James D. Cawley

Solid freeform fabrication of ceramics

In this review, the author suggests that solid freeform fabrication is an extension of conventional manufacturing technology (made possible by advances in computing and automated shaping machinery). That is, the basic unit operations and the layered-assembly strategy have long histories, what is new is the automation thereof. It is further suggested that the key process control variables that arise when SSF is applied to ceramics are of a character that is familiar to ceramic processing in general. It is speculated that thoughtful extension of green machining practices may erode the current position of assembly-based approaches to SFF. Lastly, one under-explored application area for SFF is identified as the production of test specimens with unique and highly-controlled microstructure for scientific testing.

Oxygen-18 tracer study of the passive thermal oxidation of silicon

This work focuses on the thermal oxidation of silicon near 1273 K using the double-tracer oxidation method. The results confirm that oxidation occurs by the transport of electrically neutral non-network oxygen through the interstitial space of the vitreous silica (ν-SiO2) scale. Simultaneously, self- (or isotopic-) diffusion occurs in the network, resulting in characteristic isotopic fraction distributions near the gas-scale interface. The self-diffusion coefficients calculated from these profiles agree with those reported for tracer diffusion in ν-SiO2, and the diffusion coefficient calculated from the scale growth is consistent with reported O2 permeation data. An important parameter that describes the double-oxidation behavior is the ratio of the value of Δ/√(Dnt′),where Δ is the scale thickness grown during the second oxidation, Dn is the network self-diffusion coefficient for oxygen, and t′ is the time of the second oxidation.

Depth profiling light nuclei in single crystals: A combined nuclear reaction and RBS technique to minimize unwanted channeling effects☆

Abstract The difficulties of depth profiling in single crystals, using MeV light ion beams, are discussed with particular emphasis on planar channeling effects. Depth profiling of 18 O self-diffused crystals (corundum, Al 2 O 3 ; forsterite, Mg 2 SiO 4 ) via the 18 O(p, α ) 15 N nuclear reaction is used as an illustration. The effects on the alpha spectrum of channeling the proton beam along major axes and planes are described. There is a high probability that planar channeling effects distort the spectra strongly when the proton beam is incident on the crystals off axis but with any azimuthal orientation. A simple routine procedure is presented to minimize such unwanted effects, based on a combined use of nuclear reaction depth profiling and RBS techniques.

Thick glass film technology for polysilicon surface micromachining

This paper explores the use of thick glass films as suitable alternatives to CVD oxide films for use as sacrificial, planarization, and passivation layers in polysilicon surface micro-machining processes. Such glasses can be spin-coated to produce films up to 20 /spl mu/m thick in one step and to globally planarize the wafer surface, extending the overall mechanical design capability by enabling additional device structural complexity. Glass optical constants were determined, and the film quality was evaluated using SEM, EDS, XPS, and XRD. The films were found to have low intrinsic stresses and other characteristics desirable for sacrificial layer applications. A glass chemical-mechanical polishing process with 5300/spl sim//spl Aring//min removal rate and acceptable selectivity to polysilicon was developed, along with a wet etch chemistry that preferentially etches the film at 3.24 /spl mu/m/min without affecting the silicon substrate or the structural polysilicon. The film was used to planarize up to 10-/spl mu/m-tall topographies associated with surface micromachined features through spin-on and polish-back steps, and was in addition demonstrated to be a viable protective layer for silicon wafers during extended KOH etching in silicon bulk micro-machining processes. The glass has stable constituents that do not diffuse or contaminate either the substrate or the device features during the application and firing procedures.

Computer-Aided Manufacturing of Laminated Engineering Materials (CAM-LEM) and its Application to the Fabrication of Ceramic Components Without Tooling

Advanced ceramics such as alumina, silicon carbide and silicon nitride (monolithics and composites) have properties that suggest application in gas turbine engines. However, the production of components from these materials is very different from that typical of superalloys and this has limited the range of applications for ceramics in gas turbines. The manufacturing freedom offered by the recently developed technologies termed “rapid prototyping,” RP, or equivalently, “solid freeform fabrication,” SFF, may enable a much wider range of applications to be served in the future. RP was developed to allow production of form-and-fit models without the need for tooling and has proven to be a key assel in the design of new components as well as for the implementation of design changes to existing ones. Direct SFF using engineering materials to prototype components is undergoing continued development and is expected to provide an enabling technology that promises to change design philosophies for components made from ceramics (and other powder-based materials). In this paper, the opportunities for SFF in gas turbine applications are discussed, a brief state-of-the-art overview of RP and its application to engineering ceramics is provided, and a particular process, CAM-LEM, is highlighted.Copyright

Oxygen Diffusion in Silica and Corrosion of Ceramics

It is generally acknowledged that there are two mechanisms for oxygen transport in vitreous silica: interstitial and network. Depending on the conditions employed in the experiment, the relative contribution by each mechanism, and the degree of interaction between the two mechanisms, will vary. Prior experimental studies of tracer diffusion and permeation are reviewed in this context. Sequential oxidation experiments of silicon-based ceramics have proven useful in revealing the relative importance of each mechanism. A family of analytical and numerical solutions to the diffusion equation are presented and results from the models are discussed.

Oxidation of Carbon/Carbon through Coating Cracks

Reinforced carbon/carbon (RCC) is used to protect the wing leading edge and nose cap of the Space Shuttle Orbiter on re-entry. It is composed of a lay-up of carbon/carbon fabric protected by a SiC conversion coating. Due to the thermal expansion mismatch of the carbon/carbon and the SiC, the SiC cracks on cool-down from the processing temperature. The cracks act as pathways for oxidation of the carbon/carbon. A model for the diffusion controlled oxidation of carbon/carbon through machined slots and cracks is developed and compared to laboratory experiments. A symmetric cylindrical oxidation cavity develops under the slots, confirming diffusion control. Comparison of cross sectional dimensions as a function of oxidation time shows good agreement with the model. A second set of oxidation experiments was done with samples with only the natural craze cracks, using weight loss as an index of oxidation. The agreement of these rates with the model is quite reasonab