Chemical vapor deposition of thoriated tungsten protective cups Page: 23 of 45
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To correct this situation, the material was plated on a molybdenum mandrel. The
plated tungsten adhered intimately, and the cup's inside dimensions and surfaces conformed
exactly to the outside dimensions and surface finish of the mandrel. Utilization of the moly-
bdenum mandrel made it possible to completely eliminate the need for machining or finishing
the inside surfaces of the cup. Precisely machining the mandrel's outside dimensions to the
required dimensions for the inside of a cup, as illustrated in Figure 6, resulted in an as-
deposited cup which fulfilled all of the drawing requirements for inside dimensions. It is
believed that the cooling of the top surface of the molybdenum mandrel, compared to the
stainless steel, is eliminated or greatly minimized because of the much higher thermal
diffusivity of the molybdenum. This feature maintains all surfaces of the mandrel at a
relatively constant temperature.
After vapor deposition, the molybdenum mandrels were removed from the cups by
immersion in a boiling solution composed of equal parts by volume of HNO3, HCI, and
H20. This solution dissolved the molybdenum but did not attack the thoriated tungsten. The
as-deposited chemical milled cup is illustrated in Figure 7.
The cups in the as-fabricated chem-milled condition are extremely frangible. Their
grain structure is large and columnar as shown in Figure 8. The KHN (Knoop Hardness
Number) is in excess of 800, and the material is impossible to machine by conventional metal
removal methods. This extreme hardness is probably due to the induced stresses presumably
caused by distortion of the tungsten lattice from the much larger thoria molecules. Upon heat
treatment at 20000C, the stresses are relieved, the hardness drops to around 350 KHN, and
the material is readily machined or ground by conventional methods. The above heat treatment
recrystallizes the structure into a very fine, equiaxed grained material with 2000 to 3000 grains
per square millimeter as illustrated in Figure 9. Apparently the thoria particles are either
translucent or too small, possibly molecular, to be resolved metallographically. Material
reheated to 22000C for one hour showed no significant change in grain size or structure.
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Blay, J.S. Chemical vapor deposition of thoriated tungsten protective cups, report, January 1, 1968; Pittsburgh, Pennsylvania. (https://digital.library.unt.edu/ark:/67531/metadc1026564/m1/23/: accessed March 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.