Electric arcs and electrochemical etching can be used to impart microscopic textures to surfaces of metals. In electric-arc texturing, one strikes an arc between a water-cooled electrode and a substrate that one seeks to texture. As the arc is moved across the substrate surface, a condensate from the plasma consisting of a mix of electrode and substrate species becomes redeposited on the substrate, forming an extremely rough surface layer that is intimately mixed with, and attached to, the substrate. The thermal emittances of arc-textured surfaces are considerably greater than those of the corresponding untreated surfaces. Unlike high-thermal-emittance oxide coatings, arc-textured surfaces are integral with substrates and are thus less likely to spall because of differential thermal expansion over wide ranges of operating temperatures. Arc texturing shows promise for enhancing the performances of metal components that must radiate heat; for example, anodes of x-ray tubes used in computer-assisted tomography, or waste-heat radiators on spacecraft.

Figure 1. Total Hemispherical Emittances of specimens of several alloys were increased considerably by texturing with carbon arcs.

Experiments in arc texturing were performed using (1) substrates made of various elemental metals and metal alloys; (2) electrodes made of carbon or silicon carbide; and (3) various arc currents, both dc and ac. The carbon electrodes were operated in argon; the silicon carbide electrodes were operated in air. Initial tests showed that ac arcs yielded higher emittances; thereafter, all texturing was done with sinusoidal-waveform ac arcs at (1) currents from 14 to 20 A with frequencies from 100 to 1,000 Hz in the case of carbon electrodes and (2) currents of 15 A with a frequency of 60 Hz in the case of silicon carbide electrodes. Figure 1 shows the increases in total hemispherical emittances of various alloy substrates, both untreated and treated by carbon-arc texturing.

Figure 2. Etch Pits Separated by Unetched Surface are clearly visible in this optical micrograph of specimen of electrochemically textured Ti/6Al/4V.

In electrochemical texturing, a substrate is connected as the positive electrode in an electrochemical cell, and during operation of the cell, the electrolyte is agitated. The result of this process is a random distribution of electrochemically etched pits produced on the substrate surface. Experiments in electrochemical texturing were performed on substrate rods of the alloy Ti/6Al/4V (numbers indicate weight percentages, with balance of Ti), using an aqueous solution of sodium chloride as the electrolyte. The electrolyte was agitated by an ultrasonic-bath apparatus; the stainless-steel wall of the apparatus both contained the electrolyte and served as the negative electrode of the electrochemical cell. The agitation of the electrolyte assisted in the removal of oxides forming on the surfaces of the rods. In each experiment, only the end face of the rod was exposed, at a current density of 4.2 A/cm². The resulting erosion took the form of approximately hemispherical etch pits, with diameters of about 50 µm, randomly separated by unetched metal surface areas (see Figure 2).

The sizes and density of the etched pits could be tailored to favor the in-growth of bone on the stems of an orthopedic implant. Because there would be no buildup of material on surface areas between etch pits, the outside dimensions of the orthopedic- implant stems would be unaffected by the electrochemical texturing. Thus, use of electrochemical texturing would not make it necessary to alter the dimensions of orthopedic implants.

This work was done by Bruce A. Banks and Sharon K. Rutledge of Lewis Research Center and Scott A. Snyder of Ohio Aerospace Institute. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Materials category.Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Lewis Research Center, Commercial Technology Office, Attn: Tech Brief Patent Status, Mail Stop 7- 3, 21000 Brookpark Road, Cleveland, Ohio 44135.

Refer to LEW-16605.

NASA Tech Briefs Magazine

This article first appeared in the April, 1999 issue of NASA Tech Briefs Magazine.

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