eJournals International Colloquium Tribology 23/1

International Colloquium Tribology
ict
expert verlag Tübingen
125
2022
231

Influences of roughness, moisture content and lubrication on friction of polymers against 100Cr6

125
2022
Joel Voyer
Heinz Haider
Claudia Mayrhofer
Igor Velkavrh
Tom Wright
ict2310331
23rd International Colloquium Tribology - January 2022 331 Influences of roughness, moisture content and lubrication on friction of polymers against 100Cr6 Joel Voyer V-Research GmbH, Dornbirn, Austria Corresponding author: joel.voyer@v-research.at Heinz Haider Austrian Research Institute for Chemistry and Technology, Vienna, Austria Claudia Mayrhofer Austrian Research Institute for Chemistry and Technology, Vienna, Austria Igor Velkavrh V-Research GmbH, Dornbirn, Austria Tom Wright V-Research GmbH, Dornbirn, Austria 1. Introduction The interactions in dry and lubricated tribological polymer systems on smooth metal surfaces (Rz < 4 µm) were previously extensively investigated and published, and the main parameters influencing their friction behaviour are well known: the main mechanisms determining friction forces are adhesion and deformation. Depending on the polymer and on its adhesive and mechanical properties, friction may have a strong dependency on both surface roughness and normal load. Generally speaking, adhesive friction is predominant at low normal loads, but deformative friction becomes more important at higher loads. For specific polymers having high load bearing capabilities, an increase of the normal load usually results in a decrease of the friction coefficient (COF), due to a change in the main friction mechanism. For some industrial applications, tribologically optimized smooth surfaces cannot be economically produced and therefore, rough surfaces have to be used, which produces a change of the main tribological mechanisms. In this study, the influence of surface roughness coupled with modifications of the polymer’s moisture content and test temperature will be investigated and analysed. 2. Materials and Experimental Procedure Tribological investigations were undergone using 3 different polymers (POM, PA6-GF30 and PK) against 100Cr6 steel with different surface roughness (smooth or rough) (details in Table 1). To change the polymers’ moisture content, some were dried in a furnace at 70°C while others were immersed in water at 70°C until a weight change equilibrium was reached (~10 days). Table 1: Details on investigated materials. Parameter Unit Value Polymers - POM / PA6-GF30 / PK Moisture % rel. H. 0 / 100 Steel Plate hardened 100Cr6 Rz µm Rz ~ 1 (smooth) Rz ~ 20 (rough) 2.1 Friction theory for smooth and rough surfaces For smooth surfaces, the main friction mechanism is based on adhesion effects, which are mostly influenced by surface energy and real contact area of the tribological system [1]. By introducing a lubricant, this adhesion mechanism tends to be minimized due to a decreased in the surface energy. For rough (and especially rough-lubricated) plastic-metal contacts, where surface roughness has a major significance on friction behaviour [1], adhesion effects may be neglected, since deformation becomes the predominant friction mechanism. For instance, for soft polymers under normal loads against metallic partners, roughness asperities of the metal penetrate in the polymer without any metallic asperity’s deformation [1]. 332 23rd International Colloquium Tribology - January 2022 Influences of roughness, moisture content and lubrication on friction of polymers against 100Cr6 2.2 Tribological Experiments To evaluate static and dynamic COF, tribo-tests using different surface pressures (stepwise change from 20 to 0,5 N/ mm 2 ), two different polymer moisture contents (0%/ 100%) and two lubrication conditions (unlubricated/ lightly greased) were performed. Table 2 lists general tests parameters and Figure 1a shows the test setup and sample geometry. Static and dynamic COF were calculated from 4 cycles of the raw signals. Static COF was determined as first peaks after the sample’s motion (red dots in Figure 1b) and dynamic COF was an average of raw COF values using a window of 80% of a half-cycle (Figure 1c). a) b) c) Figure 1: a) Test setup and sample geometry, b),c) determination of static and dynamic COF. Table 2: Test parameters. Parameter Unit Value Surface Pressure N/ mm 2 0,5 / 1 / 2,5 / 5 / 7,5 / 10 / 15 / 20 Stroke per Cycle mm 20 Speed mm/ s 83 Temperature °C 20 / 70 Lubricant Condition - Dry / Grease 3. Results and Discussions The pre-treatments showed clearly that the water absorption capacity of the polymers differs drastically: - POM (water absorption = 1,5 wt%) - PA6-GF30 (water absorption = 5,9 wt%) - PK (water absorption = 3,2 wt%) Thus, it is expected that the polymers’ tribological properties may be influenced by the moisture contents. 3.1 Tribological Experiments Figures 2 and 3 present static and dynamic COF for all polymers but only for 20°C. Similar results for 70°C were obtained but not presented here. First, for some of the friction curves (especially for PA6 and PK), a load dependency of COF may be observed, as previously described in [1], and is due to a nonlinear behaviour between friction and normal force, which reduces COF with increasing normal load [2]. For low loads, lower COF were usually measured for polymers with low moisture contents than for polymers with high contents. Usually, a low moisture content leads to higher E-modulus and compressive yield stress, which causes a lower penetration depth and therefore at low loads, a lower COF. For polymers with high moisture, inverse observations are encountered: lower E-modulus and compressive yield stress results in higher COF at low loads. Although roughness asperities under high loads may penetrate deeper into polymers with higher moisture, resulting shear forces may be lower, due to lower mechanical properties. The introduction of a lubricant on a smooth surface eliminates any load dependency of COF, resulting in flattened COF curves over the whole surface pressure range independently of the moisture (Figure 2), due to a reduction of adhesion combined with a separation of the contact surfaces (decrease of asperity penetration). For rough surfaces, the presented investigations have shown that independent of polymer moisture, mechanical properties determine static COF, due to the fact that the metal asperities are more pronounced, even though although lubrication is used (Figure 3). 23rd International Colloquium Tribology - January 2022 333 Influences of roughness, moisture content and lubrication on friction of polymers against 100Cr6 Figure 2: COFs of dried and water saturated polymers, smooth plate (Rz ~1 µm), dry / lubricated, 20°C. Figure 3: COFs of dried and water saturated polymers, smooth / rough plates (Rz 1 / 20 µm), lubricated, 20°C. 4. Conclusion No general conclusions but only main observations may be drawn from the results: Dry conditions - POM: no influence of temperature or moisture. - PA6: strong influence of temperature and moisture. - PK: light influence of temperature and moisture. - COF mostly determined by adhesion effects. Lubricated Smooth Surfaces - Elimination of load dependency of COF. - reduction of static and dynamic COF. - almost no influences of moisture and temperature. - asperities effects not predominant. Lubricated Rough Surfaces - roughness and deformative friction are determinant. - asperities effects predominant: polymer mechanical properties determine the friction behaviour. References [1] Erhart G.: Zum Reibungs- und Verschleißverhalten von Polymerwerkstoffen. Diss. TH Karlsruhe, 1980. [2] Sujeet S.: Polymer Tribology. Imperial College Press, ISBN-10 1-84816-202-2, 2009.