Damage-free processes of grinding of brittle materials have been widely used in industry for producing electronic and optical components with high surface integrity. It has been found that the transition threshold from ductile flow to brittle fracture during the process of material removal plays a central role in the quality control of a machined surface. However, the precise microscopic mechanism which governs the formation of dislocation structure and micro-cracking when machining a brittle material such as alumina, remains unclear. The mechanism of formation and structure of the plastic or damage zones in alumina of two different grain sizes (1 and 25 mu m) subjected to single-point scratching with sharp and blunt indenters were studied in this paper. Using transmission electron microscopy, characteristic features of the plastic/damage zone in terms of loading conditions and microstructure of the materials were carefuIly investigated. It was found that the grain size and the geometry of the indenter had a great effect on the dislocation structure of the plastic zone and that the subsurface damage could be very severe, even though the machined surfaces appeared damage-free. These results indicate that the ductile flow to brittle fracture transition in machining brittle ceramics is more complicated than previously thought and that a reliable criterion has yet to be established to predict a real damage-free grinding process.