Wear, 158 (1992) 1-13 1

Wet abrasion of polymers

S. W. Zhang East China Petroleum Institute, Dongying, P.O. Box 902, Shangdong 2010066, Beijing (China)

Abstract

Mechanisms of wet abrasion for nitrile rubber (NBR), polytetralluoroethylene (PTFE) and fluoropolymer alloy (F50-1) have been investigated. Specimens were held against a rotating steel disc. Both the specimen and the steel disc were immersed in an abrasive liquid of the type used for oil-well drilling. The abraded surfaces of samples were examined using scanning electron microscopy. It is concluded that wear occurs as a result of two different mechanisms: a local microtearing process and a general microlayering or micropolishing process. Wear rates of polymeric materials have been found to increase with increasing sand content of media and with decreasing sliding speed. Mostly, these rates of wear also increase with normal load. However, wear rates of F50-1 were found to be surprisingly insensitive to normal load at 100 C.

1. Introduction

The wear process of the operating surface of a body induced by a fluid medium containing abrasive particles between the interacting solid surfaces in relative motion may be termed wet abrasion or hydroabrasive wear. It is closely analogous to abrasive erosion. However, its leading feature is the abrasive fluid flowing between the interacting solid surfaces under load contact. Unfortunately, these two types of wear were mixed up for a long time, with the result that not much attention was paid to the consideration of wet abrasion as an independent form of wear until now. Particularly, study of the wear mechanisms of wet abrasion of polymers is almost a gap in tribology.

A number of polymeric tribocomponents used in the petroleum and mining industries are operated in liquid media containing abrasive particles. It has been shown that wet abrasion is usually the dominant wear mechanism resulting in earlier failure of these components.

This work was specifically aimed at obtaining a preliminary understanding of the basic mechanisms of polymer wet abrasion based on investigations of nitrile rubber (NBR), polytetralluoroethylene (PTFE) and fluoropolymer alloy (F50-1).

2. Literature survey

Wet abrasion of polymers is a complex process which has not been extensively studied. Hence knowledge of the basic mechanism has remained obscure, though some relevant experimental observations have been presented.

Burr and Marshek [l] have developed an empirical equatiqn for the abrasion of elastomeric O-ring materials. Wear experiments were conducted on an O-ring abrasive

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wear test machine [2]. Specimen cuts from a size number 330 O-ring, which is the size used to seal a standard size rock bit bearing and has a normal inside diameter of 54 mm and a cross-sectional diameter of 4.76 mm, were held against a rotating steel wear cylinder. Both the specimen and the cylinder were immersed in an abrasive mud used for oil-well drilling. The equation obtained may be used to predict the volume loss for varying conditions of the elastomers surface contact stress and time if the wear constant is determined by wear testing.

Simuiating the service conditions of a screw-liner pair in a Dynadrill (a downhole drilling motor), an experimental study of wear in rubber-steel friction pairs with a crescent-shaped radial clearance in an abrasive liquid has been carried out [3]. With a rubber-lined cylinder against a steel cylinder in water containing abrasives and in drilling mud respectively, experiments were conducted to determine the effects of dynamic load and its frequency, concentration of abrasives and thickness of rubber- lined layer on the wear characteristics of the screw-liner pair. At a frequency ~=20 Hz and magnitude S= & 118 N cmP2 of dynamic load, a plot of wear rates of rubber in an abrasive liquid containing 3% by volume of abrasives against dynamic load has been obtained (Fig. 1). The wear rate of rubber is seen to increase with increasing average dynamic load per unit area, in particular for p > 20 kPa. Concerning the wear mechanism of rubber under the action of dynamic load, the wear processes resulting from microcutting and microscratching are seen to be dominant. Furthermore, it has been proven that the lifetime of rubber is related to its tensile strength and tear strength.

In view of the short service life of the pumping parts in reciprocating mud pumps, Lewis 14-71 has investigated the impact of design factors on the wear characteristics of the pump cylinder liner, which incorporates a piston with a replaceable rubber seal. A new piston has been developed on the basis of functional analysis which concentrated on the wear problems associated with the replaceable seal.

The effect of various abrasive-containing liquids on the wear resistance of the rubber seal in the piston of a drilling pump has been studied [8]. It has been found that the wear resistance of the rubber for all test media is inversely proportional to the load per unit area. This conclusion is much the same as that obtained in ref. 1. Moreover, the wear mechanism is dependent on the load per unit area. It has been shown that a change from abrasive wear to corrosion wear occurs at high load per unit area (10 MPa or more). It is possible that hardly any abrasive particles enter

Fig. 1. Wear rate vs. average dynamic load per unit area, p, (A) in water-containing abrasives and (B) in drilling mud (after ref. 3).

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the ebbing surfaces under conditions of high contact load. The effect of selective transfer on the tribaiogicaf characteristics of rubber-metal frictional pairs in reciprocating motion was also investigated in ref. 8. It was observed that the addition of a small amount of cupric sulphate to the liquid considerably reduces the abrasive wear.

The effect of lubricating rubber samples with water on the abrasion rate was also examined [9] using a blade abrader as described in ref. 10. It has been found that the wear rate is dramatically reduced by at least a factor of ten, whereas the frictional force decreases only by perhaps 10%. This result is apparently in conflict with earlier theory for line contact abrasion under dry conditions [ill. Other lubricants such as talc and silicone oils show a similar effect. This may be explained by a change in deformation mode of the abrasion pattern under lubricated conditions [X2].

However, wet abrasion was not considered to be a specific type of wear in the studies in question, though most of the experiments were conducted in abrasive liquid media. In recent years a number of studies on wet abrasion of nitrile rubber, poIytetr~uoroethyle~e and ~uoropol~er alloy have been carried out by the present author and his colleagues. Some of the results wili be discussed below.

3. Experimeutal details

The wear test machine is shown schematically in Fig. 2 and has the following capabilities: (1) the normai load imposed on the specimen can be set up to 2 kN with dead-weights; (2) the sliding speed on the frictional surface can be set at any value between 0 and 10.7 m s-; (3) the frictional force can be recorded automatically; (4) the duration can be controlled automatically; (5) the test specimens can be run in a container of abrasive mud or other liquid.

Two shapes of specimen, pin (6 mm outer diameter and 12 mm length) and ring (66 mm outer diameter, 54 mm inner diameter and 8 mm thickness), were used, made of NBR, FSO-1 and PTFE respectively. Fluoropolymer alloy (F50-1) is a new polymeric

Fig. 2. Schematic drawing of wear test machine.

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material developed by Shangshi Institute of Organic Chemistry, Academia Sinica. It has the excellent properties of fluoropolymer; the main physical properties are listed in Table 1.

The experimental parameters for each test are given in Table 2. Specimens were held against a rotating steel disc with a certain normal load.

Both the specimen and the steel disc were immersed in an abrasive liquid of the type used for oil-well drilling.

The abraded surfaces of test samples were coated with gold and examined using scanning electron microscopy (SEM).

TABLE 1

Main physical properties of F50-1

Tensile strength (N cm-*) room temperature 200 C

Compressive strength (N cm-) Elongation (%)

3000 800 2600

room temperature 200 C

Compression creep (%) room temperature 70 C

Hardness (Duro) Melting point (C) Specific gravity (g cme3) Coefficient of friction Flex fatigue life (cycles) Water absorption (%)

400 500

1.36 3.13 D57-59 267-327 2.15-2.17 0.17-0.21 3x104 0.005

TABLE 2

Experimental parameters

Test code no.

Specimen material Shape Number

Normal load L (N)

Sliding speed II

(m s-)

Sand content of medium (wt.%)

Temperature (C)

Nl N2 P

NBR NBR PTFE Pin Pin Ring 1 2 1

70-200 44-59 100-200

0.33 1.1-1.8 3.13-8.8

2.81 0.02 0.02 5.11 0.13 0.13

26&3 25 25

F

F50-1 Pin 2

235-393

1.1-8.8

3

100

5

4. Results and discussion

4.1. Morphological character of abraded surfaces The abraded surface of the NRR samples was examined in detail; its basic

morphological character was found to be a number of parallel tearing traces and a microlayered (small, closely spaced flaps) surface texture (Fig. 3). As seen, the spacing of tearing traces decreases with increasing sand content in the liquid under otherwise identical conditions (Fig. 4) and the depth of traces increases with increasing normal load (Fig. 5).

Fig. 3. Three-body wet abrasion pattern (sand content 2.81 wt.%, normal load 120 N) (code Nl).

Fig. 4. Three-body wet abrasion pattern (sand content 5.11 wt.%, normal load 120 N) (code Nl).

Fig. 5. Three-body wet abrasion pattern (sand content 5.11 wt.%, normal load 180 N) (code Nl).

Fig. 6. Two-body wet abrasion pattern (normal load 120 N) (code Nl).

In contrast, it has been found that the worn surface of a rubber pin immersed in water is characterized by same, scratching traces and microlayered surface texture (Fig. 6).

As shown, the edges of the tearing traces are mostly less regular than those of the scratching traces. It seems certain that the irregular-shaped abrasive particles rubbing against the rubber surface are not simply sliding but also rotating along the direction of motion.

In general, the microlayers on the worn surfaces are formed almost at right angles to the scratches (Figs. 3 and 4). When the normal load is increasing at and above a limiting value, the microlayers are in parallel with the direction of motion {Fig. 5).

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This can be accounted for by the fact that the flowability of sands in liquids is much worsened under heavy load contact; thus the rubber specimen may be considered to be worn against a filament gauze. In this situation the microlayers are formed in the direction of sliding, just like the wear pattern for NBR worn against a polyester gauze

[131* It has been observed that the fineness of the microlayered surface texture indicates

the abrasion level of the worn surface. The surface microdelamination probably results from micromolecular fracture or repeated rupture of molecular chains under the action of mechanical stress as proposed previously 134, 151.

Consequently, the mechanism of three-body wet abrasion in question appears to involve two wear processes simultaneously: directional microtearing (minority) by coarse particles and directionless microlayering or micropolishing (majority) by fine particles.

Concerning the PTFE and F50-1 samples, the morphological character of the abraded surfaces is much the same as that for the NBR samples. However, a certain plastic defo~ation was found on the abraded surfaces of the PTFE and F50-1 samples, especially under high temperature and larger normal load conditions (Fig. 7).

4.2. Rates of wear 4.2.1. NBR The experimentally measured rates of wear are given in Table 3. As expected, the wear rateA increases with the frictional force F. Using logarithmic

scales for both axes, the experimentally measured rates of wear are described quite accurately by a linear relationship (Fig. S), which can thus be represented by the general result

A=kF 0)

where the coefficient k and exponent n are characteristic of the material being examined. The values obtained are given in Table 4,

It is of interest to note that the wear equation in question has the same form as obtained previously using a different type of test machine under dry abrasive

Fig. 7. Three-body wet abrasion pattern (sand content 3 wt.%, normal load 334 N) (code F).

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TABLE 3

Measured wear rate A at various values of friction force F {code Nl)

Sand content 2.81 wt.% F (N) 18.6 21.1 24.1 26.7 31.0 32.6 A (g rev-) X 10 0.55 0.72 0.83 0.92 1.10 1.30

Sand content 5.11 wt.% F (N) 22.8 25.2 28.7 33.5 37.4 40.8 A (g rev-)X 10 0.80 0.94 1.27 1.36 1.38 1.75

Fig. 8. Wear rate A vs. frictional force F: A, sand content 5.11 wt.%; B, sand content 2.81 wt.%) (code Nl).

TABLE 4

Coefficient k and exponent n for NBR at room temperature (code Nl)

Sand content (wt.%) kx10 n

5.11 2.0 1.2 2.81 1.0 1.4

Dry two-body abrasion [16] 6.5 x 1O-6 2.1

conditions {16]. It may be deduced that the main physical process of wet three-body abrasion is tearing in a similar way to that of dry two-body abrasion.

Using logarithmic scales for both axes, a linear relationship between wear rate A and coefficient of friction JL was also obtained based upon the experimentally measured values given in Table 5 (Fig. 9):

A=appb

The values of coefficient a and exponent b are given in Table 6.

(2)

It should be pointed out that eqn. (2) shows a correlation between wear rate and coefficient of friction in the same way as that found by Burr and Marshek under wet abrasion conditions but using a different type of test machine [l]. The rates of wear depend strongly on the coefficient of friction: at high levels of coefficient of friction the wear was light, but at lower coefficient of friction the wear was rapid.

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TABLE 5

Measured wear rate A at various values of coefficient of friction p (code Nl)

Sand content 2.81 wt.%

_Z (g rev-)X 10 0.163 1.30 0.172 1.10 0.178 0.92 0.201 0.83 0.234 0.72 0.266 0.55

Sand content 5.11 wt.%

A (g rev-)X 10 0.204 1.75 0.208 1.38 0.223 1.36 0.239 1.27 0.94 0.280 0.326 0.80

I

Fig. 9. Wear rate A ZV. coefficient of friction +: A, sand content 5.11 wt.%; B, sand content 2.81 wt.%) (code Nl).

TABLE 6

Coefficient a and exponent b for NBR at room temperature (code Nl)

Sand content (wt.%) a b

5.11 0.16 1.5 2.81 0.03 2.0

Result of Burr and Marshek [l] 0.0034-0.0045 0.025

TABLE 7

Measured coefficient of friction p at various values of normal load L (code Nl)

L (N) 70 90 120 150 180 200 p (sand content 2.81 wt.%) 0.266 0.234 0.201 0.178 0.172 0.163 /I (sand content 5.11 wt.%) 0.326 0.280 0.239 0.223 0.208 0.204

This occurs because the coefficient of friction increases with decreasing normal load imposed on the specimen. The measured coefficients of friction given in Table 7 are plotted in Fig. 10 against the normal load t using logarithmic scales for both axes:

p=CL+

Fig. 10. Coefficient of friction p ZX. normal load L: A, sand content 5.11 wt.%; B, sand content 2.81 wt.% (code Nl).

TABLE 8

Coefficient c and exponent d for NBR at room temperature (code Nl)

Sand content (wt.%)

5.11 2.81

Result of Tabor 1171

C

0.27 0.22

d

0.45 0.47

HI.33

TABLE 9

Measured wear rate (code N2)

Sand Sliding content speed v (wt.%) (m s-)

Wear rate A (g rev-) X 10

Normal load 1, (N)

44 51.5 58.9

0.13 1.1 11.0 13.3 21.4 1.8 4.17 6.20 7.24

0.02 1.1 1.0 0.95 0.60 1.8 0.88 0.87 0.44

The values of c and d are given in Table 8. Equation (3) is in agreement with the empirical relation of polymeric friction at

low load proposed by Tabor [17]. For mild experimental conditions (code N2) the experimentally measured wear

rates and friction coefficients are given in Tables 9 and 10 respectively. As seen, the wear rates and coeffcients of friction increase with decreasing sliding

speed. This might arise from an improvement in localized lubrication behaviour on the friction surface as the sliding speed is increased.

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TABLE 10

Measured coefficient of friction p (code N2)

Sand Sliding content speed v (wt.%) (m s-*)

Coefficient of friction p

Normal load L (N)

44 51.5 58.9

0.13 1.1 0.31 0.33 0.33 1.8 0.14 0.19 0.21

0.02 1.1 0.19 0.16 0.15 1.8 0.13 0.12 0.11

TABLE 11

Measured wear rate A (code P)

Sand content (wt.%)

0.13

0.02

Sliding speed v (m s-t)

3.14 6.28 8.80

3.14 6.28 8.80

Wear rate A (g rev-) X lo6

Normal toad L (N)

100 150

6.5 7.0 5.2 5.8 1.7 2.5

3.2 4.4 1.4 1.6 1.5 2.2

200

7.1 5.5 3.7

5.0 2.7 3.1

TABLE 12

Measured coefficient of friction p (L = 150 N) (code P)

Sand content (wt.%)

0.13 0.02

Coefficient of friction p

Sliding speed ZJ (m a_-*)

3.14 6.28

0.23 0.19 0.18 0.17

3.80

0.19 0.16

4.2.2. PTFE The experimental results under mild conditions (code P) are shown in Tables 11

and 12. The influence of sliding speed on wear rate and frictional coefficient is the same

as that for NBR mentioned before.

L(N) c ,* 3 -1)

Fig. 11. Wear rate A vs. normal load L: A, v =3.14 m s-l; B, v=6.28 m s-l; C, v=8.80 m s-l (code F).

Fig. 12. Wear rate A vs. sliding speed V: A, L =294 N; B, L =334 N; C, L = 393 N (code F).

In each case, as stated above, the wear rate and the coefficient of friction increase with increasing sand content. This is probably due to the increase in the number of tearing traces with increasing sand content. However, under wet abrasion conditions the effect of the number of hard particles on the mechanism of rubber friction under load contact is quite complex, which may be connected with not only the levels of boundary lubrication at the interface but the viscoelasticity of rubber as well, since the latter is considered to be a major factor complicating the mechanism of rubber friction against normal load [18].

Obviously, the wear rates of the polymers in question are also increased with increasing normal load. However, such is not the case with NBR provided that the sand content is quite small, say 0.02 wt.%, as shown in Table 9. This can be accounted for by the fact that the coefficient of friction decreases with increasing normal load under this condition.

It has been found that the wear rate of PTFE is larger than that of NBR by several orders of magnitude. It is likely that the surface of PTFE samples can easily adsorb water molecules in the medium; as a result, the surface layer is decomposed and decreased in strength and elastic modulus.

4.2.3. F50-I Figures 11 and 12 show the effects of normal load and sliding speed on the wear

rate of F50-1 at 100 C. As expected, the effect of sliding velocity on wear rate for F50-1 is much the same as that for PTFE and NBR. However, experimental results have shown that the normal load has no effect on wear rate for F50-1. This fact proves that F50-1 has excellent temperature-resistant and anti-adhesive characteristics.

5. Conclusions

Wet abrasion of polymers appears to involve two different mechanisms. One is a local microtearing process and the other is a general microlayering or micropolishing process.

The wear rates of polymeric materials have been found to increase with increasing sand content of the medium and with decreasing sliding velocity. Moreover, these

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wear rates also increase with increasing normal load in general, but only if the sand content of the medium is not too small for NBR. Concerning F50-1, the wear rates were found to be surprisingly insensitive to normal load at 100 C.

References

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Russian). 4 E. C. Lewis, in S. K. Rhee, A. W. Ruff and K. C. Ludema (eds.), Wear of Materials 1981,

ASME, p. 791. 5 E. C. Lewis, Petrol. Erg, 53 (1981) 162. 6 E. C. Lewis, Petrol. Eng., 53 (1981) 130. 7 E. C. Lewis, Petrol. Eng., 54 (1982) 136. 8 B. F. Puchugen, Machinery and Oilfield Equipment, 8 (1982) 9 (in Russian). 9 A. H. Muhr and A. G. Thomas, in S. K. De (ed.), Proc. Int. Conf on Rubber and Rubber-

like Materials, Jamshedpur, November 1986, Indian Inst. Technol., Kharagpur, India, 1986, p. 68.

10 D. H. Champ, E. Southern and A. G. Thomas, in L. H. Lee (ed.), Advances in Polymer Friction and Wear, Plenum, New York, 1974, p. 133.

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York, 1983, p. 610. 14 V. L. Dyrda, V. L. Vettegren and V. P. Nadutyi, Int. Polym. Sci. Technol., 3 (1976) T/66. 15 S. W. Zhang, Rubber Chem. Technol., 57 (1984) 755. 16 S. W. Zhang, Rubber Chem. Technol., 57 (1984) 769. 17 D. Tabor, Wear, 1 (1957-1958) 5. 18 A. D. Sarker, Friction and Wear, Academic Press, New York, 1980.