Indian Journal of Engineering & Materials Sciences

Vol. 24, October 2017, pp.362-368

Boriding kinetics and mechanical properties of borided commercial-purity nickel

A Calika, N Ucar

b*, K Delikanli

a, M Carkci

a & S. Karakas

c

aDepartment of Manufacturing Engineering, Suleyman Demirel University, 32260 Isparta, Turkey bDepartment of Physics, Suleyman Demirel University, 32260 Isparta, Turkey

cDepartment of Materials Science and Engineering, Çankaya University, 06810 Ankara, Turkey

Received 15 February 2016; accepted 17 March 2017

Kinetics of boride layer growth and tensile behaviour in borided commercial-purity nickel was investigated. Boriding

was carried out in a solid medium consisting of Ekabor-II powders at 1173, 1223 and 1273 K for periods of 3, 5 and 8 h.

Scanning electron microscopy (SEM) and optical microscopy showed column morphology in the boride layer. X-ray

diffraction (XRD) analyses indicated that the boride layer formed on the surface consisted mainly of Ni2B, with precipitates

of Ni6Si2B. A parabolic relationship between layer thickness and processing temperature was observed. The obtained results

showed that although the boride layer thickness increased with increasing boriding temperature and time, boriding

parameters had no significant effect on the hardness of the boride layer or the matrix. Tensile properties were negatively

influenced by the boriding treatment; both yield and tensile strength values decreased due to the presence of the hard yet

brittle surface coating. In addition, the growth kinetics of boride layers was also analysed. The results showed a nearly

parabolic relationship between the layer thickness and the process temperature, with activation energy of 47.3 kJ mol−1.

Keywords: Ni, Boriding, Powder pack, Boride layer, Hardness

Nickel and nickel-based alloys are widely used in

chemical, energy conversion, power production,

waste incineration, pharmaceutical and many other

industries due to their good corrosion resistance in

extreme temperatures and aqueous environments1-3

.

However, pure nickel is not considered for

applications where wear resistance is of primary

concern4. In order to improve its mechanical

properties, many surface treatments have been

suggested5,6

. One of these surface treatments is

boriding, a thermo-chemical surface hardening

treatment in which boron diffuses into the surface of

the work-piece to form hard borides with the base

material7-11

. Corresponding to this, it has been shown

that the formation of the hard metal borides on the

surface provides desirable results in terms of wear

resistance12-14

.

Mua et al.12

studied the effects of boriding time and

temperature on the boriding of 99.9% pure nickel.

Boriding was performed by means of the powder-

pack method using commercial LSB-II powders

(containing SiC as a diluent). Boride (Ni2B) and

silicide (Ni5Si2, Ni2Si) phases were detected on the

surface of the borided specimens. They also observed

that depending on boriding time and temperature, the

thickness of the coating ranged from 36 to 237 µm.

The hardness values were 832 HV0.01 for the silicide

layer, 984 HV0.01 for boride layer, and 139 HV0.01 for

the Ni substrate. The same phases were also observed

by Ozbek et al.15

in borided 99.5% purity nickel, and

the microhardness of silicides formed on the surface

of the nickel substrate were reported to reach hardness

values of up to 805 HV. These two studies indicate

that borided metals, owing to their high surface

hardness and corrosion resistance, are potential

candidate materials for various industrial applications

as well as for biomedical applications such as joint

arthroplasty. In addition to these studies, in the

literature3,12,16

, comprehensive studies have been

attempted to understand the boriding parameter

effects on the hardness, boride layer thickness and

phases formed boride layer of pure Ni samples.

However, current knowledge concerning boride layer

growth kinetics on the surface of pure Ni is limited.

In the present work, commercial-purity Ni was

borided using the powder-pack method and the effect

of boriding time on both microhardness and plastic

deformation has been studied in detail. Also, growth

kinetics such as average activation energy (Q) and

growth rate constant (K) for the diffusion of boron in

commercial purity Ni were studied by measuring the

thickness of the boride layer. ____________

*Corresponding author (E-mail: nazimucar@sdu.edu.tr)

CALIK et al.: BORIDING KINETICS AND MECHANICAL PROPERTIES OF BORIDED COMMERCIAL-PURITY NICKEL

363

Experimental Procedure The chemical composition of the commercial-purity

Ni substrate is given in Table 1. The samples were

initially cut to dimensions 10 mm×10 mm×50 mm. The

boriding of the samples was then achieved in a solid

medium using commercial Ekabor-II powder (BorTec

GmbH) at 1173, 1223 and 1273 K. The samples were

packed with the boriding powders in stainless steel

containers and borided in an electrical resistance furnace

for exposure times of 3, 5 and 8 h under atmospheric

pressure. After boriding the containers were removed

from the furnace and cooled in air.

The presence of borides on the surface of the

borided commercial-purity Ni samples was

determined using an X-ray diffractometer (Rigaku

D-MAX 2200) with Cu-Kα radiation with 0.15418 nm

wavelength. Metallographic sections were prepared

by sectioning the samples from one side, grinding up

to 1200-grid emery paper, and polishing using 3 µm

alumina paste. The specimens were then viewed in

their as-polished state using optical and scanning

electron microscopy (SEM JEOL 5600LV). The

thickness of borides was measured by means of a

digital thickness measuring instrument attached to an

optical microscope. The boride layer thickness values

given in the results section are averages of at least 20

measurements. To determine the hardness of the

borided commercial-purity Ni, a Vickers

microhardness tester with a load of 100 g was used.

Many indentations were made across each coating

under each experimental condition to check the

reproducibility of hardness data.

Borided and untreated samples were pulled in

tension with a 50 kN-capacity Instron tension-

compression testing machine, with a specimen gauge

length of 15 mm. The tests were performed at a strain

rate of 10-6

s−1

at room temperature. Load and

elongation values were recorded during tensile testing

and converted into engineering stress–strain curves.

Ultimate tensile strength (maximum stress on the

stress–strain curve), yield strength (stress at 0.2%

offset strain) and ductility values were determined.

Results and Discussion

Microstructural characterization

As seen in Figs 1-3, SEM and optical examinations

revealed that the borides formed on the surface of

commercial purity Ni samples had columnar

morphology. Moreover, two distinct regions were

identified on the surface of borided commercial-purity

Ni samples; the boride layer and the substrate.

It can be seen from Figs 1-3 that the boride layer

additionally contains small precipitates displaying

dark contrast. The precipitates appear mainly

localized at the surface and decrease in number

towards the interior. The overall boride layer

thickness reached a value of 104 µm at 3 h, while it

Table 1 — Chemical composition of the commercial-purity Ni

in mass fractions, wt%

Ni C Mn Fe Si Cu

99.5 0.08 0.2 0.1 0.08 0.13

Fig. 1 — SEM cross-section view of borided commercial-purity

Ni (a) 3 h, (b) 5 h, and (c) 8 h for 1173 K

INDIAN J. ENG. MATER. SCI., OCTOBER 2017

364

attained a value of 179 µm and 237 µm at 5 and 8 h,

respectively, at 1173 K. These values are 141, 182,

238 µm at 1223 K and 165, 185 and 240 µm at 1273

K for borided pure Ni, respectively. These results

show that longer boriding durations and higher

temperatures resulted in greater boride layer

thickness due to boron diffusion. Similar results

have been also obtained in other studies17-20

.

Figure 4 shows the time dependence of the squared

value of boride layer thickness at increasing

temperatures. The relationship is linear at all

temperatures, suggesting simple diffusion-

controlled growth behavior. This behaviour of pure

Ni is similar to that of borided steels17,20,21

.

Corresponding to this, a mathematical model of the

growth kinetics of boride layers on gray cast iron22

and AISI 316 stainless steel23

was proposed for the

powder-pack boriding. This diffusion model was

able to estimate the thickness of boride layers and

predict the boron-depth-concentration profiles for

boride phases as a function of time, temperature

and boron surface concentration.

In the present study, the kinetics of boride layer

growth was analyzed taking into account the classical

Fig. 3 — SEM cross-section view of borided commercial-purity

Ni (a) 3 h, (b) 5 h, and (c) 8 h for 1273 K

Fig. 2 — SEM cross-section view of borided commercial-purity

Ni (a) 3 h, (b) 5 h, and (c) 8 h for 1223 K

CALIK et al.: BORIDING KINETICS AND MECHANICAL PROPERTIES OF BORIDED COMMERCIAL-PURITY NICKEL

365

kinetic method based on the Arrhenius equation. Most

diffusion processes obey a parabolic law

described by

Ktd =2

... (1)

where d is the thickness of the boride layer (mm),

t is the boriding time (s) and K is the growth rate

constant, respectively. From Eq. (1), the obtained

effective growth rate constants with respect to boriding

temperatures of 1173, 1223 and 1273 K are 2.51×10−8

cm2 s

−1, 1.98×10

−8 cm

2 s

−1 and 1.74×10

−8 cm

2 s

−1,

respectively. These results indicate that the growth rate

constant decreases with increasing temperature. This is

due to the fact that the boride layer itself impedes

further diffusion of boron. The growth rate value

obtained are very close to the values obtained for pure

cobalt24

, but are lower than those of borided steels at

this temperature due to the low diffusivity of boron in

nickel compared to iron25

. It is clear from the

experimental results presented that boriding time plays

an essential role in the thickness of the boride layer as

well as the type/quantity/distribution of phases formed

within the boride layer.

Considering the activation energy for boriding

commercial-purity Ni, the growth rate constant (K)

depends on temperature and this relationship is

expressed by an Arrhenius equation:

−=

RT

QKK exp0

... (2)

Where 0K is a pre-exponential constant, Q is the

activation energy for the diffusion of boron

T is the absolute temperature (K) and R is the

universal gas constant (J mol-1

K-1

). The activation

energy Q for the diffusion of boron is determined by

the slope obtained by the plot Kln vs. T1 using

Eq. (2). In this study, the calculated activation energy

for the formation of the boride layer on the surface of

the borided commercial-purity Ni was 47.3 kJ mol−1

for our experimental temperature range of 1173-1273

K. Table 2 compares the obtained value of boron

activation energy (47.3 kJ/mol) with data available in

the literature. These results show that the activation

energy depends on the chemical composition of the

substrate, the boriding method used, and the strength

of atomic bonding within the metal.

Diffraction patterns for the samples borided at

1173 K are presented in Fig. 5. Ni2B is the

predominant phase on the surface of the boride layer

when pure Ni is borided at 1173 K and 1273 K for

3 h. However, with increased boriding time and

temperature, siliconization also occured due to the

presence of the SiC diluent in the boriding powders.

Thus, Ni6Si2B was also detected in the boride layer

due to increased diffusion of boron and silicon at 5 h

and 8 h. Corresponding to this, in Refs12,31

it has been

Table 2 — Comparison of the boron activation energy for some materials depending on the boriding method

Substrate Temperature range

(K)

Boriding medium Activation energy

(kJ/mol)

Reference

Molybdenum 1273-1673 Spark plasma sintering 218.8 23

Pure iron 1223-1323 Paste 151 24

Pure iron

AISI 304

1073-1223

1023-1323

Powder

Plasma paste

90

123

25

26

Nickel aluminide

Pure Co

1073-1223

1123-1273

Powder

Powder

188.8

231.7

27

21

Pure Ni 1123-1273 Powder 47.3 Present study

Fig. 4 — Time dependence of the square of boride layer thickness

at increasing temperatures

INDIAN J. ENG. MATER. SCI., OCTOBER 2017

366

shown that boron atoms diffuse easily into the surface

of metallic material due to their relatively small size

and high mobility. At the early stages, diffusion of

boron into the surface of pure nickel leads to the

formation of Ni2B phase. However, when the boriding

source also contains SiC, the accompanying diffusion

of silicon gives rise to formation of borosilicides32,33

.

Microhardness

In this study, the hardness of the boride layer

formed on the surface of borided commercial-purity

nickel was on average 859-1063 HV0.1, whereas the

hardness of the matrix was approximately 82-92

HV0.1. The hardness of the coating was significantly

higher than that of pure nickel substrate, which was

a consequence of the presence of hard Ni6Si2B and

Ni2B phases in the coating layer. Figure 6 shows

the hardness variation of the boride layer formed on

pure nickel borided at 1173 K for 5 h from the outer

layer to the interior.

Tensile properties

Room temperature stress-strain curves for borided

and untreated commercial-purity Ni are given in

Fig. 7. It is evident from the results that both tensile

strength and yield strength values of the borided

commercial-purity Ni samples are lower than the

values for untreated commercial-purity Ni.

Some studies34-36

, especially those conducted on

steels, have shown that yield and tensile strength

increases significantly with a boriding treatment

because of the presence of a boride layer and/or

hard boride phases. Consequently, boriding has

been used as a surface improvement process for

numerous materials. In the present study, a boriding

treatment of only 3 h decreased the tensile and yield

strength by more than 10% in commercial-purity Ni

samples. The strength values also decreased with

increasing boriding time due to the increased

thickness of the boride layer. Grain growth due to

high temperature exposure, and discontinuities due

to the precipitation of Ni6Si2B may have also

contributed to the decrease in strength. However, it

can be seen from Fig. 6 that there is no significant

difference between the tensile and yield strength

values obtained for specimens borided for 5 and

8 h. Summarizing, strength was negatively

influenced by the boriding treatment due to the

combined effects of grain growth and the presence

of the brittle boride layer. Similar results have

been obtained by Tian et al.37

They noted that

tensile properties including yield strength and

ultimate tensile strength of boronized N80 steels

were lower than the untreated alloy. The lower

tensile and yield strength was attributed to the low

ductility of boride layer, and it was concluded that

Fig. 5 — XRD patterns of borided commercial-purity Ni

Fig. 6 — View of Vickers indentations from the outer layer to the

interior of the base material in commercial-purity Ni borided at

1173 K for 5 h

Fig. 7 — Stress strain curves of commercial-purity Ni borided at

1173 K

CALIK et al.: BORIDING KINETICS AND MECHANICAL PROPERTIES OF BORIDED COMMERCIAL-PURITY NICKEL

367

the cracking of the boride layer may have induced

stress concentrations and consequently weakened

tensile properties to some extent.

When ductility is considered, the boriding

treatment increased the ductilities of commercial-

purity Ni. In some studies 38-40, it has been shown

that surface modification treatments such as boriding

may cause cracks to develop within the boride layer

as a result of coefficient of thermal expansion (CTE)

mismatch between the boride layer and the surface,

leading to embrittlement. The current study, in

contrast, reveals that ductility increases with the

boriding treatment. While the boriding process itself

was effective in increasing ductility, the increase was

not found to be linearly dependant on boriding time.

In other words, the changes in ductility were

inconsistent with regard to boriding time. The reason

for these discrepancies may be attributed to the

development of microcracks in the boride layer (as

observed in Fig. 7).

Conclusions The following conclusions can be drawn from this

study:

(i) The boriding treatment significantly increased

the surface hardness of commercially-pure

nickel due to the formation of hard boride layers

in the coatings.

(ii) XRD analyses indicated that Ni2B was the

dominant phase on the surface of the boride

layer when pure Ni was borided at 1173 K for

3 h. With increased boriding time, both borides

and borosilicides, namely Ni6Si2B and Ni2B

were detected on the surface of the borided

specimens. Ni6Si2B formed as discontinuous

precipitates within the boride layer.

(iii) The boride layer that formed on top of the

substrate had columnar morphology, and its

thickness increased with increasing treatment

time. Moreover, the borides had a hardness of

859 to 1063 HV0.1, while the substrate

unaffected by boron diffusion attained a value

of about 82 to 92 HV0.1.

(iv) The activation energy for borided commercially-

pure nickel samples was 47.3 kJmol−1

,

considerably lower than that obtained for many

borided metals and Ni alloys.

(v) Strength was negatively influenced by the

boriding treatment; yield strength and tensile

strength decreased with increasing boriding

time. Ductility, on the other hand, increased with

the boriding treatment.

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