Anisotropy mechanism of material removal and damage formation in single crystal 4H-SiC scratching

Abstract

Single crystal 4H-SiC, the third-generation semiconductor material, exhibits excellent properties in short wavelength optical components. However, the high brittleness and hardness of material itself present significant challenges for machining It is crucial to investigate the anisotropy dependence of damage formation and material removal mechanism. The scratching experiments of varying crystallographic orientations online monitored using the acoustic emission (AE) signals and force signals. The surface and subsurface morphologies of the scratch grooves on the C plane, M plane and A plane with different crystallographic orientations were investigated. The anisotropic characteristics of material removal were preliminarily revealed by examining the scratch critical depth. Fast Fourier Transform (FFT) of AE signals was utilized to extract peak frequencies associated with material removal behavior. The peak value of signal frequency at 78.71 kHz and 82.82 kHz indicated the pronounced plastic removal, while that at 82.82 kHz and 91.88 kHz exhibited the positive correlation with the salience of brittle fracture removal. The variations in scratch orientation on the same plane were reflected in the peak amplitude of the medium frequency bands. Surface morphology revealed significant differences, which were affected by the material atomic arrangement and crystal orientation characteristics, and thus affected the stress distribution and crack propagation behavior. Slip deformation was the primary mechanism for plastic flow in materials, which delayed material fracture. As the stress concentration was reached, shear stress facilitated the cleavage of Si-C bonds, which propagated along slip/twinning planes. Furthermore, the anisotropic crystal structure of the single crystal 4H-SiC resulted in significant differences in mechanical responses. The atom arrangement and bonding characteristics endowed the material deformation capabilities. Along specific crystallographic orientations on the C plane, the robust atomic bonds impeded the material ability to relieve stress through dislocation slip, which resulted in a pronounced brittle fracture tendency. The changes in friction coefficients were related to crystallographic orientation, it fluctuated most significantly on the C plane, while remained relatively stable on the M plane. According to the stress field model, compressive stress induced the nucleation of lateral cracks aligned with the direction of the compressive forces, while tensile stress facilitated the nucleation and propagation of median cracks. The subsurface crack propagation behavior was influenced by the anisotropy in the crystal structure, local stress field distribution and interatomic bond strength. The propagation direction of subsurface cracks exhibited anisotropy depending on the scratch crystallographic plane. Specifically, lateral cracks propagated horizontally and deflected by approximately 30°, while median cracks propagated vertically and deflected by approximately 30° and 60° on both sides.