Lloyd Brown

Coefficient of Friction Measurement in the Presence of High Current Density

On the microscale, armature-rail interface contact is accomplished through surface asperity interaction. In this imperfect contact model (a version of the Bowden-Tabor model), we postulate the coexistence of one or more contact regimes, such as solid-solid contact, liquid-metal lubricated contact, and arcing contact. We are developing microscopic models and experimental apparatus to study this postulate. This work, in particular, describes the experimental apparatus developed to assist in the investigation of possible contact regimes and presents preliminary data that indicate a possible decrease in coefficient of friction in the presence of high current density when compared to a no-current condition

Testing and Evaluation of Metal Fiber Brush Operation on Slip Rings and Commutators

A research apparatus was designed for use in an investigation to compare the current conduction and wear behavior of metal fiber brushes on commutators and slip rings. To this end, metal fiber brushes were operated, while conducting direct current, on gold-plated copper rotors with and without unfilled gaps to simulate slip rings and commutators, respectively. The results of an investigation to determine the wear mechanism of metal fiber brushes on commutators and slip rings are presented. Metal fiber brushes were operated, while conducting direct current, on gold-plated copper rotors with and without unfilled gaps to simulate slip rings and commutators. Wear rates on unfilled-groove commutators were found to be only modestly higher than on slip ring rotors. Three possible causes for enhanced metal fiber brush wear on commutators were considered: (1) accelerated ldquoadhesiverdquo wear controlled by contact spots, (2) fatigue induced wear, and (3) ldquofiber choppingrdquo. All observations on wear debris and brush fibers are consistent with ldquoadhesiverdquo fiber brush wear on commutators as well as on slip rings; due to interlocking contact spots. Little to no evidence was found to support the other two hypotheses of fatigue induced wear and fiber chopping. The same conclusion also follows from comparing commutator wear rates with that on slip rings. A geometrical factor to account for the local elevation of brush pressure due to commutator grooves not supporting brush fiber tips was determined. This factor, of approximately 1.2, brought the wear data for commutators and slip rings into coincidence.

Physical and Mechanical Characterization of a Nanocarbon Infused Aluminum-Matrix Composite

A new class of nanocarbon-infused materials, termed covetics, has been developed by Third Millennium Materials, LLC. In this paper we have evaluated the enhanced performance prospects for strength and electrical conductivity of a nanocarbon infused 6061 aluminum composite by comparing properties to those of 6061 aluminum. Unlike most metal matrix carbon composites, this material is unique in that the nanocarbon is so strongly bound to the metal that it is stable in the molten state. The proprietary manufacturing process is still in the early stages of development, and we had an opportunity to evaluate a limited amount of extruded 6061 aluminum covetic sample material. This paper examines the effects of covetic processing on the physical, electrical, and mechanical properties of 6061 aluminum using chemical, optical, and scanning electron microscopy, density measurement, microindentation hardness testing, electrical conductivity measurement, quasi-static tensile testing, and high strain-rate compression (Hopkinson bar) testing. In the as-extruded condition (warm worked at 227°C) the results show that the nanocarbon provides approximately a 30 % improvement in yield strength compared to baseline 6061-T0. This could be explained using electron microscopy observations which showed that the covetic 6061 was more resistant to grain growth and coarsening during extrusion. High strain rate, Split Hopkinson Pressure Bar (SHPB) tests revealed an opposite trend-the as-extruded covetic material exhibited lower stresses at equivalent strains. However, 6061 aluminum is not normally processed in the low strength as-extruded condition, so the covetic material was heat treated to the T6 condition. In the T6 condition, the strength and ductility of 6061 with and without 3 wt. % nanocarbon were approximately equal at all strain rates. Whereas the nanoscale carbon increased the electrical conductivity of 6061 by 43 % in the as-extruded condition, the conductivity only improved 15 %–19 % in the T6 condition. The nanocarbon/aluminum composite displays potential as an improved strength aluminum alloy with much higher electrical conductivity than is typical for other aluminum alloys and aluminum matrix composites. This study identified a clear need for standards development for the chemical analysis of nanocarbon in covetic materials.

Measurement of High-Strain-Rate Strength of a Metal-Matrix Composite Conductor

Castings of metal matrix composites are of potential interest as high strength, high wear resistance conductors. This paper examines the high-strain-rate strength of a tungsten-carbide (WC) filled aluminum bronze alloy (C95400) selected for its good combination of good electrical and thermal conductivity and high mechanical strength, toughness, and wear resistance. A functionally graded material with high wear resistance at the surface was fabricated by centrifugal casting which uses a rotating mold to deposit the high density WC particles at the outer surface while retaining the bulk electrical and thermal conductivity of the bronze alloy for conducting applications.

Strain Rate Dependency of Bronze Metal Matrix Composite Mechanical Properties as a Function of Casting Technique

Strain rate dependency of mechanical properties of tungsten carbide (WC)-filled bronze castings fabricated by centrifugal and sedimentation-casting techniques are examined, in this study. Both casting techniques are an attempt to produce a functionally graded material with high wear resistance at a chosen surface. Potential applications of such materials include shaft bushings, electrical contact surfaces, and brake rotors. Knowledge of strain rate-dependent mechanical properties is recommended for predicting component response due to dynamic loading or impact events. A brief overview of the casting techniques for the materials considered in this study is followed by an explanation of the test matrix and testing techniques. Hardness testing, density measurement, and determination of the volume fraction of WC particles are performed throughout the castings using both image analysis and optical microscopy. The effects of particle filling on mechanical properties are first evaluated through a microhardness survey of the castings. The volume fraction of WC particles is validated using a thorough density survey and a rule-of-mixtures model. Split Hopkinson Pressure Bar (SHPB) testing of various volume fraction specimens is conducted to determine strain dependence of mechanical properties and to compare the process-property relationships between the two casting techniques. The baseline performances of C95400 bronze are provided for comparison. The results show that the addition of WC particles improves microhardness significantly for the centrifugally cast specimens, and, to a lesser extent, in the sedimentation-cast specimens, largely because the WC particles are more concentrated as a result of the centrifugal-casting process. Both metal matrix composites (MMCs) demonstrate strain rate dependency, with sedimentation casting having a greater, but variable, effects on material response. This difference is attributed to legacy effects from the casting process, namely, porosity and localized WC particle grouping.

Mesoscale Contact Characteristics under Current Transfer

Sliding metal-on-metal wear experiments were conducted at the meso-scale within a controlled environment chamber to evaluate the influence of current density, polarity, and humidified gas on system wear behavior. A meso-scale friction tester (MFT) was modified by enclosing the apparatus with an environmental chamber used for introducing humidified gases. Additionally, a high capacity direct current precision power supply was used to supply current through the contact interface of the test materials. Wear rates were observed to decrease with increasing levels of humidity. This observation is in agreement with previous experiments of this nature, but had not been conducted within this scale of measurement. Scanning electron microscopy (SEM) analysis of the wear areas and energy dispersive spectrometer (EDS) analyses suggest the possibility of micro arcing and melting during the evolution of the wear track. The static friction forces and the periods of stick slip were found to correlate with the grain size distribution. Experimental measurement of weld force and blow off force was found to correlate poorly with the theoretically predicted values, and could be attributed to scaling effects not included in the models.

Functionally Graded Bronze/Tungsten-Carbide Castings: A Characterization and Property Study

The contact resistance and wear behavior of electrical contact surfaces is a function of hardness, applied load, and material constituents. This work presents a characterization of the mechanical and physical properties of conducting alloys fabricated as functionally graded metal matrix composites (MMCs), in particular, tungsten-carbide-filled bronze. Tungsten-carbide reinforcing particles are attractive in this application for their high hardness and concomitant wear resistance. When used as bushings, bearings, and sleeve materials, bronze has improved wear performance with the addition of tungsten-carbide particles, which improves the hardness of the contact surface. Bronze might also be attractive as a potential electrical conductor when a need exists for high-strength, wear-resistant contact surfaces as found in circuit breakers and sliding electrical contacts. An overview of two types of MMC production methods, sedimentation and centrifugal casting, is presented. MMCs fabricated using the two different methods are compared and contrasted based on physical and mechanical properties. The use of centrifugal casting provides a more effective improvement in physical and mechanical properties plus significant improvement in hardness with relatively low reduction in base-material conductivity was observed. The centrifugal casting method allows for a more tailored product in terms of locating enhanced material properties within the casting.