Comparison of microstructure and oxidation behavior of La-Mo-Si coatings deposited by two approaches

To investigate the effect of preparation methods on the La-Mo-Si (LMS) coatings, we developed a new LMS coating system using pack cementation (PC) and supersonic atmospheric plasma spraying (SAPS). Microstructure analysis showed that the SAPS-LMS coating possessed a higher porosity than that of the PC-LMS coating. Higher porosity can provide more channels to the oxidative and corrosive gasses to permeate the SAPS-LMS ceramic top-coat. After static oxidation for 150 h under 1773 K, the mass loss of SAPS-LMS coating (3.12 wt%) was much higher than that of the PC-LMS coating (0.05 wt%), and the parabolic rate constants presented faster oxidation kinetic in SAPS-LMS coating with respect to the PC-LMS coating. These results revealed that the protection effectiveness of SAPS-LMS coating was inferior to PC-LMS coating. Compared with SAPS-LMS coating, microscopic pores and cracks appeared in the PC-LMS coating with a thicker oxide film, which benefits from the formed La-Si-O-Al glass oxide with an excellent ability for crack healing. The reasons for poor antioxidant performance of SAPS-LMS coating are the higher volatility of La-Si-O glass containing Mo5Si3 and a weak interfacial interaction between coatings and substrate.