Surface and grain-boundary energies as well as surface mass transport in polycrystalline yttrium oxide

The sessile drop technique has been used to measure the temperature dependence of the contact angle, θ, of the liquid metals Ag and Cu in contact with polycrystalline yttrium oxide (yttria, Y₂O₃) at the temperature range 1,333–1,773 K in Ar/4%H₂ atmosphere. Combination of the experimental results with literature data taken for nonwetted and nonreactive oxide/liquid metal systems permit the calculation of the surface energy of Y₂O₃ as γ_sv (J/m²) = 2.278–0.391 × 10⁻³ T. For the same atmospheric conditions, thermal etching experiments on the grain boundaries intersecting the surface of the polycrystalline ceramic allow to determine the groove angles, ψ, with respect to temperature and time as well as the grain-boundary energy of Y₂O₃ as γ_ss (J/m²) = 1.785–0.306 × 10⁻³ T. Grain-boundary grooving studies on polished surfaces of Y₂O₃ annealed in Ar/4%H₂ atmosphere between 1,553 K and 1,873 K have shown that surface diffusion is the dominant mechanism for the mass transport. The surface diffusion coefficient can be expressed according to the equation D _s (m²/s) = 1.22 × 10⁻³ exp(−343554/RT).