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Wear.46 (1978) 203 - 212 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands 203 MICROSTRUCTURAL CHANGES IN SEMIMETALLIC DISC BRAKE PADS CREATED BY LOW TEMPERATURE DYNAMOMETER TESTING* T. A. LIBSCH and S. K. RHEE Research Laboratories, Bendix Corporation, Southfield, Michigan (U.S.A.) (Received July 5, 1977) Summary A combination of commercial semimetallic disc pads and a ventilated gray cast iron rotor were tested with an inertial dynamometer according to the following schedule: 100 burnish stops plus 300 stops initiated at a rotor temperature of 120 “C. Changes in friction and wear observed during this test have been related to structural and compositional changes in the pads, rotor and collected wear debris. These structural and compositional changes have been characterized by scanning electron microscopy, energy dispersive analysis of X-rays, X-ray diffraction and Knoop hardness measurements. Friction-affected zones are created in both the pads and the rotor during the initial portion of the test. In the pads the zones are denser than the bulk material; in the rotor the zones are composed of a thin layer of material permanently transferred from the pads over a thicker layer of decomposed pearlite. The removal of the rough surface of the as-ground pads and the concurrent development of friction-affected zones are responsible for the high rate of pad thickness wear observed during the initial portion of testing. Introduction A significant research and development effort is currently in progress to understand the friction and wear properties of “semimetallics” [l - 31. This class of friction materials is characterized by a very high fraction of iron and graphite. Steel fiber is used as a structural reinforcing agent in place of asbestos which is commonly used in “organic” friction materials. In inertial dynamometer tests, semimetallic friction materials exhibit equivalent friction and considerably higher wear resistance than organic fric- tion materials at temperatures above 230 “C. Below this temperature the fric- tion of semimetallics is equivalent to or higher than that of organics; the wear *Paper presented at the International Conference on the Wear of Materials, St. Louis, MO., U.S.A., April 26 - 28, 1977.

Microstructural changes in semimetallic disc brake pads created by low temperature dynamometer testing

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Wear.46 (1978) 203 - 212 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

203

MICROSTRUCTURAL CHANGES IN SEMIMETALLIC DISC BRAKE PADS CREATED BY LOW TEMPERATURE DYNAMOMETER TESTING*

T. A. LIBSCH and S. K. RHEE

Research Laboratories, Bendix Corporation, Southfield, Michigan (U.S.A.)

(Received July 5, 1977)

Summary

A combination of commercial semimetallic disc pads and a ventilated gray cast iron rotor were tested with an inertial dynamometer according to the following schedule: 100 burnish stops plus 300 stops initiated at a rotor temperature of 120 “C. Changes in friction and wear observed during this test have been related to structural and compositional changes in the pads, rotor and collected wear debris. These structural and compositional changes have been characterized by scanning electron microscopy, energy dispersive analysis of X-rays, X-ray diffraction and Knoop hardness measurements.

Friction-affected zones are created in both the pads and the rotor during the initial portion of the test. In the pads the zones are denser than the bulk material; in the rotor the zones are composed of a thin layer of material permanently transferred from the pads over a thicker layer of decomposed pearlite. The removal of the rough surface of the as-ground pads and the concurrent development of friction-affected zones are responsible for the high rate of pad thickness wear observed during the initial portion of testing.

Introduction

A significant research and development effort is currently in progress to understand the friction and wear properties of “semimetallics” [l - 31. This class of friction materials is characterized by a very high fraction of iron and graphite. Steel fiber is used as a structural reinforcing agent in place of asbestos which is commonly used in “organic” friction materials.

In inertial dynamometer tests, semimetallic friction materials exhibit equivalent friction and considerably higher wear resistance than organic fric- tion materials at temperatures above 230 “C. Below this temperature the fric- tion of semimetallics is equivalent to or higher than that of organics; the wear

*Paper presented at the International Conference on the Wear of Materials, St. Louis, MO., U.S.A., April 26 - 28, 1977.

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resistance, however, is variable. Semimetallic disc pads exhibit poorer low temperature wear resistance than organics when tested with ventilated rotors only at low tem~ratures; however, wear resistance increases at low temper- atures after testing at higher temperatures. This behavior is not observed when semimetallic pads are tested with solid rotors.

In vehicle tests, i.e. under the wide variety of conditions encountered in service, semimetallics are found to be three to five times more wear resistant than organics.

The purpose of the current investigation was twofold: (a) to explain the decreasing thickness wear rate of the pads observed during low temperature inertial dynamometer testing and (b) to present fundamental microstructural information on tested semimetallic pads and the associated rotor. Emphasis has been placed upon the semimetallic pads; collected wear debris and the rotor have been examined to a lesser extent.

Experimental procedure

Friction and wear testing‘ Commercial semimet~lic disc pads and a ventilated gray cast iron rotor

were tested with an inertial dynamometer using a brake system designed for subcompact passenger cars. The test schedule was as follows; 100 burnish stops plus 300 stops at an initial rotor temperature of 120 “C. Burnish stops were initiated at 17.9 m s-i; test stops at 22.4 m s-l. Parameters common to both burnish and test stops were as follows: (a) initial rotor temperature, 120 “C; (b) rolling radius, 28.2 cm; (c) simulated weight, 402 kg; (d) deceleration, 4.0 m s- 2. Pad weights and thicknesses (six readings) were measured before the test and after each loo-stop interval; the thickness of the rotor was measured before and after the test. Rotor temperature, pad temperature, line pressure and torque were graphically recorded during each stop.

Wear debris was collected on a set of four filters placed between a guard partially enclosing the rotor and a vacuum source; filters were renewed after each lOO-stop interval.

Me tallographic examination The scanning electron microscope (SEM) was used to examine (a} the

subsurfaces and surfaces of as-ground and tested pads, (b) the collected wear debris and (c) the subsurfaces and surfaces of as-machined and tested rotors. An energy dispersive analysis by X-rays (EDAX) unit interfaced with the SEM was used to determine compositions of selected areas.

Two samples of wear debris from the burnish interval and two samples from the last 100 stops were examined by X-ray diffraction. In each interval one sample contained debris brushed from the pads and the other was composed of a mixture of very small pieces of the four filters used to trap airborne debris. The percentage by weight and the average size of the iron and graphite particles in each sample were calculated from integrated peak intensities and line broadenings [43 .

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Additional information was gained from Knoop hardness measurements [ 51 taken on (a) iron particles at the surface and in the bulk of tested pads and (b) on 5.7” taper sections of as-machined and tested rotors.

Experimental results

Dynamometer data Wear data for the semimetallic pads are illustrated in Fig. 1. Thickness

wear during the burnish interval was greater than that during the last 100 stops despite the fact that the dynamometer parameters applied during the burnish were less severe. A detailed discussion of this observation is present- ed later. The wear of the outer pad is greater than that of the inner pad; this result is frequently observed in inertial dynamometer tests and is attributed to the brake system rather than to the properties of the friction material.

.50 c 3.2

P --2.8 L .40-- A

I 1 STOPS AT 120°C BURNISH

Fig. 1. Thickness wear and weight wear of semimetallic pads us. stops at 120 “C: 0 thick- ness wear, l weight wear; - inner pad, - - - outer pad.

The line pressure* at 120 “C (Fig. 2) increases during the burnish interval but remains relatively constant after that,

During a typical stop initiated at a rotor temperature of 120 “C, the semimetallic pad temperature increased by 78 “C and the rotor temperature increased by 28 “C. (Thermocouples were located about 0.10 cm below pad and rotor rubbing surfaces.) The temperature increases of a typical organic pad and ventilated rotor tested under the same dynamometer schedule were 11 “C and 56 “C respectively. In semimetallics a larger fraction of the heat generated at the sliding interface is conducted into the pad.because of the relatively high thermal conductivity of this material.

*For constant applied torque the line pressure is inversely related to the friction coefficient.

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~~~:-;” 0 100 200 300

tBlJRNISH+ STOPS AT 120 C

Fig. 2. Average line pressure vs. stops at 120 “C.

Metallographic data: pads The only difference observable in SEM micrographs of the subsurfaces

of pads before and after testing was the presence of a thin layer (about 0.038 cm) in which the organic components appeared to be partially decom- posed. SEM micrographs of the rubbing surfaces of as-ground and tested pads are presented in Fig. 3; components identified by EDAX are designated in Fig. 3(B). Differences in composition between the as-ground and tested pad surfaces will be examined in detail in a subsequent section.

Fig. 3. SEM micrographs of the semimetallic pad surface (A) before and (B) after inertial dynamometer testing. Identifiable components are (a) iron particle, (b) steel fiber and (c) organic ingredients or graphite.

The hardness of iron particles at the surface of tested pads (about 350 Knoop hardness number with a 20 g load) was found to be the same as the hardness of those particles in the bulk.

Metallographic data: wear debris The recovery for an interval was calculated as the weight of debris

collected* divided by the weight change of the two pads and the rotor during

*The debris was collected from the four filters and brushed from the inner and outer pad surfaces. Approximately 60 wt.% of the recovered wear debris was from the filters.

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the same interval. The percentage recoveries for the burnish interval, first 100 stops, second 100 stops and third 100 stops were 50,71,96 and 73% respectively. These values were calculated assuming zero rotor weight change.

SEM micrographs of wear debris are illustrated in Fig. 4. The wear debris particles vary greatly in size and morphology. EDAX analysis indicates (a) that the large flat particles are iron (presumably from the pad surface) and (b) that elements are present from both the pads and rotor.

Fig. 4. SEM micrographs of wear debris collected from (A) the inner pad and (B) the filters following the burnish interval. Note the filter fibers in the lower micrograph.

X-ray diffraction was employed to determine quantitatively differences between wear debris collected during the burnish interval and that collected during the last 100 stops. These results are listed in Table 1. It was observed (a) that the average size of iron particles collected during the last 100 stops was markedly smaller than that collected during the burnish interval and (b) that the fraction of iron collected during the last 100 stops was significantly larger than that collected during the burnish interval.

TABLE 1

Weight percentages and average particle sizes of iron and graphite in collected wear debris

Interval Location Wt.%

Iron Graphite

Particle size (A)

Iron Graphite

Burnish Pads 82.8 17.2 285 85 Burnish Filters 76.7 23.3 77 42 Last 100 stops Pads 95.9 4.1 188 85 Last 100 stops Filters 85.9 14.1 41 45

In this analysis, the debris was assumed to be composed totally of iron and graphite. This assumption is not unreasonable in the light of the semimetallic composition.

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Metallographic data: rotor SEM micrographs of the rubbing surface of the rotor before and after

testing are given in Fig. 5. The absence of graphite flakes and the presence of dark colored elevated areas on the tested rotor surface should be noted. EDAX analysis showed that these areas contain elements whose only source is the pad, i.e. material has been permanently transferred from the pad to the

Fig. 5. SEM micrographs of the ventilated rotor rubbing surface (A) before and (B) after inertial dynamometer testing. Typical rotor wear is 0.0076 mm for the schedule employ- ed (rotor thickness wear is l/80 pad thickness wear).

SE34 examination of the subsurface of the tested rotor showed deform- ed graphite flakes in a decomposed pearlitic layer as reported previously by several investigators [ 6, 7 ] . The relative contributions of pressure and temp- erature in the formation of the decomposed pearlitic layer are presently un- known.

A significant difference (about 100 Knoop hardness number) was observed in the hardness between the rubbing surfaces of as-machined and tested rotors. The higher hardness in the tested rotor surface is attributed to the presence of the decomposed pearlitic layer. The hardnesses of the as- machined and tested rotors converge at a depth of about 0.025 mm, an indication of the depth of the friction-affected zone present in the tested rotor. This depth is considerably greater than the depth (about 0.0025 mm) of the decomposed pearlitic layer apparent in SEM micrographs, i.e. a signif- icant fraction of the decomposed pearlitic layer is not able to be distinguish- ed visibly from the bulk. The depth of the friction-affected zone in the rotor is expected to vary considerably with the dynamometer schedule employed.

Discussion

Dynamometer data The weight loss A W for a pad was calculated for an interval from the

expression

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AW= (t-t')Ap

where the parameters t, t’, A and p represent the average pad thickness before and after an interval, the pad surface area and the actual density of the friction material* respectively. A significant difference was observed be- tween the calculated and measured weight losses of the pads during the burnish interval (Table 2). This difference is primarily attributed to the removal of the rough surface of the as-ground pads. A small fraction of the difference may be due to an increase in pad density resulting from (a) loss in porosity and (b) creation of friction-affected zones. In order to cause an increase in density the friction-affected zones in the pads must have a higher inorganic and lower organic content than the bulk material. Such microstruc- tural changes have been observed on the surface of the tested pads and are discussed later. During the last 100 stops the calculated and measured weight losses are virtually identical as expected.

The average line pressure required during a stop depends on the state of the sliding interface (pad and rotor surfaces and wear debris). The variation in line pressure observed in Fig. 2 indicates that the state of the sliding inter- face changes during the burnish interval and remains relatively constant after that. The changes in average line pressure during the burnish interval are attributed [8] to (a) a decrease in surface roughness, (b) an increase in the real area of contact, (c) the formation of wear debris and (d) the creation of friction-affected zones.

TABLE 2

Comparison of calculated and measured pad weight losses during the burnish interval and last 100 stops

Interval Pad Average thickness wear (cm)

Calculated weight loss (g)

Measured Weight loss weight loss ratio (g)

Burnish Inner 0.011 1.08 0.55 1.96 Burnish Outer 0.024 2.47 0.90 2.74 Third 100 stops Inner 0.0038 0.38 0.38 1.00 Third 100 stops Outer 0.0051 0.51 0.50 1.02

Materials evaluation In order to characterize the friction-affected zones in the pads, a two-

dimensional systematic point count [9] was conducted upon 50X SEM micrographs of tested pad surfaces using a 7.6 cm X 7.6 cm lattice contain- ing 25 points. The surface area fraction of a component was readily deter- mined as the ratio of the number of points assigned to that component to the total number of points considered (667). The area fractions of the -

*A = 30.6 cm2;p = 3.30 g cme3.

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components in the bulk of a tested pad are approximately equivalent to the volume fractions of those components in the pad composition. Differences between the area fractions of the components at the surface and in the bulk of tested pads are listed in Table 3. It is observed that the iron and steel contents have increased at the expense of organic components and graphite. Also presented in Table 3 are the differences between the area fractions of components at the surface and in the bulk of pads tested for an additional 300 stops at 290 “C. A further increase in the inorganic/organic ratio should be noticed. In the test at 290 “C the iron particle surfaces were smeared, i.e. the porosity observable within the iron particles in Fig. 3 was absent.

The authors believe that the friction-affected zones in the pads are created during the burnish interval by pyrolysis of the organic components [ 10, 111. Once created, the thickness of the zones remains constant through- out the remainder of the dynamometer test schedule, i.e. the zones penetrate deeper into the pad only to compensate for the portion of the zone worn away.

TABLE 3

Changes in the semimetallic pad surface composition created by inertial dynamometer testing

Component identification

Pig. 3(B))

Description Change in area Change in area at 120 “C at 290 “C

(%I (%I

z Steel Iron particles fibers + + 4.4 8.1 - + 21.7 1.1 C Organics, graphite -22.3 -32.5

Mechanisms of semimetallic wear Evidence for the adhesive type of wear is seen in the transferred layer

on the surface of the tested rotor. The regions comprising this layer are formed when adhesive bonds at the rubbing interface are stronger than those within the semimetallic pads.

No direct evidence of the oxidative type of wear was found in the data, yet wear by this mechanism cannot be eliminated. The repeated formation and subsequent removal of oxide films on the iron particles at the pad sur- faces could be a factor in semimetallic wear. Oxide film removal should occur easily because of the relatively weak nature of the iron-iron oxide bond [ 12, 131. At high test temperatures the oxide films are apparent with- out the aid of a microscope, and this mechanism may predominate.

Surface fatigue wear of the iron particles at the pad surface may also be a factor in semimetallic friction material wear. The morphology of the iron particles in the wear debris supports this mechanism; at the same time, how- ever, no pits are observed in the tested pad surfaces.

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The discussion above shows that the wear of semimetallics is complex in nature and is a combination of three of the main types of wear identified by Rabinowicz [ 81.

Summary and conclusions

Friction-affected zones are created in both the pads and rotor during the initial portion of the dynamometer testing. In the pads the zones are denser than the bulk material, i.e. the ratio of inorganics to organics is higher than in the bulk. The friction-affected zone in the rotor consists of two layers - a thin layer of components permanently transferred from the pad and a thicker layer of thermally and mechanically decomposed pearlite (broken cementite plates dispersed in a ferrite matrix).

The relatively high rate of pad thickness wear observed during the burnish interval is attributed to removal of the rough surface of as-ground pads and to the development of friction-affected zones.

As the thickness wear rate of the semimetallic pads decreased during the testing procedure, the average iron particle size in the wear debris decreased while the fraction of iron in the wear debris increased. At the same time, the graphite particle size remained unchanged while the fraction of graphite decreased.

The wear of semimetallic friction materials appears to be a complex mixture of the adhesive, oxidative and surface fatigue types of wear.

Acknowledgments

The authors wish to thank The Bendix Corporation for its support and its permission to publish this work. The authors are grateful to Mr. B. W. Klein for reviewing the manuscript and offering constructive suggestions. The authors wish to thank Mr. M. A. Brooks and Mr. J. G. Vanderpoorte for conducting the dynamometer test and Mr. D. G. Jones for preparation of the metallographic specimens.

References

1 B. W. Klein, Semimetallic outer pads for disc brakes, Bendix Tech. J., 2 (1969) 109 - 113.

2 F. W. Aldrich, Semimetallics: A new type of friction material, SAE Paper 710591, 1971.

3 S. K. Rhee, Wear of metal-reinforced phenolic resins, Wear, 11 (1971) 471 - 477. 4 B. D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley, Reading, Mass., 1956. 5 G. L. Kehl, The Principles of Metallographic Laboratory Practice, McGraw-Hiil, New

York, 1968. 6 S. K. Rhee, R. T. DuCharme and W. M. Spurgeon, Characterization of cast iron fric-

tion surfaces, SAE Paper 720056, 1972.

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7 S. K. Rhee and R. T. DuCharme, The friction surface of gray cast iron brake rotors, Wear, 23 (1973) 271 - 273.

8 E. Rabinowicz, Friction and Wear of Materials, Wiley, New York, 1965. 9 R. T. DeHoff and F. N. Rhines, Quantitative Metallography, McGraw-Hill, New York.

1949. 10 T. Liu and S. K. Rhee, High temperature wear of asbestos reinforced friction

materials, Wear, 37 (1976) 291 - 297. il T. Liu and S. K. Rhee, High temperature wear of semimetallic friction materials,

Wear, 46 (1977) 213 218. 12 C. Kaito, K. Fujita and T. Naiki, Electron microscopic observation on oxidation of

fine smoke particles, Jpn J. Appl. Phys., 9 (1970) 151. 13 K. Kaito, K. Fujita and H. Hoshimoto, Electron microscopic study of oxidation

processes by metal fine particles, Jpn. J. Appl. Phys., 12 (1973) 489 - 496.