Some relations for determining the wear of composite brake materials

Wear, 27 (1974) 91-97 Q Elsevier Sequoia S.A., Lausanne

91 - Printed

in The Netherlands

SOME RELATIONS FOR DETERMINING BRAKE MATERIALS

D. PAVELESCU

THE WEAR OF COMPOSITE

and M. MUSAT

Department of Machine Parts, The Polytechnic Institute of Bucharest (Romania) (Received

June

1, 1973; in final form August

2, 1973)

SUMMARY

Investigation of several types of composite brake materials showed that both the mean temperature of the friction surface and the wear rate of the materials vary non-linearly as a function of load and speed. The presence of some metal inclusions leads to a less complex non-linear relationship of the wear rate.

INTRODUCTION

Composite brake materials have been investigated to obtain materials with optimum braking characteristics. for various categories of vehicles. The properties required of friction materials include a high and constant coefficient of friction, good heat conduction and low wear of both surfaces. The wear process is complex so proceeding from experimental data obtained for various materials and several characteristic parameters an attempt is made to examine the variation of the wear and temperature of the materials and to establish any appropriate relationships. TEST INSTALLATION

AND MATERIALS

The friction pair was of the plastic-metal type with plane surfaces (pin on disc). The dry friction takes place between the specimen (1) made of the composite material (11.2 mm 0; h =20 mm) and a rotating cast iron disc (Fig. la), whose mean surface roughness was R, = 0.5pm. Uniform motion is achieved by a wheel (3). Positioning the specimen with respect to the rotation axle (4) and by means of a variator (5) a large range of velocities can be covered. Different loadings are obtained by a calibrated spring (6), control is by an elastic steel arm with a double profile (7), a pair of strain gauges placed on one of the surfaces (8) and a measurement bridge (9). Another pair of strain gauges (10). on the second profile of the elastic arm (Fig. lb) measure force and the coefficient of friction by means of an electronic potentiometer (11). TEST RESULTS

AND DISCUSSION

Test were carried out only in the dry friction regime. The cast iron surface

92

D. PAVELESCU, M. MUSAT

Fig. l(a). Sketch of the apparatus used for friction and wear measurements. Fig. l(b). The elastic arm of the device shown in Fig. 1(a).

was cleaned with felt and each specimen was run-in for 15 min. The test conditions were : tangential velocities (u) 5.5, 9.0, 12.8,’ 16 m/s loads (N) 10, 20, 30 daN pressure (p) 10, 20, 30 daN/cm’ Two types of composite specimen materials were used: (a) 3 different materials with brass inclusions: Fiat, Becorit and Bremskerl; (b) 3 different materials without metal inclusions: Jugoasbest, Necto and Cilt (one of the Romanian materials). Wear was measured by weighing the specimens with an analytical balance; the wear intensities 1, (pm/km) listed in Table I, were obtained. From these and the curves in Fig. 2 I,, increases with load according to the Archard and Hirst relation’ : V = KNut

(1)

where V is the volume of material removed by wear (cm”), N is the load (daN), v is the sliding velocity (cm/s), t is the time (s) and K the wear factor, characteristic of a certain material (cm’/daN). Dividing both sides of relation (1) by A, (nominal contact area, cm’) and noting that vt = L = the distance travelled (cm), we obtain L = KP, where I, = h/L is the wear intensity

(2) (cm/cm);

h = V/A,

is the height of the worn out

WEAR OF COMPOSITE TABLE WEAR

BRAKE

93

MATERIALS

I RATES

FOR

Material

BRAKE Speed

Load

COMPOSITE 5.5

MATERIALS

(pm/km) 12.8

9.0

16.0

(mls)

CdaN) Fiat

Becorit

Bremskerl

Necto

Cilt

Iugoazbest

10 20 30 10 20 30 10 20 30 10 20 30 10 20 30 10 20 30

8.0 16.1 22.0 8.1 13.8 26.5 4.6 8.0 12.1 8.7 10.5 16.5 4.2 8.9 13.4 11.5 14.6 17.8

8.3 15.7 22.4 1.9 16.7 26.2 4.3 8.6 12.9 7.6 11.7 15.1 4.2 8.4 12.5 10.1 14.7 18.3

15.0 23.3 35.4 9.5 18.7 34.3 6.1 10.2 18.0 10.2 17.3 30.4 5.3 11.4 19.6 15.5 24.0 36.1

20.7 33.5 50.2 10.9 22.7 38.3 7.1 14.3 22.8 16.8 43.2 81.6 9.5 26.3 60.1 23.8 52.4 19.2

13.6 25.6 36.6 9.5 20.3 31.7 6.0 12.1 18.1 11.5 19.4 26.9 6.0 12.5 19.3 16.1 25.6 34.5

19.3 36.4 52.0 11.0 23.4 36.5 7.8 15.6 23.4 18.3 40.8 71.5 10.1 27.4 51.6 25.3 48.3 13.3

25.8 43.4 66.7 12.8 28.3 44.5 8.3 16.4 30.2 28.4 101.2 22 1.3 18.5 83.4 209.7 35.6 78.3 153.4

Observation: The values listed in the first column for each speed are experimental given in the second column are calculated values using relations (3) and (4).

Pq

results and

FIAT

100

00

100

50

50

50

0

0

0

10

20

30

IdaN) ‘U

IdaY4, IUGOAZBEST

(p/km) 200

(daNI

Fig. 2. Wear rate us. load and velocity

for brake composite

materials

(samples

only).

24.2 45.6 65.1 12.0 25.7 40.0 9.0 18.0 27.1 30.3 99.0 226.0 20.3 86.0 212.2 37.9 86.8 149.6 those

94

D. PAVELESCU,

M. MUSAT

layer (cm) and pm= N/A, is the mean pressure on the nominal contact area (daN/cm2). To express I,, pm/km is generally used as unit of measure. Relation (2) shows the linear dependence of I, on load, I, being independent of velocity. The data given in Table I and Fig. 2 show, however, an increase of 1, with both load and velocity. Also, the dependence of ZUon load is not linear. The differences in wear resistance of the various materials is outwith the present paper. In all cases the variation of Z, with velocity and load is not linear, both for materials with and without metal additions. Although measured, data on the variation of the friction force are not presented in the present study. The friction coefficient differed from one material to another but the variation was reduced under the test procedure. THE WEAR RATE

Although Archard’s relation (1) can be written in the form (2) the test results given in Table I and Fig. 2 do not indicate, for composite materials, a linear dependence of 1, on N or P,,,, but a non-linear dependence. Thus, the reason for the non-linearity of IU and N and o was sought. A dependence relation of the form: lU= KN”vb

(3)

was determined. This, however, satisfies only the group of metal containing composite materials. Table II lists the values of the constants K, a and b corresponding to various metal containing composite materials; only the constant a does not differ. TABLE II THE VALUES OF CONSTANTS Constant

K a b

K. a AND b IN RELATION

(3)

Materials Fiat

Becorit

Bremskeri

0.19 0.90 1.00

0.315 1.10 0.40

0.13 1.00 0.70

Table I presents the second series of data, the values 1, obtained with the aid of relation (3). These values, close to the experimental ones, confirm the relation suggested. For the second group of materials without metal additions, a relation containing the following two terms was obtained: I”= K,N”v~+K~N”~v~=u~(K~N~+K~N‘-“~)

(4)

The wear rate of these materials is characterized by a superposition of effects, particularly at high velocities and heavy loads which required the introduction of

WEAR OF COMPOSITE

BRAKE MATERIALS

TABLE III THE VALUES OF CONSTANTS

COnstants

K,, K,, a, b, c, d IN RELATION

Materials Necto

Cilt

lugoazbest

Kz

0.60 0.013

0.17 0.0135

0.90 0.085

; c d

0.60 0.65 0.25 0.80

0.50 1.00 0.15 1.00

0.50 0.65 0.23 0.70

K,

(4)

the velocity as a power of N in the second term. Table III lists’the values of the constants K,, K,, a, b, c and d for the three composite materials; the influence of material composition may be seen. Using relation (4) and values from Table III, the calculated values 1, are also presented in Table I, (second column); the calculated values are close to the experimental values thus confirming the suggested relationship. TEMPERATURE

VARIATION

Temperature was measured 0.5 mm under the friction surface with a nickelchrome/nickel miniature thermocouple ( 0= 1 mm) (12) introduced through the lower face of the part (2) perpendicular to the friction surface, (Fig. 1). It was measured after 5 min of normal wear, immediately after the stopping of the disc which was found-to have enough thermal inertia not to introduce errors. Table IV presents the mean values of the temperature rise AT,= T,- To (“C) as compared to the temperature To initially measured. All determinations were made from the same To (30°C) by cooling the disc (2) with a blower. The value AT, = 280°C was not exceeded. A non-linear variation of AT with velocity and load was found for all materials. No differences between materials with and without metal inclusions were observed. This observation seems to be paradoxical in view of the fact that one reason for the introduction of metal in braking composite materials is to assist heat transfer through the friction material. The explanation of this behaviour is that at the test velocities only a low percentage (1%) of the frictional heat was distributed to the specimen (1). For different lower loads and velocities Furey examined the variation of the mean temperature rise AT, and the total heat quantity Q supplied by friction for a steel/constantan friction pair under point contact’ and found for dry friction a dependence of the form: AT, = kQ

(5)

where the constant k depends on the pair of metals and the experimental conditions and n varies between 0.55 and 0.63 (when AT, is expressed in “F and Q in Cal/s). This dependence was also confirmed for boundary friction, the values n=0.5

96

D. PAVELESCU,

TABLE

M. MUSAT

IV

THE MEAN

VALUES

OF TEMPERATURE Speed

Material

RISE ( C) 12.8

9.0

5.5

16.0

(niis) LOllll (daN)

Fiat

10 20 30 10 20 30 10 20 30 10 20 30 IO 20 30 10 20 30

Becorit

Bremskerl

Necto

Cilt

Iugoazbest

55 90 115 80 120 150 80 120 150 55 80 110 65 90 115 85 125 160

60 97 117 82 121 150 84 121 148 55 85 III 63 93 115 92 131 161

80 115 160 100 160 190 110 150 185 70 115 150 80 125 150 120 175 210

80 I21 156 108 158 197 109 154 IXX 75 II7 152 83 124 152 I20 168 206

100 150 190 130 190 235 130 IX5 225 90 145 190 100 150 180 135 ‘05 240

100 150 192 131 192 239 130 184 226 94 147 192 101 148 184 141 200 245

Observation: The values listed in the first column for each speed are experimental given in the second column are the results of calculation with relation (6).

115 175 220 145 215 270 140 210 250 105 170 220 110 165 210 155 230 280 results

114 172 220 148 217 271 145 206 252 108 I71 222 114 167 208 157 224 274 and those

being found for a cast iron/cast iron friction pair with plane surfaces3. For the composite materials examined, using the values given in Table the relation:

IV,

AT, = K(Nv) (6) was obtained, which is similar to (5). In relation (6) AT, is in (“C), N, (daN) and v in (m/s). Table V gives the values of the constants K and II for various composite materials. TABLE

V

THE VALUES COrl.~ttlllr

OF CONSTANTS

K AND n IN RELATION

Materials With metal inclusims

Without

metal inclusions

Becorit

Bremskerl

NIXto

Gilt

1Ll~OUdWSf

5.4

9.1

0.60

0.55

11.5 0.5

4.0 0.65

7.0 0.55

12.5 0.50

Fiut

K n

(6)

WEAR

OF COMPOSITE

BRAKE

MATERIALS

97

It may be observed that 11and K have generally different values as a function of material, n lying within 0.5 . . . 0.65 and K between 4 . . . 12.5. Taking into account these values and using relation (6) the mean temperature rise AT, (second line in Table IV) has been calculated. The results and the measured results correlate, confirming relation (6). CONCLUSIONS

For the composite materials the tested wear rate can be expressed by two non-linear relations (3) and (4) as a function of the presence or the absence of metallic inclusions. It was found that for plastic composites, the interaction of controlling factors is much more complex than for metallic materials or metal/plastic pairs. The absence of metallic inclusions complicates the analysis. Although the increase of the mean temperature of the metal counterpart is not affected by the presence or absence of metal inclusions, the latter decrease the temperature gradient at the friction surface of the composite material reduce wear particularly at high velocities and heavy loads. REFERENCES 1 J. F. Archard and W. Hirst, The wear of metals under unlubricated conditions, Proc. Roy. Sot. (London), A 236 (1956) 397410. 2 J. M. Furey, Surface temperature in sliding contact, ASLE Trans., 7 (1964) 1357146. 3 D. Pavelescu, New conceptions, calculations and applications in friction and wear of deformable solids, Ed. Acad. R. S. Romania, Bucharest (1971).