Application of Design of Experiments to Extrusion Freeform Fabrication (EFF) of Functional Ceramic Prototypes

Extrusion Freeform Fabrication (EFF) is an adaptation of the Stratasys Fused Deposition Modeling (FDM) process for the Solid Freeform Fabrication (SFF) of functional ceramic prototypes. It is a complex process involving many process variables, including parameters that are operation, machine, materials, and geometry specific. A Taguchi factorial Design of Experiments (DOE) technique was utilized to study the effects of machine specific process parameters as well as their interactions based on the mechanical and physical properties of sintered ceramics specimens. Post-processing software was developed to control and modify these parameters. This software interface wasdesigned to mimic the Quickslice™ interface for setting motion parameters based upon the material and the operation. The results of this investigation provided useful information for the experimental analysis of the machine specific process parameters. Suitable parameters were selected for the EFF process for fabricating representative ceramic prototypes. With the optimized parameters, complicated parts were successfully fabricated using both Kyocera SN282 and Starck M-11 silicon nitride powders.Mechanical Engineerin

Use of indentation technique to measure elastic modulus of plasma-sprayed zirconia thermal barrier coating

Elastic modulus of an yttria partially stabilized zirconia (YSZ) thermal barrier coating (TBC) was evaluated with a Knoop indentation technique. The measured elastic modulus values for the coating ranged from 68.4 {+-} 22.6 GPa at an indentation load of 50 g to 35.7 {+-} 9.8 at an indentation load of 300 g. At higher loads, the elastic modulus values did not change significantly. This steady-state value of 35.7 GPa for ZrO{sub 2} TBC agreed well with literature values obtained by the Hertzian indentation method. Furthermore, the measured elastic modulus for the TBC is lower than that reported for bulk ZrO{sub 2} ({approx} 190 GPa). This difference is believed to be due to the presence of a significant amount of porosity and microcracks in the TBCs. Hardness was also measured

Thermal-shock behavior of advanced ceramic/composite hot-gas filters

The thermal shock/fatigue behavior of monolithic and composite hot-gas candle filters obtained from various manufacturers was evaluated. The composite filters were made of both oxide and nonoxide materials; the monolithic filters were made only of nonoxide materials. During single-cycle thermal shock tests, composite filters show little or no strength degradation when quenched from temperatures between 900 and 1000{degrees}C. At higher quenching temperatures, slow strength degradation was observed. Regular monolithic SiC filters showed no strength degradation when quenched from temperatures up to {approx}700-900{degrees}C, whereas at higher quenching temperatures, the strength decreased at a relatively sharper rate. On the other hand, recrystallized monolithic SiC filters showed higher initial strength and retained this strength to higher quenching temperatures when compared with regular SiC filters. This finding may be related to the difference in the strength of grain boundary phases in the two filters. For thermal cycles between room temperature and 800-1000{degrees}C, composite filters show little (18-24%) strength degradation up to three cycles, beyond which the strength remains unchanged. Similar behavior, with an initial strength drop of 15-28%, was observed for monolithic filter specimens that were thermally cycled between room temperature and 800{degrees}C

Effect of fiber architecture on mechanical behavior of SiC(f)/SiC composites

We evaluated mechanical properties (first matrix cracking stress, strength, and work-of-fracture) of Nicalon-fiber-reinforced silicon carbide matrix composites with three different fiber lay-up sequences (0{degrees}/20{degrees}/60{degrees}, 0{degrees}/40{degrees}/60{degrees}, and 0{degrees}/45{degrees}) at various temperatures from room to 1300{degrees}C. Up to 1200{degrees}C, ultimate strength and work-of-fracture for the 0{degrees}/40{degrees}/60{degrees} and 0{degrees}/45{degrees} composites increased, but then declined at 1300{degrees}C. The decreases were correlated to in-situ Nicalon fiber strength and fiber/matrix interface degradation. However, for the 0{degrees}/20{degrees}/60{degrees} composites, ultimate strength and work-of-fracture reached their a minima at 1200{degrees}C. These measured ultimate strengths at room and 1300{degrees}C were correlated to the predictions made with an analytical model and to in-situ fiber strength characteristics. The large difference in room-temperature ultimate strengths between the three sets of composites is attributed to the relative contributions of the off-axis fibers to the load-bearing capacity of each composite

Effects of flaws on fracture behavior of structural ceramics

We evaluated the effects of fiber coating thickness, fiber orientation, and elevated temperature on flaw morphology and mechanical properties of Nicalon-fiber-reinforced SiC matrix composites with fiber cloth lay-up sequences of 0{degree}/45{degrees}, 0{degree}/20{degrees}/60{degrees}, and 0{degree}/40{degrees}/60{degrees} and fiber coating thicknesses of 0. 2 and 0.4{mu}m. For the three fiber cloth lay-up sequences (0{degree}/45{degrees}, 0{degree}/20{degrees}/60{degrees}, and 0{degree}/40{degrees}/60{degrees}), mechanical property (first matrix cracking stress, ultimate stress, and work of fracture) initially increase with coating thickness and reach peak values at a coating thickness of 0.2{mu}m. A further increase in coating thickness does not result in further improvements in mechanical properties; this is related to the role of coatings in protecting fibers from damage during composite processing. Measured values of strength and work-of- fracture of the above composites at elevated temperatures increased with temperature up to 1200{degrees}C, but decreased at higher temperatures. This decrease is correlated to in-situ fiber strength and fiber/matrix interface degradation. Correlations between model prediction and measured room-temperature ultimate strength of composites with 0{degree}45{degrees} and 0{degree}/40{degrees}/60{degrees} lay-up sequences were established by using in-situ fiber strength characteristics. The failure modes and degradation mechanisms in hot-gas filters and ceramic composite joints are being characterized by the mechanical and fractographic evaluation techniques established thus far. Correlations of these results with those of nondestructive evaluation can provide critical information for improved quality control

Neutrino Spectroscopy of the Early Phase of Nearby Supernovae

Neutrinos emitted during stellar core collapse up to their trapping phase carry information about the stage from which the Supernova explosion process initiates. The dominant $\nu_e$ emission mechanism is by electron capture on free protons and f-p shell nuclei and the spectrum of these neutrinos is a function of the ambient physical conditions within the core as well as the nuclear equation of state. The number of collapse phase $\nu_e$ which can be detected by Super-Kamioka and Sudbury Neutrino Observatory from a Supernova within 1 kpc, and their generic energy spectra are given.Comment: 9 pages of text and tables plus 2 pages of figures. Accepted for publication in Phys. Rev. Lett. on 11th Jul., 1997. Please e-mail Comments etc. to [email protected]

Oscillation Effects On Neutrinos From The Early Phase Of a Nearby Supernova

Neutrinos emitted during stellar core collapse leading to a supernova are primarily of the electron neutrino type at source which may undergo oscillation between flavor eigenstates during propagation to an earth-bound detector. Although the number of neutrinos emitted during the pre-bounce collapse phase is much smaller than that emitted in the post-bounce phase (in which all flavors of neutrinos are emitted), a nearby supernova event may nevertheless register a substantial number of detections from the pre-bounce phase at SuperKamiokande (SK) and the Sudbury Neutrino Observatory (SNO). The calorimetric measurement of the supernova neutrino fluence from this stage via the charge current and neutral current detection channels in SNO and the corresponding distortion of detected spectrum in SK over the no-oscillation spectrum, can probe information about neutrino mass difference and mixing which are illustrated here in terms of two- and three-flavor oscillation models

A rare case of suture material obstructing the closure mechanism of a prosthetic aortic valve: a case report

Prosthetic aortic valve dysfunction presenting as aortic regurgitation is a complication of mechanical valve replacement. We describe a case of late valve dysfunction caused by an annular suture of excessive length obstructing the closure mechanism of a bileaflet prosthetic valve