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The role of filler geometrical shape in wear and friction of filled PTFE
THE
ROLE
IN WEAR
OF FILLER
GEOMETRICAL
AND FRICTION
C. J. SPEERSCHNEI~ER
for review April
IO, rg67.;
PTFE
AND C. H. Ll
Honeywell Resewch Center, Hopkins, {Received
SHAPE
OF FILLED
iMim. (U.S.A.)
accepted
July 5, 1962)
SUMMARY Under reciprocating conditions a study has been made of the wear and friction properties of pure and AlaOa-filled PTFE mated to stainless steel. Spherical and irregular particles of comparable size (approximately 7 p) were used as the fillers. it was found thai the abrasive damagedue to Al&a was greatly reduced by using spherical particles. Particle shape also had an effect on the coefficient of friction, ,u, being the same as pure PTFE for spherical but greater for irregular filled material. Both optical and electron microscopic observations of the mating surfaces were used to interpret the role of filler particle shape.
INTRODUCTION
Because of a very low coefficient of friction, PTFE is considered a bearing material; unfortunately it also has a low resistance to wear. Many investigators have shown that by the dispersion of fillers the resistance to wear can be greatly increasedl-a. Various types of materials have been used except those which are harder than the mating shaft because of the abrasive damage. It is our belief, that to obtain maximum wear resistance with the least amount of filler, a very hard material should be used; the abrasive damage may be minimized by using a specific geometrical shape such as spherical particles. If this is the case two specimens from the same material but of different shape should show a difference in wear and friction phenomena. In addition to the usual measurements of wear and friction, both optical and electron microscopic examinations of the mating surfaces were used to evaluate the role of the filler materials. The effect of the finish of the mating surface (stainless steel in this case) on friction was studied also. EXPERIMENTAL
PROCEDURES
The PTFE used is 41-BX, an aqueous dispersion, supplied by E. I. DuPont De Nemours and Company. Filler materials of interest are spherical and irregular shaped Al203 particles. The spherical A1203 is produced in this laboratory by fusion using a proprietary technique and has a statistical average particle size of approximately 5~ detested with the optical microscope. The irregular A1203, from which the above spherical particles are formed, was supplied by the Norton Company and its statistical average particle size is 7~. The technique used to incorporate the A1203 into PTFE is that suggested by DuPont. In this technique the 41-BX was added to an AlzOs-water mixture, stirred slowly to mix thoroughly, agitated to coagulate and W6ar, 5 (1962) 392-399
WEAR AND FRICTION OF FILLED PTFE
393
dried at IIO’C to drive off excess water and at 26o’C to drive off the wetting agent. The material was then pressed at 11.25 kg/mm2 into a 6.3 cm diam. x 0.75 cm thick disk which was sintered at 380°C. In order to maintain a similar crystallite size for all specimens a cooling rate of 1$%/h was chosen. Previously we have shown that the cooling rate from the sinter temperature affects the PTFE crystallite sized. In addition for a given cooling rate we have also found that the presence of A1203 filler material had a slight effect on crystallite size, being smallest for the above cooling rate. The mating materials are stainless steel with different finishes. In order to avoid any ambiguity, the pure and the filled PTFE will be called the bearing plates or cylinders, and the stainless, the wear plates or journals. For wear studies, the desired bearing plates were machined from the sintered disk to the dimension of 0.63 x 0.63 x 1.27 cm with the bearing surface area 0.63 x 1.27 cm, while the wear plates were 7.3 x 1.27 x 0.32 cm, and ground perpendicular to their length to an 8 r.m.s. finish, maintaining the flatness within a tolerance of 5 0.0013 cm. The wear test apparatus employed the mechanism of a reciprocating motion at a speed of IOO cycles/min. The amount of wear, or more specifically, the decrease in thickness of the bearing plate was measured in situ so that periodic measurements could be made throughout the test without changing the experimental conditions. A dial gauge mounted on the reciprocating table was used to measure the distance, D, between the top surfaces of the wear and bearing plates. Thus, D = DO - Dt is the amount of wear after time t. It has to be noted that D is not an absolutemeasurement of the amount of wear for the bearing plate, except on the condition where there is absolutely no wear on the wear plate. A load of 9.0 kg was used in all wear experiments. The change in weight was not used to measure the amount of wear for two reasons : (I) in situ measurements could not be made and (2) preliminary experiments on gross amount of wear had shown an increase in the weight after testing for the IO vol.% irregular Al2Oa-filled PTFE bearing plates. In friction, the bearing cylinders were machined to 0.63 cm diameter by 1.27 cm in length, with no special attempt to control the surface finish. The journals, with the same diameter and 6.3 cm long, were prepared to two different finishes. Using
Fig. I. Surface profile of (a) “rough” and (b) “smooth” finish. a dilatometer the finish of both surfaces was measured as 5-6 r.m.s., however, microscopic examination indicated that the finishes were not similar. To illustrate this difference between the “rough” and “smooth” finish a profile of the surfaces is shown in Fig. I. The “rough” surface was obtained by centerless grinding and surface was obtained by using a Fisher Laboratory burnishing. The “smooth” Polisher, Model II. This was accomplished by high frequency vibrations of the specimen on the soft cloth with No. 3 A1203 polishing grit for 4 to 5 days. The prior Wear, 5 (1962) 392-399
394
C‘.J. SPEERSCHNEIDER, C. H. LI
surface condition in the preparation of the “smooth” finish was a 9-10 r.m.s. finish obtained by centerless grinding. Friction measurements were made on a friction pendulum similar to that of KYROPOLUS AND SHOBERT~ which consists of a stainless steel journal free to oscillate on four fixed bearing cylinders. Ignoring the effect of air drag, the equation of oscillation and the pendulum constants enable one to determine the coefficient of friction :
p-LY *
cos
e
Ao - A,, ___4%
where p = coefficient of friction a= distance from journal to center of gravity (cm) r= radius of the journal (cm) e= 38” (angle bearing cylinder makes with horizontal) A,, = original amplitude (radians) A,,= amplitude after n full periods (radians) ?Z= number of full periods The values a, Y and 8, shown in Fig. 2, are constants of the pendulum, and n is determined during the experiment. For convenience (Ao - An) has been set equal to one-half Ao, and thus one measures the number of full periods necessary for decay to one-half the original amplitude. Vibration and viscous friction effects are minimized by using a small initial amplitude of 6” or 0.105 radians. Under the experimental conditions an accuracy of & 118’ is attainable; this was sufficiently accurate to two significant figures in the value for p, the coefficient of friction. In all experiments, the weight of the pendulum was 2.25 kg.
::5 PENOULUW
WEIGH1
CENTER GRAVITY
OF
Fig. 2. Schematic drawing showing the fundamental constants of the pendulum. Wear, 5 (1962) 392-399
WEAR AND FRICTION OF FILLED EFFECT OF FILLER
GEOMETRICAL
PTFE
395
SHAPE ON WEAR
Figure 3 shows the decrease in thickness of pure and filled PTFE bearing plates with increasing number of cycles. Each curve starts with a very high but continuously decreasing wear rate (henceforth called run-in), then levels off to a constant rate
0 0
5
6
6
Ii! ._
I5 CYCLES
I6
21
24
27
30
rtb
Fig. 3. Wear rates for pure and AlaOs-filled PTFE. Pure, + ; z-1/2 vol. % spheres, A. ; spheres, 0; z-r/z vol. 0/Oirregular, 0; JO vol. y0 irregular, x .
IO vol. y.
Fig. 4. Surfaces of wear plates run against: (a) pure PTFE, (b) PTFE _t spherical Ale&, (c) PTFE f irregular AlzOa. The white debris is PTFE resin; black debris, metal (Fe), metal oxide (Fez03 and AIaOa), and PTFE. Overall length of wear plates 7.3 cm.
(henceforth called steady-state). Under the present experimental conditions, run-in took about 4,500 cycles, regardless of whether it was of pure or filled PTFE. It should be noted that an addition of merely z-112 vol. y0 Al203 greatly increased the resistance to wear. For example, the steady-state wear rate for pure PTFE was approximately g.o,Io-7 cm/cycle as compared with 1.8,10-7 cm/cycle for the z-r/z vol. y0 filled PTFE, Wear,
5
(1962)
392-399
390
S'fCEHSC‘HNEIL)ER,C. H. LI
a five-fold improvement. Additional resistance to wear was noted with higher filler content; the steady-state rate for IO vol. % filled PTFE was 1.10-7 cm/cycle. In these measurements the filler shape showed no difference in wear rate for IQ vol. y/, filled, but the irregular particles seem to have slightly better resistance to wear for z-xiz vol. T/0filled. I-Iowever, an examination of the wear plates indicated marked differences. As expected, surfaces mated to pure PTFE were coated with PTFE, and a heav) build-up of PTFE debris formed just outside the contact area. Those surfaces mated to irregular AlzOa-filled PTFE were severely damaged by abrasiveness. X-ra! diffraction studies of the weal debris showed the presence of A1203 and Fez03. Spherical Al&-filled PTFE also showed presence of Al&a and Fe&a in the debris on the wear surfaces, but the quantities were greatly reduced. In fact the 2-112 vol. y0 spherical A1203 did not record through X-ray data any Fez03 in the debris. Although the degree of abrasiveness could not be measured quantitatively, Fig. 4 shows a fair evidence of the difference.
Fig. 5. Spherical Altos-filled (2-1/2 vol. "/o) PTFE bearing surface after 30,000 cycles. Polarized light. Note the “optical cross” which characterizes the reflection from a spherical particle.
I ‘ig. 6. Spherical AlsOa-filled (IO vol. “1;) PTFE bearing surface after 30,000 cycles. Polarized light
The abrasive nature of filled PTFE could be further appreciated by examining the bearing plate surfaces. For spherical 4120a-filled PTFE the surfaces became almost saturated with spheres as shown in Figs. 5 and 6. It must be noted that the IO vol. 0/O filled PTFE had a slightly higher concentration. Due to the slight transparency of PTFE, it was lather difficult to ascertain whether these spheres were protruded from or just under the bearing surfaces. Hence the surfaces were replicated and viewed under an electron microscope. Two prominent features were revealed. First, the Wear, 5 (1962) 392-399
WEAR AND FRICTION OF FILLED
PTFE
397
surface density of spheres in many areas appeared to be less than for the same samples examined under the light microscope, indicating that some of the spheres
Fig. 7. Electron micrograph of bearing surface shows area of apparent low sphere concentration: collodion-carbonreplicashadowed with chromium.
Fig. 8. Electron micrograph of bearing surface shows worn spheres; collodion-carbon replica shadowed with chromium.
Fig. 9. Irregular AlaOs-filled (Z-I/Z vol. %) PTFE bearing surface after 30,000 cycles. Polarized light.
Fig. IO. Irregular AlaOs-filled (IO vol. %) PTFE bearing surface after 30,000 cycles. Polarized light.
Wear, 5 (1962)392-399
SPEERSCHNEIDER,
398
(‘. H. I.1
were indeed underneath the surface (Fig. 7). Secondly, there were areas showing spherical particles that lay slightly above the PTFE matrix, and a number of these had been worn (Fig. 8). In contrast, the surface concentration of A1203 for irregular filled specimens was lower and considerable Fez03 was present (Figs. 9 and IO). In general, those specimens with IO vol. “/:,filler showed distinctly a higher concentration of oxides than those with z-1/2 vol. %. Furthermore, there were areas saturated with very fine AlsOa and Fez03 particles with the sizes being much smaller than the original. For some reasons unknown, these areas were distinguishable by a “mud crack” network, (Fig. IO). It is interesting to note that the IO vol. y. irregular Alloy-filled bearing plates actually showed a weight gain after the 30,000 cycle wear test, in spite of the fact that the wear curve indicated a decrease in plate thickness. Thus, it seems apparent that caution should be taken in the analysis of wear based on data obtained from wear measurements. EFFECT OF FILLER GEOMETRICAL
SHAPE ON FRICTION
Because of low coefficients of friction in these materials, it was not possible to obtain within a reasonable degree of certainty the amplitude decay after one period. Thus for each test, the number of swings or full periods was recorded as the amplitude decayed from 6 to 3 degrees. For different bearing-journal combinations the number of swings for a given interval of decay was not the same; those with a low coefficient of friction would require a greater number. The initial, intermediate, and final coefficients of friction vcere the results obtained from the first, the tenth and the hundredth test for the same mating surfaces. It should not be interpreted, for example, that the initial coefficient of friction was obtained from the first period of swing. All the testing results were summarized in Table I. It is interesting to note that the spherical particle filled materials behaved almost identically to pure PTFE; regardless of surface conditions of the wear journal, the coefficients of friction, initial, intermediate, and final, were nearly the same. However, the n-regular particle filled materials behaved in an entirely different manner. The initial value of the rough TABLE COEFFICIENTS
OF FRICTION
I
FOR
PURE
AND
FILLED
PTFE _.~_.._
Coefficient Bearing
cylider
PTFE-pure PTFE + 10 vol. y0 Ale08 spherical PTFE + IO vol. :/0 A1203 irregular
Waw jownai surface &x&t&
Rough Smooth Rough Smooth Rough Smooth
___~f&id
0.08
offriction
h%rmeaiadate
Fi?&
0.04
0.09 0.04
Cl.05
0.08
0.09
0.07
0.03
0.04 0.06 0.08
0.05
0.05
0.06
0.08
0.15 0.rq
surface was lower than that of the smooth surface; then both values increased to a final coefficient of friction of about 0.14. This is quite close to the coefficient of friction of AlsOa (sapphire) on steels. Microscopic examinations of the mating surfaces revealed the same surface appearances as were described in the previous section. As
WEAR ANDFRICTIONOF FILLED PTFE
399
would be expected, the surface finish markings on the wear journals, both smooth and rough, could be identificable after testings for both pure and spherical particle filled PTFE, but not for irregular particle filled materials. This again illustrated the abrasiveness of irregular AlzOs. DISCUSSION In PTFE an ideal objective for fillers to achieve is the improvement in wear resistance whilst maintaining the low coefficient of friction. From the above experimental results it seems that IO vol. */Qspherical AlzOe-filled materials were not far from ideal. On the other hand, irregular shaped particles exhibited an entirely different and undesirable effect which, it could be concluded, was due to the abrasiveness. The so-called irregular shaped A1203 particles are actually alumina crystals with sharp crystallographic angular corners. These well-defined geometrical edges would penetrate into the mating surface during contact, thereby introducing plowing and gouging effects. Also, due to reciprocating motion, those crystals partially penetrating into the mating surface would be broken down into finer and finer sizes by repeated cleavage, The fine particles together with the debris (Fez08 in the present case) from plowing would impregnate onto the PTFE surface. As a result the surface concentration of foreign particles greatly increased until finally obliterating any PTFE to mating surface contact; in other words, no PTFE served as a bearing material. The spherical A1203 particles were also single crystals. Because of their shape it was more difficult for their penetration into the mating surface. In addition, since the spherical particles were produced by fusion, their surfaces had few flaws and consequently had an extremely high resistance to fracture. Thus, these particles with their uniform size could not become concentrated to the extent of obliterating the PTFE. Vvork is continuing on the application of spherical fillers to other bearing materials. ACKNOWLEDGEMENTS
The authors are grateful to Dr. T. L. JOHNSTONof the Ford Motor Company Scientific Laboratory for his contribution during the early stages of the work, and to Miss J. LUND for the preparation of replicas and subsequent electron micrographs. They also appreciate the continued interest of Dr. J. N. DEMPSEY, Director of Research. REFERENCES 1 H. S. WHITE, J. Research Nat. Bur. Standards, 54, October (1~56). f D. C. MITCHELL AND G. PRATT, Cmf. of Lubvicaticm and Wear, 1957. Inst. Mech. Eng.. 1958, p. 416. 3 R. D. TABER AND F. A. ROBBINS,Me&. Eng., 79, Sept. (1957). 4 C. J. SPEERSCHNEIDERAND C. H. LI,J. Ap$i, Physics, 33 (1962)1871. 5 S. KYROPOLUS AND E. I.SHOBERT,Rev. Sci. Instv., 8 (1937) 151. 6 F. P. BOWDEN AND D. TABOR, The Friction and Lubrication of Solids. Oxford University Preps, 1954, p. 162. Wear, 5 (1962) 392-399
ROLE
IN WEAR
OF FILLER
GEOMETRICAL
AND FRICTION
C. J. SPEERSCHNEI~ER
for review April
IO, rg67.;
PTFE
AND C. H. Ll
Honeywell Resewch Center, Hopkins, {Received
SHAPE
OF FILLED
iMim. (U.S.A.)
accepted
July 5, 1962)
SUMMARY Under reciprocating conditions a study has been made of the wear and friction properties of pure and AlaOa-filled PTFE mated to stainless steel. Spherical and irregular particles of comparable size (approximately 7 p) were used as the fillers. it was found thai the abrasive damagedue to Al&a was greatly reduced by using spherical particles. Particle shape also had an effect on the coefficient of friction, ,u, being the same as pure PTFE for spherical but greater for irregular filled material. Both optical and electron microscopic observations of the mating surfaces were used to interpret the role of filler particle shape.
INTRODUCTION
Because of a very low coefficient of friction, PTFE is considered a bearing material; unfortunately it also has a low resistance to wear. Many investigators have shown that by the dispersion of fillers the resistance to wear can be greatly increasedl-a. Various types of materials have been used except those which are harder than the mating shaft because of the abrasive damage. It is our belief, that to obtain maximum wear resistance with the least amount of filler, a very hard material should be used; the abrasive damage may be minimized by using a specific geometrical shape such as spherical particles. If this is the case two specimens from the same material but of different shape should show a difference in wear and friction phenomena. In addition to the usual measurements of wear and friction, both optical and electron microscopic examinations of the mating surfaces were used to evaluate the role of the filler materials. The effect of the finish of the mating surface (stainless steel in this case) on friction was studied also. EXPERIMENTAL
PROCEDURES
The PTFE used is 41-BX, an aqueous dispersion, supplied by E. I. DuPont De Nemours and Company. Filler materials of interest are spherical and irregular shaped Al203 particles. The spherical A1203 is produced in this laboratory by fusion using a proprietary technique and has a statistical average particle size of approximately 5~ detested with the optical microscope. The irregular A1203, from which the above spherical particles are formed, was supplied by the Norton Company and its statistical average particle size is 7~. The technique used to incorporate the A1203 into PTFE is that suggested by DuPont. In this technique the 41-BX was added to an AlzOs-water mixture, stirred slowly to mix thoroughly, agitated to coagulate and W6ar, 5 (1962) 392-399
WEAR AND FRICTION OF FILLED PTFE
393
dried at IIO’C to drive off excess water and at 26o’C to drive off the wetting agent. The material was then pressed at 11.25 kg/mm2 into a 6.3 cm diam. x 0.75 cm thick disk which was sintered at 380°C. In order to maintain a similar crystallite size for all specimens a cooling rate of 1$%/h was chosen. Previously we have shown that the cooling rate from the sinter temperature affects the PTFE crystallite sized. In addition for a given cooling rate we have also found that the presence of A1203 filler material had a slight effect on crystallite size, being smallest for the above cooling rate. The mating materials are stainless steel with different finishes. In order to avoid any ambiguity, the pure and the filled PTFE will be called the bearing plates or cylinders, and the stainless, the wear plates or journals. For wear studies, the desired bearing plates were machined from the sintered disk to the dimension of 0.63 x 0.63 x 1.27 cm with the bearing surface area 0.63 x 1.27 cm, while the wear plates were 7.3 x 1.27 x 0.32 cm, and ground perpendicular to their length to an 8 r.m.s. finish, maintaining the flatness within a tolerance of 5 0.0013 cm. The wear test apparatus employed the mechanism of a reciprocating motion at a speed of IOO cycles/min. The amount of wear, or more specifically, the decrease in thickness of the bearing plate was measured in situ so that periodic measurements could be made throughout the test without changing the experimental conditions. A dial gauge mounted on the reciprocating table was used to measure the distance, D, between the top surfaces of the wear and bearing plates. Thus, D = DO - Dt is the amount of wear after time t. It has to be noted that D is not an absolutemeasurement of the amount of wear for the bearing plate, except on the condition where there is absolutely no wear on the wear plate. A load of 9.0 kg was used in all wear experiments. The change in weight was not used to measure the amount of wear for two reasons : (I) in situ measurements could not be made and (2) preliminary experiments on gross amount of wear had shown an increase in the weight after testing for the IO vol.% irregular Al2Oa-filled PTFE bearing plates. In friction, the bearing cylinders were machined to 0.63 cm diameter by 1.27 cm in length, with no special attempt to control the surface finish. The journals, with the same diameter and 6.3 cm long, were prepared to two different finishes. Using
Fig. I. Surface profile of (a) “rough” and (b) “smooth” finish. a dilatometer the finish of both surfaces was measured as 5-6 r.m.s., however, microscopic examination indicated that the finishes were not similar. To illustrate this difference between the “rough” and “smooth” finish a profile of the surfaces is shown in Fig. I. The “rough” surface was obtained by centerless grinding and surface was obtained by using a Fisher Laboratory burnishing. The “smooth” Polisher, Model II. This was accomplished by high frequency vibrations of the specimen on the soft cloth with No. 3 A1203 polishing grit for 4 to 5 days. The prior Wear, 5 (1962) 392-399
394
C‘.J. SPEERSCHNEIDER, C. H. LI
surface condition in the preparation of the “smooth” finish was a 9-10 r.m.s. finish obtained by centerless grinding. Friction measurements were made on a friction pendulum similar to that of KYROPOLUS AND SHOBERT~ which consists of a stainless steel journal free to oscillate on four fixed bearing cylinders. Ignoring the effect of air drag, the equation of oscillation and the pendulum constants enable one to determine the coefficient of friction :
p-LY *
cos
e
Ao - A,, ___4%
where p = coefficient of friction a= distance from journal to center of gravity (cm) r= radius of the journal (cm) e= 38” (angle bearing cylinder makes with horizontal) A,, = original amplitude (radians) A,,= amplitude after n full periods (radians) ?Z= number of full periods The values a, Y and 8, shown in Fig. 2, are constants of the pendulum, and n is determined during the experiment. For convenience (Ao - An) has been set equal to one-half Ao, and thus one measures the number of full periods necessary for decay to one-half the original amplitude. Vibration and viscous friction effects are minimized by using a small initial amplitude of 6” or 0.105 radians. Under the experimental conditions an accuracy of & 118’ is attainable; this was sufficiently accurate to two significant figures in the value for p, the coefficient of friction. In all experiments, the weight of the pendulum was 2.25 kg.
::5 PENOULUW
WEIGH1
CENTER GRAVITY
OF
Fig. 2. Schematic drawing showing the fundamental constants of the pendulum. Wear, 5 (1962) 392-399
WEAR AND FRICTION OF FILLED EFFECT OF FILLER
GEOMETRICAL
PTFE
395
SHAPE ON WEAR
Figure 3 shows the decrease in thickness of pure and filled PTFE bearing plates with increasing number of cycles. Each curve starts with a very high but continuously decreasing wear rate (henceforth called run-in), then levels off to a constant rate
0 0
5
6
6
Ii! ._
I5 CYCLES
I6
21
24
27
30
rtb
Fig. 3. Wear rates for pure and AlaOs-filled PTFE. Pure, + ; z-1/2 vol. % spheres, A. ; spheres, 0; z-r/z vol. 0/Oirregular, 0; JO vol. y0 irregular, x .
IO vol. y.
Fig. 4. Surfaces of wear plates run against: (a) pure PTFE, (b) PTFE _t spherical Ale&, (c) PTFE f irregular AlzOa. The white debris is PTFE resin; black debris, metal (Fe), metal oxide (Fez03 and AIaOa), and PTFE. Overall length of wear plates 7.3 cm.
(henceforth called steady-state). Under the present experimental conditions, run-in took about 4,500 cycles, regardless of whether it was of pure or filled PTFE. It should be noted that an addition of merely z-112 vol. y0 Al203 greatly increased the resistance to wear. For example, the steady-state wear rate for pure PTFE was approximately g.o,Io-7 cm/cycle as compared with 1.8,10-7 cm/cycle for the z-r/z vol. y0 filled PTFE, Wear,
5
(1962)
392-399
390
S'fCEHSC‘HNEIL)ER,C. H. LI
a five-fold improvement. Additional resistance to wear was noted with higher filler content; the steady-state rate for IO vol. % filled PTFE was 1.10-7 cm/cycle. In these measurements the filler shape showed no difference in wear rate for IQ vol. y/, filled, but the irregular particles seem to have slightly better resistance to wear for z-xiz vol. T/0filled. I-Iowever, an examination of the wear plates indicated marked differences. As expected, surfaces mated to pure PTFE were coated with PTFE, and a heav) build-up of PTFE debris formed just outside the contact area. Those surfaces mated to irregular AlzOa-filled PTFE were severely damaged by abrasiveness. X-ra! diffraction studies of the weal debris showed the presence of A1203 and Fez03. Spherical Al&-filled PTFE also showed presence of Al&a and Fe&a in the debris on the wear surfaces, but the quantities were greatly reduced. In fact the 2-112 vol. y0 spherical A1203 did not record through X-ray data any Fez03 in the debris. Although the degree of abrasiveness could not be measured quantitatively, Fig. 4 shows a fair evidence of the difference.
Fig. 5. Spherical Altos-filled (2-1/2 vol. "/o) PTFE bearing surface after 30,000 cycles. Polarized light. Note the “optical cross” which characterizes the reflection from a spherical particle.
I ‘ig. 6. Spherical AlsOa-filled (IO vol. “1;) PTFE bearing surface after 30,000 cycles. Polarized light
The abrasive nature of filled PTFE could be further appreciated by examining the bearing plate surfaces. For spherical 4120a-filled PTFE the surfaces became almost saturated with spheres as shown in Figs. 5 and 6. It must be noted that the IO vol. 0/O filled PTFE had a slightly higher concentration. Due to the slight transparency of PTFE, it was lather difficult to ascertain whether these spheres were protruded from or just under the bearing surfaces. Hence the surfaces were replicated and viewed under an electron microscope. Two prominent features were revealed. First, the Wear, 5 (1962) 392-399
WEAR AND FRICTION OF FILLED
PTFE
397
surface density of spheres in many areas appeared to be less than for the same samples examined under the light microscope, indicating that some of the spheres
Fig. 7. Electron micrograph of bearing surface shows area of apparent low sphere concentration: collodion-carbonreplicashadowed with chromium.
Fig. 8. Electron micrograph of bearing surface shows worn spheres; collodion-carbon replica shadowed with chromium.
Fig. 9. Irregular AlaOs-filled (Z-I/Z vol. %) PTFE bearing surface after 30,000 cycles. Polarized light.
Fig. IO. Irregular AlaOs-filled (IO vol. %) PTFE bearing surface after 30,000 cycles. Polarized light.
Wear, 5 (1962)392-399
SPEERSCHNEIDER,
398
(‘. H. I.1
were indeed underneath the surface (Fig. 7). Secondly, there were areas showing spherical particles that lay slightly above the PTFE matrix, and a number of these had been worn (Fig. 8). In contrast, the surface concentration of A1203 for irregular filled specimens was lower and considerable Fez03 was present (Figs. 9 and IO). In general, those specimens with IO vol. “/:,filler showed distinctly a higher concentration of oxides than those with z-1/2 vol. %. Furthermore, there were areas saturated with very fine AlsOa and Fez03 particles with the sizes being much smaller than the original. For some reasons unknown, these areas were distinguishable by a “mud crack” network, (Fig. IO). It is interesting to note that the IO vol. y. irregular Alloy-filled bearing plates actually showed a weight gain after the 30,000 cycle wear test, in spite of the fact that the wear curve indicated a decrease in plate thickness. Thus, it seems apparent that caution should be taken in the analysis of wear based on data obtained from wear measurements. EFFECT OF FILLER GEOMETRICAL
SHAPE ON FRICTION
Because of low coefficients of friction in these materials, it was not possible to obtain within a reasonable degree of certainty the amplitude decay after one period. Thus for each test, the number of swings or full periods was recorded as the amplitude decayed from 6 to 3 degrees. For different bearing-journal combinations the number of swings for a given interval of decay was not the same; those with a low coefficient of friction would require a greater number. The initial, intermediate, and final coefficients of friction vcere the results obtained from the first, the tenth and the hundredth test for the same mating surfaces. It should not be interpreted, for example, that the initial coefficient of friction was obtained from the first period of swing. All the testing results were summarized in Table I. It is interesting to note that the spherical particle filled materials behaved almost identically to pure PTFE; regardless of surface conditions of the wear journal, the coefficients of friction, initial, intermediate, and final, were nearly the same. However, the n-regular particle filled materials behaved in an entirely different manner. The initial value of the rough TABLE COEFFICIENTS
OF FRICTION
I
FOR
PURE
AND
FILLED
PTFE _.~_.._
Coefficient Bearing
cylider
PTFE-pure PTFE + 10 vol. y0 Ale08 spherical PTFE + IO vol. :/0 A1203 irregular
Waw jownai surface &x&t&
Rough Smooth Rough Smooth Rough Smooth
___~f&id
0.08
offriction
h%rmeaiadate
Fi?&
0.04
0.09 0.04
Cl.05
0.08
0.09
0.07
0.03
0.04 0.06 0.08
0.05
0.05
0.06
0.08
0.15 0.rq
surface was lower than that of the smooth surface; then both values increased to a final coefficient of friction of about 0.14. This is quite close to the coefficient of friction of AlsOa (sapphire) on steels. Microscopic examinations of the mating surfaces revealed the same surface appearances as were described in the previous section. As
WEAR ANDFRICTIONOF FILLED PTFE
399
would be expected, the surface finish markings on the wear journals, both smooth and rough, could be identificable after testings for both pure and spherical particle filled PTFE, but not for irregular particle filled materials. This again illustrated the abrasiveness of irregular AlzOs. DISCUSSION In PTFE an ideal objective for fillers to achieve is the improvement in wear resistance whilst maintaining the low coefficient of friction. From the above experimental results it seems that IO vol. */Qspherical AlzOe-filled materials were not far from ideal. On the other hand, irregular shaped particles exhibited an entirely different and undesirable effect which, it could be concluded, was due to the abrasiveness. The so-called irregular shaped A1203 particles are actually alumina crystals with sharp crystallographic angular corners. These well-defined geometrical edges would penetrate into the mating surface during contact, thereby introducing plowing and gouging effects. Also, due to reciprocating motion, those crystals partially penetrating into the mating surface would be broken down into finer and finer sizes by repeated cleavage, The fine particles together with the debris (Fez08 in the present case) from plowing would impregnate onto the PTFE surface. As a result the surface concentration of foreign particles greatly increased until finally obliterating any PTFE to mating surface contact; in other words, no PTFE served as a bearing material. The spherical A1203 particles were also single crystals. Because of their shape it was more difficult for their penetration into the mating surface. In addition, since the spherical particles were produced by fusion, their surfaces had few flaws and consequently had an extremely high resistance to fracture. Thus, these particles with their uniform size could not become concentrated to the extent of obliterating the PTFE. Vvork is continuing on the application of spherical fillers to other bearing materials. ACKNOWLEDGEMENTS
The authors are grateful to Dr. T. L. JOHNSTONof the Ford Motor Company Scientific Laboratory for his contribution during the early stages of the work, and to Miss J. LUND for the preparation of replicas and subsequent electron micrographs. They also appreciate the continued interest of Dr. J. N. DEMPSEY, Director of Research. REFERENCES 1 H. S. WHITE, J. Research Nat. Bur. Standards, 54, October (1~56). f D. C. MITCHELL AND G. PRATT, Cmf. of Lubvicaticm and Wear, 1957. Inst. Mech. Eng.. 1958, p. 416. 3 R. D. TABER AND F. A. ROBBINS,Me&. Eng., 79, Sept. (1957). 4 C. J. SPEERSCHNEIDERAND C. H. LI,J. Ap$i, Physics, 33 (1962)1871. 5 S. KYROPOLUS AND E. I.SHOBERT,Rev. Sci. Instv., 8 (1937) 151. 6 F. P. BOWDEN AND D. TABOR, The Friction and Lubrication of Solids. Oxford University Preps, 1954, p. 162. Wear, 5 (1962) 392-399