Investigation on surface formation mechanism in elliptical ultrasonic assisted grinding (EUAG) of monocrystal sapphire based on fractal analysis method

The elliptical ultrasonic assisted grinding (EUAG) method has been proved to be promising in machining of hard and brittle materials, e.g., sapphire and silicon. However, its potential has not been sufficiently developed because the processing mechanism has not been clearly explained. This paper focuses on the surface formation mechanism in EUAG of monocrystal sapphire by investigating the fractal behavior of ground surface. The EUAG experiments on monocrystal sapphire were performed on the constructed apparatus by installing an elliptical ultrasonic vibrator on a CNC surface grinder. For comparison, the conventional grinding (CG) experiments were also carried out on the same experimental apparatus without ultrasonic vibration. The ground surfaces obtained were analyzed by fractal method, and results show that: (1) the great improvement of the surface quality with lower surface roughness is achieved in EUAG compared with those in CG. Moreover, the fractal dimension of ground surface in EUAG is larger than that in CG. It is also found that the fractal dimension is more sensitive to surface defect compared with the surface roughness. (2) The fractal dimension changes differently comparing CG with EUAG as the depth of cut increases from 0.4 to 3.2 μm. From the variations of the fractal dimension with different depth of cut, it is found that there are two variation regions, i.e., steady region and deteriorated region both in EUAG and CG, but the depth of cut in steady-deteriorated transition point of EUAG is larger than that in CG, meaning that the EUAG can achieve a deeper ductile depth of cut. (3) In EUAG, elliptic motion trajectories of adjacent abrasive grains interact with each other, causing the lower side walls of grooves and more exquisite microstructures. Thus the higher surface fractal dimension in EUAG is obtained and a better surface quality is achieved. Therefore, the surface formation characteristics in EUAG of monocrystal sapphire can be investigated by fractal analysis.