Insight into sequential multi-scratch interference mechanism on monocrystalline sapphire

Monocrystalline sapphire serves as the primary substrate for LED fabrication, and mechanical abrasion remains the dominant finishing method for its surfaces. This study examines how sequential scratching—with one, two, and three passes—affects damage evolution and scratch interaction on the A-, C-, and M-planes of sapphire. Rockwell indentation tests were performed, supported by AFM and SEM imaging of scratch grooves, acoustic emission monitoring of the contact process, and analytical modelling of the elastic stress field. The activation of individual slip/twinning and cleavage systems were also computed using critical resolved shear stress and fracture energy criteria. The experimental data show that the transition from ductile flow to brittle fracture varies markedly with crystal orientation. When a second scratch follows at 10 μm spacing, its ductile-to-brittle transition load drops by 66.0% on the A-plane, 34.3% on the C-plane, and 29.8% on the M-plane compared with a single pass. With a third scratch, the intermediate ductile-to-brittle regime disappears altogether. According to the stress analysis, tensile stresses behind the indenter promote radial and lateral cracking, whereas tensile stresses ahead of the indenter facilitate median crack nucleation. Among the three planes, the C-plane activates the greatest number of slip systems (basal twinning >0.3) and exhibits the lowest cleavage probabilities (prismatic 0.37, rhombohedral 0.15), which explains its highest ductile-to-brittle threshold (7.89 N) and enhanced formability while the lowest threshold (6.05 N) of M-plane. The quantitative measures offer direct benchmarks for selecting machining loads, inter-scratch spacings, and tool path overlaps to achieve subsurface-damage-free sapphire processing across different orientations.