tPowder mixtures of Ni, NiO and Al are ball milled for 1 and 10 h. X-ray diffractometry and differentialthermal analysis show that while ball milling for 1 h produced mechanically activated powder; 10 h ballmilling produced NiAl and Al2O3phases. Dense NiAl/Al2O3composite coatings are formed on gray castiron substrate by spark plasma sintering (SPS) technique. The effect of powder reactivity on microstruc-ture, hardness and scratch hardness of NiAl/Al2O3coatings after SPS is discussed. Results show that in thecoating sample made of mechanically activated powder in situ synthesis of NiAl/Al2O3composite coatingis fulfilled and a thicker well-formed diffusion bond layer at the interface between coating and substrateis observed. The diffusion of elements across the bond layers and phase evolution in the bond layers wereinvestigated. No pores or cracks were observed at the interface between coating layer and substrate inany of samples. Higher Vickers hardness and scratch hardness values in coating made of 10 h ball milledpowder than in coating fabricated from 1 h ball milled powder are attributed to better dispersion of Al2O3reinforcement particles in NiAl matrix and nano-crystalline structure of NiAl matrix. Scratched surfaceof coatings did not reveal any cracking or spallation at coating-substrate interface indicating their goodadherence at test conditions.

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