Session: 03-04-02: Advanced Machining and Finishing Processes

Paper Number: 144980

144980 - Ultrasonic Vibration-Assisted Scratching Process of Sapphire: The Effects of Feeding Speed 

Sapphire, a single-crystal form of α-alumina, is one of the hardest materials with a Mohs hardness of 9. It has several distinctive features, including high hardness, excellent thermal stability, chemical resistance, good mechanical strength, high dielectric strength, effective light transmission, and relatively good lattice spacing. All these unique characteristics make sapphire a good substrate material to be used in the semiconductor industry and other areas. The high hardness and low fracture toughness of sapphire makes it difficult to machine. Microcracking and chipping are also very common during traditional fine surface machining of sapphire. Ultrasonic vibration-assisted machining has been explored in many experimental and theoretical investigations and found to be useful in surface machining of sapphire. 

Scratching test using a single diamond abrasive grain is a great way to understand the material removal mechanism and deformation features of brittle materials. This test result can also be used to simulate and predict the cutting force and material removal mechanism of other multi-abrasive machining processes like grinding and lapping. Studies have shown that vertical ultrasonic vibration-assisted scratching can improve the cutting forces, scratch quality, and material removal characteristics of sapphire. However, the effect of different scratching speeds has not been studied yet. To fill this knowledge, in this experimental investigation, we are considering both the effects of scratching speed and ultrasonic vibration on scratching force, scratch quality, and material removal mechanism of sapphire. We have used four different scratching speeds ranging from very slow (0.1 mm/sec) to very fast (100 mm/sec). We have studied the cutting force data and observed the scratch-induced microscopic features and thus got some insight into the deformation patterns arising from the surface morphologies of sapphire. 

In this study, sapphire wafers were scratched by a single diamond tool at four different speeds, such as 0.1 mm/sec, 1 mm/sec, 10 mm/sec, and 100 mm/sec under conditions with and without ultrasonic vibrations. A linear ramping indentation depth from 0 to 20 µm along the scratch length of 25 mm was obtained in both cases. The tests were conducted on a rotary ultrasonic machine where the horizontal feeding was controlled by a motion controller. The cutting force was measured using a dynamometer and the scratch-induced microfeatures at different indentation depths were studied with an optical microscope.  

Scratching has two major forces: one along the vertical cutting direction and the other along the horizontal feeding direction. The preliminary test results indicate that: in both conventional scratching (CS) and ultrasonic vibration-assisted scratching (UVS), the increase in scratching depth results in the larger average values of scratching forces in the vertical direction. On the other hand, the increase in scratching speed results in a smaller average value of scratching forces in the vertical direction. However, both scratching speed and indentation depth had a very negligible effect on the horizontal scratching force in both CS and UVS processes. Under each level of scratching speed, the vertical force in the UVS process was much higher than that in the CS process due to the deeper indentation caused by the ultrasonic amplitude. The microscopic features indicate that the edge chipping at a higher scratching speed is considerably lower than that at a lower scratching speed for both CS and UVS processes. Fewer microcracks were also observed at higher scratching speeds.   

The scratching speed can be considered equivalent to the tangential speed of the multi-abrasive cutting tool in different surface machining processes. So, the result from this study can be used to predict the spindle speed and depths of the groove during fine machining operations of sapphire. This experimental investigation validates that higher scratching speed with ultrasonic vibration can lead to a better surface finish with less amount of microcracks. 

Presenting Author: Shah Rumman Ansary Texas Tech University

Presenting Author Biography: I, Shah Rumman Ansary, am a Ph.D. student in the Department of Industrial, Manufacturing, and Systems Engineering at Texas Tech University. Here I work as a research assistant in the Advanced Manufacturing lab, supervised by Dr. Weilong Cong. My research area is microfeature fabrication of brittle materials and I mostly work with rotary ultrasonic systems. I got my MS in Mechanical Engineering from Texas Tech University in 2022 and BS in Mechanical Engineering from Bangladesh University of Engineering & Technology in 2015.

Authors:

Shah Rumman Ansary Texas Tech University
Sarower Kabir Texas Tech University
Weilong Cong Texas Tech University