This article is focused on various synthetic methods and characterization techniquesof nanofluids. Factors enhancing the stability and lubrication mechanism have been delineated in detail as well.

In this study, a series of tribological tests were conducted on a pin-on-disc tester to study the lubrication mechanism of SiO₂ nanoparticles under different surface roughness considering various loads and velocities. For a comprehensive understanding of the mechanism of SiO₂ nanoparticles, base fluid was also employed as a contrast. Results show that the reductions of friction coefficients and wear scar widths increase with the decrease of surface roughness, due to the increase in rolling effect and self-repairing mechanism of SiO₂ nanoparticles. The lubrication mechanism of SiO₂ nanoparticles is the rolling effect when the height-diameter ratio (λ) is less than 6, and the self-repairing mechanism at λ of 6 and 10, whereas, there is no obvious difference by adding nanoparticles when λ is 20. When the height-diameter ratio is less than 6, surface wears show an increasing trend as the load increases due to the high hardness of nanoparticles, while it is the opposite at λ of 10 and 20 because of the self-repairing mechanism.

The tribological properties of nanofluids are influenced by multiple factors, and the interrelationships among the factors are deserving of further attention. In this paper, response surface methodology (RSM) was used to study the tribological behavior of reduced graphene oxide–Al₂O₃ (rGO-Al₂O₃) nanofluid. The interaction effects of testing force, rotational speed and nanoparticle concentration on the friction coefficient (μ), wear rate (W_r) and surface roughness (R_a) of steel disks were investigated via the analysis of variance. It was confirmed that all the three input variables were significant for μ and W_r values, while testing force, nanoparticle concentration and its interaction with testing force and rotational speed were identified as significant parameters for R_a value. According to regression quadratic models, the optimized response values were 0.088, 2.35 × 10⁻⁷ mm³·N⁻¹·m⁻¹ and 0.832 μm for μ, W_r and R_a, which were in good agreement with the actual validation experiment values. The tribological results show that 0.20% was the optimum mass concentration which exhibited excellent lubrication performance. Compared to the base fluid, μ, W_r and R_a values had a reduction of approximately 45.6%, 90.3% and 56.0%. Tribochemical reactions occurred during the friction process, and a tribofilm with a thickness of approximately 20 nm was generated on the worn surface, consisting of nanoparticle fragments (rGO and Al₂O₃) and metal oxides (Fe₂O₃ and FeO) with self-lubrication properties.

Ni60/WC composite coatings reinforced with various WC contents were fabricated on 45 steels via high-speed laser cladding. First-principles calculations were adopted to investigate the interfacial characteristics between Ni and WC. The microstructures of the coating were analyzed by a scanning electron microscope (SEM). The microhardness and wear resistance of the coatings were evaluated by a Vickers hardness tester and a friction and wear tester. Theoretical calculations indicate that the C-terminated WC (001) crystal plane achieves the most stable bonding with the Ni (111) surface through the hcp site, with an interface energy of 9.57 J·m⁻². The interface is mainly provided by Ni-W metal bonds and Ni-C covalent bonds. The microstructure results show that the WC particles have good metallurgical bonding with the Ni matrix. Thus, the good interface ensures efficient load transfer to hard WC particles. The microhardness rose markedly with the increase in WC content, reaching 760 HV_0.2 for the 40 wt.% WC, which is 1.8 times that of Ni60 coatings. Tribological experiments showed that appropriate WC content could significantly improve the wear resistance of the coating.

Nanofluids containing different concentration of TiO₂ nanoparticles were prepared. Their rheological behavior was investigated on LVDV III ultra-programmable Brookfield viscometer at different temperature and particles loading. Results show that rheological behavior of nanofluids is determinated by the concentration of TiO₂ in base fluid, nanofluids shows good Newton behavior when nanoparticles loading is low, while shear thinning becomes evident when the volume fraction of particles was continuously increased. Besides, viscosity of nanofluids shows strong nanoparticles loading and temperature dependence, but quite differs from classical viscosity model, as the theory models severely underestimate the value of viscosity, which is attributed to electroviscous effect of nanoparticles in base fluids. The primary electroviscous coefficient is evaluated at different pH values. Modified primary electroviscous coefficient is in good agreement with Booth’s theory.