composite as friction material

Composites as Friction Materials: Recent Developments in Non-Asbestos Fiber Reinforced

Friction Materials-A Review

JAYASHREE BIJWE

Industrial Tribology Machine Dynamics and Maintenance Engineering Centre (ITMMEC)

Indian Institute of Technology New Delhi-110016, India

Replacement of conventional asbestos based friction materials has been called for because of bans on the use of asbestos. Research in this direction in the last decade has led to the development of more efficient asbestos-free friction materials for automobiles. Fiber reinforced polymers show great promise for applications in modem vehicles. This review article focuses on the recent developments in the application of composites a s friction materials. The first part of the article contains brief information about brakes, their action, brake materials, their desired properties, etc. The second part deals with the recent developments in non-asbestos fiber reinforced friction materials.

1. INTRODUCTION

he most important safety aspect of an automobile T is its brake system, which must stop the vehicle quickly and reliably under varying conditions. It is composed of an actuating (hydraulic) system and a frictional system. The frictional system, in turn consists of a drum pad or disc against which a sacrificial friction material, also called a brake lining, is applied. When the pad touches the disc, the cohesion due to friction shears the frictional material attached to the back plate. Frictional systems are generally operated dry, e.g. in brakes and automotive clutches, but in some clutches and in all automatic vehicle transmis- sions, they run in lubricant. Brakes are of two types, hydraulic and pneumatic. Various types and kinds of brakes and clutches for industrial vehicles are described in Table 1 (1) . The combination of various parts in a brake system transforms the applied pressure to mechanical forces, leading to quick retarda- tion and final stopping of the vehicle (2-5). During a stop, kinetic energy of a moving vehicle is converted to heat at the sliding interface of the friction pair. This is then dissipated primarily by conduction through the drum/disc and by convection and radiation to the atmosphere and adjacent components; secondly by absorption leading to chemical, metallurgical and wear processes at the interface. Typically, normal operating temperature of the rotor is 15O"-25O0C for passenger cars and 370°C or above in front disc pads (6).

Friction material serves in a variety of ways to control acceleration and deceleration of various vehicles

and machines from large brakes for jumbo aircraft to small hand-activated brakes for bicycles. Disc brake linings are called pads whereas drum brake linings are known as segments or strips and heavy truck drum brakes are called blocks. The general appearance of these is shown in Fig. 1. Material requirements and processing technology for each type are different.

Working of Brakes

During braking the velocity and deceleration vary with time. In Fig. 2, a general pattern of variation of deceleration a s a function of time is shown. Following are the phases involved in the braking process (3, 5):

(i) Initial response phase (0-P) from beginning of the actuation force until the onset of braking force.

(ii) Pressure buildup phase (P-Q) from the onset of braking force to the moment when it reaches its stable value.

(iii) Active braking phase (Q-R) from the beginning of the stable braking force till it ceases.

(iv) Final response phase (R-S) from the deactivation to the disappearance of the braking force.

2. METHODS FOR EVALUATION OF FRICTION MATERIALS

These are broadly categorized in the following two classes: (i) physical, chemical, and mechanical char-

378 POLYMER COMPOSITES, JUNE 1997, Vol. 18, No. 3

Composites as Friction Materials

Table 1. Types and Kinds of Brakes and Clutches for Industrial Vehicles (1).

Kind Type Application Sliding Material Friction Coefficient

DRY Brake Disc

Drum Band

Clutch Disc Drum Band

WET Brake Band Clutch Disc

Running, power control Running, turning winding Running, turning winding, power

control Main, driving Main, driving, turning Running, turning winding

Running, power control Main use, draining

Semimetallic resin moldings Semimetallic resin moldings Semimetallic resin moldings

Metallic, semimetallic Resin molding Resin molding

Woven lining, resin molding metallic paper type

0.3-0.45 0.3-0.45 0.3-0.45

0.3-0.5 0.3-0.5 0.3-0.5

0.08-0.12 0.08

0.1 24.14

I I I

Fig. 1 . Various types of molded, rolled, and sintered friction materials commonly used for automotiues, railways, and industrial applications (a) brake linings (b) molded brake linings (c) woven brake linings (d) roll linings, (e) disc brake pads fl clutch facings @) clutch segments (h) friction blocks (i) brake shoes.

acterization and (ii) tribo-evaluation, i.e. friction and wear testing.

acetone extraction, liquid chromatography (LC), infra- red spectroscopy (IR) etc. Physical characterization is

2.1. Phyeical, Chemical, and Mechanical Characterization

generally done by X-ray analysis, scanning electron microscopy (SEM), measurements of density, thermal conductivity, thermal expansion coefficient, specific

Chemical characterization generally includes ther- heat, etc., while for mechanical strength characteriza- mogravimetric analysis (TGA). differential thermal tion, tensile strength, flexural strength, hardness, analysis [DTA), pyrolysis gas chromatography (PGC), etc., are determined (6).

POLYMER COMPOSITES, JUNE 1997, Vol. 18, No. 3 379

Jayashree Bijwe

t

I I 1 I

, I

I > 0 O P Q R S

time (Seconds)

Fig. 2. The four phases occurring in the braking process over time t : [iJ initial response phase (0 to PJ, (iiJ pressure buildup phase [P to QJ, (iii] active braking phase [Q to RJ, and [ivJfinal response phase (R to SJ (51.

2.2. Tribo-Evaluation of the Friction Materials

This includes friction and wear characteristics under various testing conditions, fade, recovery and squeal studies.

Terminology Used in Tribo-Evaluation of the Materials

(i) Friction coefficient-Ratio of frictional force to the applied load is called the friction coefficient, denoted by p.

(ii) Fadet -A temporary reduction of the braking effectiveness due to loss of friction between the braking surfaces, resulting from heat.

(iii) Friction peaking-An increase in friction occurring during or after high temperature operation.

(iv) Recovery-Once the brakelining cools, it should repeatedly recover its original friction coefficient-this is termed as "recovery."

(v) Water recovery-The ability to recover from loss in effectiveness due to exposure to water.

(vi) Effectiveness-A measure of stopping efficiency, expressed in a number of different ways: a s the coefficient of friction, the hydraulic or air line pressure required or the torque developed, or the distance required to stop the vehicle. The effectiveness is also measured as new or off rack (without any prior use), preburnished (after little prior use), burnished (after moderate use), and faded (after use at elevated temperature) (6).

(vii) Load and speed sensitivity-The ability to main- tain effectiveness a t various weight loadings and rubbing speeds, respectively. Most of the materials show losses in effectiveness at increased speeds (except semimetallics).

(viii) Wear-Loss in mass, or change in volume (deformation), or change in performance, is defined as wear. It is generally described by the following equation ( 1 1, 12)

W = KP"VbtC (1 )

where W is wear, K the wear coefficient, P the normal load, V the sliding speed, t the sliding time, and a, b,

' Fade is caused by thermal decomposition or the iiquescence of organic components such as phenolic resin. as a result of frictional heating 17-101.

380

and c are a set of parameters for a typical friction pair a t a given temperature. Other terms, such as green effectiveness, burnished effectiveness, delayed fade, blister fade, flash fade, and contamination fade (13), are less commonly used.

2.3. Factors Influencing the Performance of the Friction Materials

The friction and wear behavior of materials are determined by the nature of interactions between the friction surfaces. The nature and degree of such interactions depend on a great many parameters, as shown in Fig. 3 (14). Hence, brake friction and wear characteristics are sensitive to many parameters such as brake design, lining and rotor material, its prior use history, composition, surface geometry, surface energy, chemical reactivity, physical and mechanical properties of the surface and testing conditions of pressure, velocity, temperature, number of applications, environment, i.e., oily, humid, sandy, and contamination with wear debris.

Poorly understood tribological mechanisms, ob- scure compositions, and numerous brake designs make it difficult to have a purely scientific approach to this class of materials (15). Development of friction materials for a typical end use has traditionally been more of an art than a science, since the influence of ingredients cannot be predicted a priori. Hence, most of the formulations tend to be proprietary (16-19).

General Tribo-Mechanisms in Friction Materials

Friction force is a result of combined effect of the deformation of asperities, the adhesive force in the adhesive contacts, and the ploughing of the hard asperities, hard particles, and wear debris. The extent of contribution by each of these mechanisms is a function of a great many parameters, as seen in Fig. 3. Flash temperature a t the asperities can reach 1100°C within 1 ms and then cool, as others become active ( 13). Heterogeneous and highly anisotropic friction materials have been developed to avoid the ther- moelastic instabilities.

When a polymeric friction material slides against the ground or a machined cast iron surface, its surface roughness and wear decrease with the time and eventually reach a steady state. During this period, material transfer from the polymer surface onto the metal takes place in the form of a film typically of 1 to 7 pm thickness, though the reverse transfer also oc- curs to a lower extent. Beneath this friction film, a cast iron surface layer undergoes severe plastic deformation, destroying virgin pearlitic cast iron microstructure (20). At higher temperatures due to spheroidiza- tion of the rotor, wear of the rotor increases (21). Similarly, the surface of friction material is covered with a film with a composition similar to that on cast iron surface. A s the temperature of interacting surfaces increases, the organic phase decomposes and the composition of the surface becomes richer in inorganic content. The thickness of such a heat-affected

POLYMER COMPOSITES, JUNE 1997, Vol. 18, No. 3


Fig. 3. Parameters influencing friction and wear performance of the material sliding in abrasive wear mode (I 4).

O P E R A T I N G CONDITION

Contact area

*Contact pressure

*Surface topography of the

Lubricot ion

Cornpositlon Microstructure FiberlFiller-matrix Interface

,@Type of fiber and properties,

Aspect ratio Fiber orientation

layer depends on time of exposure at the elevated temperature and thermal conductivities of the two mating materials and their wear rates (20, 21). At still higher temperatures, this continuous friction film is destroyed, and wear increases exponentially. Lower- ing of temperature regenerates the film, and during this period both p and wear rate vary. The wear rate of the friction material is controlled by a thermal decomposition mechanism at higher temperatures and by abrasive. adhesive and fatigue mechanisms at lower temperatures (5, 22). as shown in Fig. 4 (15).

2.4 Methodology Involved in Mbo-Evaluation of Friction Materials

The testing assembly involves a tribological couple formed by a metallic counterpart, generally cast iron,

0'09 I 0.07

Q

THERMAL WEAR PREDOMINATES

r __---- -03 I-

v 0.02 " O.O1 t

0 ' I I I I

100 200 300 400 500 6( =F 38 93 150 205 260 315OC

ISOTHERMAL TEMPERATURE

Fig. 4 . Predominant wear mechanisms of friction materials ( 1 5).

and a polymer composite, usually attached to a metallic back-plate. In automotive applications, the frictional material is fixed on a fluctuating component, while the metallic counterpart is mobile or rotating.

To test whether the friction materials meet desired properties in terms of performance, noise, and durability, they are subjected to a series of tests on a test track and then on roads before they are released as commercial products. The tests are classified as vehicle tests and brake dynamometer tests. Depending on the type of vehicle, various test procedures are adopted by the friction material manufacturers (23). Besides performance, side-to-side and front-to-rear balance and noise properties can be determined only in the vehicle tests. Wear is determined in accelerated testing and does not reflect normal drive wear. Though vehicle tests are the ultimate in evaluation of friction materials, they are expensive, time consuming, and subject to load conditions and weather variability (6). The brake dynamometer test is fast in screening materials, less expensive, and capable of controlling the test conditions precisely. In the U.S., two types of brake dynamometers, inertial and chase, are commonly used. The former evaluates a full size brake or brake system and simulates the vehicle braking to a greater extent than the latter. The latter, however, is smaller, less expensive, and faster than the former. The analyte can be in the form of piece of pad lining, strip, or disc. The literature indicates that the data produced on the former are more reliable than the latter (23). Hence, the latter is used as a test for rapid screening and for quality control only. Other test procedures such as SAE J661a, along with SAE 5866, friction assessment and screening test (FAST) developed by the Ford Motor Company, Schedule 49 CR on chase dynamometers, Schedule 78, Schedule 11 1, Schedule 82 on inertial dynamometers are described (5, 23) and will not be discussed here.


Jaybshree Bijwe

Dgerence in Testing Conditions of Drum Brakes and Disc Brakes

Disc brakes offer faster cooling with their larger exposed surface area and better cooling geometry. They are, however, more vulnerable to contaminants, liquid or particulate. Hence, traditionally, front disc brakes and rear drum brakes are used. The requirements for disc brake friction materials are similar to those for drum brakes although the operating conditions are considerably different. Disc brakes operate a t higher temperature than drums. The pad surface, being small a s compared to disc surface, is subjected to higher pressures than the lining in the drum to achieve the same vehicle deceleration. The loading on a typical disc pad and drum lining would be -1 MPa and 0.2 MPa, respectively, to give a deceleration of -0.2g. In heavy duty brake applications, pressures >4 MPa are possible. For assessment of the energy dissipation capacity of a contact, generally the PV factor (product of pressure and velocity) is important. For evaluation of disc brake materials, typical selected speed ranges are 2-3.2 m / s and 1-6 MPa, respectively, (resulting PV factors in the range 2-24 MPa m/s ) (24, 25). The evaluating PV limits are on the higher side a s compared to the actual ones.

2.5. Expected Characteristics of Ideal Friction Materials

Automotive brake material must satisfy a certain set of consumer expectations, which include safety, durability, comfort, and reasonable cost. The exis- tence of numerous brake designs provides another level of complexity in formulating friction materials. The development of friction materials is a complex interactive process, requiring skill to optimize the following desired properties.

i) Adequate and stable friction level over a wide range of operating speeds, pressures, and temperatures, regardless of their conditioning and age.

ii) Ability to withstand frictional heat generated in contact, i.e., thermal and oxidative stability.

iii) Resistance to fade. iv) Good recovery. v) High resistance to wear.

vi) High resistance to cracking and thermal fatigue. vii) Tribological compatibility with the metallic coun-

terpart, i.e., there should not be excessive wear or grooving to metallic disc/drum.

viii) Load index should be one. ix) Strong enough to withstand the high compressive

x) Minimum sensitivity to water, moisture, oils or

xi) Should operate smoothly without noise, shudder,

xii) Easy and consistent in manufacturing.

The requirements for counterpart, i.e. drum materials, are a s follows:

and shear forces generated during usage.

corrosive, salty, and muddy environments.

vibrations.

i) Rigid enough to resist all types of non-homoge- neous stresses, but not so rigid to sustain defor- mations caused by tribological counterpart.

ii) High fatigue strength. iii) Low thermal capacity. iv) High thermal conductivity.

Generally for drums, automotive pearlitic cast iron, with a lamellar particle matrix with the Brine11 hardness range HB 170-280, is used for normal service.

Friction materials require load-carrying capacities much higher (up to a factor of 100) than all other tribo-applications where polymer composites are used. Hence, this class is unique since it demands stable and high friction coefficients and wear rates comparable to many other different wear resistance polymer composites, while working at far higher loads, temperatures, and sliding distances and harsh environmental conditions.

3. EVOLUTION IN THE BRAlCE MATERIALS

Depending on the brake service requirements of the vehicles, various classes of friction materials with specific type of performance characteristics have been developed. Details of classification of earlier organic material have been cited (15). These traditional materials were slowly replaced by new generation materials because of constraints on the brake systems due to

federal motor vehicle safety standards, ban on the use of asbestos due to its health haz- ards, need for fuel efficient vehicles, and noise level monitoring.

These changes in brake materials and design are sum- marized in Table 2.

3.1. Classification of Friction Materials

Selection of a friction material depends on the end use. Material requirements and manufacturing processes of friction materials have been detailed (6). Friction materials can be categorized in three main classes: metallic, carbon-carbon composites, and organic polymeric (resin bonded).

Metallic Brake Linings (Sintered Materials or Cerrnets)

Heavy weights and high speeds of aircraft and high- speed trains require extreme thermal stability. Copper and iron based, solid state sintered materials filled with variety of fillers such as ceramics or other inorganic powders are used in vehicles of high power input densities (23). Though they are easy to fabricate and comparatively cheaper, their high densities make the brake system less energy efficient.

Carbon-Carbon Composites

This material is replacing age old brake materials in racing cars and high tech vehicles such as high speed



Table 2. Trends in Brake Design and Brake Materials (15).

Period

Material Brake: Front-to-Rear

Design Split Fronts Rears

Up to late 1960s Drum brakes on four wheels; Front larger in area and width than rear

Late 1960s to early 1970s Initial disc brakes on heavy cars

Mid 1970s Improved disc brakes on medium weight cars

Late 1970s to early 1980s Lighter front wheel drive vehicles

1980 onwards Lighter front wheel drive vehicles

55:45 Primary and secondary Primary and secondary organic lining organic lining

60:40 Class A disc Primary-secondary

60:40 Semimetallics Primary-secondary organic combination

organic combination

75:25 Semimetallics

75:25 Semimetallics or NAO NAO combination or or non servo

non-servo

trains, main battle tanks, and military and commercial aircraft, where performance is demanding and cost is secondary. Around 63 vol% of carbon-carbon composites produced in the world are used in aircraft brake systems. These materials are prepared from carbon fibers bonded to amorphous carbon. Organic resins are either baked at high temperature or by a chemical vapor deposition process from methane applied to generate amorphous carbon binder. After processing, an essentially pure carbon composite of low porosity results (24, 27). This lightweight, thermally stable material, with reasonably high specific heat, shows excellent performance as a brake material. It is being accepted rapidly for more and more vehicles, though it is expensive. Advantages associated with this class of materials are:

high capacity, 2.5 times greater than that of steel; high strength at elevated temperature (twice that of steel); 40% lighter weight; almost double the service life; and inert to harsh environments.

Cost and oxidation sensitivity are, however, disad- vantages.

Organic Polymeric [Resin Bonded) Friction Materials

Among the friction materials most commonly used in brakes and clutches for normal duty, these usually contain 30-40 wt% organic resin. A large variety of ingredients are used, and formulations are generally patented. The typical friction material contains at least four classes of ingredients with occasional over- lap of functions, such as improving friction and wear characteristics, and processing. The selection of materials of these four classes in their optimum concen- trations is a challenging task for manufacturers (28). These classes are binders, fillers, friction modifiers, and reinforcements.

i) Binders: These are resins having high thermal stability and oxidation resistance at elevated temper-

ature. They contribute to the frictional characteristics of the brake formulation. In wet processing binder is a viscous liquid (usually resole) having characteristics suitable for thermoplastic processing. In dry processing, the binder is in powdery form (usually a novolac), which cures under pressure and temperature. Most common synthetic resins are low cost phenolics, cresylic type or cashew nut shell liquid (CNSL) based phenolics, which are modified with drying oils, rubber, epoxy, cardanol, etc. The binder is not a major component by volume or cost, but it is the most important with respect to the mechanical integrity of the composite. Less thermally stable binders degrade to produce oily degradation products, and are suited only for light duty drum brakes. Such brakes fade and wear excessively (13). Resins that form a stable char a t rubbing surfaces are more suitable for higher severity applications. Additives like cashew particles (7-1 2%) and para-aramid (2-3%) enhance wear life under stringent usage conditions.

ii) Fillers: Low cost materials such as barytes (BaSO,), calcium carbonate, and clay are used as fillers whose primary role is to cut the overall cost, and not to deteriorate the required properties of the composites.

iii) Friction modifiers: These are of two types: abrasive and nonabrasive. They have diverse roles of increasing or modifying frictional and mechanical characteristics. Abrasive powders like alumina or chromium oxide are used to increase the friction coefficient while solid lubricants such a s graphite are added to moderate it. Metallic chips/powders of brass or copper are added to improve heat dissipation. The metallic powders are reported to be beneficial for reducing fade (8). These are used in heavy-duty organic linings (6).

iv) Reinforcements: Fibrous reinforcement plays a major role in maintaining strength, thermal stability and frictional properties of the composites. On the basis of type of reinforcement, this class of materials may be subdivided as follows:


Jayashree Bijwe

Organic friction material

(a) Asbestos-based (b) Non-asbestos based generally called organic brake linings

(b) (i) Semimetallics (semimets) b (ii) *Non-asbestos or organics resin bonded metallics (NAO))

using variety of fibers

(a) Asbestos based organic friction material. Earlier friction materials, i.e., leather and cotton impregnated with asphalt/rubber, were completely replaced by the asbestos fiber reinforced formulation in 1905 because of its unique property profile, including thermal stability, relatively high friction, and reinforcing capability due to its morphology (a combination of long and short fibers), wear resistance, and retention of these properties at elevated temperature and low cost. Above 650°C, however, it loses 90% of its water of crystallization and 70% of tensile strength. Hardness also is not high at elevated temperatures (5, 29). and it is also a health hazard (30, 31). The ban on the use of asbestos by the EPA (Environmental Protection Agency) (3 1-34) has forced the friction industry to seek other fibers.

(b) Non-asbestos materials. (b)(i) Semimetallics. These asbestos-free friction materials were introduced in the late 1960s and gained wide usage in the mid- 1970s. These are generally described as a compromise between organic and sintered metallic friction materials. These were developed in Europe during World War 11, because of difficulties in importing asbestos, and appeared in the early 1950s in the MGB disc pad made by Girling in England (29). Generally they contain 50% metallic component in the form of powders, sponge iron particles, steel fiber and ceramic powder, rubber particles, graphite, and phenolic resin, etc. These materials are primarily used for disc brake pads, in down-sized and medium-sized cars with smaller brakes requiring improved frictional properties a t higher temperatures, and for heavy duty operations. They differ from NAO linings because of their restricted composition range with unique friction and wear properties. They exhibit a stable friction coefficient, 0.35-0.45 (at low temperature), 0.3-0.35 (at high temperature), low wear, excellent compatibility with rotor, improved energy absorption, higher temperature capability, premium price and quiet operation. While semimetallics are still being used in auto-

* This review article I s based on this class of brake lining.

motive disc pads, they have made no major inroads into other phases of the business.

(b)(ii) Non-asbestos fiber reinforced materials. Their development was rapid from 1983. Over 1200 different fibers, acicular materials, and other reinforcing agents have been tested to date. Most of the NAO materials use a blend of different fibers and other nonfibrous reinforcements; the number of potential formulations is staggering. The fibers a t the forefront are glass (chopped, wool, and many proprietary spe- cies), metal (generally steel), ceramic, mineral, aramid (DuPont Kevlar), cellulosic and other organics. Table 3 (35) describes some of the important properties of these fibers. Acicular materials such as wollastonite and plate-like materials, such a s attapulgite, are also reinforcing agents (1 3). The binder resin is generally of the phenolic type; both liquid and powder forms or rubber or cashew binder resin are also used. Incorpo- ration of many inorganic additives for improving overall performance is effective. However, their combined effect is unpredictable and is achieved through trial and error coupled with prior experience and testing expertise. A large number of patents have been issued on NAO in Japan, the U.K. and the U.S. (36-50).

The normal fade-free maximum operating temperatures of various friction materials are, for drum linings and clutch friction materials, 250°C; organic materials, 300-350°C; semimetallics, 400°C; and, cereme- tallics and carbon composites, 700°C (6).

4. DEVELOPMENTS IN ASBESTOS-FREE FIBER REINFORCED BRAKELININGS

In spite of the immense importance of frictional materials, little has been reported in the literature, as compared with the vast literature on antifriction materials (5 1-55). This may be partly due to the complex nature of tribo-mechanisms of multi-ingredient materials. The efforts focus on formulating and evaluating composites, rather than on investigating the underly- ing mechanisms. Limited mathematical models such as a linear or inverse rule (56, 57) to predict tribological properties in terms of properties of ingredients have been tried. Such models have, however, limita- tions, since tribo-properties are not intrinsic material properties.

A very systematic and extensive effort to investigate the influence of individual fibers or filler on the tribo- performance of a friction material has been done (5). Two characteristic groups of fibers (i) reinforcing, such a s glass, steel ceramic and mineral were added 10 and 20 wt% and (ii) fiber aiding processing (aramid, carbon and acrylic) were added in 4 and 8 wt%; Table 4 (5). Evaluation of composites containing abrasives such as finely divided powders of refractory materials, such as alumina, green chrome oxide, zirconite, and quartz, was also done. These composites containing phenolic resin, fine abrasive powders of different grain sizes, and 30 wt% steel fibers fall in another category of friction materials (semimetallics) and hence are not included in this review. Crosa and Baumvol (5) tribo-


Composites as Mtion Materials

Table 3. Properties of Fibers Commonly Used as Reinforcements (35).

Fiber Advantages Potential Problems

Aramid

Glass

Carbon

Steel

Cellulosic fiber

Thermoplastic fiber Asbestos

High strength and modulus, high thermal stability, nonaggressive, good wear, stable and steady p

High strength and modulus, cheap

High strength, modulus and high

High strength, modulus and

Infusible

High strength High strength and modulus,

thermal stability

thermal stability

thermal stability, infusible, good wear properties, acts as a filler

Expensive, extra care to avoid nonuniform fiber or pulp on mixing; fiber alone is not adequate and needs other ingredients in formulation

Melts at elevated temperature causing fade, fiber loses form in high shear mixing, molding, spring back, generally unsteady F, low wear characteristics

Loses fiber form in mixing, expensive

Heavy, corrodes, abrades disc/rotors, squeal

Low strength and modulus, low char temperature

melt, causing fade Health hazard, at higher temperature (550°C)

tribo-properties deteriorate because of loss of water of crystallization

tested fiber reinforced phenolic composites and un- filled resin in pad form, on an inertial brake dynamometer fitted with a passenger car disc brake of gray cast iron. Results are shown in Fig. 5. Tests included wear of discs also at two different PV levels and at 230°C. The SEM investigations included micrographs of individual fibers and worn surfaces of pads and discs. These studies did not aim at formulating multi- component brake linings, yet they prove to be useful in understanding the roles of individual types of fiber or abrasives on the tribology of the two component system. Following are the findings of the investigations:

Virgin resin exhibited lower p than the composites with the exception of carbon fiber composite, which indicated lubrication. This was supported by the appearance of transferred carbon particles on the disc. Fiber inclusion was thought to be the reason for increased p. Fiber, a dominating component, enhanced the ploughing component. Mineral fibers exhibited the highest p, followed by fibers of ceramic (CrF), steel (SF), glass (GF), and aramid (ArF). Inclusion of only SF, GF, and CrF resulted in higher pad wear than that of a neat phenolic resin at a high PV level. At lower PV level, however, only SF showed a marginal increase in wear of a pad. Composite pad wear was in the order SF > GF > MF > CrF > CF > ArF > AF. Except for MF, AF, CF, and neat polymer, all fibers tended to increase wear of metallic discs. Maximum damage to the disc occurred with SF in the pad, followed by AF and CrF. SEM studies indicated breakage of GF, generating pulverized particles, leading to an increase in IJ. and wear. Steel fibers did not break, but tended to lift off, leading to enhanced ploughing. MF and CrF fibers have comet-like microstructures with a tail and a head or "shot," the "shot" being the harder part. MF has a higher concentration of shots

0

that are smaller but harder than the lesser concentration of less hard shots of ceramic fibers. This is reflected in friction behavior, the former leading to higher p than the latter. Three organic fibers (ArF, CF, and AF) led to less pad wear. In case of ArF composites, though there was no transfer on a disc, inherent mechanical properties of the fibers were thought to be a reason behind its wear resistance. In the case of AF and CF composites, wear resistance was attributed to the lubricating action of films of carbon transferred on the disc.

An effort to find a replacement for asbestos was made by Washabaugh (29). EMCOR 66 (ultra short fibers) is the blend consisting primarily of mineral attapulgite, a crystalline hydrated magnesium alumi- num silicate (M&Si8O,,H,,.4H,O), which is not a known health hazard (29). It consists of short acicular particles with average fiber length 0.5 pm and an aspect ratio 20: 1, is typical of the range of short fibers and whiskers. Various NAO composites were fabricated with both dry and wet mixing procedures, the matrix ingredients being phenolic resin and a combination of different long fibers such as glass, aramid with ultra-short EMCOR fibers, and friction modifier (36). Better preforming properties with combination of aramid fibers were possibly due to interaction with the broomlike ends of aramid. Semimetallics were also fabricated from EMCOR 66, steel wool in phenolics and evaluated. Tribo-evaluation as per SAE J661a of these NAO composites, and semimetallics led to hot and cold friction coefficients in the range of 0.35-0.45, which was highly suitable for automotive and truck applications. The materials displayed ready process- ability and high temperature stability. EMCOR fibers also led to the smoothening of the friction curve.

A comparative investigation of nine non-asbestos (NAO) commercial materials and four asbestos-based materials on a full scale inertial dynamometer has been done (31). Unburnished, burnished, initial hot and final performance, fade, and recovery studies


Jayashree Bijwe

Table 4. Selected Physical Properties of the Different Fibers Used in the Work (5).

Fiber

Physical Property Chrysotile Asbestos* Aramid Acrylic Carbon Glass Steel Mineral Ceramic

Tensile strength (GPa) 2.1 2.75 0.88 1.3 3.4 0.95 1.5 1.1 Modulus of elasticity (GPa) 11.7 62.0 17.7 30.0 72.0 11.0 70.0 152.0 Mohs hardness 2.5-4.0 - - 6.0 6.5 5.0 6.0 6.0 Specific gravity (g/cm3) 2.4-2.6 1.44 1.18 2.6 2.5 7.5 2.7 1.7 Diameter (km) 1 e--30 12 10 10 10 120 5 10 Elongation to break (%) - 3.3 15 2.0 4.8 7.0 - 1 .o

* Included for comparative purposes.

were performed. Though most of the physical characteristics of these two types of materials were comparable, NAO materials were found to be less ductile and displayed lower wear rates during F.A.S.T. (friction assessment and screening test).

Loken (35) fabricated various composites with fixed composition (50% wollastonite, 20% BaSO,, 15% cashew friction particles and 15% dry phenolic resin) reinforced with Kevlar (5%) in various forms and aspect ratios. The fiber form and processing parameters influenced mechanical properties, particularly strength of the composites to a greater extent, but not friction and wear significantly. Comparative performance (Table 5) indicated Kevlar as an excellent substitute for asbestos. Figure 6 (35) contains wear data on four materials, indicating the superiority of Kevlar composites filled with thermally stable and inert dolomite (Mix c) over asbestos based materials and ap- proaching that of semimetallics. Though Mix A exhibited better wear performance than the asbestos-based material, it resulted in rough stops, indicating that lab tests do not always reflect field performance. It is observed that fiber orientation affected wear performance only in the case of aramid composites and not for glass or asbestos composites. Composites with fibers normal to the sliding direction showed better performance than those with parallel direction (35).

Commercial disc brakepads contain more than ten constituents. Kato and Magario (58) focused only on aramid fiber, Kevlar 29 (roving type 97 and length = 2mm) reinforcement in phenolics to examine the influence of the amount of fiber (0-40 wt%) on properties such as hardness, thermal conductivity, friction coefficient, and specific wear rate (volume wear per unit force, per unit sliding distance). Investigations of nine materials, including neat phenolic resin (in slider form), on a slider on-disc system in the pressure range from 0.28 to 56 MPa were made against a cast iron disc. The following observations emerged from the studies.

1) Hardness (HRR) and thermal conductivity of composites decreased linearly with increasing fiber content.

2) With increasing number of operations, p ap- proached a steady state value in the range of 0.22 to 0.28, which depended on fiber content.

3) A s seen in Fg. 7, inclusion of fibers resulted in a decrease in p from 0.5 to 0.25. It was also observed

a. m/s

F I B E R n

F I B E R

M F -MINERAL F I B E R A F - ACRYLIC FIBER

C r F -CERAMIC F I B E R A r F - ARAMID FIBER

G F -CLASS FIBER C F - CARBON FIBER

S F -STEEL F I E E R BASIC- PHENOLIC RESIN

Fig. 5. CoeffLcient of friction, wear of pads, and wear of the discs at two d@erent loadcarrying capacities for the various ftber reinforced friction materials. (The wear of the discs is the total wear for the two loadcarying capacities) (5). [Reprinted

from G. Crosa and I . J . R. BaumvollAduances in composite tribology, 1993, pp 609, with kind permission from Elseuier Science-NL, Sara Burgerhartstraat 25, 1055 KV, Amsterdam, The Netherlands.]

that increase in fiber content >15% did not reduce I*..

4) Specific wear rate of phenolics decreased drastically (forty times) on fiber inclusion.



Table 5a. Composition of Selected Composites (35).

Composition

Friction Particles (CNSL) NC 104- Phenolic Resin Kevlar Fiber Crimped

Designation Wollastonite Dolomite BaSO, 40 NC 126 and Chopped

Mix A

Mix B

Mix C

Asbestos based (-5040% asbestos - 15% BaSO, -5% other

organics) -25%

26% 26%; 100% <300 mesh

<loo mesh

<200 mesh

- 50%; 75%

- 50%; 100%

- - -

16% 15% 16% 5% (1/4"; 12 pm dia.)

15% 15% 15% 5% (1/4"; 12 pm dia.)

15% 15% 15% 5% (1/4"; 12 pm dia.)

- - - ,15%

Table 5b. Physical Properties of the Composites in Table 5a.

J 661 Test Dispersion

and Brake Disc Brake Disc Friction On Car Designation Stability Molding Machining Performance Wear Fade Performance

Mix A good good good excellent excellent good/excellent roughness Mix B poor poor poor good, p little excellent excellent excellent

Mix C good good good excellent excellent excellent excellent Asbestos excellent excellent excellent good good excellent excellent

low

based

5) The rate at which the specific wear rate decreased with an increase in fiber content slowed down rapidly for >25 vol% fiber and was almost negligible >35%, indicating fiber content range 25-35% was adequate for minimum specific wear rate and 15% for minimum p.

6) Topographical studies (roughness values) on slider surfaces indicated maximum changes for neat resin and minimum for composites with 40% fibers.

7) SEM studies indicated that enhancement in friction and wear properties due to fiber inclusion was due to stronger and more firmly attached transfer film on the counterface. Low thermal conductivity, high mechanical strength, and reduction in size of wear debris were thought to be responsible factors for adherence of the film. Abrasive action of metallic asperities on the slider became negligible once it got covered with transformed film. Owing to low surface energy of polymer film, the interaction between film and composite was a minimum.

These studies highlight that friction and wear properties desired for friction materials could not be inde- pendently controlled. For optimum friction levels ( p = 0.35). fiber content > 10 wt% was not advisable.

A more efficient and low cost alternative to asbestos fibers was claimed by Ashland Oil, USA (59). Short carbon fibers manufactured from petroleum pitch,

under the trade name Carboflex, have shown excellent performance a s compared with asbestos lining. Car- boflex fibers are claimed to have a unique combination of properties such as high strength (equal to steel), low density (%th of steel), stiffness, high heat conductivity, thermal stability up to 3050°C, oxidation stability up to 540°C, self lubricity, wear resistance, very low expansion coefficient and hence the ability to retain dimensions over a wide range of temperature, ease in handling, variation in length and diameters, and in- expensive as compared with PAN carbon fiber. A comparative study of these carbon fiber linings and asbestos linings (59) a s per SAE J 661a revealed the superiority of the former over the latter in terms of braking ability a t elevated temperatures, pressures, and speeds. It showed higher fade resistance and less braking time, which is critical in emergency stops. Table 6 highlights some of the factors of the comparative performance of these two materials. Further studies on the influence of fiber length on braking performance indicated that fiber length in the range 300 to 500 pm led to still better results. In fact, evidence of commercialization of these materials in passenger cars are reported. A high carbonizing pitch product under the trade name Aerocarb was proposed to be more effective if combined with Carboflex.

In-depth investigations on friction and wear characteristics of aramid composites were made by Briscoe


Jayashree Bijwe

f 1

1 / COMMERCIAL

/ ASBESTOS BASED CONTROL

J TRUCK BLOCK MIX WITH 2"/0

/ 7 KEVLAR /

MIX C WITH

/ P

./ 5'10 KEVLAR

Fg. 6 . High temperature wear of friction materials (35).

1 .o -I3 t 2

MI . LIC

- '4bO S b O 660 7d0 860 9bO lob0 O F

204 260 316 3 7 1 427 482 538 "C DRUM TEMPERATURE

Fig. 7. Variation in friction coefli- cient and speci@ wear rate as a function of vol% aramid fLber content (sliding speed 5.6 m / s , load 294 NI (58).

et al. (25, 60) with an emphasis on the fundamental response and effect of the aramid fibers in composites on tribo-properties, and to compare this with other fiber reinforced systems and a commercial brake material. The approach to evaluate material response was totally different from the conventional approach. The experimental setup, a steel ball on a composite flat, was selected to model the inherent frictional re-

sponse of the materials. Two combinations were used-a hot ball on a cold flat, and a hot ball on a hot flat, which could lead to a considerable difference in the surfacelbulk temperature distribution, and the overall temperature dependence of the frictional response. The selected composites are described in Ta- ble 7. The theme of the investigations was to use load index to interpret the influence of temperature on the

388 POLYMER COMPOSITES, JUNE 1997. Vol. 18, No. 3


Table 6. Comparative Properties of Non-Asbestos (Carboflex) and Asbestos Formulations (59).

SAE J 661 Test Dispersion and

Dispersion Brake Disc Brake Disc Operating Composition Stability Molding Machining P Wear Fade Performance

Asbestos good very good very good 0.32-0.42 0.010 good very good

Carboflex good very good very good 0.40-0.65 0.002 excellent very good formulation

formulation

Table 7. Composition and Tribological Properties of Selected Composites (25, 60).

Load Index (High Speed Simulation

kP.1

Material Resin Fiber Orientation Overall Initial

EPOXY Epoxy-Kevlar Phenolic-Kevlar

Phenolic-Kevlar

Phenolic Carbon

Phenolic-Glass

Brake

Epoxy-polyfBed812 - Epoxy-polyfBed812 Phenolic-Cello-Bond (35 wt%)

Phenolic-Cello-Bond (50 wt%) (50 wt%)

Phenolic-Cello-Bond Carbon, W S , high strength grade

glass, E type (35 wt%)

Kevlar 49, roving type 986 (80 ~01%)

(35 wt%) (35 wt%)

(35 wt%)

Phenolic-Cello-Bond

Phenolic-Cello-Bond -

- P N P 0 P 0 P

0 N P 0

- 0.672 1.035 0.885 0.926 0.921 1.064 0.783

0.833 1.01 0 1.095 0.922 0.990

- 1.054 1.163 1.080 1.103 1.029 1.279 1.004

0.970 1.010 1.095 1.129 0.990

N - normal, P - parallel, 0 - orthogonal.

frictional behavior of the selected composites. Mea- surements of p at various temperatures indicated ir- regular patterns for all the systems in different fiber orientations. The brake material, however, showed a decrease with increase in temperature. Values of p of the selected materials studied over the temperature range ambient to 300°C varied from 0.1 to 0.3, with the exception of phenolic-Kevlar (50%) composite, which showed values up to 0.4. It was also seen that Kevlar alone did not have the fundamental type of response essential in brake materials. In fact it performed a lubricating function in these simulations. Investigations indicated that the initial load indices* were close to unity and then decreased to different extents during experiments. For composites of Kevlar and carbon fiber this decreased to different extents as experiments proceeded. For both these composites this decrease was high as compared with glass fiber composites and asbestos brake material. This was thought to be due to the higher temperature dependency of rheology of the interfaces. Epoxy-Kevlar, phenolic-Kevlar, and phenolic-carbon systems showed pronounced evidence of third body (film) formation and lubrication of the contact with the trends in the experimental load index of pronounced curvature (de- viation from the linearity in friction force vs. load re- lationship). In the case of phenolic-glass and brake

* FF - KW" where FF is frictional force, W is load. and n is the load index. n = % indicates elastic contact and n = 1 indicates plastic deformation (61. 62).


material there was evidence of material damage of the sample accompanied by direct dependency of the friction on load. Thus, in the first group of composites, a marked reduction in p with load (i.e. fade) was due to thermally induced lubrication (film transfer) of the contact, and in the second group it did not occur, possibly because a lubricating film was not produced, or if produced, it was abraded away during sliding.

With a view to investigate the influence of aramid fiber on tribo-properties of the composites, Sinha and Biswas (63, 64) selected compression molded cast composite of Kevlar 49 (axially; 30 wt% fiber, density 1.3 g/cm3) in a phenolic resin. Friction and wear studies at ambient temperature were carried out against an EN24 steel disc on a pin-on-disc machine at various PV values. The experimental setup and counterface material was not exactly as per SAE standards, and fade and recovery were not studied.

The investigations did reveal the potential of aramid fibers as a substitute for asbestos.

The following salient features emerged from the studies.

1 ) The neat phenolic resin exhibited high steady state

2) Inclusion of Kevlar fibers resulted in p, but poor wear resistance.

a .decrease in adhesive forces at mating surfaces, b. reducing p (without fibers, 1.5-1.2, with fibers,

0.8-0.31, c. stabilizing p after a longer time, compared with

the phenolic resin, which is, however, undesir-

389

Jayashree Bijwe

able for a friction material, d. enhancing wear resistance substantially (30-40

times), e. reducing p to a steady state less drastically a s

compared with resin when water lubricated, f. increasing wear rate when water lubricated. Even

in this condition, film was observed on the wear track.

Figure 8 shows p as a function of PV factor. The steady p in the desired range at high PV values indicated the excellent potential of the composite as a brake material. In general, the smoother the counterface, the higher the p and the lower the wear rate. SEM studies revealed in situ formation of a phenolic resin film on a wear track. Fiber ends exposed at the interface trapping wear debris and enhancing smooth film formations were thought responsible for the decrease in tangential traction and hence subsurface stresses and ultimately wear rate.

Izyumova et al. (65) fabricated various composites with different processing techniques based on organic binders, i.e., phenolic resin (both novolac and resol type) and butadiene-nitrile rubber), with an intention to find a substitute for asbestos. Four types of fibrous reinforcements, i.e., basalt fiber BCTB, glass fiber CIIA, chopped glass fiber EC 130-140 P, and aramid fibers, were used and compared with a n asbestos formulation for physico-mechanical and tribological properties studied on three different rigs, leading to the following conclusions.

1) Among the three selected machines for tribo-evaluation, RANZI-LRC simulated the field conditions to the maximum extent.

2) Heat treatment and processing parameters influenced the physico-mechanical and tribological performance.

3) Among the various binders, novolac resin displayed the best performance, i.e. stable p at 350"- 420°C.

0.6 I

0

Q 0

0

" 0.0 0 10 20 30

PV f ac to r ( M P a m;')

Fig. 8. Friction coefi ient plotted against the PV factor for the aramid composite (64).

Among the selected composites, basalt fiber composite exhibited good physico-mechanical and tribological properties. The optimum concentration of fibers for a highly stable p at all selected high temperatures was 7 wt%. Only aramid composite transferred a fiber/material on the counterface. On full scale tests (dynamometer and road test) the composite containing combinations of fibers (basalt + aramid) displayed the best tribo-performance since it displayed more stable p and higher braking efficiency than the asbestos based linings. This composite was recommended for mass pro- duction for brakelining for ZIL cross-country trucks.

Investigations by Shibata et al. (66) on developed hybrid composites reinforced mainly with fibers of carbon and aramid proved the superiority of new composites, resulting in higher and stabler p, less brake noise, judder, and wear than the asbestos lining.

One group of researchers-Dharani, Blum, and Go- pal (34, 67-70)-is actively engaged in formulating and evaluating non-asbestos brakelinings. A complete and systematic evaluation of materials as per stan- dard procedure (SAE J 661a) was done on a Chase dynamometer. Friction, wear, fade, and recovery characteristics were studied under varying speeds, pressures, drum temperatures, and number of applications. Wear mechanisms were investigated using SEM and EDAX analysis. Phenol formaldehyde resin, NC 126 (structure shown in Fig. 91, and baryte (BaSO,) were common resins and fillers, respectively, in all the composites. The composites differed only in type and nature of reinforcement.

Six composites with composition 35 / 20/ 25 / 20 wt% of glass fiber/resin/filler and friction particles (CNSL) were selected (34). The milled glass fibers in each composite differed in length, diameter, and sizing. Studies of mechanical properties of selected composites indicated that the sizing of fibers influence only tensile strength, while flexural strength increased with increase in fiber length and decrease in fiber diameter. The flexural modulus increased with increase in fiber diameter. Young's modulus, however, was observed to be independent of fiber length, diameter, or sizing. The friction coefficient of the composites were found to be in the range of 0.21 to 0.33, which is reasonable but not adequate for brake linings. The mechanical properties could not be corre-

OH I OH

( d H 1 CH=CH-(CHZ)~-CHZ-CH~ 26 Fig. 9. Idealized chemical structure of partially crosslinked phenolJomaldehyde resin (341.



lated with frictional properties. The studies were useful for an in-depth understanding of the role of glass fibers in friction performance.

The details of composites selected in their further work (67-70) are described in Table 8. The following are the findings on the tribo-evaluation of the composites. A) GFRFM (glass fiber reinforced material) phenolic

composite. i) With the increase in load, speed, and tempera-

ture, both p and specific wear rate (WJ decreased. W, vs. temperature relation, however, showed a reverse trend. Microscopic studies indicated resin degradation at higher temperature and weakening of fiber-matrix adhesion, and hence the ease of fiber pullout, were responsible for this behavior.

ii) Fade characteristics were influenced by sample history. A film containing filler, glass fiber, iron, and possibly carbon also formed on the worn tips of glass and steel fibers caused further fade. Rubbing off this film with abrasive papers resulted in less fading.

iii) High, unstable p and fading tendency of the composite makes it an unsuitable friction material.

B) CFRFM (carbon fiber reinforced material) phenolic composite. i) Friction behavior of this composite was less de-

pendent on testing parameters than GFRFM. ii) Specific wear rate (W,) increased with speed and

temperature and decreased with load. iii) Conditioning the specimens with several fade-

recovery test cycles resulted in steady friction during subsequent fade tests, followed by excellent recovery. F-R (fade and recovery) characteristics of this composite were different from the GFRFM because of accumulation of wear debris at the sliding interface, influencing successive sliding.

iv) A steep decline in friction with varying operating parameters (Fig. 10) rendered this composite unacceptable for brakelinings, though the overall performance was better than that of the GFRFM.

C) Control Composites GC and SC (Table 8) containing fibers of glass and steel, and hybrid composites (GKC and SKC) containing fibers of glass -t Kevlar and, steel + Kevlar.

The hybrid composites were tailored to examine influence of inclusion of Kevlar in control composites. Ad- ditional experiments on squealing characteristics of composites were also performed. The interesting results (Figs. 11-14) were as follows:

reduction in high friction; enhanced frictional stability over a wide range of operating parameters such as pressure, speed, and temperature; substantial decrease in wear rate; elimination of squeal.

The extent of enhancement in desired tribo-properties of the composites depend on type of composite and operating parameters. Steel-Kevlar composites, however, exhibited the best combination of properties, and were least influenced by test conditions. The au- thors claimed that steel-Kevlar composites showed excellent potential for use in automobile brakelinings.

Further investigations (7 1 ) focused on understanding tribo-mechanisms of the two component system consisting of CNSL modified phenolic resin and Lep- oinus fiber, RF 5 164(volcanic rock fiber) with varying amounts of fiber (0-13.5 ~01%) . Friction and wear studies of five composites at different temperatures resulted in a low but stable p over wide temperature range. Both neat resin and high fiber loading composites showed high p. A proper amount of fiber loading (0.024 and 0.05) improved fade properties. Minimum wear rate was recorded with 0.05 fiber loading. Adhe-

Table 8. Details of the Selected Brake Material (67-70).

CNSL Friction Phenolic Resin Particles NC Filler, Baryte Secondary

Composite NC 126 104-40 (BaSOJ Primary Fiber Reinforcement

GFRFM 20 wt% 20 wt% 20 wt% milled E glass steel fiber 30 wt% I = 150 pm

carboflex I = 200 pm

milled E glass d = 15.8 prn

d = 1 3 p m 10 wt% CFRFM 25 wt% 15 wt% 20 wt% carbon P 200 steel fiber

d = 10pm 10 wt% - 20 vol% GC* glass composite 40 vol% -

GKC" (glass-Kevlar 40 vol% - 8 vol% milled E glass d = Kevlar pulp

Sc' Steel composite 40 vol%

SKC" Steel-Kevlar 40 vol%

composite) 15.8 pm 12% - 20 vol% steel S-207 coarse -

- 8 vol% steel S-207 coarse Kevlar pulp grade 40 vol%

composite arade 40 vol% 12%

* GC and SC were designated as control composites. Of GKC and SKC were designated as hybrid composites.


Jayashree Bijwe

1.2 -

- 1.0-

*E 0-8-

L

L \

U v

w 0.6-

L 0.4-

c

2

3 0.2-

n.o+

0 01

200 300 LOO 500 600 700 - Load ( N )

0

O

&I

go

0

0

0

B

L - - - - J I . I I I I I

- I

.- O-LO--

.c 0,35

5 0.30-

c U

LL

c C

. C J 0.25 0.20

U . - _'---- - D r u m Temperature C'C)

( C )

Fig. 10. Variation in friction coefliient of CFRFM (carbon f h e r reinforced material) as a function of [a) Load [at V = 5.8 m / s and T = 204°C); (b) Speed [at P = 669 N and T = 204°C); [c) Temperature [at P = 669 Nand V = 5.8 m/s ) (69).

sive and abrasive wear mechanisms were dominant for low and high fiber loading material, respectively.

For investigating the simultaneous influence of Cu powder, BaSO,, and cashew dust on friction and wear of brakepads, Handa and Kato (72) selected three series of several composites containing five ingredients. The amount of phenolic resin (20%) and aramid fibers (20%) was fixed. Among the three components, Cu powder, BaSO,, and cashew dust, the amount of one ingredient was fixed and the other two were varied from 0% to 40%. Tribo-evaluation of these composites was done on a slider-on-disc type wear tester under

L

2 0 0.41 L

L i 0 ? 6, 0.21 O

0 0 43

0

0 $03 0

0

0

0 0

0 0

00

0

0 0 p."o

- - 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Average coeff icient of fr ict ion

( b ) Q. 1 1. Friction-wear relationships at va ying temperature a, 0) and uaying speed (0, 0 ) of [a) glass composite (0, 0). glass-Keular composite (0, 0): [b) steel composite (0, 01, steel- Keular composite (0, 0 ) [clustering ofpoints as shown in the box indicates stable friction and wear ouer different operating conditions. Scatter of data points parallel to the ordinate indicates good friction stability and poor wear stability. Scatter of data parallel to the abcissa means vice versa) (70).

two sets of a conditions: first, simulating mild braking on a level road, and second, severe braking on a long downhill. Friction and wear were studied as a function of increasing amount of a typical filler (and decreasing amount of the other filler). To get a clear idea of the influence of each ingredient on fade and wear rate, multiple regression analysis(MRA) was applied to the data. The following are the conclusions from the data analysis and SEM studies.

1) Cu powder inclusion resulted in increase in fade resistance and decrease in wear resistance.



g 0.2 - L W > Q

0.0 I ' I ' I ' I ' I

Temperature ("C)

(a1

0.8

h

2 0.6 E

2 0.L 2

1 0

U v

L

0

r" 0-2

0.0 2

I ' I . I ' I '

3 220 2LO 260 280 300 3 Tempera ture ("CI

(b)

I

Fig. 12. Effect of temperature on friction coefficient (Fig. 12a) and wear rate (Fg. 12b) of uarious composites at 5.8 mls; (i) glass Composite (GC). (ii) glass-Kevlar composite [GKC), (iii) Steel composite (SCJ; and Steel Kevlar composite (SKC) (70).

2) BaSO, inclusion led to the exactly opposite behavior, i.e., a decrease in fade resistance and an increase in wear resistance and strength.

3) SEM studies indicated that a large part of a layer from the worn surface was peeled off a s a result of repetitive sliding, which roughened the surface, leading to an increase in wear when BaSO, was eliminated from the composite.

4) In the case of Cu powder-cashew dust combination, specific wear rate increased almost linearly with the increasing amount of Cu powder. Maxi- mum fade resistance and high friction properties were exhibited when both ingredients were 20 wt%.

Among the series of composites from BaSO, and cashew dust, the best friction performance and fade resistance were shown with 0% BaSO,. Minimum specific wear rate, however, was displayed by the composite containing 10% BaSO, and 30% cashew dust.

MFU results indicated that both Cu powder and cashew dust had a noticeable effect on the friction ratio.* BaSO,, however, had little effect.

Among the three composites (73), the first composite contained potassium titanate fibers, the second was a hybrid composite with potassium titanate and asbestos fibers, and the third contained only asbestos fibers. Tribo-evaluation on a dynamometer led to the conclusion that the first composite exhibited high p, high wear rate, and low tensile strength because of cracks generated on the surface. The hybrid composite, however, displayed the desired properties, indicating that potassium titanate alone cannot replace asbestos.

Three commercial non-asbestos phenolic based friction materials recommended for hydrogenerators were reported to exhibit high wear, produce more dust, and lead to thermal cracking of the metallic counterpart (74). The composition differed in the type of fibers, fillers, and friction modifiers. Fibrous reinforcements were Kevlar + glass + wollastonite + gran- ulated wool, Kevlar, and chopped glass. In-depth investigations of these composites and asbestos brakelinings led to the conclusion that no material matched the tribo-performance of asbestos lining. The investigations indicate the need for rigorous testing of friction materials before they are used in real vehicles/applications. It was also clear that Kevlar cannot replace asbestos in all situations.

In this article, the friction materials using various types of phenol formaldehyde resins and C.N.S.L. resins only are discussed. However, other resins such a s melamine and nitrile rubber (75), thermosetting poly- imides and epoxy resins (76), butadiene nitrile rubber (77, 78), aromatic nitro-compounds (79). and pitch derived condensed polycyclic aromatic hydrocarbons (80) are also reported in patents.

5. CONCLUSION

Since 1980, efforts to develop non-asbestos friction materials were initiated primarily for health and environmental safety considerations. In the last decade a large number of such friction materials have been successfully formulated and patented. This new class of non-asbestos fiber reinforced organic polymeric friction materials have completely replaced asbestos based brake materials because of their superior performance and their environmentally friendly nature. The present state indicates that future efforts should be focused more on the scientific approach for under-

p,,,, in the constant interval test pf,.., In the constant temperature test "I Friction ratio =

This represents the rate of reduction in p due lo successive severe braking.


Jayashree Bijwe

Fg. 13. Typical values of friction coefficient obtained during wear tests at 204°C and various speeds for steel composite (SC) (i.iiiii) and steel-Keular composite [SKC), (iu, u, ui). Sliding speeds, (i) and [iu) - 5.8 m/s; (ii) and (u) - 9.5 m/s , (ii) and [ui) - 11.2 m / s (70).

>r

ul C w C

Y .-

Y

I

0.0 j 0 20

0 Frequency ( k H z ) 2 5

(a 1 Fg. 14. Noise frequency spectra at 204"C, and speed 11.2 m/s: (a) glass composite (GC); (bl glass- Keular composite (GKC); (c) steel composite (SC); [d) steel-Keular composite (SKC) (70).

394

40 60 80 100

Appl icat ion Nurnber

z

ln C W

C

L .-

c

I

0 F r e q u e n c y ( k H z ) 2 5

( b)

0 Frequency ( k H z 1 2 5 0 F r e q u e n c y ( k H z ) 2 5

(c 1 ( d )



standing their tribo-mechanisms, developing wear models, and laws of mixtures for multi-ingredient formulations rather than just formulating and evaluating. Tailoring of the desired properties should be made possible by critically analyzing the simultaneous influence of various components on friction and wear. Compared with antifriction materials, this area of friction materials is almost untouched.

ACENOWLEDGIYIENT

The author is grateful for the kind permission granted for reprinting the following figures/ tables: Fig. 4 (Ref. 15, pp. 5131, Fig. 10 (Ref. 69, pp. 1981,Figs. 11-14 (Ref. 70, pp. 202-205), Table 1 (Ref. 1, pp. 258) and Table 4 (Ref. 5, pp. 605) with kind permission from Elsevier Science-NL, Amsterdam, The Nether- lands. Ftg. 8 (Ref. 64) and Fig. 9 (Ref. 34) with kind permission from Chapman and Hall, and Gordon and Breach Publishers, respectively. Fig. 6 and Ref. 35 “Reprinted with permission from SAE Paper No. 800667 0 1980 Society of Automotive Engineers, Inc.”

1.

2.

3.

4.

5.

6.

7. 8. 9.

10.

11. 12. 13.

14.

15.

16.

17.

18. 19. 20.

21.

22.

23. 24.

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Revised October 1996


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