## 3 Results and Discussions
However, at higher sliding speeds, COF increases. This is because of the increase in interface temperature due to frictional heating at higher sliding speeds. This can cause  
Figure 3: Effects of (**a**) applied normal load (at 5 Hz frequency), and (**b**) frequency (at 10 N constant load), on average coefficient of friction. All lubricated tests were conducted using synthetic gear oil. thermal softening of the material and the formation of adhesion between the two surfaces, leading to an increase in friction coefficient. Also in lubricated contact, higher sliding speeds lead to a reduction in the time available for the lubricant to form a complete film between the two sliding surfaces. As a result, the surfaces come in contact with each other, leading to higher friction. The variation of COF with test time can be observed in Figure 4. In dry contact, the COF starts high at around 0.8 and maintains this average value during the test. The COF graph for dry contact shows fluctuations and instability in the friction coefficient due to the lack of lubrication. In dry contact, the surfaces can experience adhesion, plowing, and/or deformation, which can lead to high friction and wear. The asperities on the surfaces may interlock, leading to a higher friction coefficient during initial contact [26]. As the surfaces continue to slide, the asperities may break off or wear down, leading to a reduction in friction coefficient. However, as the surface roughness continues to change over time due to wear and other factors, the friction coefficient may also experience fluctuations. This can result in a rough friction coefficient graph with many peaks and valleys over time. On the other hand, COF during the lubricated sliding wear test is significantly lower, showing the effectiveness of lubrication for the Co-Cr-Mo alloy. The results showed that the lubricant film did not break down or become depleted during the test and a low value of COF was maintained. Figure 5 shows the effects of load and sliding speed on the wear rate of an SLM-printed Co-Cr-Mo alloy. As expected, the wear rate in lubricated contact is significantly lower than in dry contact. As the applied load increases, the wear rate increases. The higher load results in higher contact pressure, and this increased pressure causes higher frictional forces between the surfaces, which can accelerate the wear rate. When a material is subjected to higher contact stress, it undergoes greater deformation, causing microcracks to form, leading to fatigue and eventual failure, which eventually increases the wear rate [27]. It can also be observed in Figure 5b that the wear rate increases at higher sliding speeds, although the effect of sliding speed is not as remarkable as the effect of applied load on the wear rate. Higher sliding speeds can generate more heat due to increased friction between the surfaces in contact. This increased heat causes softening of the material, which can increase the wear rate [28]. Figure 4: Variation of coefficient of friction with test time for SLM samples tested at room temperature, under a 10 N load, and a 5 Hz frequency. The lubricated test was conducted using synthetic gear oil. Figure 6 shows the effect of different types of lubricants on the tribological properties of an SLM-printed Co-Cr-Mo alloy. As can be seen in the figure, synthetic gear oil (MOBIL-GEAR SHC 150) with a kinematic viscosity at 40 \({}^{\circ}\)C of 150 cSt provides the best protection against wear, followed by polyphenyl ester oil (4P-3E) with a kinematic viscosity at 40 \({}^{\circ}\)C of 69.45 cSt, paraffin mineral oil with a kinematic viscosity at 40 \({}^{\circ}\)C of 18.73 cSt, and castor oil with a kinematic viscosity at 40 \({}^{\circ}\)C of 13.52 cSt. On the other hand, using deionized water as a lubricant for the Co-Cr-Mo alloy leads to the highest wear rate. Figure 5: Effects of (**a**) applied normal load (at a 5 Hz frequency), and (**b**) frequency (at a 10 N constant load), on the wear rate. All lubricated tests were conducted using synthetic gear oil.