W. C. Vassell

Atomic constraint in hydrogenated ‘‘diamond‐like’’ carbon

Carbon bonding environments (measured by nuclear magnetic resonance spectroscopy) and compressive stress in plasma‐deposited hydrogenated diamond‐like carbon (DLC) films have been examined systematically as a function of substrate bias voltage. These results are related in terms of random network theory to show that hard DLC formed in an intermediate voltage range (100–400 V) consists of small graphitic clusters linked in a random network which is stiffened by a high density of quaternary carbon.

Amorphous hydrogenated carbon films for tribological applications. I. Development of moisture insensitive films having reduced compressive stress

Abstract Although earlier investigations on the tribological behaviour of amcrphous hydrogenated carbon (AHC) films in sliding contact with steel showed encouraging results, four open issues were identified. They were: (a) dependence of friction and wear on humidity (i.e., the friction coefficient and the wear increased with humidity), (b) limitations on film thickness (i.e., films greater than 2 μm thick delaminated due to large compressive stress), (c) deposition of films on substrates other than silicon and (d) lubricant compatibility (i.e., formation of lubricant-derived antiwear films on AHC film surfaces). Steps were taken to address some of these open issues by incorporating silicon in AHC films. Friction and wear tests were conducted on AHC films containing various amounts of silicon. Incorporation of silicon in AHC films rendered the friction coefficients and the wear of a steel counterface insensitive to moisture. Silicon incorporation in AHC films also significantly reduced compressive stress. This allowed deposition of 10 μm thick films. These effects were achieved without any compromise with the friction coefficient and the film wear if the amount of silicon in the film was kept within a certain concentration range. In addition, silicon-containing AHC films were thermally more stable than silicon-free films. Experiments conducted with two lubricants resulted in significantly lower wear of the silicon-free AHC films than that obtained for unlubricated sliding. Similar friction coefficients were obtained for AHC film/steel and steel/steel combinations in lubricated sliding.

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.

Amorphous hydrogenated carbon films for tribological applications II. Films deposited on aluminium alloys and steel

Abstract This paper describes the methods for the deposition of AHC films on aluminium alloys (2024, 7075 and an additional Al-Si alloy) and AISI 4340 steel. Both unmodified and silicon modified AHC films were deposited. AHC films could be deposited on aluminium alloys without any interlayer. The deposition of AHC films on steel required an interlayer which could be aluminium, silicon or chromium. Thin films (1–2 μm) deposited on aluminium alloys and steel influenced durability of films and friction coefficients in contact with steel. These were believed to be due to plastic deformation of substrates. Deposition of a thicker coating system (interlayer + AHC) reduced friction coefficients and also improved film durability. The durability of films deposited on steel substrates was evaluated under both unlubricated and lubricated conditions for 5.5 million cycles under 4.4 N load and up to 2.5 m/s sliding speed. Although there was wear, the films survived 5.5 million test cycles under unlubricated sliding, but in the presence of two lubricants, the film wear was very small and could not be measured. It was observed that the wear of the steel counterface in contact with silicon-containing AHC films could be higher than that against an uncoated steel in the presence of certain lubricants.

Oxygen Sensing by Electrochemical Pumping

An oxygen pumping structure using ZrO2 electrochemical cells is described for measuring the percentage of O2 in a high‐temperature gaseous environment. The device output is linearly proportional to the O2 percentage, has a weak dependence on temperature, and does not require a reference atmosphere.

A high-sensitivity sensor for the measurement of combustible gas mixtures☆

Abstract An electrochemical oxygen pumping device consisting of two ZrO 2 cells is described. It is capable of measuring with high sensitivity combustible mixture concentrations over a wide composition range. The sensor output is approximately proportional to the concentration, has a weak dependence on temperature and is essentially independent of the gas pressure. This type of sensor can be used for monitoring and controlling combustion processes such as those encountered in furnaces and internal combustion engines.

Observation of electron standing waves in Mg by tunneling

Abstract Electron standing wave states in Mg films are observed by electron tunneling measurements. An electron group velocity is determined by this experiment.

Solid state NMR study of carbon bonding in amorphous hydrogenated carbon films

Carbon bonding environments in hydrogenated amorphous carbon films (a-C:H) deposited from an rf-biased methane plasma onto various substrates have been quantified by application of solid state13C NMR. A family of films were prepared by systematically varying the substrate bias voltage. Quantitative data on carbon chemistry in these films is required for modeling the impact of structure on mechanical and optical properties. A variety of NMR acquisition pulse sequences have been investigated to determine the conditions under which quantitative13C NMR data can be acquired in this system. The results indicate that data acquisition from this material requires different protocols than for the study of polymeric hydrocarbon films. With proper experimental design, NMR is an excellent technique for structural studies of these materials.

An electrochemical transistor using a solid electrolyte

Transistor action has been observed at elevated temperatures (800 °C) with a structure composed of two ceramic ZrO2 electrochemical cells separated by an enclosed volume. One cell emits O2 from an ambient atmosphere into the volume (base region) where it is collected and returned to the ambient by the opposing cell. Electrical operation can be understood in terms of the current limitation presented by the Pt cathode of the collector cell. Small signal voltage, current, and power amplification are observed with a frequency response limited by the double‐layer capacity of the collector. This approach to transistor action should be applicable to other solid‐electrolyte systems.

Absolute pressure measurements using ZrO2 electrochemical cells

An absolute pressure sensor is reported that uses electrochemical pumping of oxygen. Sensor operation uses the pressure dependence of O2 diffusion in a host gas. The device is unusual because no reference pressure or vacuum is required for absolute measurements. The main features of device operation are in agreement with calculation.