c-ZrO(2)/SiO(2) composites with various amounts of SiO(2) additions were produced by a colloidal processing and pressureless sintering at 1400 degrees C. The influence of the microstructure and SiO(2) content on the grain size, flexural strength and fracture toughness of the composites were determined. The results showed that SiO(2) had very limited solubility of 0.3 wt% in c-ZrO(2) matrix. This suggests that only small part of the SiO(2) dissolved in the c-ZrO(2) matrix and the rest of SiO(2) segregated at grain boundaries and multiple junctions as amorphous glassy phase. This glassy phase formed rounded grains and prevented the grain growth by minimizing grain boundary energy and mobility. c-ZrO(2)/SiO(2) Composites having different SiO(2) contents showed high flexural strength and fracture toughness. With the increase of SiO(2) content from 0 to 10 wt%, the grain size, flexural strength and fracture toughness changed from 5.7 to 0.5 mu m, 275 to 292 MPa and 1.5 to 3.64 MPa m(1/2) respectively. The increase in fracture toughness and flexural strength could be attributed to smaller grain size and crack deflection. (C) 2007 Elsevier Ltd and Techna Group S.r.l. All rights reserved.