Combustion co-synthesis of Si 3 N 4 -based in situ composites
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Combustion co-synthesis of Si3 N4 -based in situ composites J. T. Li, W. S. Liu, Y. L. Xia, and C. C. Ge Laboratory of Special Ceramics & PyM, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China (Received 19 January 1996; accepted 15 July 1996)
The feasibility of synthesizing silicon nitride-silicon carbide-titanium carbonitride composites by combustion reactions is demonstrated. With titanium carbonitride taken to be an ideal solid solution, its composition is determined as TiC0.36 N0.64 . Thermodynamic analysis supports the experimental results.
The need for further improvement in the mechanical reliability of Si3 N4 ceramics has recently led to the development of particulate reinforced ceramics.1 Among these additives TiN and TiC are preferred because of their high electrical conductivity, and therefore can be subjected to electrodischarge machining (EDM) for complex shapes.2,3 However, the reaction between TiC and Si3 N4 at high temperature can lead to substantial changes in the phase composition of the materials,4,5 which was the reason why in the 1990s the use of TiN and Ti(C, N) for substitution of TiC was proposed.6 In the present paper, we are going to study the possibility of in situ synthesis of Si3 N4yTi(C, N)ySiC composite powders via combustion synthesis (or Self-propagating High-temperature Synthesis, SHS). Silicon, silicon nitride, and titanium carbide powders with particle sizes of 23 mm, 1 mm, and 5 mm, respectively, were mixed in methanol medium and ball milled for 10 h; the mixed powders were dried in air followed by cold-pressing to compacts with dimensions of 20 mm in diameter and 15 mm in height. The relative density of the compacts is ,50% of theoretical value. The powder compacts with the composition of SiySi3N4yTiC 3 : 1 : 1, 3 : 1 : 2.6, and 5 : 1 : 5 are named hereafter as ST31, ST32, and ST5, respectively. The compacts were put into a SHS reaction chamber which was filled to different nitrogen pressures. The compacts were ignited by passing 20 A electrical current through the tungsten coil, which was placed about 1 mm above the compact. The combustion temperature was recorded by a W-Re3yW-Re25 thermocouple connected with a computer. The SiySi3 N4 powder compacts with different amounts of TiC as additive can be ignited at a nitrogen pressure higher than 2 MPa and synthesized in SHS mode to Si3 N4 -based composites. The XRD patterns of the above SHS products are shown in Fig. 1, from which it is found that all the products consist of a –Si3 N4 , b –Si3 N4 , Ti(C, N), and b –SiC; besides, there also exists some amount of Ti5 Si4 phase in the SHS product from specimen ST31. The diffraction peaks of 2968
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J. Mater. Res., Vol. 11, No. 12, Dec 1996
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residual Si remain in the SHS products of specimen ST5, indicating that Si cannot be nitrided completely in this condition. It has been reported by some authors that pure silicon powder compacts cannot or har
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