23rd International Colloquium Tribology - January 2022 317 Improving abrasive wear performance of polymers Helena Ronkainen VTT Technical Research Centre of Finland, Espoo, Finland Corresponding author: helena.ronkainen@vtt.fi Jani Pelto VTT Technical Research Centre of Finland, Espoo, Finland Vuokko Heino VTT Technical Research Centre of Finland, Espoo, Finland Mikko Karttunen VTT Technical Research Centre of Finland, Espoo, Finland 1. Introduction Interest for thermoplastic polymers in engineering applications has increased due to their lightweighness and low cost, combined with ease of manufacturing as compared to metals and ceramics. The range of polymeric materials is vast and by blending and by using reinforcements, composites with widely different properties can be generated [1, 2]. Thermally resistant and mechanically strong polymers are available for demanding engineering applications, but they still often lack sufficient abrasive wear resistance causing material loss or aesthetic deficiencies. At VTT different strategies have been used to improve the abrasive wear performance of neat polymers. In this study, two semicrystalline thermoplastics, a commodity polymer high density polyethylene (HDPE) and a biobased engineering polyamide (PA1010), were used as matrix polymers with differed strategies to improve their wear performance. 2. Materials High density polyethylene HDPE powder was used as the matrix polymer and the commercially available nanopowders were used for polymer composites, namely graphene oxide (GO), halloysite aluminosilicate clay hollow nanotubes (HNT), fumed alumina (f-Al 2 O 3 ), and gamma-alumina (nano-γ-Al 2 O 3 ). Vinyltrimethoxy silane and organic peroxide were used for the chemical surface treatment of the fillers. Besides using different fillers, the HDPE was also blended with the ultrahigh molecular weights polyethylene (UHMWPE). The bio-based polyamide PA1010 was used for PA composites and different microand nano-scale fillers were used as filler reinforcements. The nano-scale reinforcements studied were GO, HNT, hydrophilic fumed silica (SiO2), hydrophopic fumed silica, and the micro-scale fillers were glass beads and glass flakes (1 µm in size). The fillers were chemically surface treated to improve the dispersion and the adhesion of the fillers to the polymer matrix. The compounding of polymer composites was carried out with DSM Xplore micro-compounder and the test samples were moulded with ThermoHaake MiniJet injection moulding machine. 3. Experimental The filler surface treatment was characterised by FT-IR and SEM. The abrasive wear of polymers was evaluated with a three-body sand abrasion test, where the abrading sand particles are introduced into the sliding contact of the rubber-coated wheel and the polymer sample. The sand (grain size about 0.32 mm) was flowing into the contact point with the flow rate of 320 gmin -1 . The tests followed the ASTM 65-04 standard, and besides the standard test parameters, also lower level of loading parameters was applied to verify the influence of loading on polymer wear. In the lower loading level tests, the load and the speed were reduced to decrease the heating effect of polymers, and to provide same amount of sliding distance compared to the higher load level, the test duration was increased. The test parameters used in the tests are shown in Table 1. As a reference material for the developed polymer composites the polyether ether ketone (PEEK) was used. The wear surfaces were studied by optical and scanning electron (SEM) microscopy after the tests to reveal the wear mechanisms. The indentation modulus and hardness of polymer composites were also measured by using CSM Micro Combi Tester (Anton Paar). Table 1: The test parameters with two loading levels used in sand abrasion tests. Load level 1 Load level 2 Normal load 19 N 45 N Velocity 50 rpm ~ 0.62 ms -1 100 rpm ~ 1.23 ms -1 Test time 6 minutes 3 minutes 318 23rd International Colloquium Tribology - January 2022 Improving abrasive wear performance of polymers 4. Results The abrasive war volumes measured for the polymers and polymer composites are presented in Figure 1. The PEEK polymer showed reasonable wear in abrasive test conditions and it was used as a reference for the developed polymer composites. The neat HDPE shows high wear volumes with both loading levels. The wear could be reduced dramatically by blending the HDPE with UHMWPE and by using the fillers as reinforcements. The improvement in performance is apparent under the harsher, more destructive abrasive loading conditions. HNT composite showed reduced wear and particularly composites with GO fillers provided 50 to 60 % reduction in wear for the studied HDPE/ UHMWPE blends. The lowest wear was achieved with UHMWPE/ HDPE blend with as high as 80% of UHMWPE and 0.5 wt.-% GO, a composition which was still well processible by standard injection moulding. The neat PA1010 polymer had a rather good wear performance, comparable to PEEK as presented in Figure 1. The abrasive wear performance of PA could be further improved by adding microand nano-scale fillers. The micro-scale GFL and the combination of micro and nano-scale-fillers GB+GO reduced the wear significantly. However, even more significant improvement in performance was achieved by nano-scale fillers. HNT, GO and silica fillers could reduce the wear about 10 % for the lower load level (PA1010/ 15wt-% HNT) and up to 50 % for the upper load level tests (PA1010/ 3wt-% SiO2). Figure 1: The abrasive wear volumes of PEEK, HDPE, PA1010 and the developed polymer composites. The abrasive wear was evaluated in sand abrasion tests with two different sets of test parameters. The wear surfaces studied by optical and SEM revealed change in wear mechanisms with different filler additions. As an example, the wear surface of PA showed scratched and plasticly deformed surface whereas the composite with the addition of silica fillers showed fractured features as presented in Figure 2. 23rd International Colloquium Tribology - January 2022 319 Improving abrasive wear performance of polymers Figure 2: The wear surfaces of PA1010 and PA1010/ hydrophilic fumed silica (3 wt-%) with the load level 1 and load level tests. When comparing the abrasive wear of HDPE and PA composites to PEEK, several composited were developed with improved wear resistance compared to PEEK. Since PEEK is rather expensive specialty polymer designed for high temperature environment from +140°C up to +200°C, the HDPE and PA composites can be considered as good and reasonably priced option for many applications that require high abrasive wear resistance, however, at lower temperature range < +100°C. 5. Conclusion The abrasive wear resistance of HDPE and PA could be significantly improved by nano-scale filler reinforcements, particularly with GO and HNT. The composites also provided lower wear compared to the reference PEEK polymer. 6. Acknowledgement This project has received funding from the Finnish Academy and VTT Technical Research Centre of Finland. References [1] Briscoe, B.J., Sinha, S.K., Tribological applicationsmof polymers and their composites - past, present and future prospects. In: Tribology of Polymeric nanocomposites. pp- 1-21. [2] Friedrich, K., Fakirov, S., Zhang, Z., Polymer composites, from nanoto micro-scale. Springer 2005.