Finite element analysis (FEA) was performed on a bi-layer cylindrical structure consisting of a low-density layer on top of a high-density layer. For this model, the layers used the shrinkage behavior, viscosity, and elastic properties of barium titanate determined for the 45% and 55% green densities. The stresses predicted by FEA showed good agreement with stresses predicted using analytical equations for a linear viscous bi-layer cylinder. The model was then extended to use more complex density gradients measured by X-ray computed tomography on a bilayer compact. In this case, the shrinkage behavior and viscosity properties were extrapolated from the experimental data. In the subsequent simulation, the stresses and strains were predicted during sintering. For the bi-layer structure studied, a highly stressed region was identified on the free surface of the sintering compact and this was shown to lead to edge cracking during densification. 3027J ournal

Specimens of heated, open-cell ceramics were thermallyshocked by immersion in water or oil. It was found the strength retained after thermal shock underwent a gradual decrease with increasing quench temperature, indicative of a cumulative damage mechanism which manifests itself with increasing thermal stress. This damage could also be monitored using measurements of the elastic constants before and after quenching. The thermal shock resistance of the open-cell materials was found to be strongly dependent on cell size (increased with increasing cell size) and weakly dependent on density (increased with increasing density). Two possible sources of thermal stress were considered; one was associated with the temperature gradient across the microscopic struts and the other with the heating of the quenching medium as it infiltrates the cellular structure. Such heating was confirmed and it was concluded that this was the dominant source of thermal stress in this particular study, controlling the thermal shock resistance of the open-cell ceramics. [

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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.

Titanium and its alloys possess several attractive properties that include a high strength-to-weight ratio, biocompatibility, and good corrosion resistance. However, due to their poor wear resistance, titanium components need to undergo surface hardening treatments before being used in applications involving high contact stresses. Laser nitriding is a thermochemical method of enhancing the surface hardness and wear resistance of titanium. This technique entails scanning the titanium substrate under a laser beam near its focal plane in the presence of nitrogen gas flow. At processing conditions characterized by low scan speeds, high laser powers, and small off-focal distances, a nitrogen plasma can be struck near the surface of the titanium substrate. When the substrate is removed, this plasma can be sustained indefinitely and away from any potentially interacting surfaces, by the laser power and a cascade ionization process. This paper presents a critical review of the literature pertaining to the laser nitriding of titanium in the presence of a laser-sustained plasma, with the ultimate objective of forming wide-area, deep, crack-free, wear-resistant nitrided cases on commercially pure titanium substrates.