Abstract
The microstructure of NiAl coating formed by air plasma spraying of ball-milled NiAl30 powder, isothermally exposed at 850 °C for 6 minutes in air, is studied in this paper. An exothermic reaction starting at 490 °C was determined by DTA within the heating period performed with the rate of 10 K.min^-1. The reaction was recognized as a reaction synthesis restricted to local areas with appropriate chemical and phase composition. Metallic phases with the exception of some retained γ-Ni reacted to form NiAl phase. Average hardness of the coating increased from 130 to 450 HV 0.01. The local temperature increase induced by the exothermic reaction is responsible for the complete transformation of all amorphous Al₂O₃ oxides into crystalline oxides with spinel type structure.

Key words
NiAl coating, air-plasma spraying, microstructure, TEM studies, intermetallic phases, thermal analysis

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