Tantalum and Niobium Powders for Cold Spray Applications

Dr. Matthew Osborne, R&D Manager of Metallurgical Technology

Abraham Calvin, Sr. R&D Engineer

Cold Spray Technology

Cold spray is a variation of thermal spray technology where fine metal powders are carried by a

supersonic gas stream to strike a substrate and form a conformal coating. Unlike thermal spray,

cold spray particles impact the substrate as solids rather than liquids, and bond due to solid flow

and interlocking of the particles with each other, and with the substrate. Layers can be built to

almost any thickness and the varieties of materials and applications are numerous. The unique

bonding interface and lower temperature interaction is opening the door to new material

systems and opportunities. In many applications, heating from weld repair or traditional thermal

spray can be detrimental to the substrate, giving cold spray an advantage as an in situ build or

repair method. Many excellent references such as Practical Cold Spray[1] can provide overviews

and detailed studies of cold spray. Figure 1 shows a general schematic of the cold spray

equipment and process.

Fig. 1 General cold spray schematic [2].

Cold spray of metals onto surfaces is finding many advantages. As mentioned above, cold spray

enables the ability to perform repairs with minimal thermal impact, eliminating the need for postrepair heat treatment. Eliminating thermal treatments permits point-of-use repair, making cold spray extremely attractive in applications such as keeping military vehicles in service[3, 4]. As many legacy military programs face the challenge of component obsolescence, it is becoming

imperative to sustain critical component inventories; however, cold spray has the potential to

reduce the need to replace components with new, impacting obsolescence[5]. Cold spray is

demonstrating the unique ability to combine materials with different properties (e.g., yield

strength, melting point, corrosion resistance) to form graded layers[6] or a blended mixture such

as a hard-phase / soft-phase composite for the replacement of electroplated hard coatings[7]. Cold spray processing and materials have been developed for aerospace[8], a large number of military applications[9, 10], and to address extreme environments in energy generation[11, 12, 13]. As cold spray becomes a standard repair and build technology, the number and variety of applications and materials will increase.

Applications for Tantalum and Niobium

Tantalum (Ta) and Niobium (Nb) are well known for their corrosion resistance and refractory

properties. They are commonly used unalloyed in electronics, semiconductor manufacturing,

medical, and specialty chemical processing applications where superior properties are required.

Tantalum has a long history in medical applications[14] and its use continues growing due to its low toxicity[15, 16], osteointegration capability[17], and radiopacity[18]. In military applications, tantalum is used in durable electronics capacitors, munitions19, and has also been tested to extend the life of gun barrels[20]. Tantalum fills important roles in defense and is considered a critical strategic material[21].

A review of literature shows continued testing of cold sprayed Ta and Nb in critical military[22] and energy applications[23]. Due to the nature of these metals, they are easy to forge and roll at low temperature using traditional metal forming methods. This workability has translated to ideal

cold spray deposition performance, achieving near fully dense materials with strong, cohesive

layers with superior mechanical and corrosion resistant properties[22, 24, 25]. Forming surface

coatings via the cold spray process captures the benefit from tantalum’s and niobium’s corrosion

and thermal resistant properties while maintaining the strength, lower weight, and lesser costs

of the underlying substrates. For example, an iron-based or nickel-based plate or pipe can be

coated with Ta to allow more economical use in acid processing. Many applications for Ta and

Nb are known and the general expansion of cold spray will assist in growing the use of these

metals and their alloys.

Powder Requirements for Cold Spray

Cold spray is a straightforward process; particles impact a surface at velocities high enough to

deform and flatten the particles, expose nascent surfaces for bonding, and fill the surface

topography to prevent porosity. A number of operational and material parameters influence this

process, and need to be carefully controlled to repeatedly create strong, dense, and continuous

coatings with high deposition efficiency (DE = the amount of sprayed powder that is incorporated to the deposit; ~yield). The key powder characteristics for unalloyed metals include: 1. powder flowability, 2. particle diameter and shape, 3. particle microstructure and porosity, 4. gas impurities, and 5. the powder surface.

1. Powder flow is important for controlled feed into and through the cold spray system,

though the standard flow measurement methods are not perfect indicators of usability[26].

For a given material, powder flowability is a function of particle size distribution (PSD),

shape, and surface. Narrower size distributions flow more readily[27]; equiaxed shapes like

spheres and cubes flow easier than irregular particles[28, 29]; and rough, dirty, or damp

surfaces increase interparticle friction and inhibit motion[26, 30]. Superior cold spray

powders will have consistent PSD and particle shapes, low and controlled gas contents,

and clean, dry surfaces.

2. Particle size and shape also influence particle acceleration in the cold spray gas stream

and the impact energy at the substrate. For a given particle diameter (diameter is used,

even for nonspherical particles) there is a critical velocity[31] resulting in impact which

deforms the particle sufficiently to create fresh metal surface and fill voids, while not

being excessive and causing substrate damage. It is obvious that a very wide size

distribution will result in particles moving both faster and slower than the optimal spray

parameters resulting in less desirable deposit density and surface conditions. Particle

shape provides a similar effect with more and less aerodynamic particles having different

critical velocities (and deposition efficiencies) and causing similar variations in deposit

quality. Maintaining consistent size and shape of the particles is critical for maintaining

optimized parameters and material characteristics.

3. Particle microstructure and porosity have a measurable effect on the deposition

efficiency of cold sprayed materials. The presence of inter-particle pores can lead to lower

interfacial mixing and therefore weakening the inter-layer bonds as well as between the

deposit and substrate. The microstructure and porosity of the powder translate directly

to the deposit material and can directly influence the mechanical performance of the bulk

material[32].

4. Higher gas impurities increase the metal’s yield strength and thereby increase the critical

velocity[33, 34]. It is preferred to keep oxygen levels below 300 ppm, though the overall

measurement is a combination of internal, dissolved oxygen and the inherent surface

oxide layer. Finer powders, with a higher surface-to-volume ratio, will naturally have

higher oxygen content overall. The control of oxygen in these finer particle applications

becomes an equilibrium challenge between the particle size desired and the minimum

surface oxygen present.

5. The surface of a particle influences its flowability (see above), the deposition efficiency,

and coating properties[26, 30]. Particle surfaces with excess moisture, hydroxides and other contaminants, and extreme oxide layers will cause variation in flow through the system, lower DE, and poor coating strength due to less particle deformation and the

incorporation of contaminants, oxides, etc. in the coating. Powders for cold spray should

be processed, handled and stored to prevent contamination and surface reactions; where

possible they should also be dried and packaged under an inert atmosphere for optimal

quality and shelf life. Tantalum and niobium surface oxides are stable at room

temperature, however, additional care and preventative measures are recommended for

fine (<25 micron) and ultra-fine (<10 micron) particle sizes due to their high surface-tovolume ratio.

Global Advanced Metals Ta and Nb Powders

Global Advanced Metals (GAM) has a well-known history of producing high-quality Ta and Nb

products including flake and agglomerated powders for capacitors, and angular met-grade

powder as feed for mill products. GAM has applied the knowledge and capabilities used to

develop powders for the semiconductor and capacitor industries now to produce powders with

optimized and controlled particle size distributions and chemistry for cold spray applications. The micrographs in Figure 2 show examples of angular tantalum, tantalum-alloy, and niobium

powders developed for cold spray applications.

Figure 2. GAM angular powders designed for cold spray. Clockwise from top-left: 45-125µm

Ta, 10-25µm Ta, 10-25µm Ta-2.5W, and 15-40µm Nb.

Early cold spray development testing of GAM tantalum powders have demonstrated consistent

dense coatings with good bonding and low porosity. The powders used for these tests were

larger, spherical particles (A), larger angular particles (B), and smaller angular particles (C). Table

1 shows the powder attributes and Figure 3 shows the feed powders. Cold spray deposition was

performed by a collaborator to produce flat coupons, tensile bars, as well as coatings on interior

and exterior curved surfaces. Figure 4 shows macro photos of various part geometries. With only

preliminary parameters established, good bonding and low porosity were achieved as shown in

the SEM photos and EBSD images in Figure 5.

Table 1. Cold spray Ta powder attributes

1 Does not include gases.

2 N/A indicates powder did not flow through Hall funnel

Figure 3. Micrographs of powders A, B, and C used in cold spray testing (left to right).

Figure 4. Macro photos of Ta cold spray samples (dimensions are approximate), left to right:

50x50 mm flat coupon, 50 mm radius outer curve, 50 mm radius inner curve, 30mm diameter

cylinder.

Figure 5. SEM and matching EBSD images for (left to right): Powder A coupon section, Powder B

section, Powder C section. Growth direction is vertical in all cases.

Summary

As previous and emerging research have shown, careful control of powder properties is critical

for success in cold spray applications. Early testing has demonstrated that the positive results

seen in tantalum and niobium cold spray literature holds true for the high-quality powders

produced by GAM. GAM has extended well-established processing techniques commonly used

in the manufacturing of capacitor-grade and metallurgical-grade powders to the tantalum,

tantalum alloy, and niobium cold spray powders thus providing superior performance in the form

of consistent and controlled particle size distributions, excellent flow, and industry leading

chemistry. Further material development and cold spray testing have been performed and

additional results will be shared in future presentations and publications. GAM continues to

invest in the development and production of powders designed for cold spray and additive

manufacturing and thus will be well positioned to support the anticipated growth in this industry.

References

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2 S. Kuroda, 28 May 2021. [Online]. Available: File:Cold spray schematics.png. (2021, May 28). Wikimedia Commons, the free media repository. Retrieved Dec 11, 2021 from https://commons.wikimedia.org/w/index.php? title=File:Cold_spray_schematics.png&oldid=565605238.. [Accessed 11 12 2021].

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34 K. Murray, M. Kearns and A. Coleman. [Online]. Available: https://www.metalpowder.sandvik/cn/SysSiteAssets/metal-powder/products-applications/cold-spray/design-and-production-of-powders-tailored-to-cold-spraying.pdf. [Accessed 13 11 2012].

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Powder requirements for cold spray applications FINAL.pdf

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