Tribological characteristics of mineral and plant oil blends

Due to the biodegradability, low toxicity, high viscosity index, reasonably cost to produce and environment friendly, the bio-lubricants such as vegetable oils are being commonly used as sustainable resources. However, vegetable oils are still suffering from low wear resistance (high wear scar) and low thermal stability (low flash temperature parameter), which led to limit their applications. In this research, the evaluations of tribological characteristics as a new source of bio-lubricant, namely, cactus oil besides other different types of vegetable oils that include refined, bleached, and deodorized (RBD) palm olein, castor oil and also jatropha oil were implemented. These types of oils were employed as an extensive substitute to mineral oil in the air compressors machines and with the intention to reduce the amount of mineral oil, which these are non-degradable, toxic properties, unfriendly to the environment and also high-priced in producing, the blends of vegetable oils were also applied as a partially bio-lubricant replacement for the mineral oil. According to ASTM D4172-B standard test of four-ball tribotester, the lubricity of the oils was examined in terms of kinematic viscosity, viscosity index, coefficient of friction (Cf), wear scar diameter (WSD), and flash temperature parameter (FTP). The results of normal load, rotation speed, and oil temperature were also evaluated for blended and neat oils, and then the results were compared to the mineral oil. The observations of the wear surfaces were attained utilizing an image processing that considered as a new technique in the tribological field, as well as the ball volumetric losses were also calculated. To optimise the blended oils based on the computed parameters, a design of experiments (DOE) method was used. The tribological performance characteristics, such as the wear losses, cylinder surface temperature, and kinematic viscosity were evaluated using a reciprocating air compressor for 60 hours as an intermittent operating time. As a comparison between the vegetable oils, the neat cactus oil shows a satisfaction tribological behaviour due to lowest value of ball volume losses and comparable results for wear scar diameter and flash temperature parameter after being used as an abundant bio-lubricant. It was also found that the cactus oil fulfilled a significantly better tribological performance as a partial bio-lubricant, as an illustration, the wear losses was 0.058 g/l for the optimum cactus blend codes as (E44.24/CC55.76), while the mineral oil was 0.09 g/l. The results also demonstrated that the wear scar diameter for the optimum cactus blend under 600 N shows a 37.83% reduction and the wear losses for the optimum RBD palm olein blend shows a 97.55% reduction. Furthermore, it was also observed that the flash temperature parameter values increased to be 135.35% and 114.07% for the E53.11/RB46.89 and E44.24/CC55.76, respectively, which therefore it is able to conclude that the blending process contributed towards improving of the tribological characteristics and the air compressor performance. With regards to the results of viscosity, the neat and optimised blends oils exhibit an essential consistency based upon the viscosity grade requirement of ISO VG32. Moreover, the results of image processing possesses appropriately represented the surface wear characteristics and provided precise details about the distribution of wear depth and wear shape together with the ball volumetric losses.