Oxidation resistance, thermal conductivity, and spectral emittance of fully dense zirconium diboride with silicon carbide and tantalum diboride additives
Van Laningham, Gregg Thomas
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Zirconium diboride (ZrB₂) is a ceramic material possessing ultra-high melting temperatures. As such, this compound could be useful in the construction of thermal protection systems for aerospace applications. This work addresses a primary shortcoming of this material, namely its propensity to destructively oxidize at high temperatures, as well as secondary issues concerning its heat transport properties.To characterize and improve oxidation properties, thermogravimetric studies were per- formed using a specially constructed experimental setup. ZrB₂-SiC two-phase ceramic composites were isothermally oxidized for ∼90 min in flowing air in the range 1500-1900°C. Specimens with 30 mol% SiC formed distinctive reaction product layers which were highly protective; 28 mol% SiC - 6 mol% TaB₂ performed similarly. At higher temperatures, specimens containing lower amounts of SiC were shown to be non-protective, whereas specimens containing greater amounts of SiC produced unstable oxide layers due to gas evolution. Oxide coating thicknesses calculated from weight loss data were consistent with those measured from SEM micrographs. In order to characterize one aspect of the materials' heat transport properties, the thermal diffusivities of ZrB₂-SiC composites were measured using the laser flash technique. These were converted to thermal conductivities using temperature dependent specific heat and density data; thermal conductivity decreased with increasing temperature over the range 25-2000°C. The composition with the highest SiC content showed the highest thermal conductivity at room temperature, but the lowest at temperatures in excess of ∼400°C, because of the greater temperature sensitivity of the thermal conductivity of the SiC phase, as compared to more electrically-conductive ZrB₂. Subsequent finite difference calculations were good predictors of multi-phase thermal conductvities for the compositions examined. The thermal conductivities of pure ZrB₂ as a function of temperature were back-calculated from the experimental results for the multi-phase materials, and literature thermal conductivities of the other two phases. This established a relatively constant thermal conductivity of 88-104 W/m·K over the evaluated temperature range. Further heat transport characterization was performed using pre-oxidized, directly resistively heated ZrB₂-30 mol% SiC ribbon specimens under the observation of a spectral radiometer. The ribbons were heated and held at specific temperatures over the range 1100- 1330°C in flowing Ar, and normal spectral emittance values were recorded over the 1-6 μm range with a resolution of 10 nm. The normal spectral emittance was shown to decrease with loss of the borosilicate layer over the course of the data collection time periods. This change was measured and compensated for to produce traces showing the emittance of the oxidized composition rising from ∼0.7 to ∼0.9 over the range of wavelengths measured.