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Erodent particle characterization and the effect of particle size and shape on erosion
Wear, 138 (1990)
189 - 208
189
ERODENT PARTICLE CHARACTERIZATION AND THE EFFECT OF PARTICLE SIZE AND SHAPE ON EROSION* S. BAHADUR and R. BADRUDDIN Department
of Mechanical
Engineering,
Iowa State
University,
Ames, IA 50011
(U.S.A.)
Summary Sic, A1203 and Si02 particles in various grit sizes are characterized by image analysis in terms of their area, area diameter, width, length, width-tolength ratio W/L and perimeter squared-to-area ratio P2/A. Along with the mean, median and r.m.s. deviation of these parameters, the cumulative frequency distributions of the area and W/L are. also obtained. W/L and P2/A are used as the indicators of particle shape. The particles become more elongated and less circular as the size of Sic and A1203 particles increases and that of Si02 particles decreases. The characterized particles are used for erosion studies on MNi(250) maraging steel in aged condition in a sand-blast type rest rig. The variation of erosion is studied with changes in size and shape of these particles. Erosion rate increases with increasing particle size for Sic and A120s particles up to a certain value and decreases for Si02 particles. However, it increases with increasing P2/A and decreasing W/L for all three types of particles. Erosion behavior is analyzed considering the effect of rake angle in ploughing and microcutting as observed in erosion. It is the changing contribution from ploughing and cutting with changes in shape and size of particles that accounts for erosion variation.
1. Introduction The size and shape of particles are of fundamental importance in many areas of engineering and scientific research, and in particular in tribology. The role of these particle characteristics is well recognized in the abrasive and erosive wear processes. Figure 1 shows different approaches for particle characterization. Referring to Fig. l(a), which gives the plan and elevation views of a typical angular particle, it is seen that various dimensions may be assigned to the particle. Of these dimensions, the maximum separation L’ along a straight line between two points located on the surface of particle is the only unique dimension. The other dimensions, namely thickness T, *Paper presented at the International CO, U.S.A., April 8 - 14, 1989. 0043-1648/90/$3.50
Conference
on Wear of Materials, Denver,
@ Xlsevier Sequoia/Printed
in The Netherlands
190
MINIMUM SQUARE
PLANE OF MAXIMUM STABILITY
SPHERES OF EQUAL VOLUME AND SURFACE
Fig. 1. Illustration of parameters dimensions; (b) two dimensions;
commonly used in particle (c) three dimensions.
characterizations:
(a) limiting
width W and length L, are the minimum distances between two parallel planes. One of these planes, as in the case of thickness, is the plane of maximum stability. In the case of width, the two planes are perpendicular to the planes defining thickness, and in the case of length, perpendicular to the planes defining both thickness and width. The size of particle is sometimes expressed in terms of the diameter of a circle of equal projected area or alternatively of equal projected perimeter, as shown in Fig. l(b). The former is designated by cl, and the latter by d,. In addition, the particle size may be expressed as the diameter of a sphere of equal surface area or of equal volume, marked as d, and d, respectively in Fig. l(c). The particle size is also expressed in terms of the Martin and Feret diameters, marked as M and F in Fig. l(b), which are the statistical diameters. These are approximately equal to d, and d, respectively if the measurements are made in a number of random directions [ 11. Other dimensions used to characterize particle size are the aperture A of a sieve (Fig. 1) or equivalent round aperture. The deviation due to non-ideal shape is quantitatively expressed by means of the shape factors. When three mutually perpendicular dimensions of a particle can be determined, the following Heywood’s ratios [ 21 are used elongation ratio = L/W flakiness ratio = W/T These ratios are equal to unity for the most regular body which is a sphere, and the more elongated the particle, the greater is the elongation ratio. The reciprocal of elongation ratio, called the aspect ratio, is also used to describe
191
particle shape. Its value ranges from unity for a spherical body to zero for a body of a perfectly fibrous shape. The ratio of the square of the perimeter to the area of the particle (P*/A) also provides a quantitative measure of the deviation in shape of an irregular particle from that of a known geometry. For example, values relative to 4n and 16.0 indicate deviations in the shape of a particle from those of a sphere and a square respectively. Of the many factors that affect erosion, the effects of operating parameters such as the impingement angle and impact velocity are well established [3]. The effects of target material characteristics such as the hardness, toughness and microstructure, have also been studied by various researchers [ 3 - 61. As for the impact particle characteristics, most studies have looked at the effect of particle grit size on erosion. The abrasive and erosive effectiveness of hard particles in sizes below 100 pm is considerably reduced [ 7,8]. Vetter and Swanson [Q] have applied the morphological analysis technique [lo - 121 to the characterization of sand particles and developed an empirical equation for abrasive wear in terms of the size parameters. It has been reported that erosion is also a function of the hardness of impacting particles [8,13]. Goodwin et al. [8] related erosion rate G (defined as the mass of material eroded per unit mass of erodent impacted) by different types of sands with varying diamond pyramid hardness number H by the simple power relationship G = Kpe3, where K is the proportionality constant. It has been recognized that the situation is much more complex because of the interdependence of shape and hardness where the shape has also been known to influence erosion significantly. Raask [14] found in his erosion experiments on mild steel using relatively low velocities that 100 E.tm sharp quartz particles were 10 times more erosive than glass spheres of the same size. Winter and Hutchings [15] analyzed the oblique impact of individual angular particles and concluded that the impact process differed with the shape and orientation of particles. In their work on erosion by a variety of abrasive particles, Goodwin et al. [8] concluded that the hardest particles tended to have the sharpest profiles. It is seen from the above literature review that no comprehensive study related to the characterization of erodent particles has been reported. The reasons for this are two-fold: the lack of general availability of systems capable of quantitative characterization, and the time and expense involved in characterization work. It is the realization of the lack of data on particle characteristics and their importance in abrasive and erosive wear studies that led to the present work. The earlier part of this paper deals with particle shape characteristics and the later part examines the effect of these particle factors on the erosion process. 2. Experimental details 2.1. Particle characterization Three types of graded particles, i.e. Sic, Al203 and Si02, were used in this work both for the characterization and erosion studies. The particulars
192
of grit/micron sizes used and their suppliers are listed in Table 1. While SIC particles were available from the suppliers of graded particles (Buehler and LECO Corp.) in grit sizes 120 - 600, Al,Os particles were available in fewer grit sizes, where 240 grit was the coarsest. As such, Al,Os particles 5 and 15 pm in size were included to get a larger size variation. SiOZ particles were separated from an ungraded batch by the standard sieving procedures [ 161. The latter involved starting with a sample of 100 g and sieving it for 15 min. Since the particles tended to adhere together, a method was developed to separate them from one another. This procedure involved entrapping a small quantity of powder in the capillary of a pipette and spraying it with the help of pressurized gas onto a carbon stub 25 mm in diameter. The stubs were then placed in a Kinney vacuum evaporator and coated with a carbon layer 300 A thick.
TABLE
1
Grit or micron
sizes and suppliers
of particles
Particle material
Supplier
Grit or micron sizeb
Sic
Buehler
120 grit (142)
LECO
-412%
SiO2 a (Ottawa sand) aUngrated bNumbers
240 grit (54 - 63)
320 grit (29 - 32)
400 grit (20 - 23)
600 grit (12.5 - 17)
(15)
600 grit (12.5 _ 17)
(5)
100 grit
-
LECO
240 grit (54 - 63)
400 grit (20 - 23)
Ottawa Sand
40 grit
42 grit
sample separated with parenthesis
50 grit
after sieving into the grits indicated. indicate size range for various grits in micrometres.
The particles were photographed in a scanning electron microscope (SEM) and analyzed with a LeMont Scientific image analyzer. Figure 2 shows the units in the analyzer system. It is a software-based system designed for analyzing optical images generated by a Dage-MT1 model NC68DX television camera with a resolution of 512 pixels X 480 pixels. The system interfaces a DEC LSI 11/73 microprocessor to an RS-170 format TV generated image, an AP-512 analog processor and an FB-512 frame grabberframe buffer.
193
I
MACRO LENS
1
LIGHT MICROSCOPE
FRAME BUFFER ACTIVE IMAGE 1
t
t CPU
~&TE FLOPPY DISK
OEC
LSI
TEMPORARY MEMORY BUFFERS
11/73
1 DIGITIZER TABLET
TERMINAL 1
'
KEYBOARD
Fig. 2. Schematic system.
diagram of LeMont
Scientific
(OASYS)
automated
image analyzer
A minimum of 100 particles of each size were photographed in an SEM. The photographs were placed under a macrolens connected to a television camera. The frames were locked in position and displayed on a TV monitor. A color file was created to get sharp contrast of particles against the background. This technique involved assigning grey levels to various intensities of brightness. The colored images were filtered for background noise with a built-in interactive OASYS image analysis program. Editing and finer adjustments to the images were done for separating the contours of connected particles on a digitizer tablet. The edited images were stored in various temporary memory buffers, hard disks and floppy disks. The OASYS program was then used to perform quantitative and statistical analyses on a number of particle parameters. These parameters included area A, area diameter d,, width W, length L, ratio of width-to-length W/L and ratio of perimeter squared-to-area of particle P2/A. 2.2. Erosion experiments Using the characterized particles of Sic, Al203 and Si02 as erodents, erosion experiments were run on 18Ni(250) maraging steel in a sand-blasttype test rig described in detail elsewhere [5]. Specimens were metallographically polished and then eroded with a total of 80 g of abrasive in four exposures of 20 g each. After each exposure, the specimen was removed from its locating fixture, brushed off lightly and weighed to determine the erosive mass loss. Since the velocity of particles is dependent upon their size and density [ 171, pressure-velocity curves were established using a double-disk arrangement for each combination of particle size and material. Erosion experiments were run with an average impingement velocity of 42 m s-l for Sic and Si02 particles, and 65 m s-l for Al203 particles. A
194
higher velocity in the latter case was used because the pressures needed for generating a velocity of 42 m s-l with finer Al,Os particles were too low to be monitored accurately by pressure gauges. The angle of impingement was maintained at a fixed value of 30” to maximize erosive loss of the ductile steel [3]. Erosion specimens 25 mm X 19 mm X 6 mm in size were cut out of a 25 mm X 6 mm flat of commercial BNi(250) maraging steel. They were solution annealed at 815 “C for 30 min and aged in a salt bath at 590 “C for 30 min. The opposite flat sides of these specimens were ground parallel. Erosion test surfaces were polished in running water in steps down to 600 grade Sic paper. Final polishing was done on a rotary polishing cloth with water-suspended Al,Os particles 0.3 and 0.05 pm in size.
3. Results 3.1. Erodent particle characterization Figures 3, 4 and 5 show three sets of SEM micrographs of Sic, Al,Os and Si02 particles respectively. The particles chosen were of considerably different grit sizes with the possibility of providing a large variation in shape parameters. Extremely fine particles were excluded because these were not suitable for meaningful erosion experiments. Referring to Sic particles in Fig. 3, it is seen that 120 grit particles are the sharpest, having sharp corners and elongated features. In all the grit sizes, particles vary in shape from elongated to near-round. Figure 4 provides a visual comparison of A1,03 particles. The particles with the lowest grit number again appear to be the sharpest. Though 15 pm particles are much smaller in size, they contain a large number of sharp particles. However, 600 grit particles contain a large number of fine particles which appear to be even smaller than 5 pm particles. Figure 5 shows SiOZ particles with sizes ranging from 40 to 100 grit. These particles are much smoother and rounder (and hence less sharp) than Sic and Al,Os particles. The results of image analysis performed on these three kinds of particles are presented in Tables 2, 3 and 4. The average value is calculated here without any kind of weight and so is the arithmetic mean of all physical measurements of one kind. The average value of a derived parameter such as area diameter was not calculated from the average value of the sample but instead from the measured values of areas and derived diameters of individual particles. The median values of P*/A are not given because the analysis system was not capable of a generating histogram of this derived quantity. In Tables 2 and 3 the micrometer size range provided by the manufacturer is also included within parenthesis along with every grit number. It is noted that the measured area diameter values are quite different from the size range specified for different grits. It is seen from Table 2 for Sic particles that as the grit size increases from 120 to 600, the measured average area diameter decreases from 151.0
(a)
(b)
(d)
(e) Fig. 3. Micrographs of SIC particles: (a) 120 grit; (b) 240 grit; (c) 320 grit;(d) (e) 600 grit.
400 grit;
196
(a)
Cd)
(e) Fig. 4. Micrographs (e) 5 Pm.
of Al203 particles:
(a) 240 grit; (b) 400 grit; (c) 15 Mm;(d)
600 grit;
(a)
(b)
(cl
(d)
Fig. 5. Micrographs of SiOz particles: (a1) 40 grit; (b) 42 grit; (c) 50 grit; (d) 100 grit.
to 11.9 pm, i.e. by a factor of 12.5. There is an abrupt decrease in area diameter from grit 120 to 240 and from grit 400 to 600, while the decrease in other cases is gradual. The same type of variation is also observed for area, width and length parameters. The last two columns in the table provide W/L and P*/Awhich are the dimensionless shape parameters. Since smaller values of aspect ratio (W/L)indicate greater elongation of particles, it is noted that coarser particles are more elongated than finer particles as this ratio changes from 0.432 to 0.585. Since the mean and median values of W/L for each of the 120 and 240 grits are the same, a normal distribution of aspect ratio values is indicated. For other grits the values are somewhat different but again fairly close. As for the other shape factor, deviations in the value of P*/Afrom 47r indicate departure from circularity (or sphericity in a threedimensional context). Thus, particles with smaller grit numbers have lower sphericity than those with higher grit numbers with the exception of 400 and 600 grits which have nearly the same values of P*/A .
198
TABLE 2 Statistical values of size and shape parameters as obtained by analysis of Sic particles Grit
Statistical parameters
Area diameter (Pm)
Area (ym2)
Width (Mm)
Length (Pm)
W/L
P2/A
120 (142 Mm)
average r.m.s. deviation median
151.00 36.30
19.00 x 10s 6.53 x lo3
94.850
25.310
223.030 41.700
0.432 0.120
19.10 2.59
282.00
17.20 x lo3
92.070
219.460
0.433
-
average r.m.s. deviation median
40.03 20.02
1.59 x 103 1.49 x 103
28.637 16.762
55.458 27.779
0.528 0.184
17.90 3.65
34.50
1.23 x lo3
25.500
53.143
0.525
-
average r.m.s. deviation median
34.60 13.20
1.01 x 10s 0.92 x 10s
22.713 11.890
45.309 22.840
0.537 0.166
16.40 4.77
30.00
0.79 x 103
20.974
40.719
0.525
-
average r.m.s. deviation median
28.10 8.12
0.67 x lo3 0.39 x 103
21.158 8.190
39.666
12.653
0.557 0.195
15.80 4.70
25.80
0.66 x 10s
20.500
38.636
0.568
-
average r.m.s. deviation median
11.90 3.70
0.12 x 103 0.07 x 103
9.021 3.315
16.163 5.819
0.585 0.179
15.70 4.32
0.12 x 103
8.859
15.316
0.597
-
240 (54 - 63 pm)
320 (29 - 32 pm)
400 (20 - 23 pm)
600 (12.5 - 17 pm)
%orrect
_a
values could not be obtained due to the default setting of the class intervals.
Table 3 provides the same set of data on Al,03 particles. Here, sizes of 15 and 5 pm relate to the particles supplied by LECO Corp. under these size designations and had no grit numbers assigned to them. It is interesting to note by comparison between Tables 2 and 3 that for the same grit numbers the sizes of A1203 and Sic particles are considerably different. The area diameters of 5 E.crnand 600 grit particles are about equal and those of 400 grit and 15 pm close to each other. Due to the smaller total spread in the size of A120s particles considered, the overall variation in W/L and P2/Ais fairly small. The elongation of 240 and 400 grits and 5 pm particles is greater than that of 15 pm and 600 grit particles. The deviation from circularity is the highest for 240 grit and lowest for 600 grit. Table 4 shows results of similar analysis performed on Si02 particles of 40, 42, 50 and 100 grit sizes. The average area diameters of these particles are about an order of magnitude larger than those of Sic particles, with the exception of 120 grit, and Al*Os particles. The values of W/L for silica particles indicate much lower elongation than for Sic and Al,Os particles. Furthermore, 100 grit particles (which are the finest) have the lowest W/L
199 TABLE 3 Statistical values of size and shape parameters as obtained by analysis of A1203 particles Grit
Statistical parameters
Area $am,eter m
Area Wn2)
Wiclth Mm)
Length Mm)
W/L
PZ /A
240 (54 -63pm)
average r.m.s. deviation median
33.10 22.50
12.50 x lo2 13.60 x lo2
23.088 17.256
47.416 34.607
0.532 0.210
16.80 5.57
34.70
8.20 x lo2
20.500
41.917
0.561
-
average r.m.s. deviation median
12.40 7.82
1.69 x lo2 1.80 x lo2
8.470 5.651
17.958 13.058
0.514 0.175
16.00 5.01
-a
1.08 x 102
8.150
16.441
0.375
-
11.80 6.40
1.41 x 102 1.56 x lo2
8.079 3.849
17.325 14.331
0.564 0.193
15.00 5.71
--a
0.88 x lo2
7.795
13.906
0.600
-
6.99 x lo2 0.93 x 102
5.890 4.320
10.478 8.064
0.564 0.168
14.30 3.77
0.34 x 102
4.933
8.399
0.580
-
0.58 x lo2 0.51 x 102
5.430 2.789
10.734 6.189
0.539 0.166
15.70 3.73
0.48 x 102
5.364
10.268
0.545
-
400 (20 - 23 pm)
15 pm
600 (12.5 - 17 pm)
5E.tm
average r.m.s. deviation median average r.m.s. deviation median average r.m.s. deviation
7.79 5.33 -a
7.73 3.74 _a
=Correct values could not be obtained due to the default setting of the class intervals.
of 0.685 while the coarsest size 40 grit particles have the highest W/L value of 0.734. This is different from the observations on Sic and A1203 particles, where larger particles tended to be more elongated. With P2/A= 12.9, 40 grit particles are almost spherical in shape; its value for a sphere is 12.57. On the other end of the size spectrum, 100 grit particles with P2/A = 14.3 are less circular than a sphere but more circular than a square with P2/A = 16.0. Figure 6 provides area distribution curves for 240, 320, 400 and 600 grits of Sic particles. The distribution for Sic 120 grit is included in Fig. 7 along with A1203 240 grit because of the large difference in size. Figure 8 gives area distribution curves for the remaining grades of A1203 particles. In all the samples, 10% - 15% of particles are very fine and the remainder, with the exception of Sic 600 grit, have a fairly wide distribution. The 120 grit Sic particles (Fig. 7) are much larger than their nearest size (240 grit) as compared with the difference in sizes between the other adjacent grits. The distribution in Fig. 9 shows that Si02 particles are very well graded. Figures 10 - 12 show cumulative frequency distributions of the aspect ratios of particles. For A1203 particles, and Sic particles with the exception
200 TABLE 4 Statistical values of size and shape parameters as obtained by analysis of SiOz particles Grit
40
42
Statistical parameters
Area $am)eter m
Area (vm2)
average r.m.s. deviation median
490.00
19.20
average r.m.s. deviation median average r.m.s. deviation median
50
100
average r.m.s. deviation median
%orrect
x 104 5.00 x 104
70.40
Width Mm)
Length Mm)
W/L
P/A
429.022
584.582 80.472
0.734 0.113
12.90
76.433
2.55
469.00
18.60 x 104
461.021
564.291
0.738
-
417.00 56.80
13.90 x 104 3.53 x 104
359.300 69.423
506.216 69.533
0.714 0.133
13.40 2.49
-a
16.00 x lo4
349.466
513.232
0.732
-
406.00 78.20
13.40 x 104 4.52 x lo4
345.200 79.912
501.544 123.944
0.700 0.151
14.30 4.48
_a
14.80 x lo4
348.602
504.096
0.723
326.00 59.90
8.56 x lo4 3.16 x lo4
271.070 56.941
407.556 110.082
0.685 0.143
-a
8.01 x lo4
287.126
398.960
0.701
14.30 3.09
values could not be obtained due to the default setting of the class intervals.
++ +
A 0
A 0 0
0 0
+
A
0
+
2400 3200 4000 ooog
y 0.0
0.2
0.4 AREA.
0.0 (p,m)*
0.8
1.0
x 10’
Fig. 6. Percentage cumulative frequency distribution of areas of Sic particles.
of 120 grit, the distributions are similar. In these cases, about 10% of particles have aspect ratios of less than 0.3, which means that they are fairly elongated. Another 10% of the particles have aspect ratios between 0.78 and 0.95 which means that they are fairly regular, and the remaining 80% have aspect ratios lying between 0.3 and 0.78. As for 120 grit Sic particles, apart from a smaller median value, the aspect ratio range of the bulk of particles is narrower. The aspect ratio distributions of SiOz particles shown in Fig. 12 are different. Here, 80% of particles have aspect ratios in the range of 0.55 to 0.85, indicating that these particles are well graded.
201
0.0
0.1
0.2 AREA,
Fig. 7. Percentage cumulative 240 grit A1203 partides.
0.9 (w.m)*
0.4
0.5
x 10’
frequency
distribution
of areas of 120 grit Sic carbide and
100 60
00
40
20
0
0.0
0.2
0.4 AREA,
Fig. 8. Percentage cumulative
0.6 (p,mP
0.2
1.0
x IO*
frequency
distribution
of areas of Al203 particles.
100
30
60
40
20
0 0.0
0.1
0.2 AREA.
9. Percentage cumulative
0.3 (pm)*
0.4
0.6
x lo@
frequency
distribution
of areas of SiOz particles.
3.2. Effect of erodent particlec~a~cter~tic~ on erosion The variation of erosion with particle size, as determined by image analysis reported in the earlier section, is shown in Figs. 13 and 14. The
202
0.2
0.0
0.4 ASPECT
Fig. 10. Percentage
0.6
0.8
1.0
RATIO. W/l_
cumulative frequency
distribution
of aspect ratios of Sic particles.
100
80
80
40
20
-I
0
0.4 ASPECT
0.6
0.8
1.0
RATIO. W/L
Fig. 11. Percentage cumulative frequency distribution of aspect ratios of Al203 particles.
100
+
400 429 6Og
c’
1000
A 0
: 0.0
_I 0.4 ASPECT
0.8
0.6
1.0
RATIO, W/L
Fig. 12. Percentage cumulative frequency distribution of aspect ratios of SiO2 particles.
target material used for erosion experiments was 18Ni(250) maraging steel solution treated and aged at 590 “C for 30 min. This aging time was selected because it resulted in a rn~~urn hardness of 51R,, which was expected to provide higher erosion values. It is noted from Fig. 13 that erosion initially increases with increasing particle area diameter for both the Sic and A1,03
203
SIC
0
1200
x 2400 A 0 +
A’,%
l
3200 4000 Boog 2409
@ 4000 a 15pm l
6oog
b 5pm
41”““““““’ 0 20
40
60
00
100
120
140
AVERAQE AREA DIAMETER. da, pm
Fig. 13. Variation of erosion rate of 18Ni(250) of Sic and alumina particles.
maraging steel with average area diameters
5.0
I
3.0 300
I
I
400 450 350 AVERAQE AREA DIAMETER, da, pm
Fig. 14. Variation of erosion rate of lSNi(250) of SiOz particles.
500
maraging steel with average area diameters
particles. The erosion rate curve becomes horizontal after about 50 pm for Sic particles. Similar erosion leveling tendency is also indicated for Al,Os particles more than 35 pm in area diameter, but the horizontal part of the curve could not be established in this work because of the limiting 240 grit size. The saturation of erosion due to size effect has also been reported by other workers [ 81. Unlike the above two cases, the erosion rate of maraging steel by SiOZ particles decreases almost linearly with increasing size (Fig. 14) and the magnitude of erosion rates is considerably smaller. The variation of erosion with particle shape parameters was next examined. Figure 15 shows the variation of erosion rate with aspect ratio
204
0.3
0.3
8
0.3
I
0.1
0.1
. \ 12 InI8 :/
E .W
15
0.1
1,
mh
2 m/r
,\
A
ui
‘\
5 fj 0 5
.
12og
.
2400
x 2400
x 4009
A 3200
fi 16pm
+ 400Q
+ BOOg
0 6000
a 5pm
0.01 0.6
0.8
_ _ 0.3 ASPECT
400
s 420 n 500 + 1oog
cc)
(b)
(ai 0.01 0.3
l
0.01 0.6
0.8
RATIO.
W/L
Fig. 15. Variation of erosion rate of 18Ni(250) of (a) Sic, (b) A1203and (c) SiOz particles.
0.3
0.6
0.8
maraging steel with average aspect ratios
W/L on logarithmic axes for all three types of erodent particles. The aspect ratio values are based on our analysis and were taken from Tables 2 - 4. It is noted that the erosion rate, in general, varies inversely with the aspect ratio of particles. For example, 120 grit Sic particles with an aspect ratio of 0.432 had an erosion rate of 0.18 mg g-’ compared to 0.09 mg g-’ for 600 grit particles having an aspect ratio of 0.585. The same inverse linear relationship of erosion rate with aspect ratio on the log-log plot applies to A1203 and SiOZ particles as well. It should be recalled that, unlike the case of Sic and Al,Os particles, the elongation of SiOZ particles was found to be inversely related to the size. The variation of erosion rate with the other shape factor, i.e. P2/A, is shown in Fig. 16. Here, a direct linear proportionality on the log-log plot between erosion rate and P2/A ratio is observed for all particle materials. Considering a wide distribution of particle sizes in each grit category (Figs. 6 - 9) and the error in velocity measurement for small size particles, it is felt that the scatter is reasonable.
4. Discussion
There are three characteristics of an impacting particle, namely hardness, size and shape, that may affect erosion. The hardness of Sic and A1203 particles is 2480 and 2100 KHN respectively, which is much in excess of the hardness of target material (530 KHN). As such hardness is not
205 0.3
: E
0.1
42 m/r
.W ti i p 0 !i
f\ l
x ‘I + 0
0.01
1200 2400 3209 4009 f3OOQ
l
x A + 0
I __
10
20
(a)
I 30
I 40 60
2400 4000 l&m 6000 Sprn
42 ml8
400 x 420
l
sag + 1000 A
(b)
I
30 10 20 (PERIMETER)Z~~R~~.
Fig. 16. Variation of erosion rate of BNi(250) of (a) Sic, (b) A1203 and (c) SiOz particles.
(c)
I 40 60 P%
10
20
30
40 60
maraging steel with average Pz/A ratios
expected to make any difference in erosion by these two particle materials. The hardness of SiOZ particles is 820 KHN, which is about 55% higher than that of the target material. It is, therefore, reasonable to ignore the deformation of Si02 particles on impact as well. Furthermore, since SiOZ particles are fairly round, blunting due to deformation if any will have negligible effect in changing the shape of particles. The effect of size of erodent particles has been widely studied and the reported behavior [18] for all three types of particles is similar to that observed in this work (Fig. 13) for Sic and A1,03 particles. It should be pointed out that the variation of erosion rate with particle size of SiOZ particles observed in our work (Fig. 14) is different from that reported in the literature. Misra and Finnie [7] critically reviewed the explanations provided by research workers on the dependence of erosion (and also abrasion) on erodent (abrasive) particle size. Some of the explanations offered relating to erosion involve differences in ploughing, strain rate sensitivity of material, penetration to different depths composed of hard and soft layers etc., by different size particles. As for the mechanisms of erosion, most workers explain material removal by microcutting and ploughing leading to flake formation, as evidenced in our earlier work by scanning electron micrographs of maraging steel eroded surfaces as well [ 51. The role of microcutting is recognized to be significant in case of sharp particles and of ploughing in case of round particles. Thus, the shape effect has been qualitatively recognized in erosion but its verification outside the confines of model studies has not been possible because of the inability to quantify
206
angularity. The increasing erosion rate with increasing particle size of SiC and A1203 particles and the opposite variation for SiO, particles indicates that the shape of particles is equally, if not more, significant in the erosion process. As for the size effect, it is conceivable that extremely fine particles may not result in any erosion because their impact is limited to producing merely elastic deformation in surface layers. The limiting size for this condition will be governed by particle density, impact velocity and target properties. The conditions used in this work were such that significant erosion was produced by particles of all sizes. Winter and Hutchings [15] studied the effect of rake angle on ploughing and microcutting on lead and steel targets using flat-faced single particles. They concluded that microcutting was favored by positive or small values of negative rake angles while ploughing occurred with large negative rake angles. The rake angle is defined as the angle between the perpendicular to target surface and the leading edge of impacting particle, as shown in Fig. 17. When working with quantitative shape analysis of multiple particles as in this work, it is not possible to measure the angle at every corner of a particle. As such, the measure of particle angularity was obtained in terms of the W/L and/or P2/A, which is reasonable for polyhedral-shaped particles. As for the rake angle, it depends upon the shape of the particle and, in the case of angular particles, also upon its orientation to the target surface at the point of impact. For example, a spherical particle will always impact the target surface with a large negative rake angle (Fig. 17(a)) so that ploughing is the only plausible mode of deformation. However, an angular particle may impinge with either positive or negative rake angle. Considering random impact orientations, angular particles should be expected to produce both microcutting and ploughing. Furthermore, the rounder the particle, the greater will be the likelihood of ploughing (due to larger included angle at polyhedron comers) in comparison to microcutting. Examining the erosion behavior observed in this work from the viewpoint of the above analysis, it could be said that erosion by Si02 particles involves more ploughing than microcutting. This is so because these particles are very regular and circular in shape so that impingement takes place with large negative rake angles. With increasing grit size, there is a decrease in the regularity or circularity of particles so that microcutting tends to increase. It is this aspect of the erosion mechanism which accounts for the increasing erosion rate with increasing grit size (or finer particle size).
(a) Fig. 17. surface.
(a)
(0) Negative
and
(b)
positive
rake
angle
CYfor particle
impacting
on a target
207
Sic particles become more elongated and deviate more from circularity as grit size decreases. This accounts for an increasing contribution from cutting with increasing particle size. Whereas material in ploughing is merely displaced, resulting in pile-up mostly along groove edges (and is later removed during subsequent impacts), it is removed in microcutting instead of being displaced. As such the higher the contribution from cutting, the larger will be the erosion rate. This explains the variation of erosion rate with W/L and P2/Ain Figs. 15 and 16 for both Sic and A120s particles. The review of erosion models reported in the literature indicates that, except for the empirical models proposed by Head and co-workers [19 - 211, none includes particle shape factor as a variable. It is hoped that understanding of the variation of erosion with erodent particle characteristics presented in this work will help in a more realistic modeling of the erosion process in future.
5. Conclusions (1) It has been demonstrated that image analysis may be used for the characterization of erodent particles in terms of their size and meaningful shape parameters. (2) The measured area diameter values for Sic and A1203 particles were quite different from the size range specified in literature for different grits. The measured sizes of Sic and A120s particles for the same grit number were also different. (3) SiOZ particles were more regular in shape and closer to circularity than Sic and A120s particles. (4) Sic particles became more elongated and less circular with increasing size while the opposite was the case with Si02 particles. The general variation for A1203 particles was similar to that of Sic particles, though not that systematic. (5) All grits of commercial Sic and A1,03 particles contained 10 - 15% (by number) very fine particles and the size distributions were fairly broad. (6) Si02 particles separated into various grits from an ungraded batch by standard sieving procedures had much better gradation both for size and shape than the commercial Sic and Al,Os particles. (7) The erosion rate of maraging steel increased with increasing size of Sic and A1,03 particles until leveling in erosion due to the size effect occurred. Conversely, the erosion rate decreased with increasing size of Si02 particles. (8) The erosion rate increased with increasing P2/Aand decreasing W/L for all three types of erodent particles. (9) It is possible to explain the variation in erosion behavior with changes in shape factors in terms of the relative ploughing and microcutting contributions.
208
Acknowledgment We are grateful to the National work through Grant MSM-8413595.
Science Foundation
for supporting
this
References 1 H. Heywood, The origins and development of particle size analysis, in M. J. Groves and J. L. Wyatt-Sargent (eds.), Particle Size Analysis, The Society for Analytical Chemistry, London, 1970, pp. 1 - 186. 2 H. Heywood, The evaluation of powders, J. Pharm. PharmacoZ., 15 (1963) 56T. 3 I. Finnie, J. Walak and Y. Kabil, Erosion of metals by solid particles, J. Mater., 2 (1967) 682. 4 A. V. Levy, The solid particle erosion behavior of steel as a function of microstructure, Wear, 68 (1981) 269. 5 M. Naim and S. Bahadur, Effect of microstructure and mechanical properties on the erosion of lSNi(250) maraging steel, Wear, I12 (1986) 217. 6 L. Ambrosini and S. Bahadur, Erosion of AISI 4140 steel, Wear, 117 (1987) 37. 7 A. Misra and I. Finnie, On the size effect in abrasive and erosive wear, Wear, 65 (1981) 269. 8 J. E. Goodwin, W. Sage and G. P. Tilly, Study of erosion by solid particles, Proc. Inst. Mech. Eng. (London), 184 (1969/70) 279. 9 A. F. Vetter and P. A. Swanson, Particle morphology applied to characterizing abrasive materials, Particulate Sci. Technol., 1 (1983) 127. 10 J. K. Beddow and A. F. Vetter, A note on the use of classifier in morphological analysis of particulates, J. Powder Bulk Solids, 1 (1977) 42. 11 D. W. Luerkens, J. K. Beddow and A. F. Vetter, Structure and morphology - the science of form applied to particle characterization, Powder Technoi., 50 (1987) 93. 12 R. Lenth, J. K. Beddow, C. R. Chang and A. I. Vetter, Particle image analyzing system, in J. K. Beddow (ed.), Particulate Systems: Technology and Fundamentals, Hemisphere, Washington, 1983. 13 W. Zhu and Z. Y. Mao, Study of Erosion by Relatively Soft Particles, in K. C. Ludema (ed.), Wear of Materials - 1987, The American Society of Mechanical Engineers, New York, 1987, pp. 787 - 796. 14 E. Raask, Tube erosion by ash impaction, Wear, 13 (1969) 301. 15 R. E. Winter and I. M. Hutchings, Solid particle erosion studies using single angular particles, Wear, 29 (1974) 181. 16 ASTM standard test method for sieve analysis of granular metal powders. Designation: B214-86. Philadelphia, Pennsylvania, ASTM, 1986. 17 G. P. Tilly, Erosion caused by airborne particles, Wear, 14 (1969) 63. 18 G. P. Tilly, Erosion caused by impact of solid particles, in D. Scott (ed.), Treatise on Materials Science and Technology, Vol. 13, Academic Press, New York, 1979, pp. 287 - 319. 19 W. J. Head and M. E. Harr, The development of a model to predict the erosion of materials by natural contaminants, Wear, 15 (1970) 1. 20 W. J. Head, L. D. Lineback and C. R. Manning, Modification and extension of a model for predicting the erosion of ductile materials, Wear, 23 (1973) 291. 21 W. H. Jennings, W. J. Head and C. R. Manning, A mechanistic model for the prediction of ductile erosion, Wear, 40 (1976) 93.
189 - 208
189
ERODENT PARTICLE CHARACTERIZATION AND THE EFFECT OF PARTICLE SIZE AND SHAPE ON EROSION* S. BAHADUR and R. BADRUDDIN Department
of Mechanical
Engineering,
Iowa State
University,
Ames, IA 50011
(U.S.A.)
Summary Sic, A1203 and Si02 particles in various grit sizes are characterized by image analysis in terms of their area, area diameter, width, length, width-tolength ratio W/L and perimeter squared-to-area ratio P2/A. Along with the mean, median and r.m.s. deviation of these parameters, the cumulative frequency distributions of the area and W/L are. also obtained. W/L and P2/A are used as the indicators of particle shape. The particles become more elongated and less circular as the size of Sic and A1203 particles increases and that of Si02 particles decreases. The characterized particles are used for erosion studies on MNi(250) maraging steel in aged condition in a sand-blast type rest rig. The variation of erosion is studied with changes in size and shape of these particles. Erosion rate increases with increasing particle size for Sic and A120s particles up to a certain value and decreases for Si02 particles. However, it increases with increasing P2/A and decreasing W/L for all three types of particles. Erosion behavior is analyzed considering the effect of rake angle in ploughing and microcutting as observed in erosion. It is the changing contribution from ploughing and cutting with changes in shape and size of particles that accounts for erosion variation.
1. Introduction The size and shape of particles are of fundamental importance in many areas of engineering and scientific research, and in particular in tribology. The role of these particle characteristics is well recognized in the abrasive and erosive wear processes. Figure 1 shows different approaches for particle characterization. Referring to Fig. l(a), which gives the plan and elevation views of a typical angular particle, it is seen that various dimensions may be assigned to the particle. Of these dimensions, the maximum separation L’ along a straight line between two points located on the surface of particle is the only unique dimension. The other dimensions, namely thickness T, *Paper presented at the International CO, U.S.A., April 8 - 14, 1989. 0043-1648/90/$3.50
Conference
on Wear of Materials, Denver,
@ Xlsevier Sequoia/Printed
in The Netherlands
190
MINIMUM SQUARE
PLANE OF MAXIMUM STABILITY
SPHERES OF EQUAL VOLUME AND SURFACE
Fig. 1. Illustration of parameters dimensions; (b) two dimensions;
commonly used in particle (c) three dimensions.
characterizations:
(a) limiting
width W and length L, are the minimum distances between two parallel planes. One of these planes, as in the case of thickness, is the plane of maximum stability. In the case of width, the two planes are perpendicular to the planes defining thickness, and in the case of length, perpendicular to the planes defining both thickness and width. The size of particle is sometimes expressed in terms of the diameter of a circle of equal projected area or alternatively of equal projected perimeter, as shown in Fig. l(b). The former is designated by cl, and the latter by d,. In addition, the particle size may be expressed as the diameter of a sphere of equal surface area or of equal volume, marked as d, and d, respectively in Fig. l(c). The particle size is also expressed in terms of the Martin and Feret diameters, marked as M and F in Fig. l(b), which are the statistical diameters. These are approximately equal to d, and d, respectively if the measurements are made in a number of random directions [ 11. Other dimensions used to characterize particle size are the aperture A of a sieve (Fig. 1) or equivalent round aperture. The deviation due to non-ideal shape is quantitatively expressed by means of the shape factors. When three mutually perpendicular dimensions of a particle can be determined, the following Heywood’s ratios [ 21 are used elongation ratio = L/W flakiness ratio = W/T These ratios are equal to unity for the most regular body which is a sphere, and the more elongated the particle, the greater is the elongation ratio. The reciprocal of elongation ratio, called the aspect ratio, is also used to describe
191
particle shape. Its value ranges from unity for a spherical body to zero for a body of a perfectly fibrous shape. The ratio of the square of the perimeter to the area of the particle (P*/A) also provides a quantitative measure of the deviation in shape of an irregular particle from that of a known geometry. For example, values relative to 4n and 16.0 indicate deviations in the shape of a particle from those of a sphere and a square respectively. Of the many factors that affect erosion, the effects of operating parameters such as the impingement angle and impact velocity are well established [3]. The effects of target material characteristics such as the hardness, toughness and microstructure, have also been studied by various researchers [ 3 - 61. As for the impact particle characteristics, most studies have looked at the effect of particle grit size on erosion. The abrasive and erosive effectiveness of hard particles in sizes below 100 pm is considerably reduced [ 7,8]. Vetter and Swanson [Q] have applied the morphological analysis technique [lo - 121 to the characterization of sand particles and developed an empirical equation for abrasive wear in terms of the size parameters. It has been reported that erosion is also a function of the hardness of impacting particles [8,13]. Goodwin et al. [8] related erosion rate G (defined as the mass of material eroded per unit mass of erodent impacted) by different types of sands with varying diamond pyramid hardness number H by the simple power relationship G = Kpe3, where K is the proportionality constant. It has been recognized that the situation is much more complex because of the interdependence of shape and hardness where the shape has also been known to influence erosion significantly. Raask [14] found in his erosion experiments on mild steel using relatively low velocities that 100 E.tm sharp quartz particles were 10 times more erosive than glass spheres of the same size. Winter and Hutchings [15] analyzed the oblique impact of individual angular particles and concluded that the impact process differed with the shape and orientation of particles. In their work on erosion by a variety of abrasive particles, Goodwin et al. [8] concluded that the hardest particles tended to have the sharpest profiles. It is seen from the above literature review that no comprehensive study related to the characterization of erodent particles has been reported. The reasons for this are two-fold: the lack of general availability of systems capable of quantitative characterization, and the time and expense involved in characterization work. It is the realization of the lack of data on particle characteristics and their importance in abrasive and erosive wear studies that led to the present work. The earlier part of this paper deals with particle shape characteristics and the later part examines the effect of these particle factors on the erosion process. 2. Experimental details 2.1. Particle characterization Three types of graded particles, i.e. Sic, Al203 and Si02, were used in this work both for the characterization and erosion studies. The particulars
192
of grit/micron sizes used and their suppliers are listed in Table 1. While SIC particles were available from the suppliers of graded particles (Buehler and LECO Corp.) in grit sizes 120 - 600, Al,Os particles were available in fewer grit sizes, where 240 grit was the coarsest. As such, Al,Os particles 5 and 15 pm in size were included to get a larger size variation. SiOZ particles were separated from an ungraded batch by the standard sieving procedures [ 161. The latter involved starting with a sample of 100 g and sieving it for 15 min. Since the particles tended to adhere together, a method was developed to separate them from one another. This procedure involved entrapping a small quantity of powder in the capillary of a pipette and spraying it with the help of pressurized gas onto a carbon stub 25 mm in diameter. The stubs were then placed in a Kinney vacuum evaporator and coated with a carbon layer 300 A thick.
TABLE
1
Grit or micron
sizes and suppliers
of particles
Particle material
Supplier
Grit or micron sizeb
Sic
Buehler
120 grit (142)
LECO
-412%
SiO2 a (Ottawa sand) aUngrated bNumbers
240 grit (54 - 63)
320 grit (29 - 32)
400 grit (20 - 23)
600 grit (12.5 - 17)
(15)
600 grit (12.5 _ 17)
(5)
100 grit
-
LECO
240 grit (54 - 63)
400 grit (20 - 23)
Ottawa Sand
40 grit
42 grit
sample separated with parenthesis
50 grit
after sieving into the grits indicated. indicate size range for various grits in micrometres.
The particles were photographed in a scanning electron microscope (SEM) and analyzed with a LeMont Scientific image analyzer. Figure 2 shows the units in the analyzer system. It is a software-based system designed for analyzing optical images generated by a Dage-MT1 model NC68DX television camera with a resolution of 512 pixels X 480 pixels. The system interfaces a DEC LSI 11/73 microprocessor to an RS-170 format TV generated image, an AP-512 analog processor and an FB-512 frame grabberframe buffer.
193
I
MACRO LENS
1
LIGHT MICROSCOPE
FRAME BUFFER ACTIVE IMAGE 1
t
t CPU
~&TE FLOPPY DISK
OEC
LSI
TEMPORARY MEMORY BUFFERS
11/73
1 DIGITIZER TABLET
TERMINAL 1
'
KEYBOARD
Fig. 2. Schematic system.
diagram of LeMont
Scientific
(OASYS)
automated
image analyzer
A minimum of 100 particles of each size were photographed in an SEM. The photographs were placed under a macrolens connected to a television camera. The frames were locked in position and displayed on a TV monitor. A color file was created to get sharp contrast of particles against the background. This technique involved assigning grey levels to various intensities of brightness. The colored images were filtered for background noise with a built-in interactive OASYS image analysis program. Editing and finer adjustments to the images were done for separating the contours of connected particles on a digitizer tablet. The edited images were stored in various temporary memory buffers, hard disks and floppy disks. The OASYS program was then used to perform quantitative and statistical analyses on a number of particle parameters. These parameters included area A, area diameter d,, width W, length L, ratio of width-to-length W/L and ratio of perimeter squared-to-area of particle P2/A. 2.2. Erosion experiments Using the characterized particles of Sic, Al203 and Si02 as erodents, erosion experiments were run on 18Ni(250) maraging steel in a sand-blasttype test rig described in detail elsewhere [5]. Specimens were metallographically polished and then eroded with a total of 80 g of abrasive in four exposures of 20 g each. After each exposure, the specimen was removed from its locating fixture, brushed off lightly and weighed to determine the erosive mass loss. Since the velocity of particles is dependent upon their size and density [ 171, pressure-velocity curves were established using a double-disk arrangement for each combination of particle size and material. Erosion experiments were run with an average impingement velocity of 42 m s-l for Sic and Si02 particles, and 65 m s-l for Al203 particles. A
194
higher velocity in the latter case was used because the pressures needed for generating a velocity of 42 m s-l with finer Al,Os particles were too low to be monitored accurately by pressure gauges. The angle of impingement was maintained at a fixed value of 30” to maximize erosive loss of the ductile steel [3]. Erosion specimens 25 mm X 19 mm X 6 mm in size were cut out of a 25 mm X 6 mm flat of commercial BNi(250) maraging steel. They were solution annealed at 815 “C for 30 min and aged in a salt bath at 590 “C for 30 min. The opposite flat sides of these specimens were ground parallel. Erosion test surfaces were polished in running water in steps down to 600 grade Sic paper. Final polishing was done on a rotary polishing cloth with water-suspended Al,Os particles 0.3 and 0.05 pm in size.
3. Results 3.1. Erodent particle characterization Figures 3, 4 and 5 show three sets of SEM micrographs of Sic, Al,Os and Si02 particles respectively. The particles chosen were of considerably different grit sizes with the possibility of providing a large variation in shape parameters. Extremely fine particles were excluded because these were not suitable for meaningful erosion experiments. Referring to Sic particles in Fig. 3, it is seen that 120 grit particles are the sharpest, having sharp corners and elongated features. In all the grit sizes, particles vary in shape from elongated to near-round. Figure 4 provides a visual comparison of A1,03 particles. The particles with the lowest grit number again appear to be the sharpest. Though 15 pm particles are much smaller in size, they contain a large number of sharp particles. However, 600 grit particles contain a large number of fine particles which appear to be even smaller than 5 pm particles. Figure 5 shows SiOZ particles with sizes ranging from 40 to 100 grit. These particles are much smoother and rounder (and hence less sharp) than Sic and Al,Os particles. The results of image analysis performed on these three kinds of particles are presented in Tables 2, 3 and 4. The average value is calculated here without any kind of weight and so is the arithmetic mean of all physical measurements of one kind. The average value of a derived parameter such as area diameter was not calculated from the average value of the sample but instead from the measured values of areas and derived diameters of individual particles. The median values of P*/A are not given because the analysis system was not capable of a generating histogram of this derived quantity. In Tables 2 and 3 the micrometer size range provided by the manufacturer is also included within parenthesis along with every grit number. It is noted that the measured area diameter values are quite different from the size range specified for different grits. It is seen from Table 2 for Sic particles that as the grit size increases from 120 to 600, the measured average area diameter decreases from 151.0
(a)
(b)
(d)
(e) Fig. 3. Micrographs of SIC particles: (a) 120 grit; (b) 240 grit; (c) 320 grit;(d) (e) 600 grit.
400 grit;
196
(a)
Cd)
(e) Fig. 4. Micrographs (e) 5 Pm.
of Al203 particles:
(a) 240 grit; (b) 400 grit; (c) 15 Mm;(d)
600 grit;
(a)
(b)
(cl
(d)
Fig. 5. Micrographs of SiOz particles: (a1) 40 grit; (b) 42 grit; (c) 50 grit; (d) 100 grit.
to 11.9 pm, i.e. by a factor of 12.5. There is an abrupt decrease in area diameter from grit 120 to 240 and from grit 400 to 600, while the decrease in other cases is gradual. The same type of variation is also observed for area, width and length parameters. The last two columns in the table provide W/L and P*/Awhich are the dimensionless shape parameters. Since smaller values of aspect ratio (W/L)indicate greater elongation of particles, it is noted that coarser particles are more elongated than finer particles as this ratio changes from 0.432 to 0.585. Since the mean and median values of W/L for each of the 120 and 240 grits are the same, a normal distribution of aspect ratio values is indicated. For other grits the values are somewhat different but again fairly close. As for the other shape factor, deviations in the value of P*/Afrom 47r indicate departure from circularity (or sphericity in a threedimensional context). Thus, particles with smaller grit numbers have lower sphericity than those with higher grit numbers with the exception of 400 and 600 grits which have nearly the same values of P*/A .
198
TABLE 2 Statistical values of size and shape parameters as obtained by analysis of Sic particles Grit
Statistical parameters
Area diameter (Pm)
Area (ym2)
Width (Mm)
Length (Pm)
W/L
P2/A
120 (142 Mm)
average r.m.s. deviation median
151.00 36.30
19.00 x 10s 6.53 x lo3
94.850
25.310
223.030 41.700
0.432 0.120
19.10 2.59
282.00
17.20 x lo3
92.070
219.460
0.433
-
average r.m.s. deviation median
40.03 20.02
1.59 x 103 1.49 x 103
28.637 16.762
55.458 27.779
0.528 0.184
17.90 3.65
34.50
1.23 x lo3
25.500
53.143
0.525
-
average r.m.s. deviation median
34.60 13.20
1.01 x 10s 0.92 x 10s
22.713 11.890
45.309 22.840
0.537 0.166
16.40 4.77
30.00
0.79 x 103
20.974
40.719
0.525
-
average r.m.s. deviation median
28.10 8.12
0.67 x lo3 0.39 x 103
21.158 8.190
39.666
12.653
0.557 0.195
15.80 4.70
25.80
0.66 x 10s
20.500
38.636
0.568
-
average r.m.s. deviation median
11.90 3.70
0.12 x 103 0.07 x 103
9.021 3.315
16.163 5.819
0.585 0.179
15.70 4.32
0.12 x 103
8.859
15.316
0.597
-
240 (54 - 63 pm)
320 (29 - 32 pm)
400 (20 - 23 pm)
600 (12.5 - 17 pm)
%orrect
_a
values could not be obtained due to the default setting of the class intervals.
Table 3 provides the same set of data on Al,03 particles. Here, sizes of 15 and 5 pm relate to the particles supplied by LECO Corp. under these size designations and had no grit numbers assigned to them. It is interesting to note by comparison between Tables 2 and 3 that for the same grit numbers the sizes of A1203 and Sic particles are considerably different. The area diameters of 5 E.crnand 600 grit particles are about equal and those of 400 grit and 15 pm close to each other. Due to the smaller total spread in the size of A120s particles considered, the overall variation in W/L and P2/Ais fairly small. The elongation of 240 and 400 grits and 5 pm particles is greater than that of 15 pm and 600 grit particles. The deviation from circularity is the highest for 240 grit and lowest for 600 grit. Table 4 shows results of similar analysis performed on Si02 particles of 40, 42, 50 and 100 grit sizes. The average area diameters of these particles are about an order of magnitude larger than those of Sic particles, with the exception of 120 grit, and Al*Os particles. The values of W/L for silica particles indicate much lower elongation than for Sic and Al,Os particles. Furthermore, 100 grit particles (which are the finest) have the lowest W/L
199 TABLE 3 Statistical values of size and shape parameters as obtained by analysis of A1203 particles Grit
Statistical parameters
Area $am,eter m
Area Wn2)
Wiclth Mm)
Length Mm)
W/L
PZ /A
240 (54 -63pm)
average r.m.s. deviation median
33.10 22.50
12.50 x lo2 13.60 x lo2
23.088 17.256
47.416 34.607
0.532 0.210
16.80 5.57
34.70
8.20 x lo2
20.500
41.917
0.561
-
average r.m.s. deviation median
12.40 7.82
1.69 x lo2 1.80 x lo2
8.470 5.651
17.958 13.058
0.514 0.175
16.00 5.01
-a
1.08 x 102
8.150
16.441
0.375
-
11.80 6.40
1.41 x 102 1.56 x lo2
8.079 3.849
17.325 14.331
0.564 0.193
15.00 5.71
--a
0.88 x lo2
7.795
13.906
0.600
-
6.99 x lo2 0.93 x 102
5.890 4.320
10.478 8.064
0.564 0.168
14.30 3.77
0.34 x 102
4.933
8.399
0.580
-
0.58 x lo2 0.51 x 102
5.430 2.789
10.734 6.189
0.539 0.166
15.70 3.73
0.48 x 102
5.364
10.268
0.545
-
400 (20 - 23 pm)
15 pm
600 (12.5 - 17 pm)
5E.tm
average r.m.s. deviation median average r.m.s. deviation median average r.m.s. deviation
7.79 5.33 -a
7.73 3.74 _a
=Correct values could not be obtained due to the default setting of the class intervals.
of 0.685 while the coarsest size 40 grit particles have the highest W/L value of 0.734. This is different from the observations on Sic and A1203 particles, where larger particles tended to be more elongated. With P2/A= 12.9, 40 grit particles are almost spherical in shape; its value for a sphere is 12.57. On the other end of the size spectrum, 100 grit particles with P2/A = 14.3 are less circular than a sphere but more circular than a square with P2/A = 16.0. Figure 6 provides area distribution curves for 240, 320, 400 and 600 grits of Sic particles. The distribution for Sic 120 grit is included in Fig. 7 along with A1203 240 grit because of the large difference in size. Figure 8 gives area distribution curves for the remaining grades of A1203 particles. In all the samples, 10% - 15% of particles are very fine and the remainder, with the exception of Sic 600 grit, have a fairly wide distribution. The 120 grit Sic particles (Fig. 7) are much larger than their nearest size (240 grit) as compared with the difference in sizes between the other adjacent grits. The distribution in Fig. 9 shows that Si02 particles are very well graded. Figures 10 - 12 show cumulative frequency distributions of the aspect ratios of particles. For A1203 particles, and Sic particles with the exception
200 TABLE 4 Statistical values of size and shape parameters as obtained by analysis of SiOz particles Grit
40
42
Statistical parameters
Area $am)eter m
Area (vm2)
average r.m.s. deviation median
490.00
19.20
average r.m.s. deviation median average r.m.s. deviation median
50
100
average r.m.s. deviation median
%orrect
x 104 5.00 x 104
70.40
Width Mm)
Length Mm)
W/L
P/A
429.022
584.582 80.472
0.734 0.113
12.90
76.433
2.55
469.00
18.60 x 104
461.021
564.291
0.738
-
417.00 56.80
13.90 x 104 3.53 x 104
359.300 69.423
506.216 69.533
0.714 0.133
13.40 2.49
-a
16.00 x lo4
349.466
513.232
0.732
-
406.00 78.20
13.40 x 104 4.52 x lo4
345.200 79.912
501.544 123.944
0.700 0.151
14.30 4.48
_a
14.80 x lo4
348.602
504.096
0.723
326.00 59.90
8.56 x lo4 3.16 x lo4
271.070 56.941
407.556 110.082
0.685 0.143
-a
8.01 x lo4
287.126
398.960
0.701
14.30 3.09
values could not be obtained due to the default setting of the class intervals.
++ +
A 0
A 0 0
0 0
+
A
0
+
2400 3200 4000 ooog
y 0.0
0.2
0.4 AREA.
0.0 (p,m)*
0.8
1.0
x 10’
Fig. 6. Percentage cumulative frequency distribution of areas of Sic particles.
of 120 grit, the distributions are similar. In these cases, about 10% of particles have aspect ratios of less than 0.3, which means that they are fairly elongated. Another 10% of the particles have aspect ratios between 0.78 and 0.95 which means that they are fairly regular, and the remaining 80% have aspect ratios lying between 0.3 and 0.78. As for 120 grit Sic particles, apart from a smaller median value, the aspect ratio range of the bulk of particles is narrower. The aspect ratio distributions of SiOz particles shown in Fig. 12 are different. Here, 80% of particles have aspect ratios in the range of 0.55 to 0.85, indicating that these particles are well graded.
201
0.0
0.1
0.2 AREA,
Fig. 7. Percentage cumulative 240 grit A1203 partides.
0.9 (w.m)*
0.4
0.5
x 10’
frequency
distribution
of areas of 120 grit Sic carbide and
100 60
00
40
20
0
0.0
0.2
0.4 AREA,
Fig. 8. Percentage cumulative
0.6 (p,mP
0.2
1.0
x IO*
frequency
distribution
of areas of Al203 particles.
100
30
60
40
20
0 0.0
0.1
0.2 AREA.
9. Percentage cumulative
0.3 (pm)*
0.4
0.6
x lo@
frequency
distribution
of areas of SiOz particles.
3.2. Effect of erodent particlec~a~cter~tic~ on erosion The variation of erosion with particle size, as determined by image analysis reported in the earlier section, is shown in Figs. 13 and 14. The
202
0.2
0.0
0.4 ASPECT
Fig. 10. Percentage
0.6
0.8
1.0
RATIO. W/l_
cumulative frequency
distribution
of aspect ratios of Sic particles.
100
80
80
40
20
-I
0
0.4 ASPECT
0.6
0.8
1.0
RATIO. W/L
Fig. 11. Percentage cumulative frequency distribution of aspect ratios of Al203 particles.
100
+
400 429 6Og
c’
1000
A 0
: 0.0
_I 0.4 ASPECT
0.8
0.6
1.0
RATIO, W/L
Fig. 12. Percentage cumulative frequency distribution of aspect ratios of SiO2 particles.
target material used for erosion experiments was 18Ni(250) maraging steel solution treated and aged at 590 “C for 30 min. This aging time was selected because it resulted in a rn~~urn hardness of 51R,, which was expected to provide higher erosion values. It is noted from Fig. 13 that erosion initially increases with increasing particle area diameter for both the Sic and A1,03
203
SIC
0
1200
x 2400 A 0 +
A’,%
l
3200 4000 Boog 2409
@ 4000 a 15pm l
6oog
b 5pm
41”““““““’ 0 20
40
60
00
100
120
140
AVERAQE AREA DIAMETER. da, pm
Fig. 13. Variation of erosion rate of 18Ni(250) of Sic and alumina particles.
maraging steel with average area diameters
5.0
I
3.0 300
I
I
400 450 350 AVERAQE AREA DIAMETER, da, pm
Fig. 14. Variation of erosion rate of lSNi(250) of SiOz particles.
500
maraging steel with average area diameters
particles. The erosion rate curve becomes horizontal after about 50 pm for Sic particles. Similar erosion leveling tendency is also indicated for Al,Os particles more than 35 pm in area diameter, but the horizontal part of the curve could not be established in this work because of the limiting 240 grit size. The saturation of erosion due to size effect has also been reported by other workers [ 81. Unlike the above two cases, the erosion rate of maraging steel by SiOZ particles decreases almost linearly with increasing size (Fig. 14) and the magnitude of erosion rates is considerably smaller. The variation of erosion with particle shape parameters was next examined. Figure 15 shows the variation of erosion rate with aspect ratio
204
0.3
0.3
8
0.3
I
0.1
0.1
. \ 12 InI8 :/
E .W
15
0.1
1,
mh
2 m/r
,\
A
ui
‘\
5 fj 0 5
.
12og
.
2400
x 2400
x 4009
A 3200
fi 16pm
+ 400Q
+ BOOg
0 6000
a 5pm
0.01 0.6
0.8
_ _ 0.3 ASPECT
400
s 420 n 500 + 1oog
cc)
(b)
(ai 0.01 0.3
l
0.01 0.6
0.8
RATIO.
W/L
Fig. 15. Variation of erosion rate of 18Ni(250) of (a) Sic, (b) A1203and (c) SiOz particles.
0.3
0.6
0.8
maraging steel with average aspect ratios
W/L on logarithmic axes for all three types of erodent particles. The aspect ratio values are based on our analysis and were taken from Tables 2 - 4. It is noted that the erosion rate, in general, varies inversely with the aspect ratio of particles. For example, 120 grit Sic particles with an aspect ratio of 0.432 had an erosion rate of 0.18 mg g-’ compared to 0.09 mg g-’ for 600 grit particles having an aspect ratio of 0.585. The same inverse linear relationship of erosion rate with aspect ratio on the log-log plot applies to A1203 and SiOZ particles as well. It should be recalled that, unlike the case of Sic and Al,Os particles, the elongation of SiOZ particles was found to be inversely related to the size. The variation of erosion rate with the other shape factor, i.e. P2/A, is shown in Fig. 16. Here, a direct linear proportionality on the log-log plot between erosion rate and P2/A ratio is observed for all particle materials. Considering a wide distribution of particle sizes in each grit category (Figs. 6 - 9) and the error in velocity measurement for small size particles, it is felt that the scatter is reasonable.
4. Discussion
There are three characteristics of an impacting particle, namely hardness, size and shape, that may affect erosion. The hardness of Sic and A1203 particles is 2480 and 2100 KHN respectively, which is much in excess of the hardness of target material (530 KHN). As such hardness is not
205 0.3
: E
0.1
42 m/r
.W ti i p 0 !i
f\ l
x ‘I + 0
0.01
1200 2400 3209 4009 f3OOQ
l
x A + 0
I __
10
20
(a)
I 30
I 40 60
2400 4000 l&m 6000 Sprn
42 ml8
400 x 420
l
sag + 1000 A
(b)
I
30 10 20 (PERIMETER)Z~~R~~.
Fig. 16. Variation of erosion rate of BNi(250) of (a) Sic, (b) A1203 and (c) SiOz particles.
(c)
I 40 60 P%
10
20
30
40 60
maraging steel with average Pz/A ratios
expected to make any difference in erosion by these two particle materials. The hardness of SiOZ particles is 820 KHN, which is about 55% higher than that of the target material. It is, therefore, reasonable to ignore the deformation of Si02 particles on impact as well. Furthermore, since SiOZ particles are fairly round, blunting due to deformation if any will have negligible effect in changing the shape of particles. The effect of size of erodent particles has been widely studied and the reported behavior [18] for all three types of particles is similar to that observed in this work (Fig. 13) for Sic and A1,03 particles. It should be pointed out that the variation of erosion rate with particle size of SiOZ particles observed in our work (Fig. 14) is different from that reported in the literature. Misra and Finnie [7] critically reviewed the explanations provided by research workers on the dependence of erosion (and also abrasion) on erodent (abrasive) particle size. Some of the explanations offered relating to erosion involve differences in ploughing, strain rate sensitivity of material, penetration to different depths composed of hard and soft layers etc., by different size particles. As for the mechanisms of erosion, most workers explain material removal by microcutting and ploughing leading to flake formation, as evidenced in our earlier work by scanning electron micrographs of maraging steel eroded surfaces as well [ 51. The role of microcutting is recognized to be significant in case of sharp particles and of ploughing in case of round particles. Thus, the shape effect has been qualitatively recognized in erosion but its verification outside the confines of model studies has not been possible because of the inability to quantify
206
angularity. The increasing erosion rate with increasing particle size of SiC and A1203 particles and the opposite variation for SiO, particles indicates that the shape of particles is equally, if not more, significant in the erosion process. As for the size effect, it is conceivable that extremely fine particles may not result in any erosion because their impact is limited to producing merely elastic deformation in surface layers. The limiting size for this condition will be governed by particle density, impact velocity and target properties. The conditions used in this work were such that significant erosion was produced by particles of all sizes. Winter and Hutchings [15] studied the effect of rake angle on ploughing and microcutting on lead and steel targets using flat-faced single particles. They concluded that microcutting was favored by positive or small values of negative rake angles while ploughing occurred with large negative rake angles. The rake angle is defined as the angle between the perpendicular to target surface and the leading edge of impacting particle, as shown in Fig. 17. When working with quantitative shape analysis of multiple particles as in this work, it is not possible to measure the angle at every corner of a particle. As such, the measure of particle angularity was obtained in terms of the W/L and/or P2/A, which is reasonable for polyhedral-shaped particles. As for the rake angle, it depends upon the shape of the particle and, in the case of angular particles, also upon its orientation to the target surface at the point of impact. For example, a spherical particle will always impact the target surface with a large negative rake angle (Fig. 17(a)) so that ploughing is the only plausible mode of deformation. However, an angular particle may impinge with either positive or negative rake angle. Considering random impact orientations, angular particles should be expected to produce both microcutting and ploughing. Furthermore, the rounder the particle, the greater will be the likelihood of ploughing (due to larger included angle at polyhedron comers) in comparison to microcutting. Examining the erosion behavior observed in this work from the viewpoint of the above analysis, it could be said that erosion by Si02 particles involves more ploughing than microcutting. This is so because these particles are very regular and circular in shape so that impingement takes place with large negative rake angles. With increasing grit size, there is a decrease in the regularity or circularity of particles so that microcutting tends to increase. It is this aspect of the erosion mechanism which accounts for the increasing erosion rate with increasing grit size (or finer particle size).
(a) Fig. 17. surface.
(a)
(0) Negative
and
(b)
positive
rake
angle
CYfor particle
impacting
on a target
207
Sic particles become more elongated and deviate more from circularity as grit size decreases. This accounts for an increasing contribution from cutting with increasing particle size. Whereas material in ploughing is merely displaced, resulting in pile-up mostly along groove edges (and is later removed during subsequent impacts), it is removed in microcutting instead of being displaced. As such the higher the contribution from cutting, the larger will be the erosion rate. This explains the variation of erosion rate with W/L and P2/Ain Figs. 15 and 16 for both Sic and A120s particles. The review of erosion models reported in the literature indicates that, except for the empirical models proposed by Head and co-workers [19 - 211, none includes particle shape factor as a variable. It is hoped that understanding of the variation of erosion with erodent particle characteristics presented in this work will help in a more realistic modeling of the erosion process in future.
5. Conclusions (1) It has been demonstrated that image analysis may be used for the characterization of erodent particles in terms of their size and meaningful shape parameters. (2) The measured area diameter values for Sic and A1203 particles were quite different from the size range specified in literature for different grits. The measured sizes of Sic and A120s particles for the same grit number were also different. (3) SiOZ particles were more regular in shape and closer to circularity than Sic and A120s particles. (4) Sic particles became more elongated and less circular with increasing size while the opposite was the case with Si02 particles. The general variation for A1203 particles was similar to that of Sic particles, though not that systematic. (5) All grits of commercial Sic and A1,03 particles contained 10 - 15% (by number) very fine particles and the size distributions were fairly broad. (6) Si02 particles separated into various grits from an ungraded batch by standard sieving procedures had much better gradation both for size and shape than the commercial Sic and Al,Os particles. (7) The erosion rate of maraging steel increased with increasing size of Sic and A1,03 particles until leveling in erosion due to the size effect occurred. Conversely, the erosion rate decreased with increasing size of Si02 particles. (8) The erosion rate increased with increasing P2/Aand decreasing W/L for all three types of erodent particles. (9) It is possible to explain the variation in erosion behavior with changes in shape factors in terms of the relative ploughing and microcutting contributions.
208
Acknowledgment We are grateful to the National work through Grant MSM-8413595.
Science Foundation
for supporting
this
References 1 H. Heywood, The origins and development of particle size analysis, in M. J. Groves and J. L. Wyatt-Sargent (eds.), Particle Size Analysis, The Society for Analytical Chemistry, London, 1970, pp. 1 - 186. 2 H. Heywood, The evaluation of powders, J. Pharm. PharmacoZ., 15 (1963) 56T. 3 I. Finnie, J. Walak and Y. Kabil, Erosion of metals by solid particles, J. Mater., 2 (1967) 682. 4 A. V. Levy, The solid particle erosion behavior of steel as a function of microstructure, Wear, 68 (1981) 269. 5 M. Naim and S. Bahadur, Effect of microstructure and mechanical properties on the erosion of lSNi(250) maraging steel, Wear, I12 (1986) 217. 6 L. Ambrosini and S. Bahadur, Erosion of AISI 4140 steel, Wear, 117 (1987) 37. 7 A. Misra and I. Finnie, On the size effect in abrasive and erosive wear, Wear, 65 (1981) 269. 8 J. E. Goodwin, W. Sage and G. P. Tilly, Study of erosion by solid particles, Proc. Inst. Mech. Eng. (London), 184 (1969/70) 279. 9 A. F. Vetter and P. A. Swanson, Particle morphology applied to characterizing abrasive materials, Particulate Sci. Technol., 1 (1983) 127. 10 J. K. Beddow and A. F. Vetter, A note on the use of classifier in morphological analysis of particulates, J. Powder Bulk Solids, 1 (1977) 42. 11 D. W. Luerkens, J. K. Beddow and A. F. Vetter, Structure and morphology - the science of form applied to particle characterization, Powder Technoi., 50 (1987) 93. 12 R. Lenth, J. K. Beddow, C. R. Chang and A. I. Vetter, Particle image analyzing system, in J. K. Beddow (ed.), Particulate Systems: Technology and Fundamentals, Hemisphere, Washington, 1983. 13 W. Zhu and Z. Y. Mao, Study of Erosion by Relatively Soft Particles, in K. C. Ludema (ed.), Wear of Materials - 1987, The American Society of Mechanical Engineers, New York, 1987, pp. 787 - 796. 14 E. Raask, Tube erosion by ash impaction, Wear, 13 (1969) 301. 15 R. E. Winter and I. M. Hutchings, Solid particle erosion studies using single angular particles, Wear, 29 (1974) 181. 16 ASTM standard test method for sieve analysis of granular metal powders. Designation: B214-86. Philadelphia, Pennsylvania, ASTM, 1986. 17 G. P. Tilly, Erosion caused by airborne particles, Wear, 14 (1969) 63. 18 G. P. Tilly, Erosion caused by impact of solid particles, in D. Scott (ed.), Treatise on Materials Science and Technology, Vol. 13, Academic Press, New York, 1979, pp. 287 - 319. 19 W. J. Head and M. E. Harr, The development of a model to predict the erosion of materials by natural contaminants, Wear, 15 (1970) 1. 20 W. J. Head, L. D. Lineback and C. R. Manning, Modification and extension of a model for predicting the erosion of ductile materials, Wear, 23 (1973) 291. 21 W. H. Jennings, W. J. Head and C. R. Manning, A mechanistic model for the prediction of ductile erosion, Wear, 40 (1976) 93.