Deidre A. Hirschfeld

The effect of a simple annealing heat treatment on the mechanical properties of cold-sprayed aluminum

Cold spray, a new member of the thermal spray process family, can be used to prepare dense, thick metal coatings. It has tremendous potential as a spray-forming process. However, it is well known that significant cold work occurs during the cold spray deposition process. This cold work results in hard coatings but relatively brittle bulk deposits. This work investigates the mechanical properties of cold-sprayed aluminum and the effect of annealing on those properties. Cold spray coatings approximately 1 cm thick were prepared using three different feedstock powders: Valimet H-10: Valimet H-20: and Brodmann Flomaster. ASTM E8 tensile specimens were machined from these coatings and tested using standard tensile testing procedures. Each material was tested in two conditions: as-sprayed; and after a 300°C, 22h air anneal. The as-sprayed material showed high ultimate strength and low ductility, with <1% elongation. The annealed samples showed a reduction in ultimate strength but a dramatic increase in ductility, with up to 10% elongation. The annealed samples exhibited mechanical properties that were similar to those of wrought 1100 H14 aluminum. Microstructural examination and fractography clearly showed a change in fracture mechanism between the as-sprayed and annealed materials. These results indicate good potential for cold spray as a bulkforming process.

Effect of Surface Modification on Adhesion of a Metal Bond Coat to PMR Composites

Graphite-reinforced polyimide composites are being considered for use in advanced turbine engines, which require the application of wear- and oxidation-resistant coatings onto the composites. To ensure in-service adhesion of a hard, protective ceramic or cermet coating to these composites, it is first necessary to apply a metal bond coat This study examines the effect that a silica layer has on the adhesion of a proprietary metal bond coat to PMR-15 and PMR-II-50 composite surfaces. Previous work has shown that radio frequency (RF) oxygen plasma etching was the most effective technique for increasing the surface energy of the composites and plasma-enhanced chemical vapor deposition (PECVD) of silica using tetramethoxysilane (TMOS) produced stoichiometric silica films. In addition, UV/ozone chemical vapor deposition (UVCVD) using tetraethylorthosilicate (TEOS) was also used to deposit silica species on the composite surfaces. The surfaces of PMR-15 and PMR-II-50 samples were modified by the following methods: grit blasting; grit blasting, ultra-violet/ozone (UV/ozone) etch, UVCVD of silic; and grit blasting, RF oxygen plasma etch, PECVD of silica. Adhesion pull tests were conducted and the fracture surfaces were examined using both optical and electron microscopy. PMR-15 samples with PECVD silica showed a significant increase in adhesion when compared with both UVCVD silica and grit blasting only. PMR-II-50 showed no significant difference in the adhesion strength for any surface modification method. Electron microprobe analysis of the fracture surfaces showed no evidence that silica was present on surfaces treated by UVCVD. Analyses also showed that the silica bonded more strongly to both the PMR-15 and PMR-II-50 neat resins than to the graphite fibers. The poor surface quality of PMR-II-50 composites is believed to have a major effect on bond coat adhesion strength.

Mineralogy of Magnetic Soils at a UXO Remediation Site in Kaho'olawe Hawaii

Magnetic characteristics of soils can have a profound influence on electromagnetic sensors for the detection of unexploded ordnance (UXO) and may cause false alarms in the case of spatially variable concentrations. In particular, the performance of several electromagnetic sensors is hampered by viscous remanent magnetism, which is caused by the presence of ferrimagnetic iron oxide minerals of different sizes and shapes. Tropical soils formed on basaltic substrates commonly have large concentrations of iron oxide minerals. To improve detection and discrimination of UXO in these soils it is crucial to have a better understanding of the types of minerals responsible for the magnetic behavior, as well as their distribution in space. In this paper we present the results of recent field and laboratory studies of soil magnetic properties and soil mineralogy at the former Naval training range on Kaho’olawe Island, Hawaii. We discuss the role of environmental controls such as parent material, age and precipitation on the magnetic properties.

Surface Modification of Polyimide Composites by RF Plasma and UV/Ozone Treatments

Graphite-reinforced polyimide composites are being considered for advanced turbine engine applications, which require that the composite be coated with a wear- and oxidation-resistant coating. For this study, the surface properties of PMR-15 and PMR-II-50 composites were examined and modification techniques investigated to improve coating adhesion. RF plasma and UV/ozone treatments were used to modify the surface chemistry of the polyimide composites, and were optimized to increase the surface energy of the polyimide composites, by etching and deposition of SiO x films. Chemical and physical changes of the composite surface were characterized by contact angle analysis and Fourier transform infrared-attenuated total reflectance spectroscopy (FTIR-ATR). Both RF plasma and UV/ozone treatments effectively removed surface contaminants and modified surface chemistry as indicated by increased surface energy. PMR-II-50 and PMR-15 composites responded differently to argon plasma etching and UV/ozone etching because of their structures. Films deposited by plasma-enhanced chemical vapor deposition (PECVD), using tetramethoxysilane, were stoichiometrically closer to SiO2 than the films deposited by PECVD using tetraethoxysilane.

Optimization of anodic bonding to MEMS with self-assembled monolayer (SAM) coatings

This work describes full wafer encapsulation of released, self-assembled monolayer (SAM) coated, multi-level polysilicon surface micromachines using the anodic bonding technique. This process has been utilized to protect fragile surface micromachines from damage due to particles, moisture contamination, and post-release die handling. The anodic bonding process was optimized to ensure strong glass-to-wafer bonds, while maintaining the effectiveness of liquid-phase and vapor-phase deposited SAM coatings. The temperature, time, and voltage effects on each SAM coating was analyzed. Glass-to-silicon and glass-to-SAM coated silicon had shear strengths of approximately 18 MPa. Glass-to-polysilicon bonds had lower shear strengths of approximately 10 MPa. Bonds were hermetic to 5 X 10-8 atm-cm3/s.

Elastic Recoil Detection of Depletion Layer Formation During Anodic Bonding

Conventional elastic recoil detection (ERD) techniques have been employed in an attempt to elucidate the effects time, temperature and alkali ion content have on depletion layer formation during anodic bonding. Hydrogen and/or lithium ion concentration profiles were evaluated for both untreated and lithium‐treated sodium borosilicate glass. From in situ ERD, depletion layer formation is highly dependent on temperature and alkali ion content. Lithium‐treated sodium borosilicate glass improves depletion layer formation at low temperatures and at high temperatures increased ion mobility results in rapid depletion layer formation.