Refractory Metal Coatings
& Freestanding Structures

 

Through chemical vapor deposition, Ultramet fabricates refractory metal coatings and freestanding structures of tungsten, rhenium, tantalum, molybdenum, and niobium. Complex geometries are produced on removable mandrels, and the rapid deposition rate allows manufacture of components up to 0.25" thick. Refractory metal structures are also available in the form of highly porous, high specific stiffness open-cell foams.

 

Applications

  • Structures used at high temperatures that exceed the capability of conventional steels and superalloys and that require greater toughness and ductility than ceramics

  • Combustion chambers and hot-gas-path components (e.g. valves and tubing)

  • Textured high emittance coatings for significantly increased heat transfer away from or to the coated component, as needed

  • High reflectivity coatings

  • Thermocouple sheaths with a platinum-group surface coating for high corrosion resistance

  • Crucibles for crystal growing

  • Radiology identification components for human implant devices (e.g. catheter bands and pacemaker tags)

  • Bonding dissimilar metals or ceramics to metals in which welding or brazing is not possible

Thin- and thick-wall (<0.01–0.125") crucibles fabricated at Ultramet by chemical vapor deposition

Solid rhenium solar-thermal engines fabricated by Ultramet for NASA Marshall under the Shooting Star program. Engines are supported by 80% porous rhenium structural foam.Solid rhenium solar-thermal engines fabricated by Ultramet for NASA Marshall under the Shooting Star program. Engines are supported by 80% porous rhenium structural foam.Solid rhenium solar-thermal engines fabricated by Ultramet for NASA Marshall under the Shooting Star program. Engines are supported by 80% porous rhenium structural foam.\\\\\

 

Solid rhenium solar thermal engines fabricated by Ultramet for NASA Marshall under the Shooting Star program. Engines are supported by 80% porous rhenium structural foam

 

 

Properties of Selected Refractory Metals

All properties are approximate and are for polycrystalline, annealed material at 20ºC unless otherwise indicated.

*Room temperature to 1000ºC.

 

 

Rhenium

 

Rhenium coatings and freestanding structures enable parts to survive and outperform other materials in solid rocket motor applications at temperatures approaching 5756°F (3180°C).

 

Advantages

  • High melting point: 5756°F (3180°C)

  • No ductile-to-brittle transition temperature

  • Endothermic oxidation at high temperature

  • Resistance to water vapor

  • Zero erosion in solid propellant environments

  • Stable contact with carbon, molten alkali metals, oxides, and glasses

  • Ability to protect carbon, carbon/carbon, molybdenum, and tungsten alloys from oxidation, wear, and corrosion

  • Excellent wear resistance

 

 

High-Emittance Coatings

 

Ultramet produces textured high-emittance coatings that radiate heat at elevated temperatures and improve heat transfer. Although usually applied to hot objects that need to be kept cool, the coatings can also be applied to cool objects that need to be radiatively heated.

 

The emittance of Ultramet’s black rhenium coating is 0.82 at 1292ºF (700ºC) and increases to 1.00 at 3632ºF (2000ºC). Even after partial oxidation, high emittance is achieved.

 

Measured emittance of black rhenium coating in as-deposited state and after partial oxidation at 2372ºF (1300ºC)

 

By varying the chemical vapor deposition conditions for rhenium, Ultramet can form pyramidal dendrites, which result in a black appearance and yield high emittance. This high surface area black rhenium coating significantly increases heat transfer into or away from the coated surface.

 

 

Microstructure of black rhenium coating showing pyramidal dendrites formed by chemical vapor deposition

 

 

Black rhenium coatings are used extensively on the outside of chemical rocket engines to help keep the walls cool and minimize thermal soakback. Cool walls enable hotter gases to be used, which increases engine performance, and because the wall of the engine is kept cool, longer life is achieved.

 

For applications in oxidizing atmospheres, Ultramet’s hafnium carbide/silicon carbide coating provides high emittance and excellent oxidation protection. Ultramet developed the HfC/SiC coating system to protect graphite and carbon/carbon structures during reentry. Measured emittance of HfC/SiC varies from 0.8 to 0.9 at temperatures ranging from ambient to 3200ºF (1760ºC). At a constant heat flux, HfC/SiC keeps the surface several hundred degrees cooler than conventional carbide coatings.

 

Emissivity data generated during arcjet testing at NASA Johnson for a carbon/silicon carbide composite coated with Ultramet’s layered hafnium carbide/silicon carbide

 

 

Coatings with similar morphologies can be generated by chemical vapor deposition of tungsten (melting point 6165ºF [3407ºC]), but because tungsten dendrites are finer, the emissivity is slightly reduced.

 

 

Platinum Group Metals

 

Ultramet’s 100% dense coatings of the platinum group metals protect underlying materials from highly oxidizing and corrosive environments to temperatures approaching the melting point of the coating (e.g. 4430°F [2443°C] for iridium). These ductile coatings can provide tens of hours of useful operation in the demanding high temperature, high stress liquid rocket motor environment.

 

Advantages

  • Extremely low diffusion coefficients for interstitial elements

  • Ability to deposit on porous substrates to provide highly active catalysis without being subject to thermal poisoning effects

  • Coating thickness range: 0.1–100 µm

Iridium coatings allow a 900°C increase in operating temperature capability over conventional silicide coatings in the harsh nitrogen tetroxide/monomethyl hydrazine combustion environment. Iridium has also been applied for high temperature oxidation protection of refractory metal thermocouple sheaths.

 

 

Thermocouple Sheaths

 

Ultramet manufactures refractory metal thermocouple sheaths for ultrahigh temperature applications in inert and oxidizing environments. Crucibles and tubing can also be fabricated from all five freestanding refractory metals.

 

Properties of Materials Used for Thermocouple Sheaths

 

 

Microtubing

 

Ultramet manufactures seamless refractory metal microtubing for radiopaque marker bands used in medical devices, ring electrodes for electronics, high temperature and corrosive tubing applications, and capillary devices. Tubing is available in sizes as large as 12" diameter and 6' long.

 

Advantages

  • Stringent dimensional and material tolerances

  • Lot-to-lot consistency

  • Quick lead times

  • Small batch quantities to assist product development and prototyping

 

Typical Specifications of Ultramet Microtubing

 

Materials Tungsten/rhenium, tantalum, rhenium, tungsten, stainless steel, niobium, molybdenum
Inside diameter 0.005–0.25" (tolerance ±0.0005")
Wall thickness 0.001–0.060" (tolerance ±0.0005")
Length 0.020" to as required (tolerance ±0.003")

 

 

Rhenium microtubing and tungsten tags manufactured by Ultramet

 

 

Flange Attachments

 

Through chemical vapor deposition, Ultramet can apply flanges to materials that cannot otherwise be welded, brazed, or mechanically attached. The following photographs show a brittle tungsten component onto which ductile niobium flanges were vapor deposited, which allowed attachment using conventional compression fittings. Similarly, flanges can be deposited onto ceramic components to allow welding, brazing, and other conventional attachment methods.

 

 

Left, weldable/brazable attachment of dissimilar materials; right, tungsten heat exchanger component with niobium flange attachment fabricated by chemical vapor deposition at each end

 

 

Tantalum Diffusion Coatings

 

Ultramet diffuses highly corrosion-resistant tantalum into the surface of conventional stainless steels and nickel-based superalloys to dramatically improve resistance to acid corrosion, particularly nitric acid. Applications include valve components and piping used for the transfer of acids and other corrosive liquids.

SEM image (polished cross-section) of 2-µm tantalum diffusion coating applied to a nickel alloy substrate showing excellent conformity of coating to surface finish and minimal porosity (4000×)


Benefits of Tantalum Metal

  • Provides extraordinary corrosion resistance because a tough impermeable oxide film forms on the surface when the metal is exposed to normal atmospheric conditions
  • Exhibits a uniform grain structure
  • Is impervious to chemical attack below 302°F (150°C)
  • Is vulnerable only to hydrofluoric acid, acidic solutions containing the fluoride ion, and free sulfur trioxide
  • Melts at 5425°F (2996°C)
  • Exhibits high strength and ductility
  • Demonstrates corrosion resistance comparable with that of glass

 

Advantages of Diffusion Coatings

  • Thin (1–3 µm) surface layer that grades from pure tantalum at surface to a mixture of tantalum and substrate elements

  • High bond strength, because tantalum is diffused into the substrate surface and chemically mixes with substrate elements

  • Precise replication of intricate substrate features that requires no postprocess machining or polishing

  • Little or no change to the treated substrate physical dimensions

  • Original surface roughness maintained

  • High corrosion resistance

  • Application to various steels, chromium-coated materials, and nickel-based superalloys

  • Ability to combine effectively with a conventional chromium coating to substantially increase the lifetime of the chromium coating

  • Application to net-shape products as a final production step

The diffusion coating process itself requires no costly equipment, is readily scalable to large batches, is rapid (deposition times of <1 hour), and is economical because little tantalum is required. Ultramet developed this process under NASA funding to protect space shuttle propellant valves.

 

The tantalum diffusion coating significantly enhances the corrosion resistance and lifetime of chromium-coated components. Tantalum fills the microcracks that inevitably exist in chromium coatings and prevents corrosive liquids from reaching the component surface.

 

SEM image (polished cross-section) of 1-µm tantalum diffusion layer applied over a conventional chromium coating. Tantalum diffuses into and fills microcracks, which exist in all chromium coatings, thereby increasing corrosion resistance and lifetime (top: 1000×; bottom: 4000×)