Investigation into the anisotropy of cross-grinding surface quality in C- and M-planes of sapphire

Abstract

Sapphire is an important material for optical applications, but the anisotropy of key properties makes it difficult to obtain uniform ground surface. In this work, cross-grinding experiments were first conducted on C- and M-planes of sapphire to investigate the dependence of the ground surface quality on crystal orientation. Then, scratching tests were performed along the specific crystal orientations to explain the formation mechanism of the anisotropic grinding patterns. The results showed that the surface roughness distribution on C-plane is of a threefold symmetry, while one axis of symmetry for M-plane. The damage pits on C-plane possess lower aspect ratio than that on M-plane. By combining the scratching-induced fracture features with the contact-induced deformation theory of sapphire, it was found that the propagation of the primary cracks in sapphire is accompanied by activating different deformation systems. The material removal on C-plane is dominated by the shallow lateral cracks, which propagate parallel to the ground surface and result in shallow and wide pits. However, the deep lateral cracks play an important role on M-plane, which intersect with the ground surface and cause deep and dense defects. The non-uniformity of the ground surface is attributed to the appearance of radial cracks for C-plane, while it is associated with the number of the available deformation systems for M-plane.

Feasibility Analysis of Sapphire Compound Refractive Lenses for Advanced X-Ray Light Sources

“…After fabrication, the surface roughness of sapphire can reach several tens of nanometers. The surface roughness of the sapphire-based Fresnel zone plate prepared by Li et al achieved 12 nm [24]; Wang et al found that the surface roughness can reach about 15 nm in the anisotropy study of sapphire [25].…”

Electrostatic Self-Assembled Composite Abrasives for Chemical Mechanical Polishing of A-Plane Sapphire

“…M-plane sapphire can be used for epitaxial growth of semi polar or nonpolar GaN films, and the window/dome materials of supersonic aircraft. [4][5][6][7][8][9][10] For the above applications, sapphire substrates must have atomic smooth and planar surface without scratches and sub-surface damage. However, due to its chemical inertness, extreme hardness (Mohs hardness of 9) and brittleness, the machining of sapphire is extremely difficult which increases its processing cost and limits its applications.…”