Variable-depth single-grit scratch experiments have been conducted on three different grain size alumina ceramics. The extent of induced damage as a function of depth of groove was measured. At low depth, the scratch groove appeared smooth with minimal brittle damage, indicating a ductile mode of deformation. With increased depth, brittle cracking extended beyond the scratch groove. The transition depth from the predominantly ductile mode of deformation to the predominantly brittle mode was measured and compared with an analytical model that estimates the plastic zone size surrounding a scratch in brittle materials. It was found that the ductile to brittle transition depth increases with decreasing grain size.