N. LI et al.: PREPARATION AND HARDNESS OF A FUNCTIONALLY GRADED Ni-Al COATING
27–33
PREPARATION AND HARDNESS OF A FUNCTIONALLY
GRADED Ni-Al COATING
PRIPRAVA IN TRDOTA PREVLEKE NA OSNOVI Ni-Al
Ningning Li, Lei Xu, Liang Huang, Yuping Tong, Zhengquan Jiang
*
, Keliang Li
North China University of Water Resources and Electric Power, School of Materials Science and Engineering, Zhengzhou
Prejem rokopisa – received: 2022-10-10; sprejem za objavo – accepted for publication: 2022-12-05
doi:10.17222/mit.2022.650
A functionally graded Ni-Al coating can improve the hardness of low-carbon steel, which can then adapt to various conditions
due to a high hardness. The primary purpose of this research was the preparation of a Ni-Al coating by adopting a two-step
method including nickel plating and pack aluminizing. The coating phase and morphology were characterized with XRD, SEM
and EDS. Subsequently, the hardness of the Ni-Al coating was measured using a nanoindentation test. The results show that dif-
ferent aluminum contents in the coating surface lead to different coating phases, mainly Ni2Al3. The change in the coating
phases conformed to the Ni-Al binary phase diagram. The Ni-Al coating showed a continuous gradient structure and composi-
tion. In addition, the coating also showed continuous gradient variation, relating to the hardness.
Keywords: Ni2Al3 coating, nanoindentation, hardness, gradient
Funkcionalnim prevlekam Ni-Al se lahko po preseku spreminja sestava in temu primerno tudi fizikalne lastnosti. Osnovni
namen raziskave, opisane v ~lanku, je bila priprava prevlek Ni-Al z dvostopenjsko metodo elektro platiranja niklja in paketnega
aluminiziranja (angl.: pack aluminizing). Faze prevlek in njihovo morfologija so bile dolo~ene z rentgensko difrakcijo (XRD),
vrsti~no elektronsko mikroskopijo (SEM) in elektronsko disperzijsko spektroskopijo (EDS). Sledile so {e meritve nanotrdote
izdelanih prevlek Ni-Al s postopkom nanovtiskovanja oziroma kontroliranim vtiskovanjem nanoprizme. Rezultati analiz so
pokazali, da razli~na vsebnost Al na povr{ini prevleke vodi do nastanka razli~nih faz, predvsem faze Ni2Al3. Spremembe fazne
sestave v prevleki se ujemajo z binarnim faznim diagramom Ni-Al. Izdelane prevleke na osnovi Ni-Al ka`ejo neprekinjen gradi-
ent strukture in sestavo. Isto~asno se v skladu s tem zvezno spreminjajo tudi njihove fizikalno-kemijske lastnosti, kar se
predvsem odra`a na izmerjeni nanotrdoti.
Klju~ne besede: prevleka na osnovi Ni2Al3, nano vtiskovanje, trdota, gradient
1 INTRODUCTION
Functionally graded materials (FGMs) refer to a class
of non-homogeneous composite materials with continu-
ous and quasi-continuous changes in the structure and el-
ements, exhibiting the performance and composition
change in the gradient. Their microstructures, physical,
chemical and biological properties show continuous
changes in a single phase, or a combination of phases, to
achieve a particular function. FGMs are one of the cru-
cial topics relating to the current structural and func-
tional materials.
1
A functionally graded coating is classi-
fied according to the changes in the composition, relating
to the surface technology area, and its potential applica-
tions are primarily found in high-temperature, wear, cor-
rosion and other areas.
2,3
The preparation methods for a functionally graded
coating include vapor deposition,
4
plasma spraying,
5
la-
ser cladding
6
, etc. A vapor-deposition coating is very
thin. Plasma spraying is subject to power restriction
where the processes are complex and difficult to control,
and the coating bonding strength is poor. The high en-
ergy of the laser-cladding method may damage the prop-
erties of the matrix.
In recent years, several intermetallic compound coat-
ings have been researched due to their high melting
points, low densities and excellent corrosion resistance at
high temperatures. Ni-Al intermetallic compound coat-
ings have been applied to high-temperature alloy sur-
faces because they can easily form a dense alumina layer
at high temperatures.
7,8
Pack aluminizing is the favored
method that can be used for preparing nickel-aluminum
coatings on Ni-based alloys. To apply a nickel-aluminum
coating onto another metal surface, a two-step process
including nickel plating and powder aluminizing has
been invented.
9–11
In addition to exhibiting the advan-
tages of good quality, simple operation, minor technical
difficulties and low investment in the equipment, this
method also reduces the influence of the chemical com-
position of the substrate on forming an aluminized coat-
ing so that a low aluminized temperature can be used.
According to recent research, the low temperature can
also protect the substrate properties.
In this study, we investigated the synthesis of a func-
tionally graded Ni-Al coating on a substrate by adopting
a two-step method including plating nickel and pack alu-
minizing. The coating composition and morphology
were characterized with XRD, SEM and EDS. The hard-
ness was tested with nanoindentation.
Materiali in tehnologije / Materials and technology 57 (2023) 1, 27–33 27
UDK 669.058.4 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(1)27(2023)
*Corresponding author's e-mail:
jiangzhq@ncwu.edu.cn (Zhengquan Jiang)

2 EXPERIMENTAL PART
2.1 Materials
To carry out the research conveniently, the ordinary
Q235 steel was selected as the substrate, with dimen-
sions of (10 × 10 × 2) mm; its nominal chemical compo-
sition is shown in Table 1. Before plating, the substrate
was mechanically polished with metallographic papers,
polished and ultrasonically cleaned with acetone for
5 min, then washed with distilled water and activated in
5 w/% HCl for 20 s; finally, it was washed with distilled
water again and quickly immersed into a bath.
The basic Watts nickel composition and plating con-
ditions are shown in Table 2. The chemicals used are of
analytical grade and dissolved in distilled water. The so-
lution pH value was adjusted to 4.0 for H
2
SO
4
(20 w/%).
The electroplating power supply was a DC power supply.
For maintaining the solution homogeneity, mechanical
stirring by a magnetic stirrer located at the container bot-
tom was used. Four nickel-plated samples were prepared.
The samples were washed with distilled water and etha-
nol repeatedly, being removed from the cell.
The pack mixture is composed of the Al powder,
AlCl
3
and Al
2
O
3
powder reminder as the filler. Alumina
power needs to be used after high-temperature calcina-
tions. AlCl
3
was used without any processing after being
removed from the reagent bottle. Previous studies
showed that the content of aluminum powder has the
greatest impact on the coatings phases.
12
In order to ex-
plore the Al-powder role in the formation of a Ni-Al
functionally graded coating, a series of samples was
manufactured, and the contents of the Al power were (8,
15, 50 and 70) w/%. For the convenience of discussion,
they were defined as Sample 1, Sample 2, Sample 3 and
Sample 4, respectively. The activating AlCl
3
content was
5 w/%, and the Al
2
O
3
reminder was the inert filler. The
pack aluminizing process is described as follows: the
substrate was embedded in a packed mixture in an alu-
mina crucible and then positioned in a vacuum tube fur-
nace with argon. The furnace was heated at a rate of
10 °C/min and the dwell time was 20 h at a temperature
of 650 °C. Eventually, the samples were removed after
having been cooled down to room temperature, and alco-
hol was used to remove the adhered particles.
2.2 Hardness test
The hardness of the coatings was measured with a
nanoindentation test; the CSM instrument, which came
from Switzerland, was equipped with a diamond
Berkovich indenter. The maximum applied load was
50 mN, the holding time was 10 s, while the loading and
unloading rates were 100 mN/min.
2.3 Surface characterization
The phase composition of the surfaces of the coatings
was identified using X-ray diffraction (XRD) with a
CuK
a
radiation (D/Max-RA, Rigaku, Japan). The chemi-
cal compositions and microstructures at the cross-sec-
tions of the coatings were tested with SEM and en-
ergy-dispersive spectroscopy (EDS) (S-3400II, Hitachi,
Japan).
3 RESULTS AND DISCUSSION
3.1 Coating phases of pack aluminizing with different
Al amounts
XRD results are shown in Figure 1, namely, the
phase results for different coatings, which show that the
four samples have different phases. Sample 1 has a sin-
gle Ni
2
Al
3
phase, while Samples 2, 3 and 4 are composed
of Ni
2
Al
3
and NiAl
3
of different ratios. The formation of
varying phases will be discussed in combination with
other data in Section 3.5.
N. LI et al.: PREPARATION AND HARDNESS OF A FUNCTIONALLY GRADED Ni-Al COATING
28 Materiali in tehnologije / Materials and technology 56 (2022) 6, 27–33
Table 1: Nominal chemical composition of Q235 low-carbon steel (w/%)
Composition C Mn Si S P Fe
Content 0.140–0.220 0.300–0.650 0.300  0.050 0.045 Bal.
Table 2: Chemical composition of the Watts Ni bath and the electro-deposition conditions
Composition NiSO
4·6H2O NiCl2·6H2OH 3BO3 T pH Agitation
Parameter 240 g/L 20 g/L 30 g/L 45–60 °C 4.0 Magnetic stirrer
Figure 1: XRD patterns for different coating surfaces

3.2 Chemical morphologies and compositions of alu-
minized coating surfaces
Figure 2 represents different coating-surface mor-
phologies, while Table 3 shows the components of dif-
ferent coating surfaces.
As can be seen, the morphologies of the coatings are
different. Figure 2a represents the single Ni
2
Al
3
phase,
fabricated using an 8 w/% Al-power amount, and its sur-
face as a whole is relatively flat. The EDS result shows
that the coating components are 58.57 x/% Al, 40.99 x/%
Ni and 0.44 x/% Fe (atomic %). The XRD data analysis
consists of a phase diagram of Ni-Al.
13
Figure 2b shows
the coating fabricated with 15 w/% Al powder. Ex-
hibiting a sharp contrast, its appearance is relatively
rough. More Fe is detected with the EDS results. The Al
content is 64.78 x/%, and the phases are NiAl
3
and
Ni
2
Al
3
, combined, in the two-phase region, in the Ni-Al
diagram. This result is consistent with XRD, and when
the peak intensity of Ni
2
Al
3
is much stronger than that of
NiAl
3
, it is clear that the Ni
2
Al
3
phase prevails. Fig-
ures 2c and 2d show the Ni-Al layer fabricated with
50 w/% and 70 w/% Al powder, and the apparent differ-
ence is the Fe element that is not detected on the two
coating surfaces. The Al contents are 73.93 x/% and
74.96 x/%, which means that the NiAl
3
phase prevails.
This is also confirmed with the XRD result.
NiAl
3
and Ni
2
Al
3
are the two typical phases that are
easy to form; their melting points are 854 °C and
855–1133 °C. By comparison, NiAl
3
is not expected to
appear in the coating. However, the three layers are pre-
pared with pack Al-powder amounts of 15 w/%, 50 w/%
and 70 w/%, and they all contain a NiAl
3
phase. There-
fore, in the following discussion, this paper mainly fo-
cuses on the Ni
2
Al
3
coating to explore its microstructure
and properties.
3.3 Cross-sectional microstructure and composition of
the Ni
2
Al
3
coating
Figure 3 shows SEM and EDS results for the cross-
sectional microstructure and composition of the Ni
2
Al
3
N. LI et al.: PREPARATION AND HARDNESS OF A FUNCTIONALLY GRADED Ni-Al COATING
Materiali in tehnologije / Materials and technology 57 (2023) 1, 27–33 29
Figure 2: Surface morphologies of different coatings fabricated using different pack Al-power amounts: a) 8 w/%, b) 15 w/%, c) 50 w/%,
d) 70 w/%
Table 3: Components of different coatings (x/%)
Material components Sample 1 Sample 2 Sample 3 Sample 4
Al 58.57 64.78 73.93 74.96
Ni 40.99 23.28 26.07 25.04
Fe 0.44 11.94 0.00 0.00

coating, fabricated with an 8 w/% pack Al-powder
amount. It can be seen from Figure 3a that the alumi-
nized layer is composed of the surface Ni
2
Al
3
coating
and an interdiffusion zone in the center. The Ni
2
Al
3
layer
and substrate are clearly distinguished in Figure 3a,
while the interdiffusion layer is not easily observed on
the backscatter image. Figure 3b shows that the Ni
2
Al
3
coating thickness is approximately 30 μm with a low
Fe-element content. The Ni/Al content ratio is constant
throughout the coating thickness. This proves that the
Ni
2
Al
3
coating is a single and stable phase, which is con-
sistent with XRD. In addition, the coating cross-section
is very smooth, with no cracks at the interface, indicating
a tight bond with the substrate. It can also be seen that
the interdiffusion zone thickness is 20 μm, with three el-
ements including Al, Ni and Fe. The content of Ni and
Al decreases slowly, gradually penetrating the substrate.
At the same time, a small amount of iron is found to be
diffusing from the Q235 substrate. In addition to being
different from the Ni
2
Al
3
coating, the diffusion layer is
also different from the substrate, which is the result of
the interdiffusion between the coating and substrate,
therefore leading to the metallurgical bonding with the
substrate, and further enhancing the interfacial adhesion.
A reduction in delamination and spalling is a unique ad-
vantage of a functionally graded layer. The reason for
this is that it reduces the stress concentration and de-
creases the crack-driving force.
Another phenomenon that can be seen on Figure 3b
is the fact that the Al atoms fluctuate at the beginning
and then reach a relative stability. A possible reason for
this is the fact that the sample surface is in direct contact
with the pack cement agent during the aluminizing pro-
cess, enhancing the activity of the Al atoms, while after
aluminizing, rich Al atoms are not evenly spread, thus
resulting in a high Al content on the surface. Later the Al
atoms are stabilized throughout the coating thickness.
3.4 Hardness of the functionally graded Ni
2Al3coating
Nanoindentation is also known as a depth-of-indenta-
tion sensitive method, which has been used for precision
measuring of hardness, elastic modulus and other me-
chanical properties of various materials in recent
years.
14,15
It can directly determine a contact area in real
time through a load-displacement curve, instead of mea-
suring the surface indentation with an optical method,
thus significantly reducing errors.
16
Its theory is based on
the Oliver-Pharr method for calculating the hardness and
elastic modulus. In addition to considering the unloading
curve, this method also considers the indenter shape and
the indentation depth to calculate the contact area under
a load.
Figure 4 shows a typical load-displacement curve of
the nanoindentation test. The symbols represent the fol-
lowing: P
max
is the maximum load, h
max
is the deepest
penetration depth, S is the contact stiffness (the initial
slope of the unloading curve), and h
f
is the residual dis-
placement after complete unloading. The measurement
process is divided into three stages: loading, load holding
and unloading. Through the testing and analysis, the
hardness (H) and elastic modulus (E) values for different
materials can be obtained, to find out whether the coat-
ing can improve the substrate property.
N. LI et al.: PREPARATION AND HARDNESS OF A FUNCTIONALLY GRADED Ni-Al COATING
30 Materiali in tehnologije / Materials and technology 57 (2023) 1, 27–33
Figure 3: Cross-sectional microstructure and composition of the Ni
2
Al
3
coating
Figure 4: Typical load-penetration depth curves obtained with a
nanoindentation test

Load-penetration depth curves for the Ni
2
Al
3
coating
are shown in Figure 5. As can be seen, five turns are on
the same trend, though exhibiting low discrete variables.
Table 4 reveals specific data, verifying the change in the
hardness. Finally, the average hardness H of the Ni
2
Al
3
coating is about 15.05 GPa, and the elastic modulus E is
227.59 GPa. The value of H/E is characterized by the
material wear resistance to some extent.
17
The same method is used to measure different coat-
ing regions, to verify whether the coating improves the
property of the substrate, and the specific figures and
data are shown in Figure 6 and Table 5. For comparison,
only one group of data is selected for the analysis. It is
evident that the hardness value of the substrate is the
lowest, while its values for h
max
, h
f
and S are the largest.
In contrast, the hardness value of the surface Ni
2
Al
3
coat-
ing is the highest, while h
max
, h
f
and S are the lowest.
From the coating surface to the interdiffusion layer, and
then to the internal substrate, the value of H/E is gently
decreased, thereby a gradient change in the performance
is realized. The size of the area surrounded by the curve
indicates a plastic material. As can be seen, the nano-
indentation area of Ni
2
Al
3
is more compact than the
interdiffusion zone and substrate, including smaller re-
gions and indicating a less plastic deformation of the
coating in the loading process. This is in line with the
high hardness of the Ni
2
Al
3
layer: the greater the hard-
ness, the smaller is the plastic deformation under the
same pressure. It can also be seen that the variety in the
pressing depth of the coating is the smallest, within the
same holding time.
Figure 7 represents the morphology after the
nanoindentation. Although each area was detected five
times, only two groups appear in this image due to a lim-
ited range of the camera, but the difference can still be
N. LI et al.: PREPARATION AND HARDNESS OF A FUNCTIONALLY GRADED Ni-Al COATING
Materiali in tehnologije / Materials and technology 57 (2023) 1, 27–33 31
Figure 5: Load-penetration depth curves for the Ni
2
Al
3
coating
Table 4: Specific data for the Ni
2
Al
3
coating measured with nanoindentation
Parameter 12345 Mean
H (MPa) 16589.46 15561.99 15124.08 14169.13 13789.45 15046.82
E (GPa) 221.80 221.72 215.26 239.53 239.62 227.59
Table 5: Nanoindentation test data for varying coating regions
Material H (MPa) E (GPa) h
max
(nm) h
f
(nm) S (mN/nm)
Q235 substrate 2543.1 203.2 1044.54 974.11 0.8994
Interdiffusion zone 6614.5 175.6 676.92 482.49 0.5317
Ni
2
Al
3
coating 15046.82 227.59 481.43 329.55 0.4389
Figure 6: Load-depth curves obtained with the nanoindentation test of
different coating regions
Figure 7: Morphology of different regions measured with nano-
indentation

noticed. It can be concluded from Figure 7 that under
the same pressure, the indentation size and deformation
of the Ni
2
Al
3
coating are the smallest, and a triangular
pyramid shape can be seen. However, the indentation
size and deformation of the interdiffusion zone are in the
middle, while the matrix is the largest. A more signifi-
cant deformation indicates a lower hardness and a deeper
indentation depth. Similarly, the value of the unload-
ing-curve initial slope S is also larger. Therefore, it can
be concluded that the hardness of the Ni
2
Al
3
coating is
the highest, which is also consistent with the previous
test data from Table 5.
3.5 Formation analysis of the Ni
2
Al
3
coating
The formation of the Ni
2
Al
3
coating is analyzed be-
low, as shown in the relation
18,19
where X stands for one
element out of F, Cl or Br, n is an integer of less than 3,
and the range of y is more than 1/3, but less than 3.
Al + AlX
3
 AlX
n
(1)
AlX
n
+Ni AlNi
y
+AlX
3
(2)
For this research, X indicates Cl. A brief analysis is
as follows. The low aluminizing temperature promotes
the inward diffusion of aluminum. The AlCl
n
gas phase
is formed due to the contact between the Al powder and
activator AlCl
3
, which is a crucial step in the formation.
AlCl
n
is more thermally stable than AlCl
3
as the tempera-
ture increases. After that, AlCl
n
reacts with the nickel-
plated surface to deposit aluminum atoms. An inter-
metallic layer forms on the surface, with an AlNiy com-
position of (1/3  y  3). As the aluminum atoms are
brought to the surface by a gaseous-state process, a
solid-phase diffusion follows. The generated activator
AlCl
3
returns to the aluminum source to continue the
process. When X represents F or Br, a similar chemical
reaction occurs.
This process is different from pack aluminizing of a
nickel-base superalloy surface.
20
The general reason for
it is that there are two noticeable differences in the
chemical composition and precipitation phase between
the nickel-plated substrate and nickel-base superalloy.
The electroplated nickel layer is a single  phase, while
the nickel-base superalloy includes a  solid solution as
well as multiple precipitation phases, namely the  and
carbide phase; at the same time, solution elements Mo
and Co increase the activation energy of the vacancy for-
mation. The presence of residues reduces the effective
diffusion zone, leading to an extension of the diffusion
path. Therefore, high-temperature aluminizing is applied
on the nickel-base superalloy, while the nickel-plated
substrate can be aluminized at a low temperature to re-
duce the impact on its properties.
The microstructure and chemical composition of the
aluminized layer exhibit a gradual change. As a result,
the hardness also changes gradually, effectively reducing
the stress. This is the advantage of functionally gradient
materials. Instead of the traditional homogenous material
coatings, FGM coatings do not only improve the connec-
tion strength and reduce the crack-driving force, but they
also endue excellent friction and wear properties of ma-
terials.
4 CONCLUSIONS
From the study, the following conclusions can be
drawn:
1) A single Ni
2
Al
3
coating phase can be prepared
with an 8 w/% Al content of pack cement. There is a
transition zone between the coating and substrate. The
thickness of the Ni
2
Al
3
layer is approximately 30 μm,
with a low content of the Fe element. The interdiffusion
zone thickness is 20 μm, with three elements including
Al, Ni and Fe.
2) The average hardness of the Ni
2
Al
3
layer is about
15.05 GPa, which is higher than those of the inter-
diffusion zone and Q235 substrate that are 6.6 GPa and
2.5 GPa. The indentation size and deformation of the
Ni
2
Al
3
coating are the smallest, with a triangular pyramid
shape.
Acknowledgment
This research was funded by the Key R&D and Pro-
motion Special (Scientific Problem Tackling) Project of
the Henan Province (No. 212102210039), Science and
Technology Collaborative Innovation of the Zhengzhou
Project in 2022 and the High-Level Introduction of Tal-
ent Research Start-Up Fund of the North China Univer-
sity of Water Resources and Electric Power (No.
4001/40680).
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