Abrasive wear behavior of particle reinforced ultrahigh molecular weight polyethylene composites

Wear 225–229 Ž1999. 199–204

Abrasive wear behavior of particle reinforced ultrahigh molecular weight polyethylene composites Chaozong Liu a

a,)

, Luquan Ren b, R.D. Arnell a , Jin Tong

b

Center for AdÕanced Materials and Surface Engineering, The UniÕersity of Salford, Newton Building, Salford M5 4WT, UK b Open Laboratory for Terrain Machine, Jilin UniÕersity of Technology, Changchun 130025, China

Abstract This paper, based on orthogonal experimental design method, reports the results of abrasive wear investigations of various composites of ultrahigh molecular weight polyethylene ŽUHMWPE. reinforced with quartz powder rubbed against abrasive papers under dry conditions. The main purpose was to study the influence of such parameters such as filler particle size, load, sliding speed and abrading particle size on the abrasive wear performance of UHMWPE matrix composites. Statistical analysis was carried out to develop an equation in which the wear volume of the specimen was expressed in terms of load, abrading particle size, sliding speed, and their interactions. It was observed that wear rate was lower for reinforced composites than for the unfilled polymer. Load is the most important factor in the wear of unfilled UHMWPE specimens. However, for the wear of filler reinforced UHMWPE composites, the role of the load abates and the role of abrasive particle size increases with the increase in filler particle size. The main controlling parameter shifted from load to abrasive particle size when the filler size shifted from 0.18 to 0.35 mm in composites. Sliding speed seems to have little effect on the total wear volume. Worn surfaces were studied using a scanning electron microscope ŽSEM. to give an insight into the wear mechanisms. The results show that the hardness and plowing resistance increased with the addition of hard powder, which leads to an enhancement of abrasive resistance. q 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Abrasive wear; Ultrahigh molecular weight polyethylene; Composite

1. Introduction Innovations in modern technology have placed ever-increasing demands on advanced composite materials. Ultrahigh molecular weight polyethylene ŽUHMWPE. composites, especially particle filled composites, form an excellent class of tribo-materials because of their high specific strength and stiffness, combined with their excellent wear performance. UHMWPE composites excel in highly abrasive systems such as: chute lines in agricultural, earthmoving and mining equipment liners abraded by coal, earth and mineral ores w1–3x. However, data regarding abrasive wear investigations of particle filled UHMWPE composites are limited. Very little has been reported on the effect of filler or reinforcements on the abrasive wear performance of UHMWPE. Hence, a fundamental and comprehensive understanding of the abrasive wear mechanisms of these composites is required w4–9x. )

Corresponding author. Fax: [email protected]

q 44-161-295-5108;

e-mail:

In the present work UHMWPE and its quartz powder reinforced composites were selected. These composites are known to have excellent potential in abrasive wear applications. Wear behavior of these composites in abrasive conditions has not yet been reported. In this work, based on an orthogonal test design method, UHMWPE and its composites were abraded under different loads against various sizes of abrasive paper at different sliding speeds. Efforts were made to correlate wear volume to load, abrading particle size, sliding speed, and their interaction.

2. Experimental details 2.1. Experimental materials HMWPE was in the form of powder, with an average particle size of 330 mm. The average molecular weight is 3.15 = 10 6 , the measured density is 0.935 g cmy3 , the melting point is 1368C, and the tensile strength at 208C is 29 MPa. Two grades of quartz powders were used as filler, with average particle size range of 0.35–0.18 mm, respec-

0043-1648r99r$ - see front matter q 1999 Published by Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 9 . 0 0 0 1 1 - 3

C. Liu et al.r Wear 225–229 (1999) 199–204

200

Table 1 Properties and compositions of UHMWPE composites Specimen

Filler content Žvol.%.

Filler particle size Žmm.

Density Žg cm3 .

Tensile strength ŽMPa.

UHMWPE Composite-1 Composite-2

0 20 20

0.35 0.18

0.91 1.59 1.46

16.7 14.9 14.6

tively. UHMWPE powder was mixed with each of these two grade of fillers by mixing the materials in a V-mixer for 4 h. Room temperature compaction of the powder was carried out in a floating die tool Ž10 mm in diameter. with a dwelling time of 2 min under a pressure of 100 MPa. The cylindrical samples were free sintered at 1758C in the heater for 6 h, and cooled under a pressure of 40 MPa to below 808C, then freed from the die and followed by natural cooling to room temperature, as illustrated in Refs. w10,11x. The cylindrical pins were 20 mm in length and 10 mm in diameter, its compositions and physical properties were listed in Table 1. 2.2. AbrasiÕe wear test Wear tests were carried out on a pin-on-disk wear test rig in room temperature. The pin specimen had a diameter of 10 mm. A dead weight was applied to the pin holder. In this studies, three grade of commercial flint abrasive papers, with mean abrasive particle size of 375, 250 and 125 mm, respectively, were used and were clamped onto the rotating disk using a metal ring. During sliding, the pin was moved across the rotating disk from center out to edge so that the pin always contacted fresh abrasives, as illustrated in Ref. w12x, the single pass sliding distance was 9.06 m. Weight loss of the specimen, W, was measured using an analytical scales with a precision of "0.1 mg. Wear has been expressed as the wear volume loss, y, in cubic centimeter Žcm3 ., was calculated from the weight loss and the density of the specimen, i.e., y s weight loss, Wrdensity of the specimen, D. Each experiment was repeated for three times and the mean value was taken.

nations, a thin gold film was deposited on the worn surface. 3. Results and discussion 3.1. The test arrangement and results The effects of sliding velocity Ž Z1; m sy1 ., abrasive particle size Ž Z2 ; mm., load Ž Z3 ; kPa. and their interactions on the wear volume loss were investigated. The investigated parameters and their test levels are listed in Table 2. The orthogonal array L18 Ž2 = 3 7 . was selected to arrange the test program. The test arrangement and results are listed in Table 3. 3.2. RegressiÕe equation between Õolume loss and its affecting factors For the convenience of data processing and to calculate the regression coefficient according to code, the investigated parameters are encoded as following to give normalized variables varying within the range of wy1, 1x w13x: X 11 s 2 Ž Z1 y Z1 . rD1 s Ž Z1 y 0.523 . r0.174 X 12 s Ž Z2 y Z2 . rD2 s Ž Z2 y 250 . r125 X 13 s Ž Z3 y Z3 . rD3 s Ž Z3 y 146 . r98 X 22 s 3

X 23 s 3

2.3. Microscopy

ž ž

Z2 y Z2

D2 Z3 y Z3

D3

2

/ /

N 2y1 y 12

2

N 2y1 y 12

s3

s3

ž ž

Z2 y 250 125 Z3 y 146

3 2

2 y 3

where X 11 , X 12 and X 13 stand for the first order code of sliding velocity, abrasive particle size and load, respec-

Table 2 The investigated parameters and their test levels

Zero level: Z0 j High level: q1 Lower level: y1 Point space

/ /

2 y

Ž 1.

After wear test, the worn surface were examined using a scanning electron microscope ŽSEM.. Before the exami-

Investigated parameters

98

2

Sliding speed, Z1 Žm sy1 .

Abrasive particle size, Z2 Žmm.

Load, Z3 ŽkPa.

0.697 0.349 0.348

250 375 125 125

146 244 48 98

C. Liu et al.r Wear 225–229 (1999) 199–204

201

Table 3 Experimental arrangement and results Experiment number

Z1 Žm sy1 .

Z2 Žmm.

Z3 ŽkPa.

y1

y2

y3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

0.349 0.349 0.349 0.349 0.349 0.349 0.349 0.349 0.349 0.697 0.697 0.697 0.697 0.697 0.697 0.697 0.697 0.697

375 375 375 250 250 250 125 125 125 375 375 375 250 250 250 125 125 125

48 146 244 48 146 244 48 146 244 48 146 244 48 146 244 48 146 244

0.2135 0.4379 0.7012 0.182 0.3545 0.4996 0.1340 0.2119 0.2899 0.1878 0.4369 0.7402 0.2121 0.3521 0.5489 0.1189 0.2038 0.2447

0.0661 0.1765 0.2201 0.0518 0.1064 0.1700 0.024 0.0455 0.0481 0.0610 0.1188 0.2431 0.0475 0.0849 0.1171 0.0201 0.0346 0.0486

0.1184 0.3260 0.5760 0.1609 0.2451 0.3677 0.0429 0.0678 0.0896 0.1080 0.4287 0.4287 0.0905 0.2440 0.4601 0.0396 0.0763 0.1147

0.5973

0.0784

0.4394

2 S s Ý18 is 1 yi j y

1 18

ŽÝ18 .2 is1 yi

Note: y1 , y 2 and y 3 are the wear volume losses of UHMWPE, composite-1 and composite-2, respectively.

tively; X 22 , X 23 stand for the second order code of abrasive particle size and load, respectively. D is the level space of investigated parameters; N is the test level num-

ber of the parameter. Z is the mean value of the investigated parameter. The coding of the parameters are summarized in Table 4 with the statistical calculation and regres-

Table 4 Statistical analysis table Codes

c0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Dj Bj Ž y1 . bj Ž y 1 . Bj Ž y 2 . bj Ž y 2 . Bj Ž y 3 . bj Ž y 3 .

1 y1 y1 1 y1 1 1 1 y1 y1 1 0 y2 1 1 y1 y1 1 1 1 1 1 y1 0 y2 y1 1 0 1 y1 0 y2 0 y2 0 1 y1 0 y2 1 1 0 1 y1 1 1 y1 1 y1 1 y1 1 1 0 y2 y1 1 y1 1 1 1 1 y1 1 1 y1 1 y1 1 y1 1 1 y1 1 0 y2 y1 1 1 y1 1 1 1 y1 1 1 0 y2 y1 1 0 1 1 0 y2 0 y2 0 1 1 0 y2 1 1 0 1 1 1 1 y1 1 1 1 1 1 1 0 y2 1 1 1 1 1 1 1 1 18 18 12 36 12 36 12 6.0699 0.0209 y1.5143 y0.3777 1.9762 0.0786 y0.0807 0.3372 0.0012 y0.1262 y0.0105 0.1647 0.0022 y0.0067 1.6842 y0.1328 y0.6647 y0.0489 0.5765 y0.0159 0.0255 0.0936 y0.0074 y0.0554 y0.0014 0.048 y0.0004 0.0021 3.8063 y0.1503 y1.3762 y0.8986 1.4765 0.2387 0.2618 0.2098 y0.0084 y0.1147 y0.0250 0.1230 0.0066 0.0218

X11

X12

X 22

X13

X 23

X11 X 12

X 11 X 13

X 12 X 13

X 22 X 13

X 12 X 23

X 11 X 12 X 13

1 0 y1 1 0 y1 1 0 y1 y1 0 1 y1 0 1 y1 0 y1 12 y0.4356 y0.0363 y0.1133 y0.0094 y0.1751 y0.0146

1 0 y1 0 0 0 y1 0 1 1 0 y1 0 0 0 y1 0 1 8 y0.7584 y0.0948 y0.2835 y0.0354 y0.6565 y0.0821

y1 0 1 2 0 y2 y1 0 1 y1 0 1 2 0 y2 y1 0 1 24 0.013 0.0005 0.0131 0.0005 y0.2517 y0.0106

y1 2 y1 0 0 0 1 y2 1 y1 2 y1 0 0 0 1 y2 1 24 y0.1388 y0.0058 y0.0191 y0.0008 y0.0805 y0.0034

y1 0 1 0 0 0 1 0 y1 1 0 y1 0 0 0 y1 0 1 8 y0.0948 y0.0119 y0.0237 y0.0030 0.1653 0.0207

C. Liu et al.r Wear 225–229 (1999) 199–204

202 Table 5 Repeat test results at zero level Repeat number

y1

y2

y3

1 2 3 4 5

0.3489 0.3627 0.3559 0.3563 0.3687

0.0998 0.1110 0.0907 0.0896 0.0916

0.2401 0.2489 0.2445 0.2321 0.2657

listed in Table 5. The results of F statistical tests show that Eqs. Ž3. – Ž5. correlate well with the results at the degree of confidence of 99% Žthe confidence level being set at 0.01.. 3.3. InÕestigation of wear behaÕior of composites Worn surfaces were examined using a SEM in order to determine the predominant wear mechanisms. The SEM

sion coefficients bj . Where Bj , Dj and bj are expressed as follows: 18

Bj s

Ý Ž X ji P yi . is1 18

Dj s

Ý Ž X ji2 . is1

bj s BjrDj .

Ž 2.

Based on the calculated results in Table 4 and neglecting the items that have minor effects on the abrasive wear test results, the regression equation between wear volume loss Ž y; cmy3 . and each individual parameter as well as their interactions can be obtained. For unreinforced UHMWPE specimen, the regression equation between wear volume loss Ž y 1; cm3 . and investigated parameters were expressed as: y 1 s 0.3372 y 0.1262 X 12 q 0.1647X 13 y 0.0948 X 12 P X 13 . Ž 3. For the specimens of composite-1 Žreinforced by 20 vol.% quartz powder with a diameter of 0.35 mm., the regression expressed as: y 2 s 0.0936 y 0.0544 X 12 q 0.048 X 13 y 0.0354 X 12 P X 13 . Ž 4. For the specimens of composite-2 Žreinforced by 20 vol.% quartz powder with a diameter of 0.18 mm., the regression equation was: y 3 s 0.2098 y 0.1147X 12 q 0.123 X 13 y 0.0821 X 12 P X 13

Ž 5. then, using Eq. Ž1., Eqs. Ž3. – Ž5. can be converted into following Eqs. Ž6. – Ž8., respectively: y 1 s 0.0631 q 1.1 = 10y4 Z2 q 3.606 = 10y3 Z3 y 7.7 = 10y6 Z2 Z3

Ž 6. y5

y 2 s 0.0309 y 3.42 = 10

y3

Z2 q 1.19 = 10

Z3

y 6.7 = 10y6 Z2 Z3

Ž 7. y5

y 3 s 0.0114 q 5.82 = 10 y6

y 6.7 = 10

Z2 Z3 .

y3

Z2 q 2.935 = 10

Z3

Ž 8.

In order to estimate the experimental errors, the fitting results were tested using results obtained from five repeated tests at zero level ŽNo. 5 run.. The repeat results are

Fig. 1. SEM image of worn surface at Z1 s 0.39 m sy1 , Z2 s 25 mm and Z3 s146 kPa. Ža. Unfilled UHMWPE. Žb. Composite-1. Žc. Composite-2.

C. Liu et al.r Wear 225–229 (1999) 199–204

203

Table 6 Relative abrasion resistance coefficient of the specimens Specimen

UHMWPE Composite-1 Composite-2

Abrasive particle size Žmm.

Sliding speed Žm sy 1 .

Load ŽkPa.

125

250

375

48

146

244

0.349

0.697

1 3.11 1.60

1 3.85 1.49

1 5.52 2.88

1 4.30 2.19

1 4.03 1.96

1 4.15 1.81

1 3.88 1.98

1 4.44 2.00

micrographs of worn surface were shown in Fig. 1. Wear furrows caused by ploughing and cutting processes acting on the UHMWPE surface are visible on unfilled UHMWPE samples, typical deep and wide wear furrows are clearly seen, plastic deformation occurred on both sides of the furrow ŽFig. 1Ža..; while for the filled specimen, the wear behavior changed. Fig. 1Žb. and Žc. demonstrated that the presence of fillers tends to prevent the furrows from developing and reduce the possibility of the formation of deep furrows. The larger the filler particle size, the shallower the furrows. The relative abrasion resistance coefficients of the three composite specimen are listed in Table 6. It was observed that the wear resistance of the samples was greatly improved after reinforcement with quartz powder, an increase of about two to four times compared to unfilled UHMWPE was observed. The regression coefficient bj can be taken as a measure of the role of parameter on the wear, the stronger the role of the parameter, the higher the coefficient correspondingly. From the wear regression Eqs. Ž3. – Ž5. and Table 4, it was seen that, during the abrasive wear process, the parameter such as load Z3 , abrasive particle size Z2 , and their interaction Z2 = Z3 have certain effects on the wear behavior of samples. Among these parameters, for the wear of neat UHMWPE samples, load is the main factor, followed by abrasive particle size and interaction between load and abrasive particle size. However, for the wear of filled UHMWPE composites, the role of the abrasive particle size and load in wear becomes complicated. The role of load abates and the role of the abrasive particle size in wear increases with the increase of filler particle size. In the wear of composite-2 Žwith quartz powder of 0.18 mm in diameter., from the Eq. Ž5., it was revealed that, although the load is still the leading parameter and abrasive particle size the second, the particle size play an almost equal role to that of the load; while for the wear of composite-1 Žwith quartz powder of 0.35 mm in diameter., from Eq. Ž4. it was shown that the regression coefficient of load Ž0.048. dropped to below that of abrasive particle size Ž0.0544., i.e., with the filler particle size shifted to 0.35 mm, the abrasive particle size has become the main parameter in the wear of composites, and load the second. This shows that changes in the physical properties, such as surface hardness, filler size and toughness etc., resulted in the main parameter being shifted. In all cases, the sliding speed Ž Z1 . has little effect on the wear of the samples.

Thus, when quartz powder was combined into UHMWPE matrix, which lead to increased surface hardness of the specimen, an enhancement of ploughing and cutting resistance was observed. As a result the damage caused by ploughing and cutting was considerably reduced. At the same time, there is sufficient resistance to shear of the filler–matrix interface to prevent easy removal of the filler particles. All these effects contribute to the enhancement of wear resistance w14,15x. The smaller fillers may readily be transferred into the transfer film formed on abrasive paper, therefore the effect of wear resistance improvement in UHMWPE reinforced by smaller fillers is not as obvious as that for the larger filler particles. 4. Conclusions Ž1. The wear resistance of UHMWPE was improved significantly after reinforcement with quartz powder. It increased by about five times relative to unfilled UHMWPE. Larger particles are superior to smaller one for enhancement of wear resistance. Ž2. Load is the main parameter, and abrasive particle size plays only a minor part, in the wear of unfilled UHMWPE samples. However, for the wear of filler reinforced UHMWPE composites, the role of the load abating and the role of abrasive particle size increases with the increase in filler particle size. The main controlling parameter in the wear of specimen shifted from load to abrasive particle size when the filler size shifted from 0.18 to 0.35 mm in composites. This shows that the wear process changed due to the introduction of filler. Ž3. The wear resistance improvement of filler reinforced UHMWPE was attributed to the combination of hard particles, which prevent the formation of deep, wide and continuous furrows, and enhancement of the surface hardness. These factors combine to improve the plowing and cutting resistance. Ž4. A successful attempt has been made to describe the abrasive wear behavior of filler reinforced UHMWPE composites using regression equations. Acknowledgements Thanks are due to the National Nature Science Foundation of China and to the Chinese Foundation for Develop-

204

C. Liu et al.r Wear 225–229 (1999) 199–204

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