Evaluation of physicochemical and tribological properties of chaulmoogra (Hydnocarpus wightianus) oil as green lubricant base stock

Due to environmental concerns, natural oils are increasingly being used as an alternative to mineral oil-based products such as lubricants. However, the use of edible oil for industrial application adversely affects their availability for edible purposes. Chaulmoogra oil is a non-edible oil obtained from the seeds of chaulmoogra tree that grows in wastelands. The fatty acid constituents of vegetable oils determine their properties. Presence of long-chain cyclic fatty acids like chaulmoogric acid, hydnocarpic acid and gorlic acid distinguishes chaulmoogra oil from other oils. Research on how cyclic fatty acids influence the properties of vegetable oils and affect their efficacy as base oils for lubricants is scarce in the literature. This paper presents a preliminary evaluation of the physicochemical, viscometric, oxidative and tribological properties of chaulmoogra oil as a lubricant base stock. Gas chromatography–mass spectroscopy, Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy methods were used for structure elucidation. The viscometric analysis was conducted by using Brookfield viscometer. The tribological performance was evaluated by a four-ball apparatus as per ASTM D4172-94. Thermal and oxidative stabilities were evaluated by thermogravimetric/differential thermal analysis in nitrogen and oxygen environments. Pour point was assessed by ASTM D97 method and differential scanning calorimetry. Chaulmoogra oil has a viscosity of 9.2 cSt at 100 ℃ and a viscosity index of 166. Though the coefficient of friction was better than a commercial lubricant of similar viscosity range (SAE 20W40), the wear scar diameter was comparatively higher. Addition of antiwear additive zinc dialkyl dithiophosphate showed considerable improvement in the wear performance of chaulmoogra oil. Chaulmoogra oil showed a high pour point value around 15 ℃ which needs to be improved by appropriate pour point depressants. Environmental advantage, coupled with non-edible nature of chaulmoogra oil, makes it an excellent base stock for lubricants.

FORMULATION OF REFINED, BLEACHED AND DEODORISED PALM STEARIN WITH ZINC DIALKYL-DITHIOPHOSPHATE ADDITIVE AND ITS TRIBOLOGICAL PERFORMANCE

Vegetable oils have recently received worldwide attention for their use as a lubricant base stock that has numerous advantages, including their environmental friendliness. In this study, a refined, bleached and deodorised palm stearin was selected as the base lubricant, and its friction and wear performance were investigated with a pin-on-disk tribotester. The effect of zinc dialkyl-dithiophosphate (ZDDP) additive in concentrations of 1wt%, 3wt% and 5wt% on friction and wear performance were evaluated. Commercial semi-synthetic oil SAE 15W50 was used for comparison purposes. The experiments were conducted at a sliding speed of 1.5 m/s under a normal force of 9.81 N for 60 min. Results show that an increase in ZDDP concentrations improved both friction reduction and wear performance of the lubricant. The coefficient of friction (COF) of RBD palm stearin was reduced approximately at 71% when 5wt% of ZDDP was added and it shows that the friction reduction performance of PS+5wt% (COF=0.039) was comparable to SAE 15W50 (COF=0.035). While, wear coefficient of RBD palm stearin was reduced significantly from 2.08 × 10−3 to 8.89 x 10−5 when 5wt% ZDDP additive was added and it shows that the wear performance of PS+5wt% was better than that of SAE 15W50, 1.94 x 10−4. Further analysis of the wear worn surface with a high-resolution optical microscope was also conducted with a surface profilometer to examine the metallurgy of the pin surface and the roughness of the pin.

Mechanisms of antiwear tribofilm growth revealed in situ by single-asperity sliding contacts

Zinc dialkyldithiophosphates (ZDDPs) form antiwear tribofilms at sliding interfaces and are widely used as additives in automotive lubricants. The mechanisms governing the tribofilm growth are not well understood, which limits the development of replacements that offer better performance and are less likely to degrade automobile catalytic converters over time. Using atomic force microscopy in ZDDP-containing lubricant base stock at elevated temperatures, we monitored the growth and properties of the tribofilms in situ in well-defined single-asperity sliding nanocontacts. Surface-based nucleation, growth, and thickness saturation of patchy tribofilms were observed. The growth rate increased exponentially with either applied compressive stress or temperature, consistent with a thermally activated, stress-assisted reaction rate model. Although some models rely on the presence of iron to catalyze tribofilm growth, the films grew regardless of the presence of iron on either the tip or substrate, highlighting the critical role of stress and thermal activation.