This study deals with the role of non-oxide sintering aids such as boron carbide (B4C) or – free boron (B) plus free carbon (C) – on the Spark Plasma Sintering treatment of silicon carbide. The results so obtained clearly show that free boron plus free carbon additions lead to the higher densification rates. This favourable behaviour with regards to the densification kinetics is accompanied by the absence of any abnormal grain growth. At the opposite, boron carbide additions do not significantly raise the densification kinetic after SPS treatment of SiC in comparison to pure silicon carbide. In this case, TEM investigations point out the formation of a borosilicate vitreous phase due to the dissolution process of B4C in contact with a native superficial silica layer surrounding the SiC grains. The resulting liquid phase leads to an abnormal grain growth coupled with undensifying process.

https://hal.archives-ouvertes.fr/hal-00272217/document

Role of boron on the Spark Plasma Sintering of an α-SiC powder

Pressureless sintering process to obtain high density silicon carbide–boron carbide (5% vol) composite (BCSiC) was studied. Sintering behaviour of this material was investigated between 1950 and 2200°C and it was compared to SiC material doped with boron and carbon. Density up to 96% T.D. was reached at 2150°C. Microstructural investigation revealed that B 4 C grains, having an average size of 2 μm, were well dispersed in the SiC matrix. Mechanical properties like fracture toughness, hardness, flexural strength were determined in the range RT-1500°C and compared to other SiC-based material. BCSiC material showed same fracture toughness of SSiC, higher hardness and higher flexural strength than SSiC also at high temperature; these properties make BCSiC a very interesting material for application in which high oxidation resistance is requested, e.g. ceramic heat exchanger.

Pressureless sintering and properties of αSiC–B4C composite

A comprehensive review of electric discharge machining of advanced ceramics

Properties such as high hardness, low density, and high elastic modulus have made SiC ceramics proper choices for a variety of industrial applications. However, disadvantages such as low sinterability, and low fracture toughness have limited the fabrication of these ceramics. Past researches show that the use of Al2O3-Y2O3 additives play an important role in improving the sinterability and the properties of the composites. The use of oxide, carbide, nitride and boride additives results in improved sinterability, physical and mechanical properties. The investigations show that the microstructure, porosities, amount of additives, reaction of additives with the matrix, grain size and, finally, the sintering temperature are the most important factors affecting the properties of SiC ceramics. In this paper, the effect of using various additives, the sintering temperature and the annealing heat treatment on sinterability, microstructure and properties of the SiC matrix composites fabricated by pressureless sintering method have been investigated.

The effect of oxide, carbide, nitride and boride additives on properties of pressureless sintered SiC: A review

Silicon carbide based materials are foreseen candidates for next generation nuclear applications due to a combination of the following properties: high temperature strength, high thermal conductivity and low nuclear activation. Their main drawback lies in their too low toughness. A promising route to enhance such mechanical properties is to reduce the grain size down to the nanosize range. Enabling a quite low sintering time, the spark plasma sintering technique has been used to process nano size monolithic Silicon carbides with several grain sizes and with or without boron additives. The mechanical properties, including Young modulus, flexural strength and toughness, of these materials have been measured from room temperature to 1300 °C and compared to those of a commercially sintered α-SiC. The results are carefully discussed in correlation with the microstructure. Despite a lower density, the obtained flexural strength and toughness properties of the nano grain silicon carbide are very promising when processed without boron additives. Thus, efforts should be focused on the processing of large size nanograins SiC components by SPS without boron and with high density.

Gian Loris Minoccari

Papers

Pressureless sintering and properties of αSiC–B4C composite

Flexural strength and toughness of liquid phase sintered silicon carbide