Percolation effects on toughening in a ductile-phase reinforced ceramic coating

Inert ceramic coatings are essential to a safe, economic operation at high temperatures and chemically aggressive environments. While in service, coatings are typically subjected to large thermomechanical loads which drives fracture and subsequent coating delamination. A candidate strategy to mitigate delamination is to reinforce the brittle ceramic coating with a soft, ductile phase. This talk will focus on the design of one such ductile-phase reinforced environmental barrier coating (EBC) which mitigates the risk of particle-impact ignition in staged combustion rocket engines. The EBC comprises of an inert glass-ceramic phase reinforced with nominally pure nickel (Ni) phase. Structural characterization revealed that Ni in the coating forms a percolating network above a volume fraction of 0.3, which is in excellent agreement with the theoretical percolation threshold. Additionally, we measured the thermomechanical properties of the EBC and combined it with a thermomechanical fracture analysis to compute the energy release rate for coating delamination under the thermal transients expected in a rocket engine. Results show that cold shock at shutdown gives rise to the highest energy release rate for coating delamination, of order 100 J/m2, which will cause delamination of conventional ceramic coatings. To design the present composite EBC against delamination, we measured the coating toughness as the Ni volume fraction varied across the percolation threshold. While the initiation toughness increased linearly with Ni volume fraction, peak toughness increased sharply as the Ni content approached the percolation threshold due to enhanced crack-bridging. Moreover, the peak toughness of ~160 J/m2 provided by a percolating Ni network is 4x higher than a monolithic ceramic, and is sufficient to resist delamination under most severe thermal transients expected in service.