THE GROWTH AND CHARACTERISTICS OF CVD SIC-TIC IN-SITU COMPOSITES

SiC-TiC in-situ composites have been synthesized by low pressure chemi cal vapour deposition on graphite with SiCl4, TiCl4 C3H8 and H-2 react ion gases to improve the toughness of SiC-TiC ceramics. The composite was deposited at various temperatures (1500 similar to 1600 degrees C) , total pressure (40 similar to 300 torr) and reactant concentrations. The microstructure was investigated by scanning electron microscopy, optical microscopy and transmission electron microscopy. The morpholog ies of facet structure, nodular structure and dendrite structure appea r depending on the experimental conditions. A dense SiC-TiC deposit wi thout porosity was obtained with a maximum growth rate of 1.6 mm h(-1) at C3H8 = 25 cm(3) min(-1) and 200 torr. Only beta-SiC and TiC phases have been identified in the composite with a dramatic change of the c omposition at the interface of SiC and TiC grains. The fracture toughn ess (K-Ic) determined by the indentation method exhibits a value as hi gh as 5.9 MPa m(1/2) that could be obtained by deposition at low press ure and low C3H8 concentration. Cracks propagate with more deflection in the SiC-TiC composites than in monolithic SiC. The result is due to existing severe strain at the SiC-TiC interface observed by transmiss ion electron microscopy. (C) 1998 Elsevier Science Limited and Techna S.r.l. All rights reserved.