Proceedings of SerbiaTrib '13

Polyethylenes with a molecular weight of less than500,000 g/mol can be observed to undergo such aflow transition (Tf). However, when the molecularweight of polyethylene increases above 500,000g/mol, the entanglement of the immense polymerchains prevents it from flowing. UHMWPE doesnot exhibit a flow transition for this reason.UHMWPE is a linear, low-pressure,polyethylene resin. It has both the highest abrasionresistance and highest impact strength of anyplastic. Combined with abrasion resistance andtoughness, the low coefficient of friction ofUHMWPE yields a self-lubricating, non-sticksurface. Static and dynamic coefficients aresignificantly lower than steel and most plasticmaterials. Elastic modulus of UHMWPE isapproximately 0.69 GPa. ASTM F648-00 definesstandard specification for Ultra-High-Molecular-Weight Polyethylene powder and fabricated formfor surgical implants. To date it has proven to bethe best polymer material for use in total joints.Along with the extensive application ofUHMWPE, the understanding of polymer tribologyis becoming increasingly important. Many authorshave investigated different aspects of tribologicalperformance of UHMPWE [2-7]. The structuralfactors associated with surface mechanicalproperties (crosslinking, oxidation state, localorientation of polymer, crystallinity, etc.) can behighly variable and localized and may vary onmicron spatial scales or smaller [8]. Therelationship between UHMWPE mechanicalproperties and the in-vivo performance of afabricated form has not been determined. Whiletrends are apparent, specific property-polymerstructure relationships are not well understood. Themechanical properties are subject to variation as thefabrication process variables (such as temperature,pressure, and time) are changed. [ASTM F-648(2000)].Reciprocating sliding at different test devicesand from different aspects has been a subject ofinvestigations [9-11]. Different approaches forimprovement of existing UHMWPE materials havebeen tried. [10] investigated effects of nitrogen ionirradiation on tribological properties. [9]investigated friction and wear behavior of ultrahighmolecular weight polyethylene as a function ofpolymer crystallinity. [2, 7, 8, 13, 14, 15]investigated crosslinking and material behaviorwith different approaches to crosslinking.This paper deals with friction coefficientbehaviour during dry reciprocating sliding ofUHMWPE in contact with alumina (Al2O3), withina range of velocities typical for hip implants. Fivevalues of normal force and three values of slidingspeed have been observed.2. MATERIALS AND TRIBOLOGICAL TESTPolished rectangular flat UHMWPE sampleswere used for tests, supplied by the companyNarcissus Ada, Serbia. Sliding tests were done atball-on-flat configuration of CSM Nanotribometerin dry conditions. Alumina was used as a ballmaterial (diameter, 1.5 mm), since it is extremelyhard and chemically inert. Alumina is frequentlyused in combination with UHMWPE in artificialhip joints. Duration of each test was 3000 cycles (1cycle, 1.6 mm), which is enough to reach stabilesteady state of friction coefficient. During the test,the dynamic friction coefficients were recorded inreal time, using the built-in TriboX 2.9.0 software.Five values of normal force (100 - 1000 mN) andthree values of sliding speed (4 - 12 mm/s) havebeen tested. Maximum elastic contact stress(according to applied normal loads) were calculatedby Hertz method and compared to test conditions.Characteristics of conducted tribological tests aregiven in Table 1.Table 1. Tribological parameters.Normal load values, F nMaximum elastic contact stress(according to applied normalloads)Maximum linear speed values, v3. RESULTS AND DISCUSSION100 mN, 250 mN, 500 mN,750 mN, 1000 mN28.5 MPa, 38.6 MPa, 48.7MPa, 55.7 MPa, 61.3 MPa4 mm/s; 8 mm/s; 12 mm/sReal time diagrams of the friction coefficient as afunction of the sliding cycles (sliding distance) wererecorded for each test. Friction coefficient curve is ofsinusoid shape, whereat the opposite directions aremarked with + and - sign, denoting coefficient offriction in two different directions of sample moving.Good agreement with reported values of frictioncoefficient was obtained [6, 16, 17, 18].Dynamic friction coefficient curves exhibitedrather uniform behavior for all test conditions(Fig.3). Somewhat larger values of frictioncoefficient could be observed during the running-inperiod in case of low loads (100-250 mN) and thelowest velocity (4 mm/s), as shown in Fig. 3a. Incase of high loads and speeds, friction coefficientreached steady state values shortly after thebeginning of the test. Maximum contact pressuresin these tests were approximately from 30 - 60MPa, representing high contact stresses exhibited inhip/knee implants. Especially extreme loadingconditions are present in the knee implant system.Similar behavior (short running-in phase for thefriction coefficient) was also reported by otherauthors [18].13 th International Conference on Tribology – Serbiatrib’13 89