The rapidly advancing technological landscape is driving an increasing demand for high-performance lubrication solutions that enhance energy efficiency and operational effectiveness [1]. This is particularly critical in industries where friction and wear are primary concerns, such as automotive, aerospace, manufacturing, and energy systems. The key challenge is to develop lubricants capable of withstanding extreme pressures, temperatures, and mechanical stresses while simultaneously minimizing energy losses and component degradation [2].

While conventional lubricants are effective in many scenarios, they often fall short under severe operating conditions. They may degrade at high temperatures, lose viscosity under shear, or fail to provide sufficient anti-wear protection in heavy-load applications [3,4]. Furthermore, the growing emphasis on sustainability and reduced environmental impact necessitates lubricants that not only perform better but also align with eco-friendly standards. To address these challenges, researchers and engineers are increasingly turning to nanotechnology, with NLs emerging as one of the most promising transformative solutions.

The concept of NLs represents a significant advancement in lubrication technology, building on the foundational principles of nanotechnology. Researchers have explored the incorporation of nanosized particles, including metals (e.g., Cu, Ag, Au) [5,6], metal oxides (e.g., ZnO, TiO2, Al2O3) [7,8], and carbon-based material (e.g., multi-wall carbon nanotubes, graphene, graphene oxide) [9,10], into traditional lubricants such as mineral oils and synthetic fluids. These nanoparticles exhibit unique properties, such as high surface area to volume ratios, exceptional mechanical strength, and tunable surface chemistry, which enable them to interact synergistically with base oils and significantly enhance tribological performance [11,12]. Research has demonstrated that NLs can reduce friction coefficients by up to 50 %, improve load-bearing capacity through the formation of adaptive protective layers, and enhance thermal conductivity by 15–40 %, leading to better heat dissipation and reduced wear in mechanical systems [13]. Additionally, their ability to form durable, self-repairing tribofilms on contact surfaces helps prevent direct metal-to-metal contact, even under extreme pressures and temperatures, further extending the lifespan of critical components such as bearings, gears, and piston rings [14]. Beyond these mechanical benefits, certain nanoparticles like MoS2 [15] and hexagonal boron nitride (h-BN) [16], exhibit lamellar structures that provide low shear resistance, while others, such as diamond nanoparticles [17], offer extreme hardness for superior wear resistance. Given these advantages, NLs are increasingly being explored for applications in high-performance engines, industrial machinery, refrigeration systems, and even micro-electromechanical systems (MEMS) [[18], [19], [20]]. Recent studies also highlight their potential in renewable energy systems, such as wind turbines and electric vehicle drivetrains, where efficiency and durability are paramount [21,22]. Furthermore, the principles of nanofluids extend beyond lubrication into advanced energy harvesting. Environmental and energy assessments of advanced renewable energy systems, such as photovoltaic-thermal (PV-T) configurations combined with reflectors and nanofluid filters, highlight the critical role of thermal management and optical efficiency. In these systems, nanofluids serve not only as coolants but also as spectral splitters that selectively absorb or transmit solar radiation, thereby optimizing the utilization of the solar spectrum [4,23,24]. For example, nanofluid-based filters in CPV-T systems can reduce thermal degradation of photovoltaic cells while simultaneously improving overall system efficiency. The integration of vortex generators and heat sinks further enhances heat dissipation, mitigating hot spots and extending the lifespan of solar modules. These innovations underscore the potential of nanofluids and related nanomaterials to contribute to more efficient and sustainable energy systems.

According to the Scopus database, there has been growing interest in the use of NLs across various fields, with the number of publications on this topic increasing exponentially over the past decade. A significant number of studies have been conducted to investigate the potential benefits of NLs in greater depth, approaching the topic from different perspectives including materials science, tribology, thermal engineering, and industrial applications. These studies aim to develop a comprehensive understanding of NLs and their suitability for enhancing the performance of various base fluids, ranging from conventional mineral oils to advanced synthetic and bio-based lubricants.

However, the expanding body of literature on NLs has also led to considerable ambiguity among researchers. First, there is a notable discrepancy between experimental results and theoretical predictions, with many studies reporting inconsistent outcomes that lack clear explanations [[25], [26], [27]]. For instance, a comprehensive review of NL performance revealed variations in friction reduction (ranging from 10 % to 60 %) and wear resistance (differing by orders of magnitude) across different studies, raising questions about the reliability of the findings [28]. These inconsistencies arise from multiple factors, including variations in experimental conditions (load, speed, and temperature), nanoparticle characteristics (size, morphology, and concentration), and testing methodologies. Additionally, the rapid, exponential growth in publications over the past decade has created a surge of information that obscures clear trends, making it difficult to extract meaningful conclusions, as many studies lack standardized protocols or fail to report critical parameters. Second, challenges such as nanoparticle dispersion stability, long-term durability, and cost-effectiveness remain unresolved and must be addressed to facilitate widespread industrial adoption. For example, while laboratory-scale studies often demonstrate promising short-term results, real-world applications frequently encounter issues such as particle agglomeration, sedimentation, and degradation under prolonged operational conditions. Furthermore, the economic feasibility of large-scale NL production remains uncertain, as the costs associated with nanoparticle synthesis, surface functionalization, and quality control often outweigh the benefits for mainstream applications. Environmental and safety concerns, including potential toxicity and disposal challenges, further complicate the path to commercialization.

Despite these obstacles, ongoing advancements in nanotechnology and materials science continue to reinforce the potential of NLs as a transformative solution for next-generation lubrication needs. Emerging approaches, such as the development of hybrid nanoparticles, smart self-dispersing systems, and bio-compatible formulations, show promise in addressing current limitations.

While the preceding discussion highlights ongoing debates and gaps in NL research, and numerous existing reviews have focused primarily on reporting performance enhancements, the present study distinguishes itself through a systematic and critical approach aimed at diagnosing the root causes of persistent inconsistencies that hinder industrial adoption. Moving beyond a mere compilation of findings, this work provides a comprehensive analysis of the underlying reasons for divergent results in the literature. Its unique contributions include a critical synthesis of stability challenges, offering a unified framework for sedimentation behaviors; a quantitative analysis of the prevalence and insufficiency of current stabilization methods; and a systematic evaluation of characterization techniques that highlights methodological limitations and biases. Furthermore, this review provides a detailed examination of application-specific failure modes in real-world systems, such as gearboxes, engines, hydraulics, and refrigeration units, linking laboratory-scale results to practical operational hurdles often overlooked in prior studies. Finally, the study offers forward-looking, actionable solutions and standardized protocols for preparation, characterization, and long-term testing, designed to bridge the gap between academic research and industrial implementation. By addressing these critical aspects, this work aims to contribute to a more unified and practical understanding of NLs and to advance the field from empirical observation toward predictive and reliable science.