Stability and Ageing Behaviour of SiO2 Nanoparticle-Modified Lubricants and Biolubricants: An Experimental–Numerical Investigation

This study comparatively investigated the ageing behaviour of a conventional lubricant and a biolubricant modified with 0.25–1.00 wt.% KH570-coated SiO2 nanoparticles. Nanolubricants and nanobiolubricants were prepared using a two-step dispersion method and subjected to accelerated thermal ageing at 220 and 260 °C for exposure times ranging from 120 to 240 h. Density, kinematic viscosity at 40 and 100 °C, viscosity index, Total Acid Number (TAN), Total Base Number (TBN), Fourier Transform Infrared Spectroscopy (FTIR), and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) were used to evaluate their physicochemical and chemical degradation. The results showed that increasing temperature and exposure time generally intensified density growth, viscosity thickening, viscosity-index deterioration, and oxidation, whereas SiO2-containing formulations exhibited more moderate changes in several responses under severe ageing conditions. At 260 °C and 168 h, the viscosity at 40 °C of the conventional lubricant decreased from 193.7 mm2·s−1 without nanoparticles to 135.1 mm2·s−1 with 1.00 wt.% SiO2, corresponding to a reduction of 30.25%. For the biolubricant, viscosity decreased from 312.9 mm2·s−1 to 98.6 and 116.8 mm2·s−1 with 0.75 and 1.00 wt.% SiO2, corresponding to reductions of 68.49% and 62.67%, respectively. Under the same severe ageing conditions, oxidation in the conventional lubricant decreased by 27.19% at 1.00 wt.% SiO2, while oxidation in the biolubricant decreased by 34.11% and 22.08% at 0.75 and 1.00 wt.% SiO2, respectively. Response Surface Methodology combined with ANOVA and backward elimination identified temperature, exposure time, nanoparticle concentration, and their interactions as response-dependent controlling factors. The resulting reduced regression models provided high goodness of fit; for example, the coefficients of determination ranged from 0.9976 to 0.9981 for density and from 0.9954 to 0.9999 for viscosity at 40 °C. The response surfaces confirmed that temperature and time generally promoted degradation, whereas SiO2 addition was associated with lower excessive viscosity and oxidation increases under specific concentrations at high thermal severity, with a more pronounced effect in the biolubricants. Overall, KH570-modified SiO2 nanoparticles showed potential for mitigating changes associated with accelerated thermo-oxidative ageing, although the optimum concentration depended on the lubricant formulation and the investigated response. Because all formulations were tested under identical nominal ageing conditions, the observed differences support a comparative assessment of nanoparticle effects; however, the contribution of evaporation and concentration effects could not be independently quantified.