Thermal stability of plasma-sprayed NiAl/CrB2 composite coatings investigated through in-situ TEM heating experiment

Nickel aluminide coatings - produced using atmospheric plasma spraying method - attracted considerable attention due to their high melting temperature and oxidation resistance. What is more, an addition of CrB2 allows to significantly increase their wear resistance. Such coatings are typically used up to 500 °C, what requires thermal stability of their microstructure. Therefore, in this paper, the in-situ TEM heating of APS-deposited NiAl coating with addition of 15 wt% of CrB2 up to 800 °C was performed in order to follow the microstructure changes and eventual phase transformations. The TEM observations backed with phase analysis showed that the amorphous veins present in the NiAl matrix of as-deposited coating transform into Ni6AlB3 phase at approaching 500 °C. On the other hand, the CrB2/NiAl interface preserved its shape and remains free from any precipitates even up to 800 °C. The ex-situ TEM microstructure observations of NiAl + CrB2 coating, carried out after high-temperature wear test (500 °C, ~15 min), showed that after it amorphous channels indeed turns crystalline. In addition, the NiAl matrix presents significantly higher dislocation density, being a result of repeated loading received during the wear test.