奈米級氮化矽粉體有助於使燒結緻密的溫度降低，促使燒結體更容易達理論密度及奈米尺寸的微觀結構。其中奈米結構更可有效提升試片強度(Hall–Petch方程式)、耐磨耗性與超塑性。但也由於奈米粉體表面過多的氧殘留，而造成粉體在燒結過程中容易有第二相O’-sialon的出現，進而導致氮化矽機械性質降低。因此本研究以非晶質奈米氮化矽為起始粉末，分別利用HP和SPS兩種燒結方式，比較其燒結機制下對相組成、微觀結構與機械性質間的影響。接著利用碳熱還原反應除去奈米氮化矽表面過多的氧化物，以避免燒結體中O’-sialon相形成，並探討不同熱處理溫度下對粉體的反應機制及其燒結體的相組成、微結構、機械性質等影響。
研究結果顯示，以HP慢速升溫與SPS的快速升溫燒結製程中皆會有O’-sialon相出現，而在1800 oC的燒結溫度下，β相可佔試片最大量。微觀結構在SPS燒結中可促使試片不僅有長柱狀晶粒，基材上仍可維持次微米結構，並且有較大的視長寬軸比。因此在機械性質上可發現以SPS燒結法可有效促使氮化矽基材料提升韌性外，亦可藉由基材提供強硬度以維持良好的硬度性質。
當粉體添加5 wt%碳黑進行10小時的碳熱處理後發現，碳熱還原反應可在熱處理溫度1250 oC下有效進行，進而得到單一相氮化矽燒結體；但隨著熱處理溫度上升，氮化矽粉體中產生的液相開始阻礙碳熱還原反應。經分析發現在熱處理溫度1350 oC以上，粉體反應逐漸由碳熱還原轉變為氮化矽分解，造成在燒結體中發現大量的碳殘留與O’-sialon相。
比較碳熱還原反應機制與燒結體的機械性質發現，在熱處理溫度1250 oC下獲得單一相氮化矽，在強度上提升不大，主要是殘留碳的影響，但在韌性值方面，燒結溫度1800 oC韌性性質可提升32.3 %，而燒結溫度1600 oC韌性性質更可提升高達79.7 %，顯示獲得純β-Si3N4的優點。

Materials based on the nanocomposite concept described by Niihara, show excellent bending strength, which is accompanied by a modest increase of fracture toughness and improved creep behavior. However, fabrication of nanocomposite materials by a traditional processing route of mixing the submicrometre Si3N4 powders followed by hot pressing or pressureless sintering is sometimes problematic and often does not yield expected results. Therefore, we use amorphous and nano-Si3N4 as initial powders, and then perform carbothermal reduction and nitridation (CRN) to remove excess oxygen.
Experiment results indicate that the bulks of sintering powder by HP and SPS contain O’sialon phase before CRN and large amount of β phase, while sintered at 1800 oC. From microstructure observation, the higher aspect ratio and smaller matrix obtained by SPS synthesis not only enhance the fracture toughness but also the hardness.
According to the mechanism of CRN reaction, it avoids the O’sialon phase formation by removing the excess oxygen. We found that the carbon is effective to react with silica at 1250 oC, and consequently the pure β-Si3N4 is observed. But, when the calcination temperature increased to 1350 oC, the sintering additives (6Y8A) form liquid phase that lead to reaction of carbothermal reduction impeded. On the other hand, the liquid phase formation also promotes Si3N4 decomposed to Si2N2O. Therefore, there is residual carbon and O’sialon appears in the sintered bulk.
This residual carbon has influence on mechanical properties of the materials. The bending strength enhancement is not obvious even after getting pure β-Si3N4, because of the residual carbon affects the strength, but fracture toughness is found to increase clearly. The toughness of pure β-Si3N4 sample obtained under 1800 oC has been increased to 32.3% compared to without carbonthermal reduction, where as the powders fabricated by SPS at 1600 oC shows increase in toughness to 79.7%.

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