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To enhance the resistance to oxidation and prolong the lifetime, oxidation protection coatings were applied on Mo-9Si-8B (in at.%) alloy substrates by Si-B co-pack cementation. Subsequently, the samples were conditioned at 1450°C for 8 h in air to develop an outer 10 μm thick aluminoborosilicate scale. A multilayered microstructure of MoSi 2 , Mo 5 Si 3 , and Mo 5 SiB 2 /MoB of about 80 μm thick was observed underneath the scale. During cyclic testing between room temperature and 1300°C the samples exhibited a very low mass change of only up to +/− 0.2 mg/cm 2 within 500 h at high temperature. During oxidation the MoSi 2 phase layer was partially consumed by silica and Mo 5 Si 3 formation. No high material regression due to MoO 3-evaporation took place during testing. However, a low mass loss during the first 100 h of testing was observed, presumably due to stress cracking caused by thermal mismatch of coating and substrate accompanied by subsequent healing. The stresses were reduced by the growth texture in the [001] direction of the Mo 5 Si 3-phase. In comparison to the uncoated substrate material, the mass change was decreased by 99.8%. This points to a significantly prolonged lifetime and shows the huge potential of coated Mo-9Si-8B alloys for application at very high temperatures and under thermal cycling loads.

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