International Colloquium Tribology
ict
expert verlag Tübingen
125
2022
231
A new approach for the friction and wear characterisation of polymer fibres under dry, mixed and hydrodynamic sliding
125
2022
Justus Rüthing
Regine Schmitz
Frank Haupert
Michael Sigrüner
Nicole Strübbe
ict2310339
23rd International Colloquium Tribology - January 2022 339 A new approach for the friction and wear characterisation of polymer fibres under dry, mixed and hydrodynamic sliding Justus Rüthing Hamm-Lippstadt University of Applied Science, Lippstadt, Germany Corresponding author: justus.ruething@hshl.de Regine Schmitz Hamm-Lippstadt University of Applied Science, Lippstadt, Germany Frank Haupert Hamm-Lippstadt University of Applied Science, Lippstadt, Germany Michael Sigrüner Rosenheim University of Applied Science, Rosenheim, Germany Nicole Strübbe Rosenheim University of Applied Science, Rosenheim, Germany 1. Introduction A new approach for the friction and wear characterisation of polymer fibres for the application in fibre reinforced concrete is developed. The abrasive conditions, similar to the ones found in an industrial concrete mixing process, are simulated using an optimised pin-on-disc test rig to conduct tests under dry, mixed and hydrodynamic sliding conditions. Using this test method, it is shown, that it is possible to differentiate the friction and wear characteristics of polymer fibres based on their dimension and sliding condition. 2. Materials 2.1 Specimen - Polypropylene Fibres For this study, three polypropylene [PP] fibres, produced by a single screw extruder and post treated in a stretching unit described in [1], where characterised. By variation of the processing parameter of the stretching unit, three fibres with the draw ratios from 1: 10, 1: 14 and 1: 17 were created and used in this study. Because of the different values for the vertical and across diameters, an elliptical fibre shape is assumed. The dimensions of the fibres are shown in Table 1. For the tribotest itself, the polymer fibre is fixed to a specifically designed specimen-holder. Within this configuration a fibre-area of 20 mm in length is tested against the abrasive counterpart. Table 1 Dimensions of the tested polypropylene fibres Draw Ratio Diameter Across [µm] Diameter Vertical [µm] 1: 10 670 (± 17) 627 (± 22) 1: 14 639 (± 10) 500 (± 15) 1: 17 575 (± 36) 552 (± 34) 2.2 Counterpart - Alumina-Disc The counterpart used consists of an Alumina (Al 2 O 3 ) disc with a surface roughness [R a ] of 1,59 µm (see Figure 1). To ensure the same surface properties apply for each measurement, the surface is grinded down using a diamond grinding disc (Schmitz Metallography, grain size 0080) before each tribotest. 2.3 Pin-on-Disc Test Rig The optimised test rig, described in [2], consists of an inhouse designed and built pin-on-disc tribometer, a programmable peristaltic pump (see Figure 1) as well as an 340 23rd International Colloquium Tribology - January 2022 A new approach for the friction and wear characterisation of polymer fibres under dry, mixed and hydrodynamic sliding Figure 1: Optimised Pin-on-Disc Test Rig optimised specimen holder. Within this test rig, normal force can be applied in the range of a few Newtons to 200 Newtons (± 0,3 N) through a mechatronic controlled load unit. With a force sensor, friction force is measured. Using a stepper motor, the counterpart disc is rotated to speeds of up to 5,0 m/ s. Applied load, friction force and rotational speed are measured, recorded and analysed in real time using an in-house designed software. The programmable peristaltic-pump is capable of supplying lubricants to the discs surface using an injection attachment in the defined rates of 0,01 to 5,7 ml/ min. 3. Experimental Method The testing procedure consists of a dry-sliding-phase, in which the tested fibre is abraded to a depth of 150 µm. Thereafter, water as a lubricant is constantly added over the water injection attachment to the discs surface. To model the different sliding conditions, similar to the ones found in an industrial concrete mixing process, two lubrication rates are used: 0,5 ml/ min for 25 min to model mixed sliding conditions, and 4,0 ml/ min for 25 min to model hydrodynamic sliding conditions. To compare the tribological properties of the fibres between each other, the loading force was adjusted for each fibre to keep an equal pv-product of 0.16 MPa m/ s for each test. The counterparts disc speed was kept at a constant 0,4 m/ s. The normal forces used were 5.3 N for 1: 10, 5.1 N for 1: 14 and 4.6 1: 17. The dry sliding friction and wear data of the tested fibers were taken in the defined steady state from 100 to 150 µm of absolute wear. For the mixed and hydrodynamic sliding conditions, the wear data was taken in the last 10 min of each lubrication rate. 4. Findings The findings of the tribological characterisation of three PP fibres in this study show, that the friction and wear behaviour of the tested fibres can be determined under dry, mixed and hydrodynamic sliding using the described test method. The wear and friction data gathered in this study are presented in Figure 2. 4.1 Friction The results of the tribological characterisation of three different fibres show, that the diameters impact the coefficient of friction [COF] as the fibre with the largest diameter (1: 10) demonstrating the highest and 1: 17, with the lowest diameter, demonstrating the smallest COF. Further, a reduction in the COF can be examined across all fibers with rising addition of lubricant. The dry sliding phase resulting in the highest and the hydrodynamic sliding phase resulting in the lowest COF out of every fibre tested. The friction results are presented in Figure 2. 4.2 Wear The wear behaviour of the tested fibres show a reduction in the steady-state wear rate due to the sliding conditions. The fibre with the lowest wear rate under dry sliding, tested in this study, is the fibre 1: 10. Considering the mean error, fibres 1: 14 and 1: 17 cannot clearly be differentiated between each other. However, a difference in the wear behaviour between the fibre 1: 10 and the fibres 1: 14 and 1: 17 can be examined. For the mixed sliding conditions, fibre 1: 10 shows a higher wear compared to fibres 1: 14 and 1: 17. No difference in wear behaviour can be examined between the fibres 1: 14 and 1: 17. With further increase in the lubrication rate and therefor hydrodynamic sliding, fibre 1: 10 stills show the highest wear rate compared to the fibres 1: 14 and 1: 17. As with the wear behaviour under mixed sliding, no difference in wear behaviour can be examined between the fibres 1: 14 and 1: 17. 23rd International Colloquium Tribology - January 2022 341 A new approach for the friction and wear characterisation of polymer fibres under dry, mixed and hydrodynamic sliding Figure 2: Steady-State wear rate and coefficient of friction of the fibres 1: 10, 1: 14 and 1: 17 in variation of the lubrication rate 5. Conclusion The results show, that the fibre with the smallest draw ratio (1: 10) exhibits the better wear-resistance under dry conditions whereas the fibres with the medium (1: 14) and high (1: 17) draw ratios show superior wear-resistance under mixed and hydrodynamic sliding. For the COF, the fibre with the smallest draw ratio has the highest COF under dry sliding. The COF for fibres with the medium and highest draw ratios is comparable across all tested sliding conditions. Using this optimised pin-on-disc test equipment, it is shown, that tribotests can be performed under various lubrication rates to characterize the tribological behaviour of polymer fibres. 6. Acknowledgement The authors thank the German federal ministry of education and research (BMBF) for the funding of this study as part of the project 13FH068PB6. References [1] M. Sigrüner, D. Muscat und N. Strübbe, „Investigation on pull-out behavior and interface critical parameters of polymer fibers embedded in concrete and their correlation with particular fiber properties“, J. Appl. Polym. Sci., 2021 [2] R. Schmitz, F. Haupert, J. Rüthing, M. Sigrüner und N. Strübbe, „Tribologische Charakterisierung von Polymerfasern unter Trockenreibung, Mischreibung und Hydrodynamik mittels einer optimierten Pin-on-Disc-Prüfmethode“, TuS, 2021
