Carbon sintering aids

It is technically easier to manufacture a molded component from SiC-powder by slip casting or dry pressing, working it mechanically and then sintering pressureless at 1950 to 2000°C. Due to the low sintering activity of silicon carbides, such processes have only been recently successfully carried out with the advent of fine particulate SiC-powders (specific surface area > 5 m /g) with low oxygen-contents (< 0.2%). Boron or aluminum and free carbon or boron carbide are added as sintering aids. 

Commercially available precursor powders were used as raw materials in this study. The ZrB2 powders had a purity of >90% and an average particle size of <2.5pm. SiC powders had a purity of >99% and an average particle size of <0.5pm. As the sintering aids, B4C powders had a purity of >97%, with a particle size <1 pm, and the phenolic resin which was also used as adhesives, could produce a 30wt% pyrolyzed carbon. 

Boron carbide is among the hardest materials yielding only to diamond and boron nitride. It is also one of the most corrosion-resistant compounds at room or moderate temperatures. When considering the corrosion resistance of boron carbide materials, it is important to remember that they are rarely stoichiometric, with the carbon content varying from 9.88 to 23.4% [96] Many of them contain free carbon or sintering aids. Thus their behavior depends on the chemical composition. 

Aluminum is an effective sintering aid for B4C and SiC ceramics if combined with elemental boron and carbon. Phase relations in the B-C-Al-Si system may hence indicate suitable procedures to initiate transient liquid phase or enhanced solid-state sintering. Furthermore, liquid A1 may be used to infiltrate porous B4C bodies acting as a reinforcing phase. 

The main prerequisite conditions for achieving high densities are the use of submicron powders, temperatures in the range of 2100-2200 °C, pressures of 25-40 MPa, a dwell time of 15-20 min, and a vacuum or argon atmosphere. In order to provide carbon as a sintering aid and to resist the high applied pressures at temperatures exceeding 2000 °C, carbon dies are commonly used. 

Near stoichiometric, dense SiC fibers are produced at higher temperatures (1800 to 2300 C) with a boron bearing sintering aid they do not contain significant amounts of free carbon [34-35] [41] [53] [72]. As a result, the SiC grain size is relatively large in these fibers, e.g., 50-200... 

Pressureless sintered SiC bodies made from ultrafine SiC powders which are themselves highly milled Aecheson powders or are made from vapor-phase reactions of silanes with hydroearbons. Such ultrafine powders are usually mixed with carbon and boron sintering aids in order to achieve the necessary high densities. 

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