~-:;:-%~ ~§] ;,;l (1995), "11 11 'I'! "11 5 :§~
`Journal of KSTLE(1995) Vol. 11, No. 5, pp.144-149
`
`Frictional Characteristics of the Lubricants Formulated with
`Non-Conventional Base Stocks
`
`Woo-Sik Moon and Jong-Hun Lee
`Yukong Limited, Taejon, KOREA
`
`Abstract-Use of high-quality basestocks is increasing to produce high-performance lubricants.
`However, their tribological characteristics have not been understood clearly yet. In this study, a new(cid:173)
`ly developed basestock from a fuel hydrocracker and a poly-alpha-olefin are selected and in(cid:173)
`vestigated on the properties of lubricants formulated with them. The Lubricants are prepared by
`blending the basestocks with typical additives such as a zinc dialkyldithiophosphate, a dispersant, a
`detergent and a dispersant-inhibitor package. Frictional and wear-preventing properties are in(cid:173)
`vestigated using an oscillating-type wear-testing machine. The contact is a ball-on-disk mode and the
`resting temperature is varied from room temperature to 200°C. The results show that their frictional
`;:,roperty is varied significantly and that the non-conventional oils result in lower friction and lower
`wear compared with conventional lubricants, especially at the higher temperatures.
`
`Key words : Friction, Non-Conventional Basestock, Engine Oil, Ball-On-Disk, Wear test, Additives, In(cid:173)
`teraction
`
`1 . Introduction
`
`The requirements for engine oils have become
`more severe, following
`the
`trend to higher per(cid:173)
`formance of engine design. In order to make high(cid:173)
`performance lubricants, it is necessary to improve
`formulation technology as well as to develop better
`additive:s and base stocks. In recent years, the use of
`high-quality basestocks has increased in order to
`satisfy the requirements for high-performance lu(cid:173)
`bricants. Generally, lower volatility, good oxidation
`stability and viscometric properties have been re(cid:173)
`quired for the lubricants to function successfully und(cid:173)
`er increasingly severe operating conditions.
`Recent trend toward lower viscosity of engine oils
`is direcdy connected to the improvement of their fuel(cid:173)
`economy performance. That is to say, automotive
`manufacturers tend to recommend the use of SW /30
`multigrades to reduce engine friction losses at all
`operating temperatures. The traditional formulation
`approach for these products was to use very low
`viscosity mineral base oils. However, such products
`tend to produce high oil consumption and be ox(cid:173)
`idized and thickened rapidly because of the high vo(cid:173)
`latility of these base oils. Hence, in order to produce
`fuel-efficient engine oils with higher performance, it
`
`is generally necessary for them to be formulated us(cid:173)
`ing high-quality base oils in full or partial amount
`which include synthetic fluids and highly-refined min(cid:173)
`eral oils. Poly-alpha-olefins and hydrocracked bases(cid:173)
`tocks meet the high performance specifications such
`as lower volatility, low temperature properties and
`viscosity-temperature characteristics, because
`they
`have higher quality including high viscosity index,
`low Noack volatility and good oxidation stability
`compared with conventional base oils.
`Synthetic fluids
`like polyalphaolefin and ester
`have been used but only for limited purpose due to
`their high price. However, the so-called VHVI (very
`high viscosity index) base stocks produced from hy(cid:173)
`drocracking and/or wax-isomerization reported to be
`able to solve the problem and to have comparable
`quality [1,2,3].
`It has been demonstrated that a severely hy(cid:173)
`drocracked VHVI base oil, formulated into friction(cid:173)
`modified engine oils, displayed enhanced fuel e(cid:173)
`conomy characteristics due to superior oxidation sta(cid:173)
`bility and a lower viscosity/pressure coefficient, com(cid:173)
`pared with a conventional mineral oil blend [4]. At
`a study on the performance of MoDTC friction mod(cid:173)
`ifiers, base oils of higher saturate content have been
`found to enhance their friction-reducing performance,
`
`144
`
`Page 1 of 6
`
`ORONITE EXHIBIT 1015
`
`

`

`Frictional Characteristics of the Lubricants Formulated with Non-Conventional Base Stocks
`
`145
`
`compared with base oils containing higher levels of
`aromatic .and/or polar synthetic components. It was
`proposed that the effect of base oil composition on
`the perfonnance is by an adsorption mechanism [5].
`Now, it is well understood that tribological beha(cid:173)
`vior of base stocks depends on and is influenced by
`chemical composition and structure of their hy(cid:173)
`drocarbon and non-hydrocarbon constituents. In a
`four-ball-wear test, higher wear was produced with
`super-refined mineral oils than with conventionally
`refined mineral oils [6]. Aromatics and sulfurs con(cid:173)
`taining hydrocarbons
`in
`solvent-extracted base
`stocks have been found to provide good wear and
`friction characteristics, while saturate hydrocarbons
`usually were markedly poorer [7,8]. However, it is
`known that increased refining of mineral oils gen(cid:173)
`erally enhanced the effectiveness of friction mod(cid:173)
`ifiers [9].
`In this study, an attempt is made to expand an
`tribological property of
`understanding on
`the
`representative engine oil additives and the influence
`of base stocks. A newly developed base stock from
`a fuel hyclrocracker and a polyalphaolefin are select(cid:173)
`ed as non-conventional base stocks and investigated
`tribological properties of lubricants for(cid:173)
`on
`the
`mulated with them. The lubricants are prepared by
`blending the base stocks with typical additives such
`as a zinc dialkyldithiophosphate, a dispersant,a de(cid:173)
`tergent and a dispersant-inhibitor-package additive.
`
`2. Experimental Details
`
`2-1 . Lubricants
`Some characteristics of the base oils used in the
`experiments are given in Table 1. Four different
`base oils, which are classified into different API
`base oil grouping [10], are selected to blend lu(cid:173)
`bricants.
`Base od BOl, which is a solvent-extracted 150
`neutral, is classified as Group I. Base oil B02 pro(cid:173)
`duced from a lube-hydrocracker pertains to Groupll,
`while base oil B03 produced from a fuel hy(cid:173)
`drocracker is Group III as its viscosity index is high(cid:173)
`er than 120. As shown in the table, the VHVI base
`stock (B03) has different properties from con(cid:173)
`ventional ones (BOl, 2), containing almost no sulfur
`and much Jess aromatics. Moreover, the viscometric
`properties have been improved remarkably. That is,
`the VHVl base stock has higher viscosity at high
`
`Table 1. General properties of base oils
`
`Properties
`API Classification
`Specific Gravity
`Viscosity lOO"C,cSt
`Viscosity Index
`Viscosity @-20°C, cP
`(CCS) @-25°C
`Flash Point, "C
`Pour Point, °C
`Noack Volatility,wt%
`Sulfur, wt%
`Aro., vol% (HPLC)
`TAN, mgKOH/g
`Aniline Point, 'C
`
`B03
`BOl B02
`B04
`Gr.I
`Gr.II Gr.III Gr.IV
`0.869 0.865
`0.844
`0.827
`5.2
`5.1
`6.0
`5.9
`97
`99
`123
`135
`2100
`2000
`1550
`800
`3800
`3600
`2600
`1300
`222
`212
`226
`234
`-12
`-15
`-12
`-57
`17.0
`16.6
`7.2
`7.5
`0.58
`0.03
`0.00
`0.00
`27.7
`3.5
`0.6
`0
`0.02
`0.02
`0.01
`0.01
`104
`111
`121
`129
`
`Table 2. Composition of 803 (detail analysis by IP
`386/ASTM 02786)
`
`Saturates, %
`Alkanes
`1-Ring Naphthenens
`2-Ring Naphthenens
`3-Ring Naphthenens
`4-Ring Naphthenens
`5-Ring Naphthenens
`6-Ring Naphthenens
`
`99.2
`3 "l.65
`24.98
`23.88
`11.09
`4.48
`l.11
`0.0
`
`temperature and lower viscosity at low temperature.
`Detailed compositions of B03 are given in Table
`2. It is noted that relatively larger amount of na(cid:173)
`phthenic components exist. This fact indicates that
`the greater part of aromatics in the feed stock were
`converted into naphthenic components during the hy(cid:173)
`drocracking process.
`Among various additives contained in engine oils,
`the
`following
`representative ones are
`selected,
`ZnDDP as a dual-functional additive of wear-pre(cid:173)
`vention and oxidation-inhibition, a succinimide as
`dispersant, and an overbased calcium sulfonate.
`Their detailed properties are given in Table 3.
`One DI-package-type additive of API SH per(cid:173)
`formance grade is also blended to make engine oils.
`Two kinds of friction modifiers arc added up into en(cid:173)
`gine oils to investigate the influence of different base
`stocks. These additives of commercial grades were
`blended into different base oils with various com(cid:173)
`binations to prepare oil samples for the experiments.
`Detailed formulations of the representative oils
`
`Vol. 11. No. 5, 1995
`
`Page 2 of 6
`
`

`

`146
`
`Woo-Sik Moon and Jong-Hun Lee
`
`Table 3. Properties of additives
`Properties
`ZnDDP
`Viscosity,cSt @ 40'C
`130
`@ lOO'C
`10
`TBN, mgKOH/g
`Elements,
`
`Dispersant
`8000
`420
`27
`Zn 8.9 wt% N 1.46 wt%
`P 8.0 wt%
`
`Detergent
`2000
`64
`300
`Ca 11.9wt%
`
`Etc.
`
`Primary-
`secondary
`mixed type
`
`PIBSA-PAM
`type
`
`Overbased
`Calcium
`sulfonate
`
`Table 4. Preparation of lubricants
`
`DI package
`
`FMl
`
`125
`
`15.0
`
`FM2
`168.
`13.0
`
`Ca 0.459 wt% Mo 1.1 wt%
`Mg 0.370 wt%
`Na 0.561 wt%
`Zn 1.19 wt%
`API SH grade MoDTC type
`
`Glycerol
`oleate type
`
`load ••
`
`Fig. 1. SRV oscillating machine.
`
`testing machine (Optimol Model) is shown in Fig. 1.
`The sliding system consists of an oscillating upper
`specimen and a stationary lower specimen. The ball
`specimen of 10 mm in diameter is rigidly secured
`into the upper specimen holder. The ball slides over
`the lower disk specimen, giving the load in con(cid:173)
`tacted state. The load and friction force are measur(cid:173)
`ed using a two-phase piezoelectric-type transducer
`which is located under the disk specimen. Friction
`coefficient is averaged over each period of os(cid:173)
`cillating run and recorded continuously. Both the
`ball and the disk specimens are made of SUJ 2
`(which corresponds to AISI 52100) bearing steel.
`All the friction experiments and measurements
`were conducted in the following manner. In Table 5
`are given the testing conditions including normal
`load, stroke, frequency and temperature.
`Measurements of friction were conducted on a ball(cid:173)
`on-disk c,0nfiguration operating at 100 Newton, 1.0
`mm stroke, 50 Hz and temperature ramped from 30
`to 200°C.
`Before each run, specimens were cleaned care-
`
`Lubricants
`
`A
`Al
`A2
`A3
`B
`Bl
`82
`83
`c
`Cl
`C2
`C3
`D
`D1
`02
`03
`
`801
`801
`801
`801
`802
`802
`802
`802
`803
`803
`803
`803
`804
`804
`804
`804
`
`Formula
`Base Oil ZnDDP Detergent2 Dispersant
`5 wt%
`1 wt% wt%
`0
`0
`0
`0
`0
`0
`0
`0
`0
`0
`0
`0
`Cl
`0
`0
`Cl
`
`0
`
`0
`
`0
`
`0
`
`0
`
`0
`
`0
`
`0
`
`0
`0
`
`0
`0
`
`Cl
`0
`
`0
`0
`
`tested are summarized in Table 4.
`Lubricants A,B,C and D are single-additive sys(cid:173)
`tems containing ZnDDP by 1.0 wt% concentration
`in the base oils. Lubricants Al-Dl and A2-D2 are
`double- additive systems containing other additives
`plus ZnDDP; the former systems contain the de(cid:173)
`tergent by 2.0 wt% and the later systems contain the
`dispersant by 5.0 wt%. In lubricants A3-D3, three
`kinds of additives are contained:1.0 wt% ZnDDP, 2.
`O wt% detergent and 5.0 wt% dispersant. These
`treating rates of additives are comparable with those
`of high performance engine oils.
`
`2-2. Experiments
`A general view of the SRV oscillating-type wear-
`
`Journal of the KSTLE
`
`Page 3 of 6
`
`

`

`Frictional Characteristics of the Lubricants Formulated with Non-Conventional Base Stocks
`
`147
`
`c
`"'
`'"
`'-<
`'-
`1.,
`._,
`r.
`~
`,,
`
`()
`
`Ix
`
`0. J(l
`
`0. 14
`
`0. J 2
`
`0. I
`
`'-
`Le_ 0. OX
`
`Wt>l1ling
`
`.i_ _ _ _ 1.-
`
`- . _L - -L~ J . _ _ _i __ _L_ ___ _i_ · - - - 1_ ___ J
`
`Table 5. Experimental conditions
`Test Method
`Load
`Stroke
`Frequency
`Temperature
`
`Ball-On-Disk
`100 N
`1.0 mm
`50 Hz
`40-200°C
`
`fully in heptane, dried in a hot air stream, and then
`set in the, apparatus. The sliding speed was held con(cid:173)
`stant throughout the run, but the load were in(cid:173)
`creased to the predetennined value during a break-in
`of five minutes and kept constant thereafter. Speci(cid:173)
`men temperature was increased from room tem(cid:173)
`perature to 200°C by the speed of 20°C increase
`over 10 minutes. Friction and specimen temperature
`were continuously measured through the run.
`After a run, the specimens were removed from
`the apparatus and washed as before. Then the wear
`scar width on the ball were detennined under an opt(cid:173)
`ical microscope.
`
`3. Results and Discussion
`
`Variations of friction coefficient with increasing
`temperature are shown in Fig. 2 for experiments
`with lubricants A, B, C and D. Friction coefficient
`shows considerable differences among four oil sam(cid:173)
`ples of different base stocks blended. From 40°C to
`120°C, friction coefficient is almost the same for all
`the oils tested increasing gradually from 0.125 to 0.
`140. This suggests that the decrease in lubricant
`viscosity over this temperature range have some in(cid:173)
`fluence on friction coefficient. However, after the
`temperature reaches 140°C, friction coefficient has
`different values among the lubricant tested, presum(cid:173)
`ably due to the thennal decomposition and/or ac(cid:173)
`tivation of ZnDDP. Lubricant A, blended with BOl,
`suppresses the increase in friction coefficient upto
`200°C, while lubricant B,C and D give higher fric(cid:173)
`tion coefficient and finally result in welding of the
`specimens at the temperature of 200, 160 and 140°C,
`respective! y.
`These fiictional characteristics have a correlation
`with hydrocarbon composition of the
`lubricants.
`That is, the lubricants with higher aromatics content
`give better surface protection.
`This difference is considered to be caused by the
`additive solvency of the base oils. Because the
`
`20
`
`40
`
`GO
`
`100 120 140
`110
`'t:
`TPmperaturP,
`Fig. 2. Influence of basestocks on the friction : one·
`• , OIL A; o, OIL
`additive lubricants (ZnDDP),
`B; ..6., OIL C; X, Oil D.
`
`lhO 1110 200 220
`
`0. 18
`~
`" 0. 16
`u
`.....
`..... 0 . 14
`" 0
`u
`c: 0. 12
`0
`
`0. 1
`
`u
`'-
`LI..
`
`20
`
`40
`
`60 80 100 120 140 !(10 180 200 220
`·c
`TPmperalurP,
`Fig. 3. Influence of basestocks on the friction: two(cid:173)
`additive lubricants (ZnDDP+dispersant), • • -, OIL
`A2; ·O·, OIL B2; ·..6.-, OIL C2; -X·, Oil 02.
`
`ZnDDP is thermally decomposed at around 120°C,
`the frictional properties of the lubricant should de(cid:173)
`pend on the decomposition products of the ZnDDP
`at the higher temperature [11 ). Oil A has even lower
`friction at high temperature, as it has good solvency
`for the decomposed products which is known to
`have better frictional characteristics than ZnDDP it(cid:173)
`self [12]. However, with decreasing solvency, they
`cannot protect the sliding surfaces at higher tem(cid:173)
`perature, because the decomposition products are
`separated from the oils
`The minimum wear amount was produced with lu(cid:173)
`bricant A but other lubricants generated higher wear
`of similar levels.
`As shown in Fig. 3, addition of a dispersant to lu(cid:173)
`bricants A-D changes variations of friction coef(cid:173)
`ficient with increasing temperature. Except for lu(cid:173)
`bricant B, other lubricants have a very similar trend
`in that friction coefficient decreases gradually to a(cid:173)
`bout 0.125 over the temperature range of 100°C to
`200°C. Anyway, the dispersant improved both fric-
`
`Vol. 11, No. 5, 1995
`
`Page 4 of 6
`
`

`

`148
`
`Woo-Sik Moon and Jong-Hun Lee
`
`IJ. l x
`
`0. lb
`
`c v
`
`u
`
`~ 0. 14
`~
`'-~
`c
`
`0. 12
`
`II
`
`I
`
`0. ()~
`
`Welding
`
`0. l X
`c 0. 16
`v
`'.::
`~ 0. 14
`v
`0
`'~
`c: 0. 12
`
`(,
`
`u
`
`L
`lL
`
`0. 1
`
`---1 - - - L ---
`
`1_ ____ 1-_
`
`SAE SW/30
`
`2D
`
`40 Ml
`
`JOO 120 140
`HO
`"c
`Tcmpn,,lure,
`Fig. 4. Influence of basestocks on the friction: two·
`additive lubricants (ZnDDP+detergent), - • -, OIL A
`1; -0-, OIL A2; -A-, OIL A3; -X-, Oil A4.
`
`lt,O
`
`IHO 200
`
`ll. l x
`c 'a 0. I<,
`'"
`~ n. 14
`~
`~~
`c: 0. I 2
`-:,
`
`~
`
`L
`lL
`
`0. l
`
`0. OX
`
`20
`
`40
`
`t,O
`
`100 120 140 1(,0
`KO
`"c
`Trmpf'rature.
`Fig. 5. Influence of basestocks on the friction: three(cid:173)
`additive lubricants (ZnDDP + dispersant + de(cid:173)
`tergent), - • -, OIL A3; -0-, OIL B3; -•-, OIL C3; -
`X-, Oil 03.
`
`!XO 200 220
`
`tion coefficient and wear amount for all the lu(cid:173)
`bricants tested. This result clarifies that the disper(cid:173)
`sant has a synergistic effect on the activity of
`ZnDDP, especially at the higher temperature range,
`whatever the base oil may be.
`It is generally understood that dispersants desolve
`some ZnDDP-decomposition products giving better
`frictional and wear-preventing properties [11,12).
`Now it is clear that when dispersants are added the
`non-conventional basestocks have the same as or
`even better than conventional ones in the friction!
`properties in spite of their relatively lower solvency.
`Addition of a detergents to lubricants A-D gen(cid:173)
`erally improves friction coefficient excepting for lu(cid:173)
`bricant B2 which results in welding at 180°C, as
`given in Fig. 4. It is considered that welding at the
`higher temperature is prohibited by synergistic ef(cid:173)
`fects at sliding surface of ZnDDP and the detergent.
`When a detergent is added into lubricants Al - Dl
`to make three additive systems, friction coefficient
`
`Journal of the KSTLE
`
`0. OS
`
`20
`
`40 60
`
`__ 1 __ _1_ _ _ _ L__L_ _ _ _ _ _ i _ J _ _ _ _J____.__._.J
`XO 100 120 140 160 180 200 220
`Tempernlurr,
`°C
`Fig. 6. Influence of basestocks on the friction: full(cid:173)
`formulated engine oils,
`- • -, GROUP III; -0-,
`GROUP I;
`-A-, GROUP !+GROUP III;
`-X-,
`GROUP 1(10 W/30).
`
`responds differently for each lubricant but generally
`increases over the higher temperature range, as Fig.
`5 shows results with lubricants A3-D3. With lu(cid:173)
`bricants B3 and D3, friction coefficient increases
`over all the temperature range studied. However,
`transition in friction coefficient is found with lu(cid:173)
`bricants A3 and C3. That is, friction coefficient de(cid:173)
`creases at the initial lower temperature range but in(cid:173)
`creases rapidly around a critical temperature of
`120°C and then levels off.
`More study is necessary to understand how the
`balancing among the three additives influences the
`frictional characteristics. However, it may be pos(cid:173)
`sible to suppress friction coefficient at higher tem(cid:173)
`perature via good combination and selection of ad(cid:173)
`ditives, as we see in the following results with full
`formulated engine oils.
`Figure 6 gives variation of friction coefficient for
`full-formulated engine oils using different base
`stocks but with the same additive systems. The oil
`blended with a group III base stock has almost con(cid:173)
`stant friction coefficient over all
`the temperature
`range investigated, while other oils give friction
`coefficient increasing gradually with increasing tem(cid:173)
`perature. When the similar group II base stocks are
`used, the oil of SAE lOW-30 has lower friction coef(cid:173)
`ficient than SAE SW-30, perhaps due to the higher
`viscosity. When we add group III base stock into
`the group I base stock in order to make a SAE SW-
`30 oil, friction coefficient decreases slightly com(cid:173)
`pared with the oil blended with a full group I base
`stock, as shown in the figure.
`This result indicates that, when a well balanced
`
`Page 5 of 6
`
`

`

`Frictional Characteristics of the Lubricants Formulated with Non-Conventional Base Stocks
`
`149
`
`n I K
`
`c
`
`' 1). \(,
`, ..
`'
`<::
`
`11. 14
`
`11. 12
`
`11. I
`
`11 nx
`
`211
`
`4n
`
`1,11
`
`I 2fl 140
`I ()11
`Kil
`T1'mp,·ratur1·. c
`Fig. 7. Influence of friction modifiers on the friction:
`a full-formulated lubricant (conventional basestock),
`. • ·, GROUP I; -0 ·, MoDTC; ·A·, Ashless.
`
`\i>il
`
`I K() 200 22'!
`
`0. I K
`
`c <> n. \(,
`
`u
`
`~ 0. 14
`v
`c;. .,
`~ n. l 2
`. , ·-
`0. I
`
`LL
`
`0. OK
`
`:'.il
`
`40
`
`iii)
`
`100 121) 140
`KO
`Trmiwrnlurt·, °l:
`Fig. 8. Influence of friction modifiers on the friction:
`full-formulated
`lubricant(Group
`III),
`• • ·,
`a
`GROUP I; ·O·, MoDTC; ·A·, Ashless.
`
`l<,il
`
`!XO 200 220
`
`additive system is selected, the group III base stock
`give excellent frictional properties at all the tem(cid:173)
`perature investigated which are almost impossible
`with conventional base stocks. That is, excellent sur(cid:173)
`face adsorption and oxidation stability are originated
`from the base oil and balanced solvency and disper(cid:173)
`sancy is supported by additives.
`When we add a conventional ashless friction mod(cid:173)
`ifier (0.2 wt% concentration) to the SAE 5W-30 oil
`(group I) described in Fig. 6, friction coefficient de(cid:173)
`creases slightly over the temperature ranging from
`60°C to ll20°C, as shown in Fig. 7. Addition of a
`MoDTC
`friction modifier
`(0.2 wt%concentration)
`reduces friction coefficient considerably and is ef(cid:173)
`fective upto 200°C. However, friction coefficient of
`
`the group I oil with MoDTC is still higher than the
`untreated group III oil discussed in Fig. 6.
`As given in Fig. 8, the effectiveness of friction
`modifiers is slight when they are added to the SAE
`5W-30 oil blended with a group III base stock ,com(cid:173)
`pared with the oil blended with a group I base stock.
`Reduction in friction coefficient is larger with FM2
`and more effective at the temperature range higher
`than 140°C.
`
`4. Conclusions
`
`(1) Composition of base oils influences greatly on
`the tribological behavior of additive systems.
`(2) Ashless dispersant enhances frictional pro(cid:173)
`perties of ZnDDP-containing lubricants at the tem(cid:173)
`perature range higher than 120°c.
`(3) A high VI base stock (group III) produced
`from a fuel hydrocraker bottom feed can reduce fric(cid:173)
`tion coefficient in formulated engine oils .
`
`References
`
`1. M. Ushio, Preprint. ACS, 37, 4, 1293-1302, 1992.
`2. N. C. Yates, Preprint. ACS, 37, 4, 1303-1312,
`1992.
`3. W.S. Moon, J.H. Lee and S.K. Hahn, Proc. JAST,
`Tokyo 1995-5, 429-432.
`4. J. lgarashi, Thesis Dr. Eng, 170-187. 1993.
`5. A.J. Stipanovic and J.P. Schoonmaker, SAE Paper
`932779, 77-88, 1993.
`6. E.E. Klaus and H.E. Bieber, ASLE Trans., 7, 1-10,
`1964.
`7. R.S. Fein and KL. Kreuz, ASLE Trans., 8, 29-38,
`1965.
`8. M.T. Benchaita, R.D. Schwartz and F.E. Lock(cid:173)
`wood, STLE Trans., 33, 1, 85-95, 1990.
`9. F.G. Rounds, ASLE Trans., 8, 21-28, 1965.
`10. API base oil classification, 1994.
`11. I-Ming Feng, Wear, 3, 309-311, 1960.
`12. Kawamura, F. Fujita, Y. Esaki, and H. Moritani,
`SAE paper 852076, 1985.
`
`Vol. 11, No. 5, 1995
`
`Page 6 of 6
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