In recent years, due to the trend in designing of machines and machine elements, polymers have taken a significant role as materials for parts like cams and gears, as a substitution for conventional materials. Additionally,polymers are extensively used as materials for manufacturing of parts for mining equipment,conveyor lines, rotary valves, etc. In all these applications, polymers are exposed to abrasive wear.
Abrasive wear is a wear mechanism generated by the sliding of a hard material over a softer material under load, while surface asperities of the harder material tend to remove the softer material . In general, abrasive wear can be divided into two major groups due to the mechanisms of generation: two body abrasion,and three body abrasion. Two body abrasion occurs in direct contact of two counter surfaces when one is significantly softer than the other one, while three body abrasion occurs when hard particles get trapped between two sliding surfaces. Some estimation say that abrasive wear contributes up to 60% of total costs caused by wear.
Since polymers became important technical materials in modern engineering, numerous papers referred to these in the last decades.
The first article in history summarizing results of polymer abrasion was published in 1969. The following years brought a wide variety of experiments related to abrasive wear of polymers. Furthermore, a wide variety of experiments has been performed on polymer composites, reinforced polymers and epoxy resins. Most of these experiments have been performed on conventional pin-on-disc testing setups with sand paper as counterbody to the polymer pins (two body abrasion), as well as on the ASTM G65 abrasion tester with different testing setups (steel or rubber wheel, Three body abrasion). Attempts to investigate abrasion resistance of polymer materials on micro scale have also been done. Most of these studies can be seen as a summary in several review articles.
In this work we investigate polymer materials possibly replacing metals for dosing wheels in rotary feeders, especially for those with smaller geometry, where entire parts have to be made from bulk polymer and where the implementation of reinforcing fibers in the polymer structure is not possible. The selected polymer materials are commercially available.
Experimental procedure：
For the selected specimens, the tests were performed using the ASTM G65 abrasion tester. The decision was based on already available experiences, not only from literature (chapter 1) but as well from our own experimental work. Nevertheless, adjustments of the test system according to the application demands could be done easily to the ASTM G65 apparatus. Adjustment of the system consisted of retrofitting of the test rig with the new nozzle which was capable to maintain the sand flow within the limits defined by application. A schematic of the test rig can be seen in Figure 2. The tested samples were fixed to the sample holder lever, which was positioned in the tangential direction in relation to the rubber or steel wheel. The sample holder lever is connected to the load lever having the same pivot. The load was applied by positioning a calibrated dead weight at the end of the load lever. During testing, the examined samples were exposed to the influence of the abrasive. The abrasive was stored in the sand tank and fed into the contact zone through an appropriate nozzle. After leaving the nozzle the abrasive was entrapped in and moved through the contact zone due to the relative motion of the wheel, which was rotating in direction of the sand flow (clockwise in the schematic).
Tests were carried out at modified conditions with a load of 50 N, a rotational velocity of the wheel of 200 min-1, a sand flow rate of 3 litres/minute and testing times 1.5, 3, 4.5, and 6 hours, respectively. Both test setups – with rubber wheel and with a steel wheel – were used. Before and after testing, each test sample was cleaned in ethanol and dried.
Mass loss was determined by weighing the samples before and after the tests with an analytical balance (RADWAG XA 210/X,0.01 mg to 210 g, RADWAG Balances & Scales, Radom, Poland). Each test was repeated at least two times, and for comparison the volume loss for each selected testing sample was calculated. Wear tracks on the wheel surfaces were measured by a confocal microscope (Leica DCM 3D, Ernst Leitz Wetzlar, GmbH, Germany) for noncontacting assessment of the microscale topography. The size of the scanned area was 0.96 mm × 1.3 mm.
ASTM G65 abrasive test results：
The 1.5 hour experiments Average volume loss (mm?) as a function of the polymer hardness (HS), for the experiments of 1.5 hour lenght is presented in Figure 3. In general, for both test setups (rubber and steel wheel) the highest volume loss was detected for the Polymer A, which possess the lowest hardness. Nevertheless, it should be noticed that for the steel wheel test setup, the polymer A exhibits lower wear volume (by 65 %) when compared to the rubber wheel setup.
A similar trend was detected for Polymer B,with approximately 22 % volume loss obtained in the tests with the steel wheel setup as compared to those with rubber wheel one.
However, it can be noticed that this trend changed after an increase in hardness on Shore scale over 85 HS. Polymer C exhibited 48% lower, and Polymer D 35% lower volume losses in the tests with the rubber wheel setup in comparison to the tests with the steel one. Moreover, the lowest wear volume in the tests with two setups was obtained with Polymer C.
3–6 hour experiments：
The two best performing polymers, Polymer C and Polymer D were selected for the longer term experiments. The average volume loss(in mm?) as a function of time is presented at Figure 5. It can be seen that, by increasing the testing time, the difference between Polymer C and Polymer D became obvious. In the rubber wheel test setup (Figure 5 a), Polymer C exhibited a mild increase in wear comparing to the 1.5 hour test results, reaching a wear loss volume of about 750mm? at the volume loss at the end of the test.
On the other hand, Polymer D was completely worn off after 4.5 hours of testing. This difference was even more pronounced in the tests with the steel wheel test setup, see Figure 5 b, in which Polymer D was completely worn off after 2 hours of testing.
A negatiive trend in the wear behavior was detected for Polymer C in the time sequence between 4.5 and 6 hours; for all tested samples, the detected mass loss was lower after 6 hours of testing than after 4.5 hours.
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