In this article, we present summaries of the evolution of surface morphology resulting from the irradiation of single-crystal silicon with femtosecond laser pulses. In the first section, we discuss the development of micrometer-sized cones on a silicon surface irradiated with hundreds of femtosecond laser pulses in the presence of sulfur hexafluoride and other gases. We propose a general formation mechanism for the surface spikes. In the second section, we discuss the formation of blisters or bubbles at the interface between a thermal silicon oxide and a silicon surface after irradiation with one or more femtosecond laser pulses. We discuss the physical mechanism for blister formation and its potential use as channels in microfluidic devices.

Propagating reactions in initially planar cobalt/aluminum exothermic multilayer foils have been investigated using high-speed digital photography. Real-time observations of reactions indicate that unsteady (spinlike) reaction propagation leads to the formation of highly periodic surface morphologies with length scales ranging from 1 μm to 1 mm. The characteristics of propagating spinlike reactions and corresponding reacted foil morphologies depend on the bilayer thickness of multilayer foils.

Several binary intermetallic compounds-each containing a rare-earth (RE) element paired with a transition metal (TM)-were prepared by self-propagating, high-temperature synthesis (SHS). Thin multilayers, composed of alternating Sc or Y (RE element) and Ag, Cu, or Au (TM), were first deposited by direct current magnetron sputtering. Once the initially distinct layers were stimulated and caused to mix, exothermic reactions propagated to completion. X-ray diffraction revealed that Sc/Au, Sc/Cu, Y/Au, and Y/Cu multilayers react in vacuum to form single-phase, cubic B2 structures. Multilayers containing Ag and a RE metal formed cubic B2 (RE)Ag and a minority (RE)Ag 2 phase. The influence of an oxygen-containing environment on the reaction dynamics and the formation of phase were investigated, providing evidence for the participation of secondary combustion reactions during metal-metal SHS. High-speed photography demonstrated reaction propagation speeds that ranged from 0.1-40.0 m/s (dependent on material system and foil design). Both steady and spin-like reaction modes were observed.

Nickel/titanium nanolaminates fabricated by sputter deposition exhibited rapid, high-temperature synthesis. When heated locally, self-sustained reactions were produced in freestanding Ni/Ti multilayer foils characterized by average propagation speeds between ∼0.1 and 1.4 m/s. The speed of a propagating reaction front was affected by total foil thickness and bilayer thickness (layer periodicity). In contrast to previous work with compacted Ni–Ti powders, no preheating of Ni/Ti foils was required to maintain self-propagating reactions. High-temperature synthesis was also stimulated by rapid global heating demonstrating low ignition temperatures (Tig)∼300–400 °C for nanolaminates. Ignition temperature was influenced by bilayer thickness with more coarse laminate designs exhibiting increased Tig. Foils reacted in a vacuum apparatus developed either as single-phase B2 cubic NiTi (austenite) or as a mixed-phase structure that was composed of monoclinic B19′ NiTi (martensite), hexagonal NiTi2, and B2 NiTi. Single-phase, cubic B2 NiTi generally formed when the initial bilayer thickness was made small.

Femtosecond (fs, 10 -15 second) laser ablation of single-crystal nickel-base superalloy CMSX-4 under laser fluences ranging from 0.1 to 160 J/cm 2 has been investigated. For comparison, the ablation behaviors of Ni and Ni 3 Al as individual components of the superalloy have also been studied. Two distinct ablation regimes were observed, dependent on the incident laser fluence. The ablation threshold fluences for these two ablation regimes were determined to be / th1 = 0.30 ± 0.03 and / th2 = 5.3 ± 0.5 J/cm 2 in the superalloy. Ablation thresholds for the single-phase Ni and Ni 3 Al alloys were close to those measured for two-phase superalloy. The two distinct surface morphologies produced by ablation in these regimes were characterized in detail by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The ablation rate equations for the two regimes are presented and the corresponding ablation mechanisms are discussed. The implications for application of fs pulsed lasers to machining of superalloys are also discussed.