The surface morphology and crystal structure change of dental zirconia after hydrofluoric acid (HF) etching were evaluated. Four groups of sintered zirconia specimens were 1) control group, 2) immersion in 9.5%HF at 25 o C for 1, 2, 3, or 24 h, 3) immersion in 9.5%HF at 80o C for 1, 3, 5, or 30 min and 4) immersion in 48%HF at 25 o C for 30 or 60 min. The specimens were evaluated under SEM and XRD. The SEM analysis revealed changes in surface topography for all the HF-etched zirconia specimens. The irregularities surface increased with increasingly longer immersion times and higher etching solution temperatures. The XRD analysis of the HFetched zirconia specimens revealed the presence of a crystalline monoclinic phase along with a tetragonal form. It was concluded HF can etch dental zirconia ceramic, creating micro-morphological changes. Tetragonal-to-monoclinic phase transformation was induced on the etched zirconia surface.

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A SiC/SiC composite in a disc preform of 120 mm in diameter and 3.2 mm in thickness was fabricated by the chemical vapor infiltration (CVI) process. The composite was reinforced with plain-woven high-crystalline Tyarnno-SA fiber. Microstructure examinations and density measurements indicated a quite dense and very good space homogeneous matrix deposition in the composite. The fracture behavior and statistic reliability of the flexural strength upon three-point bending loading were investigated. The composite had an average flexural strength 597 MPa with a standard deviation 70 MPa. The statistic analysis of the strength showed good consistency among the bending specimens, with a Weibull modulus of 10.2, which is much higher than that for the Nicalon and Hi-Nicalon fibers reinforced CVI-SiC matrix SiC/SiC composites.

In this research, waste gypsum (CaSO4·2H2O), a by-product material from industrial factory, was upgraded and then used as raw material for building materials. The by-product gypsum possessed a high acidic value of 3-point pH scale and moisture content of 40 %. The two properties had an impact on setting reaction and hardening of gypsum. Therefore, the studies of gypsum phase transformation to calcium sulfate hemihydrate (CaSO4·0.5H2O) were focused on washing process and amount of calcium carbonate (CaCO3) added at 0, 1, 3 and 5 % wt. After washing, waste gypsum and washed water were reduced from high acidic value to neutralization (pH = 7) as a result of CaCO3. Next, the neutralized gypsum was heated to the optimal temperature at 160 °C for 2 hours and transformed to hemihydrate gypsum phase observed by XRD. Finally, the relationship of amount of CaCO3-mechanical property such as bending strength will be investigated.

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