# The Effect of Particle Type and Size on CoCr Surface Properties by Fine-Particle Shot Peaning
## 1 Introduction  
The need for research on cobalt-chromium (CoCr) alloys stems from their widespread use in the manufacture of medical implants, particularly for dental and orthopedic applications [1]. While titanium-based alloys such as titanium-niobium are commonly used due to their biocompatibility and mechanical properties, they may not be suitable for all applications. For example, CoCr alloys have been found to have better wear resistance and higher strength compared to titanium-based alloys [2]. Additionally, CoCr alloys have been shown to have favorable tribological properties for use in joint replacements [3]. Therefore, research on CoCr alloys is important for optimizing their properties and understanding their behavior in various medical applications, particularly in cases where the superior mechanical properties of CoCr alloys may be required. Cobalt-chromium (CoCr) alloys are widely used because of their excellent mechanical properties, high corrosion resistance, and high specific strength [4; 5; 6; 7]. The main problems with medical devices that use the metal alloy are the wear of the material, the material debris that can affect human tissue, and the resulting decrease in the longevity of the implant [8]. Therefore, increased strength and scratch resistance can improve both the longevity of the medical implant and patient outcomes. Shot peening has been widely used as a low-cost and simple method for increasing the life and wear resistance of metals [9]. Compressive stress and microstructure changes improve the surface-strengthening layer of the metal [10; 11]. The small spherical shots for the process are made of hardened cast steel, glass, or ceramic beads. After impacting the component's surface with shots at a relatively high impingement velocity, an inhomogeneous plastic deformation zone appears on the surface layer [12]. The study of shot peening strengthening usually focuses on mechanical properties and changes in the deformation layer rather than particle size and type. The peening parameter can affect material properties including particle size, particle type, distance between nozzle and material surface, peening time, or peening pressure [13; 14]. The hardness of the material determines what type of particle can be used for the peening process. The hardness of the particle should be higher than the surface material in order to change the surface microstructure and improve its surface properties. The hardness of the CoCr alloy is quite high; therefore, the particle to be used in the peening process for CoCr alloy must have a hardness high enough to modify the surface. Research on shot peening of CoCr alloys has been carried out in several studies. For example, a study by [PERSON] et al. (2017) investigated the effect of shot-blasting parameters, such as blast pressure and distance, on the surface roughness and microstructure of the CoCr alloy [15]. The study showed that increasing the blast pressure and decreasing the blast distance resulted in a higher surface roughness and more homogenous microstructure. However, another study by [PERSON] et al. (2018) found that shot blasting can cause surface damage and residual stresses in the CoCr alloy, which can reduce its fatigue strength [16]. This research highlights the need for careful optimization of shot-blasting parameters to avoid such problems. Therefore, this study of CoCr alloy shot peening is relevant, as it can provide insights into how to optimize this alloy's surface properties for various applications while simultaneously avoiding potential problems that can arise from the process. There is no recent literature that has reported the effect of particle type and size on CoCr alloy surface properties, including surface morphology, surface roughness, or surface hardness, by using the fine-particle shot peening technique. This study investigates the effect of particle type and size on the surface properties of the CoCr alloy using the fine-particle shot peening technique.