24th International Colloquium Tribology - January 2024 205 Tribological Testing for the Assessment of Friction and Metal Transfer in Sliding Contacts between Cemented Carbide and Aluminum during Metal Forming N. Cinca 1* , M. Olsson 2 , M. G. Gee 3 1 Hyperion Materials & Technologies, Polígono Industrial Roca Calle Verneda 12-24, Martorelles 08107, Barcelona, Spain 2 Dalarna University, SE-791 88, Falun, Sweden 3 Department of Materials and Mechanical Metrology, National Physical Laboratory, Hampton Road, Teddington, Middlesex TW110LW, UK * Corresponding author: nuria.cinca@hyperionmt.com 1. Introduction Cemented carbides are well-known heterogeneous materials used to manufacture tools are employed in many industries where wear resistance is required together with a proper balance of hardness and fracture toughness. Diverse wear mechanisms can occur depending on the operating conditions for the components. In the metal forming industry, a sliding friction wear mode takes place with transfer of the metal work material to the tool [1]. Different types of tribological testing can be used to characterize the friction and wear phenomena during sliding, but it is always difficult to extrapolate the behavior from lab testing to field performance. Decreasing the length scale down to the microlevel can provide some insights of initial metal transfer due to surface asperities [2, 3]. In the present study, tribological characterization of Al metal transfer in sliding motion onto two cemented carbides was performed two ways. Firstly, to evaluate the initial sticking on a polished surface, an Al tip was made to slide onto different cemented carbide grades under well-defined contact conditions. In the second test, a pin on disc system was used with the capability for continuous imaging and capture of changes to the wear surface. 2. Methodology The two commercial cemented carbide grades tested in the present work are presented in Table 1. Grade A is a plane WC-Co grade, while grade B has cubic carbides. Both tribological tests were performed under dry conditions. Table 1. Chemical Composition and Hardness Grade WC-phase [vol.%] γ-phase [vol.%] Binder [vol.%] Hardness, HV 30 [kg/ mm 2 ] A 79.6 - 20.4 Co 1294 ± 9 B 63.3 19.8 16.9 Co 1517 ± 5 In the first test, an aluminum stylus was slid over a scratch that had been made intentionally onto a well-polished surface and the coefficient of friction (COF) was recorded during the test. The testing conditions were normal load 10 N, sliding speed 10 mm/ min, sliding distance 10 mm, 1&5 passes. For the second test, a pin on disc tribometer fitted with linescan camera to obtain continuous images of wear track on disc was used. The COF was continuously measured. Lapped cemented carbide plates (Ra 1.03 mm and 0.684 mm for A and B respectively measured with Alicona InfiniteFocus) of 75 mm diameter were tested against the domed end (10 mm radius) of an Al pin of 20mm diameter sliding at 200N with a 59mm wear track. The test duration was 1 hr at a speed of 2 rev/ min. 3. Results & Discussion 3.1 Multipass Testing with Aluminum Stylus The initial metal transfer represented in this test illustrates that both cemented carbide grades present similar friction characteristics, with the friction coefficient increasing after reaching the intentionally introduced scratch and decreasing afterwards. This behaviour can be explained by the repassivation of the fresh aluminum surface that was scrapped off when passing over the scratch. However, the friction peaks after first and fifth passes are higher for grade A than grade B. Since grade A is softer, its scratch ridges of are higher, which induces that more aluminum is scraped off in connection to the scratch. However, after passing the scratch, the COF remains higher for grade B, resulting in higher transfer tendency. Figure 1. COF of aluminium pin sliding against the tested cemented cabide grades. Grey line is 1 pass and black line is 5 passes. 3.2 Pin on Disc in Situ Testing In the pin on disc tests (Figure 2), the COF continuously increased along the test time for the softest grade A, starting Tribological Testing for the Assessment of Friction and Metal Transfer in Sliding Contacts between Cemented Carbide and Aluminum during Metal Forming 206 24th International Colloquium Tribology - January 2024 at values around 0.4 and reaching 0.7 at the end of the test. By contrast, the COF for grade B was relatively constant for the whole test at about 0.6-0.7. With the linescan camera, the sticking progression of aluminum was recorded. Small areas of transferred aluminium were seen almost immediately in both tests. These were often associated with scratches of the lapped surfaces. The higher transfer tendency in grade B discussed in Figure 1 can be the reason for the higher values of friction here recorded here. Using real time in situ measurement techniques provide information on wear mechanisms related to microstructural features of tested surfaces. Figure 2. Visual and graphical representation of the change of COF with the time along the pin on disc test for both cemented carbide grades. The brighter, the higher the COF value. 4. Conclusions Two cemented carbide compositions have been tribologically tested for aluminum metal forming applications. With the multipass testing to evaluate the initial metal transfer and the ball on disc test, it has been found that the surface topography plays a key role in the aluminum sticking, which results in an increase in the friction coefficient. Both methodologies provide insights on the wear mechanisms. References [1] V. Westlund, J. Heinrichs, S. Jacobson, On the role of material transfer in friction between metals: Initial phenomena and effects of roughness and boundary lubrication in sliding between aluminum and tool steels, Tribol.Lett. 66 (2018) 66-97. [2] J. Heinrichs, M. Olsson, S. Jacobson, New understanding of the initiation of material transfer and transfer layer build-up in metal forming—In situ studies in the SEM, Wear 292-293 (2012) 61-73. [3] V. Westlund, J. Heinrichs, M. Olsson, S. Jacobson, Investigation of material transfer in sliding friction-topography or surface chemistry, Tribol.Int. 100 (2016) 213-223. [4] M. Gee, T. Kamps, P. Woolliams, J. Nunn, K. Mingard, In situ real time observation of tribological behaviour of coatings, Surf Coat Technol 442 (2022) 128233.