Fastening structures in vehicles that endure repetitive shear loads must maintain sufficient clamping forces to prevent loosening caused by joint slippage. The minimum clamping force required for controlling slippage is calculated using analytical and theoretical methods and is closely related to the static friction coefficient between the joint materials. In this study, we introduce a novel test apparatus designed to measure the static friction coefficient between two materials under high load conditions, with its experimental suitability confirmed through reliability verification. We experimentally analyzed the effects of the normal load, surface roughness, and mechanical properties on the static friction coefficient for materials commonly used in vehicle joints, including coated steel, steel, and aluminum alloys. Four machine learning algorithms—Gaussian process regression (GPR), ensemble, artificial neural network (ANN), and support vector regression (SVR)—were evaluated to develop a prediction model for the static friction coefficient. The predictive performance of each model was assessed using various evaluation metrics, and the results revealed that the GPR model achieved higher accuracy in predicting the static friction coefficient than did the other models.

This study presents a comprehensive investigation into the synthesis, dispersion behavior, and performance evaluation of surfactant-free copper oxide (CuO) nanoballs (NBs) dispersed in polyalphaolefin (PAO) oil. CuO NBs were synthesized via a modified precipitation technique and characterized via X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), confirming their monoclinic crystal structure and spherical morphology with particle sizes ranging from 25 to 132 nm. The dispersion quality and long-term stability of the nanolubricants were assessed via UV–Vis spectroscopy and zeta potential analysis, which indicated that 0.01 wt% CuO achieved the highest stability (zeta potential: 154.3 mV) and minimal sedimentation for up to 10 days. Rheological measurements revealed Newtonian behavior across all concentrations, with the highest relative viscosity observed at 0.05 wt% and 100 °C. The viscosity index improved at lower concentrations, supporting the lubricant’s thermal adaptability under dynamic shear conditions. The thermal conductivity increased with CuO addition, peaking at 0.01 wt%, primarily due to enhanced Brownian motion and reduced nanoparticle agglomeration. Tribological performance, evaluated using a reciprocating tribometer under a 10 N load and 840 m stroke length, revealed that 0.01 wt% CuO achieved a 37% reduction in the coefficient of friction (COF 0.055) and the lowest specific wear rate among all the tested samples. Surface analysis via three-dimensional (3D) profilometry and SEM-energy dispersive X-ray spectroscopy (EDS) revealed smoother contact surfaces and no evidence of CuO deposition, suggesting that a rolling friction mechanism was the dominant lubrication mode. These findings confirm that surfactant-free CuO NBs significantly enhance the tribological, rheological, and thermal properties of PAO oil, offering a cost-effective and environmentally friendly solution for high-performance industrial lubrication systems.

The use of nanofillers with high surface area and extreme purity in polymer composite is an effective strategy to obtain high performance polymeric nanocomposites. Therefore, the effect of nanofillers such as carbon nanotubes (CNT), titanium dioxide (TiO₂), and their hybrid on rubber-based composites was studied. In this study, rubber nanocomposites were fabricated by using room temperature vulcanized (RTV) silicone rubber matrix and nanofillers (i.e. CNT, TiO₂, and CNT-TiO₂) through solution casting method. Here, the purity and surface area of CNT (purity: >96% and BET surface area: 300 m²/g) and TiO₂ (purity: >98% and BET surface area: 165 m²/g) were estimated by field emission scanning electron microscopy/energy dispersive X-ray (FESEM-EDX) and adsorption isotherms. The mechanical properties of the rubber nanocomposites were enhanced by incorporating nanofillers. The compressive modulus was 2.18 MPa for unfilled composites and increased to 6.8 MPa (CNT), 3.95 MPa (CNT-TiO₂), and 2.44 MPa (TiO₂) at 5 phr, respectively. Similarly, the tensile strength was 0.54 MPa for unfilled composites and increased to 1.37 MPa (CNT), 1.33 MPa (CNT-TiO₂) and 0.61 MPa (TiO₂) at 5 phr, respectively. Further, the actuation displacement was improved with increasing input voltage and it was 2 mm for CNT, 1.6 mm for CNT-TiO₂ hybrid and 0.5 mm for TiO₂ at 10 kV. Moreover, a series of experiments show the potential application in piezoelectric actuation.