**1. Introduction**

Due to the amount of wear and friction present in an automotive vehicle, lubricants are essential for the longevity and function of cars. Greases, which are boundary lubricants, are essential to the protection of car parts, preventing them from breaking down. Not only are they formulated specifically for the purpose of preventing wear, but they are also made to withstand the conditions of a car in action. Greases prevent rusting and accumulation of debris on surfaces by forming a protective layer; in addition, greases' many properties such as their ability to flow at high temperatures while also being excellent at dissipating heat are what make them valuable and widely used [1]. A vast majority of applied greases will last throughout the entire lifetime of vehicles and do not need to be reapplied.

Greases are commonly derived from petroleum or synthetic materials. Synthetics are often preferred as they can typically function over a wider range of temperatures compared to those made from petroleum [2]. Furthermore, different types of vehicles can operate under different conditions and with either a higher or lower number of variables. For example, vehicles operating under higher/lower moisture, extreme loads, and having high speed bearings will all require different grease specifications to properly protect against corrosion and wear. This can be done through the usage of thickeners and additives such as rust inhibitors and anti-wear and friction-reducing agents [3].

In electrical vehicles (EVs), greases need to be formulated for new factors, the major ones being the increased presence of electricity, electrical currents, and noise in an EV due to the absence of an internal combustion engine (ICE) [4]. Due to the increased number of electrical components such as electric modules and sensors, the greases must be formulated to be unreactive with electricity. Usually, the noise of an engine will mask the creaks and rattles of a car, but as an EV is silent, any noise from the lack of lubrication and contact of surfaces will become much more apparent. Furthermore, a larger amount of grease must be used in an EV than in an internal combustion engine vehicle (ICEV) as the need for lubrication is higher in an EV. To further the understanding of the need of greases in EVs, this perspective paper will focus on the new variables that are present in EVs that necessitate a specified formulation of greases.

#### **2. Greases in Automotive**

#### *2.1. Types of Greases and Their Usages*

The most common types of greases are soaps, of which different options are shown in Table 1 along with their properties and applications. Lithium greases are widely used for their lubricity, shear stability, and thermal resistance [3,5]; calcium-based ones have better water resistance but worse thermal resistance, and sodium greases have high dropping points but cannot operate above 120 ◦ [6]. These greases are created through the blending of base oils and thickeners through intense mixing until the mixture becomes gelatin-like [3,7]. Overall, lithium grease has been shown to impart the advantages of high adherence, noncorrosiveness, available at high pressures, and moisture resistance, making it compatible with several OEM applications such as EVs/hybrid vehicles (HVs) [8,9]. Other types of greases include polyuria (PU), clay, and silica. PU greases' high operating temperature, oxidative protection properties, and low bleed features make them useful for permanent sealing applications [10]. Clay greases are inert and favored in the food industry [3]. Although silica-based greases are a strong thickener, they are very sensitive to heat.


**Table 1.** Greases containing mineral oil (soaps) (Adapted from [8]).

\* Depending on conditions of service.

Overall, approximately six different types of greases are present in a vehicle system— the majority of which are soap greases. Lithium greases are present in car hinges to gears. White greases are used for water sealing and surface adhesion. Aluminum greases are often used in wheel bearings, and copper greases are used in exhaust assemblies and battery pole connections. Finally, red rubber greases are present in areas with O-rings and hydraulic systems, and brake greases are used in drum and disc brakes [11].
