Multifunctionality of nonasbestos organic brake materials

CHAPTER

Multifunctionality of nonasbestos organic brake materials

17 Jayashree Bijwe

Industrial Tribology Machine Dynamics and Maintenance Engineering Centre (ITMMEC), Indian Institute of Technology, New Delhi, India

17.1  INTRODUCTION 17.1.1  TRIBOLOGICAL SITUATIONS AND ROLE OF FRICTION AND WEAR Tribology is a science of two interacting surfaces in relative motion and encompasses friction, wear, lubrication, and related design aspects. Friction is a measure of resistance to motion of two objects and is a joint phenomenon and is expressed as a joint parameter for quantification by coefficient of friction (µ). Major consequences of friction are generation of heat leading to further thermal distortion of components, noise, and vibrations. All these lead to wear and hence wastage of energy and material. Wear is a loss in weight or change in dimensions of a surface when two surfaces sliding against each other are in relative motion. It is also a joint phenomenon. However, it is expressed as an individual parameter for quantification known as wear rate/specific wear rate of each surface. Consequences of wear are shown in Figure 17.1. It is a general misconception that tribological applications need low µ and low wear. As seen in Figure 17.2, there are various combinations of these two triboparameters required in different applications. Among these, friction material (FM) used in brakes and clutches fall in the area where moderate µ and moderate wear resistance are required [1–7].

17.1.2  ROLE OF BRAKES IN AUTOMOTIVES “Brake” is one of the most important parts of the automobiles, locomotives, aircrafts, and other moving bodies because it is related to the safety of human life and machines. The safety of a vehicle lies in its efficiency of speed negotiations with Multifunctionality of Polymer Composites. ISBN: 978-0-323-26434-1 DOI: http://dx.doi.org/10.1016/B978-0-323-26434-1.00017-9 © 2015 2014 Elsevier Inc. All rights reserved.

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CHAPTER 17  Multifunctionality of nonasbestos organic brake materials

Consequences of wearing of a component

Deterioration in accuracy and surface damage

Frequent inspection and replacement of part

Generation and trapping of debris which affects µ

Fatigue and crack, debris generation, and hence failure by rupture/fracture

Material loss from surface

Misalignment and hence induced vibrations

FIGURE 17.1 Consequences of wearing of a component.

Low friction, high wear Pencil leads, grinding, polishing, and running in

Low friction, low wear Antifriction material: bearings, bushes, gears, slide, seals, cams, prosthesis, dentistry, turbine blades, etc.

High friction, low Wear

W

Brakes, clutches, tyres, ignition of fire by stone, etc.

µ W

W µ

µ µ, W

Infinite friction, zero wear

High friction, high wear

Adhesives applied to joints

µ W

µ

Erasers

W

FIGURE 17.2 Various tribo-applications with desired combinations of friction and wear in required amounts [1].

17.1  Introduction

the traffic. The basic functions of the brake systems under various operating conditions are: Slowing down the speed in congested traffic, typically called as city driving braking. ● Stopping the vehicle completely in case of emergency typically known as emergency/panicky braking. ● Holding the vehicle stationary on a slope in a hill. ●

Brake systems are classified as service brakes and secondary brakes. Service brakes are used for normal braking. Secondary or emergency brakes are used during partial brake system failure and parking [7]. Typical brake systems consist of an energy source, application system, transmission system, and brake assembly. Energy source (of various types such as muscular driver pedal effort, brake boost assist systems, power brake systems, surge brakes, drop weight brakes, electric brakes, and spring brakes) is responsible for producing, storing, and delivering the energy for braking. The second subsystem is the “energy apply system” and is used for modulating the level of braking. The third one is the “energy transmission system” (mechanical, hydraulic, pneumatic, electrical, mixed, etc.) which is used for transferring the energy from apply system to wheel brakes. Finally, “brake assembly” the fourth component is for generating the forces for retarding the motion of the vehicle. Important facts about brakes are: ● ● ● ●

Throughput ultimately depends on the emergency braking distance (EBD) Brakes should be applied as little as possible but as much as necessary Brakes must absorb energy at controlled rates Ability to stop gives freedom to speed

The FM is a sacrificial one and its lining is applied on the sliding part (pad/shoe/ block/strip) which when pressed against the rotating component (disk/drum) is fixed on a wheel and converts kinetic energy into heat energy due to friction process during braking. The heat thus generated at the sliding interface of the rotor and stator (FM) is dissipated primarily by conduction through various components of the brake, by convection to the atmosphere and by radiation to the atmosphere and adjacent components. It is also absorbed by chemical, metallurgical, and wear processes occurring at the interface. The most important function of FM is to provide adequate friction with minimal damage to the pad surface, which would otherwise affect the triboperformance in consecutive braking process. Thus, the heart of the braking device is this FM, and it is expected to continue its functioning reliably and efficiently for a long time in adverse operating conditions. The performance expectations from the friction couple, however, have changed drastically due to advances in the vehicle technology. There have been increasing demands to produce more powerful vehicles (higher speeds with larger sizes and weights) with higher performance to power ratio and better aerodynamic properties [8] and hence demands on performance of FMs are continuously increasing. Nowadays it is taken for granted that the brake system must work reliably, despite careless users, extreme speeds, and adverse environment.

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CHAPTER 17  Multifunctionality of nonasbestos organic brake materials

17.1.3  EVOLUTION IN FMs The primitive FM, i.e., leather was replaced by sintered metal based, followed by composites reinforced with asbestos fibers, ruled the world for almost 80 years because of extremely good performance. However, asbestos, after proven as a health hazard, was replaced by today’s most popular nonasbestos organic (NAO) FMs. The drive for eco-friendliness of FMs along with the raised expectations for superior performance led to the inclusion of multiple fibers based on various classes such as ceramics, mineral, metallic, and organic in FMs. Last three decades have witnessed dramatic changes in the formulations of the materials as a result of continuous efforts in this direction for more and more improved performance. FMs are of various types (Figure 17.3) with their own advantages and limitations. Each has its own application domain depending on the performance requirements and cost. The asbestos-based composites have become obsolete while semimetallic composites are also not the preferred materials because of limitations and problems, such as batch to batch variations in the properties. Metal matrix composites are used for high-speed trains; carbon–carbon composites (very expensive) are used in air crafts and formula-1 racing cars. Among these FMs, NAO FMs are quite recent (since mid-1980s) and are almost invariably used in every vehicle. The performance is significantly superior to the asbestos-based materials or semimetallics [9–12]. The formulations are developed by the industry based on trial-and-error basis or with existing expertise and most of these are in patented forms.

Friction materials

Metallic

Organic

Non asbestos

Semi metallic

FIGURE 17.3 Broad classifications of FMs.

Carbon-carbon composite

Asbestos

Low metallic

Non metallic

17.1  Introduction

17.1.4  FORMULATION OF FMs AS A MULTICRITERIA OPTIMIZATION PROBLEM Unique functional demands on FMs require unique material properties. Brakes must work under unlubricated sliding contact, while µ must be relatively constant, totally “reliable” and relatively high; the wear must be relatively mild and seizure cannot be accepted. This combination of demands needs a material with multifunctionality. Reliability, repeatability, and serviceability of FMs at an acceptable cost are of vital importance in this area of FMs. Ideal requirements of the FMs are mostly complex and extremely conflicting [9,10]. If one of them is being modified, other gets adversely affected. The friction performance attainment is believed to be a multicriteria optimization problem, which further relies upon the multiple objective decisionmaking (MODM). Few of them, for example, are as follows: Desired range of µ (≈0.35–0.45 in general; ≈0.6 for sports car) depending on the type of vehicle: If it is higher, then vehicle may get toppled up due to sudden locking of wheels. If it is lower, it will take too long to stop the vehicle at a desired spot which will be chaotic. ● Low sensitivity of µ toward variation in operating parameters (load, speed, temperature, etc.): Reduction in µ at elevated temperature is called temperature fade while deterioration in µ due to increased load is called pressure fade. The FM must have fading tendency as low as possible. Since current FMs, NAO are based on the resins which are sensitive to temperature, with increase in temperature due to frictional heating or variation in operating pressure, µ invariably reduces to some extent depending on organic contents of the FM. Revival of µ when the FM is cooled is called recovery. For ideal FM, fade should be minimal and recovery should be high. ● Low sensitivity of µ to humidity, water, and oils: µ should not be affected due to contamination of pad surface by water, oils, brake fluids, etc. ● Moderate wear resistance (WR): WR should be moderately low. During use, ingredients on the top working layers of FM get degraded leading to deterioration in performance. Generation of carbonaceous material due to charring of the resin is responsible for fade. Layer on the top surface of FM must be slowly removed for rejuvenating the µ. Otherwise fade in µ due to accumulation of charred products will be inevitable. Hence there must be some reasonable wear of the pad continuously. However, if WR is too low, then frequent replacement of pads will be required which is not advisable. ● Counterface friendliness/rotor compatibility: WR of disk/drum should be high since it is more expensive. Moreover its surface should not be damaged, scratched due to harshness of FM since it will in turn affect the overall µ. In short, wear, cracking, or grooves on rotor are not just acceptable and FM should be friendly with rotor. ● Generation of noise and vibrations: FM should not produce any type of noises such as squeal, judder, creep, groan, or low-frequency vibrations. Comfort in braking without any noise is essential to the driver as well as for the people nearby. ●

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Metal pickup (MPU): It should not pick up wear debris from the disk since it will affect overall µ in consecutive braking. ● Thermal properties: Thermal conductivity should be adequate. If it is very high, then brake fluid will be affected and start boiling, leading to “spongy” brakes. If it is low, heat accumulation on the pad surface will be more leading to degradation of ingredients and hence deterioration in µ, which is most dangerous. It is hence very complex task to strike the right thermal conductivity of the pad. ● Thermo-oxidative stability of FM should be very high since during friction flash temperature of pad is high which tends to degrade the organic constituents directly resulting in performance deterioration and also decompose heatsensitive ingredients such as metal sulfides. ● Conformability: Friction heating of FM, though undesirable but unavoidable, should be uniform. Hence contact spots should be evenly spread. Hence pad should be conformed with the disk. It should have low modulus. ● Mechanical properties: It should have adequate strength and toughness to take anticipated load. Adequate compressibility and hardness are also required. If too hard, user will lose comfort in braking and wear also will be perhaps less than anticipated apart from undesirable aspects of noise, squeal, etc. If it is too soft, it may be easily deformed and wear will be very high, though braking comfort will be high. ● No thermal fatigue or surface cracking. There are three types of stresses encountered on the FM during friction, namely, chemical, thermal, and mechanical, which may lead to deterioration in the pad surface. Frictional heat should be conducted away without damaging the disk. ● Weight: FM should be lightweight from energy saving point of view, especially for aircrafts. ● Ease in production, high repeatability, and consistency. ● It should be environment friendly since wear debris originating from FMs are extremely fine (including nanometer size) and hence are directly inhaled by the user on the road. ● Cost viability, etc. ●

The composition of surface layers on both the interacting surfaces, which in turn is controlled by very complex friction mechanism and the composition of FM itself, has resulted in the utmost complexities in tailoring the formulations. It is not surprising that this area of material development is still regarded as a black art/magic rather than a science [13]. Single ingredient has never been efficient to satisfy the above performance-related issues and hence composite material is the ideal choice [9,10]. A right combination of ingredients in right amounts, sizes, and shapes is required to optimize these multiple properties. The rotor material is also expected to have the following performance characteristics. Adequate μ with FMs. Rigid enough to resist all types of nonhomogeneous stresses; but not so rigid to sustain deformations caused by a tribo-counterpart.

● ●

17.1  Introduction

High fatigue strength. Low thermal capacity. ● High thermal conductivity and dissipativity so as to dissipate frictional heat from the surface. ● Good conformability with friction composites. ● Very high wear resistance. ● Very high fatigue resistance, both thermal and mechanical. ● No tendency of any cracking. ● High thermal stability and lightweight. ● Good abrasion resistance. ● Ease of production. ● ●

Generally for drums, perlitic cast iron with Brinell hardness HB 170–280 is used.

17.1.5  VARIOUS CLASSES OF INGREDIENTS USED IN NAO FMs The nonasbestos low metallic fiber-reinforced phenolic-based composites (NALMFRP) are currently used in almost all the vehicles on roads or locomotives. The several thousands of ingredients have been tried so far for developing NAO FMs can be categorized in four classes as shown in Figure 17.4. Many ingredients can be multifunctional also. Binder: These are polymeric resins (water soluble/oily liquid/solid), generally thermosetting (added in 6–15% by weight in general) and are regarded as heart of FMs, which bind all ingredients firmly so that they can contribute to the final

●

Classification ingredients of NAO FM

Binders

Fibers

Fillers

Friction modifiers

Binds the heterogeneous ingredients firmly

To improve thermal, wear, and strength properties

Improves some of the specific properties (TC, porosity, etc.)

To impart desired friction characteristics

Functional fillers

Space fillers

FIGURE 17.4 Classification of ingredients of NAO FMs [9].

Abrasives

Solid lubricants

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CHAPTER 17  Multifunctionality of nonasbestos organic brake materials

property spectrum. Resin also contributes to friction, wear, and all performance properties. Generally phenolic resins and their modified forms such as oilmodified resins (linseed, castor, soya bin, etc.), cashew nut shell liquid (CNSL) resins, elastomer-modified resins, cresylic resins, etc. are used in FMs. These resins have higher µ (0.6–0.7) and lower wear rate as compared to other polymers. Thermo-reactive binder system of a phenol novolac leads to crosslinking into a three-dimensional network during pad manufacturing. Fully crosslinked binder network gives the pad strength over a wide temperature range, which is dependent on the manufacturing conditions such as curing temperature, time, pressure, breathings, and postcuring such as time and temperature. For final expected pad properties, different kinds/types/combinations of resins are used. At high temperatures, all resins degrade, lead to fade, so that their physical, mechanical, and chemical properties are deteriorated. ● Fibers: These impart strength, resistance to wear, impact and thermal degradation and also contribute in other performance properties such as thermal conductivity, porosity, wear, friction, fade, recovery etc. The fibers can be of ceramic, metallic, inorganic, or organic types. The fibers that are most commonly employed are of glass, steel, aramid, carbon, rock, basalt, cellulose, etc. These impart strength, wear resistance and also contribute in other performance properties such as thermal conductivity, porosity, wear, friction, fade, and recovery. Most important parameters during fiber selection are type, aspect ratio, amount, and combination sizing which leads to resin compatability, uniform distribution, etc. Right mixing process will preserve its length and flexibility. Some comparison of properties of common fibers used in FMs is given in Table 17.1. ● Ceramic fibers’ functions: Sufficient thermal resilience (high melting point of 1430°C, but start to soften at ≈600°C); improves compressive strength and not compressibility; increases µ and µ-instability; may increase wear resistance; excess amount may abrade disk; may lead to noise, vibration, and harshness (NVH) if in excess. ● Inorganic fibers’ functions: Thermally high stable materials; contribute to the pad integrity; influence friction level and wear behavior depending on the fiber structure and chemistry; potential shot works as an abrasive, etc. Table 17.1  Some Details of Fibers Used in NAO Materials as a Replacement for Asbestos Fibers Glass Carbon Ceramic Aramid Asbestos

Service T (°C)

µ Trends

Composite Strength

Technological Compatibility

Cost

Environmental Aspects

 750  550 1650  500  600

G F G F G

G G F G G

G F F F E

G P P F E

G U U U P

P, poor; F, fair; G, good; E, excellent; U, unknown.

17.1  Introduction

Organic fibers’ functions: A special production treatment generates fibrillated fibers from the filaments; fibrillated fibers offer a process aid to avoid separation of raw material in a mix during transportation; fiber increases the strength of a pad; a high fiber content in a mix increases the bulk volume and may increase porosity of FM; at elevated temperatures organic fibers may degrade and contribute to fade, etc. ● Cellulose fibers’ functions: Quieter product; processing aid; probably give more resiliency; increase in fade and wear; cost reduction, etc. ● Chopped carbon fibers’ functions: Improvement in thermal conductivity and lubricity, wear resistance, strength, etc. ● Aramid (para) fibers/pulp functions: Very good strength and temperature resistance; very good stiffness to weight ratio; impart good processability; provide dimensional stability and strength to preforms; stop separation of ingredients after mixing; improve strength of the final product; stop cracks; increase wear resistance; better friction stability; capability of damping; reduce NVH; affect other properties’ porosity, voids, density, etc.; not aggressive to the counterface, etc. Glass fibers work only when mixed with aramid fibers/pulp, etc. ● PAN (polyacrylonitrile) fibers’ functions: Fibrillated fiber; high surface area acrylic (PAN) pulp used for combined mix homogeneity and preform strength; offer equal performance to aramid pulp at a lower cost when used as a processing aid; impart excellent frictional stability, etc. ● Potassium titanate whiskers’ functions: Thermally resilient (high melting point ≈1371°C); very hard; impart good wear resistance, etc. ● Friction modifiers: These are added to impart the desired friction characteristics to the FMs. This class includes the abrasives like alumina, silica, SiC, zirconite, MgO, and chrome oxide to boost the friction level and to clean up the pyrolyzed surface film formed on the counterface which is essential for rejuvenating original friction of FM. FM also includes lubricants like graphite, MoS2, and metal sulfides to moderate and to stabilize the friction coefficient at ambient or elevated temperatures. ● Fillers: Fillers are of two types. They are the space fillers and the functional fillers. The space fillers are added mainly to reduce cost and to ease the manufacturing. They do not contribute to performance properties significantly. Most commonly used inert fillers are barite (BaSO4) and calcium carbonate (CaCO3). The functional fillers are often incorporated to improve some of the specific properties. For example, vermiculite is added to improve the porosity, while wollastonite is for reinforcement. Some functional modifiers like cashew dust, brass swarfs, Cu powder, and Al and Zn powders are also included to improve the fade and recovery performance. Size of fillers has lot of bearing on the performance properties of FMs. ● Finer the size of fillers more is the surface area and more the requirement of resin to “wet” them and more are the problems associated with resins (fade, etc.). ● Bigger particles, however, are easily dug out during braking leading to more wear in spite of higher strength of an FM. ●

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Optimum size of the filler is thus necessary for desired performance. Nano-size, however, shows excellent properties because of improved adhesion with the matrix and fillers apart from excellent capability of modifying beneficial transfer layer on the counterface [14]. ● Rubber: Typical rubbers are nitrile butadiene (NBR), styrene butadiene (SBR), silicone rubber, etc. Major functions are as follows. ● It can be reactive and cross-link during manufacturing or nonreactive (already cured). ● These are viscoelastic materials that improve damping behavior of the pad matrix. ● At high temperature, the rubber degrades and may lead to fade. ● Friction dust: It is prepared from CNSL by its reaction with para-formaldehyde/ hexamine/formaldehyde to produce cross-linked polymer which is then baked in inert atmosphere followed by powdering to produce friction dust (more thermally stable hard particles). It is moisture resistant, low ash resilient material, and does not adhere well to other ingredients. These particles increase organic contents in FM without inviting flow problems as in the case of resins. These are black in color like coke, graphite, and tire peels and feel hard. At elevated temperature, however, it degrades. Main functions are stabilizes the friction level; assists FMs to exhibit high performance over a broad temperature range; influences pad compressibility; improves resistance to wear; generates low brake noise; improves skid resistance; helps in fast heat dissipation; suppresses brake noise, etc. ●

17.1.6  COMPLEXITY OF COMPOSITION OF FMs FMs thus encompass the above four classes of constituents making them a combination of polymeric, carbonaceous, metallic, and ceramic phases [9,10]. The performance of such multiphased composite materials is determined by the selection of the constituents (in right amounts and combinations), their shapes, sizes, orientations, and uniform distribution in the matrix. The performance defining attributes (PDAs) of such heterogeneous materials need to be predicted on the basis of micro-mechanics analyses, while taking into account some of the design variables to a useful level of accuracy [11]. The proportions of the various ingredients are to be chosen to suit the properties of a desired FM. For example, too little resin reduces its strength due to inadequate binding leading to excessive wear, whereas too much of the same contributes to friction fade at elevated temperatures [15–17]. Similarly too little of abrasive inclusion makes the frictional response inadequate whereas too much of the same causes rotor incompatibility and generates severe thermo-elastic instabilities (TEI) [18,19]. Because of these compositional and behavioral variations with respect to different operating, environmental, and thermal conditions, it is sometimes said that “FMs are just as variable in behavior as human beings” [13]. By juggling with all these variables of composition and manufacturing, a range of materials with desired friction and wear properties can be produced. The numerous brake designs employed on several types of vehicles force another level of complexity in formulating FMs. For example, the disk pads of a racing car

17.1  Introduction

need to withstand hard, intensive stresses during usage but need to last only at least for the duration of the race, whereas a colliery winder having friction brakes would last for many years and are used only in emergencies and maneuvering. Similarly, the friction requirement of passenger cars is in the range of 0.35–0.40, whereas for heavy-duty trucks (HCVs) it is in the range of 0.3–0.35. Hence, in FM development, an attempt to improve one desired feature often proves detrimental for other due to the interference of the inherently conflicting PDAs. For example, FMs should respond with a stable µ and less fade at elevated temperatures. Expecting them to be fade resistant at elevated temperatures due to frictional heating at the braking interfaces and near 100% recovery of the µ as they are cooled down is another contradicting performance criterion in ideal materials. Moreover FMs need not have a very low wear rate as in the case of bearing materials (anti-FMs). On the contrary, they are expected to have a moderate wear rate so that the surface film is removed in the course of braking and new surface is generated and to attain this, abrasives are added which clean up such films during braking action [10,14]. However, inclusion beyond a critical level may lead to grooving of the rotor disk surface. Similarly, the thermal conductivity of these materials is also expected to be moderate, as too high of the same would lead to “spongy brake” and too low of the same would accumulate heat on friction surface which is most vital and cause resin degradation leading to fade. Such delicately balanced conflicting requirements make the problem of formulation extremely challenging. The effects of most of the fillers cannot be predicted accurately not only because of insufficient fundamental knowledge about their influencing patterns in a multiphase (heterogeneous) composition but also due to their tendency of synergistic/antagonistic effects apart from very major influence of a third body called “tribo-film” on the disk; whose quality, composition, thickness, and tendency of back-transfer on the pad leading to secondary plateaus and hence affecting overall performance; Moreover, quality and type of film also depends on several parameters including compositional and operational [18,19]. Since the influence of combination of fillers, fibers etc is so complex, that most brake lining formulations are achieved through trial and error. Thus, the formulation and the composition play a decisive role in the overall performance of such materials. It is for these complexities that the FM formulation design and development is believed to be more of an art and less of science [13,20]. Apart from these with the increasing enforcements of the environment-related legislations on the FM manufacturers and users, they have to be aware of ecological compatibility of ingredients. There has always been a continuing stress on the manufactures; first asbestos, then Pb, Zn, etc. and now recently copper to be avoided [21]. Thus the development of FMs is a complex interactive task in which optimized combination of interdependent properties is sought. Hence the FM performance attainment is believed to be multicriteria optimization problem [11,20], which further rests upon MODM. Interestingly there has to be a correlation between the bulk material and ever changing friction surface and is a necessary requirement in more efficient and tailored design of FMs, which is extremely difficult task. The successful formulations so far have been confined to the friction industries. It is for this commercial sensitivity of the materials that the technology remained secluded from the scientific community for many decades.

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17.1.7  COMPLEXITY INVOLVED IN PERFORMANCE EVALUATION OF FMs The theme of performance evaluation of developed FMs in general is similar to other materials, namely, laboratory testing for screening the potential on reduced scale prototype followed by dynamometer testing under more realistic conditions and finally field testing before commercialization. However, various available standard schedules have added to the confusion in research pertaining to FMs. Several researchers have come up with novel setups, rigs, methods, and standards for reduced scale testing. Any correlation, however, has not been necessarily observed in the performance when compared with dynamometer testing. Among the testing methods, the chase and the friction assessment and screening test (FAST) inertia dynamometer are the most popular conforming to the standard SAE J 661 a. The former, however, is for the evaluation of FMs in the form of drum linings. Based on the detailed studies, it was accepted [22] that the performance evaluation on a chase dynamometer lacks correlation with that of a FAST dynamometer and did not simulate the realistic conditions of the road, environment, alpine ascents, descents, etc. and hence final outcome of results did not match. In light of the advent of modern fast moving vehicles, the FM requirements became more complex in accordance with the applied braking pressure, speed, road conditions, and environmental conditions. Hence, the old standards became obsolete. Continuously new approaches to brake testing are being introduced depending on newer vehicles introduced and also realizing that more realistic conditions are necessary for evaluation. JASO C 406, Euro, R-90 (Regulation-90 by UN), and several others have come up as per the complex vehicle dynamics requirements.

17.2  SOME HIGHLIGHTS OF RESEARCH INVESTIGATIONS Two approaches are practiced by the researches to study the influence of ingredients on performance of FMs. Some prefer to develop very simple formulations (binary, ternary, or quaternary) with a view that performance of selected filler will be surfaced out emphatically while others believe that such reduced scale composites do not represent realistic ones and hence are unable to take into account complex interaction (synergistic or antagonistic) and hence have no practical significance. They prefer to develop realistic formulations and continue to study effect of targeted ingredient changing systematically in the selected formulations. During comprehensive studies undertaken in author’s laboratory to understand how ingredients affect the performance of FMs, second approach was preferred. While developing realistic FMs containing 10–15 ingredients, various themes were selected as follows: Influence of combination of fibers, namely, organic, rockwool, PAN, carbon, and cellulose in fixed amount [23–26].

●

17.2  Some Highlights of Research Investigations

Influence of various abrasives (alumina, SiC, silica, and zirconia) in varying amounts (0%, 2%, 4%, and 6%) [23]. ● Influence of varying amount and size (micron and nanometer) of abrasives (alumina and silica) [27]. ● Influence of varying amount (10%, 12.5%, and 15%) and types of phenolic resins (straight, alkyl benzene modified, NBR modified, linseed oil modified, CNSL modified) [15–17]. ● Influence of varying amount, type (natural and synthetic), and size of graphite [28]. ● Influence of varying amount, size, shape, and type of metallic contents such as copper, brass, and steel in the form of short fibers and powders (micron and nanometer size) [29–32]. ● Influence of newly synthesized resins, etc. [33–36]. ●

In each case, parent composition was kept constant and only selected ingredient was changed. While varying the amount, difference in composition was compensated by adding equivalent amount of barite (inert filler). The FMs were developed by a particular mixing schedule of selected ingredients for selected time in a plow shear mixer, followed by compression molding in the form of brake pads under selected temperature and pressure along with intermittent breathings to expel the volatiles and finally postcuring and grinding operations. The performance evaluation was based on physical (density), chemical (acetone extraction to investigate uncured resin), mechanical (hardness, tensile strength, flexural strength, etc.), and tribological properties (reduced scale prototype, chase machine, Krauss machine, and brake inertia dynamometer as per industrial schedules).

17.2.1  INFLUENCE OF SIZE, SHAPE, AND AMOUNT OF METALLIC CONTENTS IN FMs Metals such as copper, brass, steel, and aluminum are used in FMs in various forms: physical (powders and fibers of various sizes and shapes) and chemical (as metals or their oxides or sulfides). Everything affects performance properties significantly. Basically these are added to enhance thermo-physical properties of FMs (thermal conductivity, diffusivity, specific heat, thermal expansion, and most of the times wear resistance). Among these, especially Cu and corresponding alloys offer lubricating effect at high temperature. Few efforts were done to investigate influence of size, shape, and amount of Cu in FMs [14,29–32]: a. Varying amount of Cu powder (diameter 280–430 µm) 0, 10, and 20 wt% b. Varying shape of Cu component, i.e. micron sized powder (diameter 280–430 µm) or short fibers (length 2–2.35 mm) c. Varying size of Cu powder, micron size (280–430 µm) or in combination with nano size (70–90 nm).

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Two series of composites were developed as follows: Series 1: Composites containing

● ●

0%, 10%, and 20% Cu powder 0%, 10%, and 20% Cu fibers

Series 2: Composites containing

● ●

0% and 10% Cu micro powder 0%, 10% (2% nano + 8% micro) Cu powder

Developed brake pads were evaluated on industrial inertia dynamometer as per JASO 406 standards. Essence of the results is shown in Figures 17.5 and 17.6. Results for representative series of copper-based FMs are shown in Figure 17.5 where FM with 20% Cu powder showed lowest wear and 20% Cu fibers led to highest wear. µ due to fibers was always high with lot of fluctuations (undesirable trend). Ideal material should show these curves parallel to X-axis and with increase in speed there should be very small shift in µ-values. Fibers proved poor in all these respects. Based on Effectiveness II (measures the efficiency of an FM to function more reliably under different pressures and speeds) and F&R (Fade and Recovery) (highlights the effect of temperature on µ) studies clear conclusions were drawn as follows: With addition of Cu (powder and fiber) in FMs: µ-performance, fade resistance, µ-recovery, and wear resistance of the composites improved. ● From shape point of view, Cu powder proved better than Cu fiber in selected performance properties. ● From amount point of view, 10 wt% proved better for friction point of view, while 20 wt% proved better for wear resistance point of view. ●

Further studies to investigate effect of nano-sized copper particles, one more FM was developed which contained 8% micro-powder and 2% nano-powder and the performance was compared with that containing 0% and 10% micro-powder of Cu. It was concluded that by replacing just 2% micro-powder by equivalent amount of nano-particles, performance increased significantly (Figure 17.6). While conducting in-depth studies on three types of metals, namely, copper, brass, and steel (fibrous and powdery), several composites were developed as given in Table 17.2. Detailed evaluation of these composites led to the following conclusions: Inclusion of metallic fillers led to significant enhancement in all properties and powdery fillers led to significant improvement in friction performance as compared to that by their fibrous forms. Though fibers led to higher enhancement in strength, they deteriorated friction and wear behavior significantly in general. Mechanical properties showed no correlation with friction and wear performance. ● In almost all the performance properties, copper filler proved best followed by brass, though the difference was marginal. Iron powder or steel fibers performed ●

Cu powder vs. Cu fibers: dyno test—JASO C 406 0.45 (a) 50 kmph

0.40 µ

Wear Vol (10–2) cc

250

0.43

0.38 0.35 0.33 0.30

CP1 CF2

CF1 CP2 0.1

0.2

0.3

0.4

250 225

0.6

0.7

0.8

205

200 175

0.5

WEAR

CP1

258 246

238

198

CP2

CF1 Composites

CF2

Ref0

(b) 80 kmph

350

0.40 0.38 0.35

CF1 CP2

0.33 0.30

0.1

0.2

CP1 CF2 0.3

0.4

0.5

0.6

0.7

0.8

Disc temperature (°C)

0.43

µ

Effectiveness II

Deceleration (g) 0.45

Deceleration (g) 0.45

200

CF1

150

CF2

100

F&R

0.4 µ

0.40 µ

CP2

0.42

0.43

0.38

0.38 0.36

0.35 CF1 CP2

0.33 0.30

CP1

250

50 0.44

(c) 100 kmph

Ref

300

0.1

0.2

0.34

CP1 CF2 0.3

0.4

0.32 0.5

Deceleration (g)

0.6

0.7

0.8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 No. of brakes

FIGURE 17.5 Figures in left column indicate sensitivity of µ toward pressure and speed (dynamometer test) while figures in right column indicate results on Krauss test for wear (top) and µ-temperature relation (bottom).

275

0.45

100

0.43

0.38 0.35 0.33

Effectiveness II

0.30

CN

CM

0.40

CM

CN

0.33

µ

0.40

CN

CM

CN

CRef

0.2

0.3

0.4

CRef

0.5

0.6

0.7

0.8

(a)

CM

CN

CRef

Short fiber

150 100

0.44

F&R 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

(b)

CM

CN

CRef

Nano-powder

0.41

0.38

0.38

0.35

0.35

0.33

0.32

0.30

0.1

CN

200

0.47

CRef

140

125

Micron-powder CM

250

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

CM

165

150

Composites

80

50

(c) 100 kmph

175

µ

0.43

200

85

300

CRef

232

225

100

90

WEAR

250

Deceleration (g)

(b) 80 kmph

0.35

0.45

95

75

0.38

0.30

(b)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.45 0.43

CRef

Temperature of disc

µ

0.40

Speed spread (%)

(a) 50 kmph

Wear Vol (10–2) cc

Cu nano vs. Cu-micron: dyno test—JASO C 406

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Number of braking

Deceleration (g)

FIGURE 17.6 Influence of copper powder in NAO FM performance with varying amount (0% CRef and 10% CM) with micro- and nano-size (CN contains 8% micro- and 2% nano-Cu particles).

Table 17.2  Formulation Design for Series Based on Metallic Powders and Fibers (a) Metallic Powders (Parent Formulation 60%) Powder Series Series Name Designation of composites Selected metal (wt%) Barite (inert filler) (wt%)

Brass (BP Series) BP0 0 40

BP1 10 30

Copper (CP Series) BP2 20 20

CP0 0 40

CP1 10 30

CP2 20 20

Iron (IP Series) IP0 0 40

IP1 10 30

IP2 20 20

(b) Metallic Fibers (Parent Formulation 60%) Fiber Series Series Name Designation of composites Selected metal (wt%) Barite (inert filler) (wt%)

Brass (BF Series) BF0 0 40

BF1 10 30

Copper (CF Series) BF2 20 20

CF0 0 40

CF1 10 30

CF2 20 20

Steel (SF Series) SF0 0 40

SF1 10 30

SF2 20 20

B, brass; C, copper; I, iron; S, steel; P, powder; F, fiber and subscripts 0, 1, and 2 for 0, 10, and 20 wt% of selected metal in composites, respectively. BP0, CP0, IP0, BF0 CF0, and SF0 are the same composites and during intercomparison between all composites, a special designation “Ref” was used for these 0 wt% composites.

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CHAPTER 17  Multifunctionality of nonasbestos organic brake materials

poorly except in case of recovery behavior and counterface friendliness where they excelled. Though iron powder is the cheapest metallic filler, it is not the proper choice. Thus performance order was copper > brass >>> iron which matched with their thermal conductivity order. Performance was exactly in reverse order of their cost. ● For lower µ-pressure sensitivity, 20% loading of powders proved best while for µ-temperature sensitivity, 10% loading worked best. ● In case of FMs with powdery fillers, uniform trends and good correlations were observed with few properties such as wear performance and thermal conductivity; TL behavior with thermal effusivity and compressibility; and performance µ of FMs with µ of other metals in FMs studied in similar conditions. For fiber series, no correlation emerged. More heterogeneous structure due to inclusion of fibers could be one of the reasons for the same.

17.2.2  INFLUENCE OF AMOUNT AND TYPE OF RESINS IN NAO FMs Generally phenolic resins are used in FMs as binders. There are several varieties of this class of materials and studies were taken up to investigate type and amount of resins on performance properties [15–17]. Five resins, namely, straight phenolic (S), alkyl benzene modified (A), CNSL modified (C), nitrile-butadiene rubber (NBR) modified (N), and linseed oil modified (L) were selected and five series of composites containing all identical ingredients except resin were developed. Each of these series contained same resin in three amounts, namely, 10%, 12.5%, and 15%. In depth studies led to the conclusion that No FM worked best or worst in all the performance parameters selected as priority wise such as performance µ,% fade, fade µ, wear, rise in temperature of disk, % recovery, and recovery µ. In general, 12.5% amount led to better results and proved to be optimum amount. Higher amount led to more fade and very poor friction performance while lower led to more wear. ● Alkyl benzene-modified resin proved best from friction-related parameters. However, its wear performance was poorest though strength properties were highest. ● On the contrary, linseed oil-modified FMs proved best for wear performance and poorest from strength and friction-related aspects. ●

17.2.3  INFLUENCE OF AMOUNT AND TYPE OF FIBERS IN NAO FMs Fibers are essential for replacing asbestos in FMs. In depth studies were taken up to compare contribution of various fibers such as rockwool (Lapinus), PAN, aramid pulp, carbon, and cellulose [23–26].

17.2  Some Highlights of Research Investigations

Series of composites was developed and tribo-evaluated keeping all ingredients (87%) including rockwool (10%) as constant and varying these four fibers (3%) in each FM. It was concluded that extent of contribution by these fibers was as follows: Magnitude of µ-cellulose > aramid > PAN ≥ carbon Sensitivity of µ to pressure − (lower the better) − PAN > cellulose > aramid > carbon ● Sensitivity of µ to pressure − (lower the better) − PAN ≥ cellulose > aramid > carbon ● Wear − aramid ≥ carbon ≥ PAN >>>> cellulose ● Resistance to fade − (higher the better) − carbon >>> PAN > aramid > cellulose ● Recovery − (higher the better) − cellulose > PAN > carbon > aramid ● Resistance to disk temperature rise – (higher the better) – carbon > PAN > aramid > cellulose ● ●

Thus no fiber proved best in offering all these performing parameters. Overall, one has to strike a balance of combination of these fibers to achieve delicate balance of desired properties.

17.2.4  INFLUENCE OF NEWLY DEVELOPED RESINS IN NAO FMs Phenolic resins are invariably used for developing FMs although they are associated with following serious problems including those related to environmental pollution. These include: Necessity of harsh chemicals as catalysts such as NaOH during its synthesis. Evolution of noxious volatiles (ammonia, formaldehyde) during molding of products leading to environmental pollution and voids and cracks in the molded products. ● Poor shelf life hence problems in transportation and storage. ● Shrinkage in molded products. ● ●

Keeping this in view new type of benzoxazine resins were developed which do not have above-mentioned flaws. Four types of FMs containing identical ingredients (90%) and varying four resins (10%) in each FM were developed in the laboratory. Fifth FM was developed with identical ingredients but was based on traditional resin (straight phenolic resin 10%). In depth performance evaluation and comparison with FM containing 10% led to the conclusion that these resins excelled in all performance properties desired for FMs and performed significantly better than the conventional resins. Table 17.3 provides gist of the studies based on performance parameters of the selected FMs. FM based on new resin showed all properties better than that of phenolic-based FM and also commercial material.

569

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CHAPTER 17  Multifunctionality of nonasbestos organic brake materials

Table 17.3  Performance Parameters of FMs (Developed in the Laboratory) and Commercial One

Properties

A-FM Based on New Resin

P-FM Based on Phenolic Resin

C-FM of Commercial Brake Pad (Reputed Company)

μPerformance μRecovery μFade % Fade % Recovery Wear ( × 10−6 m3) Temperature rise of disk (°C)

0.389 0.408 0.357 8 105 5.3 387

0.386 0.411 0.329 15 106 6.9 456

0.430 0.471 0.379 12 109 8.8 439

17.3  CONCLUSIONS Based on the detailed elaboration on the complexity of simultaneous influence of multiple ingredients in various sizes, shapes, amounts, and in combinations on performance properties of NAO FMs, it can be concluded that this class of multifunctionality materials is still a challenge to the researchers and practitioners in the industry. In spite of extensive efforts to understand the functioning mechanisms of these materials by the researchers, hardly any commendable knowledge has been accumulated over the several decades. It is still a weak researched area, still some type of “black-magic” or an “art” rather than fully revealed science. There is still a monopoly of practitioners or experts in the industry who knows the knack of developing successful formulations or altering the performance parameters as per newer requirements of vehicles as a consequence of innovations in vehicle technology. Nevertheless, continuous research efforts in this area may bring more transparency in the behavioral patterns of this class of materials which may use approximately 15–25 ingredients from the array of thousands of potential ingredients. That needs more systematic efforts on investigating reasons for synergism or antagonism of typical combination of materials apart from investigating their complete interaction with the counterpart whose roughness, and texture continuously changes during braking cycles leading to vicious chain of events of film transfer, back-transfer, back-back transfer, glazing, MPU, scratching, scoring etc.

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