Thermal spray coatings of Ni–10 wt-%Al composite powder synthesised by low energy mechanical milling

Ni–10 wt-%Al composite powders, as a self-bonding spray material, were prepared by low energy mechanical milling process and then sprayed by plasma and flame spray techniques. The phase identification, microstructure, hardness and adhesion strength of the coatings were investigated by X-ray diffraction, SEM, ASTM E384-99 and ASTM C633-01 respectively. The size and microstructure of powder particles are two important parameters for thermal spraying. Morphological and microstructural investigation of the Ni–10 wt-%Al powders showed that the powder particles after 23 h of milling time had the optimum properties with respect to their size and microstructure. X-ray diffraction patterns of powder particles included only the elemental Ni and Al peaks without any traces of oxides or intermetallics phases. Powder particles produced by mechanical milling had a fine lamellar structure consisting of pure Al and Ni layers. This structure encourages the Ni/Al exothermic reaction during flight in plasma spraying. The adhesive tests also showed that the plasma and flame failure were cohesive, that is, within the coating itself, because adhesive on steel in both coatings is much higher than cohesive.