No abstract

Homogeneous nucleation of diamond powder is reported. The experiments were performed in a low-pressure microwave-plasma reactor. The deposits were collected downstream of the reaction zone and subjected to wet oxidation to remove nondiamond carbons. The residues were analyzed by optical and electron microscopy, electron diffraction, and Raman spectroscopy. A variety of hydrocarbons diluted in argon, hydrogen, or oxygen gas mixtures were tested. In most cases only nondiamond materials, like graphite and carbyne, were obtained. Homogeneous nucleation of diamond was clearly observed in dichloromethane- and trichloroethylene-oxygen mixtures. The particles formed had crystalline shapes, mostly hexagonal. The largest particles were about 0.2 μm, although most of the particles were on the order of 50 nm in diameter. The powder was identified to be a mixture of polytypes of diamond.

A series of diamond polytype structures are described and their IR and Raman vibrational modes predicted. The diamond polytypes are analogous to the well-known silicon carbide polytypes. The intermediate 6H diamond polytype was recently identified by single crystal electron diffraction of vapor precipitated diamond powder. In addition, end member polytypes of 3C (cubic diamond) and 2H diamond (hexagonal lonsdaleite) have been previously established, and polytypes such as 4H, 8H, 15R, and 21R diamond are predicted, but may be difficult to isolate and identify. The various diamond polytype structures differ only in the stacking sequences of identical puckered hexagonal carbon layers. These identical carbon layers lie parallel to the cubic 3C {111} and the hexagonal 2H {001} planes. A new method for uniquely labeling the structural layers in the polytype stacking sequences is presented. Factor group analysis was used to determine the IR and Raman selection rules for five diamond polytypes with structures intermediate between those of end members diamond and lonsdaleite. Brillouin zone folding techniques were used to determine band positions, in analogy with analyses of SiC polytypes discussed in the literature. The results predict that (i) all diamond polytypes are Raman active, (ii) limiting polytypes 3C and 2H are not IR active, and (iii) polytypes 4H, 6H, 8H, 15R, and 21R have IR active modes.

The Army Research Laboratory (ARL) was asked to participate in an OSD-funded erosion effort by the Coating Technology Integration Office at Wright Patterson Air Force Base. Solid particle (sand) erosion testing was conducted by the University of Dayton Research Institute to determine the erosion resistance of materials currently used on the leading edges of Army aviation rotor blades of aircraft in Southwest Asia (SWA). This testing and evaluation was important for two reasons; first, Iraq and Afghanistan are the primary locations of our current anti-terror operations, and second, the sands within these two countries are the worst in the world from an erosion standpoint (dry conditions ? freshest grains of sand ? predominantly angular quartz grains ? blowing winds). The sand utilized herein is considered even more erosive than the sand from these two countries, since they contain a higher concentration of quartz than the SWA sand. In 2005, observations of actual SWA field failures of helicopter rotor blade protective tapes and coatings were compared to existing state-of-the-art, laboratory-based sand erosion data during a U.S. Army sponsored program. Laboratory-produced data did not match the severity of field-use damage, even under extremely high levels of particle loading. The need to test to erosive failure representative of this environment was determined to be paramount in establishing relative performance levels of erosion resistant protective systems being screened for potential field use. The goal of this effort was to provide two synthetic sand formulas capable of testing various polymer-based candidate rotor blade protective systems to failure. The test media was derived from characterization of sand and dust materials unique to SWA. The synthetic sand mixtures developed by this effort will be incorporated in a new test protocol for sand erosion to represent a truly ''worst case'' test, with extended application to other aerospace components susceptible to sand erosion damage applicable to Department of Defense activities in most dry-hot desert regions. Comprehensive post-test analysis performed by ARL included: visual examination, mass loss calculations, erosion rate determination, surface roughness testing, volume loss calculations, scanning electron microscopy characterization, and metallography. As a result of post-test analysis, many trends were observed, with the results documented herein. The results of this testing have been used as a baseline for future testing of alternative materials and coating systems, and to prepare a solid particle erosion test standard (MIL-STD-3033).