Table of Contents

Abstract ............................................................................................................................................................. 2
1.0 INTRODUCTION ........................................................................................................................................... 3
1.1 Importance of Ti6Al4V ............................................................................................................................. 3
1.2 Electroless Nickel Coating on Titanium in the Aerospace Industry ......................................................... 4
1.2.1 Aerospace Industry ........................................................................................................................... 4
1.3 Importance of Electroless Nickel Coating on Titanium ........................................................................... 4
1.3.1 Enhancing Corrosion Resistance....................................................................................................... 4
1.3.2 Improving Wear Resistance .............................................................................................................. 5
1.3.3 Enhanced Surface Finish and Lubricity ............................................................................................. 5
1.4 Applications of Titanium in Aerospace .................................................................................................... 5
1.4.1 Turbine Engines ................................................................................................................................ 5
1.4.2 Landing Gear Components ............................................................................................................... 5
1.4.3 Hydraulic Systems............................................................................................................................. 6
2.0 EXPERIMENT ............................................................................................................................................... 7
2.1 Materials and methods ........................................................................................................................... 7
2.1.1 Following steps brief the details of pre-treatment method for coating. ......................................... 7
2.2.2 Electroless bath ................................................................................................................................ 8
2.2.3 Experimental procedure ................................................................................................................... 9
3.0 Instrumentation and Measurement Techniques .....................................................................................10
4.0 Result and discussion................................................................................................................................11
4.1 Characterization of Ni-P coating ............................................................................................................11
4.1.1 Scanning Electron Microscope (SEM) analysis of Electroless Ni-P coating ....................................11
4.1.2 Elemental composition of Electroless Ni-P coating ........................................................................12
4.1.3 Adhesion Test .................................................................................................................................13
4.1.4 Thickness determination ................................................................................................................14
4.1.5 Heat Treatment ..............................................................................................................................15
4.1.6 Hardness test ..................................................................................................................................16
4.1.7 Wear Resistance Test Using the Reciprocating Method ................................................................18
4.1.8 Salt Spray ........................................................................................................................................23
5.0 Conclusion .................................................................................................................................................26
6.0 Future Studies ...........................................................................................................................................28
7.0 Reference ..................................................................................................................................................29

1
Abstract
This investigation electroless nickel-phosphorus (Ni-P) deposited over Ti6Al4V alloy substrate.
The study discussed about different aspects of the coating process, including bath composition,
plating parameters, and subsequent heat treatment to improve the lubricity and hardness of the
deposit. Then the coated specimens were examining for various characterization such as
morphology, composition, thickness, adhesion, lubricity, micro harness and corrosion. Scanning
Electron Microscopy (SEM) examination exhibited a nodular shape and indicating as amorphous
structure. Energy-dispersive X-ray spectroscopy (EDX) confirmed an 8 wt.% phosphorus content
and the rest is of nickel. Thickness of the coating were confirmed 24micron by preparing cross
sectional mould and examined under the optical microscope. The adhesion property of the coated
specimens was tested by heat quench test method as per ASTM B571 and there was no peel of or
flacking observed on the coated specimens which designates Ni-P has good adhesion on Ti alloy
substrate. Lubricity of the un-coated and coated specimens were measured using linear
reciprocating sliding wear tester, revealing that the Ni-P coating significantly reduced the wear rate,
with the coefficient of friction from 0.4157 to 0.2887. Micro hardness measurements made on Ti
alloy, as-plated and heat treated (4000C/1 hr) specimens were Ni-P coatings exhibited 600 ± 13;
550 ± 17 and 980 ± 22 VHN50 gf, respectively. Continuous salt spray test carried out on as-prepared
deposit exhibited excellent corrosion resistance without any sign of corrosion even after 460 hrs of
exposure.

2
1.0 INTRODUCTION
Titanium has emerged as critical materials in aerospace industries due to their unique combination
of good mechanical properties, low density, and operability in a number of special forming
processes. Titanium is recognized for its high strength-to-weight ratio, excellent corrosion
resistance, and biocompatibility, making it a preferred material in fields ranging from aerospace to
biomedical engineering. Ti6Al4V, one of the most widely used titanium alloys; offers enhanced
mechanical properties and superior performance in demanding environments. [1]

1.1 Importance of Ti6Al4V

Titanium Ti6Al4V is the workhorse alloy of the titanium industry, also known as Grade 5, TA6V or
Ti64. This α-β titanium-based alloy is the most commonly used of all titanium alloys and accounts
for 50% of total titanium usage in the world. Phase stabilizers are Aluminium and Vanadium. Al
reduces density, stabilizes and strengthens α while vanadium provides a greater amount of the more
ductile β phase for hot-working. [10]

Ti6Al4V alloy is known for its remarkable combination of high strength and low weight, making it
highly suitable for applications where both robust strength and reduced weight are critical, such as
in the aerospace and automotive industries. [2]. The alloy's exceptional resistance to corrosion
allows it to perform well in various environments, including exposure to seawater, making it a
prime candidate for marine applications. Moreover, Ti6Al4V exhibits excellent biocompatibility,
making it an ideal material for medical implants and devices that must interact seamlessly with
human tissue. Its superior fatigue resistance ensures that it can withstand cyclic loading, which is
particularly important for components used in the aerospace and automotive sectors. Additionally,
Ti6Al4V maintains its mechanical properties at elevated temperatures, offering high thermal
stability that is essential for high-temperature applications. However, Ti6Al4V alloy, while
possessing numerous advantageous properties, does come with certain drawbacks. One notable
disadvantage is its cost; Ti6Al4V is considerably more expensive than other commonly used
materials such as aluminium or steel. This higher cost can be a significant limiting factor for its use
in applications where budget constraints are a primary concern. Additionally, Ti6Al4V presents
challenges in terms of machinability. Its hardness and tendency to work harden during machining
processes require the use of specialized equipment and techniques, which can further increase
manufacturing costs and complexity. [3]. Furthermore, despite its much strength, Ti6Al4V has
relatively poor wear resistance. This limitation means that in environments where components are
3
subject to high levels of wear, additional surface treatments or coatings are often necessary to
enhance the durability and lifespan of the alloy.

1.2 Electroless Nickel Coating on Titanium in the Aerospace Industry

1.2.1 Aerospace Industry

In the aerospace sector, the need for coatings on Ti6Al4V and titanium is driven by the requirement
to enhance surface properties such as wear resistance, thermal stability, and corrosion resistance.
Coatings like Diamond-Like Carbon (DLC), Electroless Nickel coating and Thermal Barrier
Coatings (TBCs) are applied to improve performance and longevity of aerospace components,
including turbine blades, airframe structures, and fasteners.

Electroless nickel coating is an autocatalytic process that deposits a uniform layer of nickel-
phosphorus or nickel-boron alloy onto a substrate without the use of electrical current. This
technique provides several advantages, including uniform thickness even on complex geometries,
superior corrosion resistance, and enhanced wear resistance. When applied to titanium, a material
renowned for its high strength-to-weight ratio, excellent corrosion resistance, and biocompatibility,
electroless nickel coating further enhances its properties, making it particularly valuable in high-
performance and critical applications, such as in the aerospace industry.

1.3 Importance of Electroless Nickel Coating on Titanium

1.3.1 Enhancing Corrosion Resistance

Titanium itself is known for its excellent resistance to corrosion, particularly in harsh
environments. However, electroless nickel coating can further enhance this property, offering
superior protection against oxidation and other forms of chemical attack. In the aerospace industry,
where aircraft components are often exposed to extreme environmental conditions, including
moisture, salt, and various chemicals, this enhanced corrosion resistance is crucial. For instance,
the use of electroless nickel-coated titanium components in aircraft engines and landing gear can
significantly extend their service life by preventing corrosion-related failures.

4
1.3.2 Improving Wear Resistance

Wear resistance is another critical property for aerospace components, which are subjected to
constant friction and mechanical stress. Electroless nickel coatings provide a hard, durable surface
that can withstand these conditions better than untreated titanium. This improvement is particularly
important for parts such as turbine blades and bearings, where wear resistance directly impacts
performance and reliability. The uniformity of the coating ensures that even the most intricate parts
receive equal protection, maintaining their integrity and functionality over extended periods.

1.3.3 Enhanced Surface Finish and Lubricity

The aerospace industry demands components with high surface finish quality for optimal
performance. Electroless nickel coatings offer a smooth, lustrous finish that not only enhances the
aesthetic appeal but also reduces friction, which is beneficial in applications involving moving
parts. This reduction in friction can improve the efficiency of moving components and reduce the
wear and tear associated with high-speed operations. For example, in hydraulic systems and fuel
delivery mechanisms, a lower coefficient of friction can lead to smoother operation and increased
efficiency. [4].

1.4 Applications of Titanium in Aerospace

1.4.1 Turbine Engines

Turbine engines in aircraft are subject to extreme temperatures and harsh

environmental conditions. Electroless nickel-coated titanium components
are increasingly used in these engines to take advantage of their combined
properties of high strength, corrosion resistance, and wear resistance. For
instance, the blades and vanes in turbine engines benefit from the
enhanced surface hardness and corrosion protection provided by the
coating, leading to longer operational lifespans and reduced maintenance costs.

1.4.2 Landing Gear Components

The landing gear of an aircraft is another area where electroless nickel-

coated titanium is making significant inroads. The coating's ability to
resist corrosion and wear is particularly valuable in this application,
where the landing gear is frequently exposed to water, salt, and debris

5
during take-off and landing. By applying electroless nickel coatings, manufacturers can enhance
the durability and reliability of landing gear components, ensuring they perform optimally under
the severe conditions encountered during aircraft operations.

1.4.3 Hydraulic Systems

Hydraulic systems in aircraft rely on components

that must operate smoothly and reliably under high
pressure. Electroless nickel-coated titanium parts,
such as pistons and cylinders, offer improved
lubricity and wear resistance, which is critical for
the efficient functioning of these systems. The
coating helps reduce friction and wear, leading to
smoother operation and reduced risk of system failure. [5]

6
2.0 EXPERIMENT

2.1 Materials and methods

Test specimens of Titanium alloy, Ti6Al4V of size 24×24×2 mm were used as substrate. The
following method is used for pre-treatment and coating. All chemicals were laboratory grade and
DM water was used throughout.

2.1.1 Following steps brief the details of pre-treatment method for coating.

1. Ti alloy substrate surface were scrubbed using 600, 800, 1000, 1200, emery to remove the
passive oxide layer.

2. Alumina powder (0.5Micron) were used to polish the surface of the alloy to obtain smooth
and uniform roughness.

3. Then the polished surface was cleaned using tap water followed by DM water and dried.
Also the substrate weight was taken and noted for further measurements

4. Ultrasonically the specimens were cleaned 8mins using acetone.

5. Degreasing was carried out using mild alkaline solution containing Sodium carbonate
(Na2CO3)- 25gpl, Sodium hydroxide (NaOH)- 30gpl, Tri sodium Phosphate (Na3PO4)-
30gpl, for 13mins at 85oC temperature.

6. Further acid pickling was carried out on Ti substrate the solution contains HF (120ml/L)
and HCl (400ml/L) at room temperature for 2mins with continues stirring.

7. The acidic solution contains HCl (600ml/L) and H2SO4 (400ml/L) was used to form a
hydride conversion (hydrogenating treatment) film for 65mins at room temperature with
continues stirring [7].

7
 8. Nickel strike was carried out for 1min at 1.3 amps current at room temperature by using
Woods nickel bath to form a thin nickel layer over the Ti alloy substrate.
9. Then clean and oxide free Ti alloy substrate was immersed into the electroless plating bath
to form an adherent NiP coating during the coating using glass rod the bath were stirred to
achieve the uniform coating. There was no rinsing ware carried out after Woods nickel
strike.

The pre-treatment parameters were designed based on Refs. [6]

Every pre-treatment sequences the substrate was rinsed with tap water followed by distilled water
to ensure no residual solution on their surface.

Pictorial representation of the above pre-treatment is shown in Fig 1., which gives the information
of all the pre-treatment process.

Figure 1. The pre-treatment flow diagram before plating Ni-P coating on Ti6Al4V alloy

2.2.2 Electroless bath

After pre-treatment, the substrate is immersed in 500ml electroless nickel plating bath at 84±3 oC.
The components of electroless nickel (Ni-P) bath are listed in Table 1. The plating parameters were
preliminarily designed based on the standard technical data sheet (TDS) of Growel Ginplates Ni
632 solution. Then they were further optimized, such as adjusting the bath composition and
concentration, to adapt the titanium alloy substrate. The pH of the solution was adjusted to 5, using
10% Sodium Hydroxide and Sulphuric acid, accordingly. During electroless nickel plating, the
plating bath is stirred continuously at the speed of 500rpm. To every new substrate, a fresh plating
bath is employed to avoid variations in the compositions in the plating bath during long term

8
storage and to get consistent thickness, which could present a better wear resistance and cracking
resistance.

Bath composition ml/L

Ginplates Ni 632 A 60

Ginplates Ni 632 B 150

DM Water 790

Table 1. The components of electroless Ni-P plating bath

2.2.3 Experimental procedure

In the coating process, meticulous attention is given to each step of the pre-treatment phase to
ensure optimal results. Once the pre-treatment procedures are completed, the plating bath is
prepared according to the specified composition detailed earlier at table 1. The pH of the bath is
adjusted to a target value of 5 using either a 10% sodium hydroxide (NaOH) solution or a sulfuric
acid (H₂SO₄) solution, depending on the initial pH level of the bath.

Following the pH adjustment, the bath is maintained at a temperature of 84±3°C. This temperature
is crucial for the plating process to proceed effectively. After reaching the desired temperature, and
once all pre-treatment steps are thoroughly completed, the substrate is immersed into the plating
bath for the deposition of the coating. During the immersion, the continuous formation of bubbles
around the substrate serves as a clear indicator that the electroless nickel plating reaction is
occurring.

The plating rate, or deposition speed, within the bath is approximately 15 micrometers per hour
(μm/hr). The total duration of the coating process is determined based on the thickness of the
coating required, measured in micrometers. Thus, the number of hours the substrate remains in the
plating bath is calculated to achieve the desired coating thickness. This study approximately 2hr
coating were carried out and the measured thickness was 24microns.

9
3.0 Instrumentation and Measurement Techniques

The investigation of the surface morphology of the coatings was performed using a scanning
electron microscope (SEM). For this study, the morphological characterization and elemental
analysis of the nickel coating were conducted with a Supra 40 VP (Carl Zeiss), scanning electron
microscope, manufactured in Germany. This SEM was integrated with an energy dispersive X-ray
spectroscopy (EDX) system from Oxford Instruments Analytical, UK, which allowed for detailed
compositional analysis.

The adhesion properties of the electroless nickel coatings were assessed using a heat quench test. In
this procedure, the specimens were heated to a temperature of 250°C and maintained at this
temperature for 1 hour. Subsequently, the samples were rapidly quenched in cold water to evaluate
the adhesion characteristics of the coating under thermal stress.

To determine the thickness of the electroless nickel plating, the test coupons were subjected to
micro sectioning and metallographic polishing. The thickness measurements were then conducted
using an Olympus SZ61 optical microscope equipped with a graduated scale, which allowed for
precise assessment of the coating's thickness.

The micro hardness of the coating was measured with a Wilson Hardness Tester, Buehler, Model
VH1102. This tester employed a diamond indenter to obtain Vickers hardness numbers. Hardness
measurements were averaged from five separate indentations on each specimen, with each
indentation performed using a load of 50 gram-force (gf) applied for 10 seconds.

Corrosion resistance was evaluated by immersing the test specimens in a 10% sodium chloride
solution with a pH of 7.0. After a period of 20 days, the specimens were examined meticulously for
any signs of discoloration or corrosion, such as the formation of spots on the surface.

The tribological properties of TC4 titanium alloy and Ni-P coatings were evaluated through
experiments utilizing a linearly reciprocating ball-on-flat sliding tribometer. The testing was
conducted using the Rtec instruments, Model MFT2000A.

10
4.0 Result and discussion

4.1 Characterization of Ni-P coating

4.1.1 Scanning Electron Microscope (SEM) analysis of Electroless Ni-P coating

A detailed Scanning Electron Microscope (SEM) analysis was carried out to examine the surface
morphology of coated and uncoated titanium alloy. The SEM images provided valuable insights
into the surface texture and structural integrity of the coatings.

The surface images of the titanium alloy and its Ni-P coatings revealed distinct morphological
features. Consistent with findings from other studies, the Ni-P coating exhibited a characteristic
cauliflower-like texture. This unique surface morphology is attributed to the specific growth mode
of the electroless Ni-P coating process.

In the initial stages of deposition, the formation of nodules is observed. These nodules serve as the
foundational structures upon which the coating develops. As the deposition progresses, these
nodules grow in a columnar fashion, resulting in the distinctive cauliflower-like appearance. This
growth pattern is typical for electroless Ni-P coatings and is influenced by factors such as
deposition parameters, bath composition, and substrate characteristics.

The SEM analysis also highlighted the absence of defects such as cracks, voids, or delamination in
the as-plated coating. The smooth and continuous nature of the coating, as seen in the cross-
sectional images, underscores the quality of the electroless plating process. This defect-free coating
ensures reliable performance in service conditions, providing a robust barrier against wear and
corrosion. Figure 2 shows the nodular structure at different magnification of 500x and 1500x.

a b

11
 Figure 2. SEM images of the surface of Ti6Al4V substrate as-plated a) 500x, b) 1.50Kx

4.1.2 Elemental composition of Electroless Ni-P coating

Energy Dispersive X-ray (EDX) analysis of the as-deposited Ni-P coating on the titanium alloy reveals a
composition of 91.71 weight percent nickel and 8.29 weight percent phosphorus. These results confirm the
successful application of the electroless Ni-P coating on the titanium substrate, demonstrating the
anticipated elemental composition of the coating. The presence of 8.29% phosphorus indicates effective
deposition, achieving the desired chemical characteristics essential for enhancing the tribological
performance of the alloy. This composition aligns with the expected outcome, supporting the efficacy of
the electroless plating process used in this study. Figure 3 presents a graph depicting the expected results,
further substantiating the reliability and accuracy of the EDX analysis. The confirmation of the Ni-P coating's
successful deposition on the titanium alloy surface is critical, as it underscores the potential of this method
to improve the wear resistance and overall tribological properties of titanium alloys. This advancement is
significant for extending the applications of titanium alloys in high-demand engineering fields such as
aerospace, where enhanced surface properties are crucial for performance and durability.

Figure 3. EDX result of Electroless Ni-P coating on Ti6Al4V alloy

12
4.1.3 Adhesion Test

The heat quench test is a rigorous method used to simulate the thermal stresses that a coated
material might encounter in real-world applications. The absence of defects such as blisters or
cracks following this test demonstrates the robustness of the electroless nickel coating when
subjected to thermal cycling. This is particularly important for applications in industries such as
aerospace, automotive, and electronics, where materials frequently experience significant
temperature variations.

Additionally, the as-plated adhesion test underscores the immediate effectiveness of the electroless
nickel plating process. The lack of initial degradation in the coating indicates that the plating
process was executed with high precision, resulting in a uniform and strongly adherent coating.
This initial adhesion strength is crucial for components that may face mechanical stress or
corrosive environments shortly after the plating process.

The adhesion of electroless nickel plating on a titanium substrate was assessed using a heat quench
test. In this procedure, the titanium substrate was subjected to a heating process where it was
maintained at a temperature of 250°C for one hour. Following this, the heated substrate was rapidly
cooled by immersion in cold water, creating a thermal shock. Upon detailed inspection of the
substrate post-quenching, no blisters, cracks, or discoloration were observed. This result indicates
that the electroless nickel plating adhered well to the titanium, maintaining its integrity despite the
thermal stress. Figure 4(a, b)., shows the substrate before and after adhesion. No major difference
could be found.

a b
Figure 4, Adhesion images of a) Before coating, b) After coating

13
4.1.4 Thickness determination

To accurately determine the thickness of the electroless nickel plating on titanium substrates, a
series of meticulous procedures were performed on the test samples. The first step involved micro
sectioning, where the test samples were carefully cut to expose a cross-section of the plated layer.
Following this, the sectioned samples underwent moulding to facilitate handling and ensure that the
cross-sectional area remained intact during subsequent steps.

Once the samples were securely moulded, it was subjected to a thorough polishing process using
0.5-micron alumina powder. This step was crucial to achieve a smooth and even surface, free from
any imperfections or irregularities that could affect the accuracy of the thickness measurement.
Polishing was carried out using a series of progressively finer abrasives to ensure a high-quality
finish.

The polished samples were then analysed using an optical

microscope equipped with a graduated scale. This microscope
allowed for a detailed examination of the cross-sectional area of the
electroless nickel coating. The graduated scale provided a precise
means of measuring the thickness of the coating along the cross-
section. By taking measurements, a comprehensive assessment of
the coating's uniformity and overall thickness was achieved.

Figure5. Micro sectioned image of This methodical approach to measuring the thickness of electroless
the sample along with the
measurement nickel plating is essential for ensuring the quality and performance
of the coating. Accurate thickness measurements are critical for applications where the coating's
protective properties and adherence to specifications are paramount. The combination of micro
sectioning, moulding, polishing, and optical microscopy provides a reliable and precise means of
evaluating the coating's thickness. Figure 5., provides a clear view of the cross- sectioned image of
the sample along with the measurement.

Also the other method of calculating the thickness is employed which is through weight difference
and density. The calculation follows.

Thickness= Weight difference*104/Density of the nickel bath*surface area of the substrate

14
4.1.5 Heat Treatment

The titanium substrates were subjected to a meticulously controlled heat treatment process within a
tube furnace. They were heated to a precise temperature of 400°C for duration of 1 hour. After this
heating period, the samples were allowed to cool naturally to room temperature. The selection of
400°C was based on its significance as the crystallization temperature for Ni-P coatings, which is
essential for transforming the coating from its initial amorphous state to a well-defined crystalline
structure. The duration of 1 hour was chosen due to empirical evidence indicating that this
timeframe is adequate for the complete crystallization of the electroless Ni-P coating.

The heat treatment was conducted in an ambient air atmosphere to examine the impact of air during
the heating phase on the tribological properties of the electroless Ni-P coatings. Conducting the
process in ambient air is crucial for understanding the real-world performance and durability of the
coatings under typical environmental conditions. The experimental setup was meticulously
designed to provide comprehensive insights into how different heat treatment parameters affect the
performance characteristics of the Ni-P coatings, with a specific focus on their wear resistance and
frictional properties.

This study aims to expand the knowledge of optimizing heat treatment conditions to enhance the
tribological performance of electroless Ni-P coatings on titanium substrates. The findings from this
research are expected to contribute significantly to the development of advanced coating
techniques, potentially leading to broader applications of titanium alloys in industries where
superior surface properties are required, such as aerospace and automotive engineering. [8].

15
4.1.6 Hardness test

The micro hardness of the samples was evaluated using the Vickers micro hardness test, a standard
method for determining the hardness of materials by measuring the resistance to indentation. In this
study, the Vickers micro hardness was measured with applied load of 50 gram-force (gf). Also the
indentation was made different places of the coating on bare Ti6Al4V substrate, electroless nickel-
plated Ti6Al4V specimen, and heat-treated electroless nickel-plated Ti6Al4V specimen. The bare
Ti6Al4V sample exhibited a Vickers hardness value of 460 HV. This value is indicative of the
intrinsic hardness of the titanium alloy without any surface treatment. The as-plated electroless
nickel-coated Ti6Al4V sample indicates a Vickers hardness of 604 HV. This increment in the
hardness is attributed to the deposition of the adherent nickel-phosphorus layer, which provides a
harder surface compared as received titanium alloy.

Furthermore, the heat-treated electroless nickel-coated Ti6Al4V sample showed a remarkable

increase in hardness, with a Vickers hardness value of 985 HV. The heat treatment process,
conducted at 400°C for 1 hour in ambient air, facilitates the crystallization of the nickel-phosphorus
coating. This transformation from an amorphous to a crystalline state results in the formation of
hard phases within the coating, thereby significantly enhancing the hardness of the surface. The
substantial increase in hardness observed in the heat-treated sample highlights the effectiveness of
the heat treatment in improving the mechanical properties of the electroless nickel coatings. This
improvement is critical for applications requiring high wear resistance and durability. The results
from the Vickers micro hardness test confirm that the combination of electroless nickel plating and
subsequent heat treatment can substantially enhance the surface hardness of Ti6Al4V, making it
more suitable for demanding engineering applications. Figure 6 shows the typical images of the
device and indentation mark.

16
Figure 6. Vickers micro hardness test.

17
4.1.7 Wear Resistance Test Using the Reciprocating Method

The wear resistance test using the reciprocating method is crucial for evaluating the durability and
performance of materials under sliding contact conditions. Where materials are subject to repetitive
motion and friction, providing insights into their longevity and suitability for various applications.
By measuring parameters such as the coefficient of friction and material loss, this test helps in
assessing the effectiveness of surface treatments, like electroless nickel plating, in enhancing wear
resistance. These insights are vital for industries such as aerospace and automotive, where high
wear resistance is essential for the reliability and efficiency of components.

The wear resistance of the samples was assessed using a reciprocating wear test method. The test
employed a ϕ6.35 mm hardened steel ball as the counter material, with a load of 5 Newton’s
applied to each sample. Each test was conducted for duration of 30 minutes, with a stroke length set
at 10 mm per second. The total sliding distance for each 30-minute test was calculated to be 60
meters. The wear resistance was evaluated on three different Ti6Al4V substrates, each measuring
24 mm × 24 mm × 2 mm. The substrates tested included a bare Ti6Al4V sample, an as-plated
electroless nickel-coated Ti6Al4V sample, and a heat-treated electroless nickel-coated Ti6Al4V
sample.

Bare Ti6Al4V Substrate

The bare Ti6Al4V substrate exhibited a coefficient of friction (COF) of 0.4157. The wear test
resulted in a height loss of 0.0041 mm. This relatively high COF indicates significant frictional
resistance, which in turn suggests higher wear during the test. The height loss further confirms that
the bare Ti6Al4V substrate suffers considerable material removal under the given test conditions,
highlighting its limited wear resistance.

As-Plated Electroless Nickel-Coated Ti6Al4V

The as-plated electroless nickel-coated Ti6Al4V substrate showed an improved wear performance
compared to the bare substrate. The COF was measured at 0.3220, lower than that of the bare
Ti6Al4V. The height loss for this sample was 0.0054 mm. Although the height loss is slightly
greater than that of the bare Ti6Al4V, the reduced COF indicates better wear resistance due to the
lower friction experienced during the test. This suggests that the electroless nickel coating provides
a protective layer that reduces friction, though it may not significantly reduce material loss under
the tested conditions.

18
Heat-Treated Electroless Nickel-Coated Ti6Al4V

The heat-treated electroless nickel-coated Ti6Al4V substrate demonstrated the best wear resistance
among the three samples. The COF was the lowest, at 0.2887, indicating significantly reduced
friction during the wear test. The height loss was also the smallest, at 0.0016 mm. This remarkable
performance can be attributed to the heat treatment process, which induces crystallization and the
formation of hard phases within the nickel-phosphorus coating. The crystallized coating not only
lowers the COF but also enhances the overall hardness and wear resistance of the surface.

Comparative Analysis

Comparing the wear resistance data for the three samples provides clear insights into the
effectiveness of the electroless nickel coating and subsequent heat treatment:

Coefficient of Friction (COF):

o Bare Ti6Al4V: 0.4157

o As-Plated Electroless Nickel-Coated Ti6Al4V: 0.3220
o Heat-Treated Electroless Nickel-Coated Ti6Al4V: 0.2887

The COF data indicate that both the as-plated and heat-treated electroless nickel coatings
significantly reduce friction compared to the bare Ti6Al4V substrate. The heat-treated
coating shows the lowest COF, reflecting superior wear resistance.

Height Loss:

o Bare Ti6Al4V: 0.0041 mm

o As-Plated Electroless Nickel-Coated Ti6Al4V: 0.0054 mm
o Heat-Treated Electroless Nickel-Coated Ti6Al4V: 0.0016 mm

Although the as-plated nickel coating results in a slightly higher height loss than the bare substrate,
its lower COF indicates a reduction in wear through decreased friction. The heat-treated coating,
however, excels in both metrics, showing minimal height loss and the lowest COF.

Fig. 7, shows the wear scratch image of bare substrate of TI6Al4V Fig. 8, shows the wear scratch
image of as-plated EN coating and Fig. 9, shows wear scratch image of heat treated EN coating.

19
 Figure 7. Wear scratch image of bare substrate of Ti6Al4V

Figure 8. Wear scratch image of as-plated EN coating

Figure 9. wear scratch image of heat treated EN coating

Fig.10. shows the graph of the co-efficient of friction of the Ti6Al4V bare substrate. Fig.11. show
the graph of the total height loss of the Ti6Al4V bare substrate.

20
 Figure 10. Graph of the co-efficient of friction of the Ti6Al4V bare substrate

Figure 11. Graph of the total height loss of the Ti6Al4V bare substrate

Fig.12. shows the graph of the co-efficient of friction of the EN as-plated Ti6Al4V substrate.
Fig.13. show the graph of the total height loss of the EN as-plated Ti6Al4V substrate.

Figure 12. Graph of the co-efficient of friction of the EN as-plated Ti6Al4V substrate

21
 Figure 13. Graph of the total height loss of the EN as-plated Ti6Al4V substrate
Fig.14. shows the graph of the co-efficient of friction of the EN heat treated Ti6Al4V substrate.
Fig.15. show the graph of the total height loss of the EN heat treated Ti6Al4V substrate.

Figure 14. Graph of the co-efficient of friction of the EN heat treated Ti6Al4V substrate

Figure 15. Graph of the total height loss of the EN heat treated Ti6Al4V substrate

22
4.1.8 Salt Spray

Corrosion resistance is a critical property for materials used in various applications, particularly in
environments exposed to harsh conditions. The Ti6Al4V alloy, renowned for its high strength-to-
weight ratio, excellent corrosion resistance, and good biocompatibility, is extensively used in
aerospace, automotive, and biomedical industries. To enhance its surface properties, electroless
nickel plating is often applied, offering uniform thickness, improved hardness, and enhanced wear
resistance. To comprehensively evaluate the corrosion resistance of Ti6Al4V with an electroless
nickel coating, a salt spray test is employed, providing insights into the material's performance
under accelerated corrosive conditions.

The salt spray test, a standardized method for assessing corrosion resistance, involves exposing the
coated substrate to a saline mist environment. For this study, a 5 wt.% NaCl solution was
continuously sprayed at a constant rate, replicating aggressive corrosive environment. The Ti6Al4V
substrates, coated with electroless nickel, were placed inside the salt spray chamber and observed
over an extended period of more than 20 days. Sample were examined at regular intervals and
found no signs of corrosion, discoloration, or surface deterioration.

Initial observations revealed no signs of discoloration, corrosion, or black pits on the substrate
surface up to 251 hours of exposure. As the exposure time extended beyond 251 hours, slight
changes in the substrate's surface began to manifest. At approximately 295 hours, there was a slight
discoloration observed. Further continuing the exposure till 460 hours there were no major change
in the appearance noticed over the coated substrate which indicate good corrosion resistance.

The salt spray test, while an accelerated and somewhat aggressive method, provides valuable
predictive insights into the long-term behaviour of materials in corrosive settings. By subjecting the
Ti6Al4V electroless nickel-coated substrate to such rigorous testing, this study confirms the
material's robustness and reliability. The findings emphasize the critical role of surface treatments
in extending the lifespan and performance of engineering materials, ensuring their suitability for
demanding industrial applications. Figures 16, 17, 18. shows the image of the coated and uncoated
Ti6Al4V substrate before and after salts spray exposed specimens with different time intervals.

23
 Figure 16. TI6Al4V substrate before keeping to salt spray test

Figure 17. shows the image of the Ti6Al4V substrate in the salt spray test after 295 hours.

Figure 17. Ti6Al4V substrate in the salt spray test after 295 hours

24
Figure 18. shows the image of the Ti6Al4V substrate in the salt spray test after 460 hours.

Figure 18. Ti6Al4V substrate in the salt spray test after 460 hours

25
5.0 Conclusion
The study of electroless Ni-P coatings on titanium alloy substrates demonstrates notable
advancements in both surface and mechanical properties. The analysis reveals that the electroless
Ni-P coating significantly enhances the structural and functional characteristics of titanium alloys.
The SEM imaging indicates that the Ni-P coating forms a cauliflower-like texture, attributed to the
unique deposition growth mode. This texture, coupled with the absence of surface defects such as
cracks or voids, underscores the high quality and reliability of the coating in providing effective
wear and corrosion resistance.

The compositional analysis via EDX confirms that the Ni-P coating has a composition of 91.71%
nickel and 8.29% phosphorus. This precise composition is crucial for optimizing the tribological
properties of titanium alloys, making them suitable for demanding applications, particularly in
high-stress fields like aerospace. The rigorous adhesion testing through the heat quench method
further validates the robustness of the coating under thermal stress, highlighting its durability in
conditions that simulate real-world temperature variations.

Accurate thickness measurement of the Ni-P coating, achieved through a combination of micro-
sectioning, molding, polishing, and optical microscopy, confirms the uniformity and adherence to
specifications. This precise measurement is essential for ensuring that the coating delivers
consistent protective properties across different applications. The heat treatment process at 400°C
for one hour was shown to effectively crystallize the Ni-P coating, transforming it from an
amorphous state to a crystalline structure. This transition significantly enhances the coating's
tribological properties, optimizing its wear resistance and frictional performance under practical
conditions. The substantial improvement in hardness, as demonstrated by Vickers micro hardness
tests, reflects the effectiveness of both the Ni-P coating and the subsequent heat treatment. The
increase in hardness is indicative of the formation of hard phases within the coating, crucial for
applications that require superior wear resistance. The wear resistance tests using the reciprocating
method further support the effectiveness of the Ni-P coating, showing a notable reduction in the
coefficient of friction and minimal material loss for the heat-treated samples. These results confirm
the coating's exceptional performance in reducing friction and enhancing durability.

Overall, the electroless Ni-P coatings provide significant improvements in surface characteristics,
hardness, and wear resistance of titanium alloy substrates. This research highlights the potential of
electroless Ni-P coatings to enhance the performance and longevity of titanium alloys, making

26
them highly suitable for applications in demanding engineering sectors such as aerospace and
automotive. The findings underscore the value of the electroless plating process in advancing
material performance and extending the applicability of titanium alloys in high-stress
environments.

27
6.0 Future Studies
Future studies of titanium coatings should focus on several key areas to further advance the field of
electroless nickel coating on Ti6Al4V alloys. One crucial aspect is the long-term performance of
Ni-P coatings under cyclic loading conditions, which is essential for applications subjected to
repetitive stress, such as in aerospace and automotive industries. Evaluating how these coatings
withstand fatigue over extended periods will provide valuable data for predicting the lifespan and
reliability of coated components. Another important area of research is the behaviour of Ni-P
coatings in various environmental conditions, including high humidity, saline environments, and
extreme temperatures. Understanding the corrosion resistance and mechanical stability of the
coatings in such environments is vital for applications in marine and other harsh settings.

Additionally, exploring the effects of varying the phosphorus content in the Ni-P coatings could
offer deeper insights into optimizing the coating process. Different phosphorus levels can influence
the hardness, wear resistance, and overall performance of the coating. Investigating these variations
could lead to tailored coatings for specific applications, enhancing their effectiveness and longevity.
Further studies should also consider the development and evaluation of new pre-treatment
processes. Innovative techniques that can improve coating adhesion and uniformity on titanium
alloys are essential for maximizing the benefits of electroless nickel coatings. Comparative studies
of different pre-treatment methods will help identify the most effective approaches.

Moreover, the impact of different heat treatment atmospheres on the properties of Ni-P coatings
should be thoroughly investigated. Understanding how various atmospheres affect crystallization,
grain growth, and overall mechanical properties can lead to improved heat treatment protocols,
enhancing the performance of the coatings. Lastly, advanced characterization techniques, such as
electron microscopy and X-ray diffraction, should be employed to study the microstructural
changes in the coatings. These insights will provide a deeper understanding of the mechanisms
behind the improved tribological properties, guiding the development of next-generation coatings
for Ti6Al4V alloys.

28
7.0 Reference

1. Uma Rani, R., et al. "Studies on black electroless nickel coatings on titanium alloys for
spacecraft thermal control applications." Journal of applied electrochemistry 40 (2010):
333-339.
2. Froes, F. H. (1998). Titanium alloys and their properties. Titanium and Titanium Alloys:
Fundamentals and Applications, 69-88.
3. Ezugwu, E. O., & Wang, Z. M. (1997). Titanium alloys and their machinability—a review.
Journal of Materials Processing Technology, 68(3), 262-274.
4. https://www.nickelinstitute.org/media/1769/propertiesandapplicationsofelectrolessnickel_10
081_.pdf (Accessed: 05 August 2024).
5. Titanium – aerospace applications on Aluminium Distributing, inc. d/b/a Adi Metal
Aluminium
https://www.adimetal.com/product/titanium/titanium-aerospace-
applications#:~:text=Titanium%20is%20used%20in%20engine,alloys%20used%20in%20ai
rcraft%20applications. (Accessed: 05 August 2024).
6. Jia, Yao, et al. "Tribological Behaviors of Electroless Nickel-Boron Coating on Titanium
Alloy Surface." Chinese Journal of Mechanical Engineering 37.1 (2024): 13.
7. S Yazdani, R Tima, F Mahboubi. Investigation of wear behavior of asplated and plasma-
nitrided Ni-B-CNT electroless having different CNTs concentration. Appl. Surf. Sci., 2018,
457: 942–955.
8. S. Ghosh, S. Pal, and D. Chattopadhyay, "Influence of Heat Treatment on the
Crystallization Behavior and Tribological Properties of Electroless Ni-P Coatings," Journal
of Surface Engineering, vol. 34, no. 2, pp. 102-110, 2022.
9. Rani, Uma & Sharma, Anand & Minu, C. & Poornima, G. & Reddy. S, Tejaswi. (2010).
Studies on black electroless nickel coatings on titanium alloys for spacecraft thermal
control applications. Journal of Applied Electrochemistry. 40. 333-339. 10.1007/s10800-
009-9980-5.
10. https://www.farinia.com/blog/how-can-aerospace-benefit-3d-printed-titanium-ti6al4v

29