Effect of SiC, TaB₂ and TaSi₂ additives on the isothermal oxidation resistance of fully dense zirconium diboride

Abstract

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.

Microstructure and properties of ZrB2–SiC composites prepared by spark plasma sintering using TaSi2 as sintering additive

“…[12][13][14] The second way is selecting the low melting point additives, such as MoSi 2 (melting point, 2020 • C) or TaSi 2 (melting point, 2040 • C) to form the plastic interface phases. [15][16][17] For using TaSi 2 as sintering aid, Opila et al 16 and Peng and Speyer 18 found that the introduction of TaSi 2 into ZrB 2 -SiC could further enhance the oxidation resistance of composites. Additionally, Talmy et al 19 reported that solid solution appeared in ZrB 2 -10, 20, and 30 vol.% TaSi 2 composites sintered at 2000 • C by hot pressing, which formed the core-shell structure with the core being ZrB 2 and the shell containing both Ta and Zr as (Zr,Ta)B 2 .…”

Toward Oxidation-Resistant ZrB2-SiC Ultra High Temperature Ceramics

“…Hot Pressing Hot pressing is the conventional method for fabricating UHTCs and has been used extensively, [8][9][10]12,16,17,20,22,27,33,35,39,47,48,[50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65] with typical temperatures of~2173 K (1900°C) and applied pressures between 30 and 50 MPa. It allows full densification without the use of sintering aids, although most research employs modest amounts of sintering aids such as silicides, borides, metals (e.g., Ni), or C to reduce processing times and temperatures, thus reducing the costs associated with the production technique.…”

“…These will then leave the remaining SiC subject to additional oxidation and lead to a SiC depletion zone. Peng et al [22] argued that the SiC depleted zone is the result of a wicking process. The internally formed borosilicate liquid is transported via capillary action to the existing borosilicate liquid on the surface formed by earlier oxidation.…”

“…The addition of Ta-containing compounds improved the oxidation resistance of the materials over the entire temperature range studied, but the TaSi 2 performed better than the TaB 2 presumably because of the ability to form larger amounts of protective silica-based liquid. Peng et al [22] investigated the effect of SiC, TaB 2 , and TaSi 2 on the isothermal oxidation resistance of ZrB 2 . They found that an increase in SiC decreased the thickness of both the silica-rich glassy layer and the SiC depletion zone.…”

Oxidation of Zirconium Diboride with Tungsten Carbide Additions

“…The model, and comparison with experimental results, 5,8,12,15 revealed that the ZrO 2 scale was not protective at elevated temperatures because it was porous. [17][18][19][20][21][22][23][24][25][26][27][28] An alternate approach, increasing the density of the ZrO 2 scale, may also improve the oxidation resistance of ZrB 2 in this temperature range. 16 Oxygen transport is much faster through the resulting pores than diffusion through the ZrO 2 crystal structure would be.…”