Characteristics Biolubricant Enriched with Nanoparticle Additives: a Review

The engine movement mechanism is a system composed of machine elements that contact or rub against each other between these surfaces. Surfaces that rub against each other will cause a friction force that causes wear on the surface, so it requires energy to fight the friction force. Lubricants are needed to reduce friction and reduce wear rates, where lubricants are generally made of mineral materials. Lubricants made from synthetic materials are non-renewable and cause environmental problems. The development of biolubricants as lubricating materials based on animal fats or vegetable oils is continuously being carried out. This study aims to observe the potential of biolubricants based on reviews from various previous research journals. The use of additives in biolubricants is also studied whether the addition of these additives can improve the characteristics of biolubricants. This study shows that the addition of certain additives to biolubricants can improve the characteristics of biolubricants. Copyright © 2020. Journal of Mechanical Engineering Science and Technology


I. Introduction
Improving a friendly environment and efficiency resistance is important in the engine movement mechanism. Development of biolubricants as environmentally friendly lubricants continues to be developed to meet green tribology standards. The use of mineral-based lubricants is generally dominated by the automotive sector, which is about 80%, while the rest comes from synthetic materials and other materials [1]. The lubricating properties of mineral materials are non-renewable and can cause environmental problems. Research studies on lubricants from biological materials are continuously being carried out in order to obtain lubricants from biological materials that are better than lubricants from mineral substances. Sources of lubricants from biological materials consist of soybeans, cocoa, coconut, jatropha, palm oil, which generally can be grown in tropical areas [2]. Biolubricants play an important role in supporting the green tribology program. Green tribology is green tribology, namely the coverage of tribology related to friction, wear and lubrication to support environmental sustainability. The United States has used one-third of its energy needs to deal with energy caused by friction. The greater the frictional energy in a movement mechanism will lead to greater energy consumption and the resulting environmental impact [4]. The main review of green tribology aims to preserve and improve the quality of the environment and avoid global warming [5].
The criteria and characteristics of a lubricant must meet six basic standards, namely: viscosity index, total base number, pour point, flash point [6]. Biolubricant which has a low viscosity value can be improved by adding additives, its characteristics as a biolubricant become better. Lubricant is a substance that is applied between two surfaces in contact with each other to reduce the friction force and wear rate on the two surfaces [7]. Research on the use of cultivable biological materials as biochemical base materials continues to be studied. Lubricant substitution from biological materials as a substitute for mineral materials continues to be developed to preserve nature and to increase public awareness of the importance of a green environment [8]. Lubricating materials with nano-scale particles are very intensively developed so that the lubricants can reduce friction and wear. The lubrication mechanism is divided into two types: (a) Direct lubrication mechanism and (b) Indirect lubrication mechanism.
Nanoparticles are added to lubricating oil to improve its tribological properties, which can reduce the coefficient of friction and wear [9]. Figure 1. shows the lubrication mechanism between two contact surfaces for nano particle lubricants. The basic ingredients for the manufacture of lubricants are classified into three types, namely mineral lubricants, synthetic lubricants and also biolubricants (vegetable oils and animal oils). Biolubricant was used as a test material in this study. Biolubricant, namely animal and vegetable lubricants obtained from animal and plant fats. Lubricants are generally made from mineral oil. The difference between mineral lubricants and animal and vegetable lubricants is that animal and vegetable oils contain unsaturated compounds [10]. The advantage of biolubricant lies in its ability to lubricate and has good properties on wet and humid surfaces when compared to mineral oil. The lubricant layer will provide effective protection against friction. The use of vegetable lubricants are expected to improve the characteristics of lubricants that are environmentally friendly.

a. The Effect of Lubricant Variation on Viscosity and Coefficient of Friction and Wear Rate
The main function of a lubricant is to reduce friction and wear that occurs on moving objects rubbing against each other. The science of tribology is a very important part of knowing the characteristics and its application in the engineering field [11]. Tribology is the science of friction, wear, and lubricants. Viscosity is one of the physical properties of a lubricant which indicates the viscosity of the lubricant [12]. The viscosity of the lubricant affects the friction efficiency of the contact surface. The greater the viscosity of a lubricant, the greater the drag force. Stokes law states that a large viscosity causes a large friction force [13]. The friction force is proportional to the viscosity value of the lubricant. Based on the equations of the friction mechanism formula and the Stokes law, the coefficient of friction is inversely proportional to the friction force and viscosity [14][15]. Table 1. Variations in viscosity and coefficient of friction and wear rates [16][17][18][19][20][21] Research on the correlation between the viscosity of 100 O C (cSt) and the coefficient of friction (coF) can be seen in Figure 2. The graph shows that the greater the viscosity value of a lubricant causes the lower friction coefficient value. The results of research regarding the correlation of viscosity and friction coefficient are in line with the equation of the friction mechanism and the law of Stokes, namely the greater the viscosity value, the smaller the friction coefficient value. The results of the wear test research can be seen in Figure 3. The graph shows the same trendline as Figure 2, namely the greater the viscosity value of a lubricant, the lower the wear value [16][17][18][19][20][21].

b. Effect of Additives on Characteristics
Armuggam et al. [22] studied the effect of Rappassed Oil plus TiO2 additives and found that the power value was reduced by 15%. Biolubricant based on vegetable oil added with TiO2 additive experienced a 58.1% decrease in wear value from pure organic oil [24]. Charo et al. [23] investigated mineral oil SAE 10W30 plus additive MoS2 obtained 20% reduced wear value. Mineral Soybean Oil coupled with CuO additives resulted in a reduced wear value of 12% [24]. Paraffinic oil with the additive MoS2 decreased the coefficient of friction by 64% [25]. Sharma et al. [26] studied vegetable oil with TiO2 added to get a reduced wear value of up to 58.1% compared to pure oil. Rapessed oil with added CuO additives decreased the coefficient of friction by 40% [26]. Sgroi et al. [27] studied synthetic oil SAE 5W30 added with MoS2 additives to get a decrease in the value of the friction coefficient of 50%. Polyalphaolefins added with the additive MoS2 resulted in a 40% reduction in the strength value [28]. Singh et al. 2019 [29] examined the tribological performance of SAE 10W/40 and SAE20W/40 lubricants with the addition of titanium additives and graphene oxide. These additives can reduce the friction coefficient by 32%. Improved friction behavior of SAE 15W-40 lubricants with the addition of graphite particles. The addition of graphite to SAE 15W-40 lubricant can reduce the friction coefficient from 33-36% [29].

c. Characteristic Biolubricant
Characteristics of biolubricant can be observed from the test results. There are several characteristics in a biolubricant, namely Kinematic viscosity, Viscosity index, Total base number. The characteristic values of some biolubricants are shown in Table 3.

d. Test Methods
The method in testing that can be used to evaluate the tribological properties of a lubricant can be seen in Table 4, done by various researchers on lubricating oil.

III. Conclusions
Based on previous research studies on the use of additives in biolubricants, the following conclusions are the vegetable oil derived from plants can be used as a biolubricant, but pure biolubricant has a low viscosity so that the lubricant function as a film layer between two contact surfaces is lower when compared to mineral oil; Additives need to be added to pure biolubricants to improve their viscosity, so as to reduce the friction coefficient and wear rate on the contact surface; The ability of additives to improve biolubricant characteristics varies greatly depending on the type of biolubricant and additives used; The addition of MoS2 additives to paraffinic oil can reduce the value of the largest coefficient of friction, namely 64%, when compared to the combination of additives + biolubricants as follows: TiO2 + Vegetable Oil; CuO + Rapeseed Oil; TiO2 + Rapressed Oil; TiO2 + palm oil.