Laser-sustained plasma (LSP) and CCD imaging of reactant species were employed to investigate the role of near-surface plasma in CO2 laser nitriding of titanium in open atmosphere. Insights were gained regarding the role of plasma processes and the role of reactive nitriding species in the nitriding process. Studies of single nitrided trails have identified the following regimes, as a function of LSP off-focal distance and beam translation speed, characterized by (1) the formation of heavily oxidized surfaces, (2) the formation of titanium nitride (TiN) nanoparticulate, (3) nitride formation in the absence of a surface-struck or LSP and (4) the formation of near-stoichiometric, oxide-free TiN surfaces with a LSP. For the first time it will be shown that the LSP can access nitriding conditions beyond those achieved with surface-struck plasma (or in the absence of plasma) to produce uniform, near-stoichiometric, titanium nitride coatings.

No abstract

The crystallization kinetics of laser-annealed Lamodified Pb(Zr,Ti)O₃ (PLZT) thin films on LaNiO₃-coated silicon substrates were investigated for substrate temperatures below 400 °C. A KrF excimer laser having a ~20 ns pulse width and an energy density ~40 mJ/cm² was used to crystallize the films. The perovskite phase developed with cumulative laser pulse exposures; it was found that ~380 to 400 nm thick films could be fully crystallized for a total exposure time of 0.1 to 1 ms. Laser-crystallized films exhibited comparable dielectric and ferroelectric properties to those prepared by rapid thermal annealing at 650 °C for 1 min. The evolution of the dielectric properties as a function of the number of laser strikes suggests that once nuclei are present, they rapidly grow through the depth of the film. This is consistent with the electron microscopy results, which did not show a well-defined planar growth front that proceeds from the top to the bottom of the film. The resulting films showed comparatively large lateral grain sizes (on the order of 250 to 300 nm), with high defect concentrations. The nucleation and growth mechanisms were modeled using Avrami kinetics under rate-dependent and nonisothermal conditions. These results indicate that PLZT crystallization via laser annealing is nucleation-limited.

While brittle materials such as ceramics will clearly be at the forefront of improved energy efficiency, manufacturing problems related to shaping have proven to be troublesome. Fortunately, the usage of laser machining to shape structural ceramics is increasingly gaining acceptance as an alternative to traditional grinding and cutting methods. Despite the great promise of lasers for a variety of cutting and drilling procedures, premature fractures, poor surface quality, microscale damage, and prohibitively low cutting-speeds are still among the greatest obstacles, especially as the thickness is increased. While many factors contribute to the fractures encountered during laser machining, it is the inevitable and localized increase in temperature and the ensuing thermal stresses that usually cause the damage. As such, the minimization of heat buildup and the resulting thermal stresses often requires the slow and expensive practice of multiple pass or interrupted cutting or drilling. To help control fractures and allow faster machining, a unique method of simultaneously scoring and cutting known as “prescoring” was explored using alumina plates. This concept has now been used to refine the “controlled fracture” approach, where thermal stresses are used to drive a propagating crack along a preordained path using simultaneous CO2 lasers. Using this technique and a systematic design of experiment approach to investigate the effects of various parameters, the use of the dual-beam technique was shown to be capable of predictably controlling fractures in relatively thick alumina plates. In addition to providing a clean fracture surface, this method was also shown to be capable of machining these specimens faster and with less energy input than other laser machining procedures will allow.

Faster and damage-reduced laser cutting of thick ceramics using a simultaneous prescore approach While lasers offer many advantages when machining ceramics, costly premature fractures and related damage is the tradeoff often faced by manufacturers. This tradeoff is especially apparent for higher-speed machining of complex shapes where traditional "nail-bed" supports are not always practical. To help overcome these problems, research efforts have focused on a unique method of simultaneously scoring and cutting to help control fracture. Using a customized beam delivery system, a lower-power beam simultaneously created a shallow groove or prescore directly ahead of the higher-power cutting beam. To ensure that the prescore groove is completed before fracture is likely, beam spacing was set at half of the plate width. Results for 0.59 mm thick alumina plates under exaggerated mixed-mode conditions indicated that prescoring did indeed work with markedly improved fracture surfaces when compared to traditional single-beam cuts. In fact, a complete reduction of the mixed-mode fracture was achieved with net prescore groove as shallow as 14.6 m. Modeling via a customized finite-element algorithm that includes melting/ablation indicates that the two beams can be used in relatively close proximity of 2-4 beam diameters without any significant overlaps of the independently developing stress fields.