Tribological studies on nanoparticles have demonstrated their effectiveness in reducing friction and wear in tribosystems, offering a promising avenue for designing energy-efficient systems. However, selecting the right nanoparticle for a specific oil presents challenges, as the compatibility and impact of nanoparticles can vary depending on the oil grade. Factors such as nanoparticle affinity with base oils and their physical properties, including shape, size, nanostructure, hardness, and concentration, influence their behavior in different oils. Additionally, the introduction of nanoparticles can alter the thermo-physical properties of the oil, such as thermal conductivity, viscosity, density, and various points like flash point and pour point. These properties play a crucial role in determining the lubricant’s effectiveness, ensuring its suitability across a range of operating conditions.
Nano lubricants, colloidal solutions of oils and solid nanoparticles, are typically prepared using either a one-step or two-step method, with the latter being more common due to its simplicity and cost-effectiveness. In the two-step method, separately synthesized nanoparticles are mixed into lubricants using techniques like magnetic stirring, ultrasonication, or ball milling. However, a significant challenge with this approach is the stability of nano additives in the lubricant. Nanoparticles tend to aggregate due to their high surface energy, forming clusters that decrease stability and settle down easily. The limitations of the two-step method have prompted the development of the one-step method for preparing nanofluids. In the one-step method, nanoparticles are simultaneously prepared and mixed into the fluid using physical or chemical methods. This approach reduces the need for separate storage, transportation, and mixing of nanoparticles, thereby minimizing agglomeration, and increasing the stability of nanofluids. However, nanofluids prepared by the one-step method may contain residual reactant products in the suspension, diminishing their effectiveness. Additionally, the high cost and limited scalability of production have hindered the widespread adoption of the one-step method for nanofluid preparation.
Figure-1 Schematic illustration of lubrication mechanisms using water-based nano lubricants containing TiO2 NPs during hot steel rolling [3]
The effectiveness of nanoparticles in enhancing the tribological properties of oil is influenced by several factors, including size, morphology, chemical composition, concentration, nanostructure, and surface functionalization. Optimal size and concentration thresholds exist beyond which tribological improvements diminish, with larger particles often leading to higher settling rates that affect nanoparticle stability in oil. Dispersion stability varies across different oils, impacting nanoparticle effectiveness in reducing friction and wear. Additionally, nanoparticle shape plays a crucial role in tribological enhancement. While spherical nanoparticles, commonly used, are associated with a rolling mechanism, onion-shaped nanoparticles exhibit lower frictional coefficients and maximum wear reduction due to their lack of dangling bonds, which reduces particle-environment interaction and promotes dominant rolling action. Various lubrication mechanisms, including rolling, mending, polishing, and protective film formation, contribute to the improvement of tribological properties when nanoparticles are dispersed in base oil. These mechanisms are influenced not only by oil and nanoparticle properties but also by operating conditions such as load, velocity, temperature, and surface roughness of tribo-pairs. Consequently, nanoparticles exhibit different behaviors in different oils, leading to variations in friction and wear properties; they may either enhance or degrade tribological properties depending on the specific oil composition and operating conditions.
Figure-2 Nano lubrication mechanisms (a) rolling and mending (b) protective and polishing [1]
In recent years, there has been a notable increase in interest in the synthesis and properties of nanoparticles, driven by high expectations for their applications across various scientific and technological fields. This surge in interest is primarily due to the unique properties exhibited by nanoparticles, including catalytic, optical, semiconducting, magnetic, antifriction, and others. Nanoparticles are synthesized using three major methods: physical, chemical, and biological. Physical synthesis methods encompass techniques such as ball milling, electro-spraying, melt mixing, inert gas condensation, laser pyrolysis, and spray pyrolysis. Chemical methods involve processes like sol-gel synthesis, simple precipitation, chemical vapor deposition, hydrothermal synthesis, polyol synthesis, and solvothermal synthesis. On the other hand, biological methods utilize sustainable sources such as viruses, fungi, bacteria, and plant extracts to produce nanoparticles. These diverse synthesis methods offer researchers flexibility in tailoring nanoparticles for specific applications across a wide range of industries and disciplines.
The study of thermo-physical properties of nano lubricants emphasizes the interaction between nanoparticle-liquid molecules and nanoparticle-nanoparticle interactions, while the tribological aspect focuses more on nanoparticle-surface interactions. The effectiveness of a Nano lubrication system is influenced by both the properties of the base fluid and nanoparticles. Nanoparticle properties affecting thermal conductivity include shape, size, concentration, and thermal conductivity, while fluid properties affecting it include viscosity, thermal conductivity of the base fluid, and the ability to form layered structures at the interface. These differences in properties result in varied enhancements in thermal conductivity across different grades of lubricants, though the exact impact of nanoparticles on thermal properties remains incompletely understood. Conflicting views exist regarding the relationship between thermal properties and nano additive concentrations, with some studies reporting increases and others decreases with increasing concentration. Further research is needed for a comprehensive understanding. Viscosity variations in nano lubrication systems are attributed to factors such as particle loading, size, shape, and hardness of nanoparticles. Nanoparticles suspended between lubricant layers increase resistance, with resistance rising alongside particle concentration. However, some nanoparticles may suspend easily while others agglomerate due to chemical opposition and Brownian motion, resulting in varying viscosity increments. Higher nanoparticle agglomeration leads to greater viscosity increases and pump losses, necessitating surface functionalization of nanoparticles for industrial applications.
