Richard L. Landingham

Fine-grain tungsten by chemical vapor deposition☆

Abstract A method is described of obtaining fine-grain, non-columnar tungsten (W) by chemical vapor deposition (CVD). Typical CVD tungsten grows columnar grains perpendicular to a heated surface when deposited from its halides, generally WF or WCl 6 . The present method involves the partial nitriding of the tungsten to W 2 N. during deposition, and subsequent decomposition of the W 2 N. The presence of this second phase (W 2 N) alters the growth pattern of the depositing tungsten and eliminates columnar grain growth.

ULTRAFINE DISPERSION-STRENGTHENING OF TUNGSTEN BY CHEMICAL COVAPOR DEPOSITION.

A method of dispersion-strengthening tungsten, a refractory metal, with submicron (< 10 A) dispersoids of hafnium nitride, a stable ceramic, is described. Simultaneous deposition of the metal and the dispersoid is achieved by a chemical vapor deposition process called covapor deposition. The tungsten and hafnium are covapor deposited by hydrogen reduction of their halides (WCl6 and HfCl4); the dispersoid is formed during deposition by the reaction of hafnium with NH3 in the hydrogen gas. The microstructure of the tungsten matrix, either the large columnar grains typical of chemical vapor deposited tungsten or fine noncolumnar grains (< 1 μm), can be controlled by varying the deposition parameters. It is also possible by varying these parameters to change the number, size, and spacing of the dispersoids. The deposits were heat treated at 1700 °C for at least 1 h before hot microhardness values to 1400 °C were determined. These values were then compared with hot microhardness values determined after heat treatment for 96 and 500 h at 1700 °C. The microstructures of the deposits in these conditions were evaluated by thin film and scanning electron microscope techniques. The hot microhardness of the LRL dispersion-strengthened tungsten was superior to that of dispersion-strengthened tungstens prepared by other techniques. The possibilities of using this technique to improve the strength and creep resistance of other metals (e.g., nickel-chromium alloys for turbine blades) is also suggested. Parameters are given for the deposition of pure tungsten and hafnium nitride from their halides.