Effect of SiC, TaB 2 and TaSi 2 additives on the isothermal oxidation resistance of fully dense zirconium diboride
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The oxidation resistances of ZrB2 containing SiC, TaB2, and TaSi2 additions of various concentrations were studied using isothermal thermogravimetry at 1200, 1400, and 1500 C, and specimens were further characterized using x-ray diffraction and electron microscopy. Increasing SiC concentration resulted in thinner glassy surface layers as well as thinner ZrO2-rich underlayers deficient in silica. This silica deficiency was argued to occur by a wicking process of interior-formed borosilicate liquid to the initially-formed borosilicate liquid at the surface. Small (3.32 mol%) concentrations of TaB2 additions were more effective at increasing oxidation resistance than equal additions of TaSi2. The benefit of these additives was related to the formation of a zirconium-tantalum boride solid solution during sintering, which during oxidation, fragmented into fine particles of ZrO2 and TaC. These particles resisted wicking of their liquid/glassy borosilicate encapsulation, which increased overall oxidation resistance. With increasing TaB2 or TaSi2 concentration, oxidation resistance degraded, most egregiously with TaB2 additions. In these cases, zirconia dendrites appeared to grow through the glassy layers, providing conduits for oxygen migration.
I. INTRODUCTION
Transition metal borides including ZrB2, TaB2, and HfB2 are of interest for their ultra-high melting temperature (>3000 C), high hardness and strength, and high thermal and electrical conductivities.1–3 They are candidates for leading edges on reentry vehicles; their survival against atmospheric frictional heating is dependent on a combination of refractory ability and the ability to dissipate heat through thermal conduction away from the leading edge and radiant emission to the cold ambient. Engineering of these ceramics for oxidation resistance has focused on a two-phase microstructure of ZrB2 and SiC, in which a borosilicate viscous liquid with interdispersed ZrO2 forms as a passivating surface layer. Oxidation of single-phase ZrB2 does not form a protective surface layer since B2O3 is volatile (boiling point, i.e., 1 atm vapor pressure, of B2O3 is 1860 C). Oxidation heat treatments of ZrB2 + 20 vol% SiC at 1200 C and below have shown weight gain no less extensive than those of specimens composed of ZrB2 alone. However, above 1200 C, a borosilicate coating forms.4,5 Given the high volatility of boron oxide, the borosilicate glass surface coating might be expected to become more of a near-pure fused silica coating with increasing temperaa)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0216 J. Mater. Res., Vol. 24, No. 5, May 2009
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ture. One investigation has shown that the boron content of the oxide layer after heating to 1500 C for 30 min is less than 1 wt%.6 However, B2O3 vapor pressure is suppressed by its entering into solution with SiO2. Further, standard glass-forming practice melts, homogenizes, and fines borosilicate (e.g., Pyrex)
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