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Refractory Coatings For Tungsten

Report Number: WADD TR 60-825
Author(s): C. G. Goetzel, P. Landler
Corporate Author: New York University, Research Division
Laboratory: Materials Central
Date of Publication: 1961-03
Pages: 51
Contract: AF 33(616)-6868
Project: 7350
Task: 73500
AD Number: AD0258574

A preliminary study of the feasibility of protecting tungsten wires against oxidation at 3300°F (1815°C) was undertaken. A tungsten disilicide coat was the most promising coating for this temperature. Several siliconizing methods were given a cursory treatment, including vapor plating and dipping into molten copper-silicon bath, and the simplest, cementation, was selected for closer investigation. This operation was performed under flowing hydrogen atmosphere and such variables as temperature, time, and composition of the cementation pack were studied as a main part of our exploratory work. A single brittle tungsten-disilicide (WSi2) layer is formed by this method. The inherent brittleness of tungsten appears further aggravated by the development of micro-cracks in the layer as well as in the substrate during siliconizing. Oxidation tests using direct electrical resistance eating were performed. It was found that pack-siliconized wires can withstand oxidation at 3300°F in still air for prolonged periods up to and including 10 hrs. This is accomplished without any destruction of the coating in the high temperature zone owing to the formation of a viscous glassfilm on the surface. A pronounced tendency toward self-healing is a useful property of this type of coat. A serious drawback of the disilicide coat, however, is its low temperature oxidation ("Disilicide pest") which occurs in parts exposed at 1200° and 2280°F for prolonged periods. It causes a destruction of the disilicide layer over a period of 5 to 15 hrs, depending on coating thickness. It was the actual life determining factor in those tests where other failures caused by improper technique had been eliminated. Preliminary attempts to deposit ZrO2 on top of siliconized wires by means of plasma arc spray method remained largely unsuccessful because of poor adhesion. Where bonding was obtained, no improvement in life under high temperature oxidizing condition could be ascertained.

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