18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS DEVELOP WEAR-RESISTANT POLYMERIC COMPOSITES BY USING NANOPARTICLES L. Chang 1 * , K. Friedrich 2 1 School of Aerospace, Mechanical & Mechatronic Engineering, The University of Sydney, Sydney, 2006 NSW, Australia, 2 Institute for Composite Materials, Technical University of Kaiserslautern, 67663 Kaiserslautern, Germany * Corresponding author (li.chang@sydney.edu.au) 1 Abstract was found that the combination of nanoparticles with conventional micro-sized fillers can achieve a In the present work, the role of the nanoparticle synergistic effect on the tribological behavior of fillers on the sliding wear behaviour of hybrid polymeric materials [3, 4]. polymeric composites was investigated. It was found In the present work, attempts were made to develop that additional nanoparticles could further enhance wear resistant polymeric hybrid nanocomposites, by the wear resistance of polymer composites, using both nanoparticles and conventional micro- especially under extreme sliding conditions. The sized tribo-fillers. It was found that the additional beneficial effect of nanoparticles was mainly nanoparticles could effectively enhance the wear attributed to the reduction in the adhesion between resistance of polymer composites, especially under the transfer film and the polymeric specimen, which extreme sliding conditions. On the basis of helped the formation of continuous transfer film and microscopic observations of worn surfaces, the wear consequently protected the fibers from severe mechanisms were discussed. The tribological role of abrasive wear. The concept allows the development nanoparticle in modifying the sliding wear behavior of the new wear-resistant hybrid polymeric of polymer composites was particularly studied. composites by blending nanoparticles with traditional tribo-fillers. 3 Materials and Experimental 2.1 Materials 2 Introduction Two kinds of polymers, i.e. epoxy (EP) and Over the past decades, polymer composites have polyamide 66 (PA 66), were used as matrices, and been increasingly applied as structural materials in the short carbon fiber (SCF) and two solid lubricants the aerospace, automotive, and chemical industries, i.e. graphite and PTFE, acted as conventional tribo- providing lower weight alternatives to metallic fillers. The average diameter of SCF was ~14.5 µm, materials. A number of these applications are with an average fiber length of ~90 µm. The size of concentrated on tribological components, such as the graphite flakes and the PTFE powder particles gears, cams, bearings and seals, where the self- amounted to ~20 µm and ~4 µm, respectively. lubrication of polymers is of special advantage [1, Nano-sized TiO 2 particles were used as the 2]. To overcome the inhibited weakness of additional filler, at a volume content of 5%. The polymers, various fillers such as short carbon fibers average diameter of the particles was 300 nm. and graphite flakes have been used to develop Technical details of the fillers and matrix, as well as polymer composites for high wear resistance [1]. In the compositions and the compounding procedure particular, short fiber-reinforced polymers (SFRPs) have been reported previously [5, 6]. In terms of the have formed an important class of tribo-materials epoxy composites, a composition of 15vol%SCF + owing to their high specific strength, good load- 5vol%grahite + 5vol%PTFE was used as a carrying capacity and rapid, lower-cost benchmark for the conventional polymeric processibility. Recently, with the booming of composites, which was formulated as the optimum nanophased materials, nano-sized inorganic particles content according to the results from a series of have also come under consideration. In particular, it SCF/graphite/PTFE/epoxy-based composites [8].
For the PA 66 composites, a composition of clearly improved, which would promote the use of 15vol%SCF + 5vol%grahite was used as the these materials for tribo-applications in which more conventional SFRP composite. severe wear conditions are dominant. Figure 1b compares the friction coefficient of the 2.2 Wear Tests specimens tested under different sliding conditions. The wear tests were performed on a pin-on-disc It was found that the addition of nanoparticles could apparatus [5]. The specimen pin (4×4×12 mm 3 ) was effectively reduce the friction coefficient of SFRPs rotated against a polished steel disc, with an initial under all the testing conditions applied here. There surface roughness Ra, of ~220 nm. All tests in this is, however, no general relationship between the study were conducted under dry condition at room trends in friction coefficient and specific wear rate temperature. During the test, the friction coefficient as a function of pv-product. Nevertheless, a high was recorded and calculated by the ratio between friction force/coefficient is normally undesirable for tangential force and normal load. The reduction in polymeric materials, not only because it may specimen‟s height was measured by a displacement accelerate the wear loss of the materials, but it also transducer, which could be used to characterize the will lead to a high contact temperature due to the development of the wear process. An additional frictional heating, and thus a thermal-mechanical monitoring of the temperature rise during testing failure of the material. In the following sections, the was carried out by an iron-constantan thermocouple wear mechanisms of SFRP composites will be positioned on the edge of the steel disc. After the further discussed based on microscopic test, the mass loss of the specimen was measured for observations. In particular, the mechanisms for the the calculation of the specific wear rate i.e. the most favorable effects of nanoparticles on the wear important tribological property of the material, by behavior of SFRPs will be discussed in more detail. using the equation m w 4 Analysis of Wear Mechanisms s F L [mm 3 /Nm] (1) N It is known that the wear performance of SFRP where ∆ m is the specimen‟s mass loss, ρ is the composites is to a great extent determined by the density of the specimen, F N is the normal load properties of the fibers [7]. This is also true for applied on the specimen during sliding, and L is the hybrid SFRP composites filled with additional total sliding distance. particulate fillers, such as the materials considered in this study. Figure 2 shows the microscopic view of 3 Wear Tests Results fibers exposed on the worn surfaces of epoxy based SFRP composites filled without and with Figure 1a summarizes the wear test results of the nanoparticles. It can be observed that the fibers polymeric composites in comparison with that of the clearly stand out from the polymeric matrix and are pure polymers. The applied tribo-fillers enhanced fully exposed to the counterparts. By using an the wear resistance of the polymers by about one atomic force microscopy (AFM), the height from the order of magnitude at 1 MPa, 1 m/s. In this case, it is matrix surface to the exposed fiber can be accurately also noticed that the wear rates of the SFRP measured. It was found that the height of the composites are in the range of 10 -6 mm 3 /(Nm), exposed fiber always agreed with the original which is in agreement with results typically found surface roughness of the steel counterpart (which is for SFRPs sliding against various steel counterfaces ~ 220 nm) [6]. Hence, during the wear process, the [7]. With an increase in pv , the wear rate of the short fibers had to carry most of the load. To fully composites filled with traditional fillers explore the strengthening effect of short fibers, it is progressively increased, suggesting changes in the critical to ensure that the fibers are only gradually dominant wear mechanisms. For the composite with removed from the polymer matrix i.e. without additional nanoparticles, however, the wear rate of serious breakage. the nanocomposite was relatively stable at ~1×10 -6 On the basis of the above microscopic observations, mm3/(Nm), even under high pv conditions. This Figure 3a gives a schematic illustration of the failure means that the limiting pv of the nanocomposite was mechanism for the sliding wear of SRFP composites
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