Interface reactions of magnetron sputtered Si-based dual layer coating systems as oxidation protection for Mo-Si-Ti alloys

Mo-Si-Ti alloys show an improved oxidation behaviour at temperatures above 1000 degrees C compared to Mo-Si-B alloys by forming a SiO2-TiO2 oxide layer but at intermediate temperatures around 800 degrees C the major alloying element Mo still often leads to a catastrophic oxidation. Hence, Mo-Si-Ti alloys to be used at high temperature need a protective coating. In this study two coating systems were applied on the Mo-12.5Si-8.5B-27.5Ti-2Fe (at. %) and the Mo-21Si-34Ti-0.5B alloy. The coefficient of thermal expansion of the alloys was analysed to be about 7.8 x 10(-6) degrees C-1 at 1200 degrees C. The two coating systems consisted of a graded Mo-Si interlayer but differently sputtered Si top layers. The coating systems were investigated with respect to their deposition parameters, oxidation protection and interface reactions with the alloys. The coatings could protect the alloys successfully at 800 degrees C for 100 h. At 1200 degrees C both coating systems evolved a thermally grown oxide SiO2 forming a dense layer which can protect the alloys. At the interface between coating system and alloy an interdiffusion zone of mostly (Mo,Ti)(5)Si-3 grew with increasing testing time.