HARDNESS ANISOTROPY OF ACETAMINOPHEN CRYSTALS

The anisotropy of acetaminophen hardness was demonstrated using both V ickers and Knoop indentation hardness measurements. Based on a model o f Knoop hardness anisotropy proposed by Brookes et al. (1), it was con cluded that plastic flow in acetaminophen crystals occurs primarily as a result of slip in the (010)(001) system. This conclusion was corrob orated with the results of the Vickers indentation tests. The apparent brittleness of acetaminophen was rationalized because only one slip s ystem appeared to be operative. Under these conditions generalized pla stic flow cannot occur, since this requires the operation of at least five independent slip systems (2). The high stress concentrations that result from now lead to fracture. Therefore acetaminophen is more pre cisely classified as being semiductile. When a material deforms plasti cally as a result of slip in only one slip system, considerable crysta l realignment can occur during compaction. This in turn can facilitate capping during decompression and ejection, since the cleavage plane, (010), would become aligned with the direction of highest tensile stre ss.