Incipient plasticity during nano-scratch in Ni₃Al

The subsequent sliding behaviour of the initially elastic, submicron contacts between a diamond tip and bulk polycrystalline Ni 3Al was investigated by nanoscratch tests. Scratches are made under constant normal loads otherwise too low to trigger crystal plasticity in uniaxial nanoindentation experiments. Three types of behaviour are observed: (i) the entire scratch is purely elastic with a sliding coefficient of friction (μ) of about 0.086; (ii) a sudden strain burst in the normal direction, commonly known as pop-in, occurs at the onset of sliding and the subsequent sliding is plastic with μ, typically a few times larger than 0.086; and (iii) an intermediate case in which the scratch is initially elastic followed by a pop-in beyond which the sliding becomes plastic. The intermediate case occurs rather randomly. The μ, for elastic sliding is found to be load- or size-independent, but that for plastic sliding exhibits a strong size effect, i.e. it is small at small loads, but increases towards a steady value at larger loads.