David A. Willis

Observation of nanosecond laser-induced phase explosion in aluminum

The transition from normal vaporization to phase explosion during laser ablation of aluminum was investigated using a nanosecond Nd:YAG laser. The threshold nature of phase explosion was observed by a discontinuous jump in the ablation depth at approximately 5.2J∕cm2. Ablation was imaged using a shadowgraph technique that was capable of probing ablation with nanosecond exposure time and nanosecond time delay resolution with respect to laser heating. Images above the threshold captured a mixture of vapor and droplets generated by phase explosion, which began near the end of the laser pulse without a significant time lag.

Heat transfer and phase change during picosecond laser ablation of nickel

Abstract This work investigates heat transfer and phase change during picosecond laser ablation of nickel. In this study, ablation of nickel is studied using a mode-locked 25 ps (FWHM) Nd:YAG laser. The threshold fluence for mass removal (ablation) is experimentally determined. Numerical calculations of the transient temperature distribution and kinetics of the solid–liquid and liquid–vapor phase change interfaces are performed. The results show that evaporation is negligible at the free surface, resulting in superheating of liquid to near 0.9Tcr, at which temperature homogeneous nucleation will result in an explosive phase transformation, removing part of the molten layer.

Time-resolved dynamics of nanosecond laser-induced phase explosion

Visualization of Nd : YAG laser ablation of aluminium targets was performed by a shadowgraph apparatus capable of imaging the dynamics of ablation with nanosecond time resolution. Direct observations of vaporization, explosive phase change and shock waves were obtained. The influence of vaporization and phase explosion on shock wave velocity was directly measured. A significant increase in the shock wave velocity was observed at the onset of phase explosion. However, the shock wave behaviour followed the form of a Taylor–Sedov spherical shock below and above the explosive phase change threshold. The jump in the shock wave velocity above phase explosion threshold is attributed to the release of stored enthalpy in the superheated liquid surface. The energy released during phase explosion was estimated by fitting the transient shock wave position to the Taylor scaling rules. Results of temperature calculations indicate that the vapour temperature at the phase explosion threshold is slightly higher than the critical temperature at the early stages of the shock wave formation. The shock wave pressure nearly doubled when transitioning from normal vaporization to phase explosion.

Non-Equilibrium Phase Change in Metal Induced by Nanosecond Pulsed Laser Irradiation

Materials processing using high power pulsed lasers involves complex phenomena including rapid heating, superheating of the laser-melted material, rapid nucleation, and phase explosion. With a heating rate on the order of 10 9 K/s or higher, the surface layer melted by laser irradiation can reach a temperature higher than the normal boiling point. On the other hand, the vapor pressure does not build up as fast and thus falls below the saturation pressure at the surface temperature, resulting in a superheated, metastable state. As the temperature of the melt approaches the thermodynamic critical point, the liquid undergoes a phase explosion that turns the melt into a mixture of liquid and vapor. This article describes heat transfer and phase change phenomena during nanosecond pulsed laser ablation of a metal, with an emphasis on phase explosion and non-equilibrium phase change. The time required for nucleation in a superheated liquid, which determines the time needed for phase explosion to occur, is also investigated from both theoretical and experimental viewpoints.

Transport Phenomena and Droplet Formation During Pulsed Laser Interaction With Thin Films

This work investigates transport phenomena and mechanisms of droplet formation during a pulsed laser interaction with thin films. The surface of the target material is altered through material flow in the molten phase induced by a tightly focused laser energy flux. Such a process is useful for developing a laser-based micromachining technique. Experimental and numerical investigations of the laser-induced fluid flow and topography variations are carried out for a better understanding of the physical phenomena involved in the process. As with many machining techniques, debris is often generated during laser-material interaction. Experimental parametric studies are carried out to correlate the laser parameters with the topography and droplet formations. It is found that a narrow range of operation parameters and target conditions exists for clean structures to be fabricated. The stop action photography technique is employed to capture the surface topography variation and the melting development with a nanosecond time resolution and a micrometer spatial resolution. Numerical simulations of the laser-induced surface deformation are also performed to obtain the transient field variables and to track the deforming surface. The comparison between the numerical and experimental work shows that, within the energy intensity range investigated in this work, the surface deformation and droplet formation are attributed to the surface-tension-driven flow, and the recoil pressure effect plays an insignificant role in the surface topography development.

Thermocapillary flow and rupture in films of molten metal on a substrate

We consider fluid flow in thin films of molten metal resulting from irradiation by a Gaussian laser beam. Surface tension gradients due to nonuniform heating induce a flow of the molten liquid away from the center of the irradiated area, leading to formation of dry areas on the substrate. We develop a mathematical model of the flow under the assumption of the large ratio of laser beam radius to film thickness. The model extends the standard lubrication-type analysis to include the highly nonlinear dependence of evaporative flux on local interfacial temperature, unsteady heat conduction in the substrate, and positive disjoining pressure due to unbalanced contributions from the kinetic energy of free electrons in the metal. The latter is proportional to the inverse square of the film thickness. We identify thermocapillary stresses as the main mechanism of rapid removal of liquid metal from the irradiated area. Characteristic times of the process, as well as shapes of the molten region surface, agree with ex...

Microdroplet deposition by laser-induced forward transfer

A unique form of Laser-Induced Forward Transfer (LIFT) has been developed that is capable of depositing single micrometer-sized droplets. LIFT was performed using a 7 ns Nd:YAG laser operating at 1064 nm. One micron films of aluminum and nickel supported on glass donor substrates were used as samples. Films were irradiated at the interface between the film and donor substrate using the standard LIFT technique. At fluences slightly above the melting threshold, single droplets were transferred to the acceptor substrate, with deposit sizes between 1 and 2 microns. This is significant since the laser beam diameter (> 12 microns) is much larger than the deposited droplets. SEM images of the original donor films after laser irradiation indicated a re-solidified melt pool with a raised bump at the center, the point of ejection of the transferred droplets. The physical origins of the droplet formation and transfer are unclear, but appear to be a result of the combined effects of surface tension and volumetric expansion during the solid-liquid phase change process.

Laser micromachining of indium tin oxide films on polymer substrates by laser-induced delamination

A Q-switched neodymium : yttrium–aluminium–garnet (Nd : YAG) laser was used to ablate indium tin oxide (ITO) thin films from polyethylene terephthalate substrates. Film damage and partial removal with no evidence of a melt zone was observed above 1.7 J cm−2. Above the film removal threshold (3.3 J cm−2) the entire film thickness was removed without substrate damage, suggesting that ablation was a result of delamination of the film in the solid phase. Measurements of ablated fragment velocities near the ablation threshold were consistent with calculations of velocities caused by stress-induced delamination of the ITO film, except for a high velocity component at higher fluences. Nanosecond time-resolved shadowgraph photography revealed that the high velocity component was a shock wave induced by the rapid compression of ambient air when the film delaminated.

HEAT TRANSFER, PHASE CHANGE, AND THERMOCAPILLARY FLOW IN FILMS OF MOLTEN METAL ON A SUBSTRATE

A mathematical model has been developed for heat transfer and fluid flow in thin films of molten metal during nanosecond pulsed laser irradiation. Heat conduction in the substrate is modeled using the finite-difference approach, while description of heat transfer and viscous flow in the film is based on the assumption of the large ratio of laser beam radius to film thickness and involves numerical solution of a partial differential equation for the thickness. The model includes the highly nonlinear dependence of evaporative flux on local interfacial temperature and positive disjoining pressure due to free electrons in the metal. Thermo-capillary stresses which result from radially nonuniform heating are identified as the main mechanism of removal of liquid metal from the irradiated area. Characteristic times of the process, as well as shapes of the molten surface, agree with experimental observations.

LASER-ASSISTED SURFACE MODIFICATION OF THIN CHROMIUM FILMS

The objective is to investigate a process by which micrometer scale topographical changes are produced on thin chromium films using a pulsed Nd:YLF laser. The surface of chromium films is altered through laser-induced solid-liquid phase transformation and fluid flow. Experimental parametric studies are conducted to correlate the laser parameters with the topography of the laser irradiated surfaces. Experimental and analytical work is also performed to study the transport phenomena involved in the process. A numerical finite-element analysis is used to simulate the transient field variables. A nanosecond-time-resolution, fast photography system is constructed to capture the phase change and the fluid flow occurring at the target surface. The experimental and the numerical studies showed that the surface topography change was caused by the laser-induced surface-tension-driven flow, and the recoil pressure due to surface evaporation had negligible effect on the topography variation.