Effect of powder reactivity on fabrication and properties of NiAl/Al₂O₃ composite coated on cast iron using spark plasma sintering

Highlights: • Obtaining mechanically activated and reacted powders by different ball milling time of Ni, Al and NiO. • Fabrication of dense NiAl/Al₂O₃ composite coatings by spark plasma sintering of both reacted and reactive powder mixtures. • Investigation on phase evolution in diffusion bond layer at coating-substrate interface of samples. • Study on influence of powder ball milling time on microstructure, Vickers hardness and scratch hardness of coatings. • Achieving good adhesion of coating to substrate and clean interface between substrate and coating in samples. - Abstract: Powder mixtures of Ni, NiO and Al are ball milled for 1 and 10 h. X-ray diffractometry and differential thermal analysis show that while ball milling for 1 h produced mechanically activated powder; 10 h ball milling produced NiAl and Al₂O₃ phases. Dense NiAl/Al₂O₃ composite coatings are formed on gray cast iron substrate by spark plasma sintering (SPS) technique. The effect of powder reactivity on microstructure, hardness and scratch hardness of NiAl/Al₂O₃ coatings after SPS is discussed. Results show that in the coating sample made of mechanically activated powder in situ synthesis of NiAl/Al₂O₃ composite coating is fulfilled and a thicker well-formed diffusion bond layer at the interface between coating and substrate is observed. The diffusion of elements across the bond layers and phase evolution in the bond layers were investigated. No pores or cracks were observed at the interface between coating layer and substrate in any of samples. Higher Vickers hardness and scratch hardness values in coating made of 10 h ball milled powder than in coating fabricated from 1 h ball milled powder are attributed to better dispersion of Al₂O₃ reinforcement particles in NiAl matrix and nano-crystalline structure of NiAl matrix. Scratched surface of coatings did not reveal any cracking or spallation at coating-substrate interface indicating their good adherence at test conditions