## 3 Results and Discussions
An SEM image of the whole wear track of a sample tested in the dry condition is shown in Figure 7. Relatively smooth edges with noticeable longitudinal grooves can be seen in the SEM image, with some wear debris accumulated at the two ends of the wear track. With higher magnification SEM images in Figure 8, wear mechanisms can be identified. Figure 8a shows a wear track tested in dry contact. Grooves in the wear track show the main wear mechanism in dry contact is abrasive wear, where the hard particles (debris) slide against the test sample and plowed into the surface, causing material removal. The formation of cracks and the spalling (breaking off) of small particles or flakes from the surface occurs due to fatigue wear, where repeated cycles of stress can initiate and propagate surface cracks. Over time, the cracks can reach a critical size, causing small particles or flakes to break off from the surface. Galling can also be observed in Figure 8a, which is a type of wear mechanism that occurs when two metal surfaces in sliding contact undergo excessive friction and adhesion, resulting in localized welding and tearing of surface material. For lubricated contact, minimum wear loss can be observed in Figure 8b, with shallow grooves representing abrasive wear. Figure 6: Effects of different types of lubricants on (**a**) average coefficient of friction, and (**b**) wear rate of SLM samples. All tests were conducted at room temperature under a 10 N load and a 5 Hz frequency. To compare the COF and wear resistance of an SLM-printed Co-Cr-Mo alloy with a traditionally manufactured alloy, sliding wear tests were conducted on the Co-Cr-Mo alloy manufactured by the investment casting process. The comparison of the tribological performance of the two manufacturing processes can be observed in Figures 3a and 5a. The cast samples show a lower friction coefficient compared to SLM-printed samples at all three  
Figure 8: Wear tracks after reciprocating sliding wear tests under (**a**) dry contact; and (**b**) lubricated contact with gear oil. Both samples tested at 5 HZ and under a 10 N applied load and at room temperature. Figure 2:0 SEM image of the wear track of a sample tested under dry contact at a 5 Hz frequency, a 10 N load, and room temperature. Wear track length is 10 mm. tested loads. However, SLM-printed samples show better wear resistance, especially at higher loads. It was observed by SEM analysis that this is due to the entrapment of the wear debris inside the pores on the wear track. As can be seen in Figure 9a, the wear debris is entrapped inside pores on the wear track. As the test time increases (Figure 9b), more debris becomes embedded within the pores on the surface of the material and forms a protective layer against further wear. This mechanism leads to a lower wear rate in SLM-printed Co-Cr-Mo samples, although they show higher COF compared to cast samples. Figure 10 shows the cross-section of the wear track, for a sample tested at a 5 N load and a 5 Hz sliding frequency in dry contact. The subsurface cracks can be observed, representing fatigue wear due to cyclic loads applied to the surface of the samples during the reciprocating sliding wear test. The wear debris entrapped inside the surface pores can also be observed from the cross-section SEM image. Figure 2:1 Accumulation of wear debris inside the pores on the wear track. Samples were tested at room temperature in dry contact under a 10 N load and a 5 Hz frequency. The test stopped after (**a**) 200 m, and (**b**) 600 m total sliding distance. Figure 11 shows the EDX analysis of the wear track for a sample in dry contact under a 10 N load and a 5 Hz frequency. As expected, chromium (Cr), cobalt (Co), and molybdenum (Mo) are the primary elements present on the surface of the wear track. However, oxide particles can also be observed, with \"pt1\" having a composition of 5.86% oxygen (O).