Effect of phosphate compound on physical and mechanical properties of SiO₂ ceramic

This paper reports on the addition of Al(H₂PO₄)₃ binder solution with different vol% ratio and its effects on the mechanical behavior of the formed silicon dioxide ceramic. The starting material, commercial phosphate-bonded investment material, was analyzed by X-ray fluorescence (XRF) spectrometry. After silicon dioxide powder was mixed with a binder, the thick liquid was poured into a rubber mold and allowed to dry at room temperature. Then, the green bodies formed were sintered at 1200 °C for 5 h in a furnace in air atmosphere. Phase and impurities of all the ceramic samples were characterized by X-ray diffraction (XRD) and Raman spectroscopy. The morphology was revealed by scanning electron microscopy (SEM). The local atomic structures of some ceramic samples were investigated by extended X-ray absorption fine structure (EXAFS) to better understand the formation of phosphate compound. Compression test was carried out on the ceramic samples at room temperature. Results showed that quartz-cristobalite transformation in the silicon dioxide structure did not significantly affect the mechanical property of the ceramic samples prepared in this work. Porosity was decreased with Al(H₂PO₄)₃ diluted binder ratio. The presence of pores act as stress concentration points in SiO₂-based ceramics in which crack could initiate under loading. We strongly believe that a sharp increase in the compressive strength is due to phosphate compound and its unique morphology.