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Tribology and Lubrication Technology October 2012 : Page 28

Student POSteR AbStRAct Experimental Investigations on the Coefficient of Friction and Wear of Single Granules Martin C. Marinack Jr., Patrick S. M. Dougherty and C. Fred Higgs III (Advisor) Mechanical Engineering Department Carnegie Mellon University Pittsburgh, Pa. Note: For a closer look at Martin’s poster abstract, be sure to check out his short video presentation in the October digital version of TLT (available at www.stle.org) INTRODUCTION Granular flows are prevalent in both nature and industrial sec-tors where their nonlinear and multiphase behavior makes them difficult to understand and predict. They are of particular importance in the bulk solids processing industry, 1 where gran-ular flow phenomena can adversely affect product effective-ness, yield, and cost. Thus, the accurate prediction of particle flow inside of common solids processing equipment (e.g., hop-pers, silos, mixers, etc.) becomes important in being able to design against negative effects. Discrete modeling approaches are often used for the predic-tion and study of particle flows inside the geometries men-tioned. Principal among these approaches is the discrete ele-ment method (DEM), which provides for a rigorous physical treatment of particle interactions. The accuracy and effective-ness of these discrete models will depend on the modeling of individual particle interactions, which in turn relies on the ac-curacy of the interaction parameters being used. Prior discrete particle modeling studies 2-4 have shown that even small differ-ences in particle properties and interaction parameters such as particle shape, coefficient of restitution (COR), and coefficient of friction (COF) can have large effects on the global flow be-havior being predicted by the model. These studies serve to highlight the importance of experimentally obtaining detailed and accurate particle properties and particle interaction proper-ties, such as particle roughness and morphology, particle-parti-cle and particle-boundary COR and COF , and particle attrition and wear rates, for use as input in discrete particle simulations. The authors have already performed extensive work on the study of COR. 5 Unlike Rademacher 6 who studied COF between a surface and a granular mass, the experimental work per-formed here aims to study the COF , and in situ wear during sliding of an individual granule against a rotating disk. Specific consideration is given to the manner in which these parameters may change over time or with certain conditions such as sliding Martin C. Marinack, Jr., is a doctoral student in the Particle Flow and Tri-bology Laboratory at Carnegie Mellon University in Pittsburgh, Pa., where he works under the guidance of professor C. Fred Higgs, III. Martin received both his bachelor’s of science and master’s of science degrees in mechanical engi-neering from Carnegie Mellon in 2008 and 2010, respectively. A National Sci-ence Foundation Graduate Research Fellow, Martin’s research interests in-clude the study of granular flows from a tribological and classical perspec-tive, which has applications to the sol-ids processing, space, and fossil-fuel energy industries. You can reach him at mmarinac@andrew.cmu.edu. 28 STLE is offering CLS, OMA (I&II) and CMFS certification exams Oct. 12 and

Student Poster Abstract

Martin C. Marinack Jr., Patrick S. M. Dougherty, C. Fred Higgs III

INTRODUCTION<br /> <br /> Granular flows are prevalent in both nature and industrial sectors where their nonlinear and multiphase behavior makes them difficult to understand and predict. They are of particular importance in the bulk solids processing industry,1 where granular flow phenomena can adversely affect product effectiveness, yield, and cost. Thus, the accurate prediction of particle flow inside of common solids processing equipment (e.g., hoppers, silos, mixers, etc.) becomes important in being able to design against negative effects.<br /> <br /> Discrete modeling approaches are often used for the prediction and study of particle flows inside the geometries mentioned. Principal among these approaches is the discrete element method (DEM), which provides for a rigorous physical treatment of particle interactions. The accuracy and effectiveness of these discrete models will depend on the modeling of individual particle interactions, which in turn relies on the accuracy of the interaction parameters being used. Prior discrete particle modeling studies2-4 have shown that even small differences in particle properties and interaction parameters such as particle shape, coefficient of restitution (COR), and coefficient of friction (COF) can have large effects on the global flow behavior being predicted by the model. These studies serve to highlight the importance of experimentally obtaining detailed and accurate particle properties and particle interaction properties, such as particle roughness and morphology, particle-particle and particle-boundary COR and COF, and particle attrition and wear rates, for use as input in discrete particle simulations.<br /> <br /> The authors have already performed extensive work on the study of COR.5 Unlike Rademacher6 who studied COF between a surface and a granular mass, the experimental work performed here aims to study the COF, and in situ wear during sliding of an individual granule against a rotating disk. Specific consideration is given to the manner in which these parameters may change over time or with certain conditions such as sliding speed and normal load. Ultimately, the studies performed in this work should prove extremely useful in the production of detailed and highly accurate particle interaction models, for use in full-scale discrete particle modeling of industrially relevant equipment and geometries.<br /> <br /> EXPERIMENT DESCRIPTION<br /> <br /> The coefficient of friction (COF) and wear measurements were obtained through the use of a granule-on-disk tribometer. A newly designed granule loading piece and holding fixture which held individual granules against a rotating disk were used to perform the trials. Wear depth was measured by means of a linear variable differential transformer (LVDT) device. By means of this setup, COF and wear measurements can be obtained for various material combinations over a range of normal loads and speeds. In the studies performed as part of this work, many different granule materials, including brass, cellulose acetate (CA), glass, tool steel, and tungsten carbide were tested against stainless steel disks. The granules were all 4.76 mm in diameter except for the CA granules which were 4.85 mm in diameter. The loads used in these studies included 5.77 N, 14.67 N, and 23.57 N. The sliding speeds used were 3 m/s, 4 m/s, and 5 m/s. While these trials consist of pure sliding, one may scale this to granular flows by considering the aggregate sliding time experienced by granules comprising a full scale granular flow.<br /> <br /> Surface metrology measurements were obtained by means of a Zygo NewView 7300 optical interferometer. Surface roughness, maximum peak to valley height, and surface topography were recorded for the granules and disks. Surface measurements taken post-testing can give insight into the relative change in surface topography of the two surfaces being examined. These surface measurements, coupled with the COF and wear measurements, allow for the establishment of qualitative insights into the relationship between granular friction and wear mechanisms,7 as well as insights into the potential relationship between surface properties (such as surface roughness and topography) and tribological behavior. In terms of particle flows, changes in friction coefficient and particle shape (due to wear) can have a significant impact on how particles interact, and in turn, the global behavior of the flow. Thus by extension, these surface metrology measurements can also provide useful information for studying how tribological surface properties of individual granules affect granular flow behavior.<br /> <br /> RESULTS AND DISCUSSION<br /> <br /> For the sake of brevity, only results for the polymeric granules, cellulose acetate (CA), are shown. Figures 1(a) and 1(b) show the COF and wear depth versus time, respectively, for CA granules tested under a normal load of 5.77 N and at varying sliding speeds.<br /> <br /> The overall trend shown in Figures 1(a) and 1(b) is explained by the fact that the CA granules lay their own transfer film on the disk as they wear. During the initial period of high wear (Figure 1(b)) a protective transfer film is being laid on the disk which promotes a steady COF value (Figure 1(a)). After a sufficient film is present, wear begins to level out and the COF values increase rapidly as the initial film is rubbed away. Once the initial film is destroyed, the CA granule begins to transfer film again resulting in a larger but level COF. This process occurs faster at higher sliding speeds, as higher sliding speed leads to a faster deposition, removal, and redeposition of the transfer film. For instance, the 5 m/s trial displays a COF value which rises rapidly during the first minute, resulting in a higher COF during the first minute than the two slower sliding speeds. This is to be expected since the trial at 5 m/s will be the first to remove its self-deposited transfer film before being the first to redeposit the film and have COF values level out once again. With regards to granular flow applications, there are many instances in which the granules comprising the flow will not experience sliding times lasting as long as the tests performed here. Thus, it is important to note that the most significant quantitative differences in COF and wear depth begin to occur at very low sliding times.<br /> <br /> Figures 2(a)–(d) display surface topography measurements for the stainless steel disk and CA granule before and after a sliding test performed at 4 m/s. It can be seen that the roughness (Ra) of the granule’s surface (Figure 2(b)) starts out slightly higher than that of the disk (Figure 2(a)) before testing, and then decreases (Figure 2(d)) after undergoing sliding against the disk. This can be potentially explained by a polishing effect as the granule lays its transfer film on the disk. This transfer film can be seen on the disk post-testing as shown in Figure 2(c). It is also shown that the surface of the softer granule post-test (Figure 2(d)) exhibits a similar patterning to that of the harder disk (Figure 2(a)).<br /> <br /> CONCLUSIONS<br /> <br /> The aim of this experimental work was to obtain detailed coefficient of friction and wear measurements for individual granules of various materials. These types of experimental studies, which perform rigorous interrogation of single particle scale tribological behavior, allow for the in-depth characterization of particle-level interaction and collision physics. This type of characterization becomes essential for understanding the global behavior of flowing solids processes as well as for predicting granular flow behavior by means of discrete modeling approaches.<br /> <br /> ACKNOWLEDGMENTS<br /> <br /> This material is based upon work supported by the NSF Graduate Research Fellowship under Grant No. 0946825. The authors would like to thank Kennametal Inc., for providing stainless steel disks.

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