Calculated X-ray diffraction pattern data for diamond polytypes are presented, which provides the required parameters for the characterization of the proposed diamond polytypes. The literature on diamond polytypes contains some small but significant errors with respect to the details of the crystal structure, including space groups, the atom positions in the unit cell, and the atomic layer stacking. In this paper, the crystal structures of four different hexagonal diamond and two different rhombohedral diamond polytypes are theoretically calculated to provide correct crystallographic information with which the diamond polytypes in a given sample can be characterized. [

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The mechanical properties of composite ceramics composed of 0 to 20 vol% of titanium diboride particles dispersed in an a-alumina matrix were investigated. The alumina-titanium diboride composite powder was hot-pressed at 1470°C for 20 min to achieve over 98.8% of the theoretical composite density. The strength and fracture toughness of the twophase, hot-pressed composite were both significantly improved compared to the single-phase alumina. Results from different methods of measuring the stress intensity factor, (KIc) are compared and discussed. [

The mechanical properties of alumina have been successfully improved by adding isolated boron carbide particles of two different shapes. A K I , of 7.26 2 0.20 MPa-m"' for alumina-boron carbide whiskerlike composites and of 5.27 2 0.12 MPa. m"' alumina-boron carbide shardlike particle composites has been achieved. The fracture toughness of these composites is dependent on the volume fraction of the boron carbide particles as well as their size and shape. The flexural strength is also appreciably enhanced to a constant value with from 5 to 20 vol% boron carbide additions. The whiskerlike particles increase the flexural strength by 25% and the shardlike particles produce a 47% improvement. [