# Effect to the Surface Composition in Ultrasonic Vibration-Assisted Grinding of BK7 Optical Glass
## 1 Introduction  
Optical glass is a typical hard and brittle material. Because of its unique excellent properties, it has a wide range of applications in the fields of optics, inertial confinement nuclear fusion, aerospace, and defense [1; 2]. One of the most prominent features of optical glass materials is high brittleness and low fracture toughness. The critical cutting depth of the material is extremely small. When the instantaneous cutting depth of the abrasive grains did not reach the critical cutting depth, the material is removed by plastic deformation [3]. When the instantaneous cutting depth of the abrasive grains reached beyond the critical cutting depth, micro cracks are generated inside the material, as the micro cracks propagates onto the machined surfaces, the material was removed by brittle fracture [4] and powdering [5; 6]. Ultrasonic vibration-assisted grinding has been used for precision machining of brittle materials because of the fact that the average cutting force and cracks propagation can be significantly reduced [7; 8; 9]. The effect of the process parameters on the process performances has been investigated experimentally [10; 11; 12]. After introducing ultrasonic vibration into the machining process, impacteffect was induced while the tool cutting into the optical glass material [13]. The effect of process parameters on surface quality, such as surface roughness and machined surface composition may be different than that of conventional cutting [14]. To the best of our knowledge, there are few reports on the machined surface composition and materials removal modes. In the past few years, domestic and foreign researchers have carried out a series of research on the removal of optical glass materials. [PERSON] conducted a scratching test by varying the cutting depth, based on this, he divided the material removal process into three parts: pure plastic flow, plastic and fracture mixture, and brittle fracture. Clear cutting chips and material deformation characteristics were found in his experiments. In addition, he also explained the cracks propagation directions, it was claimed that the initial radial cracks tend to propagate along twinning planes while the lateral cracks propagate along the basal plane [15]. [PERSON] investigated the material removal and deformation characteristics by observing the scratched surface morphologies, he pointed that the scratch groove depth is deeper in ultrasonic vibration-assisted scratching [16]. [PERSON] researched the relationship between stress characteristics and cracks propagation directions based on a varied cutting-depth nanoscratching test [17], he also discussed the transition of material removal mode based on the observation of scratched grooves. In another research of [PERSON], the differences of material removal in conventional grinding and ultrasonic vibration assisted grinding (UVAG) were investigated by observing machined surface morphologies by using atomic force microscope (AFM) and scanning electron microscope (SEM) [18]. He also compared the scratched grooves width and cracks propagation of varied scratching depth by conducting actual experiments and simulating scratching process [19]. [PERSON] focused on the influence of elastic recovery on scratched grooves morphologies. He pointed that elastic recovery may be a significant factor that affects machined surface quality in the actual grinding process [20]. However, many of the present researches related to material removal are based on the observation and analysis after scratching and indentation tests, which means the factors such as the coupling effects of cracks in actual grinding process, and the variation of grinding parameters, could not be easily taken into consideration. In the ultrasonic vibration grinding process of optical glass, because of the huge number of abrasive grains on the surface of the grinding wheel, and the randomness of the size and shape, the protruding height of each abrasive grain is different, resulting in different instantaneous cutting depths during processing.