Microstructure and oxidation behavior of High Velocity Oxy-Fuel spray deposited Ti2AlC coatings

Licentiate thesis, 2008

Ti2AlC is one of the ternary carbides in the MAX phase group of materials. This group of materials have a unique combination of ceramic and metallic properties, such as machinability, high thermal conductivity, low density and oxidation resistance. The aim of this work is to produce thick coatings with good mechanical and thermal properties that can be used for example to operate as protection of structural materials against high temperature oxi- dation. Previously, Ti2AlC has only been fabricated as bulk and thin film. In this work it is shown that high velocity oxy-fuel spraying with Ti2AlC powder can be used to deposit relatively dense, 200 µm thick coatings. This study demonstrates that the Ti2AlC powder decompose during the spraying process. The resulting microstructure consists mainly of large Ti2AlC grains and regions of small TiC grains embedded in TixAly. These grains and regions are surrounded by oxidized zones. The study of the effect of process parameters reveal that it is possible to increase the amount of MAX phase present in the coatings using powder with a larger grain size. However, this can also have an adverse effect on porosity and thickness of the coatings. The reputation of good oxidation resistance for bulk Ti2AlC is reinforced in this work. The oxidation study shows that bulk Ti2AlC forms protective α-Al2O3 already at 700◦C. For the coatings the oxidation behavior is much more complex as they consist of different phases. The occurrence of Al2O3 in the coatings is observed in the investigated temperature range (700-1200◦C).However it does not form a continuous layer essential for good protection of the substrate. Further work is necessary to bring the good oxidation resistance into a coating.