*2.3. Viscosity*

Generally, the selection of viscosity is determined by bearing speed, operating temperature, bearing dimensions, and race surface finish. For roller element bearings, the focus will be on EHL conditions, which covers many of the conditions these bearings operate in.

The previous equations relate the variables needed to arrive at the optimal viscosity by determining film thickness. More specifically, they relate surface speed and required viscosity to determine the minimum EHL film thickness needed to match the composite surface roughness in the loaded contact area. From our previous discussion on load, we will focus on these variables given that the contribution of load to this determination is minimal under EHL conditions. There are graphical relationships that relate viscosity and speed to determine the optimal viscosity.

The next step is to determine the correct viscosity at the bearing operating temperature. Using the readily available viscosity curves, the ISO grade for the application can be determined.

Final confirmation of the correct operating viscosity is determined by calculating the specific film thickness:

$$\text{Lambda} = \text{Specific Film Thickness} = \frac{\text{h}}{\sigma} = \lambda$$

where h = film thickness from the previous equations and

#### *σ*= measured surface roughness (r.m.a., microns)

The choice of the correct viscosity minimizes asperity contact in the element load zone. Specific Film Thickness confirms the chosen viscosity, estimated from bearing speed and temperature, and yields the film thickness for the specific bearing surface finish characteristics. Lambda can have a significant effect on bearing fatigue life.

Lambda values below 1.5 reflect a dramatic loss of bearing fatigue life. Under this condition, the film thickness is not high enough to cover the average asperity height, leading to excessive friction and wear of the race and roller element surfaces. Lambda values between 1.5 and 3.0 contribute to an increased fatigue life, reflecting the ability of the film to cover load zone asperities. Lambda values above 3.0 exhibit increasing but diminishing improvements in fatigue life.

Determination of the correct viscosity is followed by determination of the National Lubricating Grease Institute (NLGI) grade needed for the application. The next step is determining the temperature range at which the grease will work. Care must be taken so as to not over- or underestimate the operating range, as catastrophic failure can occur. Therefore, it is always advised to measure the temperatures of the components if possible. Oftentimes, multiple greases can be found suitable to a particular application, at which point it should come down to the costs unless the item is already on hand [8].

#### *2.4. Grease Formulation Issues*

Since greases are formulated to operate within a specific range of conditions, operating outside those conditions or prolonged exposure to high temperatures will have a negative impact on the longevity of said grease. Figure 1 shows a correlation between induction time and the grease life—data obtained from the ASTM D 3527 test. A relationship is noticeable in that as the induction time increases, so does the life of the grease. Further testing in Figure 2 shows that an increase in temperature also leads to a decrease in induction time, thereby reducing the life of the grease. This is because greases with longer induction times achieve higher oxidative stability [14].

**Figure 1.** Induction time vs. grease life (ASTM D 3527 Test) [14].

**Figure 2.** Temperature effects on induction time [14].

#### **3. Performance Requirements for Grease Used in EVs and HVs**

In EVs/HVs, a new configuration was designed to use an electrical motor combined with a battery module system for generating energy power. Instead of an engine, which is lubricated with oil and transfers power to a transmission and from there to the wheels, a battery module system powers an electric motor that drives the wheels. When designing EVs, lubrication engineers must select gear oils, coolants, and greases to meet new driveline requirements. The EV/HV configuration design has created substantial impact on driveline lubrication and thermal cooling requirements in the areas of electrical and thermal transfer characteristics, energy efficiency, and the presence of electric currents and magnetic fields. In addition, more system-related requirements such as noise, vibration, and harshness (NVH), seals, and materials' compatibility are also being considered for important performance characteristics.

#### *3.1. Electrical and Thermal Characteristics for Grease Used in EVs and HVs*

In many HV or EV advanced system designs, automotive lubricants such as drivetrain fluids or grease lubricating encounter the integrated electric motor (e-motor) and thermal managemen<sup>t</sup> devices. This leads to the addition of electrical and thermal properties which must be considered on top of conventional lubricant properties. The introduction of electrification components has been targeted for energy efficiency and long-term durability. Automotive industries have asked for the implementation of specialized automotive lubricants or driveline fluids to allow for appropriate thermal cooling specifications, to present bearing protection, to ensure corrosion protection, and to offer oxidation and sludge control.

Recently Tung, Woydt, and Shah published a paper related to the future trends of HV/EV driveline lubrication and thermal managemen<sup>t</sup> [9]. They also reported that grease specifically designed for driveline lubrication of HVs and EVs should include appropriate electrical properties to guarantee protection from corrosion and be well suited with insulating materials [9,15,16]. There are two additional considerations on electrical conductivity and thermal conductivity of fluids. If the electrical conductivity of the liquids exceeds a certain value, current leaking can occur. However, if it is too low, degradation of the oil can occur because of lubricant oxidation, a direct outcome of electrical arcing in oil, and leads to a decrease in the protective ability of the fluid. Lubricants are not conductive but rather dissipative, though additives can affect the level of electrical conductivity. Conductivity might increase, however, as oxidation causes an oil to deteriorate. Additive suppliers suggested that lubricants susceptible to oxidation are potentially problematic. They also need to have appropriate thermal transfer characteristics, offer high-speed bearing protection, and provide oxidation and sludge control.

#### *3.2. Electric Field Interactions with Driveline Lubricant or Grease Used in EVs/HVs*

To understand driveline lubrication under the electric field is very crucial for EV/HV applications by automotive and lubricant suppliers. In a recent publication by Chen, Liang, and co-authors [15], they pointed out that lubricant properties under electric field interaction must be investigated. These important properties can be described as the electrostatic interaction, the electric charge distribution, the formation of transfer film/structural change, and the chemical–physical property changes. It has been found that lubrication is aided by weak electrostatic interactions. Static charges and the transient polarized charges on surfaces, which may be induced and strengthened by the externally applied field, enhance electrostatic interactions. They also claimed that at low electric potential, wear is adhesivetype dominated, while it is abrasive-type dominated when the potential is high. DC has been observed to enhance friction while the friction is reduced by AC. This interaction mechanism is due to vibration induced by the electrostatic force which is fluctuating under electric field. Structural change/oxidative transfer film formation in some material combinations (e.g., graphite–graphite, graphite–copper) has been found responsible for increased wear and reduced friction under the application of the external electric field [9,15].

In addition, Rhee, Yan, He, Xie, and Luo [14–16] indicated that chemical reactions and physical absorption occur at material interfaces under the influence of an external electric field, leading to a change in surface friction and lubrication behavior. Electric carrier (or electron–hole) charge distribution through the formation of localized quantum dots and electron–hole recombination affects interfacial mobility and surface friction properties [14–16].

#### *3.3. Electric Breakdown Mechanisms of Lubricants and Grease*

Besides the electrical properties of driveline lubricants or lubricating grease described in the above section, automotive R&D scientists [9,15–18] have reported that a highly fluctuating charged environment requires specially tailored lubricants to avoid component damage and premature failure due to improper lubrication. Major lubrication failure mechanisms explored can be classified into lubricant degradation, microbubble formation, and electrowetting. In the lubricant degradation mechanism, the lubricant base oil and

thickeners undergo chemical oxidation to form carboxyl compounds. Lubricity is lost on account of the formation of highly viscous and acidic degradation products and agglomeration of additives. Heat generation causes faster base oil evaporation. Local overheating in EV/HV lubricants can lead to microbubble formation, which may then be driven by viscous drag, pressure gradient, and dielectrophoretic forces. The formation of these microbubbles, which are unstable and coalesce, tends to destabilize lubrication upon electrical breakdown. The microbubbles form more rapidly in conditions of electrode insulation. In addition, the electric field induces interfacial stress on a non-polar lubricant confined between two metallic surfaces in the electrowetting mechanism. Due to differing dielectric properties, a two-phase dispersion of lubricant may also destabilize this mechanism and can lead to the spread and breakdown of the lubricant when the electrostatic stress is too high.

#### *3.4. Thermal Cooling Requirements for Lubricants and Grease Used in Electrical/Hybrid Systems*

The heat generation rates in engine power controllers, computer chips, and optical devices/systems are on the rise because of future development trends that favor higher speeds and smaller features for increased performance for engine components, microelectronic devices, and brighter beams for optical devices [9]. Thermal cooling has become one of the main focuses of advanced industries such as microelectronics, transportation, manufacturing, and metrology. Electric hybrid and fuel cell vehicles use power electronics to control their electric motor. Power electronics require their own cooling loop including a heat exchanger, pump, and radiator. Power densities exceeding 100 W/cm<sup>2</sup> while needing to maintain a temperature below 125 ◦C may eventually exceed 250 W/cm2. Conventional cooling methods to promote heat rejection rates apply increased surface areas such as fins and microchannels for heat dissipation. However, current thermal cooling designs have already reached their threshold. For HVs or EVs, the cooling requirements are more stringent than IC engines, especially in the case of fast charging and heavy consumption. Lithium-ion batteries and EV motor systems need to maintain the correct temperature range by cooling means. If they exceed this range, the batteries will face a "runaway", not deliver the same power, and, more importantly, they will degrade quickly. Power electronics are also very susceptible to heat, especially during recharging. Heat sinks are used to draw away heat. Solutions for the thermal managemen<sup>t</sup> of EVs or HVs have been described in the recent publication [9] by Tung, Woydt, and Shah. For example, advanced thermal managemen<sup>t</sup> approaches such as microchannels for cooling batteries or fast-charging cables or immersion of battery cells in a dielectric fluid have been commercialized in energy and automotive industries.

#### Thermal Management and Measurement of Thermal Conductivity of Driveline Grease

The future development of thermal managemen<sup>t</sup> technologies is encumbered by the urgen<sup>t</sup> demand for thermal protection and cooling of electrification components. The traditional method for thermal cooling can no longer be progressed. New requirements have been enforced for high-performance cooling in electrical vehicles, batteries, motors, and power electronics [9]. New developments in thermal managemen<sup>t</sup> of EVs/HVs are helping to extend the driving range and lifetime requirement. Global research activities using advanced coolants have the scientific merits and high potential for application in thermal cooling technologies. To meet these thermal managemen<sup>t</sup> requirements, automakers are using combinations of cooling fluids and advanced thermal cooling devices throughout the electrical/hybrid propulsion system to improve overall energy efficiency and coolant compatibility with electrification components. In the next few sections, the authors will review the state-of-the-art thermal managemen<sup>t</sup> technology used for meeting thermal cooling and extended driving range requirements of EVs/HVs.

Recently, lubricant industrial researchers [18–23] have investigated the thermal properties of lubricants or grease operated under tribological sliding conditions. Pettersson and Callen [18,19] have shown the fundamental phenomenon that the base oil molecular structure determines the thermal capacity and thermal conductivity of a lubricant [18].

The higher the number of the rotational and vibrational quantum states, the higher the thermal capacity [19]. When there are multiple vibrational and rotational states, it takes a higher energy input to increase the averaged kinetic energy, e.g., the temperature. In addition, Gedde and Jin [20,21] have indicated that the thermal conductivity of base oil was correlated to the molecular diffusivity in the fluid. The more easily the molecules of a lubricant pass through each other, the higher the lubricant thermal conductivity. This also means that there is a relationship between the lubricant viscosity and lubricant thermal properties because both the molecular quantum state density and the diffusivity closely correlate to the lubricant viscosity. This correlation can restrict the selection of the lubricant when both the tribological working condition and thermal managemen<sup>t</sup> are considered. When tribological working conditions take a higher priority, it is difficult to change the base oil thermal properties. Thus, it is quite desirable to change the lubricant thermal property with some additives.

Jin, Shaikh, and Barbés have found out that driveline lubricants can significantly increase the thermal conductivity and thermal capacity of a lubricant by adding nanoparticles to the lubricant [21,22]. Essentially, adding those dispersed nanoparticles increased the carriers of thermal energies. Adding 0.8 vol% of silica nanoparticles can double the thermal conductivity of a lubricant [21]. Polyalphaolefin (PAO) containing 0.5 vol% carbon nanotubes has a more than 50% thermal conductivity compared to neat PAO. However, nanoparticles also lower the specific heat of the lubricant [23]. In addition, Chen and Dai and colleagues also investigated the synergistic effects of nano-lubricant additives [24,25]. They indicated that this nano-lubricant additive can be used to optimize the thermal property of a lubricant to fit any specific powertrain cooling design. Moreover, the nanoparticle additive improves the tribological performance of lubricants. They have shown substantial experimental evidence using this nano-lubricant additive which can dissipate heat extremely fast in EV/HV cooling processes.

In the lubricant industry, the most common experimental method for measuring the thermal conductivity of a lubricant is called the transient hot-wire method [26,27]. The transient hot-wire experimental set-up was simple to perform and had high accuracy. This method used a Pt or Ni wire which was sealed inside a cylindrical pressure vessel filled with lubricant. The wire was heated up for a short amount of time electrically, and its temperature was monitored simultaneously by its electric resistance. The thermal conductivity and the thermal capacity of the lubricant can be calculated from the temperature change of the wire. In general, this measurement can be modeled as an axisymmetric thermal transportation problem [26]. It has an additional advantage when used to characterize lubricants, as the lubricant thermal properties are highly correlated with its pressure, and the pressurized transient hot-wire method is easy to achieve.

#### *3.5. The Other Requirements for Grease Used in EVs and HVs*

Another three major issues that are present in EVs or HVs are energy efficiency, noise, and electrification components [28]. Another important aspect of an EV is the increased presence of electrification components and how the greases will be affected by the electromagnetic fields and electrical currents. These place an even higher emphasis on corrosion prevention as the electrical currents will corrode the metallic components at a faster rate as compared to those in a regular ICE. In this perspective paper, we will discuss these important areas and their specific requirements used in EVs/HVs in the following sections.
