Boron sintering aids

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. 

The ceramic, polycrystalline siHcon carbide [409-21-2], SiC, is processed using P-siHcon carbide and boron (9). The boron is a sintering aid used at... 

The high-temperature stability of SiC-based ceramics is well-known, and therefore its composite materials have been investigated for application to high-tem-perature structural materials [19-21]. However, well-known SiC-based fibers and matrix-materials stained with alkali salt are easily oxidized at high temperatures in air [22]. This would be a serious problem when these materials are used near the ocean or in a combustion gas containing alkali elements. In particular, a silicon carbide fiber containing boron (a well-known sintering aid for SiC) over 1 wt% was extensively oxidized under the above condition. In this... 

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. 

SiC and SisNa powders have been studied extensively by XPS or x-ray-in-duced AES [42,43,48-57]. Powders, whiskers, and platelets were treated at elevated temperatures, and surface analysis was carried out by measuring Si 2p XPS or Si KLL AES intensities for Si02 and SiC (or Si3N4). Subsequently, surface oxide film growth rates were measured and surface oxidation activation energies were calculated [42,43,48-51]. In some cases, the effects of the presence of yttria, a sintering aid, and boron, an impurity, were studied [43,48,49]. The effects on surface oxidation chemistry caused by different powder processing routes were also studied [50]. 

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. 

According to Greskovich and Rosolovski [372] and Prochazka [373], the specific surface area during the initial state of sintering is mainly consumed by a coarsening of the particles and pores, which in turn reduces the driving force for densification (local chemical potential). Densification comes to an end before pore closure is obtained. This so-called terminated density has been observed for pure boron carbide, SiC and silicon nitride, when no sintering aids are present. 

Boron is a well-known sintering aid for silicon carbide, but temperatures of at least 2000 C are required for sintering polycrystalline SiC powders with a grain size of the order of 1 pm. Its effectiveness in sintering SiC based fibers at lower temperatures (1600-1800 C) is probably related to the extremely small SiC grain size of PCS based materials. 

Similarly, an experimental, quasi-stoichiometric, oxygen-free SiC fiber has been produced from a Si-AI-C-0 precursor. In this process, aluminum is introduced into the polymer as aluminum (III) acetylacetonate, and acts like boron as a SiC sintering aid during heat treatment at 1800°C [40]. 

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... 

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