A novel flexure-based biaxial compression/shear apparatus has been designed, built, and utilized to conduct tribological studies of interfaces relevant to MEMS. Aspects of our new apparatus are detailed and its capabilities are demonstrated by an investigation of two interfaces for MEMS applications. Tribological tests may be performed with normal and tangential forces in the &micro;N to N range and relative sliding displacements in the nm to mm range. In this testing range, the new experimental apparatus represents an improvement over existing techniques for tribological studies at the small scale.Singapore-MIT Alliance (SMA

Anand, Lallit

Gearing, B.P.

DSpace@MIT

A NOVEL TESTING APPARATUS FORTRIBOLOGICAL STUDIESAT THE SMALL SCALEB. P. GEARING and L. ANANDAbstract— A novel flexure-based biaxial compres-sion/shear apparatus has been designed, built, and uti-lized to conduct tribological studies of interfaces relevantto MEMS. Aspects of our new apparatus are detailed andits capabilities are demonstrated by an investigation of twointerfaces for MEMS applications. Tribological tests may beperformed with normal and tangential forces in the µN toN range and relative sliding displacements in the nm to mmrange. In this testing range, the new experimental appara-tus represents an improvement over existing techniques fortribological studies at the small scale.Keywords—MEMS, Friction, Mechanical testing.I. IntroductionTHE useful life of a variety of MEMS is to a large partdetermined by friction and wear between contactingparts in relative motion. A better understanding of con-tact mechanics at the small scale will allow for the develop-ment of improved friction models for the optimum designof MEMS. Further, a more accurate account of the physicalaspects of friction at the small scale, in particular of theaffects of the surface topography on the frictional response,may allow for the design of interfaces with optimal surfacemicro-topography for minimized friction and wear.We have developed a novel compression/shear appara-tus to study the tribological response of interfaces at thesmall scale. The “small scale” in this context is normaland tangential forces in the µN to N range, and relativesliding displacements in the nm to mm range. We beginby detailing the physical aspects of our new apparatus,and then demonstrate its capabilities by studying the fric-tional response of silicon on silicon, as well as the effects ofdiamond-like carbon coatings on friction.II. BIAXIAL COMPRESSION/SHEARAPPARATUSThe biaxial testing apparatus is shown in Fig. 1. Twoflat bodies whose interfacial response is to be investigatedare brought together in the space denoted as “specimen.”One body is attached at the base of the “flexure for nor-mal force,” and the opposing body is secured to the “sheardisplacement flexure.” Normal forces on the interface areapplied using a voice-coil actuator (BEI Sensors and Sys-tems Co., CA, Model LA13-12-000A) acting through the“flexure for normal force.” Tangential displacements atB. P. Gearing and L. Anand are with the Department of MechanicalEngineering, Massachusetts Institute of Technology, Cambridge, MA02139. E-mail:anand@mit.eduthe interface are achieved by actuating the “shear dis-placement flexure” with a piezo-electric inchworm actuator(Burleigh Instruments, NY, Model IW700). The tangen-tial force arising from the interfacial interaction is mea-sured by the resulting displacement of the “flexure forshear force.” All displacements are measured using non-contacting eddy current sensors (Kaman Instrumentaion,CO, Model SMU9000-5U) with a resolution of 20 nm overa range of 1.25 mm.Fig. 1. BIAXIAL COMPRESSION/SHEAR APPARATUS.The design incorporates compound spring flexures within amonolithic frame and possesses only a limited number of as-sembled parts. Thus, device assembly errors and backlashare minimized. The use of the double compound springs foractuation in the normal direction minimizes the potentialfor rotations or parasitic deflections due to either machin-ing imperfections or to variations in the properties of theframe material. The frame is a monolithic, light weight de-sign made of 12.7 mm thick Al7075-T6 plate and is 440 mmx 460 mm in size. Hard-stops ensure that the maximumbending stress in the spring webs does not exceed 5 MPa,which provides essentially infinite fatigue life. The normalaxis is nested within the shear load cell and has been de-signed using FEA to minimize cross-talk between normalforce application and shear force measurement. The de-sign also allows for the measurement of relative separationor approach of the surfaces in the normal direction as atest is conducted.The normal force is controlled with a resolution of 80µN over a range of 5 N, the shear force is measured with aresolution of 225 µN up to 15 N, tangential displacementis imposed in 4 nm steps up to 6.35 mm, and the relativemotion of the surfaces in the normal direction is measuredwith a resolution of 20 nm over a range of 1.25 mm.1 Im-posed displacement rates range from 0.5 µm/s to 600 µm/s.The entire system rests upon a vibration-isolated table tominimize environmental disturbances.Calibration of the flexure stiffness for each axis isachieved by measuring the displacement of the axis un-der precision weights. In the present embodiment of thedesign, the normal axis has a stiffness of 9.53 N/mm, andthe stiffness of the tangential axis is 43.2 N/mm. The elec-tronics driving the normal axis are calibrated by measuringthe normal displacement due to known computer outputvoltage ramps. The electronic system (consisting of a 16-bit data acquisition board, a precision amplifier poweredby a low-noise source, and the voice coil) has a transferfunction of 1.33 N/V. All system responses are highly lin-ear over their operating ranges. The transfer functions ofthe mechanical and electrical systems are utilized to applya precise normal load on an interface under compression.The simple, compact physical design of the apparatus maybe scaled to allow for testing over a wide range of nor-mal and shear forces, as well as a large range of imposedtangential displacements.2 Design and manufacturing is-sues concerning the compression/shear apparatus as wellas several other novel small scale testing devices recentlydeveloped at MIT may be found in Gudlavalleti et al. [1].An example of experimental data obtained with the bi-axial apparatus is shown by the tangential force-sliding dis-tance response in Fig. 2. The interface is an Al6111-T4 2mm x 2 mm coupon (with an as-received surface texture)against polished tool steel. A normal force of 1 N was im-posed on the interface, and the tangential sliding velocitywas 6 µm/s, with regions where the tangential displace-ment was reversed and then imposed again in the initialsliding direction. The negative sign on the tangential forceaxis indicates motion in the opposite direction to that ini-tially imposed on the interface. The ability to preciselymeasure the tangential stiffness of an interface (representedby the slope of the linear “unloading-reloading” regions) isa novel capability of our apparatus. These “unloading-reloading” portions are reminiscent of the classical results(at a more macroscopic scale) of Courtney-Pratt and Eis-ner [2], and serve as experimental motivation for adhering-slipping models of interface friction (e.g. Anand [3]; Gear-ing et al. [4]). The slope of the “unloading-reloading” curveis termed the tangential stiffness in adherence in Anand[3], and previously was experimentally difficult to measure.1Displacement sensors with improved sensitivity may be used forbetter force and displacement resolution.2Various aspects of the design are patent pending.The coefficient of friction µ (defined as the ratio of the tan-gential force to the applied normal force) as a function ofsliding distance is shown in Fig. 3, where we note that themeasured values of µ are consistent with previous standardpin-on-disk friction tests.0 0.5 1 1.5 2 2.5−150−100−50050100150200250300350TANGENTIAL FORCE  [mN]SLIDING DISTANCE  [mm]Fig. 2. FRICTIONAL RESPONSE OF AN AL6111/TOOL STEELINTERFACE AT A NORMAL FORCE OF 1 N.A second diagnostic test was conducted to demonstratethe range of tangential displacement rates attainable withthe apparatus. A 1 mm thick 2 mm x 2 mm coupon ofpolycarbonate (PC) was loaded against the polished toolsteel surface with a normal force of 750 mN. A sliding ve-locity of 100 µm/s was applied and the PC specimen wasdisplaced cyclically relative to the tool steel until the tan-gential force reached a steady-state value of 171 mN, whichoccurred at a total displacement of about 11 mm. At thisstate, the sliding velocity was reduced to 10 µm/s for anadditional 2 mm of sliding. The velocity was then increasedto 500 µm/s at the final stage of sliding. Figure 4 shows thetangential force as a function of sliding distance as the slid-ing rate decrement and increment are imposed. The solidlines are the average value of the tangential force at thecorresponding sliding velocity. We note that this interfaceexhibits positive rate sensitivity. This result demonstratesthe range of displacement rates that may be obtained withthe apparatus.III. TRIBOLOGICAL STUDIES AT THE SMALLSCALEThis section further demonstrates our new testing capa-bilities by investigating the frictional response of two in-terfaces of relevance to MEMS. The first interface is singlecrystal silicon (Si) contacting Si, and the second interface0 0.5 1 1.5 2 2.5−0.15−0.1−0.0500.050.10.150.20.250.30.35SLIDING DISTANCE  [mm]µFig. 3. µ FOR AN AL6111/TOOL STEEL INTERFACE AT ANORMAL FORCE OF 1 N.is a 1 µm thick diamond-like carbon (DLC) thin film onSi contacting DLC on Si. Both tests were conducted onas-fabricated 0.8 mm thick 2.2 mm x 2.2 mm diced squarecoupons against 0.8 mm thick 12.7 mm x 12.7 mm pieces.The normal load was held constant at 2 N as the inter-faces were displaced relative to each other at a velocity of 10µm/s. Fig. 5 shows the measured tangential force-slidingdistance response, and Fig. 6 shows the friction coefficientµ as a function of the imposed tangential sliding distance.At a sliding distance of 400 µm, µ is 0.32 for the Si/Si in-terface, which compares to the much lower value of 0.11 forthe DLC/DLC interface. These results serve as confirma-tion of the current trend to use DLC as low friction bearingsurfaces for MEMS. The evolution of the tangential forcewith sliding distance at imposed normal forces may be uti-lized to develop improved friction models for the design oflow friction and low wear interfaces for MEMS.The relative displacement η of the interface in the normaldirection is shown in Fig. 7, where it is noted that bothinterfaces showed a small amount of relative approach as thetangential displacement was imposed. At a sliding distanceof 400 µm, η is about 621 nm for the Si/Si interface ascompared to 200 nm for the DLC/DLC interface.IV. CONCLUSIONOur novel biaxial compression/shear apparatus is shownto provide precise, low-noise experimental data over a largerange of forces and displacements of interest for studyingthe tribological responses of interfaces of interest in MEMSapplications. Further, certain features of the frictionalresponse (e.g. the tangential stiffness in adherence andthe relative approach/separation of surfaces during slid-11 11.5 12 12.5 13 13.5100120140160180200220TANGENTIAL FORCE  [mN]SLIDING DISTANCE  [mm]100 µm/s 10 µm/s 500 µm/sFig. 4. RATE SENSITIVITY OF A PC/TOOL STEEL INTER-FACE AT A NORMAL FORCE OF 750 mN.ing) that were previously difficult to measure experimen-tally, are now readily obtained. We note that the designof the biaxial apparatus may be scaled to allow for test-ing over various ranges of forces and displacements. Thisnew experimental capability should provide the tribologicaldata necessary to design the next generation of low frictioninterfaces for the optimal performance of MEMS.AcknowledgmentsSupport for this work was provided by an NSF grant,CMS-9634265, and by the Singapore-MIT Alliance. Fruit-ful discussions with S. Gudlavalleti are acknowledged.References[1] S. Gudlavalleti, B. P. Gearing and L. Anand, To be submitted,2001.[2] J. S. Courtney-Pratt and E. Eisner, Proceedings of the RoyalSociety A, 1957, 238, 529.[3] L. Anand, Computational Mechanics, 1993, 12, 197.[4] B. P. Gearing, H. S. Moon and L. Anand, International Journalof Plasticity, 2001, 17, 237.0 50 100 150 200 250 300 350 4000100200300400500600700800900TANGENTIAL FORCE  [mN]SLIDING DISTANCE  [µm]Si/Si  DLC/DLCFig. 5. FRICTIONAL RESPONSE OF MEMS INTERFACES ATA NORMAL FORCE OF 2 N.0 50 100 150 200 250 300 350 40000.050.10.150.20.250.30.350.40.45µSLIDING DISTANCE  [µm]Si/Si  DLC/DLCFig. 6. µ FOR MEMS INTERFACES AT A NORMAL FORCE OF2 N.0 50 100 150 200 250 300 350 4000100200300400500600700800η  [nm]SLIDING DISTANCE  [µm]Si/Si  DLC/DLCFig. 7. η FOR MEMS INTERFACES AT A NORMAL FORCE OF2 N.

A Novel Testing Apparatus for Tribological Studies at the Small Scale

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A Novel Testing Apparatus for Tribological Studies at the Small Scale

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