Sintering behavior, microstructural development and mechanical properties of Si3N4 based nanocomposites by spark plasma sintering (SPS)

• Main Authors: Ching-HuanLee, 李京桓
• Other Authors: Jow-Lay Huang
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/47005373995092842699

博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 100 === Commercial nanosized β-Si3N4 based materials incorporated with conductive TiC or TiN nanopowders were sintered by spark plasma sintering (SPS). The microstructures of monolithic Si3N4, Si3N4/TiC and Si3N4/TiN based nanocomposites were controlled depending on the sintering parameters in SPS. The micro/nano-indentation characteristics and sliding wear performance for chosen materials were evaluated as well. At a slower heating rate (≦100℃/min), the nanosized grains are maintained after sintering at 1746℃; while anisotropic grain growth is accelerated above 1681℃ by applying a rapid heating cycle (200℃/min). The difference in heating rates lowers the actual sintering temperature by 60℃ at least. In addition to the dynamic Ostwald ripening that occur during the sintering process, the presence of Morié fringes and dislocations have confirmed that the grain coalescence is one of the possible mechanisms of grain coarsening. On the other hand, the β-Si3N4-based composite containing 5 wt% nano-TiC shows a larger average grain size and aspect ratio compared to monolithic β-Si3N4-based ceramic. This is possibly because of a leakage current hop across the conductive TiC based grains and causes joule heating during sintering. This is contrary to the conventional sintering behavior of Si3N4. By incorporating the nanosized TiC and TiN, the pinning effect of the titanium-based phase significantly suppresses the grain growth of β-Si3N4 matrix grains, and a series of β-Si3N4-based nanocomposites are thus fabricated successfully in the present study. The effects of microstructure on the mechanical responses and damage evolution of spark-plasma-sintered β-Si3N4 based ceramics has been evaluated through micro/nano-indentation tests. It was found that the nanoceramic and its coarse-grained counterpart exhibit similar elastoplastic behavior in their indentation responses. However, the increased hardness and ratio of elastic work to total work done in the nanoceramic suggest that resistance to plastic deformation is greater than that in the coarser-grained one. The smaller grain size in bridging ceramic not only enhances the energy dissipation by formation of a higher density of intergranular microcracks along the weak grain boundary phase (GBP), but also toughens the cracked solid through increasing resistance to frictional sliding and multi-cracks propagation. Prior to formation of micro-crack coalescence, better damage tolerance of nanoceramic is thought to be achieved. A model associated with microcracking and interfacial chemistry of the nanocomposites is proposed, based on the hypothesis of materials’ properties at their nanoscale. Spark plasma sintering of TiC/Si3N4 based nanocomposites with self-lubricating carbon were developed. Wear rate of TiC/Si3N4 based nanocomposites showed a V-shaped curve as a function of nano-TiC addition; whereas the one of their coarse-grained counterparts almost remain constant. The initial improvement in wear resistance is due to the increase of nano-TiC (or C) grain pullout when the nano-TiC addition is up to 20~30wt%, leading to enhancement of tribochemical type wear. Furthermore, because the weakest intrinsic flaws or microcracks were developed with increasing area fraction around TiC based grain boundary, the applied energy would be dissipated and then the wear resistance of materials was enhanced during wear process.