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

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 andmicrostructure 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