Sputtered oxidation protection coatings for Mo-Si-B-materials in high temperature turbine applications

Kurzfassung

To enhance the efficiency of jet engines, the turbine inlet temperature has to be increased by raising the temperature capability of the components used in the first stages of the turbine. This requires materials with higher melting points than that of the currently used Ni-base superalloys. Mo-Si-B alloys have melting points exceeding 2000 °C and possess high creep resistance. However, they suffer from poor oxidation behaviour. Therefore, in the present work, oxidation protection layers were developed and produced by magnetron sputtering. The coating systems consisted of a 2 µm thick Mo5SiB2 diffusion barrier interlayer which stabilized the 5 µm thick oxidation protection layers on top. Two oxidation protection coatings with the compositions of Mo-48Si-24Al and Mo-29Si-15B (in at.%) were deposited. The as-deposited coatings were amorphous and, therefore, subsequently annealed in vacuum to induce crystallization. In the former coating the C40-Mo(Si,Al)2 phase and in the latter the MoSi2, Mo5Si3, and MoB phases are formed. The oxidation behaviour of the coated samples was tested at 800, 1000 and 1300 °C for maximum exposure periods of 100 h. On the MoSi2-Mo5Si3-MoB coating, a dense borosilicate scale formed at all temperatures, protecting the substrate at 800 and 1000 °C for at least 100 h. At 1300 °C, however, buckling occurred in this coating. On the C40-Mo(Si,Al)2 coating, needle-like aluminium borate formed at 800 °C which provided oxidation protection to the substrate, until the coating degraded after about 60 h of exposure. When exposed at 1000 °C, the latter coating formed a dense SiO2-mullite oxide layer, which was protective up to 100 h; but it rapidly failed at 1300 °C by spallation.