Thermogravimetric analysis of the oxidation resistance of ZrB₂-SiC and ZrB₂-Sic-TaB₂-based compositions in the 1500-1900 °C range

Abstract

Theoretically dense ZrB₂-SiC two-phase microstructures 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 that were highly protective; 28 mol% SiC-6 mol% TaB₂ performed similarly. At and above 1700 °C, the composition with only 15 mol% SiC oxidized extensively because of deficient silicate liquid formation. Specimens with 60 mol% SiC were resistant to oxidation up to 1800 °C; at 1900 °C, this composition displayed periodic ruptures of the passivating layer by emerging gas bubbles. Oxide coating thicknesses calculated from weight loss data were consistent with those measured from scanning electron microscopy micrographs. A layer of ZrB₂ devoid of SiC was argued to be from preferential removal of SiC by reaction of a silica oxidation product with adjacent unreacted SiC to form escaping gases.

Appendix

For the reactions

$${\text{Zr}}{{\text{B}}_{{\text{2(s)}}}}{\text{ + }}\frac{{\text{5}}}{{\text{2}}}{{\text{O}}_{{\text{2(}}g{\text{)}}}} \to Zr{{\text{O}}_{{\text{2(}}s{\text{)}}}}{\text{ + }}\,{B_{\text{2}}}{{\text{O}}_{{\text{3(}}g{\text{)}}}}\quad,$$

and

$${\text{Si}}{{\text{C}}_{{\text{(s)}}}}{\text{ + 2}}{{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{Si}}{{\text{O}}_{{\text{2(1)}}}}{\text{ + }}\,{\text{C}}{{\text{O}}_{{\text{2(g)}}}}\quad,$$

it is assumed that boron oxide forms as an escaping gas and silica forms as a liquid/glass. These reaction stoichiometries dictate that the number of moles of ZrO₂ formed are equal to the number of moles of ZrB₂ consumed, ${n_{{\text{Zr}}{{\text{O}}_{\text{2}}} = }}{n_{{\text{Zr}}{{\text{B}}_{\text{2}}}}}$, and similarly $n{\text{Si}}{{\text{O}}_2} = n{\text{SiC}}$. Note that the latter equality holds even if the reaction produced CO as opposed to CO₂.

Let ΔM be the measured mass change after 100 min exposure to a given oxidation temperature. Then:

$${{\Delta }}M\,{\text{ = }}\,{\text{(}}{M_{{\text{Zr}}{{\text{O}}_{\text{2}}}}} - {M_{{\text{Zr}}{{\text{B}}_{\text{2}}}}}{\text{)}}\,{\text{ + }}\,{\text{(}}{M_{{\text{Si}}{{\text{O}}_{\text{2}}}}} - {M_{{\text{SiC}}}}{\text{)}}\quad {\text{.}}$$

where ${M_{{\text{Zr}}{{\text{B}}_2}}}$ and M_SiC are the masses of zirconium diboride and silicon carbide converted, respectively, and ${M_{{\text{Zr}}{{\text{O}}_2}}}$ and ${M_{{\text{Si}}{{\text{O}}_2}}}$ are the masses of zirconia and silica produced, respectively. Letting W be the molar mass:

$${{\Delta }}M\,{\text{ = }}\,{n_{{\text{Zr}}{{\text{B}}_{\text{2}}}}}{\text{(}}{W_{{\text{Zr}}{{\text{O}}_{\text{2}}}}} - {W_{{\text{Zr}}{{\text{B}}_{\text{2}}}}}{\text{)}}\,{\text{ + }}\,{n_{{\text{SiC}}}}{\text{(}}{W_{{\text{Si}}{{\text{O}}_{\text{2}}}}} - {W_{{\text{SiC}}}}{\text{)}}\quad {\text{.}}$$

We define X_SiC as the mole fraction of SiC in the original ZrB₂–SiC composition, thus:

$${n_{{\text{Si}}{{\text{O}}_{\text{2}}}}}\,{\text{ = }}\,\frac{{{{\Delta }}M}}{{\left( {\frac{{\text{1}}}{{{X_{{\text{SiC}}}}}} - {\text{1}}} \right)\left( {{W_{{\text{Zr}}{{\text{O}}_{\text{2}}}}} - {W_{{\text{Zr}}{{\text{B}}_{\text{2}}}}}} \right)\,{\text{ + }}\,{\text{(}}{W_{{\text{Si}}{{\text{O}}_{\text{2}}}}}{W_{{\text{Zr}}{{\text{O}}_{\text{2}}}}} - {W_{{\text{Zr}}{{\text{B}}_{\text{2}}}}}{W_{{\text{SiC}}}}{\text{)}}}}\quad,$$

((A1))

Let ρ be the density and V′ be the volume of oxide formed:

$$V\prime \,{\text{ = }}\,\frac{{{n_{{\text{Si}}{{\text{O}}_{\text{2}}}}}{W_{{\text{Si}}{{\text{O}}_{\text{2}}}}}}}{{{{{\rho }}_{{\text{Si}}{{\text{O}}_{\text{2}}}}}}}\,{\text{ + }}\,\frac{{{n_{{\text{Zr}}{{\text{O}}_{\text{2}}}}}{W_{{\text{Zr}}{{\text{O}}_{\text{2}}}}}}}{{{{{\rho }}_{{\text{Zr}}{{\text{O}}_{\text{2}}}}}}}$$

((A2))

Given a disk geometry for the partially oxidized specimen of diameter d and height h, with a uniform oxide coating thickness of T:

(A3)

The diameter d and height h of the disk vary during oxidation because the densities of the consumed phases and the corresponding formed oxide phases are not the same. Let ΔV be the volume of products formed minus the volume of reactants consumed; that is, the change in volume of the disk. Recognizing the equivalence of moles of ZrB₂/ZrO₂ and SiC/SiO₂:

$${{\Delta }}V\,{\text{ = }}\,{n_{{\text{SiC}}}}\left( {\frac{{{W_{{\text{Si}}{{\text{O}}_{\text{2}}}}}}}{{{{{\rho }}_{{\text{Si}}{{\text{O}}_{\text{2}}}}}}} - \frac{{{W_{{\text{SiC}}}}}}{{{{{\rho }}_{{\text{SiC}}}}}}} \right){\text{ + }}\,{n_{{\text{Zr}}{{\text{B}}_{\text{2}}}}}\left( {\frac{{{W_{Zr{O_{\text{2}}}}}}}{{{{{\rho }}_{{\text{Zr}}{{\text{O}}_{\text{2}}}}}}} - \frac{{{W_{{\text{Zr}}{{\text{B}}_{\text{2}}}}}}}{{{{{\rho }}_{{\text{Zr}}{{\text{B}}_{\text{2}}}}}}}} \right)\quad.$$

((A4))

If the dimensions of the disk before oxidation are d₀ and h₀, then after partial oxidation, the change in volume is

$$\Delta V = \pi\left(\frac{d}{2}\right)^2 h-\pi \left(\frac{d_0}{2}\right)^2 h_0,$$

Assuming that the thickness of oxide scale forming on all surfaces is the same, the change in linear dimensions from oxidation in the axial and radial directions would then be the same: d − d₀ = h − h₀, thus:

$${{\Delta }}V\,{\text{ = }}\,\frac{{{\pi }}}{{\text{4}}}\left[ {{{{\text{(}}h - {h_{\text{0}}}{\text{ + }}{d_{\text{0}}}{\text{)}}}^{\text{2}}}h - d_{\text{0}}^{\text{2}}{h_{\text{0}}}} \right]\quad.$$

((A5))

Equation (A2) (inserting into Eq. (A1) and the definition of mole fraction) may be used to determine the volume of oxide formed. Inserting this into Eq.(A3), along with calculated values of d and h from Eqs. (A4) and (A5) will yield T through numerical solution.