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Abstract--Railguns involve both static and sliding electrical contacts, which must transmit the large transient electrical currents necessary to impart high forces onto a projectile for acceleration to hypervelocity. Static electrical contacts between metals initially take place through small asperities, or "a-spots", distributed over the contact area. The voltage developed across the interface is directly related to the contact temperature and pressure, the number of aspots, the thermophysical and mechanical properties of the contacting materials, the current history, and any interfacial materials that may be present. To physically simulate some of the conditions attained within a railgun, a pulsed current static electrical contact experimental facility has been developed at the Naval Research Laboratory. This facility employs a 500 kN capacity servohydraulic load frame equipped with an electrically insulated load train to establish a contact pressure on interfaces between metals through which a pulsed current is transmitted. The time dependent evolutions of the voltage drops across the interfaces, as detected by probes pushed into the contacting materials, are recorded during a 40 kA peak current pulse having a 300 µs rise time with peak current densities on the order of 50 kA/cm 2 . The interface stack is assembled from a 12 mm outside diameter annular disk of metal with a 6.3 mm hole which is compressed between two hollow pedestals of a second metal. The evolution of the voltage drop across an interface during a pulse will be described as a function of initial contact pressures, current density, and polarity for dissimilar (Al/Cu) metal contacts. Thermal effects on the surfaces, including localized melting of the interface materials, were also investigated. Report Documentation PageForm Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

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This article describes the field of scalable nanomanufacturing, its importance and need, its research activities and achievements. The National Science Foundation is taking a leading role in fostering basic research in scalable nanomanufacturing (SNM). From this effort several novel nanomanufacturing approaches have been proposed, studied and demonstrated, including scalable nanopatterning. This paper will discuss SNM research areas in materials, processes and applications, scale-up methods with project examples, and manufacturing challenges that need to be addressed to move nanotechnology discoveries closer to the marketplace.

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Investigations into crack formation in particle injected surfaces and into microstructural evolution during cladding were some of the recent developments in laser melt/particle injection processing. Both surface modification techniques involved blowing powder particles into a melt pool formed by a laser beam. When the particulate materials were metal carbides composite surface layers were formed and the thermal stresses that developed during cooling of the carbide injected layer resulted in the formation of microcracks. It was found that modest levels of preheating were sufficient to prevent cracking and that cracking was eliminated mainly by the reduction in thermal stresses and not by modifications in microstructure. With lower melting alloy powders clad overlays were formed. It was found that the limited mixing between the clad material and the substrate resulted in complex macrostructures containing fluid-flow driven segregation bands. The degree of microstructural in-homogeneity depended upon the type of substrate material and had a direct influence on the soundness of the cladding.

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