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Chemical Interactions Between Si-Based Liquid and SiC: Thermodynamic and Kinetic Analysis
thesisposted on 10.10.2021, 16:22 by Alexander R StrayerAlexander R Strayer
Silicon carbide (SiC) is an industrially important non-oxide ceramic with an attractive combination of chemical stability, thermal fatigue resistance, high stiffness, and low density. Additionally, production of high toughness, near-net-shape, SiC/SiC composite structures is possible through liquid silicon (Si) melt infiltration into a SiC/carbon porous preform. Owing to a unique combination of properties, these composites have gained interest for applications such as gas turbines, heat shields, and automotive parts. However, dissolution of SiC into pure liquid Si is possible during infiltration leading to undesirable and inconsistent mechanical properties. Understanding the physicochemical interactions between SiC and Si-bearing liquids is necessary to mitigate the dissolution of SiC during the infiltration process. An experimental high-temperature apparatus was constructed to allow for precise vertical translation and rotation of SiC samples under a controlled atmosphere. Utilizing the apparatus, single crystal SiC wafers were immersed and rotated in Si melts of varying compositions to observe the rate-controlling mechanisms for dissolution. Careful attention was placed on maintaining precise melt composition allowing only fused silica and SiC to come in contact with the Si melts. Experimental results confirmed the rate controlling mechanism at 1450°C as being C transport away from the SiC/Si interface for the dissolution of single crystal 4H SiC in high?purity liquid Si. Values for the effective C solubility in high-purity Si melts were found to be between 66 and 119 ppmw, and effective diffusion coefficients for C in high-purity liquid Si were found to be between 8.1x10-6 and 1.3x10-4 cm2 /s. Similar experiments rotating single crystal 4H SiC wafers in 3 wt% B – containing Si melts were performed with the rate-controlling mechanism found to also be carbon transport away from the reaction interface. The effective C solubility in 3 wt% B- containing Si melts was found to be 94-173 ppmw and effective diffusion coefficients for C in 3 wt% B- containing Si liquid were found to be between 4.7x10-6 and 2.1x10-4 cm2 /s. Static immersion was also conducted on polycrystalline SiC samples prepared via hot pressing. Immersion studies revealed a relatively slow weight loss regime at short immersion times followed by an increased weight loss regime resulting from SiC grain fallout. Increased dissolution of polycrystalline SiC samples was observed after immersion in Si melts containing elements known to increase the C solubility in liquid Si and decreased dissolution was observed after immersion in Si melts containing elements known to decrease C solubility.