Reciprocating sliding friction and wear test apparatus

Reciprocating sliding friction and wear test apparatus

Polymer Testing 9 (1990) 195-211 Reciprocating Sliding Friction and Wear Test Apparatus S. M. H. Benabdallah Department of Mechanical Engineering, Ec...

795KB Sizes 5 Downloads 74 Views

Polymer Testing 9 (1990) 195-211

Reciprocating Sliding Friction and Wear Test Apparatus S. M. H. Benabdallah Department of Mechanical Engineering, Ecole Polytechnique de Montrral, PO Box 6079, Station 'A', Montreal, Quebec, Canada, H3C 3A7

(Received 4 November 1989; accepted 2 December 1989)

ABSTRACT As the friction and wear properties of polymers are very much influenced by the tribological variables, a testing system which permits the use of a wide range of test speeds, loads and environmental conditions is required. This paper describes a simple and accurate computer-aided bench machine developed to measure the dynamic coefficient and wear rate o f plastics. Plane on plane contact and sliding reciprocating motion have been chosen. The design concepts are outlined and the specifications and characteristics of the apparatus are presented. Factors determining the precision of the instrument are indicated. It is shown that the microcomputer attachment offers improved versatility in relation to operating conditions. A software flow chart showing the possible ways in which friction testing may be carried out is presented. Typical experimental results obtained with the apparatus are also presented for engineering thermoplastics rubbed against steel in dry conditions. Based on those data, empirical relationships between the coefficient of friction and the operating conditions have been found.

1 INTRODUCTION

As is well known, friction is the force resisting relative m o t i o n b e t w e e n surfaces in contact. The coefficient of friction is the ratio of the frictional force to the n o r m a l load. Consequently, the m e a s u r e m e n t of 195 Polymer Testing 0142-9418/90/$03.50 I~) 1990 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland

196

S. M. H. Benabdallah

friction involves measurement of tangential force. Although frictional resistance is related to the normal load, there is no proportionality between these parameters when plastics materials are tested. Indeed for those materials friction varies over wide ranges depending on sliding speed, applied load, roughness of the contacting surfaces, temperature, test duration and the atmosphere surrounding the contact. Friction is also influenced by the test method which is characterized by the type of contact (nominal point, line or plane) and the relative motion. For those reasons, in addition to standard testing equipment available, new testing systems allowing the measure of friction at different operating conditions have been developed. The principles governing friction measurement have been well reviewed.1 In a compilation of friction and wear devices edited by the ASLE (American Society of Lubrication Engineers),2 the various test devices are classified according to their geometry. We have considered the case of plane on plane reciprocating sliding, and have designed a tester which can be used for assessing surface friction and wear to study factors influencing friction coefficients or to explore the fundamental nature of friction. It is a very simple and accurate bench machine. A computer was used in order that strict control could be maintained over the operation of this tester, thereby increasing its versatility and the precision of its measurements. Also a data acquisition system specific to this type of test has been provided. This apparatus performs the following principal functions: --measurement and recording of short- and long-term (taking into account a running-in period) tests for a large number of loads at selected relative speeds; --measurement of wear as a function of time at a certain load and sliding speed without being obliged to replace the specimen into the apparatus to continue the test procedure.

2 DESIGN OF THE APPARATUS

2.1 Special features The design of the apparatus is based on four important features: (1)

The plane of friction between the test specimen and the friction surface (slider) coincides with the plane defined by the line of action of the driving force. Any alternative would cause a difference between the dynamic loading (during friction) and

Reciprocating sliding friction and wear test apparatus

(2) (3) (4)

197

the static loading. Neglect of this p h e n o m e n o n can lead to e r r o n e o u s friction and w e a r results. 3 The m e a s u r e m e n t of frictional force is not affected by the application o f n o r m a l load. T h e r e is a perfect plane on plane contact b e t w e e n the two c o m p o n e n t s u n d e r friction. The chosen friction speed is maintained constant during the test.

2.2 Description of the apparatus The device is designed to hold five samples which could be tested simultaneously, to supply the reciprocating m o v e m e n t to the sliders at a selected speed and to apply a n o r m a l load. T h e specifications of the device are shown in Table 1. T h e stroke, load, speed, duration of dwell at stroke end, and n u m b e r of cycles (test duration) may be controlled externally. A p a r t from the first two parameters, all the others could also be controlled by a computer. The normal load remains u n c h a n g e d during testing. It is clear that by varying the apparent area of contact of the specimen, it is possible to increase greatly the range of nominal contact pressures. T h e data acquisition system comprises an operational amplifier and 16-bits analog/digital conversion card. T h e acquisition speed varies from

TABLE 1 Specification of the Apparatus Tribological variables

Characteristics

Motion:

Wave form Stroke Speed Duration of dwell at stroke end

Ramp 0to0.2m 0 to 0.2 m/s 0 to 100 s

Forces:

Normal load (applied) Friction force (measured)

0 to 500 N 0 to 250 N

Others:

Interface conditions Atmosphere Contact geometry

Dry, with lubricant, with abrasive Laboratory atmosphere or low temperatures Plane on plane or othersa

a The contact surface of the specimen could have any configuration (hemisphere, cylinder, etc.).

198

S. M. H. BenabdaUah

225 Hz to 10 kHz. Digital outputs are used for switching on or off and controlling a valve in the hydraulic system, thus allowing the sliding speeds to be set. A detailed drawing representing the entire assembly which consists of five identical testing units is shown in Fig. 1. The device is designed to test small block specimens A, I cm x 2 cm2 of apparent area of contact. A plate B of the material, measuring 22 x 1-5 c m 2, acts as the friction test surface and is fixed to the slider C. The latter is mounted on a drawer D through pivots E permitting a free rotating movement around the horizontal axis. This drawer is driven in reciprocating motion with the help of four recirculating ball bearing units F which are positioned o n horizontal rods G firmly fixed to the main support H. The specimen is fixed to the sample support I which acts as a pendulum which can turn on a pivot J (Fig. l(a)). A dead weight is hooked to the same pivot to apply the normal loading (Fig. l(b)). The two rotational movements of the slider and the sample support ensure perfect contact between the two friction surfaces. The sample support is attached to a rigid arm K which is linked to the load cell L for measuring the friction force. It should be noted, as pointed out earlier, that the load cell measurement axis lies in the interfacial plane of the contact surfaces. Two plates could be fixed on each side of the slider, as shown in Fig. 1(c), which forms a cup able to contain any lubricant or abrasive, providing an interfacial environment different from dry. The careful spreading of a small known quantity of lubricant over a known area permits the deduction of the lubricant thickness.4 Having five independent testing units, it is possible with this apparatus to carry out wear testing as a function of five different total numbers of cycles without being constrained to replace the specimen. The reciprocating motion imposed on the sliders is provided by a combination of hydraulic and pneumatic systems. This combination has been chosen to ensure constant speed during the tests. Details of the pneumatic and hydraulic circuitry are shown in Fig. l(d). Air from a central compressor passes through a filter and pressure regulator to a pilot operated valve. The latter is piloted by two mechanically operated valves which are mounted on a support permitting an adjustment of their physical positions depending on preset stroke. Microswitches positioned at the e n d of the stroke give electrical signals to the computer for counting the number of cycles (test duration) and also to ensure dwell periods at the stroke ends if required. Two valves, the first manually operated, the second solenoid operated (energized by the control system of the computer), permit the test

Reciprocating sliding friction and wear test apparatus

199

(:

1

E o~

ij

t~

O e~ ¢.)

F

.=. 0

W ¢3

0

.=. @

=

J o~

I

200

S. M. H. Benabdallah

_=

0 g:l

,

0

(1.

0 Z W T

la.I W 0.,

~tJ °~ --) Ir'~

> n

i.--

J~ X o,,

..d t~W t.3tw

gg

~t,O

_o

\

0

I

.J

J

II)

v~

Reciprocating sliding friction and wear test apparatus

201

r'~

r~v

O.

-t-~ o~

,, 0

~111

~

z

~~ o l I II l--~ I A 0--

I

o~

t--I--t

o~-

I

I

I

O~ o<

I

I

/

V

\

I

f

z

i~5

O0

L

~: E ~

o o

~o ".~

l~l

~' r ......

"

,m

<~-

i~-l>

_o~ In

~--

,,, o~,.,

~IOI __

zz.,

II ~

~

,I

"~

I

I

'"

. . . . . . .

'~0

z-"

0

0

o

oOz

z,, Nm~

E

202

S. M. H. BenabdaUah

Fig. 2. Overallview of the machine. to be started by letting compressed air reach the single acting cylinder, and to be stopped by preventing this from happening. The rod of this cylinder is connected to that of a double acting pneumatic cylinder mounted on a closed circuit to ensure a constant speed. The pressure compensated flow control valve activated by a remote control motor permits the setting of the sliding speed. The second rod is attached to the drawer (sliders) of the apparatus. An overall view of the machine is shown in Fig. 2. 3 TEST P R O C E D U R E The software flow chart in Fig. 3(a) and (b) shows the possible ways in which the testing may be carried out taking into account the following

203

Reciprocating sliding friction and wear test apparatus

testing conditions: --normal load, W; ---sliding speed, v; - - n u m b e r of cycles (test duration); ---stroke. T h e normal load and stroke are set manually as described before. T h e

INPUT OF DATA Expedmental condticms, Specifications of materials Strc~.e. Speeds. Normal loads, Input cycles

Speedof data acquisition

[

MEASUREMENTSOF OFFSETS I

NO

2 3

INPUT OF DATA [ Speed, Load, Total cycles

Speedof data acquie~on

4

5 N~UT OF DATA Read cycles

6

IA~,.K~ATK~OF"OA~

7

( STARTOFTESTBYKEYSTROKE I

8 9

10

11 12 13 CALCULATIONS & GRAR't~ DISPLAY OR CONTINUE TESTING

14

4,

Ca) Fig. 3. Software flowchart of friction testing: (a) running-in period.

204

S. M. H. BenabdaUah

SPEEDCONTROLLED I

I~IPUTC~DATA I Numberof cycles required to reach stable state +

15 16

I START OF TEST 8Y KEVSrROK~ 17 18 I DATAACQUe~noN ~ ~ C O F ~ a I

19

CHANGEOF SPEEDOR APPLICATIONOF NEW LOAD J

20 21 22

(b) Fig. 3.---contd. (b) Short-term friction testing.

input constants characterizing each test at step 1 are: -----experimental conditions (interfacial conditions, ambiant temperature and humidity) and specifications of materials to be tested (type of materials, roughnesses); --friction speeds and data acquisition cycles for tests at constant normal load; --normal loads and input cycles for tests at constant speed; ---data acquisition speed. After the measurement of the offsets at step 2, two test procedures are possible, namely long-term or short-term tests depending on the consideration or not of a running-in period (step 3). For long-term tests, the total number of cycles (duration of running-in period), the speed, the normal load and the specific data acquisition speed should be specified as input of data at step 4. If data acquisition is required (step 5) (coefficient of friction as a function of time for constant load and speed), the read cycles are needed (step 6). After the application of load (step 7), the start of the running-in period is possible by keystroke (step 8) acting on the solenoid operated valve as introduced earlier. This period ends when the total number of cycles required is reached, then the slider is stopped (steps 12 and 13).

Reciprocating sliding friction and wear test apparatus

205

40 40 -I1

10 '~ ,e

0

~ -10

-40 -50 0.0

0.5

1.0

~.5

2.0

2,5

3.0

3.5

4.0

-60

4.5

TME (soe)

Fig. 4. T i m e d e p e n d e n c e of the displacement (speed constant at 0-1 m / s ) and friction force.

Tests at variable speeds/loads after a running-in period or short-term test may be carried out using steps 15 to 22 (Fig. 3(b)). For the latter case, the execution of the program moves directly from step 3 to step 15. The number of cycles required to reach a steady state must be specified (step 16). This means that it is possible to provide a delay between the setting of the imposed condition (load or speed) and the measurements of the friction force, allowing a stable state to be attained. It should be noted that by knowing the data acquisition speed (steps 1 and 4), it is possible to locate the position of the slider as a function of time for a given sliding speed. Figure 4 shows an example of speed profile (v = 0.1 m/s) and the corresponding friction force as a function of time. The nonreproducibility of signals for each direction of the reciprocating motion is due to the non-uniformity of the roughness of the sample. With the acquisition system, it is possible to record the friction force needed to calculate the friction force at each position of the slider or to obtain a very representative average value taking into account the edge effects at full stroke. 4 E X P E R I M E N T A L RESULTS

4.1 Specimens Three engineering thermoplastics were considered in this study, namely Ultra-High-Molecular-Weight Polyethylene (UHMWPE, Hercules 1900

S. M. H. BenabdaUah

206

supplied by Solidur Plastics), Polyoxymethylene (POM, Delrin 500, Du Pont) and Polyamide 66 (PA 66, Zytel 101, Du Pont). Small blocks of those plastics were machined from extruded sheets so that the sliding occurred in the same direction as the extrusion and perpendicular to the grooves of the machined contact surface. The slider material was cold-rolled steel, AISI 4340.

4.2. Experimental conditions Experiments were conducted under unlubricated conditions with a range of loads from 0 to 200 N, at a sliding speed of 0 to 0.1 m/s, and with a stroke of 10cm. All tests were performed at a laboratory temperature of 23 + 1 °C. The surface roughnesses were 0.1 #mc.l.a. (polished) for the slider and from 0.8 to 35 #m c.l.a. (measured in the direction of sliding) for the plastic samples. This range was obtained by varying the feed of the milling machine. Before each experiment, contamination was wiped from the steel contacting surface with a solvent. A new sample was used for each condition (normal load, sliding speed or roughness). All series of tests were repeated several times to ensure consistency of results so that each plot represents the average of five identical tests. The scatter of the measured values of the coefficient of friction was normally less than 0.02.

4.3 Results and discussion The coefficient of friction as a function of time is plotted in Figs 5 and 6. Those figures indicate a gradual increase with running-in time until a

W=80 N

z 0.30

g ,,~ 0.26(

o.221

•,?

¢~

0.18 <

8

UHMWPE 0.14

F i g . $.

o 0.85 3.2

Roughness (c.I.o. p,m) O E] 34.317'3

i

i

i

J

0

2000

4000 TIME (sec)

6000

Coefficient of friction of UHMWPE

ii

8000

as function of run-in time.

Reciprocating sliding friction and wear test apparatus

207

0.40

W=80 N

Z

o

v=0.1 m / s

0.35

c3.a.

i, 0

~_

=07.m

0.30

Z hi

g ~IM 0.25

-

0

. ~

o u.~wpc ,', PoM

,

,

,

[] PA66

2000

4000

6000

| /

[.~

0.20 0

Fig. 6.

8000

TIME (sac)

1E4

Effect of running-in on the coefficient of friction of U H M W P E , POM and PA 66 at the same conditions.

constant limiting value is reached. This basic character did not change with the operating conditions (normal load, sliding speed, and roughness of the contacting surface). This has also been found by Ellis 5 with different contacting materials and testing device. During running-in the most obvious changes are those related to surface topography. At constant load, the equation F = 1tAr suggests that the real area of contact At increases with continuous running, so that the friction force F should increase, since the shear strength r is constant. The results of the variation of the cofficient of friction as a function of the run-in time at different sliding speeds are shown in Fig. 7. The U H M W P E with the smoothest surface is taken as an example. On the basis of those experimental data, the following empirical equation has been found: /t(t) = / ~ , - / 3 v - " exp

(-et)

(1)

0.30 o I.o

0.25 0.20

hi E "

o 0.02

0.15

Speed

0.10

o

(m/s)

A 0.04 [ ] 0.06 00.1

Load W = 8 0 N

O.O5 0

Fig. 7.

2000

4000

6000

TIME (sec)

8000

1 E4

Coefficient of friction of U H M W P E as function of running-in at different sliding speeds.

S. M. H. Benabdallah

208

~0.50

§ 040

V--cq u

b 0.30

h . ILl

~- ~- 0.20 (.1 Z

I'-] 0.02 <0.10

i

0

Fig. 8.

20

50 1 O0 NORMAL LOAD (N)

200

500

Effect of normal load on coettident of friction of UHMWPE after a running-in period of 2 0 0 0 s.

where #.. is the steady-state value of the coefficient of friction. For the U H M W P E , fl = 0-18 x 10 -2, n = 1.2, e = 4 × 10 -4 and /z~ = 0.291 at W = 80 N. The solid lines of Fig. 7 have been obtained using eqn (1). In Fig. 8, data presented on a double logarithmic scale represent the effect of normal load on the coefficient of friction measured after a run-in time of 2000 s (as example). The solid lines are the best fit of the experimental data using a power law. A decrease of the coefficient of friction is shown as the load increases, regardless of the sliding speed. Based on Fig. 5, a running-in period of 8000 s at a load of 80 N and a speed of 0.1 m/s has been chosen for giving a relatively stable value of the coefficient of friction after a resonable length of time. The dependence of steady-state coefficient of friction #s, on load for the U H M W P E at different sliding speeds is shown in Fig. 9. A comparison of the three plastics is shown in Fig. 10. The value of # 0.45

z Q

0.40

~-o

0.35

[~

~ ~,,~-

00.1 A 0.08

Sp.d (m/,) u 0.06 v 0.04

O

,_ 0.30 Z

0.25 0.20

o15 0

Fig. 9.

' 3 ' 0 45 . . 60. 75 . . 90 1051 2 0' 1 3 5 1 5'0 1 6 5 15 NORMAL LOAD (N)

Effect of normal load on the steady-state coefficient of friction of UHMWPE at different sliding speeds.

Reciprocating sliding friction and wear test apparatus

209

0.65

O.55

~ 0.45 w

0.35

o 0.25 7 7 0

Fig. 10.

15 30 45 60 75 90 1 0 5 1 2 0 1 3 5 1 5 0 1 6 5 NORMAL LOAD (N)

Effect of normal load on steady-state coefficient of friction of U H M W P E , POM and P A 66.

under each load was measured after sliding for an equivalent time of ten traversals. It was found that # decreases with increasing load for the U H M W P E and POM; however, the opposite is valid for the PA 66. In the case of U H M W P E and POM, it could be assumed that friction is primarily the result of adhesion. A similar relationship has been reported for Nylon 6 by Watanabe e t al. 6

Due to the relatively high glass transition temperature of nylon (65 °C measured using Dynamic Mechanical Thermal Analysis), the amorphous areas may play a more important role in friction. The rise of friction in this case could be attributable to the brittleness of the asperities producing an increase in the real area of contact in such a manner that the friction force increases with the load. It should be noted that # would start to decrease at higher load. It was also found that in all cases any increase in the roughness of the plastic sample would decrease the friction force. An example of the effect of sliding speed is shown in Fig. 11. The increase of speed results in localized surface melting, resulting in polishing, and consequently adhesion becomes more pronounced. Finally, an attempt has been made to find an empirical relationship between # and the operating variables and surface topography considered in this study for the U H M W P E . This may be expressed as follows: tz = k W

-°13

exp (4-20)

(2)

where the value of k is given in Table 2 as a function of the roughness of the specimen.

S. M. H. BenabdaUah

210 0.45

05 A 30

= o

LOAD (N) n 70 V go

,j~,f v

0.35

w

E

0.25

0.15 0.00

Rg. 11.

0.02

0.04 0.06 0.08 SLIDING SPEED ( m / s )

0.10

Effect of sliding speed on steady-state coefficient of friction of U H M W P E at different normal loads.

TABLE 2 Values of k in eqn (2)

Roughness, c.l.a. (#m )

Value of k

0.85 3.2 17.3 34.3

0-35 0.32 0-29 0.24

Figure 12 shows a comparison between the experimental values of the coefficient of friction (symbols) and those obtained using the empirical equation (lines). Three roughnesses and three sliding speeds have been taken as examples. The maximum error (in comparison with experimental data) at the investigated ranges of v and W does not exceed -t-4.6%. Therefore, this equation could be used for practical purposes with reasonable accuracy. 0,5

. . . . . . . .

,

. . . . . . . .

,

. . . . . . . .

v (m/,,) . . . .

~0.3

0.06

--

"

~.

~.,, "'.

~ o o

,

O02m

~, . . . . . ~ . . . . .

;~ . T . . . ' ~ , , _

~-~ ~" ~,,,, _

v..~, ~=,

"..

~,,,'~'¢JD

0.2 A 3.2 [] 34.3

0.1 . . . . . . . . . . . . . . . . . . . . . . . . . 1.000E- 4 0.001 0.010 PV (MPa.m/s) 1~°

1~°

0.1 O0

Measured and calculated steady-state coefficient of friction as function of the product PV of U H M W P E at different sliding speeds and roughnesses.

Reciprocating sliding friction and wear test apparatus

211

5 CONCLUSION The software flowchart presented in this paper illustrates the versatility of this computer-aided bench machine developed to measure friction and wear of plastic materials. Since the contacting surfaces are mounted in horizontal planes, it is easy to perform tests under any interracial environment. Wear testing as a function of time can be carried out without having to replace the specimen in the apparatus. Friction tests made using this machine have yielded consistent results agreeing with those found by many authors.

ACKNOWLEDGEMENTS The author wishes to acknowledge the financial support of the National Research Council of Canada and to thank Dr J.-P. Chalifoux and Mr P. Rioux for their early work on this project.

REFERENCES 1. James, D. I. & Newell, W. G., A new concept in friction testing. Polymer Testing, 1 (1980) 9-25. 2. Benzing, R., Goldblatt, I., Hopkins, V., Jamison, W., Mecklenburg, K. & Peterson, M., Friction and Wear Devices, 2nd edn, ASLE, Park Ridge, 1976. 3. Sedov, F., The self-energizing effect in certain pin-disc test machines. Wear, 71 (1981) 259-61. 4. Moore, D. F. & Noah, S., Simple reciprocating friction and wear tester. Tribology International, 13 (1980) 11-15. 5. Ellis, B., Run-in effects on rubber friction. Wear, 93 (1984) 111-13. 6. Watanabe, M., Karasawa, M. & Matsubara, K., The frictional properties of nylon. Wear, 2 (1968) 185-91.