Tuning the tribological properties of Ni/NiO thin films by Nd:YAG pulsed laser irradiation

Abstract

Nickel (Ni) is one of the most common metal coatings used to synthesize composite
electrochemical coatings (CECs) as it is characterized by superior corrosion resistance, and
enhanced mechanical and tribological properties. Ni-based alloy coatings have been widely
used as wear materials [1]. The most employed Ni-based alloy coatings are NiFe, NiW, and
NiFeW. Ni, as the major element, provides ductility and enhances the corrosion resistance [2].
These coatings are widely used in gas turbines and in oil and steel industries, where sliding
or erosion between one or more bodies is commonly encountered. Ni-based alloy coatings
may also be more suitable than Co-based alloy coatings due to their better antiwear properties
and lower costs. Recently, graphene has been used in the electrodeposition of Ni composite
coatings known as nickel-graphene (Ni–Gr) coatings for lubrication application. Such coatings
are superior in tribological properties as compared to other hard CECs that consist of
chromium, boron nitride, zirconium dioxide, PTFE, etc [3]. Similarly, NiO is a well-known
antiferromagnetic material, and a metal-deficient p-type semiconductor with a 3.6 eV band
gap. Nickel oxide (NiO) films have a wide range of applications due to their excellent chemical
stability. They have been used as catalysts, electrochromic display devices, fuel cells and gas
sensors. NiO thin films usually exhibit p-type conductivity due to holes generated by Ni
vacancies in the lattice and therefore NiO is an interesting candidate for materials research
[4]. Although structural and electrical properties of NiO films have been studied extensively,
mechanical and tribological properties have been recently trending primarily due its ease of
deposition. Therefore, understanding the correlations between the mechanical properties and
microstructure of NiO-based films has been of great interest. In particular, it has been widely
conceived that the wide variety of methods used for fabricating NiO thin films often resulted in
very different film microstructures or even stoichiometries.
The present work aims at investigating how the nanomechanical properties and the surface
wettability of the Ni/NiO thin films deposited on glass substrates by radio-frequency magnetron
sputtering change with the post-deposition laser irradiation. A semiconductor laser based on
Nd:YAG operating at its 4th harmonic wavelength, λ = 266nm with varying laser fluence and
spot size of about 5 µm is irradiated on the Ni/NiO film. The localized heating allows for
smoothening of the NiO film along with contributions to the changes in the stoichiometry of
NiO (reduction of excess oxygen) and creation of Ni interstitials. In particular, the effects of
tuning laser fluence and the subsequent evolution of films microstructure and the associated
nanomechanical properties of the Ni/NiO thin films revealed by the mechanical tests, wear
and coefficient of friction variations for tribological tests are discussed. The hardness (H) and
the elastic modulus (E) of NiO thin films are measured by nanoindentation and their
dependence on the laser fluence is studied. Raman analysis along with XRD analysis for structural changes due to laser irradiation and EDX were carried out to validate the
dependence of laser fluence on the hardness of Ni/NiO thin films due to interstitials/ reduced
oxygen