**1. Introduction**

New technologies are invented, developed and applied following many di fferent paths. It is often di fficult to accurately describe these paths in hindsight, particularly the events critical to sustaining interest and support for the technology in the early and middle stages where its proponents are few and the ultimate use not certain. Fortunately, laser peening o ffers the opportunity to describe such a path clearly and definitively. This is possible because its invention and early development occurred within a single organization, and relatively few people and organizations were instrumental in taking it to commercial use. The insights into the phenomena vital to the success of laser shock peening can be traced to a few basic research investigations performed in the 1960s, followed by its invention in the early 1970s. It took another 40 years to become an accepted industrial process to treat metal surfaces for increasing fatigue strength and fatigue life. Along the way several critical, key events are identified. Without these events progress would have been significantly delayed or stopped completely. If any one or more of these events had not occurred, the use of laser shocks to modify material properties would still have been recognized at some point in the future, but the path would have been much di fferent. While under development, the technology was referred to as laser shock processing. It was lacking a defined target application until further understanding and development of the technology would bring one or more into focus. The first application became laser shock peening, or laser peening, to increase the fatigue strength and fatigue life of metal alloys. This was followed by laser peen forming. In the last two decades, investigations into laser shock processing have reached beyond laser peening, to include the use of laser-induced stress waves to evaluate adhesive bond strength in bonded structures and coatings, metal die forming, surface imprinting and other possible uses.

### **2. The Phenomenological Origins of Laser Processing**

After the invention of the laser, the first of the key events leading to laser shock processing was provided by Askaryan and Moroz at the P.N. Lebedev Physics Institute in 1962 [1]. In an experiment to measure the pressure exerted on a metal surface by a high intensity photon beam, they discovered that the pressure was at least several orders of magnitude greater than the calculated photon pressure. They rightfully concluded that they actually measured the vaporization recoil pressure produced by vaporization of material from the target surface by the laser beam. They further speculated that it was large enough to possibly be used to steer space vehicles.

Two years later, Neuman investigated the magnitude of the momentum transfer at constant and varying beam intensities for a number of di fferent metals, at the NASA Ames Research Center [2]. He noted that a short, 50 ns "giant" laser pulse produced a greater recoil pressure than a "normal" 1 ms laser pulse with five times the energy of the giant pulse. An observation that would later be recognized as peak pressure increasing with power density. Soon after, these findings were expanded by a number of investigators, both experimental and theoretical, pursuing studies of the creation of stress waves using lasers [3–7]. All these experiments were performed with the target residing in a vacuum chamber to avoid dielectric breakdown in the beam in air at the high power densities necessary to achieve increasing pressure. While generating high pressure laser shock waves in a vacuum was acceptable for research purposes, it would not be acceptable for industrial applications.

The path to removing this obstacle was demonstrated by the second key event, a discovery made by N.C. Anderholm at Sandia Laboratories in 1968 [8,9]. He vapor-deposited an aluminum film onto a 6 mm-thick quartz disk, irradiated this aluminum film through the 6 mm-thick quartz disk and measured the pressure profile using a piezoelectric quartz gauge pressed against the aluminum film. Irradiating the aluminum film with a 1.9 GW/cm2, 12 ns laser pulse, he measured 3.4 GPa peak pressure. Although this experiment, too, was performed in a vacuum, it clearly demonstrated that with a transparent overlay, significant shock pressures could be achieved at beam power densities not causing dielectric breakdown in air. This breakthrough observation would open the door a few years later to exploring the potential for using laser-induced shock waves as a materials processing tool.

These previous investigations were focused on studying the surface e ffects produced by the pulsed laser irradiation. Soon, investigators began looking at the e ffects of the laser-induced shock waves within the metals. In 1970, Mirkin at the M.V. Lomonosov Moscow State University realized that the higher energy, short laser pulses were capable of driving a relatively high pressure shock wave into the metal surface [10]. This suggested that the known e ffects of explosive or plate driven shock waves on metals' microstructure and hardness should also occur with laser-induced shock waves. He was the first to report the e ffects of laser-induced shocks on metal microstructure, observing twinning in steel ferrite grains located only below the laser-irradiated crater, down to a depth greater than 0.5 mm. The next year, Metz and Schmidt at the U.S. Naval Research Laboratory, investigated the e ffects of mild laser shocks, 0.18 GW/cm2, 35 ns pulse width, on annealed, 50 μm-thick nickel and vanadium foils [11]. After again annealing the irradiated foils after laser shocking, they observed vacancy voids in the nickel foils and vacancy loops in the vanadium foils. Although this irradiation condition was relatively mild, these loops were evidence of a high density of lattice vacancies created by the shock wave.

During this same period, 1968–1972, other investigators were investigating the important issue of the effect of varying the transparent overlay on the pressure enhancement observed by Anderholm. O'Keefe and Skeen at TRW Systems Group explored the use of thin volatile coatings of RTV (Room Temperature Vulcanizing) silicone adhesive and Duco cement as transparent overlays on 76 μm-thick 1100-0 aluminum targets [7]. For a 50 ns pulse of 1.8 GW/cm2, the peak pressure of the stress wave with a coating of 25 μm of the silicone adhesive was eight times higher than without the silicone coating. A 63 μm-thick coating of Duco cement increased the pressure about 15 times compared to the bare surface. With these overlays, both the plasma confinement and the vaporization of the overlay contributed to the pressure pulse. The contribution of vaporization of the overlay was deduced from the observation that increasing the curing time of the RTV, i.e., decreasing its volatility, also decreased the pressure.

### **3. The Transition to Laser Shock Processing**
