graphite composite materials in oil lubrications with EP additives

WEAR ELSEVIER

Wear198 (1996) 58-70

The tribological performance of 6061 aluminumalloy/graphite composite materials in oil lubrications with EP additives Jen Fin Lin, Ming Guu Shih, Yih Wei Chen Departmento/ Mec~nica! Engineering, National Chenggang [lniversily, 70101 7ainan. Taiwan

Received30 June 1995;accepted20 January1996

Abstract A thrust-on-washer adapter was applied to simulatesurfacecontacts under oil lubrications. The upper, •g-style specimenswere prepared using A356.0 aluminum alloy material; the lower, disk-stylespecimens were prepared using a compositematerial of 6061 aluminum alloy and graphite. The load applications were increased so that friction contacts ranged from hydrodynamic or mixed lubrication to boundary lubrication. The tribologicalmechanism and antiseizure performanceof oil with differentextremepressure (EP) additiveconcentrations was shown to relate closely to friction coefficients,Stribeckcurves,elecUicalvoltagevariations, wear mechanisms, and chemical productsin wear debris. The experimental results reveal that the occurrence of surface seizure is dependent upon the value of the oil temperature rise per second,rather than the oil temperatureitself.When the initial oil temperaturewas kept at 25 °C, raising the EP additiveconcentration shortened the time before surface seizure occurred. When the initial oil temperature was 70 °C, an increase in the EP additive concentration fully prevented surface seizure. Under boundary lubrication conditions, the EP additive alone was unable to generate chemically reactive films regardless of oil temperatures;the antiwearperformanceoccurredonly in tribosurfaceswhen operating in hydrodynamicor mixed lubrication regimes. geywords: Aluminumalloy/graphitecomposite;Oil lubrication;Stribeckcurve;Seizure

1. Introduction When lamellar solids such as graphite or molybdenum disulphide are applied to sliding surfaces, friction and wear decrease. The crystal structure of a lameltar solid has sheets or layers which are weakly bonded with other sheets but strongly bonded individually. In recent years, considerable work has been done on the metal matrix-graphite particle composites which exhibit low friction, low wear rates, and excellent antiseizing properties. In these composites, graphite presumably imparts improved tribologieal properties through the formation of a graphite rich film on b e tfih3surface. Metal-matrix and polymer-matrix composites with solid lubricating phases have been shown to have beneficial effects in a wide range of applications. However, several studies [ 14] point out that the wear rates of the composites have been too high, and the friction coefficients have been too variable. The thickness, composition, and other structural features of the lubricating film formed during wear are largely unknown. The lubricating film, if it forms, seems to be very thin; sometimes it is only a few atoms thick. This is essentially the same order of magnitude as the low temperature oxide films formed on the surface of metals exposed to air [5], 00a,3-1648/961515.00© 1996ElsevierScienceS.A. All fightsreserved Pli S 0043 - 16¢ 8 ( 96 ) 069 32-2

Sugishita and Fujiyoshi's study [6] was designed to assess cast iron as a self-lubricating, metal.based composite material. The formation of graphite films on gray cast iron surfaces was discussed in their work. Solid lubricants in sdf-lubricating materials seem more promising than liquid lubricants, particularly at hish temperatures, in vacuum environments, cast iron as a self.lubricating, ntetal-based composite mate° rial. The formation of graphite films on gray cast iron surfaces was discussed in their work. Solid lubricants in composite, stir-lubricating materials seem more promising than liquid lubricants, particularly at high temperatures, in vacuum environments, and in sealed systems [7]. Composite material consisting of a copper matrix with a solid lubricant phase of up to 70% graphite was previously studied under sliding wear conditions. An analytical model was developed that relates composite wear to the properties oftbe composite and of the interracial film [8]. But in order to fully understand the tribological behavior of metal-matrix, self-lubricating, particle eomnosite materials, we must first learn more about the changes which occur in graphite particles, the reactions between the ambient atmosphere and the matrix alloys, and the basic mechanism of the formation of lubricating films, Some of these problem have been addressed by Prasad and

59

L F. I.in et aL / Wear198 (1996)58.-70

Rohatgi [4], Kuhlmann-Wilsdorfet al. [9], and Das et al. [lO]. Basic powder metallurgy (PM) includesthe mixing,compacting, and sinteringof the metal powders that constitutethe matrix and graphite powders. The initial process of mi#.ng raw materials is an important first step, since this controls the distribution of particles and the porosity of com~sites, ~',h of which influencetribologicalbehavior.Segregationor clustering of particles is thus an important part of the mixing process. Segregation establishes the flow characteristics of different powders during mixing [11] and influences the tendency of particle agglomeration,thereby minimizingsurface energy. After the mixing operations, mixtures of powders are pressed in a die at pressures that make the mixtures adhere at contact points. This process is called compacting. The primary control paran~etersduring sintering are temperature and atmosphere. Inadequate bonding between the graphite and the matrix at sintering temperaturesbelow the melting point of metal results in poor composite strength. Techniques developed to overcome this problem include the use of mechanical alloys and the hot pressing process. It has been reported that sinteringcompactsunder pressure increase the density of composite materials up to 98% more than the theoretical density [11,12], and two aluminum-basedalloy composites were formed with flake-type graphite particles ranging in size from 75 to 180-185 I~m [ 13,14]. Research on the processing parameters related to the powder metal= lurgy of the 6061 aluminum alloy/graphite composite materials was done by Jha et al. [151, and Jha and Prasad [ 16]. In the study of Jha et al. [ 15], the wear rate increasedlinearly as the graphite content increased when the graphite in the composite material was elevated to a volume of 14%. The friction between two mutually contacting solid surfaces arises fi'om the interaction of discrete points of asperities. Wear due to adhesion, delamination, and abrasion was found to be directly proportional to the applied normal loads and the sliding distance, and inversely proportional to the hardness of the wearing body [ t7-19]. The force required to overcome friction ~onsists of the force required to shear the adhesion bond and the force required for the formationof Masticor plastic deformations.The asperitiesof the relatively softer material sliding in the path of the asperities of the relativelyharder material,the coefficientof friction,s,is given by f=f= +fd, where f¢ and fa are the coefficients of friction due to adhesion and deformation,respectively [20]. In elastic-plastic materials like composites, wear particles are generated by the following mechanisms: (a) adhesion,

deformation,and the fracture of asperities, (b) ploughingby hard entrapped particles or hard asperities at the sliding surface, (c) delaminationcausedby subsurfacecrack nucleation and propagation [21-24]. In the study of Rohatgiet al. [25 ], the friction behavior of composites of aluminum alloys and graphite was investigatedin a discussion on Lhe eff~t of ii~ matrix and the characteristicsof lubricating films, Not only does the work of Rohatgi et al. [26] provide a review of existing literature on the tribological p~operties of metal matrix-graphiteparticlecomposites,but it also offers a model for the formation of these films. The tribological behavior of aluminum matrix graphite particle composites lubricated by an engine oil with various concentrationsof extreme pressure (EP) additives has yet to be fully understood. In the present study, fine graphite particles were blendedby means of powder metallurgy with 6061 aluminumalloy powder to form 3 wt.% self-lubricatingcomposite materialwhich was then placed on the lower, disk-type specimens.The lubricationswere carried out in an engine oil at initialtemperaturcsof 25 and 70 °C. The effects of graphite particles in aluminum composites and the EP additive on tribologicalperformancewere evaluated through the analysis of Stribeck curves, electrical voltage plots, and friction coefficient and oil temperature variations for the following three regimes: hydrodynamiclubrication, mixed lubrications, and boundarylubrication.While the effectoftheelectricalcontact resistance (ECR) function and tribology is understood to be related to the formationof a thin lubricatingfilm betweenthe tribosurfaces, no evidence has been found m indicate that chemically reactive films were generated during the lubrication process, irrespectiveof oil temperature. The antiseizure performances arising from the change of the EP additive concentration were also investigated at two different initial oil temperatures.

2. Experiment Aluminumalloy premix 6061 powder and natural graphite were selected as the matrix and dispersoid,respectively.The chemical components and the mechanical properties of the aluminumalloyare listed in Table 1. The fine naturalgraphite particles were measured to have a mean diameter of 27 ± 8 Ixm, and the 6061 aluminum alloy powder was measured to have a mean diameter in the range of 30 to 60 p.m.The 6061 premix was mixed with 3 wt.% natural graphite particles without coating in a double cone blender for 12 h.

Table I The chemical element contentsand the mechanica! propertiesof the 6061 aluminum alloy

Chemicalcomposition(wt.%)

Si

F~

Cu

Mg

Mn

Zn

Ti

6.5-7,5

0.20 max

0.20 max

0.25--0.45

0.I0 max

0,I0 max

020 max

Density

Coefficientof Modulusof therm~ expansion elasticily

Melting Specific temperature

2.685g cm -3 at 20 ~C

21.5 p,m m-t K-t at20-I00 °C

$55--615°C

Tendo~ 72,4 OPa

%3 J kgat 100~

60

J.F, Linet al. I Wear 198 f1996) 58-70

Table2 Thechemicalcompositionsandthe me,'hanicalpropertiesof the A356.0alun~numalloy Chemical composition (wt.%) Si

Fe

Cu

0.4-0.8

0.7 0.15-0.4 ck~x

Density Mn

ME

0.15 0,8--I.2 max

Cr

Z,'~

0,04-0,35 025 max

Coefficientof thermalexpansion

Modulusof e!asr.ic;.ty

23.6pm m- ' K-t at 20-100 °C

Tension, 68.3 GPa

Melting

Specific

~c~peratu~

heat

582-652 °C

896J kg-' at 20 °C

Ti 0,15 2.7 g cm-3 max at 20 °C

Table 3 The engine oil properties

Specific gravity (at 15.6 °C)

0.8%

Dynamic viscosity (Pa s)

Expressionfor oil viscosity as a function of temperature

40°C

lO0°C

0.1980

0.0171

logtologlc,(u+ 0.8) = - 3.371 log~oT+ 9.644 =

=v=ldngmatic viscosity (cSt); T= absolute temperatu~ (°R).

Cylindrical pellets which were 55 mm in diameter and 120 ram in height were compacted in a rigid steel die on a double-acting hydraulic press at a pressure of 49 MPa. Then, the cylindrical pellets were placed in a closed tubular steel container to degas to 10- 3 Torr for 2 h, at a degassing temperature of 450 °C. The compacts were sintered in a tubular furnace for 3 h. The sintering temperature, which is strongly dependent upon the graphite content in the aluminum alloys, was 600 °C. The compacts were brought to '1"6 condition through solution treatment at 521 °C for 30 rain, artificial aging at 170 °C for 24 h, and annealing at a temp:r:ture of 413 °C. Hardness measurements were carried out on a Rockwell hardness testing machine using a 1.6 mm diameter steel ball under a load of 147 N ( 15 kgf) ( !5T type). The 6061 aluminum alloy was detected to have a hardness of approximately Hv (5 kgf)=44. The compacts were measured to have a theoretical density in the range of 93 to 96%. The micrograph of the composite with 3 wt.% graphite content is shownin Fig. 1. The dark regimes represent the pores or voids which were left behind by the evacuation of graphite particles from surfaces during the polishing process, Aluminum alloy A356.0 ingo~z were. selected as the upper specimen material. The chemical element contents and the mechanical properties of the A356.0 aluminum alloy are depicted in Table 2. The heat treatments included reaching the T6 condition through solution treatment at 525 °C, artificial aging at 155 °C, and annealing at a temperature of 343 °C. The hardness of the A356.0 aluminum alloy is 105 Hr.

The experiments were carried out on a wear testing machine with a thrust-on-washer adapter at normal ambient temperature; the dimensions of the upper and lower specimens are shown in Fig. 2. The lower and upper specimens were polished to obtain surface roughness between 0.2 and 0.3 p,m before testing. The upper, ring-style specimens were made of A356.0 aluminum alloy, whereas the lower, diskstyle specimens were prepared using the 6061 aluminum

Fig. 1, Microstructureof aluminum graphite particle compositewith 3 ca.% graphite content.

A •A ~ ~e'IPl

Fig. 2. Dimensionsof the upper and lower specimens,

alloy which contains fine graphite particles as a self-lubricating material. The friction and wear tests under oil lubrications were carried out in an engine oil with different EP additive concentrations. The upper and lower specimens in tribocon-

J.F, I.~n a al, I Wear 198 ( i~'~,} ~8-70

act operated in an oil container without circulation which was filled with 60 cm3 of oil. The oil was kept at a fixed temperature before testine~thr~,,gh..- ~ of an electrical heater which surrounded the oil container. The electrical heater was automatically shut offwhen testing began. The rotations were kept at aconsta~.t spr.zd of 1000 r.p.m. ( IAi m s-'),Inorder ;o test the thermal effect of frictional heat on lubrication performance, the oil before testing was set at either 25 or 70 °C. The Stribeck curves were obtained by steadily increasing load from the normal pressure of 1.684 MPa to 10.10 MPa in 40 rain, which were divided into 12 equal time steps. The lubricant properties of the engine oil are demonstrated in Table 3. The oil viscosity used to express the Hersey number was determined through the following equation: logan logan( o + 0.8) = n Iogto T+ C

(1)

61

wheti~, r is the kinematic viscosity in centistokes, Y is the absolute tem~rature ;.n degrees ~.ankine, and n and c are constant for any given oil. These two values are determined to be n = -3.371 and c=9.644 for this engine oil with viscosities given at 40 °C and 100 °C.

3. Results and discussion The friction and wear behavior of 6061 aluminum alloy containing 3 wt.% graphite particles were examined in an engine oil with various concentrations of EP additive. In this study, we address: (1) how the graphite particles affect the tribological behavior in three lubrication regimes; (2) the likelihood of forming chemically reactive films on the ale-

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Fig. 3. V~ations of friction c~fficient with sliding time in oil lubrications, with EP additive ~nccntmti~,~ of (a) 0 wt,%: (b) 3 wt,%; (c) 5 wt.%; (d) 8 wt.%. Initial oil temperature before testing was kept at 25 ~C.

62

J.F. Hn et el. I Wear 198 ( l ~J6) 58-70 IO.OO--

10,00 " a

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( a ) S]iding time, (s)

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( b ) Sliding lime, (s) IO.OQ

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Fig. 4. Plotsof electricalvoltagecreatedin oil lubricationwithEP additiveconcentrationof (a) 0 wL%;(b) 3 wt.%;(c) 5 wt.%;(d) 8 wt.%.Initialoil temperaturewassustainedat 25 °C minum alloy surface; (3) the performance of graphite particles in aluminum a~loy in impeding surface seizure. Seizure is the instantaneous stopping of relative motion as the result of interfaeial friction. Local so!id-state welding may be part of the seizure mechanism. Seizure usually involves local welding and significant damage to the mating surfaces. A common caus~ of seizure is the loss of fluid film clearance duo to the increasing load application, It is well known that both members of a me,'altic friction pair influence friction and wear behavior. The best friction couple is that which c~,mprises two metals and shows no solid solubility. The friction coefficients which were recorded every 2500 s are shown in Fig. 3(a) to Fig. 3(d) for the lubricating oil with different c,-ncentrations of the EP additive. These four additive concentrations include 0 wt.%, 3 wt.%, 5 wt.%, and 8 wt.%. The oil temperature was kept at 25 °C before testing.

In these plots, the numeral '0' marks the regime where hydrodynamic lubrication was dominant; '1' indicates the mixed lubrication regime; '2' indicates the boundary lubrication regime; and '3' indicates the regime where seizure happened with a iligh but limited or an infinitely high friction coefficient. All of these regimes wore identified through analysis of the plots of elec~ical voltage demonstrated in Fig. 4(a) to Fig. 4(d). The time from the beginning of rubbing action to establishing a fluid film of complete electrical insulation in hydrodynamic lubrication is the run-in period. The time for run-in and its wear condition on the rubbing surface is dependent upon the applied load, the material hardness, the surface roughness of the upper and lower specimens, and the EP additive concentration in the lubricant. During the initial period of run-in, the contact spot migrations generated a

63

J,F. Lin et al. I Wear 198 (1996) 58-70

0,14

--

0.131-

0.1~ -

O.lO

The EP additive e~en~'ation:O wt.% Initial oil tcmpera~:2$'C

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040

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0,06-

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0.10

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Fig. 5. Stdbeck curves createdin the oillubricationswith EP additiveconcentrationof (a) 0 wt.%; (b) 3 wt.%; (c) 5 wt,%; (d) 8 wt.%. Initialoiltemfxn'alure before testingwas sustainedat 25 °C,

rather unifomdy sheared and work-hardened surface layer. Local shearing-offofasperitiescontinued throughout sliding. This action should end when the microroughness appropriate to the pre,,ailing sliding conditions reaches equilibrium. The friction coefficients of the equilibrium state were quite stable: the concentration changes in the EP additive simply induced an insignificant effect on the run-in period because the applied load at the beginning of this period was quite low. However, the friction coefficients in the regime without voltage showed stoop decline from a high value in this short run-in period, In regime O, the friction coefficienls related to hydrodynamic lubrication fluctuated widely except for that shown in Fig. 3(d) whel'e the hydrodynamic lubrication was not entirely separated from the mixed lubrication. The duration of hydrodynamic lubrication was effectively extended through the increase in the EP additive in the base oil.

As the loading was furtherincreasedsuch that friction contactscame up with mixed lubrication,the frictioncoefficientsin thisregime declinedconstantlywhether or not the EP additivewas used in lubricatingoil.The fu~:r du~.as¢ inthefrictioncoefficienteven underincreasingappliedloads can be attributedto the combined effectof two favorable conditions:(I) the releaseof graphiteparticlesfrom the aluminum alloyin the lubricatingprocess,and (2) the formason of a thinFluidfilmbetween graphitelamei|as.The durationof the mixed lubrication~gime was reducedwl~n the ~P additiveconcentrationin the base oilwas increase. As the loadingwas furtherincreased,tribosurfacesentered the boundary lubricationregime,and the frictioncoefficients a,'isingin the lubricatingoilwith EP additiveconcentration less than 8 wt.% ascended constantly; however, the friction coefficients dropped in the oil with 8 wt.% EP additive. It

64

J,F. Linet al. / Wear198 (1996)58-70

can, therefore, be concluded that a sufficiently high concentration of the EP additive in the oil encourages the lowering of the friction coefficient in boundary lubrication. The frictional heat generated in boundary lubrication raised the contact temperature rapidly when heat was not conducted from the specimens; the frictional heat softened the material and eventually caused local asperity weldings under normal loads. A limited but high or an infinitely high friction coefficient was observed. The EP additive obviously does not impede the occurrence of surface seizure in oil lubrications at an initial oil temperature of 25 °C. The duration before the occurrence of surface seizure was shortened drastically when the additive concentration was increased in the lubricating oil. Detailed investigation reveals that all four voltage plots show both continuous and discrete voltage. Continuous voltage always appeared in the initial stage of sliding and is thought to be induced by the formation of a thin oil film between the two mating surfaces; wear particles were seldom produced in this regime. When the load was increased so that the contacts reached the mixed lubrication regime, the run-in process occurred intmediately, and wear particles created during the run-in process were entrapped between two mating surfaces. A great reduction in real contact area was obtained when the third-body wear formed a high resistance to the electric current, thus producing a high voltage electric current. When this happened, a thin fluid film was observed in the clearance area. When the load was further increased so that the entrapped particles fell down and were then scraped out of the wear tracks, the metallie contacts of two mating surfaces allowed the voltage to drop to zero. Consequently, the intermittent voltage appeared repeatedly in the mixed lubrication regime until the friction contacts reached boundary lubrication. In the boundary lubrication regime, the mating pair showed metallic contacts and the voltage became zero because the current resistance across the metallic contacts in asperities was negligibly small. There exists a boundary lubrication regime between the dash line and the point line (regime 2), where low friction coefficients were present even though the fluid film had vanished. The low friction coefficients in this regime were caused by the release of graphite particles from the aluminum/graphite composite material, However, damage of the fluid film between the two contact surfaces brought about steady increases in the friction coefficient when the EP additive concentration in the lubricants ranged from 0 wt.% to 5 wt.%, The friction coefficients in this regime dropped when the EP additive concentration was increased to 8 wt.%. A steep increase in the friction coefficient in regime 3 was caused by surface seizure. Surface seizure occurred in aluminum alloys lubricated by the oil with 25 °C as the initial oil temperature. In regimes 0 and 1, the duration in which the triboeontacts underwent hydrodynamic and mixed lubrications was related to the wear loss of the aluminum alloys. Theoretically, a lower wear loss will result when tilt time span of these two regimes is lengthened. Longer hydrodynamic and mixed lubrication regimes occurred in the oil with a 3 wt.% additive eoncentra-

tion and that without the EP additive. This shows that an appropriate additive concentration encourages the extension of hydrodynamic and mixed lubrication regimes. For insta'ace, when the lubricating oil contains 3 wt.% additive concentration, the extent of time in hydrodynamic lubrication is longer than it is when there is no EP additive. At the same time, excessive EP additive in the lubricant results in a substantial reduction in the duration of hydrodynamic and mixed lubrications regimes. It must be noted, however, that the amount of time in hydrodynamic or mixed lubrication regimes is not the sole factor governing wear loss; several other factors are also involved. The lubricating oil without the EP additive had the longest time in the boundary lubrication regime, and the use of the EP additive shortened the duration of boundary lubrication. Severe wear such as surface seizure appeared in oil lubrications when the load was increased steadily; the occurrence of seizure implies that the improper addition of graphite particles in aluminum alloy does not prevent severe wear. Fig. 5 (a) to Fig. 5 (d) portray the Stribeek curves pertinent to four different EP additive concentrations in the lubricating oil. These curves recorded the variations in friction coefficients as step loads were increased over 12 equal time steps. The letters on the Stribeck curves indicate frictional contacts in hydrodynamic and mixed lubrications. The EP additive in the lubricating oil caused relatively higher friction coefficients in the mixed lubrication regime. Large increases in friction coefficients were obtained when the lubricating oil contained one of the following additive concentrations: 0 wt.%, 5 wt.%, and 8 wt.%. However, an infinite value of friction coefficients was seen only in the oil with a 3 wt.% additive concentration. The plots of electrical voltage in oil lubrications are shown in Fig. 4(a) to Fig. 4(d) for four additive concentrations. Positive electrical voltage values only appeared in pan of the overall sliding time (2400 s). Comparisons of these four plots with Fig. 5 (a) to Fig. 5 (d) reveal that electrical voltage appeared only in hydrodynamic and mixed lubrications, and vanished when boundary lubrication dominated. There was no evidence that chemically reactive films had been created under boundary lubrication. The duration of time in hydrodynamic and mixed lubrication regimes was shortened greatly as the additive concentration in the lubricating oil was increased to 5 wt.% or 8 wt.%. The worn surface micrograph of the lower specimen (Fig. 6) shows the lubrication test that was stopped by the occurrence of seizure. The frictional contacts were carried out in a lubricating oil with 8 wt.% additive concentration. Numerous microcuttings resulted from the ploughing of hard entrapped particles which were uniformly distributed over the contact surfaces; some deep grooves were formed. Fig. 7 depicts the micrograph of the lower specimen for the lubrication test that was stopped by surface seizure, Surface delamination and plastic deformation, rather thau microcuttings, were the dominant wear mechanisms in surface seizure.

J.F. Kin et al, I Wear 198 (1096) 58-?0

Fig. 6. Mierogmph of the disk'swom surfacecreatedin oil lubrications which stoppedjustbeforesurfaceseizureoccurred:EP additiveconcenlralioninthelubricantwas 8 wt.%, and theinitialoiltemperaturewas sustained at25 °C beforetesting,

65

Fig,7. Micrographof the disk'sworn surfacecreatedin theoillubrications which stoppedas surfaceseizureoccurred',EP additiveconcentrationinthe lubricantwas 3 wt.%. and the initialoiltemperaturewas sustainedat25 °C.

034

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66

J. F. l, in ¢: al, / Wear 198 (1996) 58-70

The lubricant before testing was preheated to the prescribed temperature in order to investigate the effect of temperature on lubrication behavior in aIuminum alloys. At an initial oil temperature of 70 °C, the electrical voltage generated in the mixed lubrication regime was low regardless of additive concentration. This illustrates that (1) the fluid film thickness between two rubbing surfaces was greatly reduced due to the lowering of oil viscosity at higher temperatures, and (2) no chemically reactive films exist in the wearing process even when higher initial oil temperatures are used. The Stribeck curves in Fig. 8(a) to Fig. 8(d) show the variations of friction coefficients with four EP additive concentrations; lubrication tests started with an initial oil tomperature of 70 °C. The dash lines which are used to indicate the differencebetween mixed lubrication and boundary lubrications were positioned with the steep rise in friction coefficient as indicator of the transition between regimes. The

o i+.7 TheEPadditiveoow,,enumtion:3vd.% tnitiaioiltmpcrat~:70~

TheEPadditiveconcentration:Owt.% Initial oil tcmperatute:70"C

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Hersey numbers associated with the transition from mixed lubrication to boundary lubrication became larger when the EP additive concentration was increased, However, the use of the EP additive allowed the frictional contacts to reach the maximum friction coefficient in boundary lubrication and friction coefficients then declined as the Hersey number was further increased; furthermore,surface seizure was preven~.ed entirely. In the boundary lubrication regime, the release of graphite particles from the aluminum alloy assisted lubrication. The largest drop in friction coefficientstook place in the oil with a 5 wt,% EP additive. However, the friction coefficients in mixed lubrication with this additive concentration were also relatively higher than those of the other three additive concentrations. The variations of friction coefficients in relation to Stribeck curves are shown in Fig. 9(a) to Fig. 9(d). The dash lines indicate the transition from mixed lubrication to boundary

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(d) Slidlngtime, (s) (c) Slidinglime,{s) Fig.9. Variationsof frictionco¢fficicntwithslidingtimeinoillubricationswithBPadditiveconcentrationof (a) 0 wt.%;(b) 3 wt.%;(c) 5 wt.%:(d) 8wt.%. Theoil waspreheatedto an initialtemperatureof 70°C,

67

I.E. Lin el al. I Wear 198 (I996) ~8--70

(a)

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Fig. 10. X-ray maps for the wear debris obtained from li~ used lubricant with (a) 25 °C and (b) 70 °C as the initial nil temperature.The EP additive

concentrationin the lubricantwas 3 wt.%. iubrication; the point line is shown in Fig. 9(a) to depict the steep rise in the friction coefficient due to surface seizure. This violent increase appeared only in frictional contacts without the EP additive. When the lubricant with the EP additive was preheated toT0 °C before testing, a mild increase in the friction coefficient was obtained rather than a steep increase, and surface seizure was prevented. The chemical compounds in oil lubrications can be detmed by XRD (X-ray diffraction). Fig. 10(a) and Fig. 10(b) show the X-ray maps for the wear debris generated in the lubrication tests at a sliding speed of 1.41 m s-i under normal pressure of 10.1 MPa, but at initial oil temperatures of 25 and 70 °C, respectively. As is shown, no other chemical compounds were found other than the cl':mical elements, C and AI, which were originally in the aluminum alloy composite material. Fig. ll(a) to Fig. ll(d) show the oil temperature variations with time in the lubricating oil with four different EP additive concentrations. The solid line represents the temper-

ature variations of the oil with an initial temperature of 25 °C, while the dash line represents temperature variations for the oil with an initial temperatureofTO °C.These oil temper°tore curves show noticeably different temperature gradients in hydrodynamic and mixed lubrications, boundary lubrication, and surface seizure. The temperature gradient is here defined to be each degree of temperature rise per second of sliding. In hydrodynamic and mixed lubrications regimes, all curves show the temperature gradient value to be small, but the oil temperature gradients related to the boundary lubrication and surface seizure regimes to be quite high. Oil temperature is naturally dependent upon the frictional heat developed in tribocontacts. High friction coefficients occurring in boundary lubrication or surface seizure always result in significant temperature increases. Because of the great difference between the two initial oil temperatures, the oil temperatures on the dash curve in Fig. 1! (a) are always higher than those on the solid curve for the entire sliding time of 2400 s. In Fig. 1! (b), the oil temperatures shown by the dash line are still higher even when surface seizure occurs. A reasonable interpretation of this finding is that high friction coefficients are found much earlier under boundary lubrication. Fig. I l(c) shows that the frictional heat resulting from surface seizure is much greater than that generated in the boundary lubrication regime where low friction coefficients are obtained. A similar char~;eristie is shown in Fig. 11(d). Significant increases in oil temperature, as indicated from Fig. 11(a) to Fig. 1 l(d), were created only when boundary lubrication or surface seizure dominated. The tribological behavior demonstrated in oil lubrications was affected by both the EP additive concentration in the lubricant and the iJ-titiai oil temperature. Surface seizure occurred in the lubricant with an initial temperature of 25 °C irrespecPive of the EP additive concentration. When the lubricant was preheated to an initial temperature ofT0 °C, the EP additive in the lubricant impeded surfa~ seizure. The sole exception to this occurred in the lubricant without the EP additive. One can thus conclude that the performance of the EP additive is improved when the initial oil temperature is sufficiently high. The time durations of hydrodynamic and mixed lubrication regimes are shown in Table 4 for the initial oil temperatures

Table 4 The lubrication performancesin the oil with various EP additive concentrations Initial oil temperature before testing (°C)

EP additive concentration (~.%)

Time in hydrodynamicand mixed lubrications (s)

Staying lime without seizure happening (s)

25

70

Tempenh'uregradient in seizere regime ( ~ s "t)

0

1600

2100

0.11

3 5 8 0 3 5 8

1600 10~) 1000 t600 tO00 650 1210

1800 1420 I330 I850 2400 (no seizure) 2400 (no seizure) 2400 (no seizure)

0.105 0,094 0.093 0.104 -

68

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(d) Slidingtime,(s) Fig. l 1. v~ations of oil temperaturewithslidingtimecreatedin the oil lubricantwilhtheEPadditiveconcentrationof (a) 0 Wl+%;(b) 3 wt.%;(c) 5 wt.%; (d) 8 wt.%. of 25 and 70 °C. At an initial oil temperature of 25 °C, the duration of hydrodynamic and mixed lubrication was shortened somewhat when the EP additive concentration was increased. An even more noticeable decrease in hydrodynamic and mixed lubrication regime duration was exhibited in the oil with the EP additive concentration of 5 wt.% and 8 wt.%. At an initial oil temperature of 70 °C, the time duration of hydrodynamicand mixed lubrication regimes reached its lowest value when the EP additive was increased to 5 wt.%. This duration was then expected again when the additive concentration was increased to 8 wt.%. Since selflubricating solids in aluminum alloys, like graphite operating in oil lubrications, enhance lubrication, the delay or prevention of seizure occurs even when the film is absent. The appropriate concentration of the EP additive in the lubricant undoubtedly depends strongly on the project application. The time duration before surface seizure is simultaneously dependent upon both the EP additive concentration in the lubricant and the i,,itial oil temperature, At an initial oil temperature of 25 °C, the time duration without seizurewas short-

ened as the additive concentration was increased. However, at the initial oil temperature of 70 °(2, the duration without seizure was greatly extended as the amount of EP additive was increased. As a result, surface seizure was entirely prevented. The formation of chemical film, in general, can be roughly divided into three processes. First, if the oil temperature is sufficiently high, the EP additive in the base oil undergoes the decomposition process. Secondly, the decomposition products are absorbed onto the rubbing surfaces to form a chemisorbed film, Finally, some elements or compounds in the chemisorbed film react chemically with the iron-based subsurface. The chemisorbed film and the inmost chemical reaction film compose the main body of the chemical film, The layer above the chemical film is the gel-like material [27]. This gel layer shows little effect on antiwear performance. Normally, raising the oil temperature can improve the diffusion rate of the EP additive. At the initial oil temperature of 25 °C, the oil operated at so low temperature that the decomposition was attenuated or even was impeded fully, In

J.F. I.in et aL I Wear 198 (1996) 58-70

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(d) ++I " , -: *" , : ' •s ',"t z - * I " Fig+13.Weardepthprolilesofthelowerspecimencmm~linoillebricalinm undernormalpressureof 10.101MPa.at a slidingspeedof IJl m s-S+ overallslidingtime is up to 480 s. The EP additiveconcentrationin the lubricantwas (a) 0 wt.%;(b) 3 wt.%; (c) 5 wt.%; (d) 8 wt.%,and the initialoil temperaturewas sustainedat 25 eC.

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Slidingtime, (s) Fig. 12, Plotsof electricvoltagecreatedin oil lubricationsundernormal pressureof 10.101MPa,at a slidingspeedof IAI m s- t TheEP additive concenlmtionin the lubricantwas (a) 5 wt.%and (b) 8,.vi.%,endtheinitial oil temperaturewas sustainedat 25 ¢C, (to)

such circumstances, surface seizure is expected to occur easily if the load was sufficiently high. At an initial oil temperature of 70 °C, the EP additive perhaps underwent the decomposition process during the oil lubrications; however, the oil temperature in the entire process was insufficiently high to permit the compounds in the chemisorbed film to ic++clv+i+.',the aluminum al~ay. Nevertheless, both the chemiso~bed film and the gel layer became thicker by the increasing oil temperature, thus resulting in an enhanced antiwear performance; surface seizure was thus impeded fully. Thebe results imply that oil temperature is not the dominant factor in determining the likelihood of surface seizure. Rather, it seems that temperature gradient plays an important

role. It is interesting to note that temperature gradient corre. spending to the case of surface seizure varied in the range from 0.093 to 0.11 °C s- t irrespective of the initial oil tem+ perature. That is, surface seizure is only exhibited when temperature gradient is sufficiently high. The higher the temperature gradient, the quicker surface seizure occurs after sliding. When the oil was at an initial temperature of 70 °C, the temperature gradient was consistently low despite varia. tions in EP additive. Consequently, surface seizure never occurred. However, the temperature gradient in the lubricant without the EP additive was 0.104 °(2 s - t and surface seizure occurred. From the above discussion, the following two conclusions can be drawn: (1) increasing the EP additive concentration in the lubricant helps lower the temperature gradient, and thus delays or avoids the occurrence of surfm:e seizure; (2) increasing the initial temperature of an oil results in decreased temperature gradient values. Fig. 12(a) and Fig. 12(b) show the measure of electrical voltage in the wearing process under constant nmmal pressure of 10.1 MPa, with a fixed sliding speed of IAI m s -~. The electrical voltage was extremely low in lubricating oils with 0 wt+% and 3 wt.% additive concentrations. Therefore, only the electrical voltage plots droll with 5 wl.~, and 8 wt.% EP additive concentrations are exhibited. The electrical voltage generated in the lubricant with 5 wt.% additive concentration is obviously higher than that in the oil with 8 wt.~ additive concentration. Since the sliding time in this regime lasted only 480 s, the limited rise in oil teml~rature still permitted the tribocontacts to operate in mixed lubrication even under the normal pressure of 10.1 MPa. The rate of decomposition of the EP additive ~s aiso dependent upon its additive concentration in the lubricant. If a high loading corresponding to the normal pressure of 10.101 MPa was applied from the beginning of the oil lubn-

70

J.F. Un et aL/ Wear 198 (1996) 58-70

cation, higher concentration could increase the diffusion rate into the aluminum alloy matrix, Both the chemisorbed film and the gel-like layer grew thicker. Consequently, it took a longer time operating in the run-in process. The depth profile of the wear track on lower specimens was employed to evaluate wearloss. Fig. 13(a) toFig. 13(d) demonstrates the depth profiles of the wear tracks lubricated by the oil with various additive concentrations, The wear depths produced in the lubricants with 5 wt.% and 8 wt.% additive concentrations are quite shallow compared to those under the lubrications with 0 wt.% and 3 wt.% additive concentrations, Shallow wear tracks exhibited on worn surfaces imply that a thin lubricating film was formed in mixed lubrication, thus resulting in a smaller wear loss. This viewpoint is confirmed by measurements of electrical voltage for the oils with different additive concentrations.

4. Conduslon 1. EP additives in engine oil do not generate chemically reactive films on rubbing surfaces, irrespective of the initial oil temperature. Thus, under boundary lubrication regimes, antiwear performance is strongly dependent upon the formation of a thin oil film between the two mating surfaces and the lubricat:ion performance of graphite particles. 2. Oil temperature gradient--not oil temperature--is the critical factor in determining surface seizure. Surface seizure only occurs when oil temperature gradient values are sufficiently high. 3. At an initial oil temperature of 25 °C, increasing the EP additive concentration shortens the length of time before surface seizure, and seizure eventually occurs, At an initial oil temperature of 70 °C and with the EP additive in the oil, surface seizure can be prevented. 4. At an initial oil temperature of 25 °C, an appropriate EP additive concentration results in a lengthening of the duration of operation in hydrodynamic and mixed lubrication regimes, However, excessive amounts of the EP additive serve to reduce this duration. 5. At an initial temperature of 70 °C, the EP additive served to drastically reduce the amount of time in mixed lubrication before seizure; however, the same additive caused an extension in the duration of time in boundary lubrication before seizure.

References [ 1] J,K,LancasterandP.Moorhouse,Etched-pocketdrybearingmaterials, Tribal Int., 18 (1985) 139-148. [2] M.N. Gaxdos, Theory and practice of self.lubricatingoscillatoo beatings for high vacuumapplication,Part I: Selectionof the selflubricatingcompositematerJat~,L,br. Eng., 37 ( 1981) 641-656.

[3] D. Nath,T+K+G,Namboodhiriand A. Shanker,Wearof copperbase particularcomposiles,in Int. Wear of Materials Conference, ASME, NewYork. 1985. [4] S.V. Prasadand P,K. Rohelgi,Trihelogicalproperties of AI alloy particlecomposites,J. Metals (1987) 22-26. [5] T.L.Ban, An ESCAstudyof the terminationof the passivafionof elementalmetals,J. Phys. Chem., 82 (1978) 1801-~810. [6] J. Sugishitaand S. Fujiyoshi,The effect of cast iron graphiteson frictionandwearperforce. I: Graphitefilmformationon greycast ironsurfaces,Wear, 66 (1981) 209-221. [7] MB. Peterson and S. Ramalingam, Coatings for tribological applications,in D.A. Riguey{ed.), Fundamental~ of Friction and Wear of Marerials, ASM,MetalsPark,OH, 1981,pp. 331-372, [8] A.W.Ruffand M.B. Petarsnn,We~ mechanismsin self-tubricating metal-matrix composites, in Rohatgi et at. (eds.), Tribology of Composite Materials, ASMInternational,MaterialPark,OH, 1990. [9] D. Kuhlmann-Wilsdorf,D.D. Makel, N.A. Sondergaardand D,W. Maribo,On the two modesof operationof monolithicAg.-Cbrushes, in S.G. Fishmsn and A,K, Dhingra (eds.), Cast reitlforced metal composites, Prec. ASM International. Chicago, Sept, 3.-4,1988. [ 10] S. Dos,S.V.PrasadandT.R.Ramachandran,Micmsttuctureandwear of (AI-Sialloy)-graphitocomposites,Wear, 133 (1989) 173--187. [11] HJ. Rack, Lubrication of high performance powder-metallur~, aluminummatrixcomposites,Adv. Mater. Manor.. Prec., 3 (1988) 327-358, [ 12] A. Sharma,P,R,SoniandT.V. Rajah,Metal PowderRep., 43 (1988) 25-31. [ 13] P,K.Rohatgi,Y. Liu,M, YinandT,L.Ban, A surface-analyticalstudy of tribodefonnedaluminumalloy 319-10col.% graphite particle composite,Mater. Sci. and Eng., A123 (1990) 213-218. [ 14] P,K,Rohatgi,P,K.Lie, Y. Lie and Ban, Surface.analyticalstudyof Iribodefnrmed aluminum alloy 319-10v01.% graphite particle composite,Mater. $ci. Eng. A, 123 (2) (1990) 213--218. [ 15] A,K,Jha,SN. PmsadandG.S,Upadhyaya,Preparationandproperties of 6062 aluminumalloy/graphitecor~I~sitesby PM mute, Powder Metallurgy, 32 (1989) 309-312. [16] A.K. Jha and SN. Pmsad, Sintered 6061 aluminumalloy-solid lubricant p~icle composite: Sliding wear and mechanisms of lubrication,Wear, 133 (1989) 163-172. [17] E. Rabinowicz,Friction and Wear of Materials, Wiley,New York, 1966, [18] T. Sasada+ in Prod. Conf. on Tribology, Japanese Society of LubricationEngineers,Tokyo,1985,623. [19] P.R. Gibson,AJ. Cleggand A,A, Dos, Wear of cast AI-Si alloys containinggraphite,Wear, 95 (1984) 193-198. [20] F.P.Bowdenand D. Tabor,The lubricationby thinmetallicfilmsand the actionof bearingmetals,J. Appl. Phys., 14 (1943) 141-15I. [21] N.P.Suh,Tribophysics, Prentice-Hall,NewJersey, 1986. [22] S. Jahanmir,N.P.SuhandE.P, Abrahamson,Thedelaminationtheory of wearand the wearof a compositesurface,Wear, 32 (1975) 33-49. [23] N.P.Sub, An overviewof the delaminationtheoryof wear, Wear, 44 (1977) 1-16. [24] D,A.Rigney,LH. Chen,M.G.S.Naytorand A.R.Rosenfield,Wear processesin slidingsystems,Wear, 100 (1984) 195-219. [25] P.K.Rohatgi,Y. Liu and T.L. Ban, Modelingand characteristicsof the lubricating film formed during sliding wear of AI/graphite composites,inRohatgiet at. (eds,),TribologyofCompositeMaterials, ASMInternational,MaterialPark,Oil, 1990. [261 P.K. Rohatgt,S. Ray and Y. Lie, Tribologicalproperli~ of metal matrix-graphiteparticlescomposites,In[. Mater. Reviews, 37 (1992) 129-149. [27l H. So and Y.C, Lin, The theoryof antiwearfor ZDDP at elevated temperaturein bot~,~darylubricationcondition, Wear, 177 (1994) t05-115.