An enormous amount of energy is used up in overcoming the friction of moving objects. As a result, friction-related wear and heat can cause damage to the contact surface, material fatigue, unnecessary mechanical energy losses, noise emissions, and degraded machine efficiency [1]. Friction and wear are two fundamental causes of the breakdown of engineering parts in various structures, such as gears and valves. The price of machinery, fitting and maintenance due to frictional defects, wear and tear put immense burdens on the nation's economy. Approximately 1/3 of fuel is utilised in passenger vehicles to subdue friction in engines, transmissions, and braking [2]. A decrease in energy usage can be accomplished mainly by enhancing the tribological properties of system surfaces. The specifications for improved lubricants are increasingly challenging due to their properties' usability across a broader temperature range, higher loads, higher speed, improved reliability and service life.
Military armoured vehicles with diesel-based engines experience massive heat generation and pressure due to their extensive driving in uneven terrains with bulky equipment. In order to ensure the mechanical parts are working efficiently and to increase the service life of the vehicle's engine, diesel-based engine oil must manage friction effectively and minimise wear for engine mechanical components [3].
The anti-friction additive is critical in the tribology of diesel-based engine oil, especially for military vehicles with rapidly evolving mechanical equipment. As a result, the load on a heavy-duty vehicle engine per unit mass increases, making it difficult for traditional lubricant additives to meet the demands of extreme operating conditions in modern diesel engine components [4, 5]. Therefore, developing new and effective friction-resistant plus high-bearing lubricant additives is critical for meeting the demands of powerful machinery in extreme working conditions.
One of the 21st century's main scientific challenges is producing new lubricants that satisfy evolving criteria in various strategic fields, such as transportation, manufacturing, and defence. In recent years, researchers have established that nanotechnology can be the most innovative aspect of science in the twenty-first century [6]. Continuous advances in science and technology provide an outstanding forum for nanotechnology to evolve at a faster pace. As a result of development, researchers also discovered that lubricants' tribological properties could be improved with the inclusion of nanoparticles, which would significantly decrease the coefficient of kinetic friction in operating devices [7, 8].
Several nanoparticles consist of two adjacent layered structures, bound by weak van der Waals force, responsible for lowering the shear strength and causing sliding or lubricating effects on the system's active adjacent layer structure [9, 10]. Furthermore, two-dimensional (2D) nanomaterials have a larger specific surface area than other nanomaterial surfaces, allowing them to cover a large surface area during absorption on a substrate exterior, removing kinetic friction between two contact surfaces [11].
Due to its physical and chemical stability in lubrication, molybdenum disulfide (MoS2) is currently regarded as a high-potential 2D transition metal chalcogenide. The material is chemically balanced, resistive to most acids, and immune to irradiation. It is both semiconductor and diamagnetic in its purest form. The lubricant rate is dependent on its crystalline lamella structure, where the sulphur lamellae are linked by a weak van der Waals interaction which reduces the fiction [12]. During sliding, the crystalline layers of MoS2 would effectively slide and align parallel to the relative movement, which causes the lubrication effect. However, the powerful ionic bond between S and Mo makes the lamellar highly resistant to asperities' penetration [13]. Nanostructure research has also been on the rise in the last few years. For example, using MoS2 nanocrystals for lubrication will produce a superlubricity framework (a coefficient of friction lower than 0.01) [11]. Several theories have been proposed for this phenomenon, which has also been observed in fullerene configurations and nanotubes, where nanostructures act as nano bearings in tribological contact, lowering the mechanism's COF significantly [8, 14]. For the synthesis of MoS2 nanoparticles, various preparatory methodologies have been established, including high-temperature sulfurisation, thermal reduction, hydrothermal process, laser ablation, and even chemical vapour deposition (CVD) [15–18]. However, advanced microwave synthesis of MoS2 nanoparticles has rarely been documented, and its use in the tribology field has not been published in the literature so far.
Hydrothermal and microwave synthesis techniques have been employed to synthesise MoS2 nanoparticles at comparatively larger yields. The hydrothermal method is frequently used due to the accessibility of the processing equipment, but it suffers from a lack of even heating. However, substances can also be heated rapidly in the microwave synthesis process, producing a consistent temperature ramp relative to traditional oven-based hydrothermal processes. Besides that, the reaction Teflon vessels are translucent to the microwave, and will ensure continuous heating throughout the reaction vessels. In addition, microwave gains from rapid and accelerated heating, high-temperature homogeneity, and selective heating over traditional methods [19]. The reactions primarily depend on their precursors' ability, including solvents, to consume microwave energy efficiently. The above findings confirm that the microwave synthesis technique is superior to the hydrothermal technique due to its uniform heating, low energy consumption, higher yield, and shorter synthesis. In some papers [20–22], traditional heating in the oven that takes approximately 24 hours is employed to synthesise the MoS2 nanosheets, whereas microwave synthesis takes less than 30 minutes.
The novelty of this experiment is to investigate the optimisation of microwave-assisted synthesis of MoS2 nanoparticles for the tribological application using a response surface methodology (RSM) approach with a central composite design (CCD) model under Design Expert (Stat-Ease). Most of the previous studies were carried out using a univariate approach where only one element is varied at a time, often resulting in missing experimental data. However, with RSM and the CCD model, this optimisation approach investigates a larger experimental domain. The two vital experimental parameters for synthesis, such as temperature and time, vary together, resulting in higher optimum values. The principle purpose of this study is to identify the optimum time and temperature needed to synthesise the MoS2 via microwave, which gives the best tribological result in military-grade diesel-based engine oil. Overall, this research highlights the effects of microwave synthesised nanoparticles on the tribological criteria of engine oil.