## 1 Introduction  
The study and development of efficient and effective lubricants continue to be a focus for scientists, industries, and governments, since lubricants are substances that remain between surfaces in relative motion, mainly to reduce friction and wear [1; 2; 3; 4; 5]. Their use affects the efficiency of machinery, energy consumption, the local and global economy, and even the preservation of the environment [1; 2; 5]. Green tribology is a recent focus of tribology, which involves the classic challenges of tribology: to reduce friction and wear, but with an environmentalist approach [5]. Utilizing natural and biodegradable lubricants are two of the basic principles of green tribology that promote the use of green lubricants to prevent environmental pollution [5]. Green lubricants exhibit low toxicity, excellent biodegradability, and, usually, they are derived from natural vegetable oils [2; 5]. The most commonly employed natural oils for producing bio-based lubricants are rapeseed, sunflower, canola, soybean, palm, coconut, and castor oil, to name a few [1; 2; 5; 6; 7; 8]. Generally, vegetable oils have excellent lubricating properties, and they also present high viscosity indexes (VI) and high flash points [2; 5; 8]. Although natural oils have some limitations, such as poor cold flow properties and oxidation stability, such properties can be improved by chemical modifications (transesterification, hydrogenation, epoxidation, etc.) or by the incorporation of additives (anti-wear, anti-oxidation, friction modifiers, etc.) [2; 9]. Current advances in green lubrication have been based on improving the physical and chemical properties of vegetable oils--for instance, the study of [PERSON] et al. (2019) [10], who improved the dynamic viscosity, thermal stability, and oxidation stability of rapeseed and soybean oils by adding Pentaerythriol rosin ester. On the other hand, the enhancement of the friction response of some natural oils has been made principally by the addition of nanoparticles into the oils. One example is the investigation of [PERSON] et al. (2019) [11], in which the authors improved the friction properties of soybean, sunflower, and corn oil by adding SiO\({}_{2}\) nanoparticles, thereby enhancing the friction coefficient from 10% to 26%. Castor oil has been one of the most valuable vegetable oils for lubricating applications. Recently, [PERSON] et al. (2018) [12] prepared an ecological ricinoleic acid based ionic liquid and studied its performance as an additive in glycerol solutions. They found a remarkable improvement in the anticorrosion and lubricating performance of the green lubricant. [PERSON] et al. (2019) [13] instead enhanced the friction and adhesive wear response of Castor oil by adding MoS\({}_{2}\) nanoparticles. The previous research related to green lubricants shows the great progress that has been made in improving the friction and wear of vegetable oils as lubricants, as well as the formulation of more efficient bio-lubricants. However, as tribology also encompasses all the aspects of the interacting surfaces, it is important to characterize the surfaces of the lubricated contacts to understand their mechanisms of friction, wear, and lubrication [8]. Surface characterization by surface roughness and hardness measurements provides valuable complementary knowledge about the efficiency of the lubricated contact. Nevertheless, in the tribological performance research of green lubricants, hardness has been set aside, and this could be a crucial avenue to expand their applications. Based on the above, in this study, the effect that green lubrication has on the tribological and hardness properties of a mechanical system was investigated. For this purpose, three bio-based lubricants were studied, two of them using vegetable oil bases and mineral oil fractions and synthetic esters, and one using neat vegetable oil (Castor oil).