Wear, I49 (1991) 55-71 55

On the particle size effect in slurry erosion*

Randall S. Lynn

Mechanical Engineering Department, University of New Mexico, Albuquerque, NM 87131 (U.S.A.)

Kien K. Wong and Hector MCI. Clark Mechanical Engineering Department, University of Kansas, Lawrence, KS 66045-2234 (U.U.)

(Received February 23, 1991)

Abstract

The erosion rates of cylindrical steel specimens tested at a constant speed of 18.7 m s-’ in an erosion pot tester using 1.2 wt.% suspension of SIC in oil for particle diameters between 20 and 500 pm have been determined. The rate of particle impact on unit area of the surface at the stagnation line of erosion specimens was established as a function of particle size by short-time erosion tests, allowing a calculation of the mean mass removed for each particle impact as a function of particle size. These values were compared with the kinetic energy of particles using impact velocity values derived from a model of suspension flow. Results show that the decrease in erosion rate with decreasing particle size for suspensions of constant solids loading reflects the decrease in the proportion of particles impacting the target surface as well as the decrease in impact velocity. A value of about 24 kJ g-r is tentatively suggested for the energy of removal of PllO steel by erosion. It is concluded that for these dilute suspensions with particle sizes greater than about 100 km the erosion rate is proportional to the kinetic energy dissipated by particles during impact, but for particle sizes less than 100 pm other metal removal mechanisms become increasingly significant.

1. Introduction

The erosion process is essentially the removal of material from a surface by the repeated impact of gas-borne or liquid-borne particles. Since the early investigations of Finnie [l] there has been speculation that a relationship should exist between the amount of material removed in erosion and the work done, or energy dissipated, by impacting particles. The 1960 paper of Finnie [l] is worth quoting in part because of its succinct exposition of the problem.

The erosion of a surface by abrasive particles in an inert fluid should depend on the number of particles striking the surface, their velocity and their direction relative to the surface. These quantities are largely determined by the flow conditions and many practical examples may be found where a change in flow conditions has greatly increased or decreased erosion.

Several investigations have been conducted of the effect of changing particle size on the amount of material removed by erosion by liquid-borne particles [2-lo] and

*Paper presented at the International Conference on Wear of Materials, Orlando, FL, U.S.A., April 7-11, 1991.

0043-1648/91/$3.50 0 1991 - Elsevier Sequoia, Lausanne

56

gas-borne particles [ll-171. While these investigations generally agree that larger particles give rise to more mass loss by erosion than the same mass of smaller particles, there is no agreement on the origin of this phenomenon nor has any successful quantitative explanation of the effect been offered. Some examinations of slurry erosion [4,6] have been concerned with suspensions of relatively high solids loading (lo%-30%). Under these circumstances other wear mechanisms, such as constrained sliding of particles, may contribute to mass loss from the test specimen surface. Work by Hojo ef al [18] and Clark [lo] on slurry erosion has drawn attention to the importance of flow conditions as a dominating factor influencing the erosion process in slurries. Earlier, the work of Ahmad and Goulas [6] for slurry pumps and Tabakoff et al. [19] for gas-borne particle erosion in turbines has emphasized the usefulness of techniques of particle trajectory prediction in the erosion process in machinery subject to this type of damage.

Particularly useful in this regard is the concept of collision efficiency n (sometimes called striking efficiency) discussed by Soo [20]. The collision efficiency may be defined as the ratio of the number of particles striking unit area of the surface in unit time and the number of particles contained within the volume of suspension swept by that area in unit time. Vittal and Tabakoff [16] have calculated the collision efficiency for a cylinder (diameter 3.175 mm) exposed to air-borne quartz particles (sizes between 25 and 140 pm) at a cylinder Reynolds number of 40. Their work showed that n decreased sharply with decreasing particle size. The value of collision efficiency for glass beads between 75 and 750 pm in water-glycerin suspensions and cylindrical copper targets [lo] and for A1203 particles in oil suspensions [21] under erosion conditions has been assessed for a range of viscosities.

It was the objective of the present work to investigate the relationship between the rate of removal of material subject to erosion by a dilute suspension to the rate of dissipation of kinetic energy of impacting particles as a function of particle size.

2. Experimental details

Commercially available Sic powders were sieved into fractions, washed in distilled water and dried at 90 “C in air to give a range of powders between 20 and 500 pm of the size ranges listed in Table 1. Powders below 45 pm were prepared by air classification but were not used in all tests. Scanning electron microscope images of some of the powders are shown in Fig. 1. Typically, particles were jagged and irregular. The largest particles were somewhat more rounded than the others and for this reason particles in the size range 297-350 pm were taken as representative to assess mean particle mass and volume for powders below this size. The number of particles in a mass of 0.0204 g of 297-350 Frn powder was counted (349), allowing the mass per particle to be calculated. The corresponding mean particle volume was derived using the published density value for SIC of 3170 kg rnm3.

Taking the mean sieve size (m.s.s.) for these particles as 323.5 w, i.e. (297 + 350)/ 2, a constant K,, relating particle volume to mean sieve size was derived:

particle volume = (m.s.s.)3 XK;

where K, has the value 0.545. This relationship was used to estimate the volume and mass of all powder particles, except those in the 420-500 cwn range which were evaluated by counting the number of particles in a known mass. The value of K, is very close to the equivalent constant for spherical particles, namely ?r/6 (0.524). Final

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59

assessment of particle sizes was made photographically by reference to the scanning electron micrographs of the powders. Particle sizes used for calculation are listed as mean sieve sizes in Table 1.

Suspensions used in this investigation were made up from commercially available diesel fuel oil of density 856 kg me3 and viscosity 2.1 x 10e3 N s m-* at 40 “C.

Cylindrical erosion test specimens 46 mm long were made either from OFHC copper rod (diameter 5.17 mm) for short-time erosion tests or from API PllO casing steel (diameter 4.76 mm) for longer-time erosion rate tests. The copper specimens were annealed at 300 “C for 1 h to give a hardness from 48-54 Rockwell F and were electropolished using concentrated orthophosphoric acid at a d.c. voltage of 1.75 V for a minimum of 4 h to produce a highly reflecting, strain-free surface.

Typical mechanical properties of the PllO quenched and tempered steel are: hardness 290 BHN; yield strength 835 MPa; tensile strength 975 MPa; elongation on SO mm, 17%; reduction in area at fracture, 60%. The steel erosion specimens were either polished using 1 pm diamond paste or were tested with a previously eroded surface. No difference was detected in the rate of mass loss for these two surface treatments.

2.1. Erosion pot tester The pot tester is shown schematically in Fig. 2. It consisted of a circular section

stainless steel vessel (capacity 5.0 1) with a central vertical stainless steel shaft supporting two vertically positioned erosion test specimens. The shaft was driven by a 3.7 kW electric motor through a toothed belt to give a nominal rotation speed of 18.7 m s-r at the specimen. Temperature control of the suspension to 40*1 “C was achieved through the use of two water-cooled coils. Erosion specimens were supported by pin ends located in nylon cups of the specimen diameter to avoid the generation of corrosion couples with the support frame and to maintain the cylindrical flow pattern at the specimen ends. The progress of erosion was measured by mass loss after 10, 20, 35 and 60 min using an analytical balance weighing to 0.1 mg and was computed as the average for the two specimens.

Lower Bearing I of 2)

Copper Cooling Coil (1 of 2)

Baffle

Nylon Cup

Erosion Specimen

Stainless Steel Pot

b-1 65 4

Fig. 2. Schematic diagram of slurry erosion pot. Dimensions are in millimeters.

60

2.2. Erosion test method The pot was filled with oil and the machine operated with two dummy specimens

to heat the oil to a temperature of 40 “C. The specimens were replaced, the requisite amount of Sic powder added and the machine operated either for 120 s for short- time impact rate tests or in intervals up to 60 min for erosion rate tests.

2.3. Short-time tests Known quantities of SIC powder between 0.01 and 0.025 g were added to the

pot for these tests and were chosen so that the leading surface of the copper test specimens would be covered with impact craters after about 10 min testing. Tests were stopped after 120 s. The number of impact craters per unit area was counted along the stagnation line of the specimen using an optical microscope. Typical specimen surface areas close to the stagnation line are shown in Fig. 3. There is some uncertainty associated with the number of impacts, since the shape of craters was irregular and it was possible that a single impact event produced two separate craters as the particle rotated or that craters were superimposed.

2.4. Erosion rate tests For erosion rate tests a constant solids loading of 1.2 wt.% was used by adding

50 g of powder to the pot. This choice of solids loading represented a compromise between avoiding particle-particle interactions at higher solids loading and obtaining measurable erosion rates at small particle sizes. Mass loss was measured after 10, 20, 35 and 60 min. It was found that the rate of mass loss decreased with time, a commonly observed effect that has been ascribed to particle blunting and comminution [3].

The erosion rate as a function of time R, was assessed as the slope of the mass loss-time curve at zero time and was taken as representative of the erosion rate of the steel specimens by fresh SIC particles. For consistency the initial slope was determined using a third-order polynomial curve fit which corresponded quite well with the mass loss-time curve.

After 10 min testing, the angle subtended at the specimen center by the eroded cylinder surface (called the circumferential erosion angle 28) was recorded for most suspensions. It was observed that a sharp boundary separated the eroded from the uneroded surface, allowing evaluation of the angle with some confidence.

3. Results

Impact rate values in units of impacts per square millimeter per minute for the suspensions used in the short-time tests were used to calculate the collision efficiency 77 for the specimen as a function of powder particle size; that is, the number of impacts per square millimeter per minute divided by the number of particles calculated to be in the volume swept out by a 1 mm2 area of specimen moving at a speed of 18.7 m s-l in 1 min, assuming a uniform distribution of particles in the suspension. Values of the collision efficiency lie between zero and unity <and are given in Fig. 4 as a function of particle size. Each point is derived from between 20 and 55 determinations of the number of impacts on unit area.

Plots of erosive mass loss as a function of test time are shown for pre-eroded PllO steel specimens in Fig. 5. For these tests no damage nucleation delay was expected or observed, the curves passing smoothly through the origin. Experimentally determined values of erosion rate for each Sic particle size are shown in Fig. 6. Values of erosion

61

(4 - Fig. 3. Impact craters close to the stagnation line on copper short-time erosion test specimens: (a) 297-350 pm Sic particles; (b) 147-180 pm; (c) 75-88 Km; (d) 40 pm. Each bar represents 0.1 mm.

62

1.0

0.8

0.6

100

PARTICLE SIZE ( pm)

Fig. 4. Variation of collision efficiency TJ with particle size based on impact crater numbers in short-time erosion tests. Particle size refers to the mean sieve size of the powder. Values are the mean for between 20 and 55 determinations of numbers of impacts at each particle size.

1.5

1 .o

0.5

0.0 0 20 40 60 80

TEST TIME (min )

Fig. 5. Mass loss per square millimeter of frontal area (diameter~length) of erosian specimen with testing time for PI10 steel specimens tested in diesel oil at 40 “C, nominal test speed 18.7 m s-* for 1.2 wt.% suspensions of SIC of the mean sieve size shown (microns).

rate corresponding to each mean sieve size were taken from this line and are listed in Table 1. The corresponding values of circumferential erosion angle 28 are shown in Fig. 7.

63

(0 0 r

X m-

100

10

1

, “1 0 100

PARTICLE SIZE ( p m)

Fig. 6. Erosion rate R, of PllO steel in 1.2 wt.% suspensions of SC powders of indicated sizes in diesel oil at 18.7 m 5-I nominal test speed.

180

160

140

120

I-

/ *-

/ -

/ -

particle

10 100

PARTICLE SIZE (fl m) 1000

Fig. 7. Variation of circumferential erosion angle 28 (see text) with particle size for test conditions used in this investigation.

Using a potential flow model for a suspension about a cylinder developed by Wang and Ctark [22], values of the impact velocity at the stagnation line, VI, were calculated for the conditions used in these tests, with the assumption that the suspended particles were spherical rather than angular and irregular. These values are plotted

64

p Nominal Tesf Speed 18.7 m/s

100 1000

PARTICLE SIZE ( pm)

10000

Fig. 8. Calculated impact velocity VI as a function of particle size for spherical particles of the same density as Sic impacting the erosion test cylinder along the stagnation line [ZZ].

in Fig. 8. It is acknowledged that irregular particles will show a range of impact velocities. The usefulness of Fig. 8 lies in indicating how the mean velocity of impact would vary with particle size.

4. Discussion

Calculated values of the impact velocity of particles on the cylinder surface show a dramatic decrease with decreasing particle size, reflecting decreasing particle inertia with decreasing size. Thus smaller particles in the path of the advancing erosion specimen are more easily constrained by the liquid flowing about the cylinder to follow liquid streamlines rather than follow an undiverted cohision course with the cylinder surface. This is shown schematically in Fig. 9. Further, small particles impacting the cylinder do so with a much reduced velocity compared to larger particles.

Bperimental evidence for such particle retardation under conditions of erosion has been given for suspensions of glass beads in the size range 75-750 Frn in water or water-glycerin suspension [lo, 181. Clark [lo] found that glass beads may be decelerated to as httle as 10% of the nominal test speed and that consequently assumptions concerning the impact velocity of particles based on the nominal test speed in a slurry erosion test may be seriously in error. Similarly, Hojo et al. [34] have pointed out that even in a slurry jet, particle impact velocities and impact angles with the target vary strongly across the jet diameter.

From the data on mean particle mass m for each size range and calculated impact velocity, values of mean particle kinetic energy at impact &mfr:, were calculated (Table 1).

Collision efficiency decreases with decreasing particle size. The appropriate form of the relationship is unclear, however, and for the sake of simplicity a straight line

65

Non-Impacting Particles

Center tine

t = Collision Efficiency, q 1 18.7 mfs

Fig. 9. Schematic diagram of suspension flow about an erosion test specimen to show the changes of collision efficiency q with particle size. The circumferential erosion angle 2B is indicated for each case.

has been chosen on the semiiog plot (Fig. 4). The results of Vittal and Tabakoff [16] for collision efficiency of quartz particles in air also yield a straight line on a semilog plot, although the relationship must be asymptotic with collision efficiency values of zero and unity.

The numbers of particles impacting 1 mm2 of the steel erosion specimens each minute were calculated from the numbers of particles in the path of that area of surface each minute and the collision efficiency for those flow conditions taken from Fig. 4. These are given in Table 1 as the particle impact rate.

The plot of circumferential erosion angle 28 with respect to particle size appears to be linear in a semifog plot (Fig. 7). This result suggests that a relationship also links the collision efhciency and the circumferential erosion angle, which might yield values of collision eil’iciency indirectly but simply, thus obviating the tedious process of counting impact craters. This relationship requires further investigation but will be useful only when closely sized particle samples are used, since a wide range of particle sizes in the suspension will lead to an ill-defined boundary at the edge of the eroded area. A plot of collision efficiency with respect to circumferential erosion angle using data taken from the best-fit lines in Figs. 4 and 7 is given in Fig. 10.

Values of the erosion rate given in Fig. 6 and Table 1 are based on the projected frontal area of the erosion specimen. Since the actual area subjected to erosion changes with particle size as the circumferential erosion angle decreases, values of the erosion rate have been adjusted to reflect the concentration of damage over a smaller area as the particle size decreases by dividing erosion rate values by the corresponding value of sin e (Fig. 7). Adjusted values of erosion rate are given in Table 1.

The mean mass loss for each particle may then be calculated from the ratio of erosion rate and particle impact rate for each particle size. It is recognized that

66

CIRCUMFERENTIAL EROSION ANGLE, 28 (Deg.)

Fig. 10. Variation of collision efficiency with circumferential erosion angle for the conditions.

erosion test

intermediate impact angles are generally more effective in removing material from a ductile target, but it is assumed that the distribution of particles over the eroding surface is independent of particle size. Values of mass loss per impact are given in Table 1 and compare closely with previous calculations of the mean mass loss per impact for erosion of this steel using suspensions of 75-106 pm AlaOs in water or oil [21]. Values of the ratio of mean mass loss per impact and kinetic energy of impact are given in Table 1. Between 500 pm and about 100 pm the ratio is essentially constant, but rises with decreasing particle size below 100 pm.

A constant value of mass loss per impact divided by impact energy indicates that mass loss is proportional to impact energy, as predicted by Finnie [l], over a range of particle sizes. The value of the ratio is about 42X 10m6 g J-l. It is interesting to compare this result with the energy requirements for machining. For steels these are given as 0.35-1.225 kJ g-i for cutting and 2.1-10.5 kJ g-’ for grinding [24]. Taking the reciprocal of the mass-energy ratio yields an energy requirement of 24.0 kJ g-i (about 70 hp min ind3) for removal of PllO steel by erosion. It appears that for the conf-lguration used in the present test erosion is not a very efficient method of material removal. This value should be regarded as an upper limit. The value of kinetic energy is very sensitive to the value of impact velocity used. Those given in Fig. 8 do not incorporate consideration of boundary layer effects which may give rise to additional particle retardation.

A cylindrical test specimen in an erosive flow normal to its axis is impacted by particles at all angles between normal (90”) and glancing (W) impact. The importance of impact angle in determining the rate of metal removal is well established and the progressive change of shape of cylindrical erosion specimens has been noted by Levy [3], intermediate impact angles being most effective in metal removal from a ductile target. At normal impact much work will be done on the target material but, at least

67

for ductile metals, the rate of material removal is low. Nevertheless, the calculated energy necessary to remove unit mass of metal by erosion is high.

The higher measured values of the ratio of mass loss per impact and kinetic energy of impact for the smallest particle sizes is not understood but is believed to be due in part to changes in the mechanisms of material removal. Figure 11 shows a plot of mass loss per impact with respect to particle size for the sizes studied. The dashed line corresponds to a constant value of the ratio of mass loss per impact and particle impact energy. A number of experimental factors, discussed below, may contribute to the high values calculated for the smallest particle sizes.

The results as a whole demonstrate the necessity of obtaining very carefully sized powder samples for particle size work to be successful. A few particles significantly greater than the mean size in the sample will contribute disproportionately to erosive mass loss through a greater collision efficiency and a greater impact velocity than the mean, leading to a larger erosion rate. Values of the collision efficiency and numbers of particles are derived from relationships which are, necessarily, approximate. Sus- pensions containing 1.2 wt.% solids are rather dilute and the erosion rate produced by small particle sizes yields a mass loss in the order of a few milligrams per hour. Thus values of erosion rate for suspensions of small particles are inherently less precise than those for larger particle sizes, particularly since erosion rate is calculated as the initial slope of the mass loss-time curve.

A further complicating factor in slurry erosion that has received little attention is the trajectory of particles after impact. For example, the present results for 45-53 pm particles suggest that about 5.5 million particles impact each square millimeter of frontal area each minute or, in more understandable terms, 220 particles impact each 49 brn square of surface area each second. The coefficient of restitution for

PARTICLE SIZE f y m)

Fig. 11. Variation of mass Ioss per particle impact with particle size for erosion of PllO steel. Full line shows experimental results; dashed line is calculated from constant value of the ratio of mass loss per impact and kinetic energy of impact.

68

these particles is unknown, but it is doubtful whether they are able to rebound far from the steel surface, particularly when their impact speed is in the order of 3 m S -’ only. It is suggested that the concentration of particles close to the specimen surface is higher than in the suspension as a whole because of particle retardation on impact and that these particles may produce multiple impacts on the surface by collision with incoming particles and so enhance the erosion rate. There is some evidence for this phenomenon in that particle impacts could be detected over a circumferential erosion angle of about 175” for a diamond-polished steel specimen after testing in the 40 hrn suspension, although damage was concentrated within a narrower band about 60” on either side of the specimen stagnation line. Some particles may leave their impact point by sliding over the specimen surface and in so doing would certainly remove more material than through a single impact. It is clear that the difficulty of removing particles after impact on the specimen surface will increase as the particle size decreases, since available rebound energy will decrease and the mean distance between particles in the homogeneous suspension wili be smaller.

The relationship between mass loss in erosion and particle size for larger particle sizes (above about 200 pm for gas-borne erosion) shows an approximately constant value of mass loss. That is, above this value the erosion rate is independent of particle size. The results of Sage and Tilly [12] for gas-borne particles are typical. Below this value the erosion rate decreases and becomes vanishingly small at some small particle size, typically about 10 pm for gas-borne particles (the value will be greater for sIurries). The present analysis allows this form to be understood.

Large particles suffer little retardation before impact because of their high inertia and are also associated with values of collision efficiency close to unity. Thus the kinetic energy of impact per unit time will be essentially constant, irrespective of particle size, and the erosion rate, whether measured as a function of time, R,, or unit impacting mass, R,, will also be essentially constant [25].

For particles of smaller size, particle retardation prior to impact will become increasingly significant. At the same time the collision efficiency of the particles on the eroding surface will decrease, resulting in lower and lower dissipation of kinetic energy by impact, and the erosion rate will fall to vanishingly small values. It is believed that these considerations are applicable both to erosion by slurries as well as gas- borne particies. Certainly the work of Laitone [26, 271 in examining particle velocities and trajectories for normal impingement for gas-borne particles on a flat plate would support this view. The usefulness of measuring erosion rate as a mass ratio (mass of material eroded and mass of erodent employed) is questionable unless the coliision efficiency is known and constant. Generally the co&ion efficiency is implicitly assumed to be unity.

The present results ahow estimation of erosion rates for any particle size. Using a constant value of the ratio of mass loss per impact and particle kinetic energy at impact of 42~ 1O-6 g J-‘, the erosion rate Rt may be calculated as a function of particle size. For larger particle sizes a collision efficiency of unity was assumed and impact velocities were calculated using the method of ref. 22 (Fig. 8). Smaller particle sizes required an assumption about the number of impacts on unit area in unit time. Figure 12 shows the variation of particle impact rate with particle size for the range 2&5ooO pm. Values from 49 to 420 Km are derived from experimental data for collision efficiency using the straight line frt of Fig. 4. Above 500 pm a collision efficiency of unity is assumed, while the trend towards a limiting value of impact rate at smaller particle sizes reflects the very low values of collision efficiency for particle sizes in this range.

59

107

106

105

104

103

102

10' I I 10 100 1000 10000

PARTICLE SIZE (wW

Fig. 12. Variation of SIC particle impact rate under erosion conditions (1.2 wt.% suspension) as a function of particle size. Values from 20 to 423 pm are derived from n. Above 500 grn values are calculated from an assumed co&ion efficiency of unity.

Fig. 13. particle

Plot of erosion rate R, for PllO steel in l.‘hvt.%SiC-oil suspension as size. Full hne shows experimental results; dashed line is calculated.

bl Experimental l Calculated

t 100 1000

PARTICLE SIZE (pm}

a function of

70

The calculated erosion curve is shown in Fig. 13 together with the experimental data. The relationship shows the general form revealed by experiments on gas-solid erosion [12] and liquid-solid erosion [4] in that above 500 pm the erosion rate changes little with particle size, while at small particle sizes the rate falls sharply. At particle sizes between 100 and 500 pm the erosion rate falls as shown also by the present experimental results. The fit is exact in this portion because of the use of experimental data to derive the calculated curve. The discrepancy between predicted and experimental rates at low particle size, as noted above, is believed to be the result of a change in the mechanism of metal removal, in which direct particle impact plays a decreasingly important role as particle size decreases.

5. Conclusions

(1) Through short-time erosion tests using very dilute suspensions of Sic in oi1 and electropolished specimens it is possible to estimate the impact frequency of particles on the specimen surface and the collision efficiency of particles in the flow regime.

(2) Both the collision efficiency and the impact velocity of particles decrease with decreasing particle size under conditions of erosion testing.

(3) The mass loss rate in erosion is proportional to the rate of dissipation of kinetic energy of the impacting particles above about 100 pm in these tests.

(4) The frequently reported decrease in erosion rate with particle size is the result of the combined effect of decreasing collision efficiency of particles and decreasing impact velocity with decreasing particle size.

(5) A limiting value of erosion rate above some intermediate particle size reflects a condition in which the collision efficiency approaches unity and particle retardation prior to impact is small.

(6) For cylindrical erosion specimens, measurement of the circumferential erosion angle may provide a method of estimating the collision efficiency of particles in the suspension if particles are of uniform size.

(7) Analysis of erosion rates for suspensions of small particles (less than 100 pm) in terms of the rate of dissipation of kinetic energy of impact may be complicated by an accumulation of particles at the specimen surface caused by their retardation on impact, with a consequent change in the mechanism of material removal.

(8) Details of particle trajectories and velocities as well as an understanding of the flow regime are essential if comparisons of erosion rates are to be well founded.

Acknowledgments

The authors wish to thank Lawrence Technology, Lawrence, KS for the supply of copper rod, and Barry Smith, Metallurgy Department, RMIT, Melbourne, Australia for providing the air classification of powders.

References

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p. 512.

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3 A. V. Levy, Erosion corrosion of metals in coal liquifaction environments, Muter. Perform., 19 (1980) 45-51.

4 A. Elkholy, Prediction of abrasion wear for slurry pump materials, Wear, 84 (1983) 39-49. 5 Y. Oka and M. Matsumura, Erosive wear testing apparatus - simulation of erosion caused

by slurry of low impingement velocity, Int. Co@ on Wear of MatetiuLF 1983, ASME, New York, 1983, pp. 360-366.

6 K. Ahmed and A. Goulas, On the use of particle trajectories for the prediction of erosion in slurry pumps, Int. Con& on Slurry Flows, ASME, FED, Vol. 38, ASME, New York, 1986, pp. 125-133.

7 A. V. Levy and G. Hickey, Liquid-solid particle slurry erosion of steels, Wear, 117 (1987) 129-146.

8 J. Cruzado, R. A. Hardin and H. MCI. Clark, Resistance of oil-well casing steels to erosion by sand-oil suspensions, Proc. 7th Int. Con& on Erosion by Liquid and Solid Impact, Cambridge, 1987, Cavendish Laboratory, University of Cambridge 1987, pp. 38-l-38-7.

9 C. Lhymn and P. Wapner, Slurry erosion of polyphenylene sulfide-glass fiber composites, Wear, 110 (1987) l-11.

10 H. MCI. Clark, On the impact rate and impact energy of particles in a slurry pot erosion tester, Wear, 147 (1991) 165-183.

11 J. G. A. Bitter, A study of erosion phenomena, Wear, 6 (1963) 5-21. 12 W. Sage and G. P. Tilly, The significance of particle size in sand erosion of small gas

turbines, Aeronaut. J., 73 (1969) 427-428. 13 I. Finnie, Some observations on the erosion of ductile metals, Wear, 19 (1972) 81-90. 14 A. Misra and I. Finnie, On the size effect in abrasive and erosive wear, Wear, 6.5 (1981)

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