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Influence of atmospheric humidity on abrasive wear—i. 3-body abrasion
313
Wear, 31 (1975) 373-379
(1. Elsevier Sequoia
S.A.. Lausanne
Printed
in The Netherlands
INFLUENCE OF ATMOSPHERIC 3-BODY ABRASION
HUMIDITY
ON ABRASIVE WEAR-I.
J. LARSEN-BASSE Depurtment
(Received
of Mechanical
September
Engineering,
Universiry
of Hawaii,
Honolulu.
Hawaii
96822
(C.S. A.)
IO, 1974)
SUMMARY
Abrasive wear testing in 3-body abrasion by SIC abrasives was performed for a number of materials under controlled levels of atmospheric humidity. Under the experimental conditions’used, the wear rate increases sharply with humidity above ambient conditions while there is little effect below this point. The rate of increase is independent of backing wheel material, except for a wheel of low hardness and friction. Surface topography studies show that the effect is primarily due to moistureassisted fracture of the abrasive grains, which brings more and sharper cutting asperities into contact with the abrading surface. Metals which have a tightly adhering, strong oxide film show an additional wear increase with humidity. This is attributed to moisture-enhanced oxide deformation and fracture, resulting in enhanced deformation of the surface layers.
INTRODUCTION
Some wear data have been reported for copper abraded under controlled levels of atmospheric humidity’. The wear rate increased rapidly in the high humidity range while there was little effect below ambient conditions, Fig. 1. Testing was performed in 3-body abrasion with a polyurethane backing wheel. Similar curves were obtained for SIC and Al,03 abrasives. It was noticed that the abrasive powder in the hopper required several days’ to reach its equilibrium cutting conditions at a new humidity level, as also illustrated in Fig. 1, and that the abraded surface exhibited slight changes in topography with changing humidity. The indentations and occasional grooves formed by the abrasives were narrower and more numerous at high humidity and the grooves were longer. Further experiments showed that fracturing of SIC abrasives in shear was much more pronounced at high humidity. It was concluded that a major reason for the large effect of humidity on wear rate is moisture-assisted fracture of the abrasive grains. This phenomenon could contribute to the increase in wear at high humidity in two ways: (1) large abrasive grains, which otherwise may carry a major part ofthe applied load, fracture easily at high humidity and as a result more abrasive grains come into
771
I
( -... 0
Cu/SiC
F@.
I.
. .._I
I IO
Partial
mdicate
120
pressure
of water
vapor,
Influence of atmospheric direction
of humidity
mm
$0 ttg
humidity and of equalization time on abrasive wear of copper. Arrows change and the numbers show days elapsed since conditions were changed.
contact
with the metal surface, and (2) “self-sharpening” of cutting abrasives becomes possible and this may result in an increase of the amount of metal removed by each. It is not inconceivable that other factors also play a role. For example, there could be contributing effects from lubrication due to adsorbed moisture. as proposed by Rabinowicz et ul.‘, from Rebinder effects3 in the metal, from oxide changes, etc. Further work is necessary in order to clarify this. The purpose of this paper is to report some additional wear data which show that the moisture effect is common to a number of combinations of backing wheel and abrading material. EXPERIMEN.rAL
PROCEDIJRE
Cylindrical specimens of 6.4 mm diam. were pressed axially against the perimeter of a broader backing wheel. The applied load was 1.77 kg (55 g/mm2) and the surface velocity was 12.6 cm!s. A continuous stream of SIC abrasive grit. nominal size 120 pm, was supplied from a hopper to the contact area. For each condition the specimen was first “run in” for a period of 5-10 min and then the weight loss was determined for a testing period of 15.-30 min. The equipment was enclosed in a small chamber in which the humidity could be controlled by the use ofvarious salts: 10-l 5% r.h.. silica gel: 25-35% r.h., potassium sulfate solution ; acetate ; 45-65”/0 r.h., ambient conditions: 78-85, “/(, r.h., ammonium 98%loOo/;, r.h., distilled water. At each humidity level the abrasive powder was kept in the chamber for a period of 4 5 days before any tests were performed. This was done in order to assure equilibrium between the abrasive and the environment. When only specimen or wheel were changed, a waiting period of tl h was sufficient. The abraded materials are listed below : Zinc, commercial purity (99.97:); Nickel, annealed electrolytic cathode (99.95%); Ala.7 Mg-0.4 Si alloy, heat treated (6063-T6);
EFFECT
OF HUMIDITY
ON ABRASIVE
375
WEAR-I
Carbon steels, cold drawn, 0.18% C and 0.40% C; Borosilicate (Pyrex) glass. More tests were performed with a polyurethane backing wheel but in some experiments backing wheels of plexiglass (polymethyl methacrylate), polypropylene, or hot rolled low carbon steel were used. RESULTS
AND
DISCUSSION
The wear data are plotted in Figs. 2 and 3. All materials tested show a rapid increase in wear rate when the absolute humidity increases above 10 mm Hg, which roughly corresponds to normal ambient conditions. There is little change below this point. The wear resistance at zero humidity is proportional to the specimen bulk hardness, Fig. 4. It is possible that some of the scatter generally seen in this type of plot is due to use of data obtained at ambient conditions, where humidity may affect the wear rate of different types of materials to differing degrees. The humidity-induced wear increase, taken as the slope of the curve at high humidity, is proportional to the wear rate at low humidity, Fig. 5. A different line is obtained for each of the two groups of materials, zinc, aluminum and glass in one group and copper, nickel and carbon steels in the other. This shows that the materials in the former group are affected more by water vapor than are the metals in the latter group. Glasses and other ceramic materials are known to be susceptible to moistureassisted deformation and crack propagation, often called static fatigue. For zinc and aluminum one would expect deformation of the surface layers, as it occurs in abrasion, to depend rather strongly on the strength ofthe tightly adhering oxide film. This should not be the case for nickel and carbon steels, where oxide films form more slowly, are c 6063-T6
Br
0’ 0
’
PHeo.
’ IO
’
j
’ 20
OO-
20
mm Hg
PntO.
Fig. 2. Wear rate us. humidity
for zinc, aluminum
alloy and borosilicate
glass.
Fig. 3. Wear rate us. humidity
for nickel and 0.18% C and 0.40% C steels.
mm Hg
376
J. LARSEN-RAW!
01' 0
50
100
150
Hardness, DPN, kg/mm2
Fig. 4. Abrasion
resistance
at 0% r.h. vs. bulk hardness.
1.0.
,"/6063
0
i
OB-
;
_’
E
/ /’
2.
n\ 6 0.6.
%
/,’
E
/
:: f
0.4-
0.21
, #Pyrex Ni 0,
1’
-06”
,’ “COIFJ _-TG40 I 0 i-_-_
0
I 2 Wear Rate at 0% r.h.. mm3/hr
Fig. 5. Rate of wear increase
with humidity
3
above ambient
conditions
us. wear rate at 0% r.h.
not strongly adherent, and do not have any particular strength. Thus, it is not unexpected that zinc, aluminum and glass show a stronger dependence of wear on humidity than do nickel and carbon steels. For the latter group of materials the effect must primarily be due to changes either in the wheel-abrasive interaction or in the metal removal efficiency of the abrasives. Experiments with various backing wheels were performed. The results for nickel are shown in Fig. 6. Similar curves were obtained for the aluminum alloy and the 0.18% C steel. It is seen that for three of the four wheel materials used the rate of increase in wear of the nickel at high humidity is essentially independent of backing wheel. This indicates that the humidity effect is related to changes in abrasive-metal interaction rather than to changes in the abrasive-wheel interaction. The difference
EFFECT OF HUMIDITY
377
ON ABRASIVE WEAR-I
s t t , ~‘~Plai*la** -_o_----__._/
1
01 0
NV120 Sic 1
1
h
’
0
IO
PII.+
1
’
1
’
20
mm Ha
Fig. 6. Wear rate vs. humidity for nickel, using various backing wheel materials. TABLE I PROPERTIES
OF PLASTICS BACKING WHEELS 60% r.h. Penetration (mm) IO mm ball
Friction vs. steel relative values (contact pressure 3-13 g/mm’)
Density
Plexiglass Polyurethane Polypropylene
(glcm3)
62.5 (kg) major load
10 (kg) minor load
10% r.h.
60% r.h.
100% r.h.
1.15 1.05 0.90
0.170 0.488 0.476
0.146 0.302 0.252
0.45 0.94 0.33
0.42 0.92 0.32
0.60 1.10 0.37
in wear rate at any given humidity appears to be mainly due to differences in wheel hardness, harder wheels giving higher wear rates. It is not quite clear why the polypropylene wheel gives deviating behavior. At ambient conditions it has approximately the same hardness as the polyurethane wheel and it shows the same relative increase in friction at high humidity as the other wheels, Table I. However, its coefficient of friction is consistently much below the value for the other polymeric wheel materials and it would appear that the deviating results obtained with the polypropylene wheel are due to a combination of low hardness and low friction. A study of the abraded metal surfaces showed that for the polypropylene wheel the abrasives tended to roll more than for the other wheels and to form a “washboard” surface with rounded ridges and grooves in the direction of sliding, Fig. 7(a). For the other wheel materials this effect was rarely observed and the abraded
378
.I. LARSEN-BASSE
Fly. 7. Abraded surfaces. (a) Nickel run I‘S.polyprl 3pyleae at 4 mm Hg. (b) Aluminum at 16 mm Hg. (150x \
alloy run P.S.plexiglass
Fig. 8. Scanning r.h. (1500 x )
(a) 30”,,, r.h. (b) 100” ,,
electron
microscopy
of copper
surfaces,
abraded
vs. polyurethane.
surfaces showed a uniform distribution of craters and a few short grooves, Fig. 7(b). Scanning electron microscopy indicated considerable change in microtopography of the abraded surfaces with changes in humidity. The indentations formed by the abrasive grits are larger, more widely scattered, less uniform in size, and relatively shallower at low humidity, and some smearing is observed, Fig. 8. It
EFFECT
TABLE
OF HUMIDITY
ON ABRASIVE
379
I
WEAR
II
SIEVE ANALYSES
OF GRIT
Siere opening
9
120 Sic USED IN ABRADING
NICKEL
(STEEL BACKING
WHEEL)
of sample retained
I/cm) Unused
124 75 61 Bottom -.
35.3 60.7 2.0 1.7
pan
Used at
Used ar
Used ar
/Ol$
60”/,
1OO’;b r.h.
r.h.
7.8 85.6 3.4 3.0
7.4 85.3 3.5 3.4
r.h.
0 90.5 4.3 4.9
appears that the abrasives fracture more easily at high humidity and that this results in improved material removal efftciency because more and sharper grits indent the surface and because the smearing effect of large grits disappears. The effect of high humidity levels on fracture of the Sic abrasives was confirmed by sieve analyses of used grits, Table II. CONCLUSIONS
The previously reported rapid increase in 3-body abrasive wear rates at high humidity has been confirmed for several combinations of backing wheel and abrading material. The effect is due to a number of factors. Most important of these is moistureassisted fracture of the abrasives which results in more efftcient material removal. Another factor is moisture-enhanced oxide deformation, and thus metal surface deformation, in metals with a strong, tightly adhering oxide layer. ACKNOWLEDGEMENTS
This work was supported by the National Science Foundation GK-13685.
under Grant
REFERENCES 1 J. Larsen-Basse,
humidity on abrasive wear, Proc. 1971 Int. Con& ,Mechanical III (1972) 353-362. 2 E. Rabinowicz, L. A. Dunn and P. G. Russell, A study of abrasive wear under three-body conditions, Wear, 4 (1961) 345-355. 3 V. I. Likhtman, P. A. Rebinder and G. V. Karpenko, Eficr ofsurface-Actice Media on the Dejbrmation 01 Metals, Chemical Publ. Co., New York, 1960. Behavior
ENect of atmospheric
of Materials,
Kyoto,
Japan,
Wear, 31 (1975) 373-379
(1. Elsevier Sequoia
S.A.. Lausanne
Printed
in The Netherlands
INFLUENCE OF ATMOSPHERIC 3-BODY ABRASION
HUMIDITY
ON ABRASIVE WEAR-I.
J. LARSEN-BASSE Depurtment
(Received
of Mechanical
September
Engineering,
Universiry
of Hawaii,
Honolulu.
Hawaii
96822
(C.S. A.)
IO, 1974)
SUMMARY
Abrasive wear testing in 3-body abrasion by SIC abrasives was performed for a number of materials under controlled levels of atmospheric humidity. Under the experimental conditions’used, the wear rate increases sharply with humidity above ambient conditions while there is little effect below this point. The rate of increase is independent of backing wheel material, except for a wheel of low hardness and friction. Surface topography studies show that the effect is primarily due to moistureassisted fracture of the abrasive grains, which brings more and sharper cutting asperities into contact with the abrading surface. Metals which have a tightly adhering, strong oxide film show an additional wear increase with humidity. This is attributed to moisture-enhanced oxide deformation and fracture, resulting in enhanced deformation of the surface layers.
INTRODUCTION
Some wear data have been reported for copper abraded under controlled levels of atmospheric humidity’. The wear rate increased rapidly in the high humidity range while there was little effect below ambient conditions, Fig. 1. Testing was performed in 3-body abrasion with a polyurethane backing wheel. Similar curves were obtained for SIC and Al,03 abrasives. It was noticed that the abrasive powder in the hopper required several days’ to reach its equilibrium cutting conditions at a new humidity level, as also illustrated in Fig. 1, and that the abraded surface exhibited slight changes in topography with changing humidity. The indentations and occasional grooves formed by the abrasives were narrower and more numerous at high humidity and the grooves were longer. Further experiments showed that fracturing of SIC abrasives in shear was much more pronounced at high humidity. It was concluded that a major reason for the large effect of humidity on wear rate is moisture-assisted fracture of the abrasive grains. This phenomenon could contribute to the increase in wear at high humidity in two ways: (1) large abrasive grains, which otherwise may carry a major part ofthe applied load, fracture easily at high humidity and as a result more abrasive grains come into
771
I
( -... 0
Cu/SiC
F@.
I.
. .._I
I IO
Partial
mdicate
120
pressure
of water
vapor,
Influence of atmospheric direction
of humidity
mm
$0 ttg
humidity and of equalization time on abrasive wear of copper. Arrows change and the numbers show days elapsed since conditions were changed.
contact
with the metal surface, and (2) “self-sharpening” of cutting abrasives becomes possible and this may result in an increase of the amount of metal removed by each. It is not inconceivable that other factors also play a role. For example, there could be contributing effects from lubrication due to adsorbed moisture. as proposed by Rabinowicz et ul.‘, from Rebinder effects3 in the metal, from oxide changes, etc. Further work is necessary in order to clarify this. The purpose of this paper is to report some additional wear data which show that the moisture effect is common to a number of combinations of backing wheel and abrading material. EXPERIMEN.rAL
PROCEDIJRE
Cylindrical specimens of 6.4 mm diam. were pressed axially against the perimeter of a broader backing wheel. The applied load was 1.77 kg (55 g/mm2) and the surface velocity was 12.6 cm!s. A continuous stream of SIC abrasive grit. nominal size 120 pm, was supplied from a hopper to the contact area. For each condition the specimen was first “run in” for a period of 5-10 min and then the weight loss was determined for a testing period of 15.-30 min. The equipment was enclosed in a small chamber in which the humidity could be controlled by the use ofvarious salts: 10-l 5% r.h.. silica gel: 25-35% r.h., potassium sulfate solution ; acetate ; 45-65”/0 r.h., ambient conditions: 78-85, “/(, r.h., ammonium 98%loOo/;, r.h., distilled water. At each humidity level the abrasive powder was kept in the chamber for a period of 4 5 days before any tests were performed. This was done in order to assure equilibrium between the abrasive and the environment. When only specimen or wheel were changed, a waiting period of tl h was sufficient. The abraded materials are listed below : Zinc, commercial purity (99.97:); Nickel, annealed electrolytic cathode (99.95%); Ala.7 Mg-0.4 Si alloy, heat treated (6063-T6);
EFFECT
OF HUMIDITY
ON ABRASIVE
375
WEAR-I
Carbon steels, cold drawn, 0.18% C and 0.40% C; Borosilicate (Pyrex) glass. More tests were performed with a polyurethane backing wheel but in some experiments backing wheels of plexiglass (polymethyl methacrylate), polypropylene, or hot rolled low carbon steel were used. RESULTS
AND
DISCUSSION
The wear data are plotted in Figs. 2 and 3. All materials tested show a rapid increase in wear rate when the absolute humidity increases above 10 mm Hg, which roughly corresponds to normal ambient conditions. There is little change below this point. The wear resistance at zero humidity is proportional to the specimen bulk hardness, Fig. 4. It is possible that some of the scatter generally seen in this type of plot is due to use of data obtained at ambient conditions, where humidity may affect the wear rate of different types of materials to differing degrees. The humidity-induced wear increase, taken as the slope of the curve at high humidity, is proportional to the wear rate at low humidity, Fig. 5. A different line is obtained for each of the two groups of materials, zinc, aluminum and glass in one group and copper, nickel and carbon steels in the other. This shows that the materials in the former group are affected more by water vapor than are the metals in the latter group. Glasses and other ceramic materials are known to be susceptible to moistureassisted deformation and crack propagation, often called static fatigue. For zinc and aluminum one would expect deformation of the surface layers, as it occurs in abrasion, to depend rather strongly on the strength ofthe tightly adhering oxide film. This should not be the case for nickel and carbon steels, where oxide films form more slowly, are c 6063-T6
Br
0’ 0
’
PHeo.
’ IO
’
j
’ 20
OO-
20
mm Hg
PntO.
Fig. 2. Wear rate us. humidity
for zinc, aluminum
alloy and borosilicate
glass.
Fig. 3. Wear rate us. humidity
for nickel and 0.18% C and 0.40% C steels.
mm Hg
376
J. LARSEN-RAW!
01' 0
50
100
150
Hardness, DPN, kg/mm2
Fig. 4. Abrasion
resistance
at 0% r.h. vs. bulk hardness.
1.0.
,"/6063
0
i
OB-
;
_’
E
/ /’
2.
n\ 6 0.6.
%
/,’
E
/
:: f
0.4-
0.21
, #Pyrex Ni 0,
1’
-06”
,’ “COIFJ _-TG40 I 0 i-_-_
0
I 2 Wear Rate at 0% r.h.. mm3/hr
Fig. 5. Rate of wear increase
with humidity
3
above ambient
conditions
us. wear rate at 0% r.h.
not strongly adherent, and do not have any particular strength. Thus, it is not unexpected that zinc, aluminum and glass show a stronger dependence of wear on humidity than do nickel and carbon steels. For the latter group of materials the effect must primarily be due to changes either in the wheel-abrasive interaction or in the metal removal efficiency of the abrasives. Experiments with various backing wheels were performed. The results for nickel are shown in Fig. 6. Similar curves were obtained for the aluminum alloy and the 0.18% C steel. It is seen that for three of the four wheel materials used the rate of increase in wear of the nickel at high humidity is essentially independent of backing wheel. This indicates that the humidity effect is related to changes in abrasive-metal interaction rather than to changes in the abrasive-wheel interaction. The difference
EFFECT OF HUMIDITY
377
ON ABRASIVE WEAR-I
s t t , ~‘~Plai*la** -_o_----__._/
1
01 0
NV120 Sic 1
1
h
’
0
IO
PII.+
1
’
1
’
20
mm Ha
Fig. 6. Wear rate vs. humidity for nickel, using various backing wheel materials. TABLE I PROPERTIES
OF PLASTICS BACKING WHEELS 60% r.h. Penetration (mm) IO mm ball
Friction vs. steel relative values (contact pressure 3-13 g/mm’)
Density
Plexiglass Polyurethane Polypropylene
(glcm3)
62.5 (kg) major load
10 (kg) minor load
10% r.h.
60% r.h.
100% r.h.
1.15 1.05 0.90
0.170 0.488 0.476
0.146 0.302 0.252
0.45 0.94 0.33
0.42 0.92 0.32
0.60 1.10 0.37
in wear rate at any given humidity appears to be mainly due to differences in wheel hardness, harder wheels giving higher wear rates. It is not quite clear why the polypropylene wheel gives deviating behavior. At ambient conditions it has approximately the same hardness as the polyurethane wheel and it shows the same relative increase in friction at high humidity as the other wheels, Table I. However, its coefficient of friction is consistently much below the value for the other polymeric wheel materials and it would appear that the deviating results obtained with the polypropylene wheel are due to a combination of low hardness and low friction. A study of the abraded metal surfaces showed that for the polypropylene wheel the abrasives tended to roll more than for the other wheels and to form a “washboard” surface with rounded ridges and grooves in the direction of sliding, Fig. 7(a). For the other wheel materials this effect was rarely observed and the abraded
378
.I. LARSEN-BASSE
Fly. 7. Abraded surfaces. (a) Nickel run I‘S.polyprl 3pyleae at 4 mm Hg. (b) Aluminum at 16 mm Hg. (150x \
alloy run P.S.plexiglass
Fig. 8. Scanning r.h. (1500 x )
(a) 30”,,, r.h. (b) 100” ,,
electron
microscopy
of copper
surfaces,
abraded
vs. polyurethane.
surfaces showed a uniform distribution of craters and a few short grooves, Fig. 7(b). Scanning electron microscopy indicated considerable change in microtopography of the abraded surfaces with changes in humidity. The indentations formed by the abrasive grits are larger, more widely scattered, less uniform in size, and relatively shallower at low humidity, and some smearing is observed, Fig. 8. It
EFFECT
TABLE
OF HUMIDITY
ON ABRASIVE
379
I
WEAR
II
SIEVE ANALYSES
OF GRIT
Siere opening
9
120 Sic USED IN ABRADING
NICKEL
(STEEL BACKING
WHEEL)
of sample retained
I/cm) Unused
124 75 61 Bottom -.
35.3 60.7 2.0 1.7
pan
Used at
Used ar
Used ar
/Ol$
60”/,
1OO’;b r.h.
r.h.
7.8 85.6 3.4 3.0
7.4 85.3 3.5 3.4
r.h.
0 90.5 4.3 4.9
appears that the abrasives fracture more easily at high humidity and that this results in improved material removal efftciency because more and sharper grits indent the surface and because the smearing effect of large grits disappears. The effect of high humidity levels on fracture of the Sic abrasives was confirmed by sieve analyses of used grits, Table II. CONCLUSIONS
The previously reported rapid increase in 3-body abrasive wear rates at high humidity has been confirmed for several combinations of backing wheel and abrading material. The effect is due to a number of factors. Most important of these is moistureassisted fracture of the abrasives which results in more efftcient material removal. Another factor is moisture-enhanced oxide deformation, and thus metal surface deformation, in metals with a strong, tightly adhering oxide layer. ACKNOWLEDGEMENTS
This work was supported by the National Science Foundation GK-13685.
under Grant
REFERENCES 1 J. Larsen-Basse,
humidity on abrasive wear, Proc. 1971 Int. Con& ,Mechanical III (1972) 353-362. 2 E. Rabinowicz, L. A. Dunn and P. G. Russell, A study of abrasive wear under three-body conditions, Wear, 4 (1961) 345-355. 3 V. I. Likhtman, P. A. Rebinder and G. V. Karpenko, Eficr ofsurface-Actice Media on the Dejbrmation 01 Metals, Chemical Publ. Co., New York, 1960. Behavior
ENect of atmospheric
of Materials,
Kyoto,
Japan,