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Title: Development of (Alpha/Beta) Si-AION Nano ceramic for wear resistance Applications

Date:           7th Dec 2015

Venue:        Bld 63 room123

Abstract:

Si3N4 ceramics are known for their outstanding performance in challenging environments. However, due to the covalent boding present between Si and N, fully-dense Si3N4 ceramics are difficult to achieve under typical sintering conditions. Thus, SiAlON ceramics have been introduced into the field, in which part of Si and N are replaced by Al and O, respectively. 

The use of nano starting powder materials throughout this work  has proven its positive impact in  accelerating the reaction kinetics, which is usually presented in terms of lowered sintering temperatures and shorter holding time. Spark plasma sintering was chosen to be the sintering technique to benefit from its novelty in limiting phase transformation and unwanted grain growth, along with being an economical processing route when compared to conventional sintering techniques. Further, Ca additives were shown to enhance the densification and wetting process through the production of the liquid phase, with the least crystal distortion in α- SiAlON unit cells, which is reflected in higher stability of the latter phase.

Al metal precursor was tested in this work in the context of improving sinterability at lower temperatures, such that Al metal replaces partially AlN.  It was found that by such replacement, Ca- α- SiAlON can be sintered at lower peak temperatures with either retained or enhanced mechanical properties. Additionally, Al  metal precursors was shown to be a novel tool in securing α- SiAlON at high sintering temperatures through the hindering of alpha to beta phase transformation. Si metal precursor was examined as well in similar context, however, it did not show the same tendency due to the limited chemical reactivity and its relative high melting point when compared to Al metal.

Ba additive was examined in this study to establish a basis for comparison with Ca additives.. The formation of S-SiAlON was shown to be typical in the presence of Ba additives. Further, it was found that an increase in the amount of β-phase occurred when β-Si3N4 was introduced into the starting mixtures in place of amorphous Si3N4. The mechanical properties of the sintered Ba-SiAlON were far below those of Ca- α- SiAlON due to the ionic radius of Ba and the presence of certain phases.

As an attempt to improve the fracture toughness of sintered Ca- α- SiAlON ceramics, post- sintering heat treatment of specified samples was carried out at 1500oC for 12 hours in Ar environment. The enhancement in the fracture toughness has been shown to be multi-factored phenomena, such that crack deflection mechanism became functional when the Ca amount and N:O ratio  increased in the grain boundary as a result of the devitrification process. In addition to that, crack-bridging  showed up at certain sites in the crack way, which is thought to play a role in increasing the fracture toughness.

Some of the presentation media:

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