The effect of mo substrate oxidation on splat formation

Disk splats are usually observed when the deposition temperature exceeds the transition temperature, whereas thick oxide layer will reduce the adhesion resulting from high deposition temperature. In present study, single molybdenum splats were sprayed onto polished molybdenum substrates with different preheating processes to clarify the effect of surface oxidation on the splat formation. Three preheating processes included heating the substrate to 350 °C, 550 °C, and cooling the substrate from 550 °C to 350 °C, which were performed in argon atmosphere. The chemistry and compositions of substrate surface was examined by XPS. The cross sections of splats were prepared by focus-ion-beam (FIB), and then characterized by SEM. Nearly disc-shaped splat with small fingers in the periphery was observed on the substrate preheated to 350 °C. Perfect disc-shape splat was deposited at 550 °C. Flower-shaped splat exhibited a central core and discrete periphery detached by some voids on the substrate preheated to 350 °C (cooling down from 550 °C). The results of peeling off splats by carbon tape and morphology of FIB sampled cross-sections indicated that no effective bonding formed in the splat-substrate interface for the substrate ever heated to 550 °C, due to the increasing content of MoO₃ on preheated molybdenum surface.