# Experimental Study on ELID Grinding of Silicon Nitride Ceramics for G5 Class Bearing Balls
## 1 Introduction
To increase the strength reliability of silicon nitride ceramics for bearing balls, three types of hot-press sintering processes were proposed. Additionally, the correlation between the microstructure and strength reliability was studied to produce high-density and high-strength silicon nitride [14]. Ceramic removal machining techniques are categorized into three main methods: fixed abrasive processing utilizing grinding wheels, flow abrasive processing using abrasive slurry, and high-energy special processing employing laser or discharge phenomena. Fixed abrasive processing is a machining method that can achieve a high material removal rate and excellent shape accuracy. However, it has the disadvantage of frequently causing brittle fracture due to the aggravation of surface damage during the process. Flow abrasive processing is another machining method primarily used for ceramics, as it causes minimal surface damage [15; 16; 17; 18]. Nevertheless, it suffers from a low material removal rate, difficulty in controlling shape accuracy, and extended processing time. On the other hand, high-energy special processing is not suitable for ceramic parts requiring high shape precision and surface quality, such as ceramic bearing balls. This is because the severe degree of thermal surface damage and the complexity of controlling shape accuracy pose challenges. Ceramic bearing balls are classified into 11 grades, ranging from G3 to G200, depending on their shape and surface roughness levels [19]. G5 class ceramic bearing balls demand high shape accuracy and surface roughness, with a ball diameter variation and spherical deviation of 0.13 um or less, and a surface roughness of 0.014 Ra or less. To efficiently process brittle materials with high shape accuracy and surface quality, it is crucial to adopt processing technology that reduces processing time while achieving an excellent surface without surface damage [20]. Electrolytic In-process Dressing (ELID) grinding [15; 21; 22; 23; 24] is well suited for high-precision processing of brittle materials such as ceramic ball bearings. It combines the advantages of fixed abrasive processing with a high material removal rate and excellent shape accuracy, and flow abrasive processing, which offers superior surface roughness and low surface damage [25; 26; 27]. The ELID grinding method involves the generation of pulsed power in the electrolytic power supply, accompanied by a non-linear electrolytic phenomenon, to achieve continuous dressing during the grinding process. This balances the electrolytic characteristics of the metal bond grinding wheel and the passivation of the oxidized layer. Due to the electrolytic phenomenon, the binder in the wheel oxidizes, allowing the abrasives to protrude. In this electrolytic process, after the dissolution of a few micrometers, an insulating layer rapidly forms on the surface of the grinding wheel due to the generation of ferrous hydroxide and ferric oxide passivating agents, effectively preventing excessive dissolution. As grinding commences, the workpiece comes into contact with the passivated layer, causing it to be gradually removed as the grinding wheel wears. Consequently, the insulating properties of the passivated layer diminish, leading to the redissolution of the binder and the repeated exposure of abrasive particles, resulting in continuous dressing. This cyclic process is referred to as the ELID cycle. The continuous dressing effect during grinding has a significant impact on the machining of brittle materials such as ceramics because it helps maintain a consistent grinding pressure, significantly reducing the maximum grinding force [21; 22; 24]. This study conducted basic grinding experiments using ELID grinding technology to achieve the required surface roughness for silicon nitride ceramic bearing balls. The impact of material properties and grinding parameters on the surface quality of ground silicon nitride ceramics was investigated and compared through the assessment of surface defects and surface roughness.