## 2 Materials and Methods  
The specimens were produced using an industrial 3D printer manufactured by Renishaw (Wotton-under-Edge, UK). The printer model, Ren AM 250 SLM, was used with an upgraded laser system that can deliver a maximum laser power of 400 W. The SLM machine has a build volume of 250 mm \(\times\) 250 mm \(\times\) 300 mm (Z-axis), and the default laser spot size diameter is \(\theta\) = 140 \(\upmu\)m. To maintain optimal printing conditions, the oxygen concentration inside the building chamber was controlled to be below 50 ppm. According to the manufacturer's recommendations, the ideal powder for the printer should be spherical in shape and have a particle size range of 15 \(\upmu\)m to 45 \(\upmu\)m.  
The Co-Cr-Mo specimens with the dimensions of 40 mm \(\times\) 20 mm \(\times\) 10 mm were fabricated using a metal powder that complies with the Co-Cr-Mo ASTM F75 standard [20], which was provided by Renishaw. The powder had a composition in wt% of 64.15 Co, 28.81 Cr, 5.21 Mo, and small amounts of other elements including Ni, Fe, C, Si, Mg, W, P, S, Al, Ti, and Co (balance). Molybdenum enhances strength and heat resistance, while chromium provides resistance to oxidation and corrosion. Table 1 presents the SLM process parameters used to produce the Co-Cr-Mo samples. The values presented in the table represent the optimal process parameters as recommended by the SLM machine manufacturer for the Co-Cr-Mo powder. These parameters are designed to achieve the best balance of mechanical strength, hardness, and density, while also minimizing the occurrence of the lack of fusion during the printing process. The temperature during the SLM printing was set at 1350\({}^{\circ}\) to ensure the melting and proper fusion of the Co-Cr-Mo alloy powder layers during the SLM process. For polishing, manual rotary grinding-polishing was initially used with silicon carbide papers of 180, 500, 1200, and 2400 grit. Subsequently, the Struers TegraPol-25 automatic polishing machine with TegraForce-5 for rotation and TegraDoser-5 for liquid dispensing refined the process. Successive 3 \(\upmu\)m grit diamond polishing (\"DiaPro\") and 1 \(\upmu\)m grit colloidal silica polishing (\"OP-Suspension\") steps were used for a mirror-like finish. Samples were etched with a 10% H\({}_{2}\)SO\({}_{4}\) and CH\({}_{3}\)OH solution for 25 s at room temperature. Polished samples were subjected to hardness tests using a Leco LM-80 AT microhardness \begin{table}
\begin{tabular}{c c} \hline
**Parameters** & **Value** \\ \hline Laser powers & 320 W \\ \hline Layer thickness & 0.04 mm \\ \hline Scanning strategy & Parallel scanning \\  & 67\({}^{\circ}\) rotation after each layer \\ \hline Scan speeds & 700 mm/s \\ \hline Hatch distance & 0.09 mm \\ \hline \end{tabular}
\end{table} 
Table 1: Optimum process parameters for powder bed fusion selective laser melting of cobalt-chromium-molybdenum alloy. tester with a load of 1 kgf. The density of the samples was determined using the Archimedes' principle method. This involved measuring both the mass and volume of the samples and then calculating the density by dividing the mass by the volume. Sliding wear experiments were conducted using a Ducom Instruments TR-282 linear reciprocating tribometer. A range of normal loads (5 N, 10 N, 15 N), and sliding frequencies (2 Hz, 5 Hz, 8 Hz) were used for the sliding wear tests. The tests were conducted in both dry and lubricated conditions, with a 10 mm diameter hardened steel ball, with a roughness of Ra = 1.2 \(\upmu\)m as the counter material. Different types of lubricants (gear oil, paraffin oil, castor oil, ester oil, and deionized water) were used for the lubricated sliding wear tests. These lubricants were selected to cover a broad range of kinematic viscosities. By including lubricants with varying viscosities, we aimed to investigate how different lubrication conditions influence the tribological behavior of the system. The tribometer was set for a wear track (sliding stroke) of 10 mm, and a total sliding distance of 1000 m was used for all tests.