Sarkar, Subhasish, Baranwal, Rishav Kumar, Biswas, Chanchal, Majumdar,
Gautam and Haider, Julfikar (2019) Optimization of process parameters for
electroless Ni–Co–P coating deposition to maximize micro-hardness. Mate-
rials Research Express, 6 (4). ISSN 2053-1591
Downloaded from:
https://e-space.mmu.ac.uk/622314/
Version: Accepted Version
Publisher: IOP Publishing
DOI: https://doi.org/10.1088/2053-1591/aafc47
Please cite the published version
https://e-space.mmu.ac.uk
Optimization of process parameters for Electroless Ni-Co-P Coating
deposition to maximize Micro-hardness
1
Subhasish Sarkar,
1*
Rishav Kumar Baranwal,
2
Chanchal Biswas,
1
Gautam Majumdar,
3
Julfikar Haider
1
Department of Mechanical Engineering, Jadavpur University, Kolkata-700032, India
2
Deartment of Materials and Metallurgical Engineering, Jadavpur University, Kolkata-
700032, India
3
Division of Mechanical Engineering, School of Engineering, Manchester Metropolitan
University, Manchester, United Kingdom
*corresponding author
ABSTRACT
The present study investigates optimisation of microhardness of electroless Ni-Co-P alloy
coating over copper substrate. The microhardness of the coating was significantly higher
compared to the substrate. Three different design factors i.e., the concentration of cobalt
sulphate, concentration of sodium hypophosphite and bath temperatures were used as the
process parameters which were optimised by using Box Behnken Design (BBD) and coating
micro hardness was taken as a response factor. Vickers’ hardness test was conducted to obtain
the micro hardness values of the coated samples. From the model analysis results, it was found
15 g/L of cobalt sulphate, 25 g/L of sodium hypophosphite and a bath temperature of 85 °C
were the optimum conditions for the coating deposition in order to obtain the hardness value
of 1921 HV
10g
. After annealing at 350 °C the hardness value was further enhanced to 1990
HV
10g
. Analysis of variance (ANOVA) was carried out to find the graphical relationship
between the different process parameters. The detail surface morphology of the Ni-Co-P
coating was studied by using an optical microscope and a Scanning Electron Microscope
(SEM). The phase and elemental compositions were determined by X-Ray Diffraction (XRD)
analysis and Energy Dispersive X-Ray analysis (EDX).
KEYWORDS
electroless coating, Ni-Co-P, Microardness, Box Behnken Design (BBD), ANOVA,
Optimisation
1. INTRODUCTION
The electroless nickel coatings were first developed by Brenner and Riddell [1] in 1947. In this
coating deposition technique, electron is supplied by a reducing agent instead of electric
current, hence the name electroless. The technique is an autocatalytic process in which the
substrate is immersed in a solution, the electroless bath, which contains the source of metal
ions, reducing agent, bath stabilizer, complexing agent, buffering agent, accelerators, and
surfactants or wetting reagents. Nowadays, electroless nickel coating is widely used in many
industries [2-4] due to its improved corrosion resistance, wear resistance, magnetic and
hardness properties [5-11]. The coating has attracted much attention due to its application in
the fields of engineering, surface science, and purification technology used in automotive,
chemical and petroleum industries, electronics, food, marine, material handling,
pharmaceutical, military, mining, etc. [30]. Ni–Co-P alloy coatings are of huge importance, as
they possess high-temperature wear and corrosion resistance characteristics. Moreover, the Ni–
Co-P alloy deposition is an anomalous co-deposition and the hardness of alloy increases as
long as they possess FCC lattice structure [31]. There are a wide variety of electroless coatings,
which can be broadly classified into four categories viz., pure nickel, alloy and poly-alloy
coatings, composite coatings and electroless nano coatings. The alloy and poly-alloy coatings
can be further classified into binary, ternary and quaternary alloy coatings [12]. In binary
alloys, there are two elements in the coating deposition existing in different phases. s. Similarly,
in ternary, there are three elements present and in quaternary, there are four elements present
[13]. In ternary alloys, an additional element is added to the binary alloys to create a composite
or an alloy coating. Literature review suggests that binary coatings like Ni-P and Ni-B has
proved to have better tribological and mechanical properties [14-18]. However, the need of
having better properties is always a demand in this era and challenging to the researchers at the
same time. Hence, the use of a third element/complex like Co, TiO
2
, W, Cu, Fe, ZrO
2
, Al
2
O
3
,
PTFE, etc. [19-26] is preferred in many cases. Ni-Co-P coating is obtained when cobalt is
introduced as an additional element, in order to incorporate the property of electromagnetic
shielding, and to improve the anti-corrosion and hardness properties of the surface at the same
time [27-29, 43].
The present study focuses on synthesis of Ni-Co-P for increasing the hardness properties of
copper substrate and also to find the optimum conditions for obtaining optimized hardness
value of the coating. Vickers Hardness Test was used to determine the micro-hardness values
of the as-deposited coating. Box Behnken Design (BBD) helped determining the optimum
processing conditions. The BBD modelling suggested 17 model values which when compared
to the experimental values showed a very slight deviation. Analysis of Variance (ANOVA)
was conducted to find out the significant coating deposition parameters and their interactions
affecting the hardness of the coating.
2. EXPERIMENTAL DETAILS
2.1. Procedure of Synthesis of the Coating
2.1.1. Substrate Preparation
The substrate material chosen for this experiment was copper, as it is a very readily used
material in day-to-day life and has very common but good applications in real life scenarios.
The copper substrate was cut from a foil (99.0% pure, lobachemie) which was present in the
rolled form. In this experiment, Ni-Co-P coating was deposited on copper substrates of size
20.0×15.0×0.1 mm
3
. After cleaning with distilled water, the substrate was dipped into 3:1 dilute
HCl solution for acid pickling in order to remove oxide layer and other foreign metals. Pickling
was carried out for 10 minutes followed by cleaning with distilled water. The surfaces of the
substrates were activated using palladium chloride solution for 10-15 seconds, which was pre-
heated to 55
o
C. Finally, the substrate was prepared for dipping into electroless bath.
2.1.2. Bath preparation
In bath preparation at first NiSO
4
.6H
2
O, CoSO
4
. 7H
2
O, NaH
2
PO
2
. H
2
O, Na
3
C
6
H
5
O
7
. 2H
2
O and
(NH
4
)
2
SO
4
were taken according to Table 1. The pH Value was maintained at 5. Once the
composition was prepared then the substrates were dipped in the electroless bath. Pre-prepared
substrates were kept immersed in the bath for an hour to allow the coating to be deposited.
After that, the substrates coated with Ni-Co-P were taken out of the bath and rinsed in distilled
water. The coating depositions were carried out with different bath compositions and
temperatures along with a fixed time, bath volume and pH value. The composition of the bath
is given in Table 1.
Table 1: Bath Composition for Ni-Co-P electroless coating deposition
Bath composition
Quantity
Nickel sulphate (NiSO
4
. 6H
2
O)
25 g/L
Cobalt sulphate (CoSO
4.
7H
2
O)
10/15/20 g/L
Sodium hypophosphite (NaH
2
PO
2
. H
2
O)
20/25/30 g/L
Tri-sodium citrate dihydrate (Na
3
C
6
H
5
O
7
. 2H
2
O)
15 g/L
Ammonium sulphate ((NH
4
)
2
SO
4
)
10 g/L
pH value
5
Time
1 hour
Bath volume
250 cm
3
Bath temperature
80 °C / 85 °C / 90 °C
2.2. Experimental procedure of Micro-Hardness measurement
The coating samples were of dimensions 20.0×15.0×0.1 mm
3
. Holding a small piece as such
will be difficult in order to perform the hardness test using indentation technique. As a result,
the mounting of the coated samples were done using Bakelite as the thermosetting resin. The
mounting process was carried out at a temperature and pressure of 175 °C and 240 bar,
respectively for 5 minutes. The mounted coated samples underwent Vickers Hardness test
aided by an indenter having a diamond tip of 136°. A 10 gram load was applied, and the total
time for loading and unloading was allowed to be 15 seconds. Indentation surface area (A) and
hardness number (HV) can be calculated by Equation (1) and (2) respectively. The Vickers
microhardness Ni-Co-P coated mounted samples were measured as per ASTM standard E384-
16 using a hardness tester.
A =
≈
(1)
