Cavitation erosion studies of aluminum 6061-T6 in mineral oil and in ordinary tap water are presented. The maximum erosion rate (MDPR, or mean depth of penetration rate) in mineral oil was about four times that in water. The MDPR in mineral oil decreased continuously with time, but the MDPR in water remained approximately constant. The cavitation pits in mineral oil were of smaller diameter and depth than the pits in water. Treating the pits as spherical segments, we computed the radius r of the sphere. The logarithm of h/a, where h is the pit depth and 2a is the top width of the pit, was linear when plotted against the logarithm of 2r/h - 1

Buckley, D. H.

Rao, B. C. S.

English

NASA Technical Reports Server

NASA 
Technical 
Paper 
21 46 
AUGUST 1983 
ruEn 
Cavitation Pitting 
and Erosion 0.f 
Aluminum 606LT6 in 
Mineral Oil and 
Bezzam C. S. R a o  and 
Donald H. Buckley 
25th 5 Anniversary 
, ,  
1958-1983 
Water 
NASA 
TP 
2146 
c.1 
1 
I 
i 
https://ntrs.nasa.gov/search.jsp?R=19830027129 2020-03-21T01:13:04+00:00Z
TECH LIBRARY KAFB, NM 
NASA 
Technical 
Paper 
21 46 
1983 
National Aeronautics 
and Space Administration 
Scientific and Technical 
Information Branch 
0134983 
Cavitation Pitting 
and Erosion of 
Aluminum 606LT6 in 
Mineral Oil and Water 
Bezzam C. S. Rao and 
Donald H. Buckley 
Lewis  Research  Center 
Cleveland,  Ohio 
Summary 
Cavitation erosion studies on aluminum 6061-T6 in 
mineral  oil and in ordinary tap water are presented. The 
maximum  erosion rate  (MDPR,  or mean  depth  of  pene- 
tration  rate)  in  mineral  oil was about  four times that in 
water. The  MDPR in mineral  oil  decreased  continuously 
with time,  but the MDPR in  water  remained 
approximately  constant.  The  cavitation  pits  in  mineral  oil 
were of smaller  diameter and  depth  than  the pits  in  water. 
Treating  the  pits  as  spherical  segments, we computed  the 
radius r of  the sphere. The  logarithm  of hla,  where h is 
the pit depth  and  2a is the  top width  of the  pit, was linear 
when plotted  against the  logarithm  of 2r/h - 1. 
Symbols 
a pit radius  atsurface 
h pit depth 
P ( R )  pressure at bubble wall 
Pi(R)  total  vapor and gas  pressure in bubble 
PO ambient  pressure in liquid 
P, vapor  pressure  of liquid 
R bubble  radius 
r radius of cavitation  pit 
t time 
U bubble wall collapse velocity, dR/dt 
u surface  t nsion of liquid 
p viscosity of liquid 
Introduction 
Cavitation erosion in engine bearings has been of in- 
creasing  importance in the past  decade,  perhaps because 
of the design trends  toward  higher  rotational  speeds.  This 
phenomenon is predominantly  observed in diesel engine 
bearings and  on  rare occasions in gasoline engines 
operated under sustained overspeed or with incorrect 
ignition  timing  (ref. 1).  Many  examples of cavitation 
erosion  in  bearings are  reported by Wilson  (ref. 2), 
Summers-Smith  (ref. 3), Conway-Jones  (ref. 4), and 
James (ref. 5). A brief review of cavitation in bearings 
was recently published by Dowson and Taylor  (ref. 6). 
Hunt et al. (ref.  7) report a study  concerning the 
occurrence of cavitation between meshing  gear  teeth  in an 
oil-lubricated  gearbox.  Heathcock and  Protheroe (ref. 8) 
report  the possible occurrence  of  cavitation  in  gold- 
mining machines  operated with water-base  fluids, 
typically 5-percent-oil-in-water emulsions. 
In spite of the increasing importance of cavitation 
erosion of different  materials  in  oils,  not  many  studies on 
this  topic  have been reported in the  literature.  Endo et al. 
(ref. 9) report  cavitation  erosion  studies  of 0.09 percent 
carbon steel in  spindle  oil and  of tin-base  white  metal  in 
spindle  oil,  in  machine  oil,  and  in silicon oil. Garner  et  al. 
(ref. 1) studied the  erosion resistance of  some tin-, 
copper-,  and lead-base  bearing  materials  in Shell Rotella 
30 oil. Soda  and  Tanaka (ref. 10) report the cavitation 
erosion  patterns  and  the  pressure  distributions  over  the 
test specimen  surface  in S A E  20 oil. 
The  authors  are presently  studying the  cavitation 
erosion  of  different  bearing  metals  and  alloys  in  mineral 
oils.  This  report  presents the  variations  of weight loss, the 
surface  roughness, and  the pit diameter  and  depth caused 
by cavitation erosion on aluminum 6061-T6 in mineral 
oil D21 and water. 
Experimental  Equipment 
and Test Conditions 
The experiments were carried out in an ultrasonic  mag- 
netostrictive  oscillator  operating  at  20-kHz  frequency 
and 50-pm double amplitude. The material for the test 
specimens was commercially  pure  aluminum 6061-T6 
rod, 12.7 mrn in diameter.  The  mechanical  properties of 
aluminum 6061-T6 are presented in table I. The test 
liquids were mineral oil D21 and ordinary tap water, 
whose  physical properties  are given in table 11. 
TABLE I.  -MECHANICAL  PROPERTIES OF 
ALUMINUM 6061-T6 
Density, kg/m3 ........................................................ 2700 
Yield strength,  MN/m2 ................................................ 110 
Ultimate  tensile  strength,  MN/m2 .................................. 276 
Elastic  modulus,  MN/mZ ....................................... 71 x lo3 
Ultimate  resilience, MN/m2 ......................................... 0.54 
Elongation,  percent ...................................................... 12 
Hardness,  Bhn ............................................................ 95 
Nominal  composition ............. 0.6 Si, 0.25 Cu, 1.0 Mg, 0.25 Zn 
TABLE 11. -PHYSICAL  PROPERTIES OF 
MINERAL OIL D21 AND  WATER 
Property 
Density, kg/m3 
Kinematic viscosity at 20" C ,  
Surface  tension  at 20" C, 33.2  73.6 
Bulk  modulus,  MPa 1 . 7 ~  101 2 . 1 8 ~  1@ 
Flashpoint, "C ""_ 
Pour point, "C - 9.4 ""_ 
c s  
dynes/cm 
The test specimen was subjected to cavitation, and 
weight loss measurements were taken  at 5-min intervals 
for  the tests  in  mineral oil and  at 5-min intervals for  the 
first 35 min and  then  at 15-min intervals in water. 
Experimental  Results  and Discussion 
Weight Loss 
The variations of cumulative weight loss of the test 
specimen in mineral oil and in ordinary tap water are 
presented in figure 1. The  corresponding  mean  depth  of 
penetration  rates  (MDPR) in the  two  liquids  are 
presented in figure 2. As figure  1 shows,  aluminum 
6061-T6 eroded  much  faster in mineral oil than in water 
40 
30 
0 20 40 60 80 100 
Time, rnin 
Figure 1. - Cumulative  weight loss of aluminum 6061-T6 as a function 
of test  time. 
1 
100 
Figure 2 -Mean  depth of penetration  rate  (MDPR)  for  aluminum 
6061-T6 as a function of test  time. 
initially,  without  showing any  incubation  period. 
However,  as  the  test  time  increased,  the  rate  of weight 
loss (or MDPR) in mineral oil decreased continuously, 
but  the  MDPR in  water  remained  approximately 
constant. This was apparently  caused by the  rapid 
attenuation  of  pressure waves in  mineral oil as the  depth 
of  erosion  increased.  The  peak  MDPR in mineral oil was 
four  times  that in water.  The  eroded  surfaces  at  test times 
of 40 min  in  mineral oil and 90 min  in  water  are  shown  in 
figure 3. 
Surface Topography 
Surface  profiles of aluminum 6061-T6 tested in 
mineral oil for 4,20, and 40 min are  shown in figure 4. A 
la )  After  testing  for 40 rnin in mineral oil D21. 
(b) After  testing  for 90 rnin in water. 
Figure 3. - Eroded surfaces of aluminum 6061-T6 
L 
number  of  pits  of varying depths selected from  the 
profiles are  plotted  one below the  other in figure 5. The 
pit depth shown in this figure varies from  about 7.5 to 
125 pm,  and  the top width  (diameter) varies from  about 
55 to 450 pm. 
Surface  profiles  of  aluminum 6061-T6 tested  in  water 
for 20, 60, and 90 min are  shown  in  figure  6. A selected 
number of pits are plotted one below the other in fig- 
ure 7. The pit depth shown in this figure varies from 
about 7.5 to 240 pm,  and  the top width  (diameter)  varies 
from about 55 to 950 pm. A study of figures 4 to 7 
together with figure 3 indicates that erosion in mineral  oil 
is more  uniformly  distributed  over  the specimen surface 
than erosion in water. For the same erosion time, the 
erosion pits in water are about two times deeper than 
those in mineral  oil,  but fewer in number.  The  computed 
mean  depth of penetration  (MDP)  for  the specimen 
tested in water for 90 min  was 93 pm,  and  the  maximum 
pit depth measured was  250 pm.  The  MDP  for  the 
specimen tested in mineral  oil for 40 min  was  110 pm,  and 
the maximum pit depth  measured was 125 pm. 
The precise shape  of  the  erosion pit is of  importance in 
computing  the energy absorbed by the  material.  To 
obtain a physical appreciation of erosion pit shape,  some 
selected pits were plotted to a natural scale (unlike the 
distorted scale of  surface  profiles in figs. 4 to 7). Figures 
8 and 9 show these pits in mineral oil and in water, 
respectively. 
Discussion of Experimental Results 
Collapse times and pressures. - The  cavitation  bubbles 
grow or collapse  during  one-quarter cycle of  the 
oscillator, viz  within 12.5 psec. Vyas and  Preece (ref. 11) 
report that most bubbles in water collapse in about 5 
psec. It is known that viscous effects  alter  the  pressure at 
the bubble wall and thus reduce the effective pressure 
differential  and  consequently  the  rates  of  either  bubble 
growth  or collapse. The pressure P ( R )  at  the  bubble wall 
during  collapse  can  be expressed (ref. 12) as 
P ( R )  =Pi (R)  - - + ~ 20 4 p u  
R R  
where 
(a) After  testing  for 4 min. 
(b) After  testing  for 20 min. 
(c) After  testing  for 40 min. 
Figure 4. - Surface  profiles of erosion  on  aluminum 6061-T6 tested in mineral   o i l  D21. 
P i t  
Figure 5. -Topography of selected eros ion  p i ts   on 
a l u m i n u m  6061-T6 tested in m i n e r a l   o i l  021. 
3 
f25 pm 
4 
Figure 7. -Topography of selected pits of 
erosion  on aluminum 6061-T6 tested in 
water. 
(a)  After testing for 20 min. 
(b) After testing for 60 min. 
(c) After testing for 90 min. 
Figure 6. - Surface profiles of erosion on  aluminum 6061-T6 tested in water. 
P i ( R )  total  vapor  and  gas  pressure in bubble 
R bubble  radius 
u surface  t nsion  of  liquid 
p viscosity of  liquid 
U bubble wall collapse velocity, dR/dt  
t time 
The collapse pressures are generally computed  (ref. 12) 
by assuming  a  bubble  of  initial  radius Ro= 1.27 mm (50 
mils). The pressure generated by the collapse of such a 
bubble to a size R/Ro = 5 x 10-3 is 95  245 MPa (9.4 x 105 
atm). Vyas and Preece (ref. 11) measured a maximum 
stress  amplitude  of  about 700 MPa (- 7 x 103 atm). If we 
assume the same size bubble in mineral oil also, the 
viscosity term in equation (1) has  a negligible influence on 
the pressures generated.  However,  the  erosion pits clearly 
show  that  the collapsing bubbles are smaller  in  mineral 
oil than in water.  Also, the minimum  radius R to which 
the bubbles collapse would be different in mineral oil 
than in water. If a  bubble of initial  radius Ro=2.54 pm 
(0.1 mil) collapses to a size R/Ro = 1 x 10-2,  the viscosity 
effect due  to  the  third term on  the right side of equation 
(1) could  contribute  as much as 75 percent of the  total 
pressure generated. The pressure generated by such a 
bubble works out to 253 MPa (2.5 x 103 atm). Plesset 
(ref. 13) computed that neglecting surface tension and 
viscous effects  introduces  a 1-percent error  for  bubbles  of 
initial  radius R o r l  mm collapsing under  a pressure 
Po-p,?O.3 MPa.  The larger  initial weight loss rate  and 
the smaller pits in mineral oil observed in the present 
studies indicate that cavitation  bubbles in mineral oil are 
smaller and have  perhaps higher collapse pressures than 
cavitation  bubbles in water. 
Nature of cavitationpits. - A  cavitation pit is generally 
assumed to be  a segment of  a  sphere, as shown in figure 
10. If 2a is the  chord  diameter  and h is the pit depth,  the 
radius r of the  sphere  can be expressed as 
Using equation (2), we computed  the  radii of  the 
different pits and drew the spherical segments on the 
corresponding pits in figures 8 and 9.  These figures show 
that  the volume of the pits would be overestimated if it 
were calculated by treating them as spherical segments. 
4 
I I 
Figure 8. - Shape of pits on aluminum 6061-T6 
tested in mineral oil 021. 
The pits, in general, have a deep central portion with 
shallow, flat portions on either side. 
Also, we can express 
or 
logh+50g(; -1)=0 a 2  
Figure 1 1  shows the logarithm of h/a  to be linear when 
plotted  against  the  logarithm of 2r/h - 1 .  The 
experimental points show a slope of - 1/2 because the 
++ 
' 0.1 mm (pits 4, 6) 
,)LO. 02 mm (pits 1, 2); 
0.2 mm (pits 8, 9 )  
Pit 
I 
1 
I 1 
Figure 9. - Shape of pits on aluminum 6061-T6 
tested in water. 
Figure 10. - Theoretical cavitation pit. 
5 
. 2  c 
Test liquid 
0 Water 
0 Mineral  oil 
Ql 
Figure 11. - Variation of ratio hla with 2rlh - 1, where h is pit depth. 
a is pit radius at surface, and r is  radius of sphere. 
value of r was computed from the values of h/a. The 
values of  h/a are  of significance in figure 11. In mineral 
oil,  the h/a  of  individual  pits varied from 0.125 to 0.400; 
in water, it varied from 0.075 to 0.260. In other  words, 
for  the same depth,  the pits were wider in water than  in 
mineral oil. This perhaps is due  to  the larger microjets 
striking  the  surface in water than in mineral oil indicating 
that  cavitation  bubbles grow to a  larger size  in water than 
in mineral oil. 
Considering pits of depth less than 15 pm only, we 
found  that in mineral oil 
h 
- =0.312 
a 
and in water 
Previous pit measurements  of  cavitation  erosion in water 
by Robinson and Hammitt (ref. 14) indicated the ratio 
h/a  to be 0.20. Stinebring et al.  (ref. 15), using a 
microscope and collimated light beam, determined the 
minimum and maximum values of h/a  to be 0.068 and 
0.333, respectively, in water.  This  range  of h/a  values is 
in agreement with the present measurements. Equation 
(6) agrees closely with the  measurements  of  Robinson  and 
Hammitt. 
Summary of Results 
Cavitation erosion studies on aluminum 6061-T6 in 
minera! oil D21 and water had  the following results: 
1. The maximum weight loss rate, or mean depth of 
penetration rate (MDPR), in mineral oil D21 was four 
times that in water. However,  the MDPR decreased 
continuously with further test time in mineral oil. 
2. The  cavitation  erosion pits were of  smaller  diameter 
in mineral oil D21 than the pits in water for the same 
depth.  The pits  in  general have a deeper central  portion 
with shallow  flat  portions on either side. 
3. The ratio of pit depth to pit radius at the surface 
h/a  of individual pits varied from 0.125 to 0.400 in 
mineral oil and  from 0.075 to 0.260 in water.  Considering 
pits of depth less than 15 pm only, we found that in 
mineral oil h/a=0.312  and in water h/a=0.220. 
Lewis Research Center 
National  Aeronautics and Space  Administration 
Cleveland, Ohio, March 18, 1983 
References 
1. Garner,  D.  R.;  James,  R.  D.;  and  Warriner,  J. F.: Cavitation 
Erosion  Damage in Engine  Bearings:  Theory  and  Practice.  J.  Eng. 
Power, vol. 102, no. 4, Oct. 1980, pp. 847-857. 
2. Wilson, R. W.: Cavitation Damage in Plain Bearings. Cavitation 
and Related Phenomena in Lubrication. D. Dowson, M. Godet, 
and  C.  M.  Taylor,  eds.,  Institution  of  Mechanical  Engineers 
(London), 1975, pp. 177-184. 
3. Summers-Smith,  D.:  Discussion  on  Cavitation  Damage.  Cavitation 
and  Related  Phenomena.  D.  Dowson,  M.  Godet,  and  C.  M. 
Taylor,  eds.,  Institution of Mechanical  Engineers  (London),  1975, 
4. Conway-Jones, J .  M.:  Discussion on  Cavitation  Damage. 
Cavitation  and  Related  Phenomena.  D.  Dowson,  M.  Godet  and  C. 
M. Taylor, eds., Institution of Mechanical Engineers (London), 
5. James, R. D.: Erosion Damage in Engine Bearings. Tribol. Int., 
vol. 8, no. 4, 1975, pp. 161-170. 
6. Dowson, D.; and Taylor, C. M.: Cavitation in Bearings. Annual 
Review of  Fluid  Mechanics.  Vol. 11. M.  Van  Dyke, J .  V. 
Wehausen, and J. L. Lumley, eds., Annual Reviews, Inc., 1979, 
7. Hunt,  J. B.; Ryde-Weller, A.  J.; and  Ashmead,  F.  A.  H.: 
Cavitation Between Meshing  Gear  Teeth.  Wear,  vol.  71, 1981, pp. 
8. Heathcock, C. J.; and Protheroe, B. E.: Cavitation Erosion of 
Stainless Steels. Wear, vol. 81, 1982, pp. 311-327. 
9. Endo, K . ;  Okada, T.; and Nakashima, M.: A Study of Erosion 
Between Two Parallel Surfaces Oscillating at Close Proximity in 
Liquids. J.  Lubr.  Technol.,  vol.  89,  no.  3,  July  1967,  pp. 229-236. 
10. Soda,  N.;  and  Tanaka, Y.: Cavitation  Erosion  of  Flat  Surface  Near 
Rotating  Shaft.  Wear  of  Materials 1981. S. K .  Rhee,  A.  W.  Ruff, 
and K .  C. Ludema, eds., ASME, 1981, pp. 607-612. 
11. Vyas, B.; and  Preece,  C.  M.:  Stress  Produced  in  a Solid  by 
Cavitation.  J.  Appl.  Phys.,  vol.  47,  no.  12,  Dec. 1976, pp. 
12. Ivany, R. D.; and Hammitt, F. G.: Cavitation Bubble Collapse in 
Viscous Compressible  Liquids-Numerical  Analysis. J. Basic 
Eng., vol, 87, no. 4, Dec. 1965, pp. 977-985. 
13. Plesset, M. S.: Bubble  Dynamics.  Cavitation  in  Real  Liquids. 
Robert  Davies,  ed., Elsevier Publishing  Company, 1964, pp. 1-18. 
14. Robinson, M. J.; and Hammitt, F. G.: Detailed Damage Char- 
teristics  in  a  Cavitating  Venturi. J .  Basic Eng., vol. 89,  no. 1, Mar. 
15. Stinebring,  D.  R.;  Holl,  J.  W.;  and  Arndt,  R.  E. A.: Two Aspects 
of Cavitation  Damage  in  the  Incubation Zone-Scaling by  Energy 
Considerations and Leading Edge Damage. J. Fluids Eng., vol. 
102, no.  4, Dec. 1980, pp. 481-485. 
pp. 198-199. 
1975, pp. 201-217. 
pp. 35-66. 
65-78. 
5133-5138. 
1967, pp. 161-173. 
6 
1. Report  No. I 2.  Government Accession No, I 3. Recipient's  Catalog No. 
NASA TP-2146 1 
4. Title and Subtitle 5. Report  Date 
CAVITATION PITTING AND EROSION OF ALUMINUM 6061-T6 IN 
MINERAL  OIL AND WATER 
506-53-1B 
7.  Author(s) I 8. Performing  Organization Report  No. 
Bezzam C. S. Rao and Donald H. Buckley 
- 
9. Performing Organization Name and Address 
~. 
National Aeronautics and Space  Administration 
Lewis Research  Center 
Cleveland, Ohio  44135 
12. Sponsoring Agency Name and  Address 
National Aeronautics and Space  Administration 
Washington, D. C. 20546 
I E-1516 
-1 10. Work Unit  No. 
I 
11. Contract  or Grant No. 
I Technical Paper 
14. Sponsoring Agency Code 
1 
15. Supplementary  Notes 
Bezzem C. S. Rao, Indian Institute of Science, Bangalore, India, and National Research Council - NASA 
Research Associate; Donald H. Buckley, Lewis  Research  Center.  presented  in  part at ASME 
Cavitation and Polyphase Flow Forum, Houston, Texas, June 20-22, 1983. 
" 
16. Abstract 
Cavitation erosion  studies of aluminum  6061-T6 i n  mineral  oil and in  ordinary tap water are presented. 
The  maximum erosion rate (MDPR, o r  mean depth of penetration  rate)  in  mineral  oil was  about four 
times that in  water.  The MDPR in mineral  oil  decreased continuously with time, but  the MDPR in  water 
remained  approximately  constant.  The  cavitation  pits  in  mineral  oil  were of smaller  diameter and depth 
than the  pits in water. Treating  the  pits  as  spherical  segments, we computed the  radius r of the  sphere. 
The logarithm of h/a, where h is the pit depth and 2a is the top width of the pit, was linear when 
17. Key Words  (Suggested by Author(s1) 
Cavitation erosion Aluminum 
Mineral  oil Pitting 
Water Lubrication 
J 
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Cavitation pitting and erosion of aluminum 6061-T6 in mineral oil water

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