Effect of h-BN spray coatings on wetting and reactivity in Si/SiC system

A successful implementation of highly efficient latent heat thermal energy storage (LHTES) systems working at ultra high temperatures (up to 2000 °C) requires a proper selection of both phase change materials (PCMs) and refractories that would withstand a long-term contact heating under such extreme conditions. In the AMADEUS Project, silicon based materials have been pointed out as very interesting candidates for PCMs applications due to their very high latent heat values, that in turn should be directly reflected in high energy density of the LHTES system. Regarding an achievement of high reliability and long lifetime of the LHTES device, the most important conditions are a lack of wettability and negligible reactivity between the PCM/refractory couples. However, an extremely high reactivity of molten Si (coming from its high chemical affinity to oxygen, nitrogen and carbon) makes it hard to select a proper material for the PCM container. In fact, almost all of commonly applied ceramics are well wetted by molten Si at temperatures around its melting point. The only one exception is hexagonal boron nitride that shows non-wetting behavior. The results of wettability tests reported in literature show that contact angle values in Si/polycrystalline h-BN system are within the range of θ=95–145° (depending on testing conditions) at temperatures up to 1500°C, while there is no available information on the couple behavior at higher temperatures. On the other hand, a well developed commercially available h-BN spray slurries are available on the market (e.g. Henze HeBoCoat®). These slurries are widely applied as release agents for sintering and casting processes or as anti-adhesive coatings in glassmaking industry. Nevertheless, the performance of the boron nitride coatings has not been so far evaluated under conditions similar to that predicted in the Amadeus Project.

In this work, we experimentally examined the wettability and reactivity behavior of silicon on polycrystalline SiC substrates coated by two types of h-BN coatings (Henze HeBoCoat® 21E – polymer slurry and Henze HeBoCoat® 401E – ceramic slurry). The uncoated silicon carbide substrate was also selected as the reference sample, due to its well-recognized good wetting behavior in contact with molten Si.
The Si/h-BN coating/SiC samples were subjected to sessile drop experiments at temperatures up to 1750 °C combined with a contact heating procedure. After the test the structure and chemistry of solidified specimens were characterized by means of optical and scanning electron microscopy.