Proceedings of SerbiaTrib '13

Table 1. Overview of the results from the analysed papers (for pure PTFE and its composites at dry sliding conditions)*Ref.[7]Testrig**Ballon-disc[8] Pin-ondisc[9] Pin-ondisc[10] Pin-ondisc[11] Pin-ondiscType (amount and size) of thefiller***1. C particles (18 %; 10 – 25 μm)+ Gr flakes (7 %; 25 – 50 μm);2. E-glass fibres(15 %; 10 × 50 – 75 μm);3. E-glass fibres(25 %; 10 × 50 – 75 μm)1. Glass fibres (17 %);2. Bronze (25 %);3. C (35 %)1. Glass particles(25 %; 40 μm);2. Bronze particles(40 %; 48 μm)1. Gr flake (2 %; 10 μm);2. Gr flake (5 %; 10 μm);3. Gr flake (10 %; 10 μm)1. Bronze (25 %);2. Bronze (40 %);3. Bronze (60 %)CounterpartmaterialAISI 440Csteel ball(d = 9 mm)AISI 440Csteel discEN 32hardenedsteel discStainlesssteel discAISI 400Csteel discLoad,N (MPa)5(pointcontact)5 – 30(0.2 – 1.1)60(0.6)25(0.2)Slidingspeed,m/sSlidingdistance,mCoefficient offrictionSpecific wear rate,mm 3 /Nm × 10 –6PTFE Composites PTFE Composites0.1 1000 0.11 0.13 – 0.16 950 90 – 7000.32 –1.281152 –46080.13 –0.790.11 – 0.711.5 2500 – –476 –943app.5676 – 290app. 4.4 –3351 8000 0.24 0.20 – 0.26 1650 10 – 190app.5 – 200(0.2 – 7.1) 0.32 – 2 2000 0.12 –0.22app.0.12 – 0.181000 1 – 100* The friction and wear values in the table are approximate and can be used only as a guidance, since the authors in most cases didnot presented the results in appropriate way; ** Pin – cylindrically shaped specimen (flat contact); *** C – carbon, Gr – graphiteoccur, characterized by brittle fracture or severeplastic deformation. On the other hand, at low loadsusually mild wear occur, characterized by the localplastic flow of the thin transfer film and surfacelayers (decreasing friction), together withdelamination wear.Contact area: The contact area will determine theprojected contact stresses. If the load cannot bereduced, one way of reducing stress is to increase theprojected contact area. However, if the area ofcontact becomes too large instead of the materialflowing across the counterpart surface, it will have atendency to build up, forming ridges, which cancause high localized stresses and higher adhesion,thus higher friction and wear. It is important to designa part with correct match up of load and contact area.Sliding speed: The high sliding speeds canproduce high temperatures due to friction heating.This may cause the polymer or the polymercomposite additives to degrade. However in somecases higher temperature might be beneficial to thelubricating process. In order to develop a surfaceshear film and/or a transfer film, the molecular chainmust have time to reorient. If one slides too fast overthese un-oriented chains, instead of reorienting, theywill fracture, leading to the production of large wearparticles and high wear. Thus it is important tochoose sliding speed for each particular polymer toensure the optimum performance.Counterpart topography: If the counterpartmaterial is too rough it can abrade the compositeand not allow a shear film or transfer film to form.Therefore, it could be generally accepted that, thesmoother the counterpart the lower the wear. Thishas certain limits, since it is also found that overpolishingtend to remove the counterpart softermatrix material, leaving the harder phases and/orparticles protruded above the surface.Temperature: At lower temperature the frictionand wear properties of most polymers are not asexceptional as they are at or above the ambienttemperature. At lower temperatures polymers losetheir relaxations ability, i.e. the movement of theirmain molecule chain do not obtains adequatedegree of freedom, and thus the polymer does notobtain a great deal of plasticity. High temperaturescan affect bonding between the filler material andpolymer matrix. They can also affect the lubricatingproperties of some additives in polymer composite,since these additives might desorbs gases at certaintemperature or even decompose.3. STATE-OF-THE-ART OF PTFECOMPOSITES TRIBOLOGICALRESEARCHESTribological behaviour of PTFE and itscomposites with filler materials such as carbonparticles, graphite flakes and E glass fibres (Table 2)was investigated by Khedkar et al. [7]. Experimentswere performed under the normal load of 5 N andsliding speed of 0.1 m/s. They found that the usedfiller additions increase wear resistance in allcomposites that were studied. The highest wear136 13 th International Conference on Tribology – Serbiatrib’13