Instituto Tecnológico y de Estudios Superiores de Monterrey

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Instituto Tecnológico y de Estudios Superiores de Monterrey

CEM Campus

School of Engineering and Sciences

Master of Science in Nanotechnology

Thesis

Tribological evaluation of a graphene-based lubricant in metal-on-metal and wet clutch interfaces

by

Julio Alberto Cao Romero Gallegos ID A01327704

Mexico City, June 11^th, 2021

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Instituto Tecnológico y de Estudios Superiores de Monterrey Campus CEM

School of Engineering and Sciences

The committee members, hereby, certify that have read the dissertation presented by Julio Alberto Cao Romero Gallegos and that it is fully adequate in scope and quality as a partial requirement for the degree of Master of Science in Nanotechnology.

Dr. César David Reséndiz Calderón Tecnológico de Monterrey School of Engineering and Sciences

Committee Member Dra. Dora Iliana Medina Tecnológico de Monterrey School of Engineering and Sciences Committee Member

Engineering and Sciences

Mexico City, June 11^th, 2021

Dr. Leonardo Israel Farf á n Cabrera Tecnol ó gico de Monterrey School of Engineering and S ciences Principal Advisor

Dr. Rubé n Morales Menéndez D ean of Graduate Studies School of

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Acknowledgments

Al Instituto Tecnológico y de Estudios Superiores de Monterrey, así como al Consejo Nacional de Ciencia y Tecnología por la beca de estudios y el apoyo económico que me han otorgado, respectivamente, para poder desarrollar este trabajo de tesis.

A mi director de tesis: el Dr. Leonardo Israel Farfán Cabrera, por su consejo, apoyo, asesoría y guía durante la maestría.

Al laboratorio de Reología y Física de la Materia Blanda del Instituto Politécnico Nacional por proveer las nanopartículas de grafeno y apoyar en la preparación de las muestras de aceite utilizadas para desarrollar este trabajo de investigación.

A Arturo Ramírez Mendoza, Carlos Aguirre Miranda, José Jorge Rojas Carreón y Julio Varela Soriano del laboratorio del Centro para la Innovación en Diseño, Manufactura y Automatización por haberme apoyado y siempre tener un ambiente ameno en el laboratorio durante mi estancia en la maestría.

A mi madre Griselda Lizeth Gallegos Gutiérrez, mi padre Jesús Alfonso Cao Romero Arroyo y mi hermano Carlos Alfonso Cao Romero Gallegos, por su incondicional apoyo, consejos y motivación para superarme profesionalmente.

Al Dr. Juan José Cantú Gutiérrez, que en paz descanse, a mi abuela María Julia Gutiérrez Kanter y a mi tía María Eugenia Cao Romero Arroyo por haberme inspirado, así como aconsejado y motivado para estudiar un posgrado.

A todos mis compañeros de la materia de proyecto integrador por haberme apoyado con el diseño y los primeros pasos de la fabricación de la máquina perno sobre disco utilizada para conformar este trabajo de investigación.

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Abstract

Certain nanoparticles have been consistently demonstrated to exhibit remarkable lubricant improvements as used as additives. In particular, graphene nanosheets (GNS) added to lubricating oils have shown significant reduction of wear and coefficient of friction (CoF) in a wide range of metallic interfaces for different applications. Wet clutches from automatic transmissions are lubricated with high-performance automatic transmission fluid (ATF). Their efficiency, in terms of torque transmission by friction, depends on the tribological behavior of both the wet clutch sliding plates and ATF. However, commercial ATFs need continuous enhancement to comply new and more stringent OEM requirements. One of those requirements is to maintain the short- and long-term antishudder property of ATFs. This property, which is represented by a friction coefficient increasing with sliding speed, helps to keep reduced vibrations during clutch engagements and lock-up stages in the transmission, which is needed for driving comfort and increased durability of involved components in the transmission. Considering the tribological effectiveness of GNS in lubricants for reducing friction and wear, it can be a suitable additive for improving the antishudder property of ATFs. Although GNS have been largely studied as additives in lubricants for several metallic interfaces, their effects on wet clutches interfaces have been scarcely studied. Thus, this thesis contributes with an assessment of the tribological (friction and wear) behavior of graphene-based oils (blends of mineral base oil + GNS) at different concentrations in metal-on-metal contacts and the CoF behavior with speed (antishudder representation) in a wet clutch interphase using a pin-on-disc tester. The tests ran in metal-on-metal interfaces confirmed a significant wear rate reduction with unaltered CoF in aluminum and steel when a mineral oil with 1.0 wt.% GNS concentration was used in tests. In addition, the CoF of the wet clutch was reduced in general, while the antishudder property was enhanced by the addition of 1.0 wt.% GNS to the mineral oil.

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Introduction

Lubricating oils have always been the principal strategy to reduce material degradation and energy loses due to friction [1, 2]. In order to achieve the optimal performance of lubricants, different additives are added to mineral, synthetic or vegetable base oils. However, this formulation can be expensive, but it is negligible in comparison to the cost of the maintenance or premature failures in machinery [3]. Then, it is important to develop new and advanced lubricants for mechanical applications.

A crucial automotive component requiring of eco-friendly and high-performance lubricants is the automatic transmission (AT). ATs are complex mechanical units used to transmit torque at varied speeds in an automatic way on-demand in the vehicle. They typically use continuous slip torque converter wet clutches to improve efficiency and transmit power by friction [4]. A wet clutch is composed of an array of steel and friction material discs immersed in lubricant. They are pressed between each other to transmit power by friction engagement from one shaft to another shaft (drive shaft), as illustrated in figure 1. The lubricants used in ATs are named automatic transmission fluids (ATFs). They are advanced fluids used specifically as hydraulic fluids and for allowing the torque transfer in the wet clutches from the AT while keeping lubricated and cooled all the tribological elements involved in the AT.

Figure 1: Schematic image of a wet clutch and drive shaft. Redrawn from [5].

The friction discs are made of different composites. Among the most common is the resilient porous composite consisting of fibers, thermosetting resins and fillers attached to an inner steel disc [6]. The friction disc is a good heat insulator with a high coefficient of friction (CoF). On the other hand, the metallic discs, which are used as the countersurface due to its strength, rigidity, and high thermal conductivity, are usually made of high carbon steel. The high friction generated by the lubricated interfaces in the set of friction and steel discs is enough to provide the required transmission of power in the AT and to move the vehicle [4, 7].

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When the wet clutch operates, there is a significant difference of speeds between friction and steel discs during the clutch engagement. At idling stage, one disc (metallic disc) is static (S1) (as shown in figure 2) while the other disc (friction disc) has a rotational velocity (S2). Therefore, when the discs get in contact, the velocity (S1) will be forced by friction to reach the velocity (S2) in less than 0.25 s [8]. That means that the metallic disc will have a considerable acceleration while slip and friction is induced between the discs. This engagement process causes wear, vibrations, and heat to arise. Hence, the use of an efficient ATF is elementary since this lubricant provides an important tribological/thermodynamical improvement by reducing wear, vibrations and temperature keeping good frictional performance in the wet clutch. The relationship between frictional behavior and vibrations in wet clutches has been widely studied [9-12]. The vibrations caused by friction during wet clutch engagement are named as shudder. Shudder leads to detrimental effects for the entire transmission system producing increased fuel consumption and audible noises [9, 10, 12]. So, it is important that ATFs possess antishudder property. Kugimiya [13] reported that the use of additives with significant thermal stability can be effective in providing appropriate antishudder property, which is represented by positive slopes in CoF vs. Sliding speeds curves obtained from tribological experiments, as explained in more details in section 1.3.1.

Figure 2: Schematic image of Disc1 and Disc2 representing a static friction disc and a metallic disc rotating at the engine speed.

ATFs are expected to provide optimal power transmission and tribological characteristics not only in the short-term but also in the long-term [14]. Nonetheless, the ATF’s performance is diminished with the wet clutch long-term operation due to degradation and wear of clutch discs [10]. These lubricants are carefully formulated with different additives to reduce CoF values mainly

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being exposed to different pressures (0 - 2.9 Mpa), sliding speeds (0 − 25 m.s⁻¹) and temperatures (−40 - 200°C) [10, 16-19]. Nanoparticles with remarkable characteristics have been considered as potential substitutes to traditional additives. They are used to produce colloidal nanoparticles suspensions, also named as nano-based lubricants [1, 20, 21]. There are various materials used to produce nanoparticles as additives in lubricants, for example: metals (bismuth [22], nickel [23], copper [24, 25]), metal oxides (TiO2 [26], SiO2 [26], Al2O3 [27-30], MoS2 [31]) and carbon-based nanomaterials (multi-walled carbon nanotubes (MWCNTs) [32-34] and graphene nanosheets (GNS) [35, 36]). In general, the lubricating properties reported elsewhere for numerous nano-based lubricants have been satisfactory. In particular, MWCNTs and GNS have been demonstrated to present significant wear and CoF reduction under metal-metal interfaces [32-36]. Therefore, according to the remarkable thermal properties [37] and the reasonable tribological enhancements of GNS [35, 36], it is expected that they can have a positive effect on the antishudder property of ATFs.

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Problem definition

Shudder is a phenomenon caused by low-frequency vibrations that occur between 20 and 30 Hz in an AT [38]. They cause negative effects in the vehicle performance. It happens due to a continuous interaction/sliding of wet clutch discs during engagements and the transition between dynamic and static CoFs [39]. It is known that the shudder intensity is not totally dependent on the clutch materials and lubricant properties [40], but also it largely depends on AT´s hardware related problems [19]. However, providing good anti-shudder property to lubricants is the cheapest and fastest way to diminish shudder in ATs in contrast to modification to the AT hardware. Therefore, ATFs with better antishudder property are required nowadays for using in vehicles equipped with modern ATs. Besides, commercial ATFs, even those exhibiting higher performance, comprises additives which promotes acceptable antishudder property but are harmful for the environment [41]. Thus, eco-friendly additives, such as GNS could be a suitable option to formulate ATFs with similar or enhanced performance.

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Hypotheses and Research Questions

GNS nanoparticles added to mineral oil may reduce CoF and wear in metal-metal interface due to the effective tribofilm formed by GNS, as reported in literature.

GNS nanoparticles added to mineral oil may have a positive CoF vs. speed behavior in a wet clutch interface at operating temperatures (26°C and 100°C) due to the effective tribofilm formed by GNS, as reported in literature.

Research Questions:

• Could GNS added to the oils exhibit reduced wear rate on the base oil in metal-metal interfaces?

• Could GNS added to the oils exhibit reduced CoF on the base oil in metal-metal interfaces?

• Could GNS added to the oils exhibit positive “CoF vs. speed” curve in wet clutch interfaces at operating temperatures?

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Objectives

The aim of this thesis is to evaluate the tribological behavior of mineral oil blended with graphene nanosheets (GNS) in metal-on-metal and wet clutch interfaces.

The specific objectives of this research are:

• Adapt actual wet clutch components in a pin-on-disc tester set-up.

• Prepare oil, metallic and wet clutch samples.

• Run ball-on-disc tests using different concentrations of GNS in mineral oil on steel-steel and steel-aluminum sliding contacts.

• Carry out microscopy of the tested metallic samples for wear analyses.

• Run pin-on-disc tests using actual wet clutch materials at 26°C and 100°C.

• Carry out microscopy of the tested wet clutch samples for wear analyses.

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1 Theoretical framework

1.1 Nano-based lubricants

A nano-based lubricant is a dilute liquid suspension and engineered stable colloid of metallic or non- metallic nanoparticles (>100nm) added into a conventional heat transfer fluid as a base fluid [42].

A critical step to synthesize a nano-based lubricant is the dispersion of the nanoparticles since long- term nanoparticle dispersion stability in base fluids is one of the requirements for the use and commercialization of nano-based lubricants in different applications [42]. Therefore, the fluid- particle and particle-particle interactions, which depends on polarity, are critical in the nano- modified lubricant stability [14, 36, 42]. When the Van der Waals interactions dominate to other repulsive forces found in the suspension, the nanoparticles start to agglomerate and settle down. It generates a reduction in thermal conductivity and tribological performance [33, 42]. Moreover, if a certain vol.% or wt.% of nanoparticles is exceeded in the base oil, the presence of unsuspended particles will be visible. For instance, inorganic fullerenes of cylindrical or spherical shape with sizes between 50-150 nm have been demonstrated to not have the sufficient compatibility for dispersion in oils [1]. Nonetheless, the functionalization of those nanoparticles, which can be made through a surfactant addition such as stearic and oleic acid, can be used for improving stability [43]. Although it can be effective, there is a concern about their decomposition at high temperatures since it produces acidity of the resulting oil by its usage [1, 32, 33, 42]. For this reason, surfactant-free methods (i.e., ultrasonic probe-type disruption) have attracted attention recently [42, 44].

Quantity and size of nanoparticles added to the oil have a considerable effect on the performance of the resulting suspension [1]. The weight fraction consideration in the suspension process is important since choosing a quantity below the optimal quantity can promote undesirable properties [45], but selecting a quantity over the optimal can lead to the formation of more rapid nanoclusters in the base fluids. In addition, it may form clogs in the suspension channels for heat transfer [33, 42, 46]. Also, using the non-optimal concentration of nanoparticles can generate discontinuous oil films that leads to a reduction of the nano-modified oil´s performance [37, 46]. On the other hand, the size of nanoparticles is another factor which can affect the performance of the nano-modified lubricants since the mechanical and physic-chemical properties of the particles changes with size. As an example, the Hall-Petch law shows that hardness of nanoparticles is inversely proportional to their size (< 100nm). Hardness of nanoparticles is a matter of importance in lubricants. If nanoparticles are harder than the sliding surfaces, it can lead to increased wear caused by abrasion induced by the particles to the sliding surfaces [1]. Besides, tribofilm, coefficient of heat transfer and CoF are influenced by the nanoparticles size. For instance, if nanoparticle’s radius is larger than the size of the valleys of surface roughness, the nanoparticle will not deposit there, and it may not perform tribological enhancements by a third-body (rolling) mechanism at the interface [1]. Also, the coefficient of heat transfer has been reported to be inversely proportional to the grain size of the nanoparticle [47]. So, the use of nanoparticles as additives in lubricants can increase thermal conductivity, and consequently, rise its performance [20, 28, 32, 33, 42, 45].

Finally, in addition to select a suitable size and type of nanoparticle for the lubricants, a fluid synthesis method must be considered since it affects suspension’s stability and cost. There are two distinct types of synthesis for nano-based lubricants. The first is a one-step method, in which the nanoparticles are synthesized and dispersed within the fluid avoiding drying, transportation, storage and dispersion of nanoparticles in the process. Agglomeration is minimized while stability of the

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suspension is enhanced by using this method [48, 49]. The second is a two-step method. It starts with the preparation separately of the nanoparticles in powder form by physical or chemical methods. Then, they are dispersed in the oil in a consecutive but independent step. This is the most economic method to prepare nano-based lubricants in a large scale since the individual nanoparticle synthesis have already scaled up to industrial production levels [48].

1.2 Graphene

Graphene is a two-dimensional hexagonal lattice made of bonded carbon atoms. It has extraordinary physical properties (mechanical, chemical, electrical, thermal, and electronic) while being stronger, lighter, and stiffer than steel [37]. Its distinctive properties are provided due to their extremely tightly packed atoms and high surface area to volume ratio. Graphene is a superior material than graphite due to the lack of interlayer linkages which could significantly reduce thermal conductivity and other properties [50]. Besides, graphene and graphene oxide (GO) have been stated as environmentally safe and eco-friendly materials since they do not affect the environment and living organisms [1]. There are different techniques to produce graphene. Among the most popular, there are: mechanical exfoliation, chemical vapor deposition, thermal decomposition of SiC, unzipping of MWCNTs, CO gas reduction by Al2S3 and GO reduction by solar radiation [37].

Nevertheless, the quality of the produced graphene depends on the technique used [1, 37].

Specifically, GNS (micrographic image shown in figure 3) between 10 and 30 layers of graphene films have gained significant interest in recent years in the tribology community [50]. It has been chosen as reinforcement for producing polymer-matrix, metal-matrix, and ceramic-matrix nanocomposites for tribological applications, as well as to be used as additive in lubricants [1, 37, 50]. The last has been a successful achievement since graphene can be dispersed in oils producing stable suspensions with promising tribological performance [1, 37, 51]. Most of its extraordinary tribological performance is attributed to the protective film formed by graphene between the rubbing surfaces [20, 35, 36, 43, 51]. Nonetheless, as explained above, excessive concentration or/and bad dispersion of graphene nanoparticles can lead to increased wear and diminished performance of the lubricant [2, 37, 45]. Consequently, dispersion of GNS is necessary since these have an inherent hydrophilic nature which would cause agglomerations in oil [50, 51]. The effective dispersion method for graphene has been achieved by surface modification with surfactants and ultrasonic methods, which also have been effective with carbon nanotubes [33, 42].

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Figure 3: Scanning electron microscopy (SEM) micrograph of hydrophobic GNS. Retrieved from [52].

1.3 Tribology

Tribology is the study of friction, wear, and lubrication in the contact between two or more entities in relative motion. This discipline is principally aimed for optimization of machines, processes and products experiencing damage or energy losses by friction and wear. Regularly, the relative motion in contacts of metals, ceramics and polymers have rarely CoF lower than 0.5 [53]. It is unacceptable for engineering applications requiring low friction values since it leads to accelerated material damage and early failure by wear or heat. Therefore, lubrication is used as the principal strategy for diminishing wear, friction, and heat, and consequently, energy loses [1, 2].

In general, there are four types of lubrication: hydrodynamic, boundary, mixed, and solid-film lubrication. The lubrication regimes can be modelled by the Stribeck´s curve (as shown in figure 4).

It was modelled through experimentation with bearings. Friction starts with large values at the zone 1 (boundary lubrication) due to insufficient fluid-film thickness for keeping separated the rubbing surfaces. So, many asperities are interacting mechanically between the surfaces producing high values of friction. Afterwards, as fluid-film thickness increases by effects of increasing speed and/or viscosity and/or reducing load, the CoF reduces and passes to a transition zone 2 (mixed lubrication) from boundary lubrication (zone 1) to hydrodynamic lubrication (zone 3). In this transition some regions at the interphase are fully separated but some asperities are still interacting mechanically in other regions at the contact. The hydrodynamic lubrication regime (zone 3) occurs when the surfaces are totally separated. So, the CoF (µ) in that zone is significantly affected by the viscous shear caused by the lubricant, which increase with speed. Accordingly, the curve of µ is proportional to rotational speed (v) of the tribopair and the lubricant viscosity (η), but inversely proportional to the normal load applied of the tribopair (W). Finally, solid-film lubrication is used when extreme temperatures and pressures exist in a tribosystem and a fluid becomes unsuitable for lubricating the surfaces [1].

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Figure 4: Stribeck’s curve characterizing lubrication regime. Showing: (1) boundary friction, (2) mixed regime, (3) hydrodynamic lubrication as lambda (λ) value increases. Redrawn from [17].

The CoF (µ) is defined as the ratio of the friction force (F) and the perpendicular load (W) applied to a particular surface. It may be expressed by:

(1) In boundary lubrication regime, the friction mechanism is provided by mechanical interactions of asperity peaks between the sliding surfaces, which results in adhesive bonding, and elastic and plastic deformations in the surfaces [3, 53, 55]. In this case, the apparent area does not conform the total apparent area of the surfaces in contact. By this, the true total area is the sum of all the individual peaks that are in contact, as may be expressed by:

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Where:

F is the frictional force (N).

At is the true contract area (m²).

τ is the effective shear stress of the material (Pa).

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Wear and heat are the main consequences of friction. Wear is the volumetric or mass material removal from surfaces. It is found as appearance, volume, mass, or/and geometric changes in the surfaces of mechanical elements [55]. There are two main ways to visualize wear problems: 1) by considering a material property problem or, 2) by considering a system problem. However, the last is the most complicated to solve, but the most effective for elucidating a solution since it comprises tribopair contact characteristics, surfaces properties, relative motion of entities, dynamics of the system and the surrounding environment. The most common wear modes in metal-on-metal contacts are abrasive wear, adhesive wear, delamination wear and running-in/polishing. For polymer-metal wear, strong adhesion is the principal mechanism based on chemical interaction. In this case, Van der Walls forces have large influence on the frictional behavior of the tribopair [3].

An important factor in tribology is the consideration of most of influencing characteristics and conditions in the surfaces interaction (tribopair). Since they result in the lubrication behavior, thermal conductivity, vibrations, wear mechanisms, among others. A common metric to characterize the surfaces is roughness which is measured by a tester with a stylus with a diamond tip. The measured surfaces are defined by the orientation of surface features, stylus tip condition, stylus trace direction, cut-off speed and length. The two principal parameters obtained by roughness characterization are: Ra, called the center line average and Rq, called the root mean square roughness. Ra is the most common value used to characterize roughness while Rq values can discriminate between a softly undulating surface and a spiky surface [55]. Establish of an initial roughness in the surfaces is important to control the wear process. However, usually, when surfaces are new or unworn in a tribological interaction, an initial wear process called running-in is produced in any tribopair. It results in high quantities of wear particles and considerable reduction of roughness in the surfaces [14, 53, 55]. In this process the roughness of surfaces is self-conditioned and conformed between each other due to sliding, to then, get a transition to other mode of wear which can be more severe. The surface conditioning produced by running-in generates the predominant surface roughness during most of the service-life of components.

1.3.1 Tribology of wet clutch

In the particular case of wet clutches, the contact is represented by a friction composite-on-metal lubricated interphase. In clutch engagement, the friction disc is useful to promote an easy occurrence of boundary lubrication at the interface by the effect of permeability, high roughness, and porosity of the friction composite, which is required for the power transmission. The boundary lubrication formation occurs by moving out the lubricant from the interface as easy and quickly as possible during engagement. To facilitate this process even more, the friction discs are texturized with channels (grooves) that promote the leaving out (flushing) of the ATF by centrifugal force [7, 14, 54, 57]. The porosity of the material allows the permeation of ATF when the disks are pressed [7, 8]. It is also effective for allowing heat dissipation generated by friction produced in engagements. Hence, during clutch engagement, the lubrication of the wet clutch starts in a hydrodynamic regime when the wet clutch discs are separated, or they are not pressed to be in contact. Then, when load is applied to press the discs one against other, it transitions firstly to a

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mixed regime, and then, to a boundary regime till reach the total discs engagement [38]. The ATF existing between the discs should be moved out as fast as possible from the clutch interface to allow the wet clutch to operate in the boundary lubrication regime [6, 7, 15]. That is because the friction material surface is intended to have good oil absorption and smooth surface to provide large real contact areas and high friction [4]. In addition, during sliding of the discs, wear is produced in both discs. However, owing to the softness of the friction disc, critical wear is mainly expected at the friction material surface in the form of flatter contact regions on the top fibbers, which is considered as minor and mild wear in these materials [7]. Moreover, a shiny darkened stained area in the friction material is formed by the wet clutch operation [17]. It is principally produced by material degradation from the ATF trapped in the porous structure of the friction disc [10, 17]. This is commonly named as “glazing”. As the glaze layer grows, the ATF diffusion will diminish since the friction material surface porosity will be lower by the trapped degraded products and the wet clutch performance will decrease [10]. A severe glazing can produce high wear and temperature, as well as a torque transmission capacity reduction and further shudder due to a lower CoF produced by a thick shiny layer. Besides, pore obstruction and glazing can increase due to oil oxidation and insoluble wear particles attributed to inefficient performance of ATF additives such as: detergents and dispersants [14]. Since friction is generated during the engagement process of the wet clutch, it should be controlled to meet with desired friction range. For this, an adsorbed lubricant layer made of polar molecules through Van der Waals forces is formed on the friction materials (as shown in figure 5) even after most of ATF has been moved out in the engagement [3]. These formed layers, called tribofilms, have a relatively low CoF and are adsorbed on the friction material. They can be formed by surface energy interactions which enhances the performance of wet clutches, specially at low temperatures [3].

Figure 5: Schematic image of the tribofilm formation at the boundary of the substrate by an adsorption mechanism. Redrawn from [3].

Since friction is an inherent feature of wet clutches, tribofilms play a crucial role in controlling

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lower CoF mostly at low sliding speeds. Hence, it is useful for achieving CoF increasing with sliding speeds, which is suitable for limiting self-excited vibrations (shudder). It is explained in the following sections. The mechanism promoting CoF increasing with speed is related to the film disruption that would increase shear in the contact areas [14, 57]. For example, Zechao et al. [56] reported that there is a positive effect of tribofilm thickness regarding antishudder property. It is well known that oils with additives, namely, anti-wear packages, friction modifier packages, detergents, GNS, MWCNTs, Ni/MWCNTs, ZnO/MWCNTs, GNS/Al2O3, etc., produce tribofilms that are effective for reducing friction and promote good antishudder (CoF vs. speed) behavior [35, 36, 51, 58, 59].

However, Zhao et al. [15] argued that some additive packages used for ATFs does not present a proper long-term performance of antishudder. Although the tribofilm formation is beneficial for the tribological properties, a couple of weaknesses of these adsorbed layers can be presented. First, if asperity contact is sufficiently aggressive to remove the adsorbed layers and the underlying oxide film, the areas of nascent metallic surface will not allow an effective formation of tribofilms [3].

Second, high temperatures can lead to desorption of the protective film [1].

In order to know the most important factors influencing shudder in wet clutches and to find possible solutions for preventing this problem, a model proposed by Kugimiya et al. has been developed [11]. It provides the equations that describe forced angular displacement, forced vibrations due to pressure fluctuation and self-excited vibrations (shudder) in an AT equipped with a torque converter with slip-controlled lock-up clutch (wet clutch) system. The equations were deduced from the model shown in figure 6. This model idealizes the slip-controlled lock-up clutch system by assuming the sliding of a couple of wet clutch plates submerged in an ATF as happens in actual ATs.

Figure 6: Idealized model for obtaining equations that model vibrations in a slip-controlled lock-up clutch system when the lock-up (wet clutch) is continuously slipping. Redrawn from [11].

The equation of motion of the lock-up clutch system during engagement may be described by:

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(3) The driving torque Tq in the equation (3) can be calculated with the following equation:

Tq = µ PRS (4)

Where µ is the CoF of the clutch’s sliding surfaces, P is the contact pressure on the friction surface, R is the effective radius of the friction surface, and S is the effective area of the friction surface, which would be the contribution of all the micro-contact areas mentioned by Ingram et al.

[16].

Also, considering that the pressure fluctuates due to AT’s hardware issues related to the pressure application to the wet clutches, the pressure fluctuation may be expressed by:

P = P0 [1 + a · cos(nωt)] (5)

where P0 = the average contact pressure on the friction surface, n= frequency of pressure fluctuation during a revolution and the proportional constant “a” is the magnitude of the pressure fluctuation which is determined by the hydraulic system design.

Considering the pressure fluctuation, P, variability in slip velocity, v0, and temperature, T, the CoF, µ, may be described by the next equation.

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µ0 = CoF on the friction surface at slip velocity v0 and contact pressure P0. Solving the equations (3), (4), (5) and (6), the angular displacement θ may be expressed by:

where

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(9)

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Then,

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where A and ∅ are integral constants determined by the initial conditions.

The definitions of θ1, θ2 and θ3 are:

θ1 = constant forced angular displacement caused by F0.

θ2 = forced vibration due to the pressure fluctuation on the friction surface.

θ3 = self-excited vibration determined by inertia, rigidity and damping force in the system and friction characteristics of the friction surface.

Hence, to prevent unstable vibrations in the system, it is necessary to reduce the values of θ3

and θ2. The magnitude of the change in torque, which may produce vibrations, may be described by:

(14) when natural frequency occurs, it may be expressed by the following equation:

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In this way, resonance vibration will occur, and evidently the value of ∆Tq becomes considerably large. The resonance vibration, Tr, may be expressed by the following equation:

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To prevent vibrations due to pressure fluctuation on the friction surface, some approaches are suggested by Kugimiya et al. [11]:

(1) Decrease the factors which cause periodic pressure fluctuations.

• Reduce factor a.

(If a = 0, the periodic driving force which causes resonance vibration does not occur. a is the source of resonance vibration.)

(2) Provide the friction surface with certain characteristics which reduce resonance vibration.

Three possibilities would be:

• Lower CoF (µ0)

• Balance the temperature dependence of the CoF:

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• Increase the positive value of ^𝜕µ

𝜕𝑣 .

In addition, shudder, which is defined by θ3, can be prevented by reducing its value meeting the following expressions:

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This condition will provide antishudder. Hence, self-excited vibrations will not occur. Hence, shudder vibration is prevented majorly if ∂µ/∂v> 0, even when pressure fluctuation is present.

If the previous condition is satisfied, the sliding velocity (v) will be reduced avoiding undesired self-excited vibrations. It is desirable for any clutch-based transmission mechanism to reduce heat generation, wear and slipping. Thus, ∂µ/∂v > 0 is a needed behavior for any wet clutch requiring of high-performance, smooth coupling and durability.

Although some theoretical antishudder methods are available for overcoming this problem, it is complicated to solve the problem in practice. Certain materials, complex hardware designs and sophisticated ATFs have been developed to reach non-shudder at AT systems. There are principally two mechanisms that may provide antishudder in practice: 1) wet clutch plates morphology ensures boundary lubrication in the contact over all sliding speeds; 2) organic FMs adsorbed tribofilms provide CoF increasing with sliding speed [31]. Apart of having a CoF increasing with sliding speed, other authors [14, 40] and the JASO M348-95 standard method recommend that static CoF (as measured at the wet clutch “breakaway” at 0.72 RPM using a SAE No. 2 full-size test rig [40]) should (19)

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clutch engagement (as shown in figure 7). If this torque jump has an exponential behavior, it shows a shudder phenomenon of the wet clutch engagement [39, 60]. Shudder can appear because ATFs can have an increase of static CoF as the lubricant ages. Thus, fresh ATFs exhibit good antishudder behavior, but shudder can appear by an increase of static CoF as the lubricant is aged or degraded [40]. So, it is important to evaluate the antishudder performance of ATFs through standard or non- standard testing procedures for the critical conditions of operation [4].

Figure 7: Schematic curve representing the friction torque of carbon fabric/phenolic composites during clutch engagement. Where µi, µd, µo correspond to the initial CoFs: near the initial engagement, during sliding and consolidating contact, respectively. Redrawn from [39].

1.3.2 Automatic Transmission Fluid

ATFs are formulated and synthesized with specific properties to achieve the required transmission performance according to meet OEM requirements. Some of the most important requirements are listed in Table 1. The base stock material used to formulate an ATF can be mineral or synthetic oils.

Moreover, several additives (friction modifiers, antiwear agents, detergents, dispersants, and extreme pressure additives) in different proportions are used to provide and enhance the required properties. Most of additives like friction modifiers (FM) and antiwear agents are organic or organometallic compounds, namely, fatty acids, alcohols, glyceryl monooleate, glyceryl esters and glyceryl amines [3, 53]. These are added from 5 wt.% to a maximum of 20 wt.% [38]. The chemicals used as friction modifiers are long chain carbon molecules (minimum n=9) with a polar end and a long straight hydrocarbon molecular chain as the tail [3, 14, 38, 53]. The tail of this chain is a non- polar compound and has a couple of functions: 1) it will help it to solubilize in the lubricant; 2) enhance separation of the solid-solid contact. Their effectiveness depends on the solvation capacity of the oil [14, 38] while they are attached by adsorption to the rubbing surfaces. They can be

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chemically formed or physically adsorbed [38, 53]. Indeed, formation capacity is related to the polarity of the ATF. Tribofilms depend on polar compounds which will attach to the polar surfaces of the tribopair, and their strength will define their performance during wet clutch engagement.

Table 1: ATF properties and requirements by OEM.

There are some OEM specifications such as: Ford MERCON®, Chrysler ATF+3®, General Motors DEXRON®, Allison Type C-4, Caterpillar TO-4, etc., which establish certain fluid properties, testing procedures and criteria for the ATFs. For instance, GM DEXRON® requires an ATF that possess a higher dynamic CoF than static CoF. Also, it requires the ATF to withstand oxidation tests such as THOT (Turbo Hydramatic Oxidation Test), in which the transmission must provide an effective operation at 163°C for 300 hours [38]. Also, oxidation tests like the aluminum beaker oxidation test (ABOT) are conducted for this specification. It is used to measure the thermodynamical capacity of the ATF. In contrast, Ford MERCON® specification requires the opposite tribological behavior. It must have a lower dynamic CoF and higher static CoF. Ford MERCON® requires significant improvements in antiwear performance, low temperature fluidity, shear stability and antishudder durability. For other ATs, such as for heavy-duty equipment, an ATF with superior properties is required since the clutch must provide a significantly greater torque and fluid compatibility with different materials [9]. Thus, the use of a wrong or a deficient ATF in a transmission may result in poor shift quality and accelerated damage of internal AT´s components [9]. In addition, advanced refining processes of base oil and formulation with additives is essential to provide long-term

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severe cases [16]. Nonetheless, during long-term operation, oil aging caused mainly by high temperatures and oxidation leads to formation of sludge and further system’s malfunction [32].

Hence, high-thermal conductivity and good thermal stability of ATFs is necessary for allowing the long-term performance of clutches [1, 33]. Moreover, exhibit good resistance to chemical changes like acidification, polymerization, and catalysis by contact with metals from debris and other elements made of copper or polymers is desirable [40]. Finally, the ATFs must have a low viscosity during cold start to provide good fluidity but keeping enough viscosity to lubricate appropriately with a suitable antishudder property when it is heated up.

1.4 State-of-the-art

Nowadays, research on the antishudder performance of commercial additives has been conducted [56]. Nonetheless, some investigations have demonstrated that some commercial additives do not have suitable long-term antishudder behavior [6, 56]. Consequently, in order to find alternatives to those common additives, other authors have tested nanoparticles as potential replacements for friction modifiers and antiwear agents; among the most remarkable are GNS. Dan Zheng et al. [35]

dispersed GNS to an oil (PAO4) at 0.01 wt.% providing good results as found in a UMT-2 tribotester.

They were helpful to reduce wear by 50% compared to the base oil at 100°C. The tribopair tested consisted of a GCr15 steel ball on a RTCr2 alloy cast iron plate. In addition, the surfaces were texturized by making micro-dimples. It was even better achieving a wear reduction over 90%. It demonstrated a synergistic effect in which the micro-dimples stored graphene to promote the formation of GNS tribofilm as confirmed by energy-dispersive x-ray (EDX) and Raman spectroscopy.

Another investigation carried out by Senatore et al. [51] aimed to study the tribological behavior of graphene oxide nanosheets (0.1 wt.%) as nanoadditive in mineral oils. A Wazau TRM100 with a temperature-controlled container was used to test graphene oxide nanosheets dispersed in a group I mineral oil. A tribopair consisting of a X155CrVMo12-1 steel disc and a X45Cr13 steel ball were used in the range of 25 − 80°C. According to the results reported, CoF decreased more than 20%

while wear was decreased about 30%. Also, Raman spectroscopy evidenced the presence of a protective tribofilm made of reduced graphene oxide (rGO). Besides, Eswaraiah et al. [2] used an engine oil as dispersion medium for ultrathin graphene nanoparticles with thickness of less than 2 nm. The friction and wear tests (ASTM-D5183 standard method) were done in a four-ball tester at a rotating speed of 600 RPM under a constant load of 392 N and a temperature of 75°C for 60 min.

The results demonstrate CoF and wear reduction about 80% and 33%, respectively, as compared to an engine oil. Lin et al. [43] showed that surface modified GNS dispersed in a SN350 base oil improved the tribological performance through testing performed in a four-ball tester. The GNS had 1.2 µm as average diameter and thickness of 10-15 nm. 12.7 mm GCr15A steel balls (AISI 52100) were used for the tests according to the ASTM-D4172-82 standard method. A rotating speed of 1200 RPM, a constant load of 147 N, test time of 60 min and a temperature of 75 ± 2°C were used.

The authors found more than 50% of wear reduction and 30% of CoF decrease, which was ascribed to the protective film produced by GNS added to the oil as confirmed through EDX and scanning electron microscopy (SEM) analyses. On the other hand, other kind of nanoparticles with notable results of friction and/or wear reduction when added as additives to oils have been reported. For

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example, nanoparticles of TiO2 in mineral and semi-synthetic oil [26], SiO2 in liquid paraffin [61], SiO2 and MoS2 in engine oil [31], SiO2 in paraffin grease [44], MWCNTs in ionic liquid [34]; as well as some hybrid nanoparticles, namely, Al2O3/GNPs in cutting fluid [27], SiO2/Al2O3 in mineral oil [62], Al2O3/TiO2 in engine oil [47], SiO2/rGO in PAO oil [63] and Ni/MWCNTs in mineral oil [64]. The concentrations used for the above nano-based lubricants ranged from .01 to 20 wt.%. In addition, other positive effect found for nanoparticles, namely, Al2O3/TiO2, Al2O3, MWCNTs and graphene oxide added to vegetable oils and mineral oils, is the improvement of thermal stability [30, 33, 42, 47, 50].

From the reviewed investigations, the possible improvement of the antishudder property of oils by adding GNS nanoparticles as additives has not been explored yet. Thus, this thesis aims to determine a suitable concentration of GNS added to a mineral oil by evaluating the tribological behavior through performing ball-on-disc tests (ASTM G99-17 standard method [65]) on metal-on- metal interfaces. Then, the friction behavior (CoF vs. speed) of a wet clutch was evaluated by using the developed nano-based lubricant in a pin-on-disc tester.

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2 Methodology

2.1 Experimental set-up: Pin-on-disc apparatus

The tribological testing was done by using a pin-on-disc tester. The pin-on disc apparatus is suitable for conducting accelerated wear testing of materials since it mainly operates under boundary lubrication regime [66]. This machine allows inexpensive testing under different conditions of contact pressures, sliding velocities, different tribopairs, and temperatures as compared to other full-scale tribological testers [4, 10, 11, 18, 66]. The main details of design and building of the tester used are provided in Appendix A. The schematic diagram and photograph of the tester is shown in figure 8. The tester has a pivoted arm with a movable counterweight which provides the equilibrium to the arm prior to apply the load to the contact of pin and disc specimens. A second degree of freedom along the pivot axis allows the free movement of the arm when friction is produced between the pin and disc specimens. The friction force is measured and recorded through a PASCO®

force sensor located next to the arm and connected to a data acquisition system. The location of the pin can be moved forward or backward to provide the desired effective radius of the wear track on the disc specimen by using an extension manufactured in the arm. An electric motor with a controllable velocity is used to rotate the disc holder at the desired speed and sliding distance. An arrangement of K type thermocouples and a hot air gun heater were used to manually rise, measure, and control the temperature of the oil with a ± 6°C precision. The disc holder implemented in the tester has the special capacity for holding small or large disc samples (up to 6”) that can be actual brake discs or wet clutch discs even mounted in oil containers for allowing the testing of a lubricated environment.

Figure 8: (A) Schematic diagram of the pin-on-disc apparatus. (B) Picture of the pin-on-disc tester used for the testing.

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The tester was used for conducting two type of experiments: 1) ball-on-disc tests based on ASTM G99-17 Standard Method [65] for running friction and wear evaluations of lubricants in metal- on-metal interfaces varying materials and nanoparticle concentrations in the base oil; 2) Friction tests of a wet clutch interface (friction material-metal interface) varying sliding speeds and temperature at constant load using the nano-based lubricant with the best performance obtained from the ball-on-disc tests.

2.2 Ball-on-disc tests

The ball-on-disc tests were based on the ASTM G99-17 Standard Method [65]. It consists on generating wear on a rotating metallic disc by a fixed hard steel ball during a certain sliding speed, distance, temperature, and lubricating conditions. Friction was measured and recorded at a sampling rate of 20Hz during the test for post calculation of the mean CoF (using Eq. 1) for each condition. On the other hand, wear scar volume generated in the disc sample was calculated by Eq.

20 through the measurement of the scar width (d) by microscopy technique. R is the wear track radius, and r is the pin end radius. The test parameters used are shown in Table 2. They were selected to identify the optimal GNS concentration added to the oil exhibiting the best results in terms of CoF and wear rate. Considering the conditions listed in Table 2, a total of 36 tests were run.

The changing parameters were disc material and nanoparticle concentration in the oil. The sliding speed, distance, and load were selected according to produce measurable wear scars.

disk volume loss = 2πR [r ²sin ⁻¹(d/2r) − (d/4)(4r ²– d ²)¹^/²] (20)

Table 2: Parameters used in ball-on-disc tests to assess different concentrations of GNS for two different disc materials.

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2.2.1 Test specimens

Aluminum (AA 3003) and steel (AISI 1030) discs were used for the ball-on-disc tests. A couple of the discs used can be observed in figure 9. These have 5mm in thickness and diameters of 69.85mm and 57.15 mm, respectively. The hardness of the aluminum (AA 3003) was 70 HRB while the steel (AISI 1030) had a hardness of 83 HRB (measured in a Wilson® Rockwell 500 tester). In order to measure and characterize the roughness (Ra) of the discs surface, a TSK Surfcom 130A roughness tester was used. A roughness (Ra) of 0.117 ± 0.017µm was determined for steel and a roughness (Ra) of 0.193 ± 0.017 µm for aluminum. The ball specimens used for the tests were stainless steel balls (AISI 420) with a mean diameter of 7,93mm, a hardness of 52 HRC and maximum surface roughness (Ra) of 0.203 µm, according to the manufacturer data sheet. These materials were selected due to their high hardness and resistance to corrosion.

Figure 9: Aluminum and steel discs, and steel ball picture used for the ball-on disc tests.

2.2.2 Lubricants

The nanoparticles selected for preparing the nano-based lubricants were GNS grade C-750 provided by Sigma Aldrich (as previously shown in figure 3). These have a < 2µm nanoparticle size as given by the data sheet provided. The powder-like nanoparticles were dispersed in a common mineral base oil (ExxonMobil EHC 65), with viscosity of 42 cSt at 40°C and 6.5 cSt 100°C (ASTM D 445), in different concentrations (0.1, 0.5 and 1.0 wt.% of GNS in mineral oil) by magnetic stirring and ultrasound.

First, magnetic stirring at 1100 RPM for two hours was performed. Then, further dispersion was

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done by a Cole-Parmer 8891 ultrasonic bath (42 kHz) for 30 min to provide longer term stability.

The samples prepared can be observed in figure 10.

Figure 10: Photograph of the nano-based lubricants prepared with different concentration of GNS.

2.3 Wet clutch test

After finishing the ball-on-disc experiments and identifying the nano-based lubricant with the best tribological performance, other set of pin-on-disc tests was carried out for evaluation of friction of the wet-clutch discs interface. The trials were performed according to the test conditions given in Table 3. A sweep of 15 different sliding velocities in the range from ∼ 0.03 − 0.9 m.s⁻¹was tested for determining the behavior of CoF vs. speed that may represent the antishudder performance of the lubricants to be tested. The friction force was measured during the entire test at a sampling rate of 20Hz. Then, the mean CoF for each speed was calculated by using Eq. 1. The temperatures selected corresponded to the approach of a normal cold start operation temperature (26°C) and critical high temperature in the wet clutch of ATs (100°C) [10]. The contact pressure was selected to approach 0.9 MPa that is in the range of mean contact pressures in conventional automatic power shift transmission used in heavy vehicles [18]. In the test, the contact was immersed in lubricant during the whole testing. Since new samples were used for each test, in which a new lubricant sample was tested, surface conditioning (running-in) was required. It consisted of starting to run the test at a constant speed (∼ 0.45 m.s⁻¹) for 1 hour as exemplified in figure 11. This running- in period duration was suitable to achieve the stabilization of CoF by conditioning of the surfaces with only slight wear [53]. It promoted negligible influence of surface damage on the CoF measurement. Hence, the measured CoF represented was majorly influenced by the properties of the oil. After this running-in period, the sweep of speeds was run (as shown in figure 11). Each speed was tested for 20 s for accumulating a total of 300 s. After each test, the machine was completely

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and meticulously cleaned with acetone to remove the lubricant and possible specimens’ debris generated during the test.

Table 3: Parameters used for wet clutch tests.

Figure 11: Example of the CoF measurement during an entire friction test of the wet clutch samples using an ATF (Mercon®). It includes running-in period (1hr.) at constant sliding speed of (0.45 m.s⁻¹) and sweep of sliding speeds (15 increasing speeds).

0.16 0.17 0.18 0.19 0.2 0.21

CoF, μ

1 hr. running-in (.45 m.s⁻¹) 15 sliding speeds sweep ATF (MERCON)

26°C

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2.3.1 Specimens and test lubricants

For wet clutch friction testing, small specimens were extracted from actual Raybestos® friction discs to serve as the pin specimens while actual steel disc separators (CS70) from an actual wet clutch were used as disc specimens. Figure 12 shows the image of the specimens prepared. The dimensions, roughness and hardness from the discs are found in Table 4. It can be observed that the friction disc is more than fifty times rougher than the metallic disc, as expected for providing high friction. The pin specimens were held in a special pin holder which was manufactured to hold a flat-tipped specimen and keep it in proper position during the whole test. Moreover, to provide the compliance between both flat pin and disc specimens, the pin holder was designed to have a spherical seating to provide mobility for the pin. Besides, to hold the steel disc specimen and contain the oil to be tested, the cage from an actual wet clutch was used and modified as shown in figure 13.

Figure 12: Picture of actual wet clutch discs showing: 1.- Friction disc made of Raybestos®, 2.- Metallic disc made of high carbon steel and 3.- The extracted sample pin from the friction disc.

Table 4: Wet clutch test discs roughness and hardness data.

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Figure 13: Schematic diagram of the disc holder with the modified and adapted wet clutch cage, used as metallic disc and lubricant container during wet clutch testing.

The lubricants tested were: 1) neat mineral oil (ExxonMobil EHC 65), 2) a fully formulated ATF (MERCON®), and 3) the nano-based lubricant (mineral oil + 1.0 wt.% GNS, as the best lubricant identified from the ball-on-disc tests). The fully formulated ATF, with viscosity of 30 cSt at 40°C and 7.3 cSt 100°C (ASTM D 445), was tested as a reference for the comparison with the other oils and to visualize the actual CoF vs. speed performance of a commercial ATF.

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3 Results and discussion

3.1 Tribological behavior in metal-on-metal interfaces

The results of wear rate and CoF obtained for steel and aluminum from the ball-on-disc tests are shown in figure 14. The reported data in the plots correspond to the average values obtained from three test repetitions conducted. They also show with the corresponding standard deviation which were < 5%. It was found that GNS significantly reduced wear rate for both materials when testing all the GNS concentrations. The most significant reduction in wear rate of aluminum was found when using the 0.5 wt.% GNS concentration meanwhile the greatest wear rate reduction for steel was observed when the maximum concentration (1.0 wt.%) was tested. In the case of aluminum CoF, it presented a reduction with all the GNS concentrations added to the oils. However, the concentration producing the lowest CoF was 0.1 wt.%. On the other hand, the CoF of steel was almost unchanged by any of the GNS concentrations. Some of the most remarkable examples of the wear scar micrographs obtained are shown in figure 15. They show scars produced in steel and aluminum by using neat mineral oil and the nano-based lubricant with GNS (1.0 wt.%). Evident abrasion marks (as shown in figure 15 D and E) were found in aluminum due to its lower hardness compared to the stainless-steel balls used as counterpart in the tests. It is clearly observed that the wear scar produced with the nano-lubricant is smaller than mineral oil for both materials; the most significant difference being in steel. According to the results found, the best GNS concentration for the case of steel-on-steel interface was 1.0 wt.%, which was selected for the wet clutch friction testing since the materials involved in the wet clutch discs interface corresponds to (steel and friction material). The reduction of wear and CoF in most of cases is attributed to the adsorbed protective layer of GNS produced at the metallic interfaces, as it has been reported and evidenced by other authors [35, 43, 51]. The schematic diagram illustrated in figure 16 shows a representation of how GNS can be accommodated in the roughness valleys of the sliding surfaces as a tribofilm.

The tribofilm provides a third body interface with lower shear strength than the rubbing surfaces promoting a reduced wear rate. Also, the observed CoF reduction in some cases could be attributed to the GNS lower shear stress (τ) and high plastic flow stress (Py) which provide a lower CoF [3].

However, the mechanisms and evidence of the GNS tribofilm formation in the experiments run for this thesis were not evaluated and generated for this thesis. It will be subject of a future research in which the ball-on-disc tests will be performed at higher temperatures (> 100°C) to produce a more readably tribofilm desorption as suggested in Ref. [1]. The standard deviations found in the set of experiments are attributed mainly to factors such as GNS dispersion stability into the oils and affectations to the GNS dispersion by centrifugal forces caused by the rotational movement of the disc and oil container during test, as illustrated in figure 17.

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Figure 14: Tribological results obtained from the ball-on-disc tests for steel and aluminum using nano-based lubricants with GNS at different concentration (wt.%): A) Wear rate for aluminum; B) Wear rate for steel; C) CoF for aluminum; D) CoF for steel.

Figure 15: Micrographs obtained from the wear scars generated in the aluminum and steel discs using neat mineral oil and nano-based lubricant (1.0 wt.% of GNS): A) steel tested using neat mineral oil; B) steel tested using nano-based lubricant (1.0 wt.% of GNS); C) aluminum tested using neat mineral oil; D) aluminum tested using nano-based lubricant (1.0 wt.% of GNS).

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Figure 16: Schematic diagram of a protective layer (tribofilm) formation at a metal-on-metal interface due to an adsorbed layer of GNS.

Figure 17: Steel specimen tested with the nano-based lubricant. The image shows the affectations to the GNS dispersion by centrifugal forces caused by the rotational movement of the disc.

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3.2 Friction coefficient behavior of wet clutch interface

Considering that good antishudder property of ATFs may result if a positive slope (∂µ/∂v > 0) in a curve of CoF with sliding velocity is observed [11, 12]. The friction results from the wet clutch friction tests were plotted as CoF vs. sliding speed to visualize the possible antishudder behavior of the lubricants. The results obtained for the three lubricants tested (neat mineral oil, mineral oil + GNS 1.0% wt. and ATF (Mercon®)) in the wet clutch at 26 and 100°C are shown in figures 18 A and B, respectively. According to the results, the tested commercial ATF exhibited a good antishudder property (∂µ/∂v > 0) at 26°C as expected to comply with the MERCON® specification [4]. On the other hand, the mineral oil exhibited lower CoFs for all the speeds in contrast to the commercial ATF exhibiting a behavior ∂µ/∂v < 0. So, its anti-shudder property can be considered as more reduced than ATF. It is because the mineral oil did not have any additive to modify its frictional behavior in comparison to the ATF. Moreover, the standard deviations obtained for the mineral oil rose as the speed increased, which can be associated to the oil’s frictional instability of the mineral oil. In the case of the nano-lubricant (mineral oil + GNS 1.0% wt.), it exhibited the lowest CoFs for all the speeds at 26°C. However, in contrast to the neat mineral oil, it presented frictional stability and ∂µ/∂v > 0 almost similar to ATF. It may be ascribed to the GNS tribofilm formation and disruption, which is proportional to sliding speed, as reported by other authors [14, 57]. All oils were expected to get a drop in CoF values with sliding speed at higher temperature due to physic- chemical changes in the base oil and additives which affect the tribological performance [6, 39]. The three lubricants had similar CoF values for the different speeds (in the range of 0.12 and 0.16) at 100°C. However, the three oils presented a CoF decreasing with sliding speed (∂µ/∂v < 0). The ATF failed to maintain a positive ∂µ/∂v curve as unexpected. The mineral oil and the graphene-based fluid curves had negative slopes in the ∂µ/∂v curve, but the nano-based fluid had a slighter slope drop which can be ascribed to the GNS added. It is noteworthy that the CoF vs. sliding speed behavior of the ATF was very similar than that of the nano-lubricant at 100°C. Thus, the addition of GNS to the mineral oil resulted in an improvement of anti-shudder property of neat mineral oil. The drop of the slopes of the ATF and the graphene-based fluid may be attributed to the tribofilm desorption because of the higher temperature, as reported elsewhere [1]. Overall, the addition of 1.0% wt. of GNS to the mineral oil helped to generate the lowest CoFs increasing with sliding speed at 26°C but decreasing at 100°C, which was very similar to the commercial ATF tested.

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Figure 18: Results of CoF (µ) vs. sliding speed obtained for the wet clutch interface by using the three different oils (mineral oil, ATF (MERCON®), and mineral oil + 1.0% wt. GNS) at: a) 26°C; and b) 100°C.

Micrographs of the friction materials were obtained after the wet clutch tests. Some examples are shown in figure 19. As it can be seen, the friction material surfaces exhibited no abrasion or adhesion marks after the testing. Instead, some black regions and flattening of the upper composite fibers were identified along the surface of all the specimens. It can be ascribed to the tribofilm products absorbed into the composite and plastic deformation generated by the contact pressure.

The micrographs confirm the fact that the friction discs’ surfaces were tested under running-in conditions (initial wear stage). Thus, the time/sliding distance tested were not sufficient to produce significant wear on the plates and degradation of the lubricants.

Figure 19: Micrographs from the friction disc pins tested at 100°C tests by using: A) ATF; b) mineral oil; C) Mineral + 1.0 wt.% GNS.

Finally, although the pin-on-disc tester was successfully employed for evaluating the CoF vs.

speed behavior of the lubricants, it is important to remark the limitations of the used pin-on-disc tester to approach other actual wet clutch conditions that can have influence on the measurement of a more realistic anti-shudder property of the oils. The factors which could not be replicated in the

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different metals (i.e., iron and copper) [40] during long-term testing, application of a fluctuating varied pressure during the test, coupling of one or more pairs of entire discs with particular and actual features, and testing at higher sliding speeds. Hence, a full-scale wet clutch test rig like the SAE #2 wet clutch machine would be necessary to validate the obtained results, which provided a first insight of the hypotheses.

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4 Conclusions

From the metal-metal interface testing:

• The GNS addition to mineral oil produced a significant wear rate and CoF reduction of aluminum at 26°C, with optimal 0.5 and 0.1 wt.%, respectively. However, at 1.0 wt.%, the CoF was not significantly reduced.

• A significant wear rate reduction of steel was observed with GNS addition to mineral oil at 26°C, with optimal 1.0 wt.%. Nonetheless, all wt.% of GNS did not significantly reduced the CoF value.

From the wet clutch interface friction testing:

• The ATF (MERCON®) had the best antishudder property (∂µ/∂v > 0) and higher CoF values at 26°C tests, followed by the mineral oil + 1.0% wt. GNS with also ∂µ/∂v > 0. The neat mineral oil had poor antishudder property since ∂µ/∂v < 0. Also, the mineral oil + 1.0% wt. GNS exhibited the lowest CoF for all the speeds tested.

• All oils showed a drop in CoF values exhibiting a decrease with sliding speed at 100°C.

Nonetheless, the CoF vs. sliding speed behavior of the ATF was very similar than that of the nano- lubricant at 100°C. So, the addition of 1.0% wt. of GNS to the mineral resulted in an improvement of anti-shudder property (∂µ/∂v) of the neat mineral oil.

4.1 Future works

• Perform metal-metal tribological tests at a wider range of temperatures to complement the tribofilms hypotheses since at higher temperatures a reduction of the GNS performance is expected.

• Carry out tests in a SAE #2 to confirm the frictional behavior of the nano-based lubricant since a more realistic environment can be obtained in a full-scale wet clutch.

• Analyze the worn surfaces by using: X-ray photoelectron, Raman, infrared or/and energy dispersion spectroscopy to obtain information about the presence of a protective tribofilm made of graphene.

• Perform aging of the three oils to assess and compare the thermo-oxidation stability of the oils during wet clutch testing.

Instituto Tecnológico y de Estudios Superiores de Monterrey