# The Effect of Particle Type and Size on CoCr Surface Properties by Fine-Particle Shot Peaning
## 4 Discussion  
The main objective of this study is to examine the effect of particle types and sizes on the surface properties of the CoCr, including surface morphology, surface roughness, surface hardness, and any residual elements, for the purposes of the improving medical devices that use CoCr materials such as dental or joint replacement implants. The results are as follows:  
For the surface morphology, the results showed that the fine-particle shot peening process can change the surface morphology by creating multiple dimples on the surface material. The ceramic particle, with its higher hardness, can create clearer surface dimples than the silica particle, and for the effect of particle type, the results showed that the larger particle can create larger and clearer dimples on the CoCr surface when using both ceramic and silica particles. The results were in line with the previous study by [PERSON] et al., which reported that a higher-hardness particle can change surface morphology more clearly and deeply than a lower-hardness particle [14]. For the surface roughness, the ceramic particle can increase the surface roughness more than the silica particle when comparing the same-sized particles. The surface dimples can enhance the lubrication properties by increasing the formation of a fluid film that decreases the friction force between the motion surface. Recent literature that has studied lubricant film formation by using artificial hip joints with micro-dimples on the surface showed that the prostheses with micro-dimples significantly increased the lubricant film thickness. Using the tribology test results, [PERSON] et al. reported that the micro-dimpled, ceramic-on-ceramic interface of hip joint arthroplasty can reduce the coefficient of friction when compared with a non-dimpled surface [27]. From the results of recent literature, it can be determined that a surface with appropriate micro-dimpling can increase  
Figure 9: Residual elements of sample that was peened with silica particles (SiO\({}_{2}\)) showing the elements of silica and oxygen on the surface. the lubrication properties and decrease the friction coefficient, ultimately resulting in a decrease to the wear-and-tear process of the motion surface [28; 29]. The surface hardness is the most important factor affecting a surface material's strength and wear process. The hardness can be defined as a material's resistance to permanent deformation, and hardness can be used to determine resistance to the general wear-and-tear process, in particular scratch resistance. High-hardness materials can resist scratching that makes the surface irregular and increases the sharpness of the surface, which in turn increases the abrasive wear at the motion surface. As a result, high-hardness materials may be the best option for medical implants such as joint replacements, dental implants, or other orthopedics due to their wear- and scratch-resistance properties. In recent literature, it has been reported that the fine-particle shot peening process can increase the surface hardness of the material [30]. In this study, the results showed that fine-particle shot peening can significantly increase the surface hardness of CoCr material using both silica and ceramic particles. Due to its higher hardness, the ceramic particle can increase the surface hardness more than the silica particle, and in different particle sizes, the larger particle size can create higher surface hardness than the smaller particle size. In summary, the particle that can increase greater surface hardness is a ceramic particle of a large particle size (250-425 \(\upmu\)m). One of the most important concerns with regard to the fine-particle shot peening process on a surface material is the residual element of any particle that was used for surface modification. Residual elements left on a surface material that will be used for a medical implant may affect the cells of the human body.