Sintering Mechanism And Performance Improvements Of Recrystallized Silicon Carbide

Due to its high purity, excellent high-temperature mechanical properties, corrosion resistance, thermal stability etc, recrystallized SiC (RSiC) has been widely used in the fields with severe environment, such as high-temperature kilns, metallurgy filters, exhaust filters, heat exchanger, electric heating element. However, the inherent sintering mechanism of RSiC without shrinkage during firing leads to its limited mechanical properties and oxidation resistance, which is insufficient to meet the growing demands for application. The key manufacturing technology of RSiC still depends on only a few foreign companies, resulting in its high price. Although many efforts have been conducted in recent years, few studies focused on the sintering mechanism and effects of impurities in raw materials on the sintering characteristic of RSiC, which are closely related to the microstructure of RSiC, thereby affecting its relative properties. Moreover, the high temperature for preparing RSiC (2200-2450℃) results it extremely difficult to investigate the process in detail. Based on these status, this work started with the investigation of sintering mechanism and microstructure evolution of RSiC, next stepped into impact of impurities in raw material on the sintering mechanism of RSiC, and then focused on the property improvement of RSiC which was inadequate according to the applications.The recrystallization of SiC compactions with different raw material particle sizes and size distributions was investigated, besides the relations among microstructure evolution, sintering mechanism, raw material particle size&distribution, and sintering temperature, which was employed to the analysis of recrystallization mechanism and the suggestion how to get optimal properties by microstructure design. The results showed that SiC achieved its recrystallization by surface diffusion at low temperatures and evaporation-condensation at high temperatures. The raw material particle size&distribution and sintering temperature had a great influence on the microstructure and grain growth of RSiC. The grain growth has not been affected by sintering temperature for the single particle size, while for multi-particle size, the merging between particles was notable, and as the temperature increased, the merging ability increased, which resulting in the grain growth. As a result, the porosity&pore size and properties of RSiC could be controlled by adjusting the raw material particle-size composition and sintering temperature. The effect of SiO2, as one of the most common impurities in the SiC raw powders, on the weight loss, microstructure evolution and morphology of by-products of RSiC during sintering with different SiO2contents in raw materials under different temperatures, has been investigated. Under the sintering temperatures, SiO2, in the form of liquid state coated the fine SiC, reacted with SiC at the solid/liquid interface of SiC/SiO2, producing the gas products of SiO(g) and CO(g). The produced SiO(g) further promoted the dissociative evaporation reaction of SiC at the interface, resulting in greater weight loss than that of pure SiC during recrystallization. The recrystallization atmosphere altered from SiC2(g), Si2C(g), and Si(g) for the pure SiC to the composition containing high contents of SiO(g) and CO(g) for that with SiO2, which inhibited the effective recrystallization of SiC, extending the normal sintering time. The analysis of growth environment, morphology and growth rates of by-products of RSiC, proved the results from the presumption of weight loss and microstructure evolution reasonable.Based on the status of RSiC with relative low density and low flexural strength, one employed polycarbosilane (PCS) as precursor, and a cyclic process combining polymer impregnation-pyrolysis (PIP) and high-temperature recrystallization to increase the density of RSiC with different porosities. Despite the pure PCS is cabon-surplus for SiC, relative pure SiC product has been obtained successfully by combining a prior oxidative cross-linking treatment and recrystallization. When densified with PCS/Xylene solution, RSiC with homogeneous structure can be obtained but with a relatively low densification efficiency. However, that with SiC/PCS/Xylene slurry can notably improve the efficiency but with a structure of dense area in exterior and loose area in interior of RSiC. The PIP treatment improved the density of RSiC to a certain degree, but it was not effective by a single way of PIP cycles owing to the blocked pores. The recrystallization treatment at2400℃reopened the plugged pores and made the pores intercommunicated by the evaporation-condensation of SiC particles, which resulted in a continued PIP density increasing process. A high performance RSiC with high density and high flexural strength was obtained finally after three PIP-recrystallization cycles on the basis of commercial RSiC.Melt infiltration was employed to prepare dense RSiC-MoSi2composites to improve the oxidation resistance of RSiC which weakened by the interconnected open pores of RSiC. The MoSi2melt was relatively stable under infiltration temperatures, was wettable and compatible with SiC. However, still part of the MoSi2decomposed, forming melt rich in Mo, which reacted with SiC forming Mo4.8Si3C0.6.The difference of the thermal expansion coefficients between SiC and MoSi2resulted in cracks created, which had influence on the improvement of mechanical property. The prepared RSiC-MoSi2composites with excellent oxidation resistance, low coefficient of thermal expansion and resistivity can be used as structural material, as well as heating elements to achieve the integration of structure and function.