mH.EepubliKorea

pertieble ace speh tounderf w1tor ofresu

and hnal lt a cots inic seaplesrocat

replaced in a timely manner, but premature replacement isundesirable since it leads to machine downtime andnancial loss. Hence, it is highly desirable to predict the lifeof a seal so that catastrophic failure can be prevented and atthe same time minimize the nancial loss of prematurelyreplacing the seal.

* Corresponding author. Tel.: 82 2 2123 2822; fax: 82 2 365 0491.E-mail address: kimde@yonsei.ac.kr (D.E. Kim).

1 Currently at Woongjin Coway Co. Ltd., Gwanak-gu, Seoul, Republic ofKorea.

Contents lists available at SciVerse ScienceDirect

Polymer T

journal homepage: www.else

Polymer Testing 31 (2012) 9931000shaft seals are classied as dynamic seals [3]. If a seal isimproperly installed or used, failure will eventually occurwith time. There are several modes of failure in sealsincluding abrasion, compression set, and chemical andthermal degradation [46]. Particularly in the case ofdynamic seals, surface wear occurs gradually over timeeventually leading to opening of the gap and leakage [7].This may cause catastrophic damage to the machine and,therefore, the reliability of seals is crucial for extended lifeof machines.

elastic property of the material, unlike metals or ceramics[810]. In contact sliding conditions, elastomers experiencerelatively large elastic deformation and high internal fric-tion due to their low elastic modulus and high damping,respectively [11]. The main cause of frictional energydissipation of elastomers is due to adhesion at the contactregion and hysteresis effect of internal friction duringdeformation [1214]. If continuous stress is applied to thecontact region, the surface of the seal will eventually wearout and fail. To prevent such a failure, the seal should be1. Introduction

Seals are widely used in cylindersto prevent liquid or gas from interoperate by bearing pressure againsclose the gap between two componenmay be divided into static or dynamthe nature of their application. Examgaskets and o-rings, whereas recip0142-9418/$ see front matter 2012 Elsevier Ltdhttp://dx.doi.org/10.1016/j.polymertesting.2012.07.01ydraulic systemseakage [1]. Theyunter surface tocontact [2]. Sealsls depending onof static seals areing and rotating

Seals are usually made of relatively soft and compliantmaterial such as an elastomer to conform to the contactingsurface. As any other material, elastomers are prone towear under extended contact sliding conditions and this isknown to be the most profound cause of failure of dynamicseals. Thus, in order to optimize the design of seals forprolonged life, the tribological properties of seal materialsneed to be clearly understood. In the case of elastomers, thetribological behavior is greatly inuenced by the visco-Wear 2012 Elsevier Ltd. All rights reserved.FrictionSeal elastomer seals.Test method

Accelerated wear test of FKM elasto

S.H. Lee a,1, S.S. Yoo a, D.E. Kim a,*, B.S. Kang b,aDept. of Mechanical Engineering, Yonsei University, Seodaemun-gu, Seoul, RbKorea Institute of Machinery & Materials, Yuseong-gu, Daejeon, Republic of

a r t i c l e i n f o

Article history:Received 15 June 2012Accepted 25 July 2012

Keywords:Accelerated testingAcceleration factorElastomer

a b s t r a c t

The tribological proof developing suitawear pattern of tha test time of 1000ments performedalumina particles oaccelerated by a factest conditions. The. All rights reserved.7er for life prediction of seals

. Kim b

c of Korea

s of FKM uorocarbon elastomer were investigated with the aimcelerated wear test conditions for this type of seal material. Thecimen under normal operating conditions was monitored overbe used as a reference. The results were compared with experi-accelerated testing conditions. It was found that by mixingmm with the lubricant, the wear of a FKM specimen could be58, while maintaining similar wear behavior as that of the normallts of this work are expected to aid in the prediction of life FKM

esting

vier .com/locate/polytest

work was to provide a fundamental basis for ALT of elasto-mers such as FKM that will be helpful to the design of ALT forindustrial applications.

2. Experimental details

2.1. Experimental specimens

FKM elastomer was chosen for its wide use in industryas a seal material in hydraulic systems and various vacuumdevices. The mechanical properties of FKM elastomer areknown to be relatively good compared with othercommercial elastomers in respect of durability, heat resis-tance and chemical stability [22]. The FKM test specimenswere cut from an o-ring to a length of 20 mm to give the

S.H. Lee et al. / Polymer Testing 31 (2012) 9931000994Predicting the life of machine components is desirableacross a wide spectrum of industry. However, due to thetime and cost involved in obtaining the necessary failureinformation from eld data, accelerated testing methodsare often employed [15]. In order to develop a reliableaccelerated life testing (ALT) method, the mechanism offailure must be clearly understood. In ALT, the mainparameter that affects the life of the component is used asthe control variable [16]. The level of that particularparameter is increased or decreased to change the time offailure of the component. In some cases, there may be morethan one parameter used to accelerate the failure process.In mechanical systems, load, pressure, speed and/ortemperature are often selected as control parameters forALT [17]. In the case of tribological systems, the ALTparameter depends on the mechanism of wear which maybe due to abrasion, adhesion, fatigue or corrosion [18]. Ifcorrosion is the main mechanism of wear in a giventribological system, temperature or concentration of thecorrosive agent may be used as the ALT parameter [19]. Onthe other hand, if abrasion is the main mechanism particlesmay be added to the lubricant to increase the severity ofsurface abrasion [20].

Quantitatively, wear can be expressed by Archards wearequation as shown below [21]:

V kLx=H (1)where V is the wear volume, k is the wear coefcient, L isthe applied load, x is the sliding distance and H is thehardness of the component. It can be seen from thisequation that for given load, sliding distance and hardnessthe amount of wear is proportional to the wear coefcient.Also, load may be increased to achieve the same amount ofwear in a shorter sliding distance for givenwear coefcientand hardness. Thus, based on Archards wear equation,either the wear coefcient or load may be used as the ALTparameter. Although load may seem to be a convenientchoice for the ALT parameter, care must be taken to avoidthe undesirable effects of articially increased load duringthe accelerated wear test. Such effects include grossdeformation of the component and material propertychange due to increased temperature at the sliding inter-face. Another practical difculty of increasing the load isthat the test system has to be sufciently sturdy to endurethe high load. On the other hand, the wear coefcient canbe altered in various ways that can lead to acceleratedwear.

The approach of this work was to identify the effect ofadding particles to the lubricant on the accelerated wear ofFKMelastomer. Essentially, addition of particles is ameans ofincreasing the wear coefcient. A pin-on-plate reciprocatingtribotester was used to perform the wear tests using FKMelastomer specimens sliding against a stainless steel surfaceinboundary lubrication condition. The test represented a sealmaterial sliding against thewall of a piston cylinder. Initially,the wear characteristics of a FKM specimen under normalboundary lubrication conditions were assessed. Then, weartests were performedwith particles added to the lubricant toaccelerate the wear process. By comparing the results of thetwo experimental conditions, the acceleration factor (AF) fortheproposedALTmethodwasderived. Themotivationof thiscross sectional shape and dimensions shown in Fig. 1.The counter surface was a at stainless steel (stainless

steel 304) plate. Stainless steel (SS) was chosen to representa hard metallic surface against which seals are commonlyin contact. The mechanical properties of the specimensused in this work are given in Table 1. Since the hardness ofSS was much greater than that of the FKM elastomer, wearof the SS specimen was not expected to be a signicantconcern during the wear tests. Finally, Rando 46, which iscommonly used as lubricant in hydraulic systems, was usedas the lubricant in the wear tests.

2.2. Experimental set-up and conditions

A pin-on-plate reciprocating tribotester was used forthe wear tests. The FKM specimen was used as the pin thatwas slid against a at SS counter surface. The SS specimenwas mounted on a plate attached to a linear actuator whichreciprocated back and forth. The strokewas set to 40mm ata reciprocating frequency of 1 Hz, which resulted in thetotal sliding distance of 80 mm per second. The stroke waschosen to provide sufcient sliding distance along the axisand the speed was selected to negate the effects oftemperature increase that may occur if the sliding speed istoo high. A load of 450 gf was applied by using a deadweight placed on top of the FKM specimen holder. This loadwas selected based on the desired degree of contact areaand pressure. Essentially, a high enough contact pressurewas desired to generate a signicant amount of wearwithin a reasonable test time. However, excessive contact

Fig. 1. Cross sectional shape and dimensions of the FKM elastomer specimenused in the wear tests. The arrow indicates the surface that is in contact withthe stainless steel plate during the wear test.

alumina particles were added to the lubricant to induce

specimen since it was the peak location as viewed from thecross section of the specimen. Thus, the center region wasexpected to experience the highest contact pressure. Also,wear did not occur evenly across the contact region of theFKM specimen. Rather, wear could be found only at certainlocations along the contact region. The fact that wear didnot occur evenly supported the probabilistic nature of wear.This outcome was in accord with the original concept ofwear coefcient in the Archards wear equation that krepresents the probability of a contact point to form a wearparticle [24]. Thus, it may be stated that, due to someprobabilistic event such as particle introduction, surfaceinhomogeneity, or even system instability, wear is initiatedat particular locations along the contact region. It wasinteresting to note that as the wear test time increased, theexisting wear scars grew in size and new wear scars werealso formed.

As a means to monitor the state of sliding during the

S.H. Lee et al. / Polymer Testing 31 (2012) 9931000 995more abrasion.During the wear test, the frictional force between the

FKM and SS specimens was measured by a load cell andrecorded in real time through a PC-based data acquisitionsystem. The frictional force information was utilized tomonitor the state of sliding contact during the wear test.Fig. 2 shows a schematic diagram of the experimental set-up.

2.3. Experimental method and results

2.3.1. Wear test under normal wear conditionsInitially, the wear characteristics of FKM under normal

sliding conditions were assessed using the normal lubri-cant for 1000 h. During this test, the frictional force wasmonitored and the surface of the FKM specimen wasexamined periodically for signs of surface degradation. Thetest was stopped at approximately 100-h intervals and theFKM specimenwas removed from the tribotester for opticalmicroscope analysis of the surface. After characterizing thewear pattern of the surface, the FKM specimen was repo-sitioned in the tester to continue the wear test. Extremecare was to taken to ensure that the angle and position ofcontact between the FKM and SS specimens were consis-tent before and after the specimen mounting process. Bydoing so, the characteristics and progression of wear of thepressure may lead to secondary effects that are not repre-sentative of the natural wear process. Thus, for the shapeand properties of the FKM specimen used in this work, itwas determined that a contact pressure below 1 MPa wasappropriate. As shown in Fig. 1, for the FKM specimen tipradius of 2.5 mm, the maximum contact pressure andcontact width were estimated to be 0.65 MPa and 0.47 mm,respectively, based on the Hertzian contact theory for anelastic contact between a cylinder and a at surface [23].

The wear tests were performed under boundary lubri-cation conditions, a small amount of lubricant (0.4 g) beingplaced in the contact region. Due to the surface tension ofthe lubricant and because the sliding speed was relativelylow, the lubricant remained at the sliding interfacethroughout the wear test. For the wear test conductedunder normal condition, the lubricant was used unaltered.However, in the case of the accelerated tests, w1 mm

Table 1Mechanical properties of FKM elastomer and stainless steel.

FKM elastomer Stainless steel

Poissons ratio 0.46 0.31Hardness 71 (Shore A) 63 (HRC)Modulus of elasticity 16.5 MPa 207 MPaRoughness 0.139 mmFKM specimen could be obtained as a function of the testduration. This result was used as a reference to comparewith the results of the accelerated wear test.

Fig. 3 shows the optical microscope images of thenormal wear pattern of the FKM specimen at threedifferent locations at various time intervals. The imagesfrom left to right show the generation of wear with respectto time at a similar location on the FKM specimen. A fewinteresting observations could be made from this result.The vertical grooves are marks that were initially on theFKM specimen believed to be formed during themanufacturing process of the o-ring. Wear scars appearedas dark horizontal strips aligned in the direction of sliding.The wear pattern indicated that wear occurred at anextremely slow rate since the forming marks on the FKMspecimen could still be found after 1000 h of wear test.Wear scars were mostly found at the center region of the

Fig. 2. Schematic of the experimental set-up used for the wear tests.wear test, the frictional force was measured in real time.Fig. 4 shows the friction coefcient between FKM and SSspecimens during 1000 h of normal wear test under anapplied load of 450 gf. As can be seen from the data, thefriction coefcient was quite stable throughout the testwhich indicated that there was no signicant change at thesliding interface. Also, the average friction coefcient valuewas about 0.028 which was within the range of a typical

t threesents

S.H. Lee et al. / Polymer Testing 31 (2012) 9931000996Fig. 3. Optical microscope images of the wear pattern of the FKM specimen athe wear test against a SS specimen under normal wear condition. Arrow reprwear at a similar location on the FKM specimen.boundary lubricated system [25]. These results supportedthe extremely low wear characteristics of the FKM spec-imen as described above.

2.3.2. Wear test under accelerated wear conditionsIn order to identify the appropriate ALT method for the

FKM specimen, two types of particles were investigated forinclusion in the lubricant. First, iron particles witha randomly distributed particle size in the range of1 w 300 mm with a concentration of 1 mg per 10 mg oflubricant was used. These particles were chosen to repre-sent the wear particles and contaminants that may beintroduced to the interface of the seal and the counter

Fig. 4. Friction coefcient between FKM and SS specimens during 1000 h ofnormal wear test.e different locations at 670 h, 770 h, 895 h, and 1000 h time intervals duringthe direction of sliding. The images from left to right show the generation ofsurface. In the second set of accelerated wear tests,w1 mmalumina particles were added to the lubricant witha concentration of 3 mg per 10 mg of lubricant. Aluminaparticles were chosen due to their strong abrasive natureand small size. In identifying the appropriate ALT method,the most critical concern was to assure that the wearcharacteristics of the normal and accelerated test condi-tions were similar. This could be done by comparing thewear patterns observed through surface analyses of thenormal and accelerated wear test specimens. Otherexperimental conditions such as the sliding speed andapplied load were kept the same for both the normal andaccelerated wear tests.

Accelerated wear tests of FKM specimens were con-ducted with the lubricant with iron and alumina particlesfor 2 h and 10 h, respectively. Fig. 5 shows the opticalmicroscope images of the wear pattern of the FKM spec-imen after performing the accelerated wear test using thetwo types of particles. The wear patterns were comparedwith those of the normal wear test results. From Fig. 5(a), itcould be found that the wear patterns at the three differentlocations along the contact region of the FKM specimenafter the accelerated test using the iron particles were quitevaried. Also, the wear pattern was not similar to that of thenormal wear test. Thus, it was considered that the ironparticles used were not appropriate for the ALT of FKMelastomer.

As for the accelerated test performed with the aluminaparticles, the general morphology of the worn surface wasquite similar (Figs. 3 and 5(b)). The accelerated wear

3. Analyses of experimental results

The wear coefcient was obtained using the Archardswear equation in order to assess the severity of wear of theFKM specimens for both normal and acceleratedwear tests.The wear volume, total distance and load were obtainedexperimentally, and the hardness of the specimen wasmeasured using a Shore hardness tester. The load in theexperiment was 450 gf and the total sliding distance wascalculated from the test duration and the sliding speedused in the wear test. In the case of the normal wear test

Fig. 6. Friction coefcient between FKM and SS specimens during 10 h ofaccelerated wear test using alumina particles.

S.H. Lee et al. / Polymer Testing 31 (2012) 9931000 997surface distinctly showed the vertical grooves that wereformed during the manufacturing process of the o-ring asin the case of the normal wear test. This suggested that the

Fig. 5. Optical microscope images of the wear pattern of the FKM specimenat three different locations after (a) 2 h of wear test using Fe particles and (b)10 h of wear test using alumina particles against a SS specimen underaccelerated wear conditions. Arrow represents the direction of sliding.accelerated wear condition using the alumina particles wasnot too severe. Furthermore, it was found that withalumina particles the accelerated wear test produceddiscontinuous wear strips along the contact region as in thecase of the normal wear test. From the similar wear patternresults of the two wear tests, it was concluded that theaccelerated wear test condition using the w1 mm aluminaparticles was acceptable as a valid ALT method.

The friction coefcient during the accelerated test usingthe alumina particles was obtained to monitor the state ofsliding. Fig. 6 shows the friction coefcient between FKMand SS specimens during 10 h of accelerated wear testunder an applied load of 450 gf. The frictional behavior wasquite different from that of the normal wear test. As can beseen from Fig. 6, unlike the case of the normal wear testwhere the friction coefcient remained quite steadythroughout the test, the friction coefcient decreased froman initial value of 0.093 to about 0.069 after 10 h of accel-erated wear. Also, the range of friction coefcient wassignicantly higher than the value of 0.028 obtained in thenormal wear test. However, it was below 0.1 which sug-gested that the boundary lubrication condition also existedin the accelerated wear test (Fig. 7).

The higher friction coefcient that resulted in theaccelerated wear test compared to that of the normal weartest may be simply attributed to the presence of aluminaparticles included in the lubricant. It has been reported inthe past that particles at the sliding interface usually causethe plowing component of friction to increase [26].conducted for 1000 h at a speed of 80 mm per second, thetotal sliding distance was 288 km. The hardness of FKMspecimen was 71 Shore A. To obtain a dimensionless wearFig. 7. Optical microscope images of the wear pattern of the FKM specimenat three different locations after (a) 20 h and (b) 30 h of wear test againsta SS specimen under accelerated wear conditions. Arrow represents thedirection of sliding.

coefcient value, the Shore A hardness of 71 should beconverted to a hardness unit in MPa. Since it was notstraightforward to do so, the Shore A hardness value wasused in the Archards wear equation to obtain the effectivewear coefcient. Thus, the unit for the effective wearcoefcient is mm2$(Shore A)/N. From hereon, a symbol k isgiven to represent this unit.

The most difcult part in obtaining the wear coefcientwas in the measurement of wear volume. Since wear on theFKMspecimenoccurredat certain locationsalong the contactregion it was not possible to estimate the wear volume bysimply measuring the change in the radius of the FKM spec-imen. Hence, the wear scars on the FKM specimen wereanalyzed in detail using both 3-D and 2-D surface prol-ometers. Fig. 8 showsa typical imageof the3-Dsurfaceproleof a wear scar on the FKM specimen. The image representsone of thewear strips alreadymentioned in the discussion ofFig. 3. It clearly shows that wear occurred unevenly along thecontact region. The 3-D surface prolometer was used tomeasure the projected wear area along the contact region ofthe FKMspecimen. For the FKMspecimenof thenormalwear

S.H. Lee et al. / Polymer Testing 31 (2012) 9931000998test, the totalprojectedareaof thewornregionwasmeasuredto be about 2.25mm2. This corresponded to about 12% of thetotal contact area as calculated from the Hertzian equation.While the 3-D surface image provided useful informationregarding the totalworn area, itwas not convenient to obtainthe depth of the wear scar. A 2-D surface prolometer wasutilized for this purpose.

Fig. 9 shows the 2-D prole of the wear surface along thecontact region of the FKM specimen. From this data, theaverage depthof thewear scarwas approximated to be about1.05 mm assuming that the cross-sectional shape of the wearscar was an equilateral triangle. The average depth wasmultiplied to the projected wear area of 2.25 mm2 to obtainthe totalwear volume of 2.36103mm3. It should be notedthat the total wear volumewas estimated from the availablesurface prole data and an assumption that the cross-sectional area of the wear scar was an equilateral triangle.Nevertheless, given the extremely small amount ofwear thatoccurred irregularlyalong the contact regionof thespecimen,

Fig. 8. 3-D surface prole image of a typical wear scar on the FKM specimen.that the wear coefcient of the system is known.Based on the effective wear coefcients obtained

experimentally, the AFk can be derived from the followingequations:

VN kNLNxN=HN (2)

VA kALAxA=HA (3)where subscript N stands for normal wear test and A standsfor accelerated wear test. Also, the sliding distance can beexpressed as

xN vNtN (4)

xA vAtA (5)where v is the sliding speed and t is the test duration. Thus,given that the sliding speed for both the normal andaccelerated tests is the same,

tN=tA xN=xA (6)Essentially, the ratio of the test times given in Eq. (6) may

be considered as the acceleration factor, AFk. Thus, from Eq.(6) and the Archards equation, the following expression forAFk can be derived:

AFk xN=xA kA=kN (7)where kA and kN are the wear coefcients of acceleratedand normal tests, respectively. Finally, from the effectivewear coefcients obtained experimentally, the accelerationfactor can be calculated as follows:

AFk 7:37 109k1:27 1010kz58

Thus, by adding alumina particles to the lubricant thewear of the FKM specimen could be accelerated by a factorof 58. However, it should be noted that wear phenomenondepends very much on the system such as the contactpressure and temperature during the operation. In thisthis valuewas taken as thebestestimate for thewearvolume.Using all the variables obtained, the effectivewear coefcientof the FKM specimen in normal wear condition was calcu-lated to be an extremely small value of 1.27 1010 k.

Following the same procedure, the wear coefcient forthe 10-h accelerated wear test of the FKM specimen wasobtained. For the accelerated test, the sliding distance wasonly 1 percent of the normal sliding test or 2.88 km. Thehardness of 157 MPa was used as in the case of the normalwear test since all the FKM specimens were sampled fromthe same o-ring. Also, the same load of 450 gf was applied.Based on the averagewear depth of the FKM specimen afterthe accelerated wear test, the wear volume was calculatedto be 1.38 103 mm3. This amount of wear volumeresulted in the effective wear coefcient of 7.37 109 k.This value was signicantly higher than that of the normalwear test. Thus, the accelerated wear effect of the aluminaparticles added to the lubricant was conrmed.

Finally, the acceleration factor, AFk, of the proposed ALTmethod for the FKM elastomer was derived. For the systemwhere the life is based on the amount of wear, the Archardswear equation may be used to estimate the life provided

e FKM

S.H. Lee et al. / Polymer Testing 31 (2012) 9931000 999Fig. 9. 2-D surface prole of the wear track on thsense, ALT condition should be customized to t the systemunder consideration and the acceleration factor of 58cannot be generalized for all types of FKM seals. Never-theless, for operating conditions similar to the ones used inthis work, the acceleration factor of 58 would be valid.Furthermore, the ALT method presented in this work isexpected to serve as a good reference for development ofALT methods for other types of elastomers and seals.

4. Conclusions

The tribological properties of FKM uorocarbon elas-tomer were investigated with the aim of developing suit-able accelerated wear testing conditions for this type ofa seal material. A pin-on-plate reciprocating tribotester wasused to investigate the wear generation pattern of FKMspecimens under normal and accelerated test conditions.All the wear tests were conducted in boundary lubricationconditions. The following conclusions may be drawn fromthe results of this work:

1). Observation of the FKM specimen surface at varioustime intervals during the normal wear test showed thatthe wear scars were formed irregularly along thecontact region of the FKM specimen. The wear scarsgrew in size and new ones were formedwith increasingnumber of sliding cycles.

2). For 1000 h of normal wear test, which corresponded to288 km of sliding, the effective wear coefcient wascalculated to be 1.27 1010 k.specimen obtained after the normal wear test.3). In the accelerated wear tests, particles were added tothe lubricant to increase the probability of wear. It wasfound that adding Fe particles of random size in thelubricant was not suitable in reproducing the similarwear pattern as that of the normal wear test. Rather,adding w1 mm alumina particles to the lubricant ata concentration of 3 mg per 10 ml of lubricant produceda similar wear pattern as that of the normal wear test.

4). For 10 h of accelerated wear test with alumina particlesadded to the lubricant, the effective wear coefcientwas calculated to be 7.37 109 k.

5). The acceleration factor for ALT of FKM elastomer withalumina particles added to the lubricant was 58.

6). The results of this work is expected to aid in the designof ALT method for FKM as well as other elastomers.

Acknowledgments

This work was supported by the National ResearchFoundation of Korea (NRF) grant funded by the Koreangovernment (MEST) (No. 2012-0001232).

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S.H. Lee et al. / Polymer Testing 31 (2012) 99310001000

Accelerated wear test of FKM elastomer for life prediction of seals1. Introduction2. Experimental details2.1. Experimental specimens2.2. Experimental set-up and conditions2.3. Experimental method and results2.3.1. Wear test under normal wear conditions2.3.2. Wear test under accelerated wear conditions

3. Analyses of experimental results4. ConclusionsAcknowledgmentsReferences