Microhardness and dislocation identification studies on paracetamol singlecrystals

Purpose. To study the mechanical behaviour of paracetamol single crystals. Methods. Microhardness indentation techniques were used to study the hardne ss anisotropy of paracetamol. Solvent etching technique was used to define the range of plastic deformation and the orientation of the dislocation lin es. The orientation dependence of Knoop hardness on the {001}, {110} and {2 0 (1) over bar} surfaces was compared with calculated values of the Effecti ve Resolved Shear Stress (ERSS) for plastic deformation by specific disloca tion types. Results. The principal habit faces of single crystals using both Vickers an d Knoop indenters showed a range of hardness from 235 to 456 MPa depending on the type of indenter used and its orientation on the surface. Solvent et ching of the plastically deformed region of the crystal around the Vickers/ Knoop indentations confirmed that the slip plane was (010). ERSS analysis s uggested that the deformation occurred by the slip of dislocations of the t ypes (010)[001] and (010)[100]. Crystals doped with 0.08-0.8 w/w% p-acetoxy acetanilide showed hardness values similar to the pure material. Conclusions. The low number of distinct dislocation slip systems (two) is c haracteristic of a brittle material and is consistent with the observation that paracetamol will tolerate only deformations of 1 part in 10(6) before fracture.