Timothy J. Potter

A study of gas chemistry during hot‐filament vapor deposition of diamond films using methane/hydrogen and acetylene/hydrogen gas mixtures

The composition of the reaction gases in a hot‐filament reactor for chemical vapor deposition of diamond films was analyzed using a gas chromatograph coupled with a quartz microprobe. Concentrations of several hydrocarbons were determined as functions of filament temperature (FT) and the position of the probe relative to the filament for two feed gases, methane/hydrogen and acetylene/hydrogen. The diamond growth rate was measured as a function of FT in both feed gases. The major chemical process in these reaction systems is found to be conversion between methane and acetylene with ethane and ethylene as reaction intermediates. For FT≤1800 °C, the chemical reactivity is low, and no diamond deposition is observed. For FT≥1900 °C, nearly identical chemical composition near the filament is obtained from both feed gases (indicating possible attainment of thermodynamic equilibrium in the gas mixtures), and the measured diamond growth rates are similar. A substantial depletion of carbon in the reaction gases nea...

Optical study of silicon-containing amorphous hydrogenated carbon

Silicon-containing amorphous hydrogenated carbon films deposited by a plasma-enhanced chemical vapor deposition process were studied using both Raman and ellipsometry spectroscopies. Analyses of the experimental data from both these techniques yielded valuable information about the microstructure of the films. The silicon incorporation in amorphous hydrogenated carbon breaks down large size sp2 carbon clusters and enhances sp3 bonding. The reduction of large sp2 graphitic defects, the enhancement of sp3 bonding, and the associated microstructure changes are responsible for the desired properties of silicon-containing amorphous hydrogenated carbon.

Spatially resolved ultrasonic attenuation in resistance spot welds: implications for nondestructive testing.

Spatial variation of ultrasonic attenuation and velocity has been measured in plane parallel specimens extracted from resistance spot welds. In a strong weld, attenuation is larger in the nugget than in the parent material, and the region of increased attenuation is surrounded by a ring of decreased attenuation. In the center of a stick weld, attenuation is even larger than in a strong weld, and the low-attenuation ring is absent. These spatial variations are interpreted in terms of differences in grain size and martensite formation. Measured frequency dependences indicate the presence of an additional attenuation mechanism besides grain scattering. The observed attenuations do not vary as commonly presumed with weld quality, suggesting that the common practice of using ultrasonic attenuation to indicate weld quality is not a reliable methodology.

Laser reflective interferometry for in situ monitoring of diamond film growth by chemical vapor deposition

A simple laser reflective interferometer has been employed for in situ monitoring of diamond film growth in a hot‐filament chemical vapor deposition reactor. This method uses a low power HeNe laser beam reflected at normal incident from the substrate. The high refractive index of the diamond film and the relatively high reflectivity of the Si substrate result in pronounced and easily detected interference oscillations in the reflected beam intensity. The oscillation period provides an accurate and immediate measure of the growth rate. In addition, the variations of the extrema of the oscillations provide an estimate of the quality and surface texture of the diamond films. Significant improvement in research productivity has been realized by using this technique.

Abrasion of Steel by Ceramic Coatings: Comparison of RF-DLC to Sputtered B4C

The abrasion rates of steel balls sliding against a very smooth diamond-like carbon (DLC) coating and a rough boron carbide (B4C) coating are compared. The initial abrasiveness of the B4C coating is about 2 orders of magnitude greater than that of the DLC coating. Both coatings exhibit a rapid decrease in their abrasiveness with sliding distance, but the details of the abrasion kinetics of these coatings are quite different. The abrasiveness of B4C falls according to a simple power law, while the abrasiveness of the DLC remains constant for a duration that depends on the load and then switches rather suddenly to zero. An explanation for this different behavior is proposed. During the abrasion process the asperities on the B4C are smoothed to a startling extent.

Comparison of Metallurgical and Ultrasonic Inspections of Galvanized Steel Resistance Spot Welds

Metallurgical examination of galvanized steel resistance spot welds was used to gauge the capabilities of two ultrasonic, non‐destructive, scanning techniques. One method utilized the amplitude of the echo from the weld faying surface, while the other used the spectral content of the echo train to map the fused area. The specimens were subsequently sectioned and etched, to distinguish the fused, zinc‐brazed, and non‐fused areas. The spectral maps better matched the metallurgical maps, while the interface‐amplitude method consistently overestimated the weld size.

Ultrasonic maps of porosity in aluminum castings

The use of cast aluminum in the automotive industry has grown dramatically in recent years, leading to increased need for quantitative characterization of microporosity. As previously reported in the literature, the attenuation of ultrasound can be used to measure the porosity volume fraction and the mean pore size. An immersion ultrasound system has been built utilizing this technique to scan castings with high spatial resolution. Maps of attenuation are shown to locate areas of varying porosity readily and reliably.

The Role of Heavy Hydrocarbons in Cvd Diamond Growth

A mass spectrometric analysis of heavy hydrocarbons (HHCs) during hot-filament CVD diamond growth was performed together with in situ monitoring of the growth rate. Many HHCs were detected and tentatively identified. Of all HHCs studied, only diacetylene shows good correlation with the diamond growth rate under various deposition conditions. Its possible role is discussed.