Oxidation behavior of pack-cemented Si–B oxidation protection coatings for Mo–Si–B alloys at 1300 °C

Abstract

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 μmthick aluminoborosilicate scale. A multilayered microstructure ofMoSi2,Mo5Si3, andMo5SiB2/MoB of about 80 μmthick 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/cm2 within 500 h at high temperature. During oxidation the MoSi2 phase layer was partially consumed by silica and Mo5Si3 formation. No high material regression due to MoO3-evaporation took place during testing. However, a lowmass loss during the first 100 h of testingwas observed, presumably due to stress cracking caused by thermalmismatch of coating and substrate accompanied by subsequent healing. The stresses were reduced by the growth texture in the [001] direction of the Mo5Si3-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.