Abstract
ZrB2-SiC is of high interest for Thermal Protection Systems (TPS) for future hypersonic vehicles. Both ZrB2 and its oxidation product, ZrO2, possess high melting temperatures (Tm=3245°C and 2715°C respectively) needed for this application. SiC is added to improve the oxidation resistance. However, the oxidation resistance of ZrB2-SiC at ultra-high temperatures is poor and the oxidation mechanisms are not well understood. The aim of this work was to perform a quantitative study of the oxidation behavior in order to improve life prediction.
The oxidation behavior of ZrB2-30 vol% SiC was studied using two oxidation procedures. A box furnace was used to oxidize specimens for times between 30 seconds and 100 hours at temperatures of 1300°-1550°C in stagnant air. For ultra-high temperature testing, a resistive heating system was designed and built, which allowed oxidation at temperatures of 1300°-1800°C for times between 5 and 70 minutes in controlled oxygen atmospheres. Oxidation was quantified by measuring mass change and oxidation product layer thicknesses. A combination of scanning electron microscopy, energy dispersive spectroscopy, x-ray diffraction, x-ray photoelectron spectroscopy, inductively coupled plasma optical emission spectrometry, and time of flight secondary ion mass spectrometry was used to characterize the oxidation products.
Two oxidation regimes were identified; 1) low temperature oxidation below 1627°C and 2) high temperature oxidation at and above 1627°C. Low temperature oxidation exhibited a two-layer oxide, which consisted of a borosilicate glass layer above a ZrO2+C layer. Key findings indicate that oxygen transport in both zirconia and borosilicate glass must be considered in modeling the low temperature oxidation behavior of ZrB2-30 vol% SiC. In addition composition and thickness variations of the borosilicate glass layer must also be considered.
The transition between the low and high temperature oxidation regimes is attributed to a change in the thermodynamically favored oxidation products, considering a locally SiC-rich microstructure. High temperature oxidation, T≥ 1627°C, resulted in formation of three oxidation product layers. A borosilicate glass layer was found above a layer with ZrO2 and borosilicate glass. Beneath these was a porous layer of ZrB2 resulting from SiC depletion due to active oxidation to SiO(g). Key findings indicate that the oxidation rate is much more rapid in this oxidation regime, that the SiC depletion layer growth is best explained by parabolic gas phase diffusion in the porous layer, and again, oxygen transport in both zirconia and borosilicate glass must be considered in modeling the high temperature oxidation behavior of ZrB2-30 vol% SiC.
This work provided a quantitative analysis of the oxidation kinetics of ZrB2-30 vol% SiC. The thermodynamic and kinetic analyses of the two distinct oxidation regimes of ZrB2-30 vol% SiC enable improved life prediction.
