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

摘要

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 degrees C for 8 h in air to develop an outer 10 mu m thick aluminoborosilicate scale. A multilayered microstructure of MoSi2, Mo5Si3, and Mo5SiB2/MoB of about 80 mu m thick was observed underneath the scale. During cyclic testing between room temperature and 1300 degrees 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 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 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 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. (C) 2015 Elsevier B.V. All rights reserved.