Slurry erosion of polyphenylene sulfide-glass fiber composites

Wear, 119 (1987)

1

1 - 11

SLURRY EROSION OF POLYPHENYLENE SULFIDE-GLASS COMPOSITES

FIBER

C. LHYMN* Division of Science, Engineering and Technology, The Pennsylvania The Behrend College, Station Road, Erie, PA 16563 (U.S.A.)

State

University,

P. WAPNER Department of Mechanical Engineering and Energy Processes, University, Carbondale, IL 62901 (U.S.A.) (Received June 6.1986;

revised February 2,1987;

Southern

Illinois

accepted February 26, 1987)

Summary The erosive wear of glass-fiber-reinforced polyphenylene sulfide in a sand slurry has been investigated as a function of sand particle velocity and sand particle size. The morphology of slurry erosion failure has been examined by scanning electron microscopy and a possible equation for slurry erosion has been derived. 1. Introduction Composites are widely used as structural components and thus the erosive wear behavior of structural composites is important. The durability of structural materials, such as pipe, in a coal liquefaction plant or the coal mining industry is influenced by their slurry erosion behavior [ 1 -31. Slurry erosion has been studied for metals [4, 51 and ceramics [6 - 91 but no report has yet become available for fibrous polymer composites. The thrust of this paper is to provide such information and, further, to provide a phenomenological understanding of slurry erosion behavior. As an example of a high performance structural material, polyphenylene sulfide (PPS) reinforced with short glass fibers was used, since PPS-glass composites are known to have good chemical inertness, good high temperature performance and good mechanical strength [lo]. Hence it will be very useful to compare this new thermoplastic composite with conventional cast iron in terms of slurry erosion rate. 2. Experimental procedure The PPS matrix phase, reinforced with short glass fibers, was prepared by the injection molding process and supplied by Phillips Petroleum Com*Work partially carried out at Southern IlIinois University, Carbondale, IL 62901, U.S.A. 0043-1648/87/$3.50

@ Elsevier Sequoia/Printed in The Netherlands

2

r-----J 1

L--

c:

5 3

Fig, 1. Schematic diagram of slurry erosion test apparatus. The four specimen holders are horizontal: 1, motor; 2, specimen holder; 3, slurry tank; 4, rotating shaft; 5, slurry.

parry. The diameter of an E-glass fiber is 10 ~.trnand the average length of a fiber is around 200 pm. The slurry erosion tester is schematically shown in Fig. 1. The specimen, of about 10 mm X 20 mm rectangular surface, was exposed to the slurry at an angle of about 45” to make the comparison with the cast iron study easier 141. A preliminary study indicated that the slurry erosion rate increased as the attack angle (angle of the face normal with respect to both the rotation axis and the direction of travel) increased from 0” to 90”, leveling off at about 45” [ll], indicat~g typical brittle behavior [12]. The slurry consisted of 33.3 wt.% commercial sand particles in tap water and the sample was rotated by a variable-speed motor drive. The erosion rate was monitored by measuring the weight loss and the numerical expression for the erosion rate W, defined in the present work is

we= $

(1)

where Am is the weight loss, A is the exposed surface area of the specimen and t is the time duration of erosion. The microstructure of the eroded specimen surface was examined using a scanning electron microscope. 3. Results and discussion The specific weight loss us. erosion time data are shown in Fig. 2. The erosion rate graph is approximately linear. As the eroding sand particle size increases, the erosion rate (mg mm -2 s-l) increases drastically as can be seen

specific weight loss

100

300

200

-

400

500

600 (hours)

erosion time

Fig. 2. Erosion loss us. time for PPS-glass fiber (40 wt.%) composite. The velocity of the specimen through the slurry is 150 cm s-l.

1x10-10 fi 0.5 -_j

0.6

0.7

(ml)

sand particle size

Fig. 3. Erosion rate vs. sand particle size for PPS-glass fiber (40 wt.%) composite. velocity of the specimen through the slurry is 150 cm s-l.

The

in Fig. 3. The erosion rate US.velocity of sample rotation data are given in Fig. 4 which shows the tendency of the erosion rate to increase as the velocity increases. The eroded volume increases faster with increasing particle size than with increasing particle velocity. It was found experimentally that the slurry particles rotate together with the specimen during the rotational

4

100 --+

specimen

vekxity

150

(cmlsecf

through

slurry

Fig. 4. Erosion rate us. particle velocity .for PPS-glass fiber (40 wt.%) composite. The sand particle size is approximately 0.7 mm.

motion of the sample holder and the rotational velocity of the sand appears to be somewhat lower than that of the specimen. This may be the reason why the effect of the velocity is lower than that of the particle size for the present case. A microstructural study of the eroded surface shows that the erosion takes place in the PPS matrix area as shown in Fig. 5. The PPS matrix phase is relatively soft compared with the glass fiber and thus the material of lower hardness suffers wet erosion. Micrographs of erosive failure events, showing

Fig. 6. Scanning electron surface.

micrograph

of eroded PPS-glass

fiber composite

specimen

5

a “crack~g-~~pp~g-type” erosion, are presented in Fig. 6. The micrographs in Fig. 7 appear to indicate a “flaking-type” erosive failure. Locally, tearingextrusion-type failure is also seen as shown in the wear debris of Fig. 8 where plastic deformation is believed to be a governing mechanism.

fb)

Fig. 6. (a) Scanning electron micrograph of eroded PPS-glass fiber specimen surface. (b) Same as (a) at different location. (c) Same as (a) at another spot.

6

(4

(b)

Fig. 7. (a) Scanning electron micrograph of worn debris formation process for PPS-glass composite. (b) Scanning electron micrograph of crater formed during slurry erosion of PPS-glass composite.

Fig. 8. Scanning electron micrograph of worn debris.

A possible sequence in cracking-chipping failure appears to be the formation of microcraters by the impacting particles, development of grainy features by preferential erosion, cracking along the boundaries of grains,

7

crack widening, connecting-up of discrete microcracks and finally erosion debris formation to form a macrocrater hole. The grains observed are most probably a local row structure of uniformly oriented lamellar platelets [ 131. The flaking erosion is believed to take place by the initiation-prop~ation of subsurface microcracks to generate delamination debris as has been established in flaking wear [14,15]. Extruded (or tom-awhy) lips are visible at the rims of flakes as seen in Fig. 7(a). The mechanism of plastic deformation in extrusion or tearing is believed to be similar to that in ductile metal erosion [ 161. A possible explanation of slurry erosion can be provided phenomenol~ ogically as follows. As illustrated in Fig. 9, subsurface microcracks are assumed to grow under the external impacting force of the slurry particles to form flake-like debris. Crack growth morphology can be either lateral or vertical. Wear debris is assumed to form when a critical amount of damage has accumulated in the surface-subsurface zone. The critical damage hypothesis for the mechanism of crack propagation can be mathemati~~ly expressed by the following crack channel propagation equation [ 171: Irfivpt = L*

12)

where V, is the crack growth velocity, t is the erosion time, NF is the number impacts per unit time and L* is a characteristic damage length. Clearly, NF is proportional to the particle velocity and the linear slurry density d,. Considering the interaction between particles, up will be somewhat modified. Thus if Nf is the number of impacts per unit time period Nf = k&d,

(3)

where k, is a constant which contains the effect of velocity damping and depends on the impacting angle and system geometry. Empirically [ 181 v, = o&Q

(4)

where K, is the fracture toughness of the composite specimen, and cy and fi are material constants. L* can be represented by

r,

v-

cone

crater

formation

&

microcracksgeneratedin

the

Sp~CilWl

Fig. 9. Subsurface

crack propagation

in slurry erosion.

(5)

where AV is the eroded volume, h is the debris thickness, I* is the debris length, A, is the impacted surface area of the specimen and y is a material constant taking account of a specific crack growth morphology. Hence, from the previous equations, the following expression is derived:

&qklVpd,)t

= yy

f

(6)

c

Since AV = Am/d, where d is the specimen density cqK,PVpds t(d) = F

;

(7)

c

where ol is a new material constant and D is the particle diameter, if the solid particle is spherical. Therefore = qK/

A V,d,d 2 A

(8)

The contact area A, is assumed to be expressed by A,=k,Z

(9)

where FN is the impacting load of a particle, H is the hardness and le, is a material constant. The parameter h is assumed to be proportional to FN/(Eef), where E is the elastic modulus and ef is the failure strain. The impacting load FN is given by FN=---

1 MVp2 2

(10)

VP?,

where t, is the contact time between the particle and the specimen target, i.e. VpCtc is equal to the distance for which FN acts and c is a material constant less than unity, incorporating the effect of energy loss-velocity damping. Equation (10) is based on the energy balance. Hence k, (MVp)2

WC= a,KcP V,d,d A

=

,2&P

---

2H

O6 v d2d, - ’ t2 EG, ’ c

1

1

tc2 Eef 5-2~

where (x2 is a new parameter and G, is the fracture energy approximately

9

equal to Hef.Since, macroscopically, the fracture toughness K, is given by

v91 Kc2=G,E

(12)

and E is proportional to H v

5-2~

W,=a,KcP--

(13)

d2d,D6 KC2

where CY~ is a new parametric constant. Since not all the impacting energy is given to the target, M is somewhat modified to Mkl.Consequently, MB2 is equivalent to D6k2times a constant, i.e.

We = (~~~~~-2V~5-qp%&d

s

(14)

where cz4is a parametric constant. The effect of the particle concentration in the slurry is incorporated in the parameters k, and d,.The effect of the incident angle of the solid particles impacting the specimen is assumed to be contained in the parameters k, and 121. The parameter k 1 is eventually incorporated in the material constant (xq. There are four empirical parameters in eqn. (8) to be determined from preliminary experiments. Generally, eqn. (8) implies the following. (1) The erosion rate increases as the impacting particle mass increases. (2) The erosion rate increases as the velocity of the impacting solid particles increases. (3) The erosion rate is inversely proportional to the fracture energy. (4) The erosion rate is a function of the fracture toughness. Points (1) and (2) are borne out by the data shown in Figs. 3 and 4. Generally, /3 is less than 2 and hence the erosion rate decreases as the fracture toughness increases. For example, in the previous calculation [20], fl was found to be about 0.54 and thus W, a Kc-1*46. The effect of particle velocity on erosion has been shown in various experiments [6,12,21 - 231. Generally, the erosion rate is proportional to VP'-3for ductile target material and to VP 3-' for brittle material [ 241. The rate of erosion is almost independent of the particle diameter for ductile material [24] while for brittle material the erosion loss is proportional to Dn where n ranges from less than unity [22] to 3 - 4 [12] when the diameter of the impacting particles is less than tens of micrometers. As an example of a data-fitting calculation by the leastsquares method, the following empirical equation was obtained for the data in Fig. 3: In W, = 4.46(1n D) = 26.47

(15)

where the units of W, and D are (X 10-lomg mmp2 s-l) and (pm) respectively. For the data in Fig. 4 the following empirical equation results: In W, = 3.88(1n VP)- 16.19

(16)

where the units of W, and VP are(X lo-” mg mmp2 s-l) and (cm s-l) respectively. The sand particles are being dragged by the rotational flow of fluid

10

and consequently the exponent representing the velocity effect (3.88) is lower than that for the particle size effect (4.46). The erosion rate of the PPS-glass composite is clearly lower than that of cast iron as reported previously [4]. Using a recently modified slurry erosion tester [ 111, the erosion rates for a PPS-glass fiber (40 wt.%) composite and ordinary cast iron (hardness, 85 HRB) under identical conditions are shown in Table 1. As can be noted, there is a difference of two orders of magnitude in the erosion rates. This clearly reveals a significant reduction in slurry erosion loss for the PPS-glass fiber composite as compared with the popular cast iron. TABLE 1 Slurry erosion rates of PPS-glass fiber (40 wt.%) composite and cast iron*

Erosion data

PPS-glass b (mg mme2 6-l)

Cast iron= (mg mmm2 s-l)

3.6 x 10-g 3.2 x 1O-g 5.2 x 1O-g

1.3 x 10-7 1.1 x 10-7

PConditions, 33.3 wt.% sand. bTensile strength, 23 X lo3 lbf inp2; elongation, 1.9%; flexural modulus, 1.45 ine2; heat deflection temperature at 264 lbf inp2, 273 “C. CHardness, 85 HRB (80 000 lbf iV2).

X

lo6 lbf

4. Conclusions The following points can be noted from the present work. (1) The slurry erosion rate of PPS-glass fiber composites is very low compared with that of cast iron under comparable conditions. (2) The failure process in slurry erosion appears to be controlled by subsurface crack propagation and subsequent work debris formation. (3) A phenomenological equation (eqn. (8)) has been derived to explain the effect of particle size and particle velocity on the slurry erosion rate by utilizing the concept of crack propagation.

Acknowledgments The author is deeply grateful collecting data for Figs. 2 - 4. The building the slurry erosion tester is partially supported by the Materials University, Carbondale, IL.

for the help of Mr. M. Solverson in machine shop help by Mr. J. Hester in also greatly appreciated. This work was Technology Center of Southern Illinois

11

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