MoSi₂ Coatings on Titanium Alloy Substrates for High Temperature Oxidation Protection of Moved Parts

MoSi₂-TiB₂ coatings were applied on Ti6Al4V substrates by means of low pressure plasma spraying with and without a surface pre-treatment with plasma-transferred-arc. The coatings were characterized by optical and scanning electron microscopy, X-ray diffraction analysis and hardness measurements. No difference in the microstructure between the coatings was detectable. The microstructure shows the typical lamellar structure of thermal spray coatings with a good embedding of the titanium borides in the matrix. At the interface between matrix and some borides a reaction zone is visible. In comparison to the feedstock powder, the phase composition of the coatings has change, because a great amount of the tetragonal MoSi₂ phase transformed into the hexagonal high temperature modification. The content of titanium diboride is lower in the coating. Coatings on substrates with a pre-treatment show a good adhesion to the substrate, while the adhesion of the coatings on the pre-treated substrates is poor.