Proceedings of SerbiaTrib '13

PBT has the average values of the frictioncoefficient, , in the narrowest range, around thevalue 0.2. The increase of this average could beexplained by the elimination of the relatively bigwear particles that are characteristic for this polymer(see Figure 4). The values obtained for F = 5 N aregrouped under 0.2 for all the tested sliding speeds.The composites GB10 (PBT + 10% micro glassbeads) and GB20 (PBT + 20% micro glass beads)have the average value of the friction coefficientscattered on larger intervals, especially for thesmaller normal loads (F = 1 N and F = 2.5 N). For F= 1 N, it is hard to establish a dependency relation ofthe friction coefficient on the adding materialconcentration and the sliding speed. It could benoticed that for blocks made of GB20, there arelarger intervals.At the sliding speed of v = 0.25 m/s, the abrasivewear is predominant, the polymer being hung (torn)and drawn from the superficial layers as microvolumes,their size being greater at higher speeds(Figure 2). At the sliding speed of v = 0.75 m/s, theinfluence of the normal load on the average value ofthe friction coefficient is similar: increases from0.12 for F = 1 N, to ~0.2 for F = 5 N.the composites with same type of micro glass beadsadded in a polyamide matrix [13].The values of the friction coefficient have thetendency of being less dependent on the slidingspeed for the normal load F = 5 N; this recommendsthese materials for an exploitation regime withdifferent working speeds (differentiated speedsimposed by the technological process), withouthaving very different energy consumption levelswhen the speed is changing.The extreme values of the friction coefficient arecaused by the generation and the detaching of thewear debris, the ring passing over a bigger microglass beads, an agglomeration of micro glass beadsor fragments of some broken ones on the surface asremained after a preferential elimination of thepolymer from the superficial layer. In other studieson the polymeric composites with micro glass beads,there were no reports on fracturing the hard particles.For the composites with PBT matrix, the authorsnoticed breakings of the micro glass beads, generallythose of bigger diameters (20...40 m) being broken.Figure 3 presents four broken micro glass spheres(A, B, C and D) on an area of ~ 600 m × 600 m inthe central zone of the contact; the resultedfragments are embedded into the polymeric matrix.Such events taken place in the contact create highoscillations of the friction coefficient.Figure 2. SEM image of the block made of GB10, forv = 0.25 m/s, F = 5 N, L = 7500 mFor the blocks made of GB20, under F = 2.5 N,the scattering of the values for the friction coefficientis the largest. The probable cause would be themicro-cutting processes that will have a morereduced intensity when the sliding speed increases.There were not noticed processes of dragging themicro glass beads on the block surfaces, meaningthat the interface between the micro glass beads andthe polymeric matrix is harder to damage, ascompared to, for instance, the mobility of the microglass beads in the sliding direction, but also in thedepth of the superficial layer, as noticed in testingFigure 3. SEM image of a block made of GB10 – fourbroken micro glass beads (A, B, C and D). Testconditions: v = 0.25 m/s, F = 5 N, L = 7500 mFrom SEM images (Figure 4), the wear debriswere characterized as size and shape, many are madeespecially of polymer with only small glass debris(from fragmented micro glass beads) or small microglass beads (but rare). During the test, the weardebris adhere one to each other and are generally bigand rare (as compared to the wear debris resultedfrom other polymer in dry sliding against steel) and13 th International Conference on Tribology – Serbiatrib’13 115