Papers by Keyword: Passive Oxidation

Abstract: Comparisons of heating tests at atmospheric pressure and low pressure by using a thermal plasma torch were performed. A constant heat flux on the sample surface was applied in the study of the oxidation mechanism of C/C-SiC composite, used in thermal protection systems. The SEM and EDS analysis show an intensive glassification at the surface, which are strongly depend on the oxygen partial pressure and the sample surface temperature. For vacuum conditions, at maximum surface temperature of 1450 °C and the oxygen partial pressure of about 66 Pa, a uniform passivation layer of SiO₂ is formed. At atmospheric pressure, under an oxygen partial pressure of 2.1×10⁴ Pa, the maximum surface temperature is 400 °C higher than obtained in vacuum, reaching levels of 1850°C. Under these conditions, the protective oxide layer is partially volatilized with time, increasing the specific mass loss rate by a sublimation of the composite, directly exposed to the plasma jet. This effect is alike to what occurs in the process of transition from passive to active oxidation of SiC.

Abstract: ZrB2-SiC composites were prepared by arc melting using ZrB2 and -SiC powders as raw materials and their oxidation behavior were investigated. The eutectic composition of the ZrB2-SiC system was ZrB2-58.5mol%SiC and the melting point temperature was about 2570 K. At 1673 K mass gain was observed in the oxidation of ZrB2-SiC eutectic composition and the mass gain rate increased with decreasing Ptot. The oxide scale consisted of an amorphous SiO2 layer, a ZrO2-SiO2 eutectic-like layer and a Si-deficient layer.

Abstract: The passive oxidation mechanism of CVD SiC was discussed from experimental results with high-temperature thermogravimetry and thermodynamic analyses. The bubble formation temperature around 1900 K could be too low for an oxygen inward diffusion limited process but conform to a CO outward diffusion limited process. The parabolic rate constant (kp) had weak oxygen partial pressure (PO2) dependence, kp ∝ PO2 n where n = 0.09 to 0.12. These n values may be consistent with the CO outward diffusion limited process. The activation energy of kp obtained in the present study, 210 kJ/mol, could suggest a different mechanism from the well-approved oxygen molecule permeation limited process at lower temperatures below 1600 K. Amorphous phase was significantly contained in SiO2 scales formed in an N2-O2 atmosphere. No effect of the amorphous formation on kp was identified.