The effect of slip degassing on the microstructure and mechanical properties of slip cast and reaction bonded Si3N4 was studied. The slip was prepared by aqueous ball milling of silicon (Si) powder. Hydrogen bubbles, a result of Si oxidation during milling, were degassed from the slip using a combination of vacuum and heat. The slip was then cast into a plaster mould to obtain rectangular green bodies. The Si green samples were sintered in a nitrogen atmosphere at 1500°C to convert the Si to Si3N4. After that the nitrided samples were polished to dimensions of 3 x 4 x 30 mm. The density, porosity, flexural strength, phase content and microstructure of the sintered samples were studied. The results showed that the degassing process increased the slip density. After casting and subsequent nitridation, it was found that the average apparent density of the samples increased from 2.89 to 2.95 g/cm3, the porosity decreased from 52.9 to 49.5 %, and the flexural strength increased from 8.1 to 9.3 MPa, when the degassed slip was used. A microstructural examination showed that the pores in the samples were filled with whiskers, which most likely resulted from a vapor phase growth mechanism. The samples produced from the degassed slip tended to have fewer whiskers, due to the reduced pore size and volume. A comparison of the XRD patterns showed no phase differences between the samples. The appearance of Si2N2O, and SiC likely resulted from the reactions between O2 and C impurities with Si3N4.
This research studied the effect of composition of a glass-based high temperature adhesive on the bond strength of 96% alumina bars. The adhesives studied were composed of 40-70% glass powder, 5-30% polyvinyl alcohol (PVA), and 15-55% water. Half-lengths of green alumina bend bars were bonded together with the adhesive to form a full length bar and then sintered. The flexural strengths of the sintered bars were measured and the resultant fracture surfaces were examined by SEM. The results showed that an initial increase in the PVA and glass content increased the flexural strength to the highest value of 120 MPa, but that further increases resulted in decreased strength. The strength values fluctuated when the PVA content was above 20% and the glass content was 40% or 60%, which indicated poor adhesive homogeneity. SEM analysis of the fracture surfaces showed a separation layer between the alumina and glass adhesive when the glass or PVA content were either too high or too low, which again indicated poor adhesive homogeneity. It was concluded that the optimal composition of the adhesive was 45-55% glass and 7-15% PVA, which gave a minimum flexural strength of 80 MPa.
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