Abstract
The essence of this research was to examine the properties of Inconel 625 coated titanium grade 5
for potential applications in relevant industries. The titanium was deposited through the Radio-
Frequency (RF) magnetron sputtering technique, varying the RF powers, substrate temperatures
and the deposition time. Inconel thin film is extensively used for microelectronics, optical,
electrical, mechanical and high temperature applications, and also for surface protection of
several metals and non-metals. The motivation for the application of Inconel thin film in this
field is due to their strength, corrosion resistance, wear resistance, ready availability and
refractory ability. Titanium is also used for diverse industrial applications. However, it is often
combined with some alloying elements to enhance their properties, hence, necessitating its
coating with Inconel 625 for surface protection against wear and corrosion due to the formation
chromium and nickel oxide, especially in corrosive environment.
A comprehensive review of literature has shown that all chemical and physical deposition
techniques have been employed to deposit Inconel on metallic substrates to improve their
mechanical, microstructural, optical, corrosion resistance, microelectronics and electrical and
thermal characteristics. Nevertheless, only the physical and chemical vapour deposition techniques
have exclusively been utilized for the improvement of the surface properties of non-metallic
substrates for microelectronics optical and electrical applications. Inconel coatings have influenced
the microstructural characteristics such as topography, morphology, grain distribution, grain size,
grain growth, crystallinity, recrystallization, porosity, texture, twin and slats formation in
materials. Similarly, mechanical and tribological properties such as creep, strength, fatigue
fracture, wear, corrosion, delamination and erosion have also been reportedly affected by Inconel
coatings.
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In this research, the deposition method used for the coating of Inconel 625 on a titanium substrate
sample is the radio-frequency (RF) magnetron sputtering technique. This magnetron sputtering
deposition provides metals with minimal distortion, strong bonding of coating with the substrate
and low dilution of the substrate. In the magnetron sputtering coating technique, there is barely
any crack formation, unlike the case of some processes where the possibility of crack formation is
high as a result of rapid melt pool solidification, which could affect the microstructure and
corrosion performances of metals due to the ability of the cracks to act as regions for pitting and
other forms corrosion. The radio-frequency (RF) magnetron can also give a cleaner production
environment, improved surface quality, excellent deposition and low material wastage, which
leads to the improvement of the production economy. The formation of pores due to air bubbles
entrapment is significantly low in the radio-frequency (RF) magnetron sputtering.
The chamber utilized for the magnetron sputtering was well vacuumed at a base pressure of 1.13
x 10-5 mbar. The deposition was accomplished at a time ranging from 60 to 90 minutes. The
sputtering of the samples was carried out at the power rating ranging from 100W to 200W and
temperature ranging from 100 to 200 oC. After completing the sputtering procedure, the entire
samples were kept in the chamber to cool sufficiently and thereafter, the samples were removed
from the vacuum and cautiously machined to a dimension of 10 mm x 10 mm for characterization.
The characterizations carried out are wear, microhardness, nanoindentation, microstructural and
corrosion test. The wear test of the Inconel coated titanium samples was done using tribometer of
8.1.8 version in accord with ASTM G99 criterion. The wear characteristics of the samples were
examined with a normal load of 10 N at the target temperature, laboratory temperature, frequency,
humidity and linear speed of 24 °C, 21.48 °C, 15 Hz, 47.78%, and 0.25 cm/s, respectively. With
the aid of the tribometer, the wear rates were examined. A nanoindenter was employed to study
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the micro hardness of the samples at 400.0 Hz in accordance to ASTM A-370. The diamond hard
tip was pressed on the test samples at a peak load of 300 mN`` for loading and 900 mN/min for
unloading. The Vickers hardness (HV) of the samples was also studied using the nanoindenter.
The microstructural characterization of the uncoated and the Inconel coated samples was
performed using the scanning electron microscope (SEM), equipped with an energy dispersive
spectroscopy (EDX) and X-ray diffractometer (XRD). The morphology of the samples was
examined using the SEM, while the elemental composition and crystallographic structure of the
samples were investigated using XRD. The corrosion properties of the samples were then
examined by using the potentiodynamic polarization experiment in HCl solution.
The results revealed that the Inconel coated titanium exhibited improved wear rate, microhardness
(Vickers hardness) and nanoindentation resistance, relative to the uncoated titanium. The
Inconel coatings enhanced material loss, plasticity and elasticity, strength and nanoindentation of
the titanium substrate. Specifically, the sample coated at the temperature of 200oC, time of 90
minutes and power of 200 W exhibited the lowest wear rate, highest microhardness (Vickers
hardness) and the best indentation resistance. The images on the SEM displayed low
agglomeration as the samples were without crevices, pores, and cleft. This means that the target,
which is the Inconel thin film was homogeneously distributed on the substrate (titanium). The
result from the EDX characterization of the various samples projected the presence of some
elements like C, Si and Ti that have been found to be highly beneficial to the material properties.
The samples coated revealed high peaks at the XRD profiles. This indicated the possible
microstructural homogeneity, chemical and stability of the thin film. Also, the polarization test
indicated that the Inconel coated titanium exhibited reasonable corrosion rates and corrosion
current densities. Among the Inconel coated samples, the sample coated at the temperature of
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150oC, time of 60 minutes and power of 150 W possessed the least corrosion rate of 0.03765
mm/year and corrosion current density of 3.2404E-06 A/cm2. The behavior of the Inconel coated
titanium samples in the test solution indicated their ability to passivate due the choice of process
parameters and the presence of the Inconel thin film, which prevents the ingression of the corrosive
ions into the active sites of the titanium as a result of the formation of oxide layers of Nickel and
Chromium.
Keywords: Corrosion current density; Energy dispersive x-ray analysis (EDX); Inconel thin film;
Magnetron sputtering; Nanoindentation; Titanium.