI 52100 steel balls, commonly employed in the bearing industry, were used to slide against the surface of the borided AISI D2 steel. The balls used had a diameter of 4.75 mm and a microhardness of 850 HV. On the other hand, the stationary samples

Fig. 5—Schematic diagram illustrating the procedure used to estimate the Fe2B layer thickness

Fig. 6—The PLINT TE77 high frequency wear friction machine (1: Load meter, 2: Loading stirrup, 3: Force transducer, 4: Heater block, 5: Oscillation mechanism, 6: Motor)


236

had a disc shape with a diameter of 18 mm and a thickness of 3 mm. Before all the tests, the ball and disc were cleaned from any residue oxide layer or machining lubricant by washing in ethanol using an ultrasonic bath (Fisherbrand 11020). The disc and ball were placed as shown in the simplified schematic diagram (see Fig. 7)

Then, test parameters such as load, frequency and stroke were selected and introduced into the computer. The maximum contact pressure was selected in such a way that wear would be produced with a low number of cycles. A Labview program collects the data generated, which was basically the friction coefficient versus time. It was possible to control the test parameters and follow the progression of the friction during every test on the screen. The tests were run to 36000 cycles. This was predetermined with several preliminary tests, to know how many cycles were necessary to cause damage on the surfaces. Finally, three experiments were carried out for each test type. Table 1 shows the operating conditions of the tests conducted.

Results and Discussion

SEM observations and EDS analysis

Figure 8(a) gives the cross-section of boride layer formed on the AISI D2 steel at 1223 K for 8 h using SEM. The obtained microstructure exhibited a flat morphology because of the alloying elements present in the AISI D2 steel. Figure 8(b) shows the EDS analysis obtained at the surface of borided AISI D2 steel. It indicates the presence of three substitutional elements: Cr, V and Mn as well as Fe. In fact,

chromium element can dissolve in the Fe2B phase by occupying the substitutional sites in the Fe sublattice. Figure 8(c) provides the EDS analysis obtained at the (Fe2B/substrate) interface. It reveals that carbon and silicon are being displaced towards the diffusion zone by forming together with boron element, solid solutions like silicoborides (FeSi04B06) and Fe5SiB2) and boroncementite (Fe3B0.67C0.33)

28, 38.

X-ray diffraction analysis

Figure 9 shows the XRD pattern obtained at the surface of borided AISI D2 steel at 1273 K for 8 h. It indicates the presence of Fe2B and interstitials compounds such as CrB, Cr2B, MoB, Mo4B2 and V2B because of the affinity of boron for these substitutional elements. It is known that the growth of Fe2B layer presents a highly anisotropic nature. The [001] direction is the easiest path for the boron diffusion through the Fe2B phase, due to the tendency of boride crystals to grow along a direction of minimum resistance, perpendicular to the external surface.

Rockwell-C adhesion test

An indenter hardness tester was used to assess the Daimler-Benz Rockwell-C adhesion, as a destructive quality test for examined layers; it was employed for determination of cohesion. The well-known adhesion test prescribed by the VDI 3198 norm was used34. The principle of this method was presented in Fig. 10. 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.

Table 1—Test operating conditions.

Test atmosphere Hertzian pressure (GPa) Load(N) Frequency (Hz) Amplitude (mm) Cycles

293-296 K and 45% - 50% Relative humidity 2.01 20 10 15 36000

Fig. 7—Simplified schematic diagram of high frequency friction machine (1: Friction force transducer, 2: Ball, 3: Disc, 4: Heater block, 5: Roller, 6: Normal load, 7: Oscillator driver)


237

The damage to the boride layer was compared with the adhesion strength quality maps HF1-HF6 (see Fig. 10). In general, the adhesion strength HF1 to HF4 are defined as sufficient adhesion, whereas HF5 and HF6 represent insufficient adhesion (HF is the German short form of adhesion strength)34. SEM image of the indentation craters for the samples borided at 1273 K for 4 h is shown in Fig. 11. The indentation craters obtained on the surface of borided AISI D2 steel revealed that there were radial cracks at the perimeter of indentation craters. However, a small quantity of spots with flaking resulting from delamination was visible and the adhesion strength quality of this boride layer is related to the HF5 standard.

Tribological characterization

For tribological study, the borided sample at 1273 K for 2 h of treatment was used. The roughness of the

Fig. 8—(a) SEM micrograph of the cross-section of borided AISI D2 steel at 1223 K for 8 h, (b) EDS spectrum obtained at the surface of borided sample and (c) EDS spectrum obtained at the (boride layer/substrate) interface

Fig. 10—Principle of the VDI 3198 indentation test34

Fig. 9—XRD patterns obtained at the surface of borided AISI D2 steel at 1273 K during 8 h of treatment


238

unborided surface was 0.154 while the borided surface had a value of 0.154 µm.

The surface hardness of substrate was 287 HV while the surface microhardness reached a value of 2322 HV. Figure 12 shows the variation in the friction coefficient of the borided and unborided surfaces under dry sliding conditions against a steel ball. The image shows that the borided sample exhibits a friction coefficient lower than that of the unborided substrate. The average friction coefficient for the unborided sample was between 0.292 and 0.363 and for the borided sample, the average frictton coefficient ranged from 0.245 to 0.263. These results

are in agreement with those obtained by other researchers39- 41.

The tribological tests caused wear scars on the flat specimens (discs). There were measurable grooves on the discs. The wear depth of each groove was measured using a Mitutoyo Surftest Profilometer. Due to the depth varying along the length of the groove a number of depth measurements (transverse to the length of the groove) were taken. The experimental average depth was taken from these measurements. The volume of a ‘perfect groove’ could be calculated from this information as done in earlier studies42. The profiles shown in Fig. 13(a) and (b) show grooved features, demonstrating the two-body wear mechanism. Figure 13(a) indicates that the unborided AISI D2 steel’s surface was more severely worn compared to the borided surface as shown in Figure 13(b).

Furthermore, the measured wear volumes for the unborided and borided surfaces on AISI D2 steels show a remarkable difference, as apparent in Fig. 14. The specific wear rate of the unborided sample was 1.02×10–5 mm3/Nm, whereas the value of specific wear rate for the borided sample at 1273 K during 2 h

Fig. 11—SEM micrograph showing the indentation of VDI adhesion test on the surface of AISI D2 steel borided at 1273 K for 4 h

Fig. 12—Variation of friction coefficient of steel balls during sliding against borided surface for 2 h and unborided substrate

Fig. 13—Wear scar depth of the AISI 52100 steel’s ball on: (a) unborided AISI D2 steel and (b) borided surface at temperature 1273 K during 2 h


239

was approximately 1.56×10–6 mm3/Nm. This difference can be attributed to the low resistance of the unborided surface against sliding wear.

Figures 15(a) and (b) show the SEM images of the borided surfaces obtained at 1273 K with exposure time of 2 h. Figures 15(c) and (d) give the SEM images of unborided surfaces where wear scars are produced during sliding contact against a steel ball. In Figure 15(a) and (b) a defined wear scar is produced, which has a width of approximately 0.57 mm. Some pits and spalls are observed and common wear mechanisms like plastic deformation, cracks and scratching were formed on the borided surface. Figures 15(c) and (d) shows the wear scar formed on the unborided surface, which has a width of approximately 1.45 mm. Likewise, some pits and spalls are observed and common wear mechanisms like plastic deformation, cracks and scratching were formed on the unborided surface.

Estimation of boron activation energy

The evolution of 2Fe B layers thicknesses as a function of the boriding time is required for

estimating the boron diffusion coefficients through the 2Fe B layers by applying the suggested diffusion model. Figure 16 describes the time dependence of the squared value of 2Fe B layer thickness for different temperatures. The slopes of the straight lines

Fig. 14—Variation of wear volume against the boriding condition

Fig. 15—SEM micrographs of wear scar on surfaces of the AISI D2 steel: (a) and (b) borided surface at 1273 K for 2 h; (c) and (d) unborided surface.


240

in Fig. 16 provide the values of growth constants

2

2Fe B( 4 )Dε= . These values can be obtained by a

linear fitting through origin at each boriding temperature. Table 2 provides the estimated value of boron diffusion coefficient in 2Fe B at each temperature along with the squared normalized value of ε parameter determined from Eq. (9).

The results, which are summarized in Table 2, reflect a diffusion-controlled growth of the boride layers.

By combining the results (square of normalized growth parameter ( 2ε ) and growth constants (

2

2Fe B4 Dε )) presented in Table 2, the boron diffusion

coefficient in the 2Fe B layers (2Fe BD ) was estimated

for each boriding temperature. So, an Arrhenius equation relating the boron diffusion coefficient to the boriding temperature can be assumed. As a result, the boron activation energy (

2Fe BQ ) can be determined

from the slope of the straight line shown in coordinate system:

2Fe BlnD as a function of reciprocal boriding

temperature (see Fig. 17).

The boron diffusion coefficient through 2Fe B layers was deducted as:

( )2

2 1Fe B 4.4 10 201.5 kJmol / ,D exp RT

− −= × −

… (10)

where 314.8=R Jmol-1K-1 and T the absolute temperature in Kelvin. From Eq. (10), the pre-exponential factor ( 2 2 1

0 4.4 10 m sD− −

= × ) and the

activation energy ( 1201.5 kJmolQ −= ) values are affected by the contact surface between the boriding medium and the substrate, as well as the chemical composition of the substrate.

Figure 18 shows the optical micrograph of the cross-section of the AISI D2 steel’s sample borided at 1243 K for 5 h.

Table 3 compares the value of boron activation energy for the AISI D2 steel with the values from the literature data43-47 for some borided steels. The boron activation energies, found for the powder method are found to be comparable. The obtained value of boron activation energy for the AISI D2 steel43 by the electrochemical method is very low to the value estimated in the present study. It is concluded that the reported values of boron activation energy depend on various factors such as: (the boriding method, the chemical composition of boriding agent, the chemical composition of base steel and mechanism of boron diffusion).

The time dependence of Fe2B layer thickness is given by Eq. (11) as follows:

Table 2—The square of normalized growth parameter and growth constants as a function of the boriding temperature

Temperature (K) Type of layer ε2

(Dimensionless) 4ε2 DFe2B

(µm2s-1)

1123 1.19×10−1 1173 3.79×10−1 1223 7.32×10−1 1273

Fe2B 1.689625×10−3

16.0×−1

Fig. 16—The square of Fe2B layer thickness as a function of the boriding time

Fig. 17—Arrhenius relationship for the boron diffusion coefficient through the 2Fe B layer


241

( )( )( )

2 2

2

2 2 2 2

Fe B Fe BFe B up low1/2 1/2

Fe B Fe B Fe B Fe B0 up low 0

8v 2

/ 2

D t C CD t

ln t t C C Cε

−= =

+ −

… (11) The results obtained by Eq. (11) are in good agreement with the experimental results given in Table 4.

An iso-thickness diagram was also plotted as a function of the boriding temperature and the treatment time as shown in Figure 19. It can be used as a simple tool to select the optimum value of Fe2B layer thickness in relation with the potential applications of the borided AISI D2 steel in the industry.

As a rule, thin layers (e.g. 15-20 µm) are used to protect against adhesive wear (such as chipless 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 ceramic industry). In the case of low carbon steels and low alloy steels, the optimum boride layer thicknesses range from 50 to 250 µm, and for high alloy steels, the optimum boride

layer thicknesses are between 25 and 76 µm. In addition, this model can be extended to predict the growth kinetics of (FeB+ Fe2B) at the surface of different ferrous alloys.

Conclusions In this work, the AISI D2 steel was pack-borided in

the temperature range of 1123-1273 K for a variable treatment time ranging from 2 to 8 h. The following conclusions are drawn from this study:

(i) A single Fe2B layers was formed at the surface of AISI D2 steel for the given boriding conditions.

(ii) The growth kinetics of Fe2B layers followed a parabolic growth law.

(iii) A kinetic model was suggested to estimate the boron diffusion coefficient through the Fe2B layer.

(iv) The boron activation energy was estimated as 201.5 kJ mol-1 for the AISI D2 steel.

(v) Validation of the diffusion model was made by comparing the experimental Fe2B layer thickness with the predicted value for the

Fig. 19—Iso-thickness diagram giving the Fe2B layer thickness as a function of the boriding parameters

Table 4—Predicted and experimental values of the Fe2B layer thickness at 1243 K for 5 h of treatment

Temperature (K)

Type of layer Boride layer thickness (µm))

estimated by Eq. (11)

Experimental boride layer

thickness (µm)

1243 Fe2B 128.1 125.97±8.4

Fig. 18—Optical micrograph of the boride layer formed on the AISI D2 steel at 1243 K for 5 h Table 3—Comparison of boron activation energy of AISI D2 steel

with other steels using different boriding methods

Material Boriding method

Boron activation energy (kJ mol -1)

References

AISI D2 AISI H13 AISI 316 AISI M2 AISI H13 AISI D2

Electrochemical Salt-bath Powder Powder Powder Powder

137.66 244.37

198 207

213.92 201.5

[43] [ 44] [ 45] [ 46] [47]

Present study


242

borided AISI D2 at 1243 K for 5 h. A good concordance was obtained between the experimental result and the predicted one.

(vi) An iso-thickness diagram relating the boride layer thickness to the boriding parameters (time and temperature) was suggested for the practical use of this kind of steel. This contour diagram can serve as a simple tool to the select the optimum value of Fe2B layer thickness.

(vii) The interfacial adherence of the boride layer on AISI D2 steel (obtained at 1273 K during 4 h), by the Daimler-Benz Rockwell-C indentation technique, was found to be related to HF5 category according to the VDI 3198 norm.

(viii) The average microhardness value of the unborided surface was approximately 10 times lower than that of the borided surface at 1273 K for 2 h.

(ix) The average friction coefficient value for the unborided surface was a slightly greater than that of the borided surface.

(x) Sliding wear resistance for the borided surface was 7-6 times greater than that of the unborided surface.

(xi) The wear mechanism for the borided surface was plastic deformation, cracking and abrasion scratching lines while cracking and abrasion scratching lines were observed in the case of unborided surface.

Acknowledgements

The work described in this paper was supported by a grant of PROMEP, CONACyT México and The Tribology Group of The University of Sheffield, UK. The authors wish to thank Ing. Martín Ortiz Granillo, Director de la Escuela Superior Campus Ciudad Sahagún-Universidad Autónoma del Estado de Hidalgo for his valuable collaboration for this study.

References

1 Sinha A K, J Heat Treatment, 4 (1991) 437. 2 Davis J R, Surface Hardening of Steels Understanding the

Basics, (ASM International; USA), 2002, p. 213-223. 3 Nam Kee-Seok, Lee Ku-Hyun, Lee Deuk Yong & Song Yo-

Seung, Surf Coat Technol, 197 (2005) 51. 4 Gunes I, Taktak S, Bindal C, Yalcin Y, Ulker S & Kayali Y,

Sadhana-Acad P Eng S, 38(3) (2013) 513. 5 Kartal G, Timur S, Sista V, Eryilmaz O L & Erdemir A, Surf

Coat Technol, 206 (2011) 2005.

6 Anthymidis K G, Zinoviadis P, Roussos D & Tsipas D N, Mater Res Bull, 37 (3) (2002) 515..

7 Kulka M, Makuch N, Pertek A, Maldzinski L, J Solid State

Chem, 199 (2013) 196. 8 Keddam M, Kulka M, Makuch N, Pertek A & Małdziński L,

Appl Surf Sci, 298 (2014) 155. 9 Nait Abdellah Z & Keddam M, Mater Technol, 48 (2014) 237.

10 Campos I, Oseguera J, Figueroa U, Garcia J A, Bautista O & Kelemenis G, Mater Sci Eng A, 352 (2003) 261.

11 Keddam M, Chentouf S M, Appl Surf Sci, 252 (2005) 393. 12 Bektes M, Calik A, Ucar N & Keddam M, Mater Charact,

61 (2010) 233. 13 Campos-Silva I, López-Perrusquia N, Ortiz-Domínguez M,

Figueroa- López U, Gómez-Vargas O A, Meneses-Amador A & Rodríguez-Castro G, Kovove Mater, 47 (2009) 75.

14 Keddam M, Ortiz-Domínguez M, Campos-Silva I & Martinez-Trinídad J, Appl Surf Sci, 256 (2010) 3128.

15 Ortiz-Domínguez M, Hernandez-Sanchez E, Martinez-Trinídad J, Keddam M & Campos-Silva I, Kovove Mater, 48 (2010) 1.

16 Campos-Silva I & Ortíz-Domínguez M, Int J Microstruct

Mater Properties, 5 (2010) 26. 17 Keddam M & Chegroune R, Appl Surf Sci, 256 (2010) 5025. 18 Campos-Silva I, Ortiz-Domínguez M, Cimenoglu

H, Escobar-Galindo R, Keddam M, Elías-Espinosa M & López-Perrusquia N, Surf Eng, 27 (2011) 189.

19 Ortiz-Domínguez M, Campos-Silva I, Hernández-Sánchez E, Nava-Sánchez J L, Martínez-Trinidad, Jiménez-Reyes M Y & Damián-Mejía O, Int J Mater Res, 102 (2011) 429.

20 Mebarek B, Bouaziz S A & Zanoun A, Mater Tech, 100 (2012) 167.

21 Nait Abdellah Z, Keddam M, Chegroune R, Bouarour B, Lillia H & Elias A, Matér Tech, 100 (2012) 581.

22 Nait Abdellah Z, Keddam M & Elias A, Int J Mater Res, 104 (2013) 260.

23 23 López Perrusquia N, Antonio Doñu Ruiz M, Vargas Oliva E Y & Cortez Suarez V, Mater Res Soc Symp Proc,

1481 (2013) 105. 24 Yu L G, Chen X J, Khor K A & Sundararajan G, Acta Mater,

53 (2005) 2361. 25 Massalski T B, Binary Alloy Phase Diagrams, (ASM

International, Materials Park, Ohio, USA), 1990, p. 280. 26 Okamoto H, J Phase Equilibria Diffus, 25 (2004) 297. 27 Nait Abdellah Z, Chegroune R, Keddam M, Bouarour B,

Haddour L & Elias A, Defect Diffus Forum, 322 (2012) 1. 28 Brakman C M, Gommers A W J & Mittemeijer E J, J Mater

Res, 4 (1989) 1354. 29 Dybkov V I, Reaction Diffusion and Solid State Chemical

Kinetics, (Trans Tech Publications, Switzerland-UK-USA), 2010, p 7.

30 Jost W, Diffusion in Solids, Liquids, Gases, (Academic Press Inc; New York), 1960, p 69-72.

31 Shewmon P, Diffusion in Solids, (Minerals, Metals and Materials Society; USA), 1989, p. 40.

32 Kunst H, Schaaber O, Härterei-Tech Mitt, 22 (1967) 275. 33 Domínguez M Ortiz, Contribución de la Modelación

Matemática en el Tratamiento Termoquímico de

Borurización, Ph D thesis, SEPI-ESIME from the Instituto Politécnico Nacional, México, 2013.


243

34 Verein Deutscher Ingenieure Normen VDI 3198, (VDI-Verlag; Düsseldorf), 1991, p 1-8.

35 Vidakis N, Antoniadis A & Bilalis N, J Mater Process

Technol, 143-144 (2003) 481. 36 Taktak S, Mater Des, 28 (2007) 1836. 37 Kanakia M D, Cuellar Jr. J P & Lestz S J, Development of

Fuel Wear Tests Using the Cameron-Plint High Frequency

Reciprocating Machine, Belvoir Fuels and Lubricants Research Facility SwRI Report No. BFLRF 262, May 1989.

38 Dukarevich I S, Mozharov M V, Shigarev A S, Metalloved

Term Obrab Met, 2 (1973) 64. 39 Martini C, Palombarini G, Poli G & Prandstraller D, Wear,

256 (2004) 608. 40 Taktak S, Surf Coat Technol , 201 (2006) 2230. 41 Garcia-Bustos E, Figueroa-Guadarrama M A, Rodríguez-

Castro G A, Gómez-Vargas O A, Gallardo-Hernández E A, Campos-Silva I, Surf Coat Technol, 215 (2013) 241.

42 Cardenas E E Vera, A study of rolling contact fatigue and

reciprocated sliding wear on AISI 4320, 8620, 4140 and 01

steel substrates and hard coatings TiN, CrN and WC/C, Ph D Thesis, SEPI-ESIME from the Instituto Politécnico Nacional, México, 2009.

43 Sista V, Kahvecioglu O, Eryilmaz O L, Erdemir A & Timur S, Thin Solid Films, 520 (2011) 1582.

44 Taktak S, J Mater Sci, 41 (2006) 7590.

45 Campos-Silva I, Ortiz-Domínguez M, Bravo-Bárcenas O, Doñu-Ruiz M A, Bravo-Bárcenas D, Tapia-Quintero C & Jiménez-Reyes M Y, Surf Coat Technol, 205 (2010) 403.

46 Campos-Silva I, Ortiz-Domínguez M, Tapia-Quintero C, Rodriguez-Castro G, Jimenez-Reyes M Y & Chavez-Gutierrez E, J Mater Eng Perform, 21 (2012) 1714.

47 Xu L, Wu X & Wang H, J Mater Sci Technol, 23 (4) (2007) 525

growth kinetics and mechanical properties of fe b layers...

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