Corrosion and microstructural characterization of molybdenum cermets following hydrogen exposure up to 2630K

Ceramic-metallic (cermet) fuels are a promising fuel type for space nuclear thermal propulsion (NTP). A key feasibility issue of NTP fuels 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 high temperature exposure duration of 80 min. Following testing, 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 - <1/4> 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 particle grain boundaries in the interior of the samples at 2500-2630 K.