Microhardness and dislocation identification studies on paracetamol single crystals

Purpose. To study the mechanical behaviour of paracetamol single crystals. Methods. Microhardness indentation techniques were used to study the hardness anisotropy of paracetamol. Solvent etching technique was used to define the range of plastic deformation and the orientation of the dislocation lines. The orientation dependence of Knoop hardness on the {001}, {110} and {20 (1) over bar} surfaces was compared with calculated values of the Effective Resolved Shear Stress (ERSS) for plastic deformation by specific dislocation types. Results. The principal habit faces of single crystals using both Vickers and 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 etching of the plastically deformed region of the crystal around the Vickers/Knoop indentations confirmed that the slip plane was (010). ERSS analysis suggested that the deformation occurred by the slip of dislocations of the types (010)[001] and (010)[100]. Crystals doped with 0.08-0.8 w/w% p-acetoxyacetanilide showed hardness values similar to the pure material. Conclusions. The low number of distinct dislocation slip systems (two) is characteristic of a brittle material and is consistent with the observation that paracetamol will tolerate only deformations of 1 part in 10(6) before fracture.