Basic aspects of ion beam modifications of materials are being investigated. These studies include ion beam mixing, defect production, radiation-enhanced diffusion, and ion beam-assisted film growth. The work combines molecular dynamics computer simulation with experimental studies using MBE grown films and 1 keV to 3 MeV ion beams and various surface analysis methods. Metals, intermetallic compounds, oxide ceramics, and compound semiconductors are of interest.

Basic aspects of diffusion and radiation-induced defects in model ceramic oxide materials are investigated. Specimens especially grown by MBE methods, tailored to specific experiments, ion beam analysis, and x-ray diffraction and SIMS comprise the methods of study.

Nanophase processing is a novel technique by which metallic and ceramic materials can be produced in the form of ultrafine powders with sizes in the range of 5 to 50 nm. The resulting powder particles, called nanophase or nanocrystalline powder, can be cold compacted at or near room temperature to near theoretical density. The novel microstructure of ceramics thus produced can impart several useful engineering properties. Ongoing research on these nanophase ceramics include sintering kinetics, fracture strength and toughness, and superplastic deformation. The goal of the research program is to identify useful engineering properties of the nanophase ceramics and to understand their structure and property relationships. Ball mixing, inert gas condensation, and computer simulation are employed.

The structure of nanoparticles and their interactions with one another and with substrates are examined on an atomic scale using high-resolution transmission electron microscopy and molecular dynamics computer simulations. Sintering of small assemblies of nanoparticles, epitaxial relationships between the particles and substrates, and the dependence of size on alloy phase stability are investigated.

This research involves preparation of chemically stabilized B-crystobalite powders synthesized by the solution polymerization technique employing Pechini resin and PVA solution as a polymeric carrier and consideration of mullite-cordierite composites with B-crystobalite interfaces showing the optimum phase transformation weakening behavior at the laminate/matrix interface. We are investigating grain size effects on the B-> (a transformation of cristobalite and fabricating laminate structure by the tape-casting process including the control of thermal and co-firing conditions between them.

The greatest hindrance to the increased use of oxide-oxide composites for high-temperature applications is the failure of the matrix and reinforcement to debond adequately during fracture. To overcome this problem, several different types of oxide interfacial materials (or interphases) are being developed. The primary approach is to use a material that can undergo a stress-induced phase transformation at the interphase. When a matrix crack approaches the interphase, the stress field of the crack causes the interphase to undergo a volume decreasing phase transformation. This volume change produces local disintegra tion of the interphase and consequently debonding between the matrix and reinforcement.

High-temperature oxidation limits the applications of the strong and tough nonoxide ceramic fiber-reinforced composites. Oxide fiber/oxide matrix composites can be the solution to this problem if there is an oxidation-resistant weak interface between the fiber and matrix. In this research, the tape-cast laminates, sandwich specimens, and fiber model systems have been fabricated and studied. The preliminary results have been achieved by applying a weak interphase (LaPO₄ or YPO₄) to weaken the fiber/matrix interface. Interfacial properties have been evaluated by the indentation technique, flexural test, and fiber push-out test.

Modern ceramic fabrication techniques are being applied to inexpensive raw materials to produce strong lightweight building panels. Emphasis is on common clays, fly ash, bottom ash, and diatomaceous earth as raw materials. Lignin sulphonate, a by-product of the paper industry, is used as a foam-stabilizing agent in fabricating foamed ceramic construction materials. Sol incorporation and gellation, with careful control of pH and rheology, is similarly being evaluated in fabrication, as is phosphate bonding. Combustible additives, such as sawdust, and expansive additives, such as vermiculite, are also being studied as means of decreasing density. Extruded honeycomb structures will be produced and evaluated.

In the first phase of this investigation, several techniques for sample preparation and microanalysis were developed. In the second phase, detailed microstructural and microchemical investigation of fibers and matrices is being performed. In particular, the amorphous/crystalline nature of the materials and the presence of second phases are being studied. The effect of various processing conditions and exposure to various environments on fibers and fiber/matrix interfaces is being investigated using a variety of microanalytical techniques.

T he potential use of covalent compounds, such as AlN, BN, and BeO, as well as various transition metal carbides, nitrides, and borides, as alloying additives to hot-pressed silicon carbide and as composite-forming particulates is being investigated. STEM/EDX, Auger electron spectroscopy, and other techniques are being used to determine phase equilibria, diffusivities, and transformation mechanisms.

We are investigating mechanisms of high-temperature oxidation of hot-pressed SiC-AlN compositions. The dense materials are either composites or solid solutions, depending on processing conditions. The oxidation is strongly affected by the formation and further reaction of several layers found at the outer surface, such as SiAlON and mullite, and by micro structural factors. High-resolution and analytical TEM, as well as SEM and XRD, are used in the investigation.

Materials in the Si,Al,O,N system have potential for engineering applications at both high and low temperatures. Creating two-phase O' + ß' SiAlON materials can help achieve good mechanical strength combined with good oxidation resistance. In this project, we week to understand the microstructural features of these materials in order to facilitate control of their critical properties.