Oxidation behavior of magnetron sputtered double layer coatings containing molybdenum, silicon and boron

Protective coating systems were applied to Mo-9Si-8B (at.%) alloys to prevent oxidation at elevated temperatures. The coatings produced by magnetron sputtering and subsequent annealing consisted of an outer oxidation protection layer and an interlayer between this and the substrate. Three amorphous outer layers with different compositions were deposited: Mo-45Si-25B, Mo-55Si-10B and Mo-29Si-15B (all in at.%). The interlayer was selected to give a diffusion barrier with the composition of the Mo5SiB2 (T2) phase. All coatings were dense and well-adherent. During vacuum annealing the amorphous as-deposited coatings became crystalline exhibiting mainly the intermetallic Mo5SiB2 compound as interlayer and the MoSi2, Mo5Si3 and MoB phases in the top layers. The samples were exposed to dry laboratory air in the pesting regime at 800 °C and above, i.e. at 1000 and 1300 °C for up to 100 h under cyclic conditions. All coatings were protective at 800 and 1000 °C for at least 100 h and showed a marked improvement in mass change compared to the uncoated substrate. For protection at 800 °C higher boron content is preferential, while at higher oxidation temperatures a lower boron content provides improved oxidation protection. At 1300 °C stress induced failures like cracking, spallation and buckling occurred due to the relatively high CTE mismatch between PVD coating and substrate. Even though, the mass change was still markedly reduced as compared to the bare substrate.