Nickel monoaluminide coating on ultralow-carbon steel by reactive sintering

By sintering a mixture of nickel and aluminum powders on the surface of an ultralow-carbon steel block of 15 mm in thickness, nickel monoaluminide (Ni Al) is reaction-synthesized and simultaneously joined to the steel block. W hen the thickness of the compact powder mixture is above 5 mm, an exothermi c, self-propagating high-temperature reaction of Ni + Al --> NiAl is induce d by heating to approximately 900 K, and the temperature of the compact ris es very quickly and exceeds the melting point of NiAl (1911 K). Accordingly , an in situ NiAl coating on the steel is completed in a very short time. W hen the compact thickness is below 5 mm, on the other hand, the synthesis r eaction is incomplete because of heat loss due to absorption by the steel b lock, and intermediate products such as Ni3Al and NiAl3 remain in the compa ct. However, the in situ coating is completed by further heating to 1473 K followed by holding for 0.54 ks. Across the interface between the steel and the coating, the concentrations of the elements gradually change and, acco rdingly, the Vickers hardness continuously changes from approximately 80 in the steel to 350 in the NiAl. The joint strength evaluated from a tensile test is above 150 MPa.