Oxidation protection of Mo-Si-B alloys by magnetron-sputtered coatings

Mo-Si-B alloys with melting temperature above 2000 °C are attractive high temperature materials offering significantly enhanced operating temperatures compared to those of the currently used Ni-based superalloys. However, their oxidation behavior is poor at temperatures below 1000 °C, suffering from evaporation of MoO3. Above 1000 °C oxidation is characterized by a transient state showing considerable mass loss followed by a steady state condition with reduced mass change. To mitigate this degradation by oxidation, application of coatings is an appropriate method ensuring the formation of slowly growing protective scales. The presentation summarizes recent results on overlay coatings that were deposited on coupons of a Mo-9Si-8B (in at.%) alloy using magnetron sputtering. A double layer design was applied: a 2 µm thick intermediate Mo5SiB2 layer to prevent interdiffusion and approximately 5 µm thick protective topcoats with different chemical compositions: Mo-45Si-25B, Mo-55Si-10B, Mo-29Si-15B, Mo-48Si-24Al, Mo-71Si-8Al (all in at.%). The amorphous as-deposited coatings were annealed in a vacuum furnace. In the boron containing Mo-Si topcoats, the MoSi2 and MoB phases formed as well as the Mo5Si3 phase in the Mo-29Si-15B coating. In the annealed Mo-48Si-24Al and Mo-71Si-8Al topcoats, the C40-Mo(Si,Al)2 and C11b-MoSi2 phases were observed, respectively. The oxidation behavior of the coated samples was investigated at 800, 1100 and 1300 °C under cyclic condition in laboratory air. The dwell time at high temperature was 10 or 20 h and the samples were tested for up to 10 cycles. Microstructural examinations of the coated samples were carried out using scanning electron microscopy, energy-dispersive X-ray spectroscopy, glow discharge optical emission spectroscopy and X-ray diffraction measurements. Please click Additional Files below to see the full abstract