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Lubricated friction and wear simulation of multi-scratches in asperity-asperity contact
WEAR ELSEVIER
Wear 217 (11198) 111-I-109
Lubricated friction and wear simulation of multi-scratches in asperity-asperity contact YingiongWanga., Stephen M. Hsu b Iomega. 8fX~Tusman Drive, Milpitas, CA 95035. USA h Ceramics Dil'ision. National Institute qfStundards and Techmdogy, Gaithersburg. MD 20899, USA
Received 5 November 1997: accepted 9 January 1998
Abstract
Multi-~ratch tests of ball-on-ball ( 3.175 mm diameter 52100 steel) in lubrication were carried out to simulate the friction and wear process of the asperity-asperity contact in sliding. The results show that the friction coefficient increases with the increased scratch number from about O.I at the Ist scratch to about 0.4 at the 40th scratch. There is no plastic deformation and wear on the contact surface for the Ist scratch. However. friction coefficient rises to 0.15 at the 5th scratch and abrasive wear occurs in the contact area. Then. with the scratch number increased further, the friction coefficient continues to increase and the contact surface undergoes different wear stages: plastic smearing ( 10th ,,a:ratch), surface flaw initiation (20th scratch), and tensile crack initiation and propagation (40th scratch). © 1998 Elsevier Science S.A. All rights re,fred, K¢'~3i'ords: Asperity Contact: Lubrication: Wear: Collislom Scratch
!. Introduction
Friction, wear and lubrication are important issues in mechanical systems with moving surfaces. For most mechanical applications, lubrication is provided to minimize friction and wear of the material. Since manufactured surfaces will always have a degree of roughness, the contact between moving surfaces becomes contact between asperities from the two surfaces. Some r e . a r c h e r s I 1,21 suggest that only a very small percentage of the apparent contact area is in real con tact. Some experimental results reveal that the real contact area could be large depending on the conformity of the two surfaces 13]. Based on experimental observation, for normal manufacturing process, an asperity size is inversely proportional to the hardness of the material 14 I. The sliding process of two contact surfaces is a process of repeated scratches between local asperities of the two surfaces. The asperity-asperity elastohydrodynamic lubrication has been investigated in recent years 15-81. However. previous studies summarized in tribological books 19-131 dealt with little on how the tribological process is going on at a asperity-asperity contact in boundary lubrication from the Isl scratch to a number of scratches. Therefore. what really * C~nxcsponding author. E-mail: wangly@'iomcga.com (IO-1.3-1648/98/St9.00 ,'f~ 1998 El.~vier Science S.A, All rights reserved. PIi S 0 0 4 3 - 1 6 4 8 ( 98 t tit} 149-5
happens to the asperity-asperity contact with increased scratch numbers in boundary lubrication remains unclear. Not all sliding tribological pairs are designed to operate in the presence of a generous supply of lubricant. The nature of the working environment may make it impossible, or impracticable, to arrange for the contact to be lubricated by a full hydrostatic or hydrodynamic fluid film, for example, in satellit¢ application where any liquid lubricant would be lost or degraded by evaporation, or in food processing or chemical
¢~uiclliIml/l"~
I/ l l i i i l ' - - ! 1 1 111 Fig. I. The used Iwo-ba|l collision apparalus.
¥. Wun~t. S.M. H.m / Weur 217 ( l ~ )
104-109
105
iJ (a)
GO
0-1
02
03
0.4
G5
Scratch Distance (men) Fig. -.2.The normal force F,,. tangential force F,. friction coefficient F,IF,, vs. ~ratch distance (a) and the a~.~,nci~d ¢ o ~ c l .qa'fac¢ (5) for the kit .v,cratch. The surface is well pzotected by ~uhrication.
G~
250
~
200
.0.30 "g"
,¢rMd*
-0.25
z~ 1.o
-o~
100
- 0.15
u
- (110
50
0
(a)
O.flO
i GO
0.1
0 i2
G3 '
0.4
0.S
Sc,,-~h Distance IN)
Fig. 3, The normal force F,. tangential force F,. friction c~rfficiem F,/F. vs..~oratch di.~Imce and the as.~ociaP.-dcontact .',mrfaoefor the ~ b ~ralch: (a) L;. F,, and F,/F. curves. (b) worn surface overview, re) magnified worn surface. AIx~*;ivetrices are evidenL
[o5
E Wang. S.M. Hsa / Wear 217 (1998~ 104-109
the contact pressure and stress could increase to a maximum value and then decrease with scratch distance. A three-dimensional Swiss-made KISTLER 9251A quartz force transducer and a Labview data acquisition system were employed m display the vertical and horizontal force continuously on screen and record them. The loading range for normal load was t-800 N with precision up to 0.01 N. The sampling rate of data could be changed from lower than 1 scans/s to Eigber than 5000 scans/s. The speed and position of moving samples were controlled by x. y and z direction stages with precisions of t / z m / s for speed and I pm for position. The speed of the stage in scratch direction could be adjusted in the range of 1 ~ m / s - I m/s. A high speed (up to 500 picture/s) digital camera was used to observe the contact interface during scratching. This test setup was designed to simulate the friction and wear process in lubricated sliding asperity-asperity contact. The issue is whether a 3.175 mm diameter ball can be treated as an asperity. For a given surface, asperity reflect the roughness of the surface. On a micro~opic scale, the asperity depth
plant where contamination of either the product or the environment by any e~ape of a lubricating fluid would be unacceptable. Hence, it is very. necessary to know what happens in the asperity-asperity contact in this kind of lubrication situation. Therefore, this study intends to use multi-scratches of ball-on-ball in small size (3.175 mm in diameter) lubricated with only a drop of lubricant in the beginning of the test to simulate the above situation and find out how the lubrication, friction and wear go on with the increased scratch number.
2. Experimental details 2. I. Test equipment
The ball-on-ball test apparatus shown in Fig. 1 was used to simulate the asperity-asperity sliding contact in lubrication. A spherical ball of 3A75 mm diameter was driven to move against a similar but stationary ball with overlap so that
1 1 I
250 7- a,l~mmdtasteelballena.t75mmmaateelball 225 -I LubrtcaL-,dwtmadxoporpma/iln
[ sc,'~w.hspeedo.5mr./. 2001 ZOtln- - t c h ~
0.5 ~' 0.4
175
"';"
g,=
o. i
It. 100
0.2
0.1
0
(a)
•
~
t
~
,
,
,
,
,
J
,
0.0
0.50 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.450.50 Scratch Distance ( m m )
Fig. 4. "1"benormal fon:eF.. langenlial forceFt. frictioncoefficientb,/F, vs. ~ratch distance and the associatedcontactsurface for the lgth ~nttch: (a) F, I"~and F,IF. curves. (b) worn~rface overview.(c) magnilicdworn surface.Plasticsmearingdominatesthe wearpattern.
E Wang,$.M, H.;u/Wear 217(1998) I04-109
I07
~o / &171~wldtnmdhdell&1?Voaeudle~lill Ioi:k qmN0,5n l t
i"-t
(a)
o.~ o,o5o.lo o.15 o2o o , ~ a ~ ~ a a 1 4 o m S c r a t c h D i e t m ~ Imm)
Fig. 5. The normal fc~ce F,. mgemial f0¢¢.-,F,. friction coefheient F,IF. vs. scratch di.qance and the as.~ciatcd contact surface for Ihe 201k .scratch: (a) F,. F,, and F,Ib,, curves, (b) worn surface overview of upper ball. (c) wnnt surface overvlcw of h,wer lyail,and (d) magnilied wom.~rface of (e). Surface flaws initiation.
to width ratio is small, and a spherical tip o f a ball is a reasonable first approximation 1141. O n the other hand. the normal force in this study was 210 N. resulting in a m a x i m u m contact radius 0, I m m which is in the asperity radius range o f engineering sufface0,'OOI ram-0.1 m m [ 1 5 ] .
2.2. Test materials
The test balls were 52100 steel with hardness of Rc 63 and roughness Ra 0.025 # m . The lubricant used was light paraffin ( saybolt v i ~ o s i t y 125 / 135 ) without additi yes.
2.3. Test conditions
The multi-scratch tests were run under the following conditions.
Ball diameter Cleaning of balls beli.'a'etests
Scratch speed
3.175 mm Balls were c k~aneduhmumically with hexanc 12 min). act:tone ( 2 mln). dilul~'dMh:m ( 2 rain) and dcitmized water ( 10 limes) and dried wilh c(mVces.'.sednitrogen gas before paraffin-lubrieatedscratch test., ().5 mm/s
The maximum vertical load
2 I0 N
Maximum Hertz pressure Ma)tirnurnconlacl radius Quantity of lubricant
9.5 GPa O.I mm One drop of panwflinin the ¢imaactarea before Ihc les! and no-additional pamflin during Ibe muhi-.,,crafchtcsls Interval between two .scratches 2 rain Ambient temperature 2~C Environmental relatively 50e,f humidity The lower ball was driven to move against the upper stationary ball with a chosen overlap. Once one ~ r a t c h finishes
108
K Wang, S.M, H..;uI Wear 217 (/~8) 104-10g
the upper ball was lifted by the z-direction stage, and the lower ball was driven back to its original position. Then the upper ball was lowered to keep the same overlap as the two balls had previously to maintain the same load. Scratching was repeated until the required number of scratches was reached. Vertical force F: and ho.-'izontal force F~ were continuously monitored and recorded.
3. Experimental results Fig. 2a shows the normal load F., tangential toad F, and friction coefficient F,/F. curves vs. scratch distance of the 1st scratch of ball-on-ball at speed 0.5 mm/s with overlap 40 (corresponding to a maximum F, about 2 i 0 N, maximum Hertz pressure 9.5 GPa). The normal force F, and tangential force F, are converted from the recorded vertical force ~ and horizontal force F~ based on the contact geometry. The friction coefficient F,/Fo was about 0.1 for the Ist scratch. The corresponding contact surface after the I st scratch is given in Fig. 2b. There was no apparent plastic deformation or abrasive damage observed on the surface. It suggests that the contact surfaces were well lubricated at the 1st scratch even though the maximum Hertz stress was very high (9.5 GPa).
The tangential force F,, normal force F. and friction coefficient/:,IF, for the fifth ball on ball scratch are illustrated in Fig. 3a. The test conditions remained the same as those for the I st scratch test of Fig. 2. The friction coefficient increased gradually following each scratch and reached O. 15 at the 5th scratch, The worn surface of the 5th scratch is shown in Fig. 3b with overview and Fig. 3c with higher magnification. Micro scratch traces were clearly seen. Abrasion dominates
the wear at this stage. The friction coefficient of O.15 at the 5th scratch indicates that boundary lubrication was still present but wear had occurred. Fig. 4a, b and c present the F. F., F,IF. curves and the associated worn surface for the 10th scratch. The test conditions were identical with those for the Ist scratch test. The friction coefficient rose to O. 185. The worn surface shows plastic smearing, due to higher generated friction heat compared with the situation for the 5th scratch. The 20th scratch test results and the associated worn surfaces are illustrated in Fig. 5a,b,c and d, respectively. The friction coefficient reached 0.365, indicating a substantially reduced lubrication effect (the friction coefficient of 0.365 was still lower than the perfect dry-sliding value of O.7 when balls are cleaned ultrasonically with hexane ( 2 rain), acetone (2 min), diluted Micro (2 rain) and deionized water ( I0
Fig. 6. The normalforceF,,.tangentialforceF,. frietio, coefficientF,/F. vs. ~ratch distance and the associatedcontactsurfacefor the ~
.scratch:(a) F,,
F,,and F,/F.curres.{b ) worn surfaceoverview of upper b~ll.(c ) worn surfaceoverview of Iower ball,and {d ) magnifiedworn surfaceor (c ).Tensile¢racks propagalion.
K Wang,£M. ItsuiWear217(lf~8) 104--109
ball to simulate the lubricated friction and wear process in asperity-asperity contact.
o..o
A¢= 0,45
• The conta¢l area was w e l l l~Olected by the oil film at
-.,, 0.40
J 0.30 0 I~ 0.2S0
1tram
o.2o-
0.15-
109
U l I¢llltldl
0.10 0.050.050.05 0.05 o.lo o.~s oao o~s 0~o o~s o.,, o.,5 o.so
S c r a t c h D i s t a n c e (ram) Fig. 7. Frictioncoefficien!changes with increa~d ~ratch number.
times) and dried with compressed nitrogen gas). The worn surface shown in Fig, 5d reveals initiation of many f a i r y deep flaws on the surface. Those flaws may be induced by the repeated high stress at and beneath the contact surfaces. Fig. 6a,b,c,d gives the F,, F,, F j F , curves vs. scratch
distance and the corresponding worn surfaces at the 40th scratch. The friction coefficientincreased m about 0.4. There are short and fairly long cracks with propagation direction perpendicular to the scratch direction. These cracks seems to be produced by the repeated tensile stress in the scratch direction. If the scratch number is farther increased, the tensile cracks could be expected to propagate to a larger scale and result in more severe damage to the contact surfaces and subsurfaces. The increase of friction coefficient with increased scratch number is summarized in Fig. 7. it is noted that when the scratch number is below five, boundary lubrication dominates the sliding process. However, after the 5th scratch, the lubrication effect substantially reduced and the friction coefficient increases to a value of about 0.4 at the 40th seratch.
4. Conclusions
The above described work can be summarized as below. • This work uses the 3.175 mm diameter 52100 steel ball to scratch in single and multiple against the same size steel
the I st scratch and there was no any plastic defcemation or abrasive wear within the contact area even though the contact pressure was very high (9.5 OPa). • The friction coefficient increased with the increasing scratch number from 0.1 at the 1st seratch to aboet 0.4 at t i c 40th scratch. • When the scratch number was five, the friction coefficiem was less than 0,15 and boundary l u b r k a i o n conditions obtained. • With an increase in the number of scratches, t h e c o m a ~ surfaces exhibited the following sequential stages: well protected: abrasive wear, plastic smearing; surface flaws initiation; and tensile cracks propagation.
Refm Ill I.V. Kragelskii,N.M. Mikhin, ~ of Fri~ion Units of Machines. American Societyfor Mechanical Engine~-~'s. New York.
1988. {21 J.A. Greenwood.J.P.B. Williamson.Comaclof normallyflaE.~Mrface, Prec. R. So¢.London.Set. A .-~5 (1966) 330. [31 F.X. Wang. P. Lacey. R.S. Gates. S.M. l-lsu.A study of Ihe rehtive suffice conformitybelweentwo surfacesin slidingcnt,dacLASMEJ. Tribol. 113 ( 1991) 755-761. 14] M.F, Ashby.J. Abulawi.H,-S. Kong.Temperaure mapsfe¢ffictiomi heating in dcy sliding.Tribol.Trans. 34 (41 ( 1991) 577-587. 15i C.C, Kweh. MJ. Palchong.It.P. Evans. R.W+Snidle. SimelalJonof elas~ohy(h'odynarniccoma:ISbetweenmegh sin-faces.ASMEJ. Tdbo]., Prel:xrint,91-Tfib-36.1991. 161 C.H. Venner, W.E. ~en Napel, Su~,¢e ronghnes_~effect~ in amEHL linecontact, ASME£ TriboL 1|4 (3) (1992) 6t6-6~. [71 X. Ai, H.S. Cheng, The influenceof moving dem on poim EHL con"la~t~,STLETribe4.Trans. 7 (1994) 323--335. [ 81 X. AL H.S. Cheng.The effecL~of surfacelexlme on EHL point conlacLs. • ASMEJ. TdboL 118 (1996) 59-66. [91 F.P. Bowden, D. Tabor, The Friction and Lubricationof Solids, Oxford Univ.Press, Oxford. UK. 1964. [ I01 H. Czichos,Tribology,El~vicc Amsterdam,the Nethedamb. 1973. [ I I ] B. Bhushan.B.K.Guput.~ofTribo198y.McGraw-l.fill.New Yolk. USA. 1991. [ 121 J.A. Williams, Engineering T~ibology. Oxford Univ. Rre~s.Oxford.
UK. 1994. 1131 PJ. Blau. Friction Science and Technology. Marcel Dekkef. New York. USA. 1996. 1141 T.N. Ying, S.M. Hsu. Asperily-a.~'~rirycontact mechanismssimufa:ed by a two-ballcollisionapparalu.q,Wear 169 (1993) 33--41. 1151 M.F.A.~hby,J. Abulawi.H.-S.Ko,g, Tempet~ure ma~ for frictional heating in dPj stiding,STLETr~bol.Trans. 34 (4) ( 1991) 577-587.
Wear 217 (11198) 111-I-109
Lubricated friction and wear simulation of multi-scratches in asperity-asperity contact YingiongWanga., Stephen M. Hsu b Iomega. 8fX~Tusman Drive, Milpitas, CA 95035. USA h Ceramics Dil'ision. National Institute qfStundards and Techmdogy, Gaithersburg. MD 20899, USA
Received 5 November 1997: accepted 9 January 1998
Abstract
Multi-~ratch tests of ball-on-ball ( 3.175 mm diameter 52100 steel) in lubrication were carried out to simulate the friction and wear process of the asperity-asperity contact in sliding. The results show that the friction coefficient increases with the increased scratch number from about O.I at the Ist scratch to about 0.4 at the 40th scratch. There is no plastic deformation and wear on the contact surface for the Ist scratch. However. friction coefficient rises to 0.15 at the 5th scratch and abrasive wear occurs in the contact area. Then. with the scratch number increased further, the friction coefficient continues to increase and the contact surface undergoes different wear stages: plastic smearing ( 10th ,,a:ratch), surface flaw initiation (20th scratch), and tensile crack initiation and propagation (40th scratch). © 1998 Elsevier Science S.A. All rights re,fred, K¢'~3i'ords: Asperity Contact: Lubrication: Wear: Collislom Scratch
!. Introduction
Friction, wear and lubrication are important issues in mechanical systems with moving surfaces. For most mechanical applications, lubrication is provided to minimize friction and wear of the material. Since manufactured surfaces will always have a degree of roughness, the contact between moving surfaces becomes contact between asperities from the two surfaces. Some r e . a r c h e r s I 1,21 suggest that only a very small percentage of the apparent contact area is in real con tact. Some experimental results reveal that the real contact area could be large depending on the conformity of the two surfaces 13]. Based on experimental observation, for normal manufacturing process, an asperity size is inversely proportional to the hardness of the material 14 I. The sliding process of two contact surfaces is a process of repeated scratches between local asperities of the two surfaces. The asperity-asperity elastohydrodynamic lubrication has been investigated in recent years 15-81. However. previous studies summarized in tribological books 19-131 dealt with little on how the tribological process is going on at a asperity-asperity contact in boundary lubrication from the Isl scratch to a number of scratches. Therefore. what really * C~nxcsponding author. E-mail: wangly@'iomcga.com (IO-1.3-1648/98/St9.00 ,'f~ 1998 El.~vier Science S.A, All rights reserved. PIi S 0 0 4 3 - 1 6 4 8 ( 98 t tit} 149-5
happens to the asperity-asperity contact with increased scratch numbers in boundary lubrication remains unclear. Not all sliding tribological pairs are designed to operate in the presence of a generous supply of lubricant. The nature of the working environment may make it impossible, or impracticable, to arrange for the contact to be lubricated by a full hydrostatic or hydrodynamic fluid film, for example, in satellit¢ application where any liquid lubricant would be lost or degraded by evaporation, or in food processing or chemical
¢~uiclliIml/l"~
I/ l l i i i l ' - - ! 1 1 111 Fig. I. The used Iwo-ba|l collision apparalus.
¥. Wun~t. S.M. H.m / Weur 217 ( l ~ )
104-109
105
iJ (a)
GO
0-1
02
03
0.4
G5
Scratch Distance (men) Fig. -.2.The normal force F,,. tangential force F,. friction coefficient F,IF,, vs. ~ratch distance (a) and the a~.~,nci~d ¢ o ~ c l .qa'fac¢ (5) for the kit .v,cratch. The surface is well pzotected by ~uhrication.
G~
250
~
200
.0.30 "g"
,¢rMd*
-0.25
z~ 1.o
-o~
100
- 0.15
u
- (110
50
0
(a)
O.flO
i GO
0.1
0 i2
G3 '
0.4
0.S
Sc,,-~h Distance IN)
Fig. 3, The normal force F,. tangential force F,. friction c~rfficiem F,/F. vs..~oratch di.~Imce and the as.~ociaP.-dcontact .',mrfaoefor the ~ b ~ralch: (a) L;. F,, and F,/F. curves. (b) worn surface overview, re) magnified worn surface. AIx~*;ivetrices are evidenL
[o5
E Wang. S.M. Hsa / Wear 217 (1998~ 104-109
the contact pressure and stress could increase to a maximum value and then decrease with scratch distance. A three-dimensional Swiss-made KISTLER 9251A quartz force transducer and a Labview data acquisition system were employed m display the vertical and horizontal force continuously on screen and record them. The loading range for normal load was t-800 N with precision up to 0.01 N. The sampling rate of data could be changed from lower than 1 scans/s to Eigber than 5000 scans/s. The speed and position of moving samples were controlled by x. y and z direction stages with precisions of t / z m / s for speed and I pm for position. The speed of the stage in scratch direction could be adjusted in the range of 1 ~ m / s - I m/s. A high speed (up to 500 picture/s) digital camera was used to observe the contact interface during scratching. This test setup was designed to simulate the friction and wear process in lubricated sliding asperity-asperity contact. The issue is whether a 3.175 mm diameter ball can be treated as an asperity. For a given surface, asperity reflect the roughness of the surface. On a micro~opic scale, the asperity depth
plant where contamination of either the product or the environment by any e~ape of a lubricating fluid would be unacceptable. Hence, it is very. necessary to know what happens in the asperity-asperity contact in this kind of lubrication situation. Therefore, this study intends to use multi-scratches of ball-on-ball in small size (3.175 mm in diameter) lubricated with only a drop of lubricant in the beginning of the test to simulate the above situation and find out how the lubrication, friction and wear go on with the increased scratch number.
2. Experimental details 2. I. Test equipment
The ball-on-ball test apparatus shown in Fig. 1 was used to simulate the asperity-asperity sliding contact in lubrication. A spherical ball of 3A75 mm diameter was driven to move against a similar but stationary ball with overlap so that
1 1 I
250 7- a,l~mmdtasteelballena.t75mmmaateelball 225 -I LubrtcaL-,dwtmadxoporpma/iln
[ sc,'~w.hspeedo.5mr./. 2001 ZOtln- - t c h ~
0.5 ~' 0.4
175
"';"
g,=
o. i
It. 100
0.2
0.1
0
(a)
•
~
t
~
,
,
,
,
,
J
,
0.0
0.50 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.450.50 Scratch Distance ( m m )
Fig. 4. "1"benormal fon:eF.. langenlial forceFt. frictioncoefficientb,/F, vs. ~ratch distance and the associatedcontactsurface for the lgth ~nttch: (a) F, I"~and F,IF. curves. (b) worn~rface overview.(c) magnilicdworn surface.Plasticsmearingdominatesthe wearpattern.
E Wang,$.M, H.;u/Wear 217(1998) I04-109
I07
~o / &171~wldtnmdhdell&1?Voaeudle~lill Ioi:k qmN0,5n l t
i"-t
(a)
o.~ o,o5o.lo o.15 o2o o , ~ a ~ ~ a a 1 4 o m S c r a t c h D i e t m ~ Imm)
Fig. 5. The normal fc~ce F,. mgemial f0¢¢.-,F,. friction coefheient F,IF. vs. scratch di.qance and the as.~ciatcd contact surface for Ihe 201k .scratch: (a) F,. F,, and F,Ib,, curves, (b) worn surface overview of upper ball. (c) wnnt surface overvlcw of h,wer lyail,and (d) magnilied wom.~rface of (e). Surface flaws initiation.
to width ratio is small, and a spherical tip o f a ball is a reasonable first approximation 1141. O n the other hand. the normal force in this study was 210 N. resulting in a m a x i m u m contact radius 0, I m m which is in the asperity radius range o f engineering sufface0,'OOI ram-0.1 m m [ 1 5 ] .
2.2. Test materials
The test balls were 52100 steel with hardness of Rc 63 and roughness Ra 0.025 # m . The lubricant used was light paraffin ( saybolt v i ~ o s i t y 125 / 135 ) without additi yes.
2.3. Test conditions
The multi-scratch tests were run under the following conditions.
Ball diameter Cleaning of balls beli.'a'etests
Scratch speed
3.175 mm Balls were c k~aneduhmumically with hexanc 12 min). act:tone ( 2 mln). dilul~'dMh:m ( 2 rain) and dcitmized water ( 10 limes) and dried wilh c(mVces.'.sednitrogen gas before paraffin-lubrieatedscratch test., ().5 mm/s
The maximum vertical load
2 I0 N
Maximum Hertz pressure Ma)tirnurnconlacl radius Quantity of lubricant
9.5 GPa O.I mm One drop of panwflinin the ¢imaactarea before Ihc les! and no-additional pamflin during Ibe muhi-.,,crafchtcsls Interval between two .scratches 2 rain Ambient temperature 2~C Environmental relatively 50e,f humidity The lower ball was driven to move against the upper stationary ball with a chosen overlap. Once one ~ r a t c h finishes
108
K Wang, S.M, H..;uI Wear 217 (/~8) 104-10g
the upper ball was lifted by the z-direction stage, and the lower ball was driven back to its original position. Then the upper ball was lowered to keep the same overlap as the two balls had previously to maintain the same load. Scratching was repeated until the required number of scratches was reached. Vertical force F: and ho.-'izontal force F~ were continuously monitored and recorded.
3. Experimental results Fig. 2a shows the normal load F., tangential toad F, and friction coefficient F,/F. curves vs. scratch distance of the 1st scratch of ball-on-ball at speed 0.5 mm/s with overlap 40 (corresponding to a maximum F, about 2 i 0 N, maximum Hertz pressure 9.5 GPa). The normal force F, and tangential force F, are converted from the recorded vertical force ~ and horizontal force F~ based on the contact geometry. The friction coefficient F,/Fo was about 0.1 for the Ist scratch. The corresponding contact surface after the I st scratch is given in Fig. 2b. There was no apparent plastic deformation or abrasive damage observed on the surface. It suggests that the contact surfaces were well lubricated at the 1st scratch even though the maximum Hertz stress was very high (9.5 GPa).
The tangential force F,, normal force F. and friction coefficient/:,IF, for the fifth ball on ball scratch are illustrated in Fig. 3a. The test conditions remained the same as those for the I st scratch test of Fig. 2. The friction coefficient increased gradually following each scratch and reached O. 15 at the 5th scratch, The worn surface of the 5th scratch is shown in Fig. 3b with overview and Fig. 3c with higher magnification. Micro scratch traces were clearly seen. Abrasion dominates
the wear at this stage. The friction coefficient of O.15 at the 5th scratch indicates that boundary lubrication was still present but wear had occurred. Fig. 4a, b and c present the F. F., F,IF. curves and the associated worn surface for the 10th scratch. The test conditions were identical with those for the Ist scratch test. The friction coefficient rose to O. 185. The worn surface shows plastic smearing, due to higher generated friction heat compared with the situation for the 5th scratch. The 20th scratch test results and the associated worn surfaces are illustrated in Fig. 5a,b,c and d, respectively. The friction coefficient reached 0.365, indicating a substantially reduced lubrication effect (the friction coefficient of 0.365 was still lower than the perfect dry-sliding value of O.7 when balls are cleaned ultrasonically with hexane ( 2 rain), acetone (2 min), diluted Micro (2 rain) and deionized water ( I0
Fig. 6. The normalforceF,,.tangentialforceF,. frietio, coefficientF,/F. vs. ~ratch distance and the associatedcontactsurfacefor the ~
.scratch:(a) F,,
F,,and F,/F.curres.{b ) worn surfaceoverview of upper b~ll.(c ) worn surfaceoverview of Iower ball,and {d ) magnifiedworn surfaceor (c ).Tensile¢racks propagalion.
K Wang,£M. ItsuiWear217(lf~8) 104--109
ball to simulate the lubricated friction and wear process in asperity-asperity contact.
o..o
A¢= 0,45
• The conta¢l area was w e l l l~Olected by the oil film at
-.,, 0.40
J 0.30 0 I~ 0.2S0
1tram
o.2o-
0.15-
109
U l I¢llltldl
0.10 0.050.050.05 0.05 o.lo o.~s oao o~s 0~o o~s o.,, o.,5 o.so
S c r a t c h D i s t a n c e (ram) Fig. 7. Frictioncoefficien!changes with increa~d ~ratch number.
times) and dried with compressed nitrogen gas). The worn surface shown in Fig, 5d reveals initiation of many f a i r y deep flaws on the surface. Those flaws may be induced by the repeated high stress at and beneath the contact surfaces. Fig. 6a,b,c,d gives the F,, F,, F j F , curves vs. scratch
distance and the corresponding worn surfaces at the 40th scratch. The friction coefficientincreased m about 0.4. There are short and fairly long cracks with propagation direction perpendicular to the scratch direction. These cracks seems to be produced by the repeated tensile stress in the scratch direction. If the scratch number is farther increased, the tensile cracks could be expected to propagate to a larger scale and result in more severe damage to the contact surfaces and subsurfaces. The increase of friction coefficient with increased scratch number is summarized in Fig. 7. it is noted that when the scratch number is below five, boundary lubrication dominates the sliding process. However, after the 5th scratch, the lubrication effect substantially reduced and the friction coefficient increases to a value of about 0.4 at the 40th seratch.
4. Conclusions
The above described work can be summarized as below. • This work uses the 3.175 mm diameter 52100 steel ball to scratch in single and multiple against the same size steel
the I st scratch and there was no any plastic defcemation or abrasive wear within the contact area even though the contact pressure was very high (9.5 OPa). • The friction coefficient increased with the increasing scratch number from 0.1 at the 1st seratch to aboet 0.4 at t i c 40th scratch. • When the scratch number was five, the friction coefficiem was less than 0,15 and boundary l u b r k a i o n conditions obtained. • With an increase in the number of scratches, t h e c o m a ~ surfaces exhibited the following sequential stages: well protected: abrasive wear, plastic smearing; surface flaws initiation; and tensile cracks propagation.
Refm Ill I.V. Kragelskii,N.M. Mikhin, ~ of Fri~ion Units of Machines. American Societyfor Mechanical Engine~-~'s. New York.
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