Corrosion and Microstructural Characterization of Molybdenum-YSZ Cermets Following Hydrogen Exposure up to 2630 K

Ceramic-metallic (cermet) fuels are a promising fuel type for outer space nuclear thermal propulsion (NTP). A key feasibility issue is the hydrogen chemical compatibility of candidate fuels in the proposed extreme operating temperatures for NTP systems (> 2500 K). In this study, molybdenum matrix cermets containing 40–70 vol% yttria stabilized zirconia (YSZ) particles (as a surrogate for ceramic fuel particles) were produced via spark plasma sintering (SPS) and exposed to flowing hydrogen at high temperature (2000–2630 K). Both steady state and thermally cycled (4 cycles with intermediate cooling to room temperature) conditions were examined for a constant total hot testing time of 80 min. The Mo matrix appears robust in a Mo-YSZ cermet and exhibits acceptable mass loss (< 1 wt%) with a sublinear dependence on exposure time (1/3 - 1/2 power) based on hydrogen testing at 2500–2630 K with thermal cycling. Moderately higher mass losses were observed for thermally cycled (4 times) vs. isothermal specimens. The subsurface Mo-YSZ interface also remains intact despite indications of debonding at the surface. Significant hydrogen attack occurs on the YSZ grain boundaries in the interior of the samples at 2500–2630 K.