Effects of oxidation on reaction front instabilities and average propagation speed in Ni/Ti multilayer foils

Vapor-deposited, equiatomic Ni/Ti multilayer foils exhibit low-speed, self-propagating formation reactions that are characterized by a spin-like reaction front instability. In addition to the intermetallic reaction between Ni and Ti, reactions performed in air can also exhibit a discrete combustion wave associated with the oxidation of Ti. In general, the oxidation wave trails the complex intermetallic reaction front. Multilayers that have a large reactant layer periodicity (≥200 nm) exhibit a decrease in net reaction speed as air pressure is reduced. Oxidation has a much smaller effect on the net propagation speed of multilayers with small layer periodicity (<100 nm). The net propagation speed of the multilayers is increased when air is present, due to the added energy release of Ti oxidation. High-speed optical microscopy shows that the increased front speed is associated with an increased nucleation rate of the reaction bands that typify the spinning reaction instability of the Ni/Ti system.