Tribology and Lubrication Technology September 2012 : Page 34
stuDeNt Poster abstract editor’s note: Starting with this issue, TLT is pleased again to be publishing the winning abstracts written by student members of STLE that participated in the Student Poster Competition held at STLE’s 2012 Annual Meeting & Exhibition last May in St. Louis, Missouri (USA). This year’s contest set an all-time record with 46 posters on display from students representing major universities in the U.S. and abroad, including South Korea, China, Czech Republic and France. The quality of the submissions was top-notch and represented the hard work of the students and their professors. In addition, the society wishes to thank the poster judges: Drs. Mitjan Kalin (University of Ljublijana), Thomas Scharf (University of North Texas), Jun Qu (Oak Ridge National Laboratory), John Lucero (NASA), Daniel Nelias (INSA) and Greg Sawyer (University of Florida) for an outstanding job in evaluating all the posters, as well as Dr. Robert Erck of Argonne National Laboratory and STLE student member Sukbae Joo (Texas A&M) for organizing the event. Please feel free to contact the authors—perhaps you can offer some guidance, a mutual research or work opportunity or even em-ployment within your organization. This is your chance to review tomorrow’s ideas and talent before they become today’s products and competitors. Also, be sure to check out our short video interviews with the poster winners, which will be included in the digital version of TLT (available at www.stle.org ). Methods in Characterization of nanoscale Friction in solid lubricants Harmandeep S. Khare and Dr. David L. Burris (Advisor) Department of Mechanical Engineering, University of Delaware, Newark, Del. InTroDuCTIon The limited applicability of conventional lubricants in extreme environments has motivated the use of solid lubricant materi-als. Solid lubricants accommodate low friction and low wear sliding by forming low shear-strength surface fi lms (called tri-bofi lms). Although conventional wisdom suggests a liquid-like bulk shear process, recent studies suggest that the fi lms and shear are confi ned to within 10 nm of the surface. 1 These fi lms are too thin and have proven diffi cult to detect and even more diffi cult to probe mechanically. Scanning probe microscopy (SPM) is perhaps the only technique with suffi cient surface sensitivity. However, quantitative SPM-based indenta-tion and lateral force measurements are notoriously diffi cult due to inherent calibration challenges. The goal of this work is to leverage existing calibration strategies to improve quantita-tive reliability of SPM-based friction measurements (lateral force microscopy, LFM). eXperIMenTal Harman Khare is currently a doctoral candidate in the Materials Tribology Laboratory at the University of Delaware working under the mentorship of Dr. David Burris. His research interests include solid lubricants for extreme environments (including polymer and metal matrix composites), nanotribology and nanome-chanical surface characterization of solid lubricants. two-slope lateral force calibration The wedge method of lateral force calibration 2 utilizes calibra-tion substrates with known wedge angles to extract values of friction coeffi cient and the lateral force calibration constant. Once friction loop width and offset (W and Δ ) are experimen-tally determined ( see Figure 1 ), friction coeffi cient is evaluated using the expression ࣆ ൗࣆ ൌ οԢ ൗ ᇱ ܛܑܖ ࢻ 34 Recruit an STLE member—2nd Annual Member-Get-A-Member Campaign. Last
Student Poster Abstract
Harmandeep S. Khare & Dr. David L. Burris
<br /> Methods in Characterization of nanoscale Friction in solid lubricants<br /> <br /> INTRODUCTION<br /> The limited applicability of conventional lubricants in extreme environments has motivated the use of solid lubricant materials. Solid lubricants accommodate low friction and low wear sliding by forming low shear-strength surface films (called tribofilms). Although conventional wisdom suggests a liquid-like bulk shear process, recent studies suggest that the films and shear are confined to within 10 nm of the surface.<br /> <br /> These films are too thin and have proven difficult to detect and even more difficult to probe mechanically. Scanning probe microscopy (SPM) is perhaps the only technique with sufficient surface sensitivity. However, quantitative SPM-based indentation and lateral force measurements are notoriously difficult due to inherent calibration challenges. The goal of this work is to leverage existing calibration strategies to improve quantitative reliability of SPM-based friction measurements (lateral force microscopy, LFM).<br /> <br /> EXPERIMENTAL<br /> two-slope lateral force calibration<br /> The wedge method of lateral force calibration2 utilizes calibration substrates with known wedge angles to extract values of friction coefficient and the lateral force calibration constant. Once friction loop width and offset (W and ?) are experimentally determined (see Figure 1), friction coefficient is evaluated using the expression<br /> <br /> The two-slope modification of the wedge method is critical for eliminating photodiode cross-talk, which is always present but unknowable. A calibration standard with known geometries is used to calibrate the lateral force. The calibration is then applied to subsequent lateral force measurements as it would for any other instrument.<br /> <br /> In addition to photodiode induced crosstalk, piezo-stack misalignment and tip shear offset introduce significant but unknown errors into SPM calibration (see Figure 2). Perhaps more important, however, the traditional approach reasonably assumes that the calibration constants are constant. Nominally identical repeat measurements on three independent AFMs have revealed exceptionally poor measurement repeatability. This is due to the fact that the calibration constants are sensitive to several factors related to standard instrument and sample setup including z-piezo voltage, cantilever positioning, and the optical path of the laser. We have developed a method for simultaneous calibration and measurement to eliminate the confounding effects of standard SPM measurement variables.<br /> <br /> In situ calibration and measurement<br /> Using force equilibrium at the tip-substrate contact, (similar to the formulation in ) friction loop width and offset as a function of applied normal load (W’ and ?’) are written in terms of the wedge angle, friction coefficient and ratio of normal to lateral calibration constants Cx/Cy (see Figure 3). In addition to the wedge angle a, a misalignment angle ? is included to accommodate the ‘effective’ misalignment of the transducer (tip-shear offset, piezo-stack misalignment, etc.).<br /> <br /> Experimentally, independent friction measurements (µ1 and µ2) are made on the sample at two different wedge angles (a1 and a2), as illustrated in Figure 3. The combination of two equations (described by Figure 3) for two wedge angles yield four equations with four unknowns, namely Cx/Cy, ?, µ1 and µ2, where, Cy is calibrated directly. It is worth noting that the proposed method does not impose any restriction on the values of measured friction coefficients at the two wedge angles (i.e., µ1 and µ2 are not constrained).<br /> <br /> Performing in situ calibration on the surface-of-interest during the measurement eliminates the need for constant calibration constants. Further, incorporation of the effective transducer misalignment angle ? into the model significantly reduces the uncertainty associated with the instrument-related complexities illustrated in Figure 2.<br /> <br /> In order to validate the proposed method, the process of calibration and measurement was carried out on a well-characterized, custom-built microtribometer (see Figure 4), where calibration values for the load cell are known as a priori. Measurements were made at two wedge angles across a range of normal loads and measured values of W’ and ?’ were used to evaluate calibration constants. Evaluated values of Cx/Cy extracted from the experimental data were determined to be within 2% of the manufacturer reported value, with friction coefficients µ1 and µ2 as 0.106 and 0.099; and misalignment angle ? -0.16°.<br /> <br /> NANOTRIBOLOGICAL CHARACTERIZATION<br /> The in situ lateral force calibration method is applied to quantify changes in tribological properties of MoS2 coatings as a result of sliding-induced tribofilm formation. Figure 5 (see inset) represents a typical uncalibrated friction versus load graph used for evaluating W,’ illustrating qualitative differences in friction between worn, unworn and single crystal MoS2. Quantitative measurements performed using the in situ technique on worn and unworn coatings show reduction in friction coefficient due to sliding. Thin-ordered tribofilms are seen to mimic the perfectly-ordered microstructure of single crystal MoS2, which exhibits the lowest value of friction (see Figure 5). These results illustrate the applicability of the in situ calibration method as applied to the measurement of nanoscale friction at solid lubricant tribofilms.<br /> <br /> CONCLUSIONS<br /> While SPM-assisted friction measurements are key to characterizing solid lubricant tribofilms, inherent instrument uncertainties make existing calibration and measurement quantitatively unreliable.<br /> <br /> To overcome SPM measurement challenges, an in situ method of nanoscale lateral force measurement is presented, which performs a singlestep calibration and measurement of nanoscale friction.<br /> <br /> The method is validated using a well-characterized microtribometer. Applicability to the measurement of nanofriction in solid lubricants is illustrated by quantitative LFM friction coefficient measurements and the clear effects of surface structure and tribofilm presence on nanoscale friction.<br /> <br /> ACKNOWLEDGMENTS<br /> The authors gratefully acknowledge the AFOSR (YIP FA9550-10-1-0295) for financial support.