The role of the Al₂O₃ passivation shell surrounding nano-Al particles in the combustion synthesis of NiAl

The self-propagating combustion behaviors of Nickel (Ni) and Aluminum (Al) thermites were studied as a function of bimodal Al particle size distributions. In particular, the low melting temperature of nano-scale Al particles coupled with the low concentrations of Al₂O₃ in micron-scale Al particles were exploited in order to optimize the macroscopic properties of the final alloy. Bimodal Al size distributions ranging from 0 to 50 wt% nano-Al combined with 50 wt% Ni were studied. Laser ignition experiments were performed on pressed pellets to determine flame propagation behavior and product microstructural features as a function of Al particle size. A new imaging technique is also presented that allows visualization of the surface reaction through highly luminescent flames and more accurate evaluation of burn rates. The wear behavior of the product alloy was measured and reported. Results show that composites composed of more micron-scale than nano-scale Al particles absorb more laser energy prior to flame propagation and experience an effective preheating. When 10-30 wt% nano Al is combined with micron AI and Ni, the wear resistance of the product alloy is optimized. Electron micrographs of the alloys suggest these properties may be attributed to whisker formations that behave as binding strings improving the overall abrasion resistance of the composite.