Buckling And Stoichiometry Of Zirconium Nitride Coatings On Pyrolytic Graphite Substrates

Diversion of TRISO fuel particles from peaceful uses represents a proliferation risk. The coating of nuclear pyrolytic graphite pebbles with a thin layer of the refractory ceramic Zirconium Nitride (ZrN) may enhance spent TRISO fuel particle containment verification. To evaluate the suitability of ZrN coatings on graphite substrates, ZrN thin films are analyzed for morphological and stoichiometric properties. ZrN thin films are deposited on Highly Oriented Pyrolytic Graphite (HOPG) to simulate well oriented pyrolytic graphite in graphite pebbles. ZrN is deposited using radio frequency reactive sputtering deposition at a substrate temperature of 25 °C with a nitrogen partial pressure of 10 mTor and a 124V DC substrate bias voltage. The resulting thin film morphology is analyzed using optical profilometry, atomic force microscopy in tapping mode, x-ray photoelectron spectroscopy, and field emission transmission electron microscopy. Film thicknesses of 85 nm to 460 nm are achieved. ZrN coating surface and bulk stoichiometry is determined to be Zr rich with a Zr to N ratio of roughly 2 to 1. A ZrO2 layer is found at the thin film surface, which is attributed to oxygen surface diffusion. ZrN coating buckling and cracking is observed with increasing deposition time. An exponential growth in the size of buckles as a function of deposition time is observed. It is found that buckling is attributable to delamination within the graphite substrate rather than a delamination of the ZrN coating from the substrate.